Mantle plumes upwelling beneath moving tectonic plates generate age-progressive chains of volcanos (hotspot chains) used to reconstruct plate motion. However, these hotspots appear to move relative to each other, implying that plumes are not laterally fixed. The lack of age constraints on long-lived, coeval hotspot chains hinders attempts to reconstruct plate motion and quantify relative plume motions. Here we provide 40Ar/39Ar ages for a newly identified long-lived mantle plume, which formed the Rurutu hotspot chain. By comparing the inter-hotspot distances between three Pacific hotspots, we show that Hawaii is unique in its strong, rapid southward motion from 60 to 50 Myrs ago, consistent with paleomagnetic observations. Conversely, the Rurutu and Louisville chains show little motion. Current geodynamic plume motion models can reproduce the first-order motions for these plumes, but only when each plume is rooted in the lowermost mantle.

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Pb). Until now, deeply dredged submarine HIMU glasses have not been available, which has inhibited complete geochemical (in particular, volatile element) characterization of the HIMU mantle. We report major, trace and volatile element abundances in a suite of deeply dredged glasses from the Tuvalu Islands. Three Tuvalu glasses with the most extreme HIMU signatures have F/Nd ratios (35.6 6 3.6) that are higher than the ratio (21) for global OIB and MORB, consistent with elevated F/Nd ratios in end-member HIMU Mangaia melt inclusions. The Tuvalu glasses with the most extreme HIMU composition have Cl/K (0.11-0.12), Br/Cl (0.0024), and I/Cl (5-6 3 10 25 ) ratios that preclude significant assimilation of seawater-derived Cl. The new HIMU glasses that are least degassed for H 2 O have low H 2 O/Ce ratios (75-84), similar to ratios identified in end-member OIB glasses with EM1 and EM2 signatures, but significantly lower than H 2 O/Ce ratios (119-245) previously measured in melt inclusions from Mangaia. CO 2 -H 2 O equilibrium solubility models suggest that these HIMU glasses (recovered in two different dredges at 2500-3600 m water depth) have eruption pressures of 295-400 bars. We argue that degassing is unlikely to significantly reduce the primary melt H 2 O. Thus, the lower H 2 O/Ce in the HIMU Tuvalu glasses is a mantle signature. We explore oceanic crust recycling as the origin of the low H 2 O/Ce (50-80) in the EM1, EM2, and HIMU mantle domains.

Three coordinated measurements on the Scylla III theta-pinch yield: β, which is the ratio of plasma pressure to external magnetic field pressure; n, which is the plasma density as a function of radius; and Ti, which is the ion temperature. The three measurements are (1) a differential magnetic loop-probe measurement that determines total flux excluded by the plasma, (2) side-on-measurement of the profile of light intensity emitted by the plasma that yields the relative density profile, and (3) a side-on gas-laser interferometer measurement that gives the absolute density integrated through the plasma. Electron temperature is measured separately by x-ray absorption and it is assumed that electron and ion temperatures are not a function of radius. In the low-pressure regime, (P initial = 20 mTorr) without bias magnetic field, typical measured values are β = 0.70 ± 0.15 and ne = ni = 3.0 × 1016/cm3 on axis, Ti = 2.2 keV, and the plasma density decreases monotonically with increasing radius from the axis, reaching zero at r = 1.1 cm. In the high-pressure regime, (P initial = 100 mTorr) with reverse-bias magnetic field, typical values are β = 0.85 ± 0.15 and ne = ni = 3.5 × 1016/cm3 on axis, Ti = 1.5 keV, and the plasma density has a central core of radius ∼0.8 cm of nearly constant density.