CONTINUOUSLY CHANGING COMPOSITION OF QUARTZ-ALBITE SATURATED MELTS FROM 819 TO 330°C IN THE NA2O-AL2O3-SIO2-H2O SYSTEM AT 0.1 GPA

The haplogranite solidus established in 1958 [1], ~650°C at 0.1 GPa water saturated pressure (minimum melt point-MMP), is typically taken as the lowest temperature that melt exists in silicic magmatic systems. Yet 1950s experiments by [1,2] also showed that hydrous peralkaline melt co-existed with quartz and feldspar at temperatures as low as 300°C. Indeed, peralkaline melt with 40 wt% H₂O coexists with quartz and 2 feldspars at 400°C [3]. Do melt compositions continuously change from the MMP to temperatures <400°C?

I have performed cold seal experiments at 0.1 GPa in which a hydrated Na₂Si₂O₅ starting powder is reacted with quartz and albite at ~70°C intervals from 330° up to the qtz-ab eutectic at 819°C. Quenching melt to glass, measured compositions continuously change from rhyolitic liquids at the eutectic to a melt near Na₂Si₂O₅ composition with very small amounts of Al₂O₃ (on anhydrous basis) at 330°C. Similar sized fixed additions of water to each capsule result in a noticeable appearance of water saturation (bubbles) in experiments >600°C but vapor undersaturation below—in the <600°C runs, calculated melt water contents range from 18 to 39 wt% H₂O. Like previous work [1,2,3] water contents of these low temperature melts approach hydrothermal solutions, providing a possible answer to the question of what constitutes the magmatic-hydrothermal transition.

Are these melts relevant to the igneous world? Perhaps. Andesite with 4 wt% H₂O placed into an imposed thermal gradient evolves to form a microcrystalline metaluminous granite at <400°C [4]. The thermal gradient leads to formation of interstitial peralkaline melt that allows rapid reaction and transport. Thermal gradients are ubiquitous in upper crustal magma systems, such that low temperature melts may be prevalent as <5% interstitial melt within a granitic assemblage. At minimum, an equilibrium low temperature melt coexisting with a granitic assemblage provides a way to keep water in a magmatic system under cold storage conditions. A less conservative interpretation is that igneous differentiation to silicic compositions involves a thermal gradient mush process with melt at 500-550°C.

1. Tuttle and Bowen, 1958; 2. Friedman, 1951; 3. Lundstrom, 2016; 4. Huang et al., 2009.