Radiogenic isotopes and Hawaiian magmagenesis

Michelle Coombs

U.S. Geological Survey

Summary and discussion

We discussed the opposing paradigms of:

  1. an undepleted, primitive lower mantle and a depleted upper mantle, versus
  2. a heterogeneous upper mantle and a general lack of information about whether any significant differences exist between the upper and lower mantle.

While the idea of the latter “raisin cake” mantle has been around for over two decades, a tendency has prevailed for explaining hotspot isotopic signatures using the former scenario. At least three source components are necessary to explain the range seen in various isotope systems at Hawaii, but the length scales at which heterogeneities are distributed within the mantle are unconstrained. Do heterogeneities exist within the spatial confines of a plume, or simply distributed within the upper mantle? It seems clear that isotope data cannot constrain upper versus lower mantle sources, and that the heterogeneities sampled by Hawaiian magmas may reside wholly in the upper mantle.