During crystallization of magmas, the 87Sr/86Sr ratio in residual liquids may undergo appreciable changes, due to 87Rb self-decay and/or assimilation of materials of different Sr isotopic composition. In a liquid evolving by fractional crystallization (FC) the increase in the 87Sr/86Sr ratio depends on the Rb/Sr ratio of the liquid itself, the time-span required for its crystallization, and the bulk partition coefficients of Rb and Sr for the crystallizing mineral assemblage. Magmatic liquids with high Rb/Sr ratios and/or long crystallization time-spans may show considerable changes. Magmatic liquids may moreover significantly change their Sr isotopic composition when crystallization is accompanied by assimilation of materials characterized by different 87Sr/86Sr ratios (AFC processes). Crystals which grow from FC or AFC-evolving liquids incorporate Sr of progressively different 87Sr/86Sr ratio. The crystal growth process may be thought to occur through progressive addition of very (infinitesimally) thin shells of matter to the crystal. In each shell, Sr is characterized by an 87Sr/86Sr ratio which is the same as that in the liquid at the same instant. If the rate of diffusion of Sr within the crystal (volume diffusion) is lower than the rate of growth of the crystal, isotopic homogeneization within the crystal cannot be maintained and the crystal begins to grow isotopically zoned. The higher the rate of change of the 87Sr/86Sr ratio in the liquid with respect to the rate of crystal growth, the more isotopic zoning is evident. After finite growth, the 87Sr/86Sr ratios of the bulk crystals are integrated values. They may differ from each other and may also be different from the 87Sr/86Sr in the residual liquid, depending on parameters such as the Rb/Sr ratios of the crystals, Sr isotopic evolution in the liquid during crystal growth, and the continuity or otherwise of crystal growth. Since the volume diffusion coefficients of Sr in common crystalline phases are very low, Sr isotopic zoning and disequilibria between crystals and between crystals and residual liquids may be preserved if temperature conditions and time available are not sufficient for diffusive processes to attain isotopic homogeneization within the crystal; they may then be observed in the final rock. In the last thirty years, Sr isotopic disequilibria have been observed between coexisting glass and crystals in high Rb/Sr rhyolitic rocks. In some cases, the geochemical data strongly suggest that the crystals formed from an originary magmatic liquid which is now represented by the associated glass. The fact that mineral-glass Sr isotopic disequilibria are observed in low-temperature/high-Rb/Sr magmatic systems does suggest that changes in the 87Sr/86Sr ratio in liquids are due to 87Rb decay during processes of simple crystallization. This reveals that Sr isotopic disequilibrium is the rule rather than the exception, but it can only be observed at present in particularly favourable systems, in which rapid increases in the 87Sr/86Sr ratio are generated. In these systems, crystal growth over different ranges of the residual liquid fraction and insufficient diffusional exchange are responsible for the observed disequilibria. In the last years, Sr isotopic profiles have been obtained through microdrill sampling of single crystals (sanidine) from high-silica rhyolite lavas. These data allow us to infer information about the continuity of crystalline nucleation and, as the isotopic zoning of the crystals reflects changes in the 87Sr/86Sr ratio which occurred in the liquid, also about the kind of process which generated the change in Sr isotopic composition in the liquid. Nevertheless, the microdrill sampling technique can only be performed on suitable crystals of sufficiently large size. In most cases, the crystals are small and only purified mineral separates may be obtained. It is important, therefore, to calculate the evolution of the 87Sr/86Sr ratio in the mineral separates. The aim of this study is to illustrate a quantitative approach and to identify the information which may be inferred from these isotopic data.
Modelling Sr isotopic evolution in mineral phases growing from magmatic liquids with changing 87Sr/86Sr
Cavazzini G
2001
Abstract
During crystallization of magmas, the 87Sr/86Sr ratio in residual liquids may undergo appreciable changes, due to 87Rb self-decay and/or assimilation of materials of different Sr isotopic composition. In a liquid evolving by fractional crystallization (FC) the increase in the 87Sr/86Sr ratio depends on the Rb/Sr ratio of the liquid itself, the time-span required for its crystallization, and the bulk partition coefficients of Rb and Sr for the crystallizing mineral assemblage. Magmatic liquids with high Rb/Sr ratios and/or long crystallization time-spans may show considerable changes. Magmatic liquids may moreover significantly change their Sr isotopic composition when crystallization is accompanied by assimilation of materials characterized by different 87Sr/86Sr ratios (AFC processes). Crystals which grow from FC or AFC-evolving liquids incorporate Sr of progressively different 87Sr/86Sr ratio. The crystal growth process may be thought to occur through progressive addition of very (infinitesimally) thin shells of matter to the crystal. In each shell, Sr is characterized by an 87Sr/86Sr ratio which is the same as that in the liquid at the same instant. If the rate of diffusion of Sr within the crystal (volume diffusion) is lower than the rate of growth of the crystal, isotopic homogeneization within the crystal cannot be maintained and the crystal begins to grow isotopically zoned. The higher the rate of change of the 87Sr/86Sr ratio in the liquid with respect to the rate of crystal growth, the more isotopic zoning is evident. After finite growth, the 87Sr/86Sr ratios of the bulk crystals are integrated values. They may differ from each other and may also be different from the 87Sr/86Sr in the residual liquid, depending on parameters such as the Rb/Sr ratios of the crystals, Sr isotopic evolution in the liquid during crystal growth, and the continuity or otherwise of crystal growth. Since the volume diffusion coefficients of Sr in common crystalline phases are very low, Sr isotopic zoning and disequilibria between crystals and between crystals and residual liquids may be preserved if temperature conditions and time available are not sufficient for diffusive processes to attain isotopic homogeneization within the crystal; they may then be observed in the final rock. In the last thirty years, Sr isotopic disequilibria have been observed between coexisting glass and crystals in high Rb/Sr rhyolitic rocks. In some cases, the geochemical data strongly suggest that the crystals formed from an originary magmatic liquid which is now represented by the associated glass. The fact that mineral-glass Sr isotopic disequilibria are observed in low-temperature/high-Rb/Sr magmatic systems does suggest that changes in the 87Sr/86Sr ratio in liquids are due to 87Rb decay during processes of simple crystallization. This reveals that Sr isotopic disequilibrium is the rule rather than the exception, but it can only be observed at present in particularly favourable systems, in which rapid increases in the 87Sr/86Sr ratio are generated. In these systems, crystal growth over different ranges of the residual liquid fraction and insufficient diffusional exchange are responsible for the observed disequilibria. In the last years, Sr isotopic profiles have been obtained through microdrill sampling of single crystals (sanidine) from high-silica rhyolite lavas. These data allow us to infer information about the continuity of crystalline nucleation and, as the isotopic zoning of the crystals reflects changes in the 87Sr/86Sr ratio which occurred in the liquid, also about the kind of process which generated the change in Sr isotopic composition in the liquid. Nevertheless, the microdrill sampling technique can only be performed on suitable crystals of sufficiently large size. In most cases, the crystals are small and only purified mineral separates may be obtained. It is important, therefore, to calculate the evolution of the 87Sr/86Sr ratio in the mineral separates. The aim of this study is to illustrate a quantitative approach and to identify the information which may be inferred from these isotopic data.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
