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

tel-00344949, version 1 - 7 Dec 2008
Annexe 3: participations à des congrès internationaux
AGU Fall meeting 2005 : poster
V41‐C1460 The Pacific‐Antarctic Ridge Axis Between 41 and 51°S :
First Sampling and Geochemical Data
Cédric Hamelin, Laure Dosso, Barry Hanan, Manuel Moreira, Frauke Klingelhoefer, Jean‐Alix
Barrat, Hélène Ondréas
�Université
de Bretagne Occidentale, IUEM, UMR 6538, Place Nicolas Copernic, 29280 Plouzané,
France�
In 1998, following 2 cruises which sampled the Pacific Antarctic Ridge between 53 and
$66\deg$, geochemical data showed evidence of a large‐scale chemical and thermal division of
the Pacific mantle. The boundary was identified at the latitude of the Easter island microplate.
However those conclusions were drawn from an incomplete data set as the ridge section
between
51 and $41\deg$S had not yet been sampled. This, therefore became the main
objective of the PACANTARCTIC 2 cruise which took place during the austral summer 2004‐05.
This 1300 km‐long ridge section is characterized by a dome shape axis. It is segmented by the
Menard major transform fault near $50\deg$ and by small non‐transform offsets such as
overlapping spreading centers.
24 dredges were collected along axis at regular intervals. Preliminary geochemical data show
that the great majority of samples are depleted with �Nb/Zr�N $�$0.5. Within each segment, the
most depleted sample is found at its northern end where the ridge becomes less robust. At the
studied
section scale, a general decrease in �Nb/Zr�N ratios towards the north is observed. It is
accompanied by a decrease in bathymetry from 2500 to 2300 m.
South of Easter Island, considering available data from $66\deg$ to $41\deg$, all geochemical
parameters including He and Hf isotopes indicate a mantle source more enriched to the north of
the
Eltanin fracture zone system than to the south. The variability of the data is less north of
Eltanin FZ than south of it.
At
the South Pacific ocean scale, the new Sr‐Nd data confirms the evidence of two mantle
domains
limited by the Easter microplate.