Lower crustal magma supply amplification: A key concept for silicic magma production

Abstract:

The formation of large (>1000 km³) eruptible silicic magma bodies requires accumulation of magma in the upper crust for ≥10⁵ years. Critical information on how this occurs comes from isotope geochemical studies of magmatic systems, models for magma generation in the mantle, energy balance considerations for magma being stored in the crust, and the properties of wallrock surrounding magma chambers (e.g. Jellinek and DePaolo, JVGR, 2003). The single most important parameter is the rate and duration of magma supply to mid-upper crustal levels. This magma supply can be amplified by melting in the lower parts of thick continental crust and is critical for producing large silicic systems in subduction settings. Lower crustal magma flux amplification not only increases the supply of magma to the upper crust, but also increases the average SiO₂ content of produced magma. The amplification factor can be estimated as A_M (= 1 + a/r_ss) (see Perry et al., GSAB, 1993) and is a function of crustal temperature. Lower crustal temperatures of 700°C to 900°C result in A_M of 2 to >10. The SiO₂ amplification factor is larger; it is approximated by: A_Si (= 1 + K_Si (a/r_ss)), where K_Si is the ratio of SiO₂ in the assimilated crust versus mantle magma (about 1.3; e.g. 64% versus 49%). The effect of crystallization on SiO₂ can be modeled using solid/liquid D_Si = 0.80 ±0.05. Studies of large volume systems suggest that rhyolite production rates are 1 to 3 km³/1000 yr (e.g. White et al., G³, 2006), similar to the production rate of basaltic magma in subduction zones. Lower crustal amplification explains how the production rate of rhyolite can match or exceed the supply of basaltic magma from the mantle. Rates of generation of granitic batholiths in subduction zone environments also match the generation rate of basaltic magma (Gehrels et al., GSAB, 2009). Data from Western U.S. and central Andean ignimbrites (Perry et al., GSAB, 1993; Kay et al., JVGR,2010) illustrate that the formation of large volume rhyolite systems is favored when the silica amplification factor A_Si ≥ 2, which generally corresponds to crustal thickness of 45 to 70 km. In more mafic crust, higher temperatures are needed, and A_Si ≈ A_M. In oceanic arcs or those built on thin crust, silicic ignimbrite volumes may be limited to 100 km³, in the absence of pulses of high mantle magma production.