LOW CARBON ISOTOPE RATIOS IN HIGH-TEMPERATURE APATITE: IMPLICATIONS FOR USE AS A BIOMARKER

Trace carbon (100s of ppm) is common in apatite from igneous and metamorphic rocks, and carbon isotope isotope ratios of high-temperature apatite have been used in some studies to interpret the source of carbon in magmas and metamorphic fluids. This is based on the assumption that carbon isotope fractionations are small in igneous and metamorphic systems, an assumption we evaluate in this study. We report carbon isotope ratios for co-existing apatite and calcite from igneous, metaigneous, and metasedimentary rocks, and show that fractionations between calcite and apatite are large and variable (Δ¹³C=2 to 20‰), reflecting variability in δ¹³C of combusted apatite. This result indicates that carbon isotopes in apatite analyzed by techniques other than phosphoric acid dissolution (such as combustion, ion microprobe, and step-heating) may 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 carbon isotope fractionation systematics of apatite, rather than a reflection of incorporation of organic carbon. This unusual fractionation behavior suggests caution when interpreting the significance of low δ¹³C of inclusions in apatite, such as those from ≥3.8 Ga metamorphic rocks from Akilia Island, Greenland.