Brucite-driven serpentinite carbonation at Montecastelli (Tuscany, Italy)

Tuscany (Italy) is characterized by numerous, large outcrops of ophiolites, i.e. serpentinites, gabbros and basalts, representing remnants of the Jurassic Ligurian Tethys. At Montecastelli (Tuscany), spontaneous CO2 mineral sequestration is an ongoing process locally affecting serpentinites along fractures and at surface. Here, the ophiolite outcrops are characterized by serpentinized harzburgites hosting discrete brucite-rich serpentinized dunite bodies. Mineralogy and texture of the serpentinized dunites indicate two-stage oceanic serpentinization: an earlier hydration, leading to the formation of a Fe-rich brucite, serpentine and minor magnetite arranged in a typical pseudomorphic mesh texture (type-1a dunite), and a late veining producing serpentine, magnetite and brucite overprinting the previous texture (type-1b dunite). Carbonation of serpentinites is strongly catalyzed by the presence of brucite-rich lithotypes. Serpentinized dunites are pervasively affected by carbonation in form of: i) replacement of Fe-rich brucite by Mg-Fe LDHs (pyroaurite-coalingite) in mesh cores and rims (type 1a) and replacement of brucite by hydrous Mg carbonates (hydromagnesite with minor nesquehonite) in veins (type 1b); ii) precipitation of hydrous Mg carbonates ± Mg-Fe LDH ± aragonite in fractures and at external rock surfaces (type 1a and 1b). By contrast, the brucite-free serpentinized harzburgites are not significantly affected by carbonation. Coatings and crusts of hydrous Mg carbonates ± aragonite can form on free surfaces of this rock around clasts but they never reach the pervasive effect observed in brucite-rich serpentinized dunites. We argue, based on all our evidences, that the infiltrated meteoric water produces the dissolution of the Fe-rich brucite and brucite in the dunitic rocks, with consequentely precipitation of Fe-rich LDH and hydromagnesite. The modified rainwaters after the pervasive interaction with the serpentinized dunites - confirmed by their high pH (8.5), high Mg content (55 mg/l) and low Si content - discharge out from the outcrops, precipitating a late hydromagnesite coating at the rock (both dunite and harzburgite) surfaces.