HR: 1340h
AN: V13C-0561    [Abstracts]
TI: The Earth's mantle primordial noble gas source: Constraints from He and Ne isotopic patterns of different mantle plumes
AU: * Stroncik, N A
EM: nicole@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Haus B, Potsdam, 14473 Germany
AU: Niedermann, S
EM: nied@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Haus B, Potsdam, 14473 Germany
AU: Haase, K M
EM: kh@gpi.uni-kiel.de
AF: Institut F�r Geowissenschaften Universit�t Kiel, Olshausenstr. 40, Kiel, 24118 Germany
AB: He and Ne isotopic signatures of oceanic island basalts (OIBs) and mid-ocean ridge basalts (MORBs) indicate that remnants of primordial gases of presumably solar composition are preserved within the Earth's mantle. The nature of this primordial gas source is still a matter of debate. In this context, He and Ne isotopic compositions in conjunction with trace elements and Sr, Nd and Pb isotopes have been determined in submarine basaltic glasses from the Easter Hotspot, the Foundation Hotspot, and Grattan Seamount located at 9.73° S close to the Mid-Atlantic Ridge. The Easter and Foundation hotspot lavas show high ³He/⁴He and Pb isotope ratios but relatively low ⁸⁷Sr/⁸⁶Sr ; thus their sources are similar to those of Iceland and Galapagos. Grattan Seamount shows low ³He/⁴He but high Pb isotope ratios. Interestingly, samples from all three volcanic areas display relatively homogeneous Ne isotopic compositions (²¹Ne/²²Ne up to 0.0385, extrapolated ratio at a solar ²⁰Ne/²²Ne of 13.8), partly plotting close to the air-solar Ne mixing line in the Ne-3-isotope plot. In contrast, the He isotope ratios vary considerably within all three sample suites. The Ne isotope ratios and high ³He/⁴He of the Easter and Foundation lavas support models of deep mantle plumes underlying these hotspots, which is consistent with previous studies. The Ne isotopic signatures observed for Grattan Seamount might not to be derived from the lower mantle, as geophysical and geochemical studies suggest that Grattan Seamount is generated by a small melting anomaly residing in the upper mantle. Using the extrapolated, radiogenic ingrowths corrected ²¹Ne/²²Ne ratio of MORB in conjunction with the ²¹Ne/²²Ne radiogenic ingrowths corrected hotspot data relative concentrations of solar Ne in the plume sources can be estimated. These calculations show that the Ne isotopic patterns of all three investigated areas require similar solar Ne concentrations in their source regions being similar to e.g. Iceland or the Azores. On the other hand, the Pb-isotopic compositions require differences in the time-integrated [U+Th]/Pb between the different hotspot sources, and the Sr isotopes as well as the trace elements indicate a derivation of these plumes from slightly different mantle sources. The similar solar Ne concentrations in different mantle plume sources suggest the existence of an essentially undegassed primitive mantle component having high solar-Ne/[U+Th], which mixes with more degassed mantle to produce the observed noble gas isotopic patterns in OIBs. This primordial component seems to be heterogeneously distributed within the Earth's mantle. Similar to other studies our data show that He systematics are much more complex than Ne systematics. The decoupling of He isotopes from Ne as well as from Sr, Nd, and Pb isotopes seems to be caused by melt generation and migration processes within the plume source regions. This indicates that, similar to trace elements, melting processes have to be considered when using He as a tracer of mantle source evolution.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3615 Intra-plate processes (1033, 8415)
DE: 3621 Mantle processes (1038)
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8415 Intra-plate processes (1033, 3615)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005