Experimental hydrothermal alteration of partially metamict zircon

Abstract

We present the results of a series of hydrothermal experiments on grains from two partially metamict zircon samples from Sri Lanka in the temperature range 350 to 650 ∞C and with different solutions (2 M AlCl 3 , 2 M CaCl 2 , pure H 2 O, and a multi-cation solution). Under these conditions, sharply bounded reaction fronts penetrated into the zircon grains and developed complex lobate and rim structures that resemble structures found in natural zircon systems. The reaction zones are characterized by a marked increase in the cathodoluminescence intensity, a decrease of the back-scattered electron emission, and an increased degree of structural order, as revealed by micro-Raman and infrared spectroscopy. Sensitive high-resolution ion microprobe and electron microprobe measurements revealed that the altered areas gained solvent cations (e.g., Ca 2+ , Ba 2+ , Mg 2+ , Al 3+ ) from the solution and lost variable amounts of Zr, Si, Hf, the REE, U, Th as well as radiogenic Pb. A comparison between "dry" and "hydrothermal" annealing trends shows that the kinetics of structural recovery, including recrystallization of the amorphous phase in metamict zircon, is strongly enhanced under hydrothermal conditions. This finding suggests that water "catalyzes" structural recovery processes in metamict zircon. We found that the structure of the reacted areas does not resemble that of crystalline zircon, i.e., is still characterized by a temperature-dependent degree of disorder, which would not be expected if the reaction is controlled by a coupled dissolution and re-precipitation process. Instead, the alteration process can be described best by a diffusion-reaction-recrystallization model. In this model, it is postulated that the diffusion of water into the metamict structure is the driving force for moving recrystallization fronts. We found that the rate and the extent of solid-state recrystallization of the amorphous phase is an important factor in determining the mobility of trace elements. This interpretation is indicated by the observation that trace elements, including U and Th, were preferentially lost during the reaction with a fluid at low temperatures, where recrystallization of the amorphous material was slow or not activated at all. The observed chemical alteration patterns are believed to reflect a competition between the kinetics of long-range diffusion and ion exchange and the kinetics of the short-range diffusion necessary for the recrystallization process.

Refining the chronostratigraphy of the Karoo Basin, South Africa: magnetostratigraphic constraints support an early Permian age for the Ecca Group

“…This temperature interval is consistent with potential Pb-loss, which should not occur in zircons except at low temperatures (Schoene, 2014), such as during low temperature hydrothermal dissolution and reprecipitation (Geisler et al, 2002;Geisler et al, 2003).…”

Zircon texture and chemical composition as a guide to magmatic processes and mixing in a granitic environment and coeval volcanic system

“…It is a disequilibrium texture, manifest by the replacement of U-Th-Y-rich zircon by U-Th-Y-poor zircon, as well as abundant pores, and Th-U-rich micro-inclusions in some cases. Such disequilibrium textures have been described in zircons from metamorphic rocks (Tomaschek et al 2003;Spandler et al 2004) as well as in pegmatites and granite (Xie et al 2005), and have been reproduced experimentally (Geisler et al 2003;Tomaschek 2004). In all these cases, the texture has been attributed to fluid interaction with zircon.…”

Liandratite from Karkonosze pegmatites, Sudetes, Southwestern Poland

“…zircon and pyrochlore supergroup minerals) has been extensively investigated (e.g. Geisler et al 2003;Rayner et al 2005;Bonazzi et al 2006;Dumańska-Słowik et al 2014). In various minerals of the above-named assemblage there are different elements; however, they share a common feature-they all concentrate in the intensively radiation-damaged, amorphous domains outside the crystal structure of the host mineral.…”