HR: 08:20h
AN: V11B-02 INVITED     [Abstracts]
TI: Origin and Nature of the High 3He/4He Mantle Reservoir
AU: * Stuart, F M
EM: f.stuart@suerc.gla.ac.uk
AF: Isotope Geosciences Unit, SUERC, East Kilbride, G75 0QF United Kingdom
AU: Ellam, R M
EM: r.ellam@suerc.gla.ac.uk
AF: Isotope Geosciences Unit, SUERC, East Kilbride, G75 0QF United Kingdom
AB: The 3He/4He of many intra-plate basalts (OIB and CFB) are significantly higher than typical values of mid-ocean ridge basalts (MORB). In the prevailing model this reflects the contribution of a reservoir that is less degassed of primordial volatiles than the shallow mantle source of MORB. The trace element and isotopic composition of this deep reservoir places important constraints on the origin and evolution of Earth's mantle. Tertiary basalts from the proto-Iceland plume (PIP) define linear trends in He-Nd and He-Sr isotope space that extend to a maximum 3He/4He of 50 Ra. All OIB-CFB with olivine 3He/4He > 10 Ra either mix from low 3He/4He up to the PIP trend, but never cross it (e.g. Hawaii, Galapagos), or they define mixing arrays that appear to coincide with the high 3He/4He end of the PIP trend (e.g. Samoa). The PIP trend appears to be a mix between a high 3He/4He source and a low 3He/4He component with lower 143Nd/144Nd and higher 87Sr/86Sr than MORB. This moderately enriched end-member may better characterise the low temperature melt fraction of many OIB sources than do mantle components based on global extreme compositions. The high 3He/4He picrites from Baffin Island have no 182W anomalies, ruling out a significant contribution of core material. Nd, Sr, Os and O isotopes and trace elements in Baffin Island picrites, and major/trace elements of melt inclusions in olivine phenocrysts, are indistinguishable from North Atlantic MORB. Although counter-intuitive, this implies that the primordial 3He-rich mantle sampled by plumes is strongly depleted in incompatible elements, rather than variably enriched (e.g. PHEM, C or FOZO). Although there are several possible ways to generate a high 3He/4He-depleted mantle, perhaps the simplest is to simply mix normal depleted mantle with an undegassed, therefore primitive, mantle. In this scenario the depleted mantle major/trace element chemistry restricts the contribution of the primordial gas-rich mantle to less than 5% and places constraints on it's initial He concentration. This mixing could originate at a thermal boundary layer between convectively isolated reservoirs, or may result by melting depleted mantle with primitive mantle heterogeneities. Whatever the origin of the primordial gas-rich depleted mantle, the absence of high 3He/4He in MORB away from the influence of hotspots requires some form of mantle stratification.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005