Non-destructive markers of parent body aqueous alteration in carbonaceous chondrites based on magnetometry and X-ray diffraction of mm-sized meteorite fragments

English Abstract:

We present a methodology for studying Fe-rich alteration assemblages, and its application to the study of aqueous alteration in C-rich asteroids. The more primitive chondrites are also the most altered. Early water-rock interactions probably occurred in their asteroidal parent bodies after ice that accreted with rocky materials have melted. In CM chondrites, serpentinization processes predominate and serpentines present a large diversity of compositions in the Si-Fe²⁺-Fe³⁺-Mg-Al system. The parameters controlling the variations are still poorly understood and may involve kinetic factors. It is important to evaluate bulk mineralogical changes in chondrites as alteration proceeds in order to understand the reaction pathways during alteration. X-ray diffraction (XRD) has proven useful although it gives limited information on complex solid solutions. In a recent study we showed that low-temperature (2-300 K) magnetometry was a powerful tool for evaluating the mineralogical changes in Fe-rich serpentines, and to estimate the proportions of other Fe-rich phases. We combined this approach with XRD using a microfocus (<100 µm beam) high brilliance rotating anode X-ray generator to evaluate the mineralogical changes among 100's of µm to mm size fragments of the Paris chondrite, the least altered CM known so far. Alteration is heterogeneous in the Paris chondrite, with mm to cm size domains where primary chondritic components are better preserved. This feature gives a unique opportunity to evaluate the reactions involved by comparing more or less altered areas. Our results suggest that the (Fe²⁺,Fe³⁺)-rich serpentine cronstedtite, which formation may be kinetically favoured, is the predominant early alteration phase. They yield new constraints on Fe partitioning during alteration, which imposes an important control on dihydrogen production during serpentinization in terrestrial systems, and probably also in extra-terrestrial ones. Our results also help understanding the Al budget during alteration, which is still poorly known. Therefore, the present non-destructive approach allows 1) a quick identification of alteration products and 2) an evaluation of mass transfers during alteration, which may help understanding the fluid flow pattern in chondritic parent bodies.