Ultramafic Rocks and the Upper Mantle

Abstract

Cosmic abundances of elements and meteorite analogies indicate an ultrabasic mantle. Detailed review of ultramafic rocks in their varied associations suggests that representative mantle sample are included among oceanic and orogenic peridotites, and nodules in basalts and kimberlites. Extrapolation from rocks to mantle must be made with caution; their petrogenesis is complex and the mantle is heterogeneous. Twenty-seven estimates of mantle composition based on meteorite models, ultramafic rocks, and hypothetical peridotites are tabulated and compared with basalts in chemical variation diagrams. The three groups are separated by their alkali contents and stages of fractionation. Mantle estimates based on ultramafic rocks are low in alkalis to provide common basaltic magmas by simple subtraction, but they can yield high-pressure picritic liquids capable of fractionating during uprise to produce low-pressure basaltic types at the surface. Trace-element contents of ultramafic rocks and the upper mantle are not well determined. Models for the upper mantle are based on mineral facies determined experimentally for appropriate compositions, temperatures and pressures. Review of experimental data for the system peridotite-gabbro-water, correlated with geophysical data, suggests that the upper mantle is composed of peridotite with layers and pods of eclogite and residual or precipitated dunite and peridotite; with increasing depth, feldspathic peridotite (rarely occurring) is transformed to spinel peridotite and this to garnet-peridotite, at 50-75 kms depth. Garnet peridotite perists down to the Transition Zone at 350-400 kms, where pyroxene is dissolved by garnet, and olivine is transformed to a spinel-like phase. Eclogite is abundant from 80 to 150 kms depth. The low velocity zone begins at 75-100 kms where hornblende becomes unstable, producing traces of interstitial hydrous magma of andesitic composition within eclogite; within a peridotite mantle, interstitial hydrous magma may be alkalic (potassic).

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