GEOCHEMICAL ANALYSIS OF DEEP CARBON TRANSPORT AT THE 1.1 GA SEABROOK LAKE ONTARIO CARBONATITE COMPLEX

The deep carbon cycle, operating over geological time scales, serves to control long-term climate cycles on Earth through the sequestration of carbon at subduction zones, and atmospheric return via volcanic outgassing. Continental rifts—regions of crustal extension—represent a significant source of volcanic outgassing from the deep Earth to the atmosphere. The eruption of rare, carbonate-rich igneous rocks such as ultramafic lamprophyres (UML), and carbonatites in incipient continental rift environments provides a link between deep carbon and atmospheric return in these settings. A prominent new model suggests that deep carbon sourced from mantle outgassing may concentrate in unstable veins below the continental lithosphere where later melting, upward percolation, depressurization, and chemical immiscibility processes result in the separation and emplacement of UML and carbonatite dikes in continental rift settings. It is these latter processes that are the focus of this study.

Here we present petrographic analyses of a suite of UML, and carbonatite dikes erupted during rifting of the continental Superior Craton c. 1.1 Ga, in Seabrook Lake, Ontario. Geochemical analysis of rare earth elements (REE) within UML samples reveal significant depletion in HREE characteristic of partial melting of a peridotite source in the presence of residual garnet. Complementary enrichment of LREE within UML is suggestive of carbonate-related metasomatism of the lower-lithosphere peridotite source region prior to melting. Whole-rock bivariate analysis reveals a linear trend between elevated K₂O, Al₂O₃, and SiO₂ content within UML, and lower concentrations of the latter within carbonatite dikes. This trend corresponds to the separation of carbonatite magma from the high-K₂O and Al₂O₃ phlogopite-bearing UML at lower crustal depths. The latter REE and whole-rock geochemical trends, never before observed at Seabrook Lake, support the current model of UML and carbonatite genesis. These observations have implications for establishing a direct vector of carbon transit from the deep Earth to the atmosphere in continental rift environments.