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Formation of steep-sided volcanic domes on Venus

Presentation #507.07D in the session Venus II (Oral Presentation)

Published onOct 23, 2023
Formation of steep-sided volcanic domes on Venus

Steep-sided domes are prominent volcanic landforms on Venus that have been postulated, given Venusian surface conditions, to require eruption of highly viscous magma. Here, we use thermodynamic modelling, based on bulk compositions inferred from Venera 13 (alkaline basalt) and Venera 14 (low alkali basalt) lander data, to investigate changes in magma composition during progressive batch melting (BM) of the Venusian crust, and during fractional crystallisation (FC) of primary magma. The SiO2, H2O, and crystal content of silicate melts are critical parameters that control melt viscosity. The aim of this study is to investigate possible mechanisms for the formation of Venusian domes by constraining the silica contents and viscosity of liquids formed by BM of Venusian crust or by FC processes of possible Venusian liquids.

We performed FC and BM models using Rhyolite-MELTS software to determine melt compositions and then calculated viscosities. Based on X-ray fluorescence data from surface probes, two Venusian basalt compositions, Venera 13 and 14, were chosen for modelling FC and BM processes at different pressures (0.01-1GPa), temperatures (900-1400°C), water content (0-2 wt.%), CO2 (0-2 wt.%) and oxygen fugacities (∆FMQ= 0). Compositions are used to determine magma viscosities, and by comparison to published physical models of dome formation, assess whether either fractional crystallisation or remelting of the Venusian crust can account for dome formation. For FC, the maximum crystal-free viscosity is ~7x107 Pa∙s for Venera 14, at the anhydrous condition at 980 °C, 0.01 GPa after 90% fractionation. For Venera 13, anhydrous, BM process, at least 62 vol% crystal is needed to exceed the critical viscosity suggested by physical models for dome formation at an eruption temperature (Te) of ~1027 °C. For Venera 14, BM process, 0.2 wt.% H2O as initial water content, at least 72 wt.% crystal is needed to exceed the critical viscosity threshold for Te = 1027 °C.

Given the unrealistically high crystal content suggested above, we further probe the possibility of dehydration triggering the increase in viscosity. For Venera 14, BM, at 1 GPa, removing 5 wt.% H2O can dramatically increase the viscosity by 2 - 4 orders of magnitude. We further involve crystallinity for the viscosity model, revealing that for the dehydration models removing 5 wt.% H2O, at least 50 vol% crystals are needed to exceed the critical viscosity proposed by physical models. Therefore, this study provides different possibilities based on viscosity models that result in the formation of Venusian domes.

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