HR: 16:30h
AN: V22G-03 INVITED     [PDF]
TI: U-series Isotopes and the Time Scales of Magmatic Processes
AU: * Hawkesworth, C J
EM: c.j.hawkesworth@bristol.ac.uk
AF: University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ United Kingdom
AU: Peate, D W
EM: dwp@dlc.ku.dk
AF: Danish Lithosphere Centre, Oester Voldgade 10, Copenhagen, DK-1350 Denmark
AU: Regelous, M
EM: m.regelous@bristol.ac.uk
AF: University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ United Kingdom
AU: Turner, S P
EM: sturner@els.mq.edu.au
AF: GEMOC, Macquarie University, Sydney, NSW 2109 Australia
AU: George, R M
EM: rgeorge@els.mq.edu.au
AF: GEMOC, Macquarie University, Sydney, NSW 2109 Australia
AB: The first published record of a radioactive decay chain was 100 years ago this year, and the sequence of isotopes in the U and Th decay chains were largely determined in the following ten years. Isotopes of U, Th, Pa, Ra and Pb with half-lives in the range 75,000 to 22 years have had a major impact in our understanding of magmatic processes, because the time scales of magmatic processes are similar to the half lives of these isotopes, and physically realistic models of natural processes require information on the rates at which those processes occur. U-Th-Pa-Ra isotopes can now be measured by mass spectrometry, routinely with errors of less than ~1%. A key characteristic is that U-series isotopes can change significantly by radioactive decay while the crystals and rocks were forming. At subduction zones fluids may be transferred from the downgoing slab in a few 1000 years. In most tectonic settings the magmas and the peridotite matrix spend different lengths of time in the melt zone, and typically the observed isotope fractionation implies some form of dynamic melting process. New U-Pa isotope data for 40 young lavas from 7 different arcs worldwide have, with one exception, ($^{231}$Pa/$^{235}$U) $>$ 1, and extend to values as high as 2.48. Their U/Nb ratios are $<$ 9.0 and so $>$ 80% of the U has been added from the subducting slab, and large enrichments of Pa over U occurred during melting and melt transport. The ages of phenocrysts and the time scale of differentiation of the host magma can be different, and in a number of cases it has been shown that the phenocrysts formed after the fractional crystallisation responsible for the whole rock composition. Different approaches are therefore used to investigate the crystallisation history and the differentiation of magmatic suites: crystallisation rates are ~ 10$^{-10}$ to 10$^{-11}$ cm/s, whereas differentiation to high silica magmas may take up to 2 x 10$^{5}$ years. The ages of crystals at the time of eruption can range back to 2-3 x 10$^{5}$ years, the older ages tend to be in the more evolved rock types, and it can take 10$^{5}$ years for high silica magmas to be generated at individual volcanic centres. Thus, the generation of evolved magmas is often thermally controlled, and the rates of fractional crystallisation have, for example, been linked to volcanic power outputs. In contrast, crystallisation in response to magma degassing or decompression, may be too fast for much fractional crystallisation to take place.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1035 Geochronology
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting