1D modelling of pegmatite migration

Pegmatites and rare metal granites are granitic igneous rocks distinguished by their texture, which is dominated by crystal growth. These rocks are frequently enriched in rare elements (e.g., Li, Cs, Be, Nb, Ta) and represent economically significant deposits, classified among the critical raw materials identified by the European Commission. Our objective is to better constrain the tectonic parameters that govern the emplacement of pegmatites within the continental crust, including their migration rates and durations, with a particular emphasis on the role of temperature in these crustal migration processes.

To model those fluid migrations, two-phase flow in Julia, based on porosity waves with compressible fluid is used. The porosity is interpreted as melt [1]. To improve the yet existing codes [2], we implement temperature in our two phase flow formulation from energy conservation. Temperature allow a thermomechanical coupling with rock viscosity. A first equation with a simple exponential coupling is used to understand thermal implication on viscosities.

The model represents a continental crust with partial melting occurring within the lowermost 5 km, where temperature is maintained at 750°C due to underplating. A constant geothermal gradient is applied from this depth to the surface. A 10% porosity anomaly is introduced in the partially molten zone, while a baseline porosity of 1% is applied throughout the model to ensure numerical stability. The fluid viscosity is set at 10^4 Pa.s, while at 750°C, the rock viscosity is 10^16 Pa.s. A constant permeability of 10^-11 m² is applied throughout the model. Thermomechanical couplings of varying strength are implemented to assess their impact on migration processes. Accordingly, the rock viscosity at 450°C is varied between 10^16Pa.s and 10^21 Pa.s.

Models reveals two distinct mechanisms that halt migration. The first occurs when the thermomechanical coupling is low (soft and hot crust). Allowing rock viscosity to remain low, so melt migration can outpace thermal diffusivity. Enabling the melt to be in an undercooling state. This means that the magma can migrate beyond the point at which the surrounding rock reaches the crystallization temperature of the melt, a necessary condition for pegmatite formation. The second case arises when thermomechanical coupling is strong, causing the surrounding rock's viscosity to become too high for the magma to reach undercooling condition. In this scenario, the magma crystallizes as soon as it reaches the surrounding rock at its crystallization temperature, potentially becoming trapped by a viscous layer (of different or colder nature).

The use of a geothermal gradient more representative of a metamorphic core complex, along with an improved thermomechanical coupling, should refine the estimates of migration time and distance. Similarly, the introduction of viscous heterogeneity would help highlight the geological structures that may or may not facilitate the migration of these magmas to shallower levels of the crust.

 

References:

[1] L. Räss, T. Duretz & Y.Y. Podladchikov (2020). https://doi.org/10.1093/gji/ggz239

[2] A. Plunder & al. (2022). https://doi.org/10.1016/j.lithos.2022.106652