Source lithology and crustal assimilation recorded in low δ18O olivine from Okinawa Trough, back-arc basin

Abstract

Low δ18O magmas (with δ18O values of <5‰) are significantly volumetrically smaller than the abundant high δ18O magmas in diverse tectonic settings. We report, for the first time, low δ18O olivines within the Okinawa Trough, a nascent epicontinental back-arc basin. An integrated element and oxygen isotope study of olivines gives direct evidence for the source lithologies and the magmatic evolution of volcanic rocks in the Okinawa Trough. In general, olivines show lower Ni contents (149–1644 ppm) and Fe/Mn ratios (44–76) but higher Mn contents (1720–3339 ppm) and Mn/Zn ratios (13–19) than olivines crystallized from partial melts of pyroxenite, suggesting that the volcanic rocks were derived from a peridotite source. Olivines from middle Okinawa Trough basalts have homogeneous δ18O values of 5.25 ± 0.55‰ (2SD), which are similar to the primitive mantle values (4.8–5.5‰). Olivines from middle Okinawa Trough andesites can be divided into two groups. The first group exhibits Fo >78, with element compositions and δ18O values (4.87 ± 0.38‰, 2SD) to similar those of the olivine in basalts. The other group of olivines exhibits Fo <78 and lower δ18O values (4.13 ± 0.45‰, 2SD). These features imply that high Fo olivines may be derived from the same magma chamber as the basalts, while low Fo olivines are generated by some shallow magmatic process. The olivines in basaltic andesites from the southern Okinawa Trough have δ18O values of 4.13 ± 0.42‰ (2SD), all of which are lower than those of the primitive mantle. We suggest that the olivines in basalts were crystalized from an unaltered deep magma source, whereas the low δ18O olivines in andesites and basaltic andesites originated from assimilated high-temperature hydrothermally altered low δ18O continental crust rather than from altered lower oceanic crust. Moreover, all the whole rocks in the Okinawa Trough have high δ18O values (> 6‰), indicating that later magmatic evolution of low δ18O magma toward high δ18O values was probably a result of the leaching of high δ18O fluid from the upper continental crust. Our study of olivines provide vital information for systematic understanding of magmatism in epicontinental back-arc basins compared to that in intra-oceanic settings.