[SIT21-P19] Microvolume stable isotope measuremetns and its applicaiton for high-pressure high-temperature experimental run products
Keywords:HPHT experiments, Micro-volume stable isotope measurements, Sulfur isotopes
At Niigata University, a MAT-253 mass spectrometer (Thermo Fisher Scientific) was installed through the MEXT Grant-in-Aid for Scientific Research on Innovative Areas. Carbon and oxygen isotopic compositions are measured using CO2and multiple sulfur isotopic composition are measured using SF6gas. A new micro-volume inlet system was installed and fundamental parameters such as pressure effect and capillary flow effect were tested. Using the micro-volume inlet system the minimum volume required for analysis is 1 micro-mole sample gas, and the precession for carbon and oxygen isotopic composition are better than 0.1‰. The precession for sulfur isotopes is ±0.3‰ and ±0.01‰ (1s) for δ34S and D33S, respectively, based on the repeated analyses of standard Ag2S (IAEA S-1).Multiple sulfur isotope measurement system consists of 1) curie point pyrolyzer for rapid conversion of small volume samples to SF6gas, 2) vacuum line system for pumping out non-condensable gasses, 3) gas chromatograph for purifying the SF6gas and 4) micro-volume inlet system for introduction of sample gas to ionization chamber.
Multiple sulfur isotope ratios experimental run products were measured using the new system. Metal phases were separated and sulfur was extracted using Cr(II) reduction method . Preliminary results indicate that S isotope fractionations occur in the pressure range of 3−5 GPa, under high-temperature conditions corresponding to the magma ocean environment. The negative δ34S of mantle, can hence be explained by sulfur isotope fractionation in a magma ocean environment. Sulfur isotope measurements were also carried out on granitoid samples from the Archean with sulfide sulfur showing positive δ34S values. Mass independent isotope fractionation, expressed as Δ33S, was also observed, suggesting that sulfur in the mantle alone cannot explain these variations. The incorporation of surface sulfur from sedimentary rocks and seawater is proposed.