[S-IT16] 惑星中心核：内部構造・形成・進化

[SIT16-P04] 海洋島玄武岩におけるタングステン同位体異常の地球化学的制約

*宮嶋 郁佳¹、William McDonough² (1.東北大学、2.東北大学ニュートリノ科学研究センター)

キーワード：タングステン同位体異常、コア-マントル相互作用

The tungsten isotopic composition of the Earth's core is (i.e., μ¹⁸²W value) lighter than that of bulk silicate Earth. However, some ocean island basalts (OIBs: e.g., Hawaii, Galapagos, Samoa) show negative μ¹⁸²W values and high ³He/⁴He ratios compared with the present atmospheric composition. Although these observations have commonly been explained by core-mantle interaction, we still lack clear geochemical evidence in support of this hypothesis. Here, I test for correlation between tungsten isotope composition and the concentration of trace elements in OIBs, including those with documented μ¹⁸²W anomalies, using published geochemical data accessible in the GEOROC database. Our test include statistical evaluations (r², p-values, testing of mixing arrays using models of plausible core and mantle compositional models). The results show no correlation between tungsten isotope composition and concentration of any trace elements, including the highly siderophile elements (HSE). These finding are consistent with previous studies. Core-mantle interaction may occurred by two processes, (1)core-materials added directly to an OIB source (e.g., Mundl et al., 2017), or (2)liquid-metal and liquid-silicate were equilibrated at core-mantle boundary (e.g., Mundl-Petermeier et al., 2020). The products of process 1 should result in increased concentration of HSE, tungsten, and molybdenum correlating with μ¹⁸²W. On the other hand, no element, except for phosphorus, seems to correlate with μ¹⁸²W. The results, no correlation found from OIBs with tungsten anomalies, prefers process 2 over process 1, but is inconsistent with the amount of phosphorus.