Shock-induced dehydrogenation of polycyclic aromatic hydrocarbons with or without serpentine: Implications for planetary accretion

Polycyclic aromatic hydrocarbons (PAHs) were shocked across a peak shock pressure range of 5.8 to 36.6 GPa with or without serpentine and were then evaluated with an elemental analyzer, an X-ray diffractometer, a gas chromatograph, and a gas chromatograph-mass spectrometer. On the basis of the results, we examined the shock-induced dehydrogenation of PAHs and the matrix's effects on it. Moreover, we discussed the hydrogen amounts supplied to the Earth's atmosphere and interior during its formation, as well as the δD values of those amounts. We did not find any matrix effects on the dehydrogenation of PAHs in the pressure range used in this study. The incipient dehydrogenation of PAHs occurred at 10 GPa and dehydrogenation was almost completed at 36.5 GPa. Assuming that carbonaceous chondrites, such as the Murchison meteorite accreted on and formed the Earth, they should have started and completed dehydrogenation at a point where the growing Earth reached a radius of 1020 km and 3060 km, respectively. By the end of the accretion, most of the hydrogen delivered to the Earth had been discharged into the atmosphere. The cumulative amount of hydrogen in the Earth's interior at that point and its δD value were calculated to be 3.3×1021 kg and −47‰, respectively. These calculated data were comparable to those already established for the Earth's interior. However, to explain the isotopically light-juvenile hydrogen (D=−125‰) in the deep Earth reported by Hauri, Chem. Geol. 183 (2002) 115-141, we should consider materials that have low δD values (e.g., −850‰ of the presolar nebular) as sources of the Earth's hydrogen.