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Geology; August 2003; v. 31; no. 8; p. 693-696; DOI: 10.1130/G19469.1
© 2003 Geological Society of America
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230Th-238U disequilibrium in East Scotia backarc basalts: Implications for slab contributions

S. Fretzdorff*,1, K.M. Haase*,1, P.T. Leat*,2, R.A. Livermore*,2, C.-D. Garbe-Schönberg*,3, J. Fietzke*,3 and P. Stoffers*,3

1 Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40, 24089 Kiel, Germany
2 British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
3 Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40, 24089 Kiel, Germany

New U-series isotope data for lavas from the East Scotia backarc spreading center span a large range in (230Th/232Th) and (238U/232Th). Most of the backarc lavas have (238U/230Th) < 1, similar to the composition of mid-oceanic-ridge basalts (MORB). Lavas from two segments have (238U/230Th) > 1 and are enriched in fluid-mobile elements, implying a recent addition of a U-rich slab-derived component. The data from one segment suggest an influence from an aqueous fluid from altered MORB, whereas samples from the other slab-influenced segment show addition of a sediment melt. The slab-influenced samples form a distinct trend in the equiline diagram between aqueous fluid and sediment melt that is suggested to be a mixing line rather than an isochron.

Key Words: East Scotia Ridge • backarc • U-series isotopes • aqueous fluids • sediment melts






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