Metasomatism and fluid generation in mantle xenoliths

Abstract

The concept of metasomatism was developed in the study of crustal rocks, to describe a process of alteration that involves enrichment by new substances brought in from the outside by fluids, commonly considered to be dilute aqueous solutions or vapours rather than silicate magmas. There is abundant evidence in mantle rocks brought to the surface that chemical changes have been caused by influx or passage of fluids, and most investigators have referred to metasomatic fluids without distinction between aqueous vapours and silicate melts. Liquid and vapour are distinct phases with different compositions and properties, that can coexist. A liquid may crystallise and exsolve a separate vapour phase, and vapour passing through a rock may cross a solidus boundary causing partial melting. Every reaction involving both phases has important geochemical effects. For understanding of metasomatism in the upper mantle, it is essential to establish the conditions for the existence of, and the interactions between melts, vapours, and their host rocks. The position of the solidus for peridotite- H_2O-CO_2, relative to the position of and the changes in the geotherm, is critical in such deliberations; the uncertainties about its position are reviewed. In the depth interval between the two points of intersection of solidus and geotherm, volatile constituents are dissolved in melts, and metasomatism by vapours cannot occur. Magmas rising to the asthenosphere-lithosphere boundary, or to a ledge on the solidus near 80 km, can release vapours for metasomatic reactions. CO_2-rich vapours cannot exist below a depth of about 80 km. The solidus for harzburgite is at higher temperature than that for lherzolite. Therefore, vapours exsolved by magmas near the asthenosphere-lithosphere boundary may cause partial melting in lithosphere lherzolite, and magnesite metasomatism in adjacent harzburgite. Carbonates in mantle nodules will dissociate en route to the surface. The cratonic lower lithosphere has been metasomatised intermittently by small magmatic intrusions, and by influx of aqueous vapours.

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