SULFUR CONTENT AND ZONATION IN APATITE AS TOOL TO RECONSTRUCT OXIDIZED SULFUR BUDGETS IN SILICIC MAGMAS

Our goal is to develop apatite as tool for reconstructing the sulfur evolution of oxidized, calc-alkaline silicic magmas. This includes documenting how much sulfur is present and how it is distributed within individual apatite grains. As apatite grows from a melt, it can incorporate oxidized sulfur (S6+) forming SO42– instead of PO43– in the apatite crystal lattice. This substitution is coupled with additional substitutions (e.g. Na+ for Ca2+ or Si4+ for P5+) to balance charges. The capability to incorporate sulfur makes apatite a potential important monitor for how much oxidized sulfur was present and how it evolved with differentiation of a magma, especially in cases where other evidence is lacking, e.g. melt inclusions. We have focused on selected samples of mostly volcanic deposits from well-known centers of calc-alkaline silicic magmatism. The deposits that are currently being investigated include silicic tuffs from the San Luis Caldera, CO, the Fish Canyon Tuff, and the Yanacocha Volcanics, Peru which are associated with a porphyry copper system. In addition, samples from the rhyolitic Rattlesnake Tuff, OR have been included to obtain a benchmark for sulfur content in apatite of silicic magma at more reducing conditions. Our analytical approach includes the following. First, apatite was located in grain mounts and thin sections using standard light microscopic techniques and backscattered electron images. Apatite was then screened with help of reconnaissance electron microprobe analysis to select individual grain for element maps. Preliminary results indicate that apatite of all calc-alkaline silicic magmas are typically zoned with higher sulfur concentrations in the cores of the apatite grains decreasing outward, similar to previous results obtained on a suite of plutonic rocks from the Yerington batholith (Geology, 26: 523-526, 1998).