Fluid evolution during metamorphism of the Otago Schist, New Zealand: (II)Influence of detrital apatite on fluid salinity

Apatite occurs in the zeolite to greenschist facies metamorphic rocks of th e Otago Schist, South Island, New Zealand, as both a groundmass constituent and as a hydrothermal phase hosted in metamorphic quartz veins. Groundmass apatite from low-grade rocks, ranging from the zeolite facies to the pumpe llyite-actinolite zone, has chloride contents ranging from 0-1.4 wt%, and f luoride contents ranging from 2.2-4.2 wt%, whilst groundmass apatite from t he greenschist facies (chlorite to biotite zone) is virtually pure fluorapa tite. Vein apatite from all grades is also fluorapatite with little or no c hloride. This difference in composition is interpreted as resulting from th e preservation of the primary magmatic compositions of detrital Cl-apatite grains, out of equilibrium with the metamorphic fluid, at low grades, whils t higher-grade groundmass apatite and neoformed apatite in quartz veins hav e compositions in equilibrium with an aqueous metamorphic fluid. The presen ce of detrital Cl-bearing apatite during the early stages of metamorphism m ay constitute a significant reservoir of Cl, given the low porosities of co mpacted sediments undergoing prograde metamorphism. Calculations indicate t hat the release of Cl from detrital apatite in the Otago Schist, as a resul t of re-equilibration of apatite with the pore fluid, may have had a signif icant effect on the salinity of the metamorphic fluid.