Experimental investigation of rates and mechanisms of isotope exchange (O, H) between volcanic ash and isotopically-labeled water

Abstract

The hydrogen and oxygen isotope ratios in hydrous minerals and volcanic glass are routinely used as paleo-proxies to infer the isotopic values of meteoric waters and thus paleo-climatic conditions. We report a series of long-term exposure experiments of distal 7700BP Mt. Mazama ash (−149‰ δ2H, +7‰ δ18O, 3.8 wt.% H2O) with isotopically-labeled water (+650‰ δ2H, +56‰ δ18O). Experiments were done at 70, 40 and 20°C, and ranged in duration from 1 to 14454h (∼20months), to evaluate the rates of deuterium and 18O exchange, and investigate the relative role of exchange and diffusion. We also investigate the effect of drying on H2Otot and δ2H in native and reacted ash that can be used in defining the protocols for natural sample preparation. We employ Thermal Conversion Elemental Analyzer (TCEA) mass spectrometry, thermogravimetric analysis and a KBr pellet technique with infrared spectroscopy to measure the evolution of δ2H, total water, and OH water peaks in the course of exposure experiments, and in varying lengths of vacuum drying.Time series experiments aided by infrared measurements demonstrate the following new results: (i) It wasobserved that from 5 to >100‰ δ2H increases with time, with faster deuterium exchange at higher temperatures. Times at 15% of theoretical “full δ2H exchange” are: 15.8years at 20°C, 5.2years at 40°C, and 0.4years at 70°C. (ii) Even at extended exposure durations experiments show no net increase in water weight percent nor in δ18O in ash; water released from ash rapidly by thermal decomposition is not enriched in δ18O. This observation clearly suggests that it is hydrogen exchange, and not water addition or oxygen exchange that characterizes the process. (iii) Our time series drying, Fourier transform infrared (FTIR)-KBr and Thermogravimetric Analyzer (TGA) analyses collectively suggest a simple mechanistic view that there are three kinds of “water” in ash: water (mostly H2O) that is less strongly bonded on the surface of ash particles that are getting lost with 24–48h of drying to up to 200–300°C, bound water in glass in a range of combining proportions of SiOH to H2O, and a small reservoir of residual, tightly held water.Experimentation with vacuum drying at 130–220°C, and with TGA up to 1000°C provides a set of simple relationships and recommendations for users. Ash loses 1–1.2wt.% water with weight loss stabilizing after 48h of vacuum drying at 130°C. This ash drying removes molecular water over the hydroxyl group in a proportion of ∼80:20% resulting in relatively constant δ2H values of the remaining total water in native ash.This study demonstrates that δ2H in ash can be rapidly changed by minor diagenesis even at relatively low temperatures of 20°C. A diagenetic history of ash is needed to interpret the D/H ratio, but the δ18O values of water in ash are more robust.