Noble gas signatures of abyssal gabbros and peridotites at an Indian Ocean core complex

Abstract

We report some of the first noble gas data for in situ lower oceanic crust and shallow mantle. From a suite of gabbros and peridotites recovered from the Atlantis Bank oceanic core complex on the Southwest Indian Ridge, we measured He, Ne, Ar, Kr and Xe concentrations as well as 3He/4He and 40Ar/36Ar ratios, there by documenting the noble gas content and signature of oceanic lithosphere. Except for a single ultramylonite, the gabbros have higher 3He/4He ratios than atmospheric. Three gabbros have MORB-like bulk 3He/4He ratios higher than 6RA despite variable helium concentrations, as much as two to three orders of magnitude lower than in MORB glasses. One of these is mylonitized, demonstrating that magmatic helium can be retained despite intense high-temperature crystal-plastic deformation in the lower crust. Of the gabbros measured, green amphibole-bearing samples show relatively high helium abundances. Peridotite noble gas concentrations measured in clinopyroxene separates are dominantly lower than gabbros. Specifically, He abundances are similar to or greater than gabbros with MORB-like 3He/4He isotopic ratios. All the gabbros and peridotite clinopyroxenes show severely contaminated 40Ar/36Ar values up to 1300. Magmatic 40Ar is enriched in the oxide-olivine gabbro with the highest 40Ar/36Ar in the entire sample suit. These results suggest as an actual data that the recycling of the lower oceanic crust and shallow mantle should be considered in modeling mantle evolution at least for helium. Measured helium abundances, though lower than in basalt glasses, are greater than those in crystalline MORB. Even if entire upper crust retains primary magmatic signature, oceanic lower crust and lithospheric mantle may impact larger by recycling due to their large volumes.