Secondary Alteration Processes IN Solar system – SAPINS

The SAPINS project will follow a scientific program subdivided into four main tasks: (i) Microcharacterization of secondary alteration phases, (ii) Determination of the composition of the alteration fluids, (iii) Laboratory fluid-rock experiments and (iv) Determination of the oxygen isotopic compositions and the Mn-Cr ages of secondary alteration phases. All the results will be integrated to built a comprehensive framework of the secondary processes that affected chondrites.

The micro-characterization of CM chondrites allowed us to demonstrate that their hydrated secondary phases (tochilinite-cronstedtite associations) result from pseudomorphisme process of anhydrous crystalline silicates. This result is important because it allows us to properly constrain the chemical compositions of alteration fluids. We also showed that physicochemical conditions (temperature, oxygen fugacity) changed drasticaly during the course of alteration. The detection of fluid inclusions in calcite is difficult due to their scarity. However, we found several interesting candidates but the measurements by Raman spectroscopy are currently impossible due to fluorescence problem. We are currently working to solve this problem. Laboratory experiments are in progress using autoclaves with : (i) mineralogocal assemblages close to those observed in CM chondrites (ii) varying compositions of fluids. This task is along-term project (some experiments will last 3 years) but the first results after 6 months of experience are very promising. The secondary phases produced have varying mineralogy but some experiments with sulfur-rich fluids and under weakly alkaline conditions produced minerals similar to those observed in CM chondrites. It is necessary to wait for confirmation of these results in the context of longer experience and perform the thermodynamic modeling. The determination of the isotopic composition in oxygen carbonates CM chondrites is finished. The results are extremely interesting because they demonstrated that: (i) alteration of CM chondrites occurred at much higher temperatures (50-300 ° C) than previously proposed (0-25 ° C) and ( ii) the CM chondrites accreted have a significant amount of ice external areas of the solar system.

In addition to the laboratory experiments that are currently underway, we currently perform the determination of the carbon isotopic compositions of carbonates. The first results are promising as they suggest they change depending on the degree of alteration. Moreover, we have also initiated the synthesis of dolomite standards doped with manganese and chromium in order to determine the relative timing of precipitation of carbonates in CM chondrites. We have had some technical problems due to a heterogeneous distribution of manganese and chromium but recent syntheses are more homogeneous. We will continue our effort to achieve homogeneous standards in order to achieved this project using the ion probe.

1. Pignatelli I., Marrocchi Y., Vacher L.G., Delon R. & Gounelle M. (2016). Multiple precursors of secondary mineralogical assemblages in CM chondrites. Meteoritics & Planetary Science 51, 785-805. doi: 10.1111/maps.1262
2. Pignatelli I., Vacher L.G. & Marrocchi Y. (2015) Comment on “Hydrothermal preparation of analogous matrix minerals of CM carbonaceous chondrites from metal alloy particles” by Peng Y. and Jing Y. [Earth Planet. Sci. Lett. 408 (2014) 252-262]. Earth and Planetary Science Letters 428, 304-306, doi.org/10.1016/j.epsl.2015.07.050
3- Fujiya W., Sugiura N., Marrocchi Y., Takahata N., Hoppe P., Shirai K., Sano Y. & Hiyagon H. (2015). Comprehensive study of carbon and oxygen isotopic compositions, trace element abundances, and cathodoluminescence intensities of calcite in the Murchison CM chondrite. Geochimica et Cosmochimica Acta 161, 101-117, doi.org/10.1016/j.gca.2015.04.010.

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. 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.
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 would provide us with important constraints on the types of geological activity experienced by asteroids and on the links between the diversity of meteorite groups and the diversity of asteroids. In addition, understanding the nature of the alteration fluids could help to decipher the spatial distribution of water in the early Solar System and the conditions under which water was delivered to Earth.
The strength of the SAPINS project lies in a multidisciplinary approach that combines the analytical, experimental and thermodynamic modeling expertise of three young researchers – a team that is therefore equipped with the multiple skills needed to attack a problem such as this. In addition, SAPINS will apply powerful analytical and experimental instruments and techniques that have been developed on terrestrial rocks to the mineral assemblages observed in chondrites. On completion, this project will provide us with a new understanding of alteration conditions and processes in the early Solar System. This knowledge will be of key importance in the preparation and data interpretation of space missions devoted to the visit and/or sampling of hydrated celestial body surfaces (e.g. Rosetta, Dawn, OSIRIS-Rex, Hayabusa). ANR funding is essential for the undertaking of multidisciplinary research such as SAPINS as these projects require a combination of expertise, notably in the fields of petrography, geochemistry and experimental petrology for SAPINS. In summary, the SAPINS project will stimulate scientific advances by combining different disciplines and stimulating interaction among the different communities involved.