Relationships between organics, water and early stages of aqueous alteration in the pristine CR3.0 chondrite MET 00426

Abstract

In order to investigate the nature of organics at the time of their accretion into chondrite parent bodies, as well as their subsequent evolution with aqueous alteration, we have conducted a study of the morphologies, spatial distribution and relationships between organic particles and the surrounding matrix phases in one of the most pristine carbonaceous chondrites currently known (MET 00426, CR), which is therefore likely to contain the best preserved record of pre-accretion features. Focused ion beam sections were extracted for transmission electron microscope observations from its matrix.Organic matter (OM) shows a heterogeneous population of grains, most of them smaller than 1μm. Diverse morphologies are observed, such as compact, rounded aggregates, individual and aggregated nanoglobules, and micron- to nanometer-sized veins. A common feature is the systematic presence of cracks connected to the grains and filled with OM. The surrounding matrix groundmass consists of amorphous iron-rich silicate particles intimately mixed with phyllosilicates, sulfides and occasional tochilinite, with sizes ranging from several hundreds of nanometers to below 10nm. A close spatial relationship is commonly observed between some of the organic matter particles and alteration phases, such as tochilinite and phyllosilicates. Phyllosilicates sometimes occur intimately intercalated with organic matter at a scale below 10nm.We used TEM/EDS techniques to quantify the water concentration in the matrix amorphous silicate material and the phyllosilicates. The water contents of both materials are identical at 10 (±6)wt.% H2O and demonstrate, that the amorphous silicate material in this meteorite is hydrated. Therefore, even though these CR3.0 chondrites are the least altered objects from a mineralogical point of view, their matrices contain significant amounts of water in the amorphous silicate. This coupled in situ study of organics and aqueous alteration suggests that a significant population of the OM accreted as a mixture of soluble and insoluble molecules together with water ice grains and that the OM was mobile at the micrometer scale. The spatial distribution of the OM grains can therefore, in part, be attributed to parent body processes. We suggest that as accreted ice melted, hydration of the amorphous silicates and formation of tochilinite and phyllosilicates occurred in the immediate vicinity of the composite water ice/organic matter grains. The water-soluble component of the organics was likely transported and redistributed in the surrounding porosity (cracks, grain boundaries) as water circulated. The textural settings suggest that some of the OM material could have been polymerized during aqueous alteration and transformed into insoluble molecules, perhaps during the last stages of alteration as water was consumed by silicate hydration reactions.