The carbonate content in high-temperature apatite; an analytical method applied to apatite from the Jacupiranga alkaline complex

Abstract

We present a method to measure carbonate content of apatite that uses infrared spectroscopy, The method was calibrated against the amount of C02 released by reaction of apatite with phosphoric acid and may be used to study apatite with C02 content below I wt%. We have applied the method to apatite from carbonatite (>:::0.3-0.4 wt% C02) and metasomatic (>:::0.6-1.0 wt% C02) and alkaline silicate «0.3 wt% C02) rocks from the Jacupiranga alkaline complex. The results show that higher concentration of carbonate was found in apatite related to metasomatic processes and, in particular, to metasomatism related to the intrusion of carbonatite from J acupiranga. Major and trace elements of the analyzed apatites do not show a simple substitution mechanism that would explain the presence of carbonate in the apatite structure. In a series of exploratory laboratory experiments in which apatite was synthesized under high temperature and pressure, no clear relationship was observed between the carbonate content of the synthetic apatite and the total C content in the coexisting fluid (molecular C02 or carbonate ions in solution). However, even though these experiments were not conclusive, they suggest that apatite coexisting with carbonate ions in solution has a C02 content above 0.3 wt%, whereas apatite coexisting with molecular C02 has a C02 content <0.3 wt%. These results indicate that the C02 content of apatite depends not only on total fluid C content, but also on the C speciation in the fluid.