Origins of elements building travertine and tufa: new perspectives provided by isotopic and geochemical tracers.

18 pagesInternational audienceFluid/rock interaction represents a major process in the formation of calcitic or aragonitic travertine and tufa (CATT). In most cases, CATT is associated to limestone dissolution somewhere along the hydrogeological pathway. However, a wide array of other substratum (basalts, rhyolites, carbonatites, ultramafics, granites, dolomites, evaporites) can act as potential source of elements involved in the formation of CATT. This study reports on the evaluation of potential geochemical tracers linking CATT to their substratum, and unravelling the origin of elements. A large database was established from available literature data as well as new data acquired in the frame of this study for a set of Modern to Recent CATT (Ligurian ophiolites, Italy; the Chaine des Puys, Limagne graben and Paris Basin, France; Reunion Island, Indian Ocean; Jebel Oust, Tunisia). Four most reliable tracing methods are identified (1) δ13C and δ18O cross-plot allows distinguishing epigean (minδ13C = − 27.2 ‰, maxδ13C = 0.9 ‰, meanδ13C = − 12.3 ‰ for N = 314) from hypogean systems (minδ13C = − 4 ‰, maxδ13C = 11.7 ‰, meanδ13C = − 2.87 ‰ for N = 198). Very low δ13C values (− 4 ‰ associated to negative δ13C values are specifically indicative of an ultramafic source rock. (2) Barium and strontium cross-plot helps to discriminate different groups of source rocks amongst the hypogean CATT: (i) source rocks composed of mixed limestones, evaporites, and dolomites are characterised by low barium ( 400 ppm) contents, (ii) mafic and granitic source rocks are undifferentiated and display similar barium (from 15 to 930 ppm) and high strontium (> 200 ppm) contents, (iii) the carbonatite group is characterised by its exceptional high barium and strontium values. In epigean CATT, a pure limestone source rock usually relates to very low barium and strontium contents ( 50 ppm). (3) Relatively high beryllium content (> 30 ppm) in CATT seems to indicate a pure granitoid source. (4) High chromium concentrations (> 20 ppm) are systematically documented in Modern CATT located on an ultramafic substratum. The definition of diagnostic compositional fields for actively forming or recently formed CATT is influenced by many factors including water composition, water temperature, dissolved gas composition and concentration, biological activity, position in the sedimentary body and early diagenesis, in addition to substratum lithology. However, the results of this study illustrate that, despite these many factors, the combined use of Ba, Sr, Be, Cr, δ13C, and δ18O may be valuable to discriminate the rock lithology prevailing in the hydrogeological or palaeo-hydrogeological reservoir of CATT