Tracing mineral replacement using Raman spectroscopy and O-isotopes

Mineral replacement reactions are an important component of element redistribution within the crust and industrial processes such as building material preservation. Recent experimental work has demonstrated that Raman spectroscopy combined with O-isotope tracers can allow us to probe the mechanism and timescales of such reactions. In this presentation we will review the recent advances in this area and their implications for understanding mineral replacement reactions. Raman spectroscopy provides information about vibrating groups of atoms within materials and can be used to examine amorphous and crystalline solids, fluids and gases. For solid materials it can also provide information about crystal orientation and crystallinity, therefore Raman spectroscopy is a powerful tool in Geosciences research. Recent experimental work has utilized the sensitivity of Raman spectroscopy to changes in atomic weight, e.g., different isotopes, to examine the uptake of ¹⁸O in aqueous oxyanions and solid material [1]. For example, uptake of ¹⁸O from enriched water has been shown to occur during silicate mineral replacement by amorphous silica [2,3], indicating an interface-coupled dissolution-reprecipitation mechanism governs this reaction. In addition, differences in the extent of ¹⁸O uptake may also be useful to examine how minerals with different silica networks are broken down. Exchange of O atoms between oxyanions in solution and enriched water is a kinetically controlled process [4], resulting in differently enriched oxyanions becoming incorporated into the solid phases of the replacement rim that have a time dependent signature. Mapping of mineral replacement rims from experiments conducted with ¹⁸O-enriched water can therefore provide information about reaction mechanisms, the reaction speed and the relationship to different fluid penetration pathways. By identifying phases and examining crystal orientation, crystallinity and O-isotope incorporation using Raman spectroscopy a wealth of information can be gained about the processes controlling mineral replacement reactions [e.g., 5].