F-OH exchange equilibria between mica-amphibole mineral pairs

Fluoride-hydroxyl exchange equilibria between phlogopite-pargasite and phlogopite-tremolite mineral pairs were experimentally determined at 1,173K, 500 bars and 1,073–1,173 K, 500 bars respectively. The distribution of fluorine between phlogopite and pargasite was found to favor phlogopite slightly, G _ex ^. (1,173 K)=–1.71 kJ anion^–1, while in the case of phlogopite-tremolite, fluorine was preferentially incorporated in the mica, G _ex ^. (1,073)=– 5.67 kJ anion^–1 and G _ex ^. (1,173K)=–5.84 kJ anion^–1. These results have yielded new values of entropy and Gibbs energy of formation for fluortremolite, S _f =–2,293.4±16.0JK^–1 mol^–1 and G _f = –11,779.3±25.0 kJ mol^–1, respectively. In addition, F-OH mineral exchange equilibria support a recent molten oxide calorimetric value for the Gibbs energy of fluorphlogopite, G _f =–6,014.0±7.0 kJ mol^–1, which is approximately 40 kJ mol^–1 more exothermic than the tabulated value.This work performed in part at Sandia National Laboratories supported by the U.S. Department of Energy, DOE, under contract number DE-AC04-76DP00789     

An experimental investigation of heulandite-laumontite equilibrium at 1000 to 2000 bar P fluid

The univariant reaction governing the upper stability of heulandite (CaAl₂Si₇O₁₈·6H₂O), heulandite=laumontite+3 quartz+2H₂O (1), has been bracketed through reversal experiments at: 155±6° C, 1000 bar; 175±6° C, 1500 bar; and 180±8° C, 2000 bar. Reversals were established by determining the growth of one assemblage at the expense of the other, using both XRD and SEM studies. The standard molal entropy of heulandite is estimated to be 783.7±16 J mol^–1 K^–1 from the experimental brackets. Predicted standard molal Gibbs free energy and enthalpy of formation of heulandite are –9722.3±6.3 kJ mol^–1 and –10524.3±9.6 kJ mol^–1, respectively. The reaction (1), together with the reaction, stilbite=laumontite+3 quartz+3 H₂O, defines an invariant point at which a third reaction, stilbite=heulandite+ H₂O, meets. By combining the present experimental data with past work, this invariant point is located at approximately 600 bar and 140° C. Heulandite, which is stable between the stability fields of stilbite and laumontite, can occur only at pressures higher than that of the invariant point, for = P _total.These results are consistent with natural parageneses in low-grade metamorphic rocks recrystallized in equilibrium with an aqueous phase in which is very close to unity.     

Thermodynamic properties,low-temperature heat-capacity anomalies,and single-crystal X-ray refinement of hydronium jarosite, (H<Subscript>3</Subscript>O)Fe<Subscript>3</Subscript>(SO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH)<Subscript>6</Subscript>

Crystals of hydronium jarosite were synthesized by hydrothermal treatment of Fe(III)–SO₄ solutions. Single-crystal XRD refinement with R₁=0.0232 for the unique observed reflections (|F_o| > 4_F) and wR₂=0.0451 for all data gave a=7.3559(8) Å, c=17.019(3) Å, V^o=160.11(4) cm³, and fractional positions for all atoms except the H in the H₃O groups. The chemical composition of this sample is described by the formula (H₃O)_0.91Fe_2.91(SO₄)₂[(OH)_5.64(H₂O)_0.18]. The enthalpy of formation (H^o_f) is –3694.5 ± 4.6 kJ mol^–1, calculated from acid (5.0 N HCl) solution calorimetry data for hydronium jarosite, -FeOOH, MgO, H₂O, and -MgSO₄. The entropy at standard temperature and pressure (S^o) is 438.9±0.7 J mol^–1 K^–1, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (C_p) data between 273 and 400 K were fitted to a Maier-Kelley polynomial C_p(T in K)=280.6 + 0.6149T–3199700T^–2. The Gibbs free energy of formation is –3162.2 ± 4.6 kJ mol^–1. Speciation and activity calculations for Fe(III)–SO₄ solutions show that these new thermodynamic data reproduce the results of solubility experiments with hydronium jarosite. A spin-glass freezing transition was manifested as a broad anomaly in the C_p data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in C_p, below 0.7 K, has been tentatively attributed to spin cluster tunneling. A set of thermodynamic values for an ideal composition end member (H₃O)Fe₃(SO₄)₂(OH)₆ was estimated: G^o_f= –3226.4 ± 4.6 kJ mol^–1, H^o_f=–3770.2 ± 4.6 kJ mol^–1, S^o=448.2 ± 0.7 J mol^–1 K^–1, C_p (T in K)=287.2 + 0.6281T–3286000T^–2 (between 273 and 400 K).     

Water diffusivity in rhyolitic glasses as determined by in situ IR spectroscopy

The dehydration rate of hydrous rhyolitic glasses at 475–875 °C was measured by in situ infrared (IR) spectroscopy in order to determine the diffusion coefficient of water in rhyolitic glasses. The IR spectra of glass thin sections were obtained at 90-s intervals during 90 min at high temperatures, and the change in absorbance at 3550 cm^–1 corresponding to total water was monitored. The diffusion coefficients obtained from dehydration rates of the rhyolitic glasses are considered to be averaged value over the water-concentration profile in the sample. The averaged apparent diffusion coefficients increase with the initial total water content from 0.20 m² s^–1 for 0.7 wt% to 0.37 m² s^–1 for 2.8 wt% at 700 °C. The apparent activation energy for the diffusion of total water decreases with increasing initial water content from 112 ± 6 kJ mol^–1 for 0.7 wt% to 60 ± 17 kJ mol^–1 for 4.1 wt%. Assuming a linear relation between the diffusion coefficient of total water and the total water content, the diffusion coefficients at each initial total water content were also determined. The diffusion coefficients of total water at the water contents of 0.7 and 1.9 wt% and at 0.1 MPa were best fitted by ln D=[(12.9 ± 0.8) – (111 500 ± 6400)/RT] and ln D=[(10.6 ± 0.4) – (86 800 ± 2800)/RT], respectively, and are in agreement with previous data (D in m² s^–1, T in K). The present in situ IR dehydration experiment is a rapid and effective method for the determination of water diffusivity at high temperatures.     

MgSiO3 ilmenite: Heat capacity,thermal expansivity,and enthalpy of transformation

A new determination, using high temperature drop-solution calorimetry, of the enthalpy of transformation of MgSiO₃ pyroxene to ilmenite gives H ₂₉₈ = 59.03 ±4.26 kJ/mol. The heat capacity of the ilmenite and orthopyroxene phases has been measured by differential scanning calorimetry at 170–700 K; C_p of MgSiO₃ ilmenite is 4–10 percent less than that of MgSiO₃ pyroxene throughout the range studied. The heat capacity differences are consistent with lattice vibrational models proposed by McMillan and Ross (1987) and suggest an entropy change of -18 ± 3 J-K^-1 ·mol^-1, approximately independent of temperature, for the pyroxene-ilmenite transition. The unit cell parameters of MgSiO₃ ilmenite were measured at 298–876 K and yield an average volume thermal expansion coefficient of 2.44 × 10^-5 K^-1. The thermochemical data are used to calculate phase relations involving pyroxene, -Mg₂SiO₄ plus stishovite, Mg₂SiO₄ spinel plus stishovite, and ilmenite in good agreement with the results of high pressure studies.     

Thermodynamic properties of strontianite-witherite solid solution (Sr,Ba)CO3

Structural parameters and thermodynamic properties of strontianite — witherite solid solutions have been studied by X-ray powder diffraction, heat flux Calvet calorimetry and cation-exchange equilibria technique. X-ray study of the synthetic samples have shown linear and quadratic (for c-parameter) composition dependencies of the lattice constants in the carbonate solid solution. The thermodynamic energy parameters demonstrate the non-ideal character of strontianite — witherite solid solutions. Enthalpies of solution of the samples have been measured in 2PbO*B₂O₃ at 973 K. The new data on the enthalpy of formation H _f,298.15 ⁰ of SrCO₃ and BaCO₃ were obtained: -1231.4±3.2 and -1209.9±5.8 kJ*mol^-1 respectively. The enthalpy of mixing of the solid solution was found to be positive and asymmetric with maximum at X_Ba (carbonate)=0.35. The composition dependence of the enthalpy of mixing may be described by two — parametric Margules model equation: H _mix=X _BaX _Sr[(4.40±3.91)X _Ba+(28.13±3.91)X _Sr] kJmol^–1 Cation-exchange reactions between carbonates and aqueous SrCl₂-BaCl₂ supercritical solutions (fluids) were carried out at 973 and 1073 K and 2 kbar. Calculated Margules model parameters of the excess free energy are: for orthorhombic carbonate solid solutions W _Sr=W _Ba=11.51±0.40 kJmol^–1 (973 K) and W _Sr=W _Ba=12.09±0.95 kJmol^– (1073 K) for trigonal carbonate solid solutions W _Sr=W _Ba=13.55±0.40 kJmol^– (1073 K).     

Naturally decorated dislocations in olivine from peridotite xenoliths

Dislocations decorated by hematite and magnetite have been observed optically in the olivine grains of undeformed or highly annealed peridotite xenoliths from Hawaii and Baja California ( 5 × 10⁵ cm^–2). The observed structures include loops, low-angle boundaries, and structures produced by multiple cross-glide of [100] screws. Loops are almost invariably parallel to (001). Simple arrays of parallel dislocations lie predominantly in (100), (010) and (001) with dislocation lines subparallel to low-index directions. [100] screws pinned to (100) boundaries are frequently seen to bow out on (001). Preliminary electron petrography has confirmed that all dislocations are decorated.     

Diffusion of lead in plagioclase and K-feldspar: an investigation using Rutherford Backscattering and Resonant Nuclear Reaction Analysis

Chemical diffusion of Pb has been measured in K-feldspar (Or₉₃) and plagioclase of 4 compositions ranging from An₂₃ to An₉₃ under anhydrous, 0.101 MPa conditions. The source of diffusant for the experiments was a mixture of PbS powder and ground feldspar of the same composition as the sample. Rutherford Backscattering (RBS) was used to measure Pb diffusion profiles. Over the temperature range 700–1050°C, the following Arrhenius relations were obtained (diffusivities in m²s^-1):Oligoclase (An₂₃): Diffusion normal to (001): log D = ( – 6.84 ± 0.59) – [(261 ± 13 kJ mol ^–1)/2.303RT]Diffusion normal to (010): log D = ( – 3.40 ± 0.50) – [(335 ± 11 kJ mol ^–1)/2.303RT]Andesine (An₄₃): Diffusion normal to (001): log D = ( – 6.73 ± 0.54) – [(266 ± 12 kJ mol ^–1)/2.303RT] Diffusion normal to (010) appears to be only slightly slower than diffusion normal to (001) in andesine.Labradorite (An₆₇): Diffusion normal to (001): log D = ( – 7.16 ± 0.61) – [(267 ± 13 kJ mol ^–1)/2.303RT] Diffusion normal to (010) is slower by 0.7 log units on average.Anorthite Diffusion normal to (010): log D = ( – 5.43 ± 0.48) – [(327 ± 11 kJ mol ^–1)/2.303RT]K-feldspar (Or₉₃): Diffusion normal to (001): log D = ( – 4.74 ± 0.52) – [(309 ± 16 kJ mol ^–1)/2.303RT] Diffusion normal to (010): log D = ( – 5.99 ± 0.51) – [(302 ± 11 kJ mol ^–1)/2.303RT]In calcic plagioclase, Pb uptake is correlated with a reduction of Ca, indicating the involvement of PbCa exchange in chemical diffusion. Decreases of Na and K concentrations in sodic plagioclase and K-feldspar, respectively, are correlated with Pb uptake and increase in Al concentration (measured by resonant nuclear reaction analysis), suggesting a coupled process for Pb exchange in these feldspars. These results have important implications for common Pb corrections and Pb isotope systematics. Pb diffusion in apatite is faster than in the investigated feldspar compositions, and Pb diffusion rates in titanite are comparable to both K-feldspar and labradorite. Given these diffusion data and typical effective diffusion radii for feldspars and accessory minerals, we may conclude that feldspars used in common Pb corrections are in general less inclined to experience diffusion-controlled Pb isotope exchange than minerals used in U-Pb dating that require a common Pb correction.     

Heat-capacity measurements and absolute entropy of ɛ-Mg2PO4OH

The low-temperature heat capacity of -Mg₂PO₄OH was measured between 10 and 400 K by adiabatic calorimetry. No phase transition was observed over this temperature range. A relative enthalpy increment of 22,119 J mol^–1 and an absolute entropy value of 127.13±0.25 J mol^–1 K^–1 at 298.15 K are derived from the results. The low-temperature heat-capacity data are compared with the DSC data obtained from 143 K to 775 K and show marginal differences in the common temperature range. The latter data are fitted by the polynomial

Equation of state,structural behaviour and phase diagram of synthetic MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript>, as a function of pressure and temperature

The behaviour of synthetic Mg-ferrite (MgFe₂O₄) has been investigated at high pressure (in situ high-pressure synchrotron radiation powder diffraction at ESRF) and at high temperature (in situ high-temperature X-ray powder diffraction) conditions. The elastic properties determined by the third-order Birch–Murnaghan equation of state result in K₀=181.5(± 1.3) GPa, K=6.32(± 0.14) and K= –0.0638 GPa^–1. The symmetry-independent coordinate of oxygen does not show significant sensitivity to pressure, and the structure shrinking is mainly attributable to the shortening of the cell edge (homogeneous strain). The lattice parameter thermal expansion is described by _a0+_a1*(T–298)+_a2/(T–298)², where _a0=9.1(1) 10^–6 K^–1, _a1=4.9(2) 10^–9 K^–2 and _a2= 5.1(5) 10^–2 K. The high-temperature cation-ordering reaction which MgFe-spinel undergoes has been interpreted by the ONeill model, whose parameters are = 22.2(± 1.8) kJ mol^–1 and =–17.6(± 1.2) kJ mol^–1. The elastic and thermal properties measured have then been used to model the phase diagram of MgFe₂O₄, which shows that the high-pressure transition from spinel to orthorombic CaMn₂O₄-like structure at T < 1700 K is preceded by a decomposition into MgO and Fe₂O₃.     

An ion microprobe study of anhydrous oxygen diffusion in anorthite: a comparison with hydrothermal data and some geological implications

The diffusion rate of ¹⁸O tracer atoms in anorthite (An₉₇Ab₀₃) under anhydrous conditions has been measured using SIMS techniques. The tracer source was ¹⁸O₂ 98.4% gas at 1 bar, in the temperature range 1300° C–850° C. The measured diffusion constants are D ₀=1 _–0.6 ⁺¹ ×10^–9 m²s^–1 Q=236±8 kJ mol^–1 Comparison of these values with published data for ¹⁸O diffusion in anorthite under hydrothermal conditions shows that dry oxygen diffusivities are orders of magnitude lower than equivalent wet values at similar temperatures. The effect of these differences on oxygen isotope equilibration during cooling is discussed.     

Cation distribution in pentlandites (Fe,Ni)9S8: Dependence on pressure and temperature and kinetics of the cation exchange reaction

This paper describes the distribution of Fe and Ni between the octahedral and tetrahedral sites in pentlandite (Fe,Ni)₉S₈. The dependence of the distribution on pressure and temperature and the activation energy of the cation exchange reaction were determined through annealing experiments. Synthetic crystals were annealed at 433–723 K and pressures up to 4 GPa, and natural crystals were annealed at 423, 448 and 473 K in evacuated silica capillary tubes for various durations. The cation distributions in the synthetic crystals were determined with an X-ray powder method employing the anomalous dispersion effect of CuK. and FeK radiations, while those of natural crystals were calculated from the cell dimensions. The values of U, S and V for the Fe/Ni exchange reaction are –6818 J mol^–1, 20.52 J K^–1 mol^–1, and 6.99 × 10^–6 m³ mol^–1, respectively. The dependence of the Fe/Ni distribution on pressure (Pa) and temperature (Kelvin) was determined as lnK = 2.47+8.20 × 10² T ^–1+8.41 x 10^–7 T ^–1 P, where K = (Fe/Ni)_octahedral /(Fe/Ni)_tetrahedral. The activation energy of the cation exchange reaction was 185 kJ mol^–1.     

Calorimetric study on high-pressure transitions in KAlSi<Subscript>3</Subscript>O<Subscript>8</Subscript>

KAlSi₃O₈ sanidine dissociates into a mixture of K₂Si₄O₉ wadeite, Al₂SiO₅ kyanite and SiO₂ coesite, which further recombine into KAlSi₃O₈ hollandite with increasing pressure. Enthalpies of KAlSi₃O₈ sanidine and hollandite, K₂Si₄O₉ wadeite and Al₂SiO₅ kyanite were measured by high-temperature solution calorimetry. Using the data, enthalpies of transitions at 298 K were obtained as 65.1 ± 7.4 kJ mol^–1 for sanidine wadeite + kyanite + coesite and 99.3 ± 3.6 kJ mol^–1 for wadeite + kyanite + coesite hollandite. The isobaric heat capacity of KAlSi₃O₈ hollandite was measured at 160–700 K by differential scanning calorimetry, and was also calculated using the Kieffer model. Combination of both the results yielded a heat-capacity equation of KAlSi₃O₈ hollandite above 298 K as C_p=3.896 × 10²–1.823 × 10³T^–0.5–1.293 × 10⁷T^–2+1.631 × 10⁹T^–3 (C_p in J mol^–1 K^–1, T in K). The equilibrium transition boundaries were calculated using these new data on the transition enthalpies and heat capacity. The calculated transition boundaries are in general agreement with the phase relations experimentally determined previously. The calculated boundary for wadeite + kyanite + coesite hollandite intersects with the coesite–stishovite transition boundary, resulting in a stability field of the assemblage of wadeite + kyanite + stishovite below about 1273 K at about 8 GPa. Some phase–equilibrium experiments in the present study confirmed that sanidine transforms directly to wadeite + kyanite + coesite at 1373 K at about 6.3 GPa, without an intervening stability field of KAlSiO₄ kalsilite + coesite which was previously suggested. The transition boundaries in KAlSi₃O₈ determined in this study put some constraints on the stability range of KAlSi₃O₈ hollandite in the mantle and that of sanidine inclusions in kimberlitic diamonds.     

High-temperature enthalpy at the orientational order-disorder transition in calcite: implications for the calcite/aragonite phase equilibrium

The enthalpy of calcite has been measured directly between 973 K and 1325 K by transposed-temperature- drop calorimetry. The excess enthalpy has been analysed in terms of Landau theory for this tricritical phase transition. The zero-point enthalpy and entropy allow estimates of the parameters a and C in the Landau expansion for free energy which expresses excess free energy G as a function of the order parameter Q and temperature T: G 1/2a(T _2c–T)Q ²+1/6CQ ⁶ with a=24 J·K·mol^-1, C = 30 kJ·mol^– T _c = 1260 ±5 K. The entropy of disorder below the transition has been formulated as a function of temperature allowing the calculation of the calcite/aragonite phase boundary when taking this extra entropy into account. There is remarkable agreement between the calculated equilibrium curve and previous experimental observations. The Landau theory predicts behaviour which fully accounts for the change in slope of the calcite/aragonite phase boundary, which is thus wholly due to the R¯3c –R¯3m transition in calcite.     

Synthesis and upper stability of paragonite

The mineral paragonite, NaAl₂[AlSi₃O₁₀ (OH)]₂, has been synthesized on its own composition starting from a variety of different materials. Indexed powder data and refined cell parameters are given for both the 1M and 2M₁ polymorphs obtained. The upper stability limit of paragonite is marked by its breakdown to albite + corundum + vapour. The univariant equilibria pertaining to this reaction have been established by reversing the reaction at six different pressures, the equilibrium curve running through the following intervals: 1 kb: 530°–550° C 2 kb: 555°–575° C 3 kb: 580°–600° C 5kb: 625°–640° C 6 kb: 620°–650° C 7 kb: 650°–670° C.Comparison with the upper stability limit of muscovite (Velde, 1966) shows that paragonite has a notably lower thermal stability thus explaining the field observation that paragonite is absent in many higher grade metamorphic rocks in which muscovite is still stable.The enthalpy and entropy of the paragonite breakdown reaction have been estimated. Since intermediate albites of varying structural states are in equilibrium with paragonite, corundum and H₂O along the univariant equilibrium curve, two sets of data pertaining to the entropy of paragonite (S ₂₉₈ ⁰ ) as well as the enthalpy ( H _f,298 ⁰ ) and Gibbs free energy ( G _f,298 ⁰ ) of its formation were computed, assuming (1) high albite and (2) low albite as the equilibrium phase. The values are: (1) (2) S ₂₉₈ ⁰ 67.8±3.9 cal deg^–1 gfw^–1 63.7±3.9 cal deg^–1 gfw^–1 H _f,298 ⁰ –1417.9±2.7 kcal gfw^–1 –1420.2±2.6 kcal gfw^–1 G _f,298 ⁰ –1327.4±4.0 kcal gfw^–1 –1328.5±4.0 kcal gfw^–1.Adapted from a part of the author's Habilitationsschrift accepted by the Ruhr University, Bochum (Chatterjee, 1968).     

High-temperature heat capacity of Co3O4 spinel: thermally induced spin unpairing transition

A strong anomaly was found in the heat capacity of Co₃O₄ between 1000 K and the decomposition temperature. This anomaly is not related to the decomposition of Co₃O₄ to CoO. The measured entropy of transition, S=46±4 J mol^-1 K^-1 of Co₃O₄, supports the interpretation that this anomaly reflects a spin unpairing transition in octahedrally coordinated Co³⁺ cations. Experimental values of heat capacity, heat content and entropy of Co₃O₄ in the high temperature region are provided. The enthalpy of the spin unpairing transition is 53±4 kJ mol^-1 of Co₃O₄.     

Activities of components in omphacitic solid solutions

Available experimental data on mixing of disordered C2/c clinopyroxenes in the system diopside-jadeite-hedenbergite-acmite are reviewed and evaluated. Because the methods used to determine jadeite activity suffer from severe uncertainty at high jadeite mol fractions, these data cannot be used to infer asymmetry in the jadeite-diopside or the jadeite-hedenbergite solid solutions. If the measurement uncertainties are taken into account, a single parameter (regular, or reciprocal energy) suffices to describe the mixing properties of these two solid solutions. It is argued that a two-site entropy of mixing satisfies the experiments and is consistent with the C2/c disordered nature of the solid solutions; the data in the range 600–1300° C are consistent with a temperature-independent interaction energy, implying no discernible excess entropy. The available experimental data imply W=26±2 kJ mol^–1 for jd-di, and W=25±3 kJ mol^–1 for jd-hd, solid-solutions. Landau theory for a tricritical phase transformation (C2/c-P2/n) is in good agreement with the calorimetrically determined disordering enthalpy, and may be used to derive a simple expression for the activities in ordered omphacite solid solutions. The derived activities of jadeite at 600° C in ordered omphacites are remarkably close to those reported previously for short-range ordered pyroxenes. A simple model is presented for determining the activities of end-members in the system jadeite-diopside-hedenbergite-acmite.     

Geochemistry and argon thermochronology of the Variscan Sila Batholith,southern Italy: source rocks and magma evolution

The Sila batholith is the largest granitic massif in the Calabria-Peloritan Arc of southern Italy, consisting of syn to post-tectonic, calc-alkaline and metaluminous tonalite to granodiorite, and post-tectonic, peraluminous and strongly peraluminous, two-mica±cordierite±Al silicate granodiorite to leucomonzogranite. Mineral ⁴⁰Ar/³⁹Ar thermochronologic analyses document Variscan emplacement and cooling of the intrusives (293–289 Ma). SiO₂ content in the granitic rocks ranges from 57 to 77 wt%; cumulate gabbro enclaves have SiO₂ as low as 42%. Variations in absolute abundances and ratios involving Hf, Ta, Th, Rb, and the REE, among others, identify genetically linked groups of granitic rocks in the batholith: (1) syn-tectonic biotite±amphibole-bearing tonalites to granodiorites, (2) post-tectonic two-mica±Al-silicate-bearing granodiorites to leucomonzogranites, and (3) post-tectonic biotite±hornblende tonalites to granodiorites. Chondrite-normalized REE patterns display variable values of Ce/Yb (up to 300) and generally small negative Eu anomalies. Degree of REE fractionation depends on whether the intrusives are syn- or post-tectonic, and on their mineralogy. High and variable values of Rb/Y (0.40–4.5), Th/Sm (0.1–3.6), Th/Ta (0–70), Ba/Nb (1–150), and Ba/Ta (50–2100), as well as low values of Nb/U (2–28) and La/Th (1–10) are consistent with a predominant and heterogeneous crustal contribution to the batholith. Whole rock ¹⁸O ranges from +8.2 to +11.7; the mafic cumulate enclaves have the lowest ¹⁸O values and the two-mica granites have the highest values. ¹⁸O values for biotite±honblende tonalitic and granodioritic rocks (9.1 to 10.8) overlap the values of the mafic enclaves and two-mica granodiorites and leucogranites (10.7 to 11.7). The initial Pb isotopic range of the granitic rocks (²⁰⁶Pb/²⁰⁴Pb 18.17–18.45, ²⁰⁷Pb/²⁰⁴Pb 15.58–15.77, ²⁰⁸Pb/²⁰⁴Pb 38.20–38.76) also indicates the predominance of a crustal source. Although the granitic groups cannot be uniquely distinguished on the basis of their Pb isotope compositions most of the post-tectonic tonalites to granodiorites as well as two-mica granites are somewhat less radiogenic than the syn-tetonic tonalites and granodiorites. Only a few of the mafic enclaves overlap the Pb isotope field of the granitic rocks and are consistent with a cogenetic origin. The Sila batholith was generated by mixing of material derived from at least two sources, mantle-derived and crustal, during the closing stages of plate collision and post-collision. The batholith ultimately owes its origin to the evolution of earlier, more mafic parental magmas, and to complex intractions of the fractionating mafic magmas with the crust. Hybrid rocks produced by mixing evolved primarily by crystal fractionation although a simple fractionation model cannot link all the granitic rocks, or explain the entire spectrum of compositions within each group of granites. Petrographic and geochemical features characterizing the Sila batholith have direct counterparts in all other granitic massifs in the Calabrian-Peloritan Arc. This implies that magmatic events in the Calabrian-Peloritan Arc produced a similar spectrum of granitic compositions and resulted in a distinctive type of granite magmatism consisting of coeval, mixed, strongly peraluminous and metaluminous granitic magmas.     

Mg–Fe interdiffusion rates in wadsleyite and the diffusivity jump at the 410-km discontinuity

Mg–Fe interdiffusion rates have been measured in wadsleyite aggregates at 16.0–17.0 GPa and 1230–1530 °C by the diffusion couple method. Oxygen fugacity was controlled using the NNO buffer, and water contents of wadsleyite were measured by infrared spectroscopy. Measured asymmetric diffusion profiles, analyzed using the Boltzmann–Matano equation, indicate that the diffusion rate increases with increasing iron concentration and decreasing grain size. In the case of wadsleyite containing 50–90 weight ppm H₂O, the Mg–Fe interdiffusion coefficients at compositions of Mg/(Mg + Fe)=0.95 in the coarse-grained region (about 60 m) and 0.90 in the fine-grained region (about 6 m) were determined to be a D_Xmg = 0.95 (m² s^–1)=1.24 × 10^–9 exp[–172 (kJ mol^–1)/RT] and D_Xmg = 0.90 (m² s^–1)=1.77 × 10^–9 exp[–143 (kJ mol^–1)/RT], respectively. Grain-boundary diffusion rates were estimated to be about 4 orders of magnitude faster than the volume diffusion rate. Grain-boundary diffusion dominates when the grain size is less than a few tens of microns. Results for the nominally dry diffusion couple in the present study are roughly consistent with previous studies, taking into account differences in pressure and grain size, although water contents of samples were not clear in previous studies. We observed that the diffusivity is enhanced by about 1 order of magnitude in wadsleyite containing 300–2100 wt. ppm H₂O at 1230 °C, which is almost identical to the enhancement associated with a 300 °C increase in temperature. It is still not conclusive that a jump in diffusivity exists between olivine and wadsleyite because water contents of olivine in previous diffusion studies and effects of water on the olivine diffusivity are uncertain.     

Application of new thermodynamic data to grossular phase relations

Recent low temperature, adiabatic calorimetric heat capacity measurements for grossular have been combined with DSC measurements to give entropies up to 1000 K. In conjunction with enthalpy of solution values for grossular, these data have yielded H _f ^o (298.15K) and G _f ^o (298.15K) values of –1583.2 ± 3.5 and –1496.74 ± 3.7 kcal mol^–1 respectively. For 15 reactions in the CaO-Al₂O₃-SiO₂-H₂O system, thermodynamically calculated P-T curves have been compared with experimental reversals and have shown good agreement in most cases. Calculations indicate that gehlenite is probably totally disordered. Estimates of zoisite and lawsonite entropies are consistent with the phase equilibrium and grossular data, but estimates of the entropies of pyrope and andradite show large discrepancies when compared with experimental reversals.Contribution no. 600 from the Mineralogical Laboratory, The Department of Geology and Mineralogy, The University of Michigan, Ann Arbor, Michigan 48109, USA