High-frequency isotopic variations in the Mauna Kea tholeiitic basalt sequence: Melt zone dispersivity and chromatography

摘要: A stratigraphically controlled sequence of Sr, Nd, Pb, and He isotope ratio measurements on tholeiitic basalt cored by the Hawaii Scientific Drilling Project is analyzed to constrain dispersivity chromatographic efficiency melt-producing zone under Hawaii. The data are interpreted using a simplified model for transport isotopic signals through melt as magma separates from upwelling solid buoyancy-driven porous flow. constant lava accumulation rate assumed 620-m-thick section basalt; total duration 120 kyr based Ar-Ar dating. amplitude-“period” spectrum variations in determined Fourier analysis compared with predicted amplitude-period mantle sources, an amplitude-wavelength plume velocity 20 cm/yr. appear be substantially attenuated record. If attenuation due hydrodynamic dispersion melting zone, then estimated range 100–1000 m, depending mainly value melt-matrix contrast. Dispersivity crude measure effective grain size zone; large values, although strictly only upper limits, suggest that permeability inhomogeneous some amount channel development present. ratios 3He/4He 206Pb/204Pb Mauna Kea describe convoluted loops when plotted against one another, consistent significant separation Pb zone. evidence chromatography flow predictions indicates maintained moderately close chemical equilibrium it separates. modeling suggests generally, causes ratio-ratio relationships observed record severely out-of-phase those present source if period variability order 105 years; periods greater than 106 years being “in phase,” whereas less 104 hopelessly scrambled.