3:45 PM - 4:00 PM
[SGC45-07] Na/K ratios of sulfate-bearing saline fluid inclusions in harzburgite xenoliths from Pinatubo: Comparison with H2O-rich components in arc and back-arc basin basalts
Keywords: aqueous fluid, magma, subduction zone, fluid inclusion
Olivine-hosted melt inclusions show a positive correlation between water contents and Fe3+/Fe2+ ratios (Kelley and Cottrell 2009 Science). Presence of sulfate ions in the fluid inclsuions, which can be slab-derived fluids, can explain such sulfate-bearing fluids can provide H2O and oxidize the wedge mantle. This also provides insights into the genesis of calc-alkaline rock series characterized by a high oxygen fugacity (Miyashiro 1974 Am J Sci). Our observation solves the missing link between high-S in arc magmas (Le Voyer 2010 J Petrol) and presence of sulfate in the slab serpentinites (Alt et al. 2012) and high-pressure metamorphic rocks (Frezzotti and Ferrando 2007 Per Mineral). Current estimate of S content in the aqueous fluids in forearc (<0.5wt% S/H2O) is more than an order of magnitude lower than that in melt inclusions in arc basalts (6-60wt% S/H2O; Le Voyer 2010, J Petrol). This indicates that sulfur can be enriched during partial melting of the mantle with larger S/H2O than that of slab-derived fluids. Alternatively fluids released beneath sub-arc contain larger amount of S by increasing solubility in supercritical fluids.
The Na/K ratio was determined in the fluid inclusions (>0.02 mm) in olivine crystals using a quadrupole inductively coupled plasma-mass spectrometry equipped with a 193 nm excimer laser ablation system. The obtained Na/K ratios (Na0.73K0.27) are a little more K-rich than those of aqueous fluids in the generation of the Mariana trough back-arc magmas (Na0.82K0.18, Stolper and Newman 1995, EPSL), H2O-rich components in Mt. Shasta (Na0.77K0.23, Grove et al. 2002, CMP), and similar to those between two H2O-rich components in Mt. Shasta (Na0.70K0.30 of melt or supercritical fluids components and Na0.87K0.13 of aqueous fluids components, Le Voyer et al. 2010, J Petrol).