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 Allen de meteorite are composed exclusively of secondary minerals, including faya litic olivine (Fa(40-45)), salitic pyroxenes (Fs(10-30)Wo(50)), nepheline, sodalite, and Fe-Ni sulfides. Chondrules in the dark inclusions are replace d by fayalitic olivine and nepheline. The chondrule pseudomorphs in 4301-2 and IVa are surrounded by salitic pyroxene rims which are commonly intercon nected. Matrices of these dark inclusions are crosscut by multiple veins of salitic pyroxenes and Fe-Ni sulfides. The inclusions are surrounded by con tinuous Ca-Fe-rich rims with variable widths and show roughly symmetric min eralogic 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 kirschstei nite (CaFeSiO4). The outermost zone consists of salite-hedenbergitic pyroxe nes (Fs(15-35)Wo(48-50)) The rims are commonly intergrown with divine in th e Allende matrix and enclose fragments of forsteritic olivine chondrules an d lumps of the Allende-like matrix material. Abundant rounded objects compo sed of salite-hedenbergitic pyroxenes, andradite, and wollastonite ale obse rved 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 corr elates to the rim thicknesses, which increase in the same order. We conclude that the Allende dark inclusions experienced at least two stage s of alteration in the presence of an aqueous solution after lithification and aggregation. One episode of alteration resulted in the replacement of p rimary chondrule minerals (forsterite, enstatite, anorthitic mesostasis, ka macite +/- troilite nodules) by secondary fayalitic olivine, nepheline, sod alite, and Ni-rich sulfides. Calcium removed from the chondrules was redepo sited as salite-hedenbergitic pyroxene veins, as rounded objects in the mat rix, and as rims around chondrules. Following this alteration, the dark inc lusions were excavated from their original location (most likely the CV3 as teroidal body) and incorporated into the Allende host. The last episode of alteration took place in situ and resulted the in dissolution of Ca-Fe-rich pyroxenes in the dark inclusions and redeposition of Ca-rich minerals as r ims around dark inclusions at the interfaces with the Allende host. The pro posed mechanism of rim growth involves diffusional exchange of Ca and Fe be tween the dark inclusions and Allende, driven by the compositional differen ce between their matrix olivines. The precipitation of rim minerals started at similar to 250 degreesC at an iron-rich geochemical barrier (the Allend e host), and continued at lower temperatures until the aqueous solution dri ed out.