Sm-Nd, Sr, C and O isotope systematics in hydrothermal calcite-fluorite veins: Implications for fluid-rock reaction and geochronology

摘要

The Sm-147-Nd-143 isotope system may be used to determine the age of hydrothermal vein growth for single veins, or sets of veins, providing that the minerals in these veins formed from a hydrothermal fluid with initially homogeneous Nd-143/Nd-144. In this study, we determined the Sm-147-Nd-143, Sr-87/Sr-86, delta C-13 and delta O-18 isotopic composition of 9 calcite and fluorite veins (comprising 12 samples in total) that form part of a low-temperature (100-200 degrees C) fault and fracture hosted hydrothermal system, in the eastern Lachlan Fold Belt, Australia. The vein swarm is hosted by mid-Devonian limestones, which were folded around 350-325 Ma, with vein growth occurring during folding. The veins chosen for analysis yield homogeneous Sr isotopic compositions (Sr-87/Sr-86 = 0.70851 +/- 0.00003) and delta O-18 isotope values (22.5 +/- 0.1 parts per thousand), which are similar to those of the host limestones. The homogeneity of Sr and O isotope compositions is attributed to the progressive reaction of fluids with host rock, with sufficient reaction occurring along discrete fluid flow pathways to fully equilibrate the fluids for these isotope systems. By contrast, Nd-143/Nd-144 isotope compositions for the same samples show more variation (Nd-143/Nd-144=0.51259-0.51199), which significantly exceeds analytical error. In addition, calcite and fluorite from within the same vein have distinctly different age-corrected Nd isotope compositions. Accordingly, it is likely that fluorite and calcite precipitated from different fluids. This study indicates that apparently synchronous vein-forming fluids, as suggested by structural relationships, may have diverse isotopic characteristics even over relatively small scales, reflecting variable equilibration of the fluids with the host-rocks. Incomplete equilibration or mixing, resulting in a range of initial Nd-143/Nd-144 compositions, is likely to be a common characteristic of fracture-hosted hydrothermal systems, compromising attempts to determine ages using Sm-Nd isochrons. The formation of veins with essentially identical Sr and O isotope compositions, yet diverse initial Nd-143/Nd-144 ratios, means that Sr and O isotopes cannot be used to identify suitable samples for Sm-Nd geochronological studies.