- GeoRef, Copyright 2004, American Geological Institute.
Apatite crystals from two types of samples were analyzed by electron microprobe for 15 major and trace elements: (1) apatite in H 2 O- and S-saturated experimental charges of the 1982 El Chichon trachyandesite and (2) apatite in volcanic rocks erupted from 20 volcanoes. The SO 3 contents of the experimental apatite increase with increasing oxygen fugacity (f O2 ), from < or =0.04 wt% in reduced charges buffered by fayalite-magnetite-quartz (FMQ), to 1.0-2.6 wt% in oxidized charges buffered by manganosite-hausmanite (MNH) or magnetite-hematite (MTH). The SO 3 contents of MNH- and MTH-buffered apatite also generally increase with increasing pressure from 2 to 4 kbar and decreasing temperature from 950 to 800 degrees C. The partition coefficient for SO 3 between apatite and oxidized melt increases with decreasing temperature but appears to be independent of pressure. Apatites in volcanic rocks show a wide range of SO 3 contents (< or =0.04 to 0.63 wt%). Our sample set includes one group known to contain primary anhydrite and a second group inferred to have been free of primary anhydrite. No systematic differences in apatite S contents are observed between these two groups. Our study was initiated to define the factors controlling S contents in apatite and to evaluate the hypothesis that high S contents in apatite could be characteristic of S-rich anhydrite-bearing magmas such as those erupted from El Chichon in 1982 and Pinatubo in 1991. This hypothesis is shown to be invalid, probably chiefly a consequence of the slow intra-crystalline diffusion that limits re-equilibration between early formed apatite and the evolving silicate melt. Contributing factors 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 f O2 , temperature, and pressure.