CONTRASTING STYLES OF HYDROUS METASOMATISM IN THE UPPER MANTLE: AN ION MICROPROBE INVESTIGATION

Abstract

Trace element compositions of amphibole (+/-phlogopite) - bearing spinel lherzolite xenoliths have been investigated with regard to their oxidation state and tectonic setting to evaluate the processes associated with the formation of hydrous upper mantle. The oxidation states of these xenoliths appear to be directly related to the style of metasomatism and the tectonic environment. Suites having oxygen fugacities at or above QFM (Dish Hill, California; Ichinomegata, Japan; Mont Briancon, Massif Central, France; Dreiser Weiher, Eifel, Germany) are all modally metasomatized, containing amphibole +/- phlogopite. These suites are either from regions overlying Cenozoic continental arcs (Dish Hill and Ichinomegata) or from Cenozoic continental rifts superimposed on ancient arc terranes (Massif Central and Eifel).Although trace element enrichment does not appear to be directly linked to the oxidation state, three distinct cases of metasomatic activity can be defined for these oxidized, hydrous xenolith suites. Both Case 1 and Case 2 are exemplified by xenoliths from Dish Hill and Ichinomegata. Case 1 is a less intense form of metasomatism resulting in the formation of amphibole by the simple hydration of the mantle. The original depleted trace-element composition of the mantle is preserved. Amphiboles produced in Case 2 metasomatism are enriched in incompatible elements, but coexist with depleted clinopyroxene. The formation of enriched amphibole is consistent with crystallization from a volatile-bearing melt phase. The Eifel and Mont Briancon trace element compositions that appear to represent equilibrium crystallization of incompatible element-enriched amphibole and clinopyroxene from a migrating melt phase (Case 3). Chemical evidence suggests that phlogopite in xenoliths from Dreiser Weiher, Mont Briancon, and Ichinomegata has a secondary origin. Mica formation, therefore, represents a distinct metasomatic episode.