Assessing the origin of Sr and Nd isotopes and (REE+Y) in Middle-Upper Pleistocene travertines from the Acquasanta Terme area (Marche, central Italy) and implications for neotectonics

Pleistocene (bedded) travertines from Acquasanta Terme display ranges of Sr and (REE + Y) contents (462–1576 mg/kg and 250–13,789 ng/g, respectively), which are consistent with an evaporitic rock source. PAAS-normalized (REE + Y) patterns may support the origin of travertine-bearing fluids from marine rocks. Travertines exhibit a very narrow range of δ87Sr values (from −1.9 to −1.84 or 87Sr/86Sr ratios from 0.70782 ± 6 to 0.70786 ± 6), indicating an unchanged rock source throughout the travertine depositional time (from about >350 to 26 ka), likely represented by Burano evaporites (Upper Triassic), as suggested by previous authors. No Sr contribution from magmatic sources has been detected. Travertines also display a very narrow range of ϵNd values (from -9.1 to −7.8 or 143Nd/144Nd ratios from 0.51217 to 0.51224), indicating an unchanged source also for (REE + Y) through time, likely the same one providing Sr. Lastly, chemical and isotopic data of Acquasanta travertines have been compared with those of bedded travertines from Semproniano (Tuscany) of similar age. In particular, δ87Sr values clearly distinguish between the two groups, in agreement with their different rock sources. As a whole, these new geochemical and isotopic data on Acquasanta travertines have provided a deeper insight into the origin of their parent hydrothermal fluids, which, upwelling from significant depth, circulated through an area still affected by major seismic activity. In particular, the application of Nd isotopes has resulted useful, as, unlike Sr isotopes of fluid which tend to re-equilibrate rather quickly with rocks along upwelling pathways, Nd isotopes sluggishly do so, thus better reflecting the fluid source.