Implications of Alkali-enriched Plagioclase in Angrite-like Survivors of the Bahía Blanca Bolide

نویسندگان

  • R. S. Harris
  • P. H. Schultz
چکیده

Introduction: Melt breccias generated by a bolide collision in the vicinity of Bahía Blanca, Buenos Aries Province, Argentina at the Miocene-Pliocene Boundary (5.28 ± 0.04Ma) contain sand-sized meteoritic clasts [2], reviewed in [1]. The clasts exhibit textures and mineral assemblages (Fig. 1) most similar to quenched volcanic angrites, especially Asuka 881371 and Sahara 99555. They contain acicular intergrowths of Al, Ti diopsode-hedenbergite (fassaitic pyroxene), calcic plagioclase, calcium silicophosphate, and anhydrous whitlockite (merrillite) with larger grains of silicon-rich and sulfur-bearing fluorapatite. We also observe accessory ulvöspinel, titanomagnetite, awaruite, and Ni-metal. The high concentration of MgO (>5 wt%), and corresponding paucity of sodium, in whitlockite is only matched by Angra dos Reis [3, 4] and emphasizes the refractory character of the clasts. Despite the nearly unique similarities with the angrite suite, we were resigned to refer to these clasts as “angrite-like” principally because of obvious alkali enrichment in plagioclase. Electron microprobe analyses demonstrated albite components between 16 and 35% (An65-84). Among angrites, only disputed [5] plagioclase grains reported by Prinz et al. [3] with An86 from Angra dos Reis deviate from An99-100 [1]. Consequently, Keil [1] also suggested that the Bahía Blanca bolide should be distinct from the angrite parent body.

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تاریخ انتشار 2017