Mantle plumes, mantle stirring and hotspot chemistry

Abstract

Recent advances in understanding plume dynamics allow the role of mantle plumes, the presumed cause of volcanic hotspots and source of the hotspots' oceanic island basalts (OIBs), to be specified in more detail than hitherto. Their role in sampling a mantle with single-layer convection and an increase in viscosity with depth by two or three orders of magnitude is considered here. Plumes would sample thin tabular regions at the bottom of the mantle. This region would be expected in the assumed mantle model to have properties appropriate to OIB sources, including greater heterogeneity, greater age and less depletion of incompatible elements than the shallow mantle sampled by ocean ridges. All proposed sources of recycled material, including oceanic crust, oceanic sediments and continental lithosphere, can be accommodated in this model. However it is noted that clear evidence for a source with primitive refractory element ratios is still lacking, as is undisputed evidence for differences between MORB source and plume source 40Ar/ 36Ar and 129Xe/ 130Xe ratios. 40Ar/ 36Ar ratios do not exclude nearly complete early degassing of the mantle. High 3He/ 4He ratios in some plumes might come from some remaining less degassed mantle or from the core.