Abstract Gullies are widespread morphological features on Mars for which current changes have been observed. Liquid water has been one of the potential mechanisms to explain their formation and activity. However, under present-day Martian conditions, liquid water is unstable and should only be transiently present in small amounts at the surface. Yet little attention has been paid to the mechanisms by which unstable water transports sediment under low atmospheric pressure. Here we present the results of laboratory experiments studying the interaction between liquid water flowing over a sand bed under Mars-like atmospheric pressure ( c. 9 mbar). The experiments were performed in a Mars Simulation Chamber (at the Open University, UK), in which we placed a test bed of fine sand at a 25° slope. We chose to investigate the influence of two parameters: the temperature of the water and the temperature of the sand. We performed 27 experiments with nine different combinations of water and sand temperatures ranging from 278 to 297 K. Under all experimental conditions, the water was boiling. We investigated and compared the types and timing of sediment transport events, and the shapes, characteristics and volumes of the resulting morphologies. In agreement with previous laboratory studies we found that more intense boiling increased the volume of sediment transported for a given volume of water. We found four main types of sediment transport: entrainment by overland flow; grain ejection; grain avalanches; and levitation of saturated sand pellets. Our results showed that increasing sand temperature was the main driving parameter in increasing the sand transport and in modifying the dominant sediment transport mechanism. The temperature of the water played a negligible or minor role, apart from the duration of sand ejection and avalanches, which lasted longer at low water temperature. At low sand temperature the majority of the sand was transported by overland flow of the liquid water. At higher sand temperatures the transport was dominated by processes triggered by the boiling behaviour of the water. At the highest temperatures, sediment transport was dominated by the formation of levitating pellets, dry avalanches and ejection of the sand grains. This resulted in a transport volume about nine times greater at a sand temperature of 297 K compared with 278 K. Our heat transfer scaling shows that the boiling behaviour will be enhanced under Martian low gravity, resulting in more efficient transport of sediment by levitating sand pellets even at temperatures close to the triple point. Our results showed that the boiling intensity played an important role in the physics of sediment transport by liquid water. This implied that the amount of water required to produce morphological changes at the surface of Mars could be lower than previously estimated by assuming stable liquid water. Boiling is a critical process to be considered when assessing gully formation and modification mechanisms mobilized by liquid water. Our work could have similar implications for any water-formed landform on Mars, which could include recurring slope lineae, dark dune flows and slope streaks. Supplementary material: Videos of the experiments are available at https://doi.org/10.6084/m9.figshare.c.3990330

ABSTRACT The Mitchell Plateau of south-central Indiana is one of the iconic karst landscapes of the United States. The sinkhole-dimpled forests, fields, and farms; the extensive cave systems; and the deep windows into the groundwater system have fostered curiosity, exploration, and publication since the mid-1800s. This paper is designed to complement a field excursion to the classic features of this landscape. Included are literature reviews focused on three karst basins of the Mitchell Plateau: Mill Creek–Mosquito Creek, Bluespring Caverns, and Lost River. Geomorphic, hydrologic, and geochemical data are synthesized in the modern context of our understanding of epigenetic karst. Revealed are three styles of karst basin: (1) small, shallow karst aquifers strongly controlled by meteoric recharge and epikarst percolation; (2) intermediate-size karst aquifers with significant base flow and surface-water–groundwater interaction; and (3) regional aquifer systems with outcrop belt recharge, downdip transport into confinement with long water-rock interaction times, and artesian flow or entrainment of mineralized waters through fractures into springs or surface waters. Quaternary glaciation has greatly influenced the vertical position of base level through river incision and sediment aggradation; conduit development is controlled by proximity to the major rivers and the stratigraphic position of conduits.

Abstract Orogenic gold systems are open, flow-controlled thermodynamic systems and generally occur in mid- to upper crustal environments where there is strong coupling between fluid flow and dilatant plastic deformation. This paper considers the principles involved in such coupling, with an emphasis on the elastic and plastic volume changes and their influence on the fluid, mechanical and thermodynamic pressures. Some misconceptions regarding the magnitudes of these three distinctly different pressures and their influences on fluid flow and chemical equilibrium are addressed, with examples at both the tens of metres scale and the crustal scale. We show that the mean stress is less than twice the lithostatic stress for Mohr–Coulomb materials with cohesion and the thermodynamic pressure only has meaning under isentropic conditions and hence is less than many previously published estimates based on high mean stresses. At the crustal scale, we also include the role of critical behaviour in influencing the geometry and magnitudes of fluid pressure gradients and fluid flow velocities in open, flow-controlled systems.

Various hypotheses for the origin of alkaline sodic mafic magmas have been proposed. This diversity of models is mainly related to the various constraints used to develop them. The goal of this paper is to test these different models using petrological and geochemical constraints in an attempt to understand why alkaline sodic rocks are so similar even while their environment of formation varies from oceanic to continental rift. Incompatible trace-element contents of alkaline basalts from ocean islands and continents show that the sources of these rocks are more enriched than primitive mantle. A fundamental question then is how the sources of alkaline rocks acquire these trace-element enrichments. Recycled oceanic crust, with or without sediment, is often invoked as a source component of alkaline magmas to account for their trace-element and isotopic characteristics. However, the fact that melting of oceanic crust produces silica-rich liquids seems to exclude the direct melting of eclogite derived from mid-ocean-ridge basalt to produce alkaline lavas. Recycling oceanic crust in the source of alkaline magma requires either (1) that the mantle “digests” this component producing metasomatized CO 2 -rich peridotitic sources or (2) that low-degree melt from recycled oceanic crust reacts with peridotite in the presence of CO 2 , producing low-silica alkaline melt by olivine dissolution and orthopyroxene precipitation. These two hypotheses are plausible in terms of major elements. However, they have specific implications about the type and proportion of recycled lithologies present in the asthenosphere to explain the specific trace-element pattern of intraplate alkaline lavas. A third hypothesis for the formation of alkaline magmas is the melting of metasomatized lithosphere. In this model, the major- and trace-element signature of alkaline magma is not controlled by the asthenospheric source (i.e., the amount of oceanic crust or CO 2 present in the asthenosphere), but by the petrological process that controls the percolation and differentiation of low-degree asthenospheric melts across the lithosphere. This process forms amphibole-bearing metasomatic veins that are a candidate source of alkaline rocks. This hypothesis offers an explanation for the generation of the Na-alkaline lavas with similar major- and trace-element composition that are observed worldwide and for the generation of K- and Na-alkaline magma observed in continental settings. This hypothesis requires the formation of significant amounts of metasomatic veins within the lithosphere. Qualitative analyses of the thermal implication of the potential models for the generation of alkaline rocks demonstrate that such magma requires low potential temperature ( T p: 1320 °C to 1350 °C). If temperatures are higher, melting of the convecting mantle will erase any signature of low-degree melts produced from fertile mantle lithologies. This analysis suggests that a role for hot thermal plumes in the generation of intraplate volcanoes dominated by alkaline magmas is unrealistic.