COMPOSITION OF >4.1GA CRUSTAL BLOCKS NORTHERN HELLAS, MARS

Studying primary crust formation for some planetary bodies, such as Venus and Earth, is challenging because no direct observations of their first stable crusts have been made, and indeed may never be made. Modeling work indicates that rocky bodies the size of Mars and larger will have derived their primary crusts from “overturn melting” [1], characterized by solidification of a magma ocean into a gravitationally unstable configuration that drives overturn of the cumulate mantle to generate voluminous depressurization melts. Mars is key for understanding crust formation in an overturn-melting regime, given its size and observable record of ancient rock. The ancient martian crust is hypothesized, based on geodynamic data [2,3], to comprise an extensive low-density, perhaps feldspathic, component. This hypothesis is supported by evidence from orbital detections [4-6], rover observations [7, 8], and the ancient martian meteorite NWA 7034 and its pairs [9, 10]. All descriptions of evolved compositions have, thus far, been local instances and not regionally extensive. To test the hypothesis of an extensive feldspathic component to the ancient martian crust, we searched for evolved lithologies using the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) in crustal blocks uplifted by the Hellas basin-forming impact [11], which are the most extensive and spatially coherent outcroppings of Pre-Noachian (> ~4.1 Ga) rock. Here, we present evidence for an ancient, layered igneous complex ~2,200 by 600 km in extent in the northern Hellas region that contains feldspathic rocks, which we interpret as anorthositic. The existence of this feldspathic material raises the intriguing possibility that the Hellas-forming impact uplifted a sample of a deep, global, low-density component of the ancient martian crust.

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