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Geochemical Arguments Favoring an Hawaiian Plume J. Michael Rhodes University of Massachusetts Dominique Weis University of British Columbia Michael O. Garcia University of Hawaii Marc Norman Australian National University. I don’t intend to dwell on the obvious:-
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J. Michael Rhodes
University of Massachusetts
University of British Columbia
Michael O. Garcia
University of Hawaii
Australian National University
These cartoons illustrate the point that the Hawaiian plume is thought to be concentrically zoned in both temperature and composition. If so, in the last 300 - 500 ka Mauna Loa and Mauna Kea will have traversed about 30 - 50 km of the plume. Over this time period we might expect to see changes in magma composition reflecting changes in melting, melt supply and changes in source components.
The submarine pre-shield stage (Loihi) is characterized by low melting and magma supply. Eruption of alkalic basalts followed by tholeiites reflecting initiation of volcanism at the margins of the plume.
Shield building stage reflects increased magma supply. Eruption of tholeiites and picrites as the volcano traverses the axial zone of the plume
Post-shield stage is characterized by a return to low magma supply, eruption of alkalic lavas as the volcano nears the margins of the plume.
Results from the Hawaii Scientific Drilling Project confirm high magma supply rates, eruption of tholeiites and picrites, between 600 and 400 ka. Followed by decline in eruption rates between 300- 400 ka and onset of post-shield volcanism and eruption of alkalic basalts.
Note. Model growth curve of DePaolo & Stolper (1996) was based on a simple geometric model of a thermally zoned plume, prior to dating!
Evolution of Hawaiian volcanoes from an alkalic pre-shield stage, through a tholeiitic shield stage, to an alkalic post shield stage is consistent with movement of the Pacific plate over a thermally zoned melting anomaly.
The distance from Loihi to Hualalai (94 km) provides a constraint on its dimensions.
SiO2 TiO2 and CaO differ at a given value of MgO between Hawaiian volcanoes. This is presumably a consequence of differences in melting and melt segregation processes in different parts of the plume. Given thermal gradients we might expect to see changes in these values as a volcano transits the Hawaiian plume.
SiO2 in basalts (normalized to 17% MgO) is dependent on depth of melt segregation and on the extent of melting. Marked decrease in SiO2(17) after 320 ka reflects a decline in melting and melt production as the volcano enters the post-shield stage. Increase in incompatible trace data (e.g. Nb/Y) supports the interpretation.
In contrast Mauna Loa shows no obvious change in SiO2(17) or Nb/Y in about 400 ka. This implies that melting conditions have remained relatively uniform as Mauna Loa transits about 30 to 40 km of the Hawaiian plume.
Magma production and evolution of Hawaiian volcanoes is frequently presented like this
But perhaps the Mauna Loa data is telling us it should really be like this with a wide, hot, central core.
You can play around with olivine compositions and KD, but the results are the same – a hotter Mauna Loa magma relative to MORB, implying significant differences in mantle potential temperatures.
High-precision Pb data from Abouchami et al. (2000, 2005) and unpublished data of Weis.