Tracking the Earliest Stages of Aqueous Alteration in the Mildly Altered CM Chondrite EET 96029

The earliest stages of CM carbonaceous chondrite aqueous alteration are very poorly understood as mildly altered CMs are extremely rare. The Paris meteorite (CM2.7) [1-3] and QUE 97990 (CM2.6) [4,5] are among the least aqueously altered CMs described to date. However, neither of them contain the pristine attribute of chondrule mesostasis glass. Glass is highly reactive and so among the very first phases to undergo aqueous alteration [6]. Therefore, the CM carbonaceous chondrite EET 96029 is very unusual as it has been shown to have retained mesostasis glass in at least one chondrule [7]. According to the new CM classification scheme of [8], which is based on H content, EET 96029 has an index of 2.0 (data in [9]), meaning that it is less altered than all but one of the fifty CMs analysed by [8]. A caveat is that a low H content could be due to mild heating as well as a low degree of aqueous processing [9]. However, the bulk O oxygen isotope composition of EET 96029 (as determined by [10]) is consistent with a low degree of alteration as it is slightly closer to that of the CO3 falls (possible representatives of the anhydrous progenitors of the CMs) than even the least altered lithology of Paris (Fig. 1).<p></p> To better understand the earliest stages of CM aqueous alteration and its impact on mesostasis glass, we have undertaken a detailed study of chondrule mesostasis textures and compositions in the mildly aqueously altered CM chondrite EET 96029.<p></p&gt

Glassy chondrule mesostasis in EET 96029: a CM3 component of a minimally altered CM2 carbonaceous chondrite

No abstract available

Impact-generated hydrothermal circulation and metasomatism of the rochechouart astrobleme: mineralogy and major and trace element distribution

The energy released during a hypervelocity impact on Earth can generate high temperatures in the target rock. There are currently 170 known impact structures worldwide, of which over one-third contain fossil hydrothermal systems [1]. Results from the analysis of these hydrothermal systems have many implications for the study of the origin of life on Earth and potential thereof on Mars. Hypervelocity impacts are also of particular economic interest as they may produce, expose or concentrate high commodity resources such as hydrocarbons, precious metals and ore minerals

Impact fracturing and aqueous alteration of the CM carbonaceous chondrites

Aqueous alteration of the CM carbonaceous chondrites has produced a suite of secondary minerals, and differences between meteorites in their abundance defines a progressive alteration sequence [e.g. 1, 2]. The means by which this water gained access to the original anhydrous constituents of the meteorites is the subject of considerable debate. Studies of rock texture, mineralogy and bulk chemical composition have concluded that solutions were generated by the melting of water ice in situ, and remained essentially static as a consequence very low intergranular permeabilities [e.g. 3, 4]. By contrast, results of oxygen isotope work and modelling have suggested that the fluids moved considerable distances within the parent body [5, 6]. Given the intergranular permeability of the CMs, an extensive fracture network would be required to support such flow. Clues to how the two very different models for aqueous alteration of the CMs can be reconciled have been recently provided by Rubin [7]. He recognised a good correlation between the magnitude of impact-induced compaction of CM meteorites and their degree of aqueous processing, with the more highly deformed meteorites being more altered. Here we have asked whether compaction was accompanied by the development of fracture networks that could have provided the conduits for aqueous solutions that mediated all or some of the alteration