NSF Forms - Ridge 2000 Program

PROJECT SUMMARY
Intellectual Merit: We propose an intensive U-series study of volcanics from the Lau
back-arc basin, which is an Integrated Study Site (ISS) for the Ridge2000 program. The Eastern
Lau Spreading Center provides a natural experiment thanks to progressive change in the distance
of the spreading center from the trench from south to north. Recent cruises have led to detailed
maps, the discovery and characterization of five new vent fields, and closely spaced rock
sampling. The rock sampling has led to the best sampled back-arc basin spreading center, with
precisely located, axial samples in a well determined tectonic context. Extensive major element,
trace element, volatile element and isotopic data have been obtained for these samples. This
exceptional sample collection, the new comprehensive data set, and the unique tectonic context of
the Lau spreading centers provide new opportunities for U-series studies.
We propose measurements of U, Th and Ra isotopes with two major goals: (1) To constrain
the origin and transport time of subduction components. Excesses akin to convergent margin
lavas are a characteristic aspect of back-arc basin lavas and distinguish them from open ocean
spreading centers. Published data from convergent margins appear to require distinct time scales
from U-Th and Ra-Th isotopes, and have correlations of 238 U and 226 Ra excesses with subduction
signatures such as Ba/Th. Further from the trench at back-arcs, the time scales for fluid transport
should differ. We have the entire range of U/Th ratios at diverse distances from the arc. Do
these relationships between U-series excesses and subduction components change with arc
distance, and can they be used to constrain fluid transport times from slab to surface? Our
existing geochemical data also provide evidence for two distinct fluid compositions, and
systematic correlations between U-Th systematics and long-lived radiogenic isotopes. Thus we
have the potential to further constrain both the origin of the U-series excesses, and the timing
between creation of these excesses and surface eruption. (2) To provide the necessary age
constraints for further development and understanding of the ISS. Age constraints are essential to
relate volcanic activity to tectonics, variations in the underlying magma chamber reflector, and
temporal changes in lava compositions. They also are an essential link between the volcanic
system and the hydrothermal and biological activities that are supported and impacted by
fluctuations in volcanism. Questions such as whether segments have been recently active,
whether there is a relationship between lava age and the presence of magma chamber reflectors,
how age distribution may vary with subduction input cannot begin to be addressed without age
constraints. While “model ages” as applied on the EPR may not be applicable in this region, age
constraints from U-series can still be obtained, and are essential for a host of questions and
hypotheses for the Ridge system from mantle to microbe. We thus propose measurement of 40
samples for U-Th-Ra measurements, and 5-10 samples for 210 Pb measurements to identify
eruptions that may be