Tracing Lithospheric Thermal Evolution with High Field Strength Element Speedometry in Rutile

Abstract:

Traditionally, the thermal evolution of the lithosphere is recovered by the interpolation of discrete temperature-time points, generated by assigning estimates of nominal closure temperatures to volume-averaged radiometric ages. Whilst informative, bulk thermochronology potentially fails to yield high-resolution thermal histories capable of revealing short-lived (10²—10⁵ years) thermal events associated with advective processes such as magmatism and fluid flow. In this contribution, we show that intragranular concentration profiles of Zr, Hf, Nb and Ta (HFSE’s) in rutile have the potential to record high-resolution thermal history information originating from short-lived tectono-magmatic events occurring at temperatures in excess of 650°C. Numerical modeling shows that the sensitivity of each rutile speedometer to faithfully record high-temperature thermal events is dependent on (i.) the rate of intracrystalline diffusion, (ii.) grain boundary transport, and (iii.) the magnitude of the diffusant partition coefficient. Given the similarity in Arrhenian relations for the element pairs Nb, Ta [1] and Zr, Hf [2] in rutile, observable concentration gradients can be co-inverted to yield a unique combination of both duration and intensity of the thermal event. To illustrate the potential utility of the method we measured Nb, Ta, Zr and Hf concentration profiles from the outer 30 µm of granulite facies rutile (Ivrea Zone, Southern Alps), using LA-ICPMS in depth-profile configuration. Joint inversion of internally-consistent concentration profiles yields a thermal history that is consistent with diffusive uptake of HFSE’s from a grain-boundary fluid phase liberated during a previously unrecognized reheating event. These data confirm that rutile HFSE speedometry provides an underutilised tool to trace transient thermal events in a temperature range directly relevant to the geological evolution of the lower crust.

[1] Marschall et al. (2013) Earth Planet Sci. Lett. 375, 361-371. [2] Cherniak et al. (2007) Earth Planet Sci. Lett. 261, 267-279.