A PERFECT MANTLE STORM AND THE ORIGIN OF THE HAWAIIAN RIDGE

Geophysical evidence for mantle plumes at Hawaii and Iceland remains controversial. Here is an alternative. The low-velocity zone (LVZ) beneath oceanic portions of plates is not only a source for strongly enriched, silica-undersaturated lavas, but for those attributes to become more pronounced beneath older portions of the plates. This is because the bases of plates block the flow of buoyant and widely distributed, small, selectively enriched partial melts, which collect there and concentrate through time. The petrology of tholeiitic and mafic alkalic lavas on ocean islands indicates that they originate at about the same depth near the LVZ at both the beginning and end of volcano formation along linear island chains, but with little consistency in the geochemical identity of mantle sources (EM-1, EM-2, HiMu, FOZO, etc.) or the sequence in which they are tapped. Nor are they especially hot. Those sources are attributed to interactions with shallow processes on Earth involving the atmosphere, hydrosphere and biosphere and/or development of ancient subcontinental lithosphere, which can remain beneath ocean basins after continental breakup and contribute to isotopic variability. Convection below the mantle transition is either far more sluggish than in the upper mantle, or almost non-existent. Following Warren Hamilton1, I propose that a system of shallow return flow triggered by subduction beneath lithospheric plates is opposed, or opposite to, that of plate motion; it is unevenly concentrated as currents; that different flow rates across such currents induce vortices that can produce upward flow of mantle; that plate motion above relatively fixed vortices determines directions and ages of linear-chain volcanoes; and that vortices punch up through, thus more proportionally interact with, alkalic melts in the LVZ at initial and final stages of volcanic cycles and produce bullseye patterns of geochemical zonation. The Hawaiian ridge is the largest of such chains– the result of a perfect, long-lived mantle storm. Plate bending near trenches (e.g., Samoa; petit spots) taps enriched and undersaturated melts of the LVZ directly, without the involvement of counterflow currents.

1Hamilton, W., 2007 GSA Spec. Paper 433 Driving mechanism and 3-D circulation of plate tectonics, doi:10.1130/2007.243(01), 1-25.