Paper No. 1
Presentation Time: 1:30 PM
THE SIGNATURE OF THERMAL METAMORPHISM IN PRIMITIVE PLANETESIMALS RECORDED IN EXTRATERRESTRIAL ORGANIC SOLIDS
Primitive chondritic meteorites contain appreciable amounts of carbonaceous organic solids. It is well known that the molecular structure of these organic solids is modified in a systematic way in response to long-term thermal metamorphism associated with radiogenic heating in the planetesimal interiors. Such changes have been noted by others using Raman spectroscopy and by us using Carbon X-ray Absorption Near Edge Spectroscopy (C-XANES) as well as 13C Solid State Nuclear Magnetic Resonance (NMR) spectroscopy. In the case of C-XANES one observes, upon long-term heating, the development of a 1s-sigma* exciton, a characteristic absorption feature diagnostic of the presence of graphene domains. In the case of solid-state 13C solid state NMR, one observes the development of a large negative paramagnetic shift of the aromatic carbon resonance, with increased thermal metamorphism, over long time scales, where the paramagnetic shift indicates transition from an organic insulator into an organic conductor. Isothermal laboratory heating experiments were used to determine the kinetics of C-XANES exciton development with time; thus allowing the use of exciton intensity as an effective thermometer assuming isothermal heating. A new focus has been the application of 1H solid state NMR to follow molecular evolution of carbonaceous solids subjected to parent body thermal metamorphism. We observe systematic changes in the 1H resonance spectrum that correlate very well with both the C-XANES features and the 13C paramagnetic shifts. Finally, thermally metamorphosed organic solids derived from primitive (undifferentiated) planetesimals are compared with terrestrial anthracite coals. Anthracite coals are obviously derived from a precursor material that differs completely from carbonaceous solids contained within planetesimals. Anthracites have also experienced a vastly different metamorphic history, i.e. these coals have been subjected to significantly lower temperatures but in the presence of considerable shear. It will be shown that these different metamorphic styles as well as different precursors lead to very different molecular structural outcomes.