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<p>Oceanic arc and back-arc magmas are typically sourced from mantle metasomatized by subducted slab fluids/melts, but the role of the overriding plate is often ignored. Here we report the oxygen isotopic compositions of olivines in basalts erupted during arc, arc rifting, and back-arc spreading stages in the Mariana convergent margin. Rifting-stage olivines have high Forsterite (Fo = 85–90) and mantle-like δ<sup>18</sup>O (4.92 to 5.69‰). The δ<sup>18</sup>O values of olivines in arc and back-arc basin lavas correlate with their Fo values; high-Fo (&gt;83) olivines have mantle-like δ<sup>18</sup>O while those in low-Fo olivines are as low as 3.87 ± 0.30‰. This indicates that fractionating magmas assimilated low-δ<sup>18</sup>O oceanic crust in the hydrothermally-altered upper plate, rather than the injections of underlying subducted slab-derived melts. Considering the globally widespread distribution of low-δ<sup>18</sup>O evolved olivine, we propose that low-δ<sup>18</sup>O wall rock may be common in the shallow lithosphere and serves as an important contributor to erupted evolved magma.</p>

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Assimilation of upper plate rocks at convergent margins contributes to the low δ18O isotopic signature of erupted magma

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年7月2日

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  • Xiaohui Li,
  • Zixuan Zhang,
  • Sanzhong Li,
  • Robert J. Stern,
  • Osamu Ishizuka,
  • Huixin Yang,
  • Yanhui Suo,
  • Long Chen,
  • Zhiqing Lai,
  • Yuan Zhong,
  • Jun Hu,
  • Liangliang Wang,
  • Liming Dai,
  • Quanshu Yan,
  • Qiuli Li

摘要

Oceanic arc and back-arc magmas are typically sourced from mantle metasomatized by subducted slab fluids/melts, but the role of the overriding plate is often ignored. Here we report the oxygen isotopic compositions of olivines in basalts erupted during arc, arc rifting, and back-arc spreading stages in the Mariana convergent margin. Rifting-stage olivines have high Forsterite (Fo = 85–90) and mantle-like δ18O (4.92 to 5.69‰). The δ18O values of olivines in arc and back-arc basin lavas correlate with their Fo values; high-Fo (>83) olivines have mantle-like δ18O while those in low-Fo olivines are as low as 3.87 ± 0.30‰. This indicates that fractionating magmas assimilated low-δ18O oceanic crust in the hydrothermally-altered upper plate, rather than the injections of underlying subducted slab-derived melts. Considering the globally widespread distribution of low-δ18O evolved olivine, we propose that low-δ18O wall rock may be common in the shallow lithosphere and serves as an important contributor to erupted evolved magma.