Eclogite Xenoliths Research Articles

Thermal history of Colorado Plateau lithosphere from Sm-Nd mineral geochronology of xenoliths

The thermal history of the lower crust and upper mantle of the Colorado Plateau region is reconstructed on the basis of Nd and Sr isotopes in minerals and whole rock xenoliths hosted by Tertiary minette and kimberlite. The mineral data (garnet and clinopyroxene) indicate that lower crustal granulite and amphibolite (equilibration depth ≈ 25 km; equilibration temperature ≈ 700 °C) were last equilibrated on a mineral scale at 1345 ± 10 Ma; a Sm-Nd garnet-whole rock age for a granitoid xenolith is also 1345 ± 10 Ma. Whole rock data indicate that the crustal rocks were extracted from the mantle at ca. 1900 Ma. The mineral ages, which are 30–100 m.y. younger than crystallization ages of Proterozoic “anorogenic” granitoids from regions bordering the Colorado Plateau, are interpreted as cooling ages set following the crustal thermal maximum at 1380–1440 Ma. The granulites and amphibolites have remained at temperatures below ≈450 °C since 1350 Ma. Two eclogite xenoliths (equilibration depth ≈45–60 km; equilibration temperature ≈600 °C), which are inferred to be Precambrian as well, yield Sm-Nd garnet-clinopyroxene ages of 21.6 ± 1.2 and 21.0 ± 0.8 Ma. The eclogite mineral ages are probably the ages of the host Garnet Ridge and Moses Rock diatremes, and require that Nd isotopes were maintained in equilibrium right up to the time of entrainment. The isotopic data and the mineral textures suggest that the eclogites were undergoing active recrystallization at 21 Ma. The contrast in mineral ages between granulite and eclogite xenoliths indicates that the equilibration temperatures of the two rock types reflect different times of equilibration, and therefore cannot be considered as evidence for a negative thermal gradient at depth. The Rb-Sr mineral data from the xenoliths give variable early Paleozoic and Proterozoic ages that cannot easily be assigned to geologic events.

Graphite- and diamond-bearing eclogite xenoliths from the Bellsbank kimberlites, Northern Cape, South Africa

One diamond-bearing and eight graphite-bearing eclogite xenoliths are described from the Bellsbank kimberlites, Cape Province, South Africa. Graphite mostly occurs as discrete grains which are commonly in the form of tabular prisms. Diamond is octahedral. Both Group I and Group II eclogite varieties are represented by the graphite-bearing specimens, while the single diamond-bearing eclogite is of the Group I variety. The carbon isotopic composition of the graphite varies from δ13C=−7‰ to δ13C=−2.8‰. This is within the range of carbon isotopic compositions for inclusion-free diamonds in kimberlite from this locality, suggesting that the carbon for the eclogites as well as some of the kimberlite diamonds are derived from the same source. The present day Nd isotopic compositions of clinopyroxene from three graphite-bearing xenoliths are slightly higher than the bulk earth estimate. Sr isotopic compositions of the clinopyroxene in these xenoliths vary from 87Sr/86Sr=0.703 to 87Sr/86Sr=0.706. This could be due to derivation of the xenoliths from a protolith with variable 87Sr/86Sr isotopic composition or could be the result of mixing between a low-Sr, high 87Sr/86Sr component and a high Sr, low 87Sr/86Sr component.

Archean mantle heterogeneity and the origin of diamondiferous eclogites, Siberia; evidence from stable isotopes and hydroxyl in garnet

Data are presented for the O isotopic composition of clinopyroxene and garnet, the C isotopic composition of diamond, and the OH- content of garnet from eclogite xenoliths brought to the surface by the Udachnaya kimberlite pipe, Yakutia, Siberia. Radiogenic isotopic data suggest that the eclogites could have been derived from an ultradepleted mantle at approximately 2.9 Ga (Pearson et al., 1995; Snyder et al., in preparation). O isotopic compositions generally show equilibration between the eclogitic minerals (Δ_(cpx-Grt) = 0.11-0.41‰) and have δ^(18)O_(SMOW) for both garnet and clinopyroxene that lie near the range of accepted mantle values of 5.7±0.7‰. However, several eclogites indicate significant deviations from this range, at higher values of 6.8-7.0‰. Also, two eclogites lie at the lower end of the mantle range, at values of 4.8 and 5.0‰ (all in clinopyroxene). C isotopic compositions of diamonds all have δ^(13)C_(PDB) in the range of -1 to -7‰ and are centered at approximately -5‰, also within the range of accepted mantle values. The OH- contents of the garnet are generally between 0 and 22 ppm (as H_(2)0), although two samples exceed 70 ppm. This range of OH- is similar to eclogitic garnet from the Kaapvaal craton of southern Africa. The mantle C isotopic values of associated diamonds, the majority of O isotopic data, and the low OH- content of the minerals, although suggesting a general lack of crustal participation in the formation of the Udachnaya eclogites, do not rule out the participation of some ancient crustal material. That these eclogites include both ^(18)O-enriched and ^(18)O-depleted types suggests that the protoliths may have been overprinted by both low- and high-temperature hydrothermal events (cf. Jacob et aI., 1994). A positive correlation between δ^(18)O and ^(87)Sr/^(86)Sr allows the interpretation of these eclogites as representing a cross section of an Archean ophiolite. However, the lack of a single coherent grouping on a plot of δ^(18)O vs. ^(87)Sr/^(86)Sr suggests that, if the Udachnaya eclogites were derived from oceanic crust, they cannot be cogenetic and must represent at least two separate ophiolite sequences. Conversely, if the eclogites are found to be cogenetic, then a totally different process affected their formation and a probable metasomatic mechanism was operative. Because of the total lack of correlation of δ^(18)O with other geochemical parameters, we find no compelling evidence that all eclogites are derived ultimately from oceanic crust.

Trace-element partitioning between garnet and clinopyroxene in mantle-derived pyroxenites and eclogites: P- T- X controls

Trace-element abundances in coexisting clinopyroxenes and garnets from five suites of mantle-derived garnet pyroxenite and eclogite xenoliths and two suites of eclogite inclusion pairs in diamonds have been determined by electron microprobe (Ti) and proton microprobe (Ni, Zn, Ga, Sr, Y, Zr). The sample sets provide gamet-clinopyroxene pairs from a range of depths on contrasting geothermal gradients (40–90 mW m −2) from South Africa, and western and eastern Australia. These data are used to assess the effects of phase composition, pressure ( P) and temperature ( T) on the partitioning of each element between garnet and clinopyroxene. Ni partitioning is moderately dependent on X Mg and strongly dependent on T. D Zn ( Zn Cpx Zn Gnt ) is ∼ 0.9±0.1 and may show a slight increase with T, but is independent of P. Ga partitioning is strongly dependent on X Jd Cpx as well as P and T. Sr and Y distributions are mainly controlled by X Ca Gnt and are weakly dependent on T but not on P. Zr partitioning is mainly dependent on T; D Zr decreases with increasing X Jd Cpx and possibly X Ca Gnt but shows no P effect. D T decreases markedly with increasing P (and possibly T) and has a weak tendency to increase with increasing X Jd Cpx. These results on trace-element partitioning in natural, equilibrated systems provide data for the modelling of melting and metasomatic processes in the upper mantle. They also provide a test dataset to evaluate experimental determinations of partition coefficients, and some relationships (e.g., D Zr, D Ni) may be used to cross-check thermobarometry calculations where spurious calculated Fe 3+ Fe 2+ ratios may give invalid temperature estimates.

Archaean Re–Os age for Siberian eclogites and constraints on Archaean tectonics

CONSIDERABLE uncertainty surrounds the nature and evolution of the Earth's mantle in Archaean times (>2.5 Gyr ago). Mantle-derived eclogite xenoliths erupted by kimberlites provide important clues in this regard, because their basaltic composition suggests that they may be remnants of an early (>4 Gyr) magma ocean1,2, subducted Archaean oceanic crust3–5, or crystallized high-pressure mantle melts6–9. Better constraints on the age and origin of mantle eclogites are thus important for our understanding of early Earth processes. Here we present rhenium–osmium isotope data for diamond-bearing eclogites from the Udachnaya kimberlite pipe in Siberia, which indicate formation in the Archaean (2.9 ± 0.4 Gyr). This age is too young for the eclogites to be remnants of early Earth differentiation, but overlaps the age range for crust generation and craton stabilization on both the Aldan and Anabar shields of the Siberian craton (2.85–3.2 Gyr)10,11. These data, together with 3.1 Gyr Re–Os model ages for diamond-bearing Udachnaya peridotites12, show that the Siberian craton lithosphere was at least 150 km thick (the minimum required for diamond stability) by the Mid-Archaean.

ReOs, SmNd, and RbSr isotope evidence for thick Archaean lithospheric mantle beneath the Siberian craton modified by multistage metasomatism

A suite of peridotite xenoliths from kimberlites intruding the Siberian craton indicate the presence of lithospheric mantle over 150 km thick at 350 Ma. We report Sr-Nd isotope data for minerals from the peridotite xenoliths together with osmium isotopic compositions for whole-rocks and two olivine separates. Additionally, the osmium isotopic composition of a carbonatite from Fort Portal, Uganda, has been measured in order to evaluate the effect of carbonatite metasomatism on mantle ReOs systematics. Osmium isotope compositions of peridotite xenoliths from the Mir and Udachnaya kimberlites vary from those characteristic of the oceanic mantle, to considerably less radiogenic values ( 187 Os 188 0s , 0.16469 to 0.10812), comparable to those previously found in other cratonic peridotites. In contrast, two eclogite xenoliths from Udachnaya have extremely radiogenic Os, 187 Os 188 Os , up to 9.67. The lowest peridotite osmium isotopic compositions require Re depletion in the mid-Archaean (3.2 Ga) and this age is interpreted as the time of differentiation of the Siberian cratonic lithospheric mantle. Archaean depletion ages for spinel peridotites of relatively shallow origin and garnet peridotites and dunites containing diamond indicate that the depleted lithosphere reached from the Moho to 150 Km depth at this time and has been stable for 3 Ga. ReOs and SmNd model ages for two eclogite xenoliths are also in the range of 2.7 to 3.1 Ga and support an ancient origin for the Siberian lithosphere. The oldest peridotite depletion ages and the eclogite model ages correspond to the oldest crustal ages obtained from the Anabar Shield of the Siberian craton, and suggest that the initiation of major crust formation and stabilisation of a thick cratonic keel were coeval. In general, the Siberian low-temperature peridotites are not as enriched in incompatible elements as those from the Kaapvaal craton yet their diopsides possess similar, low Sm Nd . The low incompatible element concentrations but LREE/MREE enrichment seen in some Siberian lherzolites suggest they may be the products of disequilibrium melting. Neodymium and strontium isotopic compositions of minerals from the peridotites are extremely heterogeneous ( ϵ Nd(350), −55.1 to 491; 87 Sr 86 Sr , 0.70253 to 0.72235). Subcalcic garnets of diamond inclusion-like composition within megacrystalline peridotites have ϵ Nd(350 values varying from −55.1 to −12.1. Depleted mantle model Nd ages are as old as 3.2 Ga permitting an ancient, enriched origin similar to that suggested for diamond inclusions (Richardson et al., 1984). Alternatively, consideration of the complex garnet SmNd isotope systematics and the presence of unsupported radiogenic Sr together with marked trace element zonation (Shimizu et al., 1994) suggest that these subcalcic garnets crystallised recently (close to the time of kimberlite eruption) from ancient, LREE-enriched, high Rb Sr precursors. We propose that the isotope systematics of subcalcic garnet diamond inclusions can also be interpreted in terms of a recent origin.

Re-Os, Sm-Nd, and Rb-Sr isotope evidence for thick Archaean lithospheric mantle beneath the Siberian craton modified by multistage metasomatism

A suite of peridotite xenoliths from kimberlites intruding the Siberian craton indicate the presence of lithospheric mantle over 150 km thick at 350 Ma. We report Sr-Nd isotope data for minerals from the peridotite xenoliths together with osmium isotopic compositions for whole-rocks and two olivine separates. Additionally, the osmium isotopic composition of a carbonatite from Fort Portal, Uganda, has been measured in order to evaluate the effect of carbonatite metasomatism on mantle ReOs systematics. Osmium isotope compositions of peridotite xenoliths from the Mir and Udachnaya kimberlites vary from those characteristic of the oceanic mantle, to considerably less radiogenic values (187Os1880s, 0.16469 to 0.10812), comparable to those previously found in other cratonic peridotites. In contrast, two eclogite xenoliths from Udachnaya have extremely radiogenic Os, 187Os188Os, up to 9.67. The lowest peridotite osmium isotopic compositions require Re depletion in the mid-Archaean (3.2 Ga) and this age is interpreted as the time of differentiation of the Siberian cratonic lithospheric mantle. Archaean depletion ages for spinel peridotites of relatively shallow origin and garnet peridotites and dunites containing diamond indicate that the depleted lithosphere reached from the Moho to 150 Km depth at this time and has been stable for 3 Ga. ReOs and SmNd model ages for two eclogite xenoliths are also in the range of 2.7 to 3.1 Ga and support an ancient origin for the Siberian lithosphere. The oldest peridotite depletion ages and the eclogite model ages correspond to the oldest crustal ages obtained from the Anabar Shield of the Siberian craton, and suggest that the initiation of major crust formation and stabilisation of a thick cratonic keel were coeval. In general, the Siberian low-temperature peridotites are not as enriched in incompatible elements as those from the Kaapvaal craton yet their diopsides possess similar, low SmNd. The low incompatible element concentrations but LREE/MREE enrichment seen in some Siberian lherzolites suggest they may be the products of disequilibrium melting. Neodymium and strontium isotopic compositions of minerals from the peridotites are extremely heterogeneous (ϵNd(350), −55.1 to 491; 87Sr86Sr, 0.70253 to 0.72235). Subcalcic garnets of diamond inclusion-like composition within megacrystalline peridotites have ϵNd(350 values varying from −55.1 to −12.1. Depleted mantle model Nd ages are as old as 3.2 Ga permitting an ancient, enriched origin similar to that suggested for diamond inclusions (Richardson et al., 1984). Alternatively, consideration of the complex garnet SmNd isotope systematics and the presence of unsupported radiogenic Sr together with marked trace element zonation (Shimizu et al., 1994) suggest that these subcalcic garnets crystallised recently (close to the time of kimberlite eruption) from ancient, LREE-enriched, high RbSr precursors. We propose that the isotope systematics of subcalcic garnet diamond inclusions can also be interpreted in terms of a recent origin.

Trace elements in diamond inclusions from eclogites reveal link to Archean granites

We report trace element data for rare garnet and clinopyroxene inclusions trapped within diamonds occurring in eclogite xenoliths from the Siberian craton. These inclusions are more depleted in incompatible trace elements and have lower Mg# than the eclogite host minerals, reflecting metasomatic enrichment of the eclogites after diamond formation by high Mg# melts that are enriched in incompatible trace elements (such as kimberlites). A reconstructed whole rock composition (using inclusion data) is severely depleted in light rare earth elements and niobium but shows a marked strontium enrichment. The melt in equilibrium with this eclogite is calculated to have had very high La Yb and Sr Nd and is Nb depleted (if rutile is present), characteristics similar to those of some convergent margin magmas but most especially Archean tonalitic rocks. Experimental petrologic studies demonstrate that eclogites of the type described here are in equilibrium with silicic melts such as tonalites or trondhjemites. Thus, both our trace element data and petrological considerations are consistent with the eclogites forming in equilibrium with Archean tonalitic or trondhjemitic magmas. Our data may therefore comprise the first evidence for the fate of these voluminous residues. A corollary of our findings is that trace element and isotopic data from eclogite minerals must be interpreted with caution because of the eclogite's complex evolutionary history. Specifically, models of early Earth differentiation based on isotopic data from Siberian eclogites [1,2] require re-evaluation.

Diamondiferous eclogites from Siberia: Remnants of Archean oceanic crust

We have investigated eight diamond-bearing bimineralic eclogite xenoliths from the Udachnaya Mine, Yakutia, Siberia, in terms of major elements, 87Sr 86Sr− , 143Nd 144Nd and oxygen isotopic ratios. The β 18 O-values, measured with the new laser-fluorination technique, are different from mantle values and range between 5.19 and 7.38%. with an average error of 0.08%.. Strontium and neodymium initial isotopic ratios for cpx are between 0.70226 and 0.70699 and 0.51170 and 0.51257, respectively. Chemically and petrographically, the Siberian eclogites are very similar to the South African eclogite suite from Roberts Victor or Bellsbank, the most important similarities being the late Archean age (2.76 Ga) and the δ 18 O values that deviate from mantle values. However, differences exist in detail, as no samples with δ 18 O values lower than mantle values have yet been reported from Siberia and the cesium concentrations of the Siberian eclogites are generally lower than those of the Roberts Victor eclogite suite. The data obtained from the studied sample suite are best explained by a model proposing an origin from Archean oceanic crust that was intensely altered prior to subduction to mantle depths. Using oxygen isotopic values, the effects of seawater alteration can be shown and the composition of the unaltered protolith qualitatively estimated. We propose that mantle eclogites from kimberlites were generated by a globally operating subduction process during the late Archean and that differences between samples from different cratons are small compared to their similarities.

Dating lower crust and upper mantle events: an ion microprobe study of xenoliths from kimberlitic pipes, South Australia

Zircons separated from five xenoliths (three quartzofeldspathic granulites, one mafic granulite and one eclogite) of lower crustal and uppermost mantle derivation were dated using the SHRIMP ion microprobe. The zircon data indicate that different types of xenoliths had different origins and formed at different times. The xenoliths are from the Calcutteroo kimberlitic pipes (167–174 Ma) that outcrop on the eastern margin of the Australian craton. The combined data-set of all zircon UPb ages from all the xenoliths records four major tectonic events associated with zircon formation, recrystallization or Pb loss. These events at about 1600-1500, 780, 620 or earlier and 330 Ma, correlate with large-scale tectonic events recorded in adjacent crustal and mantle rocks: the ∼ 1600 magmatism and tectonism of the exposed Willyama Complex and Gawler Block (in New South Wales and South Australia), the 850-650 Ma rifting and faulting of the Adelaidean Fold Belt, the 650-575 Ma extension, uplift and collision episodes at the Tasman suture line and the 500-300 Ma metasomatic events in the mantle in western Victoria. However, more detail is provided by the data from individual xenolith types. A considerable number of zircons from the quartzofeldspathic granulite xenoliths grew during the granulite facies metamorphism which occurred at some time between 1600 and 1500 Ma. A small number of zircons with ages up to 1800 Ma reflect the xenoliths' prior history and other zircons grew at 610 Ma or earlier. Zircons from the mafic granulite xenolith record an event at ∼ 780 Ma, interpreted as the age of the granulite facies metamorphism of the protolith, probably shortly after the underplating and differentiation of the mantle-derived mafic magma. Zircons from the eclogite xenolith have very low U (4–13 ppm) and Th (<3 ppm) contents, similar to those observed in zircons from some exposed eclogites of N-type MORB magma derivation. Eclogite facies metamorphism not later than 620±46 Ma is inferred from the oldest 206Pb 238 U zircon age obtained from the xenolith. An event at ∼ 330 Ma caused Pb loss in zircons from all three types of xenoliths. The coincidence of the zircon age clusters with large-scale crustal or mantle events indicates the coupling of major mantle and crustal tectonic episodes in this region. The identification of multiple zircon-forming events and the inferred different times of formation of different xenolith assemblages show that SmNd isochron ages based on an assumption of a cogenetic relationship are invalid.

The characterisation and origin of graphite in cratonic lithospheric mantle: a petrological carbon isotope and Raman spectroscopic study

Graphite-bearing peridotites, pyroxenites and eclogite xenoliths from the Kaapvaal craton of southern Africa and the Siberian craton, Russia, have been studied with the aim of: 1) better characterising the abundance and distribution of elemental carbon in the shallow continental lithospheric mantle; (2) determining the isotopic composition of the graphite; (3) testing for significant metastability of graphite in mantle rocks using mineral thermobarometry. Graphite crystals in peridotie, pyroxenite and eclogite xenoliths have X-ray diffraction patterns and Raman spectra characteristic of highly crystalline graphite of high-temperature origin and are interpreted to have crystallised within the mantle. Thermobarometry on the graphite-peridotite assemblages using a variety of element partitions and formulations yield estimated equilibration conditions that plot at lower temperatures and pressures than diamondiferous assemblages. Moreover, estimated pressures and temperatures for the graphite-peridotites fall almost exclusively within the experimentally determined graphite stability field and thus we find no evidence for substantial graphite metastability. The carbon isotopic composition of graphite in peridotites from this and other studies varies from δ13 CPDB = − 12.3 to − −3.8%o with a mean of-6.7‰, σ=2.1 (n=22) and a mode between-7 and-6‰. This mean is within one standard deviation of the-4‰ mean displayed by diamonds from peridotite xenoliths, and is identical to that of diamonds containing peridotite-suite inclusions. The carbon isotope range of graphite and diamonds in peridotites is more restricted than that observed for either phase in eclogites or pyroxenites. The isotopic range displayed by peridotite-suite graphite and diamond encompasses the carbon isotope range observed in mid-ocean-ridge-basalt (MORB) glasses and ocean-island basalts (OIB). Similarity between the isotopic compositions of carbon associated with cratonic peridotites and the carbon (as CO2) in oceanic magmas (MORB/OIB) indicates that the source of the fluids that deposited carbon, as graphite or diamond, in catonic peridotites lies within the convecting mantle, below the lithosphere. Textural observations provide evidence that some of graphite in cratonic peridotites is of sub-solidus metasomatic origin, probably deposited from a cooling C-H-O fluid phase permeating the lithosphere along fractures. Macrocrystalline graphite of primary appearance has not been found in mantle xenoliths from kimberlitic or basaltic rocks erupted away from cratonic areas. Hence, graphite in mantle-derived xenoliths appears to be restricted to Archaean cratons and occurs exclusively in low-temperature, coarse peridotites thought to be characteristic of the lithospheric mantle. The tectonic association of graphite within the mantle is very similar to that of diamond. It is unlikely that this restricted occurrence is due solely to unique conditions of oxygen fugacity in the cratonic lithospheric mantle because some peridotite xenoliths from off-craton localities are as reduced as those from within cratons. Radiogenic isotope systematics of peridotite-suite diamond inclusions suggest that diamond crystallisation was not directly related to the melting events that formed lithospheric peridotites. However, some diamond (and graphite?) crystallisation in southern Africa occurred within the time span associated with the stabilisation of the lithospheric mantle (Pearson et al. 1993). The nature of the process causing localisation of carbon in cratonic mantle roots is not yet clearly understood.

Diamondiferous Eclogites from the Udachnaya Kimberlite Pipe, Yakutia

Xenoliths from the upper mantle have undergone a wide variety of processes at varied temperatures and pressures, as recorded by mineral compositions and textures. Eclogite xenoliths in kimberlites are a unique source from which to obtain information about such processes. Furthermore, eclogites that contain accessory diamond yield important compositional constraints on the deeper upper mantle. Whether eclogites have mantle or crustal origins is still a subject of controversy. Mineralogy and petrography of 29 eclogite xenoliths from the Udachnaya kimberlite in Yakutia, Siberia are presented and combined with previous studies of these eclogites by our group (Jerde et al., 1993, 1994; Snyder et al., 1993a; Sobolev et al., 1994). Five different petrographic groups are defined, based on texture, mineral color, and degree of alteration. Chemical compositions of eclogitic minerals span a complete range from high to low-jadeite content in clinopyroxenes and from pyrope-almandine to grossular in garnets. Eclogites ...

Diamondiferous Eclogites from Udachnaya: A Subducted Component in the Siberian Upper Mantle

Many years of study of South African mantle samples have demonstrated the existence of a component related to suduction which is exemplified by eclogite xenoliths which are interpreted to represent parts of metamorphosed subducted Archean oceanic ernst (e.g. Jagoutz et al., 1984; MacGregor and Manton, 1986). It is not clear if these interpretations apply worldwide because detailed studies have until very recently been restricted to South African samples. We have investigated eight diamond-bearing bimineralic eclogite xenoliths from the Udachnaya Mine, Yakutia, Siberia in terms of major elements, S7Sr/S6Sr-, 143Nd/144Ndand oxygen isotopic ratios in order to elucidate Siberian mantle history, to compare it with that of the South African craton, and to investigate the homogeneity of eclogite-forming processes on a global scale.Analytical methods:Oxygen isotope ratios were measured on mineral separates with the new laser-fluorination technique (Mattey and Macpherson, 1993). Ongoing work on mantle eclogites and diamond inclusions (Lowry et al., 1993 and Lowry et al. in prep.) has produced an 18 identical range of 8 O-values compared to the conventional method. 143Nd/144Ndand S7Sr/86Srratios were obtained from the same ultraclean mineral separates of fresh garnets and clinopyroxenes.

Exsolution of garnet within clinopyroxene of mantle eclogites: major- and trace-element chemistry

Eclogite xenoliths from the mantle have experienced a wide variety of processes and P-T conditions, many of which are recorded in the mineral compositions and textures. Exsolution of garnet from clinopyroxene is one such texture, occurring in a minority of mantle eclogites. New analyses of clinopyroxene and garnet of eclogite xenoliths from kimberlites at Bellsbank (South Africa) and Obnazhennaya (Yakutia, Russia) are presented here, and these are combined with data from the literature. Exsolution of garnet from clinopyroxene is generally lamellar, although lens-shaped garnets are also present. Major- and trace-element characteristics show a wide range of compositions and include eclogite Groups A, B, and C. Rare-earth element (REE) concentrations of garnet and pyroxene were determined by SIMS, and the REE patterns are subtly different from those in “ordinary” eclogites. Differences include the absence of prominent Eu anomalies in samples of this study and differences in the slopes of chondrite-normalized REE patterns. It is possible that these “signatures” are unique to exsolved eclogites, a result of subsolidus elemental partitioning during exsolution. Some reconstructed whole-rock compositions are aluminuous; comparison with ordinary eclogites shows only minor differences, implying a similar origin. If the immediate precursor to the exsolved eclogites was a monomineralic pyroxenite, the excess aluminium was tied up in Tschermak's molecule, although the occasional presence of kyanite exsolution lamellae is indicative of a Ca-Eskola component. Reconstructed “pyroxenes” from kyanite- and corundum-rich samples contain unrealistic amounts of aluminium for mantle pyroxenes. A protolith (or parental pyroxene) “threshold” of ∼24% Al2O3 may exist, above which (as in a plagioclase cumulate) the final assemblage is kyanite- and/or corundum-bearing.

Depth-related carbon isotope and nitrogen concentration variability in the mantle below the Orapa kimberlite, Botswana, Africa

Cubic diamonds from the Orapa kimberlite have a very restricted δ 13 C range with a mean of −5.98 ± 0.57‰, a relatively high and constant nitrogen content 897 ± 171 ppm and low nitrogen aggregation state. They are thought to have been derived from shallow mantle depth. Diamonds with peridotitic inclusions (P-Type) range in δ 13 C between−4.2 and − 18.9‰ with a major mode between −5 and−7‰. Their nitrogen content is low and state of aggregation high. Pressure-temperature ( P- T) equilibration conditions estimated for their inclusions are close to the wet peridotite solidus. Eclogitic inclusions in Orapa diamonds (E-Type) indicate higher P- T equilibration conditions than P-Type minerals. The δ 13 C values of their hosts vary from −2.6 to−18.0%. with a major mode between −13 and −15‰. The nitrogen content of E-type diamonds is significantly higher than that of P-Type diamonds. Comparison of the mineral and carbon isotopic composition of Orapa diamond eclogites with those of E-Type diamonds indicates that only part of the E-Type diamonds could have been derived from a physical breakup of diamond eclogites. 13C-depleted E-Type diamonds and eclogite xenoliths with low δ 13 C diamonds were formed in a similar and restricted P- T range. An inclusion mineral paragenesis with compositions transitional between those of peridotitic and eclogitic minerals (websteritic, W-Type) has been recognized. It is characterized by a unique combination of a relatively high chrome and Fe content. The δ 13 C values of W-Type diamonds lie, with one exception (−6.9%.), in the range of −15.2 to −22.4‰. The nitrogen content of these diamonds is significantly lower than that of E-Type and is indistinguishable from that of P-Type diamonds. The P- T equilibration conditions estimated for them are similar to those of the peridotitic paragenesis. The collective data indicate a higher frequency of occurrence of 13C-depleted diamonds, a decrease in the N content of diamonds, and an increase in the N aggregation state with increasing mantle depth. The decline in N content is not monotonic however, because P-Type diamonds tend to have lower N contents than E-Type diamonds. The combined high chrome and Fe content of the websteritic diamond inclusions are very difficult to reconcile with a subduction origin of the low δ 13 C host and indicate that 13C-depleted C may be a primary mantle feature.

Diamondiferous eclogites from Yakutia, Siberia: evidence for a diversity of protoliths

Major-element and REE compositions of 14 diamondiferous eclogites from the Udachnaya kimberlite in Yakutia, Siberia have been determined by electron microprobe and secondary ion mass spectrometer (SIMS). Based on previous clinopyroxene classification schemes (e.g., Taylor and Neal 1989), all of these eclogite xenoliths belong to Group B/C, although some of the garnet compositions and mineral REE abundances are inconsistent with the indicated groups. This demonstrates the inadequacy of the classification scheme based on African eclogites for application to Siberian samples. Because of the coarse grain size of the Udachnaya nodules, meaningful modal abundances could not be obtained. However, reconstructed REE compositions using various garnet: clinopyroxene ratios demonstrate relative insensitivity to changes in mode for common eclogitic assemblages. Many of these reconstructed REE compositions show LREE depletions. Some depletions are consistent with an origin (either directly or through partial melting) as “normal” or Type-I ocean floor basalt. Others, however, require material of eclogitic or pyroxenitic affinities to undergo partial melting; this facilitates the depletion of LREE while leaving the HREE at nearly original levels. Many of the eclogites of South Africa are consistent with a protolith of “anomalous” or Type II ocean floor basalt. This fundamental difference between the two regions is the likely cause of the inconsistencies with the chemicallybased classification.

Nd and Sr isotope chronostratigraphy of Colorado Plateau lithosphere: implications for magmatic and tectonic underplating of the continental crust

Nd-Sr isotopes and major and trace element abundances were measured in lower and upper crustal xenoliths carried in Tertiary minettes and kimberlitic diatremes of the Colorado Plateau. The objective was to establish the vertical distribution of mantle extraction ages for an undisturbed Precambrian crustal province characterized by normal thickness and low cratonic heat flow, and to thereby evaluate the importance of magmatic or tectonic underplating in crustal growth. Mid-Proterozoic-age (1.7–2.0 Ga) material is found at all levels of the crust. Although mafic granulites and amphibolites have major and trace element compositions compatible with an origin as magmatically underplated material, Nd model ages do not indicate that there has been significant mantle contribution to the lower crust by igneous processes (such as magmatic underplating) since the main crust-forming event ca. 1.85 Ga ago. The meta-igneous lower crustal xenoliths have relatively radiogenic Nd isotopic compositions and unradiogenic Sr compositions. There is little variability in Nd model ages in spite of large variability in Sm/Nd. High and variable Sm/Nd ratios were therefore established early in the history of the protolith, during igneous or metamorphic processes coincident with or shortly after extraction from the mantle. Metapelitic xenoliths, tectonically emplaced into the lower crust, have the same Proterozoic model ages as meta-igneous parts of the lower crust. These paragneisses are probably arc-associated sediments that were underthrust during terrane accretion in the Proterozoic. The isotopic data indicate that eclogite xenoliths are likely to represent Proterozoic lithosphere rather than subducted Mesozoic MORB, as previously proposed. A number of the eclogite xenoliths have experienced a metasomatic event which introduced LREE, SiO 2 and Na 2O and shifted the Nd and Sr isotopic compositions. The least-altered eclogite xenoliths have major and trace element and radiogenic Nd isotopic compositions similar to those of other Proterozoic mafic, lower crustal xenoliths. A more plausible protolith for the eclogites is Proterozoic lithosphere that was metamorphosed to eclogite facies conditions either during Precambrian continental assembly or in more recent times.

Fluid inclusion and mineral isotopic compositions (H [sbnd]C [sbnd]O) in eclogitic rocks as tracers of local fluid migration during high-pressure metamorphism

Eclogite facies metagabbros from the Monviso ophiolitic complex (Italian Western Alps) provide a unique opportunity to trace fluid migration processes in a portion of the oceanic crust that has undergone subduction at a depth of > 40 km. We have determined the omphacites δ 18O and the abundance, δD and δ 13C of hydrous and carbonaceous compounds present in whole rocks which are believed to trace the residual phases of what was mobilized in the original rocks during subduction. Prograde dehydration reactions and eclogitization of hydrothermally altered oceanic metagabbros was accompanied by approximately 90% fluid loss. The remaining fluid was trapped as primary water-rich fluid inclusions in omphacite megacrysts that developed at the expense of the precursor magmatic pyroxene. Deformation of the eclogitic rocks resulted in continuous recycling of fluid between mylonites and omphacite veins without further fluid loss from the host ductile shear zone. Oxygen isotopes of omphacite (omp) and hydrogen isotopes of water in the fluid inclusions (FI), analyzed in low-strain rocks, mylonites and undeformed/deformed veins show marked variations, δ 18O omp and δD FI values ranging from +3.0 to +5.3‰ and from −31 to −93‰, respectively. Detailed isotopic analysis of several individual vein-wallrock pairs show that the scale of isotopic equilibration is of the order of one centimeter. Therefore, the eclogitic minerals and fluids filling the veins are concluded to be locally derived. Our results argue against recent models which suggest large-scale mass flushing of isotopically homogeneous fluids during subduction zone metamorphism. On average, the H 2O contents and δD FI value are within the upper mantle range. The carbon has been inherited from the metamorphic transformation of the original carbon present in the oceanic crust and has a mean δ 13C value of −24.2 ± 1.2‰. Two carbonaceous components can be recognized, condensed carbon which represents the major carbon species and carbonate daughter crystals present in fluid inclusions. δ 18O omp values are significantly lower than those reported for mantle minerals and are similar to pyroxene and whole-rock values from hydrothermally altered oceanic crust. It is suggested that the isotopic imprint of the Monviso eclogitic minerals and fluids represents the signature of mid-ocean ridge hydrothermal alteration and subduction zone eclogitization processes. The rocks studied could be the missing link between altered oceanic gabbros and eclogitic xenoliths. In addition, the range in δ 18O omp values recorded in the different microstructural domains covers most of the δ 18O values reported in type A, B and C eclogitic xenoliths, implying that caution must be exercised when using oxygen isotopes as an indicator of the origin of eclogitic xenoliths.

Collapse of Archean orogens and the generation of late- to postkinematic granitoids

Intrusion of late-stage granitoids into Archean granite-greenstone terranes may be spatially and temporally related to late-orogenic upper-crustal extensional collapse and formation of metamorphic core complexes. The late- to postkinematic granitoid magmas are derived through partial melting of upper mantle and lower crustal sources, and are locally associated with late-stage extensional structures. Thick roots of depleted lithospheric mantle of Archean age (tectosphere) beneath Archean cratons makes melting of the upper mantle difficult. It is proposed that the Archean tectosphere is made of imbricated slabs of buoyant Archean oceanic crust and lithosphere, together with trapped wedges of mantle; this would explain several seemingly contradictory phenomena. Decompression from upper crustal extension generates significant amounts of basaltic melts from the trapped wedges of fertile mantle. These basaltic magmas intrude and partially melt the lower crust; becoming more silicic in composition, they migrate upward to solidify in the middle and upper crust, forming the late- to postkinematic granitoid suite. The tectosphere in this model is less dense and colder than surrounding asthenosphere and is consistent with eclogitic xenoliths from subducted oceanic lithosphere found entrained in kimberlites. The direction and magnitude of shear-wave anisotropy in the upper mantle is also explained in this model through the orientation of olivine crystals in the underplated slabs of oceanic lithosphere.

Single crystal infrared spectra in the range of OH fundamentals of paragenetic garnet, omphacite and kyanite in an eklogitic mantle xenolith

Single crystals of paragenetic garnet (gt), omphacite (cpx) and kyanite (ky) were isolated from an eclogite xenolith from the Zagadochnaya kimberlite, Yakutia (grospydite Z13) and studied by polarized FTIR-microspectrometry in the OH valence vibrational region and by microprobe analyses. The coexisting minerals are homogeneous with respect to major and minor elements and have compositions near gross49pyr26alm20uvar4 (gt), jad45dio47hed6kos2 (cpx) and ky>97. Single crystal spectra show one νOH-band for gt at 3630 cm−1 (halfwidth ca. 100 cm−1) which is very likely caused by vibrations of tetrahedral (OH)4-4− clusters replacing SiO44− tetrahedra. Cpx shows one strong, but weakly pleochroic band at 3464 cm−1 (halfwidth ca. 160 cm−1) and a weak satellite band centered at 3620 cm−1) with a distinct pleochroism. Ky OH spectra exhibit two sets of weak sharp pleochroic bands, a triplet, characteristic for high pressure ky, at 3439, 3410, 3387 cm−1) and a doublet at 3279, 3264 cm−1) (halfwidths ca. 10 cm−1) From integral and linear absorbances in the unpolarized spectra defect-hydroxyl contents in the three coexisting minerals were estimated and found near O.OX wt% H2O. The distribution scheme of hydrogen in the paragenetic minerals was evaluated to be cky < ccpx ≪< cgt.