Geoscience Reference
In-Depth Information
Sample
(measured)
Instrumental
fractionation
(bias)
Slope = 0.5
Sample
(corrected)
Reference
Mass fractionation line
Natural
fractionation
δ
18
O
Figure 3.8
Fractionation depending on the mass of isotopes
16
O,
17
O, and
18
O is consistent whether its cause
is natural or analytical. The
17
O
16
O ratios are shown in delta notation. All samples
from Earth lie on the same straight line of mass fractionation whose slope is equal to the ratio of
the mass difference (17
16
Oand
18
O
/
/
16), i.e. 0.5. Natural fractionation is a deviation
determined relative to an arbitrary reference sample, hence the
−
16) by difference (18
−
notation. The isotopic bias is
introduced by the mass spectrometric analysis and follows a similar mass-dependence law: it
must be corrected for each sample by appropriate techniques.
δ
fractionation. It is important to emphasize at this early stage that isotope measurements
made in the laboratory report isotopic fractionation values that are the sum of fractionation
related to the action of all natural processes
and
of instrumental fractionation, sometimes
known, except by artificially mixing pure isotopes into certified standard materials, but
since every laboratory places - by definition - reference material at the same point in the
diagram, this is something we can live with.
Let us finally note that, because we restricted ourselves to a first-order approximation,
all the isotope fractionation laws seem to be giving a unique value for the slope of the
fractionation line, e.g. 0.5 for oxygen. To higher orders, however, the data are good enough
to tell one process from another, in particular whether isotope fractionation is controlled
by quantum mechanics eff
ec
ts, with a dependence in 1/
M
, or by kinetic effects, with a
dependence in exp
√
M
, and to recognize the effects of Rayleigh distillation during
phase change. This complex topic is beyond the scope of this textbook.
−
/
1
3.3 Hydrogen
It is important to remember that hydrogen is largely held in the hydrosphere, ocean, ice
caps, and groundwater. The mantle may contain a large fraction of the terrestrial hydrogen
inventory but its local concentrations are always small (a few hundred parts per million or
