Complex mineral textures and chemistries in boninites from IODP Expedition 352

Author(s): Hill, Stephen; Scholpp, Jesse; Ryan, Jeffrey; Luna, Antonio
Author Affiliation(s): Primary:
University of South Florida, School of Geosciences, Tampa, FL, United States
Volume Title: Geological Society of America, Rocky Mountain Section, 70th annual meeting; Geological Society of America, Cordilleran Section, 114th annual meeting
Source: Abstracts with Programs - Geological Society of America, 50(5); Geological Society of America, Rocky Mountain Section, 70th annual meeting; Geological Society of America, Cordilleran Section, 114th annual meeting, Flagstaff, AZ, May 15-17, 2018. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0016-7592 CODEN: GAAPBC
Note: In English
Summary: 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.
Year of Publication: 2018
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
Key Words: 05 Petrology, Igneous and Metamorphic; Andesites; Boninite; Chemical composition; Expedition 352; IODP Site U1439; IODP Site U1442; Igneous rocks; International Ocean Discovery Program; Mineral composition; North Pacific; Northwest Pacific; Pacific Ocean; Textures; Volcanic rocks; West Pacific
Coordinates: N282427 N282435 E1424633 E1423720
Record ID: 2018083560
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States