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

tel-00344949, version 1 - 7 Dec 2008
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Geochemical Segmentation of the Pacific
Antarctic Ridge
LAURE DOSSO 1 , CÉDRIC HAMELIN 2 , BARRY HANAN 3 , MANUEL
MOREIRA 4 , JEAN-ALIX BARRAT 2
1
CNRS UMR 6538, IFREMER, BP70, 29280 Plouzané, France
[laure.dosso@univ-brest.fr]
2
CNRS UMR 6538, U.B.O.-I.U.E.M., place Nicolas Copernic, 29280
Plouzané Cedex, France [cedric.hamelin@sdt.univbrest.fr],[barrat@univ-brest.fr]
3
Department of Geological Sciences, San Diego State University,
5500 Campanile Drive, San Diego, CA 92182, U.S.A.
[bhanan@mail.sdsu.edu]
4
Laboratoire de Géochimie et Cosmochimie, Institut de Physique du
Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 5, France
[moreira@ipgp.jussieu.fr]
Samples collected during the PACANTARCTIC 2 (PAC2)
cruise fill a sampling gap from 53° to 41° S of the Pacific
Antarctic Ridge (PAR). Therefore it becomes possible to have
a picture of the geochemical variations along the entire PAR
from the Australian Antarctic Discordance (AAD) to the Juan
Fernandez microplate.
PAC 2 data together with a compilation of published Sr,
Nd, Pb, Hf and He isotopic analyses show geochemical
variations related to first order tectonic discontinuities. One
example occurs at the Eltanin fracture zone. Another example
is within the PAC2 study area at 50°S. A major transform
fault, Menard TF, marks there the boundary between two
geochemical domains. To the south of Menard TF, trace
element and isotope ratios are depleted and more variable
than to the north where the variations are more subdued and
the geochemical parameters indicate a more enriched source.
These geochemical limits illustrate large-scale chemical and
morphological divisions of the Pacific mantle.
At a smaller scale, the regular sampling of the different
segments limited by overlapping spreading centers along the
53-41°S ridge section shows regular variations in
geochemical parameters. Most of the 100-260 km long
segments are depleted with (Nb/Zr)N < 1. The centers of the
segments have higher (Nb/Zr) N and more radiogenic Sr
isotope ratios than the segment ends where the ridge becomes
less robust. This small-scale geochemical segmentation
together with observed variations of geophysical parameters
suggests small-scale mantle flow below the ridge.
However this regular small scale variation is locally
affected by the presence of nearby off-axis volcanoes which
define a pseudo-microplate. The consequences at the ridge
axis are both morphological and geochemical. They are
revealed by a slight change in the ridge axis direction
associated with geochemical atypical signatures of the ridge
basalt s.
Kinematic reorganization of the pacific antarctic plate
boundary is likely responsible for these features.
Li diffusion and isotopic fractionation in
olivines crystals
CÉDRIC HAMELIN 1 , MARC CHAUSSIDON 2 , JEAN-ALIX BARRAT 1 ,
PIERRE BECK 3 AND MARCEL BOHN 4
1
CNRS UMR 6538, U.B.O.-I.U.E.M., place Nicolas Copernic, 29280
Plouzané Cedex, France [cedric.hamelin@sdt.univbrest.fr],[barrat@univ-brest.fr]
2
CRPG-CNRS UPR 2300, 15 rue Notre-Dame des Pauvres, 54501
Vandoeuvre-les-Nancy Cedex, France [chocho@crpg.cnrsnancy.fr]
3
Geophysical Laboratory, Carnegie Institution of Washington, 5251
Broad Branch Road NW, DC 20015-1305 Washington, USA
[pbeck@ciw.edu]
4
CNRS UMR 6538, IFREMER, BP70, 29280 Plouzané, France
[bohn@ifremer.fr]
In order to constrain the behavior of lithium isotopes in
crystals during magma cooling, the study of a fresh pillowlava
with large olivine crystals has been undertaken. An ionmicroprobe
was used to measure lithium abundances and
isotopic compositions both in olivine phenocrysts and in the
pillow rim glass. Profiles in δ 7 Li conducted through olivines
show very different patterns correlated with their cooling
history. Olivines embedded in the pillow rim glass display no
variation in Li abundance ([Li] = 1.2 µg.g -1 ) and isotopic
composition (δ 7 Li = +6 ‰). However, a large Li isotopic
zoning is found from crystal cores (δ 7 Li = +6 ‰, [Li] = 1.1
µg.g -1 ) to rims (δ 7 Li = -11 ‰, [Li] = 1.4 µg.g-1) for olivines
set in microcrystalline groundmass. The cores of large olivine
phenocrysts are equilibrated with the pillow rim glass (δ 7 Li =
+6 ‰, [Li] = 3.4 µg.g-1).
Recently, detailed investigation of the Li isotopic
composition of large phenocrysts in various magmatic
environments demonstrated that microscale δ 7 Li variations
were due to fractionation of lithium isotopes during late stage
chemical diffusion (Barrat et al., 2005; Beck et al., 2006;
Jeffcoate et al., 2006). In our case, the diffusion-induced Li
isotopes fractionation is the only process able to explain such
diverse zoning in olivines from one single pillow lava. Our
study brings new constraints on the Li fractionation during
the diffusion process and suggests that the pristine δ 7 Li values
can be completely erased in small phenocrysts, even in the
case of rapidly cooled volcanics. The use of Li isotopes in
crystals for tracking mantle heterogeneities should be taken
with extreme caution.