Diamond and graphite are fundamental sources of carbon in the upper mantle, and their reactivity with H 2 -rich fluids present at these depths may represent the key to unravelling deep abiotic hydrocarbon formation. We demonstrate an unexpected high reactivity between carbons’ most common allotropes, diamond and graphite, with hydrogen at conditions comparable with those in the Earth’s upper mantle along subduction zone thermal gradients. Between 0.5-3 GPa and at temperatures as low as 300 °C, carbon reacts readily with H 2 yielding methane (CH 4 ), whilst at higher temperatures (500 °C and above), additional light hydrocarbons such as ethane (C 2 H 6 ) emerge. These results suggest that the interaction between deep H 2 -rich fluids and reduced carbon minerals may be an efficient mechanism for producing abiotic hydrocarbons at the upper mantle.