Abiotic polycyclic aromatic hydrocarbons originating from the sub-oceanic mantle.

Experimental studies have demonstrated that petroleum may form from abiotic organic compounds synthesised in the Earth's upper mantle. However, the most direct evidence of mantle-derived organics, as obtained from whole-rock analyses, indicates ambiguous origins. Although inclusions containing organic matter have been reported in minerals from kimberlite pipes, such occurrences have not previously been documented in mantle xenoliths derived from the sub-oceanic upper mantle, which represent the predominant lithology of the Earth's upper mantle. In this study, we identified polycyclic aromatic hydrocarbons (PAHs)-bearing melt inclusions within a spinel-bearing unaltered harzburgite xenolith from Tahiti Island. The inclusions consisted of platinum-group minerals, base metal sulphides, silicate glass, and C-O-H phases. Our comprehensive sub-micrometre-scale analyses revealed the presence of PAHs in the C-O-H phases. These PAHs contained few functional groups or aliphatic chains and coexisted with CO and CO₂. They likely formed under reducing conditions in deeper regions of the upper mantle without serpentinisation. Our findings demonstrate abiotic organic matter formation in the sub-oceanic mantle and suggest widespread abiotic organic synthesis in the sub-oceanic mantle.