Origin of the volatile elements in the Solar System: organic matter and clays in chondrites
Most of the volatile elements of the chondrites are located in the organic matter and in the clays. The latter were formed during the aqueous alteration and kept the isotopic signature of the water. The study consists in the characterization through different techniques of the organic matter in two distinct classes of chondrites in order to shed light on its distribution in the protosolar nebula. The organic matter and the clays are intimately mixed in the matrix of chondrites. The measurement of their isotopic compositions should allow to determine whether a link exists between these two phases. An analytical protocol was developed using the nanoscale secondary ion mass spectrometer (NanoSIMS) along with the deconvolution of the D-/H- and 16OD-/16OH- ratios measured in mixtures of known proportions of organic matter and clays. Using this protocol, measurements have been carried out on the matrix of the ordinary chondrite Semarkona. No spatial links were observed between the organic matter and clays. This observation attests the absence of isotopic exchange between these two phases. Exceptionally D-rich clays (D/H > 3000 × 10-6) are present in Semarkona. Although the origin of these clays remains puzzling, their presence in a matrix that is constituted by clays having lower D/H ratios (D/H ~ 800 × 10-6) shows the restricted spread of the aqueous alteration (few μm).