The singular geochemistry displayed by several ocean–island basalts, picrites, komatiites, and peridotites, believed to be plume-delivered from deep down earth’s mantle, has prompted geochemists to visualize the existence of a unique zone near earth’s core/mantle boundary (CMB) having noble metals and other sidero-philes as well as rare-gases in amounts quite anomalous to the mantle composition. Several views on how such a zone could have developed have been published. An early explanation, attributed this to the heterogeneous nature of the accreting bodies, the late-accreting ones being more oxidized than their predecessors.
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