Kaersutites, suboceanic low-velocity zone, and the origin of mid-oceanic ridge basalts

Abstract

The isotopic composition of Sr and the abundances of K, Rb, Sr, and Ba in seven kaersutite samples found in association with peridotite xenoliths from three different regions are reported here. The ⁸⁷Sr/⁸⁶Sr ratios show a narrow range of 0.7027 to 0.7030 unrelated to the host basalts but comparable to that exhibited by mid-oceanic ridge basalts. From textural and geochemical considerations, we infer that the kaersutites were introduced as hydrous magmas into the lithosphere and that they occur as accidental inclusions in alkalic basalts. We suggest that the hydrous, trace-element–rich phase in the mantle needed to explain certain trace-element characteristics as well as the water content of mid-oceanic ridge basalt is kaersutitic amphibole. We propose a model in which the low-velocity zone is considered to be the region where a small volume of hydrous melt of kaersutitic composition is produced because of the instability of amphibole in a spinel Iherzolite suboceanic lithosphere. The K/Rb, Ba/Sr, and ⁸⁷Sr/⁸⁶Sr ratios of this melt are comparable to those of the mid-oceanic ridge basalts, but the trace-element abundances are high as found in the kaersutite samples analyzed here. We propose that during asthenospheric upwelling at the mid-oceanic ridge, liquids produced by large degrees of partial melting of the anhydrous Iherzolite will mix with the kaersutitic melts of the low-velocity zone. The trace-element characteristics and the Sr-isotope composition of the mid-oceanic ridge basalts reflect this mixing process.