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