Constraining carbonation freezing and petrography of the carbonated cratonic mantle with natural samples

Abstract

Peridotite xenoliths from the Cretaceous Chidliak kimberlite province (SE Baffin Island, Canada) were recently studied by Kopylova et al. (2019). Here, we focus on rare textures, with orthopyroxene grains invariably rimmed by 3–20 μm coronas of clinopyroxene, while all clinopyroxenes are rimmed by equally thin monticellite coronas. Thicker, 0.1–0.5 mm texturally equilibrated clinopyroxene also mantles garnet, and there is a gradual transition from micron- to millimeter-thick clinopyroxene mantles. We investigated the origin of these rarely preserved textures using major and trace element zoning in minerals, and measured and reconstructed bulk compositions of xenoliths. Fluxes of major elements were identified based on the conserved element ratios while accounting for the closure effect due to normalization of bulk compositions to 100%. Ca dominates the absolute elemental gain, expressed in moles per 1000 mol of Fe. The observed mineralogical and compositional changes are associated with the significant metasomatic removal of Na (70% of its budget) Al, and Cr (35% loss), minor removal of Si, Mn, Mg and Ni and the gain of Ca (~ 20%), Ti, K and incompatible trace elements. The metasomatic fluid addition beneath Chidliak was likely below 10%. The fluid was very enriched and fractionated resembling volatile-rich low-degree melts like carbonatites or kimberlites. The Chidliak peridotites were affected by “carbonation freezing”, i.e. immobilization of a carbonate-rich metasomatic agent via reactions with pyroxenes. Clinopyroxene and monticellite coronas formed in decarbonation reactions, whereby ephemeral carbonatitic fluid readily gave away Ca to silicate minerals and exsolved CO2. Chidliak peridotites highlight that it would be deceptive to imagine “carbonated peridotites” storing carbon in a normal assemblage of peridotite plus carbonate. “Carbonated peridotites” are coarse peridotites with elevated modes of clinopyroxene, garnet and olivine, and with thin rims of calcic silicate minerals storing incompatible elements. The CO2-rich magmatism on cratons and the match between the temporal Ca addition to the cratonic mantle and the observed fluxes from the carbonate-rich metasomatism underscores the importance of the latter process in shaping up the lithospheric mantle and its melts.