Canali-forming Magmas: Generation of Carbonate-sulfate Melts on Venus

Venus' canali, very long meandering channels, are inferred to be products of carbonate-sulfate magmas, comparable in some ways to carbonatite magmas on Earth. In 1994, [1] inferred that alkali-rich carbonate sulfate magmas were common and abundant just beneath Venus' surface. Current understanding of Venus suggests a cooler geotherm, so that carbonate-sulfate melt can be abundant only at pressure > 1 kilobar or so (depth > ~3.5 km). Carbonate and sulfate minerals may form by chemical weathering of basalts at the Venus surface, and can be brought to that pressure by burial under more basalts or by tectonic processes. Carbonate-sulfate melts can form much closer to the surface if temperatures are above the average geo-therm, i.e., above a plume, near an igneous body, or at a meteoroid/comet impact site. Introduction: Venus' canali are meandering channels of great length (>500km) and relatively constant widths (3-5 km), with features like cutoff meanders, cut banks, overbank deposits and levees, crevasse splays, and distributory braided channels – Figure 1 [1-7]. All these features are familiar on river-cut channels on Earth, and imply that canali-forming fluids had rheological properties like those of liquid water. The most likely canali-forming fluids, with water-like rheology and consistent with Venus' present climate , are ionic melts rich in alkali-and alkaline earth carbonates and sulfates, i.e., natrocarbonatite magma [1,8-11]. Water itself is not stable now at Venus' surface (T=740K, P=96 bars), but could have been present in earlier climates [12]. Mafic silicate lavas [6] would probably cool too quickly to form canali [11,13] (but see [14]), except perhaps in an earlier hotter climate [15] or underground [16]; subsurface flows seem unlikely to yield the observed channel landforms. Liquid sulfur has water-like rheology [17] but a melting T less than that of Venus' surface [13], and so would not likely form levees or other constructions and would likely evaporate [18]. Fluid-absent granular suspen-sions have been suggested [19], but seem unlikely (to me) because most source areas lack a source of such material – i.e., a pyroclastic volcano or large cliff. Venus Geotherms and Carbonate-Sulfate Melting: If canali were likely formed by carbonate-sulfate melts, it is worth revisiting how those melts could form on Venus [1,10]. In 1994, [1] inferred that carbonate-sulfate melts would be stable just below Venus' surface in a global 'magmafer'. Since then, estimates of Venus' 'normal' geothermal gradient have declined, and there is new data on carbonate-sulfate melts. In …