Mp. Smith et Bwd. Yardley, Fluid evolution during metamorphism of the Otago Schist, New Zealand: (II)Influence of detrital apatite on fluid salinity, J METAMORPH, 17(2), 1999, pp. 187-193
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.