HR: 15:10h
AN: V43F-07 INVITED    [Abstracts]
TI: Rogue Mantle Helium and Neon
AU: * Albarede, F
EM: albarede@ens-lyon.fr
AF: Ecole Normale Superieure de Lyon, 46 allee d'Italie, Lyon cedex 7, 69364, France
AB: The canonical view of He isotope geochemistry holds that high ³He/⁴He ratios in basalts fingerprints undegassed mantle sources. Hawaiian basalts with unradiogenic He with ³He/⁴He up to 30 R_A are therefore seen as originating from parts of the mantle that is still primordial, at least much more so than MORB mantle (³He/⁴He ~ 8 R_A). This view was strongly reinforced by the discovery of solar and even planetary Ne components in oceanic basalts and gas wells. The canonical view, however, conflicts with multiple observations on ocean islands, notably Hawaiian basalts: the correlation of {187}Os/{186}Os with δ ¹⁸O combined with the presence of unusually radiogenic Hf isotope compositions for a given Nd isotope composition and the correlation between Hf and Pb isotopes are all features strongly reminiscent of ancient subducted oceanic crust and pelagic sediments in the source of the Hawaiian plume. These conflicting observations beg the question of how Hawaiian basalts, which carry the embodiment of a primordial gas signature, at the same time can provide such strong evidence of surface material recycling. I here suggest and alternative model that uses the marble cake paradigm and Shuster et al.'s data on olivine. A solution to this conundrum lies in an analogy with oil genesis: ³He and Ne do not reside in the low-melting point peridotites in which they were originally hosted but rather migrated since early in Earth history into refractory ‘reservoir' rocks. Since there can be no free gas phase percolating at pressures in excess of olivine carbonation at ~3 GPa, He must be largely redistributed by diffusion. The time scale of diffusion is the defining parameter: although over billions of years ³He diffuses across large distances, melting events are too short to efficiently strip residual refractory rocks from their high-³He/⁴He component. Assuming that melts begin forming over the uppermost 100 km with an upwelling rate of 10 m y^-1 in plume conduits and 10 cm y^-1 under mid- ocean ridges, the characteristic times of melt extraction in each of these two environments are 10,000 y and 1 My, respectively, and the maximum thickness of refractory layers contributing their He to the magmas are 10 m and 100 m, respectively. The difference in ³He/⁴He ratios of ocean-island and mid-ocean ridge basalts and the preservation of solar neon are ascribed to the reservoirs rocks being stretched to a different extent during melting. Old fragments of oceanic lithosphere, and possibly cumulates from the magma ocean, rather than primordial mantle ‘nuggets', should host most of the primordial He and Ne presently observed in oceanic basalts. Helium with high ³He/⁴He ratios may contain a component of primordial origin, but not necessarily reflect the reservoir in which it has been residing for most of the Earth's history.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting