Japan Geoscience Union Meeting 2018

Presentation information

[EE] Oral

S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Tectonophysics

[S-IT22] Interaction and Coevolution of the Core and Mantle in the Earth and Planets

Wed. May 23, 2018 10:45 AM - 12:15 PM International Conference Room (IC) (2F International Conference Hall, Makuhari Messe)

convener:Tsuyoshi Iizuka(University of Tokyo), Hidetoshi Shibuya(Department of Earth and Environmental Sciences, Faculty of Advanced Science and Technology, Kumamoto University), Taku Tsuchiya(愛媛大学地球深部ダイナミクス研究センター, 共同), Kenji Ohta(Department of Earth and Planetary Sciences, Tokyo Institute of Technology), Chairperson:Sumino Hirochika, Tateno Shigehiko, Iizuka Tsuyoshi

11:00 AM - 11:15 AM

[SIT22-20] Hf-W chronology of the Brenham pallasite

*Yoshitaka Homma1, Tsuyoshi Iizuka1 (1.Department of Earth and Planetary Science, The University of Tokyo)

Keywords:Hf-W chronology, pallasite meteorites, core-mantle segregation

Pallasites are stony-iron meteorites consisting mainly of olivine and FeNi metal. Some relations with iron meteorites are pointed out for pallasites, implying that pallasites represent the core-mantle boundaries of their parent bodies. Therefore, pallasites potentially provide a unique opportunity to study core formation and core-mantle interaction on planetesimals. Further, chronological investigations of pallasites make it possible to decide the timescales of differentiation and crystallization of the planetesimals. In this study, the Hf-W chronology was applied to the Brenham main group pallasite. We evaluate the nucleosynthetic and neutron capture effects by combined W and Pt isotopic analysis. The metal fractions yielded ε182W values of -3.43 ± 0.15 and -3.85 ± 0.21 relative to the standard solution. The obtained ε183/184W values were in agreement with the standard solution, indicating that there is no nucleosynthetic anomaly in Brenham. The Pt isotope measurement was carried out on one of the two metal fractions analyzed for W isotopes. The positive anomaly in ε 196/195Pt represents the neutron capture effect on the Brenham metal. For correction of the neutron capture effect, we apply the reported correlation slope for iron meteorites (Kruijer et al., 2014). The corrected ε 182/184W value of -3.26 ± 0.29 corresponds to the model age of 2.0 ± 2.9 Myrs after the CAI formation, which is consistent with the olivine Al-Mg age of 1.24 +0.40/-0.28 Myrs and the Mn-Cr age of ~2.5 – 4 Myrs after the CAI formation (Baker et al., 2012; McKibbin et al., 2016). The gained Hf-W model age is also consistent with the early accretion of magmatic iron meteorites and apparently older than the non-magnetic iron meteorites, indicating that the Brenham metal formed during the early metal segregation on its parent body (Kruijer et al., 2013; Markowski et al., 2006).