Strategies for high-temperature oxygen isotope thermometry: a worked example from the Laramie Anorthosite Complex, Wyoming, USA

Abstract

An understanding of the factors that affect the magnitude and direction of isotopic exchange is fundamental to the application of oxygen isotope geothermometers. This is particularly true in high-temperature rocks where retrograde exchange between minerals commonly results in the calculation of spurious isotopic temperatures. Herein we present an approach to isotope thermometry that combines numerical modelling techniques with a directed sampling strategy to retrieve peak or near-peak temperature information from rocks that have behaved as closed systems on the hand-sample scale during cooling. The isotopic composition of minerals in such samples changes in predictable fashion on cooling. These so-called isotope exchange trajectories (IET) can be calculated for any mineral given the whole-rock isotopic composition, the modal abundance of each mineral in the rock and the fractionation factors between the minerals. Model temperatures can be obtained for both single and multiple hand samples through the calculation of IETs. Temperatures for single samples are determined by locating the intersection between a mineral's calculated IET and its measured isotopic composition on plots of δ 18O vs. 1/T 2. The resulting temperature represents either the closure temperature of the mineral or the maximum temperature attained by the rock, whichever is lower. The multiple sample method relies on the premise that, at high temperatures, isotopic equilibrium is effectively achieved over a large area, typically on the scale of meters or larger. This method permits the retrieval of temperatures that are significantly higher than the closure temperature of any individual mineral. Multiple sample temperatures are calculated by determining the intersection of the IETs of a given mineral in two or more samples. The intersection of these IETs represents the temperature at which the isotopic composition of the mineral was the same in both samples (i.e., the last temperature of large-scale isotopic equilibrium). Our methods allow for a rigorous determination of uncertainties in the calculated temperatures. These numerical techniques can be used on any samples. Precise temperature estimates, however, require samples with specific mineralogic and modal characteristics. The best samples for use with the single sample method are those that contain small amounts of a slow diffusing mineral and large fractionation factors between this mineral and the other minerals in the rock. The multiple sample method requires two or more samples with widely different modal proportions of a mineral that strongly fractionates oxygen isotopes. Such samples are present in many terrains but are commonly overlooked because their mineral assemblages are not well suited for conventional thermobarometry. These methods yield excellent results when applied to rocks from the Sybille Fe Ti oxide quarry, a late magmatic segregate of the Laramie Anorthosite. Numerical modelling of single sample and multiple samples from this quarry yield temperatures of ∼ 1000 and ∼ 1080°C, respectively. The single sample temperatures record the closure temperature of ilmenite, whereas the multiple-sample temperatures correspond closely to the experimentally-determined solidus for these rocks. These model temperatures are higher than any other conventional oxygen isotope temperatures yet reported from slowly cooled rocks.