Prospects and Pitfalls in Calibrating Non-Traditional Stable Isotope Geothermometers from Metamorphic Rocks: Insights from Mt. Moosilauke

The distribution of aluminum silicate minerals in Mt. Moosilauke, New Hampshire tightly constrains the pressure and temperature conditions during peak metamorphism to be at the aluminum silicate triple point. The minerals in the pelitic schists from the region have been used to calibrate a variety of widely used mineral geothermometers and geobarometers. The chemical equilibrium among the minerals and the well constrained temperature make the region one of the best sites to look for isotopic equilibrium among minerals and to calibrate stable isotope geothermometers.

We have selected a pelitic schist from Mt. Moosilauke to study the inter-mineral Fe isotopic fractionation. The minerals were handpicked under a binocular microscope, including garnet, ilmenite, biotite, muscovite, plagioclase and tourmaline, and measured for their chemical compositions using SEM and Fe isotopic compositions using MC-ICP-MS. The δ⁵⁶Fe values of them vary from -0.2 ‰ to +0.4 ‰. Biotite has the same δ⁵⁶Fe value as the bulk within error (+0.1 ‰). Garnet and ilmenite have lighter Fe isotopic compositions than the bulk, while muscovite, plagioclase and tourmaline have heavier isotopic compositions. The isotopic compositions of the minerals most likely reflect equilibrium Fe isotopic fractionation.

The work provides a chance to cross-calibrate with the other three methods of determining inter-mineral equilibrium Fe isotopic fractionation, which include measuring the isotopic compositions of lab experimental run products where two phases are allowed to exchange isotopes, measuring the strength of the bonds in minerals by NRIXS (Nuclear Resonant Inelastic X-ray Scattering spectroscopy), and ab-initio calculations. Natural minerals generally involve equilibration over longer timescales, so they can more closely approach equilibrium than phases in lab experiments. The NRIXS method is relatively new and more studies are needed to ascertain its reliability. Ab-initio calculations may suffer from systematic biases. We have also performed NRIXS measurements of natural minerals, and ab-initio calculations are underway. We will present comparison among the four approaches and possible applications of these calibrations to interpretations of natural iron isotopic variations in rocks.