Two Types of Optical Cells for the Study of C-H-O-S Fluids at Moho and Sedimentary Basins

Dr. I-Ming Chou

954 National Center, U.S. Geological Survey, Reston, VA 20192, USA

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 [1]. 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 [2]. 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 [2, 3, 4]. 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 [5]. 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 [5, 6]. 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 [7].

[1] Mibe, K., Chou, I-Ming, and Bassett, W.A. (2008) Journal of Geophysical Research, 113, B04208
[2] Song, Yucai and Hu, Wenxuan (2009) Acta Petrologica et Mineralogica, 28, 349-363.
[3] Guo Jingfeng, O’Reilly, S., and Griffin, W.L. (1996) Contributions to Mineralogy and Petrology, 122, 368-386.
[4] Levinson, A.A. and Cook, F.A. (1994) Gems