ALBERT

Notes: Abstract A series of experimental runs has been conducted on a glass prepared from a natural island are calc-alkaline andesite from Fiji. The crystallization sequence was determined for the pressure interval 9–36 kb under anhydrous conditions and with 2, 5, and 10% by weight of water carefully added. Addition of water markedly lowers the liquidus, depresses the appearance of quartz and plagioclase in the crystallization sequence, and greatly enlarges the field of garnet-clinopyroxene crystallization above 25 kb. Amphibole crystallizes in hydrous runs up to 25 kb. Electron microprobe analyses of critical phases allows calculation of controls on crystal fractionation trends. For hydrous conditions at 5–15 kb amphibole-clinopyroxene dominate fractionation and a moderate decrease in Mg/Fe and a slight increase in K/Na occurs. At 15–25 kb garnet also affects the fractionation and a moderate decrease in Mg/Fe and an increase in K/Na results. Above 25 kb garnet-clinopyroxene control the fractionation and there is a slight decrease in Mg/Fe but a significant increase in K/Na and a pronounced silica enrichment. In terms of major element chemistry, the derivation of the Fijian dacites in the second period of eruption may be satisfactorily explained by the fractionation of hydrous andesite at pressures 〉25 kb. Alternatively the dacites may result from lower degrees of melting of the down-going hydrous lithosphere. Similarly other members of this eruptive period may be derived according to a model of eclogite-controlled fractional melting or crystallization. Models involving amphibole fractionation at lower pressures are less satisfactory for explaining compositions in the Fijian second period of eruption, but in other environments models including amphibole-controlled fractionation may form part of a continuum of melting processes in subduction zones.

Notes: Abstract Clear evidence exists for a cordierite breakdown reaction to amphibole-kyanite-quartz in high-grade metamorphic rocks of the Arunta Complex, Australia. Using the natural minerals this reaction has been duplicated experimentally. It proceeds over a divariant band with a slope of 12±4 bars/°C, occurring between 8 and 10.4 kb at 750° C and between 9.5 and 11.3 kb at 850°C. The reaction is cut off at low temperature by the appearance of talc and at high temperature by the appearance of orthopyroxene. The maximum pressure stability of the amphibole-kyanite-quartz assemblage is about 20 kb. These data suggest that the natural rock was subjected to pressures of at least 8 kb at 750–850° C for high water fugacities. Other experimental data on the hydration of hypersthene and cordierite-hypersthene stability, point to a temperature below 820° C and an upper pressure limit of 9.5 kb at 750–820°C. Experiments at $$P_{{\text{H}}_{\text{2}} {\text{O}}}$$ 〈Ptotal indicate that the breakdown of cordierite to amphibole-kyanite-quartz is a hydration reaction, and occurred in the natural rock as a result of an increase of water fugacity at constant total pressure (8–9.5 kb) and temperature (750–820°C).

Notes: Abstract The sub-solidus fields of crystallization of a spectrum of synthetic aluminous basic compositions (high-alumina basalt, anorthite-enriched high-alumina basalt, kyanite eclogite, grosspydite and gabbroic anorthosite) have been investigated at pressures of up to 36 kb. At low pressures the assemblages are characterized by abundant plagioclase, clinopyroxene and possibly minor olivine and orthopyroxene. These correspond to natural gabbroic and pyroxene granulite assemblages. As pressure is increased garnet appears and increases gradually in amount at the expense of other ferromagnesian minerals and plagioclase, until finally at pressures of 〉23 kb at 1,100° C, plagioclase disappears and high pressure clinopyroxene+garnet+kyanite±quartz assemblages equivalent to eclogite are obtained. In the eclogite stability field, with further rise in pressure, the ratio ga/cpx and the grossular content of the garnet increase. In the high-alumina basalt composition the transitional garnet granulite assemblage (clinopyroxene+plagioclase+garnet±quartz) is spread over a pressure interval of 11 kb at 1,100° C. This is a greater interval than observed for other basalt compositions and is important in considering the hypothesis that the Mohorovicic Discontinuity is a phase change from basalt to eclogite. It indicates that the change in V p would be spread over a significant depth range, and no sharp seismic velocity discontinuity could result. The first experimental synthesis of kyanite eclogite from both high-alumina basalt and kyanite eclogite compositions has been obtained, as well as synthesis of unusual grossular-clinopyroxene-kyanite assemblages (grosspydite) from grosspydite and gabbroic anorthosite compositions. The pressures needed to synthesize these assemblages are somewhat greater than the pressures needed to synthesize eclogite from basic compositions of lower alumina content at the same temperature. Experimental confirmation of the observation that there is a direct relation between Gross/Alm + Py ratio of garnet and the Jd/Di ratio of co-existing pyroxene in grosspydite and kyanite eclogite assemblages found in kimberlite pipes has also been obtained.

Notes: Abstract The occurrence of corroded megacrysts and cumulates of olivine, clinopyroxene, orthopyroxene, plagioclase, ilmenite and apatite in near-saturated hawaiites from the mid-north coast of New South Wales point to the derivation of these hawaiites at elevated pressures. In an experimental study on one of these hawaiites under conditions ranging from 5–15 kb and 0–5% H2O, orthopyroxene was identified in only one run with 2% H2O at 1040° C and 6.5 kb. In this run it was associated with olivine, clinopyroxene and Fe-Ti oxide. The early appearance of plagioclase in the “dry” experimental runs and amphibole in runs with 5% H2O, indicates that the water content in the natural hawaiite was intermediate to these values. The near-duplication of the natural megacryst-cumulate assemblage suggests that the hawaiite host precipitated these phases at a depth of approximately 20–24 km, prior to rapid eruption to higher crustal levels.

Notes: Abstract Barium partitioning between alkali feldspar and a natural trachyte liquid, enriched with barium, has been determined as a function of pressure and temperature from 10 to 25 kb and 900°–1100° C. Both long duration experiments and a “re-equilibration” experiment suggest close approach to equilibrium. Partition coefficients (D Ba) decrease as both temperature and pressure increase (e.g., D Ba changes from 8.71 at 10 kb, 900° C to 1.48 at 25 kb, 1100° C). Water activity also controls the barium partitioning with a marked decrease in D Ba af/liq for addition of less than 0.8 wt% H2O, but with no apparent additional effect for higher water contents in the bulk composition (e.g., from 0.8–4.2 wt% H2O). The composition of alkali feldspar also has a significant effect on D Ba af/liq , but the data obtained do not allow derivation of a complete D-Or relationship. These new data suggest that Henry's Law is obeyed for most of the barium concentrations examined, and the limit of Henry's Law behaviour for barium in alkali feldspar is as high as 6 wt% BaO in alkali feldspar and 1.2 wt% BaO in the melt, similar to the results of Long (1978). The experimental results broadly overlap with natural data for D Ba, determined from coexisting alkali feldspar phenocrysts and glass (or groundmass).

Notes: Abstract A large number of garnet phenocrysts from Palaeozoic rhyodacites and granodiorite porphyrites from Central and Northeastern Victoria have been analyzed using the electron microprobe. These garnets, from an area of several thousand square miles, are very uniform in composition (dominantly almandine, with subordinate pyrope and minor grossular and spessartine). They show minor zoning with a very thin outer rim slightly richer in almandine and spessartine than the remainder of the phenocryst. They are surrounded by a complex intergrowth of cordierite and hypersthene forming a reaction rim. Resorbed quartz phenocrysts are typically associated with the garnet phenocrysts. The uniform composition, the conspicuous size and the subhedral-euhedral form of the garnet phenocrysts indicate that they crystallized directly from the acid calc-alkaline magma at an early stage of its crystallization. High pressure experimental work on a natural garnet-bearing rhyodacite glass demonstrates that almandine-rich garnet and quartz are near-liquidus phases at 18 and 27 kb $$\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} 〈 P_{{\text{LOAD }}} } \right)$$ , but garnet does not appear until well below the liquidus at 9 kb. A comparison of the composition of the experimentally crystallized garnets with the natural garnets suggests that these acid calc-alkaline magmas began to crystallize at pressures between 9 and 18 kb, i.e. at depths corresponding to the lower crust or upper mantle.

Notes: Abstract A high pressure experimental study of the partial melting fields of synthetic high-alumina olivine tholeiite, high-alumina quartz tholeiite, basaltic andesite, andesite, dacite and rhyodacite under dry and wet $$\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} 〈 P_{{\text{LOAD }}} } \right)$$ conditions has been conducted in order to investigate possible origins of the calc-alkaline series from the upper mantle. Detailed analyses of crystallizing phases using the electron microprobe has enabled calculation of the liquid line of descent in these compositions at various pressures. At 27–36 kb garnet and clinopyroxene are the liquidus or near-liquidus phases in dry tholeiite, basaltic andesite and andesite, while quartz is the liquidus phases in dry dacite and rhyodacite. Under wet conditions at 27 kb garnet, not quartz, is the liquidus phase in the dacite. Qualitatively these results show that the low melting fraction of a quartz eclogite at 27–36 kb under dry conditions is of andesitic composition whereas under wet conditions it is rhyodacitic or granodioritic. At these pressures under dry conditions the andesite liquidus lies in a marked low temperature trough between the more basic and more acid compositions. Quantitatively, the calculated compositions of liquid fractionates for varying degrees of melting of the quartz eclogite bulk composition broadly follow the calc-alkaline trend. At 9–10 kb under wet conditions $$\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} 〈 P_{{\text{LOAD }}} } \right)$$ sub-silicic amphibole and pyroxenes are the near-liquidus phases in tholeiite and basaltic andesite compositions. Calcic plagioclase and garnet occur nearer the solidus. The calculated liquid fractionates follow the calc-alkaline trend and demonstrate that the calc-alkaline series may be derived by the partial melting of amphibolite at lower crustal depths under wet conditions $$\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} 〈 P_{{\text{LOAD }}} } \right)$$ , Or by the fractional crystallization of a hydrous basalt magma at similar depths. These experimental results support two complementary hypotheses for the derivation of the calc-alkaline igneous rock suite from the mantle by a two stage igneous process. In the first stage of both hypotheses large piles of basalt are extruded on the earth's surface. Subsequently this pile of basalt may, under dry conditions, transform to quartz eclogite, sink into the mantle and finally undergo partial melting at 100–150 kms depth. This partial melting gives rise to the calc-alkaline magma series leaving a residuum of clinopyroxene and garnet. Alternatively, if wet conditions prevail in the basalt pile and the geotherms remain high, partial melting of the basalt may take place near the base of the pile, at about 10 kb pressure $$\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} 〈 P_{{\text{LOAD }}} } \right)$$ . The liquids so formed constitute the calc-alkaline suite and the residuum consists of amphibole, pyroxenes and possibly minor garnet and calcic plagioclase. Both models may be directly linked to the hypothesis of sea-floor spreading.

Notes: Abstract The experimental crystallization of staurolite in two mafic compositions at high pressures (24–26 kb) and temperatures (740 °–760 ° C) is reported. This, together with a recent report of staurolite from metamorphosed basic igneous rocks from Fiordland, New Zealand, leads to the conclusion that Staurolite may be an important subordinate phase in the subducted lithosphere. The synthesized staurolites are more magnesian than those occuring in natural metapelitic assemblages and are characterized by relatively high Si, Ti, Mg, Ca, and low Al, Fe.