HR: 1330h
AN: V12C-0599    [PDF]
TI: Long-term Rates of Mafic Magma Emplacement and Implications for Heat Advection
AU: * White, S M
EM: swhite@geol.sc.edu
AF: Inst. Crust. Studies and Dept. Earth Sci., UCSB, Santa Barbara, CA 93106 United States
AU: * White, S M
EM: swhite@geol.sc.edu
AF: Dept. Geol. Sci., Univ. of South Carolina, Columbia, SC 29208 United States
AU: Spera, F J
EM: spera@geol.ucsb.edu
AF: Inst. Crust. Studies and Dept. Earth Sci., UCSB, Santa Barbara, CA 93106 United States
AU: Crisp, J A
AF: Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109 United States
AB: Rates of magmatism (magma emplacement rate) including both volcanic products and intrusive bodies were obtained for terrestrial petrotectonic systems where reliable volumes can be estimated and geochronological data exist. Approximately 50 estimates of magma emplacement rates have been extracted from the literature published between 1982 and 2003 for persistent basaltic systems with durations from 1 ka to ~5 Ma. Although the volcanic output is highly episodic, the data indicate that the mass output rate at individual hotspot volcanoes is on the order of 10$^{-3}$ km$^{3}$/yr when averaged over several thousand years. This differs from the estimated output rates of large igneous provinces, such as continental flood basalts and oceanic plateaus, which have maximal output rates on the order of 1 km$^{3}$/yr per province. For globally averaged mid-ocean ridges, the total volcanic emplacement rate is only 10$^{-6}$ km$^{3}$/yr/100 km of ridge. Ratios of intrusive to extrusive emplacement are subject to much uncertainty, but generally lie in the range 6:1 to 10:1 for most crustal mafic magma systems. Recent seismic, geodetic, and gravity work suggests that there may be large regions of underplating and storage in subcrustal magma chambers in areas of basaltic volcanism previously not widely considered in intrusive volume estimates that may increase most of these ratios to 10:1. Rates of magmatism may be translated into excess heat flows for specific magmatic provinces to obtain estimates of advected heat transport via magmatism at regional scales over magmatic province timescales. For mafic eruption rate {\it V} and an intrusive/extrusive ratio of {\it R}, the volumetric rate of magma flow into the crust is {\it RV}. The excess heat power (J/yr) associated with magma transport from mantle to crust is {\it RV}$\rho\delta$T [Cp + $\delta$h/(T$_{liquidus}$-T$_{solidus}$)] where $\delta$T is the temperature difference between the magma and host crust, $\delta$h is the enthalpy of crystallization (250-400 kJ/kg dependent on magma composition), $\rho$ is magma density, Cp is the isobaric heat capacity of the magma, and the liquidus to solidus temperature interval is pressure and composition dependant but typically equals 300 K. The excess heat power into the crust due to mafic magmatism is roughly 2e+19 J/yr for a volumetric eruption rate of 1 km$^{3}$/a. As an example, consider the Skye sub-province (area ~1600 km$^{2}$) of the British Tertiary Igneous Province (BTIP). For the estimated volume eruption rate of 2e-3 km$^{3}$/a and R=5 the average excess heat flow is ~3 e+7 J/m$^{2}$a or ~1 W/m$^{2}$. The excess heat flux is a factor of ten greater than the average terrestrial global heat flux 0.09 W/m$^{2}$. The 'excess' heat flux is associated with a crustal thickening rate of ~3 km/Ma in the time interval 60-53 Ma. We conclude that the volume flux of magma in the active years of this part of the BTIP focused heat flow about an order of magnitude above background at the regional scale for ~5 Myr. The regional energy/mass balance estimate is consistent with geochemical modeling of Skye intrusive and volcanic rocks that point to significant magma recharge during the magmatic evolution at Skye.
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
DE: 8450 Planetary volcanism (5480)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting