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

tel-00344949, version 1 - 7 Dec 2008
INTERNATIONAL RESEARCH
long N5 segment. The discontinuity is a small left-stepping
OSC with 8 km overlap and 2 km offset.
Preliminary geochemical analyses of ridge axis samples
(Figure 3) indicate, as was expected, that a majority of
samples are depleted as shown by their (Nb/Zr)N ratios
of less than 0.5. It is worth noting that the more enriched
samples are found in ridge segments that are not clearly
defi ned by the ridge morphology (such as S2 and N2)
and within segment S3, which is clearly infl uenced by the
proximal seamount activity. The most depleted sample with
(Nb/Zr)N < 0.2 is located at the northern end of the N4
segment. Within each segment, the most depleted sample
is found at the northern end of the segment where the ridge
dome becomes narrower.
At the ridge section scale, a decrease in bathymetry is
observed toward the north. It is accompanied by an
increase in the depleted character of the samples as shown
here by the general decrease of (Nb/Zr)N toward the
north.
Off-axis
The major off-axis volcanic structures were mapped and
dredged during this cruise: 9 dredges sampled the ridge
south of the Menard TF, 4 dredges sampled the ridge north
of it, and 4 dredges sampled the 42°S ridges.
Preliminary analysis of the structure of the volcanoes is
based on multibeam bathymetry maps. The backscatter
images also help determine the relative ages of the
volcanoes. A more complete analysis of the off-axis
volcanic structures will be performed using the gravity
data combined with the geochemistry of the samples and
their absolute ages.
The volcanic ridges south and north of the Menard
TF
South of the Menard TF
Nine large (average diameter is 12 km) off-axis volcanic
structures were surveyed near the major 51°S discontinuity
(Figure 4).
Two types of structures coexist:
- (I) typical volcanoes, exhibiting conical shapes with a
summit caldera. These structures are not associated with
strong backscattering in the EM12 imagery, probably
due to the presence of sediments. They are located on
older lithosphere and are elongated in a N90° direction.
Dredge samples taken near the top recovered altered and
porphyric basalts, as well as coarse-grained rocks.
- (II) highly structured and deformed volcanoes. They are
elongated in a N90° or N70° direction and are associated
with narrow, N90°-trending volcanic ridges, some of
them show strong backscattering in the EM12 imagery.
Plagioclase-bearing pillows as well as aphyric fresh and
altered pillows were dredged near their tops.
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North of the Menard TF
The volcanic ridge is located along the latitude of the
large 48°40’S OSC. The ridge was surveyed up to 220 km
off axis. The volcanoes are smaller than those described
south of the Menard TF, with an average diameter of only
6 km. They are structured and connected by N100° to
N90°-trending narrow ridges.
The volcano ‘line’ can be divided into two groups: a fi rst
group of six volcanoes near the axis, and a second group
of four volcanoes, starting 130 km off-axis. All volcanoes
show single or double calderas opened towards the east.
The westernmost part of the survey covered two small
structures, 8 km in diameter, and deeper than the other
parts of the ridge. Toward the south western end of the
map, the N15° to N20° fabric of the abyssal hills reappears,
and is aligned parallel to the axis.
The volcanoes closest to the axis show the strongest EM12
backscattering. Four dredge samples were taken on these
volcanoes. These brought back altered basalts, various
pillows and some signs of life (corals, galatheas, mussels)
closer to the ridge axis.
The 42°S volcanic ridges
Two N90°-trending ridges located near 42°S were
surveyed:
a) Near 41°55’S, between 111°40’W and 112°30’W
This area appears to be highly structured, linear, and
narrow (7 km wide). It does not seem to have been formed
Figure 3
Bathymetry of the ridge axis with indication of the identifi ed
segments South and North of the Menard TF and (Nb/Zr)N variation
along the axis (J. Etoubleau, IFREMER, analyst). Note the
increasing depth of the ridge toward the north and at the same
time the increasing depleted character of samples as shown by the
general decrease of (Nb/Zr)N toward the north. Note also the
geochemical segmentation along the ridge with a more depleted
sample toward the north end of a segment. Transitional segments
S2 and N2 present more enriched characteristics in relation with
the proximity of the seamounts on each side of the Menard TF.