International audienceBasaltic lavas collected at the Muliwai a Pele lava channel, built during 1974 as part of Mauna Ulu’s eruption on Kilauea’s east rift zone, have been studied to investigate the effect of cooling and crystallization on the rheological properties of the lava. We have quantified the viscosity-strain-rate dependence of lava during cooling and crystallization, using concentric cylinder viscometry. We measured the viscosity of the crystal-free liquid between 1600 and 1230 °C, where we observed a deviation from the expected viscosity trend, marking the liquidus. We then made rheology measurements at subliquidus temperatures of 1207, 1203, 1183, 1176, and 1169 °C, varying the applied strain rates at each temperature. While the crystal-free liquid behaved as a Newtonian fluid, crystallization changed the rheological response to pseudo-plastic behavior, even at the lowest crystal volume fraction of 0.025. Pseudo-plastic behavior was observed down to a temperature of 1183 °C, with a crystal fraction of 0.15. Between 1183 and 1176 °C, the two-phase suspension transitioned from a power-law fluid to a Herschel-Bulkley fluid. At temperatures of 1176 and 1169 °C, with crystal fractions of 0.33 and 0.42, respectively, we observed lobate surface textures on the experimental samples, which remained preserved until the end of the experiments. Measurements at these temperatures indicated yield strengths of 82 ± 16 and 238 ± 18 Pa, respectively. The yield strength resulted from the development of an interconnected crystal network of diopside and enstatite by 1176 °C. By 1169 °C, diopside and plagioclase microcrystals had also appeared, and the effective viscosity was between 2000 and 5000 Pa s, depending on the strain rate. Further cooling to 1164 °C resulted in a rapid viscosity increase, to an effective viscosity in excess of 105 Pa s that exceeded the measurement range of our apparatus. The yield strength varies with crystallinity in an exponential fashion, with yield strength in Pa given by σ y = 1.25e12.93Φ c, where Φ c is the crystal volume fraction. The physical effect of crystals on the relative viscosity of magma was assessed by removing the effects of changing residual liquid viscosity, due to changing composition and temperature. To do this, we analyzed, synthesized, and measured the most evolved residual liquid from the subliquidus experiments. The effect of crystals was best captured by the Einstein-Roscoe equation for polydisperse spherical inclusions. We also measured the viscosity of the same crystal-liquid mixtures at low temperatures and strain rates using parallel-plate viscometry. The effect of crystals on magma viscosity was slightly greater at low strain rates, in agreement with theoretical models, although no single model reproduced these results well. In our experiments, the transition from pahoehoe to `a`a occurred between 1200 and 1170 °C, at viscosities between 100 and 1000 Pa s, depending on strain rate

The articles associated with this Special Collection focus on the NASA BASALT (Biologic Analog Science Associated with Lava Terrains) Research Program, which aims at answering the question, “How do we support and enable scientific exploration during human Mars missions?” To answer this the BASALT team conducted scientific field studies under simulated Mars mission conditions to both broaden our understanding of the habitability potential of basalt-rich terrains on Mars and examine the effects of science on current Mars mission concepts of operations. This article provides an overview of the BASALT research project, from the science, to the operational concepts that were tested and developed, to the technical capabilities that supported all elements of the team's research. Further, this article introduces the 12 articles that are included in this Special Collection.

International audienceThe Muliwai a Pele lava channel was emplaced during the final stage of Mauna Ulu’s 1969–1974 eruption (Kilauea, Hawaii). The event was fountain-fed and lasted for around 50 h, during which time a channelized flow system developed, in which a 6-km channel fed a zone of dispersed flow that extended a further 2.6 km. The channel was surrounded by initial rubble levees of ’a’a, capped by overflow units of limited extent. We sampled the uppermost overflow unit every 250 m down the entire channel length, collecting, and analyzing 27 air-quenched samples. Bulk chemistry, density and textural analyses were carried out on the sample interior, and glass chemistry and microlite crystallization analyses were completed on the quenched crust. Thermal and rheological parameters (cooling, crystallization rate, viscosity, and yield strength) were also calculated. Results show that all parameters experience a change around 4.5 km from the vent. At this point, there is a lava surface transition from pahoehoe to ’a’a. Lava density, microlite content, viscosity, and yield strength all increase down channel, but vesicle content and lava temperature decrease. Cooling rates were 6.7 °C/km, with crystallization rates increasing from 0.03 Фc/km proximally, to 0.14 Фc/km distally. Modeling of the channel was carried out using the FLOWGO thermo-rheological model and allowed fits for temperature, microlite content, and channel width when run using a three-phase viscosity model based on a temperature-dependent viscosity relation derived for this lava. The down flow velocity profile suggests an initial velocity of 27 m/s, declining to 1 m/s at the end of the channel. Down-channel, lava underwent cooling that induced crystallization, causing both the lava viscosity and yield strength to increase. Moreover, lava underwent degassing and a subsequent vesicularity decrease. This aided in increasing viscosity, with the subsequent increase in shearing promoting a transition to ’a’a

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