HR: 16:50h
AN: U14A-04    [Abstracts]
TI: Mixing the mantle marble-cake: timescale constraints from Os isotopes
AU: * Parman, S
EM: stephen.parman@durham.ac.uk
AF: Durham University, Dept. of Earth Sciences South Road, Durham, DH1 5PT, United Kingdom
AU: Pearson, G
EM: d.g.pearson@durham.ac.uk
AF: Durham University, Dept. of Earth Sciences South Road, Durham, DH1 5PT, United Kingdom
AU: Nowell, G
EM: g.m.nowell@durham.ac.uk
AF: Durham University, Dept. of Earth Sciences South Road, Durham, DH1 5PT, United Kingdom
AU: van Hunen, J
EM: jeroen.vanhunen@durham.ac.uk
AF: Durham University, Dept. of Earth Sciences South Road, Durham, DH1 5PT, United Kingdom
AB: In their seminal paper, Allegre and Turcotte (1986) presented a model in which the upper mantle is a mixture of depleted, harzburgitic mantle and subducted basalt that has been mechanically mixed together, the mantle marble-cake. Since their publication, most studies of mantle heterogeneity have focused on the enriched components, which are equated with subducted basalt and/or sediments, and successfully explain OIB Sr-Nd-Pb isotopic systematics. In this talk, we will focus on a different part of the marble-cake, depleted (harzburgitic) heterogeneities. Though abundant in abyssal peridotites and ophiolites, these have been difficult to study geochemically because they have very low concentrations of typical trace elements and radiogenic isotopes, and are overprinted by any mixing with enriched mantle or melts. However, Os is compatible during mantle melting, is enriched in depleted mantle and thus is robust with respect to mixing with enriched components or metasomatism. Somewhat surprisingly, Os isotope studies of the convecting mantle show clear evidence for depleted heterogeities up to 2 billion years old, but the relative paucity of data (less than 100 analyses), makes it difficult to extract meaningful mixing information. Rapid analysis of osmiridium grains by laser-ablation inductively coupled multi-collector plasma mass spectrometry now allows large Os datasets to be acquired (100s of datapoints), which are suitable for statistical analyses (Meibom, 2002). Here we present new and published laser-ablation analyses of osmiridiums from a global collection. The data generally show an exponential decrease in heterogeneities with age, such that over 90% of heterogeneities are destroyed within 2 billion years, though rare heterogeneities as old as 2.7 Ga survive. The exponential decrease in survivorship is generally consistent with the mechanical mixing model of Allegre & Turcotte (1986). Subsequent 2-dimensional mixing models suggest that high-viscosity blobs can persist for much longer time-scales than 2 Ga (Manga, 1996). Thus the relatively fast mixing suggested by the Os data may imply that 1) the depleted heterogeneities have similar viscosities to their surrounding mantle, 2) 2D models overestimate mixing timescales or 3) other processes, such as melt infiltration, may destroy depleted heterogeneities.
DE: 1000 GEOCHEMISTRY
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Union [U]
MN: 2007 Fall Meeting