Major and trace element chemistry and D/H, 18O/16O, 87Sr/86Sr and 143Nd/144Nd ratios of rocks from the spreading center of the Okinawa Trough, a marginal back-arc basin.

Abstract

Basalts (OKTB) from the Okinawa Trough (OKT), which is a now opening marginal back-arc basin, are chemically quite similar to those from the intra-oceanic back-arc basins such as the Mariana Trough. Both olivine-normative and quartz-normative basalts occur, and major and trace element con centrations show a systematic variation, probably derived by differentiation of the magma. D/H, 87Sr/86Sr and 143Nd/'44Nd ratios are almost identical in the dacite and basalts , and the 180/160 ratio in the former is only slightly higher than in the latter, suggesting that the dacites are ultimately cognate with basalts. Concentrations of A1203, total iron, MgO, CaO, Na2O, Ni, Cr, and V in OKTB are within the range of variation in normal type mid-ocean ridge basalts (N-MORB), whereas OKTB have significantly higher K20, Rb and Sr contents and D / H, 110/160 and 87Sr / 86Sr ratios and a lower '43Nd/ 144Nd ratio than N -MORB have. These chemical and isotopic features of OKTB can be well ex plained by the concept, which has been proposed by many authors for magma genesis in the intra oceanic back-arc basins, that the H20-rich primary magma is generated from normal-type mantle peridotite modified by component from the subducted slab. Compared with those from intra-oceanic back-arc basins, the rocks from the OKT have distinctly higher 87Sr / B6Sr and lower 143Nd /'44Nd ratios, reflecting the higher contribution of continent-derived sedimentary components to the magma source. Oxygen, Sr and Nd isotopic characteristics of the rocks of the OKT are fitted well with paired two-com ponent mixing models: one for the source-mixing and another for the crustal contamination. INTRODUCTIONThe present study was designed to report geochemical aspects of rocks of the marginal back-arc basin, analyzing fresh samples from knolls and ridges of the central graben of the Okinawa Trough (OKT) collected by the submersible "SHINKAI 2000" (Figs. 1 and 2). The OKT is now opening in the south-eastern marginal part of the Asian continent behind the Ryukyu volcanic chain (the volcanic front of the Ryukyu arc) and the Ryukyu Trench (the subduc tion zone). Recent hydrothermal activity was detected by "SHINKAI 2000" in July, 1986, and K-Ar ages of the rocks from the OKT, including those from the knolls and ridges of the present study, are younger than 0.8 Ma (Shibata et al., 1984;Kimura et al., 1986). al. Kimura et al., 1986), which have chemical characteristics common to the arc-type volcanic rocks generated under an extensional en vironment (Kimura et al., 1986). It is well known that basalts from intra oceanic back-arc basins, such as the Mariana Trough (MTB), Lau Basin and Scotia Sea, are in many respects similar to normal-type mid oceanic ridge basalts (N-MORB) (e.g. Hawkins, 1976;Pineau et al., 1976;Hawkesworth et al., 1977), whereas significant differences also are noted between them in the contents of large ion lithophile elements (Hart et al., 1972;Hawkins and Melchior, 1985), water content-D/H rela tionship (Poreda, 1985) and 8'Sr/86Sr 143Nd / 144Nd 206p...

Influence of subduction components on magma composition in back‐arc basins: a comparison between the Mariana and Okinawa troughs

“…These results are similar to those of Honma et al (1991); Yu and Zhai (2004), and Huang et al (2006a, b). In the present case, the composition of the magma may have been altered by later geological processes, resulting in the compositions of individual samples deviating from the mixing line.…”

High‐potassium volcanic rocks from the Okinawa Trough: Implications for a cryptic potassium‐rich and DUPAL‐like source

“…Bathymetric and tectonic map of the middle Okinawa Trough (MOT), showing sampling locations of volcanic rocks. Rock data are from previous studies (Honma et al, ; Huang et al, ; Ishizuka et al, ; Kimura, ; Li et al, ; Ma et al, ; Shinjo et al, ; Shinjo & Kato, ; Zhai & Gan, ). Boundaries of three OT segments are according to Yan and Shi ().…”

“…Volcanic rocks in the OT can be used to understand the dynamic processes of the early stages of back‐arc spreading (Zhao, Zhai, Wang, & Yu, ). To date, many studies of OT volcanism have been conducted (e.g., Chung, Wang, Shinjo, Lee, & Chen, ; Honma et al, ; Ishizuka, Kawanobe, & Sakai, ; Z. Q. Lai et al, ; Li, Yang, & Wang, ; Liao, Huang, & Hu, ; Ma, Wang, & Jin, ; Meng, Du, Wu, & Long, ; Shinjo, Chung, Kato, & Kimura, ; Yu, Zhai, Guo, Zhou, & Zong, ; Zeng et al, ; Zhai & Gan, ), enhancing our understanding of the magmatic origin and evolution of the back‐arc basin. The OT volcanic rocks mostly fall within the low‐ to mid‐potassium series and exhibit bimodal distribution, in which basalts and rhyolites are dominant (Shinjo & Kato, ; Yan & Shi, ).…”

Light Stable Isotopic Compositions of Enriched Mantle Sources: Resolving the Dehydration Paradox

“…Metasomatism of the mantle wedge by these slab‐derived fluids results in heavy isotopic compositions characteristic of arc and back‐arc basin basalts (δD SMOW = −10 to −57 ‰) (Hochstaedter et al, ; Honma et al, ; Marty et al, ; Poreda, ; Shaw et al, ). As dehydration progresses and D is increasingly removed from the slab by a distillation process, both the released fluids and the slab itself should become isotopically lighter.…”