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Minéralogie et composition isotopique des phases d’altération des premières roches du Système Solaire

Abstract : Calcium and aluminium-rich inclusions (CAIs) are the first solid objects formed in the solar system 4.568 Ga ago. We can estimate that they formed at a temperature higher than 1200 °C in very reducing conditions near the young Sun. In contrast, secondary phases found in CAIs suggest oxidizing and/or low temperature conditions. Most of these phases were interpreted as formed lately. However, a nebular origin of some secondary phases is still debated. The purpose of the thesis is to test if some secondary phases could have formed during CAI formation in the nebula using coupled different techniques. A compound CAI, named E101.1, from the CV3 reduced chondritic meteorite Efremovka was studied. This CAI is relevant for the study because it contains FeO-rich phases enclosed in diopside enclosed itself in the host CAI. These phases were characterized as Fe-åkermanite, kirschsteinite, fine-grained assemblage associated with wollastonite. The petrologic and textural study of these phases carried out during the thesis suggests that kirschsteinite and wollastonite formed in the nebula within an anorthite and diopside-rich precursors. Fe-åkermanite likely crystallized during the precursor incorporation into the partially melted host CAI. This is consistent with the first results of petrologic experiments that were initiated. After developping NanoSIMS imaging of D/H ratio on FIB (Focused Ion Beam) sections in weakly hydrated minerals, the δD of E101.1 minerals were measured. The lowest values ever measured in a meteoritic sample were found in anorhite with a δD of -817 ± 185 ‰ (2σ). This value is consistent with a formation near the young Sun. The fine-grained assemblage has high δD values up to 1250 ± 516 ‰ (2σ). Kirschsteinite has chondritic δD value: 163 ± 201 ‰ (2σ). The high values were attributed to evaporation during the xenolith capture in agreement with petrologic obervations which implies that kirschsteinite and wollastonite formed in the nebula in a reservoir with a chondritic H isotopic composition. This means that the D/H ratio of the nebula water passed from a solar value to a nearly terrestrial value in several hundred thousand years maximum. These complementary approaches hence showed the presence of nebular alteration phases in a CAI and that a non-predicted thermodynamical oxidizing event occured in the nebula.
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Dan Lévy. Minéralogie et composition isotopique des phases d’altération des premières roches du Système Solaire. Minéralogie. Sorbonne Université, 2019. Français. ⟨NNT : 2019SORUS206⟩. ⟨tel-03139878⟩

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