STABILITY OF S-BEARING APATITE AND BIOTITE

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% SO_3, 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.