The stability fields of aluminous pyroxene peridotite and garnet peridotite and their relevance in upper mantle structure

Abstract

An experimental study of the stability fields at high pressure of garnet peridotite and aluminous pyroxene peridotite has been carried out in compositions matching estimates of the average, undifferentiated upper mantle (pyrolite). The appearance of garnet at higher pressures in the pyrolite compositions results from either of two reactions: (1) spinel + orthopyroxene ⇋ olivine + garnet (2) aluminous pyroxene ⇋ garnet + pyroxene (lower alumina). The role of spinel in the lower pressure assemblages is sensitively dependent on temperature and bulk composition. For the pyrolite composition preferred for the upper mantle, spinel is absent above 1300°C and the first appearance of garmet at pressures of 24 kb (1300°C) to 31 kb (1500°C) is due to reaction (2). In this composition garnet does not appear on the pyrolite solidus nor in its melting interval at pressures below 31.5 kb.At temperatures less than 1300°C, garnet appears at 21 kb (1100°C) to 24 kb (1300°C) and develops by reaction (1) at the expense of spinel. The amount of garnet formed by this reaction is dependent on the alumina content of the pyroxenes and throughout the temperature range 1100–1500°C the amount of garnet present increases markedly this is matched by decreasing Al2O3 content of the pyroxenes and allow preliminary estimation of P, T-dependent curves of constant Al2O3 content for orthopyroxene in garnet peridotite assemblages.The experimental data are applied to estimate density and seismic velocity variations along oceanic and continental geothermal gradients in a pyrolite upper mantle. It is emphasized that seismic velocity distributions are sensitively affected by variations in geothermal gradient and by mantle chemical composition, e.g. by variation from pyrolite to residual, refractory, dunite-peridotite. It is suggested that seismic velocity (Vs) variation in an oceanic upper mantle of pyrolite composition may be characterized by two low-velocity channels: (1) a narrow, but sharply defined low-velocity zone at 60–70 km depth caused by mineralogical zoning in the upper mantle; (2) a broader low velocity zone at 120–150 km depth defined primarily by the critical gradient for Vs in the upper mantle but accentuated by mineralogical variations in pyrolite.