Decording impact history in the early Earth

2017 Fiscal Year Final Research Report

• Project/Area Number: 15K20935
• Research Category: Grant-in-Aid for Young Scientists (B)
• Allocation Type: Multi-year Fund
• Research Field: Geology; Petrology/Mineralogy/Economic geology
• Research Institution: Yokohama National University
• Principal Investigator: Yamamoto Shinji 横浜国立大学, 大学院環境情報研究院, 准教授 (30467013)
• Research Collaborator: KOMIYA Tsuyoshi ; ISOZAKI Yikio ; SAKATA Shuhei
• Project Period (FY): 2015-04-01 – 2018-03-31
• Keywords: 初期地球 / ジャックヒルズ / 砕屑性ジルコン / 衝撃変成作用 / 後期重爆撃

Outline of Final Research Achievements

Our research proposal aimed to resolve “Late Heavy Bombardment (LHB) hypothesis” originally proposed from studies of the Moon, using detrital shocked-zircon in the metasedimentary rock, western Australia. Hugh amount of zircon separation and high-reliability age dating techniques are developed and the results are published in international journals, respectively. Related topics about the early Earth environment, we present oral presentation (invited) at international and domestic conferences.
Main research finding in our proposal is below; four types of shock-metamorphosed zircons were identified; (1) curvi-planar (non-planar) feature, (2) multiple sets of planar feature, (3) partly granular (polycrystalline) texture, and (4) fully granular texture. Of these four, multiple sets of planar feature are proved for diagnostic evidence for impact origin, and now observed as annealed (decorated) planar feature, probably due to post-impact thermal heating or regional metamorphic overprint.

Free Research Field

地質学、岩石・鉱物学、年代学