日本地球惑星科学連合2018年大会

講演情報

[EE] Eveningポスター発表

セッション記号 S (固体地球科学) » S-MP 岩石学・鉱物学

[S-MP34] Oceanic and Continental Subduction Processes

2018年5月20日(日) 17:15 〜 18:30 ポスター会場 (幕張メッセ国際展示場 7ホール)

コンビーナ:Hafiz Ur REHMAN (Department of Earth and Environmental Sciences, Graduate School of Science and Engineering, Kagoshima University)、辻森 樹(東北大学)、Chin Ho Tsai

[SMP34-P05] Boron isotope systematics of forearc serpentinites: A case study
from the California Coast Ranges

*山田 千夏1辻森 樹1木村 純一2常 青2 (1.東北大学、2.海洋研究開発機構・地球内部物質循環研究分野)

キーワード:蛇紋石、前弧蛇紋岩、ホウ素同位体、局所分析、フランシスカン変成帯、LA-MC-ICPMS

Recently, Martin et al. (2016) characterized two contrasting origins of serpentinite in central Guatemala. They proposed an idea that the tectonic origin of serpentinites can be discriminated based on boron isotopic signature. In order to test the result by Martin et al. (2016), various serpentinites from the Franciscan Complex of Northern California were investigated using in-situ spot analytical technique that had newly developed by Kimura et al. (2016). The investigated twenty serpentinites include sheared serpentinite, massive serpentinite (or serpentinized peridotite), and serpentine veins cutting fresh peridotite. The occurrences of these serpentinites are classified into serpentinite lenses within Franciscan metagreywacke, blueschist-bearing serpentinite diapir, massive serpentinized peridotite body, extremely fresh peridotite body, and various serpentinites along the San Andreas Fault. We have tested both mineralogical variations (lizardite/chrysotile vs antigorite) and textural variations (pseudomorph vs non-pseudomorph). We also made a mineralogical assessment on relict mantle minerals in serpentinites. Our study found a large variation of boron isotope (δ11B) ranging from -12.0 to +24.4‰ and boron concentrations (B=3.0-239 μg/g). Our observations together with geological context support Martin et al. (2016)’s observation in central Guatemala; in other words, boron isotope is useful for tectonic discrimination of serpentinites. In this abstract, we will introduce petrologic and geochemical diversity of forearc serpentinites in California Coast Ranges, and we also present a new insight of isotope correlations among various aspects.