Exploring the geochemical secrets of the lava tubes in La Palma: in-situ Raman and XRF spectroscopy for the study of these volcanic structures

Under the acronym PALMALAB, the University of the Basque Country is conducting several analysis campaigns on the island of La Palma, Canary Islands, Spain. The Canary Islands archipelago, located off the northwest coast of Africa, is of entirely volcanic origin and represents one of the most prominent examples of oceanic intraplate volcanism. Formed over a presumed mantle hotspot, the islands are aligned roughly east to west and exhibit a progression in age, with the eastern islands (e.g., Fuerteventura and Lanzarote) being older than the western ones (e.g., La Palma and El Hierro). This age progression, combined with the geochemical and petrological signatures of the volcanic products, supports the hypothesis of a stationary hotspot beneath the moving African Plate. Volcanism in the archipelago began approximately 20 million years ago, and it continues today, as evidenced by historical eruptions such as those in La Palma (2021) and El Hierro (2011). La Palma stands out as a geologically significant site due to its ongoing volcanic activity, well-preserved lava tubes, and diverse range of volcanic terrains, making it not only a key natural laboratory for studying terrestrial volcanism but also a high-fidelity analogue for planetary surfaces such as those of Mars and the Moon.

From a volcanic perspective, La Palma features a highly active rift zone (Cumbre Vieja) and a variety of volcanic landforms—shield volcanoes, scoria cones, fissure vents, and extensive lava fields—that reflect both effusive and mildly explosive basaltic eruptions. These processes are analogous to those observed or inferred on the Moon and Mars, where basaltic volcanism has shaped large portions of their surfaces. Notably, the morphology and internal architecture of La Palma’s lava tubes closely resemble lava tubes identified in orbital imagery of the lunar maria and Martian volcanic provinces, such as in regions like Mare Tranquillitatis or Arsia Mons. Moreover, the mineralogical composition of La Palma's basalts shares key similarities with planetary basalts, namely low silica content and the presence of olivine, pyroxene, and plagioclase. These characteristics make the island particularly valuable for testing in-situ analytical instruments like Raman spectroscopy and X-ray fluorescence (XRF), technologies already integrated into Mars rovers (e.g., Perseverance and Curiosity) and proposed for future lunar and martian missions, i.e. ExoMars. Studying La Palma's volcanic features in-situ allows simulating planetary surface exploration, refining remote sensing techniques, and calibrating analytical tools in a terrestrial environment that mimics the physical and chemical conditions expected on Mars and the Moon.

In February 2025, the first analytical campaign was carried out using portable Raman spectroscopy and XRF systems. Several lava tubes of different formation ages were analysed. From the oldest ones of the island to a lava tube from the Tajogaite volcano (2021) were studied in order to evaluate their potential as planetary analogues. The objective of studying systems of different ages was to evaluate the differences in geochemistry due to the time of exposure of the volcanic material to the Earth's atmosphere.

For the mentioned aim, two portable InnoRam (BWTEKINC, USA) Raman systems were employed for testing different excitation laser wavelengths. In this sense, 532 and 785 nm lasers were used for the analysis of the lava tubes. In addition, a handheld XRF Tracer 5 spectrometer (Bruker, EEUU) equipped with an Rh tube as the X-ray source was used.

Both techniques demonstrated perfect performance in determining the elemental and molecular composition of lava tubes. XRF did not present any inconvenience from the analysis point of view. The major elements found in all tubes regardless of age were Si, Fe, Ca, Al and, to a lesser extent, Mg. Si, Fe and Mg did not show significant differences in their relative presence from cave to cave. In contrast, Al varied by cave, with a higher relative presence of this element in older caves.  The presence of Ca varied significantly depending on the salt crust precipitation processes present in each tube. It should be noted that in the tube analyzed from the last eruption, the determined presence of Ca was lower. In general, calcium presented a significant correlation with sulfur, and to a lesser extent with P, in the tubes that presented greater precipitation of salt crusts. However, in certain caves where precipitation processes were not detected, S was not determined, being below the LOD of the technique. Thus, S is related to fumarolic processes that give rise to the precipitation of sulfate-rich salts.

The main elemental differences were observed in the minority elements. It is worth mentioning K, an element detected as a minority in the oldest caves and, on the other hand, as a majority in the most recent caves, showing a behaviour totally contrary to that of Al. Other elements that showed cave-related variability were Br, Sr, Ni, Zn and Mn.

Regarding Raman spectroscopy, considering the fluorescence and vitreous character of the samples, more difficulties were encountered in obtaining good signals from the igneous rock matrix. Some iron oxides and carbonates were detected unequivocally.

However, this technique was very useful to characterize the salt crusts and crystalline structures formed on the walls of the lava tunnels. Using low integration times, the SNR obtained was sufficient to discern between different polymorphs and accurately determine the heterogeneous composition of these structures.

This spectroscopic analysis campaign demonstrates the ability of Raman and XRF to determine the geochemistry of lava tunnels. This fact is of great relevance considering the potential of these structures for future Martian and lunar exploration missions.

Acknowledgements

This work has been supported through the PAMMAT project “Alteration processes in Mars and Moon Meteorites, and Terrestrial Analogues at different environments: Mars2020, Rosalind Franklin and Returned Samples from Mars and Moon” (Grant No. PID2022-142750OB-I00), funded by the Spanish Agency for Research (MICIU/AEI/10.13039/501100011033/FEDER/UE).