Water and diamond inventory in cratonic eclogite and pyroxenite xenoliths from the Sask and Superior Cratons linked to tectonomagmatic craton reactivation

Abstract

The Victor pipe (Attawapiskat kimberlite field, Superior Craton) and Star pipe (Fort à la Corne or FalC kimberlites, Sask Craton) host unconventional, lherzolite-associated diamond deposits. We used FTIR spectroscopy to quantify water contents in eclogite and pyroxenite mantle xenoliths from both localities in order to assess how Proterozoic tectonomagmatism, related to the Midcontinent Rift and Mackenzie Dike Swarm, affected the water inventory during craton reactivation. Fifteen eclogites and pyroxenites from Attawapiskat have C(H₂O) contents ranging from 269 to 883 (average 500) µg/g in clinopyroxene and 14 to 318 (average 70) µg/g in garnet. Clinopyroxene in seven eclogites and pyroxenites from FALC have C(H₂O) contents ranging from 270 to 502 (average 351) µg/g, and garnet from 14 to 70 (average 34) µg/g. Contrasting geochemical systematics for the two localities suggest differences in evolution. At Victor, crystal chemistry, in particular Al₂O₃ in clinopyroxene expressed as jadeite (Jd) component, appears to control water uptake for eclogites and pyroxenites. Highest H₂O contents are recorded in the more pristine jadeite-rich high-Ca eclogites, possibly pointing to an H₂O-rich environment typical of subduction zones. The more metasomatized samples (low Jd, high MgO contents, and enrichment in incompatible elements) show lower water contents, despite evidence for interaction with a comparatively hydrous (> 3 wt% H₂O) agent plausibly linked to ca. 1.15 Ga tectonism associated with the Midcontinent Rift. We attribute this to crystal-chemical modifications, particularly the reduced jadeite component in clinopyroxene, which diminishes its capacity to incorporate water. It is also possible that water activity was lowered if the metasomatic melt contained significant amounts of CO₂, as suggested by high Zr/Hf in the samples with the lowest H₂O content, resulting in a drying effect. In contrast, at Star, metasomatism appears to have overwhelmed the crystal-chemical effect of mantle metasomatism by means of high H₂O content in the metasomatic agent, as the high-Mg metasomatized samples show evidence for the greatest addition of water. It is noteworthy that in the Sask craton, where diamond formation has been linked to melt metasomatism and refertilization, the single diamond-bearing eclogite available for study has the lowest H₂O content among metasomatized eclogites. Although more data are needed, this may indicate that, if diamond formation was melt-mediated, it is more efficient at low H₂O activity that can ensue in the presence of CO₂.