HR: 16:15h
AN: V12J-02    [PDF]
TI: Crustal Accretion and Cooling of the Lower Crust at Fast-Spreading Ridges: Thermal Models Consistent with Petrological and Geophysical Observations from Active Ridges and Ophiolites
AU: * Maclennan, J
EM: maclenna@ipgp.jussieu.fr
AF: Laboratoire des Geosciences Marines, Institut de Physique du Globe 4 Place Jussieu, Paris, 75005 France
AU: Hulme, T
EM: hulme@ipgp.jussieu.fr
AF: Laboratoire des Geosciences Marines, Institut de Physique du Globe 4 Place Jussieu, Paris, 75005 France
AU: Singh, S
EM: singh@ipgp.jussieu.fr
AF: Laboratoire des Geosciences Marines, Institut de Physique du Globe 4 Place Jussieu, Paris, 75005 France
AB: We have developed thermal models of oceanic crustal accretion at fast-spreading ridges that explicitly include petrological variation and allow the distribution of crystallisation and hydrothermal cooling to be varied. These models are used to relate geophysical observations from active ridges to petrological and geological observations from ophiolites. We ran a large number of models with different distributions of crystallisation and found that this distribution is not strongly constrained by a combination of seismic tomography and large-scale petrological observations from the Oman ophiolite. When constraints from seismic reflection and compliance surveys are also included, we find that models where 25--100% of the lower crust is formed by crystallisation in the shallow melt lens and 25--100% of the lower crust crystallises at its final depth can match the observations. Therefore, further observations are required to better constrain the distribution of crystallisation at fast-spreading ridges. Different distributions of crystallisation produce different distributions of heat release, and therefore require different distributions of hydrothermal cooling within the crust. When crystallisation occurs in the lower crust, hydrothermal cooling must extend to near-Moho depths within $<$5 km of the axis in order for the predicted distribution of melt to be consistent with the geophysical observations. Both petrographic and petrological observations from gabbros of the Oman ophiolite have now been used to determine relative cooling rates within the lower crust, and such observations may provide constraints on the distribution of crystallisation and hydrothermal cooling. In order to investigate these constraints, we calculated temperature-time paths from our models and, using the temperature-dependent partitioning and diffusion of Ca between olivine and clinopyroxene, predicted the concentration profiles of Ca in olivine from lower crustal gabbros. Cooling rates inferred from the Oman observations are consistent with conductive cooling. However, we found that this apparent conductive profile can be produced even when extensive hydrothermal cooling takes place to Moho depths within 1 km of the ridge axis. This profile is generated by conduction from the mantle to the lower crust at distances of $<$ 10 km from the axis. Such heating can reset the concentrations of Ca in olivine, producing profiles that appear to be due to either conductive or bi-modal cooling.
DE: 3035 Midocean ridge processes
DE: 3210 Modeling
DE: 3640 Igneous petrology
DE: 7220 Oceanic crust
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting