09:15 〜 09:30
[BCG06-02] Hydrogen isotope compositions in early earth mantle from apatite inclusions in detrital early Archean zircons from the Jack Hills metaconglomerate, W. Australis
キーワード：水の起源、水素同位体比、アパタイト、砕屑性ジルコン
The origin of Earth’s water remains debated. The hydrogen isotope distribution in earth mantle could be attributed for the origin of Earth’s water. Several scenarios have been proposed, such as delivery from carbonaceous/enstatite chondrites and comets, and capture of solar nebula (e.g. Piani et al., 2020; Young et al., 2023). The present deep earth mantle has been predicted to be D-poor signature (Hallis et al., 2015). Hydrogen isotope compositions are the key to understanding the origin of Earth's water. But their values have not yet been determined, since the lack of terrestrial samples for hydrogen isotope analyses prior to 4 billion years ago limits the means to decipher the origin of Earth’s water. In order to constrain the hydrogen isotope distribution in the early Earth, we have measured hydrogen isotopic compositions (δD; deuterium/hydrogen (D/H) ratio) of minute apatite inclusions in detrital zircons from the Jack hills metaconglomerate, which yields zircon grains with U-Pb ages ranging 3.0 to 4.4 Ga (Wild et al., 2001). Secondary ion mass spectrometry (SIMS) was applied for hydrogen isotope analyses, and laser ablation microprobe-inductively coupled plasma mass spectrometer (LA-ICPMS) was for trace element analysis and U-Pb age analysis, respectively. We evaluated the micro-cracks in zircon to exclude the negative possibility of secondary hydrogen interaction of apatite inclusions using polarized microscopy and cathodoluminescence (CL) images. The hydrogen isotopic compositions for all apatite inclusions (n = 16) were determined with δDSMOW falling within the range from -52 to -354 ‰. The δD values of apatite inclusions with visible cracks or healed cracks observed with CL were likely to be D-rich, on the other hands, those of apatite inclusions with no visible cracks were D-poor, ranged from -130 to a maximum of -354 ‰. This indicates that the hydrogen isotope ratio of apatite is shifted toward D-rich hydrogen isotopic compositions due to the presence of cracks on host zircons, probably from interaction with terrestrial fluid. As a surprising result, the apatite inclusions in the two zircons showed D-poor values beyond the analytical error, the δD = -222 ± 2‰ and -354 ± 23‰, respectively. The Sc/Yb versus Nb/Yb distribution patterns from detrital zircons were used to distinguish those crystals having magma types of arc, mid-ocean ridge and ocean island origins (Grimes, 2015). On diagrams of Sc/Yb versus Nb/Yb, most zircon analyzed were plotted on arc type, but these two grains plotted on mantle type (mid-ocean ridge or ocean island). Therefore, these two apatite inclusions considered to represent hydrogen isotope compositions of the mantle showed δD = -222±2‰ with 3.9 Ga and -354 ± 23‰ with 3.6 Ga. These results indicate that the hydrogen isotopic compositions of mantle in the early Archean were D-poor. Similar depleted (D-poor) hydrogen isotope composition is reported from melt inclusions of 3.3 Ga komatiites (Sobolev et al., 2019) but our data resulted in tracking back as far as 3.9 Ga. We conclude that pristine apatite inclusions in detrital zircons could retain evidence of ancient δD value, and may provide new constraints on the origin of Earth’s water.
Grimes, C. B., Wooden, J. L., Cheadle, M. J. & John, B. E. “Fingerprinting” tectono-magmatic provenance using trace elements in igneous zircon. Contrib Mineral Petr 170, 46 (2015).
Hallis, L. J. et al. Evidence for primordial water in Earth’s deep mantle. Science 350, 795–797 (2015).
Piani, L. et al. Earth’s water may have been inherited from material similar to enstatite chondrite meteorites. Science 1110–1113 (2020) doi:10.1126/science.aba1948.
Sobolev, A. V. et al. Deep hydrous mantle reservoir provides evidence for crustal recycling before 3.3 billion years ago. Nature 571, 555–559 (2019).
Wilde, S. A., Valley, J. W., Peck, W. H. & Graham, C. M. Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409, 175–178 (2001).
Young, E. D., Shahar, A. & Schlichting, H. E. Earth shaped by primordial H2 atmospheres. Nature 616, 306–311 (2023).
Grimes, C. B., Wooden, J. L., Cheadle, M. J. & John, B. E. “Fingerprinting” tectono-magmatic provenance using trace elements in igneous zircon. Contrib Mineral Petr 170, 46 (2015).
Hallis, L. J. et al. Evidence for primordial water in Earth’s deep mantle. Science 350, 795–797 (2015).
Piani, L. et al. Earth’s water may have been inherited from material similar to enstatite chondrite meteorites. Science 1110–1113 (2020) doi:10.1126/science.aba1948.
Sobolev, A. V. et al. Deep hydrous mantle reservoir provides evidence for crustal recycling before 3.3 billion years ago. Nature 571, 555–559 (2019).
Wilde, S. A., Valley, J. W., Peck, W. H. & Graham, C. M. Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409, 175–178 (2001).
Young, E. D., Shahar, A. & Schlichting, H. E. Earth shaped by primordial H2 atmospheres. Nature 616, 306–311 (2023).