Clumped‐Isotope Geothermometry and Carbonate U–Pb Geochronology of the Alta Stock Metamorphic Aureole, Utah, USA: Insights on the Kinetics of Metamorphism in Carbonates

Abstract

AbstractTo assess thermal and kinetic influences on atomic mobility and mineral (neo)crystallization, clumped‐isotope abundances of calcite and dolomite were measured alongside dolomite cation ordering and U–Pb dates, across metamorphic grade within the c. 35–30 Ma Alta stock contact metamorphic aureole, Utah, USA. Average Δ47 values of dolomite inside the metamorphic aureole reflect the blocking temperature of dolomite (300°C–350°C) during cooling from peak temperatures. Dolomite Δ47 values outside the metamorphic aureole record a temperature of ∼160°C. At the talc isograd, dolomite Δ47 values abruptly change, corresponding to a decrease of ∼180°C over <50 m in the down‐temperature direction. This observed step in dolomite Δ47 values does not correlate with cation ordering in dolomite or U–Pb dates, neither of which correlate well with metamorphic grade. The short distance over which dolomite Δ47 values change indicates strong temperature sensitivity in the kinetics of dolomite clumped‐isotope reordering, and is consistent with a wide range of clumped‐isotope reequilibration modeling results. We hypothesize that clumped‐isotope reordering in dolomite precedes more extensive recrystallization or metamorphic reaction, such as the formation of talc. Dolomite U–Pb analyses from inside and outside the metamorphic aureole populate a single discordia ∼60 Myr younger than depositional age (Mississippian), recording resetting in response to some older postdepositional, but premetamorphic process.