Hydrogen emanations in nepheline-syenite massifs: possible consequences and applications
Geological Institute of the Kola Science Centre of the Russian Academy of Sciences
The last two decades saw the growing interest in manifestations, geochemistry and origin of endogenous hydrogen. The interest is determined by the necessity to understand the role of H2 in processes of magma-, petro- and ore genesis, formation of hydrocarbon fields, preparation and implementation of earthquakes and decrease of the stratospheric ozone bulk concentration. There have been attempts to reveal the relation between the lithospheric hydrogen emanations and various dangerous natural and anthropogenic phenomena and processes to define their gas-geochemical indicators and precursors. Finally, as the hydrogen energetics becomes all the more promissing, natural H2 is all the more thoroughly studied as a potential primary energy supply. Despite lots of investigations carried out, the above-mentioned scientific issues are still debated, while the applied ones have no sufficient study. Some of the reasons to it are variety of possible sources of hypogene hydrogen, lots of factors affecting its release, spatial emission dispersion and lack of long continuous observations.
The Khibiny and Lovozero alkaline massifs on the Kola Peninsula, which are characterized by high abundance of reduced gases so far are concerned magmatic complexes, are among minor places, where relatively concentrated releases of hydrogen occur. The combination of filtration (jet) and diffusive outflow from rocks is typical of spontaneous gas releases (SGR) in these massifs. Intense short-term gas blowouts from boreholes are no common. Whereas methane is dominant and H2 along with ethane are always subordinate components in gases occluded in fluid inclusions of minerals, in SGR hydrogen often (under diffusive release mostly) prevails. Usually, hydrocarbon gas generation is substantially or completely bound with processes of the rock formation, while for spontaneously released hydrogen there are different (up to the Earth’s core) sources suggested. The spatial and temporal compositional, gage pressure and release intensity variations are typical of SGR. The gas seeps spatial localization is mainly determined by the structure and tectonics of the massifs. Changes of gas emission characteristics in time are obviously stipulated by joint influence of cosmic, geodynamic, seismic, hydrogeological and anthropogenic factors.
For more than half a century study of gases in the massifs, molecular hydrogen was given far less attention than methane and its homologues. Meanwhile, owing to H2 greater reactionary capability, mobility and abundance, study of its emanations is more important in many respects. Thus, combustible and highly explosive SGR can accumulate in atmosphere of underground mines under certain conditions, disturb the technological cycle and threaten miners’ health and life. Therefore, it is vital to introduce and follow certain measures of gas regime in underground mines working out mineral deposits of the alkaline massifs. In particular, it is necessary to control the concentration of combustible components in mine air. The presence of H2 in the mixture of these components does not only decrease the explosive threshold of their concentrations in mines, but also prevents detecting methane with the most popular portable gas analyzers of interferometer-type.
The Khibiny apatite-nepheline and Lovozero rare-metal deposits are characterized, like the very massifs, by an uneven distribution of stresses and a high as well by essential natural and induced seismicity. Exploiting these, we faced the problem of evaluating the stressed-deformed state of the rocks and forecasting dynamic manifestations of the rock pressure, especially the most dangerous of them – mountain and tectonic bursts and shallow-focus anthropogenic earthquakes. The interrelation between gasometric and geomechanical features of a rock mass revealed in the course of laboratory experiments and observations in underground mines testify to parameters of gas, hydrogen first of all, release possibly vary as indicators and forerunners of the above geodynamic events (Nivin et al., 2001; 2009).
One of the current topical problems is the global decrease of stratospheric ozone and formation of the so called ozone holes. The latter are local reductions of total ozone (TO). From hypotheses explaining causes of the Earth’s ozone layer destruction technogeneous-freon one is the best known and the hydrogenous hypothesis seems to be the most reasonable. Its main idea is that ozone is being destroyed by emanations of hydrogen and, to the lesser amount methane from some geological structures (Syvorotkin, 2002). It turned out, that the most significant and continual ozone anomalies coincide with central parts of such degassing zones. One of these are presumably the Khibiny and Lovozero massifs. Hydrogen and methane emanations from the massifs could cause negative anomalies of the TO field above the Kola Peninsula and White Sea. To verify this suggestion the monitoring of the subsoil H2 variance in the Khibiny was carried out. It revealed the actual anticorrelation between gas release intensity and regional TO change (Syvorotkin et al., 2008).
The mysterious mass death of fish recorded from time to time in rather large Seidozero lake in the central part of the Lovozero massif also can be a result of the drastic increase of hydrogen and methane release at the lake bottom.
Revealing periodicity, peculiar features and factors of temporal changes in hydrogen release is only possible with carrying out long constant observations. Along with the Khibiny, such observations recently started in the Lovozero massif as well. There the time dependence of H2 concentration in underground mine working is traced. The character of gas release variations running up to order of magnitude during a day is shown on the figure below.
Examples of monthly dispersion of hydrogen concentrations in subsoil air (left column) and in undergrond working (right column) in the Khibiny and Lovozero massifs respectively (c.u. - conventional units).
The following periods of the hydrogen concentration spikes were revealed using the mathematical treatment of the Khibiny H2 time series with the flicker-noise spectroscopy method: 60.9, 34.7, 13.9, 8.5, 7.2, 6.1, 4.9, 3.1, 2.9 and 1.37 days, 24.1 and 12 hours (Syvorotkin at al., 2008). Obviously, this periodicity is mainly due to the irregularity of Earth’s rotation around its own axis and luni-solar tiding.
Further growing and in-depth mathematical analysis of the molecular hydrogen time series in alkaline massifs of the Kola Peninsula, comparing them with data gathered by other types of observation, widening the monitoring station network will help us understand the actively debated important issues relating to the mechanisms of the lithospheric-atmospheric relationships. It will also intensify working out of hydrogen-metric indicators and precursors of dangerous geodynamic processes. Well-timed forecast of these processes will help to minimize their harmful impact.
This study was financially supported by RFBR grant 09-05-00754-a.
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