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

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
� 1.33‰ �n � 4� for NAZCA glass; and for sessions with the ims 1270 we obtained �
0.7‰ �n � 6� for olivine BZ29. In several cases, the range of αinstLi is larger than the
internal error of each measurement, suggesting that small isotopic heterogeneities at the
level of 1.5‰ may be present among the different grains of the standards we use. This
might be possible if the process of isotopic fractionation described in this article and in
previous work �Barrat et al., 2005; Beck et al., 2006� occurs more generally in terrestrial
rocks. Within this range of 1‰ to 2‰ we never observed significant variations of αinstLi
due to matrix effects within our set of standards �see Fig.1 in Beck et al., 2004�. However
strong matrix effects were recently observed for olivines having varying Mg/Fe ratios
��1.3‰ per Mg# unit, Channon et al., 2007�. Because the olivines we studied in pillow
sample ARP73‐10‐03 have constant Mg/Fe ratios �i.e. Mg# � 86, see Fig. 3.6�, the issue of
matrix effect was not a concern. The precision of the Li isotopic measurements, taking into
account counting statistics, error on correction for the instrumental mass discrimination,
and external reproducibility, range from �0.6‰ �1σ� to �3.5‰ �1σ� with an average of
�0.9‰
�1σ�.
3. PETROGRAPHY
AND MINERAL CHEMISTRY
A few picritic pillowed basalts were collected on the east flank of Venus Mount
�36°50.0’N; 33°15.0’W� in the FAMOUS area of the Mid‐Atlantic Ridge. Picrites have rarely
been documented along the ridge axis. Consequently these samples have been extensively
studied �Bougault and Hekinian, 1974; Bryan, 1979; Le Roex et al., 1981; Kamenetsky,
1996; Laubier et al., 2007�. Picrite ARP73‐10‐03 contains abundant euhedral to subhedral
olivine phenocrysts �~ 20 vol% and up to 5 mm in size� set in a fine grained groundmass.
Near the pillow surface, a 1 cm thick hyaline crust is present.
Olivine phenocrysts contain numerous glass inclusions randomly distributed in their
host. The inclusions are sub‐spherical to spheroidal in shape and vary in size from a few
microns to 300 μm. They are uniformly brown and perfectly vitreous without any quench
crystals. Some shrinkage bubbles are present only in the largest inclusions ��200 μm�.
Dark‐brown euhedral chrome spinels up to 0.6 mm in size are found as isolated crystal in
the
matrix and as inclusions in olivine phenocrysts.
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