A new formulation of garnet–clinopyroxene geothermometer based on accumulation and statistical analysis of a large experimental data set

Published experimental data including garnet and clinopyroxene as run products were used to develop a new formulation of the garnet–clinopyroxene geothermometer based on 333 garnet–clinopyroxene pairs. Only experiments with graphite capsules were selected because of difficulty in estimating the Fe³⁺ content of clinopyroxene. For the calibration, a published subregular‐solution model was adopted to express the non‐ideality of garnet. The magnitude of the Fe–Mg excess interaction parameter for clinopyroxene (W_FeMg^Cpx), and differences in enthalpy and entropy of the Fe–Mg exchange reaction were regressed from the accumulated experimental data set. As a result, a markedly negative value was obtained for the Fe–Mg excess interaction parameter of clinopyroxene (W_FeMg^Cpx = ? 3843 J mol^?1). The pressure correction is simply treated as linear, and the difference in volume of the Fe–Mg exchange reaction was calculated from a published thermodynamic data set and fixed to be ?120.72 (J kbar^?1 mol^?1). The regressed and obtained thermometer formulation is as follows: where T = temperature, P = pressure (kbar), A = 0.5 X_grs (X_prp ? X_alm ? X_sps), B = 0.5 X_grs (X_prp ? X_alm + X_sps), C = 0.5 (X_grs + X_sps) (X_prp ? X_alm), X_prp = Mg/(Fe²⁺ + Mn + Mg + Ca)^Grt, X_alm = Fe/(Fe²⁺ + Mn + Mg + Ca)^Grt, X_sps = Mn/(Fe²⁺ + Mn + Mg + Ca)^Grt, X_grs = Ca/(Fe²⁺ + Mn + Mg + Ca)^Grt, X_Mg^Cpx = Mg/(Al + Fe^total + Mg)^Cpx, X_Fe^Cpx = Fe²⁺/(Al + Fe^total + Mg)^Cpx, K_D = (Fe²⁺/Mg)^Grt/(Fe²⁺/Mg)^Cpx, Grt = garnet, Cpx = clinopyroxene. A test of this new formulation to the accumulated data gave results that are concordant with the experimental temperatures over the whole range of the experimental temperatures (800–1820 °C), with a standard deviation (1 sigma) of 74 °C. Previous formulations of the thermometer are inconsistent with the accumulated data set; they underestimate temperatures by about 100 °C at >1300 °C and overestimate by 100–200 °C at <1300 °C. In addition, they tend to overestimate temperatures for high‐Ca garnet (X_grs ≈ 0.30–0.50). This new formulation has been tested against previous formulations of the thermometer by application to natural eclogites. This gave temperatures some 20–100 °C lower than previous formulations.     

The influence of Fe<Superscript>3+</Superscript> on garnet–orthopyroxene and garnet–olivine geothermometers

Applying Fe²⁺–Mg exchange geothermometers to natural samples may lead to incorrect temperature estimates if significant Fe³⁺ is present. In order to quantify this effect, high-pressure experiments were carried out in a belt apparatus in a natural system close to CFMAS at 5 GPa and 1,100–1,400 °C. The oxygen fugacity in the experiments was at or below the Re–ReO₂ buffer. This is at significantly more oxidized conditions than in previous experiments, and, as consequence, higher Fe³⁺/Fe²⁺ ratios were generated. The Fe³⁺ content of garnet in the experiments was quantified by electron microprobe using the flank method. Making the usual assumption that Fe_total = Fe²⁺, the two-pyroxene thermometer of Brey and Köhler (J Pet 31:1353–1378, 1990) reproduced the experimental temperature to ±35 °C and the garnet–clinopyroxene Fe²⁺–Mg exchange thermometer of Krogh (Contrib Miner Pet 99:44–48, 1988) overestimated the temperatures on average by only 25 °C. On the other hand, application of the garnet–olivine (O’Neill and Wood in Contrib Miner Pet 70:59–70, 1979) and garnet–orthopyroxene (Harley in Contrib Miner Pet 86:359–373, 1984) exchange geothermometers yielded an underestimation in calculated temperatures of >200 °C. However, making explicit accounting for Fe³⁺ in garnet (i.e. using only measured Fe²⁺) leads to a vast improvement in the agreement between calculated and experimental temperatures, generally to within ±70 °C for the garnet–orthopyroxene geothermometer as well as noticeable improvement of calculated temperatures for the garnet–olivine geothermometer. Our results demonstrate that the two-pyroxene and garnet–clinopyroxene thermometers are rather insensitive to the presence of Fe³⁺ whilst direct accounting of Fe³⁺ in garnet is essential when applying the garnet–olivine and garnet–orthopyroxene thermometers.     

An experimental study of the Fe oxidation states in garnet and clinopyroxene as a function of temperature in the system CaO–FeO–Fe2O3–MgO–Al2O3–SiO2: implications for garnet–clinopyroxene geothermometry

Samples with eclogitic composition in the system CaO–FeO–Fe₂O₃–MgO–Al₂O₃–SiO₂ were produced from various kinds of starting materials held in graphite-lined Pt capsules at a pressure of 2.5–3.0 GPa and temperatures of 800–1,300 °C using a piston-cylinder or Belt apparatus. Garnets and clinopyroxenes were characterized by analytical transmission electron microscopy and electron probe micro-analysis (EPMA). Fe³⁺/ΣFe ratios determined by electron energy-loss spectroscopy (EELS) decrease in clinopyroxene from 22.2 ± 3.4 % at 800 °C to 13.3 ± 5.4 % at 1,300 °C, while in garnet, they vary between 10.8 ± 1.5 and 15.4 ± 4.7 %, respectively. Temperature estimates according to Krogh (Contrib Mineral Petrol 99:44–48, 1988) reproduce the experimental temperature to ±60 °C without systematic deviations if total iron is used in the calculation. If only the Fe²⁺ content is used, which was obtained by combining EPMA and EELS results, the experimental temperature is underestimated by 33 °C on average at 800–1,200 °C and overestimated by 77 °C on average at 1,300 °C. These systematic deviations can be explained by the temperature-dependent ratio of Fe²⁺/ΣFe in garnet divided by that in clinopyroxene. Since the difference between the calculated and experimental temperature is relatively small, a Fe²⁺-based recalibration of the thermometer appears not to be necessary for the investigated system in the range of pressure, temperature and composition covered by the experiments of this study.     

The garnet–clinopyroxene Fe2+–Mg geothermometer: an updated calibration

Multiple regression analysis on an extended dataset has been performed to refine the relationship between temperature, pressure, composition and the Fe–Mg distribution between garnet and clinopyroxene. In addition to a significant dependence between the distribution coefficient K_D and X ^Grt_Ca and X ^Grt_Mg#, as shown by the experimental data, the effect of X ^Grt_Mn has also been incorporated using data from natural Mn‐rich garnet–clinopyroxene pairs. Multiple regression of data (n=360) covering a large span in pressure, temperature and composition from 27 experimental datasets, combined with 49 natural high‐Mn granulites from Ruby Range, Montana, USA, and Karnataka, India, yields the P–T –compositional relationship (r²=0.98): where K_D=(Fe²⁺/Mg)^Grt/(Fe²⁺/Mg)^Cpx, X ^Grt_Ca=Ca/(Ca+Mn+Fe²⁺+Mg) in garnet, X ^Grt_Mn= Mn/(Ca+Mn+Fe²⁺+Mg) in garnet, and X ^Grt_Mg#=Mg/(Mg+Fe²⁺) in garnet. The Fe²⁺–Mg equilibrium between garnet and clinopyroxene does not seem to be affected by variations in the sodic content of the co‐existing clinopyroxene in the range X ^Cpx_Na=0–0.51. Comparisons between the new and former calibrations of the garnet–clinopyroxene Fe²⁺–Mg geothermometer clearly demonstrate how the various parameters in each case affect the calculated temperatures. Application of the new expression gives reasonable results for natural garnet–clinopyroxene pairs from various rock types and settings, and should be preferred to previous formulations. Using the new calibration to the self‐consistent dataset of Pattison & Newton (Contributions to Mineralogy and Petrology, 1989, 101, 87–103) suggests a systematic deviation with regard to both temperature and composition between their dataset and the datasets used in the present calibration.     

Internally consistent geothermometers for garnet peridotites and pyroxenites

Mutual relationships among temperatures estimated with the most widely used geothermometers for garnet peridotites and pyroxenites demonstrate that the methods are not internally consistent and may diverge by over 200°C even in well-equilibrated mantle xenoliths. The Taylor (N Jb Min Abh 172:381–408, 1998) two-pyroxene (TA98) and the Nimis and Taylor (Contrib Mineral Petrol 139:541–554, 2000) single-clinopyroxene thermometers are shown to provide the most reliable estimates, as they reproduce the temperatures of experiments in a variety of simple and natural peridotitic systems. Discrepancies between these two thermometers are negligible in applications to a wide variety of natural samples (≤30°C). The Brey and Köhler (J Petrol 31:1353–1378, 1990) Ca-in-Opx thermometer shows good agreement with TA98 in the range 1,000–1,400°C and a positive bias at lower T (up to +90°C, on average, at T _TA98 = 700°C). The popular Brey and Köhler (J Petrol 31:1353–1378, 1990) two-pyroxene thermometer performs well on clinopyroxene with Na contents of ~0.05 atoms per 6-oxygen formula, but shows a systematic positive bias with increasing Na_Cpx (+150°C at Na_Cpx = 0.25). Among Fe–Mg exchange thermometers, the Harley (Contrib Mineral Petrol 86:359–373, 1984) orthopyroxene–garnet and the recent Wu and Zhao (J Metamorphic Geol 25:497–505, 2007) olivine–garnet formulations show the highest precision, but systematically diverge (up to ca. 150°C, on average) from TA98 estimates at T far from 1,100°C and at T < 1,200°C, respectively; these systematic errors are also evident by comparison with experimental data for natural peridotite systems. The older O’Neill and Wood (Contrib Mineral Petrol 70:59–70, 1979) version of the olivine–garnet Fe–Mg thermometer and all popular versions of the clinopyroxene–garnet Fe–Mg thermometer show unacceptably low precision, with discrepancies exceeding 200°C when compared to TA98 results for well-equilibrated xenoliths. Empirical correction to the Brey and Köhler (J Petrol 31:1353–1378, 1990) Ca-in-Opx thermometer and recalibration of the orthopyroxene–garnet thermometer, using well-equilibrated mantle xenoliths and TA98 temperatures as calibrants, are provided in this study to ensure consistency with TA98 estimates in the range 700–1,400°C. Observed discrepancies between the new orthopyroxene–garnet thermometer and TA98 for some localities can be interpreted in the light of orthopyroxene–garnet Fe³⁺ partitioning systematics and suggest localized and lateral variations in mantle redox conditions, in broad agreement with existing oxybarometric data. Kinetic decoupling of Ca–Mg and Fe–Mg exchange equilibria caused by transient heating appears to be common, but not ubiquitous, near the base of the lithosphere.     

A garnet–hornblende geothermometer: calibration, testing, and application to the Pelona Schist, Southern California

Abstract A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In K_D=Δ (X_Ca,g) where K_D is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn-poor garnet and common hornblende of widely varying chemistry metamorphosed at low a_O2. Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks. Quartzo-feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.     

Effects of cation substitutions in garnet and pyroxene on equilibrium oxygen isotope fractionations

High-grade metamorphic rocks were used to explore oxygen isotope fractionations between pyroxene and garnet, and to investigate the effects on fractionation factors of the cation substitutions Fe³⁺Al_?1 and Ca(Fe,Mg)_?1. Recrystallized, granulite facies (725 °C) wollastonite ores from the northern Adirondack highlands contain essentially only the minerals clinopyroxene (a Di–Hd solid solution)+garnet (a Grs–Adr solid solution)±wollastonite, and exhibit a systematic dependence of measured fractionations on the Fe³⁺ content of calcic garnet: Δ(Cpx–CaGrt)=(0.14±0.12)+(0.78±0.20)X_Adr and Δ(Wo–CaGrt)=(0.15±0.22)+(0.57±0.33)X_Adr. In eclogites formed at T ≤650 °C, measured compositions of Ca-poor garnet and omphacite combined with experimental data indicate that Ca-poor, Fe-rich garnet is enriched in ¹⁸O compared to both diopside and grossular: extrapolating to 1000 K, Δ(Alm–Di)≈c. 0.2 and Δ(Alm–Grs)≈c. 0.5. Orthopyroxene and clinopyroxene from Gore Mountain, New York, show a constant fractionation that is independent of rock type, as expected if they have the same closure temperature. These data imply Δ(Opx-Cpx)≈c. 0.7 at 1000 K. Measured fractionations among Ca-poor garnet, orthopyroxene, clinopyroxene and hornblende in the Gore Mountain rocks further indicate an ¹⁸O enrichment in Ca-poor garnet over Grs (≈c. 0.5 at 1000 K). The new measurements are indistinguishable from expected equilibrium values based on experiments for the minerals enstatite, diopside, grossular, wollastonite and feldspar, but consistently indicate a significant isotope effect for the simple octahedral cation substitutions Fe³⁺Al_?1 (Grs vs. Adr) and Ca(Fe,Mg)_?1 (Ca-poor garnet vs. Grs; Opx vs. Cpx). Neither cation substitution has been directly investigated for its effect on ¹⁸O/¹⁶O fractionation with experiments in silicates. Chemical characterization of minerals is required prior to petrological interpretation of oxygen isotope trends.     

Silica precipitates in omphacite from eclogite at Alpe Arami, Switzerland: evidence of deep subduction

Observations of oriented SiO₂ precipitates in omphacite from eclogite with tholeiitic basalt protolith bordering the Alpe Arami garnet peridotite massif, Ticino, Switzerland, and petrological studies of the eclogitic mineral assemblages, suggest that this rock was subjected to higher‐pressure metamorphism than previously realized. We employed various calibrations of the Fe²⁺ ? Mg exchange thermometer and calculations of equilibria with thermodynamic data, considering the calcium–Tschermak's component (CaAl₂SiO₆), of garnet‐pyroxene pairs. From these calculations, it is concluded that the eclogitic lenses have recorded at least four stages of mineral growth corresponding to the following: Stage I (prograde) c. 2.4 GPa; 700 °C; Stage IIa (maximum recorded grade) c. 7.0 GPa; 1100 °C; Stage IIb (retrograde) c. 3.7 GPa; 900 °C; Stage III (retrograde) c. 2.1 GPa; 750 °C. Because of the preservation of Stage I, a relatively rapid subduction and exhumation of Alpe Arami eclogite is suggested. The exhumation path of the eclogitic rock is in good agreement with most exhumation paths inferred for the Alpe Arami garnet lherzolite proposed previously by several authors based upon a variety of different observations, although the eclogite and peridotite exhumation paths may diverge at depths greater than 120 km.     

Mineral/melt partitioning of trace elements during hydrous peridotite partial melting

This experimental study examines the mineral/melt partitioning of incompatible trace elements among high-Ca clinopyroxene, garnet, and hydrous silicate melt at upper mantle pressure and temperature conditions. Experiments were performed at pressures of 1.2 and 1.6 GPa and temperatures of 1,185 to 1,370 °C. Experimentally produced silicate melts contain up to 6.3 wt% dissolved H ₂O, and are saturated with an upper mantle peridotite mineral assemblage of olivine+orthopyroxene+clinopyroxene+spinel or garnet. Clinopyroxene/melt and garnet/melt partition coefficients were measured for Li, B, K, Sr, Y, Zr, Nb, and select rare earth elements by secondary ion mass spectrometry. A comparison of our experimental results for trivalent cations (REEs and Y) with the results from calculations carried out using the Wood-Blundy partitioning model indicates that H ₂O dissolved in the silicate melt has a discernible effect on trace element partitioning. Experiments carried out at 1.2 GPa, 1,315 °C and 1.6 GPa, 1,370 °C produced clinopyroxene containing 15.0 and 13.9 wt% CaO, respectively, coexisting with silicate melts containing ~1–2 wt% H ₂O. Partition coefficients measured in these experiments are consistent with the Wood-Blundy model. However, partition coefficients determined in an experiment carried out at 1.2 GPa and 1,185 °C, which produced clinopyroxene containing 19.3 wt% CaO coexisting with a high-H ₂O (6.26±0.10 wt%) silicate melt, are significantly smaller than predicted by the Wood-Blundy model. Accounting for the depolymerized structure of the H ₂O-rich melt eliminates the mismatch between experimental result and model prediction. Therefore, the increased Ca ²⁺ content of clinopyroxene at low-temperature, hydrous conditions does not enhance compatibility to the extent indicated by results from anhydrous experiments, and models used to predict mineral/melt partition coefficients during hydrous peridotite partial melting in the sub-arc mantle must take into account the effects of H ₂O on the structure of silicate melts.     

Ultrahigh-pressure metamorphism and exhumation of garnet-bearing ultramafic rocks from the Lanterman Range (northern Victoria Land, Antarctica)

Northern Victoria Land is a key area for the Ross Orogen – a Palaeozoic foldbelt formed at the palaeo‐Pacific margin of Gondwana. A narrow and discontinuous high‐ to ultrahigh‐pressure (UHP) belt, consisting of mafic and ultramafic rocks (including garnet‐bearing types) within a metasedimentary sequence of gneisses and quartzites, is exposed at the Lanterman Range (northern Victoria Land). Garnet‐bearing ultramafic rocks evolved through at least six metamorphic stages. Stage 1 is defined by medium‐grained garnet + olivine + low‐Al orthopyroxene + clinopyroxene, whereas finer‐grained garnet + olivine + orthopyroxene + clinopyroxene + amphibole constitutes the stage 2 assemblage. Stage 3 is defined by kelyphites of orthopyroxene + clinopyroxene + spinel ± amphibole around garnet. Porphyroblasts of amphibole replacing garnet and clinopyroxene characterize stage 4. Retrograde stages 5 and 6 consist of tremolite + Mg‐chlorite ± serpentine ± talc. A high‐temperature (～950 °C), spinel‐bearing protolith (stage 0), is identified on the basis of orthopyroxene + clinopyroxene + olivine + spinel + amphibole inclusions within stage 1 garnet. The P–T estimates for stage 1 are indicative of UHP conditions (3.2–3.3 GPa and 764–820 °C), whereas stage 2 is constrained between 726–788 °C and 2.6–2.9 GPa. Stage 3 records a decompression up to 1.1–1.3 GPa at 705–776 °C. Stages 4, 5 and 6 reflect uplift and cooling, the final estimates yielding values below 0.5 GPa at 300–400 °C. The retrograde P–T path is nearly isothermal from UHP conditions up to deep crustal levels, and becomes a cooling–unloading path from intermediate to shallow levels. The garnet‐bearing ultramafic rocks originated in the mantle wedge and were probably incorporated into the subduction zone with felsic and mafic rocks with which they shared the subsequent metamorphic and geodynamic evolution. The density and rheology of the subducted rocks are compatible with detachment of slices along the subduction channel and gravity‐driven exhumation.     

Constraining the P–T path of a MORB-type eclogite using pseudosections, garnet zoning and garnet-clinopyroxene thermometry: an example from the Bohemian Massif

A mid‐ocean ridge basalt (MORB)‐type eclogite from the Moldanubian domain in the Bohemian Massif retains evidence of its prograde path in the form of inclusions of hornblende, plagioclase, clinopyroxene, titanite, ilmenite and rutile preserved in zoned garnet. Prograde zoning involves a flat grossular core followed by a grossular spike and decrease at the rim, whereas Fe/(Fe + Mg) is also flat in the core and then decreases at the rim. In a pseudosection for H₂O‐saturated conditions, garnet with such a zoning grows along an isothermal burial path at c. 750 °C from 10 kbar in the assemblage plagioclase‐hornblende‐diopsidic clinopyroxene‐quartz, then in hornblende‐diopsidic clinopyroxene‐quartz, and ends its growth at 17–18 kbar. From this point, there is no pseudosection‐based information on further increase in pressure or temperature. Then, with garnet‐clinopyroxene thermometry, the focus is on the dependence on, and the uncertainties stemming from the unknown Fe³⁺ content in clinopyroxene. Assuming no Fe³⁺ in the clinopyroxene gives a serious and unwarranted upward bias to calculated temperatures. A Fe³⁺‐contributed uncertainty of ±40 °C combined with a calibration and other uncertainties gives a peak temperature of 760 ± 90 °C at 18 kbar, consistent with no further heating following burial to eclogite facies conditions. Further pseudosection modelling suggests that decompression to c. 12 kbar occurred essentially isothermally from the metamorphic peak under H₂O‐undersaturated conditions (c. 1.3 mol.% H₂O) that allowed the preservation of the majority of garnet with symplectitic as well as relict clinopyroxene. The modelling also shows that a MORB‐type eclogite decompressed to c. 8 kbar ends as an amphibolite if it is H₂O saturated, but if it is H₂O‐undersaturated it contains assemblages with orthopyroxene. Increasing H₂O undersaturation causes an earlier transition to SiO₂ undersaturation on decompression, leading to the appearance of spinel‐bearing assemblages. Granulite facies‐looking overprints of eclogites may develop at amphibolite facies conditions.     

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).     

An experimental study of the effect of Ca upon garnet-clinopyroxene Fe-Mg exchange equilibria

A series of basaltic compositions and compositions within the simple system CaO-MgO-FeO-Al₂O₃-SiO₂ have been crystallized to garnetclinopyroxene bearing mineral assemblages in the range 24–30 kb pressure, 750°–1,300° C temperature. Microprobe analyses of coexisting garnet and clinopyroxene show that K _D(Fe²⁺/Mg^G+/Fe²⁺/Mg^Cpx) for the Fe-Mg exchange reaction between coexisting garnet and clinopyroxene is obviously dependent upon the Ca-content and apparently independent of the Mg/(Mg+Fe) content of the clinopyroxene and garnet. The Ca-effect is believed to be due to a combination of non-ideal Ca-Mg substitutions in the garnet and clinopyroxene. Our data and interpretation reconciles previous inconsistencies in the temperature dependence of K _D ^? values determined in experimental studies of simple systems, complex basalt, grospydite and garnet peridotite compositions. Previous differences between the effect of pressure upon K _Das predicted from simple system theory (Banno, 1970), and that observed in experiments on multicomponent natural rock compositions (Råheim and Green, 1974a) can now be resolved. We have determined K _Das a function of P, T, and X _Gt ^Ca (grossular) and derived the empirical relation $$T\left( {^\circ {\text{K}}} \right) = \frac{{3104X_{{\text{Ca}}}^{{\text{Gt}}} + 3030 + 10.86P\left( {{\text{kb}}} \right)}}{{\ln K_{\text{D}} + 1.9034}}$$ . This empirical relationship has been applied to garnet-clinopyroxene bearing rocks from a wide range of geological environments. The geothermometer yields similar estimates for garnet-clinopyroxene equilibration for neighbouring rocks of different composition and different K _Dvalues. In addition, temperature estimates using the above relationship are more consistent with independent temperature estimates based on other geothermometers than previous estimates which did not correct for the Ca-effect. An alternative approach to the above empirical geothermometer was attempted using regular solution models to derive Margules parameters for various solid solutions in garnets and clinopyroxenes. The derived Margules parameters are broadly consistent with those determined from binary solution studies, but caution must be exercised in interpreting them in terms of actual thermodynamic properties of the relevant crystalline solid solutions because of the assumptions which necessarily have to be made in this approach.     

Transformation of garnet epidote amphibolite to eclogite, western Dabie Mountains, China

Omphacite and garnet coronas around amphibole occur in amphibolites in the Hong'an area, western Dabie Mountains, China. These amphibolites consist of an epidote–amphibolite facies assemblage of amphibole, garnet, albite, clinozoisite, paragonite, ilmenite and quartz, which is incompletely overprinted by an eclogite facies assemblage of garnet, omphacite and rutile. Coronas around amphibole can be divided into three types: an omphacite corona; a garnet–omphacite–rutile corona; and, a garnet–omphacite corona with less rutile. Chemographic analysis for local reaction domains in combination with petrographical observations show that reactions Amp + Ab + Pg = Omp +Czo + Qtz + H₂O, and Amp + Ab = Omp ± Czo + Qtz + H₂O may lead to the development of omphacite coronas. The garnet–omphacite–rutile corona was formed from the reaction Amp + Ab + Czo + Ilm ± Qtz = Omp + Grt + Rt + H₂O. In garnet–omphacite coronas, the garnet corona grew during an early stage of epidote amphibolite facies metamorphism, whereas omphacite probably formed by the reactions forming the omphacite corona during the eclogite facies stage. It is estimated that these reactions occurred at 0.8–1.4 GPa and 480–610 °C using the garnet–clinopyroxene thermometer and omphacite barometer in the presence of albite.     

Biotite and garnet compositional variation and mineral equilibria in Grenville gneisses of the Otter Lake area, Quebec

Garnet-biotite gneisses, some of which contain sillimanite or hornblende, are widespread within the Otter Lake terrain, a portion of the Grenville Province of the Canadian Shield. The metamorphic grade is upper amphibolite to, locally, lower granulite facies. The atomic ratio Fe²⁺/(Fe²⁺+ Fe³⁺) in biotite ranges from 0.79 to 0.89 (ferrous iron determinations in 10 highly pure separates), with a mean of 0.86. Mg and Fe²⁺ atoms occupy 67–78% of the octahedral sites, the remainder are occupied by Fe³⁺, Ti, and Al, and some are vacant. Mg/(Mg + Fe²⁺), denoted X, in the analysed samples ranges from 0.32 to 0.65. Garnet contains 1–24% grossular, 1–12% spessartine and X ranges from 0.07 to 0.34. Compositional variation in biotite and garnet is examined in relation to three mineral equilibria: (I) biotite + sillimanite + quartz = garnet + K-feldspar + H₂O; (II) pyrope + annite = almandine + phlogopite; (III) anorthite = grossular + sillimanite + quartz. Measurements of X (biotite) and X (garnet) are used to construct an illustrative model for equilibrium (I) which relates the observed variation in X to a temperature range of 70°C or a range in H₂O activity of 0.6; the latter interpretation is preferred. In sillimanite-free gneisses, the distribution of Mg and Fe²⁺ between garnet (low in Ca and Mn) and biotite is adequately described by a distribution coefficient (KD) of 4.1 (equilibrium II). The observed increase in the distribution coefficient with increasing Ca in garnet is ln K_D= 1.3 + 2.5 × 10^?2 [Ca] where [Ca] = 100 Ca/(Mg + Fe²⁺+ Mn + Ca). The distribution coefficient is apparently unaffected by the presence of up to 12% spessartine in garnet. In several specimens of garnet-sillimanite-plagioclase gneiss, the Ca contents of garnet and of plagioclase increase in unison, as required by equilibrium (III). The mean pressure calculated from these data (n= 17) is 5.9 kbar, and the 95% confidence limits are ±0.5 kbar.     

Deep subduction of mantle-derived garnet peridotites from the Su-Lu UHP metamorphic terrane in China

Garnet peridotites from the southern Su‐Lu ultra‐high‐pressure metamorphic (UHPM) terrane, eastern China, contain porphyroblastic garnet with aligned inclusions comprising a low‐P–T mineral assemblage (chlorite, hornblende, Na‐gedrite, Na‐phlogopite, talc, spinel and pyrite). Orthopyroxene porphyroblasts show fine exsolution lamellae of clinopyroxene and minor chromite. A clinopyroxene inclusion in garnet shows some orthopyroxene exsolution lamellae. Both the rims of porphyroblastic pyroxene and garnet and the matrix pyroxene and garnet crystallized at the expense of olivine. This is interpreted as a result of metasomatism of the peridotites by an SiO₂‐rich melt at UHP conditions. A chromian garnet further overgrew on the rims of the garnet. The X_Mg values (Mg/(Mg+Fe)) of porphyroblastic garnet decrease from core to rim and vary in different peridotite samples, while the compositions of both the porphyroblastic and the matrix pyroxene are similar in terms of Ca–Mg–Fe. The Mg‐rich cores of porphyroblastic garnet and orthopyroxene record high temperatures and pressures (c. 1000 °C, ≥5.1 GPa), whereas the matrix minerals, including the rims of porphyroblasts, record much lower P–T (c. 4.2 GPa, c. 760 °C). Sm–Nd data give apparent isochron ages of c. 380 Ma and negative ε_Nd(0) values (c.?9). These dates are considered meaningless due to isotopic disequilibrium between garnet cores and the rest of the rocks. The isotopic disequilibrium was probably caused by metasomatism of the peridotites by melt/fluids derived from the coevally subducted crustal materials. On the other hand, the Rb–Sr isotopic systems of phlogopite and clinopyroxene appear to have reached equilibrium and record a cooling age of c. 205 Ma. It is suggested that the garnet peridotites were originally emplaced into a low‐P–T environment prior to the c. 220 Ma continental collision, during which they were subducted together with crustal rocks to mantle depth and subjected to UHP metamorphism. An important corollary is that at least some of the coevally subducted crustal rocks in the Su‐Lu terrane have been subjected to peak metamorphism at P–T conditions much higher than presently estimated (≥2.7 GPa, ≤800 °C).     

Metamorphic evolution of low-T eclogite from the North Qilian orogen, NW China: evidence from petrology and calculated phase equilibria in the system NCKFMASHO

Low‐T eclogites in the North Qilian orogen, NW China share a common assemblage of garnet, omphacite, glaucophane, epidote, phengite, quartz and rutile with or without paragonite. Phase relations for the low‐T eclogites can be modelled well in the system NCKFMASHO with the updated solid‐solution models for amphibole and clinopyroxene. Garnet in the eclogite typically exhibits growth zonations in which pyrope increases while grossular somewhat decreases from core to rim, which is modelled as having formed mainly in the P–T conditions of lawsonite‐eclogite facies at the pre‐peak stage. Omphacite shows an increase in jadeite component as aegirine and also total FeO decrease in going from the inclusions in garnet to grains in the matrix, and from core to rim of zoned crystals, reflecting an increase in metamorphic P–T conditions. Glaucophane exhibits a compositional variation in X(gl) (= Fe²⁺/(Fe²⁺ + Mg)) and F(gl) (= Fe³⁺/(Fe³⁺ + Al) in M2 site), which decrease from the inclusions in garnet to crystals in the matrix, consistent with an increase in P–T conditions. However, for zoned matrix crystals, the X(gl) and F(gl) increase from core to rim, is interpreted to reflect a late‐stage decompression. Using composition isopleths for garnet rim and phengite in P–T pseudosections, peak P–T conditions for three samples Q5–45, Q5–01 and Q7–28 were estimated as 530–540 °C at 2.10–2.25 GPa, 580–590 °C at 2.30–2.45 GPa and 575–590 °C at 2.50–2.65 GPa, respectively, for the same assemblage garnet + omphacite + glaucophane + lawsonite (+ phengite + quartz + rutile) at the peak stage. The eclogites suggest similar P–T ranges to their surrounding felsic–pelitic schists. During post‐peak decompression of the eclogites, the most distinctive change involves the transformation of lawsonite to epidote, releasing large amount of water in the rock. The released fluid promoted further growth of glaucophane at the expense of omphacite and, in appropriate bulk‐rock compositions, paragonite formed. The decompression of eclogite did not lead to pronounced changes in garnet and phengite compositions. Peak P–T conditions of the North Qilian eclogite are well constrained using both the average P–T and pseudosection approaches in Thermocalc. Generally, the conventional garnet–clinopyroxene geothermometer is too sensitive to be used for constraining the temperature of low‐T eclogite because of the uncertainty in Fe³⁺ determination in omphacite and slight variations in mineral compositions because of incomplete equilibration.     

Ultra high temperature metamorphism of mafic granulites from South Altyn Orogen,West China: A result from the rapid exhumation of deeply subducted continental crust

The South Altyn orogen in West China contains ultra high pressure (UHP) terranes formed by ultra‐deep (>150–300 km) subduction of continental crust. Mafic granulites which together with ultramafic interlayers occur as blocks in massive felsic granulites in the Bashiwake UHP terrane, are mainly composed of garnet, clinopyroxene, plagioclase, amphibole, rutile/ilmenite, and quartz with or without kyanite and sapphirine. The kyanite/sapphirine‐bearing granulites are interpreted to have experienced decompression‐dominated evolution from eclogite facies conditions with peak pressures of 4–7 GPa to high pressure (HP)–ultra high temperature (UHT) granulite facies conditions and further to low pressure (LP)–UHT facies conditions based on petrographic observations, phase equilibria modelling, and thermobarometry. The HP–UHT granulite facies conditions are constrained to be 2.3–1.6 GPa/1,000–1,070°C based on the observed mineral assemblages of garnet+clinopyroxene+rutile+plagioclase+amphibole±quartz and measured mineral compositions including the core–rim increasing anorthite in plagioclase (X_An = 0.52–0.58), core–rim decreasing jadeite in clinopyroxene (X_Jd = 0.20–0.15), and TiO₂ in amphibole (Ti^M2/2 = 0.14–0.18). The LP–UHT granulite facies conditions are identified from the symplectites of sapphirine+plagioclase+spinel, formed by the metastable reaction between garnet and kyanite at <0.6–0.7 GPa/940–1,030°C based on the calculated stability of the symplectite assemblages and sapphirine–spinel thermometer results. The common granulites without kyanite/sapphirine are identified to record a similar decompression evolution, including eclogite, HP–UHT granulite, and LP–UHT granulite facies conditions, and a subsequent isobaric cooling stage. The decompression under HP–UHT granulite facies is estimated to be from 2.3 to 1.3 GPa at ~1,040°C on the basis of textural records, anorthite content in plagioclase (X_An = 0.25–0.32), and grossular content in garnet (X_Grs = 0.22–0.19). The further decompression to LP–UHT facies is defined to be >0.2–0.3 GPa based on the calculated stability for hematite‐bearing ilmenite. The isobaric cooling evolution is inferred mainly from the amphibole (Ti^M2/2 = 0.14–0.08) growth due to the crystallization of residual melts, consistent with a temperature decrease from >1,000°C to ~800°C at ~0.4 GPa. Zircon U–Pb dating for the two types of mafic granulite yields similar protolith and metamorphic ages of c. 900 Ma and c. 500 Ma respectively. However, the metamorphic age is interpreted to represent the HP–UHT granulite stage for the kyanite/sapphirine‐bearing granulites, but the isobaric cooling stage for the common granulites on the basis of phase equilibria modelling results. The two types of mafic granulite should share the same metamorphic evolution, but show contrasting features in petrography, details of metamorphic reactions in each stage, thermobarometric results, and also the meaning of zircon ages as a result of their different bulk‐rock compositions. Moreover, the UHT metamorphism in UHP terranes is revealed to represent the lower pressure overprinting over early UHP assemblages during the rapid exhumation of ultra‐deep subducted continental slabs, in contrast to the cause of traditional UHT metamorphism by voluminous heat addition from the mantle.     

A Report on a Biotite-Calcic Hornblende Geothermometer

This paper presents a biotite-calcic hornblende geothermometer which was empirically calibrated based on the gamet-biotite geothermometer and the gamet-plagioclase-hornblende-quartz geobarometer, in the ranges of 560-800℃ (T) and 0.26-1.4 GPa (P) using the data of metadolerite, amphibolite, metagabbro, and metapelite collected from the literature. Biotite was treated as symmetric Fe-Mg-AlVI-Ti quaternary solid solution, and calcic hornblende was simplified as symmetric Fe-Mg binary solid solution. The resulting thermometer may rebuild the input garnet-biotite temperatures well within an uncertainty of ±50℃. Errors of ±0.2 GPa for input pressure, along with analytical errors of ?% for the relevant mineral compositions, may lead to a random error of ±16℃ for this thermometer, so that the thermometer is almost independent of pressure estimates. The thermometer may clearly discriminate different rocks of lower amphibolite, upper amphibolite and granulite facies on a high confidence level. It is assume     

Garnet-clinopyroxene geothermometer

A garnet-clinopyroxene geothermometer based on the available experimental data on compositions of coexisting phases in the system MgO-FeO-MnO-Al₂O₃-Na₂O-SiO₂ is as follows: $$T({\text{}}K) = \frac{{8288 + 0.0276 P {\text{(bar)}} + Q1 - Q2}}{{1.987 \ln K_{\text{D}} + 2.4083}}$$ where P is pressure, and Q1, Q2, and K _D are given by the following equations $$Q1 = 2,710{\text{(}}X_{{\text{Fe}}} - X_{{\text{Mg}}} {\text{)}} + 3,150{\text{ }}X_{{\text{Ca}}} + 2,600{\text{ }}X_{{\text{Mn}}} $$ (mole fractions in garnet) $$\begin{gathered}Q2 = - 6,594[X_{{\text{Fe}}} {\text{(}}X_{{\text{Fe}}} - 2X_{{\text{Mg}}} {\text{)]}} \hfill \\{\text{ }} - 12762{\text{ [}}X_{{\text{Fe}}} - X_{{\text{Mg}}} (1 - X_{{\text{Fe}}} {\text{)]}} \hfill \\{\text{ }} - 11,281[X_{{\text{Ca}}} (1 - X_{{\text{Al}}} ) - 2X_{{\text{Mg}}} 2X_{{\text{Ca}}} ] \hfill \\{\text{ + 6137[}}X_{{\text{Ca}}} (2X_{{\text{Mg}}} + X_{{\text{Al}}} )] \hfill \\{\text{ + 35,791[}}X_{{\text{Al}}} (1 - 2X_{{\text{Mg}}} )] \hfill \\{\text{ + 25,409[(}}X_{{\text{Ca}}} )^2 ] - 55,137[X_{{\text{Ca}}} (X_{{\text{Mg}}} - X_{{\text{Fe}}} )] \hfill \\{\text{ }} - 11,338[X_{{\text{Al}}} (X_{{\text{Fe}}} - X_{{\text{Mg}}} )] \hfill \\\end{gathered} $$ [mole fractions in clinopyroxene Mg = MgSiO₃, Fe = FeSiO₃, Ca = CaSiO₃, Al = (Al₂O₃-Na₂O)] K _D = (Fe/Mg) in garnet/(Fe/Mg) in clinopyroxene. Mn and Cr in clinopyroxene, when present in small concentrations are added to Fe and Al respectively. Fe is total Fe²⁺+Fe³⁺.