Résumé
Ordinary chondrites (OCs) are classified into three groups, according to their oxidation state,which increases from the H to L to LL groups. This is demonstrated by the decrease in metalcontent (H=~8 vol%, L=~4 vol%, and LL=~2 vol%), and by a positive correlation between Δ17Oand %Fa through the OC sequence. Compared to other chondrites, OCs exhibit the largestvariation in oxidation state but there is an ongoing debate on the processes that control thisvariation. To constrain the causes of the variations in the oxidation state with respect to theassociated nebular versus parent bodies processes, we investigated the elemental and isotopicvariations of germanium (moderately siderophile and volatile) in the bulk sample, as well as inthe metal, silicate and sulfide phases, over a range of petrographic types for the H, L, and LLordinary chondrites.We found that δ74/70Gemetal is a proxy for the δ74/70Gebulk composition and that each OC group isdistinguishable by their δ74/70Gemetal, which increases from –0.51±0.09‰ for H chondrites, –0.31±0.06‰ for L chondrites, and, finally, to –0.26±0.09‰ for LL chondrites (2σSD).Additionally, the OC sequence exhibited a positive correlation, from H to L to LL, betweenδ74/70Gemetal and %Fa, as well as oxygen isotopes (δ17O, δ18O and Δ17O), that was not aconsequence of a “size sorting effect” on chondrules (i.e. chondrule mixing) or metamorphicprocesses in the parent bodies but, rather, was the result of nebular processes. We propose thatthe correlation between the δ74/70Ge values and %Fa, Δ17O, δ18O can be explained by anincreasing proportion of accreted hydrated phyllosilicates, from the H, L to LL groups, with highδ74/70Ge and Δ17O. We found that 10 to 15% of phyllosilicates, with a composition of [Ge]=4–7ppm and δ74/70Ge=3–2.5‰, is needed to change the δ74/70Ge from H to LL, which corresponds toa Δ17O≈8-7‰. This value agrees with the Δ17O≈7‰ composition of the accreted nebularcomponent reported by Choi et al. (1998). During thermal metamorphism, phyllosilicatesdestabilize, liberating germanium that will be incorporated in the metal, then leading to its highδ74/70Ge signature.High-temperature metamorphism can explain the lack of δ74/70Gemetal variation with the petrologictype in the OC, even for the type 3 chondrites (T≈675°C), implying a complete reaction even atlow petrologic types. In addition, metal-silicate re-equilibration in response to thermalmetamorphism results in a decrease in Δ74/70Gemetal-silicate from 0.33‰ to 0.06‰, within the Hchondrite group, which is interpreted as the result of δ74/70Gesilicate variation. The mean positiveΔ74/70Gemetal-silicate fractionation factor of +0.22±0.36‰ (error propagation on individual error) alsodisplays a remarkable similarity to the direction of isotopic fractionation with other germaniumisotopic metal-silicate datasets, such as the magmatic iron meteorites, the Earth silicatereservoirs. We propose that the Δ74/70Gemetal-silicate and the negative δ74/70Ge values of OCs areinherited from metal-silicate melting and partial exchange before planetesimal accretion in a lightisotope-enriched gas. Finally, the δ74/70Gemetal-Δ17Osilicate correlation between the IIE ironmeteorites and OCs, provides new evidence for the existence of a highly reduced HH group.