LARGE CARBON ISOTOPE FRACTIONATIONS IN APATITE

Carbon isotope ratios were examined in co-existing apatite and calcite from igneous, metaigneous, and metasedimentary rocks, primarily from the Grenville Province. High-temperature apatite allows for the incorporation of many minor elements, including carbon at the level of 100s of ppm. Carbon substitution in apatite is commonly thought to be in the form of CO₃ substituting for PO₄ or OH, which should have a carbon isotope fractionation with co-existing calcite approximately equal to zero because of a similarity in bonding environments. We show that this is not the case for samples analyzed in this study when carbon is liberated from apatite by combustion. Apatite in studied samples has δ¹³C values that range from -21.9 to -6.7‰ (PDB) while calcite from the same rocks range from -5.2 to -1.0‰. This large and variable fractionation (Δ=1.8 to 19.6‰) is not similar to the expected fractionation of ca. 0‰ if carbon in apatite is entirely CO₃. This result indicates that carbon isotopes in apatite sampled by bulk techniques (combustion, ion probe, step-heating) do not have a straightforward interpretation. We suggest that the low δ¹³C values in apatite that have previously been used as indicators of biological activity may be the result of the fractionation behavior of apatite, rather than a reflection of incorporation of organic carbon.