Equilibrium crystallization modeling of Venusian lava flows incorporating data with large geochemical uncertainties

Abstract

Our understanding of the Venusian surface composition is limited to the in-situ bulk rock chemical analyses collected by the Vega and Venera missions. However, these analyses have exceedingly large analytical uncertainties – up to 50% by weight (1σ) for certain oxide components. In this study, we use the Venera 14 lander data and apply a Monte Carlo approach to assess how significant uncertainties affect modeling of lava solidification. Thermodynamic modeling of mineral-melt equilibria is conducted over 1,000 iterations of simulated bulk composition within the Gaussian probabilistic bounds of the reported analytical uncertainty along a cooling path from 1350°C to 950°C, with a fixed pressure of 90 bars. The results are used to calculate melt and apparent viscosity (ηmelt and ηapp, respectively). Despite the significant analytical uncertainty of the lander data, lava viscosity is tightly constrained. Mean log ηmelt increases from 1.65 ± 0.46 Pas at 1350°C to 8.57 ± 1.42 Pas at 950°C (1σ). Log ηapp is less well-defined due to increased scatter in crystal fraction with solidification, increasing to 17.7 ± 1.3 Pas at 950°C (γ=10−6 s−1). A significant increase in ηapp occurs between 1240°C and 1080°C due to a rapid increase in crystal mass fraction (Φ increases from ∼0.05 at 1250°C to 0.8 at ∼1100°C). Slow cooling through this 150°C window must occur so as to not drastically increase lava viscosity and impede flow. These results show that despite limited geochemical data and large analytical uncertainties, reasonable constraints for the physical and chemical evolution of lava solidification can be obtained. Most of the Venusian surface is composed of volcanic plains and rises, containing abundant landforms characteristic of fluid basaltic lava. Our results provide new insights into the crystallization processes in Venusian lava flows, which are fundamental for understanding Venusian igneous processes, geodynamics, and resurfacing. This work provides a necessary framework for future thermorheological flow models to determine flow volume and effusion rates.