Spectral study of hydrothermally altered volcanic deposits of the Greek island Milos as a Martian analog

For the study of the aqueous history of Mars, the knowledge of mineral characteristics for specific geologic environments is essential [1]. Previous studies of regions on Earth suitable as analogs for studying geological processes on planetary surfaces, such as the Italian island Vulcano [2] already enabled the identification of secondary sulfate minerals formed by acid alteration of volcanic deposits due to fumarolic activity. In this work, we extend this study with the spectral investigation of alteration minerals measured on the volcanic island Milos (Greece) during fieldwork performed in 2024.

Milos is a volcanic island in the Aegean Sea and a part of the subduction-related South Aegean Volcanic Arc [3]. Hydrothermal activity changed the initial composition of the rocks and led to the formation of various secondary minerals - making the island an ideal location to extend our spectral library, coupled  with geologic context information. Spectral measurements presented here were performed with a portable VIS-NIR spectrometer operating at wavelengths between 0.35 and 2.5 µm in two locations on the island: 1) a persistently active fumarolic field in the area of Kalamos Dome (Fig. 1 a + b) and 2) an abandoned sulfur mine at Paliorema (Fig. 1 c + d), where sulfur was mainly extracted from underground deposits.

Both locations reveal a variety of alteration minerals, in particular sulfates next to native sulfur. Similar to what could be identified on Vulcano, alteration minerals on Milos are dominated by Fe- and Al-bearing sulfates (Fig. 2). However, whereas sulfate minerals such as jarosite (KFe₃³⁺[(OH)₆(SO₄)₂]) and alunite (KAl₃[(OH)₆(SO₄)₂]) are dominant on Vulcano, different hydrous phases, such as copiapite (Fe²⁺Fe³⁺₄[(OH)₂(SO₄)₆] x 20H₂O) and alunogene (Al₂[SO₄]₃ x 17H₂O), are prominent in the area of the Kalamos. This also accounts for hydrated silica, the typical alteration residue. In contrast to Vulcano, where silica occur as amorphous silica gel [4], silica on Milos have a higher water content and crystallinity, mainly identified as chert and chalcedony (Fig. 2). The fossilized hydrothermal system at Paliorema displays more mineralogical diversity. There we have identified jarosite and alunite and also alunogene and copiapite (Fig. 2). Additionally, we see spectral evidence for secondary mineral phases such as melanterite (Fe²⁺SO₄ x 7H₂O) and the rare secondary minerals amarantite Fe₂³⁺[O(SO₄)₂] x 7H₂O).

The identified minerals are known to occur in the vicinity of fumaroles and are expected to result from the direct interaction between the surface material of the volcanic rocks and fumarolic gases [4]. Secondary minerals form through leaching of the volcanic rock surface, leaving silica behind. Some minerals can also directly precipitate from volcanic gases. The diversity of hydrous minerals compared to what could be observed on Vulcano [2] implies more complex aqueous settings in the formation environments and possible dehydration patterns. The study of altered volcanic deposits on Milos, with its diverse mineralogical assemblages, provides valuable analogs for understanding similar processes on Mars.

Fig. 1: a) Kalamos Dome with b) differently colored hydrothermally altered volcanic deposits and c - d) the sulfur  mine in Paliorema. See Fig. 2 for spectra taken at the locations indicated by green letters. 

                                                                                                                                                              

Fig. 2: VIS/NIR spectra acquired in areas of hydrothermal and acid alteration of volcanic deposits on a) the Greek island Milos and b) – d) the Italian island Vulcano [2].

 

For instance, the detection of sulfates, hydrated silica, and other secondary minerals on Milos mirrors mineralogical findings from Martian regions like Gale Crater and Meridiani Planum [5]. These parallels suggest that fumarolic and hydrothermal systems on Mars may have fostered similar alteration processes, offering clues about the planet's past habitability.

The ongoing work also involves detailed laboratory analyses of Milos samples using VIS-NIR, LIBS, and Raman spectroscopy to expand the mineralogical inventory and to deepen the understanding of hydrothermal alteration processes and their implications for Martian geology and habitability.

 

References: [1] King P. L. and McSween H. Y. (2005) JGR, 110, E12, doi:10.1029/2005JE002482. [2] Stephan K. et al. (2025) Earth and Space Science, doi: 10.1029/2024EA004036. [3] Fytikas M. et al. (1986) J. Volcanol. Geotherm. Res., 28, 297-317. [4] Rice et al. (2013) Icarus, 223, 499 – 533. [5] Vaniman D. T. et al. (2004) Nature, 431, 663–665.