Kbar Water Pressure Research Articles

Synthetic gedrite: a stable phase in the system MgO-Al 2 O 3 -SiO 2 -H 2 O (MASH) at 800 °C and 10 kbar water pressure, and the influence of FeNaCa impurities

Seeded, solid-media piston-cylinder runs of unusually long duration up to 31 days indicate growth or persistence of synthetic gedrite of the composition □Mg6Al[AlSi7O22](OH)2(=6:1:7), prepared from the purest chemicals available, at 10 kbar water pressure and 800 °C. Conversely, breakdown was observed at 11 kbar and 850 °C to aluminous enstatite, Al2SiO5, and a melt of the composition MgO·Al2O3·8SiO2. Thus, pure gedrite free of iron, sodium, and calcium is likely to have only a small PT stability field in the MASH system, estimated as 10 ± 1 kbar, 800 ± 20 °C, even though metastable growth of gedrite can be observed over a larger PT range. A second starting material with the anhydrous composition 5MgO · 2Al2O3 · 6SiO2 also yielded gedrite of the composition 6:1:7, together with more aluminous phases such as kyanite, corundum or sapphirine, thus suggesting that the end-member gedrite defined as □Mg5Al2[Al2Si6O22](OH)2(=5:2:6) by the IMA Commission on New Minerals and Mineral Names probably does not exist. With the use of this second starting material, which contains FeNaCa impurities, growth of 6:1:7-gedrite was observed over a still wider PT-range. Seeded runs indicate that the true stability field of such slightly impure 6:1:7-gedrites may also be larger than that of the pure MASH phase and extend at least to 15 kbar, 800 °C. There is, thus, a remarkable stabilization effect on the orthoamphibole structure by impurities amounting only to a total of less than one weight percent of oxides in the starting material. The gedrites synthesized are structurally well ordered amphiboles nearly free of chain multiplicity faults, as revealed by HRTEM. The X-ray diffraction work on the gedrites synthesized yielded the smallest cell volume yet reported for this phase. The small stability field of the pure MASH gedrite is intersected by the upper pressure stability limit of hydrous cordierite for excess-H2O conditions, thus leading to complicated phase relations for both gedrite and cordierite involving the additional phases aluminous enstatite, talc, quartz, Al2SiO5, melt and perhaps boron-free kornerupine.

Bearthite, Ca2Al(PO4)2OH: stability, thermodynamic properties and phase relations

The new phosphate bearthite, Ca2Al(PO4)2HO, found in high-pressure metamorphic rocks, has been synthesized from a stoichiometric mixture of γ-Al2O3 and CaHPO4·2H2O between 4 kbar water pressure, 485°C and 24.5 kbar, 800°C. Its upper temperature stability limit, bearthite ⇆ hydroxyapatite + corundum + berlinite + H2O, has been tightly reversed between 485°C, 1 kbar, and 850°C, 10 kbar. We extracted the following thermodynamic parameters for bearthite:H 298/o =−4327.25 kJ/mol andS 298 o =214.5 J/mol.K, using a new ΔrCp approximation for dehydration reactions. Additional experiments on the reaction trolleite ⇆ berlinite + corundum + H2O yield new thermodynamic data for trolleite, Al4(PO4)3(OH)3 (H 298/o =−6530 kJ/mol,S 298 o =252.7 J/mol.K). We derived the phase relations in the CaO−P2O5−Al2O3−H2O system considering the phosphates bearthite, trolleite, berlinite, augelite, and hydroxyapatite. Extension to the SiO2-bearing system barely affects the stability field of bearthite but reveals a chemical control on bearthite occurrence. The stability of the assemblages hydroxyapatite + aluminium silicate + (quartz or corundum), with respect to bearthite + Ca−Al-silicate, restricts bearthite to very Capoor (Dora-Maira massif) or P-rich rocks (Monte-Rosa massif; Hohe Tauern; Vastan», Sweden). The coexistence of bearthite with a Ca−Al-silicate (lawsonite) may be possible only for pressures exceeding ca. 15 kbar under unusually low gradients of 5° C/km. Bearthite is otherwise not a high-pressure index mineral but provides thermal constraints in relatively high-grade phosphate-bearing deposits or metasediments. Its stability under mantleP-T conditions makes bearthite a sink for Sr and REE in subducted Ca-poor rocks.

Boron-free kornerupine: its upper pressure stability limit in the system MgO-Al2O3-SiO2-H2O (MASH)

Synthetic boron-free kornerupine obtained at 7 kbar water pressure and 830°C has the formula (Mg 3.72 Al 5.21 ) [6] (Al 1.28 Si 3.72 ) [4] O 19.76 (OH) 2.24 . It thus lies exactly on the join enstatite-corundum, midway between the M:A:S ratios 4:3:4 and 1:1:1. By extending the work of Seifert (1975) towards higher water pressures, the upper pressure stability limit of this phase was determined to lie between about 11.5 and 13 kbar for T = 800-900°C. The high-pressure breakdown assemblage is enstatite + corundum, and the reaction curve has a flat positive slope. Thus dehydration of B-free kornerupine at this pressure proceeds toward lower temperatures. The lower temperature stability limit in the range 7-11 kbar is defined by breakdown to the more hydrated assemblage chlorite + talc + corundum between 735°C and 750°C, the reaction curve possessing a steep, probably positive slope

Melting of plagioclase-quartz assemblages at 2 kbar water pressure

The melting of plagioclase and quartz has been investigated at PH2O =2kb. A single crystal of plagioclase was surrounded by quartz powder and water. A reaction rim consisting of glass and of An-rich plagioclase developed around unchanged starting plagioclase. Microprobe determinations of melt and coexisting plagioclase compositions reveal a strong fractionation of plagioclase components between melt and new plagioclase. For example at 850° C the approximate XAn of melt is 0.3 and that of plagioclase is 0.8. The temperature interval between beginning of melting and complete melting of cotectic compositions is 100° C or more for quartz-plagioclase mixtures with plagioclases between An 40 and An 75. In comparison to the system Ab-An-H2O the plagioclase melting loop is somewhat wider in the investigated system Qz-Ab-An-H2O but the temperature interval is much smaller (100° C) than in the system Ab-An-H2O (200° C). The solidus data indicated by the new plagioclase compositions are practically identical with those observed in beginning of melting experiments. The results show that fractionation of plagioclase components between partial melts and restite plagioclase can be more pronounced in multi-component rocks than in the pure plagioclase system. This finding is important for the development of albite rich rocks from more basic compositions.

Phenocryst abundances and glass and phenocryst compositions as indicators of magmatic environments of large‐volume ash flow sheets in southwestern Nevada

The Topopah Spring, Tiva Canyon, Rainier Mesa, and Ammonia Tanks tuffs are large‐volume, silicic ash flow sheets that provide samples of four magmatic systems in southwestern Nevada. Successively erupted within a span of 2 m.y. from the same source area, they allow comparison of the sequential evolution of large, mature Cordilleran magmatic systems. Each large‐volume sheet has a rhyolitic lower zone and quartz latitic upper zone. Coeval basaltic andesite and basalt show petrochemical continuity with these sheets and may represent mantle contributions that triggered eruptions of the midcrustal silicic portions. Abundances of phenocrysts and accessory phases increase upward with whole rock Fe (FeOt) from the base of all four sheets to maximum values unique for each system. Although maximum abundances of each mineral are unique for each sheet, each maximum occupies the same relative position within each sheet. High‐temperature minerals such as plagioclase increase in abundance continuously with FeOt in each system, showing a decrease with FeOt only within basaltic andesite at the base of the Rainier Mesa system. Late crystallizing minerals such as quartz and sphene show maximum abundances at much lower FeOt, at or near the top of the rhyolitic zone. Minerals that normally form at intermediate stages of crystallization, such as sanidine, show maxima at intermediate FeOt for each sheet. A continuum of glass and phenocryst compositions occurs within the Topopah Spring and Rainier Mesa sheets. Variations in phenocryst compositions with FeOt are generally consistent with those expected for crystallization within magma reservoirs characterized by vertical thermal and compositional gradients. However, simple fractional crystallization does not adequately explain the close relationship in each sheet among the mineral chemistry, glass (magma) chemistry, and phase assemblages, which indicate a close approach to equilibrium within each magma system. Nor does fractional crystallization account for resorption textures and maximum abundances for phenocrysts, which demonstrate dissolution rather than crystal growth within the deepest parts of each magma system. We hypothesize that phenocrysts reequilibrate as they gravitationally settle into higher‐temperature magma at greater depth or dissolve if the phase becomes unstable. In either case, lower‐density liquids are produced that should buoyantly migrate upward. Their compositions suggest that glasses and minerals with the lowest FeOt, in the base of each sheet, equilibrated at 3.5 kbar water pressure and ∼660°C. Phase assemblages and mineral chemistries indicate progressively higher temperatures with increasing FeOt within each sheet. Maximum equilibrium temperatures, ∼900°C, are obtained for the most Fe‐rich assemblage, basaltic andesite, also found at the base of the Rainier Mesa sheet. The phase assemblages indicate water saturation or near saturation for the entire magmatic system and thus indicate depths of ∼10 km to the top of each magmatic system. Considering similarities in magmatic environments of the four sheets, the distinctive petrochemical differences among rhyolitic zones probably reflect chemical differences inherited from their source regions.

A hydrothermal experiment for retrograde change of gehlenite.

Phase relations in the system gehlenite (Ak0Ge100 or Ak20Ge80)–SiO2–H2O were investigated at 1 kbar water pressure and at temperatures from 350° to 750°C. Crystalline phases formed in the gehlenite end-member (Ak0Ge100)–SiO2–H2O system are bicchulite, garnet, anorthite and corundum (or boehmite), and those in the gehlenite solid solution (Ak20Ge80)–SiO2–H2O system are vesuvianite, xanthophyllite and diopside in addition to the phases formed in the end-member system. The results of the solid solution system are more consistent with the natural alteration reactions than the results of the end-member system.The coexisting crystalline phases observed in the solid solution system change successively with the fall of temperature as follows: (1) gehlenite + garnet, (2) gehlenite + vesuvianite ± garnet, (3) bicchulite + xanthophyllite + vesuvianite and (4) bicchulite + vesuvianite ± garnet. The assemblage (1) is stable at temperatures above 720°C and that of (2) is stable at temperatures between 610°C and 720°C. The stability boundary between the assemblages (3) and (4) is inclined: 340°C for no silica-addition, 450°C for the added silica/gehlenite = 0.03 and 500°C for the added silica/gehlenite = 0.06. The large amounts of silica-additions result in the assemblage of garnet + anorthite ± diop-side at temperatures between 350°C and 750°C.Cell dimensions of hydrogrossular, i.e., degrees of hydration, vary depending not only on the formation temperatures but also on silica deficiencies of the starting materials. Cell dimensions of vesuvianite vary in the same manner as grossular. A linear relationship between the cell dimensions of garnet and those of vesuvianite was observed in the present experiment.Examples consistent with the present experimental results are found in natural mineral assemblages from many localities. The alteration conditions of gehlenites at Crestmore and Kushiro, correspond to the mineral assemblage (2), whereas those at Akagane mine corresponds to the assemblage (3). The alteration conditions which were commonly observed at Fuka correspond to the assemblage (4). The experimental results may be applied to processes and conditions of natural alteration reactions of gehlenite at other localities as well. The same correlation between the cell dimensions of garnet and those of vesuvianite as in the synthetic experiments is seen in natural specimens from Fuka and Kushiro.

Calcium-free pumpellyite, a new synthetic hydrous Mg–Al-silicate formed at high pressures

The mineral pumpellyite, W4X2Y4Z6O(20+t) (OH)(8−t) where W = Ca, Mn; X = (Mg, Fe2+, Mn)2−t (Fe3+, Al)t; Y = Fe3+, Al; Z = Si (ref. 1) is a well-known constituent of very low-grade metamorphic rocks, especially of metabasic compositions. Schiffmann and Liou2 have synthesized the ideal Fe-free endmember ‘Mg–-Al’ pumpellyite', Ca4MgAl5Si6O21(OH)7, and found its upper thermal stability to lie near 375–400 °C in the range of 5–8 kbar water pressure. Here we report the high-pressure synthesis of a pumpellyite-type phase in which the position W of the above formula is solely occupied by magnesium. This ‘Mg–Mg–Al pumpellyite’, which is chemically nearly equivalent to pyrope+H2O, crystallizes at 40–55 kbar, 700–850 °C. Therefore, it qualifies as a potential hydrous mantle phase in a subduction zone regime.

フロゴパイトおよびＮａ・フロゴパイト組成間における雲母の合成

The mica minerals were synthesized in the compositional join phlogopite(Ph)-sodium phlogopite at 1 kbar water pressure and 550°C by using starting oxide mixtures. The resulting phases were phlogopite, phlogopite solid solutions and saponite-like hydrate. Phlogopite was obtained from a Ph100 composition. Phlogopite solid solutions were obtained in the compositional range from Ph35 up to Ph100. On the other hand, the hydrate was synthesized as a single phase from the Ph0 composition. In the compositional range up toPh40 the hydrate was obtained with phlogopite solid solutions. Basal spacings of phlogopite and phlogopite solid solutions were 10.168Å ±0.005Åat Ph100 and 10.068ű0.01Å at Ph35 in composition. The result from hydrothermal experiments is consistent with the composition ofphlogopite in natural occurrences.

Oxygen diffusion studies. I. A preliminary ion microprobe investigation of oxygen diffusion in some rock-forming minerals

AbstractThe self diffusion of oxygen has been studied in prepared natural crystals of albite, grossular garnet, quartz, and ruffle by isotope exchange with hydrothermal water enriched in 18O, and subsequent analysis by ion microprobe. Measured oxygen diffusion coefficients (D) in quartz (‖c) may be described by D = 1.08 × 10−11 exp(−31.5 kcal/RT) cm2s−1 at 600−750°C and 1 kbar water pressure. For grossular, D = 2.5 × 10−16 cm2s−1 at 1050°C and 8 kbar, and D = 4.8 × 10−17 cm2s−1 at 850 °C and 2 kbar. All ruffle crystals exhibited variable amounts of corrosion, and an approximate diffusion coefficient of D(‖c) = 3.16 × 10−15 cm2s−1 cm2s−1 was obtained at 1050 °C and 1 kbar. Oxygen diffusion coefficients in albite, perpendicular to (001) faces, have been determined as a function of pressure at 600 °C Between 0.5 and 8.0 kbar pressure no systematic variation in the results was observed and most of the data may be described by D = 4.1 (±0.5) × 10−15 cm2s−1. Slow oxygen diffusion rates in quartz and garnet suggest that these minerals should have high ‘closure temperatures’ for oxygen exchange, and may provide reliable oxygen isotope exchange geothermometers.

Phosphorus and the rare earth elements in felsic magmas: an assessment of the role of apatite

The solubility of fluorapatite in 17 silica-rich melts in the system Na2O-K2O-Al2O3-SiO2 (with and without CaO or CaF2) was determined at 1 kbar water pressure and 750 900°C. Apatite saturation occurs at levels of dissolved P2O5 ranging between 0.04 (± 0.02) and 0.28 (± 0.13) wt%. with only 4 values outside the 0.09–0.20 wt% range.The results demonstrate not only that apatite is a common liquidus phase in felsic melts, but also that, under most circumstances, it remains in the residue during episodes of partial fusion of the crust. Given a solubility limit of 0.14 wt% dissolved P2O5 (the mean of the experimental values) a source containing as little as 0.05% P2O5 must be 35% melted before apatite is lost from the residue and no longer buffers the melt P2O5 concentration at the saturation value. Higher abundances of P2O5 in the source postpone the loss of residual apatite to still higher degrees of melting, and if the source P2O5 content exceeds 0.14 wt%, apatite must be residual for all degrees of melting, increasing in abundance as melting proceeds.The generally secondary influence of apatite on the rare earth element (REE) patterns of melt and residue is most apparent when garnet and/or amphibole is minor or lacking in the residue. Fractional crystallization of intermediate (e.g. andesitic) magmas toward felsic compositions invariably results in saturation in apatite and some consequent depletion of REE in the melt.

The restricted stability of osumilite under hydrous conditions in the system K2O-MgO?Al2O3?SiO2?H2O

Osumilitess was synthesized as a single phase product in the model system K2O-MgO-Al2O3-SiO2 at 800° C/ 0.5 Kbar water pressure and at 800° to 840° C/1.0 Kbar total pressure with $$X_{{\text{CO}}_{\text{2}} }$$ ∼0.3 in the gas phase. The experimentally determined solid solubility range of synthetic osumilites can be expressed by the formula KMg2(Al3-xMgx) (Al2−xSi10+x)O30 with 0≦x≦0.4. A survey of sixteen chemical analyses of natural osumilites from eleven occurrences shows a solid solubility characterized by 0≦x≦0.6. Reversed stability experiments for the synthetic osumilite KMg2(Al2.75Mg0.25)(Al1.75Si10.25)O30 determined at water pressure equal to total pressure demonstrate its restriction to water pressures below 0.8 Kbar (at 0.5 Kbar, the stability range is between 765° and 800° C). At the lower thermal stability limit osumilite+H2O vapor break down to cordierite+K feldspar+phlogopitess+quartz, at the higher one to cordierite+K feldspar+phlogopite+liquid. Reduction of water fugacity will expand the stability field largely by shifting the lower and higher thermal stability limits to lower and higher temperatures, respectively. The dependence of osumilite stability on water fugacity makes osumilite a sensitive indicator mineral for “dry” conditions in rocks formed at total pressures higher than about 0.8 Kbar.

Some experimentally determined zircon/liquid partition coefficients for the rare earth elements

Partition coefficients for La, Sm, Ho and Lu (REE) between synthetic zircon and felsic, peralkaline liquid were determined at 800°C and 2 kbar water pressure by adding small amounts of REE to experimental charges and analyzing zircons in the quenched runs with an electron microprobe. The preferred zircon/liquid partition coefficients obtained by this method are: La, 1.4–2.1; Sm, 26–40; Ho, 340+; Lu, 72–126. These results confirm the strong heavy rare earth enrichment discovered by Nagasawa (1970) in zircon separates from dacites and granites, but they also introduce a modification to the supposed zircon/liquid partition coefficient pattern. The heavy REE end of the pattern is concave downward, in qualitative resemblance to some garnet/liquid and hornblende/liquid REE partitioning patterns.

Geotermometria de basaltos terciários e mesozóicos do nordeste do Brasil

Chemical data of some Tertiary olivine basaltic samples from the States of Rio Grande do Norte and Paraíba, and of Mesozoic tholeiitic basalts from the Maranhão sedimentary basin as well, allowed a preliminar estimate of the crystalization temperatures of their plagioclases through the KUDO and WELL’s geothermometer (5).In the calculation of the equilibrium temperatures between the plagioclase crystals and the liquid, the chemical composition of their edges and the composition of the rock matrix were used.In olivine basalts, temperatures rangibg from 1078 °C to 11680 ° C at 1 Kbar water pressure were recorded, in accordance with previously determined temperatures through HAKLI and WRIGHT’s (3) approach (SIAL, 8). For the tholeiitic basalts, temperature estimates ranged from 1140 °C to 1230 ° C, at 1 Kbar water pressure.

Zircon saturation in felsic liquids: Experimental results and applications to trace element geochemistry

Hydrothermal experiments were carried out at 2 kbar water pressure, 700 °–800 ° C, with the objective of determining the level of dissolved Zr required for precipitation of zircon from melts in the system SiO2-Al2O3-Na2O-K2O. The saturation level depends strongly upon molar (Na2O + K2O)/Al2O3 of the melts, with remarkably little sensitivity to temperature, SiO2 concentration, or melt Na2O/ K2O. For peraluminous melts and melts lying in the quartz-orthoclase-albite composition plane, less than 100 ppm Zr is required for zircon saturation. In peralkaline melts, however, zircon solubility shows pronounced, apparently linear, dependence upon (Na2O + K2O)/Al2O3, with the amount of dissolvable Zr ranging up to 3.9 wt.% at (Na2O + K2O)/Al2O3 = 2.0. Small amounts (1 wt.% each) of dissolved CaO and Fe2O3 cause a 25% relative reduction of zircon solubility in peralkaline melts. The main conclusion regarding zirconium/zircon behavior in nature is that any felsic, non-peralkaline magma is likely to contain zircon crystals, because the saturation level is so low for these compositions. Zircon fractionation, and its consequences to REE, Th, and Ta abundances must, therefore, be considered in modelling the evolution of these magmas. Partial melting in any region of the Earth's crust that contains more than ∼ 100 ppm Zr will produce granitic magmas whose Zr contents are buffered at constant low (< 100 ppm) values; unmelted zircon in the residual rock of such a melting event will impart to the residue a characteristic U- or V-shaped REE abundance pattern. In peralkaline, felsic magmas such as those that form pantellerites and comendites, extreme Zr (and REE, Ta) enrichment is possible because the feldspar fractionation that produces these magmas from non-peralkaline predecessors does not drive the melt toward saturation in zircon. Zircon solubility in felsic melts appears to be controlled by the formation of alkali-zirconosilicate complexes of simple (2:1) alkali oxide: ZrO2 stoichiometry.

Feldspar-paracelsian dimorphism of the synthetic compounds (Sr,Ba)[(Al,Ga)2(Si,Ge)2O8

Abstract New results on the title dimorphism are given for the systems SrGa2Si208 – SrGa2Ge208, BaGa2Si2Og – BaGa2Ge2O8 and BaGa2Ge2Og – SrGa2Ge2O8. Their end terms as well as several intermediate terms were obtained by dry syntheses (reaction in the solid state or crystallization from the melt) or hydrothermally under 1.0 Kbar water pressure at 500° C. The proxying of Ge for Si, or of Sr for Ba, is accompanied by the shrinkage of the feldspar stability field which is progressively confined to the higher temperatures at room pressure. The composition limits for obtaining this form are SrGa2(Si0.75Ge0.25)2O8 and (Ba0.25Sr0.75) Ga2Ge2O8. The paracelsian form was obtained for all the Sr and Ba gallosilicates and germanates. The general effect of the hydrothermal conditions is of stabilizing this form. The phase transition and the stability relations of the two forms are considered for all the title terms: the feldspar/paracelsian transition is enantiotropic only within the intervals: Sr(Al0.50Ga0.50)2Si2O8 – SrGa2Si2O8, SrGa2Si2O8 – (Sr0.50Ba0.50)Ga2Si2O8 and BaGa2(Si0.50Ge0.50)2O8 –BaGa2Ge208. The correlations between the stability conditions, the densities, and certain structural parameters are examined.

Synthesis of bicchulite from gehlenite solid solutions

Bicchulite was synthesized from gehlenite solid solutions with a composition range from Ak0Ge100 to Ak40Ge60. Upper limits of the stability field of bicchulite at 1 kbar water pressure were found to be at 635°C for Ak0Ge100, 610°C for Ak20Ge80, and 605°C for Ak40Ge60. Gehlenite with the akermanite components less than 20% decomposes to bicchulite±idocrase±xanthophyllite below the temperature of the stability limit, while gehlenite with the akermanite component more than 30% breaks down at a temperature higher than the temperature of bicchulite stability, 630°C for Ak30Ge70 and 680°C for Ak50Ge50, forming idocrase+xanthophyllite±monticellite. Xanthophyllite obtained in the experiments is stable only at temperatures above about 400°C. Below that temperature it changes to hydrogrossular.Some natural specimens were also studied for geological applications.

Experimental studies of granitoid rocks near the San Andreas fault zone in the Coast and Transverse Ranges and Mojave Desert, California

In western California, Mesozoic calc-alkaline plutons, consisting of approximately 39% quartz monzonite, 36% granodiorite, 23% quartz diorite and 1% gabbro-diorite, occur in terrains displaced by the San Andreas fault zone from Bodega Head south to the intersection of the Big Pine and Garlock faults. Because these granitoids can potentially be utilized as possible markers of fault movement, the results of recent geochemical, petrochemical, and geochronological studies are considered. These investigations include the water-saturated phase equilibrium relationships of granitoids from the Coast Ranges, Transverse Ranges, Mojave Desert, and Central Sierra Nevada Batholith, California, determined to 10 kbar water pressure. The data are compatible with approximately 500 km offset along the San Andreas fault zone since Cretaceous time.

Thermodynamic relations among olivine, spinel, and phenacite structures in silicates and germanates: I. Volume relations and the systems NiOMgOGeO 2 and CoOMgOGeO 2

An experimental approach is outlined to systematically obtain free energy differences among olivine, spinel, and phenacite forms of silicates and germanates from the thermodynamics of terminal solid solutions in ternary systems. This is applied to the ternary systems NiOMgOGeO 2 and CoOMgOGeO 2 at 1200°C in air and to the system NiOMgOGeO 2 at 800°C and 0.57 kbar water pressure. From the location of conjugation lines, activity-composition relations along each orthogermanate join are calculated. The free energies of transformation from the olivine to the spinel structure at 1200°C are estimated to be +1.6, −3.5, and −8.2 kcal/mole for Mg 2GeO 4, Co 2GeO 4, and Ni 2GeO 4, respectively. Volume changes for the spinel-olivine and olivine-phenacite transitions are estimated for the silicates and germanates of Mg, Mn, Fe, Co, Ni, and Zn.

The complementariness of orogenic andesite and alpine peridotite

Current concepts on the origin of the calc-alkaline andesites of orogenic belts range widely with respect to the mechanism of magma generation, the place of origin and the nature of the source material. The hypothesis which the author believes best fits the evidence is that of the derivation of andesite through fractional crystallization of a body of primary olivine basalt which rises from the mantle into geosynclinal rocks, and which during the fractional crystallization experiences an exchange with the surrounding country rock of water flowing into the magma to a total amount of probably 2 or 3 per cent, and hydrogen flowing out. The small amount of oxygen being added to the magma during fractional crystallization is necessary to develop and maintain the calc-alkaline trend of the liquid and crystal compositions. Complementary bodies of settled crystals remaining at depth in the geosynclinal rocks after the andesite liquid is driven off are alpine peridotites. Research bearing on this hypothesis includes: 1. (1) experimental work on the system, CaO-MgO-iron oxide-plagioclase-SiO 2, showing that the process is possible; 2. (2) experiments on basalts and andesites at controlled pO 2, and on a basalt at 1 kbar water pressure as well as at controlled pO 2, showing the effect of water pressure and pO 2 on temperature of crystallization of the phases and providing data on the equilibrium pO 2 of these rocks and the solubility of water in them; 3. (3) extensive chemical analyses of andesites and other geosynclinal rocks providing data on Sr isotopes, trace elements and alkali-silica ratios, all of which argue against formation of andesite by partial melting of crustal rocks or by assimilation of crustal rocks by basaltic magma; 4. (4) field and laboratory studies of alpine peridotite complexes leading to the conclusion that they represent cumulate bodies formed by crystal settling during crystallization of a magma; 5. (5) composition data on alpine peridotite complexes demonstrating their general calc-alkaline trend characteristics as opposed to the higher iron oxide trend of cumulates of the layered intrusions which do not develop calc-alkaline andesites.