MINERAL CHEMISTRY EVIDENCE FOR MAGMA MIXING IN BONINITES, SITES U1439 AND U1442, IODP EXPEDITION 352

Shipboard classification of the boninites recovered from Holes U1439A and C and U1442 A during IODP Expedition 352 was based on bulk SiO₂ contents, with Low SiO₂ (52-55% wt) boninites comprising the bottom of both sections, overlain successively by a thin layer of Basaltic boninites (50-52% SiO₂), and then High SiO₂ boninites (SiO₂≥56%). We have conducted a petrographic study of a suite of thin sections encompassing the U1439 and U1442 sequences, focusing on mineral associations and mineral chemistries, which were measured via the JEOL 8900R electron microprobe system at the Florida Center for Analytical Electron Microscopy (FCAEM) at Florida International University.

Our examined boninite samples exhibit the typical olivine + clinoenstatite mineral assemblages for Low SiO₂ boninites and the enstatite-dominated assemblages for High SiO₂ boninites described in Reagan et al (2015). Clinopyroxenes are present as late-stage groundmass minerals, along with plagioclase in more evolved samples. Olivines range from Fo 86 to 88, while pyroxenes show enstatite-dominated compositions (En87 Fs11 Wo2) in the High SiO₂ boninites, ranging into pigeonite compositions in the Low SiO₂boninites and more differentiated rocks. Alteration minerals include zeolites and less abundant calcite, both as vesicle fills and veins.

In two of our samples (U1439C15R-1, U1439C25R-1), more complex mineral chemistries were encountered. These samples include early-crystallized olivines with embayed and reacted margins, some of which also show zoning (Fo 87 cores; Fo 77 rims), and large, euhedral pyroxene crystals with zoning that is prominent in electron backscatter imaging. Many of these zoned pyroxenes had clinoenstatite cores (En86 Fs12 Wo2), and augite rims (En43 Fs17 Wo40), while a subset appear to be anomalously rich in Al₂0₃ (cores ~4.0% wt: rims ~13.5% wt). As these sharp changes in mineral chemistry cannot occur during normal crystallization, this finding indicates mixing of the existing boninite magma with another (basaltic or highly evolved boninitic) melt to produce the observed zoning. The mixing of basaltic and boninitic melts, if confirmed, indicates a complex magmatic and tectonic environment at the initiation of the Izu-Bonin subduction system.