Orlando hit the protruding outcrop with his hammer for the final time. Then, he lifted the dislodged rock fragment under the burning July sun and carefully observed the unweathered surface. Pointing to a network of fine dark veins, carved on a green glassy background, he exclaimed:
“It’s partially serpentinized!”
It was almost six in the evening. While it was still bright, there was a long drive ahead and we didn’t want to be late home. We needed to sort the samples, discuss our notes, plan for the next day, and cook something for dinner. For me and my colleague, it was the third day in the field on Monte Maggiore, a mountain on the northernmost edge of Corsica. I have sometimes often been explaining the location of our field sites to my friends by giving a thumbs up and pointing at the tip of the thumb: “Right at the edge of Corsica”.
These thin blue veins we were looking for will help us to study and reconstruct if, how, and when these rocks have chemically reacted with deep fluids to drastically change their nature, in the process generating abiotic hydrogen and methane. This particular process is known to geologists as serpentinization. As a result of such a process, where water interacts with some particular rocks, serpentinites are formed: dark greenish rocks containing the mineral serpentine that get their name for the resemblance to the touch and color of the skin of snakes (serpens, in Latin).
Our goal during these field days was to gather samples in the 4 km² area of the massif, along north-south transects. We moved towards the coast, descending the mountain from its summit, following the contact separating the serpentinites from older continental sedimentary rocks. In simple words, deep serpentinites have been pushed up onto the Earth’s surface, sliding on top of other rocks during the closing of an ancient ocean, the Tethys during the uplift of the Alps. The surface where serpentinites slide over other rocks is known as a tectonic discontinuity.
Every ~200 meters we stopped and collected a representative rock sample of the area. We would describe the rock’s mineralogical composition and structural dispositions. A name was given to the sample, and the rock was then stored with our increasingly heavier backpacks. Walking between sample sites, we would argue about our interpretations and ideas concerning the geologic mystery laying under our feet.
The sample we gathered at the beginning of this story was partly involved in the hydration process, preserving some of the minerals in initial rock before it was serpentinized. Moving away from the summit where the discontinuity is, we observed a progressive decrease in serpentine content within the rocks, suggesting that the process of serpentinization gradually lost intensity away from the geologic surface. Finding these types of structures enables us to study the incipient stages of the serpentinization process, reconstructing the chronological evolution of serpentinization events.
At the end of the campaign, which lasted from the 22nd to 27th July 2022, Orlando and I proceeded to study the collected samples in the lab with the use of an optical microscope and further methods of chemical analysis.
Together, these small puzzle pieces will help us to understand in detail the history of this locality, with possible further implications for understanding the deep Earth cycling of hydrogen and methane.
Francesco Ressico
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