Thermomineral Water and Geothermal Energy - Hydrogeology: Objectives, Methods, Applications - page 80

2 THE ORIGIN OF HEAT
2.1 Juvenile waters
Mantle rocks contain around 0.3% water, which, through degassing, can
reach the hydrosphere, carrying a characteristic assemblage of mineral
salts.
Water is returned to the mantle via subduction zones. This water can
then be returned to the surface through volcanism or it can feed deep
hydrothermal springs in mid-oceanic ridges.
2.2 Secondary heating and mineralisation
However, these juvenile waters make up only a small percentage of the
composition of thermomineral waters, which are essentially meteoric in
origin. Water can, indeed, become heated by penetrating more or less deeply
below ground, or by circulating closer to the surface in volcanic regions or
in thermally anomalous zones. Blavoux and Letolle's isotope work enabled
the proof of thermal spring water's meteoric origin (Figure 45).
Figure 45 Isotopic analyses showing a meteoric origin for a few French mineral waters (from
Blavoux & Letolle, 1995).