The Role of Peridotite for Oceanic Volcanism

Abstract

The complementary products of oceanic crust formation are compositionally variable melt-depleted peridotites. Together with the oceanic crust, the melt-depleted peridotites are continuously recycled back into the mantle. Before remelting today, after some 10⁸–10⁹ years of recirculation within the mantle, the peridotites have developed extremely variable radiogenic isotope ratios, documented by εHf extending to values of >450 in abyssal peridotites. Peridotite isotope ratios thus range far beyond those observed in their melting products, basalts formed at mid-ocean ridges or ocean islands (εHf < 25). Here, we stress that such compositionally heterogeneous, and variably dense melt-depleted peridotites are the predominant component of Earth's mantle. We discuss a first-order approach to assess how basalts reflect the chemical and isotopic signatures of the individual components of their heterogeneous mantle sources. Generally, the incompatible element compositions of basalts are dominated by melts from minor, incompatible element enriched components, foremost recycled crust. But oceanic basalts also inherit characteristic isotopic signatures from the predominant, trace element-poor, and variably dense peridotites. The covariation between variable peridotite composition and density establishes a critical link between basalt chemistry and the thermochemical buoyancy forces that control mantle upwelling and basalt production rates under mid-ocean ridges or ocean islands. Exploiting strategies to relate geochemical and geophysical observables determined by the inherent heterogeneity of peridotite will therefore lead to a new, integrative understanding of the chemical geodynamics of the Earth's mantle-crust system.