Geoscience Reference
In-Depth Information
ppm). The consequence is that the hydrogen isotope compositions of aquifers are hardly
modified by interaction with the ground, unless the proportion of groundwater held in the
pores of the rock is extremely small.
On Earth, the average D/H ratio is about 1.4
10
−
4
, or equivalently, the isotopic abun-
dance of D is 140 ppm. On Mars, the Viking landers measured a D abundance of 780 ppm.
These numbers are much larger than the 20 ppm value of the solar nebula inferred from
spectroscopic data on Jupiter and the reason is the faster escape rate of H from planetary
atmospheres with respect to the twice-as-heavy D.
The reference material is the standard mean ocean water (SMOW), a man-made compos-
ite of various samples of desalinated seawater and
×
D usually refers to SMOW. Hydrogen
isotopes fractionate very significantly between vapor and liquid and between vapor and
ice under the temperature conditions prevailing at the surface of the Earth, which makes
δ
δ
D values universally employed tracers of the hydrological cycle. The factor 2 difference
between the two isotopes is unique in the periodic table and accounts for the isotopic vari-
ability of
D that ranges up to several hundreds of per mil. Ice at the South Pole only
contains 85 ppm D (
δ
δ
=−
400). At the temperatures of interest for environmental stud-
ies, vapor/liquid D/H fractionation varies from
D
25
◦
Cto
at 0
◦
C, and
−
−
−
110
170
at
25
◦
C. As will be discussed in the next section, the isotope variability is ampli-
fied by the progressive distillation of rain and snow from the vapor transported by the
atmosphere.
Fractionation of D from H is also observed between water and hydrous minerals.
Although the experimental data are few and particularly difficult to interpret and to repro-
duce, hydrous minerals are typically depleted in deuterium by 30-50 per mil with respect to
co-existing water and
−
75
at
+
40 may be found in minerals in equilibrium with sea-
water and in magmatic rocks altered by seawater. As expected, the effect of temperature is
to reduce isotopic fractionation. The
δ
D values of
−
D values of metamorphic and igneous hydrous min-
erals (chlorite, biotite, muscovite, amphibole) are typically in the range of
δ
.
Hydrogen isotope geochemistry therefore represents a powerful means of investigating the
dehydration reactions associated with the burial of hydrous rocks, such as schists, serpen-
tinites, and amphibolites, at subduction zones in particular, and the deep water cycle in
general.
−
40 to
−
95
3.4 Oxygen
Oxygen has three stable isotopes at masses 16, 17, and 18, with average abundances
of 99.76, 0.037, and 0.204 percent, respectively. For oxygen, as for hydrogen, the most
broadly used reference material is the SMOW but low-temperature carbonate data are also
commonly reported with respect to the carbonate of a belemnite from the Pee Dee forma-
tion (PDB), which is about 30 per mil heavier than SMOW. The
18
O value of the Sun, as
measured by the solar wind implanted in metal grains from the Moon regolith (Hashizume
and Chaussidon,
2005
) and by the cometary mission Stardust is about
δ
.The
mean terrestrial value is recorded by mantle rocks, which are the largest terrestrial reservoir
−
40 to
−
60
