An Example of Consequent Mantle Metasomatism in Peridotite Inclusions from Nunivak Island, Alaska

• Published in: Journal of Petrology
• Main Authors: RODEN, MICHAEL F., FREY, FRED A., FRANCIS, DONALD M.
• Format: Text
• Language: English
• Published: Oxford University Press 1984
• Subjects: Articles; Nunivak; Nunivak Island; Alaska
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/25/2/546; https://doi.org/10.1093/petrology/25.2.546

Many of the coarse-grained peridotite inclusions in basanites from Nunivak Island, Alaska, contain amphibole and a smaller fraction also contain phlogopite and apatite. All of these peridotites have light REE/heavy REE abundance ratios greater than chondrites and many have abundances of K, Rb, Sr, Ba and light REE which exceed estimates for primitive mantle. On the basis of mineral textures and compositions we infer that the clinopyroxene, amphibole, phlogopite and apatite equilibrated with a metasomatic fluid. Isotopic (Sr and Nd) ratios and parent-daughter abundance data for the coarse-grained peridotites constrain the age of the metasomatism to be less than 200 million years. Associated amphibole pyroxenite inclusions are not metasomatized; these inclusions probably formed as crystal segregates from an alkalic magma. Both pyroxenites and coarse-grained peridotites are isotopically similar to basalts from Nunivak Island. Using these data, we propose a model in which the metasomatized peridotites were wallrocks located adjacent to the pyroxenites, and that metasomatism of these peridotites was caused by the infiltration of a residual silicate melt or volatile-rich fluid derived from the parental magma of the pyroxenites; i.e. the metasomatism was a consequence of basaltic magmatism. Furthermore, the parental magma of the pyroxenites was probably petrogenetically related to the Nunivak volcanism. REE modelling of fluids in equilibrium with clinopyroxenes from the coarse-grained peridotites is consistent with this model.