Detection of incipient aqueous alteration in carbonaceous chondrites

We discuss if the detection of aqueous alteration depends on the techniques that are used. We apply different
methods to estimate the extent of aqueous alteration on four ungrouped carbonaceous chondrites
showing limited aqueous alteration and thermal metamorphism: Chwichiya 002, El Médano (EM) 200,
Northwest Africa (NWA) 12957 and NWA 11750, classified as C3 or C3.00-ung. The aim is to propose a
reliable methodology to identify the most primitive chondrites. Chwichiya 002, NWA 11750 and NWA
12957 display very primitive matrices and could be amongst the most primitive chondrites currently
known, similar to the least altered lithologies of the CM chondrites Paris (CM2.9) and Asuka (A) 12085
(CM2.8), A 12236 (CM2.9) and A 12169 (CM3.0). The structure of organic matter and Cr2O3 in ferroan
olivines show that the four meteorites have been less heated than the least metamorphosed standard/reference
type 3 chondrite, Semarkona (LL3.00), with Chwichiya 002, NWA 12957 and NWA 11750 similar
to the CO3.0 s, Acfer 094 (C2-ung) and Paris meteorites. Chwichiya 002 and NWA 12957 show similar
alteration phases and degree of alteration, with high abundances of amorphous material with embedded
metal and sulfide, resembling Glass with Embedded Metal and Sulfide (GEMS)-like materials, and
tochilinite-cronstedtite intergrowths (TCIs) as the major alteration phases. The matrix in NWA 11750
contains aggregates of nanoscale olivine crystals and abundant carbonates, observed as micrometersized
carbonate veins surrounding chondrules, and as nanoscale carbonates mixed with the finegrained
materials. It also contains abundant grains of metal and a low abundance of phyllosilicates. El
Medano 200 shows a high abundance of magnetite ( 10 vol%), nanoscale phyllosilicates, troilite, and
organic matter. The variability of the secondary alteration phases in the meteorites suggests different
alteration mechanisms, likely depending on both the starting composition of the meteorites and the composition
of the fluids of alteration.