EEPS Colloquium: Craig Lundstrom

How low can you go: the case for Igneous processes to 300°C with implications for continental crust formation

Earth’s bimodal topography is unique in the solar system with silica-rich continental crust forming by the igneous process of granitic magmatism. For 60 years, a paradigm of igneous petrology is that granites form by injection of >700°C magmas into the crust. But major questions remain about magma differentiation and emplacement given geochronology and geophysical observations. Coming from both experimental and observational perspectives, I will present a view that “every granite was once a gabbro.” First, experiments show that liquids in equilibrium with a granitic assemblage exist from the “solidus” down temperature to a sodium-rich liquid with ~40 wt% H2O at 330°C—I suggest these liquids play an important role in magmatic evolution, facilitating reactions with gabbros and diorites. Next I will present 2 field studies from granitic plutons having mafic complexes providing evidence for granite’s prior life as gabbro: plagioclase crystals in granites ubiquitously have Sr isotope ratios of the nearby mafic rocks, with these ratios being much lower than the whole rock granite ratio.  In summary, granitic intrusions grow by a top-down sill emplacement process whereby differentiation occurs in situ; upflow of the water-rich late-stage liquids further refines the final rock that we see.

Host: Mike Krawczynski

Colloquium made possible by the William C. Ferguson Fund.

Note: Student Awards will be announced at the beginning of this week's colloquium.