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SAPINS : Secondary Alteration Processes IN Solar System

Projet ANR JC - PI : Yves Marrocchi

CRPG participants : Laurent Tissandier & Gaston Giuliani

Chondrites are solid remnants of the formation of the solar system 4.56 Ga ago. They provide a direct glimpse into the astrophysical conditions under which the Sun was formed and into the dynamics and evolution of the accretion disk. After their formation however, chondrites experienced secondary fluid alteration that caused important changes to their textures, mineralogy, and chemical and isotopic compositions (Fig. 1). Even though it is widely acknowledged that chondrites underwent this secondary modification, the physicochemical conditions under which it took place and the timing and duration of the alteration process are still only partially understood. This gap in our understanding severely restricts interpretation of meteorite data. In view of the multi-faceted complexity of the problem, three young researchers, specialists in cosmochemistry, terrestrial alteration, fluid-rock interactions and thermodynamics, have been brought together in order to merge their respective skills and resources and create a highly capable scientific team with which to tackle this issue.
Fig. 1 : SEM image of a section of the Paris chondrite. Despite being the least altered CM chondrite known so far, Paris clearly shows signs of pervasive alteration that can help us to decipher the first stages of meteoritic alteration.

Two groups of chondrites will be used as a common theme in this project. These are thought to represent the two main types of fluid alteration that occurred on asteroidal parent bodies : (i) low temperature hydrothermal alteration (CM group) and (ii) fluid-assisted metamorphism (CV group). Within these groups, specific minerals (olivines and carbonates) and mineral associations (tochilinite/cronstedtite associations and fayalitic-magnetite-troilite veins) will serve as probes with which to address fundamental first-order questions on the alteration processes : What were the compositions of the fluids that affected primitive chondrites ? Under what physico-chemical conditions did alteration take place ? How long did the alteration process last in the early Solar System ? Finally, and more generally, what is the intrinsic role of secondary alteration processes in establishing chondrite characteristics ? Resolving these questions is of primary importance as strong associations between asteroids and meteorites are beginning to emerge from observations, in-situ measurements, sample-return missions and modeling. Thus, an understanding of secondary fluid alteration processes in the early Solar System will provide important constraints regarding : (i) the geological processes that affected asteroids, (ii) the link between the diversity of meteorite groups and the diversity of asteroids and (iii) the spatial distribution of water in the early Solar System and the conditions under which it was delivered to Earth. In this context, the SAPINS project has been designed as a long-term study dedicated to better understanding the role of secondary alteration processes in CM and CV chondrites.

Fig. 2 : (a) Schematic representation of the autoclave used for fluid-rock interactions. (b) Photograph of the experimental device. (c) Schematic representation of the part of the autoclave that allows the pH and Eh of the fluid to be monitored throughout the course of the experiment.
In order to accomplish this interdisciplinary project, we have designed a scientific program that is divided into four tasks. Each task will involve petrographic, experimental and/or geochemical aspects that require the expertise of each partner.
Task 1 : microcharacterization of secondary alteration phases in CM and CV chondrites,
Task 2 : establishment of the composition of alteration fluids by studying the compositions of fluids inclusions and carbonates,
Task 3 : laboratory experiments of alteration on mineralogical assemblages characteristics of unaltered chondrites (Fig. 2),
Task 4 : determination of the isotopic compositions and the timing of formation of secondary phases.