Multiple Sources In Ultrapotassic Arc Magmas Inferred From Combined Microthermometry And In-Situ Chemical Analysis Of Melt Inclusions In Olivine And Clinopyroxene Phenocrysts From Batu Tara Volcano, Indonesia

Batu Tara is an active, leucite-normative, ultrapotassic volcano located approximately 75 km behind the main magmatic front of the eastern Sunda arc, Indonesia. Clinopyroxene, olivine, plagioclase, leucite, Ti-magnetite, minor apatite and occasional biotite occur as phenocrysts. The exact mechanism for genesis of ultrapotassic arc rocks remains elusive, particularly in island arc settings. Concise determination of primary magma chemistry is often impeded by complex histories of crystallization and magma mixing. Melt inclusions in olivine and pyroxene phenocrysts from Batu Tara have been investigated by high-temperature microthermometry, EPMA, and LA-ICP-MS to constrain parent magma composition and shed light on the genesis of ultrapotassic magmas in island arc settings. Variation in homogenization temperature, major and trace element composition, and volatile content correlate with petrographic observation of three distinct populations of olivine phenocrysts which can be distinguished by the presence or absence of apatite mineral inclusions and CO2 fluid inclusions. Clinopyroxenes commonly exhibit pronounced chemical zonation with individual zones preserving melt inclusions of unique composition, indicating that these phenocrysts were involved in complex mixing processes and repeatedly subject to end-member magma compositions. Melt inclusions preserve a record of; (1) Early spinel, olivine and clinopyroxene crystallization, followed by later crystallization of abundant apatite; (2) Exsolution of volatile species (S, CO2, Cl, H2O) during magma degassing; (3) Mixing of multiple magmas, originating from chemically and likely genetically distinct sources, and; (4) A significant but variable component of fluid addition to the magma source inferred from systematics of incompatible trace elements.