Browsing by Subject "serpentinisaatio"

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  • Silvennoinen, Joel (Helsingin yliopisto, 2020)
    High amounts of H2 have been observed worldwide in deep bedrock aquifers. Sources and interaction processes of H2 within bedrock are poorly known so far, but recognizing those are important on order to identify risks related to various deep bedrock utilizing projects, such as repository of the nuclear waste. Aim of this study was to investigate if there is a correlation between H2 or fractionation factor α (H2O–H2) and the lithology of the host rock. In addition, several proposed low temperature H2 producing processes were reviewed via literature in order to investigate if α (H2O–H2) might help to identify the source of H2. In this study, H2 isotope data collected in previous studies in Finland from Pori, Pyhäsalmi, Juuka and Outokumpu deep bore holes representing various lithologies were used. In addition to α (H2O–H2), other parameters such as relative gas volumes, isotopic composition of water, temperature data, water pH, H2/He ratio of gasses were summarized. As part of the study, in the summer of 2019 new sampling was conducted in the Pinomäki borehole, in Pori by using tube sampling method and also field measurements were applied. Samples were later analyzed with ion chromatographic and spectrometric methods. In the results, isotope geochemical analyses were applied by using H2O–H2 system as a geothermometer. Results of the water analysis from Pori borehole were consistent with previous studies thus confirming two distinct water layers. Volume of H2 (0,4 %) differed considerably when compared with the previous study (28 %). Results of the isotopic composition of the hydrogen gas revealed extremely low values from -816 to -848 ‰ relative VSMOW which are among the most depleted δ2H-H2 values ever measured in the world. According to isotope analysis, in most of the boreholes in the study, except Outokumpu, H2 is not in equilibrium with groundwater when compared to in situ temperatures. α (H2O–H2) values were partially in similar range as fractionation related to some specific hydrogen forming processes, but there wasn’t a clear connection and values from different processes were overlapping each other. H2O–H2 system re-equilibrates relatively fast in a geologic timescale, which means that the possible isotopic fingerprint of H2 origin is lost. On the other hand, since H2 was clearly in a disequilibrium state, it might indicate the presence of possible active H2 forming processes. There wasn’t any clear correlation with host rock lithology and H2 concentration nor α (H2O–H2) either, except in Juuka, where lithology, α (H2O–H2), pH, and H2/He ratio summoned with high H2 volume (12,8 %) strongly indicates serpentinization as origin of H2. There are many challenges in the sampling of deep and narrow bore holes, especially related to the intention of preserving original conditions and also preventing gas contamination. In addition to further H2 sampling, recommendations for further studies include studying H2 producing reactors to investigate the effect of different H2 forming process to the isotope fractionation of H2.