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

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
concentration gradient is present : this is the case of profile F‐G �Fig. 3.8� for which no
mineral/matrix
interface is present.
Figure 3.9 : Cooling rates, modeled using heat conduction model �see text�, of
olivine crystals located at different depths �from 5 to 60 mm� in the pillow
lava.
6.2. Modeling Li isotopic fractionation during chemical diffusion An equation of chemical diffusion �Crank, 1967� was used in a spherically
symmetrical model to approximate the lithium diffusion into olivines. Despite the fact that
this calculation does not take into account the anisotropy of the diffusion properties in the
orthorhombic olivine crystal structure, it provides however a precise first orders estimate
of
the behavior of lithium isotopes in olivine crystals during cooling.
( ) ⎟ 2
dC ⎛ dC 2 dC ⎞
= D T ⎜ + 2
dt ⎝ dr r dr ⎠
In equation �1� D is the diffusion coefficient �m 2 .s ‐1 �, C is the concentration �μg/g�, t is
the time �s� and r is the distance to the center of the sphere �m�. 6 Li and 7 Li were calculated
simultaneously to be able to obtain profiles for both �Li� and δ 7 Li. Two assumptions were
made: �i� before diffusion takes place olivines are at equilibrium with the melt and are thus
�1�
105