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

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

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.

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

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

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

Figure 6: bathymetry Image from GeoMap with arrows pointing to some of the en echelon volcanic ridges. (http://www.geomapapp.org )( Ryan et al 2009.) ...... 26

Figure 7: shows the unfiltered trends of major oxides, a,b, e-j show trends indicative of magmas that are undergoing fractional crystallization and d show the onset of cpx crystallization at 8 wt% MgO ...... 30

Figure 8: The four locations that have evidence of a magma lens. The grey boxes highlight pressures at which magma lenses might occur. The black dashed line is the zero pressure line...... 33

Figure 9: Examples of locations that show no evidences of magma lens from the

Reykjanes Ridge show pressures the cluster greater than 100 MPa. The black dashed line is the zero pressure line...... 35

Figure 10: Upper graph shows the average pressure of partial crystallization

Rekjanes Ridge. The zero line represent sea level. The upper black line is the seafloor and the lower black line is the depth of the MOHO below sea level (see discussion for creation)...... 37

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Figure 11: The Mg# shows that the magma becomes more evolved as the ridge trends toward Iceland ...... 39

Figure 12: The variation Na8.0 along the ridge. This shows that the Na8.0 decrease to the north therefore heat flow and crustal thickness increase toward Iceland. 41

Figure 13: Comparison of seismically imaged MOHO and petrological signal for the MOHO. The black line is a regression of the petrologic data for MOHO depth. The

Black squares are seismic data form Fougler and Anderson 2005...... 43

Figure 14: Map of the Juan de Fuca ridge showing volcanic segments and fracture zones. Black circles are the location of sample groups Created with GeoMap.

(http://www.geomapapp.org )( Ryan et al 2009.) ...... 51

Figure 15: Variation diagrams for major oxides trends. Plots of CaO and

CaO/Al2O3 versus MgO (c and d) provide evidence for the onset of cpx crystallization at

7.6 wt% MgO. Note the large scatter in K2O at 6-8 wt% MgO (h)...... 55

Figure 16: Major oxide vs. Latitude. There is no evidence for systemic change in the composition of the magmas erupted along most of the ridge. However, magmas erupted in and near the Blanco Fracture Zone appear to have evolved to a greater degree than those erupted elsewhere along the Ridge...... 58

Figure 17: The Two locations that have evidence of a magma lens. The red border grey boxes highlight the magma lens. The black dashed line is the zero pressure line. ... 61

Figure 18: Examples of typical results obtained for most locations along the Juan de Fuca Ridge. The black dashed line is the zero pressure line...... 62

Figure 19: Upper graph shows the average pressure of partial crystallization

(black circles with one sigma error bars). The zero line represents the seafloor and the lower black line represents the MOHO (Canales et al 2006) . Lower graph: Average depth of partial crystallization as black circles with one sigma error. The zero line represent sea level. The upper black line is the seafloor and the lower black line is the depth of the MOHO below sea level (Canales et al 2006)...... 64

Figure 20: The results from two lava flows along the Cleft segment that provide strong evidence for sub-crustal magma lenses. The grey boxes highlight the pressures of magma lenses...... 72

Figure 21: Comparison between the Kelley and Barton geobarometer (black) and the Herzberg geobarometer (grey). Average pressures of partial crystallization are plotted for both methods. The dash line represents 1 atmosphere pressure ...... 75

List of Tables

Table 1: Averaged Pressures for each location of the Reykjanes Ridge...... 36

Table 2: Averaged Pressures for each location of the Juan de Fuca Ridge...... 63

Chapter 1: Introduction

Most of the world’s magmatic events and crustal accretion occurs at Mid-Ocean

Ridges (Pan et al 2003). It has been estimated that the mid-ocean ridges account for about

50% of the total heat loss for the Earth (Pollack et al 1993). It is therefore clear that a complete understanding of the various processes that occur at Mid Ocean Ridges requires knowledge of how this large amount of heat is transported though the ridges. The thermal characteristics of ridges control important processes such as the depth and rate of accretion of new crust, and hydrothermal circulation. The hydrothermal circulation

systems are responsible for the draw down of some CO2 from the atmosphere via hydrothermal alteration, and therefore potentially impact the climate system (Brady et al

1997). In addition, hydrothermal vent systems provide the raw materials for deep sea life, and lead to the formation of sulfide ores. Understanding thermal conditions in the sub- ridge crust will give a better understanding of the above processes and provide a firmer basis for interpreting geophysical data for oceanic crust in the vicinity of active ridges

(Machetel et al 2009). Geophysical observations are based on assumed physical properties of oceanic crust, including the lateral and vertical distribution of lithologies and densities, both of which affect seismic velocities. Lithologies, densities and seismic velocities are therefore dependent on the geothermal gradient in a given location.

Because Mid-Ocean Ridges are, for the most part, located below sea-level, the intra-crustal processes operating beneath ridges are not completely understood, and many questions remain to be answered. These include: How does the thermal structure change with respect to spreading rate, heat flux, and proximity to mantle plumes? Is the crust accreted at a single depth from a single axial magma chamber, over a wide range of depths from multiple sills or from a vertically extensive crystal mush region? How does hydrothermal circulation affect the thermal structure of the upper crust?

1.2 Hypothesis

The structure of oceanic crust at mid ocean ridges has been inferred from studies of ophiolites and from interpretation of seismic data. Three major layers have been identified, and these are, from the ocean floor down, and in order of increasing thickness: the pillow basalt layer, the sheeted dike layer, and the gabbro layer. Previous work

2 suggests that warm ridge crust is characterized by relatively few fractures zones, fast spreading rates, shallow magma lenses, and high heat flux; whereas cooler ridge crust is characterized by more abundant fractures zones, slower spreading rates, deeper magma lens, and low heat flux (see Herzberg, 2004). This leads me to hypothesize that as the heat flux increases due to faster spreading, or proximity to a mantle plume, the temperature of the crust will increase allowing magma to reach higher levels prior to cooling and crystallization. This should result in shallower magma chambers along warmer, high-flux, fast-spreading ridges than along colder, low flux, slow spreading ridges. In other words, the depth of magma chambers should be directly correlated with heat flux. By constraining the depth of magma chambers at multiple locations along one slow and one intermediate spreading ridge, I will be able to place constraints on the distribution of melt in the crust and to test models for crustal accretion along mid ocean ridges.

Samples collected from two ridge segments - the intermediate spreading Juan de

Fuca Ridge and the slow-spreading Reykjanes Ridge – were selected for study. There is no confirmed mantle plume interaction with the Juan de Fuca Ridge, so any variation of magma chamber depth should, according to the hypothesis, solely reflect the effects of 3 changing heat flux due either to variation in spreading rate along the ridge or to proximity to transform faults at the northern and southern ends of the ridge. Previous workers (e.g.

Herzberg, 2004) have shown that the crust near transforms is relatively cool and suggest that this is due to hydrothermal circulation penetrating deeper into the crust and, possibly, the upper mantle. These tectonically controlled regions of cool crust should cause magma lenses to form deeper in the crust than at locations that are distant from the transforms.

The study of samples from the slow spreading Reykjanes Ridge will allow me to compare magma chamber depths from the slow-spreading southern segment of the ridge

(north of the effects of the Charlie Gibbs fracture zone) with those for the Juan da Fuca

Ridge to confirm the proposed relationship between magma chamber depth, heat flux, and spreading rate. Samples from the northern segment of the ridge where the Iceland hot spot is approached will allow the effects of a mantle plume on magma chamber depth to be examined..

Seismic studies have been used to identify and constrain the depths of magma lenses beneath several ridges, including the Juan de Fuca and Reykjanes Ridges. An additional goal of this study is to demonstrate the feasibility of using petrologic methods

4 for determining the depths of magma chambers by showing that such methods yield depths that are consistent with those inferred from seismic studies. There are several reasons why it is desirable, and in some cases necessary, to use petrologic methods to determine magma chamber depths. First, the interpretation of seismic data, as stated previously, is dependent on assumptions about physical properties of crustal materials and these are dependent on the thermal conditions at a given location. It is therefore desirable to have independent methods to test conclusions from seismic studies. Second, petrologic studies provide quantitative estimates of temperatures of partial crystallization as well as of pressures, and hence provide useful information about thermal gradients in the crust. Third, seismic observations only provide snap shots in time of the sub-ridge structure. Previous work on seismic imaging of magma lenses along the fast spreading

East Pacific Rise suggest that partially molten magma bodies may have residence times as short as 60 to 90 days (Pan et al 2002). This makes it difficult to detect magma bodies beneath ridge segments that are not undergoing active rifting and magma emplacement.

In contrast, the petrologic method is dependent only on the chemical compositions of erupted lavas, and it is possible to use analyses of multiple samples from individual

5 locations to determine the average depth of crystallization over a relatively long time period.

Chapter 2: Methods and Data Treatment

2.1 Method

A relatively simple method to estimate the pressure of partial crystallization of magmas is based on comparing the compositions of erupted melts with those of liquids lying along pressure (P)-dependent phase boundaries. Many basalts crystallize olivine

(ol) ((Mg, Fe)2SiO4), plagioclase (plag) ((Ca, Na)(Al2-1Si2-3)O8), and clinopyroxene (cpx)

2+ (Ca(Mg, Fe )Si2O6), and their compositions can be compared with those of liquids (l) lying along the l-ol-plag-cpx cotectic boundary. The effect of pressure on the latter has been determined experimentally, (e.g. O’Hara, 1968; Grove et al. 1992), and can be seen by recasting melt compositions into normative mineral components and projecting phase

relations onto pseudoternary planes in the system CaO-MgO-Al2O3-SiO2. Projection of phase relationships from plag onto the plane ol-cpx-qtz using the recalculation procedure of Walker et al. (1979) clearly shows the shift of the ol-plag-cpx cotectic towards ol with increasing P (Fig. 1).

Figure 1: Projection from plagioclase showing the cotectic shifting toward olivine as pressure as pressure increases. Figure from Walker et al 1979.

Likewise, projection of phase relationships from ol onto the plane plag-cpx-qtz shows that the l-ol-plag-cpx cotectic shifts towards plag with increasing P. The shift of the ol- plag-cpx cotectic towards ol and plag (see O’Hara, 1968; Grove et al. 1992) reflects the different pressure dependencies of cpx-liq, ol-liq and plag-liq equilibria, and results in

8 decreasing CaO and increasing MgO and Al2O3 contents of near-liquidus melts with increasing P. Yang et al. (1996) described a model to quantitatively estimate the crystallization pressure based on such relationships. This model was calibrated with experimental data, and uses three equations to describe the composition of liquids along the ol-plag-cpx cotectic as a function of P and T. This model allows calculation of pressure for any basaltic melt in equilibrium with ol, plag and cpx. Kelley and Barton

(2008) used the Yang et al. (1996) model to calculate pressures based on projections of melt compositions from plag onto the plane ol-cpx-qtz and from ol onto the plane plag- cpx-qtz. The method described by Kelley and Barton (2008) yield six values of P for each sample. The average value is taken as the pressure of crystallization, and all values are used to calculate the uncertainty (1σ) associated with the average pressure. Comparison with experimental data suggests that calculated pressures are accurate to ~126 MPa (1σ)

(See Kelley and Barton, 2008 for discussion).

All calculations of pressure reported in this thesis were carried out using the Excel spreadsheet available from. Kelley and Barton (2008)

Note that the method described above is only calibrated for magmas of basaltic composition, and cannot be used with confidence to determine pressures of partial

crystallization for silica-rich magmas (SiO2.> 52 wt. %). Also, it should be emphasized that calculated pressures represent the lowest pressure at which the melt was in equilibrium with ol, plag, and cpx. Any chemical signature of an earlier stage of magma crystallization at higher pressures may be completely obliterated or overprinted by partial crystallization at lower pressures. However, determination of pressures from multiple samples from a single locality may allow the polybaric evolutionary history of magmas to be deciphered (Kelley and Barton, 2008). The chemical signature of high-pressure partial crystallization may be preserved in some magmas from a particular suite that have ascended sufficiently rapidly to avoid pausing and partially crystallizing at lower pressure, for example in a shallow magma lenses. On the other hand, calculation of pressures for multiple samples from the same location is useful as the results may provide evidence for isobaric crystallization in a discrete magma lens as opposed to polybaric crystallization in dikes or in a vertically extensive crystal mush. (see later discussion).

2.2 Data Filtering based on Chemical Composition

The method used to calculate pressure relies on comparison of natural melt compositions with the compositions of liquids lying along the pressure-dependent ol- plag-cpx-l cotectic. Glass analyses are preferable to whole-rock analyses for calculating the crystallization pressure, because glasses represent samples of quenched melts.

Therefore, glasses formed from liquids in equilibrium with ol, plag and cpx should have compositions that lie exactly on the cotectic at the pressure of crystallization. Some whole-rock samples represent melts, but others represent mixtures of crystals and melt. It cannot be assumed that the latter formed by closed system crystallization, because the crystals may be of accumulative origin or may represent xenocrysts (e.g. Trønnes, 1990;

Hansteen, 1991Hansen and Grönvold, 2000). The erroneous assumption that whole-rock samples represent melts can lead to errors up to 1 GPa in calculated pressures (Kelley and

Barton, 2008). Therefore, only glasses were used in this study. Analyses of fresh glasses were downloaded from the online petrologic database www.petdb.org (Wood et al 1978,

Sigurdsson 1981, Eaby et al. 1984, Fine and Stolper 1985, Dixon et al 1986, Liias 1986,

Karsten 1988, Karsten et al. 1990, van Wagoner and Leybourne 1991, Smith et al 1994,

Melson and O’Hearn 2002, 2003 , Stakes et al. 2009). The analyses used for this study

11 are available electronically by contacting [email protected] or [email protected]

To ensure that samples from the database gave compositions suitable for obtaining meaningful pressure estimates, the analyses were initially filtered using two criteria. The first criterion is that all glasses represent basalts so that all glasses with SiO2

<45 wt % and >52 wt % were filtered out of the data base and are not considered further.

The second criterion is that all samples have compositions consistent with crystallization of olivine, plagioclase, and clinopyroxene. This was accomplished using several variation diagrams on which selected major oxides are plotted as a function of

MgO. I chose to plot MgO on the abscissa, because as MgO shows a relatively large decrease in concentration in basaltic magmas as crystallization (most notably of olivine) proceeds (Cox et al. 1979). Crystallization of both plagioclase and clinopyroxene (along

with olivine) will lead to decreasing concentrations of CaO and Al2O3 as basalts evolve leading to positive correlations between these oxide and MgO on variation diagrams (see

Figure 2 b and c).

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.

Since both plagioclase and clinopyroxene can remove CaO and Al2O3 from evolving basalt, it is necessary to determine if one or both of these phases are crystallizing. A plot

of CaO/Al2O3 is useful for this purpose. Crystallization of plagioclase (along with

olivine) removes more Al2O3 than CaO from the liquid yielding a strong positive

correlation between Al2O3 and MgO, and a neutral or negative correlation between

CaO/Al2O3 and MgO. Crystallization of clinopyroxene removes more CaO than Al2O3 from the liquid yielding a strong positive correlation between CaO and MgO, and a

positive correlation between CaO/Al2O3 and MgO (see Figure 2 d). Only those glasses with compositions that plot along and within well-defined data arrays with trends consistent with simultaneous crystallization of ol, plag and cpx on variation diagrams were used in this work. Plots of all glass analyses from a particular ridge results in some scatter on variation diagrams (Fig. 3) indicating that the basalts do not evolve along a single liquid line of descent (LLD). This might indicate crystallization along ol-plag-cpx cotectics at different pressures (polybaric crystallization), but other explanations are also possible including complex evolutionary histories involving operation of processes such as mixing and assimilation in addition to crystallization (see below).

Figure 3: Show a single LLD from 61.10°N overlain on the multiple LLD for the entire Reykjanes Ridge.

Nevertheless, samples with compositions clearly inconsistent with crystallization of ol, plag and cpx, compositions that lie outside of the array defined by the vast majority if the data (anomalous compositions, or outliers) can be identified and filtered out of the data base.

Magmas erupted along an entire ridge are not likely to have evolved in identical plumbing systems. The next stage of filtration involved examining chemical variations for subsets of samples in the data base. These subsets were created by dividing the samples into groups that are most likely to have been erupted from the same magma lens or magma plumbing system and are referred to as locations. The determination of each location is accomplished by plotting the samples along the strike of the ridge and identifying natural groupings or clusters separated geographically by a relatively large spacing. The analyses for samples from each location were plotted on variation diagrams to define tighter arrays of cotectic liquid compositions, and to allow identification of outliers with anomalous compositions. The latter were filtered out of the data base and these compositions are not further considered.

The final stage of filtration involved removing any sample from each location with compositions that are anomalous with respect to chemical components other than

Al2O3, CaO and CaO/Al2O3. Variations in the concentrations of the incompatible

elements, TiO2, K2O, P2O5, were used for this purpose. As basaltic magmas evolve via crystallization, these oxides preferentially remain in the liquid yielding tight negative correlations on plots versus MgO. Processes such as mixing and assimilation enrich magmas in these components leading to compositions that are anomalous compared to those produced by crystallization. Samples with such anomalous compositions were filtered out of the data base and are not further considered.

2.3 Filtering of Results for Calculated Pressures

Pressures were calculated for all glasses, but some results are considered unreliable. The average uncertainty in P calculated for the majority of samples is <126

MPa, and is similar to that estimated from experimental data (Kelley and Barton, 2008).

Uncertainties in pressures calculated for some samples are substantially greater than this value. These pressures are considered to be unreliable, and are excluded from further consideration. This has no effect on the conclusions reached in this work, because similar

17 but more reliable estimates of P are obtained for other samples from the same locations.

The pressures calculated for some other samples are negative. Negative pressures between 1 bar and -126 MPa can be reasonably attributed to crystallization at 1 bar

(within the uncertainties inherent in the method of calculation). Pressures more negative than 126 MPa are clearly unrealistic and were filtered out of the results.

2.4 Conversion of Pressure to Depth

The depths of magma chambers may be calculated from pressures of cotectic crystallization using an appropriate value for crustal density and for the depth of the sea water column. A value of 2900 kg m-3 (appropriate for basalt) was used for crustal density, and depths below sea level were taken from the PetDB data base. The correction for the seawater column is necessary for meaningful comparison with seismic results. It is normal practice that seismic interpretation of crustal structure in marine settings have the velocity signal for seawater removed. Since the pressures for each sample reflect loading by both the water column and the rock column, a simple calculation using 1000 kg m-3 for the density of the seawater was used to removes the pressure due to the water column, yielding depths that are directly comparable to those reported in seismic studies.

2.4 Criteria for Defining Magma Lenses at a specific Location

In order to place constraints on magma plumbing systems, it is necessary to develop criteria for identifying partial crystallization in a magma lens (i.e. a sill-like body) at a given location. Magmas that partially crystallize in a discrete lens can be identified from the relationship between pressure and MgO and/or between pressure and latitude/longitude. The first criteria rests on the requirement that for crystallization in a lens/sill, the pressure remains relatively constant (e.g. within the error of the method used to calculate pressure) as the magmas evolve. For basaltic magmas, MgO can be used to monitor magma evolution, so that in this case pressure remains constant on a plot of pressure versus MgO for a batch of samples from a particular locality. Ideally, the MgO contents of magmas erupted from a lens should vary by more than ~1 wt. % (i.e. greater than the analytical uncertainty) reflecting significant cooling and crystallization during residence of the magma in the lens (Fig. 4c ).

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.

A second criterion rests on the fact that samples erupted over a relatively narrow geographic area are likely to have been erupted from a common magma lens. This can be

revealed by plotting pressure versus either latitude or longitude. (Fig. 4 a , b). This criterion is of limited usefulness because samples returned from ocean-floor sampling expeditions are usually collected from an extremely restricted geographic area so that it is not possible to examine variations in pressure with either latitude or longitude.

The results obtained for some locations may not satisfy the criteria described above, and may indicate crystallization over a relatively wide range of pressures.

Interpretation of such results depends on whether any correlation exists between calculated pressure and MgO for a suite of samples. As magma rises through a ridge system (in dikes, for example), some batches of magma may pause and partially crystallize at different levels during ascent. In this case, pressure will be positively correlated with MgO. However, if a suite of samples yields a relatively wide range of pressure that is not correlated with MgO, the simplest interpretation is that these magmas were erupted from a plumbing system with multiple magma lenses at different depths, or from a magma mush zone. In such cases, only an average, maximum and minimum depth of partial crystallization can be established.

Chapter 3: The Reykjanes Ridge

3.1 Geologic Setting

The Reykjanes ridge is part of the Mid-Atlantic ridge system, and a significant length of the ridge is located on a large scale bathymetric anomaly that is centered on

Iceland. Within this anomaly, the sea floor is 2 to 3 km higher than similarly aged oceanic crust elsewhere along the normal ridge system (Foulger and Anderson 2005).

(Fig. 5). The large-scale bathymetric anomaly is widely considered to be the result of a mantle plume that intersects the mid-Atlantic Ridge and is centered under Iceland . A number of inconsistencies with observations made on other likely mantle plumes (the lack of a hot spot track, lower than expected thermal anomalies, lack of a seismically well imaged plume in the mantle) led Foulger and Anderson 2005 to hypothesize that the bathymetric anomaly does not reflect abnormally hot underlying mantle, but rather is due to the presence of recycled subducted Caledonian crust in the upper mantle.

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.)

Melting of mantle fertilized with sunducted crust is thought by these workers to account for the eruption of ocean island basalt on Iceland. More recent geophysical and petrological work on Iceland has resolved some of the problems raised by Foulger and

Anderson (2005) with the plume model. There is evidence of significant thermal expansion in the mantle beneath the bathymetric anomaly ( Leftwich et al 2005), suggesting that the mantle is abnormally warm, high mantle potential temperatures for

Icelandic magmas (1440-1460OC versus 1280-1400 OC for MORB) are consistent with derivation from a plume (Herzberg et al. 2007; Herzberg and Asimov, 2008; Herzberg and Gazel 2009), and a whole mantle plume has been imaged beneath Iceland ( Zhao

2007).

The Reykjanes Ridge is ~1000 km long and terminates at the Charlie Gibbs

Fracture Zone to the south at 53°N. The northern termination occurs where the ridge becomes sub-aerial at the Reykjanes Peninsula of Iceland at about 64°N. Sea floor depth along the ridge ranges from 2.4 km in the south to 0 at the Iceland coast.

The Reykjanes Ridge is a slow spreading ridge with an average half spreading rate of 10 mm/yr (spreading rates calculated using NUVEl-1 Plate Parameters described

24 by Argus and Gordon 1991). Spreading is oblique to the ridge axis at between 28° to 30°

(off orthogonal) (van Wijk et al 2007). There is no well-defined rift valley as is characteristic of other slow-spreading ridges. Rather there is an axial high that is a more common feature of fast spreading ridges. The ridge crest is about 50 km wide and 1 km high (Luyendyk et al 1979). En echelon volcanic ridges occur along the ridge crest

(Figure 6). The strike defined by the midpoints of these ridges is close to orthogonal to the plate spreading direction and the tips of the ellipsoidal ridge segments are rotated counter-clockwise and overlap (van Wijk et al 2007).

Figure 6: bathymetry Image from GeoMap with arrows pointing to some of the en echelon volcanic ridges. (http://www.geomapapp.org )( Ryan et al 2009.)

The Reykjanes Ridge is unusual in that it is characterized by southward directed V- shaped ridges, which are thought to be the result of interaction with the Iceland Hotspot

(Foulger and Anderson 2005). The V-shaped ridges run southwards from Iceland for1000 kilometers, and the apices converge at about 56.5°N. This portion of the Reykjanes Ridge lacks off-setting fracture zones, whereas that portion of the ridge between ~56.5°N and the Charlie Gibbs Fracture Zone is cross-cut by offsetting fracture zones. The absence of fracture zones in the northern segment of the ridge is a unique feature of the Reykjanes

Ridge, the rest of the Mid-Atlantic Ridge is typically off-set by fracture zones spaced at about 50 km intervals (Luyendyk et al 1979).

The V-shaped ridges were originally thought to result from southward flowing asthenospheric pulses of the Iceland plume moving beneath the ridge ( Vogt 1971).

However, this model provides a good explanation for the V-shaped ridges if the

Reykjanes is a symmetrical spreading ridge. Recent observations of asymmetrical spreading, together with more recent studies of the spreading history of the Reykjanes ridge, suggests that a series of propagating rifts and failed rifts could create the asymmetrical V shaped ridges (Hey et al 2010).

3.2 Results

3.2.1 Samples

A total of 864 samples for the Reykjanes Ridge were retrieved from the

PetDB database. The samples in the unfiltered data set are characterized by SiO2 = 48.5 –

52.8, TiO2 = 1.7 – 2.9, Al2O3 = 11.6 – 15.7, total FeO = 9.23 - 17.37; MgO = 4.6 – 9.2;.

CaO = 10.3 – 13.1, Na2O = 0.95-2.3, K2O = 0.2 - 0.26, P2O5 = 0.03 - 0.31 (all values in wt %, ∑Fe=FeO).

After the filtering the data set, 519 samples remained. The samples in the filtered data set

are characterized by. SiO2 = 48.5 – 52.0, TiO2 = 1.0 – 2.3, Al2O3 = 12.6 – 15.0, FeO =

10.3 – 15.2, MgO = 5.3 – 7.9 CaO = 10.3 – 12.7, Na2O = 1.7 – 2.7, K2O. = 0.03 -0 .26

and P2O5= 0.06 - 0.31, (all values in wt %, ∑Fe=FeO). These compositional ranges are only slightly narrower than for the unfiltered data set. ) All glasses show strong

enrichment in FeO as MgO decreases, whereas SiO2 remains approximately constant as

MgO decreases. These trends are characteristic of tholeiitic suites generated by

crystallization of ol-plag-cpx under relatively reducing conditions (Fig. 7 b,c,d). Al2O3,

CaO, and CaO/Al2O3 show positive correlations with MgO at MgO<8 wt %. These chemical variations can be qualitatively explained by crystallization of ol-plag-cpx

(±spinel) . The trends of CaO and CaO/Al2O3 versus MgO show a change slope from positive to negative at MgO = 8.0 wt.%, and this value of MgO therefore marks the onset of cpx crystallization. This is consistent with the observation and conclusions of

Langmuir et al. (1992) and Michael and Cornell (1998) based on analysis of variations in

CaO versus MgO in global MORB data sets Highly incompatible (TiO2, K2O, P2O5) element concentrations all show negative correlations with MgO as expected for magmas evolving via crystallization. (Fig. 7 f,h,i)

Continued

Figure 7 Continued

Continued

3.2.2 Pressures of Partial Crystallization

The samples were grouped into 21 locations. Only three locations occur along the southern part of the Ridge and account for only 5% of the total samples. This reflects preferential sampling of the ridge in the vicinity of Iceland to better understand ridge- plume interaction. Pressures were calculated for all samples, and the results obtained for all locations were examined for evidence of the existence of a magma lens. Only four locations yield results that cluster within 126 MPa. These are 58.42° N, 61.60°N,

62.08°N, and 62.79°N, all of which have 10-30 samples per location with the exception of 62.79°N. (Fig. 8) The calculated pressures for other localities show a relatively wide range (over 126 MPa) without significant clustering at any particular pressure (see Figure

9). The average pressures for all locations (table 1), decreases from 102 ± 33 MPa at the

Charlie Gibbs Fracture Zone to 21 ± 12 MPa at 56°N, then increases to 367 ± 68 MPa as

Iceland is approached (figure 10). The increase in average pressure as Iceland is

32 approached correlates with the thickening of the crust towards Iceland (Foulger and

Anderson 2004)

Figure 8: The four locations that have evidence of a magma lens. The grey

boxes highlight pressures at which magma lenses might occur. The black dashed

line is the zero pressure line.

Figure 8

Figure 9: Examples of locations that show no evidences of magma lens from the Reykjanes Ridge show pressures the cluster greater than 100 MPa. The black dashed line is the zero pressure line.

Table 1: Averaged Pressures for each location of the Reykjanes Ridge.

Location Name Latitude °N Longitute °E Corrected Pressure MPa Pressure σ Depth km Depth σ Elevation km 53.41°N 53.41 -35.24 102 33 3.6 114.8 -2400 54.76°N 54.76 -35.22 100 55 3.5 195.0 -2075 55.67°N 55.67 -34.87 21 12 0.7 40.7 -1975 58.42°N 58.42 -31.62 70 24 2.5 82.7 -1457 59.76°N 59.76 -29.82 98 64 3.4 223.9 -956 60.02°N 60.01 -29.42 78 11 2.7 39.6 -911 60.45°N 60.45 -28.88 139 42 4.9 146.9 -922 60.73°N 60.73 -28.42 76 29 2.7 103.3 -707.5 61.10°N 61.1 -27.89 119 68 4.2 240.2 -784 61.60°N 61.6 -27.07 167 27 5.9 94.5 -760 61.98°N 61.98 -27.12 234 64 8.2 226.8 -631 62.08°N 62.08 -26.29 191 28 6.7 99.1 -667 62.33°N 62.33 -25.94 153 61 5.4 214.0 -648 62.62°N 62.62 -25.95 196 55 6.9 194.2 -832 62.70°N 62.7 -25.21 248 22 8.7 76.7 -580 62.79°N 62.79 -25.16 155 38 5.4 135.2 -598 63.19°N 63.19 -24.45 273 50 9.6 176.9 -341 63.29°N 63.29 -24.23 261 68 9.2 240.7 -99 63.42°N 63.42 -23.87 293 47 10.3 165.7 -114 63.55°N 63.55 -23.68 187 58 6.6 202.4 -100 63.64°N 63.64 -23.41 367 68 12.9 240.9 -111

Figure 10: Upper graph shows the average pressure of partial crystallization

(black circles with one sigma error bars) along the Reykjanes Ridge. The zero line represents the seafloor and the lower black line represents the MOHO Lower graph: average depth of partial crystallization (filled circles with one sigma errors) along the Rekjanes Ridge. The zero line represent sea level. The upper black line is the seafloor and the lower black line is the depth of the MOHO below sea level (see discussion for creation).

3.3 Discussion

3.3.1 Geochemical Variations and Crustal Thickness.

The molecular ratio MgO/(MgO+FeO) (or Mg#, calculated with ∑Fe=FeO) is, widely used in petrologic studies as an index of the degree of magma evolution. The variation in

Mg# along the Reykjanes Ridge suggest that magma undergoes more extensive evolution beneath the northern end of the ridge compared with magma erupted at the southern end of the ridge (Fig. 12). This suggests that magma pauses for a longer time during ascent, and undergoes more extensive crystallization, beneath the northern part of the ridge. This might reflect the increase in crustal thickness from 57°N latitude to Iceland, providing an increased opportunity for magma to pond in the crust during ascent (note that the buoyancy driving force is lower for basalt magma traversing basalt crust than for basalt magma ascending through the mantle).

Figure 11: The Mg# shows that the magma becomes more evolved as the ridge trends toward Iceland

In order to investigate a possible correlation between magma composition and the

thickness of oceanic crust, variations in Na8.0 along the ridge were examined. Na8.0 is the

39 weight percentage of Na2O corrected for fractionation to 8% MgO, and is calculated using the method described in Langmuir et al, (1992), and Michael and Cornell (1998),

Values of Na8.0 provide information about two related parameters. One is the extent of

melting in the manle beneath the ridge. The Na2O contents of mantle melts (and hence

values of Na8.0) decrease as the degree of melting increases (Langmuir et al 1992).

Variations in the extent of melting, as recorded by variations in Na8.0, can be therefore used as a proxy for variations in the thermal state of the mantle beneath the ridge.

Second, Na8.0 provides information about crustal thickness because the thickness of oceanic crust is directly correlated with the extent of melting (Langmuir et al. 1992;

Michael and Cornell 1998). Therefore, a decrease in Na8.0 indicates an increase in both the amount of melting in the sub-ridge mantle, and in the thickness of oceanic crust. By the same token, crustal thickness is often used as a proxy for the magma flux (Foulger et

al 2005.) As Iceland is an approached, Na8.0 decreases (Fig.13) indicating that the mantle beneath the Reykjanes Ridge becomes hotter and the crust increases in thickness.

Figure 12: The variation Na8.0 along the ridge. This shows that the Na8.0 decrease to the north therefore heat flow and crustal thickness increase toward

Iceland.

The depth of the MOHO along the Reykjanes Ridge has been relatively well established using seismic methods in the vicinity of Iceland and between 57 to 58 N. This provides an opportunity to compare crustal thicknesses estimated from seismic data with

41 those inferred from petrologic data. The rationale for using petrological methods to infer crustal thickness was discussed by Kelley and Barton (2008), and is based on estimates of the pressures of partial crystallization of magmas. All of the glasses recovered from the

Reykjanes Ridge represent samples of evolved magma, that is, magmas that have changed composition after segregating from the mantle source region and ascending towards the surface. Magmas can pond, cool and partially crystallize in the mantle and in the crust (Herzberg, 2004, and references therein), but because the buoyancy driving force is lower for basalt magma traversing oceanic crust than for basalt magma ascending through the mantle, it is likely that magma frequently ponds at the MOHO. Accordingly, the maximum pressures of partial crystallization calculated for samples from a particular location may indicate the depth o the MOHO beneath that location. Agreement between petrologically inferred MOHO depths and MOHO depths inferred from seismic data are very good for that part of the ridge between 55ON and Iceland (Fig. 14).

Figure 13: Comparison of seismically imaged MOHO and petrological signal for the MOHO. The black line is a regression of the petrologic data for MOHO depth. The Black squares are seismic data form Fougler and Anderson 2005.

Regression of maximum depths of partial crystallization with latitude provides an estimate of MOHO depths along the entire ridge. The petrologic results indicate that

43 crustal thickness along the Reykjanes Ridge remains approximately constant at about 7 km from near the Charlie Gibbs fracture zone to 59°N, close to the global average thickness estimate of 7.1±0.9 km (White et al. 1992; Bown and White, 1994). The crustal thickness then increases to around 15 km as Iceland is approached. This agrees well with crustal thicknesses inferred from gravity data for the ocean basin south of

Iceland of 6 to 13 km (Leftwicth et al 2005) The crustal thickness inferred for the most northerly location on the ridge (15 km) agrees well with the petrologically inferred crustal thickness beneath for Geitafell (17.8±0.9 km) on the Reykjanes Peninsular of

Iceland (Kelley and Barton, 2008). Therefore, both petrologic and seismic signals indicate an increase in crustal thickness towards Iceland and this explains the geochemical signal for a greater degree of magma evolution along the northern segment of the Reykjanes Ridge.

3.3.2 Pressures of Partial Crystallization and Crustal Accretion

The strongest evidence for a magma lens is at 62.79°N, at a pressure 146.54 ±

22.9 MPa, or at a corresponding depth of 5.16 ± .80 km below sea floor. At this location, pressures are approximately constant over a small range of latitude and

44 longitude, but there is little variation of MgO with Pressure. These results suggest that at this location magma ponded during ascent, but did not reside for a sufficiently long period of time for significant partial crystallization to occur. There is similar evidence for magma lenses at other three locations, also with little variation of MgO with Pressure.

Therefore, these results also suggest magma ponding during ascent, but that the residence time in the magma lens is too short for significant partial crystallization to occur. The calculated pressures of partial crystallization in magma lenses at these locations are, from south to north: 70.05 ± 23.5 MPa, 167.24 ± 26.9MPa, and 190.76 ± 28.2. The corresponding depths for the magma lenses are: 2.46 ± .83 km, 5.88 ± .95 km, and 6.71 ±

.99 below sea floor respectively. The depth of the magma lens at 58.42 N correlates well with seismically imaged magma lens at 2.5 km in depth at about 50 km south of this location. (Peirce et al 2007). The conclusion that the residence time in magma lenses beneath the Reykjanes Ridge is too short for significant partial crystallization to occur in consistent with the work of Pan and Batza 2002 , who calculated short residence times of

30-90 days for magma in MOR chambers.

The results obtained for other locations along the ridge do not provide evidence for the existence of discrete magma lenses in the crust. Rather, the results suggest partial 45 crystallization over a range of pressures (depths), and although the presence of discrete, melt-rich lenses at some of these localities cannot be ruled out (see discussion in Kelley

Barton, 2008), it seems almost certain that crystallization mostly occurred in dikes, in multiple lenses (or sills) at different depths, or in a crustal “mush” zone (melt pockets dispersed within a largely but not necessarily entirely solid crust). The average depth of partial crystallization for magmas erupted at these locations increases as the crust thickness towards Iceland. This is consistent with the concept that there is greater opportunity for magma to pause and partially crystallize during ascent through thicker crust. The ascent of magmas is apparently slowed and can be arrested at greater depth in thicker crust. This increase in depth of both likely magma lenses and in the average depth of crystallization probably reflects an increase in temperature of the deeper crust to the north (reflecting the influence of the Iceland plume), so that magmas become neutrally buoyant at greater depths.

Despite the fact that there is evidence for relatively shallow magma lenses at some localities, the results of this study lend support to the “many sills” model for crustal accretion along the Reykjanes Ridge. If the gabbro glacier model was appropriate for this ridge then most samples from each locality would provide unequivocal evidence for 46 partial crystallization in magma lenses – that is, calculated pressures for magmas erupted at most localities would be similar over a range of concentrations of MgO. It is noteworthy that at those localities where there is evidence for a magma lens, the range of

MgO concentrations in the erupted lavas is small implying limited partial crystallization in the lens. This effectively rules out the “gabbro glacier” model for crustal accretion along the Reykjanes Ridge.

Several different models for crustal accretion are possible based on the results obtained in this study. One is partial crystallization of batches of magma during ascent through dike-like feeder systems. However, for most localities, there is no clear correlation between MgO and pressure, and this seems to rule out a simple model of partial crystallization during ascent through dikes. The results therefore appear to be consistent with crystallization in multiple sills or lenses located over a range of depths, or crystallization in mush zones. The results do not allow discrimination between these two models. Recently, White et al (2011) have described seismic evidence for the movement of magma from one sill to another over a vertical distance of less than 3 km in the crust beneath Iceland. This distance is well within the error of the pressure calculations reported here, so that the results obtained from petrologic calculations cannot 47 unambiguously distinguish between partial crystallization in closely spaced magma lenses and in crystal mush zones. Interestingly, at the northernmost location, 63.42°N, calculated pressures clusters around two values (283.50 ± .25 MPa and 408.94 ± .35

MPa) (fig. 15). Although these pressures are extremely similar (they agree within the uncertainties of the method), it is tempting to suggest that the results for this location provide evidence for the presence of two magma lenses that are within 2 km of each other, providing direct support for the many sills model. However, the distinction between partial crystallization in multiple sills at different depths in the crust, and partial crystallization in a vertically extensive mush-zone is somewhat arbitrary. The heat generated by partial crystallization in multiple, closely spaced, vertically stacked magma lenses would be sufficient to keep intervening selvedges of ”solid” crust partially molten.

Moreover, the cumulates formed by crystallization along the walls and floor of the magma lenses would contain trapped liquid, so that the crust would have the overall characteristics of a sponge-like mush zone with melt rich regions interspersed with regions that are largely (though not completely) solid. Published studies also lend support to this conclusion. As already noted, (Pan and Batza 2002) showed that the residence time for magma in magma lenses is short, and noted that this favors the many sills model

48 over the gabbro glacier model. However, they also note that plagioclase forms rigid crystal networks in gabbros from oceanic crust, and these rigid frameworks can be interpreted as forming in mushy zones in magma chambers (Pan et al 2003). Kelley and

Barton (2008) also concluded from petrological studies that Icelandic lower crust beneath the active rift zones could best be described as a mush. Geothermal gradients for

Icelandic crust in the active rift zones (Maclennan et al 2002; Kelley, 2009) are high enough to allow the existence of melt pockets in the lower half of the crust.

Chapter 4: The Juan de Fuca Ridge

4.1 Geologic Setting

The Juan de Fuca Ridge is part of the East Pacific Rise and separates the Pacific

Plate from the Juan de Fuca Plate. The ridge is terminated at the southern end by the

Blanco Fracture Zone at 44.5°N (Wanless et al 2010) and runs north for 490 km to a triple junction with the Sovance fracture Zone and the Nootka fault at 48.78°N. The elevation is quite constant varying from 2344 m below sea level in the south to about

2500 m below sea level in the north. However, there is a topographic high at the Axial

Seamount which lies about 1700 m below sea level. Previous workers have divided the

Ridge into seven segments based on first order segmentation features or large offsets of the ridge. These are from south to north, the Cleft, Vance, Axial Seamount, Coaxial,

Cobb, Endeavor, and West Valley segments.(Hooft and Detrick 1995) The Axial

Seamount and Coaxial segments have been combined in this thesis as there is no clear offset between these segments.

Figure 14: Map of the Juan de Fuca ridge showing volcanic segments and fracture zones. Black circles are the location of sample groups Created with

GeoMap. (http://www.geomapapp.org )( Ryan et al 2009.)

Two northwest-southeast trending seamount chains intersect the Juan de Fuca

Ridge. These are the Cobb-Eickelerg seamount chain, which terminates on the ridge at the Axial Seamount, and the Heckle seamount chain, which terminates on the ridge at the

Endeavor Segment. (Fig 15).

The Juan de Fuca Ridge is an intermediate spreading ridge that is not influenced by any know mantle plume. Calculated half-spreading rates (calculated using NUVEl-1

Plate Parameters described by Argus and Gordon 1991) for each segment from south to north and are: 29.8 mm/yr for the Cleft, 29.8 mm/yr for the Vance, 30.0 mm/yr, for the

Axial volcano and Coaxial, 30.2 mm/yr for the Cobb, 30.4 mm/yr for the Endeavor, and

30.5 mm/yr for the West Valley. The spreading rates are therefore essentially constant, but spreading direction is asymmetrical along the Cleft, Coaxial, Endeavor, and West

Valley segments and symmetrical along the Vance and Cobb segments. There is also a difference in the morphology of the ridge axis among the different segments. The ridge axis at the Cleft is an axial high that is broad and has a 2 to 3 km wide axial rift. The

Vance has an 8 km wide axial valley with an axial volcanic ridge. The Cobb segment is similar to the Cleft but the axial rift is deeper and narrower (1 to 2 km wide). The

Endeavour has an axial trough 2 to 3 km wide, which is heavily faulted. The most recent 52 volcanic activity occurred in the Cleft segment and the oldest activity occurred in the

Endeavor and West Valley segments. The different morphologies and ages of volcanism are thought to reflect different stages of cycling between magmatism and tectonism

(Carbott et al 2006). This is in contrast with the axial morphology of the Reykjanes Rides that consists of an axial high with second and third order segmentation or offsets between axial volcanic ridges. Seismic studies indicate that the crustal thickness of the Juan de

Fuca Ridge varies from 7.65 km to 6.5 km from the Blanco Fracture Zone to the northern end of the Cleft segment, and remains at a constant at 6.5 km along the remainder of the ridge. ( Canales et al 2006).

4.2 Results

4.2.1 Samples

A total of 910 samples of fresh glasses from the Juan de Fuca Ridge were retrieved from the petdb.org database. These ranged in composition from basalt to dacite.

In the unfiltered data set the range of composition, expressed as wt. % oxides, is: SiO2 =

46.8 – 63.3; TiO2 0.76 – 3.97; Al2O3 = 6.71 – 17.95; FeO 8.13 – 26.9; MgO = 0.44 –

9.94; CaO 3.67 – 13.7; Na2.O = 2.03 – 4.53; K2O 0.01 – 1.07 and. P2O5 = 0,03 – 1.15.

The range of composition for basaltic samples in this data set is: SiO2 = 46.8 – 52; TiO2

0.79 – 2.95; Al2O3 = 6.71 – 17.95; FeO 8.13 – 26.9; MgO = 1.7 – 9.94; CaO 3.67 –

13.66; Na2.O = 2.03 – 3.34; K2O 0.01 – 1.07 and. P2O5 = 0,03 – .52. Compositional data

for the basaltic glasses show considerable scatter on variation diagrams (fig. 16). Al2O3

and CaO are positively correlated with MgO , whereas SiO2, FeO, TiO2 Na2O, K2O, and

P2O5 show a negative correlation with MgO. The CaO/Al2O3 ratio is negatively correlated with MgO at MgO contents greater than ~7.6, but is positively correlated with

MgO at MgO contents less that ~7.6 Wt % MgO. The scatter shown by the data on all of the variation diagrams is inconsistent with magma evolution via crystallization at a single pressure. The scatter in K2O at MgO=6-8 wt.% (Fig. 16h) is inconsistent with magma evolution at any pressure.

After the filtering the data set, 519 samples remained. These basalts ranged in

composition SiO2 = 48.8 – 51.8; TiO2=1.15 – 2.59; Al2O3 = 13.0– 15.5; FeO =9.95–

14.1; MgO = 5.66 – 7.77; CaO =10 – 13.3; Na2.O = 2.14 – 3.18; K2O =0.03 - 0.35 and.

P2O5 = 0.06 - 0.31. Chemical variations shown by these glasses can be qualitatively explained by crystallization of ol-plag-cpx (±spinel).

Continued

Figure 15: Variation diagrams for major oxides trends. Plots of CaO and

CaO/Al2O3 versus MgO (c and d) provide evidence for the onset of cpx

crystallization at 7.6 wt% MgO. Note the large scatter in K2O at 6-8 wt% MgO (h).

Figure 15 continued

There are positive correlations between MgO and Al2O3, CaO, and CaO/Al2O3, and these trends are consistent with the glasses representing liquids in equilibrium with ol, plag,

and cpx SiO2, contents increase slightly as MgO decreases, whereas the TiO2, FeO, Na2O,

K2O, and P2O5 contents increase strongly as MgO decreases. The strong negative

correlation between MgO and both TiO2 and FeO indicates that near-liquidus crystallization of Fe-Ti oxides was not important during magma evolution. These trends,

coupled with the slight negative correlation between SiO2 and MgO, indicate that the liquids follow a tholeiitic trend generated by crystallization of ol-plag-cpx under relatively reducing conditions. There is no evidence for significant variation in composition with latitude, ie. For along ridge compositional variations.(Fig. 17).

Continued

Figure 16 continued

4.2.2 Pressures of Partial Crystallization

The samples were grouped into 22 locations. Sampling density is greatest in the

Cleft segment (see appendix D). Samples from only two locations yield pressures that cluster within 126 MPa with respects to latitude, longitude, or MgO, and that therefore provide firm evidence for magma lenses (Fig. 18). The results obtained for samples from all other localities indicate crystallization over a range of pressures greater than 126 MPa, with no significant clustering at any particular pressure (Fig. 19). The average pressures of each location (table 2), decreases from south to north along the Cleft from 268 ± 79

MPa to 103 ± 90 MPa. Average pressures remain fairly constant along the Vance, Axial,

Coaxial, and Cobb segments at 100 ± 4.2 MPa with the exception of samples from

45.77°N on the flanks of the Axial volcano that yield a pressure of 53 ± 58 MPa, and from 46.89°N, at the offset between the coaxial and Cobb segments, that yield a pressure of 242 ± 98 MPa Along the Endeavor and West Valley segments the average pressure is

41 ± 2.8 MPa while at the termination of the ridge at the Sovanco Fracture Zone the pressure increases to 137 ± 68 MPa. (Fig. 20)

Figure 17: The Two locations that have evidence of a magma lens. The red border grey boxes highlight the magma lens. The black dashed line is the zero pressure line.

Figure 18: Examples of typical results obtained for most locations along the

Juan de Fuca Ridge. The black dashed line is the zero pressure line.

Table 2: Averaged Pressures for each location of the Juan de Fuca Ridge.

Location Name Latitude °N Longitute °E Corrected Pressure MPa Pressure σ Depth km Depth σ Elevation km 44.42°N 44.42 -130.09 210 66 7.4 2.3 -2406 44.59°N 44.59 -130.39 263 79 8.5 2.8 -2242 44.66°N 44.66 -130.34 178 52 6.3 1.8 -2230 44.76°N 44.76 -130.33 180 64 5.5 2.3 -2360 44.85°N 44.85 -130.30 133 43 4.7 1.5 -2313 44.98°N 44.98 -130.23 103 90 3.6 3.2 -2179 45.12°N 45.12 -130.13 99 50 3.5 1.8 -1492 45.29°N 45.29 -130.11 108 71 3.8 2.5 -2400 45.44°N 45.44 -130.05 86 45 3.0 1.6 -2094 45.60°N 45.60 -129.97 147 84 5.2 3.0 -1623 45.77°N 45.77 -130.02 53 58 1.9 2.0 -2169 45.98°N 45.98 -130.05 127 25 4.5 0.9 -1551 46.13°N 46.13 -129.85 103 16 3.6 0.6 -2066 46.31°N 46.31 -129.67 85 29 3.0 1.0 -2270 46.53°N 46.53 -129.57 116 42 4.1 1.5 -2408 46.89°N 46.89 -129.28 242 98 8.5 3.4 -2374 47.01°N 47.01 -129.27 148 119 5.2 4.2 -2558 47.23°N 47.23 -129.08 52 42 1.8 1.5 -2592 47.71°N 47.71 -129.24 52 39 1.8 1.4 -2492 47.92°N 47.92 -129.39 46 56 1.6 2.0 -1852 48.41°N 48.41 -129.05 24 56 0.8 2.0 -2703 48.76°N 48.76 -128.60 137 68 4.8 2.4 -2149

Figure 19: Upper graph shows the average pressure of partial crystallization

(black circles with one sigma error bars). The zero line represents the seafloor and the lower black line represents the MOHO (Canales et al 2006) . Lower graph:

Average depth of partial crystallization as black circles with one sigma error. The zero line represent sea level. The upper black line is the seafloor and the lower black line is the depth of the MOHO below sea level (Canales et al 2006).

4.3 Discussion

4.3.1 Geochemical Variations

One of the most interesting aspects the Juan de Fuca geochemical data is the range in composition from basalt to dacite in composition. Samples showing this range were recovered from the end of the Blanco Fracture Zone at the beginning of the ridge itself. Although most of these samples are not, strictly speaking, from the ridge, they do potentially provide insight in to how magma compositions and thermal structure change at ridge-transform intersections. The question of how silica-rich magmas are produced is important because it bears upon models for basalt evolution in fracture zones, and it is essential to understand the evolutionary history of the magmas to correctly interpret the significance of calculated pressures for the basalts. Previous studies of basalt-dacite suites at other ridges have shown that fractional crystallization can produce much of the range of composition that is seen in the samples from the Blanco fracture zone.(Clague and

Bunch 1976) However, detailed studies have shown that fractional crystallization alone cannot create magmas of dacitic composition from a basalt parent, and that these compositions are produced by fractional crystallization combined with assimilation of the

65 crust. (Wanless et al 2010). Thermal considerations indicate that the crust will only melt allowing assimilation to occur if it is altered by hydrothermal activity or by interaction with seawater. Wanless et al (2010) suggest that this is the case at the intersection of the

Blanco Fracture zone and the Juan da Fuca Ridge. They also point out that the heat needed to melt the crust for assimilation to occur would be provided by the latent heat released by the high degree of crystallization of basalt parent magmas that is the first step in generating these high silica magmas from MORB. This latter fact is important in that it implies evolution of the basalts via crystallization, allowing non-basaltic samples to be filtered out of the data set, and providing confidence that calculated pressures for the filtered basalt data set represent the pressures of partial crystallization.

The samples from 45.77°N on the flanks of the Axial Volcano that yield anomalously low pressures of 53 ± 58 MPa, and it is important to confirm that they have compositions appropriate for the method used in this study. The Cobb-Eickelerg seamount chain terminates at the Axial Volcano topographic high along the Juan de Fuca.

This seamount chain has been hypothesized to reflect the presence of a “whole” mantle plume interacting with the ridge (Zhao 2007). However, comparison of geochemical data of samples from the Axial volcano provide no evidence (such as anomalous isotopic 66 ratios or enrichments in highly incompatible elements compared with the rest of the ridge) to support the present of a mantle plume (Hooft and Detrick 1995). Furthermore the Cobb-Eickelerg Seamount is no different from other seamount chains terminating on mid-ocean ridge segments like those along the East- Pacific Rise, with the exception that the Cobb-Eickelerg seamounts seem to be larger than normal. Therefore, the Juan de

Fuca Ridge can be considered as a ridge without the influence of a plume.

As noted previously, a number of samples were filtered from the data set because

they are characterized by a most notably anomalously high K2O concentrations. Similar

anomalies are also evident in variation diagrams for other (FeO, Na2O, P2O5) major oxides, although these anomalies are not examined in detail here. All of the samples with anomalous compositions were recovered from the Endeavor segment. The cause of the

anomalous enrichment in K2O needs to be addressed to ensure that the calculated pressures for samples from the Endeavor segment represent the pressures of partial crystallization. There are three possible explanations for the anomalous enrichments in

K2O. One is that the samples had undergone alteration after eruption onto the seafloor.

Such alteration would change the composition of the glass so that pressures calculated from that composition would have no significance. This possibility is discounted because 67 only fresh glasses were included in the database used in this study, and because seawater-

rock interaction would affect the concentrations of other oxides (such as SiO2, MgO, and

CaO) in addition to K2O (Nakamura et al 2007). There is thus no evidence that the concentrations of these oxides have been affected by seawater-glass interaction. Although the Endeavor has a vigorous cross-ridge hydrothermal system (Delaney et al 1992,

Johnson et al 2010), the enrichment of the K2O is not a normal result of high temperature hydrothermal alteration (Nakamura et al 2007), and there is therefore no evidence that the glass compositions have been affected by alteration. Another possibility for the cause of

the anomalous K2O concentrations is that the samples are derived from magmas that were generated from an anomalous mantle source such as a plume (or plume-affected MORB mantle). In this case, calculated pressures would represent the pressures of partial crystallization. However, as already discussed above there is no evidence for a mantle

plume along the ridge. The third possibility for the cause of the anomalous K2O concentrations is mixing of batches of melt in the plumbing system. In this case, interpretation of pressures is complicated, and dependent on the extent to which the hybrid composition produced by mixing is displaced from the ol-plag-cpx cotectic (see discussion by Kelley and Barton, 2008). Magma mixing (including mixing of basalt with

68 partial melts of oceanic crust) is the simplest explanation for the geochemical anomalies that occur in samples recovered along the Endeavor segment (Karsten et al 1989).

However, given the uncertainty in interpreting the cause of the anomalies, samples with anomalous compositions were filtered out of the database and are not further considered.

4.3.2 Pressures of Partial Crystallization and Crustal Accretion

Results obtained for samples from only two localities provide evidence for the presence of melt lenses. The pressure for the inferred magma lens at 45.98°N is 127 ± 25

MPa, whereas that for the inferred magma lens at 46.53°N is 116 ± 42 MPa. The corresponding depths of these lenses are 4.47 ± .89 km and 4.08 ± 1.5 km. At both of these locations, pressures vary over a range of less than 126 MPa with respect to longitude and latitude, but show little variation with MgO. As discussed for melt lenses beneath the Reykjanes ridge, these results imply that magma ponded during ascent, but that the residence time in the magma lens is too short for significant partial crystallization to occur.

When compared to the seismically imaged MOHO, most of the average crystallization pressures obtained in this study yield depths that occur within the crust

69 with the exception of pressures calculated for samples from 44.5°N and 46.89°N, that yield depths consistent, within error, of the base of the crust. The most southerly location occurs at Blanco Fracture Zone, whereas the location to the north occurs at the offset between the Coaxial and Cobb segments. In contrast to the Reykjanes Ridge, it is not possible to use the petrologic method utilized in this work to predict crustal thickness along the Juan da Fuca ridge. This implies that in the more dynamically active plumbing systems of intermediate spreading ridges, the chemical signature of crystallization near the base of the crust is, for the most part, overprinted by processes such as crystallization and mixing in the shallower crust.

Although there is evidence for magma lenses at two localities, the results for most localities indicate no clear correlation between MgO and pressure, and thus the results appear to be consistent with crystallization in multiple sills or lenses located over a range of depths, Therefore, the results support the many sills model over the gabbro glacier model for crustal accretion. One location of particular interest is that at 44.66°N, in the Cleft segment. A large number of samples have been recovered from this location and the calculated pressures for these samples range over 300 MPa.(Fig 17c) The MgO contents of the samples range over almost 2 wt. %, but there is not apparent correlation 70 between P and MgO. In light of the recent conclusion of White et al (2011) that seismic evidence reveals movement of magma from one sill to another over a vertical distance of less than 3 km in the crust beneath Iceland, it is pertinent to inquire whether there is any evidence for crystallization in sills at 44.66°N, In fact there is, based on analyses obtained by Stakes et al (2006) for samples from nine individual lava flows from the Cleft segment. Calculated pressures for samples from two of these lava flows provide strong evidence for magma lenses at 44.66°N and 44.77°N (Figure 21). The pressures calculated for samples from these flows cluster in a narrow range (<126 MPa), and are obtained from samples that show a range of MgO of 1.27 and 1.52 wt% respectively. The pressures of the lenses are 140±22 MPa and 123 ± 30 MPa from south to north. The calculated depths for these lenses are at depths 4.91± .77km and 4.33 ± 1.07 km respectively. These depths are shallower than the average depths for samples from

44.66°N, but the depths correlate very well with the depths to seismically imaged sills at between 4 and 5 km (Canales et al 2009). The results for these lava flows together with the fact that other samples from 44.66°N show evidence for crystallization over a relatively wide range of pressure strongly suggests that melt lenses represent the shallowest features of at least some MOR plumbing systems.

Figure 20: The results from two lava flows along the Cleft segment that provide strong evidence for sub-crustal magma lenses. The grey boxes highlight the pressures of magma lenses.

It is possible that multiple vertically stacked magma lenses occur at this locality, but that these cannot be imaged with the method used here because the spacing of magma lenses

72 is within the error of the pressure calculations. Nevertheless, the results obtained for samples from individual lava flows suggests that future studies designed to establish the depths of magma lenses using petrologic methods should utilize strategically collected samples for which the geologic relations are known.

Samples from seven of the flows studied by Stakes et al (2006) show no evidence of partial crystallization in magma lenses. When these results are combined with results from other localities along the Juan da Fuca Ridge, it is clear that magma plumbing systems are complex, and that evidence for vertical magma transport is preserved in the composition of glasses. Based on the work of White et al (2012) in Iceland, as well as studies of ophiolites, it is likely that much of this vertical transport occurs in dikes. It is likely that some partial crystallization occurs along the walls of these dikes during pauses in eruptive activity, accounting for the range of pressures recorded in the compositions of glasses sampled from many locations along the Juan da Fuca Ridge. Complex sub- volcanic plumbing systems suggest extremely complex magma dynamics during eruptions, and this explains the complexities observed in the petrology and geochemistry of mid ocean ridge basalt. Additional work is required before the cause of these complexities can be unraveled. 73

Chapter 5: Comparison of Petrological Methods to Determine the Pressures

of Partial Crystallization

An additional goal of this study was to compare results obtained with the Kelley and Barton geobarometer with those obtained using the geobarometer described by

Herzberg (2004). The latter is based on the same theory and a principle as that described by Kelley and Barton, but uses a different projection scheme as the basis for calculation.

Calculating pressures using both methods for the same samples (the filtered data set of glasses from the Reykjanes and Juan da Fuca Ridges) provide a good basis for such a comparison. The Herzberg method yields lower pressures for most samples from both the

Reykjanes and Juan de Fuca ridges. The average difference (118 MPa or 4.2 km) between pressures calculated using both methods for the Reykjanes Ridge samples was mostly within the reported errors for both methods. The average difference (65 MPa or 2.3 km) between pressures calculated using both methods for the Juan de Fuca samples are also within reported errors for each method (Fig 22)

The source of the difference between pressures calculated using these methods (this difference can be 200-300 MPa for samples from Iceland) is poorly understood but appears to reside in the recalculation schemes used to determine the mineral components used in projecting natural compositions into pseudoquaternlry phase diagrams.

The comparative study reported here does not provides conclusive evidence as to which method yields the most accurate estimate of pressure, Despite this, I believe that the Kelley and Barton geobarometer is the better of the two as it returns pressures that are in better agreement with the results of seismic studies for the depth of the MOHO along the Reykjanes and for sills beneath both ridges. The Herzberg method also yields negative pressures for a far larger number of samples than the Kelley and Barton method

Chapter 6: Conclusions

The hypothesis as the heat flux increases due to faster spreading, or proximity to a mantle plume, the temperature of the crust will increase allowing the magma to reach shallower levels prior to cooling and crystallization, leading to shallower magma chambers than in colder, low flux, slow spreading ridges has been proven false. This leads me to suggestion that as the heat flow though the crust and crustal thickness increases, the warmer crust and the magma have a smaller density contrast. This decrease in density contrast might possibly neutralize the positive buoyancy force that magmas need to both rise through the crust and to fracture the overlying rocks and create pathways to shallower depths and, eventually, to the surface.

Results from this study show that the many sills or crystal mush model of accretion best explains the range of pressures obtained for samples from most localities along the Reykjanes and Juan de Fuca Ridges. It is suggested that currently available

77 petrological methods may not be able to distinguish between the many sills and the magma mush models if the sills, or melt lenses are closely spaces (ie. vertical spacing less than ~3 km). The final conclusion is that petrologic methods may by used to constrain the depth of the MOHO beneath slow spreading ridges even when little to no seismic data are available.

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