on a geological timescale is dominated by the formation of evaporites (Section
6.4.2).
Hydrothermal reactions as major ion sourcesThe chemistry of hydrothermal fluids indicates that basalt–seawater interactions
are a source of some elements that have been stripped from ocean crust and
injected into seawater. Data from hydrothermal fluids show that both Ca^2 +and
dissolved silica are concentrated in the hydrothermal waters compared with sea-
water (Table 6.6). Calcium is probably released from calcium feldspars (anorthite)
as they are converted to albite by Na+uptake, a process called albitization. Silica
can be leached from any decomposing silicate in the basalt, including the glassy
matrix of the rock. Globally, basalt–seawater interaction seems to provide an
additional 35% to the river flux of Ca^2 +and silica to the oceans.
Hydrothermal reactions involving sulphurSeawater sulphate is removed from hydrothermal fluids, mainly by the precipi-
tation of anhydrite (CaSO 4 ) as the downward-percolating seawater is heated to
temperatures around 150–200°C.
eqn. 6.18This reaction probably consumes all of the seawater-derived Ca^2 +in the fluid, and
about 70% of the SO 42 - ; if more calcium is added to the fluid from albitization
reactions (see above), even more SO 42 - is consumed. Anhydrite formation limits
heat
Ca SO CaSO
anhydrite()aq aq
+
()+ - Æ
()
( ∞
)
2
4(^2) –– 4
at temperatures >150 C the equilibrium
for this reaction lies well to the right
The Oceans 213
Table 6.6Changes in seawater major constituents upon reacting with mid-ocean ridge basalt
at high temperature. Hydrothermal fluid data are typical ranges from Von Damm (1995).
Seawater* Hydrothermal fluids D
Constituent (mmol l-^1 ) (mmol l-^1 ) (mmol l-^1 )
Mg^2 + 53 0 - 53
Ca^2 + 10 10–100 0–90
K+ 10 15–60 5–50
SO 42 - 28 0–0.6 - 28
H 4 SiO 4 0.1 5–23 5–23
- Data from Table 6.1.
D, difference between typical range in hydrothermal water and seawater.