In situ growth of Ca-rich rims around allende dark inclusions
Four heavily altered dark inclusions (4301-2, IV-1, 3b-1, IVa) in the Allende meteorite are composed exclusively of secondary minerals, including fayalitic olivine (Fa(40-45)), salitic pyroxenes (Fs(10-30)Wo(50)), nepheline, sodalite, and Fe-Ni sulfides. Chondrules in the dark inclusions are replaced by fayalitic olivine and nepheline. The chondrule pseudomorphs in 4301-2 and IVa are surrounded by salitic pyroxene rims which are commonly interconnected. Matrices of these dark inclusions are crosscut by multiple veins of salitic pyroxenes and Fe-Ni sulfides. The inclusions are surrounded by continuous Ca-Fe-rich rims with variable widths and show roughly symmetric mineralogic zoning. The innermost zone consists of diopside-salitic pyroxene (Fs(10-15)WO(48-50)) The central zone is composed of hedenbergite (Fs(48-50)Wo(48-50)), wollastonite (CaSiO3), andradite (Ca3Fe2Si3O12), and kirschsteinite (CaFeSiO4). The outermost zone consists of salite-hedenbergitic pyroxenes (Fs(15-35)Wo(48-50)) The rims are commonly intergrown with divine in the Allende matrix and enclose fragments of forsteritic olivine chondrules and lumps of the Allende-like matrix material. Abundant rounded objects composed of salite-hedenbergitic pyroxenes, andradite, and wollastonite ale observed in the Allende matrix near the Ca-rich rims. There are progressive Ca depletions from 4301-2 to IV-1 to 3b-1 to IVa. The degree of depletion correlates to the rim thicknesses, which increase in the same order.