38
B. Fractionation Mechanisms of Selected Elements
minations can be used as satisfactory calibrations of geothermometers, it
is clearly necessary that the discrepancies be removed.
v. Selenium
Since selenium is, to some extent, chemically similar to sulfur, one
might expect to find some analogous fractionations of the selenium isotopes in nature.
Six stable selenium isotopes are known with the following abundances (BAINBRIDGE and NIER, 1950):
74Se:
0.87% ,
76Se :
9.02%,
77Se:
7.58% ,
78Se:
23.52% ,
80Se:
49.82% ,
82Se:
9.19% .
82Sej16Se ratios have been determined by KROUSE and THODE (1962).
Selenium was extracted in its elemental form from natural samples and
was then fluorinated to SeF6 and introduced into the mass-spectrometer.
From theoretical calculations KROUSE and THODE (1962) deduced
that 76Se and 82Se differ in their chemical properties to the extent that
isotope fractionations up to 6% are predicted for 76SerSe exchange
processes, provided that mechanisms are available for such exchanges.
The 82 Sej1 6 Se ratio of 16 natural samples analyzed by KROUSE and
THODE (1962), varied by 15%0 (Fig. 16). In this connection it is interesting
to note that in addition to sulfate-reducing bacteria, anaerobic bacteria
Meteoric Massive Pitchblerde Other Volcanic Plant
Soil
selenium sulfide
containing ores
elemental materials
sulfide
vein
selenium
-12
-10
-
~
-
-8 -
0
-6
0
-
-4 -
Qj
~ -2
'" 0.() 0
2
-
~H ~
~
---+--- -~f~T -i--r--
-
§
4 -
§
6
Fig. 16. Variations in the 82Se/6Se ratio found for natural samples. (After KROUSE
and THODE, 1962)