An Introduction to Environmental Chemistry

Hydrothermal reactions as major ion sinks

Of the major elements, the case for magnesium removal from seawater during

hydrothermal cycling at mid-ocean ridges is most convincing. Experimental work

and data from many black smokers (Fig. 6.16) suggests that the hydrothermal

fluids exiting from the crust have essentially zero magnesium concentration. This

implies that magnesium is removed from seawater by reaction with basalt at high

temperature. The precise chemistry of this reaction is not known, but it can be

represented as the generalized reaction:

eqn. 6.17

The basalt, represented here by iron-rich olivine (fayalite), is leached of its iron

and hydrated by seawater, whilst Mg^2 +from seawater is used to form the mag-

nesium clay mineral (sepiolite in eqn. 6.17), which represents altered basalt. The

formation of magnesium clay mineral also removes OH-from water to make the

(OH) 4 component of the clay mineral in equation 6.17. In laboratory experiments,

it is this removal of OH-from H 2 O that leaves the fluid enriched in H+, explain-

ing the acidity of hydrothermal fluids. The H+does not show up in the products

of equation 6.17 because the equation is a summary of a number of processes

going on over time (see Section 2.4). However, acidity generation is an impor-

tant feature that, along with complexation by Cl-anions (Box 6.4), enhances iron

solubility. The reaction (eqn. 6.17) also predicts the formation of iron oxide

(Fe 3 O 4 ), iron sulphide (FeS 2 ) and silica, all of which are found at hydrothermal

vent sites. Although it is difficult to quantify the amount of Mg^2 +removed from

seawater by this process, it is probably the most important Mg^2 +sink in the

modern ocean. There is also uncertainty about the fate of Mg^2 +in altered basalt

(sepiolite in eqn. 6.17) as it moves away from the ridge axis during seafloor spread-

ing. There is evidence that Mg^2 +is leached from altered basalt by cold seawater.

If large amounts of Mg^2 +are resupplied to seawater by such low-temperature

basalt–seawater interactions, then mid-ocean ridge processes may not cause net

Mg^2 +removal from seawater.

Sodium is by far the most abundant cation in hydrothermal fluids, simply

because the fluid is sourced from seawater (Table 6.1). It has long been

thought that Na+must be removed from seawater at mid-ocean ridges, mainly

because the global Na+budget does not otherwise balance. The existence of Na+-

enriched basalts (spilites), believed to have formed by reaction with seawater at

high temperature, is tangible evidence that Na+removal from seawater occurs

during mid-ocean ridge hydrothermal activity. The formation of sodium-feldspar

(albite) probably accounts for the removal process from the fluid to the altered

basalt, but data to quantify fluxes are scant. Hydrothermal activity probably

accounts for no more than 20% of the total Na+removal from seawater, which

11 2 2 2

78

24 2

2

4

2

23 6 4 3 42 2

Fe SiO H O Mg SO

fayalite seawater

Mg Si O OH Fe O FeS SiO

sepiolite magnetite pyrite silica

slaq aq

s ss aq

() () ()+ -()

() () () ()

+++

() ()

Æ ()+++

()( )()()

212 Chapter Six