GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 276-7
Presentation Time: 3:25 PM


LUNDSTROM, Craig C., Dept of Geology, Univ of Illinois, 1301 W Green St, Urbana, IL 61801

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.