Geoscience Reference
In-Depth Information
Metamorphic
rocks
Weathered-
oceanic crust
Clays
Carbonates
Rain/snow
Granites
Seawater
Andesites
Mantle
and basalts
−
60
−
50
−
40
−
30
−
20
−
10
0
10
20
30
Mantle
18
O
δ
Figure 3.9
18
O values in rocks and natural water. Notice seawater at 0
‰
and the Earth's
mantle at 5.5
‰
. It is the equilibration of sedimentary and metamorphic rocks with seawater at
low temperature that raises the
Distribution of
δ
18
O of these rocks and lowers that of seawater. The broad
isotopic variation of meteoric water results from atmospheric precipitation, which occurs at low
temperature.
δ
presence in their source material of rocks and minerals that have been subjected to low-
temperature interaction with a hydrous fluid, such as seawater, groundwater, or meteoric
water.
Conversely, upon interaction of rocks with seawater at temperatures of
300
◦
C, the
≈
reversal of albite-water
18
O/
16
O fractionation drags these rocks towards lower
18
O values.
δ
18
O values of the gabbroic section of ophiolites,
widely regarded as the deep section of the oceanic crust exposed to circulating seawater at
such temperatures.
Marine alteration, especially the interaction of seawater with mid-ocean ridges, is thought
to be responsible for the current
18
O/
16
O ratio of seawater: throughout the Earth's his-
tory, seawater has circulated in submarine hydrothermal systems and its oxygen has been
brought into isotopic equilibrium at average temperatures of the order of 275
◦
C with the
oxygen of basalts erupted by the volcanic systems of mid-ocean ridges. To these purely
thermal effects must be added that of interaction with
18
O- and D-depleted meteoric water,
which, as discussed below, is quite spectacular in modern and fossil peri-magmatic geother-
mal fields. Oxygen and hydrogen isotope evidence on shallow intrusions indicates that
even the most fresh-looking plutonic rocks may have interacted with fluids contaminated
by meteoric fluids.
This is also clearly visible in the low
δ
