Apatite as a Tool for Tracking Magmatic CO2 Contents

Abstract:

CO₂ plays a fundamental role in the evolution of magmatic and volcanic systems, but its low solubility in silicate melts means that direct records of magmatic CO₂ concentrations remain elusive. The phosphate mineral apatite is unique among igneous minerals in its capacity to accommodate all major magmatic volatiles (H₂O, F, Cl, CO₂ and S). Although interest in apatite as a tool for tracking magmatic volatile contents (namely H₂O, F, and Cl) has increased in recent years, its potential as a record of magmatic CO₂contents remains untapped.

We present the results of high-temperature, high-pressure experiments investigating the partitioning behaviour of CO₂ between apatite and basaltic melt. Experiments were run in piston cylinder apparatus at 1 GPa and 1250 °C, with a slow initial cooling ramp employed to facilitate crystal growth. Each charge contained the starting basaltic powder doped with Ca-phosphate and variable proportions of H₂O, CO₂, and F. Run products are glass-rich charges containing 15–25 vol% large, euhedral apatite crystals (± cpx and minor biotite). Experimental apatites and glasses have been characterised by BSE imaging, electron microprobe, and ion microprobe.

Apatites range in composition from near-endmember fluorapatite (3.0 wt% F), to near-endmember hydroxyapatite (1.7 wt% H₂O), to carbon-rich apatite containing up to 1.6 wt% CO₂. Apatite compositions are stoichiometric if all anions (F^–, OH^–, and CO₃^2—) lie in the channel site, suggesting an “A-type” substitution under these conditions (i.e. CO₃^2— + [] = 2X^—, where X is another channel anion and [] is a vacancy; e.g. Fleet et al. 2004). Importantly, CO₂ partitions readily into apatite at all fluid compositions considered here. CO₂ is also more compatible in apatite than water at our run conditions, with calculated H₂O–CO₂ exchange coefficients close to or greater than 1. Our results indicate that when channel ions are primarily occupied by H₂O and CO₂ (i.e. F- and Cl-poor magmatic systems), apatite can preserve higher absolute CO₂ concentrations than the surrounding melt. In this way, apatite may prove more sensitive than other direct records of pre-eruptive volatile contents, such as phenocryst-hosted melt inclusions. We suggest that apatite has the potential to be a useful tool for tracking magmatic CO₂ abundances.