The Importance of Lithology in the Interpretation of Apatite U-Th/He Ages

The low closure temperature of the apatite U-Th/He system makes it useful for understanding the long-term post-tectonic history of decaying orogens. However, apatite U-Th/He ages, which reflect the specifics of the mineral's cooling history, must be interpreted within the context of both regional tectonic setting and local geologic factors. The retention of radiogenic helium within an apatite is the result of several geologic factors that affect the apatite host rock's cooling history. In areas lacking substantial heat advection or production (tectonic activity or volcanism), rocks that have been exhumed to the surface have thermal histories that are linked to the local geothermal gradient. Generally a geothermal gradient of 25 C/km is assumed for a tectonically passive continental crust; but, the geothermal gradient is a construct of two major factors: (1) basal heat flow, and (2) the thermal conductivity of the rocks below the surface. Radiogenic heat production in the crust is an additional, but secondary, factor. While basal heat flow may not vary significantly across a region, the thermal conductivity of rocks in the upper crust may differ by more than 100%. To evaluate the effect of variable thermal conductivities on the cooling history of an apatite's host rock, we modeled thermal histories for samples slowly exhumed from crust having two very different steady-state geothermal gradients, using the thermal properties of shales, sandstones, and limestones from the Appalachians. The two geothermal gradients were constructed by stacking these lithologies in different orders. Thermal histories were developed for each model, by "eroding" the surface unit and re-equilibrating the geotherm. Two different "erosion" rates were applied, one at 50 meters/m.y. and another of 10 meters/m.y., rates that bracket the upper and lower limits respectively of generally accepted values of Appalachian denudation. These rates are sufficiently slow so that heat is not being advected by rapid erosion. The thermal histories for these two cases were entered into a helium ingrowth-diffusion model for 130-micron apatites to predict the U-Th/He ages. The results for the ingrowth-diffusion model show that U-Th/He ages for the more rapid denudation rate thermal histories (50 m/m.y.) varied by <10%. The U-He ages for the protracted denudation rate thermal histories (10 m/m.y.) varied by 20%. Considering that denudation is never completely uniform, the addition of lithologic variability makes assumptions about the long-term rate of exhumation very difficult to constrain. However, it does help resolve why there can be dramatic along-strike variations in age from proximal samples in apatite U-Th/He and fission track ages from the central Appalachians and northern Blue Ridge. In tectonically active regions, the effect of variable thermal conductivity may be small on the final U-Th/He age, but in areas of slow cooling, the effect might be significant.