Géochimie isotopique du lithium dans les basaltes-Géochimie des ...

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
the observed 7 Li enrichment in the inclusions is fully consistent with the expectations of a
faster diffusion loss from the inclusion of 6 Li than of 7 Li. It is thus rather clear that the use
of Li isotopic composition in inclusions hosted by phenocrysts should be seen with extreme
caution: their limited volumes imply fast diffusion‐induced modifications of the pristine
δ7Li
values.
compositions 110
7.2. Effect of microscale diffusion‐induced fractionation on bulk Li isotopic
The modifications of the δ 7 Li values of crystals due to Li diffusion during the cooling
of magmatic rocks tend to create strong Li isotopic variations at the micrometer scale.
Depending on the amount of analyzed sample, whole‐rock Li isotopic measurements could
be affected by these small scale heterogeneities. In order to evaluate this effect, one can
consider the case of a magmatic rock composed of two phases �named A and B� randomly
distributed and which display a difference in δ 7 Li of 10‰. For a given volume of this rock,
the probability for the measured δ 7 Li value to be X can be approximated by the binomial
law:
7
n!
P(δ
Li = X ) = (
k!
( n − k)!
f A
7
7
with: = ( δ Li ∗k
+ ( 1−
k)
∗δ
Li ) / n
X A
B
and:
n = ( m/
ρ)
/ d
3
k
) ( 1−
f
)
1−k
A
In equation �2�, fA is the fraction of the crystal A �for our calculation fA� 0.5�, n is the
number of crystals present in a given volume of this rock, m is the sample mass, d is the
crystal size and ρ is their density. The isotopic variability induced by the sub‐millimeter
scale heterogeneity can be approximated by the variance of this binomial law of
distribution. These theoretical estimates were calculated �Fig. 3.12� for different grain radii
�50, 250 and 500 μm�. This approach shows that for instance if Li isotopic heterogeneities
are present at a scale of 250μm, then heterogeneity of �1‰ is expected when aliquots of
10 mg of rock are analyzed. This should be taken into account for bulk rock Li isotope
measurements.
�2�