Boron
39
are known to reduce selenates and selenites. A kinetic isotope effect of
15%0 has been found in the reduction of selenite ion to elemental selenium. Summarizing, we may state that selenium isotope fractionations
should be similar to those found for sulfur.
VI. Boron
Boron has two stable isotopes (BAINBRIDGE and NIER, 1950):
lOB:
18.98% ,
lIB:
81.02% .
Boron is known to be a highly mobile element geochemically and therefore may well reveal variations in its isotopic composition. For example,
boron minerals of pegmatites and hydrothermal veins (tourmaline) on
one side, and hydrated borate minerals (borax) on the other side, might
display isotope fractionation. The possible fractionations in biological
systems should also be seriously considered. UREY (1947) has calculated
the equilibrium constants of boron halogenides which differ from unity.
The corresponding data on boron isotope variations partly contradict each other: PARWEL et al. (1956), MELTON et al. (1956), LEHMANN and
SHAPPIRO (1959), FINLEY et al. (1962), SHIMA (1963), McMULLEN et al.
(1961), SHERGINA et al. (1968), SCHWARCZ et al. (1969) and AGYEI and
McMULLEN (1968).
These contradicting results might be due mainly to erroneous measurements, because boron is difficult to handle in mass-spectrometric
measurements. This occurs when boron is used both as a gaseous compound (BF 3 may cause memory effects, MELTON et al. (1956)) and as a
solid compound (inherent errors in absolute ratio determinations).
Recent investigations by AGYEI and McMULLEN (data to be published) suggest that the lIBrB ratios in terrestrial materials fell within a
narrow range of around 4.0, whereas the seawater value fell far outside
this range (see Fig. 17).
Assuming that the lIBrB ratio of about 4.0 is the mean composition
of boron being fed into the oceans by erosion and volcanic eruption,
SCHWARCZ et al. (1969) were surprised to find that the mean isotopic
composition of ocean water is approximately 5% enriched in liB with
respect to its presumed source. To explain this observation, SCHWARCZ
et al. (1969) suggested that light boron is preferentially incorporated into
clay minerals, especially illite.
SCHWARCZ et al. (1969) tested this hypothesis using illite and have
found a fractionation of about 3 to 4% enriching the clay in lOB. This is
in approximate agreement with a calculated value, assuming that the
ocean is at an isotopically steady state.
39
are known to reduce selenates and selenites. A kinetic isotope effect of
15%0 has been found in the reduction of selenite ion to elemental selenium. Summarizing, we may state that selenium isotope fractionations
should be similar to those found for sulfur.
VI. Boron
Boron has two stable isotopes (BAINBRIDGE and NIER, 1950):
lOB:
18.98% ,
lIB:
81.02% .
Boron is known to be a highly mobile element geochemically and therefore may well reveal variations in its isotopic composition. For example,
boron minerals of pegmatites and hydrothermal veins (tourmaline) on
one side, and hydrated borate minerals (borax) on the other side, might
display isotope fractionation. The possible fractionations in biological
systems should also be seriously considered. UREY (1947) has calculated
the equilibrium constants of boron halogenides which differ from unity.
The corresponding data on boron isotope variations partly contradict each other: PARWEL et al. (1956), MELTON et al. (1956), LEHMANN and
SHAPPIRO (1959), FINLEY et al. (1962), SHIMA (1963), McMULLEN et al.
(1961), SHERGINA et al. (1968), SCHWARCZ et al. (1969) and AGYEI and
McMULLEN (1968).
These contradicting results might be due mainly to erroneous measurements, because boron is difficult to handle in mass-spectrometric
measurements. This occurs when boron is used both as a gaseous compound (BF 3 may cause memory effects, MELTON et al. (1956)) and as a
solid compound (inherent errors in absolute ratio determinations).
Recent investigations by AGYEI and McMULLEN (data to be published) suggest that the lIBrB ratios in terrestrial materials fell within a
narrow range of around 4.0, whereas the seawater value fell far outside
this range (see Fig. 17).
Assuming that the lIBrB ratio of about 4.0 is the mean composition
of boron being fed into the oceans by erosion and volcanic eruption,
SCHWARCZ et al. (1969) were surprised to find that the mean isotopic
composition of ocean water is approximately 5% enriched in liB with
respect to its presumed source. To explain this observation, SCHWARCZ
et al. (1969) suggested that light boron is preferentially incorporated into
clay minerals, especially illite.
SCHWARCZ et al. (1969) tested this hypothesis using illite and have
found a fractionation of about 3 to 4% enriching the clay in lOB. This is
in approximate agreement with a calculated value, assuming that the
ocean is at an isotopically steady state.