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Cooling and Accretion of the Lower Oceanic Crust at Fast-Spreading Mid-Ocean Ridges

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Cooling and Accretion of the Lower Oceanic Crust at Fast-Spreading Mid-Ocean Ridges Cooling and Accretion of the Lower Oceanic Crust at Fast-Spreading Mid-Ocean Ridges Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften an der Fakultät für Geowissenschaften der Ruhr-Universität Bochum vorgelegt von Kathrin Faak (Bochum) Bochum, im Oktober 2012 Gutachter Prof. Dr. Sumit Chakraborty Prof. Dr. Jörg Renner Prof. Dr. Bernd Marschner Tag der mündlichen Prüfung 20. November 2012 Cooling and Accretion of the Lower Oceanic Crust at Fast-Spreading Mid-Ocean Ridges Doktoral Thesis by Kathrin Faak born in Bochum Faculty of Geosciences Ruhr-Universität Bochum Bochum, October 2012 Thesis committee Prof. Dr. Sumit Chakraborty Prof. Dr. Jörg Renner Prof. Dr. Bernd Marschner Defense of doctoral thesis November 20, 2012 Declaration of Authorship I hereby declare in lieu of an oath that I have written this thesis independently and autonomously using only the sources indicated. Location, Date Signature Abstract Magmatism along mid-ocean ridges (MORs) is estimated to account for 75 % of the recent global magmatic budget and involves the emplacement of ~20 km 3 of magma per year. The processes involved in cooling and accretion of this magma to form new oceanic crust are a principal mechanism of heat removal from the Earth’s interior. Circulation of seawater through newly formed crust extracts magmatic heat and produces hydrothermal fluids enriched in base metals and nutrients that form massive sulphide deposits, and feed chemosynthetic ecosystems on the seafloor. Additionally these hydrothermal systems have a profound influence on the composition of the oceans. However, the processes involved in the formation of oceanic crust by cooling and crystallization of the magma, and therefore providing the heat for the hydrothermal circulation, are poorly understood. The existing end-member models of crustal accretion along fast-spreading mid-ocean ridges (the ‘ gabbro glacier ’ and the ‘ sheeted sill ’ model) 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. As a consequence, this implies different variations of cooling rate as a function of depth. The present study determines cooling rates of natural rock samples of the lower oceanic crust, formed along three different segments of the fast-spreading East Pacific Rise (EPR). Since the individual samples of each location were collected from different depth, the results presented here include information about the variation of cooling rates as a function of depth in the lower oceanic crust. In turn, this allows testing the different models and provides additional constraints for the development of a revised model. To obtain cooling rates from the natural rock samples, a new ‘ Mg-in- plagioclase geospeedometer ’ was developed, which is based on the diffusive exchange of Mg between plagioclase (Pl) and clinopyroxene (Cpx) during cooling. Calibration of this tool required detailed investigation of the diffusion coefficient of Pl Mg in plagioclase ( DMg ) and the partition coefficient of Mg between plagioclase and Pl / Cpx clinopyroxene ( K Mg ) in the compositional range of the lower oceanic crust. The Pl Pl / Cpx diffusion coefficient DMg and the partition coefficient K Mg were determined experimentally as a function of temperature ( T) , anorthite-content in plagioclase (XAn ) and the silica activity of the system ( a ). SiO 2 Pl Pl / Cpx Reliable results for DMg and K Mg were obtained in a temperature range of 1100 to 1200°C and a compositional range of XAn =0.5 to 0.8. At these conditions, Pl / Cpx K Mg was found to (i) decrease with decreasing T, (ii) increase with increasing XAn in plagioclase and (iii) increase with increasing a . The diffusion coefficient D Pl SiO 2 Mg was found to (i) decrease with temperature following an Arrhenian relationship and Pl (ii) to increase with increasing a . No significant dependence of D on XAn in SiO 2 Mg plagioclase was observed. Application of the ‘ Mg-in-plagioclase geospeedometer ’ on the different natural samples suites of the EPR yield cooling rates in the range of 5 °C/year to 0.0001 °C/year, and a general trend of decreasing cooling rate as a function of depth is observed. The observation of fast cooling at the top of the lower oceanic crust and decreasing cooling rates at greater depth is consistent with a ‘ gabbro glacier ’ type model of crustal accretion. The results derived in this study provide a new geothermometer based on Mg exchange between Pl and Cpx with wide application to terrestrial and extraterrestrial rocks containing these two minerals. Furthermore, the developed ‘Mg-in-plagioclase geospeedometer ’ may be applied to these rocks to reconstruct their cooling history. The vertical distribution of cooling rates in the lower oceanic crust obtained in this study provides new information about the thermal structure along fast-spreading MORs, which is an important step in understanding the processes during cooling and accretion of new oceanic crust. Contents Contents 1. INTRODUCTION 1 1.1 Objectives of this study 1 1.2 Structure of this thesis 3 1.3 The lower oceanic crust at fast-spreading mid-ocean ridges 4 1.4 The existing models for crustal accretion at fast-spreading ridges and their constraints 14 1.4.1 The development of different models 14 1.4.2 Thermal constraints on the models of crustal accretion 18 1.4.3 Summary of the differences of the two end-member models 19 1.5 The approach of this study - Testing models of lower crustal accretion using diffusion calculations and ‘geospeedometry ’ on natural rock samples 21 1.6 The investigated natural sample suites 27 1.6.1 Hess Deep 28 1.6.2 Pito Deep 32 1.6.3 IODP Site 1256 35 1.7 References 37 2. EXPERIMENTAL DETERMINATION OF THE TEMPERATURE DEPENDENCE OF MG EXCHANGE BETWEEN PLAGIOCLASE AND CLINOPYROXENE 49 Abstract 49 i Contents 2.1 Introduction 50 2.2 Theoretical background and previous work on the diffusive exchange of Mg between plagioclase and clinopyroxene 54 2.2.1 Exchange of Mg between plagioclase and clinopyroxene 54 2.2.2 Diffusion of Mg in plagioclase 60 2.3 Experimental setup and run conditions 62 2.3.1 General experimental setup, starting materials and run conditions 62 2.3.2 Special experimental setups 66 2.3.3 Sample preparation after the experiment 67 2.4 Electron microprobe (EMP) analyses 67 2.5 Experimental results and discussion 69 2.5.1 General observations 69 Pl / Cpx Pl 2.5.2 Extracting K Mg and DMg from the experiments 71 Pl / Cpx Pl 2.5.3 Experimental results on K Mg and DMg 73 2.5.4 Uncertainties and error estimation 77 Pl / Cpx 2.5.5 Variation in ln K Mg as a function of Cpx T, X An , a , and X 82 SiO 2 CaSiO 3 Pl / Cpx 2.5.6 Discussion of the experimental results on K Mg 91 2.5.7. A new thermometer based on the exchange of Mg between plagioclase and clinopyroxene 94 Pl 2.5.8 Variation in D with T, X An and a 95 Mg SiO 2 Pl 2.5.9 Discussion of the experimental results on DMg 98 2.6 Conclusions 105 2.7 References 108 ii Contents 3. COOLING RATES WITH DEPTH IN THE LOWER OCEANIC CRUST DERIVED BY DIFFUSION MODELLING OF MG IN PLAGIOCLASE 113 Abstract 113 3.1 Introduction 114 3.2 Diffusion profiles of Mg in plagioclase and the extraction of cooling rates 118 3.3 The diffusion model 119 3.4 Model parameters and input conditions (for the diffusive exchange of Mg between plagioclase and clinoyproxene and the investigated sample suite) 121 3.4.1 Diffusion coefficient 121 Pl / Cpx 3.4.2 Initial profile determined from K Mg 124 Pl / Cpx 3.4.3 Boundary conditions determined from K Mg 127 3.5 Evolution of concentration profiles of Mg in plagioclase in contact with clinopyroxene during linear cooling 128 3.6 Uncertainties, robustness and sensitivity of the approach 131 3.6.1 A test of robustness and sensitivity of the model 133 3.7 Application to natural sample suites of rocks from different depths within the lower oceanic crust 140 3.7.1 Analytical techniques 141 3.7.2 The sample suites 142 3.8 Results from the Hess Deep (North wall) samples 144 3.8.1 Shapes of Mg-profiles in plagioclase with increasing depth 144 3.8.2 Cooling rates and their vertical distribution 145 3.9 Results from the Pito Deep samples 148 3.9.1 Shapes of Mg-profiles in plagioclase with increasing depth 148 3.9.2 Cooling rates and their vertical distribution 150 3.10 Results from the IODP 312 1256D samples 152 iii Contents 3.11 Discussion 154 3.11.1 Implications for the constraints on the cooling history of each sample 154 3.11.2 Comparison of the different sample suites 158 3.11.3 Comparison of cooling rates obtained from Mg-in-plagioclase and from Ca-in-olivine 161 3.11.4 Interpretation and discussion of the vertical distribution of cooling rates 162 3.11.5 Geological implications 167 3.12 Conclusions 168 3.13 References 169 4. CONCLUSIONS AND FUTURE WORK 175 4.1 Summary of the results from this study 175 4.2 Future work and perspectives 178 4.3 References 184 APPENDIX Appendix I - Table A1: Summary of the petrography Appendix II - Table A2: Summary of the measured profiles Appendix III - Table A3: EMP measurement conditions Appendix IV - Figure A4: Plots of all fitted Mg-concentration profiles Appendix V - Fortran code of the diffusion model Appendix VI -Organization of the Electronic Appendix ACKNOWLEDGEMENTS CURRICULUM VITAE iv 1.
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