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Thesis

Reference

Magma fluxes, timescales and petrological diversity in volcanic plumbing systems: new perspectives from Nevado de Toluca

(Mexico)

WEBER, Gregor

Abstract

The risk emanating from volcanic hazards today is the highest in recorded human history, which stresses the need for improved forecasts of volcanic activity. This thesis provides further insights in this respect by an integrated approach at the interface of petrology, geochronology and geophysical modelling, using Nevado de Toluca volcano in Mexico as a case study. Analyzing the long-term history of the volcano reveals that eruptions are preceded by a recurrent pattern of magma hybridization processes. The present state of the volcano is investigated by a new zircon age inversion technique, which shows that long-dormant volcanoes can reawake and produce large eruptions over short timescales. The question why some volcanoes erupt the same magma chemistry and others sample the entire igneous spectrum is examined by thermochemical modelling. These calculations reveal a relationship between magma fluxes, compositional diversity and the size of magmatic systems that resembles the structure of natural data.

WEBER, Gregor. Magma fluxes, timescales and petrological diversity in volcanic plumbing systems: new perspectives from Nevado de Toluca (Mexico). Thèse de doctorat : Univ. Genève, 2020, no. Sc. 5467

URN : urn:nbn:ch:unige-1398581

DOI : 10.13097/archive-ouverte/unige:139858

Available at:

http://archive-ouverte.unige.ch/unige:139858

Disclaimer: layout of this document may differ from the published version.

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Département des Sciences de la Terre Directeur: Prof. Dr. Luca Caricchi

Magma fluxes, timescales and petrological diversity in volcanic plumbing systems: New perspectives from Nevado de Toluca (Mexico)

THÈSE

présentée à la Faculté des sciences de l’Université de Genève pour obtenir le grade de Docteur ès Sciences, mention sciences de la Terre

par Gregor Weber

de

Andernach (Allemagne)

Thèse N°5467

GENÈVE Imprimerie Fornara SA

2020

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PETROLOGICAL DIVERSITY IN VOLCANIC PLUMBING SYSTEMS : N EW PERSPECTIVES

FROM N EVADO DE T OLUCA (M EXICO )

Gregor Weber

Department of Earth Sciences University of Geneva

2020

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A

BSTRACT

The risk emanating from volcanic hazards today is the highest in recorded human history, which stresses the need for improved forecasts of volcanic activity. In order to evaluate possible eruption harbingers and the hazard potential of individual volcanoes, a quantitative

understanding of the dynamics of subvolcanic plumbing systems is required. Yet, essential aspects such as the rates, timescales and chemical diversity of magma reservoirs are not well constrained. This thesis provides further insights in this respect by an integrated approach at the interface of petrology, geochronology and geophysical modelling, using Nevado de Toluca volcano in Mexico as a case study.

A petrological analysis of the 1.5 million years’ spanning history of the volcano reveals a systematic pattern: Eruptions are fed from a reservoir at about 6 km depth, which is recharged from two distinct deep magma sources. Interaction of the deep mafic recharge magmas with a shallow silicic reservoir is recorded by crystal zonation pattern and indicates that input of fresh magma batches into the shallow reservoir is required to trigger the eruptions. Using diffusion chronometry, the timing of this process is determined to last between years to centuries. In order to better constrain the present day status of the volcano, a novel approach based on zircon analyses and thermal modelling is presented, by which the rate of crustal magma input can be estimated with unprecedented resolution. These calculations show that only a few percent of the supplied magma erupted, which led to the accumulation of up to 350 km3 melt currently residing under the volcano. This shows that volcanoes, dormant for several millennia and not perceived as active by most of the population, can reawake and produce large eruptions within a few years, due to the thermal maturity of such systems. The regularities in eruption triggering processes, as well as the large melt volume residing under the volcano today, indicate that large future

eruptions from Nevado de Toluca are possible at any time without long precursory warning time.

Signs of deep crustal magma movement, such as broad ground deformation pattern and seismicity, may emerge as precursor to renewed activity of this volcano.

A remarkable feature of Nevado de Toluca is that it has essentially erupted the same dacitic magma throughout its eruptive history. The question why some volcanoes monotonously erupt the same magma chemistry and others sample the entire igneous spectrum is investigated by a new thermo-chemical modelling approach. These calculations reveal a relationship between magma fluxes, compositional diversity and the size of magmatic systems that resembles the structure of natural data. Relatively low rates of magma input into deep or hot crust leads to

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smaller geochemical variety compared to rapid injection of large magma volumes into shallower or colder crust. These findings indicate that the hazard potential, resulting from the amount of magma at depth, can be mapped for different volcanoes based on the chemical diversity of volcanic rocks at the surface of the Earth.

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Z

USAMMENFASSUNG

Das Risiko, welches von Vulkangefahren ausgeht, ist heute das höchste seit Beginn der

Menschheitsgeschichte, weshalb es dringend erforderlich ist das Verhalten von Vulkanen besser vorhersagen zu können. Um mögliche Vorboten von Ausbrüchen und das Gefahrenpotenzial einzelner Eruptionsherde besser zu verstehen, muss die Dynamik magmatischer Systeme in der Erdkruste quantitativ erfasst werden. Jedoch sind wesentliche Aspekte wie die Prozessraten, Zeitskalen und chemische Diversität in Magmakammern nur ansatzweise verstanden. In dieser Arbeit werden diesbezüglich neue Erkenntnisse vorgestellt, die anhand von Fallstudien am dazitischen Nevado de Toluca Vulkan in Mexiko gewonnen wurden, basierend auf der Verbindung von Petrologie, radiometrischer Altersbestimmung und geophysikalischen Computersimulationen.

Eine detaillierte petrologische Analyse der 1.5 Million Jahre langen

Eruptionsgeschichte des Toluca Vulkans weist systematische Muster auf: Ausbrüche werden aus einem Reservoir in circa 6 km Tiefe gefördert, welches von zwei unterschiedlichen, tiefer

gelegenen Magmaquellen gespeist werden. Die Interaktion der tiefen mafischen Schmelzen mit dem oberflächennahen, hochdifferenzierten Reservoir wurde in Kristallzonierungen

dokumentiert. Die Ergebnisse weisen darauf hin, dass ein Zufluss von frischen Magmachargen in das oberflächennahe Reservoir nötig ist um Ausbrüche auszulösen. Mittels

Diffusionschronometrie lässt sich der Zeitrahmen dieses Prozesses auf wenige Jahre bis Jahrhunderte vor den Eruptionen festlegen. Um den aktuellen Status des Vulkans besser zu verstehen, wurde eine neue Methode basierend auf der Analyse von Zirkonen und numerischen Modellen entwickelt, wodurch sich die Magmainjektionsrate in der Erdkruste mit bisher

unerreichter Präzession bestimmen lässt. Damit lässt sich zeigen, dass nur wenige Prozent des injizierten Magmas über die Zeit eruptierte, wodurch sich ein Volumen von bis zu 350 km3 Schmelze derzeit unter dem Vulkan angesammelt hat. Diese Studie zeigt, dass Vulkane wie Nevado de Toluca, die für Jahrtausende ruhen und von der Bevölkerung oft als erloschen

angesehen werden, innerhalb weniger Jahre erwachen und große Ausbrüche produzieren können.

Die gefundenen Regelmäßigkeiten in eruptionsauslösenden Prozessen, sowie das potenziell große Volumen der Schmelze derzeit unter dem Vulkan deuten darauf hin, dass große zukünftige Eruptionen nach nur kurzer Vorwarnzeit jederzeit möglich sind. Insbesondere Anzeichen für Magmabewegungen in tiefen Bereichen der Erdkruste, wie weitläufige Bodendeformation oder

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Seismizität, könnten als Vorbote künftiger Eruptionen dienen. Dementsprechend sollten Überwachungssysteme ausgebaut werden.

Eine bemerkenswerte Eigenschaft des Toluca Vulkans ist, dass über die gesamte Eruptionsgeschichte hinweg dazitische Magmen gefördert wurden. Die Frage, warum manche Vulkane monoton Magma mit bestimmter Zusammensetzung eruptieren und andere das gesamte Spektrum an Magmachemie abdecken, wurde mittels neuen thermochemichen

Modellrechnungen untersucht. Die Ergebnisse deuten auf einen bisher unbekannten

Zusammenhang zwischen Magmainjektionsrate, der Größe magmatischer Reservoire und der chemischen Diversität der eruptierten Magmachemie hin, welcher mit der Struktur natürlicher Daten übereinstimmt. Relativ niedrige Injektionsraten in tiefe oder heißere Krustenbereiche resultieren in weniger diversen eruptierbaren Magmazusammensetzungen im Vergleich zu hohen Magmainjektionsraten in kältere, oberflächennahe Bereiche der Erdkruste. Diese Resultate deuten darauf hin, dass sich das Gefahrenpotenzial, ausgehend von der Menge an Magma im Untergrund, für verschiedene Vulkane anhand der chemischen Diversität von Vulkangesteinen an der Erdoberfläche kartieren lässt.

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R

ÉSUMÉ

Les risques volcaniques de nos jours sont les plus élevés de l’histoire de l’humanité. Pour cette raison il est absolument nécessaire qu’on puisse prédire au mieux le comportement des volcans.

Pour mieux comprendre les signes précurseurs de possibles éruptions et le danger potentiel de certains foyers d’éruption, il est nécessaire de recenser de façon quantitative l’intensité de la dynamique de systèmes magmatiques dans la croûte terrestre. Pourtant, actuellement, des aspects essentiels comme les taux de traitement, la notion d’échelles de temps et la diversité chimique des chambres magmatiques sont mal maîtrisés. Cette thèse présente de nouveaux résultats obtenus lors de l’étude de cas du volcan dacitique Nevado de Toluca au Mexique, en s’appuyant sur des données pétrologiques, géochronologiques et des modélisations géophysiques.

Une analyse pétrologique de l'histoire du volcan, qui s'étend sur 1,5 million d'années, révèle un modèle systématique: les éruptions sont alimentées par un réservoir à environ 6 km de profondeur, rechargé par deux sources magmatiques distinctes provenant d’une couche plus profonde. L'interaction des magmas mafiques profonds avec un réservoir silicique peu profond est enregistrée par le schéma de zonation des cristaux et indique que l'apport de pulses de

nouveaux magmas dans le réservoir peu profond est nécessaire pour déclencher les éruptions. En utilisant la chronométrie de diffusion, la durée de ce processus a été estimée à de quelques années à plusieurs siècles. Afin de mieux contraindre l'état actuel du volcan, une nouvelle approche basée sur des analyses de zircon et une modélisation thermique est présentée, par laquelle le taux d'entrée de magma crustal peut être estimé avec une résolution sans précédent.

Ces calculs montrent que seulement quelques pour cent du magma fourni est entré en éruption, ce qui a conduit à l'accumulation de près de 350 km3 de matériel fondu résidant actuellement sous le volcan. Cela montre que les volcans, dormants depuis plusieurs millénaires et non perçus comme actifs par la plupart de la population, peuvent se réveiller et produire de grandes

éruptions en quelques années, en raison de la maturité thermique de ces systèmes. Les régularités dans les processus de déclenchement d'éruptions, ainsi que le grand volume de matériel fondu résidant sous le volcan aujourd'hui, indiquent que de grandes éruptions futures du Nevado de Toluca sont possibles à tout moment et sans long délai d'avertissement préalable. Des signes de mouvement profond du magma crustal, tels qu'un large schéma de déformation du sol et de la sismicité, peuvent apparaître comme étant des précurseur d'une nouvelle activité du volcan.

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Une caractéristique remarquable du Nevado de Toluca est qu'il a essentiellement émis le même magma dacitique tout au long de son histoire éruptive. La question de savoir pourquoi certains volcans émettent systématiquement des magmas de même chimie et pourquoi d'autres échantillonnent tout le spectre de chimie de magmas est étudiée par une nouvelle approche de modélisation thermochimique. Ces calculs révèlent une relation entre les flux de magma, la diversité de composition et la taille des systèmes magmatiques qu’on trouve dans la nature. Des taux relativement faibles de magma dans la croûte profonde ou chaude conduisent à une plus petite variété géochimique par rapport à l'injection rapide de grands volumes de magma dans une croûte moins profonde ou plus froide. Ces résultats indiquent que le danger potentiel, résultant de la quantité de magma en profondeur, peut être cartographié pour différents volcans en fonction de la diversité chimique des roches volcaniques affleurant à la surface de la Terre.

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CKNOWLEDGEMENTS

My principle thanks must go to my supervisor Luca Caricchi for his enduring and proactive support, scientific generosity and good spirit over the last four years. Some say academic research is a lonely trade. This is certainly not true for those working with Luca. Thank you for the support!

Many thanks also to Adam Kent, Joël Ruch, Zoltan Zajacz and José Luis Arce for taking part in the committee of my PhD defense.

I would like to express my gratitude to Guy Simpson, interacting with whom inspired me to dive deeper into numerical modelling. A big thanks for the support and especially for helping out, when I’ve got stuck with coding.

This thesis would not have been possible without the work of José Luis Arce, who established much of the stratigraphy at Nevado de Toluca. Thank you very much for organizing the field logistics and sharing of your knowledge about the volcano. I would also like to thank Line Probst, Martin Gander, Adam Curry and Adhara Avila Ortiz for support during fieldwork.

Alexey Ulyanov is thanked for collaboration and optimum support with the LA-ICP-MS

analyses. Many thanks to Axel Schmitt for collaboration on the zircon project and welcoming me in Heidelberg to do SIMS U-Th analysis. Martin Robyr, Aaron Hantsche, Marion Grosjean and Max Jensen are thanked for technical support and advice on many aspects of the electron microprobe. I am grateful to Massimo Chiaradia for analytical support and for interesting discussions on volcano geochemistry and ore deposits. Special thanks to Felix Marxer and Peter Ulmer for welcoming me in Zürich to cook magma at high pressures and temperatures. Kalin Kouzmanov, Urs and Maria Schaltegger gave valuable advice during my PhD, which is very much appreciated. Many thanks also to Jon Blundy and Othmar Müntener for discussing research directions and approaches.

Many thanks go to the former and current volcanic petrology group members: Tom, Eva, Line, Adam, Max, Ollie, Alessandro and Corin. Being surrounded by nice folks made this journey much more enjoyable. Many thanks also to all friends and colleagues in the Department of Earth Sciences.

Last but of foremost importance, I would like to thank my wife Anna, my family and friends for

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CONTENTS

1 INTRODUCTION ... 1

1.1AIMS AND OBJECTIVES ... 2

1.2DYNAMICS AND CHEMICAL DIVERSITY OF CRUSTAL MAGMATIC SYSTEMS ... 3

1.3NEVADO DE TOLUCA,TRANS MEXICAN VOLCANIC BELT ... 5

1.4STRUCTURE OF THIS THESIS ... 11

2 THE LONG-TERM LIFE CYCLE OF NEVADO DE TOLUCA VOLCANO ... 12

ABSTRACT... 12

2.1INTRODUCTION ... 13

2.2METHODS ... 18

2.2.1 Bulk rock analytical techniques ... 18

2.2.2 In-situ mineral and glass geochemistry ... 19

2.2.3 Thermobarometric calculations ... 20

2.2.4 Thermal modelling ... 21

2.3RESULTS ... 22

2.3.1 Bulk-rock and glass geochemistry ... 22

2.3.2 Petrography ... 26

2.3.3 Mineral chemistry ... 28

2.4DISCUSSION ... 29

2.4.1 Petrogenesis and deep structure of the plumbing system ... 29

2.4.2 The role of shallow crustal hybridization ... 35

2.4.3 Bulk-rock, magma recharge and hybridization history ... 39

2.4.4 Controls on long-term geochemical trends ... 46

2.4.5 Petrologic modes of Mexican stratovolcanoes ... 50

2.5CONCLUSIONS ... 53

3 DETERMINING THE CURRENT SIZE AND STATE OF SUBVOLCANIC MAGMA RESERVOIRS ... 54

ABSTRACT... 54

3.1INTRODUCTION ... 55

3.2MATERIALS AND METHODS... 58

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3.2.1 Zircon separation and imaging ... 58

3.2.2 Trace element analysis ... 58

3.2.3 U-Th geochronology ... 59

3.2.4 U-Pb geochronology ... 60

3.2.5 Thermal and zircon crystallization modelling ... 61

3.3RESULTS AND DISCUSSION ... 64

3.3.1 Zircon ages and geochemistry... 64

3.3.2 Thermo-chemical modelling ... 68

3.3.3 Zircon age distributions and thermal records... 70

3.3.4 Long-term magma fluxes ... 74

3.3.5 Current state of the magmatic system and eruptive potential ... 77

4 MAGMATIC PATTERNS AND TIMESCALES PRIOR TO PLINIAN ERUPTIONS ... 80

ABSTRACT ... 80

4.1INTRODUCTION ... 81

4.2SAMPLES AND ANALYTICAL METHODS ... 82

4.3RESULTS ... 84

4.3.1 Whole rock and glass geochemistry ... 84

4.3.2 Mineral textures and compositions ... 86

4.4DISCUSSION ... 94

4.4.1 Architecture of the plumbing system ... 94

4.4.2 Magma recharge ... 97

4.4.3 Melt evolution and diversity ... 101

4.4.4 Timescales of pre-eruptive processes ... 104

4.4.5 Implications for volcano monitoring ... 108

4.5CONCLUSIONS ... 111

5 THERMO-CHEMICAL PERSPECTIVES ON MAGMA DIVERSITY ... 112

ABSTRACT ... 112

5.1INTRODUCTION ... 113

5.2THERMAL AND PETROLOGICAL MODELLING ... 115

5.3RESULTS AND DISCUSSION ... 118

5.3.1 Temporal evolution of eruptible magma chemistry... 118

5.3.2 Compositional variability and recharge regimes ... 125

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5.4.2 Petrologic calculations ... 132

5.4.3 Data compilation ... 133

5.4.4 Whole rock analysis ... 134

6 CONCLUSIONS AND OUTLOOK ... 135

6.1SYNOPSIS ... 135

6.2FUTURE DIRECTIONS ... 137

6.3CONCLUSIVE REMARKS ... 138

7 REFERENCES ... 140

8 APPENDICES ... 161

APPENDIX 1 SUPPLEMENTARY MATERIALS FOR CHAPTER 2 ... 162

APPENDIX 2 SUPPLEMENTARY MATERIALS FOR CHAPTER 3 ... 165

APPENDIX 3 SUPPLEMENTARY MATERIALS FOR CHAPTER 4 ... 172

APPENDIX 4 SUPPLEMENTARY MATERIALS FOR CHAPTER 5 ... 193

APPENDIX 5 ADDITIONAL PUBLISHED CONTRIBUTIONS ... 203

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L

IST OF

F

IGURES

FIGURE 1.1.TOPOGRAPHY AND GEOLOGY OF NEVADO DE TOLUCA A)SATELLITE IMAGE AND DIGITAL ELEVATION MODEL ... 7 FIGURE 1.2.SELECTED PHOTOGRAPHS OF NEVADO DE TOLUCAS CRATER AREA FROM VARIOUS

PERSPECTIVES.. ... 9 FIGURE 1.3.SELECTED OUTCROP PHOTOGRAPHS SHOWING THE RANGE OF ERUPTIVE STYLES AND

STAGES OF NEVADO DE TOLUCA. ... 10 FIGURE 2.1.LOCATION OF NEVADO DE TOLUCA VOLCANO (NT) AND PETROLOGICAL DIVERSITY OF

STRATOVOLCANOES IN THE TRANS MEXICAN VOLCANIC BELT. ... 16 FIGURE 2.2.RELIEF, GEOLOGICAL MAP AND SAMPLING LOCATIONS.. ... 17 FIGURE 2.3.COMPILATION OF STRATIGRAPHIC RELATIONS FOR NEVADO DE TOLUCA VOLCANO. . 24 FIGURE 2.4.SUMMARY OF MAJOR AND TRACE ELEMENT SYSTEMATICS OF NEVADO DE TOLUCA

VOLCANO.. ... 25 FIGURE 2.5.REPRESENTATIVE SELECTION OF PETROGRAPHIC FEATURES OF BASALTIC ANDESITE TO DACITIC VOLCANIC ROCKS IN THE NEVADO DE TOLUCA AREA. ... 27 FIGURE 2.6.SYSTEMATICS OF ENSTATITE (MOL.%)(EN) CONTENT AND CR2O3(WT.%) IN NEVADO DE TOLUCA CLINO- AND ORTHOPYROXENES.. ... 32 FIGURE 2.7.EVIDENCE FOR MAGMA HYBRIDIZATION IN ORTHOPYROXENE CRYSTALS. ... 33 FIGURE 2.8.EVIDENCE FOR MAGMA HYBRIDIZATION IN PLAGIOCLASE... 34 FIGURE 2.9.GEOTHERMOBAROMETRY AND CRYSTALLINITY ... 37 FIGURE 2.10.TIME VARIATION OF SIO2(WT.%),V(µG/G), AND NB (µG/G) BULK-ROCK

COMPOSITIONS. ... 42 FIGURE 2.11.TIME VARIATION OF PLAGIOCLASE COMPOSITIONS SHOWN AS TWO-SIDED VIOLIN

PLOTS. ... 43 FIGURE 2.12.TEMPORAL VARIATION OF AMPHIBOLE COMPOSITIONS (A AND B) AND AMPHIBOLE

EQUILIBRIUM MELTS (C AND D). ... 44 FIGURE 2.13.VARIATION OF ORTHOPYROXENE COMPOSITIONS WITH TIME. ... 45

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FIGURE 2.14.CONTROLS ON PETROLOGIC MODES OF COMPOSITIONALLY RESTRICTED VOLCANOES..

... 49 FIGURE 2.15.SIO2(WT.%) DENSITY DISTRIBUTIONS OF WHOLE-ROCK DATA ... 52 FIGURE 3.1.LOCATION AND STUDIED SAMPLES OF NEVADO DE TOLUCA VOLCANO. ... 57 FIGURE 3.2.ZIRCON AGE SPECTRA FROM NEVADO DE TOLUCA VOLCANO. ... 66 FIGURE 3.3.TRACE ELEMENT CONTENTS AND RATIOS OF NEVADO DE TOLUCA ZIRCONS SHOWN AS BIVARIATE PLOTS. ... 67 FIGURE 3.4.TIME-TEMPERATURE EVOLUTION OF MODELLED MAGMATIC INTRUSIONS BUILT BY

DIFFERENT RECHARGE RATES.. ... 69 FIGURE 3.5.EXAMPLE OF SYNTHETIC ZIRCON AGE POPULATION COMPUTED FROM THERMAL

MODELLING. ... 71 FIGURE 3.6.NATURAL ZIRCON AGES AND TI-IN ZIRCON TEMPERATURES.. ... 73 FIGURE 3.7.RELATIONSHIP OF TEMPERATURE, ZIRCON AGE SPAN AND MAGMA FLUX IN MODEL

AND NATURAL DATA.. ... 76 FIGURE 3.8.TIME EVOLUTION OF ERUPTIBLE MAGMA VOLUMES. ... 79 FIGURE 4.1.RELIEF MAP DISPLAYING THE MOST PROMINENT VOLCANIC STRUCTURES IN THE

CENTRAL PART OF THE TRANS-MEXICAN VOLCANIC BELT (TMVB). ... 83 FIGURE 4.2.MAJOR AND TRACE ELEMENT COMPOSITION OF BULK-ROCK SAMPLES (FILLED

SYMBOLS), INTERSTITIAL AND MELT INCLUSION GLASSES (OPEN SYMBOLS) FROM PLINIAN

NEVADO DE TOLUCA ERUPTIONS AND PERIPHERAL CONES. ... 85 FIGURE 4.3.ANORTHITE AND FEO CONTENT OF PLAGIOCLASE CRYSTALS. ... 88 FIGURE 4.4.SELECTION OF BSE IMAGES AND ORTHOPYROXENE CHEMISTRY.. ... 91 FIGURE 4.5.CHEMICAL COMPOSITIONS OF ORTHOPYROXENE COMPOSITIONS IN PLINIAN NEVADO

DE TOLUCA DEPOSITS AND COMPARISON TO EXPERIMENTAL ORTHOPYROXENE COMPOSITIONS) ... 92 FIGURE 4.6.AMPHIBOLE CHEMISTRY AND THERMOBAROMETRY FOR PLINIAN NEVADO DE TOLUCA ERUPTIONS. ... 93 FIGURE 4.7.BACK SCATTERED ELECTRON (BSE) IMAGES WITH CORRESPONDING MAJOR (AN AND

FEOT) AND TRACE ELEMENT (SR,MG,TI,LA AND BA) PROFILES THROUGH NEVADO DE

TOLUCA PLAGIOCLASE.W ANALYTICAL TRAVERSE THROUGH THE CRYSTAL. ... 99

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FIGURE 4.8.CALCULATED MELT CONCENTRATIONS OF BA (µG/G),LA (µG/G) AND SR (µG/G) FROM PLAGIOCLASE COMPOSITIONS USING THE COMPOSITION DEPENDENT PARTITION COEFFICIENTS OF BINDEMAN ET AL.,(1998). ... 103 FIGURE 4.9.DIFFUSION MODELLING OF MG IN PLAGIOCLASE FOR PLINIAN NEVADO DE TOLUCA

PLAGIOCLASE.. ... 105 FIGURE 4.10.DIFFUSION MODELLING OF FE-MG EXCHANGE IN PLINIAN NEVADO DE TOLUCA

ORTHOPYROXENE. ... 107 FIGURE 4.11.COMPARISON OF OVERPRESSURE GENERATED DUE TO MAGMA INJECTION AND THE

DEPTH RANGE OF THE SILICIC NEVADO DE TOLUCA RESERVOIR.. ... 109 FIGURE 5.1.DIFFERENCES IN THE GEOCHEMICAL VARIABILITY OF ARC VOLCANOES.. ... 114 FIGURE 5.2.NUMERICAL MODEL TO SIMULATE THE TEMPORAL EVOLUTION OF TEMPERATURES IN A CRUST UNDERGOING PULSED MAGMA INJECTIONS. ... 117 FIGURE 5.3.COUPLING OF THERMAL MODELLING OUTPUT TO EXPERIMENTAL PHASE PETROLOGY.

... 119 FIGURE 5.4.TEMPORAL EVOLUTION OF EXTRACTABLE MAGMA CHEMISTRY. ... 120 FIGURE 5.5.IMPACT OF INJECTION FREQUENCY ON MAGMA VARIABILITY. ... 122 FIGURE 5.6.IMPACT OF MAGMA FLUX VARIATION ON TRENDS AND VARIANCE OF AVERAGE

EXTRACTABLE MAGMA CHEMISTRY.. ... 124 FIGURE 5.7.RELATION OF EXTRACTABLE MAGMA DIVERSITY TO INJECTION RATES AND THERMAL

CONDITIONS OF THE CRUST.. ... 126

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L

IST OF

A

BBREVIATIONS AND

A

CRONYMS

An Anorthite

BAF Block and Ash Flow

BP Before Present

BSE Back scattered electron

Bt Biotite

CL Cathodoluminescence

Cpx Clinopyroxene

DEM Digital Elevation Model

DRE Dense Rock Equivalent

E:I Extrusive to Intrusive ratio

En Enstatite

EPMA Electron Probe Microanalyzer

fO2 OxygenFugacity

GPS Global Positioning System

Hbl Hornblende (amphibole)

HREE Heavy Rare Earth Elements

Ilm Ilmenite

InSAR Interferometric Synthetic Aperture Radar

IQR Inter Quartile Range

Ka Kilo annum

LA-ICP-MS Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry LILE Large Ion Lithophile Elements

LOI Loss on Ignition

LREE Light Rare Earth Elements

LTP Lower Toluca Pumice

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Ma Mega annum

mol.% Mole percent

MSWD Mean Square Weighted Deviation

mt Magnetite

MTP Middle Toluca Pumice

NNO Nickel-Nickel-Oxide buffer

ol Olivine

opx Orthopyroxene

ox Oxide

p.f.u per formula unit

PDC Pyroclastic Density Current

PhD philosophiae doctor (Doctor of Philosophy)

plag Plagioclase

qtz Quartz

REE Rare Earth Elements

RSE Relative Standard Error

SD Standard Deviation

SRM Standard Reference Material

TMVB Trans Mexican Volcanic Belt

UTP Upper Toluca Pumice

WAEMC Weighted Average Eruptible Magma Composition

wt.% Weight Percent

X Chemical Composition or Fraction

XRF X-ray Fluorescence

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L

IST OF

A

PPENDICES

APPENDIX 1SUPPLEMENTARY MATERIALS FOR CHAPTER 2 162 APPENDIX 2SUPPLEMENTARY MATERIALS FOR CHAPTER 3 165 APPENDIX 3SUPPLEMENTARY MATERIALS FOR CHAPTER 4 172 APPENDIX 4SUPPLEMENTARY MATERIALS FOR CHAPTER 5 193 APPENDIX 5ADDITIONAL PUBLISHED CONTRIBUTIONS 203

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1 I NTRODUCTION

This chapter serves as a general introduction to the research presented in this thesis. First, the aims and objectives of the research are introduced, followed by a review of the current state of knowledge on the dynamics and petrological diversity of volcanic plumbing systems.

Subsequently, a summary of previous research on Nevado de Toluca volcano, which serves as a case study for much of the work, is presented. During the course of this PhD, two consecutive field studies have been carried out in Mexico with are briefly described. Finally, the structure of this thesis is laid out.

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1.1 Aims and objectives

The risk emanating from volcanic hazards to mankind is today higher than at any time in

recorded history, as close to 800 million people live within radius of 100 km to an active volcano (Brown et al., 2015). Consequently, the principle goal of volcanology is to improve forecasting abilities, which requires a mechanic understanding of the dynamical processes and rates

operating in magmatic plumbing systems (Sparks and Cashman, 2017; Edmonds et al., 2019).

Despite large advances over the last 30 years in the understanding of the architecture and compositional diversity of subvolcanic magma plumbing systems (recently summarized by Sparks et al., (2019)), many facets of how volcanoes work remain enigmatic.

This thesis explores different aspects of the rates, pace and chemical diversity in

subvolcanic reservoirs based on case studies at Nevado de Toluca volcano in the central Mexican Highland (Macias and Arce, 2019). The two main aims of the presented research are: 1)

Developing new tools to infer critical magmatic system variables based on geological archives, and 2) improve forecasting abilities for volcanic eruptions by linking petrological records to geophysical models. The concrete objectives which are addressed in this thesis are:

(I) To investigate which processes dominantly generate the erupted magma geochemistry over the long-term history of the volcano and constrain how the composition of rocks and minerals varies with time.

(II) Evaluate the current physico-chemical state and eruptive potential of Nevado de Toluca by further advancing a novel inversion technique based on zircon

geochronology and thermal modelling (Caricchi et al., 2016).

(III) To determine how fast long-dormant volcanoes, such as Nevado de Toluca, can transition into unrest and eruption, and to examine differences and similarities in the processes that led to different eruptions.

(IV) Constrain the controls on the focus and range of chemical composition erupted by volcanoes over time.

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1.2 Dynamics and chemical diversity of crustal magmatic systems

Hidden from direct observation, magmas move through and accumulate in the Earth’s crust at largely unknown rates (White et al., 2006). However, the pace of magma input and storage is anticipated to modulate the variety of chemical compositions that give rise to the diversity of igneous rocks (Kent, 2013; Wörner et al., 2017; Till et al., 2019), the pressurization of

subvolcanic reservoirs initiating volcanic eruptions (Jellinek and DePaolo, 2003; Gregg et al., 2013; Degruyter and Huber, 2014), and the temporal evolution of the physico-chemical state of magmatic systems (Caricchi and Blundy, 2015). Understanding these mass and energy fluxes is therefore important for a range of problems in Earth Science, including the differentiation of the Earth’s crust (Ducea et al., 2017), the formation of magmatic ore deposits (Chelle-Mishou et al., 2017) and the inner workings of volcanoes (Caricchi et al., 2014a), which is the primary focus of this thesis.

Igneous plumbing systems are at present envisioned as highly complex networks of sub- vertical dykes, horizontal sills and irregularly shaped magma bodies (Gudmundsson et al., 2014;

Tibaldi, 2015; Cruden et al., 2017; Breitkreuz et al., 2018; Magee et al., 2018), which are

transiently interconnected with each other during short episodes of instability and reorganization that may culminate in volcanic eruptions (Sparks et al., 2018). Field relations, petrological and geophysical evidence suggests that the anatomy of such structures may extend through the entire Earth crust (Ruprecht and Plank, 2013; Christopher et al., 2015; Cashman et al., 2017), yet a shallow magma storage region at pressures of 2±0.5 kbar is evident for most volcanic systems, which may reflect a favorable magma accumulation depth due to crustal rheology and the pressure dependent exsolution of volatile elements from melts (Menand, 2011; Huber et al., 2019).

While not universally accepted (Adam et al., 2016), most would agree that the compositional diversity of magmas is generally obtained at deep to mid crustal levels, during incremental assembly of magma batches into larger reservoirs over prolonged periods of time (Annen and Sparks, 2002, Dufek and Bergantz, 2005; Annen et al., 2006). Pulsed magma

injection has been extensively recorded in petrological archives, such as crystal zonation textures and chemistry (Singer et al., 1995; Tepley III et al., 2000; Cashman and Blundy, 2003;

Humprehys et al., 2006; Ginibre et al., 2007; Ruprecht and Wörner, 2007; Smith et al., 2009;

Kent et al., 2010) and is evident from geophysical and geodetic signals, observed during unrest

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2014; Hickey et al., 2016). Open system heat and/or mass exchange (i.e. mixing/mingling processes) between recharging magmas and pre-existing reservoirs are therefore widely recognized as one of the major agents in driving the geochemical diversity of volcanic rocks (Eichelberger et al., 2000; Reubi and Blundy, 2009; Kent, 2013; Laumonier et al., 2014). Partial fusion or complete digestion of mostly deep crustal rocks by intruding magmas (Hildreth and Moorbath, 1988; Martin and Sigmarsson, 2007; Solano et al., 2012; Clemens and Stevens, 2016), gravity driven fractionation of crystals and melt (Grove et al., 2003; Lee and Bachmann, 2014;

Nandedkar et al., 2014; Ulmer et al., 2018; Müntener and Ulmer, 2018), as well as the

segregation of magmatic liquids (Bachmann and Bergantz, 2004; Dufek and Bachmann, 2010;

Solano et al., 2012; Floess et al., 2019; Holness, 2018) e.g. by reactive porous melt flow (Spiegelman and Kelemen, 2003; Lissenberg and MacLeod, 2016; Jackson et al., 2018), are all believed to contribute to the compositional diversity of magmas, but their relative importance is still widely debated. While the chemical diversity observed in genetically related rock suits at individual volcanoes reflects magma differentiation processes, some system erupt magmas covering a wide spectrum of geochemical compositions, other volcanoes produce magmas with monotonous chemistry throughout their entire lifespan. Such differences may be attributed to physical filtering processes, such as density or viscosity barriers, that may prevent certain compositions from being erupted (Stolper and Walker, 1980; Marsh, 1981; Pinel and Jaupart, 2000). Such barriers might be overcome by hybridization of mafic and silicic compositions during recharge into crustal magma reservoirs (Kent et al., 2010; Kent, 2013). Alternatively, differences in the variety of erupted magma compositions may reflect local differences in the architecture of magmatic systems and rates of magma throughput (Wörner et al., 2017).

Volcanic plumbing systems also undergo substantial time evolution (Annen et al., 2006;

Melekhova et al., 2013; Annen et al., 2015; Caricchi and Blundy, 2015). Over the last 30 years, detailed mapping and dating of volcanic successions have revealed life-cycles of hundreds of thousands of years up to several million years for composite volcanoes in Arc settings (Hildreth and Lanphere, 1994; Frey et al., 2004; Bacon and Lanphere, 2006; Jicha et al., 2006; Hildreth, 2007; Hora et al., 2007; Singer et al., 2008; Gertisser and Keller, 2003; Thouret et al., 2005;

Muir et al., 2015; Godoy et al., 2018). Numerical models, simulating the large scale thermal and mechanical evolution of magmatic reservoirs, as well as mirco-scale geochemical and

petrological records, show that such systems tend to homogenize in time and spend most of their lifespans in a highly crystallized state (e.g. Bachmann and Bergantz, 2008; Annen, 2009; Huber et al., 2011; Allan et al., 2013). This is corroborated by crystallization age distributions of zircon crystals, which typically record protracted silicic melt presence on the order of 104-106 years

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(Kent and Cooper, 2018; Cooper, 2019 and references therein. These long timescales are contrasted by estimates from modelling of diffusive elemental relaxation between adjacent crystal zonation boundaries that indicate years to millennial duration of associated processes, such as magma recharge (e.g. Costa et al., 2019). Taken together, the diversity of chemical compositions erupted by volcanoes and the large range of timescales derived by radiometric dating and kinetic modelling, point towards a multitude of nested processes (Spera and Bohrson, 2018) that need to be deciphered in order to improve mitigation of the impact of volcanic

eruptions.

1.3 Nevado de Toluca, Trans Mexican Volcanic Belt

Nevado de Toluca (19 06 30N; 99 45 30 W; 4680 m above sea level), also known as volcán Xinantéctal, is a long-lived stratovolcano located in the central Mexican highland about 80 km southwest of Mexico City (Macias and Arce, 2019). It is part of the Trans Mexican Volcanic Belt, a 1000 km long east-west trending volcanic arc of Miocene to Holocene age that developed in response to the subduction of the oceanic Cocos and Rivera plates beneath the North

American continental lithosphere at different angles (Pardo and Suárez, 1995; Ferrari et al., 2012). The volcanic edifice was built from various short silicic lava flows and domes with intercalated pyroclastic deposits, with the most prominent feature of the volcanoes morphology being a 2.5 ×1.5 km2 horseshoe shaped crater that opens towards the East in direction in the direction of the City of Toluca (Fig. 1.1, Fig. 1.2; Norini et al., 2004).

Detailed stratigraphic and radiometric dating has shown that volcanic activity at Nevado de Toluca started in the Early Pleistocene at 1.5 Ma and was continuous since then, frequently with long periods of dormancy of several thousand years between eruptions (Bloomfield and Valastro, 1974; Garcia-Palomo, 2002; Arce et al., 2003, 2005; Capra et al., 2006; Torres-Orozco et al., 2017). The interplay of three fault systems that intersect beneath the volcano potentially influenced the location and eruptive history of the volcano (Garcia-Palomo et al., 2000; Bellotti et al., 2006), especially several cone destruction episodes by sector collapses that gave rise to a set of widely dispersed debris avalanche deposits around the volcano (Capra and Macias, 2000;

Caballero and Capra, 2011). The volcanos history can be divided into several stages based on geomorphology and age relations (Fig. 1.1b; Torres-Orozco et al., 2017). During the early ‘Old

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(Fig. 1.3a, b) between 1.5 and 0.27 Ma in the periphery of the main volcano. Only a few

monogenetic systems have been dated in the area (Bloomfield, 1975; Arce et al., 2013a; Torres- Orozco et al., 2017), but available data point towards a long age range between at least 0.86 Ma to 8.5 ka. The younger eruptive history can be dived into the 0.57 – 0.009 Ma ‘Recent Nevado’

stage, mostly characterized by silicic lava effusion in the crater area, and a sequence of

pyroclastic deposits ‘Young PD stage’ spanning ca. 0.114 to 0.003 Ma (Macias et al., 1997; Arce et al., 2006).

The young pyroclastic deposits (Fig. 1.3c-f) record a complex series of dome

destruction events that are preserved in block and ash flow deposits (Garcia-Palomo et al., 2002), as well as by a sequence of at least three Plinian eruptions (Arce et al., 2003, 2005, 2006; Capra et al., 2006). The oldest and least voluminous of these Plinian events is the 0.85 km3 DRE Lower Toluca Pumice (LTP) that was dated at 26 ka calibrated 14C years BP (Capra et al., 2006). In the following 13 ka activity at Nevado de Toluca was again dominated by dome formation and destruction, notably at about 16500 cal. 14C years BP when a massive block and ash flow of 0.5 km3 was produced (Arce et al., 2006). Renewed activity commenced at approximately 14

thousand years ago when a Plinian eruption of 1.8 km3 DRE gave rise to the Middle Toluca Pumice (MTP) that was distributed mainly to the north-east of the volcano (Arce et al., 2005).

The so far largest and last Plinian-type eruption from Nevado de Toluca, the 8 km3 DRE Upper Toluca Pumice (UTP), was erupted at about 12.5 ka and blanketed the area today occupied by Mexico City within the 10 cm isopach (Arce et al., 2003), demonstrating the large potential hazards emerging from this volcano. Following this event, the dacite dome ‘El Ombligo’ (Fig.

1.2c) was emplaced inside the crater at about 9.5 ka (Arce et al., 2003; Bernal et al., 2014). The last eruptive activity at Nevado de Toluca was described as a phreatomagmatic surge and dated by Macias et al. (1997) with a calibrated 14C age of 3768 years BP.

Nevado de Toluca is a remarkably uniform volcano in terms of its compositional diversity, which is restricted to minor andesite and subalkaline dacite throughout its lifetime (Torres-Orozco et al., 2017). Several detailed petrological studies have been conducted on the Plinian deposits (Arce et al., 2003, 2005, 2006; Martínez-Serrano et al., 2004; Capra et al., 2006;

Smith et al., 2009 Arce et al., 2013), which are summarized here as these eruptions are a major focus of this study. Petrographic analysis of the Plinian pumice showed that the UTP and MTP deposits are identical in terms of their mineral phase assemblage plag>opx>hbl>>ilm+mt+bt.

The LTP rocks, however, lack orthopyroxene as a major phenocryst phase. Notably, this eruption also differs from the preceding events by the presence of schist fragments that are not found in

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the MTP and UTP, possibly reflecting different fragmentation or reservoir depth (Arce et al., 2013). Estimates of pre-eruptive conditions (P-T-X-fO2) for all three eruptions have been obtained by experimental phase equilibria and Fe-Ti oxide thermometry (Arce et al., 2006, 2013).

Figure 1.1. Topography and geology of Nevado de Toluca a) Satellite image and digital elevation model (DEM) of the Nevado de Toluca area (source: Google Earth). B) Geological map overlaying the DEM showing the major eruptive stages of the volcano (modified from Torres-Orozco et al., 2017).

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Relatively high pre-eruptive temperature conditions have been estimated for the LTP oxide pairs ranging between 861–870°C, which are about 10 to 20°C higher than estimates for the UTP and MTP magmas (Arce et al., 2013). Experimental constraints on storage pressures are similar for the UTP and LTP (between 150 and 200 MPa) at water saturated conditions and are with 200 to 300 MPa slightly higher for the MTP magma (Arce et al., 2006, 2013). This shallow part of the magmatic system evolves as an open system, where fresh pulses of dacitic and

andesitic magma enter the preexisting reservoirs (Smith et al., 2009). Martínez-Serrano et al., (2004) used isotopic (Sr, Nd, Pb) and trace element evidence to show that even though Nevado de Toluca sits on thick continental crust of about 50 km, crustal melting plays only a marginal role in silicic magma genesis. Interestingly, trace element patterns of Nevado de Toluca and the neighboring monogenetics show an adakite-like signatures. The wealth of information from field, geochronologic and petrological studies obtained on Nevado de Toluca makes it an ideal target for more advanced studies to understand magma dynamics in volcanic arcs.

Two consecutive field campaigns have been carried out at Nevado de Toluca during the course of this study in August 2016 and January 2018. The principle goal of this fieldwork was to collect samples representative of the entire eruptive history of the volcano and surrounding monogenetic cones. Further, we focused on stratigraphic relations and sampling of young pyroclastic deposits. A total of 134 samples were collected, of which 97 were analyzed for this thesis. A description of the collected samples and their locations is attached in the electronic supplementary materials (Table E2.1).

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Figure 1.2. Selected photographs of Nevado de Tolucas’ crater area from various perspectives.

a) View from south-west showing the main crater domes U31 and U30 and the peripheral dacite dome U27. b) Crater of Nevado de Toluca and Cerro Gordo dome seen from west. c) View into the crater looking southwards. The interior of the crater holds the rounded ‘Ombligo’ dome and two lakes: Laguna de la Luna and Laguna del Sol.

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Figure 1.3. Selected outcrop photographs showing the range of eruptive styles and stages of Nevado de Toluca. a) Typical peripheral dacitic lava dome. b) Lava dome U27 in vicinity of the crater. c) Block-and-ash flow deposit. d) Sequence of wet surges and block-and-ash flow

deposits. e) Massive pyroclastic density current deposit (WQ) underlying a fallout layer (line above persons’ heads). e) Plinian fallout (upper coarse white layer), massive and dilute PDCs with cross bedding of the Upper Toluca Pumice eruption.

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1.4 Structure of this thesis

This PhD thesis is organized into six main chapters, a bibliography of the referenced literature, and an appendix of supplementary materials to all main research chapters (2-5). Data tables are presented in the electronic supplement to this thesis. In addition, three published articles of the author that are not a direct part of this work are included as a supplement. Chapters 2-5 have been submitted for publication to scientific journals and are either published or currently under peer-review.

Chapter 2 investigates the long-term petrological history of Nevado de Toluca volcano by detailed analysis of bulk-rock and mineral chemistry. A range of thermobarometric

calculations are presented to constrain intensive magmatic system variables. Based upon results from thermal modelling, the life cycle of Nevado de Toluca and the petrologic monotony of Mexican stratovolcanoes are discussed.

Chapter 3 integrates the zircon crystallization and trace element record of Nevado de Toluca with thermal modelling of pulsed upper crustal magma assembly. Matching synthetic and natural zircon age populations and temperature distributions, magma fluxes and

extrusive:intrusive (E:I) ratio are constrained. These results allow to characterize the physico- chemical state and size of the subvolcanic reservoir currently residing beneath Nevado de Toluca.

Chapter 4 provides detailed mineral chemistry for a sequence of Plinian eruptions from Nevado de Toluca. Using the systematics of mineral zoning pattern, a recurrent pattern of magma recharge prior to these events is identified. Using diffusion chronometry on plagioclase and orthopyroxene, the timescale of these processes are investigated.

Chapter 5 explores the question why some volcanoes erupt a large range of chemical compositions and others, like Nevado de Toluca, produce the same monotonous magma chemistry throughout their lifespan. Using a novel integration of thermal and petrological modelling, this study shows that the thermal structure of the crust and magma recharge rates modulate the diversity and temporal evolution of magma bodies.

Chapter 6 provides a general discussion and synopsis of the presented research and recommends directions for future research.

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2 T HE LONG - TERM LIFE CYCLE OF N EVADO DE T OLUCA

VOLCANO

Abstract

The petrologic diversity of volcanic rocks reflects the dynamics of magma reservoirs and the temporal evolution of magma chemistry can provide valuable information for hazard assessment.

While some stratovolcanoes monotonously produce intermediate magmas (55-68 wt.% SiO2), dominantly erupted magma types (e.g. basaltic andesite, andesite or dacite) frequently differ even between neighboring volcanoes. If such differences arise due to thermal maturation processes over time or are predetermined by other properties of magmatic systems remains poorly understood. This study helps to elucidate the underlying factors modulating the chemistry of the magma preferentially erupted by Nevado de Toluca volcano in Central Mexico. We present a new dataset of bulk-rock and mineral chemistry spanning the entire 1.5 Million years of the volcanos’ eruptive history. The results reveal that Nevado de Toluca dacites and minor andesite originate in a stable configuration of pre-eruptive processes and plumbing system architecture by hybridization between an upper crustal silicic mush and deeper sourced basaltic andesite magmas. Yet, a subtle trend towards increasing silica content with time (2 wt.% in 1.5 Ma) and episodicity in magma hybridization conditions are observed. We use thermal

simulations of pulsed magma injection to probe the controlling variables on the temporal variation and compositional mode of magma geochemistry. The results show that the subtle temporal trend towards increasing bulk-rock SiO2 content is plausibly explained by slightly

Gregor Weber1, Luca Caricchi1, José Luis Arce2

1 Department of Earth Sciences, University of Geneva, Geneva, Switzerland.

2 Instituto de Geología, Universidad Nacional Autónoma de México, Coyoacán, México.

Submitted to Frontiers in Earth Sciences on 18 May 2020.

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dropping recharge rates and continued upper crustal reservoir growth. Our modelling also shows that the dominant composition of eruptible magmas (“petrologic mode”) can shift as a function of magma flux, extrusive:intrusive ratio and temperature of the recharge magma. A comparison of SiO2 whole rock distributions for monotonous Mexican stratovolcanoes and their peripheral cones shows that their petrologic modes vary in concert, indicating that the recharge magma chemistry or temperature is a major control on the preferentially erupted magma composition for these volcanoes.

2.1 Introduction

Over the last 30 years, combined geological mapping, dating and geochemical analyses have revealed that the range of erupted magma chemistry differ significantly for individual volcanic centers (Hildreth and Lanphere, 1994; Singer et al., 1997; Gertisser and Keller, 2003; Frey et al., 2004; Thouret et al., 2005; Bacon and Lanphere, 2006; Hildreth et al., 2007; Hora et al., 2007;

Singer et al., 2008; Fierstein et al., 2011; Jicha et al., 2012; Walker et al., 2013; Muir et al., 2015;

Rivera et al., 2017). While some volcanoes sample the entire spectrum from basalt to rhyolite, others erupt magmas with monotonous chemistry through time (Kent, 2014; Wörner et al., 2017). In this contribution, we focus on systems producing a restricted range of geochemical compositions, intermediate in silica content (here defined as 55 to 68 wt.% SiO2). While stratovolcanoes in continental arc settings, such as the Trans Mexican Volcanic Belt (TMVB;

Fig. 1), are often compositionally monotonous, differences exist in the mode of erupted

compositions (Schaaf et al., 2005; Schaaf and Carrasco-Núñez, 2010; Torres-Orozco et al., 2017;

Crummy et al., 2019). The reasons why some volcanoes preferentially erupt andesite (e.g.

Popocatépetl; Schaaf et al., 2005) and others repeatedly produce dacite (e.g. Nevado de Toluca;

Torres-Orozco et al., 2017) remains unclear.

Differences in the “petrologic mode” occur even between neighboring volcanoes (Klemetti and Grunder, 2008; Hildreth, 2007; Kent et al., 2010; Wörner et al., 2017), which indicates that large-scale tectonic parameters (e.g. age or slope of the subducting slab) or crustal thickness are not sufficient alone to explain these variations. Most agree that local modulations in magma flux, crustal and/or magma properties are some of the likely variables governing the preferential eruption of specific compositions (Carmichael, 2002; Klemetti and Grunder, 2008; Caricchi and

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and inhibit volcanic eruptions of such compositions (Stolper and Walker, 1980; Marsh, 1981;

Pinel and Jaupart, 2000; Moran et al., 2011). If such barriers are effective in preventing the eruption of very mafic and silicic magma compositions, they may be overcome by

mixing/mingling processes induced by mafic magma recharge in crustal mush reservoirs and produce compositionally monotonous intermediate volcanoes (Reubi and Blundy, 2009; Kent et al., 2010; Kent, 2014). Alternatively, magma properties such as the initial water content or non- linearities in melt fraction-temperature relations can modulate the variety of compositions present in a magmatic system (Melekhova et al., 2013; Nandedkar et al., 2014, Caricchi and Blundy, 2015b; Hartung et al., 2019; Huber et al., 2019).

Thermal modelling of incremental magma reservoir assembly shows that magmatic systems evolve thermally in time with large temperature fluctuations in early stages of a system history and more homogeneous distribution of temperatures later in the life cycle of the magmatic system (Petford and Gallagher, 2001; Annen and Sparks, 2002; Dufek and Bergantz, 2005;

Annen et al., 2006; Annen, 2009; Gelman et al., 2013; Melekova et al., 2013; Karakas et al., 2017). Thus, progressively more homogeneous magma compositions may be erupted with the progressive thermal maturation of volcanic plumbing systems. Likewise, mechanical modelling predicts an increase of the duration of magma storage during the lifetime of volcanic systems because of the more efficient relaxation of overpressures in thermally primed wall-rocks (Jellinek and DePaolo, 2003; Gregg et al., 2012; de Silva and Gregg, 2014; Karlstrom et al., 2017). This could favor magma homogenization in the plumbing system and lead to more homogeneous erupted compositions. Thermal and chemical heterogeneities in magma reservoirs may decay with time by convective overturn in association with fresh recharge pulses and/or gas exsolution (Huber et al., 2009, 2012; Ruprecht et al., 2008; Burgisser and Bergantz, 2011). In highly crystalline reservoirs, melt segregation or reactive flow through vertically extensive crystal piles can occur (Rabionwicz and Vigneresse, 2004; Solano et al., 2012; Cooper et al., 2016; Holness, 2018; Bachmann and Huber, 2019; Floess et al., 2019; Jackson et al., 2018), which may be important in buffering melts to specific compositions governed by the chemistry of cumulate rocks.

Predictions of existing numerical models, relating magmatic processes and the spectrum of chemical compositions erupted by volcanoes can be tested based on long-term eruptive records, but can also help to illuminate physical controls on geochemical distributions and temporal trends. Here we present a dataset of 52 new and 45 previously published (Weber et al., 2019, 2020) whole-rock analyses and 6247 new spot analyses on minerals of temporally resolved bulk-

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rock major and trace element analyses and mineral chemistry spanning the 1.5 Ma life cycle of the dacitic Nevado de Toluca volcano in Central Mexico. We use mineral zonation systematics and thermo-barometric calculations to constrain the origin of magmas at Nevado de Toluca and investigate the temporal evolution of bulk eruptive products and their crystal cargos over the entire lifespan of the volcano. We finally discuss the origin of the observed long-term temporal trends in magma chemistry, as well as the origin of petrologic modes of Mexican

stratovolcanoes, based on our dataset, existing data and using thermal modelling to provide a quantitative framework to our interpretations.

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Figure 2.1. Location of Nevado de Toluca volcano (NT) and petrological diversity of stratovolcanoes in the Trans Mexican Volcanic Belt. The light grey shaded area marks the extent of the Neogene Trans Mexican Volcanic Belt in Central Mexico after Ferrari et al., (2012). The dashed white lines indicate the inferred depth of the subducting oceanic slab (Parado and Suárez, 1995). Red triangles mark the location of major volcanic systems:

Ceboruco (Ce), Volcán Tequila (Tq), Colima Volcanic Complex (Co), Michoacán-Guanajuato monogenetic field (MG), Nevado de Toluca (NT), Popocatépetl (Po), Los Humeros Caldera (H), Pico de Orizaba (Or). For orientation, the locations of Mexico City (MC) and Guadalajara (Gdl) are shown by white squares. The 3 subpanels below the map show density distributions of bulk-rock SiO2 (wt.%) content for the Colima Volcanic complex (left, data from GEOROC), Popocatéptl volcano (right, data from GEOROC), and Nevado de Toluca (center, this study).

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Figure 2.2. Relief, geological map and sampling locations. A) Shaded relief of the Lerma basin and valley of Mexico with major volcanic structures indicates by labels. The location of major cities is marked by white areas. Sampled monogenetic cones in the greater Nevado de Toluca area are shown by orange triangles. B) Geological map of Nevado de Toluca modified from Torres-Orozco et al., (2017). The map is divided into 6 major groups: Pre-Nevado lava flows (grey), Monogenetic volcanism (orange), Domes (gold), Old Nevado stage (darkbrown), Recent Nevado stage (red), and Young Pyroclastic deposits (beige). Sampling locations are indicated by red dots and numbers indicate the sampled unit. Calotepec dome (Ca), Cuescontepec (Cu), La Galera (Ga), Cerro Gordo (Go), Lower Toluca Pumice (LTP), Middle Toluca Pumice (MTP), Ochre Pumice (OP), Las Palomas (Pa), Pyroclastic Flow 970 (PF), Pink Pumice (PP), Sabanillas (Sa), Tenango (T), Tepehuisco (Te), Tlacotepec cone (Tl), Upper Toluca Pumice

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2.2 Methods

2.2.1 Bulk rock analytical techniques

Samples were collected in the field to represent the entire eruptive history of the volcano, with particular emphasis on units that have previously been dated by radiocarbon or 40Ar/39Ar- geochronology (Macias et al., 1997; Torres Orozco et al., 2017). A description of the sampled units and locations, including coordinates, can be found in the electronic supplementary

materials (Table S1). All samples were first cleaned from adherent alteration crusts, washed with water and dried in an oven at 50°C before further processing. Lava and pumice samples were then reduced in size using a steel jaw-crusher, a hydraulic press and sieved to grain sizes < 2 mm. Lava samples were ground to fine powders in an agate mill for 30 minutes, while pumice samples, due to their higher porosity, were milled for 20 minutes. In order to monitor the

alteration state of the samples, 2g aliquots were separated from the powders to determine the loss on ignition (LOI). The aliquots were weighted into ceramic crucibles and heated to 1050°C for 24 hours. The LOI was then calculated by weight difference before and after heating.

Subsequently, glass beads for X-ray fluorescence analysis (XRF) were prepared. Precisely 6 g of Li tetraborate were mixed for 3 minutes in a hand mortar with 1.2 g of each of the powdered samples. The so obtained mixtures were then loaded in Pt crucibles and fused at 1200°C. Major element analysis were carried out at the University of Lausanne using a PANanalytical Axiosmax X-ray fluorescence spectrometer. BHVO-2 and several in-house standard materials were used for quality control during the analytical session.

Trace element contents in the whole rock samples were obtained on the same fused glass beads by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). After XRF analysis, glass beads were crushed and mounted on adhesive tape with the side of the bead not analyzed by XRF facing to the top. We used a quadrupole spectrometer Agilent 7700x coupled to an UP-193FX ArF eximer ablation system at the University of Lausanne. The instrument was optimized by ablating the NIST SRM 612 reference glass in linear scan mode with 20 Hz

repetition rate, 75µm beam size, and 5.0 J cm-2 on-sample energy density. Ablation of the sample glass discs was carried out under a helium atmosphere, using a 12 Hz repetition rate, 5.0 J cm-2 energy density and 150 µm diameter beam size. Relative sensitivity factors were determined by analysis of NIST SRM 612 during the analytical session. On each glass bead, we collected and averaged 3 spot analyses. Data were reduced with the MATLAB based software SILLS

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(Guillong et al., 2008), using the bulk-rock Ca contents from XRF analysis for quantification.

The relative uncertainties (RSE) for the 3-point analysis are for most analyzed elements < 2%, with Rare Earth Elements (REE) generally showing RSE < 5%. The largest differences for averaged spots were obtained on Mo (RSE: 12.8 %), Cu (7.4 %) and Ni (6.5 %).

2.2.2 In-situ mineral and glass geochemistry

In this study we present mineral and glass chemistry for 19 eruptions from Nevado de Toluca, which were selected to cover the entire eruptive history of the volcano. Mineral, groundmass and glass compositions were determined on polished 100 µm thick petrographic sections by electron probe microanalyzer (EPMA). All analyses considered here were carried out, using a JEOL 8200 superprobe at the University of Geneva. Prior to analysis, the petrographic sections were coated with a 20 nm thick layer of carbon to ensure electrical conductivity. For each of the studied eruptions, we analyzed one petrographic section, for which between 8 and 15 BSE images each were obtained in order to reveal textures and internal mineral zonation features. Typically, we analyzed linear rim to core profiles of 10 to 20 crystals for each mineral phase in the individual petrographic sections. The spacing between points was variable but between 5 and 20 µm for most grains.

Mineral phases (plagioclase, pyroxenes, Fe-Ti oxides and amphiboles) were analyzed using the wavelength-dispersive x-ray spectrometer of the EPMA. Measuring conditions were set to 15 kV acceleration voltage, 20 nA beam current and focused beam size of 2 µm. The peak counting times were set to were 30 s (Si kα TAP), 20 s (K kα PETH), 20 s (Na kα TAP), 30 s (Fe kα LIFH), 30 s (Al kα TAP), 30 s (Ca kα PETJ), 30 s (Ti kα LIFH), 30 s (Mg kα TAP) , 30 s (Cr kα PETJ) and 30 s (Mn kα LIFH). Raw analysis for plagioclase and pyroxenes that showed totals outside of the range 100 ± 1.5% were rejected from further analysis. The raw amphibole totals outside the range 95.5-99.5% were filtered out. Compositions that indicated mixed analysis of silicate minerals and glass were also discarded. In total, 2883 plagioclase, 2232 pyroxene and 1128 amphibole analyses are presented in this study. Matrix and melt inclusion glasses, as well as groundmass analyses, were carried out on the same petrographic sections as the mineral analyses and we used a reduced beam current of 8 nA and 15 kV acceleration voltage. The beam size was set to 10 µm for matrix glass and melt inclusion analysis, while for the mostly

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pyroxene or amphibole. Counting times on the peak position were 30 s (Na kα TAP), 20 s (Si kα TAP), 60 s (S kα PETH), 20 s (K kα PETJ), 30 s (Mg kα TAP), 30 s (Ca kα LIFH), 30 s (Al kα TAP), 30 s (Ti kα PETJ) and 30 s (Fe kα LIFH). Alkali element measurements were performed first in the sequence to mitigate diffusive loss.

2.2.3 Thermobarometric calculations

The mineral, glass and groundmass compositions analyzed in this study were used to estimate the pressures, temperatures and oxygen fugacity of Nevado de Toluca magmas using a range of geothermobarometric techniques. Amphibole crystallization temperatures and pressures were calculated for euhedral outermost amphibole rim compositions using Equation 7b from Putirka (2016) by iterative solution, which requires input of amphibole and liquid composition, as well as an estimate of water content. We used average groundmass for each of the studied eruptions to approximate the input liquid composition. In order to assess equilibrium between the assumed liquid and outermost rim amphibole, we discarded amphibole-liquid pairs with mineral- melt exchange coefficient KD(Fet-Mg) outside the range 0.17 and 0.39 (Putirka, 2016). 39% of the analyzed amphibole rims were found to be in equilibrium. The impact of the assumed liquid H2O content was assessed by performing calculations with 4, 6 and 8 wt.% H2O. However, the resulting average differences in pressures and temperature calculations of 0.4 kbar and 0.3°C are much smaller than the uncertainties of amphibole thermobarometry (±4 kbar and ±30°C for single spot analyses; Putirka; 2016). Amphibole crystallization temperatures can also be

estimated independent of liquid composition and pressure, not necessarily with worse precision and accuracy when compared to amphibole-liquid thermometers (Ridolfi and Renzulli, 2011;

Putirka, 2016). We calculated liquid-independent amphibole temperatures using eqn. 5 presented in Putirka (2016) on outermost rim compositions in order to compare them to the amphibole- liquid pair estimates. This thermometer has been calibrated over a temperature range applicable to intermediate magmas and produces calibration data with SEE of ±53°C (Putirka, 2016).

Plagioclase-amphibole temperatures were calculated using the calibration based on the edenite + albite = richterite + anorthite exchange reaction on analyses corresponding to average outermost rim composition within 2SD in samples that showed euhedral crystal textures (Holland and Blundy, 1994). Several samples contain both clino- and orthopyroxenes, but equilibrium

between outermost rim compositions based on an Kd(Fe-Mg) of 1.09±0.14 was only achieved in

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