Infrared Spectral Evidence for K‐Metasomatism of Volcanic Rocks on Mars.

Potassium metasomatism is a common geologic process occurring in a range of environments on Earth. K‐metasomatism can dramatically change the chemistry of large volumes of crust resulting in formation of abundant K‐rich feldspar while preserving original rock textures. Remote sensing of Mars has revealed the presence of rare but striking felspar‐rich terrains as well as K‐rich phyllosilicates such as illite or muscovite. Furthermore, the Curiosity Rover has detected evidence for K‐enrichment in rocks at Gale Crater. In this study, we use spectroscopic and remote sensing analyses of K‐metasomatized rocks on Earth as a comparison to K‐rich and/or possible granitic rocks on Mars. The results suggest that evidence for K‐feldspar rich, quartz‐bearing, and illite‐bearing rocks on Mars is consistent with K‐metasomatism. K‐rich rocks in Gale Crater have themselves likely not been metasomatized, but the abundant feldspar within them might have been derived from metasomatized crust. Plain Language Summary: Mars is a volcanic planet with a surface dominated by basalt and sedimentary rocks derived from basalt, similar to oceanic crust on Earth (though Mars' crust is thicker). For decades, Mars has been considered a "one plate planet" where plate tectonics never occurred and where continental type rocks never formed. Recent observations have, however, revealed enticing evidence that felsic "continental type" rocks containing K‐feldspar and quartz are present. Such rock units were recognized in Gale Crater based on their high potassium abundances. Granite‐type rocks detected from orbit using infrared data would also presumably be rich in potassium. However, rocks of the same composition can be made through aqueous alteration rather than from magma in the crust. This study shows a striking example of potassium enrichment and mineralogical changes in basaltic rocks on Earth caused entirely by low‐temperature hydrothermal brine in the crust. We use analogous remote sensing measurements to study these potassic rocks on Earth and compare the results to Mars. It appears that at least some of the felsic rock compositions on Mars were caused by metasomatism. Key Points: Subsurface brines can deposit significant potassium in volcanic rocks while preserving original texturesSpectral signatures of K‐metasomatism include K‐feldspar and illiteFelsic compositions on Mars might occur because of low‐temperate alteration by subsurface brines [ABSTRACT FROM AUTHOR]