HR: 11:25h
AN: U12A-05    [Abstracts]
TI: Mantle Dynamics Inferred From the Samoan Hotspot: Implications for Recycling and the Preservation of Hemispheric-Scale Geochemical Anomalies.
AU: * Jackson, M G
EM: mjackson@whoi.edu
AF: MIT/WHOI Joint Program, 266 Woods Hole Rd, Woods Hole, MA 02543, United States
AB: The standard model for the geochemical evolution of the mantle assumes that much of the chemical diversity is a result of subduction, a tectonic process that introduces enriched oceanic crust and sediment into a primitive (or slightly depleted) mantle. Do surface materials survive the "blender" that we call the mantle, to be recycled and erupted as ocean island basalts (OIBs) that bear resemblance to the original subducted material? Do OIBs offer hints that there are regions isolated from the convective vigors of the mantle, where heterogeneities can be preserved for long periods of earth's history? Recently discovered lavas from Samoa show the strongest evidence yet for continental sediment recycling through the mantle, and exhibit 87Sr/86Sr > 0.7216 [Jackson et al., 2007a]. In spite of the large quantities of sediment having entered the mantle at subduction zones over geologic time, recycled sediment (EM2, enriched mantle 2) signatures are rare in OIBs [Hofmann, 1997]. If large quantities of sediment have survived subduction zones and now occupy the deep mantle, either 1.) the mantle is so dynamic that the recycled sediment signatures are diluted and difficult to discern in OIBs, or 2.) the sediments have accumulated and become isolated in the deep mantle and are rarely sampled at the surface in OIBs. The observation of ancient, large-scale isotope anomalies may offer an important insight. The DUPAL (of Dupré and Allègre fame) anomaly, a global-encircling feature of isotopic enrichment observed primarily in southern hemisphere OIBs, has survived mantle convection for as long as 3 Ga [Hart, 1984]. New high 3He/4He lavas (33.8 times atmospheric, Ra) from Samoa are consistent with a DUPAL signature in the high 3He/4He, or FOZO (Focus Zone [Hart, 1992]), reservoir [Jackson et al., 2007b]: The highest 3He/4He samples from each southern hemisphere high 3He/4He hotspot (i.e., > 11 Ra) exhibit lower 143Nd/144Nd ratios than their counterparts in the northern hemisphere (excluding lavas erupted in continental, back-arc, and submarine ridge environments). Like DUPAL, the FOZO reservoirs are ancient. The existence of these long-lived, large-scale anomalies suggests that the mantle does not always efficiently destroy heterogeneities. Where in the mantle are these long-lived geochemical anomalies preserved from convective mixing? Among geochemists, the deepest part of the lower mantle—the core mantle boundary—is a favorite location for the FOZO and DUPAL anomalies, and many geochemical models require that this region be isolated and immune to convective stirring. A recent discovery demonstrated that the 142Nd/144Nd ratios in all measured terrestrial mantle rocks are 20 ppm higher than chondrites, suggesting that the mantle hosted a superchondritic Sm/Nd reservoir during the short lifetime of 146Sm [Boyet and Carlson, 2005]; if bulk silicate earth is chondritic, then a complementary "hidden enriched reservoir" (HER) with subchondritic 142Nd/144Nd must also exist in the deep earth. The implication is that the DUPAL, FOZO, EM2 and HER reservoirs are all suggested to occupy the same location in the mantle. Surely, therefore, they must interact: If the signature of one deep reservoir escapes to the surface in plumes, the signatures of the others will be detectable as well? Further 142Nd/144Nd measurements on OIBs from plume-fed hotspots will provide an important test of this hypothesis, as mixing with the REE-rich EER may perturb the 142Nd/144Nd of the other deep mantle reservoirs. Additionally, coupling observations of surface geochemical anomalies with increasingly higher resolution seismological techniques may provide insights into the home of these (deep?) reservoirs.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 3621 Mantle processes (1038)
SC: Union [U]
MN: 2007 Fall Meeting