[S-GC52] [EE] Volatile cycles in the Earth - from Surface to Deep Interior

[SGC52-04] Sulfur-rich primitive arc magmas and possible sulfur recycling beneath volcano

*Michael Zelenski¹、Vadim Kamenetsky²、Andrey Gurenko³、John Mavrogenes⁴ (1.Institute of Experimental Mineralogy RAS, Chernogolovka 142432, Russia、2.Earth Sciences and CODES, University of Tasmania, Private Bag 79, Hobart, TAS 7001, Australia、3.Centre de Recherches Pétrographiques et Géochimiques (CRPG), UMR 7358, Université de Lorraine, 54501 Vandoeuvre-lès-Nancy, France、4.Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia)

キーワード：Sulfur-rich magma, Sulfur recycling, Sulfide, Kamchatka

Tephra of high-Mg basalts (8.9 wt% MgO) from the 1941 lateral eruption of Plosky Tolbachik volcano (Kamchatka) contains olivine and chrome-spinel phenocrysts with numerous naturally quenched melt inclusions (MI) 20–100 µm, sometimes up to 350 µm in size. Volatile contents in such inclusions measured by the electron and ion microprobes vary from near zero to exceptionally high values, containing up to 5.1 wt% H₂O, 1200 ppm CO₂, 0.30–0.35 wt% S and 0.28 wt% Cl. This includes the most primitive MI in Cr-spinel (15.6 wt% MgO in the melt) with ~0.3 wt% S. The original CO₂ content was possibly 3–4 times higher because the majority of the original CO₂ can be lost to a shrinkage bubble (e.g., Wallace, 2015). The d³⁴S of the inclusion glasses vary strongly, ranging from +6 to +11‰ and show positive correlation with S contents. Meanwhile, there is a population of MI with anomalously high sulfur (0.5 – 1.08 wt% S), which is close to the maximum sulfur abundance measured in island arc magmas (Wallace and Edmonds, 2011). The origin of anomalous sulfur was attributed to wall rock assimilation, which is supported by positive correlation between δ³⁴S and total S. Considering the volcanic environment, assimilation of hydrothermally altered rocks rich in pyrite, gypsum, alunite and native sulfur seems plausible. High total sulfur suggests that a major fraction of sulfur in the magma was dissolved as sulfate, whereas S^2– contents were just below or slightly above the level of sulfide saturation. The latter was surpassed in some volumes of magma, resulting in segregation of Ni-rich sulfide melt, droplets of which were also trapped in olivine phenocrysts. At the same time, some melt inclusions contain crystals and aggregates of anhydrite CaSO₄. The origin of anhydrite is controversial. Some large aggregates of anhydrite could have been trapped from the melt. On the other hand, subaerial oxidation of melt inclusions containing large sulfide droplets could produce enough sulfate to form anhydrite in situ. In summary, primitive arc magma from Tolbachik demonstrates high volatile contents close to or exceeding the highest ever measured in an arc setting. Additional sulfur was possibly recycled from hydrothermally altered wall rocks and gave rise to sulfide saturation of magma. The presence of anhydrite in primitive melt inclusions requires further investigations.

References:

Wallace, P.J., Kamenetsky, V.S., and Cervantes, P., 2015, Melt inclusion CO2 contents, pressures of olivine crystallization, and the problem of shrinkage bubbles. American Mineralogist, 100, 787-794.


Wallace, P.J., and Edmonds, M., 2011, The sulfur budget in magmas: evidence from melt inclusions, submarine Glasses, and volcanic gas emissions. Reviews in Mineralogy and Geochemistry, 73, 215–246.