Abstract the 1630 Ad Eruption of Furnas Volcano, São
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ABSTRACT THE 1630 AD ERUPTION OF FURNAS VOLCANO, SÃO MIGUEL, AZORES (PORTUGAL): CHEMICAL VARIATIONS AND MAGMATIC PROCESSES by Andrea Rowland-Smith Furnas volcano, an active stratovolcano 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 rock, glass and individual sanidine crystals, suggest that fractionation was accompanied by assimilation of seawater altered syenite from the magma 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 mineral chemistry…………………………………………..5 4.3 Isotope systematics…………………………………………………………...5 Section 5. Discussion…………………………………………………………………….6 5.1 Formation of the Furnas 1630 AD trachytes………………………………….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 minerals from selected samples…..………16 Table 4. Isotopic data for whole-rock pumice, glass, and sanidine samples …….……...17 Table 5. Major element modeling from trachyandesite to least evolved trachyte………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 quartz-nepheline- 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 caldera-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 mantle plume 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