ISOTOPIC INSIGHTS INTO THE PLUTONIC-VOLCANIC RELATIONSHIP WITH PROPOSAL OF A NEW ERUPTION MECHANISM
This work covers 2 themes: in theme 1 ) we present new non-traditional isotope ratio data (Fe, Si) for plutonic and volcanic rock differentiation suites (Cedar Butte volcano (ID) Finland granophyre (MN)). In both suites, d56Fe forms upwardly curving trends with increasing silica whereas d30Si increases linearly with SiO2. These identical behaviors with differentiation affirms that silicic plutonic and volcanic rocks reflect the same magma differentiation process. While the origin of these isotopic variations remains debated, the observations are consistent with prediction of a top-down thermal migration zone refining (TMZR) process.1
Assuming that a granitic mush is built top down by TMZR, theme 2) proposes that eruption of 100% mush occurs when an instability is triggered by immiscibility between peralkaline melt and water at the base of the mush. Experiments show that wet andesite evolves to granite at the cold (400°C) end of a thermal gradient2 with hydrous peralkaline melt existing interstitially. Previous work3 shows that such melt is retrograde immiscible, segregating into a water-rich melt and water poor-melt as temperature is raised. As the mush builds downward, it crosses the immiscibility boundary triggering release of a water-rich phase from the entire granitic mush. The ascending water rich melt dissolves the granite--hydrous peralkaline melt (25mg silicate + 25mg H2O) dissolves >35mg of granite at 600°C--producing aphyric ignimbrite. Importantly this scenario avoids the problem of too little latent heat to melt a mush.This model while speculative has critical hazards implications since it suggests that calderas with no seismically visible blobs of melt could generate large eruptive volumes in relatively short timescales.
1) Lundstrom GCA 73, 2009; 2) Huang et al. GCA 73,2009; 3) Friedman and Tuttle, JACS 70, 919, 1948