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The Sheahan Diamond Literature Reference Compilation - Scientific and Media Articles based on Major Keyword - Eclogite
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcements called the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Keyword Index
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
Each article reference in the SDLRC is tagged with one or more key words assigned by Pat Sheahan to highlight the main topics of the article. In an effort to make it easier for users to track down articles related to a specific topic, KRO has extracted these key words and developed a list of major key words presented in this Key Word Index to which individual key words used in the article reference have been assigned. In most of the individual Key Word Reports the references are in crhonological order, though in some such as Deposits the order is first by key word and then chronological. Only articles classified as "technical" (mainly scientific journal articles) and "media" (independent media articles) are included in the Key Word Index. References that were added in the most recent monthly update are highlighted in yellow.
Eclogite is a mafic metamorphic rock formed at pressures higher than found in the earth's crust and likely created when basaltic oceanic crust is subducted so that it underplates a craton. It is a primary source rock for diamonds that is distinct from the other main source rocks, harzburgite and lherzolite. "Ecologitic" garnets formed within eclogite have chemistry distinct from that of "pyrope" garnets formed within harzburgite and lherzolite (the G9-G10 series). Xenoliths of ecologite found within kimberlite have demonstrated a much higher diamond grade than xenoliths of harzburgite or lherzolite. Diamonds from harzburgitic or lherzolitic source rocks have an Archean formation age and a narrow carbon isotope range that excludes C13 which requires exposure to sunlight to form. Because ecologite is formed from subducted oceanic crust it will have derived some of its carbon from ocean floor sediments that include limestones, whereas the diamonds in harzgburgite would have had to scavenge primeval carbon from the peridotite host. "Ecologic" diamonds can be much younger than "peridotitic" diamonds, and, may be formed deeper because the crustal source rock was subducted underneath a craton. Because a kimberlitic magma starts its ascent even deeper, it is possible for the magma to entrain both ecologite and harzburgite/lhzerloite during its ascent. Kimberlites dominated by eclogitic chemistry tend to be found at craton margins which reflects the unlikelihood that subducting crust would end up underneath the thickest part of the craton which preserved peridotitic diamonds formed during the Archean period.
Garnet pyroxene equilibration temperatures in the system CaO MgO Al2O3 SiO2(CMAS)prospects for simplified T-independent lherzolite barometry and an eclogitebarometer
Contributions to Mineralogy and Petrology, Vol. 92, No. 4, pp. 448-455
Samarium-neodymium (Sm-Nd) systematics in eclogite and garnet peridotite nodules fromkimberlites: implications for the early differentiation of the earth #1
Proceedings of the Fourth International Kimberlite Conference, Held, No. 16, pp. 285-287
Oxygen isotopes in coexisting garnets, clinopyroxenes and phlogopites of Roberts Victor eclogites: implications for petrogenesis and mantleMetasomatism
Earth and Planetary Science Letters, Vol. 83, pp. 80-84
Eclogite facies ultramafic xenoliths from Colorado plateau diatremebreccias: comparison with eclogites in crustal environments, evaluation of thesubduction
Eclogites and eclogite-facies rocks, D.C. Smith ed., Elsevier, Dev. in, Chapter 7, pp. 387-450
Samarium-neodymium (Sm-Nd) systematics in eclogite and garnet peridotite nodules fromkimberlites: implications for the early differentiation of the earth #2
Geological Society of Australia Inc. Blackwell Scientific Publishing, No. 14, Vol. 2, pp. 864-876
Dating the cratonic lower crust by the ion microprobe SHRIMP: an U-Th-lead isotopic study on zircons from lower crustal xenoliths from kimberlite pipes
Proceedings of Fifth International Kimberlite Conference held Araxa June 1991, Servico Geologico do Brasil (CPRM) Special, pp. 45-48
Early to mid-Tertiary inverted metamorphic gradient under the ColoradoPlateau: evidence from eclogite xenoliths in ultramafic microbreccias, Navajo volcanic field
Journal of Geophysical Research, Vol. Paper # 91JB00284
Low temperature eclogites and eclogitic schists in Mn-rich metabasites in Ward Creek, California: Mn and iron effects on the transition blueschist andeclogites
Journal of Petrology, Vol. 32, No. 2, April pp. 275-302
Carbon stability in eclogites in the earth's mantle
Geological Association of Canada (GAC), Geological Association of Canada (GAC)/Mineralogical Association of Canada (MAC) Annual Meeting, Abstract, Abstract Vol. p. A61.
Eclogite facies relics and a multistage breakdown in metabasites of the KTB pilot hole, northeast Bavaria: implications for the Variscan tectonometamorphic evol
Contributions to Mineralogy and Petrology, Vol. 112, pp. 261-278
high pressure metamorphic rocks of a new type: the lherzolite-gabbro-granulite series in the base of the ultramafic section in the Voykar ophiolite allochthon Polar
Doklady Academy of Sciences USSR, Earth Science Section, Vol. 318, No. 6, pp. 148-153.
Russia, Commonwealth of Independent States (CIS), Polar Urals
Carboniferous and Triassic eclogites in the Western Dabie Mountains east central Chin a: evidence for protracted convergence of the North and South Chin a Blocks.
Journal of Metamorphic Geology, Vol. 20, 9, pp. 873-886.
Ultra high pressure metamorphism in western Tianshan, China: part I. Evidence from inclusions of coesite pseudomorphs in garnet and from quartz exsolution lamellae i
Quartz exsolution in clinopyroxene is not proof of ultra high pressures: evidence from phase equilibration temperatures and eclogite from the eastern Blue Ridge, southern Appalachians.
Geological Society of America Annual Meeting ABSTRACTS, Nov. 7-10, Paper 195-3, Vol. 36, 5, p. 453.
Subsidence and exhumation dynamics of eclogites in the Yukon-Tanana Terrane, Canadian Cordillera: petrological reconstructions and geodynamic modeling.
Immiscible transition from carbonate rich to silicate rich melts in the 3 GPa melting interval of eclogite + CO2 and genesis of silica undersaturated Oceanic lavas.
Journal of Petrology, Vol. 47, 4, April pp. 647-671.
Geochemistry of late Mesozoic adakites from the Sulu belt, China: magma genesis and implications for crustal recycling beneath continental collisional orogens.
Prograde pressure temperature path of jadeite bearing eclogites and associated high pressure low temperature rocks from western Tianshan, northwest China.
Hetenyl, G., Cattin, R., Brunet, F., Bollinger, L., Vergne, J., Nabalek, J.L., Diament, M.
Density distribution of the India plate beneath the Tibetan plateau: geophysical and petrological constraints on kinetics of lower crustal eclogitization
Earth and Planetary Science Letters, Vol. 264, 1-2, pp. 226-244.
Jianxin, Z., Jingsui, Y., Fabcong, M.,Yusheng, W., Huimin, Li., Cailai, W.
U Pb isotopic studies of eclogites and their host gneisses in the Xitishan area of the North Qaidam mountains, western China: new evidence HP-UHP belt.
Journal of Asian Earth Sciences, Vol. 28, 2-3, Nov. 15, pp. 143-150.
Schmadicke, E., Okrusch, M., Rupprecht-Gutpwski, P., Will, T.M.
Garnet pyroxenite, eclogite and alkremite xenoliths from the off-craton Gibeon kimberlite field, Namibia: a window into the upper mantle of Rehoboth Terrane.
High temperature, high pressure granulites ( retrogressed eclogites) in the central region of the Lewisian NW Scotland: crustal scale subduction in the Neoarchean.
Multiphase solid inclusions in zoisite bearing eclogite: evidence for partial melting of ultrahigh-pressure metamorphic rocks during continental collision.
Korikovsky, S., Kotov, A., Salnikova, E., Aranovich, L., Korpechkov, D., Yakovleva, S., Tolmacheva, E., Anisimova, I.
The age of the protolith of metamorphic rocks in the southeastern Lapland granulite belt, southern Kola Peninsula: correlation with the Belomorian mobile belt in the context of the problem of Archean eclogites.
Liu, Y-C., Deng, L-P., Gu, X-F., Groppo, C., Rolfo, F.
Application of Ti in zircon and Zr in rutile thermometers to constrain high temperature metamorphism in eclogites from the Dabie Orogen, central China.
Abstract: There is a growing body of evidence for the origin of cratonic mantle eclogite xenoliths by low-pressure formation in now-recycled ocean floors. Because they have protoliths ultimately derived from the convecting mantle, their study can potentially yield unprecedented insights into as yet little-understood palaeo-geodynamic regimes, once primary (fractional crystallisation, accumulation, mixing) and secondary processes (kimberlite infiltration, metasomatism) affecting their compositions are understood. This is achieved using diagnostic concentrations or ratios of the analytically and geologically most robust elements (major and minor elements, transition metals, REE), and aided by comparison to natural and modelled analogues. Here, mineral compositions taken from the literature were used to reconstruct bulk rocks and assign the samples to eclogites (further divided into high-Mg, low-Mg and high-Ca types), pyroxenites and their gabbroic (Eu* > 1.05) counterparts. Various protolith types - formed predominantly by < 1 GPa crystallisation from broadly picritic magmas leaving garnet-poor mantle sources - are identified: (1) Many high-Mg eclogites lie on modelled crystallisation trends between 0.5 and < 1 GPa. Some have elevated FeO contents with lower SiO2 and CaO possibly requiring Fe-rich pyroxenite heterogeneities in their mantle source. (2) Many high-Ca eclogites may be the differentiated (higher Na2O, TiO2 and FeO at lower MgO) equivalents of high-Mg eclogites, following modelled crystallisation trends at somewhat lower pressure (0.05 to 0.5 GPa). Other high-Ca eclogites with low FeO were produced during interaction with fluids and melts in mélange-type settings. (3) Low-Mg eclogites, with intermediate MgO content, are too FeO-rich to be intermediary crystallisation products of the same parental melt and are ascribed to melting out of Fe-rich lithologies possibly related to recycling of eclogite and/or contamination with ferromanganese sediments. (4) The positive Eu anomalies in gabbroic eclogites require accumulation of substantial amounts of plagioclase, consistent with their low FeO and TiO2 contents, but their simultaneously low MgO contents suggest that they interacted with residual melts. (5) The elevated CaO and low Al2O3 in pyroxenite may indicate clinopyroxene-rich high- or low-pressure cumulate protoliths, but high Cr2O3 and MgO, combined with low HREE and high LREE in many of these samples, suggests formation by hybridisation of eclogite-derived melt with peridotite.
Contributions to Mineralogy and Petrology, Vol. 171, pp. 101-
Technology
Eclogite
Abstract: Clinopyroxene is an essential mineral in eclogitic rocks. It commonly contains minor amounts of the defect-bearing Ca-Eskola (CaEs, Ca0.5?0.5AlSi2O6) component, with higher concentrations generally considered to indicate a high-pressure origin at least within the coesite stability field. Changes in pressure and temperature conditions can lead to exsolution of this component as a free SiO2 phase, which may have a number of petrological implications. This makes it important to understand the factors that maximize CaEs incorporation in clinopyroxene. We have undertaken a series of experiments at high pressures and temperatures (4-10 GPa and 1000-1350 °C) to further investigate the systematics of CaEs incorporation in eclogite-like clinopyroxene and the factors responsible for maximizing CaEs contents. Two simple chemical systems were chosen that allow unambiguous interpretation of the results: (1) CMAS + H2O and (2) two compositions in the NCMAS system. All experimental products contained clinopyroxene and garnet along with either a free SiO2 phase or a silicate melt. Coexisting garnet is grossular-rich, generally with Xgr ? 0.67. Compositional variations are attributable to the presence or absence of melt and changes in modal amounts of garnet at different pressure-temperature conditions. Even small amounts of H2O lower the solidus temperature and the presence of a melt reduces the SiO2 activity, which destabilizes the CaEs component in clinopyroxene. The CaEs and the Ca-Tschermaks (CaTs, CaAl2SiO6) components in clinopyroxene decrease with increasing jadeite mole fraction, which is also a function of pressure and bulk Al content. Modeling X-ray powder diffraction data yields a molar volume for the CaEs endmember of VCaEs = 60.87(63) cm3, which reasonably agrees with a literature value that was estimated from natural samples. In the presence of coexisting coesite, the CaEs and CaTs do not vary independently of each other, being controlled by the internal equilibrium 2CaEs = CaTs + 3SiO2 (coesite). This relation, observed in simple systems (i.e., CMAS ± Na), is also obeyed by clinopyroxene in more complex, natural analog bulk compositions. An assessment of available experimental data reveals a maximum of 15-18 mol% CaEs in eclogitic clinopyroxene at conditions corresponding to 130-180 km depth. CaEs contents are maximized at high temperatures; i.e., at or near the solidus in the presence of coesite. Thus, this study supports the role of CaEs exsolution in contributing to melt generation during upwelling of eclogite bodies in the mantle, albeit with some caveats. Somewhat higher maximum CaEs contents (~20 mol%) are found in Ca and Al-rich bulk compositions, such as grospydite xenoliths. Such bulk compositions also seem to require the coexistence of kyanite. Other Ca and Al-rich rock types, like rodingites, should have the potential of containing CaEs-rich clinopyroxenes, except that they are SiO2-undersaturated. This emphasizes the further role of bulk composition, in addition to high temperatures, in achieving maximum CaEs contents in high-pressure clinopyroxene.
Abstract: Evidence for chemical and lithological heterogeneity in the Earth’s convecting mantle is widely acknowledged, yet the major element signature imparted on mantle melts by this heterogeneity is still poorly resolved. In this study, a recent thermodynamic melting model is tested on a range of compositions that correspond to potential mantle lithologies (harzburgitic to pyroxenitic), to demonstrate its applicability over this compositional range, in particular for pyroxenite melting. Our results show that, despite the model’s calibration in peridotitic systems, it effectively reproduces experimental partial melt compositions for both Si-deficient and Si-excess pyroxenites. Importantly, the model accurately predicts the presence of a free silica phase at high pressures in Si-excess pyroxenites, indicating the activation of the pyroxene-garnet thermal divide. This thermal divide has a dominant control on solidus temperature, melt productivity and partial melt composition. The model is used to make new inferences on the link between mantle composition and melting behaviour. In silica-deficient and low-pressure (olivine-bearing) lithologies, melt composition is not very sensitive to source composition. Linearly varying the source composition between peridotite and basaltic pyroxenite, we find that the concentration of oxides in the melt tends to be buffered by the increased stability of more fusible phases, causing partial melts of even highly fertile lithologies to be similar to those of peridotite. An exception to this behaviour is FeO, which is elevated in partial melts of silica-deficient pyroxenite even if the bulk composition does not have a high FeO content relative to peridotite. Melt Al2O3 and MgO vary predominantly as a function of melting depth rather than bulk composition. We have applied the thermodynamic model to test the hypothesis that Fe-rich mantle melts such as ferropicrites are derived by partial melting of Si-deficient pyroxenite at elevated mantle potential temperatures. We show that the conspicuously high FeO in ferropicrites at a given MgO content does not require a high-Fe mantle source and is indeed best matched by model results involving around 0-20% melting of silica-deficient pyroxenite. A pyroxenite source lithology also accounts for the low CaO content of ferropicrites, whereas their characteristic low Al2O3 is a function of their high pressure of formation. Phanerozoic ferropicrites are exclusively located in continental flood basalt (CFB) provinces and this model of formation confirms that lithological heterogeneity (perhaps recycled oceanic crust) is present in CFB mantle sources.
Journal of Metamorphic Geology, In press available
Europe, Sweden
Eclogite
Abstract: Conditions of the prograde, peak-pressure and part of the decompressional P-T path of two Precambrian eclogites in the eastern Sveconorwegian orogen have been determined using the pseudosection approach. Cores of garnet from a Fe-Ti-rich eclogite sample record a first prograde and syn-deformational stage along a Barrovian geothermal gradient from ~670 °C and 7 kbar to 710 °C and 8.5 kbar. Garnet rims grew during further burial to 16.5-19 kbar at ~850-900 °C, along a steep dP/dT gradient. The pseudosection model of a kyanite-bearing eclogite sample of more magnesian bulk composition confirms the peak conditions. Matrix reequilibration associated with subsequent near-isothermal decompression and partial exhumation produced plagioclase-bearing symplectites replacing kyanite and clinopyroxene and is estimated at 850-870 °C and 10-11 kbar. The validity of the pseudosections is discussed in detail. It is shown that in pseudosection modelling the fractionation of FeO in accessory sulphides may cause a significant shift of field boundaries (here displaced by up to 1.5 kbar and 70 °C) and must not be neglected. Fast burial, exhumation and subsequent cooling are supported by the steepness of both the prograde and the decompressional P-T paths as well as the preservation of garnet growth zoning and the symplectitic reaction textures. These features are compatible with deep tectonic burial of the eclogite-bearing continental crust as part of the underthrusting plate (Eastern Segment, continent Baltica) in a collisional setting that led to an effectively doubled crustal thickness and subsequent exhumation of the eclogites through tectonic extrusion. Our results are in accordance with regional structural and petrologic relationships, which demonstrate foreland-vergent partial exhumation of the eclogite-bearing nappe along a basal thrust zone and support a major collisional stage at c. 1 Ga. We argue that the similarities between Sveconorwegian and Himalayan eclogite occurrences emphasize the modern style of Grenvillian-aged tectonics.
Abstract: Minerals recovered from the deep mantle provide a rare glimpse into deep Earth processes. We report the first discovery of ferric iron-rich majoritic garnet found as inclusions in a host garnet within an eclogite xenolith originating in the deep mantle. The composition of the host garnet indicates an ultrahigh-pressure metamorphic origin, probably at a depth of ~200 km. More importantly, the ferric iron-rich majoritic garnet inclusions show a much deeper origin, at least at a depth of 380 km. The majoritic nature of the inclusions is confirmed by mineral chemistry, x-ray diffraction, and Raman spectroscopy, and their depth of origin is constrained by a new experimental calibration. The unique relationship between the majoritic inclusions and their host garnet has important implications for mantle dynamics within the deep asthenosphere. The high ferric iron content of the inclusions provides insights into the oxidation state of the deep upper mantle.
Abstract: Mantle potential temperature (TP) and composition are crucial parameters that regulate terrestrial dynamics and geochemical cycles, ranging from controls on the peridotite solidus and consequent geochemical differentiation, to plate stiffness conducive to the operation of plate tectonics, and the recycling efficiency of volatiles in subduction zones. Earth’s mantle has been cooling, but there is little agreement on the rate at which this proceeded. It is also unclear whether the Archaean ambient mantle was similar to, or more or less depleted than that giving rise to modern MORB. Since the ambient convecting mantle is most reliably sampled at spreading ridges, ancient kimberlite-borne eclogite xenoliths with low-pressure oceanic crustal protoliths, together with orogenic eclogites and (meta)basalts from allochtonous greenstone belts, may be used to constrain some characteristics of the convecting mantle sources from which their protoliths were ultimately derived. Carefully screened eclogite suites up to 3 Ga in age have TiO2-REE relationships consistent with fractionation of olivine±plagioclase during formation of picritic protoliths from a melt that separated from a garnet-free peridotite source, implying intersection of the solidus at ?3.0 GPa. Low melt fractions (F<0.25), calculated from samples with the least fractionated protoliths using the batch melting equation, further argue against deep intersection of the mantle solidus. This is contingent on correctly identifying the mantle source (C0) as depleted, which is supported by depleted initial 176Hf/177Hf in 2.9 to 2.6 Ga orogenic eclogite suites. Inversion of melt fractions for temperature suggests moderately elevated TP of ~1420-1470º C, significantly lower than some estimates for the ambient convecting mantle at that time. If these results are accurate, the unusual degree of melt depletion experienced by cratonic lithospheric mantle (F = 0.3-0.5) underpinning Earth’s oldest continental cores requires formation at excess TP and/or from fertile mantle which sustains longer melting columns and higher melt productivity, either during plume-ridge interactions or plume subcretion. This matches increasing evidence that ancient continental crust formed by melting at the base of oceanic plateau-like enriched oceanic crust. A moderate Mesoarchaean TP also argues for early plate strengthening that would support plate tectonics and topography.
Abstract: Mantle-derived eclogite xenoliths are key for studying the evolution of the cratonic lithosphere, because geochemical evidence suggests that they typically represent fragments of Archean and Proterozoic oceanic lithosphere [1]. Recently, it has been suggested that eclogite xenoliths can serve as redox sensors of the Precambrian upper mantle using V/Sc as a redox proxy [2]. However, metasomatism can change the original oxidation state of the cratonic mantle [3], thereby limiting its use for monitoring mantle redox evolution. Circa 1.8–2.2 Ga eclogite xenoliths erupted with Jurassic kimberlites of the northern Slave craton have geochemical features that indicate oceanic crust protoliths [4, 5]. Such Paleoproterozoic ages are common for Slave craton mantle eclogites [6], linking eclogite formation with 1.9 Ga subduction-collision events at the western craton margin. The eclogites studied here have highly variable Fe3+/?Fe (0.019 – 0.076 ±0.01), with logfO2 (?FMQ-4 to +2 ±0.5) that are both relatively oxidized and reduced compared to Slave mantle peridotite xenoliths [3]. Also, eclogite fO2 positively correlates with some indicies of metasomatism, such as elevated TiO2 in garnet. In addition to considering the time gap between eclogite formation and kimberlite eruption, the highly variable fO2–depth systematics of the eclogites studied here illustrate the drawbacks of using averaged eclogite fO2 to define the redox evolution of the upper mantle. Despite this, the ca. 2 Ga northern Slave craton eclogites have an average depth-corrected logfO2 of ?FMQ-0.5±1.3 (1?) that overlaps with modern MORB, and complies with the upper mantle redox evolution trend predicted using V/Sc ratios of mantlederived melts [2]. However, given the debate around the secuarity of mantle redox [7], further research into the suitability of mantle eclogites as redox sensors is warranted.
Abstract: Eclogite xenoliths brought to the surface by kimberlites are high pressure mafic rocks whose origin (magmatic vs crustal) remains debated. In addition to disagreement on how to interpret eclogite compositions, mantle metasomatism overprints the mineralogy and geochemistry of some of these rocks, making the question of their protolith undoubtedly more complex. In this contribution we aim to test the robustness of multiple S-isotope signatures in highly metasomatized eclogitic sulfides. We selected 12 interstitial sulfides from Mir (n=4) and Udachnaya (n=8) eclogites, intergrown with garnet and omphacite. We analysed their lead (including Pb204) and S-isotope (32S, 33S, 34S and 36S) compositions, insitu, using a Cameca ims 1280. The samples consist of complex assemblages of pyrrhotite pentlandite intergrowth with K- and Cl-rich sulfides (djerfisherite) invaded by veinlets of alteration minerals (mainly chlorite). All our samples display internal zoning in Pb concentration (118 ppm to 4.2 wt%) but are homogeneous in isotopic compositions (e.g. 208Pb/204Pb = 38.09 ± 0.35‰). Pb-Pb ages of eclogitic sulfides are modern and undoubtedly reflect the metasomatic overprint by a Cl- and K-rich kimberlitic melt (consistent with the presence of djerfisherite). Sulfur isotope signatures of these sulfide (G34S = -1.3‰ ±2‰) fall within the canonical mantle range and cannot be distinguished from the composition of sulfides in the kimberlite (-1.4 ±2.2‰, Kitayama et al., 2016). Furthermore, Mir and Udachanaya eclogitic sulfides carry the largest mass independant fractionation (MIF) ever reported in mantle rocks. The overall trend reveals negative ?33S (down to - 1.1‰) associated to positive ?36S (up to 3‰). This observed correlation between ?33S and ?36S is consistent with the composition of sulfate aerosols formed in the Archean by photolysis reactions and likely dissolved in the ocean [4]. Our results indicate that multiple sulfur isotopes survive intense metasomatism (because isotope fractionation does not create S-MIF), and provide further evidence that the protoliths of Siberian eclogites were mafic rocks altered by seawater in the Archean.
Earth and Planetary Science Letters, Vol. 479, pp. 1-17.
Mantle
eclogite
Abstract: The conventional wisdom holds that metamorphic reactions take place at pressures near-lithostatic so that the thermodynamic pressure, reflected by the mineral assemblage, is directly correlated with depth. On the other hand, recent field-based observations and geodynamic simulations suggest that heterogeneous stress and significant pressure deviations above lithostatic (overpressure) can occur in Earth's crust. Here we show that eclogite, normally interpreted to form at great depths in subduction zones and Earth's mantle, may form at much shallower depths via local overpressure generated in crustal shear zones. The eclogites studied crop out as lenses hosted by felsic paragneiss in a sheared thrust slice and represent a local pressure and temperature anomaly in the Taconic orogenic belt, southern New England. Sharply-defined chemical zones in garnet, which record ?5 kbar pressure rise and fall accompanied by a temperature increase of 150-200?°C, demonstrate extremely short timescales of diffusion. This requires anomalously fast compression (?500 yrs) and decompression. We use coupled phase equilibria and garnet diffusion forward modeling to fit the observed garnet profiles and test the likely paths using a Monte Carlo-type approach, accounting for off-center sectioning of garnet. The simulation shows that a ?5 kbar pressure increase after the temperature peak is necessary to reproduce the garnet zoning. Remarkably, this post-peak-T compression (from 9 kbar to 14 kbar) lasted only ?500 yrs. If the compression was due to burial along a lithostatic pressure gradient, the descent speed would exceed 30 m?yr?1, defying any observed or modeled subduction rates. Local overpressure in response to partial melting in a confined volume (Vrijmoed et al., 2009) caused by transient shear heating can explain the ultra-fast compression without necessitating burial to great depth.
Abstract: Lenses of retrogressed eclogites occur in a 100 km wide zone of the Nyong Complex, a remnant of the Eburnian-Transamazonian orogen, marking a Palaeoproterozoic suture between the Congo and São Francisco Cratons. The eclogites show trace element pattern (depleted in LREE) similar to those of mid-ocean ridge basalts, indicating that the precursor melts formed in a depleted mantle source and the eclogites formed from oceanic crust. Despite numerous plagioclase ‘exsolutions’ up to 25 mol% jadeite component is preserved in omphacite and points to minimum pressures of 16 kbar at c. 800 °C. Pressures may have been 18-20 kbar as indicated by estimated compositions of peak omphacite. The age of eclogite metamorphism has been constrained by U-Pb SHRIMP dating of zircon at 2093 ± 45 Ma. The eclogites are associated with 2.05 Ga old charnockites and mafic granulites containing textures characteristic for near-isobaric cooling. These rocks may represent the plate above a subduction zone in which the eclogites were tectonically emplaced. With an age of 2.09 Ga the eclogites of the Nyong Complex are older than other subduction related Palaeoproterozoic eclogites of the Ubendian (1.88 Ga) and Usagaran belts (2.0 Ga) at the southern border of the Tanzania Craton. They are also older than eclogites in the Belomorian province (1.9 Ga; Russia) and thus represent the oldest known eclogites outcropping in an orogenic belt. The African eclogites (all with MORB chemistry) indicate that during the formation of the Nuna supercontinent the Palaeoproterozoic oceanic lithosphere around the Congo-Tanzania Craton was thick, cold and rigid enough to become subducted similar to cold oceanic lithosphere in the modern plate tectonic regime. However, apparent geothermal gradients of 12-14 °C/km for the Palaeoproterozoic eclogites are higher than those of Neoproterozoic and Phanerozoic eclogites and are interpreted as the result of warm subduction in a hotter Palaeoproterozoic Earth.
Russian Geology and Geophysics, Vol. 58, pp. 1305-1316.
Africa, Angola
eclogites
Abstract: We studied the Sm-Nd, Rb-Sr, and Re-Os isotope compositions of mantle xenoliths (eclogites and peridotites) from diamondiferous kimberlites of the Catoca cluster of the Kasai Craton. In the eclogites, the primary strontium isotope composition 87Sr/86Sr varies from 0.7056 to 0.7071, and the neodymium isotope composition eNd, from 1.8 to 2.6. The 187Re/188Os and 187Os/188Os ratios range from 135 to 80 and from 1.3110 to 1.9709, respectively, which indicates a significant portion of radiogenic Os: yOs = 129-147. These isotope values exceed the values assumed for model reservoirs (primitive upper mantle (PUM) and bulk silicate Earth (BSE)) and those of chondrites. The isotope composition of the studied systems indicates the formation of eclogites from a rhenium-enriched source, namely, the subducted oceanic crust transformed as a result of metasomatism and/or melting under upper-mantle conditions.
Abstract: Lenses of retrogressed eclogites occur in a 100 km wide zone of the Nyong Complex, a remnant of the Eburnian-Transamazonian orogen, marking a Palaeoproterozoic suture between the Congo and São Francisco Cratons. The eclogites show trace element pattern (depleted in LREE) similar to those of mid-ocean ridge basalts, indicating that the precursor melts formed in a depleted mantle source and the eclogites formed from oceanic crust. Despite numerous plagioclase ‘exsolutions’ up to 25 mol% jadeite component is preserved in omphacite and points to minimum pressures of 16 kbar at c. 800 °C. Pressures may have been 18-20 kbar as indicated by estimated compositions of peak omphacite. The age of eclogite metamorphism has been constrained by U-Pb SHRIMP dating of zircon at 2093 ± 45 Ma. The eclogites are associated with 2.05 Ga old charnockites and mafic granulites containing textures characteristic for near-isobaric cooling. These rocks may represent the plate above a subduction zone in which the eclogites were tectonically emplaced. With an age of 2.09 Ga the eclogites of the Nyong Complex are older than other subduction related Palaeoproterozoic eclogites of the Ubendian (1.88 Ga) and Usagaran belts (2.0 Ga) at the southern border of the Tanzania Craton. They are also older than eclogites in the Belomorian province (1.9 Ga; Russia) and thus represent the oldest known eclogites outcropping in an orogenic belt. The African eclogites (all with MORB chemistry) indicate that during the formation of the Nuna supercontinent the Palaeoproterozoic oceanic lithosphere around the Congo-Tanzania Craton was thick, cold and rigid enough to become subducted similar to cold oceanic lithosphere in the modern plate tectonic regime. However, apparent geothermal gradients of 12-14 °C/km for the Palaeoproterozoic eclogites are higher than those of Neoproterozoic and Phanerozoic eclogites and are interpreted as the result of warm subduction in a hotter Palaeoproterozoic Earth.
Abstract: Strontium and Pb isotopic compositions of clinopyroxene (cpx) in selected samples from three well-characterised eclogite suites with oceanic crustal protoliths (Lace/Kaapvaal craton, Orapa/Zimbabwe craton and Koidu/West African craton) were acquired by high-precision isotope dilution thermal ionisation mass spectrometry (ID-TIMS) and in situ multicollector-laser ablation-inductively-coupled plasma mass spectrometry (MC-LA-ICPMS). The aims of this study are twofold: (1) assess their utility to obtain formation or resetting age constraints and identify elemental signatures that enhance the chances of successful age dating, and (2) to confirm the veracity and utility of results obtained by novel MC-LA-ICPMS techniques. Strontium-Pb isotope systematics of eclogitic cpx measured in this study are decoupled and may reflect addition of unsupported radiogenic Sr during seawater alteration or interaction with oceanic sediments in subduction mélanges, and/or disturbance due to mantle metasomatism, to which the more incompatible Pb is more susceptible. Despite a complex history, subsets of samples yield meaningful model dates. Clinopyroxene fractions from Lace with high Pb contents (36?ppm), unradiogenic Pb isotopic compositions (206Pb/204Pb?=?13.5713.52) and low 238U/204Pb (1.01.5) give single-stage model Pb dates of 2.902.84?Ga. In contrast, samples from Orapa plot to the right of the Geochron and do not yield meaningful Pb model ages. However, these data do define secondary isochrons that can be modelled to yield minimum age constraints on major events affecting the cratonic lithosphere. Within the uncertainties, the resultant 2.18?±?0.45?Ga age obtained for Koidu eclogites reflect disturbance of the Pb isotope system due to subduction beneath the craton linked to the Eburnean orogeny, while they retained their unradiogenic 87Sr/86Sr (0.7016). Similarly, the age for samples from Orapa (2.20?±?0.54?Ga) is interpreted as an overprint age related to Palaeoproterozoic accretion at the western craton margin. Gabbroic eclogites (Eu/Eu*?>?1) with plagioclase-rich protoliths having low time-integrated Rb/Sr and U/Pb retain the least radiogenic Sr and, in part, Pb. High model ? (9.0 to 9.1) for several eclogites from Lace with elevated LREE, Th and Pb abundances reflects ca. 3.0?Ga addition of a sedimentary component, possibly derived from reworking of a high-? basaltic protocrust, as observed on other cratons. We suggest that sample targeting can be usefully guided by fast-throughput in situ LA-ICPMS techniques, which largely yield results identical to ID-TIMS, albeit at lower precision, and which can further help identify kimberlite contamination in the mineral separates used for solution work.
Abstract: Subduction of calcium carbonate, sequestered in the oceanic crust by hydrothermal metamorphism and biogenic action, accounts for a significant flux of carbon into the mantle, where it contributes to the genesis of carbonatitic and silica-undersaturated melts. However, the reported phase relations in the system CaCO3, notably the transition boundary from disordered calcite (calcite V, here ccv) to aragonite (ara), vary considerably among different studies. Moreover, the thermodynamic properties of ccv and of liquid CaCO3 (CaCO3L) remain to be determined. In order to address the dearth of experimental data on phase relations, and to determine a set of internally consistent thermodynamic properties for ara, ccv and CaCO3L, multi-anvil experiments were performed at 3-6?GPa and 1300-1750?°C. By re-evaluating all experimental data, the transformation of ccv-ara fits the equation Tccv-ara?=?397.6?+?320.17?×?P and the melting curve Tm?=?1578.9?+?139.65?×?P???11.646?×?P2, where pressure is in GPa and temperature in K. Thermodynamic properties retrieved for calcite V and liquid CaCO3 are used to compute phase diagrams of relevance for chemical compositions representative of eclogite heterogeneities of the astenospheric mantle, and compared with experimentally derived phase relationships. Aragonite represents a carbonate of major abundance in carbonated eclogites at high temperature, close to the solidus; its ability to fractionate REE and Ba-Sr contributes to the peculiar geochemical signatures of silica undersaturated magmas. The relatively refractory nature of aragonite impacts on our understanding of the deep carbon cycle.
Earth and Planetary Science Letters, Vol. 505, pp. 162-172.
Mantle
eclogites
Abstract: Some high-Mg eclogite xenoliths, entrained by kimberlites from the mantle lithospheres of ancient continental cores, and rare orogenic eclogites and ophiolites, exhumed or obducted during the closure of palaeo-ocean basins, have elemental and isotopic compositions indicative of protoliths that formed as little-differentiated melts erupted in ancient ocean floors. Despite metamorphism and, in part, partial melt loss, these samples of ancient mid-ocean ridge basalt and picrite retain a memory of the chemical and physical state of their protoliths' ambient convecting mantle sources. Published data show that, when filtered to exclude specimens with cumulate protoliths or showing evidence for later enrichment (metasomatism), the samples lack Y or Al 2 O 3 depletion relative to TiO 2 and MgO. This indicates melt segregation of the protolith predominantly from a garnet-free peridotite source and implies intersection of the solidus at low pressures (?3 GPa). Given the dependence of melt composition and volume on source composition (assumed to be similar to modern depleted mantle) and mantle potential temperature (T P), we calculate moderate average melt fractions F (?0.22 ± 0.01) from the Ti contents of the least differentiated samples in three sample suites with 2.6 to 2.9 Ga ages. This converts to T P of ?1410 ± 10 • C assuming a final pressure of melting of 0.5 GPa, melt productivity of 10%/GPa and mantle adiabat of 0.4 • C/km, and using a mantle solidus parameterisation. Though model-dependent, the results are in agreement with recent work advocating moderate Archaean mantle T P. Estimates drop to F = 0.19 and T P = 1380 • C at 1.9 Ga and F = 0.12 and T P = 1310 at 0.6 Ga, corresponding to a decrease in T P of only ?100 • C over the last 3 Ga. A less depleted mantle source yields higher F and T P , but the above estimates are in better agreement with qualitative evidence from Al 2 O 3 and Y, and with Nd-Hf and Sr isotope compositions of orogenic eclogite and granulite suites and mantle eclogites, respectively, which indicate that portions of the Meso-to Neoarchaean mantle were depleted. Moderate T P supports early plate strengthening and a possible transition to plate tectonics in the Mesoarchaean if not earlier. Moreover, moderate temperatures in Archaean subduction zones may have facilitated deep recycling of volatiles that would otherwise have been lost from subducting slabs at shallow depths.
Abstract: Subduction of calcium carbonate, sequestered in the oceanic crust by hydrothermal metamorphism and biogenic action, accounts for a significant flux of carbon into the mantle, where it contributes to the genesis of carbonatitic and silica-undersaturated melts. However, the reported phase relations in the system CaCO3, notably the transition boundary from disordered calcite (calcite V, here ccv) to aragonite (ara), vary considerably among different studies. Moreover, the thermodynamic properties of ccv and of liquid CaCO3 (CaCO3L) remain to be determined. In order to address the dearth of experimental data on phase relations, and to determine a set of internally consistent thermodynamic properties for ara, ccv and CaCO3L, multi-anvil experiments were performed at 3-6?GPa and 1300-1750?°C. By re-evaluating all experimental data, the transformation of ccv-ara fits the equation Tccv-ara?=?397.6?+?320.17?×?P and the melting curve Tm?=?1578.9?+?139.65?×?P???11.646?×?P2, where pressure is in GPa and temperature in K. Thermodynamic properties retrieved for calcite V and liquid CaCO3 are used to compute phase diagrams of relevance for chemical compositions representative of eclogite heterogeneities of the astenospheric mantle, and compared with experimentally derived phase relationships. Aragonite represents a carbonate of major abundance in carbonated eclogites at high temperature, close to the solidus; its ability to fractionate REE and Ba-Sr contributes to the peculiar geochemical signatures of silica undersaturated magmas. The relatively refractory nature of aragonite impacts on our understanding of the deep carbon cycle.
Abstract: The dynamical evolution and exhumation mechanisms of oceanic?derived eclogites are controversial conundrums of oceanic subduction zones. The previous studies indicated that density is the primary factor controlling the exhumation of oceanic rocks. To explore their density evolution, we systematically investigate the phase relations and densities of different rock types in oceanic crust, including mid ocean ridge basalt (MORB), serpentinite, and global subducting sediments (GLOSS). According to the density of eclogites, these currently exposed natural eclogites can be classified into two categories: the self?exhumation of eclogites (?MORB < ?Mantle) and the carried exhumation of eclogites (?MORB > ?Mantle). The depth limit for an exhumation of oceanic?derived eclogites solely driven by their own buoyancies is 100-110 km, and it increases with the lithospheric thickness of the overriding plate. The parameters of carried?exhumation, that is, KGLOSS and KSerp, are defined in order to quantitatively evaluate the assistance ability of GLOSS and serpentinites for carrying the denser eclogites. KGLOSS is mainly controlled by pressure, whereas KSerp is dominantly affected by temperature. Using 2?D thermomechanical models, we demonstrate that the presences of low?density, low?viscosity GLOSS and seafloor serpentinites are the prerequisites for the exhumation of oceanic?derived eclogites. Our results show that oceanic?derived eclogites should be stalled and exhumed slowly at the Moho and Conrad discontinuities (named Moho/Conrad stagnation). We propose that oceanic?derived eclogites should undergo a two?stage exhumation generally, that is, early fast exhumation driven by buoyancy at mantle levels, and final exposure to surface actuated by tectonic exhumation facilitated by divergence between upper plate and accretionary wedge or by rollback of lower plate.
Abstract: The eclogites of the North Muya complex (Eastern Siberia) are located within the Early Neoproterozoic metasedimentary and felsic rocks of the Baikal-Muya Fold Belt (BMFB). The eclogites show subduction-related affinity, with large-ion lithophile (LILE) and light rare-earth element (LREE) enrichment and high field-strength element (HFSE) depletion signatures, similar to the exposed plutonic and volcanic rocks of the Early Neoproterozoic (Early Baikalian) subduction setting in the BMFB. Coupled Nd (?Nd(T) of +6 to ?1.4) and Sr (87Sr/86Sr ratio of 0.705-0.708), along with key trace-element indicators, imply progressive crustal recycling (up to 5-10%) from the Early Precambrian continental rocks to a depleted mantle source or equivalent crustal contribution via intracrustal contamination. Mineral ?18O data (+3.9???+11.5) indicate that the contaminant or recycled crustal substrate might be represented by rocks altered at both low and high-temperature, or result from variable fluid-rock interaction in the subduction channel. Pseudosection modelling of eclogites, coupled with zircon UPb geochronology (~630?Ma) suggest that the Ediacarian high-pressure metamorphic event for different rocks shared a maximum depth corresponding to 2.5-2.7?GPa with variable temperature range (560-760?°C), reflecting their potential relation to distinct slices of the subducted crust. The estimated metamorphic conditions for both the burial and exhumation of rocks indicate a continental subduction setting, but with a relatively cold geotherm (~20-25?°C/kbar). These conditions resulted from the continental subduction of the Baikal-Muya composite structure beneath the relatively thin and immature overlying arc lithosphere of southern Siberia. Some carbonate-bearing eclogites and garnet-pyroxene rocks, metamorphosed under T below 700?°C and a minimum P up to 1.4?GPa, exhibit LREE-enriched patterns and low ?Nd(T) values of ?7 to ?16. These rocks have Paleoproterozoic to Archean model ages and may support the existence of a Paleoproterozoic or older lithosphere in the Baikal-Muya Fold Belt, but their subduction history and origin remain uncertain due to geochemical and isotopic signatures probably overprinted by carbonate metasomatism.
www.minsocam.org /MSA/Centennial/ MSA_Centennial _Symposium.html The next 100 years of mineral science, June 20-21, p. 27. Abstract
Mantle
eclogites
Abstract: Omphacite is a clinopyroxene solid solution of Fe-bearing diopside and jadeite, and is stable up to about 500 km depth in the Earth’s interior. It is also a major mineral component of eclogite (up to 75 vol%). Basalt, which makes up most of the Earth’s oceanic crust, transforms into eclogite at the depth > ~60 km. Due to the ~20% higher density of eclogite, it is considered one of the main driving forces for the slab subduction. Subducted eclogite is also an important source of the chemical heterogeneities in the Earth’s mantle, which are the potential reservoirs for the enriched geochemical components. Thus, studying the geophysical properties of omphacite at elevated pressure-temperature conditions is of great interest for both the geophysical and geochemical community. Previous studies have proposed to utilize the unique anisotropic seismic properties of eclogite to identify possible subduction channels and eclogite-rich regions in the Earth’s interior. Due to the elastically isotropic nature of garnet and the relatively small proportion (< 10 vol%) of the silica minerals in eclogite, the seismic anisotropy of eclogite is primarily caused by the lattice preferred orientation of omphacite. Thus, in this study, in addition to determining the densities, and isotropic velocities of omphacite at the high pressuretemperature condition, we also paid special attention to the elastic anisotropy of omphacite. We combined the synchrotron single-crystal X-ray diffraction at Advanced Photon Source, Argonne National Laboratory with offline Brillouin spectroscopy experiments at University New Mexico to investigate the anisotropic thermoelastic properties of omphacite. Incorporated with the preexisting thermoelastic database of other relevant mantle mineral phases, we compared the anisotropic seismic properties of eclogite (slab crust) with pyrolite (ambient mantle) along mantle geotherms down to 500 km depth. The maximum isotropic and anisotropic velocities contrast between pyrolite and eclogite is at 310-410 km, making it an optimal depth range for seismologists to search for eclogite-rich heterogeneities in the Earth’s interior. The ~5%-7% velocity difference between eclogite and pyrolite also needs to be taken into account when estimating the slab temperatures between 310-410 km depth. Otherwise, the slab temperature could be underestimated by a few hundred K without considering the possible lithology difference.
Geochemistry, Geophysics, Geosystems, Vol. 20, 7, pp. 3116-3133.
mantle
eclogites
Abstract: How rocks deform at depth during lithospheric convergence and what are the magnitudes of stresses they experience during burial/exhumation processes constitute fundamental questions for refining our vision of short?term (i.e., seismicity) and long?term tectonic processes in the Earth's lithosphere. Field evidence showing the coexistence of both brittle and ductile deformation at high pressure?low temperature (HP?LT) conditions particularly fuels this questioning. We here present 2D numerical models of eclogitic rock deformation by simple shear performed at centimeter scale. To approximate the eclogite paragenesis, we considered the deformed medium as composed of two mineral phases: omphacite and garnet. We run a series of models at 2.0 GPa and 550 °C for different background strain rates (from 10?14 s?1 to 10?8 s?1) and for different garnet proportions (from 0% to 55%). Results show that whole rock fracturing can occur under HP?LT conditions for strain rates larger than ~10?10 s?1. This suggests that observation of brittle features in eclogites does not necessarily mean that they underwent extreme strain rate. Care should therefore be taken when linking failure of eclogitic rocks to seismic deformation. We also explore the ranges of parameters where garnet and omphacite are deforming with a different deformation style (i.e., frictional vs viscous) and discuss our results in the light of naturally deformed eclogitic samples. This study illustrates that effective stresses sustained by rocks can be high at these P?T conditions. They reach up to ~1 GPa for an entirely fractured eclogite and up to ~500 MPa for rocks that contain fractured garnet.
Americam Geophysical Union Fall meeting, 1p. Abstract
Europe, Norway
eclogites, peridotites
Abstract: The preservation of Archean cratons is typically attributed to the presence of a highly-depleted and buoyant sub-continental lithospheric mantle (SCLM) that is equally old or older than its overlying crust. Time constraints on the formation and petrological evolution of the SCLM are key to investigating its long-term evolution and role in the formation and preservation of the continental crust. Nevertheless, such constraints are difficult to obtain as well-preserved samples of the SCLM are rare and typically lack conventional chronometric minerals. The history of SCLM rocks is typically inferred on the basis of model ages, many of which indicate an Archean origin; however, these dates are difficult to link to specific mineral assemblages or chemical signatures, and the petrological and dynamic processes that these represent. Garnet Lu-Hf geochronology is one of the few chronometers that could overcome this limitation. In this study, a refined method in Lu-Hf garnet chronology was applied to fragments of the Laurentian SCLM that are now exposed as orogenic peridotites in the ultrahigh-pressure domains of the Western Gneiss Complex, Norway. The peridotite bodies comprise a variety of unusually well-preserved rock types-from dunites that record decompression and melting at >350 km depth to fertile lithologies produced by melting and fluid metasomatism. Our internal isochron results from pyrope (after exsolution from majorite) in dunite samples yielded identical Neoarchean ages; these are the first-ever obtained for mantle garnet. The ages coincide with a time interval during which there was voluminous juvenile crust formation, indicating a link between this global process and the deeply sourced mantle upwellings that these samples represent. Internal isochrons from websterite-and clinopyroxenite-hosted pyrope yielded Meso-to Neoproterozoic ages that exactly match two distinct supercontinent break-up events in the overlying continental crust. Together, the new Lu-Hf results indicate that since its extraction during a period of widespread Archean crustal growth, the Laurentian SCLM appears to have largely been at petro-physical and chemical stasis and evolved only during short pulses that ran in sync with the supercontinent cycle.
Nature Research Scientific Reports, Vol. 9:20190 doir.org/10.38 /s41598-019-55743-1, 11p. Pdf
Mantle
eclogite
Abstract: Oxygen fugacity (ƒO2) is an intensive variable implicated in a range of processes that have shaped the Earth system, but there is controversy on the timing and rate of oxidation of the uppermost convecting mantle to its present ƒO2 around the fayalite-magnetite-quartz oxygen buffer. Here, we report Fe3+/?Fe and ƒO2 for ancient eclogite xenoliths with oceanic crustal protoliths that sampled the coeval ambient convecting mantle. Using new and published data, we demonstrate that in these eclogites, two redox proxies, V/Sc and Fe3+/?Fe, behave sympathetically, despite different responses of their protoliths to differentiation and post-formation degassing, seawater alteration, devolatilisation and partial melting, testifying to an unexpected robustness of Fe3+/?Fe. Therefore, these processes, while causing significant scatter, did not completely obliterate the underlying convecting mantle signal. Considering only unmetasomatised samples with non-cumulate and little-differentiated protoliths, V/Sc and Fe3+/?Fe in two Archaean eclogite suites are significantly lower than those of modern mid-ocean ridge basalts (MORB), while a third suite has ratios similar to modern MORB, indicating redox heterogeneity. Another major finding is the predominantly low though variable estimated ƒO2 of eclogite at mantle depths, which does not permit stabilisation of CO2-dominated fluids or pure carbonatite melts. Conversely, low-ƒO2 eclogite may have caused efficient reduction of CO2 in fluids and melts generated in other portions of ancient subducting slabs, consistent with eclogitic diamond formation ages, the disproportionate frequency of eclogitic diamonds relative to the subordinate abundance of eclogite in the mantle lithosphere and the general absence of carbonate in mantle eclogite. This indicates carbon recycling at least to depths of diamond stability and may have represented a significant pathway for carbon ingassing through time.
Abstract: The Precambrian is marked by the stabilization of subcontinental mantle lithosphere and associated crystallization of lithospheric diamonds from slab-derived carbon [1,2]. These features and higher nitrogen contents are consistent with diamond fluid delivery from the crustal (eclogitic) part of the slab and keel growth by lateral accretion/advective thickening [e.g. 3-5] in shallow and reducing mantle wedge settings. Such diamonds are rare to non-existent in the Phanerozoic. Sublithospheric diamonds are also slab-derived but from nitrogen-poor fluids/melts. These diamond fluids were delivered from deeper carbonated crustal or serpentinized mantle portions of cold slabs that subducted into the mantle transition zone regions and warmed up. While the age of such sublithospheric diamonds is poorly known, current mantle tomogrpahy shows that these conditions are a feature of the modern Earth and are consistent with the few ages that show these diamonds are younger. Aside from depth differences, the conditions for the crystallization of slab-associated lithospheric vs sublithospheric diamonds are fundamentally different in slab temperature, fluid composition, and fluid source within the slab source-rock. We hypothesize that these differences between lithospheric and sublithospheric diamonds may also be of temporal significance. If so, slab-derived diamond petrogenesis could be a key to understanding why cratonic keel formation is prevalent in the early Precambrian. Slabderived diamonds provide evidence for a transition from Precambrian conditions of shallow-devolatilization and warmer, more buoyant plates, that would facilitate lithosphere thickening to Phanerozoic conditions of deeperdevolatilization, cooler, and less buoyant plates, that are less conducive to lithosphere thickening.
Journal of Petrology, in press available, 89p. Pdf
Mantle
eclogite
Abstract: Despite its accessory mineral status in metabasaltic rocks, rutile controls the whole-rock Ti, Nb and Ta budget. These are key elements used to trace fluid- and melt-mediated mass transfer across the mantle-crust boundary. Rutile also contains significant amounts of the redox-sensitive element V, which is increasingly used to estimate oxygen fugacity. Kimberlite-borne mantle eclogite xenoliths, which are frequently rutile-bearing, have been interpreted as residues from the extraction of silicic partial melt similar in composition to the average continental crust. Published mineral compositions for eclogite xenoliths from various cratons combined with geothermobarometrical calculations show that TiO2 contents in garnet and clinopyroxene increase with increasing temperature of last residence in the lithospheric mantle, while apparent clinopyroxene-garnet distribution coefficients decrease. This implies that (1) increasing TiO2 contents in eclogitic garnet or clinopyroxene are not a signature of increasing metasomatism with depth, (2) whole-rock eclogites reconstructed without rutile will increasingly underestimate TiO2, Nb and Ta contents with decreasing temperature, and (3) low-temperature eclogites are more likely to contain free rutile. Only about a third of the ?250 samples considered here would have whole-rock TiO2 contents (reconstructed with calculated rutile modes) required for rutile saturation during subduction and partial melting. If there is a role for subducting oceanic crust now sampled as mantle eclogite, the characteristic Ti-Nb-Ta depletion in continental crust may require fluid-dominated processes, where these elements are not efficiently mobilised.
Abstract: Paleoproterozoic retrogressed eclogite (retroeclogite) occurs in the Itaguara Sequence included in the suture zone formed by collision between the Archean Divinópolis and Campo Belo/Bonfim Complexes in the southern São Francisco Craton, which represents the South American counterpart of the African Congo Craton. The Itaguara retroeclogite contains scarce omphacite and phengite but abundant garnet porphyroblasts embedded in a fine-grained, amphibole, biotite and quartz-bearing matrix. The 2.20 ± 0.05 Ga eclogitization event (garnet and whole rock Sm-Nd isochronic age) of the E-MORB protolith (TDM ~ 2.47 Ga) is recorded by omphacite formation during high-pressure prograde stage in amphibole eclogite facies due to ~70 km depth subduction process. Amphibole eclogite facies metamorphic peak stage of 17-20 kbar and 600-700 °C occurred during ~2.1 Ga continental collision. Tectonic exhumation-related decompression during collision probably triggered partial melting of the eclogitic rock. Finally, decompression late stage estimated between 5 and 8 kbar and 550-650 °C under amphibolite facies overprint during orogenic collapse was responsible for appearance of kelyphitic reaction rims (symplectite) around garnet crystals. As its Paleoproterozoic contemporary analogues from Congo Craton, the Itaguara retroeclogite is one of the oldest records of the modern-style plate tectonics.
European Journal of Mineralogy, Vol. 31, pp. 889-803. pdf
China
eclogite
Abstract: A combined study of detailed petrographic observation, mineral chemistry analysis and phase equilibrium modeling indicates that the high-temperature eclogites from the Dabie orogen, central China, experienced two episodes of anatexis: the first is phengite dehydration melting during the exhumation of deeply subducted slices, and the second is heating melting related to the post-orogenic collapse. Petrographic evidence and clues of the anatectic events include biotite + plagioclase + garnet ± amphibole intergrowth in matrix and biotite + plagioclase intergrowth within amphibole porphyroblast. Pressure-temperature (P-T) pseudosection and modal variation diagram indicate that the biotite + plagioclase + garnet ± amphibole in matrix was formed by the reactions phengite + clinopyroxene + quartz = melt + sanidine + garnet + plagioclase and later melt + sanidine + garnet = biotite + plagioclase, while the biotite + plagioclase intergrowths within poikiloblastic amphibole were formed by the reaction amphibole + muscovite + epidote = biotite + plagioclase + melt. In addition, the combination of petrological observations and P-T estimates suggests that the first melting event occurred at the late Triassic, while the second is related to the early Cretaceous mountain-root removal and subsequent asthenospheric upwelling and heat input. As the P-T paths of high-temperature/ultrahigh-pressure rocks have high probabilities to cross-cut phengite-melting curves, phengite melting during decompression may be a common process in these rocks. Moreover, the coexistence of multiple episodes of anatexis in a single tectonic slice suggests caution when identifying and dating partial melting in high-temperature/(ultra)high-pressure rocks.
Abstract: Some seismic models derived from tomographic studies indicate elevated shear?wave velocities (?4.7 km/s) around 120-150 km depth in cratonic lithospheric mantle. These velocities are higher than those of cratonic peridotites, even assuming a cold cratonic geotherm (i.e., 35 mW/m2 surface heat flux) and accounting for compositional heterogeneity in cratonic peridotite xenoliths and the effects of anelasticity. We reviewed various geophysical and petrologic constraints on the nature of cratonic roots (seismic velocities, lithology/mineralogy, electrical conductivity, and gravity) and explored a range of permissible rock and mineral assemblages that can explain the high seismic velocities. These constraints suggest that diamond and eclogite are the most likely high?Vs candidates to explain the observed velocities, but matching the high shear?wave velocities requires either a large proportion of eclogite (>50 vol.%) or the presence of up to 3 vol.% diamond, with the exact values depending on peridotite and eclogite compositions and the geotherm. Both of these estimates are higher than predicted by observations made on natural samples from kimberlites. However, a combination of ?20 vol.% eclogite and ~2 vol.% diamond may account for high shear?wave velocities, in proportions consistent with multiple geophysical observables, data from natural samples, and within mass balance constraints for global carbon. Our results further show that cratonic thermal structure need not be significantly cooler than determined from xenolith thermobarometry.
Abstract: Eclogite xenoliths, together with garnet pyroxenites and some mafic garnet granulites, found in kimberlites located along the southern margin of the Kaapvaal craton in southern Africa have been analysed by electron microprobe and mass spectrometry techniques to determine their geochemical characteristics. The majority of eclogites are bimineralic with garnet and omphacitic clinopyroxene in subequal proportions, with rutile as the main accessory phase; a few contain kyanite. Based on K2O in clinopyroxene and Na2O in garnet, the eclogites can be classified as Group II eclogites, and the majority are high-Ca in character. Garnet pyroxenites comprise garnet clinopyroxenites and garnet websterites. Major and trace element concentrations and isotope ratios of reconstituted bulk rock compositions of the eclogites and garnet pyroxenites allow constraints to be placed on depth of origin and likely protolith history. Calculated Fe-Mg exchange equilibration temperatures for the eclogites range from 815 to 1000?°C, at pressures of 1•7?±?0•4?GPa as determined by REE partitioning, indicating that they were sampled from depths of 50-55?km; i.e. within the lower crust of the Namaqua-Natal Belt. The garnet pyroxenites show slightly lower temperatures (686-835?°C) at similar pressures of equilibration. Initial 143Nd/144Nd and 87Sr/86Sr ratios (calculated to time of kimberlite emplacement) of both lithologies overlap the field for lower crustal samples from the Namaqua-Natal Belt. Further evidence for a crustal origin is found in the similar REE patterns shown by many of the associated garnet granulite xenoliths. Garnet pyroxenites are interpreted to have a similar origin as the associated eclogites but with the mafic protolith having insufficient Na (i.e. low modal plagioclase) to allow for development of omphacitic pyroxene. Metamorphism of the mafic protoliths to these eclogites and garnet pyroxenites is inferred to have occurred during crustal shortening and thickening associated with the collision of the Namaqua-Natal Belt with the Kaapvaal craton at 1-1•2?Ga.
Abstract: One of the first appearances of eclogite-facies mineral assemblages in the geological record occurs in the c. 2000 Ma Palaeoproterozoic Usagaran Belt in central Tanzania, where the extended margin of the Tanzanian Craton is interpreted to have been subducted. Mafic rocks are interpreted to have contained the mineral assemblage garnet + omphacite + rutile + quartz ± amphibole. This high-pressure assemblage has been overprinted by a secondary mineral assemblage containing clinopyroxene + plagioclase + hornblende + ilmenite ± orthopyroxene. Mineral equilibria forward modelling indicates that the eclogite-facies assemblages reached minimum peak pressure-temperature (P-T) conditions of ~17 kbar and ~700 °C. Inclusions in garnet document a prograde P-T history consistent with burial through upper amphibolite-facies conditions and possible partial melting. Petrological and compositional evidence from garnet suggests that following peak metamorphism, the eclogite-facies rocks were heated while stalled at approximate peak pressures. Temperature estimates derived from Zr concentrations in interpreted texturally retrograde rutile support a near-isothermal post-peak P-T evolution for the eclogite-facies rocks - an evolution that terminates at retrograde P-T conditions of approximately 7.6-8.2 kbar and 680-790 °C. The relict eclogite domains form part of a larger assemblage with enclosing migmatitic metapelitic lithologies (the Isimani Suite). The metapelitic gneisses contain garnet + kyanite + biotite + staurolite + hornblende + plagioclase + muscovite + rutile + quartz and preserve minimal evidence of a high-pressure history, conceivably due to post-peak mineralogical recrystallisation. P-T modelling, inclusion assemblages and compositional zonation patterns in porphyroblastic garnet suggests the metapelitic gneisses — similarly to the relict eclogites — experienced burial to minimum peak pressures of approximately 16.5-17 kbar. Compositional zoning patterns in eclogitic garnet suggest the Isimani system was buried, reached peak metamorphic conditions, and was subsequently exhumed within a timeframe of up to 20 Myr. A tectonic regime involving crustal thickening and subduction, followed by extensional exhumation of the entire Isimani Suite is our preferred model for the development of the c. 2000 Ma Usagaran Belt.
Abstract: Investigating the seismic properties of natural eclogite is crucial for identifying the composition, density, and mechanical structure of the Earth’s deep crust and mantle. For this purpose, numerous studies have addressed the seismic properties of various types of eclogite, except for a rare eclogite type that contains abundant olivine and orthopyroxene. In this contribution, we calculated the ambient-condition seismic velocities and seismic anisotropies of this eclogite type using an olivine-rich eclogite from northwestern Flemsøya in the Nordøyane ultrahigh-pressure (UHP) domain of the Western Gneiss Region in Norway. Detailed analyses of the seismic properties data suggest that patterns of seismic anisotropy of the Flem eclogite were largely controlled by the strength of the crystal-preferred orientation (CPO) and characterized by significant destructive effects of the CPO interactions, which together, resulted in very weak bulk rock seismic anisotropies (AVp = 1.0-2.5%, max. AVs = 0.6-2.0%). The magnitudes of the seismic anisotropies of the Flem eclogite were similar to those of dry eclogite but much lower than those of gabbro, peridotite, hydrous-phase-bearing eclogite, and blueschist. Furthermore, we found that amphibole CPOs were the main contributors to the higher seismic anisotropies in some amphibole-rich samples. The average seismic velocities of Flem eclogite were greatly affected by the relative volume proportions of omphacite and amphibole. The Vp (8.00-8.33 km/s) and Vs (4.55-4.72 km/s) were remarkably larger than the hydrous-phase-bearing eclogite, blueschist, and gabbro, but lower than dry eclogite and peridotite. The Vp/Vs ratio was almost constant (avg. ? 1.765) among Flem eclogite, slightly larger than olivine-free dry eclogite, but similar to peridotite, indicating that an abundance of olivine is the source of their high Vp/Vs ratios. The Vp/Vs ratios of Flem eclogite were also higher than other (non-)retrograded eclogite and significantly lower than those of gabbro. The seismic features derived from the Flem eclogite can thus be used to distinguish olivine-rich eclogite from other common rock types (especially gabbro) in the deep continental crust or subduction channel when high-resolution seismic wave data are available.
Precambrian Research, doi.org/10.1016/j.precamres.2020.105879in press available, 80p. Pdf
Russia
eclogites
Abstract: Competing evolutionary models and age of eclogite facies metamorphism, Mesoarchaean, Neoarchaean or Palaeoproterozoic, of the subducted Mesoarchaean oceanic crust (Salma association, Belomorian Eclogite Province) are discussed on a basis of systematic analysis of previously known and newly obtained data. Four main types of zircons were distinguished in eclogites: porous crystals with numerous inclusions from eclogite-metagabbro; wide-rimmed zircons with relict porous cores similar to previous type separated from garnetites; round-oval zircons from eclogite-metagabbronorite that are characteristic for granulite facies rocks and zircons with euhedral oscillatory zoning cores and oval grains that are characteristic for the eclogite facies pillow basalts. Regular changes in REE patterns and in crystallization-recrystallization temperatures of certain domains of the porous zircons display sequence of magmatic and metamorphic events. The???2.9?Ga domains retain magmatic-type REE patterns. Low- and medium-temperature inclusions of prenite, pumpelliite, albite, actinolite, chlorite, diaspore and saponite in garnet and abundant microinclusions of the prenite-pumpelliite and greenschist facies in zircons with LREE-MREE enrichment indicate hydrothermal metamorphism in the spreading ridge and ocean floor at 2.9-2.82?Ga. Disappearance of Ce positive anomaly from REE pattern in zircon, change negative to positive Eu anomaly and LREE-MREE enrichment caused by plagioclase removal and replacement of rutile with sphene evidence eclogite facies metamorphism linked with subduction at 2.82-2.78?Ga. Temperatures in the 700-900?°C range of the round-oval zircons from eclogite-metagabbronorite records the Neoarchaean granulite facies overprint at 2.77-2.70?Ga. Series of the high temperature Palaeoprpoterozoic events was terminated by 2.1-1.7?Ga event marked by the rims with lowest REE that frame all types of zircons. Change from positive to negative Eu anomaly, retrieval of negative Ce anomaly indicate the presence of plagioclase, reduction type of fluids and low water activity characteristic of high-temperature metamorphism under stretching condition and mantle-plume activity. The deep reworking of the Sm-Nd isotope system in the Belomorian tectonic province at???1.9?Ga, including the Salma eclogite association, was caused by the enormous crustal heating that spread from the Lapland granulite belt southward. Radiogenic 176Hf enrichment of 1.9?Ga zircon indicates recrystallization of a long-existed garnet with release of significant amount of 176Hf.
Petrology, geochemistry and Sm-Nd systematics of the Paleoproterozoic Itagurra retroeclogite from Sao Francisco/Congo craton: one of the oldest records of the modern-style plate tectonics.
Journal of Petrology, in press available, 52p. Pdf
United States, Colorado Plateau
eclogite
Abstract: The Colorado Plateau has undergone as much as 1.8?km of uplift over the past 80?Ma, but never underwent the pervasive deformation common in the neighboring tectonic provinces of the western USA. To understand the source, timing and distribution of mantle hydration, and its role in plateau uplift, garnets from four eclogite xenoliths of the Moses Rock diatreme (Navajo Volcanic Field, Utah, USA) were analyzed in situ for ?18O by secondary ion mass spectrometry. These garnets have the largest reported intra-crystalline oxygen isotope zoning to date in mantle-derived xenoliths with core-to-rim variations of as much as 3‰. All samples have core ?18O values greater than that of the pristine mantle (?5.3‰, mantle garnet as derived from mantle zircon; Valley et al., 1998; Page et al., 2007) consistent with an altered upper oceanic crust protolith. Oxygen isotope ratios decrease from core to rim recording interaction with a low-?18O fluid at high temperature, likely derived from serpentinite in the foundering Farallon slab. All zoned samples converge at a ?18O value of ?6‰, regardless of core composition, suggesting that fluid infiltration was widely distributed. Constraints on the timing of this fluid influx, relative to diatreme emplacement, can be gained from diffusion modeling of major element zoning in garnet. Modeling using best-estimates of peak metamorphic conditions (620ºC, 3.7?GPa) yield durations of?
Earth and Planetary Science Letters, Vol. 554, doi.org/10.1016/ j.epsl.2020. 116664 13p. Pdf
Mantle
eclogite
Abstract: Results from numerical modelling and experimental petrology have led to the hypothesis that partial melting was important in facilitating exhumation of ultrahigh-pressure (UHP) metamorphic rocks from mantle depths. However, the melting reactions responsible are rarely well-documented from natural examples. Here we report microstructural features and compositional data that indicate in situ partial melting dominated by breakdown of omphacite in UHP eclogite from the Sulu belt, China. Diagnostic microstructures include: (i) the presence of in situ leucosome pockets composed of plagioclase, euhedral amphibole, minor K-feldspar and epidote within host zoisite- and phengite-bearing eclogite; (ii) skeletal omphacite within the leucosome pockets that has a lower jadeite content (25-45 mol.%) than rock-forming omphacite (39-54 mol.%); and, (iii) seams of Na-rich plagioclase that extend along grain boundaries separating phengite, quartz and zoisite, and which commonly exhibit low dihedral angles where they terminate at triple grain-boundary junctions. Major oxide proportions of 57 leucosome pockets, calculated using mineral modes and compositions, yield leucodiorite bulk compositions characterized by intermediate SiO2, high Al2O3 and Na2O, and low K2O contents. In primitive mantle-normalised trace element diagrams, the leucosome pockets show enrichment in large ion lithophile elements, U, Pb, Zr, Hf and Ti, but depletion in Th and Ta, patterns that are similar to those of rock-forming omphacite. Rather than forming predominantly by breakdown of phengite and/or zoisite, as widely proposed in the literature, the leucosome pockets have petrographic characteristics and major oxide and trace element compositions that are consistent with partial melting dominated by omphacite breakdown. Based on conventional thermobarometry, the eclogite was exhumed from pressure-temperature (P-T) conditions of 3.6-3.1 GPa and 900-840 °C. Partial melting led to the formation of the leucosome pockets, which equilibrated with the rims of surrounding rock-forming garnet and pyroxene during crystallisation. Conventional thermobarometry using rim compositions yields P-T conditions of 1.6-1.2 GPa and 780-690 °C, broadly consistent with calculated phase equilibria and Ti-in-zircon temperatures from zircon overgrowths. Weighted mean ages of ca 217-214 Ma from thin overgrowths on zircon are interpreted to record melt crystallisation. This study provides insight into an overlooked mechanism by which eclogites partially melt during exhumation from UHP conditions, and permits a better understanding of the processes that assist deeply-subducted continental crust to return to shallower depths.
Russian Geology and Geophysics, Vol. 62, pp. 525-546. pdf
Europe, Finland, Sweden
eclogites
Abstract: The Belomorian Province (BP) of the Fennoscandian Shield is a high-grade belt composed of Meso- to Neoarchean tonalite- trondhjemite-granodiorite (TTG) gneisses with subordinate supracrustal complexes. The Belomorian crust is underlined by a thick mantle keel, a structural element typical of Archean cratons. Belomorian rocks were metamorphosed under conditions of mainly high-pressure amphibolite to granulite facies in both Archean and Paleoproterozoic times. The TTG gneisses contain numerous blocks of almost completely retrogressed eclogite (eclogite-1). This paragenetic association of eclogite-1 and gneisses can be classified as an Archean eclogite-TTG gneiss mélange, a component of the Belomorian continental crust produced by subductional, accretionary, and collisional processes of the Belomorian collisional orogeny 2.9-2.66 Ga. The Paleoproterozoic history of the BP comprises of two prominent tectonic periods: (i) early Paleoproterozoic (~2.5-2.4 Ga), related to a superplume, and (ii) late Paleoproterozoic (2.0-1.85 Ga), resulted from crustal reworking during the Lapland-Kola collisional orogeny that produced strong penetrative metamorphic and local deformational overprint. The Paleoproterozoic highest-grade metamorphic overprint is represented by patches of eclogites (eclogite-2) in Paleoproterozoic mafic dikes and eclogite-1. Field relations between eclogite-1 and eclogite-2 are described in the Gridino area of the western coast of the White Sea. So, the BP is a high-grade polymetamorphic belt formed by a superposition of the Neoarchean Belomorian and Paleoproterozoic Lapland-Kola orogenies, whose characteristic features are eclogites produced by subduction and collision.
Earth and Planetary Science Letters, Vol. 572, 13p. Pdf
Mantle
eclogites
Abstract: Multiple sulphur isotopic compositions of sulphides from Kaapvaal craton mantle eclogites allow to elucidate the recycling of sulphur into the deep Earth and to differentiate between recycled crust and mantle origins of eclogite-hosted sulphides, including the precious metals that they capture. We present multiple sulphur isotope ratio measurements by secondary ion mass spectrometry for sulphides from a collection of mantle-derived eclogite xenoliths from Proterozoic and Mesozoic kimberlite occurrences in South Africa (Premier, Roberts Victor, Jagersfontein). Previous work established that the host eclogites have elemental and oxygen isotopic compositions in support of seawater-altered oceanic lithosphere protoliths, and for many of these xenolith suites Archean ages have been suggested. The eclogite-hosted sulphides have ?34S values from -5.7 to + 29 ‰, with the upper end of this wide range representing the highest-ever recorded ?34S composition of material derived from the Earth's mantle. The ?33S values range from -0.29 to + 0.18 ‰ and do not record significant mass-independent sulphur isotope fractionation, i.e., there is no compelling S-MIF signature. Most of the sulphide grains have ?34S values that fall within a range between -6 and + 4 ‰, and they probably retain an isotopic record of sulphides that formed originally within altered oceanic crust. In contrast, the highly positive ?34S values from +13 to + 29 ‰ detected in sulphide grains from a single eclogite xenolith are similar to those of marine sulphates, which were probably a minor sulphur component of the oceanic crustal protolith. The lack of a significant S-MIF signature in the eclogitic sulphides that show ?34S evidence for a recycled crust origin implies that this sulphur component stems from a < 2.4Ga post-Archean surficial reservoir. This finding suggests that the cratonic mantle eclogites may have formed from post-Archean oceanic crust (e.g., Paleoproterozoic eclogite protoliths), or - as is preferred here - the 'surficial' sulphur was introduced into the cratonic root during relatively young metasomatic events and is thus unrelated to eclogite petrogenesis and Archean continent formation.
Abstract: Igneous and metamorphic rocks contain the mineralogical and geochemical record of thermally driven processes on Earth. The generally accepted thermal budget of the mantle indicates a steady cooling trend since the Archean. The geological record, however, indicates this simple cooling model may not hold true. Subduction-related eclogites substantially emerge in the rock record from 2.1 Ga to 1.8 Ga, indicating that average mantle thermal conditions cooled below a critical threshold for widespread eclogite preservation. Following this period, eclogite disappeared again until ca. 1.1 Ga. Coincident with the transient emergence of eclogite, global granite chemistry recorded a decrease in Sr and Eu and increases in yttrium and heavy rare earth element (HREE) concentrations. These changes are most simply explained by warming of the thermal regime associated with granite genesis. We suggest that warming was caused by increased continental insulation of the mantle at this time. Ultimately, secular cooling of the mantle overcame insulation, allowing the second emergence and preservation of eclogite from ca. 1.1 Ga until present.
Earth and Planetary Science Letters, Vol. 580, 11p. Pdf
Mantle
eclogites
Abstract: Zinc isotopic compositions (ZnJMC-Lyon) of low-MgO (<13 wt.%) and high-MgO (>16 wt.%) eclogites from the Koidu kimberlite complex, Sierra Leone, West African Craton, help constrain the origins of cratonic eclogites. The Zn of low-MgO eclogites range from MORB-like to significantly higher values (0.21‰ to 0.75‰), and correlate inversely with Zn concentrations. Since marine carbonates are characterized by higher Zn and lower Zn concentration than basaltic rocks, the low-MgO eclogites are suggested to originate from altered oceanic crustal protoliths that underwent isotopic exchange with carbonates within the crust during subduction. Compared to low-MgO eclogites, all but one of the high-MgO eclogites also have high Zn (0.35‰ to 0.95‰), but they have lower Zn concentrations and Zn/Fe ratios, both of which are negatively correlated with MgO contents. These features point to formation of high-MgO eclogites via metasomatic overprinting of low-MgO eclogites through addition of secondary clinopyroxenes crystallized from infiltrating ultramafic melts. Thus, both low-MgO and high-MgO eclogites bear the imprint of subducted carbonate-bearing oceanic crust. Our study shows that the distinctively high-Zn signatures of marine carbonates can be retained in deeply subducted oceanic crust that may contribute to mantle sources of intraplate alkali basalts with elevated Zn and Zn/Fe. Therefore, Zn isotopes provide a viable means to trace carbonate recycling in the mantle.
Geochimica et Cosmochimica Acta, in press available, 53p.
China
eclogite
Abstract: Alkaline basalts occur widely in intraplate settings and carbonate-bearing mantle sources such as carbonated peridotites are increasingly regarded to play a key role in their formation. Carbonated eclogites, most likely the products of subducted carbonate-bearing altered oceanic crust, are probable alternative ingredients in the mantle sources of many intraplate alkaline basalts, highlighting the importance of the subduction-driven deep carbon cycle. However, this widely proposed hypothesis remains enigmatic because the recognition of low-MgO primitive alkaline basalts predicted by experiments is scarce. Here we show that Cenozoic continental intraplate alkaline basalts occurring above the stagnant oceanic slab in the mantle transition zone beneath the Hannuoba region, eastern China, display geochemical features consistent with their origin as low-degree partial melts of carbonate-bearing eclogites. Their MgO contents correlate positively with CaO, Ba/Th and Ti/Eu, but negatively with Dy/Yb and ?Nd. Remarkably, the most primitive alkaline basalts are characterized by low MgO (<5.25 wt.%), low heavy rare earth elements and Sc contents, low CaO/Al2O3 (<0.41), low Ti/Eu (<3380), but Dy/Yb (>7.1) higher than those of ocean island basalts (OIBs). These features cannot be ascribed to differentiation from high-MgO alkaline basalts because significant amounts of crystallization of clinopyroxene and garnet did not occur during ascent. Differentiation also cannot account for the correlations of time-integrated Sr-Nd isotopes with MgO, Dy/Yb and Ba/Th. Instead, the linear correlations mainly reflect strong interaction between ascending primitive alkaline melts and the lithospheric mantle. The compositions of primitive alkaline basalts reflect the key control of garnet and clinopyroxene in the mantle residue (eclogites), and the Ti, Zr and Hf anomalies further indicate the critical effect of carbonates in the eclogite source. Partial melting of the carbonate-bearing eclogites likely occurred in the uppermost asthenosphere. The production of alkaline basalts with low MgO contents by partial melting of carbonate-bearing eclogite below the peridotite solidus in an intraplate setting has been overlooked and the magmas were instead often considered to be highly evolved. Recycled altered oceanic crust thus may not only cause metasomatism of the deep mantle but may also serve as a direct source of mafic melts. These results on natural rocks support the experiment-based model for subducted altered oceanic crustal material and also indicate its diverse fate in the mantle.
Abstract: The origin of early continental lithosphere is enigmatic. Characteristics of eclogitic components in the cratonic lithospheric mantle (CLM) indicate that some CLM was likely constructed by stacking of subducted oceanic lithosphere in the Archean. However, the dynamic process of converting high-density, eclogite-bearing subducted oceanic lithosphere to buoyant CLM remains unclear. We investigate this process through numerical modeling and show that some subducted and stacked eclogites can be segregated into the asthenosphere through an episodic viscous drainage process lasting billions of years. This process increases the chemical buoyancy of the CLM, stabilizes the CLM, and promotes the preservation and redistribution of the eclogites in the CLM, explaining the current status of early subduction relicts in the CLM revealed by geophysical and petrological studies. Our results also demonstrate that the subduction stacking hypothesis does not conflict with the longevity of CLM.
