Compositions of Hawaiian Double Track Volcanoes: Shallow or Deep Controls?
Abstract
At least three mechanisms have been proposed to explain the compositional differences between the two parallel chains of Hawaiian volcanoes known as Loa and Kea trends, respectively: (1) The Hawaiian plume is concentrically zoned, and Loa trend volcanoes tap the central portions, whereas Kea trend volcanoes tap more peripheral portions of the plume. (2) Variations in lithospheric thickness control the depth and extent of melt extraction, and therefore the specific compositional mix between enriched and depleted components distributed uniformly throughout an otherwise unzoned plume. (3) The two tracks sample a large-scale compositional gradient initially located in the plume source, the thermal boundary layer near the base of the mantle. Model (1) is inconsistent with the observation that both pre-shield and post-shield Loa-trend volcanoes, which sample the periphery of the plume, have Loa-type, not Kea-type, isotopic characteristics. Model (2), as proposed by Ballmer et al. (2011), invokes systematically higher extents of melting for Kea-trend volcanoes. This conflicts with geochemical evidence, such as La/Yb ratios that are consistently higher in Kea-trend shield volcanoes than in corresponding Loa-shields, indicating lower, not higher extents of melting for Kea-shields. It also conflicts with radiogenic 208Pb*/206Pb* ratios of pre-shield Loihi, which, in spite of a large difference in melt fraction, are similar to Loa-shield lavas. Model (3) has recently been linked to the isotopic DUPAL anomaly and the edge of the large, lower-mantle shear-wave velocity anomaly in the Pacific (Weis et al., 2011). Using geodynamic modeling, we show how an isotopic gradient in the lower-mantle thermal boundary layer is drawn into the plume conduit and sampled by the two volcano tracks. An isotopic gradient or sharp boundary, even when initially located away from the plume conduit by hundreds of kilometers, will nevertheless ultimately be drawn into the center of the conduit. This can explain why isotopically distinct volcanic double chains are also found elsewhere in the Pacific Ocean (Huang et al., 2011). The isotopic evolution of volcanoes from pre- to post-shield stages depends on the angle between the azimuths of the deep-mantle gradient and the lithospheric motion. We compare predicted and observed isotopic evolutions along the Hawaiian double track and find reasonable correspondence between the two. Ballmer, M.D., Ito. G., van Hunen, J., Tackley, P.J. (2011) Nature Geosci. 4, 457-460. Weis, D., Garcia, M.O.,Rhodes, J.M., Jellinek, M., Scoates, J.S. (2011) Nature Geosci. 4, 831-838. Huang, S., Hall, P.S., Jackson, M.G. (2011) Nature Geosci. 4, 874-878.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMDI41B..01H
- Keywords:
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- 1025 GEOCHEMISTRY / Composition of the mantle;
- 1033 GEOCHEMISTRY / Intra-plate processes;
- 8121 TECTONOPHYSICS / Dynamics: convection currents;
- and mantle plumes;
- 8137 TECTONOPHYSICS / Hotspots;
- large igneous provinces;
- and flood basalt volcanism