HR: 15:30h
AN: V43F-08    [Abstracts]
TI: The origin of high 3He/4He values in oceanic lavas: arguments against a heterogeneous upper mantle source
AU: * Castillo, P R
EM: pcastillo@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, CA 92093-0212, United States
AU: Hahm, D
EM: dhahm@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, CA 92093-0212, United States
AU: Hilton, D R
EM: drhilton@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, CA 92093-0212, United States
AB: One of the contentious issues fuelling the debate on the origin of intraplate magmatism is the source of high 3He/4He ratios in oceanic lavas. The contemporary hotspot hypothesis posits that the high 3He/4He signature is coming from a distinct source reservoir that lies relatively deep in the mantle whereas others argue that there is no such a reservoir. For example, the SUMA (Statistical Upper Mantle Assemblage - Meibom & Anderson, EPSL 17, 2003) hypothesis argues that the compositional variability of oceanic lavas results from sampling upon melting of the heterogeneous upper mantle. It claims that extreme isotopic signals, such as high 3He/4He ratios, of some oceanic lavas are due to small degrees of partial melting that preferentially sample dispersed, anomalous mantle components. In contrast, large degrees of partial melting sample a larger volume, and hence many components of the heterogeneous mantle; these also promote more effective mixing of the resulting melts, producing a relatively homogeneous composition typified by that of N-type MORB. To constrain the source of high 3He/4He lavas, we analyzed a ~dozen representative samples from the axis and near-ridge seamounts in the 11o45'N to 15o00'N segment of the EPR. Axial lavas are N- to T-type MORB that came from a heterogeneous mantle source (Castillo et al., G3 1, 2000). Seamount lavas extend the compositional variability of axial lavas to both higher and lower Sr, Nd and Pb isotope values, suggesting that seamount lavas indeed result from smaller degrees of partial melting of the heterogeneous mantle beneath the region. Our preliminary results, however, also show that the majority of both axial and near-ridge seamount lavas have indistinguishable, MORB-like 3He/4He values, at 8 +/- 1 RA, over a range of He concentrations. The remainder have 3He/4He < 8 RA. Thus we conclude that the heterogeneous Pacific upper mantle cannot provide the high 3He/4He signature in oceanic lavas.
DE: 1000 GEOCHEMISTRY
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 3600 MINERALOGY AND PETROLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting