A Monogenetic Alkali Basalt Field East of the Andean

The Pennsylvania State University

The Graduate School

College of Earth and Mineral Sciences

A MONOGENETIC ALKALI BASALT FIELD EAST OF THE ANDEAN

ARC BETWEEN 34° AND 35° S: IMPLICATIONS FOR MANTLE

COMPOSITION

A Thesis in

Geosciences

Timothy T. Murray

Submitted in Partial Fulfillment of the Requirements for the Degree of

Master of Science

August 2013

The thesis of Timothy T. Murray II was reviewed and approved* by the following:

Maureen D. Feineman Assistant Professor of Geosciences Thesis Advisor

Tanya Furman Professor of Geosciences Assistant VP & Assoc Dean for UG Education

Peter LaFemina Assistant Professor of Geosciences

Chris J. Marone Professor of Geosciences Associate Department Head of Graduate Programs

*Signatures are on file in the Graduate School

ii ABSTRACT

This study analyzes a series of Quaternary basaltic lavas sampled ~50km east of the arc front of the Northern Southern Volcanic Zone (NSVZ) of the Andean arc, as well as two basaltic andesites from Casimiro parasitic cone in the Diamante Caldera, located on the arc front itself, for major element concentrations, trace element concentrations, and radiogenic isotope ratios of Sr, Nd, Pb, and Hf. The basaltic field has previously been classified as a northern portion of the Payenia Volcanic Complex, the more southerly parts of which are interpreted to have resulted from adiabatic melting associated with changes in slab dip during the late Oligocene through Miocene. The retro-arc basalt samples are alkaline basalts, enriched in fluid mobile elements such as Cs, Ba, Pb, Sr, and Li, with moderate relative depletions in Nb and Ta. These characteristics are typical of rear-arc basalts from subduction zones. We propose that these basalts should be classified as retro-arc basalts associated with the Quaternary volcanic arc rather than as an extension of the more extensive Payenia Volcanic field to the south.

It has been well documented that arc lavas in the NSVZ have distinctly higher 87Sr/86Sr and lower εNd values than arc magmas found south of 34.5°S. This chemical distinction has been interpreted as indicating a significant crustal contribution to rising magmas within the NSVZ. The mechanism for this crustal contribution, however, has been largely debated. Two models have been proposed to explain the driving force for this increased crustal signature: 1) increased assimilation of crustal material with elevated 87Sr/86Sr isotope ratios and lower εNd values due to thicker, older crust in the NSVZ, or 2) the subducting Nazca plate is eroding crustal material with elevated 87Sr/86Sr isotope ratios and lower εNd values and subducting it beneath the arc, where it mixes with and alters the composition of the mantle wedge, i.e. subduction erosion. If the elevated 87Sr/86Sr isotope ratios and lower εNd values observed in the arc front lavas from the NSVZ were due to crustal contamination of the mantle source from subduction erosion, it can reasonably be expected the isotopic evidence for crustal contamination would be observable in the retro-arc basalts of the NSVZ, located a few tens of kilometers west of the arc front. Conversely, if the isotopic evidence for crustal contamination were due to assimilation of thicker, older crust beneath the arc, then one would not expect to see similar isotopic evidence for crustal contamination in the retro-arc basalts. The retro-arc basalts from this study have 87Sr/86Sr isotope ratios of 0.7037 – 0.7043, which is statistically different than 87Sr/86Sr isotope ratios found in arc lavas of the NSVZ (87Sr/86Sr = 0.7046-0.759). Similarly, εNd values retro-arc basalt samples from this study (εNd = 1.4 to 4.4) is statistically different than εNd values found in arc lavas of the NSVZ (εNd = -1.8 to -0.19). This evidence suggests that the mantle beneath the NSVZ does not contain the elevated 87Sr/86Sr isotope ratios and lower εNd values observed in arc lavas of the NSVZ. Therefore, subduction erosion cannot be the driving mechanism for crustal contribution to ascending magmas in the NSVZ. Instead, assimilation of crustal material within the older, thicker crust beneath the NSVZ must be the driving force for elevated 87Sr/86Sr isotope ratios and lower εNd values observed in NSVZ arc lavas.

iii

TABLE OF CONTENTS

List of Tables …………………………………………………………………………...v List of Figures…………………………………………………….……………………vi Acknowledgements………….…………………………………………………………ix

1 INTRODUCTION…………………………………...…………………….……….1 1.1 Classification of retro-arc basalts…….……………………………………....1 1.2 Continental crustal signature in the Northern Southern Volcanic Zone ...... 2

2 GEOLOGIC BACKGROUND….………………………………………………....6

3 METHODS……………………………………………………..………………….13 3.1 Sample Preperation…..………………………………………………….…..13 3.2 Major Element Concentrations…………………………………………...... 13 3.3 Trace Element Concentrations………………………………………...…….14 3.4 Isotope Ratio Analysis……………………………………………………...15

4 RESULTS …………………………………………………………………...18 4.1 Major Elements...... 18 4.2 Trace Elements...... 18 4.3 Isotope Ratios………………………………………...…………………….19

5 DISCUSSION……………………………………………………………………..29 5.1 Chemical Variability of retro-Arc Basalts…………………………………..29 5.2 Crustal Contamination and the State of the Mantle in the NSVZ…………..29 5.3 Crustal Contamination within the Retro-Arc Basalt Suite…………………..33 5.4 Pb Isotope Ratios…………………………………………………………...35 5.5 Classification of Retro-arc Basalt Field……………………………………..36

6 CONCLUSION…………………………………………………………………....47

REFERENCES………………………………………………………………………....50

APPENDIX A: Sample Descriptions………………………………………….53

APPENDIX B: Rock Digestion Procedures for ICP-MS Trace Element Analysis………………………………………………………………………....54

iv TABLE OF CONTENTS

APPENDIX C: Dissolution Procedure at Lamont-Doherty Earth Observatory….60

APPENDIX D: Ion Exchange Procedure used at Lamont-Doherty Earth Observatory……………………………………………………………………...... 62

APPENDIX E: Maipo Volcano Figures…………………………………………..65

APPENDIX F: Data Tables for Maipo Volcano and Basement Rock…………...67

v LIST OF TABLES

Table 1. Sample location and descriptions. Ages listed are from Ramos and Fulguera 2011………………………………………………………………………………………12

Table 2. Major element concentrations (wt. %), trace element concentrations (ppm) and isotope ratios for behind the arc basalts at 34°S and Casimiro……………………………….21

vi LIST OF FIGURES

Figure 1. Figure from Stern 2004 showing schematic map of the four volcanic zones of the Andes Mountains...... 4 Figure 2. Image altered from Ramos and Folguera 2011 showing major volcanic fields for Payenia Volcanic Province. Dashed black box represents the field area for this study.

…………………………………………………………………………………………………………………………………………...5

Figure 3. Area map showing sampled region within the Southern Volcanic Zone and relative to the Central Volcanic Zone (after Sruoga et al., 2005). The Southern Volcanic Zone is often divided into four segments, the Northern Southern Volcanic Zone (NSVZ), Transitional Southern Volcanic Zone (TSVZ), Central Southern Volcanic Zone (CSVZ), and Southern Southern Volcanic Zone (SSVZ) as shown. Dashed lines represent depth to the top of the subducting Nazca plate...……………………………………………………………………..…..9

Figure 4. Tectonic map showing sample locations relative to the Cordillera Frontal to the west and the San Rafael Block to the east (after Folguera and Ramos, 2011). Eruptive centers can be seen in line with previously mapped normal faults in the area..…….….10

Figure 5. Figure altered from Gilbert et al 2005 showing crustal thickness contours. Red dashed box represents the field area for this study. Black lines are depth to slab contours from Cahill and Isacks 1992.………………………………………………………………………...…11

Figure 6. Total alkalis (Na2O + K2O) vs. silica (SiO2) diagram after LeBas et al., 1992. Solid line represents delineation between alkaline (above line) and subalkaline (below line) compositions (after Irvine and Baragar, 1971). The basaltic samples (filled circles) from behind the arc plot within the alkaline field, while the Casimiro samples (open diamonds) plot within the subalkaline field. NSVZ field from Maipo volcano analyzed as part of this project. ………………………………………………………………………………………………………….23

Figure 7. Trace element concentrations normalized to N-MORB (after Sun and McDonough, 1989). Solid lines represent retro-arc basalts while the dashed lines represent basaltic andesites from Casimiro parasitic cone..……………………….……………...24

Figure 8. Chondrite-normalized Rare Earth Element (REE) diagram (values from McDonough and Sun, 1995). Solid lines represent retro-arc basalts while the dashed lines represent basaltic andesites from Casimiro parasitic cone. …….…………………..….25

87 86 87 86 Figure 9. Sr/ Sr plotted versus εNdCHUR values. Values of εNd and Sr/ Sr are negatively correlated. Circles represent retro-arc basalts while open diamonds represent basaltic andesites from Casimiro parasitic cone. The basaltic flow sampled northwest of Cerro Sepultura, with the highest εNd value and lowest 87Sr/86Sr value, is represented by

vii an open circle while the more silica-rich sample from Gaspar is represented with a star. MORB field altered from Sun and McDonough 1989…………………...... 26

Figure 10. εHfCHUR values plotted versus εNdCHUR. Values of εNdCHUR and εHfCHUR are positively correlated. Circles represent retro-arc basalts while open diamonds represent basaltic andesites from Casimiro parasitic cone. The basaltic flow sampled northwest of Cerro Sepultura, is represented by an open circle while the more silica-rich sample from Gaspar is represented with a star. Mantle Array calculated from Vervoort et al 1999……………………………………………………………………...... 27

Figure 11. a) 206Pb/204Pb vs 207Pb/204Pb and b) 206Pb/204Pb vs 208Pb/204Pb. Circles represent retroarc basalts and open diamonds represent Casimiro basaltic andesites. Both 208Pb/204Pb and 207Pb/204Pb are positively correlated with 206Pb/204Pb. The basaltic flow sampled northwest of Sepultura, with the highest εNd value and lowest 87Sr/86Sr value, is represented by an open circle while the more silica rich sample from Gaspar is represented with a star. NHRL calculated from Hart 1984…..…………………………….....28

Figure 12. (A) Chondrite normalized Sm/Yb values plotted by latitude. A positive trend exists from south to north. This trend is more pronounced in the heavy rare earth elements, represented by chondrite normalized La/Yb in (B) than in the light rare earth elements, represented by chondrite normalized La/Sm in (C). Normalizing values from McDonough and Sun, 1989…………………………………………………………………………………………39

87 86 Figure 13. Sr/ Sr plotted versus εNdCHUR values. See legend (inset) for data description. Trend 1 represents a mixing trend between primitive mantle and Cenozoic continental crust, while Trend 2 represents further assimilation of Paleozoic upper continental crust. Values for Paleozoic crust from Lucassen et al. 2004.…………….40

Figure 14. εHfCHUR plotted versus εNdCHUR values. See legend (inset) for data description. Trend 1 represents a mixing trend between primitive mantle and Cenozoic continental crust, while Trend 2 represents further assimilation of Paleozoic upper continental crust. ………………………………………………………………………………………………………..41

Figure 15. Variation of Sr isotopic ratios with latitude. The gray field between ~35°S and ~39°S represents typical values for the CSVZ and TSVZ (Costa et al., 2002; Davidson et al., 1987; Feeley et al., 1998; Hickey et al., 1996; Hilton et al., 1993; Reubi et al., 2011; Tormey et al. 1995). The NSVZ field was delineated using average values from (Futa and Stern 1988; Hildreth and Moorbath 1988; Lopez-Escobar et al., 1985). The open squares represent unpublished data from Maipo Volcano that were analyzed during this project and by Drew (2010)……………………………………………………………………42

Figure 16. Sr isotopic ratios versus SiO2. The field labeled CSVZ and TSVZ represents typical values for the CSVZ and TSVZ (Costa et al., 2002; Davidson et al., 1987; Feeley

viii et al., 1998; Hickey et al., 1996; Hilton et al., 1993; Reubi et al., 2011; Tormey et al. 1995). The NSVZ field was delineated using average values from (Futa and Stern 1988; Hildreth and Moorbath 1988; Lopez-Escobar et al., 1985). The open squares represent unpublished data from Maipo Volcano that were analyzed during this project and by Drew (2010). Retro-arc samples represent a common end member for both arc segments, with Casimiro samples intermediate between the retro-arc basalts and NSVZ.………...43

Figure 17. a) 206Pb/204Pb vs 207Pb/204Pb and b) 206Pb/204Pb vs 208Pb/204Pb. Huincan andesite is shown as a representative of Cenozoic continental crust. Trend 1 represents mixing from the flow NW of Sepultura (representing the NSVZ mantle) towards less radiogenic Cenozoic crust. Trend 2 represents mixing towards a more radiogenic Paleozoic upper crustal end member...... …………………………………………………………………....44

Figure 18. a) Trace element concentrations normalized to N-MORB from Sun and McDonough (1989). b) Chondrite-normalized REE diagram (values from McDonough and Sun (1995)). Solid lines represent retro-arc basalts while the dashed lines represent basaltic andesites from Casimiro parasitic cone. The grey field represents the range of previously published values for Payenia Volcanic Complex (Germa et al., 2010; Hemando et al., 2012; Kay et al., 2004; Pasquare et al., 2008). .………………………………....45

Figure 19. a) Trace element concentrations normalized to N-MORB from Sun and McDonough (1989). b) Chondrite-normalized REE diagram (values from McDonough and Sun (1995)). Solid lines represent retro-arc basalts while the dashed lines represent basaltic andesites from Casimiro parasitic cone. The grey fields represents the range of previously published values for the NSVZ (Futa and Stern 1988, Hildreth and Moorbath 1988, Lopez-Escobar et al., 1985; this study) and the CSVZ and TSVZ (Costa et al., 2002; Davidson et al., 1988; Deruelle 1982; Dungan et al., 2001; Feeley and Dungan 1996; Feeley and Dungan 1998; Hickey et al., 1986; Lopez-Escobar et al., 1977; Lopez- Escobar et al., 1981; Reubi et al., 2011)………….……………………………………………………………..46

Figure 20. Schematic cross-section of the NSVZ at ~34.5°S. Magma’s below both the arc front and retro-arc are assimilating some portion of Cenozoic lower crustal material. Only magma’s below the arc front are assimilating Paleozoic upper crust from the Cordillera Frontal. Thick skinned faulting in the foreland basin has created NW-SE trending lineaments that create pathways for magma’s below the retro-arc to ascend with little upper crustal interaction……………………………………………………………………………………….....49

ix ACKNOWLEDGEMENTS

I’d like to acknowledge my advisor, Maureen Feineman, for her continued patience, guidance, and advice throughout my time in the program and my committee members, Tanya Furman and Peter LaFemina for their wonderful advice and feedback during the entire process. I’d also like to thank Steve Goldstein and Jason Jweda at Lamont-Doherty Earth Observatory for imparting amazing amounts of time and insight pouring over data, and assistance during long nights analyzing samples. Additionally, I’d like to thank Patricia Sruoga for her invaluable knowledge of the NSVZ, and teaching me how to be a better field geologist.

Special thanks to Mario Rosas, Manuela Elissondo, Melanie Saffer, Henry Gong, Dana Drew and Waleska Castro for their assistance during this project.

This research was funded in part by:

 NSF grant #EAR- 0911430 awarded to Maureen Feineman.

I’d like to thank all of my friends and my sister Vanessa for their support throughout this entire process. I’d especially like to thank my parents for their guidance, wisdom, patience, encouragement, and love throughout everything. Without them, I could not have finished this project. Finally, I’d like to thank my younger brother, Michael David Murray, for always being an inspiration and teaching me how to laugh. Even though you are no longer with us, you continue to inspire me and I couldn’t have gotten here without you.

x 1 INTRODUCTION

The Andes Mountains are the longest mountain chain in the world, extending over 7,500 km along the western margin of the South America plate as shown in figure 1(Stern

2004). With over 200 potentially active Quaternary volcanoes (Stern 2004), the Andean subduction zone serves as the archetypal continental arc setting. Due to the extensive range and number of volcanic edifices, as well as the relative inaccessibility of parts of the arc, some processes contributing to the current petrological, tectonic, and geochemical state of the crust-mantle system remain poorly understood.

The Northern Southern Volcanic Zone (NSVZ) of the Andean Volcanic Belt, located between 33° and 34.5°S, results from the subduction of the Nazca plate beneath the South

American plate. This study focuses on a suite of Quaternary basalts from behind the volcanic front between 34.5° and 34.77° S, and two basaltic andesite samples from

Casimiro parasitic cone located on the wall of the Diamante Caldera at the same latitude.

The goal of this study is to illuminate the geochemical composition of the subarc mantle beneath the NSVZ.

1.1 Classification of retro-arc basalts

The isolated monogenetic basaltic lavas located ~50 km east of the Quaternary arc front between 34.5° and 34.77° S have previously been classified as the northern segment of the extensive Payenia Volcanic Province (Figure 2; Ramos and Folguera 2011). Previous studies have focused on the more extensive central and southern segments of the Payenia

(sometimes called Payunia) Volcanic Province, which includes the large shield volcano

Payun Matru at 36.42° S and 69.2°W (Kay and Copeland 2006). Among studies of the

1 monogenetic basalts of this northern area, most have focused on either dating of volcanic events (Stern et al. 1984; Folguera et al. 2009) or tectonic setting (Folguera et al. 2009).

1.2 Continental crustal signature in the Northern Southern Volcanic Zone

The NSVZ segment of the Andean arc is geochemically distinguished from the rest of the

Southern Volcanic Zone (SVZ) by its elevated 87Sr/86Sr ratios, lower 143Nd/144Nd ratios, increased K20, Ba, and Ce concentrations, and higher Ce/Yb and Hf/Lu ratios (Hildreth and Moorbath 1988). Hildreth and Moorbath (1988) attribute these features to a proposed model of Mixing, Assimilation, Storage, and Homogenization (MASH) in which rising magmas produced from fluid-induced partial melting of the mantle wedge stall at the base of the continental crust and assimilate lower crustal material. An alternate hypothesis in which the subducting oceanic plate is eroding and subducting continental material has been proposed by other authors (Stern, 1991; Kay et al., 2005).

In this model, the subducted crustal material mixes with and changes the geochemical composition of the mantle wedge. Subduction erosion is supported in the Central

Volcanic Zone (CVZ), north of the study area, by the disappearance of pre-Andean

Paleozoic basement, along-strike disappearance of a Jurassic Andean plutonic belt, and the eastward shift of Andean plutonic belts in time (Stern 1991). Kay et al. (2005) link erosion of crustal material by the subducting slab to episodes of crustal shortening and thickening and eastward shifts of the arc front in the NSVZ. Since both processes may produce similar geochemical signatures within the arc, it has been difficult to resolve the dominant process in the NSVZ.

2 Since the retro-arc basalts from this study are located east of the arc front, where the crust is thinner, and are chemically more primitive than rocks found at the arc front, they present an opportunity to examine mantle composition and processes at these latitudes in samples that have undergone less interaction with crustal material than have the arc front lavas. The Casimiro basaltic andesites are among the most primitive arc front lavas in the

NSVZ. They may represent a middle member between the retro-arc basalts and the more evolved rocks of the Andean arc front.

Ten samples from nine basaltic cinder cones, lava flows, and maar deposits, as well as two samples from Casimiro parasitic cone, were analyzed for major element concentrations, trace element concentrations, and radiogenic isotope ratios of Sr, Nd, Pb and Hf, in order to shed light on the composition of the mantle in this region.

Figure 1. Figure from Stern 2004 showing schematic map of the four volcanic zones of the Andes Mountains.

Figure 2. Image altered from Ramos and Folguera 2011 showing major volcanic fields for Payenia Volcanic Province. Dashed black box represents the field area for this study.

5 2 GEOLOGIC BACKGROUND

The Andean Volcanic Belt extends discontinuously along the western margin of the

South American plate from Colombia to the southern tip of Argentina. This volcanic belt is a result of subduction of the Nazca plate (and in the south, a portion of the Antarctic plate) beneath the South American plate. Subduction of these two plates continues today at a rate of 7-9 cm/year (Cembrano and Lara 2009) and an average dip of 30° (Ramos et al 1996). The Andean arc is separated into four zones of active volcanism: the Northern

Volcanic Zone (NVZ, 5°N-2°S), Central Volcanic Zone (CVZ, 14-27°S), Southern

Volcanic Zone (SVZ, 33-46°S), and Austral Volcanic Zone (AVZ, 49-55°S) (Figure 1).

Each of these zones may be subdivided into smaller volcanic arc segments based on differences in geological and/or tectonic setting (Stern 2004).

The SVZ can be sub-divided into several segments based on geologic offsets along the current arc; however, previous studies do not all agree upon naming or delineation of the segments. In this work, the divisions of Stern (2004) are used: the Northern Southern

Volcanic Zone (NSVZ, 33-34.5°S); the Transitional Southern Volcanic Zone (TSVZ,

34.5-37°S) the Central Southern Volcanic Zone (CSVZ, 37-41.5°S); and the Southern

Southern Volcanic Zone (SSVZ, 41.5-46°). The relatively thin crust (<30 km thick) in the

SSVZ and CSVZ thickens northward to >45 km thick in the NSVZ as shown in figure 5

(Zandt 2005). Similarly, the subducting slab shallows northward to an average interpreted dip of 20° beneath the NSVZ (Stauder, 1973; Barazangui and Isacks, 1976).

This change in dip angle may be related to along-arc differences in the trench to arc gap, which may in turn be a primary reason for the observed offsets in the arc between segments (Stern 2004). Wagner et al. (2005) observe that the slab goes from a flat-slab

6 north of the NSVZ, to a normally subducting slab within 2° latitude, with an unresolved geometry in this transitional area.

The NSVZ encompasses three main volcanic complexes as well as numerous minor volcanic features including parasitic cones, cinder cones, maars, and flow basalts. The three stratovolcano complexes, Tupungato-Tupungatito, Marmolejo-San Jose, and

Diamante-Maipo, are comprised of dominantly andesites with basaltic andesites, dacites, and rhyolitic ignimbrites (Stern 2004). Other minor volcanic edifices of primarily basaltic composition are present east of the main arc in the retro-arc foreland basin.

Payenia is a large volcanic province located behind the arc front in the foreland basin of the SVZ. This province encompasses multiple volcanic fields containing more than 800 volcanic centers of primarily basaltic composition located from 33°30’°S to 38°S. This province can be subdivided into three segments: a northern segment containing isolated monogenetic basaltic fields (sampled in this study), a central segment containing three significant volcanic fields with extensive lava flows, and a southern segment containing two significant volcanic fields, but lacking extensive lava flows (Ramos and Folguera

2011). The central and southern segments of Payenia have been the focus of multiple studies because they contain the largest volcanic fields, including Payun Matru and

Llancenelo (e.g., Ramos and Folguera 2011, Kay and Copeland 2006, Jordan et al. 1983,

Ramos and Barbieri 1988). Kay and Copeland (2006) interpreted much of the volcanic activity in this area to be the result of early Miocene extension followed by a contractional regime and slab shallowing. Samples from early Miocene eruptions show little to no evidence of an arc-like trace element signature, while more recent (middle

7 Miocene) lavas show an increasing, but still relatively small, arc-like signature (Kay and

Copeland, 2006).

This study focuses on the northern segment of the Payenia volcanic province, which is primarily comprised of small monogenetic cones, maars, and flows of alkaline basaltic composition. Previous studies have examined the tectonic state of the area (Sruoga and

Cortes, 1998; Folguera et al. 2009) and dated several of the volcanic centers (Folguera et al. 2009); however, little published data exists relating major and trace element concentrations and isotope ratios to the state of the Andean arc. Stern et al. (1990) analyzed a suite of basalts from ~34°S to ~52°S, two of which are located in the retro-arc region of the NSVZ, for major and trace element concentrations, and Sr, Nd, Pb, and O isotopic compositions, but their study focused primarily on basalts in the intermediate arc and retro arc farther south than the current study area. From a tectonic viewpoint, these volcanic centers reside in the foreland basin between the Andean Cordillera and the uplifted San Rafael Block located to the east. They are typically found in NNW to NW linear chains that follow tectonic structures in the area associated with the thick-skinned

Malargue foreland fold-and-thrust belt and early Miocene extension. These structures are interpreted by Cortés (2000) as piedmont fault scarps and bedrock escarpments that correspond to reactivated normal faults associated with NW-trending transtensional lineaments. In addition to determining the geochemical composition of the mantle in this region, a secondary goal of this study is to characterize these volcanic centers within the current and historic NSVZ tectonic setting.

11 Table 1. Sample location and descriptions. Ages listed are from Ramos and Fulguera 2011.

Sample Location Description Latitude Longitude Age (Ma) (°S) (°W) MD-109-13 Sepultura cinder cone 34.27906 69.09439 0.07 ±0.004 MD-109-14 flow NW of flow basalt 34.27433 69.12033 Sepultura MD-109-15 Arroyo Honda cinder cone 34.50119 69.22733 0.49 ±0.03 MD-109-16 Maar NW of maar 34.47044 69.25258 0.434 ±0.3 Arroyo Honda MD-109-17 La Leña flow basalt 34.76633 69.42108 MD-109-21 Las Bolas flow basalt 34.61500 69.02136 0.495 ±0.03 MD-109-22 Agua del Toro flow basalt 34.59197 69.03506 at dam MF-212-15 Gaspar cinder cone 34.28352 69.04361 0.106 ±0.01 MF-212-18 el Pozo cinder cone 34.32798 69.12115 0.092 ±0.01 MF-212-19 el Pozo cinder cone 34.32798 69.12115 0.092 ±0.02 G0100112-1 Casimiro parasitic cone 34.21594 69.92058 G0100112-2 Casimiro parasitic cone 34.21638 69.92742

12 METHODS

3.1 Sample Preparation

Massive (i.e., bomb and flow) samples were cut into slabs to ensure that all exposed surfaces were removed. Once cut, they were then polished using alumina sandpaper to remove saw marks before being cleaned with ethanol and milli-Q water and then placed in a ceramic jaw crusher. About fifteen grams of the resulting chips were hand-picked under a stereo microscope to ensure the selection of chips free of surface contamination and secondary mineralization. The samples were then ground to less than 150 µm by use of an agate mortar and pestle and/or an agate ball mill. Tephra samples were ultrasonically cleaned in milli-Q water then ground using an agate mortar and pestle.

3.2 Major Element Concentrations

Analyses of major element concentrations were conducted by Inductively Coupled

Plasma Atomic Emission Spectrophotometry (ICP-AES) on a Perkin-Elmer Optima

5300DV ICP-AES in the Laboratory for Isotopes and Metals in the Environment (LIME) at the Pennsylvania State University. Dissolution of the samples was performed by lithium metaborate fusion. One hundred mg of sample powder were added to 1 g of lithium metaborate powder for each sample and hand mixed by inversion and shaking.

The mixtures were then added to graphite crucibles and placed in an oven at 1000°C for ten minutes. The resulting melts were poured into Teflon beakers containing 100 ml of

0.113 N nitric acid and stirred by magnetic stir bars for thirty to forty minutes. The resulting solution was diluted and analyzed using ICP-AES. Weight percent of volatiles was determined by gravimetric loss on ignition in the LIME laboratory at Pennsylvania

13 State University. Between 0.5 g and 1 g of sample were placed in a porcelain crucible and heated overnight in an oven at 900°C, then cooled in a dessicator to prevent adsorption of atmospheric water. The samples were carefully weighed both before and after being placed in the oven, and loss on ignition (LOI) was calculated as LOI = (M1-

M2)/M1*100, where M1 is the mass before heating and M2 is the mass after heating.

3.3 Trace Element Concentrations

Trace element concentrations were determined by Inductively Coupled Plasma Mass

Spectrometry (ICP-MS) using the LIME facilities at the Pennsylvania State University.

Digestion of the samples was completed using an HF-HNO3-HCl digestion procedure

(see Appendix 1). The resulting solutions (diluted 2000x) were run on a Thermo X-

Series II Quadrupole ICP-MS. A solution containing 50 parts per billion (ppb) indium was used as an internal standard to correct for drift of the machine during each run. Five rock standards were used to generate a calibration line for my analyses: BHVO-1, BIR-1,

BCR-1, BR, and JA-1. Calibration lines for each element analyzed were examined in detail in order to ensure a correlation coefficient of at least 0.999.

A typical analytical batch consisted of 8-10 samples, two method blanks, two duplicates, and two rock standards (BHVO-1 and BIR-1) dissolved as unknowns. One to two batches were run at a time on the ICP-MS. Instrumental background was monitored with an acid blank consisting of ~2% nitric acid in Milli-Q water. The typical setup for a run would begin with an acid blank, followed by the full suite of five standards, an acid blank run as an unknown, followed by 9-15 samples. If multiple batches were analyzed, the block of blanks and standards would be run again, followed by the remaining 9-15

14 samples. In each run at least two analytical duplicates were analyzed to ensure consistency of the instrument during the run. The highest accuracy and precision were achieved using calibration lines generated using the standard set analyzed immediately preceding each batch of samples.

3.4 Isotope Ratio analysis

Acid digestion, ion exchange chromatography, and isotope ratio analysis for strontium, neodymium, lead and hafnium were conducted in the Isotope Geochemistry Lab facilities at Lamont-Doherty Earth Observatory. Three batches of 9 to 12 samples were analyzed over multiple visits following LDEO standard operating procedures. A method blank and rock standard (BCR-2) run as an unknown were included with each batch. Leaching and acid digestion procedures are outlined in Appendix 2, and ion exchange chromatography procedures are outlined in Appendix 3.

A VG Sector 54 multi-collector thermal ionization mass spectrometer (TIMS) was used to analyze Sr isotope ratios for each sample. Two µL of sample was loaded onto a degassed tungsten filament, followed by 1 µL of Sr loader. The sample and loader were dried then held at enough current to produce a light glow in order to burn off any excess

Rb. The filaments were then loaded into the TIMS and left overnight with the vacuum pump on. Samples were run on the TIMS with a target of 4.5 volts on mass 88. Eight blocks of 20 cycles were run for each sample. The Sr isotopic standard NBS 987 was analyzed multiple times during each run; with no more than four samples being analyzed

15 between standards. Samples were corrected using a 87Sr/86Sr known value of 0.71024 for

NBS 987 using a simple correction of:

( )

Equation (1) where the average measured NBS 987 value is an average of all NBS 987 87Sr/86Sr ratios measured during that batch. The standard deviation of each batch for measured NBS 987

87Sr/86Sr values was below 0.000025. Two analytical duplicates were included in each run as well as two to three filament duplicates.

The first two batches of Nd were analyzed using a VG Axiom multi-collector (MC)-ICP-

MS, while the third batch of Nd and all Pb and Hf isotope ratios were analyzed using a

Thermo Neptune MC-ICP-MS. All samples were diluted 200 times after their respective column chemistry elutions. Lead samples were spiked with 462.5 ppb thallium during dilution in order to provide an internal mass bias monitor. Isotope ratio measurements were carried out using sample-standard bracketing, with the following standards: NBS-

981 for Pb, Jndi for Nd, and Alfa Specpure 13843 for Hf, an in house standard cross- calibrated with JMC 475. The following values were used as accepted known ratios:

0.512115 for 143Nd/144Nd in Jndi, 0.282160 for 176Hf/177Hf in Alfa Specpure 13843,

16.9356 for 206Pb/204Pb in NBS 981, 15.4891 for 207Pb/204Pb in NBS 981, and 36.7772 for

208Pb/204Pb in NBS 981. Samples were corrected using Equation (1) for each respective standard.

At the beginning of each run the standards were run at least 12 times, and analysis of unknowns did not begin until at least 10 consecutive standards were within error to ensure stability of the instrument. On the Neptune a 2σ value below 100 ppm was

16 considered to be within acceptable error, while on the Axiom a 2σ value below 200 ppm was considered to be within acceptable error.

17 RESULTS

4.1 Major elements

The analyzed basalts display a fairly restricted major element compositional range (Table

2). With one exception, the basalts have between 44 and 48 wt. % SiO2. The one outlier

(MF-212-15, Gaspar) falls above this range at ~53 wt. % SiO2 (Table 2). Total alkalis

(K2O+Na2O) range from ~4 to 5 wt. %. When plotted on a total alkali versus silica

(TAS) diagram, all of the retro-arc samples plot within the alkaline field (Figure 6) except MF-212-15, which plots just below the line. Most samples have Mg# (MgO/

(FeO*+MgO)) between 0.60 and 0.74, with one outlier at 0.54 (MD-109-17). Both

samples from Casimiro have higher SiO2 values (~55 and ~56 wt. %), total alkali values within the same range (~5 wt. %), and lower Mg# (0.53 and 0.54). When plotted on a

TAS diagram, both Casimiro samples plot within the subalkaline field (Figure 6).

4.2 Trace elements

When plotted on a mid-ocean ridge basalt (MORB)-normalized spider diagram (Figure

7), all samples are enriched in Cs, Ba, Pb, Sr, and Li and depleted in Nb and Ta relative to the overall trend. In general, the trace element characteristics of the basalts do not show large variations from sample to sample, although the one sample (MF-212-15,

Gaspar) with 53 wt. % SiO2 is characterized by lower high field strength element (HFSE) and heavy rare earth element (HREE) concentrations relative to the lower-SiO2 basalts.

All samples are depleted in HREEs relative to MORB. When plotted on a MORB- normalized spider diagram (Figure 7), both samples from Casimiro are enriched in Cs,

Ba, Pb, Sr, and Li and depleted in Nb and Ta relative to the overall trend. In general, the

18 Casimiro samples are characterized by lower HFSE and HREE concentrations relative to the basalts.

The chondrite-normalized plot of rare earth elements (REE) for both the retro-arc basalts and Casimiro basaltic andesites show a decreasing trend from La to Dy that flattens out between Dy and Lu (Figure 8). A slight to negligible negative Eu anomaly is observed in all samples, with Eu/Eu* values from 0.94 to 0.99.

4.3 Isotope ratios

For retro-arc basalt samples, values of 87Sr/86Sr range from 0.703730 to 0.704312 while

143Nd/144Nd values range from 0.512762 to 0.512854 (Table 2). Calculated εNd values

= 143 144 143 144 (εNd [ Nd/ Nd)Sample / ( Nd/ Nd)CHUR – 1]*10000) range from 1.2 to 4.2 A strong linear correlation is present between 87Sr/86Sr and εNd (Figure 9). Hafnium isotope ratios range between 0.282873 and 0.282986, and calculated εHf values (εHf =

176 177 176 177 [ Hf/ Hf)Sample / ( Hf/ Hf)CHUR – 1]*10000) range from 2.6 to 7.6. A linear correlation between εNd and εHf is also observed (Figure 10). Casimiro values of

87Sr/86Sr range from 0.704412 to 0.704532 while 143Nd/144Nd values range from 0.512659 to 0.512701. Calculated εNd values are lower than the retro-arc samples (0.4 and 1.2) while hafnium isotope ratios range between 0.282838 and 0.282877 with calculated εHf values of 2.3 and 3.7. Chrondritic uniform reservoir (CHUR) values for both Hf

(0.282785) and Nd (0.512630) are from Bouvier et al. (2008).

Values of 206Pb/204Pb range from 18.482 to 18.593 while 207Pb/204Pb values range from

15.583 to 15.596 and 208Pb/204Pb values range from 38.379 to 38.498 for the retro-arc basalt samples. Casimiro values are higher for all Pb isotope ratios; 18.6424 and 18.659

19 for 206Pb/204Pb, 15.611 and 15.613 for 207Pb/204Pb and 38.566 and 38.591 for 208Pb/204Pb

Figure 11 shows a positive correlation amongst all samples for both 207Pb/204Pb and

208Pb/204Pb vs 206Pb/204Pb.

20 Table 2. Major element concentrations (wt. %), trace element concentrations (ppm) and isotope ratios for behind the arc basalts at 34°S and Casimiro.

Sample MD- MD-109- MD-109- MD-109- MD-109- MD-109- MD-109- MF-212- MF-212- MF-212- G010011 G010011 109-13 14 151 16 17 21 221 15 18 19 2-1 2-2

SiO2 47.49 46.10 47.53 46.74 46.98 46.28 47.43 52.59 46.94 44.29 56.20 55.42 TiO2 1.26 1.39 1.31 1.32 1.78 1.54 1.57 0.78 1.42 1.39 0.98 0.92

Al2O3 16.09 15.61 15.60 14.14 17.32 14.78 14.39 16.36 14.81 12.64 17.23 17.63

Fe2O3 9.79 11.13 10.02 10.70 10.52 11.20 11.36 5.99 10.38 10.88 7.57 8.11 MnO 0.16 0.18 0.17 0.17 0.18 0.17 0.16 0.10 0.18 0.17 0.11 0.13 MgO 8.49 10.52 9.80 12.65 6.18 9.69 9.80 4.73 10.80 15.82 4.38 4.95 CaO 10.93 11.35 10.08 9.68 10.83 9.27 9.08 9.36 10.18 10.09 6.93 7.75

Na2O 2.93 2.85 2.90 2.68 3.26 3.07 3.14 4.32 3.08 2.84 3.68 3.76

K2O 1.37 1.06 1.44 1.21 1.61 1.16 1.00 1.19 1.27 1.03 1.50 1.20

P2O5 0.45 0.50 0.47 0.39 0.55 0.46 0.36 0.25 0.50 0.40 0.22 0.23 LOI 0.15 0.62 0.66 0.10 0.00 0.00 0.00 1.73 <.02 <.02 0.03 <.02 Total 99.12 101.34 99.98 99.78 99.22 97.62 98.29 97.40 99.56 99.55 98.84 100.11 Mg# 0.63 0.65 0.66 0.70 0.54 0.63 0.63 0.61 0.67 0.74 0.53 0.55 Li 7.24 5.04 8.20 7.02 7.03 8.50 7.39 13.0 8.62 6.35 12.0 11.2 Be 1.25 1.08 1.41 1.17 1.35 1.27 1.26 1.37 1.38 1.09 1.64 1.54 V 232 224 223 220 243 184 194 147 275 263 180 193 Rb 28.4 19.9 36.9 28.5 32.3 25.9 23.5 21.9 29.8 22.0 42.3 31.1 Sr 926 700 716 614 714 546 514 900 804 656 613 598 Y 17.8 18.0 20.9 17.8 23.3 20.2 20.4 13.6 22.9 18.9 15.7 16.9 Zr 120 104 125 105 135 110 109 126 147 112 153 119 Nb 8.50 8.42 10.7 7.50 12.2 10.3 8.8 5.33 12.1 10.2 5.73 4.52 Cs 1.13 0.670 1.77 1.58 1.38 0.973 0.725 0.827 1.52 1.16 1.46 0.662 Ba 597 367 466 446 520 364 306 489 559 440 421 338 La 27.9 25.3 24.5 19.5 26.9 18.9 16.2 17.4 26.8 20.4 20.0 16.0 Ce 54.7 50.3 47.8 40.2 53.3 37.6 33.0 36.2 57.2 42.3 42.2 34.1 Pr 7.02 6.58 6.39 5.40 6.94 5.00 4.52 4.86 7.15 5.71 5.54 4.62 Nd 28.6 27.3 26.4 22.7 28.8 21.1 19.6 19.9 29.5 24.1 22.2 19.2 Sm 5.85 5.65 5.63 4.92 6.10 4.82 4.70 4.07 6.20 5.28 4.54 4.12 Eu 1.74 1.70 1.68 1.47 1.87 1.53 1.52 1.22 1.88 1.60 1.30 1.20

21 Table 2 (continued). Major element concentrations (wt. %), trace element concentrations (ppm) and isotope ratios for behind the arc basalts at 34°S and Casimiro.

Sample MD- MD-109- MD-109- MD-109- MD-109- MD-109- MD-109- MF-212- MF-212- MF-212- G010011 G010011 109-13 14 151 16 17 21 221 15 18 19 2-1 2-2

Gd 5.10 5.04 5.16 4.51 5.73 4.68 4.59 3.19 5.27 4.58 3.62 3.40 Tb 0.665 0.666 0.708 0.623 0.790 0.670 0.679 0.455 0.758 0.657 0.520 0.502 Dy 3.55 3.61 3.97 3.47 4.47 3.85 3.91 2.43 4.21 3.61 2.84 2.87 Ho 0.657 0.670 0.753 0.657 0.856 0.734 0.746 0.461 0.785 0.678 0.522 0.557 Er 1.70 1.71 1.96 1.71 2.24 1.92 1.94 1.24 2.04 1.80 1.37 1.51 Tm 0.240 0.240 0.284 0.244 0.322 0.279 0.281 0.184 0.283 0.251 0.194 0.217 Yb 1.53 1.50 1.80 1.54 2.05 1.77 1.79 1.15 1.82 1.54 1.25 1.43 Lu 0.224 0.216 0.263 0.220 0.302 0.259 0.259 0.174 0.266 0.228 0.184 0.211 Hf 3.14 2.77 3.20 2.80 3.37 2.77 2.82 3.19 3.48 2.81 3.47 2.89 Ta 0.649 0.618 0.805 0.573 0.913 0.763 0.673 0.398 0.819 0.744 0.419 0.370 Pb 8.46 4.19 7.72 8.08 7.39 5.08 4.87 6.33 9.01 6.85 8.32 5.87 Th 6.85 5.49 5.89 4.36 5.94 3.80 3.17 2.62 5.53 4.36 5.26 3.33 U 1.21 1.18 1.60 1.16 1.48 1.06 0.91 0.659 1.34 1.09 1.20 1.02 87Sr/86Sr 0.703942 0.703730 0.704025 0.704312 0.704143 0.704030 0.704127 0.704227 0.704220 0.704072 0.704532 0.704412 143Nd/144N 0.512819 0.512854 0.512813 0.512762 0.512770 0.512802 0.512778 0.512701 0.512746 0.512786 0.512659 0.512701 d176 Hf/177Hf 0.282964 0.282986 0.282934 0.282894 0.282911 0.282907 0.282873 0.282845 0.282882 0.282924 0.282838 0.282877 206Pb/204Pb 18.566 18.593 18.573 18.555 18.562 18.533 18.517 18.503 18.482 18.513 18.624 18.659 207Pb/204Pb 15.591 15.603 15.595 15.596 15.593 15.584 15.586 15.583 15.585 15.588 15.611 15.613 208Pb/204Pb 38.452 38.498 38.472 38.466 38.460 38.391 38.390 38.381 38.379 38.405 38.566 38.591 εNd 3.7 4.4 3.6 2.6 2.7 3.3 2.9 1.4 2.3 3.0 0.6 1.4 εHf 6.3 7.1 5.3 3.9 4.5 4.3 3.1 2.1 3.4 4.9 1.9 3.2

1 Trace element concentrations for these samples are an average of multiple runs. 2 All Fe calculated as Fe2O3 3 CHUR values for Nd (0.51263) and Hf (0.28279) from Vouvier 2008.

2 BHVO-1 and BIR-1 analyzed as unknowns

27 A

28 5 DISCUSSION

5.1 Chemical Variability of Retro-arc Basalts

With a single exception (MF-212-15), all of the retro-arc lavas studied are nepheline- normative, olivine-bearing alkaline basalts. Chondrite normalized La/Yb ratios in the basalts increase from south to north (Figure 12), which could be an indication of decreased partial melting to the north. However, since most of this change appears to be in the HREEs instead of the LREEs (Figure 12) this could also indicate a deeper zone of partial melting in which mantle garnet is present.

5.2 Crustal Contamination and the State of the Mantle in the NSVZ

It has been well documented that arc magmas of the NSVZ are chemically different from those found south of 34.5˚S (e.g., Hildreth and Moorbath 1988, Stern 1991, Kay et al.

2005). In particular, distinctly higher 87Sr/86Sr isotope ratios in arc lavas erupted north of

34.5˚S have been interpreted as indicating a significant crustal contribution to magmas within the NSVZ (Hildreth and Moorbath 1988, Stern 1991, Kay et al. 2005). The processes leading to this elevated crustal signature are, however, debated. Two models have been proposed to explain the driving force for this increased crustal signature: 1) increased assimilation of crustal material with elevated 87Sr/86Sr due to thicker, older crust in the NSVZ; or 2) the subducting Nazca plate is eroding crustal material with elevated 87Sr/86Sr and subducting it beneath the arc, where it mixes with and alters the composition of the mantle wedge.

The first model was proposed by Hildreth and Moorbath (1988) as a two stage process in which crustal material is incorporated into the ascending magma. In the first stage,

29 ascending magma stalls beneath the thicker continental crust beneath the NSVZ in a zone of Melting, Assimilation, Storage, and Homogenization (MASH). Lower crustal material that is more radiogenic than the ascending magma is assimilated into the melt, increasing the 87Sr/86Sr isotope ratios and decreasing 144Nd/143Nd isotope ratios. After further ascension, the magma undergoes a second stage of concurrent assimilation and fractional crystallization (AFC) in which upper crustal material containing still higher 87Sr/86Sr and lower 144Nd/143Nd isotopic ratios is assimilated in the melt, resulting in the radiogenic signatures seen in the evolved magmas erupted at the arc front. More recent studies have refined the model of magma hybridization in a lower crustal hot zone (eg., Annen et al.

2006), with similar implications for explaining the elevated 87Sr/86Sr in the NSVZ arc lavas.

In the subduction erosion model (Stern 1991; Kay et al. 2005) both upper and lower crustal materials are eroded and transported beneath the continental margin by the subducting Nazca plate. This crustal material mixes with the overlying mantle wedge, contaminating it with a radiogenic Sr signature not seen in lavas erupted to the south of

34.5˚S. Because the mantle wedge itself has an increased crustal signature, primary mantle melts would also contain elevated 87Sr/86Sr isotopic ratios and decreased

144Nd/143Nd isotopic ratios, regardless of interaction with the continental crust as it ascends.

It is difficult to distinguish these two processes on the basis of geochemistry alone, which is the primary reason this issue remains unresolved. Evidence in favor of the MASH zone hypothesis includes increasing crustal thickness from south to north in the SVZ (to a maximum of ~50-60 km at 32˚S, Gilbert et al., 2006; Alvarado et al., 2007); increasing

30 trace element evidence for garnet in the source region (higher La/Yb, Sr/Y) to the north

(Hildreth and Moorbath, 1988; Kay et al., 2005), and an apparent lack of basaltic magmas erupted on the volcanic front in the NSVZ. In addition, the Frontal Cordillera of the

Andes, which is composed of Paleozoic granitic and sedimentary rocks, pinches out south of 34.5˚S, removing a potential source of isotopically distinct material to the south of this latitude (Figure 4). Evidence for subduction erosion includes the disappearance of pre-

Andean Paleozoic basement west of the arc front, along-strike disappearance of a Jurassic

Andean plutonic belt, and the eastward shift of Andean plutonic belts over time. This evidence is strongest in the CVZ (14-27°S) where the along-strike differences are apparent, but is somewhat more ambiguous in the NSVZ. Kay et al. (2005) link erosion of crustal material by the subducting slab to episodes of crustal shortening and thickening with eastward shifts of the arc front. They argue that subduction erosion is a punctuated, rather than continual, process of erosion over time.

Elevated 87Sr/86Sr is one of the core pieces of evidence for a continental crustal signature within the NSVZ. Figure 15 shows that this signature increases gradually between 37˚S and 35˚S, then increases abruptly in arc magmas near 34.5°S. The Sr isotopic composition of the retro-arc basalts (87Sr/86Sr = 0.7037 to 0.7043), on the other hand, is statistically different from the basaltic andesites and andesites located on the arc front of the NSVZ, which have 87Sr/86Sr = 0.7048 to 0.7055. Instead, these values closely match those found on the volcanic front further south (the CSVZ and TSVZ), which are generally not considered to show evidence of similar crustal contamination (Hildreth and

Moorbath 1988; Stern 1990; Kay et al 2005).

31 This change in Sr isotopic values between the two volcanic segments may not be attributed simply to the presence of more evolved lavas (and lack of primitive lavas) in the NSVZ, as can be seen in Figure 16. A positive correlation between silica content and

Sr isotopic ratios is clear in arc rocks from the NSVZ, which supports the interpretation

87 86 that crustal material, with both higher Sr/ Sr and higher SiO2 content, was incorporated in the melt at some point during the ascent of the magma. The lack of a similar correlation in the CSVZ and TSVZ suggests that there is little assimilation of continental crust with radiogenic Sr values, reflecting either extremely limited overall crustal assimilation, or the lack of Paleozoic crust (hence limited isotopic contrast between the crust and mantle) south of 34.5˚S.

If the elevated 87Sr/86Sr observed in arc front lavas from the NSVZ were due to crustal contamination of the mantle source, as proposed by Stern (1991) and Kay et al. (2005), then one would expect to observe elevated 87Sr/86Sr in primitive basalts as well as more evolved andesites and dacites. Unfortunately, no primitive basalts are found on the arc front in the NSVZ. This study has determined 87Sr/86Sr in primitive basalts sampled just a few tens of kilometers east of the NSVZ volcanic front. It can reasonably be expected that if the mantle beneath the NSVZ were broadly contaminated with eroded crustal materials, the isotopic evidence of this contamination would be observable in the retro- arc basalts as well. At the same time, the retro-arc basalts lie off of the Andean

Cordillera in the foreland basin. If the elevated 87Sr/86Sr in NSVZ magmas were due to assimilation of Paleozoic rocks of the Frontal Cordillera, then one would not expect to find similarly elevated 87Sr/86Sr in the retro-arc basalts. Intriguingly, the data presented here shows that the retro-arc basalts are isotopically much more similar to lavas erupted

32 south of 34.5˚S than they are to NSVZ arc front lavas. This evidence suggests that the mantle beneath the NSVZ is similar to that beneath the rest of the SVZ, and the elevated

87Sr/86Sr is introduced in the crust.

5.3 Crustal Contamination within the Retro-arc Basalt Suite

The lava flow NW of Sepultura (MD-109-14) appears to be the most primitive sample on plots of 87Sr/86Sr versus εNd (Figure 13), and εHf versus εNd (Figure 14), and has the second lowest SiO2 content (Table 2). I interpret MD-109-14 to be the sample that has undergone the least crustal interaction, and as the one that is closest to representing the isotopic signature in the underlying mantle. Gaspar (MF-212-15) shows the most radiogenic crustal signature on these same plots, and has the highest SiO2 content (Table

2). Sample MF-212-15 is interpreted to be the sample that has undergone the most crustal assimilation, This is consistent with the observation that another bomb from this same volcanic center contains numerous large (1-3 cm) granitic xenoliths (this second sample was not included in the geochemical analyses due to its obvious heterogeneity).

The basaltic andesites from Casimiro parasitic cone plot close to MF-212-15 on Figures

13 and 14.

A mixing line may be drawn between the most isotopically primitive basaltic sample, the flow NW of Sepultura (MD-109-14), and the sample from Gaspar (MD-212-15), on plots of εNd versus 87Sr/86Sr and εHf versus εNd (Trend 1 on Figures 13 and 14). All basaltic samples fall reasonably close to this mixing line on both plots, and the Casimiro arc front samples also plot along the line. The line extends towards Cenozoic Huincan andesite and dacite samples analyzed in this study (see appendix 5). A second trend (Trend 2 on

33 figures 13 and 14) exists within the NSVZ that appears to originate at an isotopic composition near that of Casimiro, and points to a more radiogenic crustal signature.

I propose a model in which the lower continental crust of both the Andean Cordillera and the foreland basin of the NSVZ contain Cenozoic plutonic complexes. Miocene volcano- plutonic complexes at this latitude are well documented by Kay et al. (2005) to the west of the Quaternary arc, and Stern et al. (1991) infer that the eastern extent of the Cenozoic arc is located east of the retro-arc basalt field, encompassing the study area. We use the

Cenozoic Huincan andesite from the Diamante Caldera wall as a proxy for this Cenozoic crustal material. The basement of the NSVZ is believed to be dominantly composed of

Choiyoi Formation, a Paleozoic granitoid, which together with Devonian graywackes

(Lagunitas Formation) comprise the Cordillera Frontal. I interpret this Paleozoic material as representative of the middle-to-upper crust.

In this model, ascending magmas in the NSVZ interact with and assimilate a lower continental crustal material with isotopic signatures similar to those seen in the Cenozoic

Huincan andesites and move along the mixing line shown by Trend 1 (Figures 13 and 14) to a composition similar to that of Gaspar and Casimiro. Magmas ascending through the

Andean Cordillera assimilate additional crustal material with a more extreme isotopic signature (Paleozoic crust) during AFC processes in the middle to upper crust. The magmas ascending below the retro-arc, on the other hand, do not experience AFC in the upper crust in part due to normal faulting associated with thick-skinned faulting in the foreland basin. These volcanic centers are aligned with the NW-SE lineaments, which may locally provide a preferential pathway for rapid magma transport to the surface

(Ramos and Folguera 2011).

34 A model of magma stalling and mixing at the base of thick crust, followed by further assimilation in a shallower magma chamber would allow for the presence of a “normal”

SVZ mantle composition, extending northward beneath the NSVZ. This would explain the presence of low 87Sr/86Sr ratios in the retro-arc basalts compared to the arc andesites and Casimiro basaltic andesites in the NSVZ, since the crust is significantly thinner beneath the rear-arc and volcanic features appear along tectonically controlled NW-SE lineaments, allowing the unadulterated mantle composition to be expressed. Ascending magmas would have some degree of interaction with the lower continental crust, but would likely have very little interaction upon reaching the preferential pathways that the lineaments provide. Conversely, if subduction erosion is the driving force for the trace element and isotopic signatures we see in the arc magmas of the NSVZ, then the subarc mantle must be decoupled from the mantle feeding the retro-arc foreland basin. Since this decoupling would have to take place over a distance of less than 50 km I find it more likely that the retro-arc basalts are indicative of the entire mantle wedge at 34°S, and subduction erosion is not a driving factor for crustal contamination signatures seen in the

NSVZ.

5.4 Pb Isotope Ratios

The trends of multi-level crustal assimilation by basalts in the retroarc and intermediate magmas in the volcanic front are slightly more complex in terms of Pb isotopes (Figure

17). The lava flow NW of Sepultura is more radiogenic than Gaspar in terms of

206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb. This is the opposite of the trend seen in Sr, Nd, and Hf isotope ratios. The NSVZ does follow a trend of increasing radiogenic Pb with

35 the increasing radiogenic Sr, and decreasing εNd, and εHf, but Casimiro shows more elevated radiogenic Pb signatures than arc rocks from the NSVZ.

A mixing line (Trend 1 on Figure 17) can be drawn between the lava flow NW of

Sepultura, and the Huincan Cenozoic andesite, which is characterized by less radiogenic

Pb isotope ratios. Gaspar again plots close to the crustal end-member composition. A second mixing line (Trend 2 on Figure 17) can be drawn on both plots between Gaspar and the field representing the Paleozoic granitoid radiogenic Pb values (Hildreth and

Moorbath, 1988; this study). These mixing trends are consistent with a model in which ascending magmas of the Andean Cordillera and foreland basin are assimilating

Cenozoic lower crust similar in composition to Huincan, followed by additional assimilation of older (Paleozoic) crustal material for magmas ascending through the

Andean Cordillera (Casimiro and Miapo samples). Since Casimiro Pb concentrations are lower than arc rocks from the NSVZ, Pb concentrations (6-8 ppm compared to 10-20 ppm) these magmas may be more affected by assimilation of radiogenic Pb, even if the amount of Paleozoic crust assimilated is less. This could explain the apparently anomalous position of Casimiro on the mixing line, closer to the Paleozoic crust end member than the more evolved andesites and dacites from NSVZ stratovolcanoes.

5.5 Classification of Retro-arc Basalt Field

The retro-arc basalts in this study have previously been classified as a northern portion of the Payenia Volcanic Complex (Ramos and Folguera, 2011), despite the fact that the rest of Payenia Volcanic Complex is characterized by more voluminous magmatism over a greater period of time (early Miocene/Late Oligocene to historic; Stern et al., 1991; Kay

36 and Copeland, 2006). The two segments of Payenia located to the south are also more geographically expansive, extending to over 200 km from the active volcanic arc, which is consistent with intra-cratonic volcanism with minimal slab-fluid influence. The retro- arc basalts from our study are located <70 km from the Holocene arc, which is more typical of volcanism resulting from fluids released from a subducting slab. Kay and

Copeland (2006) describe a history of adiabatic melting with only a faint arc-like signature in 24-20 Ma basalts with an increasing, but still weak, arc-like signature in 19-

15 Ma basaltic to trachyandesitic lavas. Figure 18 displays an N-Morb normalized extended trace element diagram and chondrite normalized REE diagram for the retro-arc basalts and Casimiro samples. Typical ranges from previously published Payenia data

(Germa et al., 2010; Hemando et al., 2012; Kay et al., 2004; Pasquare et al., 2008) are displayed as a field. Our samples show stronger peaks in fluid mobile elements such as

Cs, Ba, Pb, Sr, and Li and stronger depletions in Nb and Ta than other Payenia magmas.

These trends are typically interpreted as influence of subduction related processes, or an arc-like signature. The retro-arc basalts and Casimiro samples generally fall within the range of typical Payenia values on a REE diagram (Figure 18). Compared to typical values of the NSVZ and typical values of the CSVZ and TSVZ, shown in Figure 19, our samples have similar enrichments and depletions. Overall N-MORB normalized values of our samples fall in between typical NSVZ values and typical CSVZ and TSVZ values.

It is unlikely that the retro-arc basalts are related to the volcanic fields from the rest of the

Payenia Volcanic Field given the stronger arc-like signature, lack of older (Miocene through late Oligocene) eruptive events seen in the southern sections, and are not located

37 as far eastward geographically. Instead, we believe the retro-arc basalts display more typical retro-arc characteristics and should be classified separately as a suite of retro-arc basalts within the NSVZ.

38 A B

C Figure 12. (A) Chondrite normalized Sm/Yb values plotted by latitude. A positive trend exists from south to north. This trend is more pronounced in the heavy rare earth elements, represented by chondrite normalized La/Yb in (B) than in the light rare earth elements, represented by chondrite normalized La/Sm in (C). Normalizing values from McDonough and Sun, 1989

Figure 16. Sr isotopic ratios versus SiO2. The field labeled CSVZ and TSVZ represents typical values for the CSVZ and TSVZ (Costa et al., 2002; Davidson et al., 1987; Feeley et al., 1998; Hickey et al., 1996; Hilton et al., 1993; Reubi et al., 2011; Tormey et al. 1995). The NSVZ field was delineated using average values from (Futa and Stern 1988; Hildreth and Moorbath 1988; Lopez-Escobar et al., 1985). The open squares represent unpublished data from Maipo Volcano that were analyzed during this project and by Drew (2010). Retro-arc samples represent a common end member for both arc segments, with Casimiro samples intermediate between the retro-arc basalts and NSVZ.

43 A

44 A

45 A

46 6 CONCLUSION

The monogenetic basaltic centers east of the arc in the NSVZ that have been previously classified as being the northernmost portion of the Payenia Volcanic Complex are more likely the result of retro-arc processes associated with the subduction of the Nazca plate beneath the South American plate. They contain elevated arc-like signatures, such as relatively higher concentrations of fluid-mobile elements and steeper REE fractionation patterns, and are closer in age to volcanics of the Quaternary arc, without any evidence of the much older magmatism observed elsewhere in the Payenia Volcanic Complex.

Radiogenic Sr and Nd isotope ratios show a clear relationship between these basalts and average values from the CSVZ and TSVZ.

Since Sr isotopic values of the retro-arc basalts have similar 87Sr/86Sr ratios as the CSVZ and TSVZ volcanic rocks from the modern arc, which do not show a noticeable increase

87 86 in Sr/ Sr ratios with increasing SiO2 content, we interpret these values as representative of mantle values beneath the SVZ of the Andean volcanic arc. The retro-arc basalts are located in the NSVZ, which does show an increase in radiogenic Sr values with increasing SiO2 content, and which has largely been attributed to either subduction erosion processes or crustal assimilation in a MASH (or equivalent) zone. Lack of significant assimilation of continental crust by rising retro-arc magmas is supported by the thinner crust (30-35 km) found beneath the retro-arc compared to the modern arc of the NSVZ (>45 km), the fact that these monogenetic centers form over structural lineaments that allow an easier ascent of magma, and the primitive nature of the basalts.

In order for subduction erosion to be the driving force of elevated 87Sr/86Sr ratios in the

NSVZ, this process would have to be reconciled with a tight decoupling and rejuvenation

47 of mantle asthenosphere beneath the retro-arc basalts, but not the modern arc. Since this decoupling of mantle material would need to occur at length scales less than 50 km, I find this scenario to be unlikely, and propose that crustal assimilation of Paleozoic crust is the driving force for increased 87Sr/86Sr ratios in the NSVZ in arc magmas.

While the sampled retro-arc basalts do not show significant crustal contamination signatures, there is variability within the basalts that further enlighten crustal assimilation processes below the Quaternary arc and foreland basin of the NSVZ. The retro-arc basalts closely follow a mixing line formed by the most primitive basalt, MD-109-14, and a Cenozoic lower crustal end member for Sr, Nd, and Hf isotopes (Trend 1 on Figures 13 and 14). A second mixing line is shown between the hybridized magma formed by Trend

1, and a Paleozoic crustal end member (Trend 2 on Figures 13 and 14). The two

Casimiro samples plot between the two trends. I propose a model in which ascending magmas in the NSVZ interact with and assimilate varying degrees of Cenozoic plutons in the lower crust (Figure 20). Magmas along the arc front undergo a second stage of crustal assimilation in an AFC zone with the Paleozoic upper crust of the Cordillera

Frontal, while the magmas below the foreland basin are able to ascend without significant upper crustal interaction along the NW-SE lineaments related to faulting in the foreland basin.

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52 APPENDIX A

SAMPLE DESCRIPTIONS

Samples include basalt flows, bombs, and tephra lapilli. Coordinates of sample locations are given in Table 1. All sampled volcanic centers are shown in Figure 4.

MD-109-13: Bomb from Cerro Sepultura containing ~ 20% phenocrysts of plagioclase, clinopyroxene, and olivine.

MD-109-14: Basalt flow from northwest of Cerro Sepultura containing ~10% phenocrysts of olivine and clinopyroxene.

MD-109-15: Tephra from Arroyo Hondo cinder cone.

MD-109-16: Tephra from a maar northwest of Arroyo Hondo

MD-109-17: Basalt flow from La Leña containing ~5% phenocrysts of olivine and clinopyroxene

MD-109-21: Basalt flow from Las Bolas containing ~ 10% olivine phenocrysts with few pyroxene phenocrysts present

MD-109-22: Basalt flow from near the dam at Agua del Toro containing ~16% phenocrysts with ~4% oxides present.

MF-212-15: Blocky basaltic sample from Gaspar containing 5-10% olivine phenocrysts up to 3mm. Xenocrysts showing reaction rims present at mm-scale. Ghost crystals may be seen in thin section. A bomb sample collected in the same location (not analyzed) contains abundant granitic xenoliths up to 4cm in size.

MF-212-18: Bomb from northwestern edge of Cerro el Pozo crater containing less than 1% phenocrysts of plagioclase and olivine

MF-212-19: Basaltic flow from northwestern edge of Cerro el Pozo crater containing

~10% phenocrysts of mostly olivine up to 2-3 mm.

53 APPENDIX B

Rock digestion procedure for ICP-MS trace element analysis*

Note: Unless otherwise specified, all acids used are ultrapure grade. All acid is 8N

HNO3; concentrated HF; 6N HCl.

Step 1. Redundancy Cleaning. One day before digestion.

1. Add around 12 ml 8N HNO3 to 25ml Teflon beakers (About half full,

measurement does not need to be precise) and seal tightly. For this step only it is

sufficient to use “plus” grade HNO3.

2. Heat on hot plate between 130°C and 140°C for 4-12 hours.

3. Turn off hot plate and allow beakers to cool.

4. Uncap in hood and dump contents back into 8N HNO3 cleaning acid container.

5. Triple rinse beakers and lids using 18.2 mΩ filtered water (Milli-Q system).

6. Dry in hood or drying rack. If you are using the drying rack, use a clean tray

lined with plastic wrap and be sure to clearly label the shelf with your initials and

date.

Step 2. Weighing

1. Carefully clean all surfaces inside scale hood, as well as the scale, with 18.2 mΩ

filtered water.

2. Label all beakers AND lids with a working lab number.

3. Line a section of the bottom of hood with aluminum foil to create a clean,

disposable, working area.

54 4. Hold clean beaker (with cap on) in front of ion fan for 30-60 seconds.

5. Weigh beaker (with cap on) and record.

6. Carefully add 0.100 grams of powdered sample to beaker and seal tightly. Record

total weight.

7. Dispose of aluminum foil, and clean all utensils.

8. Repeat steps 2 through 6 for each sample.

Step 3. Digestion

HNO3 + HF step

1. Carefully add 6 ml HNO3 and then 2 ml HF to each beaker. Put lid on tight, tilt,

and rotate vial so that any sample on the vial wall is washed off.

2. Place in a sonic bath for 20-30 minutes at 60°C. Once the sonic bath is complete

wipe off the outside of each beaker with a chem-wipe.

3. Place beakers on a hot plate between 130°C and 140°C for 12-24 hours. When

samples are dissolved sample should be clear with no dark particulates remaining

(a milky white gel at the bottom is okay).

4. If no dark particulates remain proceed to step 5. If dark particulates remain,

lightly tap each beaker on a hard, clean surface and then place in a sonic bath for

30 minutes at 60°C. Place back on hot plate at 140°C until sample is dissolved.

5. Rotate the beaker on its side and then return it to an upright position slowly to

ensure that all condensation is removed from the sides. Carefully remove the lid

55 and hold over a dark surface to ensure no dark particulates remain. Again, a

milky white gel is okay.

6. Place a layer of plastic wrap flat in the hood (about the size of the hotplate).

7. Place each beaker on the hot plate carefully, and place its lid in a corresponding

position on the plastic wrap, face down. Be careful not to reach over an open

beaker or lid.

8. Samples will remain on the hot plate until they are dry. You may start with

temperatures between 130°C and 140°C, but reduce the temperature to around

100°C as samples near completion. Be aware that some samples will dry faster

than others if the hot plate is not distributing heat evenly. Once a sample is dry

you should cap it and remove it from the hot plate to cool. This could take several

hours.

HNO3 step

1. Carefully add 6 ml HNO3 to each beaker. Put lid on tight, tilt, and rotate vial so

that any sample on the vial wall is washed off.

2. Place in a sonic bath for 20-30 minutes at 60°C. Once the sonic bath is complete

wipe off the outside of each beaker with a chem-wipe.

3. Place beakers on a hot plate between 130°C and 140°C for 12-24 hours. When

samples are dissolved sample should be clear with no dark particulates remaining

(a milky white gel at the bottom is okay).

4. If no dark particulates remain proceed to step 5. If dark particulates remain,

lightly tap each beaker on a hard, clean surface and then place in a sonic bath for

30 minutes at 60°C. Place back on hot plate at 140°C until sample is dissolved.

56 5. Rotate the beaker on its side and then return it to an upright position slowly to

ensure that all condensation is removed from the sides. Carefully remove the lid

and hold over a dark surface to ensure no dark particulates remain. Again, a

milky white gel is okay.

6. Place a layer of plastic wrap flat in the hood (about the size of the hotplate).

7. Place each beaker on the hot plate carefully, and place its lid in a corresponding

position on the plastic wrap, face down. Be careful not to reach over an open

beaker or lid.

8. Samples will remain on the hot plate until they are dry. You may start with

temperatures between 130°C and 140°C, but reduce the temperature to around

100°C as samples near completion. Be aware that some samples will dry faster

than others if the hot plate is not distributing heat evenly. Once a sample is dry

you should cap it and remove it from the hot plate to cool. This could take several

hours.

HCl step

1. Carefully add 2 ml 18.2 mΩ filtered water and then 1 ml HCl to each beaker. Put

lid on tight, tilt and rotate vial so that any sample on the vial wall is washed off.

Do not let sample solution get under the lid.

2. Place in a sonic bath for 20-30 minutes at 60°C. Once the sonic bath is complete

wipe off the outside of each beaker with a chem-wipe.

3. Rotate the beaker on its side and then return it to an upright position slowly to

ensure that all condensation is removed from the sides. Carefully remove the lid.

4. Place a layer of plastic wrap flat in the hood (about the size of the hotplate).

57 5. Place each beaker on the hot plate carefully, and place its lid in a corresponding

position on the plastic wrap, face down. Be careful not to reach over an open

beaker or lid.

6. Samples will remain on the hot plate until they are dry. Begin drying around

120°C, but reduce the temperature to around 100°C as samples near completion.

This drying step will not take as long (a couple hours) as previous drying steps so

watch samples very carefully so as not to burn them.

Step 4. Pick up

1. Carefully add 6 ml HNO3 then 6 ml 18.2 mΩ filtered water to each beaker. Put

lid on tight, tilt and rotate vial so that any sample on the vial wall is washed off.

Do not let sample solution get under the lid.

2. Place in a sonic bath for 20-30 minutes at 60°C. Once the sonic bath is complete

wipe off the outside of each beaker with a chem-wipe.

3. Place beakers on hot plate between 80°C and 100°C until samples have dissolved

(typically 5-12 hours). Double check to see that the sample is dissolved by

carefully removing lid and holding over a dark surface. Some white matter is

okay as this typically dissolves in a more dilute solution when placed in a sonic

bath.

Step 5. Transfer

1. Get an acid-cleaned (see cleaning procedure) and dry 250 ml HDPE or LDPE

bottle and label with your working lab number.

2. Weigh sealed bottle on balance and record wt. (this is easiest to do in the hood).

58 3. Carefully transfer samples into 250 ml bottle and rinse Teflon beaker and lid three

times each with 18.2 mΩ filtered water, pouring each rinse into the 250 ml bottle.

4. Place the bottles in a sonic bath for 30 minutes at 60°C. Once done let them sit

for 30-60 minutes then tilt bottles under a bright light to make sure totally clear

and free of any precipitates. Sometimes a white “smoke” appears which may

represent insoluble fluorides. If you see any cloudiness, place back in a sonic

bath for another 30-60 minutes. If smokiness persists, make a note of this, and

run it anyway. The sample may need to be dissolved again in another batch.

Step 6. Dilution

Dilute samples to around 2000 times the original weight using the following

steps.

1. For the first bottle, carefully add 18.2 mΩ filtered water until the total weight of

the solution is equal to about 200g (assuming 0.1g of sample). The total weight

on the balance should read 200g + original weight of empty bottle plus lid.

Record total weight.

2. For successive samples it is easiest to add 18.2 mΩ filtered water from the milli-Q

system using the first bottle as a rough guideline for how much water to add. Be

sure to stop well before reaching the target volume. “Top off” each sample using

18.2 mΩ filtered water from a squirt bottle in the hood. Record all total weights.

3. On the morning before you run on the ICP-MS transfer about 10 ml from 250 ml

bottle to a 15 ml centrifuge tube and place in a sonic bath for 30 minutes.

59 APPENDIX C

Dissolution procedure at Lamont-Doherty Earth Observatory – modified from Isotope

Geochemistry Procedures by Jason Jweda

All acids and water used are double distilled.

Step 1. Leaching of volcanic rock powders

1. Weigh an empty pre-cleaned Teflon beaker (wrap in Al foil and shoot with

electrostatic gun) and then re-zero the balance.

2. Add ~0.2 g of volcanic powder to beaker.

3. Weigh the beaker with the sample.

4. Slowly add DD 6 N HNO3 (or HCL) enough to cover rock powder (0.5 ml).

5. Cap samples and place in a sonic bath for ~10 minutes.

6. Heat the beakers on a hotplate for ~1 hour at 100°C.

7. Turn off the hotplate and allow the beakers to cool.

8. Pipette solution into bullet and centrifuge for ~15 minutes.

9. Pipette off supernatant and save leachate.

10. Rinse leachate by adding 0.5 ml DD water, place in a sonic bath for ~10 minutes,

and then centrifuge for ~15 minutes. Note: Save the HNO3 and first DD water

supernatant.

11. Pipette off supernatant and save leachate.

12. Repeat steps 10 and 11 four times.

13. Save leachate.

Step 2. Dissolution of Silicate Rock Powders

60 1. Transfer the leachate to the original beakers by adding 1 mL of DD concentrated

HNO3 and , and add another 2 mL of DD concentrated HNO3 to the original

beakers. Cap the beakers.

2. Add 1.5 mL of DD concentrated HF to the samples and then cap.

3. Place in a sonic bath for ~15 minutes to break up the sample.

4. Heat on a hotplate for ~ 12 hours (overnight) at 120°C.

5. Check to see if dissolution is complete (dissolved samples should be white in

color).

6. If dissolution is complete, place in a sonic bath for ~20 minutes.

a. If dissolution is incomplete, continue to rotate between sonic baths and

placing on heat as necessary to dissolve samples.

7. Un-cap beakers and air dry beakers on a hotplate at 120°C for ~3 hours

8. When dry, allow the samples to cool and then add 1 mL of DD concentrated

HNO3 to the cap of the beakers and carefully pour into beakers.

9. Add 1 mL of DD concentrated HNO3 to each beaker, place in a sonic bath for ~10

minutes, and then heat on a hotplate for ~12 hours (overnight) at 120°C.

10. Un-cap beakers and air dry on hotplate at 120°C for ~3 hours.

11. Add 2 mL of DD 6 N HCl + 0.2 N HF, cap beakers, and then place in a sonic bath

for ~10 minutes to break up the samples.

12. Heat on a hotplate at 120°C for ~ 3 hours.

13. Un-cap beakers, wipe droplets from outside of beakers, and dry on a hotplate at

120°C for ~3 hours.

14. When dry, cap the beakers and allow them to cool.

61 APPENDIX D

Ion exchange procedures used at Lamont Doherty Earth Observatory

Column 1 (Pb purification). The dried residue following dissolution was brought back up in (how much?) 0.7 N HBr and placed in an ultrasonic bath for 10 minutes before being placed on a hotplate for 2-3 hours. Samples were then centrifuged to separate any material that did not dissolve in the HBr. All excess material was saved and stored in acid cleaned polypropylene bullets. Cleaned 1.3 mL Teflon columns containing 100 µL of Bio-rad Agl-8x 100-200 mesh resin in chloride form were conditioned using 480 µL of

0.7 N HBr. Once the samples were loaded on the columns, 1.6 mL of 0.7 N HBr was added to elute rare earth elements (REEs) and then 240 µL of 2 N HCl to elute Fe. The

REE cut was dried and saved in acid-cleaned Teflon beakers for the next ion exchange column. Lead was eluted by adding 1.5 mL of 6 N HCl, 0.5 mL at a time.

Column 2 (separation of Sr from REEs). Three drops of concentrated HCl and 1 drop of

HNO3 were added to each beaker from the REE cut in order to drive off the HBr. Once the reaction was complete the sample was brought up in 5 drops of concentrated HNO3 and dried again. One mL of 1 N HNO3 was then added to each sample, which was subsequently placed on a hotplate for 2 hours before being placed in a centrifuge for 25 minutes to separate any undissolved particles. Cleaned 1.3 mL Teflon columns containing 100 µL of Eichrom Tru-Spec resin were then conditioned with 900 µL of 1 N

HNO3. Then 1.5 mL of 1 N HNO3 was used to elute Rb and Sr, and the eluent was dried and stored in teflon beakers for the next column chemistry step. Rare earth elements

62 were eluted using 990 µL of 1 N HCl, added 330 µL at a time, and stored in a Teflon beaker.

Column 3 (Sr purification). The cut containing Rb and Sr was dissolved in 0.5 mL of 3N

HNO3 on a hotplate for 2 hours before being placed in a centrifuge for 20 minutes to separate any undissolved particles. Cleaned 0.5 mL Teflon columns containing 30 µL of

Eichrom Sr-spec resin were conditioned using 0.5 mL of 3 N HNO3. Rubidium and any other remaining cations were eluted by adding 990 µL of 3 N HNO3 to the columns, dried and stored in Teflon beakers. The purified Sr was eluted in 1.5 mL of double distilled water. The resulting Sr cut was dried, and then 2 drops of concentrated HNO3 was added. The sample was placed in an ultrasonic bath (for how long?) and then dried to incipient dryness. Eight µL of 6N HCl was added to the small sample droplet and the resultant sample solution was stored in a Teflon beaker.

Column 4 (Nd purification). The REE cut from Column 2 was dried to insipient dryness and brought back up with 100 µL of 0.22 N HNO3 and then placed in an ultrasonic bath for 20 minutes. Cleaned 5 mL Teflon columns containing 1 mL of Eichrom ln-spec resin were conditioned using 1 mL of 0.22 N HNO3. Heavy rare earth elements (HREE) were eluted using 2.4 mL of 0.22 N HNO3. Neodymium was then eluted in 6 mL of 0.22N

HNO3 and dried to insipient dryness. Six hundred µL of 3%, by volume, HNO3 were added to this drop and placed on a hotplate until dissolved. The resulting solution was then stored in an acid-cleaned 2 mL polypropylene centrifuge tube.

Column 5 (Hf purification). Next, the cut containing Rb and other cations from the Sr column chemistry was combined with the HREE cut from the Nd column chemistry as

63 well as all particulates from the centrifuge steps of Columns 1-4. The resulting mixture was dried at 100°C overnight. It was then dissolved by adding 5 mL of 3 N HCl and heated overnight at 80°C to obtain full dissolution. The samples were added to 15 mL polypropylene centrifuge tubes while still warm. Then 0.4 mL of 1 N ascorbic acid

(C6H8O6) was added in 0.1 mL increments until the yellow color that was produced faded, signaling the reduction of Fe3+ to Fe2+. Once this was done, the samples were centrifuged for 15 minutes. Cleaned 12 mL Teflon columns containing 2 mL of Eichrom ln-spec resin were conditioned with 10 mL of 3 N HCl. After the sample was loaded, 20 mL of 3 N HCl was added to wash out major elements, followed by 40 mL of 6 N HCl to wash out the remaining REEs, and then 10 mL of double distilled water to wash away the preceding acids. About 40-50 mL of a mixture (made by adding 15 mL of 0.45 N HNO3,

45 mL of 0.09 N citric acid, 17 mL of 1%, by volume, H2O2 to 423 mL of double distilled water) of 0.09 N citric acid (C6H8O7), 0.45 N HNO3, and 1%, by volume, H2O2 were then added to wash out oxides and Ti. Fifteen mL of double distilled water was then added to ensure no preceding acids remained in the columns. Hafnium was eluted with 12 mL of 6 N HCl and 0.2 N HF and then dried. Twenty µL of concentrated HF and

60 µL of concentrated HNO3 were used to dissolve the dried samples and then dried to ¼ of a drop. Six hundred µL of double distilled water were added to this drop and stored in an acid-cleaned 2 mL centrifuge tube.

64 APPENDIX E

Maipo figures

Figure 21. Trace element diagram for samples from Maipo volcano normalized to N- MORB from Sun and McDonough (1989).

Figure 22. Rare Earth Element (REE) diagram for samples from Maipo volcano normalized to chondritic values from McDonough and Sun (1995).

66 APPENDIX F

Major element concentrations (wt. %), trace element concentrations (ppm) and isotope ratios for Maipo Volcano, Huincan andesite, and Choiyoi granite.

L8-07 MF-212- M15* M8* MF-212- L8-01* L8-03* MD-109- MD-109- MD-109- L8-04 4 8 2 3 4 Maipo Maipo Maipo Maipo Maipo Maipo Maipo Maipo Maipo Maipo Maipo

SiO2 58.04 56.40 N/A 60.20 55.57 54.30 56.70 60.05 60.68 57.06 66.19

TiO2 1.04 1.00 N/A 0.95 0.84 0.93 0.92 0.86 0.86 0.95 0.49

Al2O3 17.73 17.45 N/A 17.40 15.81 18.20 17.50 16.69 16.29 17.49 16.12

Fe2O3 6.52 6.17 N/A 6.23 8.20 7.55 7.47 6.32 6.42 7.47 3.15 MnO 0.10 0.10 N/A 0.10 0.12 0.12 0.11 0.10 0.10 0.11 0.08 MgO 2.55 2.57 N/A 2.37 7.06 4.14 4.33 3.41 3.58 4.27 1.04 CaO 6.82 6.38 N/A 5.76 7.16 7.48 7.29 6.15 5.94 7.43 2.92

Na2O 3.98 4.00 N/A 4.27 3.63 3.68 3.79 3.67 3.74 3.64 4.41

K2O 2.08 2.02 N/A 2.61 1.46 1.65 2.05 2.43 2.44 1.89 3.24

P2O5 0.27 0.24 N/A 0.27 0.18 < .05 0.26 0.22 0.21 0.22 0.16 LOI 0.31 <.02 N/A -0.11 <.02 0.00 0.11 0.07 0.00 0.01 1.32 Total 99.44 96.34 N/A 100.05 100.03 98.05 100.53 99.98 100.27 100.54 99.14 Mg# 43.651 45.208 N/A 42.975 63.021 52.068 53.452 51.640 52.498 53.102 39.609 Li 10.1 13.5 11.6 17.9 10.3 10.7 12.8 15.1 16.2 11.4 21.9 Be 1.90 2.03 1.70 1.90 1.42 1.58 1.84 2.20 2.24 1.82 2.64 V 163 190 200 185 168 197 192 138 145 166 44.4 Rb 66.6 72.5 74.0 91.2 42.0 48.7 70.0 87.6 91.3 60.6 112 Sr 539 577 635 545 551 564 559 473 475 518 327 Y 18.7 19.9 17.9 18.7 13.1 19.8 20.8 19.5 20.7 19.0 16.1 Zr 175 185 185 205 128 144 175 200 208 158 109 Nb 7.18 7.47 14.9 19.6 4.68 5.88 7.34 8.12 8.50 6.34 8.29 Cs 2.28 1.65 1.80 3.10 0.78 1.19 2.19 2.10 2.25 1.53 3.35 Ba 495 494 555 565 485 413 468 552 571 447 741 La 26.1 25.1 26.6 28.4 17.6 20.0 23.9 29.4 31.6 23.8 35.1 Ce 53.9 54.7 54.0 58.3 36.3 41.1 50.2 58.7 64.4 49.4 65.3 Pr 6.63 6.57 6.30 6.60 4.73 5.27 6.23 7.23 7.73 6.11 7.28 Nd 25.9 25.5 25.3 26.0 18.9 20.8 24.4 27.7 29.6 24.2 25.3 Sm 5.21 5.14 5.00 5.00 3.87 4.38 5.00 5.44 5.79 4.92 4.35

67 Eu 1.39 1.35 1.40 1.30 1.15 1.25 1.29 1.33 1.37 1.31 1.03 Gd 4.62 4.19 4.80 4.80 3.14 3.75 4.13 4.77 5.13 4.42 3.78 Tb 0.63 0.61 0.61 0.62 0.45 0.57 0.61 0.64 0.69 0.62 0.49 Dy 3.56 3.41 3.30 3.30 2.49 3.28 3.46 3.61 3.87 3.55 2.80 Ho 0.69 0.65 0.61 0.62 0.46 0.65 0.67 0.69 0.75 0.69 0.55 Er 1.84 1.76 1.80 1.90 1.20 1.80 1.86 1.88 2.02 1.88 1.55 Tm 0.27 0.26 0.23 0.25 0.17 0.27 0.28 0.28 0.31 0.28 0.24 Yb 1.78 1.68 1.60 1.70 1.12 1.74 1.77 1.85 2.01 1.87 1.64 Lu 0.27 0.25 0.23 0.25 0.17 0.26 0.27 0.28 0.30 0.28 0.25 Hf 4.78 4.35 4.10 4.50 3.23 3.41 4.08 5.27 5.59 4.31 3.59 Ta 0.68 0.63 0.76 1.20 0.39 0.51 0.78 0.79 0.81 0.60 0.83 Pb 12.7 11.5 9.90 11.9 7.86 8.25 11.0 12.4 13.4 10.9 18.9 Th 9.72 8.78 7.50 9.30 5.04 6.09 7.88 11.89 12.56 8.49 15.38 U 2.56 2.27 2.10 2.70 1.32 1.36 2.01 3.10 3.25 2.23 3.60 87Sr/86Sr 0.704828 N/A 0.704855 0.704923 N/A 0.705025 0.705033 0.705062 0.705014 0.704993 0.705514 143Nd/144Nd 0.512605 N/A 0.512577 0.512564 N/A 0.512568 0.512549 0.512576 0.512598 0.512597 0.512564 176Hf/177Hf 0.282791 N/A N/A N/A N/A N/A N/A N/A N/A 0.282788 0.282756 206Pb/204Pb 18.588 N/A N/A N/A N/A N/A N/A N/A N/A 18.595 18.620 207Pb/204Pb 15.605 N/A N/A N/A N/A N/A N/A N/A N/A 15.606 15.608 208Pb/204Pb 38.533 N/A N/A N/A N/A N/A N/A N/A N/A 38.543 38.580 εNd -0.5 N/A N/A N/A N/A N/A N/A -1.1 -0.6 -0.7 -1.3 εHf 0.2 N/A N/A N/A N/A N/A N/A N/A N/A 0.1 -1.0

68 MD- L8-09 MF-212- M30* MD- L8-08* MF-208- M31* MD- L8-10 MF-212- 109-5 9 109-9 6* 109-6 3 Maipo Maipo Maipo Maipo Maipo Maipo Maipo Maipo Maipo Maipo Maipo SiO2 65.40 64.64 56.35 61.00 60.54 65.40 61.30 N/A 58.26 56.47 58.35 TiO2 0.69 0.67 0.95 0.79 0.93 0.65 0.79 N/A 0.83 0.93 0.92 Al2O3 15.88 15.68 17.74 16.30 17.88 15.80 16.80 N/A 16.51 16.92 16.80 Fe2O3 4.41 4.43 7.30 5.62 5.43 4.51 5.92 N/A 6.51 7.27 6.83 MnO 0.07 0.07 0.11 0.09 0.09 0.07 0.10 N/A 0.10 0.11 0.11 MgO 2.04 2.04 4.23 2.82 1.81 2.05 2.82 N/A 4.51 4.54 4.46 CaO 3.87 3.93 8.01 4.98 6.01 3.94 5.15 N/A 6.37 6.99 7.00 Na2O 3.86 3.76 4.05 4.05 4.32 3.95 4.07 N/A 3.49 3.56 3.66 K2O 3.29 3.51 1.53 2.89 2.57 3.81 2.96 N/A 2.25 1.98 2.05 P2O5 0.15 0.21 0.21 0.27 0.27 0.21 0.27 N/A 0.19 0.24 0.21 LOI 0.09 0.01 0.10 0.11 0.66 0.11 0.11 N/A 0.45 0.29 0.03 Total 99.72 98.93 100.57 98.92 100.50 100.50 100.29 N/A 99.46 99.29 100.43 Mg# 47.795 47.756 53.425 49.851 39.783 47.382 48.551 N/A 57.828 55.308 56.417 Li 19.3 18.9 12.5 18.4 17.6 24.7 18.6 25.4 14.2 13.9 14.2 Be 2.89 2.82 1.60 2.24 2.24 2.60 2.40 2.50 1.93 1.83 1.89 V 66 83 197 126 108 100 128 86.9 143 170 173 Rb 145 140 47.5 105 85.1 160 110 165 74.5 63.9 71.5 Sr 363 369 615 468 492 450 490 410 436 510 530 Y 19.6 19.9 16.8 19.7 21.3 19.3 22.2 19.1 18.8 18.5 19.3 Zr 215 214 133 209 204 240 233 205 160 167 177 Nb 10.15 10.34 5.06 8.56 8.45 23.0 9.31 22.9 6.94 6.82 6.98 Cs 3.07 4.31 1.53 2.97 2.97 5.40 3.11 5.60 2.61 2.20 2.29 Ba 656 635 398 599 584 715 610 715 458 472 471 La 37.4 38.3 18.2 30.7 30.8 39.0 31.8 38.5 25.6 24.5 24.1 Ce 71.7 75.7 37.7 65.0 63.0 76.7 68.4 76.2 51.3 48.9 49.8 Pr 8.29 8.63 4.89 7.46 7.62 8.20 7.80 8.20 6.32 6.19 6.24 Nd 30.0 31.3 19.7 27.7 29.4 30.2 29.1 30.2 24.4 24.2 24.3 Sm 5.51 5.73 4.14 5.24 5.81 5.30 5.54 5.40 4.86 4.88 4.90

69 Eu 1.16 1.17 1.19 1.24 1.45 1.20 1.28 1.20 1.18 1.26 1.25 Gd 4.80 5.00 3.46 4.14 5.15 5.20 4.42 5.20 4.32 4.35 4.05 Tb 0.63 0.66 0.50 0.61 0.71 0.63 0.65 0.63 0.59 0.60 0.60 Dy 3.53 3.70 2.83 3.31 3.98 3.30 3.56 3.40 3.42 3.37 3.36 Ho 0.68 0.71 0.54 0.63 0.78 0.62 0.69 0.63 0.67 0.65 0.64 Er 1.86 1.95 1.46 1.77 2.12 1.90 1.95 1.90 1.83 1.75 1.75 Tm 0.29 0.30 0.21 0.26 0.32 0.25 0.29 0.26 0.28 0.26 0.26 Yb 1.91 2.00 1.37 1.68 2.09 1.70 1.90 1.80 1.84 1.73 1.67 Lu 0.29 0.30 0.20 0.26 0.31 0.26 0.29 0.26 0.28 0.26 0.25 Hf 5.91 6.02 3.10 4.88 5.52 5.20 5.24 4.80 4.39 4.35 4.24 Ta 1.17 1.20 0.40 0.79 0.80 1.40 0.83 1.50 0.68 0.63 0.60 Pb 17.7 20.9 8.28 14.4 15.9 16.8 14.5 17.7 13.3 11.8 11.3 Th 19.70 20.34 5.54 12.20 11.72 15.30 12.11 16.40 10.19 8.85 8.67 U 5.78 6.01 1.39 3.02 3.12 4.70 3.08 5.10 2.75 2.34 2.22 87 86 Sr/ Sr 0.705425 0.705447 N/A 0.705101 0.705008 0.705436 0.705138 0.705472 0.705067 0.704880 N/A 143 144 Nd/ Nd 0.512546 0.512539 N/A 0.512538 0.512598 0.512507 0.512540 0.512485 0.512578 0.512607 N/A 176 177 Hf/ Hf 0.282758 N/A N/A N/A N/A N/A N/A N/A 0.282777 N/A N/A 206 204 Pb/ Pb 18.574 N/A N/A N/A N/A N/A N/A N/A 18.577 N/A N/A 207 204 Pb/ Pb 15.606 N/A N/A N/A N/A N/A N/A N/A 15.604 N/A N/A 208 204 Pb/ Pb 38.520 N/A N/A N/A N/A N/A N/A N/A 38.506 N/A N/A εNd -1.6 -1.8 N/A N/A -0.6 N/A N/A N/A -1.0 -0.5 N/A εHf -1.0 N/A N/A N/A N/A N/A N/A N/A -0.3 N/A N/A

70 M14* MF-208- L8-06* MD-109- MF-208- MF-208- MF-208- 212-10 MF-212- MD-109- 5* 12 7* 8* 9* 1 11 Huincan Choiyoi Huincan BHVO- Maipo Maipo Maipo Maipo Maipo Maipo Maipo andesite granite dacite 1** SiO2 N/A 57.40 57.70 58.94 59.40 66.80 68.50 60.40 76.69 63.70 49.50 TiO2 N/A 0.92 0.89 0.97 0.95 0.65 0.51 0.64 0.04 0.54 2.76 Al2O3 N/A 16.00 17.00 18.48 18.70 15.90 15.10 17.60 12.92 16.33 13.55 Fe2O3 N/A 7.05 6.91 5.89 5.82 4.49 3.33 4.59 0.66 3.66 12.13 MnO N/A 0.11 0.11 0.09 0.09 0.07 0.06 0.09 0.02 0.07 0.17 MgO N/A 4.05 4.17 2.09 1.92 1.91 1.33 2.06 0.00 1.58 7.21 CaO N/A 6.31 6.59 5.80 6.04 4.03 2.90 5.21 0.22 4.69 11.48 Na2O N/A 3.69 3.81 4.11 4.39 3.96 3.89 4.45 4.17 4.48 2.25 K2O N/A 2.35 2.28 2.29 2.64 3.87 4.16 2.17 4.30 2.19 0.48 P2O5 N/A 0.28 0.27 0.25 0.32 < .05 0.16 0.28 0.01 0.21 0.29 LOI N/A 0.11 0.33 1.04 0.33 0.22 0.55 1.24 0.51 1.63 0.00 Total N/A 98.27 100.06 99.95 100.60 101.90 100.49 98.72 99.54 99.09 99.82 Mg# N/A 53.228 54.453 41.323 39.524 45.732 44.173 47.064 -0.756 46.037 0.54 Li 18.4 15.5 14.3 16.7 17.3 23.6 28.5 6.67 N/A N/A 4.62 Be 1.90 2.04 1.92 2.15 2.25 2.40 2.70 1.97 N/A N/A 1.09 V 135 178 173 125 140 96.3 67.4 93.1 N/A N/A 324 Rb 110 86.0 76.7 79.2 92.4 155 190 50.6 N/A N/A 9.64 Sr 525 502 545 520 604 440 350 917 N/A N/A 402 Y 19.7 21.7 20.0 20.0 19.9 18.6 19.6 15.1 N/A N/A 26.8 Zr 230 205 188 191 211 230 190 142 N/A N/A 175 Nb 18.2 8.12 9.08 7.88 12.5 22.0 24.0 7.62 N/A N/A 16.7 Cs 3.40 2.58 2.35 2.82 2.85 5.10 6.30 1.45 N/A N/A 0.102 Ba 700 521 499 544 564 675 740 721 N/A N/A 130 La 32.4 26.6 25.3 29.3 28.5 36.5 41.2 24.9 N/A N/A 15.4 Ce 65.9 58.3 55.2 58.6 62.0 72.0 80.6 52.8 N/A N/A 36.7 Pr 7.50 6.90 6.47 7.39 7.21 7.80 8.50 6.40 N/A N/A 5.45 Nd 28.8 26.6 25.0 29.1 27.5 28.7 30.6 24.4 N/A N/A 24.8

71 Sm 5.40 5.38 5.01 5.77 5.42 5.10 5.40 4.50 N/A N/A 6.28 Eu 1.40 1.28 1.25 1.44 1.38 1.20 1.00 1.31 N/A N/A 2.06 Gd 5.20 4.38 4.06 5.12 4.26 5.00 5.10 3.36 N/A N/A 2.69 Tb 0.65 0.64 0.61 0.68 0.61 0.60 0.62 0.48 N/A N/A 4.34 Dy 3.50 3.58 3.31 3.78 3.34 3.20 3.30 2.62 N/A N/A 5.22 Ho 0.67 0.69 0.65 0.72 0.63 0.59 0.62 0.50 N/A N/A 0.954 Er 2.00 1.90 1.78 1.91 1.72 1.80 1.90 1.36 N/A N/A 2.39 Tm 0.27 0.28 0.27 0.28 0.25 0.24 0.26 0.20 N/A N/A 0.325 Yb 1.80 1.82 1.71 1.81 1.59 1.60 1.80 1.33 N/A N/A 1.95 Lu 0.27 0.27 0.26 0.27 0.24 0.24 0.26 0.20 N/A N/A 0.272 Hf 5.00 4.59 4.39 5.15 4.72 4.80 4.40 3.51 N/A N/A 4.27 Ta 0.95 0.68 0.70 0.77 1.05 1.40 1.60 0.55 N/A N/A 1.29 Pb 12.8 11.7 11.4 14.5 13.3 15.6 19.0 11.5 N/A N/A 2.11 Th 10.60 9.36 8.91 11.58 10.47 14.00 17.70 4.03 N/A N/A 1.27 U 2.90 2.40 2.23 3.12 2.72 4.30 5.50 1.01 N/A N/A 0.410 87 86 Sr/ Sr 0.705137 0.704928 0.704912 N/A 0.704831 0.705423 0.705722 0.704708 N/A 0.704610 0.703463 143 144 Nd/ Nd 0.512539 0.512570 0.512509 N/A 0.512571 0.512485 0.512488 0.512591 0.512421 0.512601 0.512998 176 177 Hf/ Hf N/A N/A N/A N/A N/A N/A N/A 0.282762 0.282654 0.282777 N/A 206 204 Pb/ Pb N/A N/A N/A N/A N/A N/A N/A 18.448 18.697 18.533 N/A 207 204 Pb/ Pb N/A N/A N/A N/A N/A N/A N/A 15.580 15.584 15.593 N/A 208 204 Pb/ Pb N/A N/A N/A N/A N/A N/A N/A 38.358 38.277 38.420 N/A εNd N/A N/A N/A N/A N/A N/A N/A -0.8 -4.1 -0.6 N/A εHf N/A N/A N/A N/A N/A N/A N/A -0.8 -4.6 -0.3 N/A

BIR-1**

SiO2 46.84

TiO2 0.92

Al2O3 14.92

Fe2O3 11.12 MnO 0.17 MgO 9.50 CaO 13.18

Na2O 1.75

K2O -0.03

P2O5 0.04 LOI 0.00 Total 98.40 Mg# 0.63 Li 3.04 Be 0.098 V 319 Rb 0.183 Sr 107 Y 15.7 Zr 14.4 Nb 0.60 Cs 0.004 Ba 6.61 La 0.614 Ce 1.82 Pr 0.383 Nd 2.41 Sm 1.15

73 Eu 0.526 Gd 1.74 Tb 0.357 Dy 2.59

Ho 0.572 Er 1.63 Tm 0.255 Yb 1.68 Lu 0.253

Hf 0.590 Ta 0.076 Pb 3.69 Th 0.032 U 0.010 87Sr/86Sr 0.703053 143 144 Nd/ Nd N/A 176Hf/177Hf N/A 206Pb/204Pb N/A 207Pb/204Pb N/A

208Pb/204Pb N/A εNd N/A εHf N/A

* Analyzed by Dana Drew at Boston University as part of her undergraduate thesis for Penn State University

** BHVO-1 and BIR-1 analyzed as unknowns