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[BCG05-P12] Titanium and Chromium isotope analyses of spherules in the Barberton greenstone belt
Keywords:Archean, Barberton greenstone belt, chromium, titanium, spherule layer, accretionary lapilli
Introduction: The earliest record of major impact events on Earth may be the eight sedimentary layers (S1~S8) of the early Archean (3.47~3.24 Ga; [1]) spherule beds that have been identified in the Barberton greenstone belt of South Africa. However, the area has undergone extensive metamorphism and alteration, making it difficult to constrain the nature of the original projectile. Some studies have even questioned the meteorite impact origin of the spherules (e.g., [2]) due to the petrological similarity of silicate droplets formed by volcanic eruptions (accretionary lapilli) and by meteorite impacts. To address these issues, here we use the nucleosynthetic 50Ti and 54Cr anomalies, which provide information about the genetic relationship between the planetary materials [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. We have applied this method to several spherical particles extracted from sandstones and accretionary lapillistone of the S3 layer, and from conglomerate of the S2 layer.
Results & Discussion: About 15-45 mg of the spherules were extracted from a few centimeters square of rocks using a diamond drill equipped with a micro-mill at the University of Tokyo. Chemical separation and purification of Ti and Cr from the samples were made following a sequential separation procedure [4]. The Ti and Cr isotope ratios were determined using Neptune Plus MC-ICP-MS at the University of Tokyo and Thermo Fisher Scientific TRITON Plus TIMS at Japan Agency for Marine-Earth Science Technology, 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 = –0.06 ± 0.09 ~ 0.14 ± 0.12 (2σ), ε54Cr = 0.61± 0.13 ~ 0.77 ± 0.20 (2σ) for the spherules from S3 sandstone, and ε53Cr = –0.09 ± 0.11, ε54Cr = 0.00 ± 0.18 for the spherules from S3 accretionary lapillistone. The Ti and Cr isotope analyses of S2 samples are now in progress.
The absence of anomalies in both Ti and Cr isotopes in the S3 accretionary lapillistone spherules strongly reflects that they are solely derived from volcanic eruptions. On the other hand, S3 spherules show different results for Ti and Cr isotopes, which may reflect differences in carrier minerals; for S3 sandstone spherules, the main carrier minerals for Ti are rutile and anatase formed during metamorphic and alteration processes, while for Cr, the carrier mineral is almost exclusively Ni-rich spinel, which is relatively unaffected by metamorphism and alteration. Considering that this spinel shows a distinctly different compositional range in the NiO-MgO plot from the spinel of the Barberton komatiite [5], it is strongly suggested that the Ti isotopes of the S3 sandstone spherules do not reflect the composition of the projectile, but the Cr isotopes preserve the original composition of the projectile to some extent. Therefore, the resolved 53Cr and 54Cr excesses in all the S3 sandstone spherules indicate the carbonaceous chondrite-like projectile (characterized by 53Cr and 54Cr excesses). The 54Cr vs 1/Cr plot suggests that the composition of the S3 sandstone spherules is best explained by a contribution of about 85% CM-CV-CO chondrites. This suggests that in the early solar system (~3.24 Ga), these chondrites that accreted in the outer part of the system may have been transported to the early Earth in the inner solar system.
[1] Byerly et al. (2002) Science, 297, 1325–1327, [2] Hofman et al. (2006) Special Paper of the Geological Society of America, 405, 33–56., [3] Warren (2011) EPSL, 311, 93–100, [4] Hibiya et al. (2019)GGR, 43, 133–145, [5] Maehara (2020) thesis at the YNU.
Results & Discussion: About 15-45 mg of the spherules were extracted from a few centimeters square of rocks using a diamond drill equipped with a micro-mill at the University of Tokyo. Chemical separation and purification of Ti and Cr from the samples were made following a sequential separation procedure [4]. The Ti and Cr isotope ratios were determined using Neptune Plus MC-ICP-MS at the University of Tokyo and Thermo Fisher Scientific TRITON Plus TIMS at Japan Agency for Marine-Earth Science Technology, 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 = –0.06 ± 0.09 ~ 0.14 ± 0.12 (2σ), ε54Cr = 0.61± 0.13 ~ 0.77 ± 0.20 (2σ) for the spherules from S3 sandstone, and ε53Cr = –0.09 ± 0.11, ε54Cr = 0.00 ± 0.18 for the spherules from S3 accretionary lapillistone. The Ti and Cr isotope analyses of S2 samples are now in progress.
The absence of anomalies in both Ti and Cr isotopes in the S3 accretionary lapillistone spherules strongly reflects that they are solely derived from volcanic eruptions. On the other hand, S3 spherules show different results for Ti and Cr isotopes, which may reflect differences in carrier minerals; for S3 sandstone spherules, the main carrier minerals for Ti are rutile and anatase formed during metamorphic and alteration processes, while for Cr, the carrier mineral is almost exclusively Ni-rich spinel, which is relatively unaffected by metamorphism and alteration. Considering that this spinel shows a distinctly different compositional range in the NiO-MgO plot from the spinel of the Barberton komatiite [5], it is strongly suggested that the Ti isotopes of the S3 sandstone spherules do not reflect the composition of the projectile, but the Cr isotopes preserve the original composition of the projectile to some extent. Therefore, the resolved 53Cr and 54Cr excesses in all the S3 sandstone spherules indicate the carbonaceous chondrite-like projectile (characterized by 53Cr and 54Cr excesses). The 54Cr vs 1/Cr plot suggests that the composition of the S3 sandstone spherules is best explained by a contribution of about 85% CM-CV-CO chondrites. This suggests that in the early solar system (~3.24 Ga), these chondrites that accreted in the outer part of the system may have been transported to the early Earth in the inner solar system.
[1] Byerly et al. (2002) Science, 297, 1325–1327, [2] Hofman et al. (2006) Special Paper of the Geological Society of America, 405, 33–56., [3] Warren (2011) EPSL, 311, 93–100, [4] Hibiya et al. (2019)GGR, 43, 133–145, [5] Maehara (2020) thesis at the YNU.