Publication Date:
2022-05-26
Description:
Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 142 (2014): 501-521, doi:10.1016/j.gca.2014.07.015.
Description:
Glasses produced from decompression experiments conducted by Fiege et al. (2014a) were used to
investigate the fractionation of sulfur isotopes between fluid and andesitic melt upon magma
degassing. Starting materials were synthetic glasses with a composition close to a Krakatau dacitic
andesite. The glasses contained 4.55 to 7.95 wt% H2O, ~140 to 2700 ppm sulfur (S), and 0 to 1000
ppm chlorine (Cl). The experiments were carried out in internally heated pressure vessels (IHPV) at
1030°C and oxygen fugacities (fO2) ranging from QFM+0.8 log units up to QFM+4.2 log units (QFM:
quartz-fayalite-magnetite buffer). The decompression experiments were conducted by releasing
pressure (P) continuously from ~400 MPa to final P of 150, 100, 70 and 30 MPa. The decompression
rate (r) ranged from 0.01 to 0.17 MPa/s. The samples were annealed for 0 to 72 h (annealing time, tA)
at the final P and quenched rapidly from 1030°C to room temperature (T).
The decompression led to the formation of a S-bearing aqueous fluid phase due to the relatively large
fluid-melt partitioning coefficients of S. Secondary ion mass spectrometry (SIMS) was used to
determine the isotopic composition of the glasses before and after decompression. Mass balance
calculations were applied to estimate the gas-melt S isotope fractionation factor αg-m.
No detectable effect of r and tA on αg-m was observed. However, SIMS data revealed a remarkable
increase of αg-m from ~0.9985 ± 0.0007 at 〉QFM+3 to ~1.0042 ± 0.0042 at ~QFM+1. Noteworthy, the
isotopic fractionation at reducing conditions was about an order of magnitude larger than predicted by previous works. Based on our experimental results and on previous findings for S speciation in fluid
and silicate melt a new model predicting the effect of fO2 on αg-m (or Δ34S g-m) in andesitic systems at
1030°C is proposed. Our experimental results as well as our modeling are of high importance for the
interpretation of S isotope signatures in natural samples (e.g., melt inclusions or volcanic gases).
Description:
This project was supported by the German Science Foundation (BE1720/25-1 to H. Behrens), by the
German National Academic Foundation, and by Collaborative Research Grants from the U.S. National
Science Foundation (EAR-0838482 to C. W. Mandeville, EAR-0838436 to N. Shimizu, and EAR-
0838328 to K. A. Kelley).
Keywords:
Sulfur isotopes
;
SIMS
;
Isotopic fractionation
;
Fluid-melt
;
Magma degassing
;
Andesite
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
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