Deducing a reducing mantle

Abstract

Increasingly sophisticated techniques are being used to persuade ancient rocks to yield information about conditions on and in the early Earth — for instance, about the oxidation state of the mantle. The origin of komatiites, volcanic rocks formed by unusually extensive melting of mantle rocks, mainly during the Archaean, is the subject of much debate. They are thought to have been produced either by anhydrous melting of anomalously hot mantle, or by hydrous melting at temperatures only a little higher than those of today. Berry et al. have now determined the oxidation state of iron in pristine samples of a 2.7 billion-year-old komatiitic magma from Belingwe, Zimbabwe. Their findings are consistent with near-anhydrous melting of a source with an oxidation state similar to that of present-day ocean-floor basalt. The results suggest that the Belingwe melt was a product of high mantle temperatures of about 1,700 °C, rather than melting under hydrous conditions, confirming the existence of anomalously hot mantle in the Archaean.