Understanding the alteration of the Tagish Lake meteorite through mineralogy, geochemistry and oxygen isotopes.

摘要

Our understanding of the origin and formation of the various components of meteorites, and ultimately understanding the formation of the early Solar System, comes from our knowledge of the asteroidal alteration processes and modifications that such processes induce on these components. The majority of our knowledge in this area comes from the meteorites that have been subjected to some kind of terrestrial modification which has partly or completely erased any prior history of its components. Meteorites unaffected by terrestrial modification are rare but invaluable. Tagish Lake is one such meteorite; due to its fall history it is considered to be the best candidate for studying the alteration history of the asteroidal parent body.;Four samples (TL5b, TL11h, TL11i, and TL11v) from the pristine collection of the Tagish Lake meteorite were studied to characterize and understand its alteration history. Based on petrological and mineralogical observations I conclude that the Tagish Lake parent body was a heterogeneous mixture of anhydrous precursors of nebular origin, which were brecciated and aqueously altered to various degree. The degree to which the studied samples experienced aqueous alteration is in the order TL5b < TL11h < TL11i. Specimen TL11v, which consists of disaggregated material, is heterogeneous on the microscale and encompasses the petrologic characteristics of other three specimens as seen through EPMA observation. The whole-rock geochemistry confirms the inferred alteration sequence observed through mineralogy. There is a positive correlation with aqueously mobile trace elements, such as K, Ba, and Br, which appear to be controlled by an increase of phyllosilicates from least to most altered samples. Yet, the homogeneity of other elements suggests that elemental mass transfer occurred on a localized scale and aqueous alteration was isochemical for these elements. Oxygen isotopic results from my study corroborate the conclusion that the range of variations observed in the Tagish Lake meteorite covers the oxygen isotopic composition seen in all other carbonaceous chondrites. Such variation found in one meteorite suggests that the Tagish Lake parent asteroid sampled the material from different oxygen isotope reservoirs, perhaps 16O-rich and 17O-rich gaseous reservoirs.