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Geochemical measurements from the MESSENGER mission indicate distinct geochemical terranes on the surface of Mercury. We report chemical compositions and derived mineralogy for four geochemical terranes, as well as Mercury’s average surface composition. The geochemical terranes share higher Mg and S, and lower Al, Ca, and Fe, than terrestrial oceanic basalts. The low Fe and high S concentrations suggest that all terranes formed under highly reducing conditions. All terranes are enriched in plagioclase. Heating melted the silicate shell of Mercury and produced a global magma ocean in which stratification developed during crystallization, with basal ultramafic material grading to incompatible-element-enriched material near the surface. Later differentiation began with partial melting as result of mantle convection and heating from the decay of radioactive elements. These high-Mg, high-temperature partial melts were exceptionally fluid and produced thin, laterally extensive flows. The largest impacts excavated into the upper layers of the mantle and deposited distinctive material, including remnants of a graphite-rich flotation crust from the magma ocean, at the top of the crust. Smooth plains deposits originated as laterally extensive flood basalts that efficiently covered pre-existing layers. Distinct source compositions suggest that convection was insufficient to homogenize the mantle at ~3.8–3.9 Ga.
Mercury is a volcanic world: the planet has experienced a geological history that included partial melting of the interior and the transport of magma to, and eruption onto, the surface. In this chapter, we review Mercury’s volcanic character, first in terms of effusive volcanism (as characterized by lava plains, erosional landforms, and spectral characteristics), next in regard to the planet’s explosive volcanic activity, and then from the perspective of intrusive magmatism. We also visit the planet’s ancient yet spatially expansive intercrater plains and the prospect that they, too, are volcanic. We combine the observations of and inferences for Mercury’s smooth and intercrater plains to propose a model for the planet’s crustal stratigraphy. The extent of our understanding of the petrology of surface materials on Mercury is then discussed, including compositions and lithologies, mineral assemblages, physicochemical properties, and volatile contents. We then describe in broad terms the history of effusive and explosive volcanism on the planet, before addressing the influence that the planet’s lithospheric properties and tectonic evolution have played on volcanism. We finish by listing some major outstanding questions pertaining to the volcanic character of Mercury, and we suggest how those questions might best be addressed.
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