241-6 Physical and Chemical Evolution of Plume Magmatism and Mantle Sources on Mars

Session: Petrology, Volcanology, and Mantle Plumes across the Solar System, Part I

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Comparison of ancient (Noachian and Hesperian) igneous rocks analyzed by rovers and modern (Amazonian) basaltic meteorites reveal temporal changes in the composition and thermal state of the Martian mantle. Secular global cooling of the mantle over geologic time is predicted by magmatic models and supported by geophysical arguments. FeO/MgO/SiO₂ systematics of both ancient and modern rocks rule out melting in subduction zones and thus indirectly support magma formation in plumes. Melting of mantle rocks having relatively high contents of water, with attendant high oxidation states, may account for the alkali-rich compositions of pre-Amazonian volcanic rocks, as well as higher low-Ca pyroxene/total pyroxene ratios observed in remote-sensing data, although elevated pressures of melting may also affect these parameters. Analyses of the meteorites indicate that modern mantle sources, unaffected by tectonic recycling, have been depleted by melt extraction and are heterogeneous in terms of radiogenic isotopes, abundances of incompatible trace elements and H₂O, and oxidation states. Identification of meteorite source craters suggests that magmas erupting from a Tharsis plume have depleted sources, and an Elysium plume sampled enriched mantle. The global evolutionary pattern that emerges is decompression melting within long-lived plumes of a progressively drying mantle on a cooling, one-plate planet. On Earth, plumes evolve – uplift and extension with vast outpourings of tholeiitic flood basalt produced by high degrees of melting at modest pressures as plume heads approach the surface, followed by subsidence and compression with alkalic magmas in superimposed volcanic edifices produced by lower degrees of partial melting at higher pressures in plume tails. Geological and geophysical studies of older and younger plume terrains on Mars document surface uplift with extensive volcanism from fissures, followed by subsidence and wrinkle ridges formed by compression, with later superimposed volcanoes. The Noachian/Hesperian volcanic plains analyzed by Mars rovers may represent the flood basalt stage, and martian meteorites were derived from later Amazonian volcanoes. If correct, the physical evolution of Mars plumes mimics that of Earth, but the compositional evolution of martian plume magmas is curiously reversed.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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