Physics
Scientific paper
May 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusm.v42a..05l&link_type=abstract
American Geophysical Union, Spring Meeting 2005, abstract #V42A-05
Physics
1025 Composition Of The Mantle, 1065 Trace Elements (3670), 8125 Evolution Of The Earth, 8147 Planetary Interiors (5430, 5724)
Scientific paper
The debate over the origin of "hotspots" has recently been reinvigorated. In the traditional view, "hotspots" represent the surface manifestations of thermal plumes arising from deep thermal boundary layers. Alternatively, "hotspots" may derive from preferential melting of more fertile and hence more fusible zones in the mantle without calling upon anomalously high temperatures and hence no need for thermal plumes. This view has been fueled by the emerging acceptance that eclogite, characterized by a low temperature solidus, may be widespread in hotspot source regions. These eclogite zones could represent fragments of oceanic crust that have been recycled into the convecting mantle through subduction zones. Eclogite pods could also exist in the thermal plume scenario, but in that case, the eclogite pods are simply entrained in hot upwelling plumes. Both scenarios, however, come with their own paradoxes. In the thermal plume scenario, the hotter temperatures would result in higher degrees of melting. Because hotspot magmas are already highly enriched in incompatible trace elements compared to MORBs, the high degrees of melting in turn imply source regions with somewhat unreasonable enrichments in incompatible elements. In the alternative view, the problem is that eclogite is much denser than normal mantle (e.g., pyrolite) so that once eclogitized oceanic crust sinks to the deep mantle, it is unlikely to return on its own accord unless it is aided by a thermal upwelling itself! Both paradoxes might be easily reconciled if previously melt-depleted mantle, such as the harzburgitic residuum of oceanic crust generation, is involved in some manner. Harzburgite has a higher melting temperature than normal mantle, eliminating the requirement for high degrees of melting in the plume model and relaxing the requirement for highly enriched source regions. In addition, it has a lower density than normal mantle and eclogite, perhaps allowing it to return to the uppermost mantle by virtue of its own chemical buoyancy and in so doing entrain eclogite pods along the way. Given the evidence for recycled oceanic crust in hotspot source regions, the presence of harzburgite seems almost inescapable since the proportion of complementary harzburgite in oceanic lithosphere is much greater than that of oceanic crust itself. The question is whether this harzburgitic residuum really exists in hotspot source regions. Most trace element and isotopic systems are based on incompatible element systems and hence are sensitive to metasomatic or crustal components but not residual mantle. Only mildly incompatible to compatible elements can be used to track residual mantle. Such elements include the first series transition metals, V, Sc, Cr, Mn, Fe, Co and Ni. We show that many hotspot magmas are characterized by high Ni, Cr, Fe, and Co, low Sc and variable V contents. While any one of these features can be explained by a number of hypothetical scenarios, only small degree melting of harzburgite can successfully explain the group systematics. Hotspot magmas may represent a mixture of high degree melts of eclogite with small degree melts of harzburgite. These observations suggest that entire sections of oceanic lithosphere may be present in some hotspot source regions. Our geodynamic models show that large sections of oceanic lithosphere, if sufficiently dry, can be preserved in the convecting mantle and sink to the lower mantle by their negative thermal buoyancy. However, upon thermal re-equilibration with the ambient mantle, these coherent sections can return to the uppermost mantle by entrainment in thermal upwellings or possibly by intrinsic chemical buoyancy alone.
Lee Chaohong
Lee Zonghoon
Lenardic Adrian
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