Melt Inclusions and Their Implications of an Interconnected Magma Chamber Beneath Cerro Negro Volcano and Las Pilas-El Hoyo Complex, Nicaragua

MELT INCLUSIONS AND THEIR IMPLICATIONS OF AN INTERCONNECTED MAGMA CHAMBER BENEATH CERRO NEGRO VOLCANO AND LAS PILAS-EL HOYO COMPLEX, NICARAGUA

by Swetha Venugopal B.Sc. Hons, Simon Fraser University, 2014

Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Bachelors of Science, Honours

in the Department of Earth Sciences Faculty of Science

1! ! APPROVAL

Name: Swetha Venugopal

Degree: Bachelors of Science, Honours

Title: Melt inclusions and their implications of an interconnected magma chamber beneath Cerro Negro Volcano and Las Pilas-El Hoyo Complex, Nicaragua.

Examining Committee: Chair: Dr. Shahin Dashtgard Associate Professor

Dr. Glyn Williams-Jones Senior Supervisor Associate Professor

Dr. Séverine Moune Supervisor Visiting Scholar

Kevin Cameron Internal Examiner Senior Lecturer

Date Defended/Approved:

2! ! PARTIAL COPYRIGHT LICENCE

3! ! ABSTRACT

Melt inclusions are tiny parcels of melt entrapped within crystals growing in a magma chamber. Their composition provides insight into the evolutionary history of the magma from the initial melt to eruption. Olivine- and pyroxene-hosted melt inclusions were analyzed from Cerro Negro Volcano and the Las Pilas-El Hoyo Complex in Nicaragua in order to determine their subsurface linkage. These systems are 1 km apart and have shown historical activity of coupled eruptions. Previous geochemical and geophysical studies have suggested that a shallow connection is likely; however, melt inclusion data will provide the deeper geochemical link. Data from the two volcanoes produce a linear compositional trend with similar magma sources, indicating an interconnected magma chamber. Cerro Negro data consistently define the primitive, volatile-rich end member of the trend whereas Las Pilas data are relatively evolved and represent the late stage, degassed melt. In order to accommodate these two compositions, four scenarios are proposed, each comprising a magma chamber extending to dyke that diverges towards the edifice of Cerro Negro and Las Pilas. The primary orientation of the dyke is towards Las Pilas as it is older than Cerro Negro; the branching of the dyke towards Cerro Negro follows the mechanism of dyke capture, where ascending magma preferentially follows the direction of minimum principle stress. Implications of a branching dyke beneath Cerro Negro include its proper classification as a young stratovolcano and an increase in eruptive intensity.

Keywords: Melt inclusions; Nicaragua; Cerro Negro; Las Pilas-El Hoyo

This thesis is dedicated to my hero Venugopal Ganesan, my role model Sandhya Venugopal and my sidekick Ishaan Ranjith.

5! ! Acknowledgments

I would like to thank my family for their unyielding support. Without your encouragement, devotion and late night coffee runs I would not know how to function. To my sister, I am grateful for your uncanny ability to know exactly what to say. A special thank you to my dad for trying to make me smile by cracking random jokes in typical dad-like fashion. To my supervisors Glyn Williams-Jones and Séverine Moune, thank you for giving me this wonderful opportunity and helping me every single step of the way. Your guidance means the world to me. To my friends, you each have your own amazing way of making me laugh and keeping me grounded. Without you, I would be a much more normal person leading a boring life; who would want that anyways? Lastly, I would like to thank Roger Waters, David Gilmour, Syd Barrett, Richard Wright, Nick Mason, Eric Church and Ella Fitzgerald for keeping me company during endless nights of writing, thinking, deleting and rewriting.

6! !

Table of Contents

Approval ...... 2 Partial Copyright Licence ...... 3 Abstract ...... 4 Acknowledgments...... 6 Table of Contents ...... 7 List of Tables ...... 8 List of Figures ...... 8 Introduction ...... 9 1.1 Background of the Central American Volcanic Belt ...... 9 Methodology ...... 21 2.1 Melt inclusions ...... 21 2.2 Sample description ...... 22 2.3 Melt Inclusion description ...... 24 2.4 Sample preparation ...... 26 Results ...... 32 3.1 Cerro Negro ...... 32 3.2 Las Pilas ...... 40 Discussion ...... 48 4.1 Major oxides and volatiles of Cerro Negro and Las Pilas ...... 48 4.2 Relevant prior work ...... 52 4.3 Magma composition during evolution ...... 56 4.4 Mechanisms of an interconnected magma chamber ...... 58 4.5 Implications for future eruptions at the Cerro Negro-Las Pilas-El Hoyo Complex ... 62 Conclusion ...... 63 5.1 Further research ...... 63 References ...... 64 Appendix 1 ...... 67 Uncorrected values for olivine-hosted inclusions from Cerro Negro ...... 67 Appendix 2 ...... 68 Additional data for Cerro Negro ...... 68 Additional whole rock data for Las Pilas ...... 74

7! ! List of Tables

Table 1 ……………………………………………………………………………………...… 27

List of Figures

Figure 1 The Central American Volcanic Belt………..………………………….....…….8 Figure 2 Regional variations along the CAVB.……..……………………………...…….9 Figure 3 Location of Cerro Negro within the Maribios Range in Nicaragua….…...11 Figure 4 Cerro Negro and its associated lava fields. ………..…………………...…...12 Figure 5!The Las Pilas-El Hoyo Complex ………………………………...………...…...14 Figure 6!Major oxides from whole rock analyses. McKnight (1995)…………...…...15 Figure 7 Location of positive density anomalies. MacQueen (2013)…...……...…...17 Figure 8 3D view of the modeled plumbing system beneath Cerro Negro and Las Pilas. MacQueen (2013).…………………………….……………….………………...…...18 Figure 9 Representative thin section from Cerro Negro, 1999. ………………...….. 21 Figure 10 Representative thin section of the 1528 eruption at Las Pilas….…...… 22 Figure 11 Olivine host with melt inclusions from Cerro Negro. ….……………...… 23 Figure 12 Pyroxene host with melt inclusions from Las Pilas....………………...… 24 Figure 13 Ternary and alkalinity plot of Cerro Negro and Las Pilas whole rocks. 32 Figure 14 Major oxides of melt inclusions versus the Fo content of host olivines ………………………………………………………………………………………………...34-5

Figure 15 Volatiles vs K2O wt % of melt inclusions and groundmass..……………37 Figure 16 Comparison of major oxides at Cerro Negro…………….……………...... 39

Figure 17 Volatile variations versus K2O…………...………………………………... 41-2 Figure 18 Comparison of major oxides at Las Pilas…………...…………………...... 44 Figure 19 Comparing the Mg# against FeOT and MgO…………..………………...… 45

Figure 20 Comparison of the major oxides vs K2O of all melt inclusions, groundmass and whole rock data from this study…………….…………………...… 48 Figure 21 Comparison of volatile contents of melt inclusions, groundmass and whole rock between Cerro Negro and Las Pilas…………….……………………...… 49 Figure 22 Major oxide comparison for all Cerro Negro and Las Pilas Data (including data)…………………………………………………………………………....… 52 Figure 23 Volatile contents of all Cerro Negro and Las Pilas data.……………...… 53

Figure 24 Comparison of MgO and K2O between all data compiled at Cerro Negro and Las Pilas…………….………………………………………………………………...… 55 Figure 25 The 4 cases proposed by Portnyagin et al. (2012) to explain the compositional variability between Cerro Negro rocks and melt inclusions…...… 56 Figure 26. Schematic models illustrating the potential plumbing system beneath Cerro Negro and Las Pilas.……………………………………………………………. 58-60

8! ! Introduction

1.1 Background of the Central American Volcanic Belt

The complex tectonic setting of Central America arises from the interaction between the Cocos, Caribbean and Nazca Plates and the Panama Block (Figure 1). At present, Central America is attached to the stable Caribbean plate to the east and bounded by the Middle American Trench, comprising the Cocos Ridge, to the west. The near-constant and oblique northeasterly subduction of Cocos Plate beneath the Caribbean plate at 10 cm/year leads to the Central American Volcanic Belt (CAVB). Extending from Guatemala to northern Panama, the 1100 km long belt contains 39 Quaternary stratovolcanoes, cinder cones, vent complexes and calderas (Figure 1; Walker and Carr, 1986). Overall, the Central American Volcanic Belt is atypical of most convergent margins. The volcanic chain is composed of many right stepping linear segments that are 100-300 km in length (Carr et al, 2003). Each segment trends approximately north-south and is characterized by a roughly linear chain of closely spaced volcanoes as well as near consistent regional topography and volcanic structure (Carr et al, 2003). One of the most distinguishing features of the Central American Volcanic Belt is a geochemical gradient along the volcanic front. This gradient comprises along strike differences in magmatic composition, density and edifice heights (Carr, 1984). The leading cause of the geochemical gradient is regional variations in crustal thickness.

9! !

Figure 1. The Central American Volcanic Belt. The Cocos Plate subducts beneath the Caribbean Plate at a rate of 10 cm/year to the northeast. Volcanoes are very closely spaced, on average 27 km, which distinguishes the CAVB from other convergent margin arcs. Adapted from Carr et al. (2003).

The Central American Volcanic Belt erupts andesites and basalts as well as the infrequent dacite. Andesites along this arc are likely derived from the prolonged crystal fractionation of basalts (Carr, 1984). In terms of magma composition, the highest silica contents are found at either end of the arc in Guatemala and Costa Rica with Nicaragua erupting the most mafic magmas (Carr, 1984). These variations can be explained by the non-uniform crustal thickness along the arc (Figure 2). Voluminous intrusive and extrusive magmas comprise the approximately 40-50 km thick crust beneath Guatemala and Costa Rica (Carr, 1984). Conversely, Tertiary and Cretaceous aged volcanics form the 20-30 km thick crust below Nicaragua (Carr, 1984). Assuming a common magmatic source, fluids exsolving off the subducting Cocos slab cause the reduction of the mantle peridotite solidus, which initiates melting. The initial magnesium-rich melts rise through the mantle wedge and become trapped at the Moho due to a sharp density difference between the uprising magma and the lower crust. The high-density magma ponds at the Moho and begins to differentiate and progressively decreases in density. When the density is sufficiently lowered, the magma rises through the crust, collects in a chamber and may eventually erupt. The degree of fractionation at the Moho

10! ! is what causes the geochemical gradient along the arc. Beneath Guatemala and Costa Rica, the thicker crust requires a highly differentiated, lower density, silica rich magma whereas a thinner crust beneath Nicaragua allows a less differentiated, denser mafic magma to rise; Nicaraguan magmas are 2.66 g/cm3 while Guatemalan magmas are 2.62 g/cm3 (Carr, 1984). Magmas beneath Nicaragua are then likely to be trapped by the upper crust and thus form shallow level intrusions and surficial lava lakes. Therefore, it can be concluded that crustal thickness leads to varying degrees of fractionation of magma and thus produces the magmatic diversity along the Central American Volcanic Belt.

Figure 2. Regional variations along the CAVB. Illustrating the regional variations in magma density, silica content, edifice heights and crustal thickness. Adapted from Carr (1984).

11! ! Nicaragua

Nicaragua is distinctive along the arc as having the most primitive and mafic magmas, the lowest degree of mantle melting, low edifice heights and smaller volcano volumes. In fact, Nicaraguan magmas record the global maximum of slab signals that trace the hemipelagic sediments of the Cocos slab, such as 10Be/9Be and Ba/La (Bundschuh, et al, 2007). Previous studies have attributed Nicaraguan magmas as having a distinct enriched mid-ocean ridge basaltic signature (Carr, 2003). Hydrostatic equilibrium is a major factor that balances Nicaragua’s thin crust, low edifice heights and dense magma. This equilibrium is maintained by the expansion of subsurface conduit systems with each explosive eruption (Carr, 1984). Another major factor that contributes to Nicaragua’s distinct geochemistry is the presence of extensional faults and fissures that allow for the rapid upwelling of volatile-rich magma through the crust. The Maribios Range, a volcanic segment in northwestern Nicaragua, hosts 38 volcanic edifices that appear to be structurally controlled. The oblique and rapid subduction of the Cocos Plate results in an east-west tensional regime that forms NNW trending normal faults (Bundschuh et al, 2007). Many volcanoes that lie along these extensional faults record tholeiitic fractionation patterns. Moving to the northwest and southeast, the olivine field decreases and is correlative with the increase in crustal thickness and edifice height (Carr, 1984). Nicaragua, based on its distinct mafic compositions and slab signals, is regarded as the primitive end member of the Central American Volcanic Belt.

Cerro Negro

Cerro Negro is the youngest volcano in the Central American Volcanic Belt. This polygenetic basaltic cinder cone first erupted in 1850 and has erupted numerous times since. Twenty four Strombolian to Subplinian eruptions have constructed a 250 m high black basaltic cone with an associated lava field that extends to the north-northeast (Figure 3 & 4; Hill et al, 1998). Past eruptions have been classified as having a Volcano Explosivity Index (VEI; Newhall and Self, 1982) of 0-3 and were frequent, at most occurring every 6 to 7 years. Cerro Negro lies in close proximity to villages and cities such as Leòn, which is Nicaragua’s second largest city and is located 20 km to the west (McKnight, 1995). Scattered within a 10 km radius of Cerro Negro are smaller villages and farmlands totaling to 100,000 people (Connor et al, 2001). Proximal to Cerro Negro, potential hazards include tephra fall, blocks and bombs, lava flows and carbon dioxide emissions. Historically, ash fall has been directed westwards towards Leòn due to dominant easterly trade winds. Ash accumulation can lead to building collapse,

12! ! mechanical damage and adverse health effects. Carbon dioxide levels have recently been monitored and determined to be passively degassing from the crater, fissures and soil but have yet to reach toxic levels (e.g., Salazer et al, 2001).

Figure 3. Location of Cerro Negro within the Maribios Range in Nicaragua. Amongst this tract of volcanoes, Cerro Negro (indicated) is the youngest and one of the most active. Taken from Portnyagin et al. (2012).

The classification of Cerro Negro as either a polygenetic cinder cone or a young stratovolcano has been debated in the past. A cinder cone is significantly shorter lived than a stratovolcano, but a stratovolcano would gradually increase its eruption intensity over time. Walker and Carr (1986), on the basis of mass eruption rates, regard Cerro Negro as a cinder cone on the Las Pilas-El Hoyo complex. Conversely, McKnight and Williams (1997) found that, based on cone morphology and particularly explosive eruptions, Cerro Negro should be categorized as a young stratovolcano. They stated that Cerro Negro would not achieve the typical stratovolcano height due to the thin crust beneath Nicaragua. Further lines of evidence for a stratovolcano lie in the tectonic environment, cone composition and dominant eruptive products. Cerro Negro has been built by its own scoria deposits and dominantly emits tephra and carbon dioxide gas as a result of its volatile rich magma. Overall, for long term studies, Cerro Negro should be classified as a young stratovolcano based on its increasing eruption intensity. In the short term, however, Cerro Negro should be regarded as a parasitic cinder cone

13! ! and potentially the newest conduit of the Las Pilas-El Hoyo Complex (McKnight and Williams, 1997; Hill et al., 1998).

Figure 4. Cerro Negro and its associated lava fields. The crater is at an elevation of 390 m.a.s.l. The 1999 flow is not shown but was relatively small scale. Taken from Hill et al. (1998).

Recent eruptive history

The 1992 eruption at Cerro Negro recorded the highest eruption column to date with a 7.5 km sustained ash column depositing ash as far as 30 km southwest from the main crater (Salazar et al, 2001). The total tephra fall volume was 2.3 x 107 m3 thus categorizing this as a VEI 3 Subplinian eruption (Connor et al, 2001). Heavy ashfall in Leòn resulted in building collapse and the evacuation of 12,000 people from the outskirts as well as neighboring villages (Connor et al, 2001).

The 79-day long eruption in 1995 comprised two phases of explosive activity, separated by a 95-day long period of unrest (Hill et al, 1998). Frequent eruptions from a new vent in the

14! ! 1992 crater culminated in a phreatomagmatic eruption that produced a small phreatic-driven pyroclastic surge. The second phase of the eruption produced a 300-m high ash column with occasional incandescent bombs ejected to an altitude of 200-300 m (McKnight, 1995). A 5 m high cone formed at the new vent and began to erupt basaltic lava that migrated northward at a rate of 10-30 m/hour (Hill et al, 1998). The estimated total volume of tephra fall and dense rock equivalent basalt was 2.16 x 106 m3 and 1.33 x 106 m3, respectively, and thus is classified as a VEI 2 Strombolian eruption.

The most recent eruption on August 5, 1999 was short-lived, having lower energy than past eruptions and preceded by large magnitude earthquakes, seismic swarms and noticeable surface ruptures (La Femina et al, 2004). Activity began with three earthquakes occurring in the vicinity of Cerro Negro. Seismic swarms then followed these earthquakes and were localized along the 1992 and 1995 vents. A 200-m long, 1 m wide fissure formed to the south of three new vents on the southern flank. Two new 40 m tall scoria cones were built by lava fountaining along the fissure. Small volume lava flows extruded from these cones and resulted in a combined volume of 6 x 105 m3 of basaltic lava. The total tephra fall volume was 8.4 x 105 m3, which categorizes this as a VEI 1 Strombolian eruption (Connor et al, 2001).

Geology & Geochemistry

Cerro Negro has the highest values of Ba/La, U/Th and the lowest La/Yb in Central America, implying a high degree of mantle melting (Portnyagin et al, 2012). The eruptive products at Cerro Negro have comparatively higher phenocryst content than other regional basalts (Walker and Carr, 1986). The phenocrysts are large, between 1-7 mm, and dominated by plagioclase, olivine and pyroxene. The basaltic magma can contain up to 6% water content, which creates explosive eruptions (Portnyagin et al, 2012). Basalts are effusive and low viscosity, allowing for the segregation of gases and the settling of phenocrysts from the magma. This results in gas rich explosions from the main vent and gas poor effusions from fissures along the flanks of the volcano. Through its evolution, there have been no significant compositional changes, which could imply a stable magma chamber (Walker and Carr, 1986).

Las Pilas- El Hoyo Complex

Las Pilas is the main edifice of the Las Pilas-El Hoyo Complex, which lies 1 km SE of Cerro Negro and hosts many adjacent and overlapping eruptive centers (Figure 5; McBirney, 1955). With only 3 confirmed eruptions, between VEI 1-2, the lack of activity at Las Pilas

15! ! juxtaposes its location within one of the most active volcanic arcs in the world. The 1952 eruptions at Las Pilas, the first since its apparent formation in 1528, followed an eruption at Cerro Negro (McBirney, 1955). Although no unusual activity was documented around the complex, on October 23, 1952 the summit of Las Pilas was divided by a 1 km long fissure trending due north-south. From the fissure erupted dense clouds of steam, gas and dust (McBirney, 1955). Around parts of the fissure were fresh deposits of yellow sulphur as well as fumaroles emitting low temperature gases. Over time, no vertical or horizontal displacement was recorded along the fissure. Interestingly, the 1954 eruption at Cerro Negro preceded another explosion at Las Pilas by a few months. This eruption was mild, sending ash 20 km to the west in Leon (McBirney, 1955). It has been suggested that the1954 eruption at Cerro Negro caused a pressure increase within the magma chamber, thus causing Las Pilas to erupt once more (McBirney, 1955). The pattern of coupled eruptions at Cerro Negro and Las Pilas hint at a common magma chamber with extending dykes and conduits feeding each system (McKnight, 1995).

Figure 5. The Las Pilas-El Hoyo Complex Illustration of the Las Pilas-El Hoyo Complex and its numerous volcanic edifices. Cerro Negro can be seen to the northwest. Adapted from Bice (1980).

16! ! Interconnected plumbing system

McKnight (1995) investigated the potential for a link between Cerro Negro and Las Pilas using geochemical evidence. He reasoned that since Cerro Negro is located at the intersection of the regional N70W tensional regime and a shorter volcanic lineament trending N25W, eruptions must be structurally controlled and the eruptive state of Cerro Negro is dependent upon changes in the stress regime. Based on whole rock data from 1850 to 1971, a linear trend of major oxides was noticed between Las Pilas, Cerro Negro and Cerro La Mula, which lies 1 km north of Cerro Negro. Based on major oxide diagrams of SiO2, MgO, K2O and Al2O3, the general trends of the Las Pilas intersect at high angles to the Cerro Negro trend, with the intersection occurring near the mafic end member (Figure 6). This intersection, as well as the similar contents of incompatible elements, suggests that Cerro Negro is part of the Las Pilas complex and the two share a similar magma source. However, Cerro Negro data consistently plots within the mafic end member of the trend, implying a unique fractionation process controlling the final composition of Cerro Negro (McKnight, 1995). This process was proposed to be phenocryst sorting occurring by relative density within the conduit. During magma ascent, less dense plagioclase crystals would rise to the top of the conduit whereas denser olivine, pyroxene and other plagioclase crystals would settle near the bottom. Phenocryst sorting is plausible within low viscosity basaltic magmas and is also dependent upon eruption rate. A higher eruption rate, such as the 1992 eruption, would inhibit phenocryst sorting during rapid ascent. Conversely, a more effusive eruption, such as in 1968, would allow for a greater degree of phenocryst sorting and lead to a greater compositional difference (McKnight, 1995). Overall, based on a shared magmatic source, McKnight (1995) concluded that Cerro Negro should be considered the newest conduit on the Las Pilas complex. He also reasoned that the increase in eruptive frequency at Cerro Negro after 1947 could be due to the development of a more efficient conduit that allows larger volumes of melt to ascend.

17! !

Figure 6. Major oxides from whole rock analyses. Major oxide diagrams from McKnight’s (1995) analyses of whole rocks from Cerro Negro; Cerro Negro rocks consistently plot towards the mafic end member. Las Pilas rocks (indicated by X’s) appear to intersect the Cerro Negro trend (large open circles) at high angles.

Recent work by MacQueen (2013) provides geophysical support for the petrogenetic link proposed by McKnight (1995). Bouguer gravity data from Cerro Negro was collected in order to image the subsurface structures. Modelling of these data revealed local density anomalies related to the magmatic plumbing system. Local anomalies include a 2 km3 positive density anomaly 1 km to the SE of Cerro Negro, connected to a 1 km3 positive density anomaly below Cerro La Mula and to a 7 km3 positive density anomaly beneath Las Pilas (MacQueen, 2013). Referring to the progressive appearances of these density anomalies, the base of Cerro Negro is located at 450 m above sea level (Figure 7). Down to a depth of 1500 m, the density anomalies of Cerro Negro and Las Pilas are independent and robust (Figure 7d). Below this depth, however, the two anomalies unite, indicating the presence of an interconnected system at 2 km (Figure 7 a, b, c). The Cerro Negro anomaly also plots along the regional NNW trend of fractures, which supports the proposal of structurally controlled dykes.

18! !

Figure 7. Location of positive density anomalies. Changes in the behavior of the positive density anomalies with depth between Cerro Negro and Las Pilas. From MacQueen (2013).

Overall, these findings further suggest that Cerro Negro and Las Pilas tap an interconnected magma chamber beneath Las Pilas and that the density anomalies represent shallow intrusive complexes feeding each volcanic edifice (Figure 8; MacQueen, 2013). The depth of the Cerro Negro and Las Pilas anomaly is in agreement with the 1-2 km magma storage depth of the 1995 magma proposed by Roggensack (1997). This correlation implies that the chambers imaged by the density anomalies may actually be storage sites for magma sourced from depth (MacQueen, 2013). Therefore, the shared anomalies, extending to 2 km depth, could represent a shared pathway for magma before separating along two different paths towards each edifice. The original magma source must be deeper than 8 km, which is the maximum depth imaged by the survey.

19! !

Figure 8. 3D view of the modeled plumbing system beneath Cerro Negro and Las Pilas A model of density anomalies and its implication of a proposed secondary magma site beneath Las Pilas. Cerro Negro is indicated by a red triangle. From MacQueen (2013).

The focus of this study will be to investigate the proposals by McKnight (1995) and McQueen (2013) regarding an interconnected magma chamber beneath Las Pilas by using geochemical evidence to determine the deeper, lower crustal link between Cerro Negro and Las Pilas.

20! ! Methodology

2.1 Melt inclusions

The imperfect growth of phenocrysts during magma crystallization results in the capture of small amounts of melt. These inclusions are smaller than the host mineral, between 1 to 300 µm in size, and vary from spherical to elongate in form (Roedder, 1984). Ideally, melt inclusions, once entrapped, are protected from changes in temperature and pressure by their host mineral and thereby represent a closed system. Therefore, melt inclusions provide information about the temperature, pressure and composition of the magma at depth and also during ascent. Subsequent eruption will cause the inclusions to quench quickly, thus preserving the composition and volatile content of the inclusion’s silicate melt prior to any fractionation or degassing processes. Melt inclusions differ from groundmass, which is glass that represents quenching of the late stage, degassed and modified melt during eruption.

There are two main types of melt inclusions, primary and secondary. Primary inclusions form during initial crystal growth whereas secondary inclusions are trapped later and within annealed fractures (Roedder, 1984). Most studies regarding melt inclusions rely on primary inclusions since their composition is directly related to the melt from which the host minerals crystallize and thus provide information to considerable depths.

Once entrapped, melt inclusions can undergo modification, which is mainly controlled by temperature. Due to the thermal contraction of the melt relative to the host mineral, shrinkage bubbles form within the inclusion. Shrinkage bubbles are smaller than the inclusion itself and usually account for less than 3 % by volume (Metrich and Wallace, 2008). As the magma evolves and cools, the internal pressure within the shrinkage bubble may change, causing the bubble to fill with exsolved H2O or CO2 gas. The condensation of these gases within the melt during magma ascent may react with the inclusion, causing further modification. Additional changes may arise due to the fracturing of phenocrysts during explosive eruptions. These fractures can intercept melt inclusions and facilitate leakage of volatile elements and gases. During effusive eruptions, however, slower cooling rates can cause crystallization of the host mineral along the inclusion rim and of daughter minerals within the inclusion itself. Finally, a diffusive exchange of elements between the host mineral and the melt inclusions can also lead to a compositional change within the inclusion and the host mineral.

21! ! Overall, melt inclusions can form in most crystals during magma crystallization. However, as the magma differentiates progressive crystal formation will trap a more evolved melt. Therefore, mafic minerals, such as olivine and pyroxene, will trap a more primitive melt than later-stage minerals. Primary inclusions of primitive melts closely represent the composition of the parental, volatile bearing magma and are thus invaluable for melt inclusion studies. Knowing the composition of the primitive melt and the late-stage melt from groundmass provides information about the evolutionary pathway of the magma and its degassing history.

This study will focus on primary melt inclusions trapped within olivine crystals, since they are the first phenocrysts to crystallize from a relatively primitive magma. In cases where no viable melt inclusions are found in olivine crystals, pyroxene-hosted inclusions will be studied. Though pyroxene crystallizes later than olivine and evidently traps more evolved melt, these inclusions will nonetheless provide valuable information about the evolution of magma and its intermediary composition between primitive and eruptive.

2.2 Sample description

Whole Rock

Whole rock samples from both Cerro Negro and Las Pilas were collected in 2013 by Jeffery Zurek and Patricia MacQueen. Cerro Negro samples were from the 1999 eruption whereas Las Pilas samples are believed to be from the 1528 eruption.

Cerro Negro

The samples are fresh, dark grey to black, vesicular subalkaline basalts (48.5 wt % SiO2) that are distinctively low in K2O (0.44 wt %) and high in MgO (7.27 wt %). Phenocrysts are 1-4 mm and dominated by plagioclase (An70, 15 % by volume), olivine (Fo82 – Fo77, 10 %) and pyroxene (10 %). The average amount of phenocrysts in the sample is estimated at 25 %. Most phenocrysts are fragmented, with some being subhedral and occurring as groups (glomeroporphyritic). Both plagioclase and pyroxene phenocrysts are twinned and also occur as microlites in the finer grained groundmass. Some pyroxenes exhibit weak light brown to pale green pleiochroism. The presence of small magnetite grains was evident in the crushed sample.

22! !

Figure 9. Representative thin section from Cerro Negro, 1999. Phenocrysts are labeled. Olv = olivine, plag = plagioclase, pyx = pyroxene. Cross polars (XPL); magnification 5x; width of the photo is 3.02 mm.

Las Pilas

Las Pilas tephras were collected close to the edifice from the 1528 eruption. These samples are weathered, brown vesicular subalkaline basalts (50.94 wt % SiO2) with a much larger grain size than Cerro Negro. Phenocrysts are 1-7 mm and dominated by plagioclase

(An33- An37, 20 % by volume), pyroxene (En64 – En82, 15 %) and olivine (1-5 %). The average amount of phenocrysts by volume is estimated at 35 %. Phenocrysts are glomeroporphyritic and the groundmass contains plagioclase microlites.

23! !

Figure 10. Representative thin section of the 1528 eruption at Las Pilas Representative thin section from Las Pilas 1528. Phenocrysts are labeled. Olv = olivine, plag = plagioclase, pyx = pyroxene. Cross polars (XPL); magnification 2x; width of the photo is 7.57 mm.

2.3 Melt Inclusion description

Cerro Negro

Melt inclusions from Cerro Negro olivines occur either as single inclusions or as groups with a larger inclusion found among several smaller ones. All inclusions are glassy and range in size from a few microns up to 200 microns. Larger inclusions are more likely to have a fine wall faceting than smaller ones. On average, inclusions occupied a volume of less than 3 % and are elliptical to oblate in shape. Almost all inclusions are distinctively darker than their host olivine and most contain a single shrinkage bubble either near the center of the inclusion or near the margin. Some inclusions contain 2 smaller bubbles but only one sample contained a rim of up to 5 smaller shrinkage bubbles. The shrinkage bubbles occupied less than 2 % by volume and on average 1 %. Melt inclusions with fractures and daughter minerals were sparse and avoided.

24! !

Figure 11. Olivine host with melt inclusions from Cerro Negro. Sample 99C. Fo content of the olivine host is 78. Numerous melt inclusions are present, however, the top 2 were analyzed.

Las Pilas

Since olivine phenocrysts did not contain any noticeable melt inclusions, pyroxene- hosted melt inclusions were studied instead. A plethora of small inclusions are available in most pyroxene crystals. All inclusions are glassy, of similar colour to the host pyroxene and a few microns in diameter on average. Inclusions are irregularly shaped, but those sampled were elongate and usually contain a single shrinkage bubble near the margin. The shrinkage bubbles occupy less than 0.8 % by volume. Daughter minerals were rare and melt inclusions with such minerals were not sampled.

25! !

Figure 12. Pyroxene host with melt inclusions from Las Pilas. Sample LP-E. En content of the pyroxene is 70. Numerous melt inclusions are present, however only a few were large enough for analysis.

2.4 Sample preparation

Melt inclusions were prepared at the Cordilleran Geology Lab in the Department of Earth Sciences at Simon Fraser University. Sample preparation was the same for Cerro Negro olivines and Las Pilas pyroxenes.

Bulk rocks were crushed to a median grain size of 3 mm. Olivine crystals from Cerro Negro and pyroxene crystals from Las Pilas were picked from these grains using a binocular microscope. Crystals were then placed within a mound of liquid silica onto glass slides; each glass slide contained up to 18 crystals. Once cooled, the glass slides were examined under a petrographic microscope. Inclusions were chosen in host minerals free of any fractures or daughter minerals. Inclusions tended to be small (200 microns or less in size) and difficult to detect. Once all inclusion-bearing minerals were marked, they were carefully removed from the glass slide using a heating plate. These crystals were then washed with acetone and placed within a brass holder using epoxy. The surfaces of the holders containing the crystal were then

26! ! carefully and individually polished using a range of 30 to 1 µm pads. Once a representative exposure of the desired inclusions was obtained, the holders were placed into stainless steel pucks with 8 slots. All of the holders were set such that the surface of the puck was even and contained all exposed inclusions. Each puck was then polished using a 0.3 µm alumina polishing compound.

Sample analysis

Electron microprobe

The major and volatile elemental compositions of melt inclusions, host crystals and matrix glasses were analyzed at the Laboratoire Magmas et Volcans (LMV, Clermont-Ferrand, France) using a SX-100 CAMECA electron microprobe (EMP) and a 15 kV accelerating voltage. Calibration was based on a mixture of glasses and synthetic or natural mineral standards. Mineral analyses were done using a 1 µm focused beam, which widened to a 5-10 µm during glass analyses to reduce Na loss. Volatile analyses were measured with an 80 nA sample current and a 50 s acquisition time using the LPET diffraction crystal for Cl and S and a 300 s acquisition with TAP crystals for F. To reduce volatile loss during analyses, measurements were taken at 20 second intervals. During sulphur analysis, variations in the wavelength of sulphur Kα X-ray, a function of its oxidation state in silicate glasses, were considered. The precision of the

EMP is better than 5 % for major elements excluding MnO, Na2O, K2O and P2O5, which had an EMP precision < 10 %. The approximate 2σ precision for Cl, S and F is 7 %, 4 % and 28 %, respectively. Major element compositions of the whole rocks were analyzed by ICP AES at LMV. The associated errors are approximately 10 %.

Every melt inclusion, host crystal and matrix glass was measured more than once to ensure a reliable range of data. Melt inclusions were sampled at least twice; most were sampled three times. Host crystals were also analyzed at least twice to ensure compositional homogeneity; one analysis was done near the rim of the inclusion and the other away from the inclusion. If the host crystal contained more than one inclusion, as was the case for some olivines and all pyroxenes, each inclusion corresponded to two analyses of the host crystal. Groundmass measurements were preformed five times and whole rocks were measured once. Analyzed values are displayed in Table 1.

27! ! Post entrapment modifications

Melt inclusions were chosen such that they exhibited no obvious compositional changes following entrapment. However, modification due to crystallization of the host mineral and diffusive exchanges is difficult to avoid and must be compensated by performing calculations in order to restore the inclusion to its original composition. These calculations are well known and established for olivine-hosted inclusions and will be applied here. However, calculations based on pyroxene-hosted inclusions have yet to be perfected as they contain large errors and thus will not be preformed. Nonetheless, these inclusions still provide valuable information of magma evolution.

Olivine-melt equilibrium constant

To determine the original composition of the melt inclusion, the inclusion is corrected for any olivine host crystallization. This calculation is based on KD, the olivine-melt equilibrium constant between ferrous iron and magnesium, which is well established at 0.3 ± 0.03 (Roeder and Emslie, 1970). Prior to any corrections, the KD values of our melt inclusions ranged between 0.15 – 0.34. By incrementally adding 0.1 wt % of olivine into the melt, the equilibrium composition between the host and inclusion was determined. The amount of post-entrapment crystallization (PEC) of olivine ranged between 0 and 11 %, with most inclusions less than 5 %. PEC corrected values and corrected melt inclusion compositions are found in Table 1.

Diffusive exchange

Further modifications arise due to diffusive exchange of Fe and Mg during crystallization and equilibration of an olivine overgrown along the inclusion rim. However, such compositional changes would result in KD values significantly higher than 0.3, which was not the case. Additionally, total FeO (FeOT) of melt inclusions would be much lower than the host rock and the expected composition of the initially trapped inclusion. Within Cerro Negro rocks, Fe loss was minimal as shown by the high FeOT content of the melt inclusion (8.9-14.1 wt %) encompassing the FeOT of the whole rock (11.14 wt %).

28! ! Table 1 Cerro Negro All oxides are reported in weight percent.

T Melt Inclusion SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 S (ppm) Cl (ppm) F (ppm) Cl/K2O Mg# Kd PEC % CN-1 99A MI-1 46.258 0.767 16.511 11.695 0.050 7.110 11.121 1.976 0.313 0.198 1449.90 1111.32 24.44 0.355 51.997 0.30 10 CN-1 99A MI-2 45.514 0.713 16.183 12.827 0.182 7.626 10.847 1.824 0.522 0.091 - - - - 51.439 0.30 10 CN-1 99B 48.061 0.907 16.348 10.669 0.122 7.398 10.666 2.210 0.461 0.247 998.64 96.82 60.42 0.021 55.269 0.30 5 CN-1 99C MI-1 46.517 0.921 16.483 10.821 0.135 7.161 10.751 1.958 0.358 0.152 1213.63 756.56 146.02 0.211 54.109 0.30 2 CN-1 99C MI-2 46.334 0.793 16.791 10.634 0.276 6.928 11.046 1.881 0.378 0.269 1225.00 794.19 64.68 0.210 53.720 0.30 2 CN-1 99D 46.001 0.795 17.080 10.918 0.234 8.082 11.435 1.845 0.253 1.285 1157.85 699.55 189.72 0.276 56.876 0.30 7 CN-1 99E MI-1 45.318 0.778 17.216 11.323 0.319 7.179 11.258 1.786 0.242 0.105 1435.97 845.57 117.12 0.349 53.043 0.30 4 CN-1 99E MI-2 46.247 0.703 17.751 11.215 0.184 7.182 11.719 1.675 0.265 0.070 1462.23 795.76 226.31 0.301 53.295 0.30 1 CN-1 99F MI-1 45.619 0.811 16.448 14.113 0.230 7.434 10.260 2.105 0.313 0.085 1517.09 1291.39 204.88 0.413 48.414 0.30 11 CN-1 99G 45.324 1.130 18.269 9.253 0.168 6.512 11.284 2.145 0.336 0.208 1378.92 796.40 266.33 0.237 55.631 0.30 0.1 CN-1 99H 46.462 0.723 16.496 11.907 0.315 7.346 10.471 1.693 0.341 0.171 1541.98 952.01 67.90 0.279 52.362 0.30 7.5 CN2 99J MI-1 46.929 0.788 18.521 8.986 0.155 6.470 11.428 2.017 0.312 0.119 1392.60 761.40 79.20 0.244 56.194 0.31 0 CN2 99J MI-2 44.580 1.666 17.348 10.751 0.162 7.678 11.202 1.981 0.378 0.309 1110.84 783.18 155.01 0.207 55.995 0.30 3

CN2 99K 46.402 0.775 16.777 10.451 0.137 7.509 11.220 1.356 0.255 0.128 1364.16 802.62 138.18 0.315 56.143 0.30 2

CN2 99L 46.468 0.760 17.047 10.568 0.072 6.295 10.294 1.929 0.307 0.172 1611.77 721.95 237.72 0.235 51.485 0.30 0.2

CN2 99N 46.634 0.791 17.119 9.730 0.192 6.678 11.091 1.952 0.360 0.139 1277.20 810.40 17.00 0.225 55.014 0.34 0 CN2 99O 46.017 0.789 17.547 10.294 0.100 7.500 10.808 2.068 0.312 0.101 1264.86 663.48 89.10 0.212 56.487 0.30 10 T Host Olivine SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Fo content Mg#

CN-1 99A 39.362 0.011 0.022 20.247 0.274 40.533 0.190 0.002 0.011 78.111 78.103

CN-1 99B 39.190 0.010 0.031 17.813 0.373 41.682 0.217 0.002 0.001 80.662 80.655 CN-1 99C 39.302 0.019 0.036 20.364 0.344 40.459 0.195 0.000 0.004 77.972 77.974 CN-1 99D 39.739 0.011 0.030 17.342 0.332 43.078 0.173 0.006 0.001 81.577 81.570 CN-1 99E 39.251 0.004 0.016 19.344 0.302 41.057 0.195 0.015 0.000 79.095 79.087

CN-1 99F 38.487 0.004 0.044 21.817 0.312 38.397 0.187 0.033 0.002 75.830 75.821

CN-1 99G 39.284 0.000 0.040 17.742 0.138 42.147 0.194 0.000 0.000 80.896 80.889 CN-1 99H 39.238 0.000 0.000 19.882 0.357 40.843 0.176 0.020 0.001 78.550 78.542 CN-2 99J 39.004 0.015 0.023 18.261 0.252 42.790 0.191 0.000 0.001 80.684 80.677 CN-2 99K 39.292 0.018 0.002 18.049 0.290 43.066 0.197 0.000 0.000 80.964 80.958

CN-2 99L 38.466 0.006 0.030 20.242 0.322 40.774 0.184 0.006 0.000 78.216 78.209

CN-2 99N 38.919 0.012 0.018 20.035 0.238 40.879 0.225 0.014 0.000 78.435 78.427 CN-2 99O 38.602 0.018 0.021 17.681 0.291 42.534 0.178 0.002 0.011 81.090 81.083

! 29!

T Groundmass SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Cl (ppm) F (ppm) S (ppm) GM1 56.641 1.7259 12.5373 13.9301 0.2422 3.3994 7.7782 2.8333 1.039 0.2738 1146 479.4 7.2 GM2 54.877 1.5921 12.5126 14.6382 0.4343 3.5324 8.0152 2.2539 0.9781 0.2488 1076 537.6 12.8 GM3 55.408 1.5951 12.7688 13.6135 0.3343 3.2629 7.6307 3.2918 1.1145 0.2645 1221.8 454.4 23.2 GM4 56.039 1.6363 12.4317 14.0556 0.3341 3.8269 7.6319 2.7703 1.0259 0.2805 1350.2 526.8 5.6 GM5 54.869 1.7168 12.8924 14.4865 0.459 3.4224 7.9068 2.1738 1.0436 0.1613 1187.2 460.6 23.8

T Whole Rock SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 WR-1 48.5 0.72 17.59 11.14 0.19 7.27 12.23 1.86 0.44 0.11

Las Pilas

T Melt Inclusion SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total Mg# LPA MI-1 52.332 1.235 15.727 12.757 0.145 3.762 7.864 2.967 0.991 0.186 97.967 34.445 LPA MI-2 51.208 1.034 18.482 10.039 0.331 3.849 6.813 3.030 0.572 0.261 95.617 40.583 LPA MI-3 55.265 1.327 17.115 9.387 0.179 2.688 6.297 3.475 1.440 0.398 97.570 33.782 LPA MI-4 54.695 1.513 16.634 11.142 0.185 2.366 6.390 2.855 1.283 0.326 97.389 27.449 LPB MI-1 55.622 1.381 13.527 11.010 0.282 3.973 8.111 2.597 1.197 0.315 98.014 39.132 LPB MI-2 51.683 1.348 13.326 13.195 0.354 4.841 8.707 2.433 0.844 0.233 96.964 39.528 LPB MI-3 55.215 1.208 14.031 10.489 0.346 4.331 8.682 2.888 0.958 0.138 98.285 42.383 LPB MI-4 52.697 1.151 15.078 9.898 0.184 4.805 8.826 3.174 0.988 0.218 97.019 46.376 LPC MI-1 51.529 1.243 15.778 12.199 0.279 4.534 8.505 2.600 0.803 0.201 97.669 39.839 LPD MI-1 54.541 1.384 13.145 13.895 0.555 4.056 8.430 2.663 1.200 0.357 100.224 34.213 LPD MI-2 52.435 1.545 11.879 17.630 0.313 3.106 7.387 2.473 0.908 0.136 97.812 23.889 LPE MI-1 55.167 1.300 13.480 12.708 0.312 3.482 7.079 3.196 1.339 0.242 98.304 32.808 LPE MI-2 56.221 0.976 14.529 10.320 0.360 3.637 7.091 3.440 1.501 0.308 98.383 38.572 LPE MI-3 51.778 1.203 14.575 12.507 0.392 4.122 8.225 2.942 0.990 0.266 96.999 36.994 LPF MI-1 51.953 1.156 14.984 11.045 0.256 4.074 8.048 2.937 1.117 0.342 95.912 39.657 LPF MI-2 52.220 1.043 14.811 12.099 0.065 4.744 8.578 2.723 0.981 0.258 97.523 41.129 LPG MI-1 58.417 1.034 14.694 8.865 0.208 2.986 6.504 3.591 2.201 0.315 98.815 37.506 LPH MI-1 49.965 1.668 13.129 16.562 0.334 3.873 8.293 2.749 0.611 0.112 97.294 29.409 LPH MI-2 50.317 1.578 14.888 15.136 0.606 3.724 7.913 2.570 0.663 0.145 97.539 30.476 LPH M-3 49.992 1.544 15.325 14.867 0.418 4.365 7.888 2.504 0.527 0.205 97.635 34.343

! 30! Melt Inclusion Cl (ppm) F (ppm) S (ppm) LPA MI-1 1064.2 472.8 643.6 LPA MI-2 1113.600 521.600 1273.800 LPA MI-3 1227.800 362.200 375.200 LPA MI-4 1177.400 337.200 335.000 LPB MI-1 991.200 310.800 366.800 LPB MI-2 863.600 263.800 473.000 LPB MI-3 835.400 197.200 453.600 LPB MI-4 987.200 285.000 594.200 LPC MI-1 859.400 240.200 859.800 LPD MI-2 922.400 310.600 352.800 LPE MI-1 1399.000 343.800 547.000 LPE MI-2 1543.400 564.200 567.600 LPF MI-2 821.400 254.400 440.600 LPG MI-1 1769.400 696.600 370.400 LPH MI-2 919.200 538.800 712.400

T Host Pyroxene SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Mg# En content LPA 51.402 0.352 3.043 6.218 0.154 16.264 21.263 0.219 0.004 15.878 82.315 LPB 51.246 0.417 2.048 12.157 0.337 14.789 18.234 0.229 0.000 14.020 68.409 LPC 51.404 0.460 4.482 6.445 0.123 15.892 21.130 0.220 0.004 15.442 81.466 LPD 51.428 0.516 1.753 13.843 0.484 14.101 17.761 0.281 0.000 13.130 64.485 LPE 51.636 0.465 2.045 10.998 0.433 15.045 18.935 0.282 0.009 14.150 70.918 LPF 51.047 0.484 3.122 9.347 0.325 15.885 18.995 0.251 0.000 15.087 75.183 LPG 51.121 0.456 2.107 10.111 0.328 14.833 19.728 0.231 0.005 13.998 72.337 LPH 51.479 0.486 2.487 9.925 0.299 15.225 19.786 0.249 0.002 14.442 73.152

T Groundmass SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Cl (ppm) F (ppm) S (ppm) GM1 54.405 1.3666 14.2492 10.3816 0.3048 4.6273 8.3919 3.0364 1.4039 0.3492 98.5167 749.2 562.6 GM2 53.869 1.2479 14.868 11.4399 0.2155 4.6577 8.7724 3.3245 1.3183 0.1942 99.9076 744.8 575.4 GM3 53.892 1.2982 14.731 11.7217 0.2953 4.9708 8.9335 2.7428 1.1563 0.2345 99.9762 764.8 268.6 GM4 53.41 1.3275 15.026 11.4212 0.2549 4.9243 8.9456 2.6671 1.1776 0.162 99.3193 699.2 427.8 GM5 53.148 1.3281 14.9477 11.5676 0.1413 4.8469 8.9927 2.7518 1.1546 0.1937 99.0732 706.4 615.8

T Whole rock SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 WR-1 50.94 0.88 16.64 10.63 0.19 5.94 10.98 2.45 0.8 0.19

! 31! Results

3.1 Cerro Negro

In total, 17 melt inclusions from 13 minerals were analyzed from Cerro Negro. Melt inclusion compositions corrected for post entrapment crystallization can be found in Table 1; uncorrected values are shown in Appendix 1.

The sampled whole rock from Cerro Negro has major oxide content of 48.5 wt % SiO2,

7.27 wt % MgO and a total alkali content (Na2O + K2O) of 2.30 wt %, thereby classifying the rocks as relatively primitive, subalklaine basalts (Figure 13). Inclusion bearing olivines comprise a narrow Forsterite range (Fo75- Fo81.5), which implies crystallization occurred within a limited temperature interval (Portnyagin et al, 2012).

Figure 13. Ternary and alkalinity plot of Cerro Negro and Las Pilas whole rocks. a) Ternary plot of whole rock data from Cerro Negro (blue) and Las Pilas (red). A= total alkalis (Na2O wt % + K2O wt %). F= total iron (Fe2O3 wt % + FeO wt %). M= total magnesium (MgO wt %). b) Both Cerro Negro and Las Pilas are classified as subalkaline magmas, specifically calc-alkaline. Ternary plot template from Marshall (1996).

32! ! b) Cerro#Negro# Las#Pilas#

!12!! !11!! !10!! !9!!

O%(wt%%)% !8!! 2 !7!! !6!! O%wt%%+%K 2 !5!! Na !4!! !3!! !2!! 35# 40# 45# 50# 55# 60# 65# SiO %(wt%%)% 2 Figure 13. Continued from previous page

Melt inclusions from the same olivine host do not vary considerably in major element and volatile contents, the variations are within errors of 4 % S, 7 % Cl and 28 % F. This suggests that melt inclusions sufficiently represent the melt trapped at the time of mineral crystallization. Volatile contents of inclusions do not show an obvious relationship with the presence of a shrinkage bubble, which implies that the volatile content was not modified by post entrapment diffusion into the shrinkage bubble. However, the two inclusions CNJ-1 and CNJ-2 show slight variations in the host Fo content (78.5 vs 80.6), major oxides (46.9-44.5 wt % SiO2, 0.78-1.66 wt T % TiO2, 18.5-17.34 wt % Al2O3, 89-10.7 wt % FeO ) and in F concentration (79.2-115 ppm).

Additionally, CNJ-1 also contained the largest shrinkage bubble and the highest SiO2 and Al2O3 T but the lowest TiO2, FeO and F content (Table 1). The cause for this variation is a compositional zonation and irregular growth within the host olivine, causing entrapment of two different melts; CNJ-1 likely was the later melt since its composition is relatively evolved.

Melt inclusions have well constrained oxide contents and are basaltic in composition: T 44.5-48 wt % SiO2, 16.1-18.5 wt % Al2O3, 8.9-14.1 wt % FeO , 6.2-8.1 wt % MgO and 0.24-0.52 wt % K2O. The Fo content of host olivines and the MgO content of melt inclusions correlate positively with Al2O3 and CaO and negatively with SiO2, Na2O, K2O and P2O5 of inclusions. Since the latter oxides increase in concentration with magmatic evolution, this pattern implies that more primitive olivine hosts (Fo>78) have trapped more primitive melts (MgO >7 wt%). A

33! ! strong negative correlation exists between the Fo content of the olivine host and FeOT content of the inclusion (Figure 14). The most primitive olivine host (Fo81.5) contains some of the lowest

FeO, CaO and K2O contents sampled (17.34 wt %, 0.173 wt % and 0.0011 wt %, respectively). The corresponding melt inclusion contains some of the highest MgO and CaO contents (8.08 wt % and 11.43 wt %, respectively). Conversely, the composition of the melt hosted within the least primitive olivine (Fo75) contains some of the lowest content of most oxides; 75 wt % SiO2, 16.4 T wt % Al2O3, 7.43 wt % MgO and 10.3 wt % CaO but contain the highest FeO (10.4 wt %).

a) Portnyagin#MI# CN#1999#MI# PRIM# EVOL#

56!

54!

52! (wt %) (wt 2 50!

48! Melt SiO Melt

46!

44! 68# 70# 72# 74# 76# 78# 80# 82# 84# Olivine Fo

b) 16!

15!

14!

13!

12!

11! Melt FeO (wt %) (wt FeO Melt 10!

8! 68# 70# 72# 74# 76# 78# 80# 82# 84# Olivine Fo

34! ! c) 14!

13!

12!

11!

10!

Melt CaO (wt %) CaO(wt Melt 9!

7! 68# 70# 72# 74# 76# 78# 80# 82# 84# Olivine Fo

d) 1!

1! O (wt %) (wt O 2 0! Melt K Melt

0! 68# 70# 72# 74# 76# 78# 80# 82# 84# Olivine Fo

Figure 14. Major oxides of melt inclusions versus the Fo content of host olivines. Melt inclusion data, including data from Portnyagin et al. (2012). Graphs a, b and d exhibit a negative correlation between Fo content and SiO2, FeO and K2O. This shows that primitive olivines (Fo>78) trap less evolved melts. Graph c shows a positive correlation exists between the Fo content of the host olivine and the CaO content of the melt inclusions. The values of PRIM and EVOL represent the most primitive and evolved melt inclusion compositions, respectively (discussed below).

Volatile concentrations (S, Cl and F) of inclusions range between 998-1611 ppm, 96- 1291 ppm, and 17-266 ppm, respectively. The relative enrichment, especially of sulphur,

35! ! supports the fact that the melt inclusions represent either a partially degassed or non-degassed magma as S, Cl and F are incompatible elements and thus increase in concentration within the melt during differentiation. The most primitive inclusions, hosted in olivines with Fo>78, contain an average of 1325 ppm S, 759 ppm Cl and 125 ppm F. The most primitive melt inclusion, hosted in olivine Fo81.5, contain lower S and Cl (1157 ppm and 699 ppm, respectively) and higher F (189 ppm) than average. There is a rough negative correlation between inclusion contents of Cl and host olivine Fo such that the inclusion with the highest Cl content was trapped within the least primitive olivine (1291 ppm Cl and Fo75). Additionally, variations of volatile contents relative to the K2O content of melt inclusions and groundmass illustrate the process of degassing between primitive and evolved melts during ascent and olivine crystallization (Figure 15). Similar to Figure 14, a trend can be delineated from relatively volatile rich melt inclusions towards the degassed and evolved groundmass. Referring to Figure 15a, the primitive melt, represented by the melt inclusions, are initially sulphur rich and progressively deplete during magma evolution. Figure 15b, c show the opposite; the primitive melt becomes enriched in Cl and F with evolution indicating incompatible behavior. The average Cl/K2O ratio is 0.25, which is typical of Nicaraguan arc basalts (Ezra et al, 2007). The decrease in sulphur and the increase in chlorine and fluorine during differentiation are due to respective solubilities. The solubility of Cl increases with the ratio of Na + K/Al whereas F is known to be very soluble in silicate melts (Wallace and Anderson, 2000).

a) CN99_MI CN99_GM

250# S (ppm) (ppm) S

1# 0# 0.2# 0.4# 0.6# 0.8# 1# 1.2# 1.4#

K2O (wt %)

36! ! b) 2000#

1500#

1000# Cl (ppm) (ppm) Cl

500#

0# 0# 0.2# 0.4# 0.6# 0.8# 1# 1.2# 1.4#

K2O (wt %)

c) 700#

600#

500#

400#

300# F (ppm) (ppm) F

200#

100#

0# 0# 0.2# 0.4# 0.6# 0.8# 1# 1.2# 1.4#

K2O (wt %)

Figure 15. Volatiles vs K2O wt % of melt inclusions and groundmass a) S (log scale, ppm) vs K2O wt %. Melt inclusion values are between 1000ppm and 1610 ppm. The primitive melt (melt inclusion) is volatile rich compared to the degassed evolved melt (groundmass). b) Cl (ppm) vs K2O wt %. c) F (ppm) vs K2O wt %. Enrichment of both Cl and F during magma evolution illustrates their incompatible behavior. Error bars are 2σ.

Groundmass values plot within the expected trend such that they are the most evolved amongst whole rock and melt inclusion data (Table 1). Groundmass composition vary from 54.8- T 56.6 wt % SiO2, 12.4-12.8 wt % Al2O3, 13.6-14.6 wt % FeO , 3.2-3.8 wt % MgO, 7.6-8 wt % CaO and 0.97-1.11 wt % K2O; these narrow ranges imply that the melt has a uniform composition.

Compared to the whole rock, groundmass glass has higher contents of SiO2 and CaO but lower

37! ! T values of MgO, Al2O3, FeO and K2O. Compared to melt inclusions, groundmass glass has higher T contents of SiO2, FeO and K2O but lower concentrations of Al2O3 and MgO. The average S content (14 ppm) is lower than the least primitive olivine host, implying considerable sulphur degassing from the melt during fractionation. However, the average Cl and F content, 759 ppm and 491 ppm, respectively, are higher than primitive olivine hosts (Fo>78), implying minimal Cl and F fractionation during magma ascent.

Major oxides of Cerro Negro

Figure 16 compares the major oxides against K2O contents of melt inclusions, whole rock and groundmass at Cerro Negro. A strong correlation can be seen between all oxides such that the melt inclusion data represents the primitive end member of magmatic evolution. Groundmass values consistently plot towards the evolved, fractionated portion of the trend. Whole rock data deviates from its theoretical plot near groundmass data and instead plots near the melt inclusions. A distinct magmatic pathway from primitive to evolved can be discerned from the melt inclusions and whole rock towards the groundmass.

38! !

Figure 16. Comparison of major oxides at Cerro Negro. Melt inclusions (blue diamond), groundmass (cyan triangles) and whole rock (orange square) compared to K2O wt%. There is a distinct compositional difference between the melt inclusion and whole rock trend T with the groundmass data. The groundmass is enriched in SiO2, TiO2 and FeO relative to melt inclusions (a, b, c). Conversely, melt inclusions are enriched in Al2O3. In each graph, the groundmass is more evolved (i.e. K2O rich) than the melt inclusions (i.e. K2O poor). Melt inclusions consistently define the primitive end member of the compositional trend. Whole rock data plot close to melt inclusions, suggesting primitive (or high Mg) basalt. Error bars are 2σ. See Table 1 for details.

39! !

Figure 16. Continued from previous page. Melt inclusions are enriched in CaO and MgO compared to groundmass (e, g). Both melt inclusions and groundmass contain similar Na2O and P2O5 contents. Melt inclusions consistently define the primitive end member (i.e. K2O poor) compared to the evolved groundmass. 3.2 Las Pilas

In total, 20 melt inclusions from 8 pyroxene crystals were analyzed; compositions are shown in Table 1. Las Pilas whole rock data contain 50.94 wt % SiO2, 5.94 wt % MgO and a total alkali content of 3.25 wt %, classifying Las Pilas as an evolved subalkaline basalt (Figure

13). Pyroxene hosts have a relatively broad range of enstatite and Mg#, En64-En82 and 23-46 respectively, suggesting a wide crystallization interval during magma ascent.

T Melt inclusions contain 49.9-58.4 wt % SiO2, 11.8-18.4 wt % Al2O3, 8.8-17.6 wt % FeO ,

2.4-4.8 wt % MgO, 6.3-8.8 wt % CaO and 0.5-2.2 wt % K2O. The melts are basaltic, similar to T the whole rock. The En content of the host mineral is positively correlated with SiO2, FeO , CaO and K2O but negatively with Al2O3 and MgO. The former increases in concentration with evolution whereas the latter deplete. Thus this pattern shows that more primitive pyroxenes

40! ! (En>72) trap a more primitive melt (MgO >3 wt %). The highest En content host (En82) does not appear to have any relative enrichments or depletions of oxides, which could be a result of post entrapment crystallization.

Volatile contents of melt inclusions ranged from 335-1273 ppm S, 821-1769 ppm Cl and 197-696 ppm F. The average volatile contents of inclusions trapped within least modified pyroxenes (En>72) were 1189 ppm Cl, 612 ppm S and 433 ppm F. Inclusions within evolved pyroxenes (En<70) contained 919 ppm Cl, 448 ppm S and 273 ppm F, on average and the inclusion with the highest S content was hosted within the pyroxene with the greatest En content (Table 1). As can be seen, decreasing the En and MgO wt % content of the host pyroxene correlates to a decrease in volatiles within melt inclusions (Figure 17). The melt inclusion LP-G contained the highest Cl and F values as well as the highest SiO2 (58.4 wt %) and K2O (2.2 wt %) and lowest MgO (2.9 wt %) contents suggesting minimal Cl and F fractionation during magma ascent.

Variations of volatile contents relative to the K2O wt % content illustrate the degassing history tracked by both melt inclusions and groundmass (Figure 17). Referring to S (ppm) versus K2O wt % (Figure 17a), there is a distinct drop in sulphur content from melt inclusions to groundmass. A similar trend of decreasing volatile content during magma evolution is shown by the Cl (ppm) content versus K2O wt % (Figure 17b). However, unlike sulphur there is no distinct jump but rather a smooth decrease in Cl (ppm) from melt inclusions to groundmass. Fluorine data is more variable but appears to be in similar concentrations between melt inclusions and groundmass.

41! ! a) LP_MI LP_GM 1400#

1200#

1000#

800# S (ppm) (ppm) S 600#

400#

200#

0# 0.4# 0.6# 0.8# 1# 1.2# 1.4# 1.6# 1.8# 2# 2.2# 2.4# 2.6# K O (wt %) 2 b) 1800#

1500#

1200#

900# Cl (ppm) (ppm) Cl

600#

300#

0# 0.4# 0.6# 0.8# 1# 1.2# 1.4# 1.6# 1.8# 2# 2.2# 2.4# 2.6#

K2O (wt %)

Figure 17. Volatile variations versus K2O. a) S (ppm) vs K2O wt % b) Cl (ppm) vs K2O wt % c) F (ppm) vs K2O wt % of Las Pilas melt inclusions (green triangle) and groundmass (red circle). Degassing trends can be delineated based on S and Cl content (a and b). Melt inclusions are S and Cl rich and progressively deplete towards the groundmass values. The relatively similar F concentrations between melt inclusion and groundmass data imply that there was minimal F fractionation during magmatic evolution.

42! ! c) 700#

600#

500#

400#

F (ppm) (ppm) F 300#

200#

100#

0# 0.4# 0.6# 0.8# 1# 1.2# 1.4# 1.6# 1.8# 2# 2.2# 2.4# 2.6#

K2O (wt %)

Figure 17. Continued from previous page.

Melt inclusions hosted in the same pyroxene crystal show minimal variations in volatile or major elements contents. The exception is pyroxene LPA, which hosts 4 melt inclusions. Variations are shown in FeOT content (9.3-12.7 wt %) and Mg# (27.4-40.5), but the most significant is the S content varying from the highest value recorded to the lowest (335 ppm and 1273 ppm S), contained within one crystal (Table 1). This pyroxene crystal is likely zoned and trapped different melts during growth. Additionally, inclusions without shrinkage bubbles (LPB and LPF) contain the lowest Cl and F contents (821 ppm and 197 ppm, respectively). In order to determine whether post entrapment diffusion is an important process, the volatile contents should be lowest in inclusions with a shrinkage bubble. Since this is not the case, diffusion likely did not significantly modify volatile contents within inclusions.

The range of major oxides within the groundmass are: 53.1-54.4 wt % SiO2, 10.3-11.7 wt T % FeO , 4.6-4.9 wt % MgO, 8.3-8.9 wt % CaO and 1.2-1.4 wt % K2O. This narrow range shows a homogeneous composition. Comparing to whole rock values, groundmass values are relatively enriched in SiO2 and K2O but depleted in MgO and CaO. Compared to melt inclusions, the groundmass is relatively enriched in SiO2, Al2O3, FeO and CaO and depleted in MgO and

K2O. Unlike Cerro Negro, Las Pilas whole rock, groundmass and melt inclusion oxides are all very similar (Figure 18). However, the volatile contents within the groundmass are distinct: 19- 59 ppm S, 699-764 ppm Cl, and 268-615 ppm F.

43! ! Major oxides of Las Pilas

Figure 7 compares the major oxides of melt inclusion, groundmass and whole rock data against K2O wt % at Las Pilas. The strongest positive correlation is seen between CaO wt % and K2O wt %. This correlation is interesting because the melt inclusion, groundmass and whole rock data are scattered along the trend, unlike Cerro Negro, which shows two distinct clusters. A rough negative correlation can be delineated based on the contents of SiO2, TiO2 and K2O wt % versus K2O wt % (Figure 18). Referring to SiO2 wt % versus K2O wt %, this trend shows that the pyroxenes have trapped progressively differentiated melt.

Figure 18. Comparison of major oxides at Las Pilas. Melt inclusion (green triangle), groundmass (red circle) and whole rock (red triangle) at Las Pilas. The melt inclusion data define the relatively primitive end member of the compositional trend (i.e. K2O poor). Since groundmass data represent the late stage, fractionated melt, areas where groundmass and melt inclusion data overlap suggest the host pyroxenes crystallized from an evolved melt. Error bars are 2σ.

44! !

Figure 18. Continued from previous page.

Comparing the MgO wt% and FeOT wt% against the Mg# of melt inclusions, groundmass and whole rock data reaffirms the trends seen from the major oxide diagrams: many melt inclusions plot away from groundmass and whole rocks data towards the evolved end member (Figure 19). This suggests that inclusions with MgO concentrations of 3 wt% or less and Mg# less than 37 have undergone compositional modifications after entrapment or were trapped at a later time. The difference between groundmass and the melt inclusion trend implies that post entrapment crystallization is a dominant factor in the variability within melt inclusions and that their apparent evolved state is not only due to later entrapment. Conversely, the group of melt inclusion near the groundmass and whole rock data could also reflect the compositional variability of the melt during entrapment. This implies that pyroxene was able to continue crystallizing from an evolving and degassing melt.

45! ! a) MI! GM! WR! 7! 6.5! 6! 5.5! 5! 4.5! 4! MgO (wt %) (wt MgO 3.5! 3! 2.5! 2! 20# 25# 30# 35# 40# 45# 50# 55# Mg#

b) 20!

18!

16! (wt %) (wt T 14! FeO 12!

10!

8! 20# 25# 30# 35# 40# 45# 50# 55# Mg#

Figure 19. Comparing the Mg# of melt inclusions against FeOT wt % and MgO wt % of melt inclusions (blue diamond), groundmass (orange square) and whole rock data (green triangle) at Las Pilas. The melt inclusions that plot near the groundmass and whole rock data are in equilibrium. Melt inclusions that plot away from this cluster (with Mg# <37 and MgO <3 wt%) likely have undergone post entrapment modifications. Error bars are 2σ.

Do the melt inclusions represent undegassed magma at depth?

The olivine-hosted melt inclusions from Cerro Negro represent a partially degassed to an undegassed melt based on the correlation between the volatiles and incompatible element abundances such as K2O. The relatively high average sulphur content (1325 ppm) within olivine-hosted melt inclusions supports the undegassed state of the melt (Streck and Wacaster,

2006). The average Cl/K2O ratio at Cerro Negro is 0.25, which is similar to the Cl/K2O ratio of

46! ! Nicaragua (0.2-0.4; Sadofsky et al., 2008). Pyroxene-hosted melt inclusions, however, crystallized from a partially degassed melt.

Additional data

Additional whole rock, groundmass and melt inclusion data from the 1867, 1971 and 1992 Cerro Negro eruptions are taken from Roggensack (2001), Sadofsky et al. (2008) and Portnyagin et al. (2012). These data are PEC corrected using the models of Danyushevsky and Plechov (2011) and Borisov and Shapkin (1990) and reported in Appendix 2. Additional whole rock data for Las Pilas was available from the database of Carr et al. (2013) and are shown in Appendix 2.

47! ! Discussion

4.1 Major oxides and volatiles of Cerro Negro and Las Pilas

Comparing the major oxides and volatile contents between these two systems reveals a similar pathway and evolutionary trend for ascending magma. Figure 20 illustrates the variations in oxide contents plotted against K2O wt % for all melt inclusions, groundmass and whole rock data for Cerro Negro and Las Pilas. A very strong trend can be delineated from a primitive melt at < 0.6 K2O wt % towards a more evolved, K2O-enriched melt. The most striking pattern is the consistency of melt inclusions from Cerro Negro to represent the mafic end member while Las

Pilas melt inclusions define the evolved end member. This is best seen on the plot of SiO2 wt % and MgO wt % versus K2O wt %. Since the Cerro Negro melt inclusions define a tight cluster, the system likely stopped crystallizing olivine past a threshold of 0.6 wt % K2O and began crystallizing pyroxene. The few overlapping melt inclusions from Cerro Negro and Las Pilas provide the general compositional range of the melt at the point of co-crystallization of olivine T and pyroxene: 0.71-1.66 wt % TiO2, 1.8-3.01 wt % Na2O, 10-16.6 wt % FeO and 15.3-18.1 wt

% Al2O3.

Sulphur degassing is well tracked by this system showing a clear trend of sulphur rich, olivine-hosted melt inclusions towards a sulphur poor, degassed melt represented by pyroxene –hosted inclusions. Additionally, the sulphur content within the groundmass of both Cerro Negro and Las Pilas are exceedingly similar. Overall it can be seen that the Las Pilas data fills in the gap between Cerro Negro melt inclusions and groundmass values, implying that Las Pilas represents the intermediary composition. In terms of volatile contents, the degassing trend is clearly shown between the olivine-hosted melt inclusions towards the groundmass of Las Pilas (Figure 21). The major oxides and volatile contents of both olivine-hosted and pyroxene-hosted melt inclusions define a robust trend of magma fractionation from depth to the surface. Therefore, Cerro Negro and Las Pilas magmas likely have the same primitive source. However, Cerro Negro has compositional variability between its melt inclusion, groundmass and whole rock data that is not seen within Las Pilas.

48! !

Figure 20. Comparison of the major oxides vs K2O of all melt inclusions, groundmass and whole rock data from this study. Cerro Negro melt inclusions consistently plot at the primitive end member whereas the evolved end member is represented by Las Pilas groundmass and melt inclusion data. The overlap between the most evolved Cerro Negro melt inclusions and the least evolved Las Pilas melt inclusions could provide the composition of the melt at the point where olivine stopped crystallizing and pyroxene began. Cerro Negro groundmass data have a similar composition as melt inclusions, groundmass and whole rock data of Las Pilas, implying the same evolved melt composition. Error bars are 2σ.

49! !

Figure 20. Continued from previous page.

50! ! 1750# CN99_MI LP_MI LP_GM CN99_GM 1500# 1250# 1000# 750#

S (ppm) (ppm) S 500# 250# 0# 0.2# 0.4# 0.6# 0.8# 1# 1.2# 1.4# 1.6# 1.8# 2# 2.2# 2.4# 2.6# K O (wt %) 2 2000#

1500#

1000#

Cl (ppm) (ppm) Cl 500#

0# 0.2# 0.4# 0.6# 0.8# 1# 1.2# 1.4# 1.6# 1.8# 2# 2.2# 2.4# 2.6# K O (wt %) 2 700# 600# 500# 400# 300#

F (ppm) (ppm) F 200# 100# 0# 0.2# 0.4# 0.6# 0.8# 1# 1.2# 1.4# 1.6# 1.8# 2# 2.2# 2.4# K O (wt%) 2 Figure 21. Comparison of volatile contents of melt inclusions, groundmass and whole rock between Cerro Negro and Las Pilas. Increasing K2O wt% indicates further magmatic evolution. Cerro Negro melt inclusions consistently define the degassed, primitive end member whereas Las Pilas groundmass marks the evolved, degassed end member. The gap between Cerro Negro melt inclusions and groundmass is filled in by Las Pilas data, implying they track the same magmatic evolution (similar to Figure 20). Error bars are 2σ.

51! ! 4.2 Relevant prior work

Olivine-hosted melt inclusions from Cerro Negro have been previously studied by Portnyagin et al. (2012). In this study, samples from the 1992 eruption were analyzed and reported with past data from Sadofsky et al. (2008) and Roggensack (2001). The dataset for the 1867, 1971, 1992 and 1995 tephra and lava samples will be used to compare with the 1999 data from this study.

Portnyagin et al. (2012) examined major oxides and volatile content from olivine-hosted melt inclusions. The olivines were slightly more primitive (Fo72 – Fo82) than those used in this study. Crystallization was estimated to begin at 450 MPa, which corresponds to a depth of 14 km, and continued until near-surface depths beneath Cerro Negro (Portnyagin et al, 2012). Similar to our findings, the whole rock data defined a separate array that intersected the melt inclusion data and ranged from high Mg to low Mg basalts. Essentially, the whole rock composition was not represented by the melt trapped within the inclusions.

The magma pathway is not constant for Cerro Negro, as shown by the 1992 and 1995 eruptions during which magma was emplaced within the mid-lower crust and the upper crust, respectively (Roggensack et al, 1997). Therefore, H2O-induced crystallization during decompression is adopted in order to explain the trend of crystallization at decreasing pressure and increasing liquidus temperatures.

Cerro Negro

Cerro Negro data from this study strongly align with that of Portnyagin et al. (2012) showing a temporally homogeneous magma beneath Cerro Negro. In terms of an array of high Mg and low Mg basalts, the whole rock data from this study plots towards the high Mg end member (Figure 22). The olivine hosts in this study, though slightly more evolved, entrap melts that plot within a narrow range of compositions (Figure 22). Progressive fractionation and degassing follow very similar trends towards the evolved and degassed groundmass data. Volatile contents differ slightly such that Portnyagin et al. (2012) noted a gradual decrease in sulphur content from the 1867 to the 1971 and 1992 eruptions, indicating lower oxygen fugacity and greater magmatic degassing with time (Figure 23). The 1999 data plot within the range of 1971 data, reflecting a return to a volatile rich system.

Las Pilas

52! ! The Las Pilas data fit the trend such that the melt inclusion and groundmass data remain at the evolved end member of the cumulative Cerro Negro trend (Figure 22). With a greater spread of melt inclusion data from Portnyagin et al. (2012), there is greater overlap between melt trapped within olivine and pyroxene hosts. The depletion of sulphur within the pyroxene- hosted melt inclusions support the notion that sulphur was considerably degassed prior to pyroxene crystallization (Figure 23). The groundmass data of Las Pilas plot within the range of Cerro Negro but is slightly more evolved (Figure 22 & 23). Las Pilas whole rock plots within the melt inclusion-groundmass trend and thus do not exhibit the same compositional variability seen within Cerro Negro.

Figure 22. Major oxide comparison for all Cerro Negro and Las Pilas Data (including Portnyagin et al. (2012) and additional Las Pilas whole rock). The similarity between the 1999 Cerro Negro samples from this study and the 1867, 1971, 1992 and 1995 samples indicate that the magma chamber beneath Cerro Negro is invariant with time. A distinct compositional trend can be seen between Cerro Negro and Las Pilas. Error bars are 2σ.

53! !

Figure 22. Continued from previous page.

54! ! a) CN99_MI Port_MI LP_MI Port_GM LP_GM CN99_GM 2500# 2250# 2000# 1750# 1500# 1250#

S (ppm) (ppm) S 1000# 750# 500# 250# 0# 0# 0.2# 0.4# 0.6# 0.8# 1# 1.2# 1.4# 1.6# 1.8# 2# 2.2# 2.4# K2O (wt %)

b) 2500#

2000#

1500#

1000# Cl (ppm) (ppm) Cl 500#

0# 0# 0.2# 0.4# 0.6# 0.8# 1# 1.2# 1.4# 1.6# 1.8# 2# 2.2# 2.4#

K2O (wt %)

c) 700# 600# 500# 400# 300#

F (ppm) (ppm) F 200# 100# 0# 0# 0.2# 0.4# 0.6# 0.8# 1# 1.2# 1.4# 1.6# 1.8# 2# 2.2# 2.4# K2O (wt %)

Figure 23. Volatile contents of all Cerro Negro and Las Pilas data. The data from Portnyagin et al. (2012) define the least degassed end member in terms of S (ppm) and Cl (ppm). The magmatic degassing trend is distinct with Las Pilas (specifically the groundmass data) defining the evolved and degassed end member of the trend. Error bars are 2σ.

55! ! 4.3 Magma composition during evolution

As can be seen, the magma chamber beneath Cerro Negro has remained relatively invariant since at least 1867 and possibly since its formation in 1850 (Figure 22 & 23). Characterizing the system is a unique compositional variability between its melt inclusion and whole rock data. This variability is not seen within the whole rock sample from Las Pilas.

Portnyagin et al. (2012) calculated the composition of the parental and evolved melt beneath Cerro Negro. The same method will be applied here and compared with Las Pilas. Referring to Figure 24, the average of all olivine-hosts with Fo content greater than 80 yields the most reasonable composition of the primitive melt (PRIM). The PRIM composition is estimated at 7.33 wt % MgO and 0.29 wt % K2O. The evolved melt (EVOL) is the average of evolved inclusions hosted in olivines with Fo <77. The EVOL composition is estimated at 6.45 wt % MgO and 0.49 wt % K2O.

The EVOL composition is comparable to the average melt composition within pyroxene- hosted inclusions at Las Pilas, which is 3.8 wt % MgO and 1.05 wt % K2O. Reported in Figure 24 are the melt inclusion, groundmass and whole rock compositions from Cerro Negro and Las Pilas as well as previous data from Cerro Negro. The Las Pilas composition could be explained by the same differentiation trend at Cerro Negro, with Las Pilas melts being more evolved.

56! !

Figure 24. Comparison of MgO and K2O between all data compiled at Cerro Negro and Las Pilas. The primitive melt composition, PRIM, is defined by the average melt inclusion composition trapped within primitive olivines (Fo>80). Error bars are 2σ. The evolved melt composition, EVOL, is defined by the melt inclusion composition trapped within evolved olivines (Fo<77). PARM is the intersection between the whole rock array and the evolutionary path. Las Pilas and Cerro Negro data have the same magmatic source and thus are shown to originate from the same primitive composition and pass through the same EVOL composition. Since the whole rock and melt inclusion data exhibit compositional variability, there must be some mechanism controlling these compositions i.e. phenocryst sorting. This mechanism is only seen within Cerro Negro data.

57! ! 4.4 Mechanisms of an interconnected magma chamber

Figure 25 represents the primitive magma chamber beneath Cerro Negro and Las Pilas. The pathway from chamber to edifice can be considered as two separate dykes. Along the Cerro Negro pathway, Portnyagin et al. (2012), based on the work by Walker and Carr (1986) and Carr and Walker (1987), reasoned that the settling velocities and subsequent redistribution of phenocrysts within ascending magma controls the contrasting compositions between whole rock and melt inclusion data.

Four different cases are proposed by Portnyagin et al. (2012) to describe the processes occurring within the dyke beneath the Cerro Negro edifice (Figure 25). Entering the dyke in each case is magma similar to the PRIM composition. The magma ascends, decompresses and releases water, which thereby induces crystallization. Each case varies based on an open, closed or intermediate system crystallization trend and is dependent upon the relative settling (or floatation) velocities of major phenocrysts.

Figure 25. The 4 cases proposed by Portnyagin et al. (2012) to explain the compositional variability between Cerro Negro rocks and melt inclusions. Case 1 involves closed system crystallization where all phenocrysts are suspended until eruption and erupted magma composition is similar to PRIM. Case 2 is an open crystallization system where all phenocrysts are removed from the melt, possibly by accumulating along the dyke wall. Case 3 involves the evolution of magma from PRIM via the partial removal of phenocrysts from the magma during initial fractionation, producing a composition similar to PARM. Case 4 involves compositional zoning of magma within the dyke due to the flotation of plagioclase phenocrysts and the settling of olivine and pyroxene. From Portnyagin et al. (2012).

58! ! The first case is closed system crystallization where all phenocrysts remain suspended until eruption. The erupted magma composition is similar to PRIM. This is likely not the case beneath Cerro Negro since the PRIM composition does not lie along the whole rock trend. The second case is an open system crystallization such that all phenocrysts are separated from the melt by either settling or accumulating along the dyke wall. This is also unlikely as the resultant magma is EVOL in composition and aphyric. The most reasonable mechanism is therefore an intermediate between an open and closed system, represented by the third and fourth cases. The third case involves the evolution of magma from PRIM to EVOL due to the partial removal of phenocrysts from the magma during the initial stages of crystallization. The composition following early stage fractionation is PARM, which is slightly more evolved than PRIM. The composition of PARM is defined as the intersection of the whole rock array and the melt inclusion trend, containing 6.77 wt % MgO and 0.39 wt % K2O. Further evolution of PARM results in the flotation of newly crystallized phenocrysts. This is not the case beneath Cerro Negro because this trend does not explain the variability of high Mg and low Mg basalts. The cause of Mg variability within whole rocks relies upon the dominance of Fe-Mg phenocrysts such as olivine and pyroxene over plagioclase. Therefore, the only difference between case 3 and 4 is a vertical compositional zoning of magma within a dyke due to the flotation of plagioclase phenocrysts and the settling of olivine and pyroxene at shallow depths. Based on plagioclase hosted fluid inclusions, Portnyagin et al. (2012) determined that plagioclase phenocrysts were able to rise in evolved Cerro Negro magmas at pressures and depths below 200 MPa and 7 km. The bulk magma composition is similar to PARM and either deviates towards the leading edge of the dyke to A (low Mg, enriched in plagioclase) or to deeper portions of the dyke to B (high Mg, enriched in Fe-Mg silicates). In the case of the 1999 eruption (this study) the composition deviates towards B and becomes relatively enriched in Fe-Mg silicates. As a result, the re-distribution of phenocrysts within this dyke produces the array of whole rock compositions seen at Cerro Negro. Case 4 therefore represents the most reasonable mechanism within the dyke beneath Cerro Negro.

The mechanism controlling the composition of Cerro Negro does not account for the Las Pilas melt compositions based on the relative compositional homogeneity between melt inclusions and whole rock. Using the four cases proposed by Portnyagin et al. (2012), Las Pilas is well represented by case 1, a closed system fractionation trend with suspended phenocrysts of plagioclase, pyroxene and olivine and predicts that whole rock data plots within the same field as melt inclusion data, which is the case at Las Pilas.

59! ! A number of scenarios are proposed in order to investigate these compositionally different dykes (Figure 26). The following diagrams represent the subsurface structures beneath Cerro Negro and Las Pilas. Surrounding the interconnected magma chamber is a series of shallow intrusive complexes, some of which are likely inactive conduits.

Figure 26. Schematic models illustrating the potential plumbing system beneath Cerro Negro and Las Pilas. Each scenario comprises a 14 km deep magma chamber. Scenarios 1 and 2 include dykes sourced at the same depth from the magma chamber and extend towards the edifice. Scenario 3 and 4 illustrate branching dykes that diverge at 2 km depth. At 7 km depth phenocrysts begin to separate. The dyke beneath Cerro Negro involves phenocryst sorting whereas the dyke beneath Las Pilas does not.

The location of the magma chamber and feeding dykes follow the work by Portnyagin et al. (2012) and MacQueen (2013). Based on H2O and CO2 contents, the depth of the magma chamber is estimated at 14 - 15 km beneath Cerro Negro and plagioclase phenocrysts begin to float and separate from sinking olivine and pyroxene crystals at 7 km (Portnyagin et al., 2012). The geophysical work by MacQueen (2013) showed a possible connection at 2 km depth. This is initially interpreted to represent the depth at which the dyke branches.

60! ! Scenarios 1 and 2 comprise dykes sourced at the same depth from the magma chamber toward the edifice of Cerro Negro and Las Pilas (Figure 26). This would suggest that, theoretically, the whole rock compositions should be similar and that Las Pilas should have a greater proportion of olivine crystals. As this is not the case, these two diagrams are likely not a correct representation of the subsurface structures.

Scenarios 3 and 4 thus compensate for the considerable difference in whole rock compositions by introducing a dyke that diverges at 2 km depth. At 7 km depth within the shared conduit, phenocrysts begin to separate. Scenario 3, however, illustrates that the package of magma that diverts into the dyke beneath the Las Pilas edifice travels a shorter distance prior to eruption and would be compositionally homogeneous. The magma that remains in the dyke beneath Cerro Negro travels further and thus experiences a greater degree of phenocryst sorting prior to eruption. The depth at which the Las Pilas conduit branches could also be the depth within the Cerro Negro conduit at which the phenocryst proportions match the values seen within the Las Pilas whole rock.

Scenario 4 is a slightly different with the dominant dyke beneath Las Pilas and the area prior to divergence is larger and extends from 2 km to 7 km depth. Additionally, this expanse could be interpreted as a smaller, secondary magma chamber or an area of active and inactive dyke swarms. Within this 5 km area, phenocrysts would begin to separate, establishing a distinct compositional stratification within the conduit. This magma would preferentially rise towards Cerro Negro since the dyke is oriented parallel to the direction of minimum principle stress (La Femina et al, 2002). Therefore, only a small proportion of magma would divert towards Las Pilas since the conduit is essentially oriented parallel to maximum stress.

The occurrence of only one magmatic eruption documented at Las Pilas places significant temporal restrictions on the subsurface morphology. Since the last eruption was 300 years before Cerro Negro formed, it is more likely that a conduit was initially established beneath Las Pilas, thus discounting Scenario 3.

Scenario 4 is further supported by the concept of dyke capture by Gaffney et al. (2007). Dyke capture best occurs when intrusive conduits preferentially follow steep faults that propagate in resistant rocks. In accordance with the work by La Femina et al. (2002), who proposes east-west extension within blocks along NNW trending structures, dyke capture along a high angle normal fault beneath Cerro Negro is likely its dominant conduit. Finally, the concept of diverging and converging dyke intrusions beneath volcanic complexes, initially proposed by

61! ! Petronis et al. (2013), could explain the lack of activity at Las Pilas. Shifting along the local stress regime would influence whether a dyke diverges or becomes blocked. In the case of Las Pilas, subtle shifts within the tectonic regime following the 1954 eruption could have caused the divergence of the conduit to the northwest and the manifestation of the new dyke was the formation of Cerro Negro.

4.5 Implications for future eruptions at the Cerro Negro-Las Pilas-El

Hoyo Complex

The implications of an interconnected magma chamber include the proper classification of Cerro Negro and the future eruptions within this complex. Since the two systems are connected, Cerro Negro should be considered as the newest edifice within the Las Pilas-El Hoyo Complex. Similar to a volcanic field, this complex has no prominent peak and contains edifices distributed over a large area. However, future eruptions at Cerro Negro may cause it to build with time, allowing it to become the prominent center for activity. Proposed mechanisms such as dyke capture and converging and diverging dykes suggest that new conduits may develop and create cinder cones within the complex.

62! ! Conclusion

Olivine- and pyroxene-hosted melt inclusions from Cerro Negro and the Las Pilas-El Hoyo Complex suggest an interconnected plumbing system. Water and carbon dioxide contents from previous studies indicate the depth of the magma chamber is 14-15 km whereas geophysical data suggest that a connection likely exists at 2 km depth (Portnyagin et al, 2012; MacQueen, 2013). Melt inclusion, groundmass and whole rock data from Cerro Negro and Las Pilas define a linear evolutionary trend from the primitive melt, represented by Cerro Negro, towards the evolved and degassed melt represented by Las Pilas. Crystallization is believed to be induced by the release of water from ascending magma. Beginning at 7 km depth, phenocryst settling is a proposed mechanism that accounts for the compositional variability between melt inclusions and whole rocks at Cerro Negro (Portnyagin et al, 2012). Las Pilas data do not exhibit the same variability, therefore branching dykes must be present beneath these systems. The most likely scenario is a dyke that originally connected the magma chamber to Las Pilas but due to local stress changes and dilation, subsequent dyke branching formed Cerro Negro in 1850. The proper classification of Cerro Negro as the newest edifice on the Cerro Negro-Las Pilas-El Hoyo Complex implies that the eruptive intensity will increase. This poses significant hazards to local towns and villages who, in the past, have been devastated by ashfall causing infrastructure and agricultural damage. The lack of eruptions at Las Pilas implies the conduit is likely blocked and that future eruptions are improbable. However, dyke branching and divergence may occur in the future and create new vents. The model proposed here also has global implications such that volcanic fields comprised of closely spaced edifices could potentially share a magma chamber and that eruptions at one edifice could precede others nearby.

5.1 Further research

In order to better constrain the composition of the common magma source and the depth and divergence of the dyke, trace elements, H2O and CO2 contents of melt inclusions should be analyzed. Additionally, olivine-hosted melt inclusions from Las Pilas should be used in order to compare primitive melt compositions with melts from Cerro Negro. This supplementary data will allow for the more precise determination as to whether Cerro Negro and Las Pilas do, in fact, share the same magmatic source.

63! ! References

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Bice, D.C. 1980. Tephra stratigraphy and physical aspects of recent volcanism near Managua, Nicaragua. United States.

Bundschuh, J. and Alvarado Induni, G.E. 2007. Central America: Geology, resources and hazards. Taylor & Francis, New York.

Carr, M.J. 1984. Symmetrical and segmented variation of physical and geochemical characteristics of the Central American volcanic front. Journal of Volcanology and Geothermal Research, 20: 231-252.

Carr, M.J., Feigenson, M.D., Patino, L.C., and Walker, J.A. 2003. Volcanism and geochemistry in Central America; progress and problems. Geophysical Monograph, 138: 153-174.

Carr, Michael J.; Feigenson, Mark D.; Bolge, Louise L.; Walker, James A.; Gazel, Esteban. RU_CAGeochem v.2, a database and sample repository for Central American volcanic rocks at Rutgers University, doi:10.1594/IEDA/100403, obtained from the EarthChem Library (http://www.earthchem.org/library).

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66! ! Appendix 1 Uncorrected values for olivine-hosted inclusions from Cerro Negro All oxides are given in weight percent.

Melt inclusion SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 S ppm Cl ppm F ppm Mg# CN-1 99A MI 47.126 0.852 18.345 10.790 0.056 3.377 12.335 2.195 0.348 0.220 1611.0 1234.8 271.6 35.8 CN-1 99A MI-2 46.206 0.792 17.981 11.957 0.202 3.989 12.032 2.027 0.580 0.101 0.0 0.0 0.0 37.3 CN-1 99B MI-2 48.532 0.955 17.208 10.296 0.129 5.598 11.217 2.327 0.485 0.260 1051.2 1019.2 63.6 49.2 CN-1 99C MI-1 46.660 0.940 16.819 10.654 0.137 6.459 10.966 1.998 0.366 0.155 1238.4 772.0 149.0 51.9 CN-1 99C MI-2 46.473 0.810 17.134 10.464 0.282 6.222 11.267 1.919 0.385 0.275 1250.0 810.4 66.0 51.4 CN-1 99D MI 46.472 0.855 18.366 10.435 0.251 5.448 12.283 1.984 0.272 0.138 1245.0 752.2 204.0 48.2 CN-1 99E MI1 45.577 0.811 17.933 10.989 0.332 5.768 11.719 1.861 0.252 0.110 1495.8 880.8 122.0 48.3 CN-1 99E MI2 46.316 0.711 17.930 11.133 0.186 6.840 11.835 1.692 0.267 0.071 1477.0 803.8 228.6 52.3 CN-1 99F MI-1 46.500 0.911 18.481 13.160 0.258 3.607 11.505 2.365 0.352 0.096 1704.6 1451.0 230.2 32.8 CN-1 99G MI-1 45.331 1.131 18.287 9.246 0.168 6.476 11.295 2.147 0.337 0.208 1380.8 797.2 266.6 55.5 CN-1 99H MI 47.048 0.782 17.833 11.261 0.340 4.630 11.306 1.831 0.369 0.185 1667.0 1029.2 73.4 42.3 CN2 MI99J 46.929 0.788 18.521 8.986 0.155 6.470 11.428 2.017 0.312 0.119 1392.6 761.4 79.2 56.2 CN2 MI99J-2 44.748 1.718 17.885 10.517 0.167 6.588 11.543 2.042 0.389 0.319 1145.2 807.4 159.8 52.7 CN2 MI99K 46.547 0.791 17.119 10.296 0.140 6.783 11.445 1.384 0.260 0.131 1392.0 819.0 141.0 54.0 CN2 MI99L 46.484 0.761 17.081 10.549 0.072 6.226 10.314 1.933 0.308 0.172 1615.0 723.4 238.2 51.3 CN2 MI99N 46.634 0.827 17.258 9.730 0.192 6.678 11.091 1.952 0.360 0.139 1277.2 810.4 17.0 55.0 CN2 MI99O 46.841 0.876 19.497 9.473 0.111 3.608 11.989 2.298 0.347 0.112 1405.4 737.2 99.0 40.4

67! ! Appendix 2 Additional data for Cerro Negro All oxides are reported in weight percent.

Melt Inclusion

Data source Eruption PEC% SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Fo host Mg# MI Kd Cl ppm S ppm Roggensack, 2001 1867 1.00 47.80 0.77 18.55 10.98 0.15 7.20 12.09 1.93 0.21 0.17 80.00 52.82 0.28 800 2133 Roggensack, 2001 1867 3.00 49.25 0.69 17.48 10.77 0.18 7.49 11.71 1.93 0.19 0.16 81.00 52.09 0.26 600 1467 Roggensack, 2001 1867 3.00 50.45 0.75 17.76 9.86 0.00 6.74 11.79 2.03 0.36 0.15 81.00 51.29 0.25 800 2467 Roggensack, 2001 1867 5.00 48.09 0.69 17.55 11.25 0.22 7.75 12.29 1.70 0.24 0.08 81.00 49.58 0.23 800 2467 Roggensack, 2001 1867 0.00 49.57 0.72 18.47 10.32 0.15 6.64 11.83 1.93 0.25 80.00 53.43 0.29 600 Roggensack, 2001 1867 6.00 48.91 0.75 17.90 10.25 0.00 7.23 12.36 2.13 0.23 0.09 82.00 48.18 0.20 900 2133 Roggensack, 2001 1867 6.00 49.06 0.76 17.41 10.61 0.00 7.59 12.38 1.83 0.23 82.00 48.96 0.21 800 2133 Roggensack, 2001 1867 1.00 49.51 0.60 18.02 9.86 0.18 7.20 12.38 1.89 0.25 82.00 55.46 0.27 400 1467 Roggensack, 2001 1867 2.01 47.62 1.27 17.90 10.74 0.19 7.54 11.84 2.24 0.29 0.22 82.00 53.37 0.25 600 1800 Roggensack, 2001 1867 0.00 50.16 0.80 17.76 9.77 0.16 7.13 11.71 2.02 0.27 0.10 82.00 56.55 0.29 500 2000 Roggensack, 2001 1867 2.01 47.89 0.88 17.52 12.61 0.19 6.40 11.63 2.24 0.28 0.17 76.00 45.19 0.26 900 1333 Roggensack, 2001 1867 2.01 50.08 0.92 16.85 11.76 0.33 6.35 9.91 2.93 0.53 0.15 77.00 46.65 0.26 1100 Roggensack, 2001 1867 3.00 48.73 0.70 18.09 10.48 0.17 7.44 11.97 1.95 0.29 0.05 82.00 52.52 0.24 600 2333 Roggensack, 2001 1867 5.00 48.42 0.76 17.43 10.28 0.12 7.79 12.94 1.79 0.25 0.09 83.00 51.79 0.22 600 2467 Roggensack, 2001 1867 0.00 49.56 1.00 18.02 10.05 0.16 6.78 11.31 2.20 0.38 0.41 81.00 54.61 0.28 500 2000 Roggensack, 2001 1867 0.00 48.64 0.72 18.39 10.53 0.29 7.07 12.08 1.93 0.21 81.00 54.47 0.28 800 2333 Roggensack, 2001 1867 0.00 49.40 0.74 18.30 9.98 0.13 7.35 11.58 2.03 0.25 0.12 82.00 56.77 0.29 600 2133

Data source Eruption PEC % SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Fo host Mg# MI Kd Cl ppm S ppm Roggensack, 2001 1867 5.00 49.11 0.81 17.78 10.97 0.18 7.13 11.34 2.18 0.28 0.05 80.00 47.69 0.23 600 2133 Roggensack, 2001 1867 3.00 50.85 1.13 16.03 13.03 0.31 5.63 9.39 2.62 0.58 0.21 73.00 39.84 0.24 1300 Roggensack, 2001 1867 0.00 51.01 1.08 16.29 12.03 0.24 5.60 10.00 2.77 0.55 0.24 74.00 45.35 0.29 900 2000 Roggensack, 2001 1867 1.00 48.01 0.75 17.48 12.29 0.24 7.09 11.75 1.95 0.20 0.07 78.00 49.63 0.28 700 2333 Roggensack, 2001 1867 4.01 48.74 0.75 17.98 10.70 0.18 6.95 12.17 2.11 0.18 0.10 80.00 48.83 0.24 900 2133 Sadofsky et al., 2008 1971 2.03 47.85 0.76 18.14 10.88 0.18 7.41 12.46 1.79 0.26 0.13 81.06 52.60 0.26 960 1231 Sadofsky et al., 2008 1971 1.02 48.44 1.15 18.21 11.26 0.18 6.76 11.20 2.12 0.33 0.21 78.98 50.52 0.27 776 1221 Sadofsky et al., 2008 1971 4.07 48.94 0.56 18.12 10.40 0.23 7.62 11.78 1.81 0.25 0.16 81.93 51.92 0.24 689 1031 Sadofsky et al., 2008 1971 2.79 49.18 0.70 17.52 10.09 0.17 7.69 12.27 1.81 0.29 0.16 82.57 54.56 0.25 749 1255 Sadofsky et al., 2008 1971 2.10 49.72 0.66 17.85 9.72 0.17 7.47 12.04 1.78 0.28 0.19 82.55 55.48 0.26 945 1292 Sadofsky et al., 2008 1971 6.56 49.46 0.57 17.23 11.13 0.16 7.61 11.43 1.78 0.28 0.21 80.70 46.69 0.21 604 899 Sadofsky et al., 2008 1971 5.20 48.06 0.75 17.44 11.21 0.19 8.01 12.13 1.66 0.24 0.15 81.65 50.03 0.22 706 1453 Portnyagin et al, 2012 1971 6.41 50.90 0.82 16.12 11.85 0.23 7.17 10.45 1.80 0.37 0.13 78.32 43.81 0.22 1054 951 Portnyagin et al, 2012 1971 6.84 49.08 0.80 17.24 12.34 0.18 6.86 10.67 1.99 0.31 0.36 77.19 40.79 0.20 824 1508 Portnyagin et al, 2012 1971 4.99 49.08 0.90 17.98 11.49 0.18 6.89 10.90 1.92 0.33 0.18 78.63 45.38 0.23 830 1576 Portnyagin et al, 2012 1971 5.65 48.18 0.72 18.25 10.88 0.19 7.32 12.27 1.73 0.25 0.08 80.74 47.31 0.21 794 1685 Portnyagin et al, 2012 1971 3.05 48.87 0.93 18.13 11.43 0.24 6.67 10.90 2.09 0.39 0.20 78.29 47.31 0.25 1166 1367 Portnyagin et al, 2012 1971 5.11 49.44 0.48 17.95 10.46 0.19 7.73 11.86 1.55 0.15 0.08 81.69 50.84 0.23 314 721 Portnyagin et al, 2012 1971 6.05 48.01 1.21 17.73 11.03 0.18 7.84 11.86 1.65 0.26 0.10 81.72 48.48 0.21 616 1422 Portnyagin et al, 2012 1971 0.90 50.48 0.77 18.02 9.74 0.20 6.66 11.82 1.77 0.31 0.11 80.52 53.91 0.28 813 1479 68! !

Data source Eruption PEC % SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Fo host Mg# MI Kd Cl ppm S ppm Portnyagin et al, 2012 1971 3.23 49.47 0.73 17.67 10.36 0.15 7.66 11.88 1.66 0.23 0.06 81.86 53.28 0.25 741 1356 Portnyagin et al, 2012 1971 2.94 49.25 0.73 17.65 10.36 0.20 7.56 12.10 1.73 0.22 0.07 81.81 53.25 0.25 759 1414 Portnyagin et al, 2012 1971 1.81 49.45 0.75 18.26 10.42 0.20 7.00 11.57 1.90 0.24 0.08 80.37 52.41 0.27 516 993 Portnyagin et al, 2012 1971 2.42 49.41 0.81 18.34 9.47 0.19 7.02 12.14 1.85 0.38 0.27 82.12 54.07 0.26 728 1475 Portnyagin et al, 2012 1971 4.87 49.54 0.90 17.71 10.60 0.18 7.31 11.41 1.82 0.28 0.12 80.87 49.14 0.23 658 1359 Portnyagin et al, 2012 1971 1.67 49.84 0.80 17.52 11.21 0.21 6.95 10.87 1.95 0.34 0.15 79.04 50.64 0.27 768 1429 Portnyagin et al, 2012 1971 2.63 49.38 0.85 17.57 11.48 0.21 7.16 10.92 1.87 0.29 0.13 79.15 49.71 0.26 762 1434 Portnyagin et al, 2012 1971 7.21 50.20 0.84 17.49 10.53 0.20 7.29 11.16 1.73 0.30 0.12 80.69 45.46 0.20 779 1439 Portnyagin et al, 2012 1971 4.49 52.27 0.89 17.00 9.45 0.17 6.57 10.91 2.04 0.43 0.16 80.71 49.25 0.23 910 910 Portnyagin et al, 2012 1971 4.70 49.76 0.70 17.32 10.47 0.21 7.80 11.55 1.71 0.25 0.10 82.02 51.64 0.23 715 1402 Portnyagin et al, 2012 1971 2.33 49.43 0.96 17.89 10.34 0.18 7.11 11.49 2.01 0.33 0.14 81.03 52.38 0.26 706 1545 Portnyagin et al, 2012 1971 2.69 47.99 1.21 18.51 11.00 0.19 6.94 11.46 1.97 0.40 0.17 79.94 49.72 0.25 763 1632 Portnyagin et al, 2012 1971 5.36 47.99 1.35 18.19 11.08 0.23 7.15 11.60 1.78 0.35 0.12 80.22 46.61 0.22 785 1541 Portnyagin et al, 2012 1971 7.22 47.59 1.45 17.74 11.81 0.13 7.53 11.43 1.68 0.34 0.12 80.06 43.75 0.19 792 1667 Portnyagin et al, 2012 1971 2.25 48.28 0.85 19.15 9.53 0.17 6.38 12.99 2.13 0.29 0.11 81.02 51.48 0.25 830 1569 Portnyagin et al, 2012 1971 1.19 50.36 0.85 18.11 9.85 0.14 6.87 11.28 1.97 0.32 0.12 81.02 54.04 0.28 751 1450 Portnyagin et al, 2012 1971 2.74 49.13 0.91 19.22 8.83 0.19 6.12 12.45 2.34 0.52 0.19 81.48 51.40 0.24 731 1500 Sadofsky et al., 2008 1992 6.73 49.11 1.04 15.12 14.34 0.25 7.55 9.40 2.28 0.53 0.15 76.58 40.57 0.21 1284 838 Sadofsky et al., 2008 1992 2.88 46.70 0.94 17.58 11.77 0.17 7.85 12.90 1.50 0.24 0.19 80.99 51.24 0.25 579 255 Sadofsky et al., 2008 1992 4.40 45.90 1.62 16.82 12.95 0.21 8.41 11.07 1.58 0.53 0.71 80.63 49.04 0.23 660 396

Data source Eruption PEC % SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Fo host Mg# MI Kd Cl ppm S ppm Sadofsky et al., 2008 1992 0.45 48.08 0.45 18.43 10.24 0.17 7.54 13.18 1.52 0.12 0.16 82.02 56.32 0.28 289 356 Sadofsky et al., 2008 1992 0.85 48.73 0.91 16.98 12.53 0.24 6.56 11.15 2.17 0.40 0.14 76.52 47.34 0.28 879 1192 Sadofsky et al., 2008 1992 5.50 49.41 1.11 15.82 14.06 0.22 5.81 9.97 2.50 0.58 0.16 72.00 37.03 0.23 1421 748 Portnyagin et al, 2012 1992 4.07 50.52 0.84 15.28 13.69 0.31 7.10 9.55 1.98 0.43 0.08 75.57 43.59 0.25 1042 917 Portnyagin et al, 2012 1992 6.95 51.45 0.99 15.04 13.45 0.21 6.29 9.29 2.29 0.62 0.15 73.81 36.26 0.20 1139 403 Portnyagin et al, 2012 1992 1.53 48.16 0.80 16.91 13.16 0.24 7.06 11.48 1.65 0.27 0.08 76.57 47.28 0.27 887 1449 Portnyagin et al, 2012 1992 2.50 47.44 0.68 17.94 10.84 0.18 8.07 12.77 1.63 0.23 0.09 82.58 54.40 0.25 926 1536 Portnyagin et al, 2012 1992 0.00 50.51 0.83 17.33 10.74 0.20 6.61 10.94 2.13 0.42 0.16 78.00 52.33 0.31 1050 1198 Portnyagin et al, 2012 1992 2.35 49.55 0.63 17.10 12.14 0.21 7.06 10.68 1.90 0.36 0.21 77.94 48.32 0.26 622 440 Portnyagin et al, 2012 1992 4.52 49.13 0.89 17.06 11.81 0.18 7.28 11.14 1.80 0.35 0.20 78.96 46.99 0.24 908 1135 Portnyagin et al, 2012 1992 3.48 52.06 0.97 15.39 13.11 0.18 5.78 8.93 2.55 0.66 0.16 72.67 39.66 0.25 1239 655 Portnyagin et al, 2012 1992 6.17 50.55 0.91 15.33 14.17 0.23 6.33 9.26 2.32 0.53 0.14 72.86 36.66 0.22 1161 960 Portnyagin et al, 2012 1992 2.76 48.55 0.88 16.44 13.54 0.28 6.98 10.51 2.09 0.41 0.10 76.10 44.89 0.26 1066 1231 Portnyagin et al, 2012 1992 2.51 52.02 1.11 15.13 12.28 0.18 5.80 10.01 2.47 0.65 0.15 74.34 42.57 0.26 1149 621 Portnyagin et al, 2012 1992 0.57 49.88 0.72 16.89 13.05 0.26 6.13 10.43 1.96 0.39 0.11 73.74 44.97 0.29 1008 912 Portnyagin et al, 2012 1992 1.28 50.24 1.07 16.08 12.99 0.29 6.30 9.92 2.29 0.49 0.12 74.80 44.91 0.27 1029 1059 Portnyagin et al, 2012 1992 1.94 49.85 0.86 16.38 12.95 0.25 6.74 9.89 2.32 0.40 0.15 76.15 46.01 0.27 986 1299 Portnyagin et al, 2012 1992 1.91 50.33 0.87 16.27 12.72 0.26 6.63 9.88 2.36 0.38 0.11 76.08 46.03 0.27 1310 1593 Portnyagin et al, 2012 1992 5.04 48.11 1.02 15.91 13.94 0.26 6.58 10.95 2.44 0.45 0.11 74.95 39.48 0.22 1136 1162 ! 69! ! Data source Eruption PEC % SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Fo host Mg# MI Kd Cl ppm S ppm Portnyagin et al, 2012 1992 3.98 50.91 1.22 15.87 12.82 0.26 6.83 8.89 2.31 0.57 0.13 76.47 44.08 0.24 2036 1372 Portnyagin et al, 2012 1992 1.08 54.83 0.62 15.49 10.83 0.16 5.74 8.59 2.63 0.73 0.22 75.90 47.23 0.28 1561 628 Portnyagin et al, 2012 1992 1.58 48.16 0.84 17.95 11.11 0.20 7.67 11.90 1.70 0.25 0.08 81.14 53.50 0.27 836 1458 Portnyagin et al, 2012 1992 2.44 52.68 0.78 16.34 11.66 0.21 5.80 8.60 2.69 0.87 0.18 75.16 43.86 0.26 1119 1344 Portnyagin et al, 2012 1992 10.37 49.89 1.09 16.18 14.39 0.22 6.24 8.29 2.67 0.65 0.12 72.82 28.21 0.15 1623 1029 Portnyagin et al, 2012 1992 5.64 49.02 0.69 18.16 10.34 0.15 7.24 12.09 1.82 0.29 0.08 81.34 48.23 0.21 1089 1640 Portnyagin et al, 2012 1992 2.98 46.58 2.05 18.60 10.95 0.17 6.75 11.48 2.47 0.63 0.14 80.69 48.46 0.22 1387 925 Portnyagin et al, 2012 1992 7.01 47.87 0.84 16.13 13.44 0.15 7.59 11.58 1.78 0.29 0.12 77.92 41.71 0.20 1006 954 Portnyagin et al, 2012 1992 3.93 49.58 1.05 15.61 13.91 0.28 6.84 9.95 1.99 0.31 0.27 74.68 42.35 0.25 795 1410 Groundmass

Source Year SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Roggensack et al, 1997 1992 50.58 1.21 15.26 13.52 0.23 4.53 9.18 2.69 0.68 0.15 Roggensack et al, 1997 1992 54.49 1.44 14.43 14.58 0.27 3.46 9.44 2.91 0.85 0.18 Roggensack et al, 1997 1995 55.66 1.54 12.8 14.18 0.25 3.51 8.3 2.62 0.87 0.25 Roggensack, 2001 1867 53.94 1.68 13.29 14.73 0.19 3.41 8.97 2.83 0.77 0.19 Roggensack, 2001 1867 52.90 1.22 14.38 13.36 0.28 3.86 9.81 3.21 0.74 0.22 Portnyagin et al, 2012 1971 56.19 1.64 12.87 14.04 0.29 2.72 7.72 2.55 1.11 0.30 Portnyagin et al, 2012 1971 56.85 1.68 12.84 14.69 0.38 3.03 7.77 2.34 1.11 0.32 Portnyagin et al, 2012 1971 54.14 2.05 12.11 15.88 0.23 3.88 7.16 2.96 1.02 0.25 Portnyagin et al, 2012 1971 55.40 1.67 11.82 15.29 0.35 3.68 7.70 2.51 1.08 0.25 Portnyagin et al, 2012 1971 56.27 1.69 12.82 14.32 0.21 2.89 7.49 2.60 1.02 0.26 Portnyagin et al, 2012 1971 55.90 1.72 12.08 14.61 0.25 2.78 7.57 2.58 1.09 0.32 Portnyagin et al, 2012 1992 55.56 1.59 12.58 13.84 0.25 3.08 7.53 2.78 1.13 0.23 Portnyagin et al, 2012 1992 55.47 1.57 12.96 13.78 0.30 3.03 7.59 3.29 1.03 0.26 Portnyagin et al, 2012 1992 55.76 1.63 12.77 14.28 0.29 3.03 7.64 3.10 1.05 0.18 Portnyagin et al, 2012 1992 55.39 1.58 12.59 14.60 0.33 3.22 7.76 2.79 1.06 0.23 Portnyagin et al, 2012 1992 54.85 1.55 12.40 14.75 0.33 3.42 7.77 3.20 1.08 0.23 Portnyagin et al, 2012 1992 56.05 1.59 12.67 14.18 0.23 3.07 7.59 2.83 1.12 0.23 Portnyagin et al, 2012 1992 56.25 1.62 12.36 13.55 0.33 3.07 7.15 3.88 1.18 0.23 Whole Rock Source Year SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Roggensack et al, 1997 1992 50.12 0.78 19.03 10.17 0.19 5.17 11.75 2.2 0.48 0.11 Roggensack et al, 1997 1992 48.84 0.74 18.02 10.11 0.19 6.07 11.79 2.02 0.43 0.1 Roggensack et al, 1997 1995 50.17 0.77 17.99 10.14 0.2 6.46 11.44 2.25 0.46 0.11 Portnyagin et al, 2012 1971 47.5 0.61 15.74 10.31 0.19 10.03 12.44 1.5 0.27 0.1 Portnyagin et al, 2012 1992 48.86 0.68 21.26 8.86 0.16 4.1 12.45 2.12 0.4 0.13 Portnyagin et al, 2012 1971 47.50 0.61 15.74 11.45 0.19 10.03 12.44 1.50 0.27 0.10 Portnyagin et al, 2012 1992 48.86 0.68 21.26 9.84 0.16 4.10 12.45 2.12 0.40 0.13 Portnyagin et al, 2012 1999 49.19 0.75 18.41 11.25 0.19 5.98 12.05 2.10 0.42 0.12 ! 70! ! Additional whole rock data for Las Pilas

Source SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Carr et al, 2013 53.6 1.67 16 9.8 0.18 4.75 8.7 3.03 0.85 0.22 Carr et al, 2013 50 0.76 17.3 9.4 0.18 6.27 12.2 2.14 0.63 0.16 Carr et al, 2013 53.08 0.86 16.67 9.26 0.184 5.61 10.91 2.6 0.908 0.196 Carr et al, 2013 53.61 1.59 16.19 9.84 0.175 5.65 9.17 2.94 0.924 0.233 Carr et al, 2013 52.71 1.52 15.95 9.69 0.17 5.8 9.04 2.9 0.853 0.224 Carr et al, 2013 60.7 0.72 16.37 6.49 0.16 2.08 5.7 3.7 2.49 0.28 Carr et al, 2013 60.05 0.71 16.29 6.5 0.16 2.14 5.67 3.8 2.27 0.263 Carr et al, 2013 64.71 0.64 15.16 5.15 0.162 1.17 3.79 3.93 2.94 0.241 Carr et al, 2013 51.62 0.87 16.4 9.48 0.183 6.4 11.3 2.47 0.728 0.171 Carr et al, 2013 52.45 0.92 16.98 9.62 0.186 5.48 11.05 2.56 0.812 0.184 Carr et al, 2013 54.83 0.85 17.03 8.99 0.18 4.75 9.63 2.84 1.05 0.2 Carr et al, 2013 54.74 0.87 17.09 9.08 0.19 4.74 9.66 2.82 1.05 0.19 Carr et al, 2013 54.5 0.83 17.68 8.94 0.18 4.54 9.62 2.92 1.06 0.2 Carr et al, 2013 62.1 0.89 15.66 7.33 0.21 1.96 5.46 4.18 1.78 0.3 Carr et al, 2013 63.38 0.87 15.87 7.25 0.2 1.96 5.27 4.26 1.85 0.32 Carr et al, 2013 55.24 0.84 17.09 8.94 0.18 4.42 9.88 3 1.15 0.22 Carr et al, 2013 55.23 0.85 17.33 9.11 0.18 4.77 9.9 2.95 1.11 0.21 Carr et al, 2013 54.78 0.84 16.48 9.04 0.18 4.59 9.71 2.83 1.09 0.2 Carr et al, 2013 62.16 0.84 16.01 6.84 0.19 1.74 4.8 4.37 1.86 0.3

71! !