HR: 0800h
AN: V41D-1500    [Abstracts]
TI: The Scale and Origin of the Osmium Isotope Heterogeneity in Mid-Ocean Ridge Basalts
AU: * Gannoun, A
EM: a.gannoun@open.ac.uk
AF: Dep. Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AU: Burton, K W
EM: k.w.burton@open.ac.uk
AF: Dep. Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AU: Alard, O
EM: o.alard@open.ac.uk
AF: Dep. Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AU: Parkinson, I J
EM: i.j.parkinson@open.ac.uk
AF: Dep. Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA United Kingdom
AB: The decay of 187Re to 188Os provides an exceptional tracer of recycled crustal materials in the Earth's convective mantle, and radiogenic Os isotope compositions of many mid-ocean ridge basalts (MORB) (with187Os/188Os isotope ratios ranging from 0.127 to 0.164) have been taken by some to indicate the presence of recycled oceanic crust in the upper mantle [1]. Others, however, have attributed the data to seawater contamination [2] or an age effect (due to the decay of 187Re following igneous crystallisation) [3]. Despite the controversy surrounding the MORB glass data, no attempt has been made to reproduce these measurements. This study presents new U-Th-Ra and ultra low blank Re-Os isotope data for MORB glass from the Atlantic, Pacific and Indian Oceans (including 18 samples previously measured [1]). Coupled Th and Ra excesses rule out an age effect, but surprisingly the new Os isotope measurements do not reproduce the radiogenic compositions previously observed. These data were most likely due to an analytical artefact. The range of 187Os/188Os is significantly smaller than that observed in previous work varying from 0.126 to 0.144 with an average value of 0.133 that is indistinguishable from estimates for the modern primitive upper mantle, and within the range of abyssal peridotites. Os isotope ratios are not correlated with Os abundances or with 187Re/188Os and hence no evidence for binary mixing can be inferred. The small but resolvable variations that do exist can not be easily explained by age or seawater contamination. Rather co-variations in Os concentration with rare earth elements, Cl and B, are most readily attributed to fractional crystallisation. While subtile but systematic covariations with Sr and Pb isotopes, and distinctive signatures between the Atlantic-Pacific and Indian Oceans, are consistent with some degree of chemical heterogeneity in the MORB mantle. [1] P. Schiano, J.-L. Birck, C.J. Allegre, EPSL 150, 363-379 (1997). [2] S.B. Shirey, R.J. Walker, Ann. Rev. EPS, 26, 423-500 (1998). [3] E.H. Hauri, Phil. Trans. Roy. Soc. Lon. 360, 2371-2382 (2002)
DE: 1025 Composition of the mantle
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005