HR: 14:55h
AN: V43E-06    [Abstracts]
TI: Are There Primitive Reservoirs in a Geochemically Heterogenous Mantle?
AU: * Jacobsen, S B
EM: jacobsen@neodymium.harvard.edu
AF: Harvard University Department of Earth and Planetary Sciences, 20 Oxford St., Cambridge, MA 02138 United States
AU: Mukhopadhyay, S
EM: sujoy@eps.harvard.edu
AF: Harvard University Department of Earth and Planetary Sciences, 20 Oxford St., Cambridge, MA 02138 United States
AB: It has been argued that ³He in the deep Earth is a "primitive" component, with the same origin as the indigenous noble gas component in primitive carbonaceous chondrites. However, Earth's volatile depletion signature suggests that its building blocks should be strongly depleted in ³He relative to primitive carbonaceous chondrites. It is possible to infer the ³He abundance in the deep Earth using isotope systematics. A high-³He reservoir is clearly seen in the low ⁴He/³He ratios of large mantle plumes and leads to the surprising conclusion that this reservoir has a ³He concentration similar to that in CI and CM carbonaceous chondrites, requiring some early enrichment mechanism such as ingassing. Comparison of He isotope systematics of MORBs and plumes indicate that the plume source must be a lower mantle feature, in agreement with geophysical inferences favoring the origin of plumes at the core-mantle boundary. The simplest explanation for the high-³He source is a gas-rich deep mantle reservoir entrained by rising plumes at some point during ascent. The surface expression of plume-driven melting is, however, generally not isotopically primitive for Nd, Sr and Pb isotopes. The "primitive" He signal is strong because ³He is about 100-200 times more abundant in the ³He rich source compared to the depleted upper mantle source. The concept of a "primitive" source is often meant to imply mantle material that has not previously experienced partial melting and associated degassing. Any mantle plume source cannot be primitive in the meteoritic sense because it has experienced both accretion and core formation. The high-³He mantle is likely a result of a complex history involving: melting, degassing, ingassing, and convective mixing with deeper mantle material. It is thus "primitive" only in the sense of being a preserved early Earth feature which has been resident in the deep mantle over Earth history. The high-³He mantle and the depleted upper mantle appear to have been separate reservoirs over all of Earth history.
DE: 1000 GEOCHEMISTRY
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005