Publikationsdatum:
2013-08-24
Beschreibung:
We investigate the relationship between sulfur degassing and oxygen fugacity at Erta Ale and Masaya volcanoes. Oxygen fugacity was assessed utilizing Fe 3+ /∑Fe ratios and major element compositions measured in olivine-hosted melt inclusions and matrix glasses. Erta Ale melts have Fe 3+ /∑Fe of 0.15 to 0.16, reflecting f O 2 of ΔQFM 0.0±0.3, which is indistinguishable from f O 2 calculated from CO 2 /CO ratios in high temperature gases. Masaya is more oxidized at ΔQFM +1.7±0.4, typical of arc settings. Sulfur isotope compositions of gases and scoria at Erta Ale (δ 34 S gas -0.5‰; δ 34 S scoria +0.9‰) and Masaya (δ 34 S gas +4.8‰; δ 34 S scoria +7.4‰) reflect distinct sulfur sources, as well as isotopic fractionation during degassing (equilibrium and kinetic fractionation effects). Sulfur speciation in melts plays an important role in isotope fractionation during degassing and S 6+ /∑S is 〈0.07 in Erta Ale melt inclusions compared to 〉0.67 in Masaya melt inclusions. No change is observed in Fe 3+ /∑Fe or S 6+ /∑S with extent of S degassing at Erta Ale, indicating negligible effect on f O 2 , and further suggesting that H 2 S is the dominant gas species exsolved from the S 2- -rich melt (i.e. no redistribution of electrons). High SO 2 /H 2 S observed in gas emissions is due to gas re-equilibration at low pressure and fixed f O 2. Sulfur budget considerations indicate that the majority of S injected into the systems is emitted as gas, which is therefore representative of the magmatic S isotope composition. The composition of the Masaya gas plume (+4.8‰) cannot be explained by fractionation effects but rather reflects recycling of high δ 34 S sulfate through the subduction zone.
Digitale ISSN:
1525-2027
Thema:
Chemie und Pharmazie
,
Geologie und Paläontologie
,
Physik
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