Experimental metasomatic incorporation of sulfur into fluorapatite as a function of coupled substitutions involving sodium, silicon, iron, and cerium

Here, we report results from hydrothermal alteration and crystallization experiments at 800 °C and 1 GPa that constrain the partitioning of S, Fe, Sr, and Ce (as a proxy for the REE) between fluorapatite and hydrothermal fluids of variable compositions. The data from these experiments demonstrate that S can be incorporated in apatite by the two known coupled substitutions S⁶⁺ + Na⁺ = P⁵⁺ + Ca²⁺ and S⁶⁺ + Si⁴⁺ = 2P⁵⁺. The data also demonstrate that the presence of Sr in hydrothermal fluids promotes the incorporation of Na and S in apatite via the coupled substitution S⁶⁺ + Na⁺ = P⁵⁺ + Sr²⁺. Our data also reveal a previously unknown intrinsic relationship between Fe and S, and Ce and S in metasomatized fluorapatite that can be explained by the coupled substitutions Ca²⁺ + P⁵⁺ = Fe³⁺ + S⁴⁺ and Ca²⁺ + P⁵⁺ = Ce³⁺ + S⁴⁺, respectively. The concentrations of Cl, OH, and S in run-product apatite are positively correlated with each other, which indicates that the presence of Cl and OH can play a determinative role in the incorporation of S in apatite. Overall, the data from these metasomatism experiments, involving S and apatite, demonstrate that incorporation of sulfate and sulfite into apatite during metasomatism depends on the abundance of charge-balancing cations in the fluid.