Geochronology and Thermochronology of Lower Crustal Xenoliths: Creating a Temporal Record of North American Lithosphere Thermal Evolution

The EarthScope initiative is working to create a comprehensive 4-D model of the North American lithosphere with the deployment of large seismic networks used to image lithospheric velocity structure. This approach yields little insight into the age of lithospheric structures and thus limits our understanding of the origin of imaged mantle structures. Adding temporal constraints to the modeled temperature field that directly influences the seismic velocities of the lithosphere will greatly aid in the interpretation of seismic data. Our approach works to constrain the age and model the thermal evolution of the lithosphere using geochronology and thermochronology of lower crustal xenoliths. Lower crustal xenoliths provide us with samples of lower crust erupted to the surface long after lithospheric stabilization occurred enabling a direct link to be made between geophysical observation of the deep lithosphere and surface geology. These xenoliths often contain U-bearing accessory minerals (titanite, apatite, rutile) with a range of closure temperatures for Pb-diffusion (~650-<400 °C) and permit construction of time-temperature paths, which can in turn be related to the thermal history of the underlying lithospheric mantle. Preliminary results from samples along a N-S trending transect from Northern Montana to Arizona provide a temporal record of lithosphere thermal evolution which can be directly compared to high-resolution images of P-wave heterogeneity in the lithosphere and upper mantle. Lithospheric velocity models are determined via a travel-time tomography inversion using USArray and global data, yielding depth and lateral resolution of upper mantle velocity structures to as little as 100 km. Our goal is to link lithospheric thermal history to velocity models to better understand the how the time-temperature evolution of the Laurentian lithosphere relates to the seismic images produced by EarthScope.