Sulfur isotopic zoning in apatite crystals: A new record of dynamic sulfur behavior in magmas

The mobility and geochemical behavior of sulfur in magmas is complex due to its multi-phase (solid, immiscible liquid, gaseous, dissolved ions) and multi-valent (from S-2 to S6+) nature. Sulfur behavior is closely linked with the evolution of oxygen fugacity (fO(2)) in magmas; the record of fO(2) evolution is often enigmatic to extract from rock records, particularly for intrusive systems. We apply a novel method of measuring S isotopic ratios in zoned apatite crystals that we interpret as a record of open-system magmatic processes. We interrogate the S concentration and isotopic variations preserved in multiple apatite crystals from single hand specimens from the Cadiz Valley Batholith, CA via electron microprobe and ion microprobe. Isotopic variations in single apatite crystals ranged from 0 to 3.8% delta S-34 and total variation within a single hand sample was 6.1% delta S-34. High S concentration cores yielded high isotopic ratios while low S concentration rims yielded low isotopic ratios. We discuss a range of possible natural scenarios and favor an explanation of a combination of magma mixing and opensystem, ascent-driven degassing under moderately reduced conditions: fO(2) at or below NNO+1, although the synchronous crystallization of apatite and anhydrite is also a viable scenario. Our conclusions have implications for the coupled S and fO(2) evolution of granitic plutons and suggest that in-situ apatite S isotopic measurements could be a powerful new tool for evaluating redox and S systematics in magmatic systems. (C) 2017 Elsevier Ltd. All rights reserved.