2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 10
Presentation Time: 4:25 PM


CORE, Daniel P.1, ESSENE, Eric2, KESLER, Stephen2 and ALT, Jeffrey C.3, (1)Geological Sciences, Univ of Michigan, 2534 C. C. Little Building, 425 E. University Ave, Ann Arbor, MI 48109, (2)Geological Sciences, Univ of Michigan, 2534 C.C. Little Building, 425 E. University Ave, Ann Arbor, MI 48109, (3)Univ Michigan - Ann Arbor, 425 E University Ave, Ann Arbor, MI 48109-1063, dcore@umich.edu

The behavior of S during crystallization of felsic magmas is important to understanding S-rich volcanic eruptions and formation of ore deposits. S is present in magma as sulfide and sulfate; sulfide dominates at low fO2 and sulfate dominates at high fO2. Sulfides and anhydrite, the most common S-bearing phases in igneous rocks, are easily destroyed during degassing or low temperature alteration. However, significant amounts of S substitute in apatite and biotite, which can be used to indicate the behavior of S during crystallization, particularly the appearance of a S-bearing mineral or S-rich vapor phase.

         Biotite in felsic magmas can contain up to 0.1% ppm S, which is buffered by reactions such as:

2KFe3AlSi3O10(OH)(HS) + 2O2=2KAlSi3O8 + 2Fe3O4 + S2 + 2H2O

indicating that S contents of biotite decrease with increasing fO2 or decreasing fS2. Biotite from the oxidized Last Chance stock near Bingham (UT) has no detectable S whereas biotite from the reduced Clayton Peak stock (UT) contains about 400 ppm S. The presence of S-rich biotite indicates that sulfide is the dominant form of S in the magma and that the magma may have saturated with respect to sulfide.

Apatite in felsic intrusive rocks contains up to 1 wt% SO3, which is commonly zoned in sulfate as shown by S X-ray mapping. The S content of apatite is controlled by:

12Ca5(SO4)1.5(SiO4)1.5(OH)+20Fe3O4+102SiO2=60CaFeSi2O6+36S2+37O2+6H2O (1) and

4Ca5(SO4)1.5(SiO4)1.5(OH)+7S2+21O2=20CaSO4+6SiO2+2H2O (2).

In the absence of anhydrite the S content of apatite will decrease with decreasing fO2, fS2 and fH2O. Reaction (2) indicates that sulfate in apatite is buffered at a high level in the presence of anhydrite. The Hanover-Fierro stock (NM) contains magmatic anhydrite in deep parts of the stock. Apatite in this part of the stock is unzoned and contains 1 wt % SO3. The upper part of the stock has no magmatic anhydrite but has zoned apatite with high S cores (0.7 wt. % SO3) and low S rims (<0.04 wt. % SO3). The observed zoning in apatite in the upper part of the stock is likely caused by fluctuations in the fO2-fS2-fH2O conditions due to removal of a vapor phase rather than anhydrite saturation. These data show that sulfur content and zoning of biotite and apatite can provide important information on saturation of a S-bearing mineral or vapor phase.