Subject: Two Types of Optical Cells for the Study of C-H-O-S Fluids at Moho and Sedimentary Basins
Time: 16:30pm April 28, 2010
Place: New Geology Building 3325 Room
Reporter: Dr.I-Ming Chou
Hydrothermal diamond-anvil cell (HDAC), together with in-situ Raman spectroscopy, were used to study the structure of synthetic subduction-zone fluids (KAlSi3O8 — H2O system) at temperatures (T) up to 900°C and pressures (P) up to 2.3 GPa. Near this maximum P-T condition, corundum was observed to be stable with a supercritical fluid for the sample containing ~60 wt% of water. On the other hand, Raman spectroscopic identification of carbonate- and sulfate-bearing primary melt inclusions in corundum megacrysts, from Changle basalts in China, indicates the presence of C and S in the fluids from which the corundum crystallized . In similar corundum deposits worldwide, most researchers agree that corundum occurs as xenoliths, rather than phenocrysts, in alkali basalts, but the origin of corundum remains controversial. To solve this problem, we need to add C and S into the synthetic subduction-zone fluids, so that the effects of C and S on the stability of corundum can be identified. We also need to modify HDAC, so that routine experiments at the P-T conditions near the Moho are possible.
The fused silica capillary capsule (FSCC) technique was used to form synthetic fluid inclusions containing C, H, O, S, and others . This technique is particularly useful for the study of the pyrolysis of hydrocarbons and thermochemical sulfate reduction (TSR) at the P-T conditions of sedimentary basins . The advantages of FSCC technique for the study of C-H-O-S fluids are: (1) it is simple and inexpensive; (2) in combination with the USGS heating-cooling stage, in-situ observation and Raman characterization are possible at T up to 600°C and P up to 100 MPa; (3) long-duration (days to months) experiments can be performed in cold-seal pressure vessels with or without external Ar pressure, and the same sample can further react at different P-T conditions after non-destructive analysis; and (4) in combination with the oxygen buffer technique, it is possible to control redox state of the sample externally due to high permeability of fused silica to hydrogen.