Clumped isotope thermometry of calcite and dolomite in a contact metamorphic environment

Clumped isotope compositions of slowly-cooled calcite and dolomite marbles record apparent equilibrium temperatures of roughly 150-200 °C and 300-350 °C, respectively. Because clumped isotope compositions are sensitive to the details of T-t path within these intervals, measurements of the Δ_(47) values of coexisting calcite and dolomite can place new constraints on thermal history of low-grade metamorphic rocks over a large portion of the upper crust (from ∼5 to ∼15 km depth). We studied the clumped isotope geochemistry of coexisting calcite and dolomite in marbles from the Notch Peak contact metamorphic aureole, Utah. Here, flat-lying limestones were intruded by a pluton, producing a regular, zoned metamorphic aureole. Calcite Δ_(47) temperatures are uniform, 156 ± 12 ˚C (2σ s.e.), across rocks varying from high-grade marbles that exceeded 500 °C to nominally unmetamorphosed limestones >5 km from the intrusion. This result appears to require that the temperature far from the pluton was close to this value; an ambient temperature just 20 ˚C lower would not have permitted substantial re-equilibration, and should have preserved depositional or early diagenetic Δ_(47) values several km from the pluton. Combining this result with depth constraints from overlying strata suggests the country rock here had an average regional geotherm of 22.3–27.4 ˚C/km from the late Jurassic Period until at least the middle Paleogene Period. Dolomite Δ_(47) in all samples above the talc+tremolite-in isograd record apparent equilibrium temperatures of 328^(+13)_(-12) °C (1σ s.e.), consistent with the apparent equilibrium blocking temperature we expect for cooling from peak metamorphic conditions. At greater distances, dolomite Δ_(47) records temperatures of peak (anchi)metamorphism or pre-metamorphic diagenetic conditions. The interface between these domains is the location of the 330 ˚C isotherm associated with intrusion. Multiple-phase clumped isotope measurements are complemented by bulk δ^(13)C and δ^(18)O dolomite-calcite thermometry. These isotopic exchange thermometers are largely consistent with peak temperatures in all samples within 4 km of the contact, indicating that metamorphic recrystallization can occur even in samples too low-grade to produce growth of conventional metamorphic index minerals (i.e., talc and tremolite). Altogether, this work demonstrates the potential of these methods to quantify the conditions of metamorphism at sub-greenschist facies.