14:15 〜 14:30
[SGC33-03] 深部マントル由来火山岩のクロムーチタン安定同位体分析
キーワード：クロム、チタン、コマチアイト、巨大火成岩岩石区、海洋島玄武岩、深部マントル
Introduction: Recent studies have shown that the primordial signatures (e.g., those characterized by nebular 20Ne/22Ne and D/H ratio; [1, 2]) of the Earth-forming materials are preserved in the deep mantle and that they are chemically and isotopically distinct from those of the ambient upper mantle. However, it is still unclear exactly which type of planetary materials those primordial signatures correspond to, and when those materials were accreted to proto-Earth. To address these issues, we use the nucleosynthetic 54Cr and 50Ti anomalies, which provide information about the genetic relationship between the planetary materials especially when the two systems are combined [3]. In addition, we use the stable 53Cr, a decay product of a short-lived radionuclide 53Mn, which provides chronological information about the first ~20 Ma of solar system history. Attention is focused on the basalts from the Ontong Java plateau (OJP) and Samoan ocean islands (OIBs), and komatiites from the Singhbhum Craton, because these are suggested to record the Archean to modern plume-dominated environments [4-6].
Results & Discussion: We prepared OJP drill core samples provided by the Ocean Drilling Program (ODP Leg 192), basalts from the Samoan Island of Tutuila, and komatiites from the Singhbhum Craton, Eastern India. The chemical separation and purification of Cr and Ti from the samples were made following the procedure described by [7]. The recovery rates of both Cr and Ti were 90–100% for all samples. We have measured Cr and Ti stable isotope compositions of the purified samples using Thermo Fisher Scientific TRITON Plus TIMS and Neptune Plus MC-ICP-MS, respectively. The Ti isotope analyses yielded no resolvable 46, 48, 50Ti excesses or deficits for all samples. On the other hand, the Cr isotope analyses yielded ε53Cr = 0.02 ± 0.10 ~ 0.12 ± 0.07 (2σ), ε54Cr = 0.10 ± 0.12 ~ 0.25 ± 0.07 (2σ) for the OJP basalts, ε53Cr = 0.01 ± 0.02 ~ 0.03 ± 0.09, ε54Cr = –0.02 ± 0.08 ~ 0.16 ± 0.16 for the Samoan basalts, ε53Cr = –0.04 ± 0.08 ~ 0.13 ± 0.10, ε54Cr = 0.02 ± 0.07 ~ 0.21 ± 0.05 for the Singhbhum komatiites. The resolved 53Cr and 54Cr excesses in some sources potentially reflect the survival of carbonaceous chondrite-like (characterized by 53Cr and 54Cr excesses) materials from mixing by mantle convection over geological history. However, the exponential-law corrected ε53Cr and ε54Cr of respective samples are slightly correlated with a slope of ~1–4.8. Similarly, we found the residual correlation between ε53Cr and ε54Cr for the Cr standards calculated relative to the mean of all 463 measurements over a period of two years, yielding a slope of ~1.3 (R ~ 0.84). Although the slope ~1.3 is significantly shallower than that of the residual linear mass fractionation correlation line (slope ~2) in [8-10], it is indistinguishable from the slope obtained recently for the terrestrial basalts and peridotites by [11]. We are in the process of exploring the reason for the positive correlation observed between 53Cr and 54Cr.
[1] Williams and Mukhopadhyay (2019) Nature 565 78–81, [2] Hallis et al. (2015) Science 350 795–797, [3] Warren (2011) EPSL 311 93–100, [4] Rizo et al. (2016) Science 352 809–812, [5] Jackson et al. (2014) Nature 514 355–358, [6] Chaudhuri et al. (2017) Precambrian Res. 298 385–402, [7] Hibiya et al. (2019) GGR 43 133–145, [8] Lugmair and Shukolyukov (1998) GCA 62 2863–2886, [9] Trinquier et al. (2006) EPSL 241 780–788, [10] Qin et al. (2010) GCA 741122–1145, [11] Mougel et al. (2018) EPSL 481 1–8.
Results & Discussion: We prepared OJP drill core samples provided by the Ocean Drilling Program (ODP Leg 192), basalts from the Samoan Island of Tutuila, and komatiites from the Singhbhum Craton, Eastern India. The chemical separation and purification of Cr and Ti from the samples were made following the procedure described by [7]. The recovery rates of both Cr and Ti were 90–100% for all samples. We have measured Cr and Ti stable isotope compositions of the purified samples using Thermo Fisher Scientific TRITON Plus TIMS and Neptune Plus MC-ICP-MS, respectively. The Ti isotope analyses yielded no resolvable 46, 48, 50Ti excesses or deficits for all samples. On the other hand, the Cr isotope analyses yielded ε53Cr = 0.02 ± 0.10 ~ 0.12 ± 0.07 (2σ), ε54Cr = 0.10 ± 0.12 ~ 0.25 ± 0.07 (2σ) for the OJP basalts, ε53Cr = 0.01 ± 0.02 ~ 0.03 ± 0.09, ε54Cr = –0.02 ± 0.08 ~ 0.16 ± 0.16 for the Samoan basalts, ε53Cr = –0.04 ± 0.08 ~ 0.13 ± 0.10, ε54Cr = 0.02 ± 0.07 ~ 0.21 ± 0.05 for the Singhbhum komatiites. The resolved 53Cr and 54Cr excesses in some sources potentially reflect the survival of carbonaceous chondrite-like (characterized by 53Cr and 54Cr excesses) materials from mixing by mantle convection over geological history. However, the exponential-law corrected ε53Cr and ε54Cr of respective samples are slightly correlated with a slope of ~1–4.8. Similarly, we found the residual correlation between ε53Cr and ε54Cr for the Cr standards calculated relative to the mean of all 463 measurements over a period of two years, yielding a slope of ~1.3 (R ~ 0.84). Although the slope ~1.3 is significantly shallower than that of the residual linear mass fractionation correlation line (slope ~2) in [8-10], it is indistinguishable from the slope obtained recently for the terrestrial basalts and peridotites by [11]. We are in the process of exploring the reason for the positive correlation observed between 53Cr and 54Cr.
[1] Williams and Mukhopadhyay (2019) Nature 565 78–81, [2] Hallis et al. (2015) Science 350 795–797, [3] Warren (2011) EPSL 311 93–100, [4] Rizo et al. (2016) Science 352 809–812, [5] Jackson et al. (2014) Nature 514 355–358, [6] Chaudhuri et al. (2017) Precambrian Res. 298 385–402, [7] Hibiya et al. (2019) GGR 43 133–145, [8] Lugmair and Shukolyukov (1998) GCA 62 2863–2886, [9] Trinquier et al. (2006) EPSL 241 780–788, [10] Qin et al. (2010) GCA 741122–1145, [11] Mougel et al. (2018) EPSL 481 1–8.