HR: 10:45h
AN: V31F-02    [PDF]
TI: No Core Contribution in Mantle Plumes: New Evidence From Tungsten Isotopes
AU: * Scherst\'{e}n, A
EM: anders.schersten@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol Wills Memorial Building Queen's Road, Bristol, BS8 1RJ United Kingdom
AU: Elliot, T
EM: Tim.Elliott@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol Wills Memorial Building Queen's Road, Bristol, BS8 1RJ United Kingdom
AU: Hawkesworth, C J
EM: C.J.Hawkesworth@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol Wills Memorial Building Queen's Road, Bristol, BS8 1RJ United Kingdom
AU: Norman, M D
EM: Marc.Norman@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AB: Os isotope ratios provide major constraints on the sources of ocean island basalts, but are currently interpreted in two dramatically different models. One implies interaction with the outer core, and as a corollary of coupled $^{186}$Os-$^{187}$Os anomalies requires early inner core formation, whilst the other involves a source of recycled oceanic crust. Tungsten isotope data on the same set of picrites used to establish $^{186}$Os-$^{187}$Os anomalies for Hawaiian picrites have $\epsilon$W= -0.06 to 0.04 with errors of $\pm$0.11 to $\pm$0.31. Similarly a suite of selected South African kimberlites, which have also been proposed to contain a core contribution, have $\epsilon$W= -0.07 to 0.03 with errors of $\pm$0.10. The isotope budget of tungsten in the Earth is well characterised. Given that bulk Earth is chondritic ($\epsilon$W = -2) and the silicate Earth has $\epsilon$W = 0 (by definition), then the core $\epsilon$W = -2.1. Tungsten is siderophile during core formation and incompatible during mantle melting, which leaves it depleted in the mantle (1.9-15 ppb) but strongly enriched in the core ($\sim$500 ppb) and the crust (850-1100 ppb). Thus, tungsten is a sensitive tracer of core contributions in the source of mantle melts. We explore two different models of simple core-mantle mixing where mantle W-depletion and inner core crystallisation history are the two most important parameters. None of the samples analysed have the negative W-isotope values of -0.2 to -0.6 $\epsilon$W predicted from our preferred core contribution model. Extreme combinations of all critical input values have to be assumed to achieve any overlap between modelled and measured $\epsilon$W. Hence a simple core-mantle mixing scenario appears to be ruled out, and other explanations need to be sought. Mn-nodules have high $^{190}$Pt/$^{188}$Os $\sim$0.2-0.5, low Re/Os $\sim$0.1-0.5 but high [Os] $\sim$1-3 ppb and $^{187}$Os/$^{188}$Os $\sim$0.5-1. $\sim$2% of recycled Mn-nodule in the peridotite source reproduces the observed Os isotope trends and it would be associated with an increase of just $\sim$0.5 wt% Mn.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 9355 Pacific Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting