Effusion Rate of Basalts from the Oceanus Procellarum: Constraints from Lava Flow Numerical Modelling

Estimations of eruption rate (time averaged eruption volume) of mare basalts on the Moon are controversial, from a peak eruption rate of ~0.3-0.4 km3/myr at 3.4-3.5 Ga (Hiesinger et al., 2011; Du et al., 2018) to eruptions rate of 6.5-203 km3/myr from individual lava flows from Oceanus Procellarum and Mare Serenitatis (Weider et al., 2010). These estimations of eruption rate are remarkably lower than the effusion rate of terrestrial basalts (1-1000 m3/s, or 3-3000 × 104 km3/myr, Harris and Rowland, 2009). To constrain the effusion rate on the Moon more precisely, we developed a computing program based on a Bingham fluid model to simulate how fast the magma effused to the Lunar surface to form the lava flow in Oceanus Procellarum. We use the aspect ratio, the ratio of the horizontal extent to the thickness of a lava flow (Walker 1973), to describe the shape of the lava flow. Our research is intended to investigate how effusion rates influence the shape of the lava flow. We tested lava flows formed at different effusion rates and on a constant slope to verify their influences. An effusion rate over 10000 m3/s is impossible because the lava flow would be so wide that its aspect ratio would over 1700, which is seven times of the aspect ratios (~240) of lava flows mapped in the Oceanus Procellarum. By contrast, at an effusion rate of 500 m3/s, the lava flow turns out to be a sheet flow and its aspect ratio decreases to 570, which is more consistent with the lava shape in the Oceanus Procellarum. If we lower the effusion rate further, 100 m3/s, the lava flow will maintain a small width near the crater and the aspect ratio will decrease to 230, which is close to the aspect ratio of the lava flow in the Oceanus Procellarum. However, the shape of modelled flow at 100 m3/s effusion rate is irregular, suggesting that 100 m3/s may be the lower limit of the effusion rate of mare basalts in the Oceanus Procellarum Therefore, by modelling the lava shape at different effusion rate, we find that the lava flow will have a higher aspect ratio with an increase of the effusion rate, and 100 - 500 m3/s will be the best range of instant effusion rate of the Oceanus Procellarum mare basalts.