Data from experimental runs, coexisting phases in ultramafic rocks and phenocryst-matrix pairs in volcanic rocks have been used to compile a table of mineral-liquid distribution coefficients for Ti, Zr, Y, and Nb for basic, intermediate and acid melt compositions. These values have then been used to interpret variations of these elements, first in basalts and second, during fractional crystallization from basic to acid magmas. For basalts, petrogenetic modelling of Zr/Y, Zr/Ti, and Zr/Nb ratios, when used in conjunction with REE, Cr and isotopic variations, suggests that: (1) the increase in Zr/Y ratio from mid-ocean ridge to within plate basalts and the low Zr/Nb ratios of alkalic basalts are due to (fluid controlled) source heterogeneities; (2) the low Zr and Zr/Y ratio of volcanic arc basalts results from high degree of partial melting of a depleted source; and (3) the high Zr and similar Zr/Y ratio of basalts from fast spreading relative to slow spreading ridges results from open-system fractional crystallization. Modelling of fractionation trends in more evolved rocks using Y-Zr, Ti-Zr and Nb-Zr diagrams highlights in particular the change in crystallizing mafic phases from island arcs (clinopyroxene-dominated) to Andean-type arcs (amphibole±biotite-dominated). These methods can be applied to altered lavas of unknown affinities to provide additional information on their genesis and eruptive environment.

Petrogenetic implications of Ti, Zr, Y, and Nb variations in volcanic rocks

This paper reports the results of a detailed experimental investigation of fractionation of natural basaltic compositions under conditions of high pressure and high temperature. A single stage, piston-cylinder apparatus has been used in the pressure range up to 27 kb and at temperatures up to 1500° C to study the melting behaviour of several basaltic compositions. The compositions chosen are olivine-rich (20% or more normative olivine) and include olivine tholeiite (12% normative hypersthene), olivine basalt (1% normative hypersthene) alkali olivine basalt (2% normative nepheline) and picrite (3% normative hypersthene). The liquidus phases of the olivine tholeiite and olivine basalt are olivine at 1 Atmosphere, 4.5 kb and 9 kb, orthopyroxene at 13.5 and 18 kb, clinopyroxene at 22.5 kb and garnet at 27 kb. In the alkali olivine basalt composition, the liquidus phases are olivine at 1 Atmosphere and 9 kb, orthopyroxene with clinopyroxene at 13.5 kb, clinopyroxene at 18 kb and garnet at 27 kb. The sequence of appearance of phases below the liquidus has also been studied in detail. The electron probe micro-analyser has been used to make partial quantitative analyses of olivines, orthopyroxenes, clinopyroxenes and garnets which have crystallized at high pressure.

The genesis of basaltic magmas

The anhydrous melting behaviour of two synthetic peridotite compositions has been studied experimentally at temperatures ranging from near the solidus to about 200° C above the solidus within the pressure range 0–15 kb. The peridotite compositions studied are equivalent to ‘Hawaiian’ pyrolite and a more depleted spinel lherzolite (Tinaquillo peridotite) and in both cases the experimental studies used peridotite −40% olivine compositions. Equilibrium melting results in progressive elimination of phases with increasing temperature. Four main melting fields are recognized; from the solidus these are: olivine (ol)+orthopyroxene (opx)+clinopyroxene (cpx)+Al-rich phase (plagioclase at low pressure, spinel at moderate pressure, garnet at high pressure)+liquid (L); ol+opx+cpx+Cr-spinel+L; ol+opx+Cr-spinel +L: ol±Cr-spinel+L. Microprobe analyses of the residual phases show progressive changes to more refractory compositions with increasing proportion of coexisting melt i.e. increasing Mg/(Mg+Fe) and Cr/(Cr+Al) ratios, decreasing Al2O3, CaO in pyroxene. The degree of melting, established by modal analysis, increases rapidly immediately above the solidus (up to 10% melting occurs within 25°–30° C of the solidus), and then increases in roughly linear form with increasing temperature. Equilibrium melt compositions have been calculated by mass balance using the compositions and proportions of residual phases to overcome the problems of iron loss and quench modification of the glass. Compositions from the melting of pyrolite within the spinel peridotite field (i.e. ∼ 15 kb) range from alkali olivine basalt (<15% melting) through olivine tholeiite (20–30% melting) and picrite to komatiite (40–60% melting). Melting in the plagioclase peridotite field produces magnesian quartz tholeiite and olivine-poor tholeiite and, at higher degrees of melting (30–40%), basaltic or pyroxenitic komatiite. Melts from Tinaquillo lherzolite are more silica saturated than those from pyrolite for similar degrees of partial melting, and range from olivine tholeiite through tholeiitic picrite to komatiite for melting in the spinel peridotite field. The equilibrium melts are compared with inferred primary magma compositions and integrated with previous melting studies on basalts. The data obtained here and complementary basalt melting studies do not support models of formation of oceanic crust in which the parental magmas of common mid-ocean ridge basalts (MORB) are attributed to segregation from source peridotite at shallow depths (≦ 25 km) to leave residual harzburgite. Liquids segregating from peridotite at these depths are more silica-rich than common MORB.

Anhydrous melting of peridotite at 0–15 Kb pressure and the genesis of tholeiitic basalts

Summary Plate theory has successfully related sea floor spreading to the focal mechanisms of earthquakes and the deep structure of island arcs. It is used here to calculate the temperature distribution in the lithosphere thrust beneath island arcs, and to determine the flow and the stress elsewhere in the mantle. Comparison with observations demonstrates that earthquakes are restricted to those regions of the mantle which are colder than a definite temperature. The flow and the stress heating in the mantle can maintain the high heat flow anomaly observed behind island arcs. Plate theory also suggests a new approach to the convection problem. The most obvious mechanism causing surface motion is the force on the plates due to the sinking lithosphere. This does not appear to be the way in which the motions are maintained. However, the input of large volumes of cold material can control convection and cause general downward movements in the mantle near island arcs. This input of cold lithosphere must cease when the island arc tries to consume a continent, since the light continental crust cannot sink through the denser mantle. Attempts to assimilate continental crust in this way can produce fold mountains, and also permit a rearrangement of convection cells.

Speculations on the Consequences and Causes of Plate Motions

This chapter has four main aims. Provide a comprehensive picture of the composition of volcanic rocks from subduction-related magmatic arcs. Review evidence in favor of the existence of andesitic, as well as basaltic primary magmas in arcs. Present new data on the composition of arc lower crust, based mainly on our work on the Talkeetna arc section in southcentral Alaska. Summarize evidence from arc lower crustal sections that a substantial proportion of the dense, lower crustal pyroxenites and garnet granulites produced by crystal fractionation are missing.

One View of the Geochemistry of Subduction-Related Magmatic Arcs, with an Emphasis on Primitive Andesite and Lower Crust

The origin of high-alumina basalts and their relationships to quartz tholeiites and alkali basalts

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.

Origin of garnet phenocrysts in calc-alkaline rocks

Origin of the calc-alkaline igneous rock suite

Crystallization of basalt and andesite under high pressure hydrous conditions

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.

Trevor H. Green

Papers

The origin of high-alumina basalts and their relationships to quartz tholeiites and alkali basalts

Origin of garnet phenocrysts in calc-alkaline rocks

Origin of the calc-alkaline igneous rock suite

Crystallization of basalt and andesite under high pressure hydrous conditions

An Experimental Investigation of Sub-solidus Assemblages Formed at High Pressure in High-alumina Basalt, Kyanite Eclogite and Grosspydite Compositions