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

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
7. IMPLICATIONS OF LI DIFFUSION ON LI ISOTOPIC STUDIES OF
MAGMATIC ROCKS
7.1. Li systematic in melt inclusions
If Li diffusion is taking place between olivine crystals and their surrounding matrix, a
similar effect is expected between melt inclusions and their olivine hosts. Previous studies
have shown that large variations exist for δ 7 Li in glass inclusions. Gurenko and Schmincke
�2002� reported significant variations in δ 7 Li for glass inclusions hosted by olivine or
orthopyroxene crystals in pillow lavas from the Iblean volcanic province �δ 7 Li � ‐3.4 to
�1.2 ‰�. Kobayashi et al. �2004� found a huge range of isotopic compositions in melt
inclusions hosted by olivines from Hawaii �δ 7 Li � ‐10.2 to �8.4 ‰� and Barrat et al.
�2005� reported a extremely high δ 7 Li value for inclusions hosted in olivines from a lunar
basalt �δ 7 Li � �11.2 and �14.5 ‰�. The petrogenetic significance of δ 7 Li variations in
glass inclusions was first questioned for inclusions hosted in plagioclase and
orthopyroxene from the Plat Pays Volcanic Complex in southern Dominica �Gurenko et al.,
2005� based on observations that �i� the large Li isotopic variations found in the inclusions
�δ 7 Li � �4 to �15 ‰� do not correlate with B isotopes and with any other geochemical
tracers and that �ii� the Li concentrations and the δ 7 Li values of the inclusions differ
according to the host mineral �inclusions in plagioclase have higher δ 7 Li values and lower
Li contents than inclusions in orthopyroxene�. Gurenko et al. �2005� suggested that this
could reflect a 7 Li‐enrichment of the inclusions during Li diffusion loss to the surrounding
crystals, in agreement with the fact that Li diffusion in plagioclase is a priori faster than in
orthopyroxene.
The present pillow lava sample contains numerous glassy inclusions which have been
extensively studied by Kamenetsky �1996� who showed that the trapped melt had a
composition similar to that of the parental magma of the olivine phenocrysts. Thus no
difference in δ 7 Li values is anticipated between the melt inclusions and the olivine
phenocrysts since Li isotopic fractionations during magmatic differentiation are less than
one ‰ �Tomascak et al., 1999�. At variance the two melt inclusions analyzed �Table 3.2�
have significantly higher δ 7 Li values ��10.7 �1.0 ‰� than the pillow rim glass ��6.5 ‰�.
Because of the large difference in Li concentration between the Li‐rich glassy inclusions
��Li� � 3.2 � 0.8 μg/g� and the surrounding Li‐poor olivines ��Li� � 1.2 � 0.1 μg/g, n � 38�,
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