Serpentinization may provide a unique environment for the abiotic formation of condensed carbonaceous matter. This could support the deep biosphere and contribute to the deep carbon cycle, and may have provided the first building blocks for life. However, thus far, condensed carbonaceous matter has been found only in association with the minor mineral constituents of serpentinites. In contrast, here we show the direct association between carbonaceous matter and the dominant Fe oxide in serpentinites, magnetite. Our samples were recovered from the Yap Trench, western Pacific Ocean, with a human-occupied vehicle at a depth of 6413 m below sea level. The carbonaceous matter coincides with some micron-sized magnetite grains, but particularly with nanosized Fe oxides within serpentinite nanopores. Vibrational spectroscopy reveals that the condensed carbonaceous matter contains both aliphatic and aromatic compounds, but there is no evidence for functional groups typical for biological organics. Based on these observations, we suggest that physicochemical phenomena in serpentinite nanopores and nanosized catalytically active minerals may play a key role in the abiotic synthesis of complex carbonaceous matter.

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