Evidence for magmatic evolution and diversity on Mars from infrared observations

Compositional mapping of Mars at the 100-metre scale with the Mars Odyssey Thermal Emission Imaging System (THEMIS) has revealed a wide diversity of igneous materials. Volcanic evolution produced compositions from low-silica basalts to high-silica dacite in the Syrtis Major caldera. The existence of dacite demonstrates that highly evolved lavas have been produced, at least locally, by magma evolution through fractional crystallization. Olivine basalts are observed on crater floors and in layers exposed in canyon walls up to 4.5km beneath the surface. This vertical distribution suggests that olivine-rich lavas were emplaced at various times throughout the formation of the upper crust, with their growing inventory suggesting that such ultramafic (picritic) basalts may be relatively common. Quartz-bearing granitoid rocks have also been discovered, demonstrating that extreme differentiation has occurred. These observations show that the martian crust, while dominated by basalt, contains a diversity of igneous materials whose range in composition from picritic basalts to granitoids rivals that found on the Earth. High-resolution imaging of the martian surface has allowed the construction of morphologic maps showing considerable diversity at local and regional scales. However, petrology has not kept pace and so virtually no information exists about mineralogic or geochemical variations at comparable spatial scales. Remote sensing data from the Mars Global Surveyor Thermal Emission Spectrometer (TES) and visible/near-infrared (IR) spectroscopy from spacecraft-based and Earth-based telescopes have revealed mineralogic variations between regional-scale units 1–4 , but martian igneous and sedimentary processes almost certainly resulted in much more diversity than is presently recognized. Volcanic units are probably composed of materials formed by different degrees of partial melting and modified by fractional crystallization and interaction with crustal compositions during magma ascent and eruption. Similarly, sedimentary units presumably experienced mineral fractionations during transport and deposition, and erosion, tectonic activity and impact may have excavated different subsurface materials at local scales.