Istituto di Geoscienze e Georisorse     
Geloni C., Gianelli G. A preliminary geochemical model of the fluid-rock interaction processes forming the eastern Liguria ophicalcite (northern Apennines, Italy),. In: Periodico di Mineralogia, vol. 76 (2) pp. 137 - 154. Dipartimento di Scienze della Terra, UniversitÓ degli Studi di Roma "La Sapienza", 2007.
The Northern Apennine ophicalcites are likely the Jurassic equivalent of the present-day Lost City and Rainbow submarine hydrothermal systems, where complex tectonic, sedimentary and hydrogeochemical processes occur. The aim of this research work is to simulate the interaction of a peridotite of the Northern Apennines ophiolites with a reconstructed Jurassic seawater, and to verify whether the ophicalcite formation is enhanced or hampered. The Jurassic seawater has been reconstructed on the basis of literature data. The seawater during the Lower Jurassic was characterised by higher Ca/Mg ratios, lower SO4 and higher HCO3 contents, when compared with the present-day composition. The simulations have been conducted using the program REACT of the Geochemist Work Bench package, in a temperature range of 25-200 ░C, and TOUGHREACT, at 25░ and 100 ░C for reactive flow modelling. The thermodynamic modelling results indicate that the alteration products are antigorite, brucite, chlorite, carbonates and andradite, whereas anhydrite does not reach conditions of oversaturation, and, therefore, precipitation. The indication is that the Jurassic seawater enhances the formation of calcite and does not allow a significant precipitation of anhydrite, which is formed when the present seawater composition is used. The occurrence of andradite, even at low temperature, is in agreement with the petrographic data of the Ligurian ophiolite. This opens the possibility of an ocean-floor metamorphism origin also for the garnet phase. Reactive flow modelling indicates that significant amount of calcite (up to approximately 15 vol%) can precipitate in a fractured serpentinized peridotite at low temperatures, compatible with the conditions of the uppermost part of the Levanto Breccia, close to a deep sea sedimentary environment and therefore permeated by cold waters.
URL: http://periodicodimineralogia.it/index.php/mineralogia/index
Subject ophiolite
northern Apennines
peridotite carbonation
reactive flow modelling

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