Calcium isotopic fractionation during travertine deposition under different hydrodynamic conditions: Examples from Baishuitai (Yunnan, SW China)

摘要

Revealing the potential of Ca isotopes in the research of travertine formation processes is the principal goal of this sludy. To achieve this, the hydrochemistry, travertine precipitation rales, and variations in Ca isolopic compositions (delta Ca-44/40) of present-day endogenic (Lhermogenic) travertine in two differem hydrodynamic systems (canal and pool) are studied at Baishuilai (Yunnan, SW China). In the canal, where the travertine precipitation is faster, delta(44/40) Ca values of both solution and travertine show a downstream increase, with Ca2+ isotopic fractionation (Delta(44/40) CaCaCo3-aq) equal to similar to-1.6 parts per thousand. Compared to the canal, Ca isotopic fractionation between travertine and aqueous Cat is smaller (Delta Ca-44/40(CaCO3-aq) approximate to -1 2 parts per thousand) in the pools where travertine precipitation rates are lower.The spatial variations in delta Ca-44/40 values of solution and travertine are related to the amount of Ca removed from the solution when delta Ca-44/40 values of spring water remain stable. In addition, the results confirm the control of precipitation rates on Ca isotopic fractionation between calcite and parent aqueous Ca2+ using natural samples. The observed negative correlation between Delta Ca-44/40(CaCO3-aq) and travertine precipitation rates can be explained by the steady-state kinetic surface reaction model proposed by DePaolo (2011), with equilibrium and kinetic fractionation factors of alpha(eq) = 1.0000 +/- 0.0001 and alpha(f) = 0.9983 +/- 0.0002, respectively. An important consequence of this study is that Ca isotopes in travertine systems are good tools to have access to variations of delta Ca-44/40(aq) which are directly linked to the amount of precipitated Ca. This could then be useful for reconstructing past hydrodynamic conditions when applied to travertine core data. (C) 2016 Elsevier B.V. All rights reserved.