Environmental Engineering Reference
In-Depth Information
rock-magma reactions occur in a number of different environments and styles,
determined by the thermal environment. Even without mantle convection,
cold sea-water
weathering
occurs at the sea bed. As expected, this occurs mostly through hydration,
which produces hydrated aluminosilicate clay minerals, but oxidation also occurs,
forming oxide films on Fe and Mn minerals relevant to
red clays
, described below.
At temperatures above 200° C, hydration may form the lowest-grade metamorphic
facies,
zeolite
. Precipitation of solutes also occurs here by
reverse weathering
, involving
hydrothermal minerals themselves together with dissolved minerals sourced elsewhere.
High temperature/low water volume reactions drive metamorphism, particularly in
subduction zones. This is quite distinct from mineralization, associated with high water
volumes circulating at mid-ocean ridges, with water : rock ratios of 10-100:1.
Metasomatism takes large volumes of basaltic and other minerals into solution, infusing
new species into the crust, depositing others around and beyond the vents and taking yet
others out of solution. Three associations are found, reflecting the peridotite → gabbro →
basalt layering towards the ocean floor and parallel temperature/pressure decline.
Serpentine
- Mg
3
Si
2
O
5
(OH)
4
- forms on the hydration of olivine in peridotite above 400°
C.
Hornblende
- (Na,Ca)
2
(Mg,Fe,Al)
5
O
22
(OH)
2
- forms in gabbro at 200-400° C, and a
whole cluster of new minerals form at similar temperatures in basalt, including
albite
-
NaAlSi
3
O
8
-
chlorite
- (Mg,Fe,Al)
3
(Si,Al)
2
O
5
(OH) - and
epidote
-
Ca
2
(Al,Fe)
3
Si
3
O
12
(OH) (see Figure 12.8). They are key minerals of the metasomatic
facies
serpenti-nite
,
amphibolite
and
greenschists
.
These reactions portray
par excellence
the vital contribution of hydration (OH), the
magmatic minerals Fe, Mg, Al, Si, Ca and Na, and solid solutions to ocean and oceanic
crust geochemistry. Sea water becomes enriched by chloride and the soluble minerals Mn
and Fe but depleted of Mg. Hydrogen sulphide (H
2
S), barium sulphate (BaSO
4
),
anhydrite (CaSO
4
) and insoluble metal sulphides of Cu, Fe and Zn are precipitated -
particularly by
black smokers
or hot plumes of suspended and dissolved minerals - to
form important mineral ore deposits. Vents also stimulate their own chemosynthesizing
ecosystems and hydrothermal circulation exhales magmatic gases, including hydrogen
(H), helium (He), methane (CH
4
) and carbon monoxide (CO) (Figure 12.17).
MARINE SEDIMENTATION
Marine sediments vary according to their composition, source of materials and location in
either the offshore slope system or the abyssal plain. Terrigenous sources of eroded and
transported continental erosion products are mainly minerogenic in nature, and their
abundance and
calibre
(particle size) diminish seawards. They are mostly more stable
minerals like quartz, potassium feldspars and biotite flushed with dissolved minerals by
rivers on to the continental shelf at global annual rates of 1·8 × 10
13
kg
