NSF Forms - Ridge 2000 Program

There is thus a rich array of problems that can be addressed by detailed study of ELSC samples
that (1) are sampled with the benefit of high resolution side scan and bathymetry and therefore are
very likely to be young and the best candidates to preserve U-series disequilibria; (2) exhibit large
and systematic variations in fluid signature as recorded by U/Th, Ba/La and Pb isotopes, all
pertinent to the investigation and interpretation of U-series data; and (3) have as large variations in
( 238 U/ 232 Th) that correlate with water contents in the northernmost ridge segment ELSC 1, as are
present in the south. Specific problems we will be able to elucidate include:
• Do 226 Ra excesses correlate with Ba/La and Ba/Th for our sample suite? Is the spike in Ba
observed at 21°S associated with an increase in 226 Ra excess? If found, this would be evidence of
control of Ra excess by a subduction component, even in the back-arc. This would provide a new
constraint on the timing hypotheses for Ra excesses, because the back-arc is much farther from a
source of fluids that would lead to correlated Ra and Ba excesses. If such a correlation exists, then
one needs to imagine very rapid transport independent of distance from the volcanic front, which
would be problematic, possibly necessitating alternative models for the Ra excess occurring just
beneath the sites of magmatism. Thus we can test the hypothesis that the Ba- 226 Ra/ 230 Th correlation
should disappear in the back-arc setting, and that back-arc 226 Ra excesses would be associated with
melting column effects as at open ocean ridges.
• U-excesses for the Tonga volcanic front are interpreted as reflecting a processing age in the
mantle wedge of 30,000years. We have a large variation in U/Th in ELSC 1, much farther from the
arc. Do these samples lie on the same trend as the arc, suggesting a mixing origin for all of the
data? Do they show an older apparent age, which might be expected for a longer fluid residence
time with increasing distance form the trench? Are they instead dominated by melting column
processes, and hence show no correlation between U-excess and other indicators of a slab
component, or possibly even show Th excess at the same time as the U-enrichment from a slab
component? Depending on the results, these data will impact the understanding and interpretation of
the apparent ages of fluid transport inferred from arc U-series studies.
• For samples in the north with no apparent slab component, are Th and Ra excesses
consistent with the melting column processes of open ocean ridges, or do they differ, suggesting a
different melting regime environment? This will test the arguments of Kelley et al., [in press] and
Langmuir et al., [in press] for differences in the melting regime in the back-arc as compared to open
ocean environment.
All these interpretations will be furthered by the extensive geochemical data sets obtained on the
same samples. Our sample set combines exceptional spatial and tectonic control with high quality
data from our on-going studies of major elements [Bezos et al., 2004], trace elements [Bezos et al.,
2005], radiogenic isotopes [Escrig et al., 2005], helium isotopes [Goddard et al., 2005] and volatiles
[Michael et al., 2005]. Our proposed work will provide the most exhaustive and complete data set in
a back-arc environment, and may lead to significant new constraints on the interpretation of U-
series data at convergent margins, and our understanding of the nature and timing of formation and
delivery of subduction components to the surface.
We are well aware that one of these questions requires very young samples that preserve their
eruptive 226 Ra disequilibria. Such recent volcanism has often been the case at ocean spreading
centers, and simple geological considerations are supportive of this possibility for our samples as
well. Based on the high-resolution side scan and bathymetry, the precise and short dredge tracks,
and existence of many very well located rock core samples with sufficient glass, we believe that
most sample locations are within the zone of youngest volcanism. Certainly within the areas where
we have SM2000 bathymetry and where Jason samples were collected, we are within such a very
D-10
0649641