COMPLEX MINERAL TEXTURES AND CHEMISTRIES IN BONINITES FROM IODP EXPEDITION 352

Boninites recovered at Sites U1439 & U1442 during IODP Expedition 352 to the Izu-Bonin forearc have been categorized by their SiO₂ contents (Reagan et al 2017). The recovered boninite samples suggest considerable complexity in their evolution when looking in detail. Previous work on low-silica boninite samples from Unit 6 in Hole U1439C (sections 15R and 25R) uncovered evidence for an open-system processes, indicated by augitic clinopyroxene overgrowths on enstatites, and fayalitic rims on olivines (Scholpp et al 2017). Additional samples from Hole U1439C have been examined petrographically and analyzed for mineral chemistries using the EPMA (JEOL 8900R Superprobe) at Florida International University in Miami, Florida.

Common phenocryst assemblages encountered included embayed texture olivines ranging from Fo⁸⁸ - Fo⁹¹ and orthopyroxene from En⁸⁰ - En⁸⁵. Frequently found enclosed within olivine phenocrysts were Cr-Al rich spinels (50-58 wt.% Cr₂O₃; 8-10 wt.% Al₂O₃). Spinel crystallization pressures estimated using the olivine-spinel barometer of Ballhaus and Berry (1990) ranged from 1.4-2.2 GPa. Groundmass crystals were clinopyroxenes and (in uncommon evolved samples) plagioclase.

Two additional samples, U1439C-19R2 (Unit 6) and U1439C-2R3 (a high-silca boninite from Unit 2A) preserve evidence in the form of heavily embayed olivines, rims and overgrowths on olivine and orthopyroxene phenocrysts. Rim compositions on the orthopyroxenes shift drastically toward clinopyroxene (42-45 Wo, up to 20 wt.% CaO), with associated increases in Al contents (6-12 wt.% Al₂O₃). Such abrupt changes in crystallizing mineral compositions are not possible during normal fractional crystallization processes, leading us to suggest that mixing with a more Ca-Al rich melt was part of the evolution of these boninites.