Combined VIS-NIR, LIBS and Raman investigation of altered volcanic deposits of Vulcano island/Sicily

The ongoing volcanic activity by subareal and submarine fumaroles on Vulcano (Eolian Islands, Italy) provides extreme acid alteration conditions of volcanic deposits, which also have been a key process at local and regional scales throughout Martian geologic history. This makes the study of volcanic deposits on Vulcano a perfect spectral analog to constrain the conditions of acidic alteration with respect to Mars. During the fifth International Summer School held on Vulcano (Eolian Islands, Italy) in June 2019 lava rocks that are still actively altered by volcanic sulfur-rich gases emanating through cracks and caves pervading the study area were investigated using a portable spectrometer working in the visible and near-infrared (VIS-NIR) wavelength range (Stephan et al., 2022). Spectral measurements of different phases of altered lava rock from relatively un-weathered lava rocks to fully altered portions displaying a reddish, yellowish to whitish surface and particular crystal growth revealed characteristic alteration minerals such as jarosite and alunite. These minerals are believed to directly result from the interaction between the surface of the lava rock with the volcanically-derived acid fluids by remobilization of material from the lava rock constituting the main elements in the new mineral phases. Similarly, the residual siliceous material is progressively reddened by iron and the calcium of the lava rock is consumed in the formation of gypsum.

This study has now been extended by the analysis of corresponding data acquired by laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy, a triple combination, which significantly improved our results. In general, LIBS data confirm the presence of most of the chemical elements that make up the minerals detected at the wavelength range of the VIS-NIR spectrometer such as Si, Fe, Na, K, Ca, Al and S. Even more, the relative intensities of the detected emission lines generally support the interpretation of the VIS-NIR spectra. Thus, for example, LIBS data show that Fe (II) emission lines are strongest, where Fe-rich sulfate jarosite or silica residuals dominate the corresponding VIS-NIR data. Regarding the Al (I) emissions, no clear trend can be recognized. Possibly, alunite is not that dominant in locations, where the mineral is implied by VIS-NIR spectra. Nonetheless, both Na and K are confirmed by emission lines in the LIBS spectra and thus support the findings derived from the VIS-NIR as well as Raman data that jarosite and alunite occur in both varieties. Only LIBS data, however, show that both, Na and K are already existent in the un-weathered lava rock, which cannot be recognized in neither the VIS-NIR nor Raman spectra. LIBS data also indicate the existence of Ca (II) already in places, where gypsum has not been exclusively identified by VIS/NIR spectra. Intriguingly, LIBS data also nicely show the presence of Sr and Ti implying that Sr and Ti-bearing minerals such as Celestite and Anatase, respectively, could exist in the study area. Such minerals could not be detected in the VIS-NIR spectra. These minerals are relatively transparent in this wavelength range and thus their spectral signature could possibly be easily masked by the identified sulfates. Indeed, both minerals could be identified in the data acquired by the Raman instrument. Finally, both, LIBS as well as Raman data show no hint of the occurrence of the phosphate mineral apatite in the study area, such as indicated by the VIS-NIR spectra. An explanation could be that apatite only occurs very locally and is only detectable by the VIS-NIR spectrometer because of the largest field of view of its measurements. Thus, not only the different spectral range, but also the different field of view of the different instruments have to be considered when interpreting the data. 

In summary, the results of every instrument nicely complement each other and enable to reach a more complete view of the geologic evolution of the study area. Even more, the multi-instrument approach helps to evaluate the usage of each instrument with respect to the exploration of specific geologic settings. In order to avoid misinterpretation of spectral variations, however, special care has to be taken to align the measurement locations of the different spectral instruments due to different IFOVs of the instruments and to ideally scan the same area with all spectral instruments. In contrast to field measurements of planetary analogs on Earth this can be more easily applied to planetary mission data with the knowledge of geometric information of each instrument of the spacecraft’s payload.