A high-temperature and high-pressure Raman spectroscopic study of CaGeO3 garnet

 High-pressure and high-temperature Raman spectra of CaGeO₃ tetragonal garnet have been collected to 11.5 GPa and 1225 K, respectively, in order to investigate possible intrinsic anharmonic behaviour in this phase. The Raman peak positions were observed to vary linearly with pressure and temperature within the ranges studied, with the higher-energy peaks showing larger P- and T-induced shifts than the low energy modes. The observed T-induced shifts are similar to those reported for grossular and andradite, while the observed P-induced shifts are generally larger than those of aluminosilicate and MgSiO₃ majorite garnets (Gillet et al. 1992; Rauch et al. 1996) due to the larger bulk modulus of CaGeO₃ garnet. The observed mode shifts of CaGeO₃ garnet were used to determine the isothermal and isobaric mode Grüneisen parameters for this phase. These parameters are similar in value to those reported previously for grossular and andradite (Gillet et al. 1992). The calculated intrinsic anharmonic parameters, a _i , for CaGeO₃ garnet were determined to be nonzero, indicating significant anharmonic behaviour for this phase. These values, which range from −3.8 × 10⁻⁵ K⁻¹ to −1.3 × 10⁻⁵ K⁻¹, are also similar to those reported for andradite and grossular, but smaller than those determined for pyrope (Gillet et al. 1992). Hence, we expect MgSiO₃ majorite to show greater anharmonicity than the germanate analogue studied by us. The anharmonic parameters determined for CaGeO₃ tetragonal garnet may now be introduced into quasiharmonic vibrational heat capacity models to account for the observed anharmonic behaviour. Received: 21 April 1999 / Revised, accepted: 11 September 1999     

The crystal-chemical basis for Ar retention in micas: inferences from interlayer partitioning and implications for geochronology

 Cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length/strength in these minerals. These data therefore provide a useful crystal-chemical perspective on relative mass-transfer kinetics of radiogenic isotopes, and account for the observation that biotite is generally less retentive of ⁴⁰Ar and ⁸⁷Sr than coexisting muscovite. Partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite. First, the preferences of large (Rb, Cs)⁺ in biotite and of small La³⁺ and Na⁺ in muscovite indicate a relatively spacious interlayer volume in biotite (suggesting a longer mean K−O bond). Second, the preference of interlayer vacancies in biotite (with some/all possibly H₂O/H₃O⁺-filled) suggests that its adjacent 2:1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite. Third, the relative exclusion of large Ba²⁺ from biotite despite its large interlayer sites is attributed to O−H bonds pointing into the interlayer cavity sub-normal to (001); (K⁺, Ba²⁺)-H⁺ repulsion thereby induced by the bare proton both destabilizes Ba²⁺ and weakens K−O bonds. In contrast, muscovite offers a more favorable electrostatic environment for Ba²⁺ substitution since its O−H bonds are directed into the vacant M ₁ octahedral site sub-parallel to (001). This hypothesis is supported by the observation that progressive F(OH)₋₁ exchange enhances Ba²⁺ partitioning into biotite/phlogopite relative to coexisting muscovite. These crystal-chemical differences between biotite and muscovite are mirrored in calculated values of “ionic porosity”, Z _i , defined here as the percentage of their interlayer unit-cell volume not occupied by ions. A monitor of ionic packing density and geometry, Z _i is inversely correlated with K−O bond strength, which appears to be the rate-determining “kinetic common denominator” for a variety of processes affecting micas – including those responsible for loss of radiogenic isotopes in biotite and muscovite. Accordingly, the relatively longer/weaker K−O bonds of biotite are envisioned as being more easily stretched (during volume diffusion) or broken (during recrystallization or retrograde alteration). This in turn accounts for common observations of enhanced radiogenic Ar/Sr loss and younger ⁴⁰Ar/³⁹Ar and Rb/Sr ages in natural biotite (high Z _i ) relative to coexisting muscovite (lower Z _i ). Significantly, this pattern may arise irrespective of isotopic loss mechanism (diffusion or recrystallization, etc.), and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field, petrographic, and petrologic constraints. Extension of this partitioning/porosity-based synthesis leads to prediction of corollary age-retentivity-composition effects among chemically diverse trioctahedral and dioctahedral micas, which are best field tested in terranes that cooled slowly under dry, static conditions. Pressure effects on argon retention are also inferred from the porosity model. Received: 9 February 1995 / Accepted: 8 September 1995     

The influence of crystal field stabilization energy on Fe2+ partitioning in paragenetic minerals

In order to explore possible quantitative relations between crystal field stabilization energy, CFSE, and partitioning behaviour of the 3d ⁶-configured Fe²⁺ ion, a suite of 29 paragenetic rock-forming minerals from 12 high-grade metamorphic rock samples of the Ukrainian shield, including the parageneses garnet/orthopyroxene/clinopyroxene (2x), orthopyroxene/clinopyroxene, garnet/clinopyroxene, garnet/orthopyroxene/biotite, garnet/biotite, garnet/cordierite, garnet/cordierite/biotite, garnet/orthopyroxene/clinopyroxene/Ca-amphibole, Ca-amphibole/biotite (retrograde), was studied by electron microprobe analysis to obtain the respective K _D Fe^{2+ (Ph1/Ph2)} values and by polarized single crystal electronic absorption spectroscopy to evaluate the respective CFSE_Fe2+ values. Other than in the case of Cr³⁺, a clear quantitative relation between K _D ^(Ph1/Ph2) and the ΔCFSE^(Ph1/Ph2) was only observed when geometrical factors, mainly the volume of crystallographic sites and ionic radii of ions competing in the partitioning process, are similar in the respective two paragenetic phases to within 15–20%. In such cases, the ΔCFSE_Fe2+ contribution to K _D ^(Ph1/Ph2) amounts to 0.1 to 0.2 log K _D per 100 cm⁻¹ΔCFSE. The conclusion is that ΔCFSE_Fe2+ plays only a secondary role after geometrical factors, in the partitioning behaviour of Fe²⁺. The reason for this is seen in the facts that, compared to the 3d ³-configured Cr³⁺ ion, CFSE of the 3d ⁶-configured Fe²⁺ amounts only to 20–25%, and that the former ion enters only octahedral sites with similar geometrical properties in the paragenetic mineral phases. Received: 17 November 1998 / Accepted: 28 June 1999     

Calibration of the garnet–biotite–Al2SiO5–quartz geobarometer for metapelites

A garnet–biotite–Al₂SiO₅–quartz (GBAQ) geobarometer was empirically calibrated using more than 700 natural metapelites with a broad compositional range of garnet and biotite under P–T conditions of 450–950°C and 1–17 kbar. In the calibration, activity models of garnet and biotite identical to those in the garnet–biotite (GB) geothermometer of Holdaway [American Mineralogist 2000, 85: 881–892] were used. Therefore, the GBAQ geobarometer and the GB geothermometer can be simultaneously applied to iteratively estimate metamorphic P–T conditions. Successful applications of the GBAQ geobarometer to natural metapelites certify its validity. Most importantly, when plagioclase is absent or CaO components in garnet and/or plagioclase are deficient, this geobarometer may prove useful for estimating metamorphic pressures. The random error of the present GBAQ geobarometer is inferred to be around ±1.8 kbar. An electronic spreadsheet is available as Table S4 to apply the GBAQ geobarometer in combination with the GB geothermometer.     

In search of the mixed derivative ?2 M /? P ? T ( M = G , K ): joint analysis of ultrasonic data for polycrystalline pyrope from gas- and solid-medium apparatus

Elastic wave velocities for dense (99.8% of theoretical density) isotropic polycrystalline specimens of synthetic pyrope (Mg₃Al₂Si₃O₁₂) were measured to 1,000 K at 300 MPa by the phase comparison method of ultrasonic interferometry in an internally heated gas-medium apparatus. The temperature derivatives of the elastic moduli [(∂Ks/∂T) _P = −19.3(4); (∂G/∂T) _P = −10.4(2) MPa K⁻¹] measured in this study are consistent with previous acoustic measurements on both synthetic polycrystalline pyrope in a DIA-type cubic anvil apparatus (Gwanmesia et al. in Phys Earth Planet Inter 155:179–190, 2006) and on a natural single crystal by the rectangular parallelepiped resonance (RPR; Suzuki and Anderson in J Phys Earth 31:125–138, 1983) method but |(∂Ks/∂T) _P | is significantly larger than from a Brillouin spectroscopy study of single-crystal pyrope (Sinogeikin and Bass in Phys Earth Planet Inter 203:549–555, 2002). Alternative approaches to the retrieval of mixed derivatives of the elastic moduli from joint analysis of data from this study and from the solid-medium data of Gwanmesia et al. in Phys Earth Planet Inter 155:179–190 (2006) yield ∂² G/∂P∂T = [0.07(12), 0.20(14)] × 10⁻³ K⁻¹ and ∂² K _S /∂P∂T = [−0.20(24), 0.22(26)] × 10⁻³ K⁻¹, both of order 10⁻⁴ K⁻¹ and not significantly different from zero. More robust inference of the mixed derivatives will require solid-medium acoustic measurements of precision significantly better than 1%.     

Hydroxyl defects in garnets from mantle xenoliths in kimberlites of the Siberian platform

A suite of more than 200 garnet single crystals, extracted from 150 xenoliths, covering the whole range of types of garnet parageneses in mantle xenoliths so far known from kimberlites of the Siberian platform and collected from nearly all the kimberlite pipes known in that tectonic unit, as well as some garnets found as inclusions in diamonds and olivine megacrysts from such kimberlites, were studied by means of electron microprobe analysis and single-crystal IR absorption spectroscopy in the v _OH vibrational range in search of the occurrence, energy and intensity of the v _OH bands of hydroxyl defects in such garnets and its potential use in an elucidation of the nature of the fluid phase in the mantle beneath the Siberian platform. The v _OH single-crystal spectra show either one or a combination of two or more of the following major v _OH bands, I 3645–3662 cm⁻¹, II 3561–3583 cm⁻¹, III 3515–3527 cm⁻¹, and minor bands, Ia 3623–3631 cm⁻¹, IIa 3593–3607 cm⁻¹. The type of combination of such bands in the spectrum of a specific garnet depends on the type of the rock series of the host xenolith, Mg, Mg-Ca, Ca, Mg-Fe, or alkremite, on the xenolith type as well as on the chemical composition of the respective garnet. Nearly all garnets contain band systems I and II. Band system III occurs in Ti-rich garnets, with wt% TiO₂ > ca. 0.4, from xenoliths of the Mg-Ca and Mg-Fe series, only. The v _OH spectra do not correspond to those of OH⁻ defects in synthetic pyropes or natural ultra-high pressure garnets from diamondiferous metamorphics. There were no indications of v _OH from inclusions of other minerals within the selected 60 × 60 μm measuring areas in the garnets. The v _OH spectra of pyrope-knorringite- and pyrope-knorringite-uvarovite-rich garnets included in diamonds do not show band systems I to III. Instead, they exhibit one weak, broad band (Δv _OH 200–460 cm⁻¹) near 3570 cm⁻¹, a result that was also obtained on pyrope-knorringite-rich garnets extracted from two olivine megacrysts. The quantitative evaluation, on the basis of relevant existing calibrational data (Bell et al. 1995), of the sum of integral intensities of all v _OH bonds of the garnets studied yielded a wide range of “water” concentrations within the set of the different garnets, between values below the detection limit of our single-crystal IR method, near 2 × 10⁻⁴ wt%, up to 163 × 10⁻⁴ wt%. The “water” contents vary in a complex manner in garnets from different xenolith types, obviously depending on a large number of constraints, inherent in the crystal chemistry as well as the formation conditions of the garnets during the crystallization of their mantle host rocks. Secondary alteration effects during uplift of the kimberlite, play, if any, only a minor role. Despite the very complex pattern of the “water” contents of the garnets, preventing an evaluation of a straightforward correlation between “water” contents of the garnets and the composition of the mantle's fluid phase during garnet formation, at least two general conclusions could be drawn: (1) the wide variation of “water” contents in garnets is not indicative of regional or local differences in the composition of the mantle's fluid phase; (2) garnets formed in the high-pressure/high-temperature diamond-pyrope facies invariably contain significantly lower amounts of “water” than garnets formed under the conditions of the graphite-pyrope facies. This latter result (2) may point to significantly lower f _H2O and f _O2 in the former as compared to the latter facies. Received: 25 November 1997 / Accepted: 9 March 1998     

Heat capacities of Tschermak substituted Fe-biotite

Six members of the annite–siderophyllite join were synthesized in a three step process – crystallization of biotite from gels, decomposition of the fine-grained biotite under oxidizing conditions and resynthesis of Fe-Al biotite with planned compositions from these products – producing biotite crystals with thicknesses of up to 10 μm. The biotite was characterized by microprobe, electron microscopy and X-ray diffraction. Heat capacities of these biotites were measured with a DSC (differential scanning calorimeter) over the temperature range 143 to 623 K. Using a least-squares technique, the data were fitted to a c_p-polynomial, c _p =k ₀+k ₁ T ^−0.5+ k ₂ T ⁻²+k ₃ T ⁻³. In the temperature range 143 to 250 K, heat capacities of the different annite–siderophyllite members decrease linearly with increasing Al content. At higher temperatures, however, the c_p-polynomial of biotites with intermediate composition (except Ann₇₉Sid₂₁) exhibit a steeper slope than those of other biotites. This produces positive excess heat capacities in the annite–siderophyllite join at higher temperatures. The activity-composition data of the same binary derived from phase equilibrium experiments (Benisek et al. 1996) and the data of this study suggest two compositional regions along this join, with different extent of deviation from ideality. One at X _Sid < 0.3, characterized by a small deviation, one at X _Sid > 0.3 showing a higher nonideality, resulting in a discontinuity visible at this composition. Powder IR-spectra of these solid solutions were measured with a FTIR-spectrometer and used to calculate heat capacities according to the vibrational model of Kieffer (1979). The comparison of the vibrational function with the c_p-polynomials shows that the vibrational function reproduces well the DSC-data of the siderophyllite-poor and -rich members, but deviates for intermediate compositions, where the excess heats of mixing occur. With increasing Tschermak vector, the tetrahedral rotation angle α increases from 0 to 13° for annite to siderophyllite, respectively. At the composition of the discontinuity, this rotation angle α reaches a value of ∼8^∘. The processing of ∼300 chemical data of natural biotites indicates that over 90% of them have a tetrahedral rotation angle that lies between 7 and 9°. It would appear that biotites with these structural characteristics are most stable. Received: 27 August 1998 / Accepted: 10 November 1998     

Phosphorus – an omnipresent minor element in garnet of diverse textural types from leucocratic granitic rocks

Summary Elevated P contents of up to 0.086 apfu (1.21 wt.% P₂O₅) were found in garnet from leucocratic granitic rocks (orthogneisses, granites, barren to highly evolved pegmatites) in the Moldanubicum and Silesicum, Czech Republic, and in complex granitic pegmatites from southern California, USA, and Australia. Minor concentrations (0.15–0.55 wt.% P₂O₅) appear ubiquitous in garnet from leucocratic granitic rocks of different origins and degrees of fractionation. Concentrations of P are not related to Mn/(Mn + Fe) that vary from 0.12–0.86 and to textural types of garnet (i.e., isolated anhedral to euhedral grains and nodules, graphic and random garnet–quartz aggregates, subsolidus veins of fine-grained garnet). Garnet compositions exhibit negative correlations for P/Si and P/R²⁺ where R²⁺ = Fe + Mn + Mg + Ca, while Al is constant at ∼2.05 apfu. Concentrations of Na are largely below 0.02 apfu but positively correlate with P. The main substitution may involve A-site vacancy and/or the presence of some light element(s) in the crystal structure. The substitution □P₂ R²⁺ ₋₁Si₋₂ and/or alluaudite-type Na□P₃ R²⁺ ₋₁Si₋₃ seem the most likely P-incorporating mechanisms. The partitioning of P among garnet and associated minerals in granitic systems remains unclear; however, it directly affects the distribution of Y and REEs.     

Experimental determination of activities in FeTiO3-MnTiO3 ilmenite solid solution by redox reversals

 Solid solutions of (Fe,Mn)TiO₃ were synthesized, mostly at 0.10 X_Mn intervals, at 1 bar, 900°C and log f _{O 2} = –17.50. Analysis by EMP indicate an ideal stoichiometry for the Fe-Mn ilmenites with (Fe+Mn) = Ti = 1.000 when normalized to 3 oxygens. Their unit cell volume increases linearly with X_Mn. The composition of Fe-Mn ilmenite coexisting with metallic Fe and rutile was reversed at 1 bar, 700–900°C and fixed f _{O 2} in a gas-mixing furnace. Oxygen fugacity was controlled by mixing CO₂ and H₂ gas and was continuously monitored with an yttrium-stabilized zirconia electrolyte. Solution properties of Fe-Mn ilmenite were derived from the experimental data by mathematical programming (Engi and Feenstra, in preparation) including notably the results of Fe-Mn exchange experiments between ilmenite and garnet (Feenstra and Engi, submitted) and anchoring the standard state properties to the updated thermodynamic dataset of Berman and Aranovich (1996). The thermodynamic analysis resulted in positive deviations from ideality for (Fe,Mn)TiO₃ ilmenite, which is well described by an asymmetric Margules model with W^H _FeFeMn = –9.703 and W^H _FeMnMn = –23.234 kJ/mol, W^S _FeFeMn = –19.65 and W^S _FeMnMn = –22.06 J/(K·mol). The excess free energy for Fe-Mn ilmenite derived from the redox reversals is larger than in the symmetric ilmenite model (W^G _FeMn = +2.2 kJ/mol) determined by O'Neill et al. from emf measurements on the assemblage iron-rutile-(Fe,Mn)ilmenite. Received: 10 January 1996 / Accepted: 11 July 1996     

Heat capacity,entropy and phase equilibria of stishovite

The low-temperature heat capacity (C _P) of stishovite (SiO₂) synthesized with a multi-anvil device was measured over the range of 5–303 K using the heat capacity option of a physical properties measurement system (PPMS) and around ambient temperature using a differential scanning calorimeter (DSC). The entropy of stishovite at standard temperature and pressure calculated from DSC-corrected PPMS data is 24.94 J mol⁻¹ K⁻¹, which is considerably smaller (by 2.86 J mol⁻¹ K⁻¹) than that determined from adiabatic calorimetry (Holm et al. in Geochimica et Cosmochimica Acta 31:2289–2307, 1967) and about 4% larger than the recently reported value (Akaogi et al. in Am Mineral 96:1325–1330, 2011). The coesite–stishovite phase transition boundary calculated using the newly determined entropy value of stishovite agrees reasonably well with the previous experimental results by Zhang et al. (Phys Chem Miner 23:1–10, 1996). The calculated phase boundary of kyanite decomposition reaction is most comparable with the experimental study by Irifune et al. (Earth Planet Sci Lett 77:245–256, 1995) at low temperatures around 1,400 K, and the calculated slope in this temperature range is mostly consistent with that determined by in situ X-ray diffraction experiments (Ono et al. in Am Mineral 92:1624–1629, 2007).     

Coexisting garnets and biotites from Precambrian gneisses of the south coast of Western Australia

Garnet-biotite (-cordierite) phase relations in high-grade gneisses of the south coast of Western Australia reflect at least two metamorphic episodes. Chemical uniformity of the interiors of garnet and cordierite grains suggest thorough equilibration during a major phase of metamorphism. Narrow Mg-depleted rims on garnet grain boundaries in contact with biotite or cordierite, and complementary Mg-enriched rims on contiguous cordierites are the result of subsequent retrograde re-equilibration. The absence of reaction zoning in biotites suggests more complete retrograde modification of this mineral.Comparison between granulite and amphibolite facies garnet-biotite pairs shows that Mn contents of both minerals are higher, and Ti contents of the biotites are lower, in the lower-grade rocks. These differences, although not entirely unrelated to grade, are more directly controlled by variations in host rock chemistry and modal amounts of garnet and biotite.Partitioning of Mg, Fe²⁺ and Mn between garnet and biotite is fairly uniform, with no clear differences between granulite and amphibolite facies pairs. Application of the Mg-Fe²⁺ distributions to the geothermometers devised by Perchuk, Thompson, and Goldman & Albee yields variable T estimates of 600–680°C, 580–780°C, and 475–715°C respectively, for the main metamorphism. These estimates are low compared with the T indicated for the granulite facies rocks by other evidence (i.e. > 750°C at 5 kb P_T). The Mg-Fe²⁺ distributions between contiguous garnet-biotite rims suggest that retrograde re-equilibration occurred at least 20–140°C below the T of the main metamorphism.     

Fe–Mg partitioning between ringwoodite and magnesiowüstite and the effect of pressure, temperature and oxygen fugacity

 The partitioning of Mg and Fe between magnesiowüstite and ringwoodite solid solutions has been measured between 15 and 23 GPa and 1200–1600 ^∘C using both Fe and Re capsule materials to vary the oxidation conditions. The partitioning results show a clear dependence on the capsule material used due to the variation in Fe³⁺ concentrations as a consequence of the different oxidation environments. Using results from experiments performed in Fe capsules, where metallic Fe was also added to the starting materials, the difference in the interaction parameters for the two solid solutions (W _FeMg ^mw−W _FeMg ^ring) is calculated to be 8.5±1 kJ mol⁻¹. Similar experiments performed in Re metal capsules result in a value for W _FeMg ^mw−W _FeMg ^ring that is apparently 4 kJ higher, if all Fe is assumed to be FeO. Electron energy-loss near-edge structure (ELNES) spectroscopic analyses, however, show Fe³⁺ concentrations to be approximately three times higher in magnesiowüstite produced in Re capsules than in Fe capsules and that Fe³⁺ partitions preferentially into magnesiowüstite, with K _{D Fe3+} ^ring/mw estimated between 0.1 and 0.6. Using an existing activity composition model for magnesiowüstite, a least–squares fit to the partitioning data collected in Fe capsules results in a value for the ringwoodite interaction parameter (W _FeMg ^ring) of 3.5±1 kJ mol⁻¹. The equivalent regular interaction parameter for magnesiowüstite (W _FeMg ^mw) is 12.1±1.8 kJ mol. These determinations take into account the Fe³⁺ concentrations that occur in both phases in the presence of metallic Fe. The free energy change in J mol⁻¹ for the Fe exchange reaction can be described, over the range of experimental conditions, by 912 + 4.15 (T−298)+18.9P with T in K, P in kbar. The estimated volume change for this reaction is smaller than that predicted using current compilations of equation of state data and is much closer to the volume change at ambient conditions. These results are therefore a useful test of high pressure and temperature equation of state data. Using thermodynamic data consistent with this study the reaction of ringwoodite to form magnesiowüstite and stishovite is calculated from the data collected using Fe capsules. Comparison of these results with previous studies shows that the presence of Fe³⁺ in phases produced in multianvil experiments using Re capsules can have a marked effect on apparent phase relations and determined thermodynamic properties. Received: 13 September 2000 / Accepted: 25 March 2001     

Experimental determination of Mn-Mg mixing properties in garnet,olivine and oxide

We have measured the mixing properties of Mn-Mg olivine and Mn-Mg garnet at 1300° C from a combination of interphase partitioning experiments involving these phases, Pt-Mn alloys and Mn-Mg oxide solid solutions. Activity coefficients of Mn dilute in Pt-Mn alloys at 1300° C/1 atm were measured by equilibrating the alloy with MnO at known f _{O 2}. Assuming that the log f _{O 2} of the Mn-MnO equilibrium under these conditions is-17.80 (Robie et al. 1978), we obtain for _Mn: log_Mn = –5.25 + 3.67 X_Mn + 11.41X² _Mn Mixing properties of (Mn,Mg)O were determined by reversing the Mn contents of the alloys in equilibrium with oxide at known f _{O 2}. Additional constraints were obtained by measuring the maximum extent of immiscibility in (Mn,Mg)O at 800 and 750° C. The data are adequately described by an asymmetric (Mn,Mg)O solution with the following upper and lower limits on nonideality: (a) W_Mn = 19.9kj/Mol; W_Mg = 13.7kj/Mol; (b) W_Mn = 19.9kj/Mol; W_Mg = 8.2kj/Mol; Olivine-oxide partitioning was tightly bracketed at 1300° C and oxide properties used to obtain activity-composition relations for Mn-Mg olivine. Despite strong M2 ordering of Mn in olivine, the macroscopic properties are adequately described by a symmetric model with: W^ol = 5.5 ± 2.5 kj/mol (1-site basis) Using these values for olivine, garnet-olivine partitioning at 27 kbar/1300° C leads to an Mn-Mg interaction parameter in garnet given by: W^gt = 1.5 ± 2.5kJ/mol (1-site basis) Garnet-olivine partitioning at 9 kbar/1000° C is consistent with the same extent of garnet nonideality and the apparent absence of excess volume on the pyrope-spessartine join indicates that any pressure-dependence of W^Gt must be small. If olivine and garnet properties are both treated as unknown and the garnet-olivine partitioning data alone used to derive W^Ol and W^Gt, by multiple linear regression, best-fit values of 6.16 and 1.44 kJ/mol. are obtained. These are in excellent agreement with the values derived from metal-oxide, oxide-olivine and olivine-garnet equilibria.     

TC/EA-MS online determination of hydrogen isotope composition and water concentration in eclogitic garnet

A continuous flow method, by a combination of thermal conversion elemental analyzer (TC/EA) with isotope ratio mass spectrometry (MS), is presented for determination of both H isotope composition and H₂O concentration of garnet from eclogite. Together with biotite NBS-30, the garnet was tested by preheating mineral grains at different temperatures. Preheating at 90°C for 12 h was found to be capable of eliminating adsorption water on sample surface. This results in constant δD values and total H₂O contents for the garnet, with weighted means of −93 ± 2‰ and 522 ± 11 ppm (wt), respectively. The garnet that was preheated at 350°C for 4 h also gave constant δD values of −86 ± 6‰ and H₂O contents of 281 ± 13 ppm (wt). The latter result for the H₂O contents agrees with the H₂O contents 271 ± 58 ppm (wt) measured by Fourier transform infrared spectroscopy for quantitative analysis of structural hydroxyl in the same garnet. Stepwise-heating TC/EA-MS analyses for the garnet show that the molecular H₂O are depleted in D relative to the structural OH and has higher mobility than the structural OH. Therefore, the TC/EA-MS method can be used not only for quantitative determination of both H isotope composition and H₂O concentration of hydrous and anhydrous minerals, but also for the concentration of structural hydroxyl after high-T dehydration.     

Thermodynamic mixing behavior of synthetic Ca-Tschermak–diopside pyroxene solid solutions: I. Volume and heat capacity of mixing

The unite cell parameters and heat capacities of a series of synthetic clinopyroxenes on the join Ca-Tschermak (CaTs)−diopside (Di) were measured using X-ray powder diffraction and calorimetric methods, respectively. The volume of mixing at 298 K shows a negative asymmetric deviation from ideality. A two-parameter Margules fit to the data yields W _CaTs−Di ^V = −0.29 ± 0.11 cm³ mol⁻¹ and W _Di−CaTs ^V = −1.14 ± 0.14 cm³ mol⁻¹. Heat capacities were determined between 5 and 923 K by heat-pulse at 5−302 K and differential-scanning calorimetry at 143−923 K. The precision of the low and high temperature C _p data is better than ±1%. Polynomials of the form C _p = a + bT ^−1/2 + cT ⁻² + dT ⁻³ were fitted to the C _p data in the temperature range between 250 and 925 K. Thermal entropy values [S ₂₉₈ − S ₀] and [S ₉₀₀ −S ₀] as well as enthalpies [H ₂₉₈ − H ₀] and [H ₉₀₀ − H ₀] were calculated for all members of the solid solution series. No significant deviation from ideal mixing behavior was observed.     

P–V–T equation of state of Ca3Al2Si3O12 grossular garnet

The thermoelastic parameters of synthetic Ca₃Al₂Si₃O₁₂ grossular garnet were examined in situ at high-pressure and high-temperature by energy dispersive X-ray diffraction, using a Kawai-type multi-anvil press apparatus coupled with synchrotron radiation. Measurements have been conducted at pressures up to 20 GPa and temperatures up to 1,650 K: this P, T range covered the entire high-P, T stability field of grossular garnet. The analysis of room temperature data yielded V _0,300 = 1,664 ± 2 ?³ and K ₀ = 166 ± 3 GPa for K^￠₀ K^{\prime}_{0} fixed to 4.0. Fitting of our P–V–T data by means of the high-temperature third order Birch–Murnaghan or the Mie–Grüneisen–Debye thermal equations of state, gives the thermoelastic parameters: (∂K _0,T /∂T) _P  = −0.019 ± 0.001 GPa K⁻¹ and α _0,T  = 2.62 ± 0.23 × 10⁻⁵ K⁻¹, or γ ₀ = 1.21 for fixed values q ₀ = 1.0 and θ ₀ = 823 (Isaak et al. Phys Chem Min19:106–120, 1992). From the comparison of fits from two different approaches, we propose to constrain the bulk modulus of grossular garnet and its pressure derivative to K _T0 = 166 GPa and K^￠_T0 K^{\prime}_{T0}  = 4.03–4.35. Present results are compared with previously determined thermoelastic properties of grossular-rich garnets.     

Fe-Mn partitioning between garnet and ilmenite: experimental calibration and applications

A new mineralogic geothermometer based on the partitioning of Fe and Mn between garnet and ilmenite has been calibrated by reversal experiments in the P-T range 600–900° C, 2 and 5 kbars and for fO₂=QFM. The results constitute a sensitive geothermometer applicable over a broad range of composition and conditions. Garnetilmenite thermometry has advantages relative to existing geothermometers because of its accurate calibration, marked temperature sensitivity and the chemical and structural simplicity of the crystalline solutions involved. Application to natural assemblages reveals that the garnet-ilmenite geothermometer yields temperatures that agree well with other estimates. The reactivity of, and relatively rapid Fe-Mn diffusion in ilmenite may lead to retrograde resetting of high temperature partition values, but these factors may be useful for estimating rock cooling rates. Analysis of the experimental data indicates minor positive deviations from ideality for Fe-Mn garnets and ilmenites. Absolute magnitudes of interaction parameters (W _AB) derived from a regression analysis are subject to considerable uncertainty. The partition coefficient is, however, strongly dependent on the difference between solution parameters. These differences are well constrained with a magnitude of W _FeMn ^ilm –W _FeMn ^gar 300 cal mol^–1. The accuracy and applicability of garnet-ilmenite thermometry will improve with the availability of better thermodynamic data for garnet crystalline solutions.Abbreviations and symbols used in text R universal gas constant (cal/mol/°K) - T absolute temperature (°K or °C) - P pressure (kbars) - V ⁰ volume change of reaction (1) - H _{1, T} ⁰ standard state enthalpy change of reaction (1) at 1 bar and the T of interest, in cal/mole - S _T ⁰ entropy change of reaction (1) at T of interest, in cal/mole/°K - G _P,T ⁰ standard free energy change of reaction (1) at the T and P of interest, in cal/mole - distribution coefficient for Fe-Mn partitioning between garnet and ilmenite - K apparent equilibrium coefficient for reaction (1) - _i ^j activity of component i in phase j - W _A-B binary A-B interaction (Margules) parameter - gar garnet - ilm ilmenite - biot biotite - ol olivine - opx orthopyroxene     

Hydrogen partitioning between melt,clinopyroxene, and garnet at 3 GPa in a hydrous MORB with 6 wt.% H<Subscript>2</Subscript>O

To understand partitioning of hydrogen between hydrous basaltic and andesitic liquids and coexisting clinopyroxene and garnet, experiments using a mid-ocean ridge basalt (MORB) + 6 wt.% H₂O were conducted at 3 GPa and 1,150–1,325°C. These included both isothermal and controlled cooling rate crystallization experiments, as crystals from the former were too small for ion microprobe (SIMS) analyses. Three runs at lower bulk water content are also reported. H₂O was measured in minerals by SIMS and in glasses by SIMS, Fourier Transform infrared spectroscopy (FTIR), and from oxide totals of electron microprobe (EMP) analyses. At 3 GPa, the liquidus for MORB with 6 wt.% H₂O is between 1,300 and 1,325°C. In the temperature interval investigated, the melt proportion varies from 100 to 45% and the modes of garnet and clinopyroxene are nearly equal. Liquid composition varies from basaltic to andesitic. The crystallization experiments starting from above the liquidus failed to nucleate garnets, but those starting from below the liquidus crystallized both garnet and clinopyroxene. SIMS analyses of glasses with >7 wt.% H₂O yield spuriously low concentrations, perhaps owing to hydrogen degassing in the ultra-high vacuum of the ion microprobe sample chamber. FTIR and EMP analyses show that the glasses have 3.4 to 11.9 wt.% water, whilst SIMS analyses indicate that clinopyroxenes have 1,340–2,330 ppm and garnets have 98–209 ppm H₂O. D _H ^cpx−gt is 11 ± 3, D _H ^cpx−melt is 0.023 ± 0.005 and D _H ^gt−melt is 0.0018 ± 0.0006. Most garnet/melt pairs have low values of D _H ^gt−melt, but D _H ^gt−melt increases with TiO₂ in the garnet. As also found by previous studies, values of D _H ^cpx−melt increase with Al₂O₃ of the crystal. For garnet pyroxenite, estimated values of D _H ^{pyroxenite−melt} decrease from 0.015 at 2.5 GPa to 0.0089 at 5 GPa. Hydration will increase the depth interval between pyroxenite and peridotite solidi for mantle upwelling beneath ridges or oceanic islands. This is partly because the greater pyroxene/olivine ratio in pyroxenite will tend to enhance the H₂O concentration of pyroxenite, assuming that neighboring pyroxenite and peridotite bodies have similar H₂O in their pyroxenes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Evidence for the influence of fCH 4 on the crystallinity of disseminated carbon in greenschist facies rocks,Rhode Island,USA

Disseminated carbon has been extracted from 19 samples of arkosic rocks from the chlorite, biotite and garnet zones in the Narragansett basin, and analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD of these samples yields only the (002) peak which may be as much as four times wider than the silicon internal standard peak and is always skewed to low angles 2θ. All (002) peaks were digitized and the four moments calculated. The results suggest that width at half-height (W_1/2) provides a poorer estimate of peak shape than coefficients of skewness and kurtosis. The positive correlation of these coefficients with grain size determined by SEM in the range of 0.1–100 μm suggest a direct relationship between peak shape and grain size. No variable measured in this study correlates with metamorphic grade. However, W_1/2, skewness and kurtosis of the (002) diffraction peak of the carbonaceous material in the biotite and garnet zones correlates inversely with modal carbon. This suggests that the volatilization reaction: C+2H₂=CH₄ may be an important step in the mechanism of graphitization. Variable fugacities of H₂ and CH₄ and/or variable permeabilities may be responsible for the non-uniform development of graphite crystallinity in the greenschist facies rocks in Rhode Island.     

Benthic nutrient fluxes in the intertidal flat within the Changjiang （Yangtze River） Estuary

In an annual cycle from March 2005 to February 2006, benthic nutrient fluxes were measured monthly in the Dongtan intertidal flat within the Changjiang （Yangtze River） Estuary. Except for NH4^＋, there always showed high fluxes from overlying water into sediment for other four nutrients. Sediments in the high and middle marshes, covered with halophyte and consisting of macrofauna, demonstrated more capabilities of assimilating nutrients from overlying water than the low marsh. Sampling seasons and nutrient concentrations in the overlying water could both exert significant effects on these fluxes. Additionally, according to the model provided by previous study, denitrification rates, that utilizing NO3- transported from overlying water （Dw） in Dongtan sediments, were estimated to be from -16 to 193 μmol·h^-1·m^-2 with an average value of 63 μmol·h^-1·m^-2 （n=18）. These estimated values are still underestimates of the in-situ rates owing to the lack of consideration of DN, i.e., denitrification supported by the local NO3^- production via nitrification.