Hydrothermal circulation and geochemical processing on carbonaceous chondrite parent bodies.

摘要

Carbonaceous chondrites are a suite of primitive meteorites with bulk chemical compositions that closely resemble solar values for non-volatile elements. On this basis, carbonaceous chondrites are considered among the most primitive materials available for study and, therefore, a critical source of information on the formation and evolution of our planetary system. However, despite carbonaceous chondrites' primitive traits, mineralogical and isotopic observations indicate that most have been modified by geochemical processing of their parent bodies. Therefore, although they possess a primitive bulk chemistry, it is clear that carbonaceous chondrites are not pristine samples. In particular, the compositions of the CI and CM chondrite groups provide abundant evidence of processing by aqueous alteration. Detailed studies of these chondrites reveal a complex formation history, involving periods of mineral dissolution, precipitation, and oxygen isotope exchange due to interactions with fluids. Consequently, the primary mineralogy and oxygen isotopic compositions of these objects have been erased. Deciphering the information provided by carbonaceous chondrites requires a thorough understanding not only of their current characteristics, but also of how these characteristics were produced. Traditionally, alteration models have been founded upon an assumption of uniform alteration in a closed system. In these models, there is no fluid circulation and different chondrite groups must originate on separate parent bodies. Until recently, little work has been done to investigate the consequences of fluid flow and chemical reactions in carbonaceous chondrite parent bodies. This study presents a new numerical model of hydrothermal circulation in carbonaceous chondrite parent bodies that tracks fluid flow, transport, geochemical reactions, and isotope exchange. The results support the idea that distinct chondrite groups could form at different locations within the same body and are not required to originate on separate parent bodies. These findings suggest that the origin of carbonaceous chondrites and the a priori assumption of closed system alteration should be re-evaluated by future studies.