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

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
homogeneous for their Li concentrations and isotopic compositions and �ii� an infinite
reservoir of lithium �in the glass or the matrix� is considered for diffusion. According to
these two assumptions, the following initial Li concentrations and isotopic compositions
were used for olivines ��Li� � 1.2 μg/g, δ 7 Li � �6.4 ‰) and glass ��Li� � 3.4 μg/g, δ 7 Li �
�6.4 ‰�. In order to solve correctly equation �1� in a temperature / time dimension, the
cooling rate for each olivine must be known. This cooling rate was calculated as a function
of the depth of the olivine in the pillow using a simple heat conduction model �Jaeger,
1961�. The pillow surface temperature was kept at 2°C and the glass was considered to be
initially at 1200°C, according to the relationship which exists between the liquidus
temperature of basalts and their MgO contents �Jones, 2003�. The thermal evolution of the
two
selected olivines during the cooling of the pillow lava is shown in Fig. 3.9.
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Despite several parameters being well constrained, namely the shape of the olivine
crystals, the initial compositions of olivine and glass and the thermal history of the pillow,
solving equation �1� requires knowledge of the diffusion parameters for Li in olivine. These
are �i� the diffusion coefficient D of Li in olivine, which depends directly on the
temperature according to an Arrhenius law �D � D0 exp�‐Ea /RT�� where D0 is the diffusion
coefficient at infinite temperature, T is the temperature, Ea is the activation energy and R is
the ideal gas constant� and �ii� the ratio between the diffusion coefficients of 7 Li and 6 Li.
This ratio is expressed as D7/D6 � �m7/m6� β , where D6 and D7 are the diffusion coefficients,
m6 and m7 are the atomic masses for 6 Li and 7 Li, respectively. β ranges from 0 to 0.5 and
characterizes the difference in the mass dependence of Li diffusivity �Richter et al. 2003�. A
high value of this parameter implies a faster diffusion for 6 Li compared to 7 Li and leads to a
large isotopic fractionation. In previous studies �Barrat et al., 2005; Beck et al., 2006�, the
β‐value has been approximated by the value obtained during experimental diffusion of
lithium in silicate melt �β � 0.215, Richter, 2003�. Unfortunately, the parameters D0, Ea and
β have not yet been experimentally determined for olivine.