the amount of SO 42 - entering the higher-temperature (>250°C) parts of the
hydrothermal system. At these high temperatures the sulphate is reduced by reac-
tion with FeS compounds in the basalt, and by oxidation of Fe^2 +compounds,
forming hydrogen sulphide (H 2 S) or hydrogen bisulphide (HS-). Anhydrite also
forms as part of the black smoker chimney system when the hot vent fluids
exit at the seabed. The hot fluids heat the surrounding seawater, which causes
anhydrite to form as predicted by equation 6.18.
Most of the CaSO 4 formed in the crust (and around vents) probably redis-
solves in the ocean bottom waters as the crust ages and cools; it thus has little
effect on the overall SO 42 - budget of the oceans. It is well known that H 2 S pre-
cipitates as iron sulphide in venting hydrothermal fluids, giving rise to extensive
zones of sulphide mineralization and to the ‘black smoke’ (Plate 6.1, facing
p. 138). However, the total removal of SO 42 - from seawater by this mechanism is
again likely to be small, since evaporite and sedimentary sulphide formation ade-
quately removes the river flux of SO 42 - on geological timescales. This suggests
that over long timescales much of the hydrothermal sulphide is oxidized on, or
just below, the seabed.6.4.8 The potassium problem: balancing the seawater major ion budget
major ion budget
The major ion budget for seawater (Table 6.2) is quite well balanced (i.e. inputs
equal outputs) for all elements except K+. Laboratory studies predict that K+
behaviour will change with temperature in hydrothermal fluids. Above 150°C, in
the hotter part of hydrothermal systems, K+should be leached from basalt
(Table 6.6), representing an input to the seawater budget. However, in
cooler parts (<70°C) of hydrothermal systems, K+adsorption on to altered
basalt may be important, resulting in the formation of clay-like minerals such
as celadonite (illitic) and phillipsite (a zeolite mineral). As there is no well-
documented major removal process for K+from seawater, it is generally believed
that ridge flank low-temperature hydrothermal activity removes all of the high-
temperature hydrothermal K+input to seawater and probably some of the river
flux also.
A process that might affect the K+budget in a small way is K+fixation during
ion-exchange reactions on clay minerals. Laboratory experiments have shown
that degraded micas and illites (see Section 4.5.2), stripped of their K+during
weathering, but which retain much of their layer charge, are able to fix, irre-
versibly, K+from seawater. The process involves the replacement of hydrated
cations for dehydrated K+in the interlayer site, fixing the K+in its ‘mica’ site (see
Section 4.5.2). Globally, this process might remove another 10–20% of the K+
river flux to the oceans (Table 6.3).
The imbalance in the K+budget and small imbalances in other budgets may
be nullified by a number of processes. One possibility is the concept of ‘reverse
weathering reactions’. In reverse weathering, highly degraded clay minerals react
with cations, HCO 3 - and silica in seawater to form complex clay mineral-like
silicates. An example reaction addressing the K+problem would be:214 Chapter Six