Organic matter and thermal metamorphism in primitive chondrites

The Organic Matter of primitive chondrites is inherited from the presolar cloud, but the chondritic<br />OM have suffered from secondary processes inside the proto-solar nebula and on their asteroidal parent<br />body (Aqueous Alteration, Thermal Metamorphism), that may erase or transform the initial characteristics.<br />The TM has been approached here from OM, which is irreversibly transformed by TM. More than<br />30 chondrites (CV3, CO3, UOC) have been studied : structure of the OM has been quantified by Raman<br />spectroscopy, the texture by HRTEM. Petrographic studies have also been done on each chondrite. A new<br />interclass metamorphic tracer is defined : consistent with other ones available in the litterature, independant<br />from the AA and sensitive to the Peak Metamorphic Temperature. For the most metamorphosed<br />chondrites, the PMT is evaluated from the structural grade of the OM : 330°C (Allende), consistent with<br />other available thermometers. The limitations of the commonly defined petrographic scale are shown<br />and a new one based on a Raman spectral parameter is defined. A precise evaluation of the metamorphic<br />grade of the chondrites allows to discriminate the nebular from the asteroidal effects induced on the<br />petrography. This work gives also some constraints to the models of formation of the chondritic OM. It<br />shows that the organic precursor from ordinary and carbonaceous chondrites are chemically and structurally<br />closed and that the understanding of the isotopic characteristics from the OM (D, 15N) requires to<br />take into account the effects induced by the secondary processes. This work favors an interstellar origin<br />of the precursor of the chondritic OM.