SOME ADDITIONAL ASPECTS OF THE EVOLUTION OF BRINES IN CRYSTALLINE ROCKS GAINED FROM ISOTOPE GEOCHEMISTRY
Deeper groundwaters in crystalline rocks tend to be different in both their chemical and isotopic signatures. The chemistry of these fluids can be combinations of the cations of calcium and sodium (occasionally enriched with magnesium) and the anions chloride and sulphate. Similar to sedimentary environments, dissolved loads greater than 100 g l-1 are common. The stable isotopic signatures (18O and 2H) of the most concentrated fluids are very different than those found in sedimentary formation fluids and age dating techniques have suggested that deep groundwaters are geologically old. A number of recent studies by the authors and others have used a wide variety of stable isotope techniques to attempt to unravel the evolutionary history of these deep brines. Not surprisingly combinations of chlorine, strontium and oxygen isotopes show components related to allocthonous probably paleo seawater intrusions in some cases, as well as the overprinting of rock/water interaction. In the latter case chlorine isotopes have been very useful in showing mixing of different water masses in the subsurface.