Chronometry in plutonic rocks: cooling rates of ancient oceanic crust
The formation of new oceanic crust continuously resurfaces two-thirds of our planet and is a principal mechanism of cooling of the Earth’s interior. However, because the possibility to study modern oceanic crust in situ is rare and natural exposures are lacking, the mechanisms by which it forms are poorly constrained. The observation that only a small (~1 km wide and ~50 m deep) axial magma lens (AML) overlies a zone of low seismic velocities (LVZ), that is interpreted to be a crystal mush zone, at fast spreading oceanic ridges (Detrick et al., 1987; Dunn et al., 2000), has posed a conundrum: how is the large volume of the plutonic section of the oceanic crust (~4km) generated from such a small magma chamber?
Two prominent end-member models – the gabbro-glacier model (Quick and Denlinger, 1993; Henstock et al., 1993; PhippsMorgan and Chen, 1993) and the sheeted sill model (Kelemen et al., 1997; Korenaga and Kelemen, 1997) were proposed early on to try to address this problem; a number of hybrid models have been proposed since then. One way to test these models (and the underlying processes of heat removal) is to determine the timescale of cooling, i.e. the cooling rate. The sheeted sill model requires fast, efficient (hydrothermal) cooling over the entire plutonic sequence, whereas the gabbro glacier model predicts fast cooling rates at the top of the plutonic sequence and cooling rates that become slower with increasing depth as conduction becomes the dominant mechanism of heat removal.
Our recent studies suggest that cooling of the deeper crust was dominated by conduction. However, there are clear field evidence of fluid flow affecting the deeper crust as well. To explore the thermal effects of such fluid flow, we aim to systematically sample regions in the neighborhood of focused fluid flow zones (FFFZ) in sections of the fossil spreading zone exposed in Oman, and determine the spatial distribution of cooling rates. The extent of fluid flow that affected a given body of rock would be determined simultaneously. At the same time as addressing this geological problem, the project would provide a means of field-testing the different diffusion data and models that are generated in the other subprojects of this research unit.
Investigators
Kathrin Faak, Ruhr-Universitaet Bochum
Maria Kirchenbaur, Leibniz Universitaet Hannover
supported by
Jürgen Koepke, Leibniz Universitaet Hannover
References
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