Water-rock interactions in the early Solar system: clues from Fe-serpentines in CM chondrites – CM2cron

We developed a methodology based on magnetic mineralogy for evaluating the transformations of serpentines with the degree of alteration. In that respect, CM chondrites, the more frequent carbonaceous chondrites falls, are interesting because they represent different stages of alteration. This approach, which allows for a global characterization using magnetometry, is made possible by a study of the magnetic properties of Fe-rich serpentines by magnetometry, neutron diffraction and x-ray magnetic circular dichroism (XMCD). The reactions involved in the formation / transformation of serpentines are studied in parallel at a nanoscale, using transmission electron microscopy and x-ray absorption coupled with x-ray spectromicroscopy (STXM). Finally, this approach is combined with experimentation and modeling, which are required for understanding the thermodynamic and kinetic parameters controlling the formation of Fe-rich serpentines in the context of the alteration of asteroids.

We propose a strong analogy between serpentinization in the parent body of meteorites and the serpentinization of abyssal peridotites (magnetic study - ref. 1, 3 & 4 - and nanoscale study -ref. 2 & 5). Indeed, our results suggest that the incorporation of Fe to serpentines is higher during the earlier stages of alteration - cronstedtite forming step. Later stages yielded assemblages that are un-equilibrated from the structural, chemical and redox point of views, similar to the polysomatic assemblages observed in terrestrial hydrothermal systems.
Analytic (ref. 1 & 2) and experimental data (PhD thesis in progress) suggest a kinetic control on the formation of Fe-rich serpentines at early stages of alteration. The associated oxidation processes of Fe would be analogous to terrestrial processes, and would imply the production of hydrogen. This would have possible consequences on the long term evolution of the redox state of the parent body and/or on prebiotic organo-synthesis (ref. 2).
From a more fundamental point of view, the interpretation of the results from the magnetometry study of chondrites is based on the study of the magnetic properties of cronstedtite. The observation of spin glass properties in cronstedtite is one of the main outputs of the project.

Few studies have documented so far the variations of the magnetic properties of Fe-rich serpentines. However, they potentially bear rich crystal-chemical information, as within the compositional field covered here (greenalite – cronstedtite – berthierine - chamosite), antiferromagnetism or magnetism frustrated at various degrees (ref. 1) are encountered. Further understanding this information, by a neutron diffraction and XMCD study in progress, aims at determining the parameters controlling magnetic order/disorder in serpentines, and at understanding global crystal-chemical variations of chondritic serpentines. This approach is a required in order to estimate their conditions of formation, combined with experiments and modeling.

Publications (peer reviewed):
Ref. 1: Elmaleh, A., Tarantino, S.C., Zema, M., Devouard, B. and Fialin, M. (2012) The low-temperature magnetic signature of Fe-rich serpentine in CM2 chondrites: comparison with terrestrial cronstedtite and evolution with the degree of alteration. Geophys. Geochem Geosystems 13, Q05Z42.
Ref. 2: Elmaleh, A., Bourdelle, F., Caste, F., Benzerara, K., Leroux, Devouard, B. Formation of Fe-rich serpentine by aqueous alteration in asteroids: a nanoscale study of the Murray chondrite, submitted to Geochim. Cosmochim. Acta.

Others:
Ref. 3: Elmaleh, A., Tarantino, S.C., Zema, M., Zanda, B., Devouard, B. and Fialin, M. (2011) Low- T magnetic properties of 5 CM2 chondrites: comparison with the signature of cronstedtite and with preliminary results from the Paris chondrite Meteor. Planet. Sci 46, Special Issue-Supplement 1, A64-A64.
Ref. 4: Elmaleh, A., Zanda, B., Hewins, R., Göpel, C., Fieni, C., Pont, S. and Humayun, M. (2012) Early Stages of hydrothermal alteration in anomalous primitive CCs NWA 5958 and Paris. Meteor. Planet. Sci 47, Special Issue-Supplement 1, A127-A127.
Ref. 5: Elmaleh, A., Benzerara, K., Leroux, H., Devouard, B., Tarantino, S.C., Bernard, S., Wang, J. (2012) Nanoscale study of Fe-serpentines & associated secondary phases in the matrix of CM2 carbonaceous chondrites. Meteor. Planet. Sci 47, Special Issue-Suppl. 1, A126-A126.
Ref. 6: Cournede, C., Gattacceca, J., Rochette, P., Elmaleh, A. and Jenniskens, P. (2012) Magnetic properties of Sutter’s Mill compared to CM2, C2 and C3. Meteor. Planet. Sci., 47, Special Issue-Supplement 1, A110-A110.

Invited talk:
“Interactions fluide-roche dans les astéroïdes: formation des serpentines chondritiques”.
A. Elmaleh, F. Bourdelle, F. Caste, K. Benzerara, S. C. Tarantino, H. Leroux, B. Devouard, M. Zema” RST / Groupe Français des Argiles, Pau, oct. 2014.

2 Masters (Suraj Pushparaj and Florent Caste) et one ongoing PhD thesis (Florent Caste).

Chondrites are the most pristine Solar System samples and provide a unique opportunity to study the physical and chemical processes that occurred around the young Sun. Carbonaceous chondrites of the CM and CI families, the most pristine samples identified yet, can contain up to 10 wt% water, in the form of hydrated and hydroxylated minerals. These samples testify of an early action of water (liquid?) that likely occurred on their parent bodies. Deciphering the conditions of aqueous alteration is essential for understanding the formation of small Solar System bodies that might have significantly contributed to the terrestrial water budget.Fe-serpentines are ubiquitous products of the multi-stage alteration that has affected CM2 chondrites, and they are major mineralogical phases of these meteorites (up to 50 wt%). Their complex and variables crystal-chemistry encloses keys to a better understanding of the hydration processes.
The main objective of the project is to draw bounds to the conditions of alteration on the parent body of CM2 chondrites, by combining observations, experiments, and calculations focused on iron-rich serpentines and their conditions of stability.
Fine determination of the crystal-chemistry of Fe-serpentines in CM2 chondrites is at the center of this project, so one of our major aim is to develop appropriate tools for that purpose. We will rely on high-resolution analytical methods (SEM on FIB sections, ATEM), as well as on methods taking advantage on the low-temperature magnetic properties of the mineral (bulk magnetometry, XMCD). The latter are potentially able to yield bulk estimates of the oxidation state and valence of iron of Fe-serpentine in a series of variously altered meteorites, provided that the dependence of the properties on crystal-chemistry are well understood. That is why the analysis of mineralogical analogs has an important place in the project, as well as the synthesis of cronstedtite under variable conditions which will provide the required references, complementary to natural ones.
Interpreting the mineralogical data acquired in CM2 chondrites in terms of 1) the environmental conditions that prevailed during the aqueous alteration of the parent body of CM2 chondrites and/or 2) variations of initial anhydrous compositions requires both an experimental study and thermodynamical constrains. The synthesis of cronstedtite will be performed with a range of starting materials, and under a range of experimental conditions. In particular, experiments under a controlled atmosphere (pH2) will bring important clues to the stability of Fe-serpentines of varying crystal-chemistries. Finally, establishing a correspondence between the parameters controlling the experimental formation of cronstedtite and the environmental conditions of the aqueous alteration of the parent body of CM2 chondrites requires that both the thermodynamic equilibrium conditions and out of equilibrium chemical transfers are estimated, using fine scale chemical mapping of the alteration assemblages.