Hydrogen and carbon abundances and isotopic ratios in apatite from alkaline intrusive complexes, with a focus on carbonatites

Abstract

We report H and C contents and δD and δ^(13)C values of apatites from 15 alkaline intrusive complexes ranging in age from 110 Ma to 2.6 Ga. Sampling focused on carbonatites, but included silicate rocks as well. Heating at temperatures up to 1500°C is needed to extract fully H_2O and CO_2 from these apatites. Apatites from carbonatite-rich intrusive complexes contain 0.2–1.1 wt% H_2O and 0.05–0.70 wt% CO_2; apatites from two silicate-rich alkaline complexes with little or no carbonatite are generally poorer in both volatile components (0.1–0.2% H_2O and 0.01–0.11% CO_2). D/H ratios in apatites from these rocks are bimodally distributed: group I (δD = −51 to −74‰) and group II (δD = −88 to −104‰). We suggest that the δD values of group I apatites represent primitive, mantle-derived values and that the group II apatites crystallized from degassed magmas, resulting in lower H_2O contents and δD values. Although many factors influence the extent of degassing, the depth of emplacement could represent a major control. In contrast to H_2O contents and δD values, CO_2 contents and δ^(13)C values of gas released at high temperatures from multiple aliquots of these apatite samples are variable. This suggests the presence of more than one C-bearing component in these apatites, one of which is proposed to be dissolved carbonate; the other, with δ^(13)C ∼<−25‰, could be associated with hydrocarbons. Group I apatites have δD values similar to those of primitive, mantle-derived basaltic magmas and overlap with (but cover a narrower range than) mantle-derived mica, amphibole, and whole rocks. δ^(13)C values also overlap typical upper mantle. These results suggest that igneous apatites can retain their primary δD and δ^(13)C values.