Low carbon isotope ratios in apatite: An unreliable biomarker in igneous and metamorphic rocks

Carbon isotope ratios of apatite have been used to interpret the source of carbon in magmas and metamorphic fluids, using the assumption that carbon isotope fractionations between phases are small in igneous and metamorphic systems. Measured carbon isotope fractionations between co-existing apatite and calcite from igneous, metaigneous, and metasedimentary rocks are large and variable (Δ13C = 2.3 to 20.8‰), reflecting variability in δ13C(Ap) sampled by combustion. This result indicates that carbon isotopes in apatite sampled by common techniques (combustion, ion microprobe, step-heating) do not have a straightforward interpretation. These data suggest that the low-δ13C values in apatite that have previously been used as indicators of biological activity may be the result of the carbon isotope fractionation systematics of apatite, rather than a reflection of incorporation of organic carbon. This unusual fractionation behavior may require reconsideration of the significance of low-δ13C carbonaceous inclusions in apatite from ≥ 3.8 Ga metamorphic rocks from Akilia Island, Greenland.