“Andesitic water”: a phantom of the isotopic evolution of water-silicate systems. Comment on “Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin” by W.F. Giggenbach |
| |
| Authors: | Peter Blattner |
| |
| Institution: | aInstitute of Geological and Nuclear Sciences Ltd, P.O. Box 31312, Lower Hutt, New Zealand |
| |
| Abstract: | On account of the low porosity of the lithosphere, intracrustal fluids behave very differently from surface fluids, in that they are changing their geochemical and isotopic labels according to the geological environment. Given a heat source, meteoric waters can be supplied plentifully and their rates of throughput in geothermal systems are sufficiently high to exhaust the compositional signals of a given rock buffer. In contrast, fluids exsolved from magma, and subducted fluids, would be supplied at less than about one tenth of the meteoric rate over the life time of a system. Based on up-to-date flow models, the isotopic evolution of meteoric water interacting with crustal rock follows a curved to L-shaped track in the δD versus δ18O plot. Instantaneous (present-day) tie-lines between recharge and discharge are secants of such tracks and can have a range of slopes. At the start of an interaction, waters have δD and δ18O values approaching equilibrium with the original rock composition (water “W1”). Using known hydrogen isotope fractionation factors, W1 values generally plot in the region of “andesite” or “andesitic” waters of various authors. Since the W1 waters have δD values that are on average more positive, and also less variable than those of the meteoric recharges, most tracks and tie-lines have positive slopes, and the plotting of a large number of tie-lines will produce a focus on the field of W1 waters, regardless of the original water source. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
