Clumped isotope constraints on secondary mineral formation and thermal overprinting in the Michigan Basin, Southwestern Ontario

Abstract

Sections of Michigan Basin sediments were exposed to hydrothermal fluids during the Ordovician. Secondary calcite and dolomite occur as fracture-filling veins/vugs and replacement minerals in host Ordovician limestones – which occur at depth beneath the Bruce Nuclear site (near Tiverton) in southwestern Ontario. To provide insights on the evolution of diagenetic fluids responsible for secondary mineral growth and the effects of temperature-dependent alteration in the basin, clumped isotope geothermometry was applied to these carbonates. Ordovician calcite in veins/vugs have δ¹⁸O_C values of +20.0 to +23.4 ‰ (VSMOW) and are estimated to have formed at 65–83 °C from various waters (δ¹⁸O_w = +0.1 to +4.4 ‰) including Ordovician to modified (an evolved ¹⁸O-enriched) seawater, and mixed seawater and basinal brines at depth. Calcite veins/vugs in the lower Cambrian units formed at 74–91 °C, have lower δ¹⁸O_C values (+14.9 to +17.0 ‰) and precipitated from ¹⁸O-depleted hydrothermal brines (δ¹⁸O_w = −4.6 to −3.1 ‰). Calcite in the matrix of host limestones yielded higher apparent temperatures (T(Δ₄₇) = 49–82 °C)) that are inconsistent with precipitation from Ordovician seawater and burial diagenesis of the sediments but instead, reflect a solid-state isotope reordering due to a Late Devonian-Mississippian rift-related basinal heating. Diagenetic dolomitization in the basin occurred during shallow to intermediate burial, where dolomite primarily replaces calcite in the limestone matrix or occur as secondary infill in fractures and vugs. The clumped isotopic signature in dolomite is more resistant to low-temperature heating than calcite, whose ∆₄₇ composition reflects a 367–322 Ma regional heating event at 125–150 °C during burial, in agreement with, but more precise than previous estimates based on fluid inclusion microthermometry in secondary quartz and saddle dolomite.