HR: 08:40h
AN: V11B-03 INVITED     [Abstracts]
TI: Evolution of Helium Isotopes in the Earth's Mantle
AU: * Class, C
EM: class@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Rt. 9W, Palisades, NY 10964 United States
AB: The presence of primordial ³He in ocean island basalts (OIB) has been considered the primary evidence for the contribution of a primitive, undegassed reservoir in the lower mantle to upwelling plumes and layered convection. A new global data compilation relating helium isotopes of OIB and mid-ocean ridge basalts (MORB) to Sr-Nd-Pb isotopes and trace element abundances yields new insights into mantle dynamics. The compilation is based on data from the online GEOROC and PetDB databases. Oceanic islands are divided into ³He/⁴He groups based on the highest ³He/⁴He ratios measured in mineral separates or glass: (1) ``low`` (³He/⁴He <7 R_A); (2) ``MORB-like`` (8±1 R_A); (3) ``moderately high`` (9-15 R_A); (4) ``high`` (>15 R_A). These designations enable us to follow the geochemical characteristics of mantle plume sources based on their ³He/⁴He ratios despite the global paucity of combined He-Sr-Nd-Pb isotope and chemical data. The compositions of the four groups of OIB are not distributed randomly within the global range but show the following systematics: (i) lower ¹⁴³Nd/¹⁴⁴Nd ratios are associated with lower ³He/⁴He, (ii) for a constant ¹⁴³Nd/¹⁴⁴Nd ratio, higher Pb isotope ratios are associated with lower ³He/⁴He, and (iii) low ³He/⁴He ratios are accompanied by high Th, U abundances independent of partial melting effects. The new compilation shows there is a global relationship between the ³He/⁴He groups and Th+U contents of OIB. Thus, ³He/⁴He in OIB appear to reflect the production rates of ⁴He from recycled oceanic crust plus sediment variably enriched in (Th+U) in plume sources. The range of ³He contents in plume sources must be small compared to that of (Th+U) in order for the correlation to persist. Island groups displaying the strongest primordial ³He/⁴He signal are chemically and isotopically (i.e. Sr, Nd, Pb) most like mid-ocean ridge basalts, indicating a common history over geological time. We show through modeling that helium isotopes in the mantle can be explained through continuous, incomplete degassing by continent and ocean crust formation. It appears that the high ³He/⁴He component in plume sources is compatible with whole mantle convection and represents "old" depleted mantle, isolated from convection and upper mantle degassing for 1-2 billion years.
DE: 1000 GEOCHEMISTRY
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005