An equation of state of solute silica in NaCl brines at 500 to 900°C and 4 to 15 kbar is formulated by making use of two experimentally determined properties of quartz solubility: the silica molality decreases in direct proportion to the logarithm of the NaCl mole fraction (X(NaCl)) at pressures approaching 10 kbar, and the relative silica molality (molality at a given NaCl mole fraction, mx, divided by the molality in pure H2O at the same P and T, mo) is independent of temperature in the evaluated range. These two properties are expressed in the relation: log(mx/mo)∗ = A + BX(NaCI), where log(mx/mo)∗ denotes the logarithm of the ideal molality ratio, and A and B are functions of pressure, but not temperature or salinity, such that B = −1.730 − 1.431 × 10−3P + 5.923 × 10−4P2 −9.243 × lO−5P3, and A = 0 at P>10 kbar, whereas A = 0.6131 − 0.1256P + 6.431 × 10−3P2 at P≤10 kbar, as derived from fits to experimental data (Newton and Manning, 1999). The parameter A decreases from 0.214 to 0 from 4 to 9.5 kbar, and remains zero to 15 kbar; B decreases from −1.373 to −1.571 from 4 to 15 kbar. With the above relationship defining a variable X(NaCl)-T-P standard-state of solute silica, the activity of SiO2 can be replaced by its molality for calculations of mineral-fluid equilibria over most of the conditions for metasomatism in the deep crust and upper mantle. Significant departures from ideality occur only at the lowest pressures, and low salinities. Calculations on peridotite mineral stability in the simple system CaO-MgO-SiO2-H2O-NaCl at high T and P show that antigorite, brucite, and diopside are stable at 500°C and pressures of 5 to 15 kbar in the presence of concentrated NaCl solutions at low SiO2 activities. At 700°C, anthophyllite is stable over a wide range of salinities at 5 kbar with tremolite but not with diopside. The presence of anthophyllite buffers silica solubility at a high, salinity-independent value close to quartz saturation. At 10 and 15 kbar and 700°C, talc replaces anthophyllite as the stable hydrate, and talc-trem-olite assemblages buffer SiO2 fluid concentrations at high values nearly independent of salinity. At 900°C hydrates are unstable and diopside again becomes stable and coexists with enstatite in peridotites. These stability calculations correspond well to the observed progressive metamorphic sequence in peridotite bodies in the Central Alps. This method of analysis may be useful in interpretation of metamorphosed ultramafic bodies in general, including the basal portions of obducted ophiolitic mantle lithosphere and the mantle wedge above subduction zones. More detailed calculations, including rocks containing feldspars, must take into account the more soluble major components of rocks, especially alkalis, as these will affect the activity coefficient of SiO2 in NaCl solutions. The solubility of silica in the presence of minerals containing these components must be determined by additional measurements.
The metastable kyaniteandalusite equilibrium in the Al(2)SiO(5) system has been reversed at 700 degrees , 750 degrees , and 800 degrees C at elevated water pressures, with a variety of natural and synthetic kyanites and andalusites as starting materials. Sillimanite, the stable form of Al(2)SiO(5) under these conditions, did not appear. The value of the transition pressure at 750 degrees C is 6.6 +/- 0.4 kilobars, several kilobars below pressures given by several convergent previous determinations. The Al(2)SiO(5) pressure-temperature triple point now indicated lies far from the points found by others. The revised aluminum silicate phase diagram indicates that many rocks crystallized at lower pressures than formerly thought possible.
Dehydration (fluid-absent) melting of garnetiferous amphibolite is now widely believed to have given rise to the tonalite-trondhjemite-granodiorite (TTG) suite of plutonic rocks that is the major component of all large Precambrian Shield terranes. The presence of garnet as a residual mineral of melting is implicated by geochemistry and for efficient gravitational segregation and disposal of the ultramafic residuum. The great volume of the ancient TTG rocks suggests that early large-scale continental accretion involved a conveyor belt process that delivered hydrated oceanic basalts to depths in the earth where garnet amphibolite is stable at the melting temperature. This process became active at some time during secular cooling of the earth. Experimental definition of the maximum (hence, oldest) geothermal gradients that could have sustained dehydration melting of garnet amphibolite has been hampered by the difficulty of achieving chemical equilibrium in unfluxed (fluid-absent) silicate systems at the low temperatures (∼800°C) where melting first occurs. An alternative approach, termed "geo-experimental," attempts to define the PT stability field of garnet amphibolite on the basis of quantitative geothermometry-geobarometry of actual mafic rocks bearing the assemblage aluminous hornblende-almandine-rich garnet-plagioclase-quartz. The results show that garnet is stable to much lower pressures than previously thought. The garnet-in line has a positive dP/dT slope in both subsolidus and melt-present fields. A subduction temperature gradient as high as 36°C/km could have accompanied melting of garnet amphibolite and generation of TTG magmas in the earlier Archean. The near parallelism of the garnet-in line and geothermal curves could have acted like an on switch during cooling of the earth, resulting in a surge of felsic continent, with accumulation of nearly the present volume before the end of the Archean.
The relative stabilities of orthorhombic zoisite, $$Ca_{2}Al_{3}Si_{3}O_{12}OH$$, and its monoclinic polymorph, clinozoisite, have been examined in three ways: (a) Comparison of the P-T relations for the reaction \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$${\mathrm{2\ (clino)zoisite + quartz \rightleftarrows 4\ anorthite + H_{2}O}}$$\end{document} in which synthetic zoisite and synthetic clinozoisite were used, (b) Direct relative stability experiments on mixtures of zoisite and clinozoisite, and (c) Thermody-namic analysis of occurrences of analyzed coexisting ferrian zoisite and clinozoisite in metamorphic rocks. At 500°C and 550°C and in the region of 6-7 kbar, the boundary curve for the clinozoisite-bearing reaction is clearly shifted to lower temperatures and higher pressures than that of the zoisite-bearing reaction. In this P-T range clinozoisite is less stable than zoisite, with \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$\Delta{\mathrm{G\ (Zo \rightarrow Czo) \sim 4\ kJ/mol}}$$\end{document}. Between 350°C and 600°C zoisite was observed to grow at the expense of clinozoisite in runs on mixtures. Below 350°C the reaction rate becomes prohibitively slow. These experiments show that, if Fe-free clinozoisite has a temperature range of stability relative to zoisite, it must be at temperatures less than 350°C. The Fe-contents of natural coexisting zoisite and clinozoisite increase with increasing metamorphic grade. By assuming ideal substitution of Al and $$Fe^{3+}$$ in one of three distinct octahedral M-sites, and by assigning appropriate broad temperature ranges to the different metamorphic occurrences, a plot of the log of the ideal activity constant of the reaction clinozoisite = zoisite against reciprocal temperature shows a general decrease of $$\Delta G^{circ}$$ with temperature and predicts that Fe-free clinozoisite becomes stable relative to zoisite at ~200°C. The \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{wasysym} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document}$$\Delta{\mathrm{S}}^{\circ}$$\end{document} of the reaction is inferred to be of the order of 2 cal/mol-K. Additional analytic data on coexisting natural zoisite-clinozoisite pairs are needed to refine this value.
The fusion curve of cesium metal has been studied up to 50 000 atmospheres. The curve is unique among elements studied, in that it shows two maxima, one at approximately 22.5 kbar and 197\ifmmode^\circ\else\textdegree\fi{}C, and a second at approximately 30 kbar and 198\ifmmode^\circ\else\textdegree\fi{}C. Two triple points have been located. At 195\ifmmode^\circ\else\textdegree\fi{}C, cesium has four different melting-freezing points and possibly another one at still higher pressures.
The amphibolite-facies to granulite-facies transition at the southern margin of the Dharwar craton has been studied in the Krishnagiri, Satnur-Halaguru, and Kushalnagar areas of southern India. In all three areas the transition appears to be a progressive metamorphic overprint on cratonal gneisses and their mafic and metasedimentary enclaves, without major structural interruption. In the Kabbal-Satnur-BR Hills section of Karnataka, a high-grade charnockite massif with pronounced Rb depletion is the culmination of an apparently continuous increase of metamorphic grade southward. In this and the Kushalnagar areas, increase of paleopressure from near 6 to near 8 kbar with increasing grade indicates a depth-zone relationship of the amphibolite and granulite facies. Incipient charnockite replacing Peninsular Gneiss first appears along N-S shears parallel to the regional grain of the craton. Low-P(), high vapors were instrumental in the creation of orthopyroxene. Introduction from a deep source, either a decarbonating mantle, basaltic underplate, or deeply buried sediments, was facilitated by the N-S deformation system. A deformed continental margin or infracontinental basin in the latest Archean is a plausible setting for the metamorphism. Great crustal thickening, perhaps with entrainment of shelf or basin sediments, was effected by overthrusting, the record of which may be preserved in an early isoclinal foliation and in fold-interference patterns in the southern cratonal margin. Subsequent transcurrent shearing facilitated outgas-sing of the deep crust and upper mantle, and rising vapors transported heat, , and K to middle levels of a thickened crust, which resulted in anatectic melting. Eventual uplift and erosion exposed a 6-8 kbar paleopressure surface at the southern cratonal fringe. The weakened and segmented continental shelf or platform south of the present craton was prone to later remobilization and retrogression, in the manner of a typical "mobile belt." This view of the relationship of the Dharwar craton to the southern high-grade terrain provides no support for the concept that granulite facies terrains of the general aspect of the South Indian massifs everywhere underlie the interior of the craton. The 2-3 kbar terrain of the cratonal interior was not greatly thickened by overthrusting nor magmatic underplating during the 2.6 Ga high-grade event that affected the southern cratonal margin. It is, however, possible that granulite-grade roots of an older metamorphic cycle underlie the cratonal interior.
Abstract Fluid-mediated calcium metasomatism is often associated with strong silica mobility and the presence of chlorides in solution. To help quantify mass transfer at lower crustal and upper mantle conditions, we measured quartz solubility in H2O-CaCl2 solutions at 0.6–1.4 GPa, 600–900 °C, and salt concentrations to 50 mol%. Solubility was determined by weight loss of single-crystals using hydrothermal piston-cylinder methods. All experiments were conducted at salinity lower than salt saturation. Quartz solubility declines exponentially with added CaCl2 at all conditions investigated, with no evidence for complexing between silica and Ca. The decline in solubility is similar to that in H2O-CO2 but substantially greater than that in H2O-NaCl at the same pressure and temperature. At each temperature, quartz solubility at low salinity (XCaCl2 < 0.1) depends strongly on pressure, whereas at higher XCaCl2 it is nearly pressure independent. This behavior is consistent with a transition from an aqueous solvent to a molten salt near XCaCl2 ~0.1. The solubility data were used to develop a thermodynamic model of H2O-CaCl2 fluids. Assuming ideal molten-salt behavior and utilizing previous models for polymerization of hydrous silica, we derived values for the activity of H2O (aH2O), and for the CaCl2 dissociation factor (α), which may vary from 0 (fully associated) to 2 (fully dissociated). The model accurately reproduces our data along with those of previous work and implies that, at conditions of this study, CaCl2 is largely associated (<0.2) at H2O density <0.85 g/cm3. Dissociation rises isothermally with increasing density, reaching ~1.4 at 600 °C, 1.4 GPa. The variation in silica molality with aH2O in H2O-CaCl2 is nearly identical to that in H2O-CO2 solutions at 800 °C and 1.0 GPa, consistent with the absence of Ca-silicate complexing. The results suggest that the ionization state of the salt solution is an important determinant of aH2O, and that H2O-CaCl2 fluids exhibit nearly ideal molecular mixing over a wider range of conditions than implied by previous modeling. The new data help interpret natural examples of large-scale Ca-metasomatism in a wide range of lower crustal and upper mantle settings.
