HIGH-FREQUENCY ISOTOPIC VARIATIONS IN THE MAUNA-KEA THOLEIITIC BASALTSEQUENCE - MELT ZONE DISPERSIVITY AND CHROMATOGRAPHY

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 the dispersivity and chroma tographic efficiency of the melt-producing zone under Hawaii. The data are interpreted using a simplified model for the transport of isotopi c signals through the melt zone as magma separates from the upwelling solid by buoyancy-driven porous flow. A constant lava accumulation rat e is assumed for the 620-m-thick section of analyzed basalt; a total d uration of lava accumulation of 120 kyr is based on Ar-Ar dating. The amplitude-''period'' spectrum of the isotopic variations in the lava s equence is determined by Fourier analysis and compared with a predicte d amplitude-period spectrum for the mantle magma sources, based on an assumed amplitude-wavelength spectrum for the mantle and a plume upwel ling velocity of 20 cm/yr. The isotopic variations in the mantle appea r to be substantially attenuated in the lava record. If the attenuatio n is due to hydrodynamic dispersion in the melting zone, then the melt zone dispersivity is estimated to be in the range 100-1000 m, dependi ng mainly on the value assumed for the melt-matrix velocity contrast. Dispersivity is a crude measure of effective grain size in the melt zo ne; the large values, although strictly only upper limits, suggest tha t permeability in the melt zone is inhomogeneous and some amount of ch annel development is present. The ratios He-3/He-4 and Pb-206/Pb-204 i n the Mauna Kea section describe convoluted loops when plotted against one another, consistent with significant chromatographic separation o f He and Pb in the melt zone. The evidence for chromatography of isoto pic signals is consistent with the porous how model predictions and in dicates that the melt is maintained moderately close to chemical equil ibrium with the solid as it separates. The modeling suggests that gene rally, chromatography in the melting zone causes isotope ratio-ratio r elationships observed in the lava record to be severely out-of-phase w ith those present in the mantle source if the period of the variabilit y is of order 10(5) years; for periods greater than 10(6) years the ra tios are close to being ''in phase,'' whereas for periods less than 10 (6) years the lava record is hopelessly scrambled.