Akkretion und Abkühlung der unteren ozeanischen Kruste im Bereich von schnell-spreizenden Mittelozeanischen Rücken
Final Report Abstract
The existing end-member models to explain the formation of oceanic lithosphere at fastspreading mid-ocean ridges mainly differ in the proportion of crystallization at different depths within the lower oceanic crust. Therefore, these models predict different thermal evolution, and most significantly, different depths to which hydrothermal fluids circulate in the oceanic crust. The aim of this project was the determination of cooling rates of natural rock samples obtained from different depths within the lower oceanic crust formed along three different segments of the fast-spreading East Pacific Rise (EPR). In turn, this allows testing different thermal models for the lower oceanic crust and provides additional constraints for the development of a revised model on formation of the oceanic lithosphere. We developed and tested a new tool to determine temperatures and cooling rates from plagioclase and clinopyroxene bearing rocks (i.e. a geothermometer and a geospeedometer). This included the calibration of the partition coefficient of Mg between plagioclase and clinopyroxene ( KMg^Pl/Cpx) and the diffusion coefficient of Mg in plagioclase DMg^PI as a function of T, XAn and aSiO2. Application of the new Mg-in-plagioclase geospeedometer on the natural rocks from our sample suite allowed us to obtain the cooling rate as a function of sample depth. Our results show a significant decrease of cooling rate with increasing depth in the lower oceanic crust. The fact that this observation is very robust for three different locations along the EPR implies a very comparable, and near steady-state, thermal structure in the off-axis region along the EPR. Both, the absolute cooling rates determined from the deeper samples (>300 m below DGB), and the large decrease in cooling rate with depth, are inconsistent with thermal models that include substantial cooling by off-axis hydrothermal circulation. Instead, our data is consistent with thermal models in which the lower crust cools conductively in the off-axis, implying that most of the heat is removed by hydrothermal circulation at the top of an axial melt lens and heat conduction becomes the dominant process of heat removal with increasing depth and away from the ridge axis. Our observational results provide important constraints for modelling calculations of thermal structure of fast spreading ridge systems, and should help in the development of the next generation of models.
Publications
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(2011) Evaluation of the variation in cooling rate with depth in the lower oceanic crust at fast-spreading ridges using a newly developed Mg in plagioclase geospeedometer. – Eos Trans AGU, Fall Meet. Suppl., Abstract V13F-04
Faak, K., Chakraborty, S. and Coogan, L.A.
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(2013) Mg in plagioclase: Experimental calibration of a new geothermometer and diffusion coefficients. – Geochimica et Cosmochimica Acta 123, 195-217
Faak, K., Chakraborty, S. and Coogan, L.A.
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(2014) A new Mg-in-plagioclase geospeedometer for the determination of cooling rates of mafic rocks. – Geochimica et Cosmochimica Acta 140, 691-707
Faak, K., Coogan, L.A. and Chakraborty, S.
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Near conductive cooling rates in the upper-plutonic section of crust formed at the East Pacific Rise. – Earth and Planetary Science Letters, Volume 423, 1 August 2015, Pages 36-47
Faak, K., Coogan, L.A. and Chakraborty, S.
