[SIT26-P07] オントンジャワ海台およびサモア海洋島玄武岩のクロム同位体から検証する核マントル相互作用
キーワード：クロム同位体、核マントル相互作用、巨大火成岩岩石区、海洋島玄武岩
Introduction: Basaltic rocks from large igneous provinces (LIPs) and from ocean islands (OIBs) are thought to be derived from the deep-rooted mantle upwelling. Recent studies have shown that some of these rocks, including OIBs from Samoa and Hawaii, are characterized by 182W deficits that are well resolved from the ambient upper mantle [1, 2]. This negative 182W signature has recently been proposed to originate from the outer core which has a μ182W of –220 [3-5]. If so, the 182W anomaly may be accompanied by deficits of 53Cr, a decay product of a short-lived radionuclide 53Mn, because Cr is more siderophile than Mn at high temperature [6, 7]. However, the Cr isotope variations of LIPs and OIBs are still poorly known. Here, we report the mass independent Cr isotopic data for basaltic rocks from the Ontong Java plateau (OJP) and Samoan OIBs, to further examine the core-mantle interaction that can cause the systematic isotopic variability between LIPs and OIBs.
Results & Discussion: We prepared OJP drill core samples provided by the Ocean Drilling Program and basalt samples from the Samoan Island of Tutuila. The chemical separation and purification of Cr from the samples were made following the procedure described by [8]. The recoveries of Cr were 90-100% for all samples. We have measured Cr stable isotope compositions of the samples processed through the separation scheme by thermal ionization mass spectrometry (TIMS). The external reproducibilities achieved here were ± 5 ppm and ± 11 ppm for 53Cr and 54Cr, which are markedly smaller than isotope variation among planetary materials (53Cr ~1ε; 54Cr ~2ε). The Cr isotope analyses so far yielded ε53Cr = 0.03 ± 0.05, ε54Cr = 0.02 ± 0.10 (2SE) for the OJP basalts, and ε53Cr = 0.03 ± 0.05, ε54Cr = –0.03 ± 0.11 ~ 0.05 ± 0.11 for the Samoan basalts. The results for both the OJP and Samoan basalts reveal that there is no resolvable 53, 54Cr excess or deficit in their source. Our results suggest that there’s no systematical contribution to Cr isotope signatures in the source of OJP and Samoan islands from the material with the anomalous Cr isotope compositions. Therefore, the chemical interaction between the core and the LIPs and OIBs sources is not detectable in terms of the Cr isotopes. This implies that, unlike W, the difference in the Cr concentrations of the core and the mantle might not be so large, or that Cr in the core does not necessarily diffuse in the mantle.
[1] Mundl et al. (2017) Science, 356, 66-69, [2] Willbold et al. (2011) Nature, 477, 195-199, [3] Rizo et al. (2019) GRL, 11, 6-11, [4] Mundl et al. (2017) Science, 356, 66-69. [5] Kleine et al. (2009) GCA, 73, 5150-5188, [6] McDonough et al. (2003) ToG2, 568, 559-575, [7] Chabot and Agee (2003) GCA, 67, 2077–2091, [8] Hibiya et al. (2019) GGR, 43, 133-145.
Results & Discussion: We prepared OJP drill core samples provided by the Ocean Drilling Program and basalt samples from the Samoan Island of Tutuila. The chemical separation and purification of Cr from the samples were made following the procedure described by [8]. The recoveries of Cr were 90-100% for all samples. We have measured Cr stable isotope compositions of the samples processed through the separation scheme by thermal ionization mass spectrometry (TIMS). The external reproducibilities achieved here were ± 5 ppm and ± 11 ppm for 53Cr and 54Cr, which are markedly smaller than isotope variation among planetary materials (53Cr ~1ε; 54Cr ~2ε). The Cr isotope analyses so far yielded ε53Cr = 0.03 ± 0.05, ε54Cr = 0.02 ± 0.10 (2SE) for the OJP basalts, and ε53Cr = 0.03 ± 0.05, ε54Cr = –0.03 ± 0.11 ~ 0.05 ± 0.11 for the Samoan basalts. The results for both the OJP and Samoan basalts reveal that there is no resolvable 53, 54Cr excess or deficit in their source. Our results suggest that there’s no systematical contribution to Cr isotope signatures in the source of OJP and Samoan islands from the material with the anomalous Cr isotope compositions. Therefore, the chemical interaction between the core and the LIPs and OIBs sources is not detectable in terms of the Cr isotopes. This implies that, unlike W, the difference in the Cr concentrations of the core and the mantle might not be so large, or that Cr in the core does not necessarily diffuse in the mantle.
[1] Mundl et al. (2017) Science, 356, 66-69, [2] Willbold et al. (2011) Nature, 477, 195-199, [3] Rizo et al. (2019) GRL, 11, 6-11, [4] Mundl et al. (2017) Science, 356, 66-69. [5] Kleine et al. (2009) GCA, 73, 5150-5188, [6] McDonough et al. (2003) ToG2, 568, 559-575, [7] Chabot and Agee (2003) GCA, 67, 2077–2091, [8] Hibiya et al. (2019) GGR, 43, 133-145.