Sulfur Isotope Variation in Basaltic Melt Inclusions from Krakatau Revealed by a Newly Developed Secondary Ion Mass Spectrometry Technique for Silicate Glasses
Abstract
Sulfur is a ubiquitous element with variable valance states (S2-, S0, S4+, S6+) allowing for its participation in a wide variety of chemical and biogeochemical processes. However, its potential as an isotopic tracer in magmatic processes has not been fully developed and is crucial to understanding of sulfur recycling in subduction zones and between Earth's major reservoirs, mantle, lithosphere and coupled hydrosphere-atmosphere. Previous studies of silicate glasses and melt inclusions have been hampered by lack of an in situ isotopic measurement technique with spatial resolution of 10 to 100 microns. We have developed a new secondary ion mass spectrometry (SIMS) analytical technique for measurement of 34S/32S ratios in silicate glasses utilizing the IMS 1280 at Woods Hole Oceanographic Institution. A beam of 133Cs+ ions with 13 keV energy and current of 1-2 nA is focused onto a 10 micron spot and rastered over 30 × 30 microns. A Normal Incidence Electron Gun was used to compensate excess charge. The rastered beam is then centered to the optical axis of the machine, and a mechanical aperture is placed on the image plane to limit the area of analysis to the central 15 × 15 microns. The energy slit width was adjusted to 50 eV. A mass resolving power of 5500 was sufficient for eliminating mass interferences. A suite of synthetic and natural glasses with δ34SVCDT values spanning from - 5.6‰ to 18.5‰ with SiO2 from 44-72 weight % were measured. Magnitude of the instrumental mass fractionation (α) for 34S/32S ratios is 0.991 and is constant for all the glasses measured despite their compositions. Precision of individual measurements of 34S/32S ratios is 0.4 ‰, or better. Preliminary δ34S measurements of olivine-hosted basaltic melt inclusions in pre- 1883 basaltic scoria from Krakatau volcano Indonesia vary from -5.6 to 7.9‰ with sulfur concentrations from 490 to 2170 ppm, respectively. Host olivines are Fo77-80 and inclusions generally need minor to no post-entrapment corrections using KDFe-MgOl-liquid = 0.30. Most inclusions are basaltic though a few range up to basaltic andesite. Sulfur X-ray wavelength scans of melt inclusions indicates 60% of dissolved sulfur present as SO4. Dissolved H2O (by FTIR) ranges from 2.7 to 4.1 weight %, and CO2 concentrations are currently being determined. Dissolved Cl in melt inclusions ranges from 600 to 1100 ppm. Possible correlations of δ34S values with dissolved volatiles (by SIMS, and FTIR), and trace element concentrations are being evaluated.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V13F..07M
- Keywords:
-
- 1031 Subduction zone processes (3060;
- 3613;
- 8170;
- 8413);
- 1041 Stable isotope geochemistry (0454;
- 4870);
- 3630 Experimental mineralogy and petrology;
- 3694 Instruments and techniques;
- 8413 Subduction zone processes (1031;
- 3060;
- 3613;
- 8170)