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Petrological Constraints on Magma Plumbing Systems Along Mid-Ocean Ridges

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Petrological Constraints on Magma Plumbing Systems Along Mid-Ocean Ridges Petrological Constraints on Magma Plumbing Systems along Mid-Ocean Ridges Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Jameson Lee Scott, B.S. Graduate Program in Geology The Ohio State University 2011 Thesis Committee: Michael Barton, Advisor W. Berry Lyons Wendy Panero Copyright by Jameson Lee Scott 2011 Abstract Plate spreading at the mid-ocean ridges is accompanied by intrusion of dikes and eruption of lava along the ridge axis. It has been suggested that the depth of magma chambers that feed the flows and dikes is related to the heat flux – the higher the heat flux the shallower the magma chamber. To examine this hypothesis, I determined the depths of magma chambers beneath the intermediate spreading Juan de Fuca Ridge (JdF) in the northeast Pacific and the slow spreading Reykjanes Ridge (RR) south of Iceland. Pressures of partial crystallization were determined by comparing the compositions of natural liquids (glasses) with those of experimental liquids in equilibrium with olivine, plagioclase, and clinopyroxene at different pressures and temperatures using the method described by Kelley and Barton (2008). Chemical analyses mid-ocean ridge basalts glasses sampled from along the RR and JdF were used as liquid compositions. Samples with anomalous chemical compositions and samples that yielded pressures associated with unrealistically large uncertainties were filtered out of the database. The calculated pressures for the remaining 519 for the RR and 479 samples for the JdF were used to calculate the depths of partial crystallization and to identify the likely location of magma chambers. The RR results indicate that the pressure of partial crystallization decreases from 102 ± 33 MPa at the Charlie Gibbs Fracture Zone to 21 ± 12 MPa at 56°N, then ii increases to 367 ± 68 MPa as Iceland is approached. Four magma lenses were identified at depths of 2.5±.8km, 5.2±.8km , 5.9±1km, and 6.7±1. The magma lens at 2.46±.83 km agrees very well with seismically imaged sill at 2.5 km (Peirce et al 2007). The JDF results indicate that the pressure of partial crystallization decreases from 200 to100±50 MPa from the Blanco fracture zone to the north along the Cleft segment of the ridge. Calculated pressures remain approximately constant at 87±.53MPa along ridge segments to the north of the Cleft. Two magma lenses were identified at depths of 4.47±.89km and 4.08±1.5km. Pressures calculated for samples from single lava flows along the Cleft segment described by Stakes et al (2006) allow identification of two magma chambers at depths of 4.91±.77km and 4.33±1.07km which agree well with the depth of 5 to 6 km for a seismically imaged sill (Canales et al 2009). The average depth of partial crystallization of both ridges increase with increasing heat flux. While calculated pressures provide evidence for some crystallization in axial melt lenses, results obtained for some samples from virtually every locality also suggest partial crystallization in the crust beneath these lenses, and therefore the results support the many sill or crystal mush models for accretion of oceanic crust for both ridges.. The average difference between pressures calculated with both methods within the uncertainty in the calculation. The Herzberg method returns slightly lower pressures for most samples. iii Dedication Dedicated to Jessie, Barbra, and James Scott iv Acknowledgments Special thanks to Dr. Barton Michael Barton for all the advice and editing and to Dr. W. Berry Lyons and Dr. Wendy Panero for all the help. v Vita May 2005 .......................................................................................Sikeston High School 2009......................................................................B.S. Geology, The Ohio State University 2009 to Present ....................................................Graduate Teaching Associate, School of Earth Science, The Ohio State University Fields of Study Major Field: Geological Sciences vi Table of Content ABSTRACT ............................................................................................................ II DEDICATION ...................................................................................................... IV ACKNOWLEDGMENTS ..................................................................................... V VITA ..................................................................................................................... VI TABLE OF CONTENT ....................................................................................... VII TABLE OF FIGURES ............................................................................................ II LIST OF TABLES ................................................................................................ VI CHAPTER 1: INTRODUCTION ........................................................................... 1 1.2 Hypothesis ............................................................................................................................ 2 CHAPTER 2: METHODS AND DATA TREATMENT ....................................... 7 vii 2.1 Method ................................................................................................................................. 7 2.2 Data Filtering based on Chemical Composition ................................................................... 11 2.3 Filtering of Results for Calculated Pressures ....................................................................... 17 2.4 Criteria for Defining Magma Lenses at a specific Location .................................................. 19 CHAPTER 3: THE REYKJANES RIDGE .......................................................... 22 3.1 Geologic Setting .................................................................................................................. 22 3.2 Results ................................................................................................................................ 28 3.2.1 Samples ............................................................................................................................. 28 3.2.2 Pressures of Partial Crystallization .................................................................................... 32 3.3 Discussion ........................................................................................................................... 38 3.3.1 Geochemical Variations and Crustal Thickness. ................................................................ 38 3.3.2 Pressures of Partial Crystallization and Crustal Accretion ................................................. 44 CHAPTER 4: THE JUAN DE FUCA RIDGE ..................................................... 50 4.1 Geologic Setting .................................................................................................................. 50 4.2 Results ................................................................................................................................ 53 4.2.1 Samples ............................................................................................................................. 53 viii 4.2.2 Pressures of Partial Crystallization .................................................................................... 59 4.3 Discussion ........................................................................................................................... 65 4.3.1 Geochemical Variations ..................................................................................................... 65 4.3.2 Pressures of Partial Crystallization and Crustal Accretion ................................................. 69 CHAPTER 5: COMPARISON OF PETROLOGICAL METHODS TO DETERMINE THE PRESSURES OF PARTIAL CRYSTALLIZATION ...................... 74 CHAPTER 6: CONCLUSIONS ........................................................................... 77 REFERENCES ..................................................................................................... 79 ix Table of Figures Figure 1: Projection from plagioclase showing the cotectic shifting toward olivine as pressure as pressure increases. Figure from Walker et al 1979. ......................... 8 Figure 2: Variation diagram of the most useful oxides to determine if a series of erupted basaltic melt compositions are consistent with crystallization of olivine, plagioclase, and clinopyroxene. ........................................................................................ 13 Figure 3: Show a single LLD from 61.10°N overlain on the multiple LLD for the entire Reykjanes Ridge. .................................................................................................... 15 Figure 4: Theoretical examples of trends created by the data points that would indicate a magma lens. A and B show pressures that varies by less than 126 MPa over a range of latitude and longitude. C shows expected trends of MgO wt% of magma that evolves during ascent and after ponding in a magma lens. ............................................... 20 Figure 5: Map of the Reykjanes Ridge showing the V-shaped ridges. The Charlie Gibbs fracture zone, marking the southerly termination of the ridge, is labeled. Black circles are the samples localities. Created with GeoMap. (http://www.geomapapp.org )( Ryan et al 2009.) ............................................................................................................... 23 ii Figure 6: bathymetry
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