Geoscience Reference
In-Depth Information
Among the processes that fractionate oxygen isotopes at low temperature, probably the
most important is between water and the different forms of dissolved and crystallized car-
bonates. This was understood in 1947 by Urey and his students who first identified the
potential of this technique for paleothermometry. Kim and O'Neil
(1997)
made a very
careful investigation of
18
O/
16
O fractionation between calcite and water and suggested the
expression:
18030
T
18
O
16
O
calcite
/
1000 ln
α
water
=
−
32.42
(3.38)
/
Fractionation is prone to effects of salinity. The different carbonate ions H
2
CO
3
,HCO
3
,
and CO
2
3
18
O values, decreasing in that order. Today, the oxygen
isotope geochemistry of foraminifera has become the essential tool for establishing tem-
perature and salinity in the ancient oceans. Seawater
18
O/
16
O changes only by addition of
melt water, which is meteoric water with very different isotopic properties, and we will
see that this is actually used to trace the evolution of the ice caps. If this effect can be
neglected, the
have very different
δ
18
O of a marine calcite fossil may be used to infer the temperature of the
seawater in which the animal grew. Fractionation of oxygen isotopes in phosphates (e.g.
especially tooth enamel) are used in the same way.
Because of prominent temperature effects, oxygen isotope geochemistry provides
unique constraints on the origin of igneous and metamorphic rocks:
1. Magmatic differentiation of basaltic rocks takes place at temperatures of about 1100
◦
C
and therefore does not entail any significant fractionation of oxygen isotopes (
δ
<
0.4 per
18
O values of most fresh basaltic rocks derived from either glass
or olivine fall in the range 5.2-5.8
δ
which is within 0.3 per mil of mantle values.
2. Partial melting at temperatures in excess of 800
◦
C creates no or minimal isotope
fractionation.
3. Large deviations of
18
O values from the mantle values require that either some
source rocks were once involved in low-temperature processes (they contain sedi-
ments or altered rocks) or that the samples was exposed to hydrothermal alteration or
weathering.
4. The order of decreasing
δ
18
O at equilibrium should be quartz, feldspar, Fe-Mg
silicates, magnetite. The commonest rocks in the continental crust derive largely
from the metamorphic transformation of sediments (schist, gneiss) and their melting
(granite).
all the other curves. A first implication is that exchanged silicates at hydrothermal and
surface temperatures have
δ
18
O values much higher than the co-existing water: clay min-
erals are enriched by about 12-25 per mil with respect to seawater in the temperature
range of 150-25
◦
C. This shift is clearly visible in the high
δ
18
O values of altered oceanic
basalts. It also explains the role that the sedimentary cycle plays with respect to the con-
tinental crust. The consistently high
δ
18
O values (typically
) of gneiss and
mica-schist, but also of the anatectic granites which form by melting of these metamor-
phic rocks reflects this so-called metasedimentary origin. Such high
δ
+
7to
+
15
18
O values of silicic
δ
metamorphic and magmatic rocks with respect to the mantle value (
≈+
5.5
) require the
