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ABSTRACT

THE 1630 AD ERUPTION OF , SÃO MIGUEL, (): CHEMICAL VARIATIONS AND MAGMATIC PROCESSES

by Andrea Rowland-Smith

Furnas volcano, an active on the island of São Miguel, Azores, is considered one of Europe’s most hazardous volcanoes. This work constitutes the first detailed petrographic, compositional, and isotopic study of the Furnas 1630 AD eruptive deposit. The eruptive products of the Furnas 1630 AD deposit are almost exclusively trachytic, with limited major element variations but large trace element variations that can be attributed to extensive fractional crystallization. Constant Nd and Pb but variable Sr isotopic signatures in the 1630 AD eruptive products, including whole , glass and individual crystals, suggest that fractionation was accompanied by assimilation of seawater altered from the chamber walls. Analysis of stratigraphically controlled samples from throughout the Furnas 1630 AD deposit indicate systematic but non-monotonic variations in the composition of the eruptive products that reflect complex magma chamber geometry and/or multiple magma chambers.

THE 1630 AD ERUPTION OF FURNAS VOLCANO, SÃO MIGUEL, AZORES (PORTUGAL): CHEMICAL VARIATIONS AND MAGMATIC PROCESSES

A Thesis

Submitted to the

Faculty of Miami University

in partial fulfillment of

the requirements for the degree of

Master of Science

Department of Geology

by

Andrea Rowland-Smith

Miami University

Oxford, Ohio

2007

Advisor ______Dr. Elisabeth Widom

Reader______Dr. John Rakovan

Table of Contents

Title………………………………………………………………………………………...i

Table of Contents………………………………………………………………………….ii

List of Tables….………………………………………………………………………….iii

List of Figures...…………………………………………………………………………..iv

Acknowledgements………………………………………………………………………..v

Section 1. Introduction……………………………………………………………………1

Section 2. Geologic Setting and Background………………………………………...... 2

Section 3. Sampling and Analytical Techniques…………………………………………3

Section 4. Results…………………………………………………………………………4

4.1 Major and trace elements……………………………………………………..4

4.2 Petrography and chemistry…………………………………………..5

4.3 Isotope systematics…………………………………………………………...5

Section 5. Discussion…………………………………………………………………….6

5.1 Formation of the Furnas 1630 AD ………………………………….6

5.2 Origin of chemical variations among the Furnas 1630 AD trachytes………...7

5.3 Evidence for open-system processes…………………………………………8

5.4 Comparison of the recent Furnas and Fogo magmatic systems……………..10

5.5 Petrogenetic models for the evolution of the Furnas 1630 AD magmatic system……………………………………………………………………11

Section 6. Conclusions…………………………………………………………………..12

Tables…………………………………………………………………………………….14

Figures……………………………………………………………………………………21

References………………………………………………………………………………..33

ii List of Tables

Table 1. Major and trace elements concentrations from the 1630 Furnas AD deposit….14

Table 2. Rare earth element concentrations of selected samples………………………..15

Table 3. Average chemical compositions of from selected samples…..………16

Table 4. Isotopic data for whole-rock , glass, and sanidine samples …….……...17

Table 5. Major element modeling from to least evolved ………18

Table 6. Major element modeling from least evolved trachyte to most evolved trachyte 19

Table 7. Trace element modeling from least evolved trachyte to most evolved trachyte 20

iii

List of Figures

Figure 1. Map of the Azores archipelago and São Miguel island……………………….21

Figure 2. Stratigraphic column of the Furnas 1630 AD deposit with a schematic representation of the eruptive phases………………………………………22

Figure 3. Detailed map of Furnas volcano……………………………………………....23

Figure 4. Alkalis vs. silica classification diagram………………………………………24

Figure 5. Major element variation diagrams…………………………………………… 25

Figure 6. Trace element variation diagrams…………………………………………….26

Figure 7. Chondrite normalized rare earth element diagram……………………………27

Figure 8. Photomicrographs……………………………………………………………..28

Figure 9. Sr, Nd, Pb isotope variations…………………………….……………………29

Figure 10. Ternary diagram representing the phase diagram of the -- kalsilite system……………………………………………………………..30

Figure 11. Zr concentration versus relative stratigraphic position……………………...31

Figure 12. Cartoon diagrams illustrating magma chamber configurations……………...32

iv

Acknowledgements

I am sincerely grateful to the many people who have facilitated my study of geology. My advisor, Dr. Elisabeth Widom, instilled in me a respect for rigorous research standards. Her attention to fine detail encouraged a very critical approach to research methods. Her personal attention to the writing process was extremely valuable. The technical assistance I received from Zu Watanabe, Dr. John Morton, Dr. Dave Moecher, and Dr. Darin Snyder made this research possible. I appreciate the help I received from Dr. Dave Kuentz, Dr. Kendall Hauer, and Bill Wilcox for assistance in graphic design. Financial support for this research was covered by my advisor’s National Science Foundation grants (NSF EAR #0207529 and NSF MRI #0116033) and funding from the geology department.

v 1. Introduction

The island of São Miguel is the most populous of the nine Azores islands with approximately 150,000 inhabitants, the majority of whom live within 15 km of one of three active stratovolcanoes (Sete Cidades, Fogo, and Furnas). Together, these three volcanoes have produced at least five major -forming eruptions within the past 50,000 years (Moore, 1990; Moore, 1991). Recent acknowledgment of the volcanic hazards on this island prompted the U.S. Agency for International Development and the U.S. Geological Survey to engage in a detailed study of the volcanic geology and eruption frequency on São Miguel (Moore, 1991). Furnas volcano, the youngest stratovolcano on São Miguel, has been active for the past ~100 ka, and is thought to have produced at least two caldera forming eruptions (Guest et al., 1999). At least 10 explosive intracaldera eruptions have occurred over the past 3200 years, the most recent being the 1630 AD eruption which killed almost 200 people (Booth et al., 1978; Moore, 1990; Moore, 1991). The average dormant period over the past 3200 years has been approximately 350 years, although 5 eruptions have occurred over the past 1.1 ka, suggesting a more recent average dormant period of less than 200 years. In either case, the implication is that Furnas may be overdue for an eruption given that 377 years have passed since the last eruption. The frequent explosive eruptions in recent times, coupled with the large population that lives within the caldera and in neighboring towns, makes Furnas volcano a serious hazard; even a small eruption would be likely to cause significant fatalities (Guest et al., 1999). Furnas is thus considered to be one of the most hazardous volcanoes in Europe, and in the early 1990's Furnas was included as one of six European Laboratory Volcanoes selected for volcanological research in the International Decade for Natural Disaster Reduction (Duncan et al., 1999; Guest et al., 1999, Ghazi et al. 1997). Recent studies associated with this initiative have focused primarily on gravity and deformation measurements (Jónsson et al., 1999; Trota et al., 2006; Montesinos et al., 1999), fumerolic gas emissions (Ferreira and Oskarsson, 1999; Notcutt and Davies, 1999), and hazard and risk assessment (Cole et al., 1999; Jones et al., 1999; Dibben and Chester, 1999; Pomonis et al., 1999; Baxter et al., 1999). In addition, a few recent volcanological studies have documented the eruptive history of Furnas volcano over the past 30 ka (Guest et al., 1999; Cole et al., 1999) including a detailed investigation of the 1630 AD deposit (Cole et al., 1995). However, despite the recent focus on Furnas volcano, relatively little is known about the magmatic processes or timescales of magma evolution leading to the highly explosive eruptions that characterize this volcano (Moore, 1991; Oskarsson et al., 1998). This study presents the first detailed petrographic, geochemical and isotopic analyses of the Furnas 1630 AD eruptive products, aimed at unraveling the magmatic processes leading to the eruption and developing a framework for a future related study of the timescales of magma evolution. The results of this study show for the first time that there is significant compositional variability throughout the 1630 AD deposit that, when integrated with the stratigraphic and volcanological results of Cole et al. (1995), requires a complex petrogenetic evolution of the pre- eruptive Furnas 1630 AD magmatic system. The petrogenetic evolution of the Furnas 1630 AD pre-eruptive magmatic system is similar to but distinct from magmatic processes beneath the neighboring Fogo volcano.

1 2. Geologic Setting and Background

The regional tectonics of the Azores are quite complex, as the archipelago spans the Mid- Atlantic Ridge in the vicinity of the triple junction between the North American, African and Eurasian plates (Fig. 1). In addition, both geochemical and tomographic data indicate the presence of a beneath the Azores (Schilling, 1975; Moreira et al., 1999; Ritsema & Allen, 2003; Montelli et al., 2004). It is postulated that the mantle plume is responsible for generating the excess melting that has resulted in the formation of the thickened crust of the Azores platform and the island volcanism. The island of São Miguel, one of the easternmost of nine islands that comprise the Azores Archipelago, is itself tectonically complex, as it lies at the intersection of two tectonic lineaments. The NE-SW trending Terceira rift, which connects the Mid-Atlantic Ridge to the strike-slip Gloria fault and separates the Eurasian and African plates, is an active extensional zone (Searle, 1980; Luis et al., 1998; Vogt and Jung, 2004) that cuts through the western end of the island. The E-W trending lineament, approximately parallel to the inactive East Azores Fracture Zone, is dominant in the eastern two thirds of São Miguel. It is thought that these lineaments control the expression of volcanism on the island, which is dominated by three active trachytic stratovolcanoes (from west to east: Sete Cidades, Fogo, and Furnas) separated by regions of active basaltic volcanism (Moore, 1991). Furnas, the easternmost active stratovolcano on São Miguel, is located approximately 10 km to the east of Fogo volcano and is adjoined on its east side by the extinct trachytic stratovolcano Povocão. Furnas volcano contains at least two , including a younger, well- defined inner caldera, which is contained within an older and less pronounced outer caldera (Cole et al., 1995). The oldest known deposits from Furnas volcano are approximately 93 ± 9 ka, with the two caldera forming eruptions inferred to be ~30 ka and 12 ka, respectively (Moore, 1991; Guest et al., 1999). Since then, numerous intracaldera eruptions have occurred, including at least 10 explosive trachytic eruptions over the past ~3 ka. and intermediate (including alkali and basanite through and trachyandesite) have only erupted on the flanks of the volcano surrounding the trachytic vents, and within an extensional region between Furnas and Fogo volcanoes. The exclusion of mafic and intermediate magmas from the trachytic vent region (~8 km X 5 km) may indicate the presence of persistent, shallow, trachytic magma chambers beneath the stratovolcanoes that prevent the ascent of mafic magmas (Moore, 1990; Moore, 1991). However, the occurrence of trachytic eruptions at the Congro on the eastern flank of Fogo volcano as well as minor trachytic eruptions interspersed with the primarily basaltic activity in the region between the two stratovolcano flanks (Moore, 1991), also raises the possibility of interconnected trachytic plumbing systems between these volcanoes. The most recent eruption of Furnas volcano, which occurred in 1630 AD, was a highly explosive eruption that produced ~0.65 km 3 (dense rock equivalent) of predominantly pumice and ash layers (Cole et al., 1995). The eruption began early on the morning of September 3, 1630 with explosive activity that fluctuated between subplinian and phreatomagmatic activity, lasting for three days. After the main explosive activity subsided, a dome grew over a period of about two months, until November 2, 1630. The resulting deposit has been mapped and described in detail by Cole et al. (1995), who subdivided the deposit into multiple stages including early erupted lapilli (L1-L5) and ash layers (A1-A5) that represent magmatic and phreatomagmatic activity, respectively, later erupted lapilli and ash (Lf), and the final dome formation phase (Fig. 2). Two samples analyzed by Cole at al. (1995) were both trachytic and of

2 nearly identical composition, leading to speculation that the 1630 AD deposit may be nearly uniform in composition (Cole et al., 1995).

3. Sampling and Analytical Techniques

Samples were collected throughout the entire 1630 AD deposit. Due to the changing wind directions during the eruption (Cole et al., 1995), several different localities had to be sampled in order to acquire the complete eruptive sequence (Fig. 3). Samples from the earliest eruptive phases, L1-L3, were collected at locality #1, located ~ 1km SW of the 1630 eruptive center on the outer slope of the older caldera. Samples from L4/L5 were collected at localities #2 and #3 on the coast SE of the caldera, near the town of . Sampling localities #4 and #5, to the W and NW of the eruptive center, provided from the early and middle stages of the Lf eruptive phase, respectively. The final phases of the eruption, including the last stages of pumice fall and the dome formation, were sampled at localities #6 and #7 in the SE region of the 1630 eruptive center. At each locality composite and/or individual large pumices were collected, with the exception of the dome site (locality #7), where dome fragments were sampled. All samples were processed initially by thorough cleaning in deionized water, including multiple sessions in the ultrasonic bath and rinsing in between. After drying in an oven, the samples were chipped in a Sepor alumina jaw crusher, and powdered in a Spex shatterbox using a high purity alumina vessel. Major and trace element measurements of whole rocks were acquired at Miami University using a Beckman SpectraSpanV Direct-Current Plasma Atomic Emission Spectrometer (DCP-AES) following the procedures of Katoh et al. (1999). Selected samples were also analyzed for rare earth element (REE) concentrations by inductively coupled plasma mass spectrometry (ICP-MS) using a Varian quadrupole instrument at Miami University. Bulk REE were separated and purified using cation exchange chemistry prior to analysis, following sample preparation and measurement methods described by Bondre et. al (2006). Mineral analyses of selected samples were performed using an ARL-SEMQ electron microprobe with a wavelength dispersive system at the University of Kentucky, following methods described by Anderson and Moecher (2007). Sr, Nd and Pb purification chemistry and isotopic analyses were done at Miami University. In order to remove contaminant Sr that may have accumulated on sample surfaces during post-eruptive alteration, all samples were pre-treated with a leaching procedure developed for similar samples from Fogo volcano, and documented to effectively remove post-eruptive Sr (Snyder et al., 2007). Whole rock and glass samples were cleaned in 1N HCl for 10 minutes in an ultrasonic bath, and individual sanidine crystals were cleaned for ~45 seconds with 2.5 N HCl, followed by thorough water rinses, prior to digestion. For Sr, Nd, and Pb isotope analyses whole rock powder or glass was dissolved in concentrated HF-HNO 3 and taken up in HBr or (if for Sr analyses only) in HNO 3. Procedures for Sr, Pb, and bulk rare earth separations are described in Snyder et al. (2007). Pb was separated by anion exchange chemistry using HBr and HNO 3. Some whole-rock Sr samples were purified using large (13 ml) cation columns following techniques of Walker et al. (1989), but most Sr samples were purified using small (0.25 ml) EiChrom Sr-Spec resin columns, following the methods of Deniel and Pin (2001). Nd separations from the bulk REE cuts were performed using EiChrom Ln-Spec resin, following the methods of Pin and Zalduegui (1997).

3 The Sr, Nd, and Pb isotopic compositions and Sr concentrations by isotope dilution were measured by thermal ionization mass spectrometry on a Finnigan Triton at Miami University. Due to the low concentrations of Sr and small sample sizes, samples were loaded on single Re filaments with TaF 5 activator for enhanced ionization, following methods of Charlier et al. (2006). Pb samples were loaded on single Re filaments with silica gel and phosphoric acid, and run at 1300 °C. Nd samples were loaded onto double Re filaments with phosphoric acid, and measured as the metal. Strontium isotope ratios were corrected for mass fractionation using 86 Sr/ 88 Sr=0.1194. Ninety measurements of NBS987 demonstrate a long term reproducibility of ± 0.000016 (2 SD) about a mean of 0.710238. Neodymium isotope ratios were corrected for mass fractionation using 144 Nd/ 146 Nd = 0.7219. Seventy-two measurements of La Jolla gave an average value of 143 Nd/ 144 Nd= 0.511846 ± .000006 (2 SD). 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb were corrected for fractionation by 0.11% per amu, based on deviations of the measured ratios in NBS981, following Todt et al. (1996). Seventy-five measurements of NBS981 yield reproducibilities of ±0.013, ± 0.017, ±0.054 (2 SD) on 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb, respectively.

4. Results

4.1. Major and trace elements

Whole-rock samples from the throughout the Furnas 1630 AD deposit show relatively limited compositional variation. With the exception of one sample which is a trachyandesite (SiO 2 = 58%), all of the samples are trachytes, with ~63% SiO 2 (Table 1 and Fig. 4). In addition to the near constant SiO 2 concentrations, the trachytes exhibit extremely limited variation in concentrations of Al 2O3, K 2O and Fe 2O3 (Fig. 5). In contrast, the concentrations of P 2O5, MgO, CaO, TiO 2 and Na 2O in the trachytes show distinct variations (ranging in magnitude from 200% to 19%, respectively) throughout the deposit. With decreasing MgO, concentrations of P 2O5, CaO and TiO 2 decrease, and Na 2O concentrations increase (Fig. 5). The trachyandesite sample differs significantly from the trachyte samples in all major element concentrations. In addition to lower SiO 2, the trachyandesite has significantly lower Al 2O3, K 2O and Na 2O and higher MgO, CaO, and P 2O5, TiO 2, and Fe 2O3 compared to the trachytes. Trace element abundances show significant variability throughout the deposit. Concentrations in the trachytes of Zn, Rb, Nb, Y and Zr are high (tens to thousands of ppm) and variable throughout the deposit, with concentration variations ranging from 59% to 86% (Table 1). Zn, Rb, Nb, Y and Zr all exhibit increasing concentrations with decreasing MgO in the trachytes (Fig. 6). In contrast, Sr concentrations in the trachytes are extremely low (4 ppm to sub-ppm levels), and exhibit a positive correlation with MgO. The trachyandesite has trace element concentrations that are within the range of the trachytes for all of these trace elements except Zr and Sr which are slightly lower and significantly higher, respectively, in the trachyandesite. REE abundances (Table 2) also vary significantly within the Furnas 1630 AD deposit. Concentrations of REE elements in the trachytes show variations of 37% to 59%, with the heavy REE (HREE) exhibiting greater variability than the light REE (LREE). Concentrations of REE generally decrease with increasing MgO, with the exception of Eu which exhibits the opposite

4 trend. All samples are characterized by strong LREE enrichments (Fig. 7) and strong negative Eu anomalies that increase in magnitude with increasing HREE and LREE abundances. The trachyandesite has concentrations for most REE that are within the range of the trachytes, but it has slightly lower concentrations of La and Ce and a significantly higher concentration of Eu and less pronounced negative Eu anomaly.

4.2 Petrography and mineral chemistry

The samples from the 1630 eruptive sequence exhibit a variety of colors and textures (Fig. 8). Most of the trachyte pumices have light grayish-tan glassy, vesicular matrices with very low contents (a few % or less by volume). The predominant phenocryst phases include sanidine, , , ulvospinel, and . Sanidine is by far the most abundant phenocryst phase (~90% of by volume), and ranges in size from microphenocrysts (< 0.5 mm) to 5mm. Inclusions of biotite and needle-shaped apatite in the sanidines are common. The trachyandesite is darker grey and denser than the trachyte pumices, with smaller and less prevalent vesicles, and a higher phenocryst content (~5 %). The trachyandesite groundmass is primarily composed of randomly oriented sanidine microlites with minor glass, forming a hypocrystalline texture. The microphenocrysts form a weakly diktytaxitic texture. The trachyandesite contains the same phenocryst assemblage as the trachytes, but the sanidines show disequilibrium textures including reaction rims and resorption features. The trachytic dome samples are cream colored and contain very few vesicles, making these samples distinctive compared to the other trachytes. The is holocrystalline with a pilotaxitic texture. Similar to the trachyandesite, the sanidine phenocrysts in the dome samples are embayed and exhibit reaction rims. Mineral chemistry was obtained by electron microprobe for selected samples, and representative analyses for sanidine, biotite, augite and ulvospinel are presented in Table 3. Although a limited number of mineral analyses were made due to the very low crystal content of most samples, only minor compositional variation is observed for crystals of a given mineral phase, and there is no apparent systematic variation in mineral composition throughout the deposit.

4.3 Isotope systematics

Sr, Nd, and Pb isotopic compositions and Sr concentrations by isotope dilution have been determined for whole rocks, glass separates, and sanidines from select samples spanning the Furnas 1630 AD eruptive sequence (Table 4; Fig. 9). Leaching experiments were performed on samples of the two most evolved trachyte pumices (SM1630-20 and SM1630-3) in order to confirm that the leaching procedure developed previously for similar samples from Fogo volcano (Snyder et al., 2004) would be appropriate for removing post-eruptive Sr in Furnas samples. For both SM1630-20 and SM1630-3, 87 Sr/ 86 Sr ratios of the unleached whole rock pumices are significantly more radiogenic than the respective samples leached with 1N and 2N HCl (Table 4). Data for sample SM1630-20 demonstrates that the 87 Sr/86 Sr ratio and Sr concentration are the same within measurement error whether a 1N or 2N HCl leach is performed, confirming that the 1N HCl leach effectively removes the radiogenic post-eruptive Sr. This is consistent with the results of the more extensive leaching experiments performed previously on Fogo pumices (Snyder at el., 2004).

5 The five whole rock pumices show significant isotopic variability in 87 Sr/ 86 Sr (0.70491- 0.70533) that correlates negatively with Sr concentration (Fig. 9). In contrast, no variation is observed in Nd isotope ratio ( 143 Nd/ 144 Nd =0.51272±1). Pb also exhibits essentially constant isotope signatures with 206 Pb/ 204 Pb= 19.92 ± 0.02, 207 Pb/ 204 Pb= 15.74 ± 0.02, and 208 Pb/ 204 Pb= 40.12 ± 0.08. Although the Pb isotope measurements vary slightly outside of our reproducibility on the standard NBS981, we attribute this to slightly greater variability in mass fractionation during sample measurements compared to standard measurements due to matrix effects and less efficient ionization in the former. For three of the pumice samples, glass separates and individual sanidine crystals were also analyzed for Sr isotopes and Sr concentration (Fig. 9). Compared to the whole rock pumice samples, the respective glass separates are distinctly less radiogenic, but the glass separates nevertheless also exhibit a range in 87 Sr/ 86 Sr (0.70498-0.70508). As was found for the whole rock pumices, 87 Sr/ 86 Sr in the glass separates is negatively correlated with Sr concentration. The sanidine crystals also exhibit variability in Sr isotope ratios, with 87 Sr/ 86 Sr ranging from 0.70492- 0.70521. Of the five sanidine crystals analyzed, four are less radiogenic than either their host glass or the respective whole rock pumice. One sanidine crystal (from sample SM1630-20) is more radiogenic than its host glass, but less radiogenic than the respective whole rock pumice.

5. Discussion

5.1. Formation of the Furnas 1630 AD trachytes

Previous studies of São Miguel magmatic suites have demonstrated that the trachytes can be produced by extensive fractional crystallization of mafic parent magmas. (Moore at al., 1991; Widom et al., 1992; Oskarsson et al., 1998; Renzulli and Santi, 2000; Beier et al., 2006). A variety of mafic magma types including ankaramite, alkali basalt and basanitoid have been erupted on the flanks of the stratovolcanoes and in the intervening extensional zones, and could represent potential parental magma compositions, although it is likely that the ankaramites are accumulative relatives of the basanites (Moore, 1991; Beier et al., 2006). Least squares modeling calculations of Sete Cidades magmatic suites indicate that the Sete Cidades trachytes can be produced by via ~91% crystallization of a slightly nepheline normative with removal of olivine, clinopyroxene, , ilmenite, , apatite, alkali , , and biotite (Renzulli and Santi, 2000; Beier et al., 2006). Major element compositional trends within the Sete Cidades magmatic suites from alkali basalt (MgO ≈ 13%) through trachyandesite (MgO ≈ 2%) are relatively well modeled by thermodynamic equilibria calculations (Melts program; Ghiorso, 1997) at a pressure of 5 kbar (log fO2 = 10, H 2O and CO 2 = 0.5%), thus supporting the notion that the intermediate to evolved magmas are produced primarily via fractionation of mafic parental magma (Beier et al., 2006). Although there is a complete compositional continuum between the basaltic and trachytic endmembers, thermobarometric data indicate polybaric fractionation with the basaltic melts stagnating at the Moho at ~15 km depth, and the more evolved magmas fractionating in the upper crust at depths of ~3 km (Renzulli and Santi, 2000; Beier et al., 2006). Furnas volcano has erupted mildly alkaline volcanic products that follow very similar major element trends to those observed in Sete Cidades, with the exception that Furnas volcanic products are slightly more potassic (Oskarsson et al., 1998; Moore, 1991). Thus, the Furnas

6 trachytic magmas are likely to have evolved via similar mechanisms to those at Sete Cidades, with the exception of slightly more potassic parental magmas at Furnas. Although no primitive magmas were erupted in association with the Furnas 1630 AD eruption, mafic magmas from the vicinity of Furnas can be used as proxies for modeling the evolution of the trachyandesite, the least evolved juvenile product associated with the 1630 AD eruption. Based on fractionation trends predicted by Melts (Ghiorso 1997) for Furnas basanites (Oskarsson et al.; 1998), the Furnas 1630 trachyandesite could be produced with ~75% fractional crystallization of a basanite parent magma via a trachybasaltic at a pressure of 5 kbar, over a temperature drop of ~1250 - 1100 °C. Oskarsson et al. (1998) extend their modeling to trachytic compositions that, when evolved from trachybasalt at 5 kbar, approximate those of the Furnas 1630 AD deposit. In contrast, shallower (2 kbar) fractionation trends produce compositions that are too rich in TiO 2 at the appropriate SiO 2 content. However, it is known that the thermodynamic models of the Melts program (Ghiorso, 1997) are not reliable at high silica contents, hence a polybaric fractionation model with the evolved magmas fractionating at shallower depth, akin to Sete Cidades, may be a more appropriate model. Unfortunately, compositions in the Furnas 1630 AD samples are not amenable to crystallization pressure determinations as they have a zero calculated jadeite component (Putirka et al., 2003). Least squares major element fractional crystallization models using Igpet05 (Carr, 2005) with the Furnas 1630 AD trachyandesite (SM1630-7) as an immediate parent suggest that ~60% fractional crystallization of a sanidine dominated assemblage with lesser clinopyroxene, ulvospinel, apatite and biotite can reproduce the composition of the least evolved Furnas 1630 AD trachyte (SM1630-31) reasonably well, with a residual sum of squares (RSOS) of <1 (Table 5). However, note that the trachyandesite erupted in the 1630 AD eruption cannot be the actual parent to the trachytes that make up the bulk of the deposit, because some incompatible trace element concentrations in the trachyandesite are comparable to or higher than the least evolved trachytes (e.g. Zr, Rb, and some REE).

5.2. Origin of chemical variations among the Furnas 1630 AD trachytes

Compositional variation in the Furnas 1630 AD trachyte suite is relatively limited for several major elements. Concentrations of SiO 2, K 2O and Fe 2O3 are essentially constant, and with the exception of one sample (SM1630-28) that may have accumulated sanidine, the same is true of Al 2O3. However, concentration variations of ~20% to ~200% are observed for P 2O5, MgO, CaO and Na 2O. Decreasing P 2O5, CaO and TiO 2 with decreasing MgO are qualitatively consistent with fractionation of apatite, clinopyroxene and ulvospinel, three of the phases in the observed phenocryst assemblage of the trachytes. Although Na 2O concentrations increase modestly and Al 2O3 and K 2O concentrations remain approximately constant with decreasing MgO, trace element variations indicate the importance of fractionation of sanidine, which is by far the dominant observed mineral phase in the trachytes. Extremely low Sr concentrations (sub-ppm levels in some cases), as well as pronounced negative Eu anomalies that increase in magnitude with decreasing MgO, require a dominant role of sanidine fractionation during the trachyte magma evolution. Increasing concentrations of other trace elements with decreasing MgO, including Zn, Rb, Nb, Zr and Y, are also consistent with fractional crystallization, as these elements typically display incompatible behavior during magma evolution. A nearly 2-fold increase in Zr (912-

7 1697 ppm) with decreasing MgO requires a minimum of 46% fractional crystallization assuming Zr behaves as a perfectly incompatible element, and a greater amount of crystallization if D Zr > 0. The combination of relatively limited major element variations coupled with large trace element variations is a well-known but relatively poorly understood phenomenon associated with highly evolved magmatic systems (e.g. Hildreth, 1981; Worner et al., 1984; Wolff et al., 1990; Rogers et al., 2004, Snyder et al., 2004). One possible mechanism for generation of large compatible and incompatible trace element concentrations with limited major element abundance variations is via fractionation of a near-eutectic liquid. As has been proposed for other trachytic systems (Widom et al., 1992; Rogers et al., 2004), it is suggested that the composition of the Furnas 1630 AD trachytes close to the thermal minimum on the Ab-Or join in the system quartz- kalsilite-nepheline (Fig. 10; Petrogeny’s Residua System) may dictate pseudo-eutectic behavior of the trachytic magma. In this situation, cooling and crystallization of the trachytic magma is dominated by fractionation of sanidine whose major element composition is very close to that of the crystallizing liquid, hence little major element variation is produced despite prolonged fractionation. Least squares regression modeling performed using Igpet05 (Carr, 2005) is consistent with chemical variations among the Furnas 1630 AD trachytes resulting from extensive fractional crystallization. The model results are furthermore consistent with fractionation of a mineral assemblage similar to the observed phenocryst assemblage, and whose bulk composition is similar to the trachytic liquid, especially for SiO 2, K 2O, Fe 2O3 and Al 2O3. Table 6 illustrates model results for fractionation from the least evolved trachyte (SM1630-31) to the most evolved trachyte (SM1630-3) using average mineral compositions determined for the Furnas 1630 AD trachytes. The model results indicate that the major element compositional range within the Furnas 1630 AD trachyte suite can be well modeled (RSOS = 0.02) by ~43% crystallization of a mineral assemblage with 91% sanidine, 4% clinopyroxene, 3% ulvospinel, 1% biotite, and 0.3% apatite (Table 6). Trace element modeling using the relative mineral phases determined by Igpet and mineral-trachyte partition coefficients from the literature ( Mahood & Stimac, 1990; Villemant, 1988; Luhr et al., 1984; Nagasawa, 1973; Nabelek, 1980) is consistent with this result, although a slightly greater extent of crystallization with a slightly higher percentage of apatite (1%) is indicated in order to simultaneously model the nearly 2-fold increase in Zr concentration with a significantly less than 2-fold increase in REE abundances during fractionation (Table 7).

5.3. Evidence for open-system processes

Although the major and trace element modeling is quantitatively consistent with evolution by purely closed system fractional crystallization, isotopic evidence argues for a more complex magmatic evolution of the Furnas 1630 trachytes. As described previously, Nd and Pb isotope signatures in whole rock trachyte pumice samples are essentially constant, but Sr isotope signatures vary among the whole rock pumices, glass separates and single sanidine separates (Fig. 9). Differences in the Sr isotopic composition between individual sanidine crystals from within a given pumice (e.g. SM1630-20 and SM1630- 3) as well as differences between sanidine crystals and their respective host glasses (SM1630-20, SM1630-3 and SM1630-14) suggest that the sanidine crystals are xenocrysts. Whole rock pumice samples are also isotopically variable in Sr, most likely due to incorporation of

8 xenocrystic sanidine. The sanidine crystals generally have Sr concentrations ~10X higher than their respective host glasses (Table 4), thus even minor fractions of xenocrystic sanidine in a whole rock pumice can influence its Sr isotopic composition. Although one whole rock sample (SM1630-20) is more radiogenic than any individual sanidine analyzed in this study, we attribute this to the limited number of sanidine crystals analyzed (only 5 from 3 pumice samples). Clearly the whole rock must have an isotopic composition equivalent to the sum of the glass and the crystals, and sanidine is by far the most abundant and, other than trace apatite, should be by far the most Sr-rich of the phenocryst phases. A similar phenomenon of Sr isotopic disequilibria between sanidine crystals and host glass was observed in trachyte pumices from the ~5 ka Fogo A eruption (Snyder et al., 2004), and was attributed to incorporation in the trachyte magma of sanidine from altered syenite wallrock comprising the magma chamber walls. Abundant syenite xenoliths are found in trachyte pumice deposits associated with both Fogo and Furnas volcanoes (Oskarsson et. al., 1998; Moore et al., 1991), hence this is a possible process affecting the Sr isotopic variability in the Furnas 1630 AD trachytes as well. The negative correlation between Sr isotopic composition and Sr concentration in the Furnas whole rock pumices (Fig. 9) is consistent with this process, as more evolved trachytes with lower Sr concentrations would be more strongly affected by minor xenocryst incorporation than would be less evolved trachytes with higher Sr abundances. Although no syenite xenoliths from the Furnas deposits have been analyzed for Sr isotopes, the isotopic compositions of the sanidine crystals from the Furnas 1630 AD trachytes overlap with the isotopic compositions of from the neighboring Fogo volcano (Widom et al., 1993). Sr isotopic variability in the trachyte glass separates, however, cannot be explained simply by incorporation of xenocrystic sanidine. The glass separates are distinctly less radiogenic than the respective whole rock pumices, but nevertheless exhibit measurable Sr isotope variations that display a negative correlation between 87 Sr/ 86 Sr and Sr concentration (Fig. 9). Two possible explanations for Sr isotopic variations in trachyte glass separates include pre- eruptive aging of a chemically zoned magma that has variable Rb/Sr ratios (Christensen and DePaolo, 1993) or crustal assimilation (Snyder et al., 2004). Because of the high Rb and variable but generally extremely low Sr concentrations in the Furnas 1630 AD trachytes, they exhibit nearly an 8-fold variation in Rb/Sr ratios (from 37 to 291). If the trachyandesite is included (Rb/Sr = 0.27), the sample suite displays >1,000-fold variation in Rb/Sr. Given this extreme range in Rb/Sr ratios, relatively short amounts of time would be required to produce the range of Sr isotopic compositions observed in the Furnas trachyte samples. For example, if we assume that the Furnas 1630 AD trachyte magma has an 87 Sr/ 86 Sr of 0.70491 initially (discussed below), the radiogenic ratios found in the glass separates of the most evolved samples (e.g. SM1630-20 glass = 0.70508) could be produced solely by radioactive ingrowth of 87 Sr over a period of only ~9 ka. This timescale is on the same order as the ~4.6 ka pre-eruptive magma residence time calculated for the Fogo A magma based on U- series disequilibria data (Snyder et al., 2007), hence is not out of the question. However, the Furnas 1630 AD samples do not produce a strictly isochronous relationship on an Rb-Sr isochron diagram, thus alternative processes are more likely responsible for the Sr isotopic variability of the trachtye glasses. Future U-series disequilibria studies of the Furnas 1630 AD eruptive products will further address this possibility by constraining the pre-eruptive magma residence time. Crustal assimilation is an alternative process that could result in variable Sr isotope signatures of the Furnas trachyte glasses. As was demonstrated for trachyte glasses from Fogo A

9 (Snyder et al., 2004), minor assimilation of local marine sediment, altered oceanic crust, and aged and/or seawater-altered volcanic edifice or syenite wallrock are all viable processes for generating variations in Sr isotope ratios of trachyte magmas with no measurable effect on Nd or Pb isotope ratios. In the case of Fogo A, correlations between Th and Sr isotope ratios in the trachyte glasses allowed Snyder et al. (2004) to conclude that assimilation of seawater-altered syenite wallrock was the most likely assimilant. Given the evidence for the incorporation of xenocrystic sanidine in the Furnas whole rock pumice samples, assimlation of syenite via melting of local magma chamber wallrock is a likely explanation for the variable Sr isotope signatures in the Furnas 1630 AD trachyte glasses. Approximately 1% assimilation by mass of seawater altered syenite similar to those associated with Fogo volcano could account for the Sr isotope variations observed in the Furnas 1630 AD trachytes.

5.4. Comparison of the recent Furnas and Fogo magmatic systems

The Furnas and Fogo volcanoes are in close proximity to one another, with only ~10 km separating their respective calderas (Fig. 1), and thus it is feasible that they could share a common mantle source, and perhaps even have interrelated magmatic plumbing systems. from the vicinity of Fogo and Furnas exhibit a range in Sr-Nd-Pb isotopic compositions, although there is no clear distinction between basalts that may be more closely associated with Fogo versus Furnas. However, all of the basalts in the Fogo-Furnas region are distinctly more radiogenic in Sr, Nd and Pb isotopes than basalts erupted to the west of Fogo volcano, which become progressively less radiogenic westward (Widom et al., 1997; Beier et al., 2006). The Sr, Nd and Pb isotopic compositions of the Furnas 1630 AD eruptive products are all within the range of basalts from the vicinity of Fogo and Furnas volcanoes (Fig. 9). The Nd and Pb isotope signatures of the 1630 AD trachytes most likely represent that of the parental basalt from which they fractionated, as the very high concentrations of Nd and presumably Pb in the trachytes are difficult to overprint by crustal assimilation. The Sr isotopic compositions of the more evolved Furnas 1630 AD eruptive products are clearly affected by assimilation of a radiogenic contaminant, as demonstrated above. However, the trachyandesite and the least evolved trachyte (samples SM1630-7 and SM1630-31, respectively) have identical Sr isotopic compositions to one another despite having significantly different Sr concentrations (593 vs. 4 ppm, respectively). The extremely high Sr concentration in the trachyandesite cannot be easily overprinted by contamination, hence the Sr isotope signature of these samples ( 87 Sr/ 86 Sr = 0.70491) is also likely to represent that of the parental basalt. The uncontaminated Furnas 1630 AD trachytes and trachyandesite have Sr, Nd and Pb isotopic compositions identical to those of the ~ 3.8 ka Congro trachyte erupted on the eastern flank of Fogo volcano <5 km from Furnas caldera, suggesting that these trachytic magmas derived from basaltic parents that formed from melting of the same mantle source. The Furnas 1630 AD deposit is also identical to that of the ~4.6 ka Fogo A trachyte in Sr and Nd isotopes, and to the most recent Fogo 1563 AD deposit in Nd isotopes, but is slightly less radiogenic in Pb than these Fogo deposits (Fig. 9). This difference in Pb isotopic composition indicates that the Furnas 1630 AD magma emanated from a mantle source that was similar to, but compositionally distinct from, that of the Fogo magmas. The isotopic similarity between Furnas and Congro magmas, and the distinction between these and magmas erupted from the Fogo caldera, suggests a link between the plumbing system and/or mantle source beneath Furnas and the eastern flank of Fogo volcano, that is separate from that more directly beneath Fogo caldera.

10 In addition to the minor isotopic distinction between the recent Fogo and Furnas trachytes, major and trace elements in the Furnas 1630 AD deposit exhibit some features that are distinct compared to those in the Fogo A and 1563 AD trachyte deposits. Most notably, the Furnas 1630 AD trachytes range from silica saturated to slightly nepheline normative, whereas the Fogo trachytes range from silica saturated to slightly quartz normative (Fig. 10). Based on SiO 2 versus MgO trends, it is clear that this distinction in silica saturation is due to shallow fractionation processes rather than to a compositional difference between the respective basaltic parent magma compositions, as at higher MgO (>0.4%) the Fogo and Furnas trachytes trends converge in MgO-SiO 2 space (Fig. 5). The main difference in modeled fractionating assemblages between the Furnas 1630 AD and Fogo A deposits is a lower fraction of biotite in the fractionating assemblage of the Furnas trachytes, which serves to elevate the SiO 2 content of the fractionating assemblage thus reducing the increase in SiO 2 of the residual liquid. Less biotite in the fractionating assemblage of Furnas trachytes relative to Fogo trachytes could be attributed to lower volatile contents and/or deeper fractionation for the Furnas magmas. Another distinctive feature of the Furnas 1630 AD trachytes compared to the Fogo trachytes is the extremely low Sr concentration in the Furnas trachytes even at relatively high MgO (>0.4%). As with the difference in SiO 2, the difference in Sr is attributed to shallow level fractionation processes, as the Furnas trachyandesite has high Sr abundaces that are consistent with an extension of the Sr-MgO trend observed for the Fogo A trachytes (Fig. 6). The anomalously low Sr abundances in the Furnas trachytes are suggestive of more extensive sanidine fraction earlier in the trachyte magma evolution, perhaps linked to the lesser role of biotite.

5.5. Petrogenetic models for the evolution of the Furnas 1630 AD magmatic system

Many large silicic eruptions produce chemically zoned deposits in which there is a monotonic progression with depth in the deposit from the least evolved to most evolved magma compositions (e.g. Hildreth, 1981; Worner et al., 1984; Wolff et al., 1990; Widom et al., 1992; Watanabe et al., 2005). Such progressions are generally inferred to approximate the inverse of compositional stratification within the pre-eruptive magma chamber, resulting from relatively simple top down emptying of a magma chamber in which the most evolved and least dense magmas reside at the top of the chamber (e.g. Hildreth, 1981; Johnson, 1989; Wolff et al., 1990). The Furnas 1630 AD deposit exhibits a more complex but nevertheless systematic relationship between composition and stratigraphic position (Fig. 11). The first erupted magmas, produced during the alternating magmatic/phreatomagmatic phases (L1-L5), were overall intermediate in composition. However, within this period, the L1-L2 phases produced normal chemical zoning tapping more to less evolved magmas with time, followed by a period of reverse zoning during phases L3-L5, in which the magmas became progressively more evolved with time. This was followed by the Lf phase, a second period of normal zoning, in which the most evolved trachytes to the least evolved trachyte and trachyandesite magmas were erupted sequentially. The very final phase of magmatic activity involved the formation of the dome, similar compositionally to the least evolved Lf trachyte pumice samples. The complex temporal variation in erupted magma compositions exhibited in the Furnas 1630 AD deposit is in distinct contrast to the monotonic variations exhibited in the Fogo A and Fogo 1563 deposits (Widom et al., 1990, Snyder et al., 2004, Watanabe et al., 2005). The composition versus depth relationships observed in the Furnas 1630 AD deposit, however, have

11 the potential to help constrain models for the nature of the magmatic plumbing system and eruption mechanisms. We propose that irregular magma chamber geometry and/or multiple magma chambers could explain the complex but systematic eruptive sequence recorded in the Furnas 1630 AD deposit in which intermediate composition magmas erupted prior to the most evolved magmas. Figure 12 illustrates magma chamber geometries that could potentially produce the observed compositional sequence, including an irregular lobed chamber or a chamber whose roof geometry is controlled by faulting associated with prior caldera collapse, both of which provide pockets of more highly evolved magma that would be tapped after the main eruption is initiated assuming concentric withdrawal volumes (e.g. Blake, 1981; Johnson and Fridrich, 1990). Alternatively, scenarios involving multiple compositionally zoned chambers could also produce this effect. For example, two distinct magma bodies with different compositional ranges could have been tapped sequentially. In this scenario, the reverse zoning in phases L4-L5 may reflect mixing between magmas of two normally zoned reservoirs (one zoned from compositions L1- L3, another zoned from compositions of lower Lf - upper Lf). The possible existence of two magma bodies is supported by the observation of two nested pumice rings, which could be indicative of two vent locations for the Furnas 1630 AD eruption (Cole et al., 1995). The crystal-rich nature of the latest Lf and dome samples, and the petrographic disequilibrium textures in the trachyandesite and least evolved trachytes likely reflect intrusion of a less evolved magma into a crystal rich mush zone of an overlying trachytic chamber, which may have triggered the Furnas 1630 AD eruption (Fig. 12).

6. Conclusions

The eruptive products of the Furnas 1630 AD deposit range in composition from trachyte to trachyandesite, but most of the deposit volume consists of trachytes with relatively limited major element abundance variations. Essentially constant SiO 2, Al 2O3, K 2O and Fe 2O3 with moderate variations in MgO, P 2O5, CaO, Na 2O and TiO 2 concentrations and large variations in incompatible and compatible trace element concentrations are consistent with evolution of the trachytes via extensive crystallization near the pseudo-eutectic point on the Ab-Or join in the Quartz-Kalsilite-Nepheline system. Major and trace element modeling demonstrates that compositional variations within the trachyte suite are primarily controlled by fractional crystallization of a sanidine-dominated mineral assemblage with lesser clinopyroxene, biotite, ulvospinel and apatite, similar to the observed phenocryst assemblage. Constant Nd and Pb but variable Sr isotopic signatures in the 1630 AD eruptive products, including whole rock, glass and individual sanidine crystals, demonstrate that open system processes have also operated. In analogy to processes documented in the neighboring Fogo volcano, the Sr isotope heterogeneity is attributed to disaggregation and partial melting of seawater altered syenite from the magma chamber walls. Non-monotonic but systematic variations in composition with stratigraphic position documented in the Furnas 1630 AD deposit suggest a complex pre-eruptive magmatic history. The eruption of intermediate composition magmas prior to more evolved magmas, with period of reverse zoning separated by two phases of normal zoning, could be explained by irregular magma chamber geometry and/or multiple chambers. Late erupted less evolved crystal-rich trachytes and a trachyandesite whose composition is non-consanguineous with the trachytes,

12 suggests that the eruption may have been triggered by the intrusion of a trachyandesite magma into a crystal mush zone at the base of a chemically zoned trachytic magma chamber. Identical Sr, Nd and Pb isotopic signatures in the uncontaminated Furnas 1630 AD eruptive products and those from the recent Congro maar deposits indicate a shared mantle source and/or plumbing system for these volcanoes. In contrast, the Furnas 1630 AD trachytes have distinct Pb isotopic compositions compared to the recent trachytes from neighboring Fogo caldera, suggesting that the mantle source and magmatic plumbing system feeding the Fogo caldera is separate from that feeding Furnas. Differences in shallow magmatic fractionation trends between eruptive products from the two stratovolcanoes are also consistent with distinct plumbing systems.

13 Table 1. Major and trace element concentrations of pumice samples from the 1630 Furnas deposit. Major elements are in weight percent. Trace elements are in parts per million.

Sample 11 14 12 15 13 16 17B 17A 33 34 32 18 2 1 19 3 Unit L1 L1 L1 L2 L2 L2 L3 L3 L4/L5 L4/L5 L4/L5 Low Lf Low Lf Low Lf Low Lf Low Lf

SiO 2 63.39 62.82 63.52 63.43 63.45 63.19 63.50 63.41 63.35 63.24 63.45 63.56 63.74 63.00 63.56 63.58 TiO 2 0.50 0.49 0.51 0.50 0.50 0.51 0.51 0.50 0.51 0.51 0.51 0.47 0.46 0.46 0.48 0.46 Al 2O3 17.34 17.36 17.20 17.10 17.35 17.24 17.09 17.15 17.42 17.39 17.28 16.88 17.05 17.55 17.01 17.23 Fe 2O3 3.87 3.88 3.84 3.84 3.82 3.79 3.84 3.83 3.88 3.82 3.83 3.99 3.95 4.01 3.99 3.96 MnO 0.26 0.26 0.26 0.25 0.25 0.24 0.24 0.25 0.24 0.24 0.24 0.28 0.29 0.27 0.27 0.28 MgO 0.30 0.37 0.32 0.38 0.31 0.38 0.38 0.38 0.38 0.37 0.38 0.35 0.27 0.28 0.36 0.26 CaO 0.69 0.80 0.73 0.81 0.70 0.83 0.82 0.80 0.85 0.86 0.85 0.77 0.67 0.63 0.78 0.62 Na 2O 7.71 7.95 7.72 7.73 7.67 7.70 7.58 7.67 7.53 7.63 7.53 7.86 7.83 7.95 7.79 7.85 K2O 5.85 5.97 5.81 5.87 5.85 6.00 5.93 5.90 5.72 5.81 5.79 5.74 5.66 5.76 5.68 5.69 P2O5 0.09 0.09 0.09 0.10 0.10 0.10 0.11 0.10 0.12 0.12 0.12 0.09 0.08 0.10 0.09 0.08

Zn 185 144 181 179 177 133 154 191 181 138 161 183 175 165 168 211 Zr 1326 1244 1240 1190 1177 1067 1203 1145 1243 1202 1254 1429 1541 1554 1386 1697 Nb 232 222 229 231 229 184 220 214 221 221 235 261 272 259 249 271 Sr _ 1.12 ______0.95 Rb 228 222 202 200 198 187 196 191 193 201 207 231 238 243 242 271 Y 72 71 69 69 68 63 66 65 67 65 68 77 80 77 74 82

Sample 20 4 21 22 23 30 29 8 27 25 24 7 6 28 31 9 Unit Low Lf Low Lf Mid Lf Mid Lf Mid Lf Up Lf Up Lf Up Lf Up Lf Up Lf Up Lf Up Lf Up Lf Up Lf Dome Dome

SiO 2 63.21 62.85 63.38 63.12 63.25 63.05 63.68 63.40 63.38 63.53 63.92 57.59 63.67 63.13 63.21 63.11 TiO 2 0.46 0.45 0.47 0.47 0.53 0.52 0.52 0.52 0.54 0.53 0.53 1.75 0.52 0.61 0.60 0.61 Al 2O3 17.26 17.57 17.21 17.54 17.61 17.59 17.30 17.44 17.35 17.36 17.41 16.62 17.42 18.50 17.52 17.70 Fe 2O3 4.01 4.00 4.04 3.98 3.94 3.86 3.85 3.88 3.85 3.84 3.81 6.84 3.85 3.50 3.86 3.86 MnO 0.28 0.27 0.28 0.28 0.26 0.25 0.25 0.25 0.25 0.26 0.30 0.23 0.24 0.19 0.22 0.22 MgO 0.34 0.25 0.34 0.33 0.39 0.39 0.38 0.32 0.40 0.41 0.42 1.94 0.32 0.36 0.44 0.43 CaO 0.75 0.60 0.76 0.72 0.87 0.83 0.81 0.73 0.88 0.88 0.88 3.91 0.71 0.86 0.97 0.92 Na 2O 7.93 8.14 7.82 7.84 7.35 7.56 7.37 7.46 7.47 7.29 6.87 5.72 7.18 6.86 7.12 7.01 K2O 5.69 5.77 5.61 5.66 5.67 5.83 5.73 5.90 5.78 5.79 5.76 4.74 6.00 5.86 5.92 6.03 P2O5 0.08 0.09 0.09 0.07 0.12 0.11 0.12 0.10 0.11 0.11 0.10 0.66 0.10 0.12 0.14 0.11

Zn 177 171 186 207 145 190 176 165 152 172 151 146 162 134 137 157 Zr 1404 1606 1518 1332 1203 1238 1267 1149 1172 1046 1174 855 1168 1047 912 959 Nb 268 269 270 259 214 241 226 219 218 209 214 174 218 202 166 202 Sr 0.76 ______596.77 _ _ 4.25 _ Rb 221 255 238 217 190 206 208 211 210 210 182 160 200 167 159 210 Y 77 81 81 79 69 68 70 65 66 64 64 62 67 48 45 46

14 Table 2. Rare earth element concentrations of selected samples. Concentration in parts per million. Samples listed in order from most evolved to least evolved. Sample La Ce Nd Sm Eu Gd Dy Er Tm Yb Lu 20 196.5 373.2 132.9 20.8 1.0 18.5 14.3 8.0 1.1 7.2 1.1 3 196.7 367.6 133.5 21.3 1.0 18.9 14.6 8.1 1.2 7.2 1.1 14 175.4 332.8 122.5 19.6 1.1 17.4 13.3 7.4 1.0 6.5 1.0 14 rep 168.1 327.2 114.1 18.3 1.1 15.8 12.2 6.8 1.0 6.0 0.9 25 165.0 307.2 114.0 17.8 1.2 15.7 11.9 6.5 0.9 5.8 0.9 31 143.1 282.8 105.0 16.7 1.5 14.2 10.3 5.3 0.7 4.5 0.7 7 136.0 264.3 108.2 17.8 3.1 15.6 11.2 5.8 0.8 4.8 0.7

15 Table 3. Average chemical compositions of phenocrysts from selected samples. Major elements are in weight percent. # analyses represents the number of samples used to calculate the average chemical compositions. (san: sanidine; bte: biotite; cpx; clinopyroxene-augite; ox: oxide-ulvospinel) Trachyandesite Least evolved trachyte Intermediate Trachyte Evolved Trachyte SM1630-7 SM1630-31 SM1630-14 SM1630-3 mineral san bte cpx Ox san cpx ox san bte cpx ox san bte ox # analyses 5 2 4 6 5 9 3 3 2 1 3 5 2 3

SiO 2 65.71 37.46 52.35 0.17 66.59 51.90 0.15 65.64 37.06 51.77 0.17 67.24 36.31 0.18

TiO 2 0.03 5.98 0.45 18.49 0.05 0.48 6.06 0.03 5.72 0.49 16.37 0.04 5.48 16.14

Al 2O3 19.64 12.00 0.92 1.44 19.87 0.98 0.12 19.41 11.57 0.97 0.99 19.43 11.59 0.83 FeO 0.48 16.30 10.72 71.56 0.34 11.56 80.41 0.27 16.65 11.83 73.08 0.28 17.24 74.03 MnO 0.01 0.57 1.32 2.57 0.01 1.47 6.77 0.01 0.64 1.46 2.68 0.00 0.69 2.87 MgO 0.00 13.84 12.56 1.73 0.02 11.68 1.42 0.01 14.36 12.24 1.09 0.02 14.02 1.66 CaO 0.38 0.03 21.06 0.02 0.62 20.58 0.04 0.43 0.00 24.03 0.01 0.40 0.02 0.06

Na 2O 6.80 0.96 0.86 0.18 6.77 1.05 0.04 6.80 0.81 1.06 0.02 6.80 0.84 0.02

K2O 7.02 8.52 0.01 0.01 6.34 0.01 0.01 6.91 8.12 0.00 0.01 6.77 8.08 0.01 total 100.09 95.64 100.24 96.18 100.61 99.72 95.02 99.52 94.91 103.83 94.44 100.98 94.27 95.80

16

Table 4. Isotopic data for whole-rock trachytic pumice, glass, and sanidine samples from the Furnas 1630 AD deposit, Sao Miguel, Azores. Sr concentrations were determined by isotope dilution. Samples are arranged from most evolved to least evolved in descending order. WR, whole pumice; GL, glass; S, sanidine, UNL-unleached, 1N- 1N HCl leach, 2N- 2N HCl leach. Sample Sr 87 Sr/ 86 Sr 143 Nd/ 144 Nd 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb (ppm) SM1630-20 WR UNL 1.2 0.706056 SM1630-20 WR1 1N 0.75 0.705332 0.512722 19.937 15.761 40.196 SM1630-20 WR2 1N 0.705275 SM1630-20 WR3 1N 0.83 0.705277 SM1630-20 WR 2N 0.80 0.705324 SM1630-20 GL 0.47 0.705075 SM1630-20 S1 1.1 0.705210 SM1630-20 S2 6.7 0.704933 SM1630-20 S3 5.4 0.705030

SM1630-3 WR1 UNL 0.706056 SM1630-3 WR1 1N 0.705171 0.512718 19.900 15.719 40.050 SM1630-3 WR2 1N 0.705199 SM1630-3 WR3 1N 0.93 0.705146 SM1630-3 GL 0.55 0.705074 SM1630-3 S1 4.1 0.704949

SM1630-14 WR1 1.08 0.705016 0.512716 19.919 15.743 40.131 SM1630-14 WR2 19.921 15.746 40.140 SM1630-14 GL 0.81 0.704976 SM1630-14 S1 7.1 0.704918

SM1630-31 WR1 4.2 0.704922 0.512730 19.923 15.748 40.149 SM1630-31 WR2 0.704913

SM1630-7 WR1 593 0.704908 0.512725 19.893 15.713 40.026 SM1630-7 WR2 0.512729 19.906 15.729 40.087

17 Table 5. The calculated fractionating mineral assemblages and relative mineral proportions determining the major element modeling for the trachyandesite to least evolved trachyte using Igpet 2005 (Carr, 2005). Major elements are in weight percent. Total Fe was converted from Fe 2O3 ( from DCP-AES analysis) to FeO. Mineral abbreviations are explained in table 3; %FC percentage of fractional crystallization; RSOS resididual sum of squares; % Mineral percentage of fractionating mineral assemblage; calc: calculated parent acquired by Igpet from observed daughter. Least Trachyandesite  evolved trachyte (LET) % FC 58% RSOS 0.96

% Mineral san 67.8% cpx 19.1% ox 5.8% bte 1.7% ap 2.8%

Parent obs calc Daughter

SiO 2 57.99 58.49 63.46 TiO2 1.76 1.80 0.60 Al 2O3 16.73 15.33 17.59 FeO 6.20 6.20 3.49 MnO 0.23 0.51 0.22 MgO 1.95 1.83 0.44 CaO 3.94 3.93 0.97 Na 2O 5.76 5.80 7.15 K2O 4.77 5.34 5.94 P2O5 0.66 0.74 0.14 Total 99.99 99.97 100.00

18

Table 6. The calculated fractionating mineral assemblages and relative mineral proportions determining the major element modeling for the least evolved trachyte to most evolved trachyte using Igpet 2005 (Carr, 2005). Major elements are in weight percent. Total Fe was converted from Fe 2O3 ( from DCP-AES analysis) to FeO. (Abbreviations explained in table 3 & 5). Least evolved trachyte  Most evolved trachyte (SM1630-31) (SM1630-3) % FC 43% RSOS 0.02

Mineral % san 91.4% cpx 4.1% ox 3.2% bte 1.0% ap 0.3%

Parent obs calc Daughter

SiO 2 63.46 63.35 63.83 TiO 2 0.60 0.55 0.46 Al 2O3 17.59 17.61 17.30 FeO 3.49 3.50 3.58 MnO 0.22 0.23 0.28 MgO 0.44 0.44 0.26 CaO 0.97 1.00 0.62

Na 2O 7.15 7.20 7.88

K2O 5.94 6.02 5.71

P2O5 0.14 0.10 0.08 Total 100 100 100

19 Table 7. Results of trace element modeling. Formula to calculate Bulk D values: Bulk D = ∑ X a * (Kd)a where a= mineral phase; X= fraction of mineral in total crystallizing assemblage; and Kd=solid-liquid partition coefficient for the mineral. Kd values are taken from the literature (Mahood and Stimac, 1990; Villemant, l o (D-1) 1988; Luhr et al., 1984; Nagasawa, 1973; Nabelek, 1980). Formula to calculate the concentration of the element in the liquid after crystallization C = C * F where C o =concentration of element in initial liquid. SM1630-31 Measured; F= fraction of liquid remaining. (mineral abbreviations explained in table 3) Least evolved trachyte (SM1630-31) to Evolved trachyte (SM1630-3) Bulk D F SM1630-31 SM1630-3 SM1630-3 Kd san cpx bte Ox ap Bulk D Measured Calculated Measured Zr 0 0.05 0.09 0.25 2 0.03 Zr 0.03 0.571 900 1551 1670 Rb 0.25 0 0.82 0 0.4 0.24 Rb 0.24 0.571 157 240 246 Sr 3.5 0 0.49 0 8 3.26 Sr 3.26 0.571 3.45 0.97 0.99 La 0.1 0.69 0.57 0.07 27 0.40 La 0.40 0.571 143.1 200.3 196.7 Ce 0.15 0.48 0.032 0.07 31 0.47 Ce 0.47 0.571 282.8 380.6 367.6 Nd 0.1134 2.35 0.034 2.5 29 0.57 Nd 0.57 0.571 105.0 133.6 133.5 Sm 0.1 3 0.03 0.08 38 0.60 Sm 0.60 0.571 17.8 20.9 21.2 Eu 1.5 1.65 0.56 0.08 25.2 1.69 Eu 1.69 0.571 1.5 1.0 1.0 Gd 0.11 1.41 0.03 0.3 32 0.49 Gd 0.49 0.571 14.1 18.8 18.9 Dy 0.05 1.58 0.03 3 24 0.45 Dy 0.45 0.571 10.3 14.0 14.6 Er 0.05 1.5 0.03 1.6 14.1 0.30 Er 0.30 0.571 5.3 7.8 8.1 Yb 0.05 1.4 0.04 0.05 10 0.20 Yb 0.20 0.571 4.5 7.0 7.2 Lu 0.0032 2.68 0.21 0.08 7 0.19 Lu 0.19 0.571 0.7 1.1 1.1 FC 90.7% 4.1% 1.0% 3.2% 1.0% 100.0%

20

Figure 1. Map of the Azores archipelago and São Miguel island, which is located to the east of the MAR in the south-eastern region of the Azores platform (after Moore, 1991 and Snyder et al. 2004). The four stratovolcanoes on São Miguel (indicated by hachured curves representing calderas) include from west to east the active Sete Cidades, Fogo, and Furnas volcanoes, and the easternmost extinct Povoacoa volcano. Also shown is the location of the Congro maar, on the eastern flank of Fogo volcano. The waist zone between Fogo and Sete Cidades, as well as the region between Fogo and Furnas, are extensional zones of predominantly basaltic volcanism. basaltic volcanism (Moore, 1990). Recognized tectonic lineaments (linear dashed regions) include the NW-SE trending extension of the Terceira rift that passes through Sete Cidades, and an E-W lineament that passes through Fogo and Furnas (Moore at al., 1991; Beier et al., 2006).

21

Figure 2. Stratigraphic column of the Furnas 1630 AD deposit with a schematic representation of the eruptive phases (after Cole et al., 1995). Interbedded pumice and ash layers (L1-A5), represent the alternating magmatic and phreatomagmatic activity prior to the final explosive phase (Lf) which was purely magmatic. The final phase of the eruption involved dome extrusion.

22

Figure 3. Detailed map of Furnas volcano (after Cole at al., 1995) including the sampling sites of this study. The 1630 eruptive center and dome are indicated by the cross-hatched pattern. Sampling sites are indicated by stars with corresponding numbers. Samples from the L1-L3 phase were collected at site #1 (37º44.120´N, 25º20.222´W); samples from the L4/L5 eruptive phase are from localities #2 and #3 (37º43.93´N, 25º18.265´W and 37º44.649´N, 25º18.124´W); samples from the early and middle Lf phase are from localities #4 and #5 (37º45.806´N, 25º17.198´W and 37º45.940´N, 25º17.492´W), respectively; and the final phases of the eruption, including the latest Lf and dome were sampled at localities #6 and #7 (37º44.369´N, 25º19.287´W and 37º44.556´N, 25º19.664´W).

23

Figure 4. Alkalis vs. silica classification diagram (after LeBas et al., 1986) illustrating the composition of the Furnas 1630 AD eruptive products (blue circles). All of the samples plot in the trachyte field with ~63% SiO 2, with the exception of one trachyandesite sample with SiO 2=58%. Fogo 1563, Fogo A, and Congro fields are shown for comparison (data from Widom et al., 1992 and Watanabe et al., 2005).

24

Figure 5. Major element variation diagrams. The larger diagrams show the compositional variations of the Furnas 1630 AD trachytes, with insets that extend the compositional range to include the one trachyandesite sample. The trachyte samples exhibit measurable variations in CaO, Na 2O, and P2O5 that correlate with MgO, but no significant variation in SiO 2, Al 2O3, or K2O. The compositional ranges of the Fogo A, Fogo 1563 AD and Congro trachyte deposits are shown for comparison (data from Widom et al., 1992, Watanabe et al., 2005 and Watanabe et al., unpublished).

25

Figure 6. Trace element variation diagrams. The larger diagrams show the compositional variations of the Furnas 1630 AD trachytes, with insets that extend the compositional range to include the one trachyandesite sample. The trachyte samples exhibit large variations in incompatible and compatible trace element concentrations that correlate with MgO. Sr concentrations in the Furnas 1630 AD trachytes are extremely low compared to those of the Fogo and Congro deposits (data for Fogo A, Fogo 1563 and Congro from Widom et al., 1992, Watanabe et al., 2005 and Watanabe et al., unpublished).

26

Figure 7. Chondrite normalized rare earth element (REE) diagram showing Furnas 1630 AD samples with Fogo A trachyte samples for comparison. All samples are strongly light rare earth element (LREE) enriched with negative Eu anomalies. REE concentrations and the magnitude of the negative Eu anomalies increase with decreasing MgO in the Furnas 1630 trachyte samples. Chondrite normalization values based on Nakamura (1974). Fogo A data from Widom et al. (1992).

27

Figure 8. Photomicrographs illustrating petrographic characteristics of the Furnas 1630 AD samples. (A) A euhedral sanidine crystal in a glassy, vesicular matrix from one of the most evolved trachyte samples (SM-1630-3). (B) A sanidine exhibiting a reaction rim in a hypocrystalline matrix with randomly orientated microlites (trachyandesite sample SM-1630-7). (C) Dome material showing an embayed sanidine crystal in a holocrystalline matrix with pilotaxitic texture (sample SM-1630-31). Biotite inclusions can be seen in this sanidine.

28

Figure 9. Sr, Nd, Pb isotope variations in the 1630 Furnas samples. (A) Sr isotope variations in whole rock, glass separates and single sanidine crystals for five samples as indicated on the X- axis. (B) Sr isotope signatures versus Sr concentration for glass and whole pumice samples (symbols as in A). (C) – (E) Sr, Nd and Pb isotope ratios versus Zr concentrations for Furnas 1630 AD whole rock samples, with Fogo A, Fogo 1563, and Congro trachytes shown for comparison. The Furnas 1630 AD samples are similar to Congro samples, but slightly less radiogenic in 206 Pb/ 204 Pb than Fogo samples. The grey bars on diagrams D and E represent the ranges of the respective isotope ratios for basalts flanking Fogo and Furnas volcanoes. In diagram C, the range of Sr isotopic compositions of the flanking basalts spans the full range. Data for flanking basalts, Fogo A, Fogo 1563 and Congro from Widom et al., 1997; Snyder et al., 2004, Watanabe et al., 2005 and Watanabe et al., unpublished.

29

Figure 10. Ternary diagram representing the phase diagram of the quartz-nepheline-kalsilite system (after Schairer, 1950). Furnas 1630 AD samples plot close to the Ab-Or divide, in the vicinity of the temperature minimum that results in a pseudo-euctectic. The Furnas 1630 AD samples extend to slightly nepheline-normative compositions, in contrast to the Fogo trachytes which range from saturated to slightly quartz normative. Data for Fogo A, Fogo 1563 and Congro from Widom et al., 1992, Watanabe et al., 2005 and Watanabe et al., unpublished.

30

Figure 11. Zr concentration versus relative stratigraphic position for the Furnas 1630 AD samples. Eruptive phases are denoted on the right side of the diagram. From the bottom of the deposit upward, the deposit is “normally” zoned with samples become less evolved (lower Zr) upward during phase L1-L2; “reverse” zoning with an upward decrease in Zr is observed during phases L3-L5. The Lf phase again exhibits “normal” zoning from the most evolved to least evolved eruptive products, including the dome and trachyandesite samples in the last stages. The complex but systematic compositional zonation in the Furnas 1630 AD deposit is distinct from the monotonic normal zoning observed in the Fogo A and Fogo 1563 AD trachyte deposits (Widom et al., 1992 and Watanabe et al., 2005).

31

Figure 12. Cartoon diagrams illustrating magma chamber configurations that could yield the complex chemical zoning pattern observed in the Furnas 1630 AD deposit. Concentric tapping (e.g. Blake, 1981; Johnson and Fridrich, 1990) of irregularly shaped magma bodies (A) or magma bodies constrained by projections from previous caldera collapses (B) could result in eruption of intermediate composition magmas prior to more and subsequently less evolved magmas. Alternatively, multiple magma bodies, each compositionally zoned but with different compositional ranges, could also produce this effect. Cartoon A illustrates the injection of trachyandesite magma into a shallower zoned trachytic magma chamber with a basal crystal mush zone, which is inferred to explain the more mafic and crystal-rich products erupted in the late stages of the Furnas 1630 AD eruption.

32 References

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36