¹Hunter Vannier, ¹Briony H. N. Horgan, ²Michael Phillips, ¹Michael Eddy, ³Rebecca Greenberger, ⁴Arya Udry
Journal of Geophysical Research: Planets, 131, e2026JE009705
Open Access Link to Article [DOI: 10.1029/2026JE009705]
¹Purdue University, West Lafayette, IN, USA
²University of Arizona, Tucson, AZ, USA
³California Institute of Technology, Pasadena, CA, USA
⁴University of Nevada Las Vegas, Las Vegas, NV, USA

Published by arrangement with John Wiley & Sons

Widespread plagioclase detections on Mars using orbital visible to near infrared (VNIR) reflectance spectra have led to multiple interpretations of “feldspathic” terrain, ranging from ancient crustal material and evolved plutons to volcanic lava flows. A systematic, laboratory-based study of plagioclase detectability in whole igneous rocks is needed to contextualize these detections. We analyzed 42 feldspar-bearing igneous rocks (∼50–77 wt.% SiO₂; ∼25–60 vol.% plagioclase) using VNIR and mid-infrared (MIR) laboratory spectra, bulk chemistry/mineralogy, microscopic analyses of rock texture, and plagioclase chemistry to assess the characteristics influencing plagioclase signatures in whole-rock spectra. We find that 1.25 μm absorption bands are common in intrusive samples, and that plagioclase can be spectrally detected at abundances <40 vol.%, even in the presence of ∼20 vol.% mafic hydrous minerals. We find that large (mm-scale) grain size is a driver of plagioclase detection, and that oxides are highly effective at subduing absorption bands. Plagioclase with a wide range of compositions (An_25–47,90) can produce a 1.25 μm absorption, unless iron-depleted (<0.13 wt.% FeO). Alteration can obscure the 1.25 μm absorption, causing rocks with similar plagioclase abundances and chemistries to exhibit different bulk VNIR spectra. Unambiguous detection of feldspathic rocks on Mars is only likely for: (a) plagioclase-rich crust (e.g., anorthosites or other plagioclase cumulates), (b) intrusive felsic rocks with limited alteration and plagioclase with iron >0.13 wt.%, and (c) plagioclase-phyric effusive rocks with little or no mafic phenocrysts. MIR spectra can distinguish felsic (granite, granodiorite, monzonite) and mafic (anorthosite, basalts) rocks with similar 1.25 μm absorptions.