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

Trace element compositions of amphibole (+/-phlogopite) -bearing spine l Iherzolite xenoliths have been investigated with regard to their oxi dation state and tectonic setting to evaluate the processes associated with the formation of hydrous upper mantle. The oxidation states of t hese xenoliths appear to be directly related to the style of metasomat ism and the tectonic environment. Suites having oxygen fugacities at o r above QFM (Dish Hill, California; Ichinomegata, Japan; Mont Briancon , Massif Central, France; Dreiser Weiher, Eifel, Germany) are all moda lly metasomatized, containing amphibole +/- phlogopite. These suites a re either from regions overlying Cenozoic continental arcs (Dish Hill and Ichinomegata) or from Cenozoic continental rifts superimposed on a ncient are terranes (Massif Central and Eifel). Although trace element enrichment does not appear to be directly Linked to the oxidation sta te, three distinct cases of metasomatic activity can be defined for th ese oxidized, hydrous xenolith suites. Both Case 1 and Case 2 are exem plified 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-ele ment composition of the mantle is preserved. Amphiboles produced in Ca se 2 metasomatism are enriched in incompatible elements, but coexist w ith depleted clinopyroxene. The formation of enriched amphibole is con sistent with crystallization from a volatile-bearing melt phase. The E ifel and Mont Briancon trace element compositions that appear to repre sent equilibrium crystallization of incompatible element-enriched amph ibole and clinopyroxene from a migrating melt phase (Case 3). Chemical evidence suggests that phlogopite in xenoliths from Dreiser Weiher, M ont Briancon, and Ichinomegata has a secondary origin. Mica formation, therefore, represents a distinct metasomatic episode.