HR: 0800h
AN: T41E-1358    [Abstracts]
TI: Igneous Cooling Rate constraints on the Accretion of the lower Oceanic Crust in Mid-ocean Ridges: Insights from a new Thermo-mechanical Model
AU: * Garrido, C J
EM: carlosg@ugr.es
AF: Dpto. Mineralogia y Petrologia, Universidad de Granada, Facultad de Ciencias, Fuentenueva sn, Granada, 18002 Spain
AU: Machetel, P
EM: Philippe.Machetel@dstu.univ-montp2.fr
AF: ISTEEM, Laboratoire de Tectonophysique, Universite de Montpellier 2, Place Eugene Bataillon s/n, Montpellier, 34095 France
AB: We report the results of a new thermo-mechanical model of crustal flow beneath fast spreading mid-ocean ridges to investigate both the effect of deep, near off-axis hydrothermal convection on the thermal structure of the magma chamber and the role of variable number of melt intrusions on the accretion of the oceanic crust. In our model the melt is injected at the center of the axial magma chamber with a 'needle' with adjustable porosity at different depths allowing the simulation of different arrangements of melt injection and supply within the magma chamber. Conversely to previous models, the shape of the magma chamber -defined as the isotherm where 95% solidification of the melt occurs- is not imposed but computed from the steady state reached by the thermal field considering the heat diffusion and advection and the latent heat of crystallization. The motion equation is solved for a temperature and phase dependent viscosity. The thermal diffusivity is also dependent on temperature and depth, with a higher diffusivity in the upper plutonic crust to account for more efficient hydrothermal cooling at these crustal levels. In agreement with previous non-dynamic thermal models, our results show that near, deep off-axis hydrothermal circulation strongly affects the shape of the axial magma by tightening isotherms in the upper half of the plutonic oceanic crust where hydrothermal cooling is more efficient. Different accretion modes have however little effect on the shape of the magma chamber, but result in variable arrangements of flow lines ranging from tent-shape in a single-lens accretion scenario to sub-horizontal in "sheeted-sill" intrusion models. For different intrusion models, we computed the average Igneous Cooling Rates (ICR) of gabbros by dividing the crystallization temperature interval of gabbros by the integrated time, from the initial intrusion to the point where it crossed the 950 °C isotherm where total solidification of gabbro occurs, along individual flow lines. The distribution of ICR of gabbros along each flow line is then computed at their final off-axis emplacement as it is now observed in ophiolites. The main result of our model is that the variation of ICR with depth strongly constrains the accretion mode of the oceanic crust. The bimodal distribution of ICR with depth inferred from the crystal size distribution studies of gabbros from the Oman ophiolite (Garrido et al., 2001) can be only reproduced by accretion models with at least two melt lenses. The location of the jump in the bimodal distribution of ICR with depth observed at ca. 4 km above the MTZ in the Oman ophiolite implies that ca. 50% of the oceanic crust is accreted in an upper magma lens, while the 50% lower half is either accreted in one lens located at the MTZ or in several melt lenses with alike melt supply and evenly distributed along the lower half of the plutonic oceanic crust. Garrido, C. J., Kelemen, P. B. & Hirth, G.. G-cubed. 2, doi: 10.1029/2000GC000136 (2001).
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3600 MINERALOGY AND PETROLOGY
DE: 8000 STRUCTURAL GEOLOGY
DE: 8400 VOLCANOLOGY
SC: Tectonophysics [T]
MN: Fall Meeting 2005