Water-Rich Melt Inclusion as "Frozen" Samples of the Supercritical State in Granites and Pegmatites Reveal Extreme Element Enrichment Resulting Under Non-Equilibrium Conditions

Abstract

In this contribution, we show that in miarolitic pegmatites during the crystallization of water-rich melts, samples of these mineral-forming melts were trapped in the form of water-rich melt inclusions, preserved primarily in quartz. The bulk concentration of water and the temperature are the system-determining parameters since from their analysis it follows that these melt inclusions depict pseudo-binary solvus curves in the coordinates of temperature and water concentration. Furthermore, using reduced coordinates (H2O/H2Ocrit vs. T/Tcrit) most melt inclusions of the studied pegmatites plot very well in a standardized and reduced solvus curve. The existence and formation of such uniform solvus curves is an expression of crystallization processes under nearly equilibrium conditions. However, many trace and some principal elements of the melt inclusions trapped near the solvus crest [H2O/H2Ocrit from 0.5 to 1.5 and T/Tcrit > 0.95] show unusual distributions, with very well-defined Gaussian and/or Lorentzian curves, characterized by defined area, width, offset, and height. This has been shown in many natural examples obtained from pegmatites. Only the offset values represent near-equilibrium conditions and corresponding element concentrations, which are equivalent to the regional Clarke number (Clarke number or Clark is the relative abundance of a chemical element, typically in the Earth's crust). We interpret these distributions as explanation for some extraordinary-chemical properties in this critical region: principally extremely high diffusion rates, low dynamic viscosity and extremely low surface tension. Near the critical point, we have both space and time-related non-equilibrium and equilibrium processes close together. Furthermore, we can show that the Gaussian and Lorentzian distribution are first approximations of the specific element distribution because at the critical point the enrichment of some elements reaches such an extent that the Gaussian and/or Lorentzian curves degenerate into a vertical line (are asymptotic to the concentration axis), which is determined by the maximum solubility of a species in the supercritical melt-water system. The highest concentration of Be, as an example, was observed in Ehrenfriedersdorf melt inclusions: 71490 ppm Be.