GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 9-5
Presentation Time: 10:25 AM


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

Earth is unique among planets having a bimodal crust. Warren Hamilton wrote many influential papers on batholiths and subduction zones, connecting water to formation of silicic continents. For 60 years, granitoid batholiths have been assumed to reflect emplacement of >700°C melt, at or above the solidus of haplogranite. Yet new observations imply igneous systems contain melt at ~500°C. Here I present new experiments that show that a low temperature melt (LTM) with 40 wt% H2O (+60% alkali di-silicate) co-exists with quartz and 2 feldspars down to 330°C at 0.1 GPa. The viscosity of this melt will be 107x lower than water saturated rhyolitic melt at 700°C and have a density ~66% of average crustal rock. Thus, this melt, once formed, will buoyantly rise through the crust, playing a major role in several Earth processes. With 30-40% H2O, LTM upwardly advecting by porous flow (φ<5%) at cm-m/yr rates will control crustal heat flow in magmatic settings. In the ocean crust, deep circulation of this melt near the MOHO mines heat from the lower crustal gabbro body explaining non-conductive heat flow patterns of ocean lithosphere. This process would produce the ubiquitous greenschist alteration of the ocean crust—I suggest long term CO2 regulation and climate control of Earth may reflect this igneous process. In the continental setting, the production of 35-50 km thick crust with granitoid-like seismic velocities is consistent with trans-crustal reactive flow of LTM homogenizing and silicifying the many plutons making up a convergent margin batholith. Observed cooling profiles of such plutons show rapid cooling from 900 to 550°C but prolonged cooling (10 Myr) below 350°C. Heat flow models show that incremental emplacement of sills to build plutons results in this same rapid cooling to 550 followed by extended durations at 350-550°C, consistent with the presence of LTM. I suggest that differentiation by upflow of LTM is effectively marked by the Daly Gap on Harker diagrams with silica variations from granodiorite to granite basically reflecting down temperature precipitation of quartz by LTM. The steady upflow of LTM through the convergent margin crust provides a mechanism for forming monstrous Cu porphyries and rapidly delivering water to upper crust magma chambers, fueling explosive volcanic eruptions.