Compositional Mapping of the Nili Patera Feldspathic Unit: Extent and Implications for Formation.

Decades of study of the igneous martian crust concluded that it was primarily basaltic, but a range of new investigations find evidence of evolved compositions. Foremost of these is a highly feldspathic unit within the Nili Patera caldera of Syrtis Major, the only detection with preserved volcanic context but which nonetheless remains ambiguous in exact composition and formation. We conduct compositional mapping of this feldspathic unit via near‐infrared spectroscopy from the Compact Reconnaissance Imaging Spectrometer for Mars instrument and find that the unit occupies at minimum 104 km2 at high confidence and an additional 41 km2 at moderately high confidence, meaning the unit is locally significant. We compare our mapping with that inferred from geomorphology and find that while texture and albedo are useful proxies, they are not perfectly reliable as substitutes for thorough compositional investigation. Study of the boundary between the feldspathic unit and surrounding mafic rock indicates the former formed early and may extend locally in the subsurface. We consider what compositional mixtures could explain the conflicting interpretations derived from visible/near‐infrared and thermal infrared spectroscopy, concluding it is likely due to thermophysical differences between the light‐toned feldspathic unit and the infilling dark mafic sand. We discuss proposed plutonic and volcanic formation scenarios for the feldspathic unit, considering Earth analogs and implications for the parent magmatic system, and offer observations in rock texture and composition that would clarify. Plain Language Summary: For many years, the surface of Mars derived from cooled magma or lava was thought to be predominantly a low‐silica rock called basalt. Recent observations have produced evidence of high‐silica rock that is rich in a mineral called feldspar (termed feldspathic), which was long thought not to occur on Mars. For our study, we focus on a detection site in Nili Patera, a crater at the summit of the Syrtis Major volcano. Using the Compact Reconnaissance Imaging Spectrometer for Mars instrument, we study how reflected sunlight interacts with the martian surface to deduce rock composition at the surface and thereby map the extent of this feldspathic rock. We find that the unique rock covers 104 km2 at high confidence and an additional 41 km2 at moderately high confidence. This indicates that the unit is locally significant and, as this is the only detection site with a preserved volcanic context, worth further study to understand its formation. We conclude with discussion of potential sources for the disagreement in compositional interpretation of the feldspathic unit and how potential formation mechanisms might be distinguished with further observations or laboratory measurements. Key Points: Nili Patera is the only feldspathic site on Mars with its volcanic context intactCompositional mapping of the unit reveals it is locally significant and occupies 145 km2 at minimumConsidering all data sets holistically optimizes the interpretation of this remotely sensed composition and its magmatic history [ABSTRACT FROM AUTHOR]