Evidence from chromium abundances in mantle rocks for extraction of picrite and komatiite melts

Abstract

ONE of the more important aspects of understanding the geochemical differentiation of the Earth is an accurate knowledge of the compositions of magmas produced by direct melting of the upper mantle, which are called primary magmas¹. It is clear that a variety of primary magma compositions can develop within the mantle, depending on the source composition, the presence of volatiles, the extent of melting and the pressure of melt segregation^1–4. Proposed primary magma compositions span a wide composition range, from basalts with ∼8–ll% MgO, through picrites with ∼11–20% MgO, to komatiites with ∼20–28% MgO; there is no agreement as to which is the dominant type of primary magma. We suggest here that the abundances of chromium in samples of the Earth's upper mantle indicate that, of the wide variety of possible primary liquids produced in experimental studies of mantle melting, the dominant type of primary liquid actually extracted from the mantle has been picritic to komatiitic, with ≳;15% MgO.