VARIABILITY IN BASALTIC WEATHERING RINDS FROM A SINGLE WEATHERING ENVIRONMENT: INTERPRETING COLOR DIFFERENCES ON WEATHERED ROCKS ON EARTH AND MARS

Weathering rinds alter the outward appearance of rocks and record interactions between rocks and their surrounding environments. A notable feature of weathered basaltic rocks is that their weathered rinds can have an array of appearances, seen as color and texture variations that occur among a population of rocks from the same locality, and at times, on the same rock. The question is why do rocks develop an array of appearances when weathered ostensibly in the same weathering environment? Furthermore, do these different colored rocks have infrared spectral differences corresponding to significant mineralogical differences? And how do these differences impact interpretations of weathering environment when studying weathering rinds on Earth and Mars?

We are investigating weathered rocks of the Columbia River Basalt (CRB) Group to determine the spectral, textural, mineralogical and chemical variations that can occur in weathering rinds of the same rock formation. We have begun by studying two different looking weathering rinds on CRBs from the Frenchman Springs Member of the Wanapum Flow collected at Frenchman Coulee, Washington. One rock rind was red, the other a lighter-toned yellow. We measured visible/near-infrared (VNIR) and thermal infrared (TIR) spectra of these two weathering rinds and respective rock interiors. VNIR and TIR spectra were similar for both rock interiors. Spectra of the red weathering rind showed relatively minor differences compared to the rock interior, whereas the yellow rind displayed deeper hydration bands in VNIR and in TIR spectra, loss of rock-forming mineral absorptions and a prominent clay-like absorption feature. These observations indicate that the yellow rind has more advanced weathering compared to the red rind. This relationship was born out in SEM imaging of the weathering rinds, which showed that weathering-related porosity was significantly more developed in the yellow rind compared to the red rind. We conclude that basic color differences between weathering rinds can indicate significant differences in weathering. We continue to work toward understanding why these differences occur in the same rock type in the same weathering environment. The rind differences potentially reflect different stages of weathering or indicate important microenvironmental differences.