A Discussion on volcanism and the structure of the Earth - Plutonic xenoliths and their relation to the upper mantle

Abstract
The best samples for chemical and petrological studies of the upper mantle are the ultramafic xenoliths carried up during volcanism, and those ultramafic rocks that may be tectonic slices of mantle transplanted into the crust. A geochemical survey of lherzolite xenoliths from a number of different localities fndkates that regional differences do occur in upper mantle material and that dxese differences may be explained by partial fusion and extraction of the liquid phase. In some localities the concentrations of K and U in the xenoliths may be high enough for the xenoliths to represent primitive mantle material There is some evidence that isotopic equilibration between co-existing minerals in the mantle is not attained over very long periods. This might lead to isotopic anomalies in rocks derived from the mantle.

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