Analogues for Martian crustal and aqueous processes: Lessons learnt from mineralogy and geochemistry of rocks in the PTAL collection.

Planetary Terrestrial Analogue Library (PTAL) is a newly built collection of rocks and spectral data aiming to support interpretation of data from Mars and small bodies of the Solar System [1]. It contains spectral data by NIR, Raman and LIBS, validated by XRD and microscopic characterization [2,3,4]. Since September 2021, the PTAL library is public and freely accessible (www.ptal.eu), and the physical collection of witness rocks is available for further studies. PTAL is collection of natural rocks and not individual minerals what gives better insight into mineralogy and geochemistry as e.g. overlapping vibrational absorption features of minerals are included. Although the spectral analogy never implies exact parallelism in processes of deposit formation, reliable identification of specific minerals can shed light on recorded evolution and alterations. Here we present the overview of analogues and assess their fidelity in terms of information about composition (mineralogy and geochemistry) of Martian crust and alteration environments.

PTAL consists of 106 rock samples from 19 diverse localities on Earth [1]. The collection contains a variety of volcanic rocks, from picrobasalts to phonolites. Sampling sites include tholeiitic basalts from Iceland, ferropicrites from Rum (Scotland), alkali-rich rocks of metasomatized origin from Canary Islands and Tenerife, basaltic tuffs and ash-fall deposits from the Granby formation (USA) with clay-infilled amygdales, as well as serpentinised peridotites from the Leka ophiolite complex (Norway). Collected rocks are good analogues for processes of martian mantle-plume fed volcanism as well as for evolution of alkali-rich crustal units on Noachian Mars. Additionally they record processes of metasomatism, deuteric and hydrothermal alteration. They are never perfect geochemical analogues to Martian crust, which is a consequence of inherited differences between the two planets, e.g. Fe and Mn content or volatile abundances. PTAL initial studies show, however, that studies of alteration pathways as a function of protolith composition are possible with these rocks, despite geochemistry mismatches [5].

PTAL contains also samples from diverse surface alteration environments and from range of climatic environments, including hot and cold deserts: John Day Formation in Oregon (USA), Dry Valleys in Antarctica, Otago Formation (New Zealand), Jaroso Ravine and Rio Tinto (Spain). These analogues contain minerals such as jarosite, hematite, or Fe-rich vermiculite and therefore good geochemical and mineralogical analogies to Mars targeted sites can be obtained. However, care is needed when processes of formation inferred [e.g. 5], as peculiar conditions leading to formation of minerals can be obtained in a spectrum of environments. PTAL strength is that it samples a sequence of alteration products, allowing detailed mineralogical and geochemical comparative analyses within alteration environment to shed light on the potential parallelism of formation process [5].

Acknowledgement: PTAL was funded by the EU Horizon 2020 Research and Innovation Programme (Grant Agreement 687302).

[1] Dypvik et al., 2021. Planetary and Space Science 208, 105339

[2] Lantz et al., 2020. Planetary and Space Science 189: 104989

[3] Loizeau et al., 2022. Astrobiology, accepted.

[4] Veneranda et al., 2020. Journal of Raman Spectroscopy 51: 1731-1749

[5] Krzesinska et al. 2021. Astrobiology 21: 997-1016