Distinguishing cooling histories via Diffusion Geospeedometry

Abstract

Extracting apparent timescales or cooling rates of rocks via inverse diffusion modelling allows the testing of geodynamic consequences on the petrological record.  Indeed, inverse modelling of diffusion in minerals such as garnet has been used extensively to constrain thermal history timescales [1] and/or cooling rates [2]. Although inherently such models do not have unique solutions, [3], they can be used to place constraints on the thermal history of rocks and regions [4]. This is interesting because different geodynamic processes will have thermal histories with different attributes [5], in particular qualtiative differences in the shape of cooling curves. In this contribution, we investigate aspects of different apparent cooling rates extracted via inverse diffusion modelling in garnet. Our results suggest a clear distinction in thermal history depending on wether the rocks experienced “active” (driven from tectonic forces) or “passive” (purely conductive) cooling [5]. We emphasize that active cooling implies slow cooling rates at high-grade conditions whereas passive cooling can have very large cooling rates at high-grade conditions (Fig. 1). For petrologic systems with relatively high closure temperatures (>400 °C), the spatially varying aparent cooling rates allow the identification of local heat sources such as intrusions or heat-producing shear zones (Fig. 1). Our results help to identify processes that have transient and local characters such as the thermal affects of heat-producing shear zones and magmatic intrusions.   Figure 1 – (a) Thermal histories from rocks in the vicinity of a shear zone, note the higher temperatures experienced by the shear-zone rocks. Such thermal histories will lead to spatial gradients of diffusion relaxation. (b) Absolute cooling rates as a function temperature for the curves shown in (a). The cooling rate versus temperature curve was calculated considering the thermal histories shown in (a) for the time period after 1Myr. (after [2]).   REFERENCES [1] Chakraborty, S. Diffusion in Solid Silicates: A Tool to Track Timescales of Processes Comes of Age. Annu. Rev. Earth Planet. Sci. 36, 153–190 (2008). [2] Burg, J.-P. & Moulas, E. Cooling-rate constraints from metapelites across two inverted metamorphic sequences of the Alpine-Himalayan belt; evidence for viscous heating. J. Struct. Geol. 156, 104536 (2022). [3] Moulas, E. & Bachmayr, M. Petrology as an ill-posed inverse problem. in 73 (Mainz Institute of Multiscale Modelling, 2023). [4] Braun, J., Beek, P. van der & Batt, G. Quantitative Thermochronology: Numerical Methods for the Interpretation of Thermochronological Data. (Cambridge University Press, 2006). doi:10.1017/CBO9780511616433. [5] Stüwe, K. & Ehlers, K. Distinguishing Cooling Histories using Thermometry. Interpretations of Cooling Curves with some Examples from the Glein-Koralm Region and the Central Swiss Alps. Mitteilungen Österr. Geol. Ges. 89, 201–212 (1998).