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

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
study: The first one is within the glassy quenched part of the pillow, and the second one is
an internal fragment located 5 cm below the pillow natural surface �i.e., 5 cm below the
interface sea water/lava�. Three olivine crystals were chosen for the detailed chemical and
isotopic study: in the first section, an euhedral crystal, 5 mm in size, located in the glassy
rim of the pillow �crystal Ol4‐11, see Fig. 3.7� and, in the second section, two joined olivine
crystals embedded in the microcrystalline groundmass �3 mm in size for the larger one:
crystal Ol5‐20, see Fig. 3.8�. Major elements, Li concentrations and Li isotopic compositions
were measured by electron and ion microprobes. Electron microprobe analyses for major
elements were carried out at Ifremer �Centre de Brest� with a Cameca SX50. The analyses
were obtained at 15 kV accelerating voltage with a sample current of �12 nA using a set of
oxide
and silicate standards.
2.1. Li concentrations measurements
The lithium concentrations and isotopic compositions were measured by ion
microprobe at CRPG‐CNRS �Nancy� using the analytical procedures which were developed
on the small radius �Cameca ims 3f� and the large radius �Cameca ims 1270� instruments,
as previously described �Chaussidon and Robert, 1998; Beck et al., 2004, 2006; Barrat et
al., 2005�. On both instruments, carbon‐coated polished samples were sputtered with an O ‐
beam of approximately 25 μm size. The secondary 6 Li � and 7 Li � ions were accelerated at
4.5 kV for the ims 3f and at 10kV for the ims 1270 and were counted in mono‐collection
mode on an electron multiplier using magnetic peak switching. The background on the two
multipliers was monitored during the different sessions: it was below 0.05 cps �count per
second� for the ims 3f and below 0.02 cps for the ims 1270. The count rates, which were in
the range from ≈ 50 000 cps to ≈ 250 000 cps on 7 Li � , were corrected for a dead time of 44
ns.
The Li concentration were measured with the ims 3f �the greater transmission of the
ims 1270 is not required in this case� by classical energy filtering �‐60V�10V�. The ion
yield of Li relative to Si was determined on a silicate glass standard �UTR 2 having 63.4
μg/g Li and 74.12 wt% SiO2, courtesy of John Stix�. Eight analyses of this standard gave an
ion yield of Li �yield Li � �Li � /Si � �/�Li/Si�� of 1.48 � 0.03. The reproducibility of this yield
on one of our olivine standards �Ol BZ 29� having a much lower content �3.1 μg/g� was of
4% relative. The Li concentrations are thus given in Table 3.2 with an error of � 5%
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