In order to provide additional constraints on models for partial melting of common metasediments, we have studied experimentally the melting of a natural metapelite under fluid-absent conditions. The starting composition contains quartz, plagioclase, biotite, muscovite, garnet, staurolite, and kyanite. Experiments were done in a halfinch piston-cylinder apparatus at 7, 10, and 12 kbar and at temperatures ranging from 750° to 1250° C. The following reactions account for the mineralogical changes observed at 10 kbar between 750° and 1250° C: Bi+Als+Pl+Q=L+Gt+(Kf), Ky=Sill, Gt+Als=Sp+Q, Gt=L+Sp+Q, and Sp+Q=L+Als. The compositions of the phases (at T>875° C) were determined using an energy-dispersive system on a scanning electron microscope. The relative proportions of melt and crystals were calculated by mass balance and by processing images from the SEM. These constraints, together with other available experimental data, are used to propose a series of P-T, T-XH2O, and liquidus diagrams which represent a model for the fluid-present and fluid-absent melting of metapelites in the range 2–20 kbar and 600°–1250° C. We demonstrate that, even under fluid-absent conditions, a large proportion (≈40%) of S-type granitic liquid is produced within a narrow temperature range (850°–875° C), as a result of the reaction Bi+Als+Pl+Q=L+Gt(+/-Kf). Such liquids, or at least some proportion of them, are likely to segregate from the source, leaving behind a residue composed of quartz, garnet, sillimanite, plagioclase, representing a characteristic assemblage of aluminous granulites. The production of a large amount of melt at around 850° C also has the important effect of buffering the temperature of metamorphism. In a restitic, recycled, lower crust undergoing further metamorphism, temperature may reach values close to 1000° C due to the absence of this buffering effect. Partial melting is the main process leading to intracontinental differentiation. We discuss the crustal cross-section exposed in the North Pyrenean Zone in the context of our experiments and modelling.

Experimental determination of the fluid-absent melting relations in the pelitic system

The ultrapotassic rocks: Characteristics, classification, and constraints for petrogenetic models

Pyroxene reflectance and transmittance spectra have been examined in a search for systematic relationships between spectral features and compositional variations and to assess the applicability of reflectance spectroscopy to pyroxene geothermometry. Orthopyroxenes containing up to about 11 percent Wollastonite show a positive correlation between Fe(2+) content and wavelength positions of the major absorption bands. Aluminum-rich orthopyroxenes display absorption bands at lower than expected wavelengths. Spectral-compositional relationships are more complex for clinopyroxenes, showing both positive and negative correlations between band positions and major cation abundances. These relationships are further complicated by the presence of significant amounts of other transition series elements such as Ti and Cr and by the presence of exsolved phases and compositional zonations. Contours of the wavelength positions of band minima projected onto the pyroxene tetralateral generally exhibit concave downward shapes. The orientations of the contours relative to pyroxene geotherms are such as to effectively preclude the use of spectroscopy to significantly constrain the temperatures of formation of pyroxenes.

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Pyroxene spectroscopy revisited - Spectral-compositional correlations and relationship to geothermometry

Nickel content of basaltic magmas: identification of primary magmas and a measure of the degree of olivine fractionation

This paper attempts to review those salic eruptives in which Si02 >60%, but specifically excluding peralkaline and silica-undersaturated types The approach is based on a compilation of major and trace elements, plus phenocryst occurrence and phenocryst modal data; some 2,890 sets of rock data are included The data are tabulated into five chemical groupings (60-63, 63-66, 66-69, 69-72, >73% SiO2); further breakdown of data is based on geographic and/or tectonic subregions, and further chemical subdivision into low-K, calcalkaline, and high-K series from each subregion In this paper, the general terms siliceous andesite, dacite, and rhyolite are applied to volcanic rocks with SiO2% of 60-63, 63-69, and >69%, respectively, except lavas specified as trachytes, for which a separate set of data are compiled The majority of salic volcanic magmas have been erupted within orogenic zones; ie, the circum-Pacific regions, Indonesia, the Mediterranean arcs, and to a lesser extent, the Caribbean and Scotia arcs An additional group of silicic magmas, dominated by rhyolites, are those of the bimodal mafic-silicic magma associations; examples are drawn from Yellowstone, Mono, Medicine Lake, and Salton Sea in the western USA: Iceland; western Scotland; and southern Queensland (Australia) Further comparative data are provided from the Hawaiian, Galapagos, and Canary Islands, and the unusual high soda-low potash lavas of Deception and Fedarb Islands Cluster analyses of averaged data for the various subregions suggest close correlations between the orogenic silicic magmas of the western USA (excluding Cascades), western South America, and Mediterranean regions Further clustering is found between the eruptives of the Cascades-Aleutians, Middle Americas, and Indonesian subregions The clustering tends to separate these orogenic magmas from the oceanic rhyolites and dacites, and the rhyolites of the bimodal associations The trachytes exhibit no clear correlations with any of these groupings Mineralogically, the calc-alkaline orogenic salic volcanics are characterized by phenocryst assemblages comprising plagioclase, quartz, augite, hypersthene, hornblende, cummingtonite, Fe-Ti oxides, biotite, and rarely sanidine The low-K eruptives lack biotite and sanidine, while the high-K eruptives contain much more abundant sanidine, quartz and biotite, and commonly also sphene and allanite Detailed phenocryst assemblages are tabulated in the text Fe-Ti oxide temperatures range from 640-780° in high-K rhyolites of western USA (excluding those of the bimodal associations); 725-900° in calc-alkaline rhyolites from the SW Pacific; and 830-1120° for orogenic dacites from various circum-Pacific regions These data also suggest that the orogenic silicic magmas from the western USA have equilibrated on a slightly higher fO2 buffer (between NNO and HM buffers) than the orogenic magmas from other regions It is also shown that the averaged compositions of the magmas from western USA, western South America, and Mediterranean regions, and indeed the high-K eruptives from other regions, trend towards and terminate close to the lower pressure “ternary minima” in the quartz-feldspar system, and the 2-feldspar cotectic surface in the feldspar system This is not found for the calc-alkaline and low-K series Rhyolites of bimodal associations often exhibit distinctive phenocryst assemblages and compositions, most notably the progressive development of strongly Fe-enriched pyroxenes, olivines, and where present, amphiboles and biotites Fe-Ti oxide data indicate relatively high equilibration temperatures (800-985°), and equilibration between the QFM and NNO fO2 buffers (ie, lower than most orogenic salic magmas) These rhyolites also frequently exhibit very fractionated trace element abundance patterns

A Review of the Mineralogy and Chemistry of Tertiary-Recent Dacitic, Latitic, Rhyolitic, and Related Salic Volcanic Rocks

The 1944 Eruption of Volcano Usu in Hokkaido, Japan

The system acmite-diopside and its bearing on the stability relations of natural pyroxenes of the acmite-hedenbergite-diopside series

Phase-equilibrium studies of this system, at and just below temperatures where a liquid phase is present, show one ternary compound, the iron analog of cordierite (2FeO. 2Al2O3. 5SiO2), which decomposes at 1210°±l0°C. to mullite, tridymite, and liquid. The fields of stability of corundum, mullite, hercynite, iron cordierite, cristobalite, tridymite, fayalite, and wustite have been delineated. The following invariant points (the first two eutectics and the remainder reaction points) were located: Fayalite+wustite+hercynite+liquid 1148°±5° Fayalite+iron cordierite+tridymite+liquid 1083°±5° Fayalite+iron cordierite+hercynite+liquid 1088°±5° Hercynite+iron cordierite+mullite+liquid 1205°±10° Iron cordierite+mullite+tridymite+liquid 1210°±10° Corundum+mullite+hercynite+liquid 1380°±5° Cristobalite+tridymite+mullite+liquid 1470°±10° Iron cordierite crystallizes with some reluctance, and the metastable invariant points fayalite+tridymite+spinel+liquid at 1073°±5°, and mullite+hercynite+tridymite+liquid at 1205°±10° can be realized. No ferrosilite or almandine garnet could be crystallized from the melts at any temperature even when melts were seeded with these crystalline phases. Natural almandine from Botallack, England (91. 3 per cent almandine), when heated begins to decompose at an appreciable rate as low as 900°C. and yields a mixture of hercynite, iron cordierite, and fayalite. The bearing of these results on petrology and slags is discussed.

The system FeO-Al2O3-SiO2

Kosmochlor (NaCrSi2O6) was synthesized by the flux method from melts along the join Na2O·2 SiO2-Na2O·Cr2O3·4 SiO2 at 1000° C in air, and isolated by dissolving the glassy matrix with hydrofluoric and perchloric acids. The join CaMgSi2O6-NaCrSi2O6 was studied at 1 atmosphere in air by the quenching technique at temperatures between 900° and 1450° C, using mixtures of kosmochlor and diopside crystals or diopside glass as starting materials. The phases are diopside solid solution, kosmochlor, spinel (Mg-chromite), eskolaite (Cr2O3) and glass. The maximum solubility of kosmochlor in diopside is 24 wt percent at 1140° C, while diopside is not soluble at all in kosmochlor, resulting in the existence of a wide range of immiscibility. Petrologic significance of the results is discussed.

Kenzo Yagi

Papers

The 1944 Eruption of Volcano Usu in Hokkaido, Japan

The system acmite-diopside and its bearing on the stability relations of natural pyroxenes of the acmite-hedenbergite-diopside series

The system FeO-Al2O3-SiO2

Synthesis of kosmochlor and phase equilibria in the join CaMgSi 2 O 6 -NaCrSi 2 O 6

Crystallization of chondrules in ordinary chondrites