Polarizing Microscope Discussion.

Introduction The volcanic sections exposed on the fore arcs of the islands of Bonin in the Izu-Bonin forearc and the data recovered from the International Ocean Discovery Program Expedition 352 drill sites have been used to study the petrogenesis of early stage magmatism in the Izu–Bonin–Mariana arc. The expedition revealed more information on the sedimentology, biostratigraphy, fluid geochemistry, petrology, rock geochemistry, downhole logs, structural geology, physical properties, and paleomagnetism of drill sites U1439, U1440, U1441, and U1442 (Whattam et al., 2020). The mineral chemistry and petrographic results of the four sites were also documented. Low-calcium pyroxine (enstatite and clinoenstatite) + olivine in a groundmass of later-formed acicular crystals of clinoenstatite, pigeonite, and highcalcium pyroxine or crystalline matrix are the most prevalent phenocryst assemblages observed. Some enstatite crystals are observed surrounding small olivine grains; some high-silicon boninite samples lack olivine, consisting exclusively of low-calcium pyroxenes and groundmass clinopyroxene in a crystalline matrix. The major crystallizing phases in highly differentiated samples composed of high-magnesium andesites and dacites include augitic clinopyroxene and plagioclase, with uncommon grains of ilmenite and apatite. At lower pressures, partial resorption associated with the formation of enstatite and clinoenstatite caused the olivine grains to become rounded and embayed. Red to purple chromium-rich spinel, often found encased within larger olivine and enstatite and clinoenstatite crystals, as well as free grains within the sample matrix is commonly observed as an accessory mineral.Mineral chemistry revealed chromium-spinel grains containing 8-10 wt% aluminium oxide (Al2O3), 11-13 wt% magnesium oxide (MgO), 18-24 wt% ferous oxide (FeO) and 5660% wt% Chromium (III) oxide (Cr2O3) (Scholpp et al., 2020). Background Boninite is a rock type that is frequently related to a given present-day tectonic environment but had been formed in more than one tectonic setting with several potential modes of origin, and thus it is hypothesized to be linked to the formation of the geology of early Earth (Pearce & Reagan, 2017). These refractory volcanic rocks are almost exclusively found in fore-arc environments, such as in primitive island arcs and ophiolite complexes that are assumed to represent past fore-arc settings or at least in nascent subduction zones that are most typically associated with embryonic arc volcanism occurring after intraoceanic subduction (Shervais et al., 2021; Pearce & Reagan, 2017). Therefore, the presence of boninites in terranes has been suggested as key evidence that subduction operated in a certain geologic location (Pearce & Reagan, 2017). The International Ocean Discovery Program Expedition 352 documented the Izu-Bonin-Mariana (IBM) system as it is the type locality for studying oceanic crustal accretion immediately following subduction initiation. The IBM arc system formed as a result of subduction of the western Pacific plate hence its location suggests a prospective rich source of boninites (Pearce & Reagan, 2017). The IBM subduction zone was thought to have formed as part of a hemispheric-scale foundering of old, dense lithosphere in the western Pacific (Bloomer et al., 1995). The onset of largescale lithospheric subsidence is thought to have occurred before the IBM fore arc’s igneous basement age of 51–52 Ma. The succession of initial magmatic products has been consistent throughout the fore arc, implying a significant outburst of asthenospheric upwelling and melting coupled with magmatism and seafloor spreading over a zone hundreds of kilometers wide and thousands of kilometers long. This phase occurred 52 million years ago, and was followed by a period of shallow hydrous melting lasting 44–45 million years ago, according to comprehensive geochronology for IBM fore-arc rocks (Ishizuka et al., 2006). The Expedition 352 chose the Bonin fore arc as a drill site because it was in the same location as Chichijima, the type locality for the important boninite rock type. The drill sites are part of a complete ophiolite section which also had been sampled by dredging and diving, and has been researched with full site survey data (Ishizuka et al., 2011). However, during the expedition, the expected stratigraphy was not observed. There was no fore-arc basalt (FAB) at the base of the Bonin fore-arc volcanic succession, and no boninite-series lavas were present atop these FAB. Dikes at the base of the sections at sites U1439 and U1440 were present instead. This indicates that these lavas are underlain by their own conduit systems, and that the FAB and boninite group lavas had most likely offset horizontally to the west rather than vertically down the drill site. This inconsistency might be due to the seafloor spreading associated with subduction initiation migrating from east to west after subduction initiation and during early arc development (Reagan et al., 2015). Considering that the fore-arc stretches far and wide beyond the expedition site, boninites may be sourced in other horizontal adjacent locations in the arc. References Bloomer, S.H., Taylor, B., MacLeod, C.J., Stern, R.J., Fryer, P., Hawkins, J.W., and Johnson, L. (1995). Early arc volcanism and the ophiolite problem: a perspective from drilling in the western Pacific. In Taylor, B., and Natland, J. (Eds.), Active Margins and Marginal Basins of the Western Pacific. Geophysical Monograph, 88:1–30. Ishizuka, O., Kimura, J.-I., Li, Y.B., Stern, R.J., Reagan, M.K., Taylor, R.N., Ohara, Y., Bloomer, S.H., Ishii, T., Hargrove, U.S., III, and Haraguchi, S. (2006). Early stages in the evolution of Izu-Bonin arc volcanism: new age, chemical, and isotopic constraints. Earth and Planetary Science Letters, 250(1–2):385– 401. Ishizuka, O., Tani, K., Reagan, M.K., Kanayama, K., Umino, S., Harigane, Y., Sakamoto, I., Miyajima, Y., Yuasa, M., and Dunkley, D.J., 2011. 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