Mantle cryptology

Abstract

A group of anhydrous peridotites from Peridot Mesa, Arizona, document isotopic and trace element heterogeneity in the source mantle. LREE enrichments in two spinel peridotites may have occurred immediately prior to entrainment through interaction with a melt similar to the host basanite. One sample, MORB-source-like in its trace element and isotopic character, has escaped this enrichment and may have evolved via a single melt depletion event acting on a relatively unfractionated parental composition ~900 m.y. ago. Detailed characterization of inclusion-free peridotite phases, and washed and unwashed whole-rock samples, verifies the presence of a ubiquitous secondary contaminant which derives from interaction of the peridotites with local ground waters and host magma. Once the veil of this contamination is removed, coexisting phases are found to be in isotopic equilibrium. Further, a comparison of washed whole rocks and calculated clean-bulk compositions documents the occurrence of an important intragranular fluid-hosted trace element component. For the very incompatible elements (K, Rb, Cs, and Ba, and probably U, Th, Pb and gaseous components as well) this component dominates the nodule budget for two of the three samples studied in detail. Production of basaltic magmas from “fertile” but incompatible-element-depleted peridotite (such as PA-65G and PA-15A, two of the samples studied here) requires the action of melting processes such as those recently proposed by McKenzie (1985) and O'hara (1985). The distinctive feature of these models is that they call on effectively larger source volumes for more incompatible elements. In this context, depletions of incompatible trace elements in MORB source mantle will be more extreme than has heretofore been suspected. This would essentially preclude the long-term total isolation of a MORB source mantle above the 670 km seismic discontinuity.