Thursday, December 6, 2018
4:00 pm

Geology Club Seminar

Sulfur systematics illuminate the elusive record magma redox evolution
Rita Economos, Assistant Professor, Dedman College of Humanities & Sciences, Southern Methodist University

Sulfur is an important element in igneous systems due to its impact on magma redox, its role in the formation of economically valuable ore deposits, and the influence of catastrophic volcanogenic sulfur degassing on global climate. The mobility and geochemical behavior of sulfur in magmas is complex due to its multi-valent (from S2- to S6+) and multi-phase (solid, immiscible liquid, gaseous, dissolved ions) nature. Sulfur behavior is closely linked with the evolution of oxygen fugacity (fO2) in magmas; the record of fO2 evolution is often difficult to extract from rock records, particularly for intrusive systems that undergo cyclical magmatic processes and crystallize to the solidus. We apply a novel method of measuring sulfur isotopic ratios via secondary ion mass spectrometry (SIMS) in zoned apatite crystals that we interpret as a record of open-system magmatic processes. We analyzed the sulfur 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 at UCLA. Isotopic variations in single apatite crystals ranged from 0 to 3.8‰ δ34S and total variation within a single hand sample was 6.1‰ δ34S. High S concentration cores yielded high isotopic ratios while low S concentration rims yielded low isotopic ratios. We favor an explanation of a combination of magma mixing and open-system, ascent-driven degassing under moderately reduced conditions: fO2 at or below NNO +1, although the synchronous crystallization of apatite and anhydrite is also a viable scenario. These findings have implications for the coupled sulfur and fO2 evolution of granitic plutons and suggest that in-situ apatite sulfur isotopic measurements could be a powerful new tool for evaluating coupled redox and sulfur behavior in magmatic systems.

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