Partition coefficients for REE between garnets and liquids: implications of non-Henry's Law behaviour for models of basalt origin and evolution

Partition coefficients for the rare earth elements (REE) Ce, Sm and Tm between coexisting garnets and hydrous liquids have been determined at high pressure and temperatures (30 kbar and 1300 and 1500°C). Two synthetic systems were studied, Mg₃Al₂Si₃O₁₂-H₂O and Ca₃Al₂Si₃O₁₂-H₂O, in addition to a natural pyrope-bearing system.Deviations from Henry's Law behaviour occur at geologically relevant REE concentrations. At concentrations < 3 ppm Ce, < 12 ppm Sm, < 80 ppm Tm in pyrope and < 100 ppm Ce, < 250 ppm Sm, < 1000 ppm Tm in grossular (at 30 kbar and 1300°C), D^{garnet liquid}_REE increases as the REE concentration in the garnet decreases. At higher concentrations, D_REE is constant. D^{grossular liquid}_REE also constant when the garnet contains less than about 2 ppm Sm or Tm. The REE concentration at which D_REE becomes constant increases with increasing temperature, decreasing REE ionic radius and increasing Ca content of the garnet.Partitioning behaviour of Ce, Sm and Tm between a natural pyrope-rich garnet and hydrous liquid is analogous to that in the synthetic systems and substantiates the substitution model proposed by Harrison and Wood (1980).Values of D_REE^{garnet/liquid} for which Henry's Law is obeyed are systematically higher for grossular than for pyrope (D^{pyrope/liquid} = 0.067(Ce), 0.108(Sm), 0.155(Tm) and D^{grossular/Liquid} = 0.65(Ce), 0.75(Sm), 4.55(Tm).The implications of non-Henry's Law partitioning of REE for models of basalt petrogenesis involving garnet are far-ranging. Deviations from Henry's Law permit refinements to be made to calculated REE abundances once basic model parameters have been defined.     

An experimental investigation of the partitioning of REE between garnet and liquid with reference to the role of defect equilibria

The partitioning of samarium and thulium between garnets and melts in the systems Mg₃Al₂-Si₃O₁₂-H₂O and Ca₃Al₂Si₃O₁₂-H₂O has been studied as a function of REE concentration in the garnets at 30 kbar pressure. Synthesis experiments of variable time under constant P, T conditions indicate that garnet initially crystallizes rapidly to produce apparent values of D _Sm (D _Sm=concentration of Sm in garnet/concentration of Sm in liquid) which are too large in the case of pyrope and too small in the case of grossular. As the experiment proceeds, Sm diffuses out of or into the garnet and the equilibrium value of D _Sm is approached. Approximate values of diffusion coefficients for Sm in pyrope garnet obtained by this method are 6 × 10^–13 cm² s^–1 at 1,300 ° C and 2 × 10^–12 cm² s^–1 at 1,500 ° C, and for grossular, 8.3 × 10^–12 cm² s^–1 at 1,200 ° C and 4.6 × 10^–11 cm² s^–1 at 1,300 ° C. The equilibrium values of D _Sm have been reversed by experiments with Sm-free pyrope and Sm-bearing glass, and with Sm-bearing grossular and Sm-free glass.Between 12 ppm and 1,000 ppm Sm in pyrope at 1,300 ° C and between 80 ppm and >2 wt.% Tm in pyrope at 1,500 ° C, partition coefficients are constant and independent of REE concentration. Above 100 ppm of Sm in garnet at 1,500 ° C, partition coefficients are independent of Sm concentration. At lower concentrations, however, D _Sm is dependent upon the Sm content of the garnet. The two regions may be interpreted in terms of charge-balanced substitution of Sm₃Al₅O₁₂ in the garnet at high Sm concentrations and defect equilibria involving cation vacancies at low concentrations. At very low REE concentrations (< 1 ppm Tm in grossular at 1,300 ° C) D_REE garnet/liquid again becomes constant with an apparent Henry's Law value greater than that at high concentrations. This may be interpreted in terms of a large abundance of cation vacancies relative to the number of REE ions.The importance of defects in the low concentration region has been confirmed by adding other REE (at 80 ppm level) to the system Mg₃Al₂Si₃O₁₂-H₂O at low Sm concentrations. These change D _Sm in the defect region, demonstrating their role in the production of vacancies.Experiments on a natural pyropic garnet indicate that defect equilibria are of importance to REE partitioning within the concentration ranges found in nature.     

Experimental determination of REE partition coefficients between zircon,garnet and melt: a key to understanding high‐T crustal processes

The partitioning of rare earth elements (REE) between zircon, garnet and silicate melt was determined using synthetic compositions designed to represent partial melts formed in the lower crust during anatexis. The experiments, performed using internally heated gas pressure vessels at 7 kbar and 900–1000 °C, represent equilibrium partitioning of the middle to heavy REE between zircon and garnet during high‐grade metamorphism in the mid to lower crust. The D_REE (zircon/garnet) values show a clear partitioning signature close to unity from Gd to Lu. Because the light REE have low concentrations in both minerals, values are calculated from strain modelling of the middle to heavy REE experimental data; these results show that zircon is favoured over garnet by up to two orders of magnitude. The resulting general concave‐up shape to the partitioning pattern across the REE reflects the preferential incorporation of middle REE into garnet, with D_Gd (zircon/garnet) ranging from 0.7 to 1.1, D_Ho (zircon/garnet) from 0.4 to 0.7 and D_Lu (zircon/garnet) from 0.6 to 1.3. There is no significant temperature dependence in the zircon–garnet REE partitioning at 7 kbar and 900–1000 °C, suggesting that these values can be applied to the interpretation of zircon–garnet equilibrium and timing relationships in the ultrahigh‐T metamorphism of low‐Ca pelitic and aluminous granulites.     

Experimental rare earth element partition coefficients for garnet,clinopyroxene and amphibole coexisting with andesitic and basaltic liquids

The partitioning of La, Sm, Dy, Ho and Yb between garnet, calcic clinopyroxene, calcic amphibole and andesitic and basaltic liquids has been studied experimentally. Glasses containing one or more REE in concentrations of 500–2000 ppm were crystallized at pressures of 10–35 kbar, and temperatures of 900–1520°C. Water was added to stabilize amphibole and to allow study of partition coefficients over wide temperature ranges. Major element and REE contents of crystal rims and adjacent glass were determined by EPMA, with limits of detection for individual REE of 100–180 ppm. Measured partition coefficients, D_REE^Cryst-liq, are independent of REE concentration over the concentration ranges used.D-values show an inverse dependence on temperature, best illustrated for garnet. At a given temperature, they are almost always higher for equilibria involving andesitic liquid. Garnet shows by far the greatest range of D-values, with e.g. D_La < 0.05 and D_Yb ~ 44 for andesitic liquid at 940°C. D_Yb falls to ~ 12 at 1420°C. D_Sm^Ga-liq also correlates negatively with temperature and positively with the grossular content of garnet. Patterns of D_ree^Cryst-Liq for calcic clinopyroxenes and amphiboles are sub-parallel, with D-values for amphibole generally higher. Both individual D-values and patterns for the crystalline phases studied are comparable with those determined for phenocryst-matrix pairs in natural dacites, andesites and basalts.D-values and patterns are interpreted in terms of the entry of REE³⁺ cations into mineral structures and liquids of contrasted major element compositions. The significance of the partition coefficients for models of the genesis of andesitic and Hy-normative basaltic magmas is assessed. Most magmas of these types in island arcs are unlikely to be produced by melting of garnet-bearing sources such as eclogite or garnet lherzolite.     

Anhydrous P-T phase relations of near-primary high-alumina basalt from the South Sandwich Islands

Anhydrous P-T phase relations, including phase compositions and modes, are reported from 10–31 kbar for a near-primary high-alumina basalt from the South Sandwich Islands in the Scotia Arc. The water content of natural subduction-related basalt is probably <0.5 wt.% and thus, these results are relevant to the generation of primary basaltic magmas in subduction zones. At high pressures (>27 kbar) garnet is the liquidus phase followed by clinopyroxene, then quartz/coesite at lower temperatures. At intermediate pressures (17–27 kbar), clinopyroxene is the liquidus phase followed by either garnet, quartz, plagioclase, then orthopyroxene or plagioclase, quartz, garnet, then orthopyroxene depending on the pressure within this interval. At all lower pressures, plagioclase is the liquidus phase followed at much lower temperatures (100° C at 5 kbar) by clinopyroxene. The absence of olivine from the liquidus suggests that the composition studied here could not have been derived from a more mafic parent by olivine fractionation at any pressure investigated, and supports the interpretation that it is primary. If so, these results also preclude an origin for this melt by partial melting of olivine-rich mantle periddotite and suggest instead that it was generated by partial melting of the descending slab (quartz eclogite) leaving clinopyroxene, garnet, or both in the residue. The generally flat REE patterns for low-K series subduction related basalts argue against any significant role for garnet, however, and it is thus concluded that the composition studied here was extracted at 20–27 kbar after sufficiently high degrees of partial melting (50%) to totally consume garnet in the eclogite source. Melting experiments on three MORB composition, although not conclusive, are in agreement with this mechanism. Results at 30 kbar support an origin for tonalite/trondhjemite series rocks by lower degrees of melting (15–30%), leaving both garnet and clinopyroxene in the residue.     

REE concentrations of garnet and omphacite in eclogites from the Dabie Mountain, central China

Distributions of the rare-earth elements (REE) in omphacite and garnet and REE behaviors during metamorphic processes were discussed. The REE concentrations of garnet and omphacite in six eclogite samples from the Dabie Mountain, central China, were measured by inductively coupled plasma-mass spectrometry (ICP-MS). The correlation of δEu ratios between garnet and omphacite indicated that chemical equilibrium of REE distribution between garnet and omphacite could be achieved during ultra-high pressure (UHP) metamorphism. Most of the partition coefficients (Kd=CiOmp/CiGrt) of light rare-earth elements (LREE) are higher than 1. However the partition coefficients of heavy rare-earth elements (HREE) are lower than 1. This indicated that the LREE inclined to occupy site M2 in omphacite, but the HREEs tended to occupy eightfold coordinated site in garnet during the eclogite formation. The REE geochemistry of the eclogites indicated that LREE could be partially lost during the prograde metamorphic process of protolith, but be introduced into the rocks during the symplectite formation. LREE are more active than HREE during the UHP metamorphism. The results are favorable to highlighting the REE behavior and evolution of UHP metamorphic rocks.     

Petrology and Rare Earth Elements Mineral Chemistry of Chadegan Metabasites (Sanandaj-Sirjan Zone,Iran): Evidence for Eclogite-Facies Metamorphism during Neotethyan Subduction

The metabasites of Chadegan, including eclogite, garnet amphibolite and amphibolite, are forming a part of Sanandaj–Sirjan Zone. These rocks have formed during the subduction of the Neo–Tethys ocean crust under Iranian plate. This subduction resulted in a subduction metamorphism under high pressuremedium temperature of eclogite and amphibolites facies condition. Then the metamorphic rocks were exhumed during the continental collision between the Afro–Arabian continent and the Iranian microcontinent. In the metabasite rocks, with typical MORB composition, garnet preserved a compositional zoning occurred during metamorphism. The magnesium (X_Mg) gradually increases from core to rim of garnets, while the manganese (X_Mn) decreases towards the rim. Chondrite–normalized Rare Earth Element patterns for these garnets exhibit core–to–rim increases in Light Rare Earth Elements. The chondrite–normalized REE patterns of garnets, amphiboles and pyroxenes display positive trend from LREEs to Heavy Rare Earth Elements (especially in garnet), which suggests the role of these minerals as the major controller of HREE distribution. The geochemical features show that the studied eclogite and associated rocks have a MORB origin, and probably formed in a deep–seated subduction channel environment. The geothermometry estimation yields average pressure of ~22 kbar and temperature of 470–520°C for eclogite fomation. The thermobarometry results gave T = 650–700°C and P ≈ 10–11 kbar for amphibolite facies.     

A revision of the garnet-clinopyroxene Fe2+-Mg exchange geothermometer

A comprehensive experimental dataset was used to analyse the compositional dependence of the garnet-clinopyroxene Fe²⁺/Mg partition coefficient (K _d). The Mg no. of garnet was found to have a significant effect on the K _d, in addition to calcium content of garnet. An empirical model was developed to relate these effects with equilibrium temperature and pressure in the form of a conventional geothermometer, T(K) = { – 1629[X^Gt _Ca]² + 3648.55[X^Gt _Ca] – 6.59[Mg no. (Gt)] + 1987.98 + 17.66P (kbar)}/(In k_d + 1.076). Application of this thermometer produced reasonable temperature estimates for rocks from the lower crust (garnet amphibolites, granulites and eclogites) and the upper mantle (eclogite and lherzolite xenoliths in kimberlites, mineral inclusions in diamonds).     

Tale of the Kulet eclogite from the Kokchetav Massive,Kazakhstan: Initial tectonic setting and transition from amphibolite to eclogite

The Kulet eclogite in the Kokchetav Massif, northern Kazakhstan, is identified as recording a prograde transformation from the amphibolite facies through transitional coronal eclogite to fully recrystallized eclogite (normal eclogite). In addition to minor bodies of normal eclogite with an assemblage of Grt + Omp + Qz + Rt ± Ph and fine‐grained granoblastic texture (type A), most are pale greyish green bodies consisting of both coronal and normal eclogites (type B). The coronal eclogite is characterized by coarse‐grained amphibole and zoisite of amphibolite facies, and the growth of garnet corona along phase boundaries between amphibole and other minerals as well as the presence of eclogitic domains. The Kulet eclogites experienced a four‐stage metamorphic evolution: (I) pre‐eclogite stage, (II) transition from amphibolite to eclogite, (III) a peak eclogite stage with prograde transformation from coronal eclogite to UHP eclogite and (IV) retrograde metamorphism. Previous studies made no mention of the presence of amphibole or zoisite in either the pre‐eclogite stage or coronal eclogite, and so did not identify the four‐stage evolution recognized here. P–T estimates using thermobarometry and Xprp and Xgrs isopleths of eclogitic garnet yield a clockwise P–T path and peak conditions of 27–33 kbar and 610–720 °C, and 27–35 kbar and 560–720 °C, respectively. P–T pseudosection calculations indicate that the coexistence of coronal and normal eclogites in a single body is chiefly due to different bulk compositions of eclogite. All eclogites have tholeiitic composition, and show flat or slightly LREE‐enriched patterns [(La/Lu)_N = 1.1–9.6] and negative Ba, Sr and Sc and positive Th, U and Ti anomalies. However, normal eclogite has higher TiO₂ (1.35–2.65 wt%) and FeO (12.11–16.72 wt%) and REE contents than those of coronal eclogite (TiO₂ < 0.9 wt% and FeO < 12.11 wt%) with one exception. Most Kulet eclogites plot in the MORB and IAB fields in the 2Nb–Zr/4–Y and TiO₂–FeO/MgO diagrams, although displacement from the MORB–OIB array indicates some degree of crustal involvement. All available data suggest that the protoliths of the Kulet eclogites were formed at a passive continent marginal basin setting. A schematic model involving subduction to 180–200 km at 537–527 Ma, followed by slab breakoff at 526–507 Ma, exhumation and recrystallization at crustal depths is applied to explain the four‐stage evolution of the Kulet eclogite.     

High-precision high field strength element partitioning between garnet, amphibole and alkaline melt from Kakanui, New Zealand

The high field strength elements (HFSE: Zr, Hf, Nb, Ta, and W) are an important group of chemical tracers that are increasingly used to investigate magmatic differentiation processes. Successful modeling of these processes requires the availability of accurate mineral-melt partition coefficients (D). To date, these have largely been determined by ion microprobe or laser ablation-ICP-MS analyses of the run products of high-pressure, high-temperature experiments. Since HFSE are (highly) incompatible, relatively immobile, high-charge, and difficult to ionize, these experiments and their analysis are challenging. Here we explore whether high-precision analyses of natural mineral-melt systems can provide additional constraints on HFSE partitioning.The HFSE concentrations in natural garnet and amphibole and their alkaline host melt from Kakanui, New Zealand are determined with high precision isotope dilution on a multi-collector-ICP-MS. Major and trace element compositions combined with Lu-Hf isotopic systematics and detailed petrographic sample analysis are used to assess mineral-melt equilibrium and to provide context for the HFSE D measurements. The whole-rock nephelinite, ∼1 mm sized amphiboles in the nephelinite, and garnet megacrysts have similar initial Hf isotope ratios with a mean initial ¹⁷⁶Hf/¹⁷⁷Hf_{(34 Ma)} = 0.282900 ± 0.000026 (2σ). In contrast, the amphibole megacrysts are isotopically distinct (¹⁷⁶Hf/¹⁷⁷Hf_{(34 Ma)} = 0.282830 ± 0.000011). Rare earth element D values for garnet megacryst-nephelinite melt and ∼1 mm amphibole-nephelinite melt plotted as a function of ionic radii show classic near-parabolic trends that are in excellent agreement with crystal lattice-strain models. These observations are consistent with equilibrium between the whole-rock nephelinite, the ∼1 mm amphibole grains within the nephelinite and the garnet megacrysts.High-precision isotope dilution results for Zr and Hf in garnet (D_Zr = 0.220 ± 0.007 and D_Hf = 0.216 ± 0.005 [2σ]), and for all HFSE in amphibole are consistent with previous experimental findings. However, our measurements for Nb and Ta in garnet (D_Nb = 0.0007 ± 0.0001 and D_Ta = 0.0011 ± 0.0006 [2σ]) show that conventional methods may overestimate Nb and Ta concentrations, thereby overestimating both Nb and Ta absolute D values for garnet by up to 3 orders of magnitude and underestimating D_Nb/D_Ta by greater than a factor of 100. As a consequence, the role of residual garnet in imposing Nb/Ta fractionation may be less important than previously thought. Moreover, garnet D_Hf/D_W = 17 and D_Nb/D_Zr = 0.003 imply fractionation of Hf from W and Nb from Zr upon garnet crystallization, which may have influenced short-lived ¹⁸²Hf-¹⁸²W and ⁹²Nb-⁹²Zr isotopic systems in Hadean time.     

Partial fusion of basic granulites at 5 to 15 kbar: implications for the origin of TTG magmas

Partial fusion experiments with basic granulites (S6, S37) believed to represent the lower crust beneath the Eifel region (Germany) were performed at pressures from 5 to 15 kbar. Water-undersaturated experiments were carried out in the presence of 1 wt% H₂O plus 2.44 or 0.81 wt% CO₂ equivalent to mole fractions of H₂O/(H₂O + CO₂) of 0.5 and 0.75, respectively, of the volatile components added. At temperatures from 850 to 1100 °C the weight proportions of melt range from 7 to 30 %. Melt compositions change from trondhjemitic over tonalitic to dioritic with increasing degree of partial melting. Crystalline residua are plagioclase/pyroxene dominated at 5 kbar to garnet/pyroxene dominated at 15␣kbar. Dehydration melting was studied in granulite S35 similar in composition to S6. The magmatic precursors of the granulite xenoliths used in this study had geochemical characteristics of cumulate gabbro (metagabbro S37) and evolved melts (metabasalts S6, S35), respectively. Melts from granulite S37 match the major element compositions of natural trondhjemites and tonalites. At 5 kbar, their Al₂O₃ is relatively low, similar to tonalites from ophiolites. At 15 kbar, Al₂O₃ in the melts is high due to the near absence of plagioclase in the crystalline residua. The Al₂O₃ concentrations in 15 kbar melts from S6 (˜20 wt%) are higher than in natural tonalites. Depth constraints on the formation of tonalitic magmas in the continental crust are provided by REE (rare earth element) patterns of the synthetic melts calculated from the known REE abundances in metagabbro S37 and metabasalt S6 assuming batch melting and using partition coefficients from the literature. The REE patterns of tonalites from active continental margins and Archean trondhjemite-tonalite-granodiorite␣associations low in REE with La_N (chondrite normalised) from 10 to 30 and Yb_N from 1 to 2 are reproduced at pressures of 10 and 12.5 kbar from metagabbro S37 which displays a slightly L(light)REE enriched pattern with La_N = 8 and Yb_N = 3. Natural tonalites with La_N from 30 to 100 require a source richer in REE than granulite S37. At 15 kbar, H(heavy)REE_N in melts from granulite S37 are depressed below the level observed in natural tonalites due to the high proportion of garnet (>30 wt%) in the residue. Melts from metabasalt S6 (enriched in REE with La_N = 38 and Yb_N = 16) do not match the REE characteristics of natural tonalites under any conditions. Received: 1 July 1994 / Accepted: 11 September 1996     

Sorption of lanthanides on smectite and kaolinite

Experiments were carried out to investigate the sorption of the complete lanthanide series (Ln or rare earth elements, REE) on a kaolinite and an a Na-montmorillonite at 22°C over a wide range of pH (3-9). Experiments were conducted at two ionic strengths, 0.025 and 0.5 M, using two different background electrolytes (NaNO₃ or NaClO₄) under atmospheric conditions or N₂ flow (glove box). The REE sorption does not depend on the background electrolyte or the presence of dissolved CO₂, but is controlled by the nature of the clay minerals, the pH and the ionic strength. At 0.5 M, both clay minerals exhibit the same pH dependence for the Ln sorption edge, with a large increase in the sorption coefficient (K_D) above pH 5.5. At 0.025 M, the measured K_D is influenced by the Cation Exchange Capacity (CEC) of the minerals. Two different behaviours are observed for smectite: between pH 3 and 6, the K_D is weakly pH-dependent, while above pH 6, there is a slight decrease in log K_D. This can be explained by a particular arrangement of the particles. For kaolinite, the sorption coefficient exhibits a linear increase with increasing pH over the studied pH range. A fractionation is observed that due to the selective sorption between the HREEs and the LREEs at high ionic strength, the heavy REE is being more sorbed than the light REE. These results can be interpreted in terms of the surface chemistry of clay minerals, where two types of surface charge are able to coexist: the permanent structural charge and the variable pH-dependent charge. The fractionation due to sorption observed at high ionic strength can be interpreted either because of a competition with sodium or because of the formation of inner-sphere complexes. Both processes could favour the sorption of HREEs according to the lanthanide contraction.     

Separate or shared metamorphic histories of eclogites and surrounding rocks? An example from the Bohemian Massif

Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite‐dominated volcano‐sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite‐bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet–staurolite–kyanite–biotite–plagioclase–muscovite–quartz–ilmenite ± rutile ± silli‐manite and prograde‐zoned garnet includes chloritoid–chlorite–paragonite–margarite, staurolite–chlorite–paragonite–margarite and kyanite–chlorite–rutile. In pseudosection modelling in the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite + paragonite (with muscovite + quartz + H₂O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet–staurolite–biotite–muscovite–quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet–amphibole–plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610–660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase–clinopyroxene symplectites, epidote and late amphibole–plagioclase domains. Garnet commonly includes rutile–quartz–epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H₂O‐undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote–amphibole–plagioclase indicating that decompression occurred under decreasing temperature into garnet‐free epidote–amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km^?1, respectively, to metamorphic pressure peaks that are 6–7 kbar apart. The eclogite–orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared.     

Mafic xenoliths in Proterozoic kimberlites from Eastern Dharwar Craton,India: Mineralogy and P–T regime

Mafic xenoliths of garnet pyroxenite and eclogite from the Wajrakarur, Narayanpet and Raichur kimberlite fields in the Archaean Eastern Dharwar Craton (EDC) of southern India have been studied. The composition of clinopyroxene shows transition from omphacite (3–6 wt% Na₂O) in eclogites to Ca pyroxene (<3 wt% Na₂O) in garnet pyroxenites. Some of the xenoliths have additional phases such as kyanite, enstatite, chromian spinel or rutile as discrete grains. Clinopyroxene in a rutile eclogite has an X_Mg value of 0.70, which is unusually low compared to the X_Mg range of 0.91–0.97 for all other samples. Garnet in the rutile eclogite is also highly iron-rich with an end member composition of Prp_26.5Alm_52.5Grs_14.7Adr_5.1TiAdr_0.3Sps_1.0Uv_0.1. Garnets in several xenoliths are Cr-rich with up to 8 mol% knorringite component. Geothermobarometric calculations in Cr-rich xenoliths yield different P–T ranges for eclogites and garnet pyroxenites with average P–T conditions of 36 kbar and 1080 °C, and 27 kbar and 830 °C, respectively. The calculated P–T ranges approximate to a 45 mW m^?2 model geotherm, which is on the higher side of the typical range of xenolith/xenocryst geotherms (35–45 mW m^?2) for several Archaean cratons in the world. This indicates that the EDC was hotter than many other shield regions of the world in the mid-Proterozoic period when kimberlites intruded the craton. Textural and mineral chemical characteristics of the mafic xenoliths favour a magmatic cumulate process for their origin as opposed to subducted and metamorphosed oceanic crust.     

Garnet + liquid equilibrium

New experiments were performed to determine saturation conditions for garnet and silicate liquid. Starting compositions were natural basalt powders ranging from komatiite to nephelinite, which were partially melted at pressures between 25 and 100 kbar. Rounded grains of natural pyrope or grossular were added to some experiments to induce garnet saturation, and to aid the segregation of liquid pools for microprobe analysis. Simple expressions describing K _eq as a function of P, T and liquid composition were calibrated by linear least squares analysis of the data from this, and other, studies. Since garnets do not often occur as phenocrysts, equations were designed to predict garnet compositions when P, T and a silicate liquid composition are given. The regression data have a pressure range of 20–270 kbar, and compositions as diverse as nephelinite and komatiite. These models should thus apply to a broad range of geological problems. The majorite component in garnet was found to increase with increasing P, but compositional effects are also important. A garnet saturation surface applied to liquids with chondritic compositions shows that such liquids crystallize garnet with Mj contents of 0.27–0.42 at 200 kbar. Models of Earth differentiation thus need to account not only for fractionation of majorite, but also for Fe-, Ca-, Na- and Ti-bearing garnet components, which occur in non-trivial quantities at high pressure. Since many models of igneous petrogenesis rely on mineral-melt partition coefficients for the minor elements Na, Ti, and Cr, partition coefficients for these elements were also examined. The K _d ^gar/liq for Na was found to be P-sensitive; Na contents of basalts may thus potentially yield information regarding depths of partial melting. Received : 28 May 1997 / 25 November 1997     

Rare earth element partitioning between clinopyroxene and silicate liquid at moderate to high pressure

Experimental determination of over seventy sets of clinopyroxene/silicate liquid (glass) partitition coefficients (D) for four rare earth elements (REE — La, Sm, Ho, Lu) in a range of REE-enriched natural rock compositions (basalt, basaltic andesite, andesite and rhyodacite) demonstrate a convex upward pattern, favouring the heavy REE (Ho, Lu) and markedly discriminating against the light REE (La). These patterns are consistent with previously documented clinopyroxene D values reported from natural phenocryst/matrix pairs and from experimental work using either REE-enriched compositions and electron microprobe analytical techniques (as in the present study) or natural or synthetic undoped compositions and mass spectrometric, ion probe or X-ray autoradiographic analytical techniques. However, the large data base in the present study allows evaluation of the effect of compositional and physical parameters on REE partitioning relationships. Considering D_Ho, it is shown that (1) D increases 6-fold with increasing SiO₂ content of the coexisting liquid from 50 to 70 wt% SiO₂ (2) D increases 4-fold with decreasing temperature from 1,120°C to 900° C (3) D increases 2-fold with increasing pressure from 2.5 to 20 kb. (4) D increases 2-fold fO₂ increases from approximately that of the MW buffer to the HM buffer (5) D remains unchanged within experimental error as the water content of the melt changes from 0.3 to 10% by weight H₂O.The absolute REE content of the clinopyroxene shows no consistent trend with temperature, but decreases slightly with increasing pressure, paralleling an increase in the jadeite component of the pyroxene. Thus the increase in D with increasing pressure is attributed to changes in the silicate liquid structure, which discriminate against accommodation of REE with increasing pressure. The clinopyroxene REE content increases with increasing fO₂, and in this case the increase in D with increasing fO₂ may be attributed mainly to this change in the clinopyroxene composition. Application of the present results to geochemical modelling allows a more appropriate choice of D values, according to the liquid composition and physical conditions applicable in the modelled system. They may also be used to evaluate cognate or xenocrystic relationships between clinopyroxene megacrysts and their host matrix.     

Uncoupled U/Pb and REE response in zircon during the transformation of eclogite to mafic and intermediate granulite (Blanský les,Bohemian Massif)

An eclogite–mafic granulite occurs as a rare boudin within a felsic kyanite–K‐feldspar granulite in a low‐strain zone. Its boundary is marked by significant metasomatism–diffusional gain of potassium at the centimetre‐scale, and probable infiltration of felsic melt on a larger scale. This converted the eclogite–mafic granulite into an intermediate‐composition, ternary‐feldspar‐bearing granulite. Based on inclusions in garnet, the peak P–T conditions of the original eclogite are 18 kbar at 850–950 °C, with later matrix re‐equilibration at 12 kbar and 950 °C. Four samples from the transition of the eclogite–mafic granulite through to the intermediate granulite were studied. In the eclogite, REE patterns in the garnet core show no Eu anomaly, compatible with crystallization in the absence of plagioclase and consistent with eclogite facies conditions. Towards the rim of garnet, LREE decrease, and a weak negative Eu anomaly appears, reflecting passage into HP granulite facies conditions with plagioclase present. The rims of garnet next to ternary feldspar in the intermediate granulite show the lowest LREE and deepest Eu anomalies. Zircon from the four samples was analysed by LASS (laser ablation–split‐stream inductively coupled plasma–mass spectrometry). It shows U–Pb ages from 404 ± 4.0 to 331 ± 3.3 Ma, with a peak at 340 ± 4.0 Ma corresponding to the likely exhumation of the rocks to 12 kbar. Older ages from zircon with steep HREE patterns indicate the minimum age of the protolith, and ages <360 ± 4.0 Ma are interpreted to correspond to the eclogite facies metamorphism. Only some zircon grains ≤350 ± 4.0 Ma have flat HREE patterns, suggesting that these are primarily modified protolith grains, rather than new zircon crystallized in the eclogite‐ or granulite facies. The metasomatic processes that converted the eclogite–mafic granulite to an intermediate granulite may have facilitated zircon modification as zircon in the intermediate granulite has flat HREE and ages of 340 ± 4.0 Ma. The difference between the oldest and youngest ages with flat REE patterns indicates a 16 ± 5.6 Ma period of zircon modification in the presence of garnet.     

Transformation of enstatite — diopside — jadeite pyroxenes to garnet

The high-pressure stability of enstatite(En)-diopside(Di)-jadeite(Jd) pyroxenes has been investigated experimentally with a split-sphere anvil apparatus (USSA-2000). On the enstatite-pyrope join, the compositions of garnet coexisting with enstatite were determined at 100–165 kbar and 1450–1850° C. The results indicate complete solubility between enstatite and pyrope. In the system CaO-MgO-Al₂O₃-SiO₂ (CMAS), the compositions of coexisting pyroxenes and garnet were determined at 100–165 kbar and 1250–1750° C. At 157 kbar, 1650° C, garnet with the composition En₇₉Di₂₁ (mol%) forms on the En-Di join. In the system Na₂O-MgO-Al₂O₃-SiO₂ (NMAS), the compositions of coexisting pyroxenes and garnet were determined at 60–160 kbar and 1200–1850° C. On the En-Jd join, the first garnet has the composition En₄₈Jd₅₂ at 135 kbar, 1650° C, and En₅₃Jd₄₇ at 140 kbar, 1500° C. On the Di-Jd join, the first garnet with the composition Di₆₃Jd₃₇ forms around 170 kbar, 1650° C. In the En-Di-Jd system, the first appearance of garnet with the composition En₄₂Di₉Jd₄₉ is estimated at 133 kbar, 1650° C. The new pyroxene with the composition NaMg_0.5Si_2.5O₆ (NaPx) transforms to garnet at 154 kbar, 1650° C. The experimental results indicate that the transformation of a twopyroxene assemblage to garnet and residual pyroxene in the En-Di-Jd system could occur at pressures consistent with the 400 km seismic discontinuity and in a pressure interval of 0–3 kbar.     

Conditions of diamond growth: a proton microprobe study of inclusions in West Australian diamonds

Crystalline primary inclusions in diamonds from the Argyle and Ellendale lamproites have been analyzed for Mn, Ni, Cu, Zn, Ga, Pb, Rb, Sr, Y, Zr, Nb, Ta, Ba and Mo by proton microprobe. Eclogite-suite inclusions dominate at Argyle and occur in equal proportions with peridotite-suite inclusions at Ellendale. Eclogitic phases present include garnet, omphacitic clinopyroxene, coesite, rutile, kyanite and sulfide. Eclogitic clinopyroxenes are commonly rich in K and contain 300–1060 ppm Sr and 3–70 ppm Zr: K/Rb increases with K content up to 1400 at 0.7–1.1% K. Rutiles have high Zr and Nb contents with Zr/Nb=1.5–4 and Nb/Ta 16. Of the peridotite-suite inclusions, olivine commonly contains > 10 ppm Sr and Mo; Cr-pyropes are depleted in Sr, Y and Zr, and enriched in Ni, relative to eclogitic garnets.Eclogite-suite diamonds grew in host rocks that were depleted in Mn, Ni and Cr, and enriched in Sr, Zn, Cu, Ga and Ti, relative to Type I eclogite xenoliths from the Roberts Victor Mine. Crystallization temperatures of the eclogite-suite diamonds, as determined by coexisting garnet and clinopyroxene from single diamonds, range from 1085 to 1575° C. Log K _D (C _i ^cpx /C _i ^gnt ) varies linearly with 1/T for Zr, Sr and Ga in most of the same samples. This supports the validity of the temperature estimates; Argyle eclogite-suite diamonds have grown over a T range 400° C. Comparison with data from eclogite xenoliths in kimberlites suggests that K _D ^Sr and K _D ^Zr are mainly T-dependent, while K _D ^Ga may be both temperature-and pressuredependent. K _D ^Ni , K _D ^Cu and K _D ^Zn show no T dependence in these samples.In several cases, significant major-and/or trace-element disequilibrium is observed between different grains of the same mineral, or between pyroxene and garnet, within single diamonds. This implies that these diamonds grew in an open system; inclusions trapped at different stages of growth record changes in major and trace-element composition occurring in the host rock. Diamond growth may have been controlled by a fluid flux which introduced or liberated carbon and modified the composition of the rock. The wide range of equilibration temperatures and the range of composition recorded in the inclusions of single diamonds suggest that a significant time interval was involved in diamond growth.     

Prograde and retrograde history of the Junction School eclogite,California, and an evaluation of garnet–phengite–clinopyroxene thermobarometry

Quantitative thermobarometry of inclusions in zoned garnet from a Franciscan eclogite block record a counter-clockwise P–T path from blueschist to eclogite and back. Garnet retains prograde zoning from inclusion-rich Alm₅₂Grs₃₀Pyp₆Sps₁₂ cores to inclusion-poor Alm₆₂Grs₂₅Pyp₁₂Sps₁ mantles, with overgrowths of highly variable composition. Barometry using the Waters–Martin version of the garnet–phengite–omphacite thermobarometer yields conditions of 7–15 kbar, 400–500°C (garnet cores), 18–22 kbar, ∼550°C (mantles), and 10–14 kbar, 350–450°C (overgrowths), in agreement with clinozoisite–sphene–rutile–garnet–quartz barometry. These pressures are ∼10–15 kbar less than those obtained using more recent, fully thermodynamic calibrations of the phengite–omphacite–garnet thermobarometer. Low early temperatures suggest that the block was subducted in a thermally mature subduction zone and not at the inception of subduction when prograde temperature is expected to be higher. Franciscan high-grade blocks likely represent crust subducted throughout the history of this convergent margin, rather than only at the inception of the subduction zone.