Mantle composition derived from the chemistry of ultramafic lavas

Abstract

ESTIMATES of the composition of the Earth's mantle have been based on both geochemical and geophysical studies (refs 1–3). As might be expected, there are some discrepancies between the estimates particularly for Na₂O and TiO₂. Such discrepancies in part arise from uncertainties concerning the degrees of melting which many volcanics represent and, possibly, the restricted tectonic settings of the ultramafic nodules studied. High MgO basalts and ultramafic lavas, some of which have been termed komatiites⁴ occur among the Archaean volcanic rocks. They represent exceptionally high degrees of melting (up to 50% or more) of peridotite source regions. Potentially such volcanics may be used to make a precise estimate of the composition of their mantle source region. High MgO lavas are well preserved in the Archaean Belingwe greenstone belt of southern Rhodesia, where they form part of a sequence deposited unconformably on granitic crust⁵. We assume that the occurrence of pillow lavas in this sequence containing both skeletal olivine spinifex (Fo_92.5 in a rock with 28% MgO, E. G. Nisbet, M. J. Bickle and A. Martin, unpublished) and skeletal olivine microphenocrysts indicates that these particular lavas have a composition close to that of the liquid.