COMPOSITION OF THE PRIMARY IGNEOUS CRUST OF MARS

Carle Pieters’ work with remotely sensed data and fundamental laboratory measurements has established many key aspects of the igneous composition of the primary lunar crust: the upper megareglith overlying an anorthositic crust, a lower noritic crust, and the deepest sections showing increasing mafic compositions that in the extreme depths of excavation may tap the mantle. With the explosion in global remotely sensed spectroscopic data for Mars over the 15 years, a structured understanding of the Martian crust from observations is evolving, but it is very different than the Moon. Thermal Emission Spectrometer and Gamma Ray Spectrometer data show that Mars is dominantly basaltic, with rare evolved magmas. However these measurements are dominated by the surface exposures that are commonly post-Noachian volcanic flows. A recent survey of high-resolution OMEGA and CRISM observations of Noachian-aged exposures in scarps and central peaks of impacts shows a greater diversity of compositions. Anorthosite has been detected on Mars, but it is extremely rare, only confirmed in one location. Many exposures of ancient crust are enriched in low-Ca pyroxene but commonly are accompanied by olivine. High-Ca pyroxene-bearing rocks are less common, more typically associated with post-Noachian volcanic terrains. While olivine-rich deposits are widespread on Mars, the locations of the strongest concentrations are between the inner and outer rings of the Argyre and Isidis impact basins. High-resolution imaging of the central peak and scarp exposures show the rocks are commonly breccias with extremely large blocks of igneous rock embedded in a matrix of finer-grained rocks that in places contain signatures of phyllosilicate alteration. There are widespread phyllosilicate-bearing outcrops in the Noachian terrains but the extent of alteration (i.e. % alteration) is not yet known. The emerging view of Mars’ igneous crust is one of Late-Noachian through Hesperian cover of basaltic flows over an extensively brecciated ancient crust dominated by low-Ca pyroxene and olivine-rich lithologies. Like the Moon, Mars has a megaregolith but it is enriched in pyroxene and olivine and is lacking in plagioclase. We are comparing these results with planetary evolution and magma ocean models for Mars.