Linear island chains in the Pacific: Result of thermal plumes or gravitational anchors?

Recent studies of Hawaiian tholeiitic volcanic suites and of xenoliths of deep-seated material beneath Hawaii lead to the conclusion that the source materials are not pyrolitic. Rather, the source materials have average 100 Mg/Mg + Fe ratios of about 82 and average densities of about 3.30 g cm^-3. Residues from melting have average 100 Mg/Mg + Fe ratios of between 84 and 85 and average densities of at least 3.40 g cm^-3. The Hawaiian melting anomaly appears to be located in somewhat fractionated asthenospheric source material whose fusion produces material more dense than either its parent or underlying material beneath the asthenosphere. We propose that the volcanic activity at the ends of the Hawaiian, Tuamoto, and Austral chains is not a result of the fortuitous location of thermal plumes but rather is a consequence of shear melting caused by plate motion. Once such melting begins, a dense residuum is formed and sinks. This downwelling ultimately forms gravitational anchors that stabilize the anomalies and cause inflow of fresh partially fractionated parental materials into the source areas for the basalts. These anchors do not drive the plates but rather represent pinning points for melting anomalies at both the top and the bottom of the asthenosphere. Gravitational and bathymetric data, as well as heat flow patterns and anomalous seismic velocities for wave paths through the deep mantle, all appear to support this hypothesis.