Fine-grained dust rims in the Tagish Lake carbonaceous chondrite: Evidence for parent body alteration

Abstract

The Tagish Lake carbonaceous chondrite consists of heavily aqueously altered chondrules, CAIs, and larger mineral fragments in a fine-grained, phyllosilicate-dominated matrix. The vast majority of the coarse-grained components in this meteorite are surrounded by continuous, 1.5 to 200 m wide, fine-grained, accretionary rims, which are well known from meteorites belonging to petrological types 2 and 3 and whose origin and modification is still a matter of debate. Texturally, the fine-grained rims in Tagish Lake are very similar throughout the entire meteorite and independent of the nature of the enclosed object. They typically display sharp boundaries to the core object and more gradational contacts to the meteorite matrix. Compared to the matrix, the rims are much more finegrained and characterized by a significantly lower porosity. The rims consist of an unequilibrated assemblage of phyllosilicates, Fe,Ni sulfides, magnetites, low-Ca pyroxenes, and forsteritic olivines, and are, except for a much lower abundance of carbonates, very similar to the Tagish Lake matrix. Electron microprobe and synchrotron X-ray microprobe analyses show that matrix and rims are also very similar in composition and that the rims differ significantly from matrix and bulk meteorite only by being depleted in Ca. X-ray elemental mapping and mineralogical observations indicate that Ca was lost during aqueous alteration from the enclosed objects and preferentially crystallized as carbonates in the porous matrix. The analyses also show that Ca is strongly fractionated from Al in the rims, whereas there is no fractionation of the Ti/Al-ratios. Our data suggest that the fine-grained rims in Tagish Lake initially formed by accretion in the solar nebula and were subsequently modified by in situ alteration on the parent body. This pervasive alteration removed any potential evidence for preaccretionary alteration but did not change the overall texture of the Tagish Lake meteorite.