Bimineralic Eclogites Research Articles

Linking granulites, intraplate magmatism, and bi-mineralic eclogites with a thermodynamic-petrological model of melt-solid interaction at the base of anorogenic lower continental crust

This study reports results from thermodynamic phase equilibrium and trace element modelling of mafic magmatic underplating and solid-liquid interaction in the lower continental crust (LCC) in intraplate settings. The arrival of underplating basalt sills into thin (∼30 km at 8 kbar) and thicker (∼45 km at 12 kbar) andesitic and basaltic LCC precursors was simulated with heating and batch-melting to yield refractory residues. Continued magmatic invasion of the LCC was then modelled at the same pressures with hybridisation between the residual solids and basaltic and picritic liquids (at 80:20 and 50:50 proportions). The first finding is that hybridisation with basalt increases the stability fields of 2-pyroxene-plagioclase and garnet-clinopyroxene-plagioclase granulites, by far the dominant LCC xenoliths found in anorogenic settings. The second finding pertains to situations when the liquid proportion of the hybrid system is lower than the fraction of the incoming mafic liquid. The resulting liquids share many characteristics with silicic volcanic rocks in continental flood basalt (CFB) provinces, including low Al and high Ti-concentrations. Stoichiometric melting reactions from 80:20 residual andesite:basalt hybridisation reproduce trace element patterns of silicic eruptives with potential for decoupling of Sr-Nd-Pb isotope systematics, as found in CFB rhyolites and LCC xenoliths. Incongruent melting of hornblende and garnet (the A in AFC) from the residual solid and dominant peritectic fractional crystallisation of plagioclase (the FC in AFC) from the liquid occur. A notable feature of LCC-picrite hybridisation is that solid residues after felsic liquid extraction, when subjected to higher pressure, are buffered to olivine and quartz-free compositions. Consequently, if these high-pressure granulite residues delaminated, they would convert to the bi-mineralic high-Mg eclogites found as xenoliths in kimberlites. The modelled residues also have the Sr-enrichment and positive Eu-anomalies of eclogite xenoliths. Solid-liquid interaction in intraplate settings has been and remains a significant process in crustal growth and evolution.

Fe3+ Distribution and Fe3+/ΣFe-Oxygen Fugacity Variations in Kimberlite-Borne Eclogite Xenoliths, with Comments on Clinopyroxene-Garnet Oxy-Thermobarometry

Abstract The valence state of iron in mantle-derived melts, such as mid-ocean ridge basalt, is a useful proxy for oxygen fugacity (ƒO2). On subduction, oceanic crust undergoes metamorphic reactions that alter its initial ƒO2, generating compositional and redox heterogeneity in the supra-subduction zone and in the convecting mantle source of oceanic basalts. Kimberlite-borne eclogite xenoliths with ancient oceanic crustal protoliths represent an important archive to trace these processes in deep time. We determined, by Mössbauer spectroscopy, Fe3+/ΣFe for garnet (0.03–0.15, average 0.08; n = 13) and clinopyroxene (0.05–0.37, average 0.23; n = 11) in bimineralic eclogite xenoliths from the Udachnaya kimberlite pipe (Siberian craton), combined the results with high-quality literature data to estimate bulk Fe3+/ΣFe and ƒO2, assessed associated uncertainties and discussed petrological implications. The incorporation of Fe3+ in, and distribution between, eclogite minerals is controlled by chemical composition (X), temperature (T), pressure (P) and ƒO2. Therefore, Fe3+/ΣFe in garnet alone is an imperfect proxy for bulk Fe3+/ΣFe. Values for the distribution of Fe3+/ΣFe between clinopyroxene and garnet (D(Fe3+/ΣFe)cpx-grt) range from 1.4 ± 0.1 to 20 ± 13 and correlate strongly with the abundance distribution of another polyvalent element, vanadium (r2 = 0.80, n = 28), probably because both vary as a function of X-T–P-ƒO2. This allows to estimate Fe3+/ΣFe in clinopyroxene and bulk rocks (0.01–0.32, average 0.06, n = 159) for the majority of literature data where only garnet Fe3+/ΣFe is known. Low Fe3+/ΣFe is retained in many high-Al2O3 eclogites with plagioclase-rich cumulate protoliths (Eu/Eu*> > 1), despite extended residence in the cratonic lithosphere. Bulk Fe3+/ΣFe increases during mantle metasomatism but is not particularly linked to enrichment in highly incompatible elements. Low grossular content in garnet ensuing from mantle metasomatism, especially at low temperature, limits uptake of Fe3+, which is accommodated in clinopyroxene instead, leading to very high D(Fe3+/ΣFe)cpx-grt. Eclogite oxybarometry requires that Fe3+/ΣFe of only garnet be known, whereby the input temperatures should be calculated with all Fe as Fe2+. For temperatures projected onto regional steady-state geothermal gradients, ƒO2 values relative to the fayalite-magnetite-quartz buffer (∆logƒO2(FMQ)) range from −5.9 to −0.2 (average − 3.1, n = 174), and are too low to stabilise oxidised sulphur in all and oxidised carbon in the vast majority of samples. Thus, ancient oceanic crust was a sink rather than a source of oxygen. In particular CaO-rich eclogite xenoliths occasionally are corundum-bearing and SiO2-undersaturated, typically with silica-deficient clinopyroxene with cations per formula unit <1.97 for 6 O anions, resulting in overestimated ƒO2. For the remaining samples, cryptic metasomatism, with enrichment in highly incompatible elements, is clearly focused in the ƒO2 interval of FMQ−4 to FMQ−3, indicating that mantle metasomatism can have both an oxidising and a reducing effect on eclogite. Unmetasomatised bimineralic eclogites have lower ƒO2 than coesite-bearing ones, reflecting reduction during metamorphism and silica-consuming partial melting of ancient subducted oceanic crust.

Neoproterozoic Eclogite-to Granulite-Facies Transition in the Ubendian Belt, Tanzania, and the Timescale of Continental Collision

AbstractIn collision-type orogens, where high-pressure and ultrahigh-pressure (HP–UHP) metamorphism usually occurs, deeply subducted continental slabs with eclogitized mafic rocks often undergo recrystallization/overprinting with various geothermal gradients after the peak conditions at lower-to-middle-crustal levels. During the crustal stabilization, the transition from eclogite-to granulite-facies is common. We conducted metamorphic petrology and zircon geochronology on (1) bimineralic and (2) partially granulitized eclogites from the Neoproterozoic Ufipa Terrane (Southwestern Tanzania). Microtextural relationships and mineral chemistry define three metamorphic stages: eclogite metamorphism (M1), HP granulite-facies overprinting (M2), and amphibolite-facies retrogression (M3). The bimineralic eclogite has a basaltic composition and lacks M2 minerals. In contrast, the kyanite eclogite is characterized by a gabbro-dioritic whole-rock composition and contains inherited magmatic zircon. Although the matrix is highly granulitized, garnet and kyanite contain eclogite-facies mineral inclusions. Phase equilibria modeling revealed P–T conditions of 2.1–2.6 GPa and 650–860°C for the M1 stage and 1.4–1.6 GPa and 750–940°C for the M2 stage. Zircon with eclogite-facies mineral inclusions from the bimineralic eclogite lacks Eu anomaly in the REE patterns and yielded the M1 eclogite metamorphic age of 588 ± 3 Ma. Zircon overgrowths surrounding the inherited Paleoproterozoic magmatic cores in kyanite eclogite yielded 562 ± 3 Ma. A weak negative Eu anomaly in the REE patterns and the absence of eclogitic mineral inclusions suggest the zircon growths at the M2 HP granulite-facies metamorphic stage. These new data indicate an eclogite-to granulite-facies transition time of 26 ± 4 million years (Myr), suggesting a rate of HP rock exhumation toward a lower crustal level of 0.7–1.5 mm/year. Furthermore, the density evolution model indicates that buoyant host orthogneiss with low-density gabbro-dioritic eclogite plays an important role in carrying high-density basaltic eclogite. Our 2D thermomechanical modeling also suggests that a slab break-off with a lower angle subduction of <20° triggers the exhumation of the HP slab sliver with 20–30 Myr eclogite-to granulite transition time of large HP–UHP terranes in major collision zones.

Relationships Between Eclogite‐Facies Mineral Assemblages, Deformation Microstructures, and Seismic Properties in the Yuka Terrane, North Qaidam Ultrahigh‐Pressure Metamorphic Belt, NW China

AbstractTo understand the relationships between eclogite‐facies mineral assemblages, deformation microstructures, and the seismic properties of subducting oceanic crust, eclogites from the Yuka terrane, North Qaidam ultrahigh‐pressure metamorphic belt, NW China, were studied. Observations of mineral textures, deformation microstructures, and petrofabrics in the eclogites indicate that garnet, omphacite, and phengite were deformed by intracrystalline deformation (i.e., dislocation creep) during prograde metamorphism. In contrast, amphibole, which was formed by the topotactic replacement of omphacite at fluid‐present conditions, is considered to have been deformed by diffusional flow (dissolution‐precipitation creep) during amphibolite‐facies retrogression associated with exhumation. Based on the petrofabrics in the samples, the seismic properties of the eclogites were calculated depending on eclogite‐facies mineral assemblages such as garnet + omphacite, garnet + omphacite + phengite, garnet + omphacite + phengite + lawsonite, garnet + omphacite + phengite + amphibole, and garnet + omphacite + amphibole. We found that the seismic signatures of each of the eclogite‐facies mineral assemblages were different. In particular, phengite‐bearing eclogites (the garnet + omphacite + phengite/garnet + omphacite + phengite + amphibole assemblages), depending on phengite content, produced the strongest seismic anisotropy (AVp and AVs), with a strong polarization anisotropy, that was at least three times higher than bimineralic (phengite‐absent) eclogites (garnet + omphacite assemblage). Our results indicate that phengite, as a stable phase at high‐pressure and high‐temperature conditions, can play an important role in the creation of trench‐parallel seismic anisotropy in the eclogite‐facies mineral assemblages found in subduction zones.

Subduction relics in the subcontinental lithospheric mantle evidence from variation in the δ18O value of eclogite xenoliths from the Kaapvaal craton

Mantle eclogites are commonly accepted as evidence for ancient altered subducted oceanic crust preserved in the subcontinental lithospheric mantle (SCLM), yet the mechanism and extent of crustal recycling in the Archaean remains poorly constrained. In this study, we focus on the petrological and geochemical characteristics of 58 eclogite xenoliths from the Roberts Victor and Jagersfontein kimberlites, South Africa. Non-metasomatized samples preserved in the cratonic root have variable textures and comprise bimineralic (garnet (gt)–omphacite (cpx)), as well as kyanite (ky)- and corundum (cor)-bearing eclogites. The bimineralic samples were derived from a high-Mg variety, corresponding to depths of ~ 100–180 km, and a low-Mg variety corresponding to depths of ~ 180–250 km. The high-Al (ky-, cor-bearing) eclogites originated from the lowermost part of the cratonic root, and have the lowest REE abundances, and the most pronounced positive Eu and Sr anomalies. On the basis of the strong positive correlation between gt and cpx δ18O values (r2 = 0.98), we argue that δ18O values are unaffected by mantle processes or exhumation. The cpx and gt are in oxygen isotope equilibrium over a wide range in δ18O values (e.g., 1.1–7.6‰ in garnet) with a bi-modal distribution (peaks at ~ 3.6 and ~ 6.4‰) with respect to mantle garnet values (5.1 ± 0.3‰). Reconstructed whole-rock major and trace element compositions (e.g., MgO variation with respect to Mg#, Al2O3, LREE/HREE) of bimineralic eclogites are consistent with their protolith being oceanic crust that crystallized from a picritic liquid, marked by variable degrees of partial melt extraction. Kyanite and corundum-bearing eclogites, however, have compositions consistent with a gabbroic and pyroxene-dominated protolith, respectively. The wide range in reconstructed whole-rock δ18O values is consistent with a broadly picritic to pyroxene-rich cumulative sequence of depleted oceanic crust, which underwent hydrothermal alteration at variable temperatures. The range in δ18O values extends significantly lower than that of present day oceanic crust and Cretaceous ophiolites, and this might be due to a combination of lower δ18O values of seawater in the Archaean or a higher temperature of seawater–oceanic crust interaction.

Diamond formation during metasomatism of mantle eclogite by chloride-carbonate melt

A xenolith of bimineralic eclogite from the Udachnaya kimberlite pipe provides a snapshot of interaction between mantle rocks and diamond-forming fluids/melts. The major-element composition of the eclogite is similar to that of N-MORB and/or oceanic gabbros, but its trace-element pattern shows the effects of mantle metasomatism, which resulted in diamond formation. The diamonds are clustered in alteration veins that crosscut primary garnet and clinopyroxene. The diamonds contain microinclusions of a fluid/melt dominated by carbonate and KCl. Compared to the worldwide dataset, the microinclusions in these diamonds fall in middle of the range between saline fluids and low-Mg carbonatitic melts. The fluid/melt acted as a metasomatic agent that percolated through ancient eclogitic rocks stored in the mantle. This interaction is consistent with calculated partition coefficients between the rock-forming minerals and diamond-forming fluid/melt, which are similar to experimentally-determined values. Some differences between the calculated and experimental values may be due to the low contents of water and silicates in the chloride-carbonate melt observed in this study, and in particular its high contents of K and LILE. The lack of nitrogen aggregation in the diamonds implies that the diamond-forming metasomatism took place shortly before the eruption of the kimberlite, and that the microinclusions thus represent saline carbonate-rich fluids circulating in the basement of lithospheric mantle (150–170 km depth).

Fluid-induced transition from banded kyanite- to bimineralic eclogite and implications for the evolution of cratons

Heterogeneous, modally banded kyanite-bearing and bimineralic eclogites from the lithospheric mantle, collected at the Roberts Victor Diamond mine (South Africa), show a reaction texture in which kyanite is consumed. Geothermobarometric calculations using measured mineral compositions in Perple_X allowed the construction of a P-T path showing a steep, cool prograde metamorphic gradient of 2°C/km to reach peak conditions of 5.8GPa and 890°C for the kyanite eclogite. The kyanite-out reaction formed bimineralic eclogite and is probably an integral part of the mineralogical evolution of most archetypal bimineralic eclogites at Roberts Victor and potentially elsewhere. The kyanite-out reaction occured at close to peak pressure (5.3GPa) and was associated with a rise in temperature to 1380°C. Mass balance calculations show that upon breakdown, the kyanite component is fully accommodated in garnet and omphacite via a reaction system with low water fugacity that required restricted fluid influx from metasomatic sources. The δ18O values of garnets are consistently higher than normal mantle values. Each sample has its characteristic trend of δ18O variance between garnets in the kyanite-bearing sections and those in the bimineralic parts covering a range between 5.1‰ and 6.8‰. No systematic change in O-isotope signature exists across the sample population. Differences in garnet trace element signatures between differing lithologies in the eclogites are significant. Grossular-rich garnets coexisting with kyanite have strong positive Eu-anomalies and low Gd/Yb ratios, while more pyrope-rich garnets in the bimineralic sections have lost their positive Eu-anomaly, have higher Gd/Yb ratios and generally higher heavy rare earth element contents. Garnets in the original kyanite-bearing portions thus reflect the provenance of the rocks as metamorphosed gabbros/troctolites. The kyanite-out reaction was most likely triggered by a heating event in the subcratonic lithosphere. As kyanite contains around 100ppm of H2O it is suggested that the kyanite-out reaction, once initiated by heating and restricted metasomatic influx, was promoted by the release of water contained in the kyanite. The steep (high-P low-T) prograde P-T path defining rapid compression at low heating rates is atypical for subduction transport of eclogites into the lithospheric mantle. Such a trajectory is best explained in a model where strong lateral compression forces eclogites downward to higher pressures, supporting models of cratonic lithosphere formation by lateral collision and compression.

Formation of diamondiferous kyanite–eclogite in a subduction mélange

Diamond- and kyanite-bearing eclogites from the Lace kimberlite on the Kaapvaal craton have common picritic to gabbroic oceanic protoliths with bimineralic eclogites, lying on arrays of Eu∗ and ∑REE that are consistent with accumulation and fractionation of plagioclase and olivine. However, they also show significant compositional differences, such as more grossular-rich garnet and aluminous clinopyroxene (cpx), which require the operation of additional processes. Their nature is elucidated using mineral major- and trace-element compositions, as well as Sr isotope ratios determined by in situ techniques.Highly variable major-element compositions across the co-genetic eclogite suites exert a strong effect on the trace-element distribution between garnet and cpx, whereby Sc, Ge, Sr, Y, Cd, REE, Th and U partition more strongly into garnet with increasing grossular-content. Thus, significant differences between the trace-element compositions of garnet can ensue from crystal-chemical effects alone, making their use as petrogenetic indicators potentially ambiguous. After correcting for these compositional effects, garnet in kyanite-/diamond eclogites, and in eclogites devoid of accessory minerals but with similar signatures, shows depletion (or dilution) in Sc, Ge, Y, In, Zr, Hf and the HREE, and enrichment in the LREE and Th compared to garnet in bimineralic eclogites. This is interpreted as the signature of a pelite-derived melt, which was transferred by addition of aluminous cpx that later exsolved kyanite and garnet, as observed in other aluminous eclogite suites. Continental input can explain initial (at 2.9Ga) 87Sr/86Sr⩾0.714 measured in cpx in eleven samples with low 87Rb/86Sr (<0.01). The association of diamond with kyanite suggests that diamond formation is also linked to this event, possibly due to diamond formation by oxidation of reduced carbon, such as methane, and attendant reduction of Fe3+ in garnet.This model of sediment melt–oceanic crust interaction reconciles evidence for both low- and high-pressure igneous processes in some aluminous eclogites. We suggest that a subduction mélange is a favourable setting for the transfer of a sediment-derived signature into oceanic crust, leading to formation of diamondiferous kyanite–eclogites from bimineralic eclogites. Diapirism, fluxed by the presence of partial melt, may have facilitated dispersal of the eclogites in the lithosphere column, consistent with their widely varying equilibration pressures ranging from ∼5 to 8GPa.

Experimental calibration of a garnet–clinopyroxene geobarometer for mantle eclogites

Thermodynamic parameters have been calibrated for a geobarometer suitable for use on eclogitic mantle xenoliths. The barometer is based on the incorporation of tetrahedrally coordinated aluminum in clinopyroxene coexisting with garnet and has been calibrated using the results of piston cylinder and multi-anvil experiments performed between pressures of 3 and 7 GPa and temperatures from 1,200 to 1,550 °C. Starting materials were hydrous and anhydrous synthetic mixtures of basaltic bulk compositions that yielded homogeneous bimineralic garnet–clinopyroxene phase assemblages. The experimental data set was expanded by employing results from previous experimental studies conducted in eclogitic systems, which widened the range of applicable conditions and compositions. The calibration reproduces experimental pressures of bimineralic eclogite assemblages, in addition to SiO2-saturated and kyanite-bearing eclogites, to within 0.4 GPa at the 95 % confidence interval. The barometer was then used to examine equilibration pressures recorded by natural mantle eclogites from various xenolith locations covering a wide pressure, temperature, and compositional range.

Constraining peak P–T conditions in UHP eclogites: calculated phase equilibria in kyanite‐ and phengite‐bearing eclogite of the Tromsø Nappe, Norway

AbstractKyanite‐ and phengite‐bearing eclogites have better potential to constrain the peak metamorphic P–T conditions from phase equilibria between garnet + omphacite + kyanite + phengite + quartz/coesite than common, mostly bimineralic (garnet + omphacite) eclogites, as exemplified by this study. Textural relationships, conventional geothermobarometry and thermodynamic modelling have been used to constrain the metamorphic evolution of the Tromsdalstind eclogite from the Tromsø Nappe, one of the biggest exposures of eclogite in the Scandinavian Caledonides. The phase relationships demonstrate that the rock progressively dehydrated, resulting in breakdown of amphibole and zoisite at increasing pressure. The peak‐pressure mineral assemblage was garnet + omphacite + kyanite + phengite + coesite, inferred from polycrystalline quartz included in radially fractured omphacite. This omphacite, with up to 37 mol.% of jadeite and 3% of the Ca‐Eskola component, contains oriented rods of silica composition. Garnet shows higher grossular (XGrs = 0.25–0.29), but lower pyrope‐content (XPrp = 0. 37–0.39) in the core than the rim, while phengite contains up to 3.5 Si pfu. The compositional isopleths for garnet core, phengite and omphacite constrain the P–T conditions to 3.2–3.5 GPa and 720–800 °C, in good agreement with the results obtained from conventional geothermobarometry (3.2–3.5 GPa &amp; 730–780 °C). Peak‐pressure assemblage is variably overprinted by symplectites of diopside + plagioclase after omphacite, biotite and plagioclase after phengite, and sapphirine + spinel + corundum + plagioclase after kyanite. Exhumation from ultrahigh‐pressure (UHP) conditions to 1.3–1.5 GPa at 740–770 °C is constrained by the garnet rim (XCaGrt = 0.18–0.21) and symplectite clinopyroxene (XNaCpx = 0.13–0.21), and to 0.5–0.7 GPa at 700–800 °C by sapphirine (XMg = 0.86–0.87) and spinel (XMg = 0.60–0.62) compositional isopleths. UHP metamorphism in the Tromsø Nappe is more widespread than previously known. Available data suggest that UHP eclogites were uplifted to lower crustal levels rapidly, within a short time interval (452–449 Ma) prior to the Scandian collision between Laurentia and Baltica. The Tromsø Nappe as the highest tectonic unit of the North Norwegian Caledonides is considered to be of Laurentian origin and UHP metamorphism could have resulted from subduction along the Laurentian continental margin. An alternative is that the Tromsø Nappe belonged to a continental margin of Baltica, which had already been subducted before the terminal Scandian collision, and was emplaced as an out‐of‐sequence thrust during the Scandian lateral transport of nappes.

Petrology and SHRIMP U–Pb dating of Xitieshan eclogite, North Qaidam UHP metamorphic belt, NW China

Normal bimineralic and phengite-bearing eclogites have been recognized within mafic boudins (or lenses) enclosed in granitic and pelitic gneisses in the Xitieshan terrane, NW China. Bimineralic eclogite contains the primary M1 mineral assemblage Grt + Omp + Rt + Qtz+/−Zrn. Inclusions in M1 omphacite may be coesite pseudomorphs. Phengite eclogite consists of the M1 mineral assemblage Grt + Omp + Phn + Qtz + Rt+/−Zrn. Most eclogites are retrogressed to various degrees and contain hornblende, diopside and plagioclase, which commonly form symplectitic and/or kelyphitic intergrowths. The peak M1 P–T conditions of the Xitieshan eclogite are calculated to be P = 2.71–3.17 GPa and T = 751–791 °C, consistent with the presence of coesite pseudomorphs in omphacite. In addition, we have identified two stages of retrograde metamorphism; an early isothermal decompression stage (M2) followed by a later stage (M3) that developed under decreasing temperatures and pressures. During M2 internally-derived fluids played an important role, whereas during M3 and below externally-derived fluids dominated. The composition of garnet in M1changed little during M2 isothermal decompression (ITD), although pyrope contents in garnet locally decrease from core to rim where the grains have amphibole and pargasite + plagioclase coronas. Also the jadeite content (56%) of M1 omphacite decreased in the presence of active fluids during retrograde metamorphism. M1 phengite is replaced by the M2 assemblage Ms + Bt accompanied by K-feldspar + quartz. Amphibole compositions changed from magnesio-katophorite in M2 to pargasite in M3. Zircon U–Pb SHRIMP dating shows that the bimineralic eclogite has a magmatic protolith age of 877 ± 8 Ma and a metamorphic age of 439 ± 8 to 461 ± 4 Ma. We conclude that the Xitieshan metabasic rocks formed in the late Proterozoic and were subsequently subducted to depths of about 100 km, where they were metamorphosed to UHP eclogite during the late Ordovician (440–460 Ma). Based on these results, we suggest that the Xitieshan eclogites, together with other UHP terranes exposed in the western part of China, form a coherent part of the huge North Qaidam UHP metamorphic belt.

Li as a barometer for bimineralic eclogites: Experiments in natural systems

Experiments performed in eclogitic systems show that the partitioning of Li between garnet and clinopyroxene is pressure dependent in natural eclogitic bulk compositions. This supports experimentally the hypothesis, based on the analysis of Li in eclogitic xenoliths and inclusions in diamond, that the partitioning of this particular element between clinopyroxene and garnet is very sensitive to changes in pressure and could be calibrated as a barometer for bimineralic eclogites. In order to calibrate this pressure dependence into a barometer, 16 multianvil experiments were performed from 4 to 13 GPa and 1100–1500 °C using starting materials sourced from a well preserved Mg-rich eclogitic xenolith from the Roberts Victor kimberlite pipe (South Africa). Our findings reinforce the general trend we observed in the CMAS system, that D Li cpx–grt for Li decreases with increasing P, and that at P ≥ 12 GPa, garnet is able to incorporate more Li than clinopyroxene. Multiple linear regression was applied to our experimental results to create the barometer: P = ( 0.000963 * T − ln D Li cpx−grt + 1.581 ) / 0.252 where P is pressure in GPa, T is temperature in °C and D Li cpx–grt is defined as the partitioning coefficient of Li obtained by dividing the concentration of Li in cpx by the concentration of Li in garnet. This barometer reproduces the experimental conditions to ± 0.2 GPa. It is applicable to Mg-rich eclogitic xenoliths, and to peridotitic and eclogitic inclusions in diamond. Application of the barometer to diamond-bearing xenoliths results in pressures in the diamond stability field. Clinopyroxene is easily corrupted in xenoliths and also preferentially takes in Li during short lived metasomatic processes. Care must be taken therefore to analyse primary, unaltered clinopyroxene. Our preliminary application to natural samples shows that the barometer can be applied beyond the experimental range to pressures down to 3.5 GPa. This is higher than the pressures encountered in crustal eclogites and thus the barometer is not applicable to these eclogites.

Recent progress in Scandian ultrahigh-pressure metamorphism in the northernmost domain of the Western Gneiss Complex, SW Norway: continental subduction down to 180–200 km depth

Abstract: Earlier workers established 20 kbar and 750–800 °C as being the maximum Scandian metamorphic grade for the Caledonized basement units in the northernmost ultrahigh-pressure metamorphic (UHPM) domain of the Western Gneiss Region, SW Norway. These early pressure estimates were entirely based on jadeite isopleths applied to bimineralic eclogites; low Al values in orthopyroxene from opx-eclogites were interpreted to be due to ’disequilibrium effects'. After the initial discovery of microdiamond and majoritic garnet in the area, the low Al 2 O 3 isopleths in opx (cores), ranging from 0.15 to 1.50 wt% in newly discovered and already known occurrences of ’internal' and ’external' opx eclogites–websterites, were reinvestigated and taken as the first indicator that metamorphic grade, on a regional scale, occurred at much higher pressures than previously thought. Regional-scale diamond-grade UHPM conditions were subsequently demonstrated by in situ microdiamond discoveries in two widely separated orthopyroxene eclogite–websterite localities included in Mg–Cr (mantle-derived) and Fe–Ti type (crust-derived) peridotite–websterite bodies, leading to a simple strategy for in situ microdiamond recognition, described here for the first time. All in situ microdiamonds are associated with multiphase solid-inclusion assemblages interpreted to be crystallized from Scandian supercritical, dense, COH-rich subduction zone fluids. In addition, the recent discovery at Fjørtoft of a second, Scandian, majoritic garnet generation (in garnet websterites enclosed in garnet peridotite), having enriched light REE (LREE) patterns (in clinopyroxene and garnet), contrasts strongly to the Archaean–mid-Proterozoic LREE-depleted majoritic garnet patterns (and exsolved pyroxenes) discovered in garnet peridotites from Otrøy. The latter is strong evidence that Scandian continental subduction (and eduction) occurred down to 180–200 km depths, making the northernmost UHPM domain by far the deepest subducted (and educted) part of the Western Gneiss Complex. Such a conclusion is also consistent with (some) geothermobarometric results from internal and external orthopyroxene eclogites and garnet websterites showing results indicative of pressure–temperature estimates deep in the diamond-eclogite facies (850–950 °C and 5.5–6.5 GPa ). Supplementary material: EMP mineral analyses of Mageroy and Raknestangen external opx eclogites, and diagrams showing EMP Grt, Opx and Cpx analyses of Mageroy and Raknestangen opx eclogite bodies are available at http://www.geolsoc.org/SUP18356 .

Towards a Li barometer for bimineralic eclogites: experiments in CMAS

An eclogite barometer has profound importance in the study of upper mantle processes and potential application to diamond prospecting. Studies on the partitioning of Li between clinopyroxene (cpx) and garnet (grt) in natural samples have shown that this particular element is very sensitive to changes in pressure and could be calibrated as the barometer demanded for bimineralic eclogites. Experiments were performed from 4 to 13 GPa and 1,100–1,400°C in the CMAS (CaO, MgO, Al2O3, SiO2) system with Li added as Li3PO4 to quantify this pressure dependence into a barometer expressed in the following equation: P = (0.00255T – ln K d)/0.2351 where P is in GPa, T is in °C and K d is defined as the partition coefficient of Li (in ppm) between clinopyroxene and garnet. The experimental pressures are reproduced to ±0.38 GPa (1σ) by this equation. This barometer is strictly applicable only to CMAS. Experiments at 1,300°C, 8–12 GPa showed that Henry’s Law is fulfilled for Li partitioning between cpx and grt in the concentration range of approximately 0.01–1 wt% Li. Direct application of the equation to experiments in natural systems performed at 1,300°C from 4 to 13 GPa consistently overestimates pressures by approximately 2 GPa.

Partial melting experiments of bimineralic eclogite and the role of recycled mafic oceanic crust in the genesis of ocean island basalts

Bimineralic eclogite, which consists solely of garnet and clinopyroxene, is a likely component of some of the ancient recycled crust residing in basalt source regions. It may originate during subduction of altered mid-ocean ridge basalt (MORB) crust, owing to extraction of small degree partial melts or siliceous hydrous fluids. It may also originate by fractional removal of early-formed partial melts from recycled crust or from pyroxenite originating by other processes. We have performed high-pressure experiments on a bimineralic eclogite (B-ECL1) and its mixture with olivine (B-ECL1-OL) at 3 and 5 GPa. Degrees of melting are slightly higher for B-ECL1-OL than for B-ECL1 at given temperatures, suggesting that addition of small amounts of olivine enhances melt productivity of bimineralic eclogite. Solidus and liquidus temperatures of B-ECL1 are slightly higher than those of B-ECL1-OL and MORB-like pyroxenite, but are lower than those of high-MgO pyroxenite and peridotite, suggesting that bimineralic eclogite is not necessarily refractory compared to other likely mantle lithologies. Partial melts of B-ECL1 and B-ECL1-OL are nepheline-normative. Because garnet and clinopyroxene in these compositions partially melt at a eutectic-like minimum with a composition that is nepheline-normative, a wide range of bimineralic eclogite compositions, including that of subducted-crust origin, that consist of garnet and clinopyroxene with compositions similar to those of B-ECL1 can produce nepheline-normative (= alkali − basaltic) liquids. Thus, in contrast to the common assumption that partial melting of recycled oceanic crust produces silicic magmas, we conclude that such lithologies can produce nepheline-normative partial melts if they first experience fractional removal of fluids or melts. The partial melts from B-ECL1 are too low in MgO to be parental to many alkalic OIB, but have low Al 2O 3 and high FeO comparable to those of alkalic OIB, suggesting that bimineralic recycled crust is a potential source for a low-Al 2O 3 and high-FeO component that is necessary for the genesis of alkalic OIB.

Petrology of highly aluminous xenoliths from kimberlites of Yakutia

Highly aluminous xenoliths include kyanite-, corundum- and coesite-bearing eclogites, grospydites and alkremites. These xenoliths are present in different kimberlites of Yakutia but have most often been found in Udachnaya and other pipes of the central Daldyn–Alakitsky region. Kimberlites of this field also contain eclogite-like xenoliths with kyanite and corundum that originate in the lower crust or the lower crust–upper mantle transition zone. Petrographic study shows that two rock groups of different structure and chemistry can be distinguished among kyanite eclogites: fine- to medium-grained with mosaic structure and coarse-grained with cataclastic structure. Eclogites with mosaic structure are characterized by the occurrences of symplectite intergrowths of garnet with kyanite, clinopyroxene and coesite; only in this group do grospydites occur. In cataclastic eclogites, coarse-grained coesite occurs, corresponding in size to other rock-forming minerals. Highly aluminous xenoliths differ from bimineralic eclogites in their high content of Al 2O 3 and total alkali content. Coesite-bearing varieties are characterized by low MgO content and higher Na/K and Fe 2+/Fe 3+ ratios, as well as high contents of Na 2O. Geochemical peculiarities of kyanite eclogites and other rocks are exhibited by a sloping chondrite-normalized distribution of rare earth elements (REE) in garnets and low Y/Zr ratio, in contrast to bimineralic rocks. Coesite is found in more than 20 kyanite eclogites and grospydites from Udachnaya. Grospydites with coesite from Zagadochnaya pipe are described. Three varieties of coesite in these rocks are distinguished: (a) subhedral grains with size of 1.0–3.0 mm; (b) inclusions in the rock-forming minerals; (c) sub-graphic intergrowths with garnet. The presence and preservation of coesite in eclogites indicate both high pressure of formation (more than 30 kbar) and set a number of constraints on the timing of xenolith cooling during entrainment and transport to the surface. Different ways of formation of the highly aluminous eclogites are discussed. Petrographic observations and geochemistry suggest that some highly aluminous rocks have formed as a result of crystallization of anorthosite rocks in abyssal conditions. δ 18O-estimations and other petrologic evidence point out the possible origin of some of these xenoliths as the result of subduction of oceanic crust. Diamondiferous samples have been found in all varieties except alkremites. Usually these eclogites contain cubic or coated diamonds. However, two sample corundum-bearing eclogites with diamonds from the Udachnaya pipe contain octahedra that show evidence of resorption.

Petrology of highly aluminous xenoliths from kimberlites of Yakutia

Highly aluminous xenoliths include kyanite-, corundum- and coesite-bearing eclogites, grospydites and alkremites. These xenoliths are present in different kimberlites of Yakutia but have most often been found in Udachnaya and other pipes of the central Daldyn–Alakitsky region. Kimberlites of this field also contain eclogite-like xenoliths with kyanite and corundum that originate in the lower crust or the lower crust–upper mantle transition zone. Petrographic study shows that two rock groups of different structure and chemistry can be distinguished among kyanite eclogites: fine- to medium-grained with mosaic structure and coarse-grained with cataclastic structure. Eclogites with mosaic structure are characterized by the occurrences of symplectite intergrowths of garnet with kyanite, clinopyroxene and coesite; only in this group do grospydites occur. In cataclastic eclogites, coarse-grained coesite occurs, corresponding in size to other rock-forming minerals. Highly aluminous xenoliths differ from bimineralic eclogites in their high content of Al2O3 and total alkali content. Coesite-bearing varieties are characterized by low MgO content and higher Na/K and Fe2+/Fe3+ ratios, as well as high contents of Na2O. Geochemical peculiarities of kyanite eclogites and other rocks are exhibited by a sloping chondrite-normalized distribution of rare earth elements (REE) in garnets and low Y/Zr ratio, in contrast to bimineralic rocks. Coesite is found in more than 20 kyanite eclogites and grospydites from Udachnaya. Grospydites with coesite from Zagadochnaya pipe are described. Three varieties of coesite in these rocks are distinguished: (a) subhedral grains with size of 1.0–3.0 mm; (b) inclusions in the rock-forming minerals; (c) sub-graphic intergrowths with garnet. The presence and preservation of coesite in eclogites indicate both high pressure of formation (more than 30 kbar) and set a number of constraints on the timing of xenolith cooling during entrainment and transport to the surface. Different ways of formation of the highly aluminous eclogites are discussed. Petrographic observations and geochemistry suggest that some highly aluminous rocks have formed as a result of crystallization of anorthosite rocks in abyssal conditions. δ18O-estimations and other petrologic evidence point out the possible origin of some of these xenoliths as the result of subduction of oceanic crust. Diamondiferous samples have been found in all varieties except alkremites. Usually these eclogites contain cubic or coated diamonds. However, two sample corundum-bearing eclogites with diamonds from the Udachnaya pipe contain octahedra that show evidence of resorption.

Eclogite xenoliths from the Kuruman kimberlites, South Africa: geochemical fingerprinting of deep subduction and cumulate processes

The mineralogically diverse eclogite xenolith suite sampled by the >1.6 Ga Zero kimberlite in the Kuruman group, close to the western edge of the Kaapvaal craton consists of bimineralic eclogites, orthopyroxene-bearing eclogites with and without sanidine, and kyanite eclogites. Garnet exsolutions in clinopyroxene occur in all Zero eclogite types, and orthopyroxene and, where present, sanidine are also exsolved from clinopyroxene in orthopyroxene-bearing eclogites. Mineral thermobarometry indicates that the last P– T condition prior to the kimberlite eruption was 900–950 °C at 3.5–4.0 GPa. However, inclusions of quartz, rutile and rarely K-feldspar, plus former fluid inclusions in garnets, indicate a complex earlier history. δ 18O values of 5.18–5.47 are all mantle-like. Trace element concentrations in minerals of the Zero eclogite suite have been analysed by Laser-ICP-MS, and whole-rock compositions for both major and trace elements were reconstructed from mineral compositions and modal proportions. Results show that the eclogite protoliths consisted of varying proportions of clinopyroxene, plagioclase and olivine with minor spinel. Abundances of mafic minerals are constrained by transition elements and plagioclase by ratios of Sr and Eu to other rare earth elements. Calculations show that <25% plagioclase may not result in a noticeable Eu anomaly in the rock pattern. Forward modelling of mineral accumulation from melts with the program pMELTS shows that almost the entire eclogite suite can be explained by fractionation of a picritic parental magma at lower crustal depths in thick Archean oceanic crust. Sanidine–orthopyroxene eclogites appear to be cogenetic with the other eclogite types, and carry more evidence for the P– T– t history of the suite. They are interpreted to originate from phlogopite or phengite-bearing protoliths which were displaced to depths of 200–250 km during subduction or collision, where the hydrous minerals dehydrated. The released potassium was taken up in clinopyroxene, whereas evidence for fluid and its solutes is entrapped in garnet, later forming quartz. Tectonic relaxation caused uplift of the eclogites from the 7–8 GPa indicated by K solubility in clinopyroxene to 3.5–4.5 GPa, during which sanidine, orthopyroxene and garnet exsolved from clinopyroxene. The fluid inclusions in garnet decrepitated at a late stage.

Experimental constraints on major and trace element partitioning during partial melting of eclogite

Isobaric partial melting experiments were performed on an Fe-free synthetic composition to simulate partial melting of subducted oceanic crust. Nominally anhydrous experiments at 3.0 GPa yielded melts in equilibrium with garnet (13 to 16 mol.% grossular) and aluminous clinopyroxene (14 to 16 wt.% Al 2O 3). Melt compositions show decreasing Si and alkalis and increasing Ca, Mg, and Ti contents with increasing temperatures. Experiments at 1200 and 1300°C were rutile saturated, whereas experiments at 1400°C contained no residual rutile. We argue that during the initial stages of subduction, accessory rutile is likely to be stable in subsolidus eclogites of average midocean ridge basalt composition and that only large degrees of partial melting will eradicate rutile from an eclogitic source. At 3 GPa, any eclogites with a bulk TiO 2 content of ≥1.5 wt.% rutile will produce rutile-saturated partial melts, except at very high degrees of melting. At higher pressures, all bulk Ti may dissolve in clinopyroxene and garnet, leaving no accessory rutile. Trace element partition coefficients for 24 trace elements between clinopyroxene, garnet, and melt were determined by secondary-ion mass spectrometry analysis of experimental run products at 1400°C and 3 GPa. Partition coefficients for the rare earth elements agree well with previous studies and have been evaluated using the lattice strain model. Partitioning data for high–field strength elements indicate complementary D Zr/D Hf for clinopyroxene and garnet. Partial melting of an eclogitic component of different modal compositions may therefore explain both subchondritic and superchondritic Zr/Hf ratios. Superchondritic Zr/Hf has recently been observed in some ocean island basalts (OIB), and this may be taken as further evidence for components of recycled oceanic crust in OIB. The data also indicate slight Nb/Ta fractionation during partial melting of bimineralic eclogite, which is not, however, sufficient to explain some recently observed Nb/Ta fractionation in island arc rocks. Accessory rutile, however, can explain such fractionation.

Significance of Eclogitic and Related Parageneses of Natural Diamonds

Eclogitic (E-type) and related parageneses of natural diamonds are represented by suites of diamond inclusions and xenoliths of diamondiferous eclogites. Major-element data are presented for 32 coexisting minerals forming 19 bimineralic and trimineralic inclusions from diamonds, including omphacite-orthopyroxene (1 sample), garnet-omphacite (5 samples), garnet-coesite (5 samples), omphacite-coesite (2 samples), garnet-picroilmenite (2 samples), garnet-kyanite (1 sample), omphacite-phlogopite (2 samples), and garnel-omphacite-phlogopite (1 sample). Major-element variations of coexisting minerals are typical of corresponding eclogites. Omphacite with 5.02 wt% Na2O, inter-grown with orthopyroxene with Mg# 83.7, represents the first example of a diamondiferous websterite paragenesis including Na-clinopyroxene. This indicates a broader range in mineral compositions of E-type-related websteritepyroxenite-associated diamonds than known previously. This unique websterite-pyroxenitic mineral assemblage represents a transitional paragenesis between peridotitic or ultramafic (U-type) and E-type parageneses. Bimineralic eclogites, ilmenite eclogites, coesite + corundum + kyanite eclogites, and grospydites occur not only as sets of inclusions in diamonds but, with a few exceptions (ilmenite and coesite eclogites), also as diamondiferous eclogite xenoliths. The coesite eclogite paragenesis is a significant inclusion suite in diamonds, and was detected in about 15 diamond occurrences worldwide. It represents from 15% to 22% of all E-type diamonds in several occurrences, and thus should not be considered as rare.