Impact delivery of organic matter on the acapulcoite–lodranite parent-body deduced from C, N isotopes and nanostructures of carbon phases in Acapulco and Lodran

The structure and nanostructures of carbon phases from the Acapulco and Lodran meteorites and their carbon and nitrogen isotopic composition were investigated at the nanometer and micrometer scale using a systematic combination of Raman microspectrometry, high-resolution transmission electron microscopy and secondary ion mass spectrometry to determine their origin and thermal evolution. Several morphological types were recognized belonging to roughly two isotopic and structural families: coarse carbon grains and rosettes, only found in Acapulco, and vein-like carbon occurrences present in both Acapulco and Lodran. Carbon phases in Acapulco are highly graphitized, and show a genetic relationship with metal indicative of metal-assisted graphitization. By contrast, carbon phases in Lodran are exclusively disordered mesoporous turbostratic carbons, in spite of their inclusion in metal and the higher peak temperature experienced by the Lodran parent body. delta super(13)C values range between -59ppt and +37ppt in Acapulco and between -38ppt and -1ppt in Lodran and show in both cases a peak in their distribution at the value of chondritic insoluble organic matter (IOM, -10ppt to -15ppt). N concentrations together with delta super(15)N values indicate a mixing between a component akin to chondritic IOM in Lodran with a delta super(15)N value around +10ppt to +20ppt and a component akin to that in the most N-poor Acapulco graphites. The latter are systematically depleted in super(15)N with a delta super(15)N value constant at similar to -140ppt for N concentrations below similar to 1.4 wt%. These observations can be explained if carbon phases in Acapulco and Lodran result from the late impact introduction of CI-CM like IOM, after significant cooling of the parent-body, and subsequent carbonization and graphitization of IOM by interaction with FeNi metal by the heat wave induced by the impact. Temperatures probably reached 900 degree C in Acapulco, enough to achieve metal-assisted graphitization but were not significantly higher than 650 degree C in Lodran. Carbon phases in Lodran would have been formed by the secondary carbonization of hydrocarbon fluids released during the primary carbonization of IOM. In the framework of this model, the C isotopic compositions can be reproduced using Rayleigh distillation at each carbonization step and the N isotopic compositions can be understood as resulting from the variable loss and preservation of super(15)N-rich nitriles (