A GEOCHEMICAL APPROACH TO MANTLE AND CRUSTAL DYNAMICS IN CENTRAL ANATOLIA

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A GEOCHEMICAL APPROACH TO MANTLE AND CRUSTAL DYNAMICS IN CENTRAL ANATOLIA

ABSTRACT

This dissertation provides new geochemical analyses of Quaternary mafic and intermediate products erupted in south-Central Anatolia. We explore the petrogenesis of primitive mafic cinder cone products erupted from the Hasandağ Cinder Cone Province (HCCP) and the adjacent Karapınar Volcanic Field (KVF) as well as intermediate compositions (basaltic andesites, andesites and dacites) erupted contemporaneously from the Hasandağ stratovolcano.

 

I begin by using whole rock geochemical analyses alongside Sr-Nd-Hf-Pb isotopic data to explore the mechanisms of mantle melting that generated the HCCP and KVF lavas and compare their petrogenesis to those observed in other Anatolian volcanic provinces. In doing so, I document changes in magma chemistry that reflect the complex and evolving tectonic regime, build upon previously proposed mechanisms of lithospheric removal, and clarify the evolution of the mantle beneath Central Anatolia in order to better understand the complex regional geodynamic history. Major and trace element geochemistry indicate melting of the spinel-bearing lithosphere to produce individual small mafic magma batches of heterogeneous melts and further suggest contribution from metasomatic phases. Isotopic analyses require contributions from the metasomatized Anatolian subcontinental lithosphere along with input from a spatially heterogeneous sublithospheric mantle. The mafic lavas record an increase in degree of melting with depth accompanied by gradual reduction in the contribution from metasomatic phases, which is indicative of lithospheric drip melting. This geochemical signature is also observed in Western Anatolian late Miocene and Quaternary mafic lavas, but is not exhibited in the Quaternary East Anatolian lavas or the Sivas Basin basalts and basanites in north-Central Anatolia. Our findings demonstrate a complex interplay between lithospheric and asthenospheric source domains and suggest drip melting was initiated after delamination of the subducted NeoTethys slab, resulting in upwelling asthenosphere and destabilization of the remaining lithosphere which melted to produce the observed mafic volcanism at the HCCP and KVF.

 

 

Next, I examine the crustal dynamics of Central Anatolia by assessing the genesis of intermediate compositions erupted during the most recent episodes of volcanism at the Hasandağ stratovolcano (Neo- and Mesovolcano, <1 Ma). My research is motivated by questions about the physical state of magma reservoirs and the processes that operate within them to produce eruptible bodies of intermediate magma. I implement whole rock major and trace element geochemistry, Sr isotopic analyses, petrographic observations and detailed zoning transects of mineral compositional data to evaluate the contributions from fractional crystallization, magma mixing and mafic recharge processes in a shallow crustal chamber, ultimately establishing a petrogenetic model for the abundant intermediate lavas (i.e., basaltic andesites, andesites and dacites). I find that the Hasandağ plumbing structure exhibits a homogenous and shallow (~4 km) rhyodacitic magma chamber with a highly viscous crystalline framework composed of abundant An40 plagioclase crystals. This framework is fractured and remobilized through periodic recharge by mafic magma resulting in heterogeneous crystal populations, distinct textures and numerous mineral disequilibrium features. Similar observations at the Erciyes stratovolcano just 150 km north of Hasandağ suggest magma mixing is regionally ubiquitous in Central Anatolian stratovolcanoes.

 

I complement our observations of magma mixing via kinetic diffusion modeling of MgO in plagioclase in order to evaluate mixing to eruption timescales. I employ a forward in time finite difference scheme to assess the diffusional relaxation of magnesium across the core-rim boundaries of plagioclase crystals indicative of interaction with a mafic recharge magma. I find that the intermediate samples exhibit a bimodal distribution where half of the results are best fit by hour to day timescale calculations and the other half indicate mixing to eruption takes place on the order of weeks to months. I interpret these results to indicate mixing at very shallow depths, possibly within the conduit and during eruption, or at greater depths within the magma reservoir, respectively. Individual thin sections often document both short and long timescales, suggesting these crystals were gathered together in the same magma batch prior to eruption.

I end my dissertation with a discussion on undergraduate idea development with regard to the paradigm of Plate Tectonics. I build upon the learning progression previously developed for K-12 students and add hypothetical levels to two of the three progress variables which document new ideas expressed by the geoscience majors. Interviews with 13 upper-level undergraduate students enrolled within a newly developed plate tectonics course both pre- and post-instruction focus our efforts on the implementation of complex systems analysis, a teaching methodology that is now stressed by the geoscience community for the development of successful geoscientists. My findings reveal the importance of expert terminology (e.g., lithosphere and asthenosphere) to the construction of sophisticated explanatory models for the complex subsystems of plate tectonics. I develop an upper anchor that focuses on the oceanic plate cycle as opposed to phenomena which result from plate motion to encourage the implementation of expert terminology via the explanation of partial melting. I urge college-level instructors to consult this upper anchor and the additional levels of the hypothetical learning progression to develop curriculum which requires model building and complex systems analysis to best prepare geoscience undergraduates for diverse career paths.

 

 

 

TABLE OF CONTENTS

LIST OF FIGURES ……………………………………………………………………………………..x

LIST OF TABLES ………………………………………………………………………………………xvii

ACKNOWLEDGEMENTS …………………………………………………………………………..xviii

Chapter 1  Introduction …………………………………………………………………………………1

1.1 A Brief Overview of Anatolian Tectonics and Volcanism ……………………….1

1.2 Origin of Mafic Lavas at the Hasandağ Cinder Cone Province and Karapınar

Volcanic Field in south-Central Anatolia………………………………………………3

1.3 Intermediate Magma Petrogenesis at the Hasandağ Stratovolcano …………….5 1.4 Mixing to Eruption Timescales …………………………………………………………..7

1.5 Development of a Learning Progression for Undergraduate Geoscience

Majors around the Theory of Plate Tectonics ……………………………………….8

1.6 References Cited…………………………………………………………………………………11

1.7 Figures………………………………………………………………………………………………16

 

Chapter 2  Post-Delamination Magmatism in South-Central Anatolia …………………..17

Abstract ……………………………………………………………………………………………….17

2.1 Introduction …………………………………………………………………………………….18

2.1.1 Tectonic Setting and Geologic History of Anatolia………………………..19

2.2 Methods ………………………………………………………………………………………….22

2.2.1 Samples and Analytical Techniques ……………………………………………22

2.3 Results ……………………………………………………………………………………………24

2.3.1 Petrography and Mineral Chemistry of Hasandag Cinder Cone

Province Mafic Lavas ………………………………………………………………..24

2.3.2 Major Element Geochemistry…………………………………………………….25

2.3.3 Trace Element Geochemistry …………………………………………………….25

2.3.4 Sr, Nd, Pb and Hf Isotopes ………………………………………………………..27

2.4 Discussion ………………………………………………………………………………………28

2.4.1 Source Regions of South-Central Anatolian Volcanism………………….29

2.4.2 Crustal Contamination ……………………………………………………………..30 2.4.3 Conditions of Melting Beneath South-Central Anatolia  …………………31

2.4.4 The Effects of Slab Removal ……………………………………………………..32

2.5 Conclusion………………………………………………………………………………………36

2.6 References Cited ………………………………………………………………………………38 2.7 Figures……………………………………………………………………………………………47

2.8 Tables …………………………………………………………………………………………….58 Chapter 3 Recycling and Recharge Processes at the Hasandag Stratovolcano,

Central Anatolia: Insights on Intermediate Magma Petrogenesis via

Plagioclase Textures and Zoning Patterns  …………………………………………………64

Abstract ……………………………………………………………………………………………….64 3.1 Introduction …………………………………………………………………………………….65

3.2 Geologic Background ……………………………………………………………………….66

3.3 Methods and Analytical Techniques ……………………………………………………68

3.3.1 Major and Trace Element Methods …………………………………………….68

3.3.2 Electron Microprobe Analysis ……………………………………………………69

3.4 Results ……………………………………………………………………………………………70

3.4.1 Major Element Geochemistry…………………………………………………….70

3.4.2 Trace Element Geochemistry …………………………………………………….70 3.4.3 87Sr/86Sr Isotopes …………………………………………………………………….71 3.4.4 Petrography ……………………………………………………………………………71

3.4.5 Plagioclase Morphologic, Textural and Compositional

Characteristics ………………………………………………………………………….73

3.5 Discussion ………………………………………………………………………………………75

3.5.1 Processes Recorded in Plagioclase Growth Patterns ………………………75

3.5.2 The Role of Fractional Crystallization and Magma Mixing a

Hasandağ…………………………………………………………………………………….77

3.5.3 Mixing to Eruption Timescales ………………………………………………….79

3.5.4 Crustal Structure ……………………………………………………………………..81

3.5.4.1 Magmatic Conditions inferred from the Hasandağ and HCCP

Mineral Assemblages…………………………………………………………………81

3.5.4.2 Dynamics of the Hasandağ Magmatic System ……………………..83

3.6 Conclusion………………………………………………………………………………………85 3.7 References Cited ………………………………………………………………………………87

3.8 Figures……………………………………………………………………………………………93

3.9 Tables …………………………………………………………………………………………….106

Chapter 4  Timescales of Magma Mixing ………………………………………………………..117

Abstract ……………………………………………………………………………………………….117 4.1 Introduction …………………………………………………………………………………….118

4.2 Diffusion Chronometry ……………………………………………………………………..119

4.2.1 Basic Theory and Approach ………………………………………………………119 4.2.2 Previous Work in Diffusion Chronometry ……………………………………120

4.2.3 Aplication to Plagioclase …………………………………………………………..121

4.3 Modeling Plagioclase Crystals ……………………………………………………………121

4.3.1 Estimating the Initial Concentration Distribution of Mg in

Plagioclase ………………………………………………………………………………121

4.3.2 Boundary Conditions ……………………………………………………………….122

4.3.3 Numerical Modeling ………………………………………………………………..122

4.4 Uncertainties ……………………………………………………………………………………124

4.5 Results ……………………………………………………………………………………………126

4.5.1 Sample Selection………………………………………………………………………..126

4.5.2 Element Maps …………………………………………………………………………128

4.5.3 Timescales of Mixing to Eruption ………………………………………………128

4.6 Discussion ………………………………………………………………………………………129

4.6.1 Distribution of Timescale Calculations………………………………………..129 4.6.2 Effects of Temperature ……………………………………………………………..130

4.6.3 Effects of Anisotropy ……………………………………………………………….131

4.7 Conclusion………………………………………………………………………………………132

4.8 References Cited ………………………………………………………………………………134 4.9 Figures……………………………………………………………………………………………138

4.10 Tables …………………………………………………………………………………………..149

Chapter 5  Mapping Undergraduate Understanding of Plate Tectonics: A Learning

Progressions Approach …………………………………………………………………………..150

Abstract ……………………………………………………………………………………………….150 5.1 Introduction …………………………………………………………………………………….151 5.2 Learning Progressions ……………………………………………………………………….153

5.3 Methodological Approach ………………………………………………………………….155

5.3.1 Initial Hypothetical Learning Progression ……………………………………155

5.3.2 Interview Protocol and Analysis …………………………………………………156

5.4 Results ……………………………………………………………………………………………158

5.5 Discussion and Implications ………………………………………………………………162

5.5.1 Terminology as a Barrier to Complex Systems Thinking ………………..163

5.5.2 Effective Teaching Practices ……………………………………………………..164

5.5.3 Teaching the History of Science …………………………………………………166

5.6 Conclusions …………………………………………………………………………………….167 5.7 References Cited ………………………………………………………………………………168

5.8 Figures……………………………………………………………………………………………171

5.9 Tables …………………………………………………………………………………………….172

Chapter 6  Summary of Conclusions ……………………………………………………………….176

6.1 Generation of Mafic Magmas at the HCCP and KVF ……………………………..176

6.2 Generation of Intermediate Magmas at the Hasandağ Stratovolcano …………177

6.3 Mixing to Eruption Timescales Recorded in the Hasandağ Intermediate

Lavas …………………………………………………………………………………………….178

6.4 Developing a Learning Progression for the Theory of Plate Tectonics at

the Undergraduate Level …………………………………………………………………..179

6.5 References Cited ………………………………………………………………………………181

 

Appendix A MATLAB Script for Mg Diffusion Modeling and Accompanying

Mineral Composition Data Collected from the EPMA ……………………………….182 Appendix B Interview Protocol for Undergraduate Interviews and Student

Leveling Information for Each Progress Variable ……………………………………….205

Chapter 1

 

Introduction

This dissertation investigates mafic and intermediate magma genesis in Central Anatolia in context of the geodynamic history of the eastern Mediterranean region, and includes a chapter detailing research on student explanatory model building around the theory of plate tectonics. In Chapters 2 through 4, I examine both mantle and crustal processes with a specific focus on Hasandağ. These processes are responsible for the array of eruptive styles and compositions within the Quaternary Central Anatolian volcanics. In Chapter 5, I focus on science education work, which is centered on undergraduate idea development around the complex system of plate tectonics.

1.1 A Brief Overview of Anatolian Tectonics and Volcanism

The Anatolian microplate comprises a small, tectonically complicated portion of the Alpine-Himalayan orogenic belt located at the juncture of Eurasia, Africa and Arabia. The land masses which comprise present-day Turkey were once positioned at the collisional boundary of two mega continents, Gondwana and Laurentia that were amalgamated during closure of the Tethys oceans in the Cretaceous and Tertiary (Bozkurt and Mittwede, 2001). Modern tectonics are a manifestation of multiple geologic eras of intracontinental convergence and the geologically recent (~5 Ma) deformation related to escape tectonism (Bozkurt et al., 2001).

 

The geologic evolution of the eastern Mediterranean region is governed by the fate of the Tethyn Oceans. By the early Mesozoic, the Paleotethys Ocean was closed, and by the late Triassic, rifting began in the eastern Mediterranean to form the Mesozoic Neotethys Ocean (Bozkurt, 2001). Rifting ceased in the middle Jurassic, and by the late Cretaceous convergence between the African and Eurasian plates resulted in continuous closure of the Neotethys Ocean basins. In the late Eocene the northern portion of the Neotethys Ocean was closed, culminating in the collision of Eurasia with the Anatolian-Iranian Platform along the Izmir-Ankara-Erzincan suture zone (Adamia et al., 1981; Sengör and Yilmaz, 1981; Figire 1-1). The trench then relocated south of Cyprus where inferred horizontal subduction began and final collision of Arabia with Eurasia occurred in the middle Miocene along the Bitlis-Zagros Suture (McKenzie, 1969; Dewey et al., 1973; Hempton, 1987; Yilmaz, 1993; Sengor et al., 2003; Bartol and Govers, 2014; Figure 1-1). Steepening of the Neotethys slab resulted in southward roll-back of the subducted oceanic crust, potentially along with the overlying mantle lithosphere from the Izmir-Ankara-Erzincan suture zone to the southern margin of the Anatolide-Taurides (Bartol and Govers, 2014; Figure 1-1).

The volcanics erupted in East, West and Central Anatolia can be explained via the evolution of the regional plate boundaries and the history of the Tethyn slabs. In the East Anatolian contractional province, anomalously thin lithosphere (Sandvol et al., 2003; Angus et al., 2006), an average elevation of ~2 km above sea level, and extensive volcanism from the mid-Miocene to the present (Pearce et al., 1990; Keskin, 2003) suggest decompression melting of upwelling asthenosphere above a steepening and broken slab (Pearce et al., 1990; Zor 2008; Keskin, 2007). In West Anatolia, the current tectonics are governed by extension related to rollback of the Aegean slab which began in the midMiocene (Lonergan and White, 1997). Lavas erupted in the late Miocene through Quaternary include contributions from the metasomatized lithospheric mantle and a sublithospheric MORB-like source, resulting from the onset of rollback followed by decompression melting of the upwelling asthenosphere (Chakrabarti et al., 2012). The Central Anatolian region has also undergone extensional thinning as a result of ~15 mm/yr WSW migration of the microcontinent (e.g., Burke and Sengör, 1986), yet the geometry of the slab and its involvement in volcanism has been more challenging to constrain. Evidence for >1 km of uplift (Cosentino et al., 2012) and widespread volcanism since the midMiocene (e.g., Le Pennec et al., 2005) are interpreted as a result of the sinking and retreat of the near-horizontal Neotethyan slab inferred across both East and Central Anatolia ~2015 Ma (Bartol and Govers, 2014).

This dissertation probes the origin of Quaternary volcanism in Central Anatolia and further clarifies the mechanisms of and source components involved in melt production with respect to the complex tectonic regime and the delaminated Neotethyn slab. I explore mafic cinder cone products at the Hasandağ Cinder Cone Province (HCCP) and Karapınar Volcanic Field (KVF) in south-Central Anatolia as well as intermediate compositions (basaltic andesites – dacites) at the adjacent Hasandağ stratovolcano. In the following pages, I introduce each research chapter and provide an overview of my motivations for the study, as well as the major findings and interpretations.

1.2 Origin of Mafic Lavas at the Hasandağ Cinder Cone Province and Karapınar Volcanic Field in south-Central Anatolia

The Anatolian microplate is one of the most tectonically dynamic regions in the world, offering an ideal location in which to study the effects of mantle processes on mafic volcanism. The Anatolian lithosphere has experienced a complex tectonic history with periods of convergence during closure of the Tethys Ocean followed by post-Miocene extension. The current tectonic regime reflects west-southwest movement of Anatolia in response to collision between the Afro-Arabian and Eurasian plates along the Bitlis-Zagros Suture Zone (McKenzie, 1969; Dewey et al., 1973; Şengör and Kidd, 1979; Hempton, 1987; Yilmaz et al., 1993), which is accommodated by microplate-scale strike-slip and transtensional faulting. In Central Anatolia, late Miocene through Quaternary volcanism has developed under this extensional regime and the formation of pull-apart extensional basins associated with regional oblique-slip faults resulted in a complex suite of volcanic products. Plio-Quaternary volcanic cones outcrop along faults and lineations and major Miocene and Pliocene stratovolcanoes were preferentially emplaced in extensional basins.

 

Chapter 2 focuses specifically on the petrogenesis of mafic volcanic products erupted at the Quaternary Hasandağ Cinder Cone Province (HCCP) and adjacent Karapınar Volcanic Field (KVF). Continental basalts act as compositional and thermal indicators of their mantle sources and provide evidence about the processes that occur within both the lithosphere and the convecting mantle. I use whole rock major and trace element geochemistry and Sr-Nd-Pb-Hf radiogenic isotope compositions to explore the following unresolved questions: 1) by what mechanism(s) and to what degree do the south-Central Anatolian mafic lavas sample asthenospheric and lithospheric source domains, and 2) how does the petrogenesis of south-Central Anatolian mafic lavas compare to those of other Anatolian mafic volcanic products. I clarify the evolution of the mantle beneath Central Anatolia in conjunction with the complex regional geodynamics.

 

My findings require a complex interplay between lithospheric and asthenospheric processes and source domains and build upon the previously proposed mechanism and consequences of lithospheric removal. Results from trace element and isotopic data show that contributions are required from the metasomatized Anatolian subcontinental lithosphere and the spatially heterogeneous sub-lithospheric mantle domain. Moreover, a systematic decrease in degree of melting with depth is documented within the geochemistry of the HCCP and KVF lavas which, accompanied by a gradual reduction in the contribution from metasomatic phases, suggests drip magmatism as the mechanism for mafic magma generation (Elkins-Tanton, 2007; Holbig and Grove, 2008). These observations are interpreted in light of seismological and geodynamic evidence for lithospheric delamination of the subducted NeoTethys slab (Bartol and Govers, 2014) and suggest that removal of the slab caused density instability of the remaining lithosphere, ultimately resulting in localized drip melting of the south-Central Anatolian lithosphere. Geochemical evidence for this mechanism is also apparent in late Miocene and Quaternary mafic lavas from Western Anatolia, but the signature of drip magmatism is not exhibited in the Quaternary East Anatolian lavas, nor is it ubiquitous across Central Anatolia (e.g., the Sivas

Basin). Ultimately, the HCCP and KVF magmas are products of physical mixing of the lithospheric melts produced via drip magmatism and asthenospheric melts produced via flux melting as the drip devolatilized.

 

This chapter is in preparation for submission (Gall et al., in prep) to Earth and

Planetary Science Letters with Barry Hanan, Biltan Kürkçüoğlu, Kaan Sayit, Tekin Yürür, Megan Pickard, Erdal Şen, Pinar Şen and Tanya Furman as the co-authors.

1.3 Intermediate Magma Petrogenesis at the Hasandağ Stratovolcano

In Chapter 3, I redirect my research to focus on intermediate magma genesis and the processes operating to produce these compositions within a shallow magma chamber in the Anatolian crust. I focus on the youngest eruptive products (<1 Ma) from the Hasandağ stratovolcano and perform abundant mineral-scale analyses on a suite of samples comprising basaltic andesite, andesite and dacite compositions. This research is motivated by important questions regarding the physical state of magma reservoirs, the assemblage of magma bodies, and the importance of recharge magmas in the initiation of eruptions (Cashman and Blundy, 2013; Eichelberger et al., 2000; Sparks et al., 1977).

 

The 13+ Ma Hasandağ stratovolcano has experienced four stages of edifice construction; here I focus on the Meso- and Neo-Hasandağ stages (<1 Ma – 6ka) which are dominated by andesite and dacite lava dome flows, and are significantly different in style and composition from the adjacent HCCP and KVF mafic compositions discussed in Chapter 2. The intermediate compositions show abundant evidence for magma mixing and thermochemical disequilibrium in hand sample and thin section, which prompt investigation of the dominant mechanisms that control magma genesis and eruption at arc volcanoes.

 

Previous researchers document similar features in the Hasandağ intermediate compositions and explore magma mixing via preliminary petrographic observations, isotopic and trace element data, and quantitative modeling (Aydar and Gourgaud, 1998; Deniel et al., 1998; Dogan et al., 2008). I build upon this previous research and use detailed micro-texture, compositional zoning and mineral diffusion analysis to elucidate crystal recycling and magma recharge processes. I focus specifically on plagioclase feldspar because it crystallizes over nearly the entire compositional and temperature range of igneous differentiation and shows pronounced compositional zoning. MgO concentrations within select plagioclase traverses in all lava types are further used in diffusion modeling to assess mixing to eruption timescales. Detailed thermobarometry calculations in several mineral systems are carried out across the compositional spectrum in order to construct a model of the Hasandağ plumbing structure and better constrain where the processes of recharge and recycling operate.

 

I complement these micro-analyses with preliminary Sr isotopic analyses as well as bulk rock major and trace element geochemical data and linear least squares modeling results to compare the relative contributions of fractional crystallization and magma mixing to the petrogenesis of the Hasandağ intermediate compositions. I use the HCCP basaltic cinder cone products as a proxy for the parental magma compositions, which both fractionated and mixed to form the observed basaltic andesites, andesites and dacites. Finally, I compare our results to the Erciyes stratovolcano, roughly 150 km to the northeast of Hasandağ in order assess the ubiquity of these processes at the regional scale.

 

My findings suggest that Hasandağ is underlain by a homogenous shallow (~4 km) rhyodacitic magma chamber in which An40 plagioclase crystals are abundant. The highly viscous crystalline framework is remobilized through periodic recharge by hotter, more mafic magma leading to the distinct textures and mineral disequilibrium features as evidenced in our samples. This process occurs on the order of days to months, confirming that mixing and eruption processes are intimately linked (Cooper, 2017; Cooper and Kent, 2014; Kent et al., 2010). Similar plagioclase textures are seen within the Erciyes dacites (Dogan et al., 2013, 2011), and Erciyes basaltic andesite compositions overlap those erupted from Hasandağ in major and trace element plots, although basaltic equivalents of the HCCP are rare to absent at Erciyes. Taken together, these observations indicate that magma mixing is a ubiquitous process occurring at the major stratovolcanoes within the Central Anatolian region.

 

This chapter is in preparation for submission (Gall et al., in prep) to the Journal of

Volcanology and Geothermal Research with Jacob Cipar, Biltan Kürkçüoğlu, Katherine Crispin and Tanya Furman as the co-authors.

1.4 Mixing to Eruption Timescales

In Chapter 4, I expand on the mixing to eruption timescale results presented in Chapter 3. Crystal zoning patterns have been used to examine a large range of magmatic processes such as mixing, degassing, transport and fractionation (e.g., Costa et al., 2008; Ginibre and Wörner, 2007; Ruprecht and Wörner, 2007; Singer et al., 1995). The improvement of precision and spatial resolution of in situ measurements of mineral compositions has provided the opportunity to experimentally determine the rates at which elements diffuse. These advances have allowed for the extraction of temporal information from compositional zoning profiles in order to discern the timescales of individual processes (e.g., Chakraborty, 2008; Costa et al., 2008; Turner and Costa, 2007).

 

In this chapter, I provide a more detailed look into the kinetic modeling of major and trace element zonation profiles, focusing specifically on magnesium diffusion in plagioclase because it is best able capture the short timescales of magma recharge, mixing and eruption due to its fast diffusivity (i.e., the diffusion coefficient for magnesium (DMg) in anorthite is ~15 times greater than DCa and ~120 times greater than DSr; LaTourrette and Wasserburg, 1998). I begin with a brief overview of diffusion chronometry theory and the major uncertainties associated with these calculations. I also elaborate on the equations and finite difference scheme used to forward model the diffusion of magnesium in plagioclase. The results include 60 plagioclase core to rim analytical traverses collected from eight basaltic andesite, andesite and dacite thin sections. Mineral major and minor element compositions were collected on the electron microprobe along with back-scattered electron images of each crystal and select element maps, which guided in boundary identification and characterization of zoning patterns. Fourteen traverses were suitable for quantitative diffusion analyses after consideration of point spacing and data quality.

 

The results show a bimodal distribution, where half of the profiles are fit by hour to day timescales, while the other half return values of weeks to months. Though uncertainty associated with diffusion modeling calculations is not insignificant, the resulting error does not change the bimodal distribution exhibited by the results. I suggest that the shortest timescale results indicate mixing within the conduit, while the longer timescales indicate mixing within the magma reservoir. Each thin section includes a heterogenous crystal population documenting both the shorter and longer timescales that must have been gathered together in the same magma batch prior to or during eruption. To confirm these results, diffusion modeling of other crystal systems should be implemented within the Hasandağ intermediate compositions (e.g., Fe-Mg diffusion within olivine).

 

This chapter will be combined with chapter 3 in preparation for submission (Gall et al., in prep) to the Journal of Volcanology and Geothermal Research with Jacob Cipar, Biltan Kürkçüoğlu, Katherine Crispin and Tanya Furman as the co-authors.

1.5 Development of a Learning Progression for Undergraduate Geoscience Majors around the Theory of Plate Tectonics

In Chapter 5, I focus on Earth Science education, particularly at the undergraduate level and with regard to the theory of plate tectonics. I build upon research carried out at the K-12 level, which resulted in the construction of a learning progression around the Big Idea of plate tectonics. Learning progressions are used in science education research to describe how students grow in sophistication towards a Big Idea in science (Corcoran and Mosher, 2009). I expand the K-12 learning progression presented in McDonald et al. (2018) to include hypothetical levels of sophistication around student explanations of tectonic processes and associated phenomena at the undergraduate level.

 

The geosciences community stresses that the application of complex systems analysis is a key learning outcome at all levels of the earth science curriculum, recognizing the importance of complex systems thinking to becoming a successful geoscientist in academia, industry, the government and the private sector. In this contribution, I explore the implementation of complex systems analysis around the paradigm of plate tectonics. By interviewing 13 undergraduates enrolled in an experimental geoscience course on plate tectonics to identify learning trajectories and mental models held by upper-level geoscience majors. Coupled pre- and post-instructional interviews allow me to identify emergent themes not expressed by the K-12 students and organized them into levels to develop the undergraduate plate tectonics hypothetical learning progression.

 

The three progress variables (i.e., Plate Movement, Mechanism of Plate Motion and Plate System) developed within the K-12 study also encompass the themes discussed at the undergraduate level, but the more sophisticated and nuanced ideas expressed by the undergraduate geoscience majors requires the addition of higher levels within the Plate Mechanism and Plate System progress variables. Student explanations of the Plate System progress variable remained the messiest, often coinciding with the incorrect usage of expert terminology such as lithosphere and asthenosphere.

 

Perhaps the most significant and unexpected finding is that of the importance of terminology. Students who allowed terminology to hold explanatory power used it successfully to guide their understanding of processes and their construction of causal, system-level models. The terms lithosphere and asthenosphere have perhaps the greatest explanatory power; students with the most advanced explanations of plate tectonics also provided normative descriptions of these terms and implemented them within their explanations of tectonic processes. I suggest that focused instruction on partial melting at the mid-ocean ridges can promote the synthesis of plate tectonics terminology, specifically with regard to the terms lithosphere and asthenosphere, ultimately aiding in understanding of process and construction of explanatory models.

 

I construct an upper anchor for plate tectonics instruction at the college level which focuses on the oceanic plate cycle as opposed to phenomena which result from plate motion that are highlighted in the K-12 upper anchor (volcanoes, earthquakes and mountains). By implementing this upper anchor within their classrooms, consulting the learning progression to see where student explanations of plate tectonic subsystems most often break down, and emphasizing the explanatory power of terminology, instructors will better prepare their students to think of plate tectonics as a complex system, and ultimately for successful careers as geoscience researchers or practitioners.

 

This chapter is in preparation for submission (Gall et al., in prep) to the Journal of Geoscience Education with Erica Pitcavage, Scott McDonald and Tanya Furman as the coauthors.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1.6 REFERENCES CITED

Adamia, S.A., Chkhotua, T., Kekelia, M., Lordkipanidze, M., Shavishvili, I., Zakariadze, 1981. Tectonics of the Caucasus and adjoining regions: implications for the evolution of the Tethys ocean. J. Struct. Geol. 3, 437–447. doi:10.1016/01918141(81)90043-2

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1.7 FIGURES

 

Figure 1-1. Tectonic map of Anatolia and surrounding regions (after Altunkaynak & Dilek, 2013). The blue box denotes the study area and black arrows indicate relative plate motions. NAFZ – North Anatolian Fault Zone; EAFZ – East Anatolian Fault Zone; TZ – Tuz Gölü Fault; EF – Ecemiş Fault; I-A-E Suture – Izmir-Ankara-Erzincan Suture; IT Suture – Intra Tauride Suture.

A GEOCHEMICAL APPROACH TO MANTLE AND CRUSTAL DYNAMICS IN CENTRAL ANATOLIA

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