FACTORS CONTROLLING SULFUR CONCENTRATIONS IN VOLCANIC APATITE

Apatite crystals from two types of samples were analyzed by electron m icroprobe for 15 major and trace elements: (1) apatite in H2O- and S-s aturated experimental charges of the 1982 El Chichon trachyandesite an d (2) apatite in volcanic rocks erupted from 20 volcanoes. The SO3 con tents of the experimental apatite increase with increasing oxygen fuga city (fO(2)), from less than or equal to 0.04 wt% in reduced charges b uffered by fayalite-magnetite-quartz (FMQ), to 1.0-2.6 wt% in oxidized charges buffered by manganosite-hausmanite (MNH) or magnetite-hematit e (MTH). The SO3 contents of MNH- and MTH-buffered apatite also genera lly increase with increasing pressure from 2 to 4 kbar and decreasing temperature from 950 to 800 degrees C. The partition coefficient for S O3 between apatite and oxidized melt increases with decreasing tempera ture but appears to be independent of pressure. Apatites in volcanic r ocks show a wide range of SO3 contents (<0.04 to 0.63 wt%). Our sample set includes one group known to contain primary anhydrite and a secon d group inferred to have been free of primary anhydrite, No systematic differences in apatite S contents are observed between these two grou ps. Our study was initiated to define the factors controlling S conten ts in apatite and to evaluate the hypothesis that high S contents in a patite could be characteristic of S-rich anhydrite-bearing magmas such as those erupted from El Chichon in 1982 and Pinatubo in 1991, This h ypothesis is shown to be invalid, probably chiefly a consequence of th e slow intra-crystalline diffusion that limits re-equilibration betwee n early formed apatite and the evolving silicate melt. Contributing fa ctors include early crystallization of most apatite over a relatively small temperature interval, common late-stage magmatic enrichment of S , progressive oxidation during magmatic evolution, and strong controls on S contents in apatite exerted by fO(2), temperature, and pressure.