High-temperature annealing of amoeboid olivine aggregates: Heating experiments on olivine-anorthite mixtures

• Komatsu Mutsumi

  The University Museum, University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-0033, Japan

• Mikouchi Takashi

  Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

• Miyamoto Masamichi

  Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

Abstract

Amoeboid olivine aggregates (AOAs) are composed of forsteritic olivine, Fe,Ni-metal, and Ca,Al-rich nodules consisting mainly of Al-diopside, spinel, and anorthite. Although the textures, shapes, and mineralogy of AOAs are consistent with their being aggregates of nebular condensates, some textures are in conflict with equilibrium condensation calculations, indicating that AOAs were not produced by a simple one-stage condensation. To examine the origin of the constituent minerals within AOAs and their textural relationships, we performed heating experiments using mineral mixtures analogous to those in AOAs. Isothermal and cooling experiments on forsterite + anorthite mixtures reveal that a high-Ca pyroxene phase forms via the incipient melting of the two minerals. Comparative studies of heating experiments performed using the mineralogy of AOAs suggest that Al-diopside in AOAs can be produced from a small degree of melting of forsterite and anorthite. The formation of Al-diopside in this way is consistent with the annealing textures observed in AOAs, and it may account for the discrepancy between the observed mineralogy of AOAs and the results of equilibrium condensation calculations, the occurrence of two types of diopside (Al,Ti-rich diopside and Al-diopside), and the variable Al_2O_3 content of Al-diopside.

Journal

• Polar science

  Polar science 3 (1), 31-55, 2009-06

  National Institute of Polar Research

Keywords

• Carbonaceous chondrites
• CAIs
• Amoeboid olivine aggregates
• Heating experiments

Details

• CRID: 1571135652537954816
• NII Article ID: 110007121876
• NII Book ID: AA12240481
• ISSN: 18739652
• Text Lang: en
• Data Source:
  • CiNii Articles