Abstract
Banded Ca-carbonate veins in travertine deposits are efficient recorders of the compositional fluctuations of geothermal fluids flowing (or flowed) from deep reservoirs up to the surface, within fault zones. In this view, these veins represent key tools for decoding those factors that influenced the geochemical variations. We have analyzed veins developed in fractures channeling geothermal fluids forming travertine deposits. The studied veins cut a fossil travertine fissure ridge, near the Larderello geothermal area (Iano area, southern Tuscany) where geothermal fluid circulation is favored by NE-trending strike-to-oblique-slip faults and their intersections with NW-trending normal ones. U-Th dating indicates that fluid circulation occurred from (at least) 172 ka to 21 ka. In this time span, the geothermal fluid changed in composition, and the banded Ca-carbonate veins recorded these variations in terms of mineralogical and stable isotope composition and temperature ( T ) of deposition. We also documented for the first time the occurrence of Mn-rich black tree-shaped structures within the veins. Mineralogy coupled with stable and clumped isotope measurements allows the reconstruction of some features (i.e., crystal texture, temperature, and CO2 origin) and the inference of the processes (i.e., pH, T, and pCO2 variations) that have controlled the fluid evolution through time. Multiple-stage and one-stage deposition processes have played an important role in modifying the stable isotope composition of banded Ca-carbonate veins; temperature coupled with pCO2 also influenced their mineralogical composition. Interpreted in the context of the tectonic setting, the data show that the NW-trending faults have mainly controlled travertine deposition. Their intersection with NE-trending faults, interpreted as transfer faults, highlights the important role of transfer zones in channeling the geothermal fluids.
Highlights
Faults and associated fractures are discontinuities of the upper crust able to control the circulation of geothermal fluids, favoring combined convective-advective heat propagation processes [1]
If geothermal fluids are stored in carbonate reservoirs, or interact with carbonate rocks, fluid composition is enriched in Ca2+ and CO3- while the saline content depends on the amount of dissolved CO2, fluid temperature, and pH [2,3,4,5,6,7,8]
Travertine deposits can act as tectonic markers in geothermal areas [16,17,18,19,20] and potentially provide key geochronologic constraints since carbonate deposits can be dated using different radiometric methods [7, 13, 15, 21]
Summary
Faults and associated fractures are discontinuities of the upper crust able to control the circulation of geothermal fluids, favoring combined convective-advective heat propagation processes [1]. If fluids reach the surface through permeable faults and their damaged volumes, thermal springs develop [9], depositing masses of CaCO3 (i.e., travertine deposits; [5]) mainly due to the CO2 leakage [5, 8, 10, 11] In these settings, the rise of geothermal fluids and travertine deposition are assumed to be contemporaneous with the activity of faults channeling the fluids [3, 12,13,14,15]. A fissure ridge-type travertine deposit consists of a ridge made of bedded travertine, from gently to steeply dipping away from the apical part, where the central fissure is located, corresponding to the fault trace intersecting the substratum From such a fissure, the geothermal fluids flow out, and travertine deposition takes place, growing the ridge
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