The transition from peraluminous to peralkaline granitic melts: Evidence from melt inclusions and accessory minerals

Abstract

Fractional crystallization of peraluminous F-and H 2 O-rich granite magmas progressively enriches the remaining melt with volatiles. We show that, at saturation, the melt may separate into two immiscible conjugate melt fractions, one of the fractions shows increasing peraluminosity and the other increasing peralkalinity. These melt fractions also fractionate the incompatible elements to significantly different degrees. Coexisting melt fractions have differing chemical and physical properties and, due to their high density and viscosity contrasts, they will tend to separate readily from each other. Once separated, each melt fraction evolves independently in response to changing T/P/X conditions and further immiscibility events may occur, each generating its own conjugate pair of melt fractions. The strongly peralkaline melt fractions in particular are very reactive and commonly react until equilibrium is attained. Consequently, the peralkaline melt fraction is commonly preserved only in the isolated melt and mineral inclusions.We demonstrate that the differences between melt fractions that can be seen most clearly in differing melt inclusion compositions are also visible in the composition of the resulting ore-forming and accessory minerals, and are visible on scales from a few micrometers to hundreds of meters.

Solubility of tin in (Cl, F)-bearing aqueous fluids at 700°C, 140MPa: A LA-ICP-MS study on synthetic fluid inclusions

“…A potential mechanism for Sn extraction at the final stages of magma crystallization is through alkaline, F-bearing melts similar to those observed in our NaFbearing experiments. Evidence for the existence of such melts was found in several granites and pegmatites around the world (Thomas et al, 2006).…”

“…Most of these inclusions are highly peraluminous (ASI = 1.2-1.3). However, recent evidence from the Erzgebirge occurrences and other granites in the world suggests that a second, even more hydrous melt of peralkaline character (ASI = 0.5-0.6) was coexisting with the peraluminous melt at high degrees of melt fractionation (Thomas et al, 2005(Thomas et al, , 2006. Synchrotron-XRF analyses on corresponding melt inclusions revealed that the peralkaline melt contained about twice as much Sn than the coexisting peraluminous melt (Thomas et al, 2005(Thomas et al, , 2006, demonstrating that a significant amount of Sn was transported by a highly peralkaline phase at the very final stage of magma crystallization.…”

The missing link between granites and granitic pegmatites

“…(2002) demonstrated an unusually high solubility of the aluminosilicate components in the fluid at high temperatures: at 830 °C there is about 26 melt % (g/g) dissolved in the fluid (fluid density 0.5 and melt density 2.0 g/cm 3 respectively) -see also fig. 7 in Thomas et al (2006). The viscosity is very low, as shown by the rapid movement of small particles, and the diffusivity of all components is very high, indicated by the very rapid adjustment of the melt volumes with temperature changes.…”

Water- and Boron-Rich Melt Inclusions in Quartz from the Malkhan Pegmatite, Transbaikalia, Russia

“…To obtain the necessary information the experiments are performed at different run temperatures within the expected solvus boundaries (see [13,21,25]). To prevent decrepitation the experiments must be performed at higher pressures.…”

“…As a rule the samples contain two different melt inclusion types, one more peraluminous and less water-rich melt (type-A) and the second more peralkaline and very water-rich melt (type-B) using the nomenclature of Thomas et al [25] and Thomas and Davidson [8]. In melting experiment at the lowest temperature homogenized inclusions of both these types represent the extremes with respect to total water concentration, and Raman spectroscopy is used to quantify that amount.…”