Oxygen and Al-Mg isotopic constraints on cooling rate and age of partial melting of an Allende Type B CAI, Golfball

Noriyuki Kawasaki, Shoichi Itoh, Naoya Sakamoto, Steven B. Simon, Daiki Yamamoto, Hisayoshi Yurimoto

研究成果: ジャーナルへの寄稿学術誌査読

6 被引用数 (Scopus)

抄録

Coarse-grained, igneous Ca-Al-rich inclusions (CAIs) in CV chondrites formed through multiple melting events. We conducted in situ O-isotope analysis and Al-Mg systematics by secondary ion mass spectrometry of relict and overgrown minerals from a partial melting event in an Allende Type B CAI, Golfball. Golfball has a Type B CAI bulk composition and a unique structure: a fassaite-rich mantle enclosing a melilite-rich core. Many of the blocky melilite crystals in the core have irregularly shaped, Al-rich (Åk515) cores enclosed in strongly zoned (Åk3070) overgrowths. Since the Al-rich melilite grains could not have formed from a melt of Golfball, they are interpreted as relict grains that survived later melting events. The O-isotopic compositions of the blocky melilite crystals plot along the carbonaceous chondrite anhydrous mineral line, ranging between Δ17O ~ −14‰ and −5‰. The Al-rich relict melilite grains and their overgrowths exhibit the same O-isotopic compositions, while the O-isotopic compositions are varied spatially among melilites. We found that the O-isotopic compositions steeply change across several melilite crystals within few tens of micrometers, indicating the O-isotopic compositions of the melt could not have been homogenized during the partial melting in that scale. According to the time scale of O self-diffusivity in the melt, the cooling rate of the partial melting event is calculated to be >6 × 104 K h−1. Al-Mg isotope data for core minerals plot on a straight line on an Al-Mg evolution diagram. A mineral isochron for Golfball gives initial 26Al/27Al of (4.42 ± 0.20) × 10–5 and initial δ26Mg* of −0.035 ± 0.050‰. The chemical and O-isotopic compositions of melilite and those initial values imply that its precursor consisted of fluffy Type A and/or fine-grained CAIs. The partial melting event for Golfball may have occurred in very short order after the precursor formation.

本文言語英語
ページ(範囲)1224-1239
ページ数16
ジャーナルMeteoritics and Planetary Science
56
6
DOI
出版ステータス出版済み - 6月 2021
外部発表はい

!!!All Science Journal Classification (ASJC) codes

  • 地球物理学
  • 宇宙惑星科学

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