KEROGEN-RICH MICROMETEORITES AND CRUDE PETROLEUM IN HADEAN TIME. M.Maurette,

Delivery of micrometeorite kerogen to the Hadean oceanic crust. Kerogens are abundant byproducts of incomplete combustion, pyrolysis and radiation reprocessing of almost any kinds of organic precursors. Therefore they are observed in a wide variety of environments, including: ▬ terrestrial shales, ▬ the fine-grained matrix of hydrous-carbonaceous chondrites (HCCs) such as Murchison; ▬ the interstellar medium; ▬ the exhaust pipes of cars. In the absence of microorganisms that can digest it, kerogen is one of the most durable and insoluble classes of organic compounds known. In particular it resists to both strong acids and ordinary organic solvents. The acid resistant residue of Murchison is essentially made of kerogen with an approximate formula, C100H48N1.8S2012 [1]. On Earth, important substances are derived from some parent kerogen. They include petroleum and varieties of activated carbon. The ion microprobe analysis of 28 Antarctic micrometeorites (AMMs) yielded their average C content of about 2.5 wt.% (see Fig. 5, in Ref. 2). The nuclear microprobe measurements of the C and N contents of AMMs and HCCs [3] indicated that their major carbonaceous components are rather similar, being composed of kerogen with a C/N ratio varying from ~25 to ~50. This is also compatible with previous studies including: (i) analyses of much larger aliquot of HCCs by GCMS [1]; (ii) microanalyses of AMMs and CM-, CIand CV-types carbonaceous chondrites with a double laser desorption-ionization mass spectrometer that revealed the mass spectra of the building blocks of their constituent kerogen, which are polycyclic aromatic hydrocarbons (i.e., PAH-moieties) [4]; (iii) microanalyses of AMMs with a Fourier Transform IR microscope [5]. In our EMMA scenario (Early micrometeorite Accretion) a huge mass influx of juvenile micrometeorites delivered about 5 × 10 g of micrometeorites during the first ~200 Ma of the post-lunar period of the Late Heavy Bombardment (LHBomb) [2, 6] –i.e., a value twice as high as the total mass of the current asteroidal belt. The study of our new collection of unweathered micrometeorites collected in surface snow from Central Antarctica shows that about 20 wt.% of the incoming micrometeorites survive unmelted upon atmospheric entry. A large fraction of them was deposited on the early oceanic crust, which was formed very soon after the formation of the Moon, about 4.4 Ga ago [7]. Assuming that about 50% of the early Earth's surface was covered with oceans, the total amount of kerogen deposited in the deep sea (sediments) (~1.6 × 10g) was equivalent to a ~30 m thick global layer, whereas the value estimated for organics trapped in the present day biosphere is about 10 cm. What was the fate of this kerogen? Could it have been involved in the prebiotic chemistry of life even though it is considered as one of the most inert organics?