Deep carbon through time: Earth's diamond record and its implications for carbon cycling and fluid speciation in the mantle

Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle fO(2) over a vast portion of Earth's history. Diamonds' inertness and antiquity means their carbon isotopic characteristics directly reflect their growth environment within the mantle as far back as similar to 3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from similar to 3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total delta C-13 data range from -17.1 to -1.9 parts per thousand (P = -8.4 to -1.9 parts per thousand; E = -17.1 to -2.1 parts per thousand) and N contents range from 0 to 3073 at. ppm (P 0 to 3073 at. ppm; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since > 3 Ga. The mode in delta C-13 of peridotitic diamonds has been at similar to 5 (+/- 2) parts per thousand since the earliest diamond growth similar to 3.5 Ga, and this mode is also observed in the eclogitic diamond record since similar to 3 Ga. The skewness of eclogitic diamonds' delta C-13 distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent.