Decoupling of Serpentinization and Prehnitization in Lower East Pacific Rise Crust at Hess Dee

Our down-hole mineralogical and geochemical analyses from the East Pacific Rise fast-spreading lower oceanic crust indicate that alteration of olivine to serpentine and of plagioclase to prehnite were independent, and neither alone monitors the total “alteration.” The results are based on representative channel sub-samples recovered from every Hole J core during IODP Expedition 345 to the Hess Deep tectonic window. Samples have been analyzed for trace element, Sr isotopic, and quantitative mineralogical compositions (the latter by Rietveld refinement using X-ray diffraction data).

Hole J is the most representative rock succession drilled at the Hess Deep as it penetrated the two principle plutonic lithologies: an upper gabbro and a lower troctolite. Units are significantly distinguished by XRD modal mineralogy and trace element abundances. The more heterogeneous gabbro contains 23-32 wt% clinopyroxene (cpx), 34-54 wt% plagioclase (plag), and <4 wt% olivine (ol). The troctolite contains 3-11% cpx, 14-36% plag, and ≤6% ol. Alteration minerals comprise together 18-31% in the gabbro versus 55-80% of the troctolite. The most abundant alteration products are prehnite and chlorite.

Gabbro samples with lowest abundances of alteration minerals (18-20 wt%) preserve ⁸⁷Sr/⁸⁶Sr ratios (0.70275-0.7028) consistent with unaltered mantle. The abundance of plag in the gabbro, the major host for Sr, suggests retention of mantle Sr isotopic compositions there is due to the large reservoir of magmatic Sr. ⁸⁷Sr/⁸⁶Sr ratios of 0.70300-0.70342 in the troctolite samples indicate seawater interaction, even where olivine is most abundant, and serpentine is at or below the ~1% detection limit by XRD. Significant alteration of the deep crust by seawater thus predates the first appearance of serpentine. These data suggest that the timing and operation of prehnite- and serpentine-producing alteration reactions are independent.