Abstract

We analyzed the first samples of Neoarchean mantle fluid captured as inclusions in fibrous diamonds from the 2.701–2.697 Ga Wawa metaconglomerate (Canada). The K-, Na-, and Cl-rich carbonate-bearing saline fluid inclusions bear a strong resemblance to those of Phanerozoic fibrous diamonds. This similarity in major and trace elements, volatile components, and nitrogen characteristics implies the uniformitarian extension of some mantle processes, including the formation of fluid-rich fibrous diamonds, back to the Neoarchean. Low initial Sr isotope ratios (87Sr/86Sri = 0.700–0.702) for the diamonds at 2.7 Ga are within error of estimates of Archean depleted mantle or bulk silicate Earth at 2.7 Ga, consistent with a non-enriched, convecting mantle/group I kimberlite-like source of the fluid. Eu anomalies occur in both Neoarchean and Phanerozoic saline fibrous diamond fluid, but not in silicic or carbonatitic fluid varieties. This restriction suggests that interaction with Cl-rich fluid may lead to Eu anomalies in mantle rocks, which therefore should not be automatically assigned to a subduction origin when a petrogenetic history involving saline fluids is suspected.

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