HR: 10:40h
AN: U12A-02    [Abstracts]
TI: Mantle Reservoirs From a Noble Gas Perspective
AU: * Mukhopadhyay, S
EM: sujoy@eps.harvard.edu
AF: Sujoy Mukhopadhyay, Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, United States
AB: The noble gases provide unique insight into mantle structure and the origin of the different mantle reservoirs. Many OIBs, such as Hawaii and Iceland, have ³He/⁴He ratios that are a factor of 4 to 6 higher than the canonical MORB value of 8±1 R_A. The high ³He/⁴He ratios in OIBs are conventionally viewed as evidence for the existence of a primitive mantle reservoir. Such a view, however, is frequently challenged on the grounds that noble gas abundances in OIBs are an order of magnitude lower than in MORBs, an observation that traditional models of magmatic degassing cannot explain. The apparent concentration paradox has been resolved by incorporating kinetic fractionation of the noble gases during magmatic degassing of the erupting magma and it can be shown that higher CO₂ and H₂O content of OIBs, compared to MORBs, leads to more extensive degassing of He in OIB magmas (Gonnermann and Mukhopadhyay, 2007). In contrast to Hawaii and Iceland, some ocean islands, such as the Cook-Austral Islands and Canary Islands (HIMU ocean islands) have ³He/⁴He ratios of 4-7 R_A, lower than the MORB range. The low ³He/⁴He ratios are attributed to the addition of radiogenic ⁴He from recycled slabs. Surprisingly, recent high-precision neon isotopic measurements made at Harvard in olivine phenocrysts from the Cook-Austral Islands indicate that HIMU neon is less nucleogenic than the MORB source. The He and Ne systematics from the Cook-Austral's demonstrate that the noble gas signature of HIMU basalts cannot arise either from simple diffusive equilibration of a recycled slab with a MORB source, or result from mixing of melts that are derived from recycled slabs and the MORB mantle. The He-Ne systematics, however, can be quantitatively modeled as a mixture of recycled slab and a primitive mantle reservoir. The scenario is consistent with He-Os and He- Nd correlations seen in the Cook-Austral basalts. Thus, both low and high ³He/⁴He OIBs incorporate the same primitive mantle reservoir, although in varying proportions. The notion of a reservoir that is primitive in its volatile content and sampled at ocean islands is very much alive. In spite of whole mantle convection, it appears that part of the Earth's mantle has remained largely undegassed. While significant progress has been made with respect to understanding the geochemical implications of He and Ne isotopic composition measured in MORBs and OIBs, our knowledge of Xenon in the mantle remains poor. Since ¹²⁹Xe and ¹³⁶Xe have been produced by the now extinct nuclides, ¹²⁹I and ²⁴⁴Pu respectively, Xe isotopic composition of the mantle can be used to test models of atmosphere formation and provide unique clues to the volatile history of the Earth's mantle. Some of the outstanding issues that still need to be resolved are whether the Earth's mantle has solar or chondritic heavy noble gases, whether OIBs and MORB have the same Xe isotopic composition, and what fraction of the ¹³⁶Xe is from ²⁴⁴Pu vs. ²³⁸U fission. Addressing these issues will require not only high precision measurements but also innovative experimental techniques to reduce air contamination that is ubiquitous in mantle-derived samples. High precision Xe isotopic measurements made at Harvard indicates that Samoa (a high ³He/⁴He ocean island) and MORBs have exactly the same proportion of radiogenic ¹²⁹Xe to ¹³⁶Xe. Although this result needs to be verified from other OIBs, it suggests that a single mantle reservoir supplies the excess ¹²⁹Xe and ¹³⁶Xe to both the MORB and OIB mantle source. The primitive mantle reservoir is the most likely carrier of the xenon isotopic anomaly.
DE: 1000 GEOCHEMISTRY
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Union [U]
MN: 2007 Fall Meeting