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

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
However, high mobility and large isotopic fractionations of Li, may also be viewed as
severe potential limitations to the use of its isotopes as a reliable tracer in mantle
petrology. One obvious indication of these limitations is the fact that, at the micrometer
scale, the Li isotopic variations are often larger by at least one order of magnitude than
they are in bulk for a given rock �Fig. 3.5�. In the case of meteoritic components such as
refractory inclusions and chondrules �not shown in Fig. 3.5�, a great deal of complexity is
added to the Li isotopic variations by the fact that early solar system irradiation processes
or recent exposure to galactic cosmic rays may have produced �i� Li with a 7 Li/ 6 Li ratio of
�2 �instead of �12.1 for chondrites� and �ii� short‐lived 7 Be which decays to 7 Li
�Chaussidon and Robert, 1998; Chaussidon and Gounelle, 2006; Chaussidon et al., 2006�.
Despite these processes which cannot affect terrestrial rocks, theoretical considerations
indicate that the fast diffusivity of Li in melts and minerals �Jambon and Semet, 1978;
Lowry et al., 1981; Giletti and Shanahan, 1997; Richter et al., 2003� may induce strong Li
isotopic variations at the micrometer scale because 6 Li is diffusing faster than 7 Li �Jambon,
1980; Richter et al., 2003�. These transient Li isotopic variations may be quenched in
minerals at the micrometer scale in the case of specific cooling paths and may obscure
isotopic
variations resulting from earlier geological events.
The search for such isotopic effects in minerals was originally carried out in
differentiated meteorites because it was anticipated that they would have favorable �i.e.
fast� cooling histories and, in the case of lunar rocks, would not have been subjected to any
secondary alteration by fluids. Large δ 7 Li variations �� 30‰ over 500 μm� were initially
found in pyroxene grains from the shergottite NWA 480 and were first interpreted as
resulting from Li isotopic fractionations taking place during the degassing of martian melts
�Beck et al., 2004�. However subsequent studies on the lunar meteorite NWA 479 showed
similar Li isotopic variations which, because of the dry character of lunar melts, can only be
due to a Li redistribution �and to the associated Li isotopic fractionation� by diffusion
between Li‐poor pyroxene phenocrysts and Li‐rich matrix upon cooling after emplacement
�Barrat et al., 2005�. A detailed investigation of the Li isotopic composition of pyroxene and
olivine phenocrysts in several nakhlites demonstrated that microscale variations of Li
abundances and isotopic compositions in martian rocks were due to Li diffusion �Beck et
al., 2006; Treiman et al., 2006�. Similar effects were recently found in terrestrial minerals
from a peridotite �Jeffcoate et al., 2006� and from primitive arc lavas from Solomon Islands
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