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Helium measurements of pore fluids obtained from the San Andreas Fault Observatory at Depth (SAFOD, USA) drill cores |
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| Authors: | S Ali M Stute T Torgersen G Winckler B M Kennedy |
| Institution: | 1. Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY, 10964, USA 2. Department of Earth and Environmental Sciences, Columbia University, 2960 Broadway, New York, NY, 10027, USA 5. Lawrence-Berkeley National Laboratory, 1 Cyclotron Road, MS70A-4418, Berkeley, CA, 94720, USA 3. Department of Environmental Science, Barnard College, 3009 Broadway, New York, NY, 10027, USA 4. Department of Marine Sciences, University of Connecticut, 1080 Shennecossett Road, Groton, CT, 06340, USA 6. National Science Foundation, 4201 Wilson Boulevard, Arlington, VA, 22230, USA |
| Abstract: | 4He accumulated in fluids is a well established geochemical tracer used to study crustal fluid dynamics. Direct fluid samples are not always collectable; therefore, a method to extract rare gases from matrix fluids of whole rocks by diffusion has been adapted. Helium was measured on matrix fluids extracted from sandstones and mudstones recovered during the San Andreas Fault Observatory at Depth (SAFOD) drilling in California, USA. Samples were typically collected as subcores or from drillcore fragments. Helium concentration and isotope ratios were measured 4?C6 times on each sample, and indicate a bulk 4He diffusion coefficient of 3.5?±?1.3?×?10?C8 cm2?s?C1 at 21°C, compared to previously published diffusion coefficients of 1.2?×?10?C18 cm2?s?C1 (21°C) to 3.0?×?10?C15 cm2?s?C1 (150°C) in the sands and clays. Correcting the diffusion coefficient of 4Hewater for matrix porosity (??3%) and tortuosity (??6?C13) produces effective diffusion coefficients of 1?×?10?C8 cm2?s?C1 (21°C) and 1?×?10?C7 (120°C), effectively isolating pore fluid 4He from the 4He contained in the rock matrix. Model calculations indicate that <6% of helium initially dissolved in pore fluids was lost during the sampling process. Complete and quantitative extraction of the pore fluids provide minimum in situ porosity values for sandstones 2.8?±?0.4% (SD, n?=?4) and mudstones 3.1?±?0.8% (SD, n?=?4). |
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