Journal of Volcanology and Geothermal Research 325 (2016) 211–224

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Journal of Volcanology and Geothermal Research

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Investigating the subsurface connection beneath Cerro Negro volcanoand the El Hoyo Complex, Nicaragua

Swetha Venugopal a,b,⁎, Séverine Moune a, Glyn Williams-Jones b

a Laboratoire Magmas et Volcans, Campus Universitaire des Cézeaux, 6 Ave Blaise Pascal, TSA 60026 –CS 60026, 63178 Aubiere, Franceb Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada

http://dx.doi.org/10.1016/j.jvolgeores.2016.06.0010377-0273/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 December 2015Received in revised form 26 May 2016Accepted 6 June 2016Available online 07 June 2016

Cerro Negro, the youngest volcano along the Central American Volcanic Belt (CAVB), is a polygenetic cinder conewith relatively frequent basaltic eruptions. The neighbouring El Hoyo complex, of which Las Pilas is the dominantedifice, is amuch larger and older complexwithmilder and less frequent eruptions. Previous studies have suggesteda deep link beneath these two closely spaced volcanoes (McKnight, 1995; MacQueen, 2013). Melt inclusions werecollected from various tephra samples in order to determine whether a connection exists and to delineate the fea-tures of this link. Major, volatile, and trace elemental compositions reveal a distinct geochemical continuum withCerro Negro defining the primitive endmember and El Hoyo representing the evolved endmember. Magmatic con-ditions at the time of melt inclusion entrapment were estimated with major and volatile contents: 2.4 kbar and1170 °C for Cerro Negro melts and 1.3 kbar and 1130 °C for El Hoyo melts with an overall oxygen fugacity at theNNO buffer. Trace element contents are distinct and suggest Cerro Negro magmas fractionally crystallise while ElHoyomagmas are amix between primitive Cerro Negromelts and residual and evolved El Hoyomagma.Modellingof end member compositions with alphaMELTS confirms the unique nature of El Hoyo magmas as resulting fromincremental mixing between Cerro Negro and residual evolvedmagma at 4 km depth. Combining all available liter-ature data, this study presents amodel of the interconnected subsurface plumbing system. Thismodel considers themodern day analogue of the Lemptégy cinder cones in Massif Central, France and incorporates structurally con-trolled dykes. Themain implications of this study are the classification of Cerro Negro as the newest conduit withinthe El Hoyo Complex as well as the potential re-activation of the El Hoyo edifice.

© 2016 Elsevier B.V. All rights reserved.

Keywords:Melt inclusionsMagmatic processesPlumbing systemCerro NegroEl Hoyo

1. Introduction

The occurrence of an interconnected, shallow (2 km) positive den-sity anomaly beneath Cerro Negro volcano and the El Hoyo complexhas prompted the geochemical investigation for a subsurface linkagebeneath these two closely spaced systems (MacQueen, 2013; Fig. 1).Moreover, comparison of whole rock geochemistry from these volca-noes has revealed a distinct evolutionary path whereby Cerro Negro ba-salts represent the primitive endmember while El Hoyo basalts definethe evolved endmember (McKnight, 1995).

In order to fully characterize the nature of the subsurface connection,optimal data comes in the form of melt inclusions, which are tiny blebsof melt trapped within growing crystals in a magma reservoir. Melt in-clusions behave as closed systems and are protected fromdegassing andcompositional changes during magma ascent (Schiano, 2003). Uponeruption, inclusions will quench to glass and preserve the pre-eruptivecomposition and conditions thus providing valuable informationabout the temperature, pressure and chemistry of the magma at thetime of inclusion entrapment.

The objective of this study is to sample olivine- and pyroxene-hostedmelt inclusions from Cerro Negro and El Hoyo in order to determine the

composition of primitive and slightly evolved melts. By analyzing themajor, volatile and trace element compositions of melt inclusions,coupled with alphaMELTS modelling (Smith and Asimow, 2005;Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998), the magmaticconditions of the intricate subsurface plumbing system and the possiblelink between these two systems can be delineated.

2. Geological context

2.1. Cerro Negro

Cerro Negro is the youngest volcano along the Central American Vol-canic Belt (CAVB; Fig. 1a). Located within the Maribios Range, it is ablack, polygenetic basaltic cinder cone that formed in 1850. Since its for-mation, at least 23 Strombolian to Sub-Plinian eruptions have built the250 m tall cone and associated lava field that extends to the north(Fig. 1b; Hill et al., 1998).

In this study, we examine tephra from the most recent eruption atCerro Negro, which occurred in 1999. This Strombolian eruption was ar-guably the lowest energy (VEI 1) of eruptions at Cerro Negro with tephradeposits totaling 5.0 × 104 m3 (DRE) of basaltic magma (La Femina et al.,

Fig. 1. Location of Cerro Negro and the El Hoyo Complex within the Central American Volcanic Belt (CAVB). A) Location of Cerro Negro and El Hoyo within the outlined Maribios Rangewithin the CAVB. From Portnyagin et al. (2012). B) Illustration of Cerro Negro's lava fields. The crater is at an elevation of 390 m a.s.l. The most recently formed lava field from the1999 eruption is indicated in purple, with the location of the cinder cones in red. Modified from Hill et al. (1998). C) Overview of the El Hoyo Complex and its location relative to CerroNegro (indicated). Modified from Bice (1980). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

212 S. Venugopal et al. / Journal of Volcanology and Geothermal Research 325 (2016) 211–224

2004). Itwas precededby seismic swarms localized in older vents andno-ticeable surface ruptures (La Femina et al., 2004). A 200m long, 1mwidefissure opened to the south of Cerro Negro's southern flank. Lavafountaining along this fissure formed two new scoria cones from whichminor volumes of lava (6 × 105 m3) were extruded, creating a smallflow field to the north (La Femina et al., 2004). Based on the composi-tional similarity of the 1995 and 1999 whole rocks, it is believed thatthe 1999 basalt represents residual magma from the 1995 eruption (LaFemina et al., 2004).

2.2. Cerro Negro's unique geochemistry

Cerro Negro erupts primitive magmas as a result of dykes parallelingthe NNW extensional regime (La Femina et al., 2002). Along the CAVB,Cerro Negro exhibits some of the lowest concentrations of light rareearth elements (LREE) and the highest concentrations of fluid-mobile el-ements, which implies a high flux of fluids from the Cocos slab (Geilertet al., 2012; Carr, 1984). Furthermore, the lavas contain the highest Ba/La, U/Th ratios and the lowest La/Yb, implying a high degree of mantlemelting (Portnyagin et al., 2012). Cerro Negro lavas are also anomalouslyphenocryst rich, earning the term “sticky basalts” (Walker and Carr,1986). Historically, Cerro Negro's lavas have contained elevated watercontents, especially from the 1992 eruption (up to 6.0 wt% water;Roggensack et al., 1997). Water-rich magma is one of the main reasonsthat explain the particularly explosive eruptions at Cerro Negro(Roggensack et al., 1997; Portnyagin et al., 2012). The variable nature oferuptions can also be characterized by the segregation of gases and thephenomenon of phenocryst settling within effusive, low viscosity basaltsat great depth. This segregation results in gas-rich eruptions from themain vent and gas-poor effusions from fissures along the flanks (Walkerand Carr, 1986). Amongst these variable eruptions, however, there hasbeen no significant compositional change, which argues for a stablemagma reservoir beneath Cerro Negro.

2.3. El Hoyo complex

The El Hoyo massif lies 4.5 km to the southeast of Cerro Negro andhas oneof the largest lavafields inNicaragua (Fig. 1c). The complex con-tains numerous overlapping eruptive centres such as El Hoyo and LasPilas, which is themain edifice at 1088mhigh.Within the El Hoyo Com-plex, there are only 3 confirmed eruptions: 1528, 1952 and 1954(Siebert and Simkin, 2002-). The lack of activity at El Hoyo not only con-trasts its position within one of the most active volcanic arcs in the

world, but also juxtaposes the frequent activity of its much smallerneighbour.

Based on indigenous records, the earliest known eruptionwithin theEl Hoyo complex occurred in 1528 (McBirney, 1955). However, this ageshould be approached with caution since the vent is not well locatedand could be found on either the El Hoyo or Las Pilas edifice. The mostrecent eruptions within the El Hoyo complex occurred in 1952 and1954. The 1952 eruption began with the appearance of a fissure alongthe summit of El Hoyo emitting dense clouds of steam and gas(McBirney, 1955). This kilometre long fissure trended north-south andwas structurally controlled, paralleling the direction of recent faulting.Apart from numerous flank vents and maars, fresh sulphur depositswere found along the fissure and around fumaroles emitting low tem-perature gases (McBirney, 1955).

The 1954 eruption occurred a fewmonths after an eruption at CerroNegro. Given the proximity of the two volcanoes, the preceding erup-tion at Cerro Negro may have acted as a trigger to the 1954 eruptionthrough a decrease in magmatic pressure (McBirney, 1955). This erup-tion at El Hoyo emitted ash nearly 20 km to the west towards León andconsisted mostly of steam and ash as well as a small volume of juvenilematerial (McBirney, 1955). Since this eruption, the El Hoyo Complex hasonly shownminor fumarolic activity and gas plumes that emanate fromthe 1952 fissure (Siebert and Simkin, 2002-).

A geochemistry study of El Hoyowhole rocks by Geilert et al. (2012)found that the low Ba/Th and Zr/Nb content implies minor magma con-tamination by carbonate sediment from the subducting slab and an in-creasing influence of a less-depleted mantle in the magma source,respectively. Overall, there are few studies of El Hoyo and therefore littleis known regarding the geochemical characteristics of its magmasource.

3. Methods

3.1. Sample descriptions

Lapilli-sized tephra samples from Cerro Negro were collected in2013 and represent the most recent eruption in 1999 (Fig. 1b;Table 1). The samples are fresh black vesicular basalts with large pheno-crysts (1–4mm) dominated by plagioclase (15–20% by volume), olivine(10–15%), and smaller pyroxene crystals (5–10%). Phenocrysts aresubhedral and commonly glomeroporphyritic (Fig. 2a). Plagioclaseand pyroxene also occur as microlites within the finer grainedgroundmass.

213S. Venugopal et al. / Journal of Volcanology and Geothermal Research 325 (2016) 211–224

Primary olivine-hostedmelt inclusions are glassy (no daughtermin-erals), sub-rounded to elliptical and randomly distributed throughoutthe host. Most inclusions are 20–50 μm and contain a shrinkage bubble,which occupy less than 3% by volume (Fig. 2b).

Two different El Hoyo tephra samples were collected andwe believethese represent the earliest known eruption of this complex (approxi-mately 1528) and the most recent eruption in 1954 (Table 1). The1528 tephra are light brown, weathered, vesicular basalts. Thesubhedral and commonly glomeroporphyritic phenocrysts are largerthan Cerro Negro basalts (1–9 mm) and dominated by plagioclase(20%) and pyroxene (15%; Fig. 2c). Plagioclase microlites also occur inthe finer grained matrix. Olivine crystals were not evident withinhand samples but were visible in thin sections as fractured, fine-grained phenocrysts smaller than 200 μm and lacking intact melt inclu-sions. The 1954 samples are dark grey, nonweathered basalts with 1–4 mm phenocrysts dominated by plagioclase (15%), pyroxene (20%)and olivine (10%).

Primary pyroxene-hosted melt inclusions were selected from the1528 tephra and olivine-hosted melt inclusions from the 1954 tephra.All pyroxene and some olivine hosts are zoned with the range of Mg#and Fo content listed in Table 1. Pyroxene hosts contain numerous in-clusions and at least two inclusions per host were chosen: one fromthemore primitive core and one from the evolved rim. These inclusionsare irregularly shaped, 10–50 μm, glassy (no daughter minerals) andcontain shrinkage bubbles that occupy less than 3% by volume(Fig. 2d). Olivine hosts from the 1954 sample are smaller than those ofCerro Negro, approximately 1–2 mm in diameter and contain roundedglassy inclusions that are 15–40 μmand are commonly free of shrinkagebubbles.

3.2. Microprobe

Following the method of Moune et al. (2012), the major and vol-atile elemental compositions of melt inclusions, host crystals, andmatrix glasses were analyzed at the Laboratoire Magmas et Volcans(LMV) using a SX-100 CAMECA electron microprobe (EMP) and a15 kV accelerating voltage. Mineral analyses were performed usinga 15 nA focused beam that was adjusted to 5 or 10 μm defocusedbeam during glass analyses to reduce Na loss. In order to collect themost precise data and reduce volatile loss during analysis, thebeamwas blanked regularly with a Faraday cup and 5measurementswere taken at 20 s intervals. Volatile analyses were measured withan 80 nA sample current and a 50 s acquisition time using the LPETdiffraction crystal for Cl and S. Sulphur speciation was measured atleast 3 times in most melt inclusions in order to estimate the S6+/ST ratio, which can be used to calculate the oxygen fugacity followingthemethod of Jugo et al. (2005). The precision of the EMP (2σ) is bet-ter than 5% for major elements excluding MnO, Na2O and K2O, whichhad an EMP precision b 10%. The approximate 2σ precision for S andCl is 4% and 7%, respectively. Major element compositions of thewhole rock samples were analyzed by ICP-AES at LMV (Table 1).The associated errors are approximately 10%.

In total, 17melt inclusionswere analyzed from the1999 CerroNegrosample; 20 inclusions from the 1528 El Hoyo sample and 30 inclusionsfrom the 1954 El Hoyo sample. Results are presented in Table 1.

3.3. Secondary ion mass spectrometer (IMS)

Water and CO2 contents of melt inclusions were analyzed using theCAMECA IMS 270 Ion Probe (SIMS) at the Centre de RecherchesPétrographiques et Géochimiques (CRPG) in Nancy, France, with a15 μm beam size for all analyses. Some crystals were transferred fromepoxy to indium sample holders while others were transferred directlyto indium from crystal bond in order to avoid carbon-coating contami-nation. Samples were gold coated at CRPG and held in a gas chamberfor degassing and cleaning (“pre-sputtering”). The glasses were

sputtered with a 10 kV Cs + primary beam of 10–15 nA. For samplesthat had undergone previous carbon coating, the pre-sputtering timewas increased in order to sufficiently clean the surface and avoid anycarbon contamination. Before and after each analytical session, a seriesof well characterized basaltic and andesitic glasses were used for cali-bration of water and carbon contents. Increased errors are due to signaldrift.

3.4. Laser ablation inductively coupled-plasma mass-spectrometry (ICP-MS)

In-situ trace element analysis of melt inclusions were also per-formed at LMV with a Resonetics M50 EXCIMER Laser and a 193 nmwavelength coupled to an Agilent 7500cs ICP-MS (LA-ICP-MS). Rawdata were processed with Glitter Software using CaO content as an in-ternal standard. The standard glasses used were NIST 612, NIST 610and BCR2-G. Analytical precision and accuracy of measurements werehighly variable but most elements were better than 15% at a 95% confi-dence level; these elements are indicated in bold in Table 2. Due to theirlow contents, other elements have higher error (up to 40%) and thuswill not be considered in this study.

4. Results

4.1. Cerro Negro

Whole rocks from the 1999 eruption are vesicular subalkaline ba-salts with 48.5 wt% SiO2, 7.27 wt% MgO and 2.30 wt% total alkalis.Groundmass values are uniform and represent the evolved, late stagemelt with 54–57 wt% SiO2, 0.97–1.11 wt% K2O and 0.01–0.06 wt%H2O (Table 1). Cerro Negro melt inclusions are hosted by olivine phe-nocrysts with a narrow forsterite range, Fo75-Fo82 (Table 1). Usingthe average FeOT and MgO contents and the olivine-melt Fe-Mg equi-librium constant of 0.30 (Roeder and Emslie, 1970), olivines with aforsterite content of 59 are in equilibrium with the groundmasswhereas olivines with a forsterite content of 78 are in equilibriumwith the whole rock.

Melt inclusions from the same olivine host do not vary considerablyin major element contents suggesting these melts represent the melttrapped at the time of host crystallisation. Melts have well constrainedoxide contents and are basaltic in composition: 47.2–49.9 wt% SiO2,9.25–13.2 wt% FeO*, 4.46–6.30 wt% MgO and 0.10–0.59 wt% K2O(Table 1; Fig. 3). The most primitive melt, CN-D with Fo82, contains47.9 wt% SiO2, 10.4 wt% FeO*, 6.30 wt%MgO, and 0.28wt% K2O. Volatileconcentrations of olivine-hosted melt inclusions range between 1051–1704 ppm S and 723–1451 ppm Cl. Water and CO2 contents were ana-lyzed in five inclusions trapped within relatively primitive hosts(Fo78–Fo81; Fig. 4). Analyzed water contents range from 2.48–4.27 wt% and CO2 contents vary from 453–1269 ppm.

Trace element compositions have typical arc-like patterns with lowconcentrations of LREE such as Nb and Zr and enrichment in U, Pb andBa (Fig. 5). Values shown in Fig. 5 are those indicated in bold inTable 2 and are normalised to the primitive mantle (Sun et al., 1989).The large compositional variability is typical of Cerro Negro inclusionsand past studies have found similar patterns from previous eruptions(Sadofsky et al., 2007; Portnyagin et al., 2012). Of the observed trends,the most significant is the elevated Ba values and the depleted, near-primitive mantle values of Th. Sample CN-N shows the highest valuesfor Pb and Ta as well as one of the highest values for Ba (4.3 ppm,0.5 ppm and 286 ppm, respectively).

4.2. El Hoyo

Whole rocks from both the 1528 and 1954 eruption have similarmajor oxide contents of 50wt% SiO2, 5.0–6.0 wt%MgO, and a total alkalicontent of 2.7–3.3 wt% thereby classifying them as relatively primitive,

Table 1Cerro Negro and El Hoyo whole rocks and corrected melt inclusion compositions including host Fo and Mg content, KD and post entrapment crystallisation (PEC%). Major oxides & H2Ogiven inwt%, S, Cl & CO2 given in ppm. Total Fe expressed as FeO*. For El Hoyo olivines, stars (*) indicate zoned hostswith the range of Fo content shown in brackets. Bdl=belowdetectionlimit.

Sample Fo Host KD PEC (%) SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O S Cl H2O CO2

Cerro Negro olivine-hosted MI'sCN-1 99A MI 78 0.20 3.43 48.9 0.859 18.5 10.8 0.061 5.23 12.5 2.22 0.354 1611 1235 – –CN-1 99A MI-2 78 0.22 2.10 47.7 0.802 18.3 12.0 0.203 5.70 12.2 2.06 0.589 – – – –CN-1 99B MI-2 81 0.30 0.03 49.9 0.976 17.7 10.3 0.134 6.05 11.5 2.39 0.503 1051 1019 – –CN-1 99C MI-1 80 0.36 3.92 49.1 1.022 18.3 10.7 0.152 5.82 11.9 2.17 0.402 1238 772 3.4 1043CN-1 99C MI-2 80 0.35 3.61 48.8 0.877 18.5 10.5 0.303 5.68 12.2 2.08 0.422 1250 810 – –CN-1 99D MI 82 0.27 0.26 47.9 0.884 18.9 10.4 0.257 6.30 12.6 2.04 0.277 1245 752 – –CN-1 99E MI1 79 0.32 2.84 47.8 0.869 19.2 11.0 0.354 5.55 12.6 2.00 0.268 1496 881 – –CN-1 99E MI2 79 0.38 3.84 48.0 0.759 19.2 11.1 0.203 5.66 12.6 1.81 0.289 1477 804 – –CN-1 99F MI-1 76 0.20 3.75 47.2 0.897 18.2 13.2 0.256 5.76 11.3 2.34 0.099 1705 1451 – –CN-1 99G MI-1 81 0.38 5.16 48.1 1.25 20.3 9.25 0.188 5.25 12.5 2.38 0.377 1381 797 4.27 1269CN-1 99H MI 79 0.26 0.86 48.9 0.805 18.4 11.3 0.351 5.87 11.7 1.89 0.382 1667 1029 – –CN2 MI99J 81 0.26 6.26 49.5 0.866 20.3 11.3 0.167 4.63 12.7 2.23 0.344 1393 761 2.48 578CN2 MI99J-2 81 0.35 10.2 47.6 2.00 20.8 10.5 0.198 4.46 13.4 2.37 0.454 1145 807 3.81 592CN2 MI99K 81 0.37 4.25 49.2 0.864 18.7 10.3 0.153 6.03 12.5 1.51 0.284 1392 819 – –CN2 MI99L 79 0.38 4.35 49.7 0.841 18.9 10.6 0.077 5.59 11.4 2.14 0.343 1615 723 – –CN2 MI99N 78 0.44 6.92 49.5 0.933 19.4 9.73 0.214 4.75 12.5 2.19 0.405 1277 810 4.12 453CN-1 99O MI1 81 0.21 2.53 48.8 0.898 19.9 9.47 0.112 5.48 12.2 2.35 0.357 1405 737 – –

El Hoyo olivine-hosted MI'sLP30 MI 77 0.34 0.18 54.2 1.08 15.9 9.98 0.153 5.56 8.31 3.07 1.23 352 959 2.09 485LP31MI 72 0.37 1.50 53.3 1.19 15.1 11.6 0.218 4.91 9.34 2.71 1.03 340 894 – –LP32MI 72 0.31 0.38 52.5 1.13 15.6 11.9 0.173 5.00 8.88 3.08 1.16 451 1182 – –LP33 MI1 72 0.35 0.95 52.1 1.09 15.0 12.9 0.175 5.36 9.22 2.66 0.90 315 809 – –LP33 MI2 72 0.37 1.37 53.4 1.17 14.9 12.0 0.268 5.08 9.00 2.64 1.08 274 951 – –LP34MI 74 0.30 2.08 51.7 1.05 15.1 12.5 0.255 5.59 9.46 2.70 1.08 456 1000 0.38 267LP35 MI 72 0.33 0.49 53.0 1.11 14.8 12.1 0.323 5.16 8.91 2.72 1.08 297 885 – –LP36 MI 73 0.36 0.17 54.0 1.19 14.7 11.6 0.215 5.29 8.54 2.64 1.16 338 929 – –LP37 MI 73 0.36 0.79 53.0 1.11 15.5 11.7 0.145 5.18 8.98 2.77 1.10 415 977 – –LP38 MI1 71 *(71–73) 0.34 0.28 53.4 1.21 14.7 12.2 0.257 4.98 8.78 2.82 1.12 385 1057 bdl 42LP38 MI2 73 *(71–73) 0.31 0.81 52.6 1.16 15.2 12.1 0.152 5.29 9.11 2.78 1.05 297 913 – –LP39 MI1 72 *(72–73) 0.32 0.37 52.6 1.10 15.1 11.9 0.183 5.02 9.42 2.93 1.08 336 979 – –LP39 MI2 73 *(72–73) 0.30 1.44 52.4 1.11 15.0 12.4 0.279 5.40 9.10 2.78 1.02 358 1046 – –LP40 MI1 79 *(76–79) 0.30 0.39 47.6 0.89 17.8 11.6 0.264 6.12 12.6 2.21 0.49 1142 834 – –LP40 MI2 77 *(76–79) 0.30 2.70 47.1 0.86 18.0 12.7 0.236 5.81 12.9 1.86 0.35 1171 704 – –LP41 MI 73 0.31 1.48 51.7 1.14 14.9 12.8 0.23 5.63 8.51 2.94 1.19 672 1274 – –LP42 MI 73 0.37 1.44 54.5 1.19 14.4 11.4 0.239 5.12 8.89 2.73 1.09 329 939 – –LP2a 15 82 *(80–82) 0.30 0.87 47.4 0.83 18.4 10.3 0.135 6.36 14.0 1.89 0.37 1289 793 – –LP2a 21 84 *(80–84) 0.31 0.97 47.8 0.91 19.7 8.78 0.209 6.31 13.2 2.39 0.44 930 638 – –LP50 mi1 79 *(72–79) 0.34 1.56 52.9 1.22 15.5 11.3 0.243 4.91 8.90 2.91 1.16 1190 752 – –LP51 MI1 72 *(72–79) 0.39 2.09 52.8 1.09 14.9 12.1 0.14 5.07 9.48 2.75 1.01 496 864 1.68 555LP51 MI1B 72 0.38 2.34 47.1 0.77 18.3 12.3 0.223 5.89 13.8 1.89 0.36 496 864 1.68 591LP52 mi1 72 0.37 2.15 52.6 1.10 15.1 11.6 0.161 5.13 9.48 2.58 0.99 298 898 2.02 769LP53 mi1 74 0.37 1.47 52.8 1.13 16.1 10.7 0.293 4.93 9.29 3.05 1.15 463 864 1.84 903LP54 mi1 72 0.37 0.70 54.1 0.73 15.4 12.2 0.285 5.35 7.71 3.31 0.58 1224 1534 0.24 129LP55 mi2 72 0.34 0.66 52.0 1.04 15.0 12.9 0.245 5.36 9.36 2.45 0.90 625 943 1.79 648LP55 mi1 72 0.37 1.28 53.2 1.20 15.1 12.0 0.181 5.10 8.82 2.74 1.08 381 968 1.13 1231LP56 mi1 72 0.35 0.60 52.4 1.09 14.9 12.6 0.211 5.22 9.39 2.59 0.99 266 823 1.65 1239LP56 MI2 72 0.33 0.09 52.1 1.10 15.1 12.6 0.287 5.21 9.41 2.67 0.95 288 802 – –

El Hoyo pyroxene-hosted MI'sLP2 MI1 70 0.27 – 54.8 1.01 14.3 9.83 3.28 6.88 0.151 4.25 8.40 3.05 1.39 332 921LP2 MI2 70 0.27 – 53.0 1.20 14.1 10.9 3.64 7.65 0.222 4.82 8.73 2.93 1.21 397 942LP5 MI1 87 0.28 – 48.4 0.782 16.0 9.03 3.01 6.32 0.104 7.39 11.0 2.23 0.353 1141 692LP8 MI2 86 0.26 – 51.4 1.15 15.3 12.2 4.06 8.52 0.269 5.03 8.34 2.98 0.723 409 651LP8 MI3 87 0.27 – 51.5 0.964 15.7 10.3 3.43 7.19 0.222 5.11 9.01 2.50 0.760 474 828LP AA MI1 70 0.26 – 56.5 1.06 13.5 10.9 3.63 7.63 0.324 3.52 7.03 3.49 1.57 405 1376LP AA MI2 69 0.26 – 57.1 1.08 12.5 11.4 3.81 7.99 0.285 3.31 7.07 3.32 1.50 440 1240LP 2a 8 MI1 74 0.26 – 55.9 0.970 14.0 10.4 3.47 7.28 0.280 3.51 6.93 3.45 1.58 487 1526LPA MI-1 84 0.26 – 52.3 1.23 15.7 12.8 4.25 8.93 0.145 3.76 7.86 2.97 0.991 644 1064LPA MI-2 86 0.25 – 51.2 1.03 18.5 10.0 3.35 7.03 0.331 3.85 6.81 3.03 0.572 1274 1114LPA MI-3 86 0.24 – 55.3 1.33 17.1 9.39 3.13 6.57 0.179 2.69 6.30 3.47 1.44 375 1228LPA MI-4 86 0.24 – 54.7 1.51 16.6 11.1 3.71 7.80 0.185 2.37 6.39 2.85 1.28 335 1177LPB MI-1 64 0.27 – 55.6 1.38 13.5 11.0 3.67 7.71 0.282 3.97 8.11 2.60 1.20 367 991LPB MI-2 73 0.27 – 51.7 1.35 13.3 13.2 4.40 9.24 0.354 4.84 8.71 2.43 0.844 473 864LPB MI-3 73 0.27 – 55.2 1.21 14.0 10.5 3.50 7.34 0.346 4.33 8.68 2.89 0.958 454 835LPB MI-4 73 0.27 – 52.7 1.15 15.1 9.90 3.30 6.93 0.184 4.80 8.83 3.17 0.988 594 987LPC MI-1 81 0.26 – 51.5 1.24 15.8 12.2 4.07 8.54 0.279 4.53 8.50 2.60 0.803 860 859LPD MI-1 64 0.27 – 54.5 1.38 13.1 13.9 4.63 9.73 0.555 4.06 8.43 2.66 1.20 353 922LPD MI-2 65 0.26 – 52.4 1.55 11.9 17.6 5.88 12.3 0.313 3.11 7.39 2.47 0.908 353 922LPE MI-1 71 0.26 – 55.2 1.30 13.5 12.7 4.24 8.90 0.312 3.48 7.08 3.20 1.34 547 1399LPE MI-2 71 0.26 – 56.2 0.976 14.5 10.3 3.44 7.22 0.360 3.64 7.09 3.44 1.50 568 1543LPE MI-3 71 0.26 – 51.8 1.20 14.6 12.5 4.17 8.75 0.392 4.12 8.22 2.94 0.990 568 1543LPF MI-1 75 0.26 – 52.0 1.16 15.0 11.0 3.68 7.73 0.256 4.07 8.05 2.94 1.12 441 821

214 S. Venugopal et al. / Journal of Volcanology and Geothermal Research 325 (2016) 211–224

Table 1 (continued)

Sample Fo Host KD PEC (%) SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O S Cl H2O CO2

LPF MI-2 75 0.27 – 52.2 1.04 14.8 12.1 4.03 8.47 0.065 4.74 8.58 2.72 0.981 441 821LPG MI-1 72 0.25 – 58.4 1.03 14.7 8.86 2.96 6.21 0.208 2.99 6.50 3.59 2.20 370 1769LPH MI-1 79 0.26 – 50.0 1.67 13.1 16.6 5.52 11.6 0.334 3.87 8.29 2.75 0.611 370 1769LPH MI-2 67 0.26 – 50.3 1.58 14.9 15.1 5.05 10.6 0.606 3.72 7.91 2.57 0.663 712 919LPH M-3 67 0.26 – 50.0 1.54 15.3 14.9 4.96 10.4 0.418 4.36 7.89 2.50 0.527 712 919

Sample Location (UTM zone 16) SiO2 TiO2 Al2O3 FeO⁎ MnO MgO CaO Na2O K2O

Whole rocksCerro Negro 1999 532,396.00 m E 1382129.00 m N 48.5 0.720 17.6 12.4 0.190 7.27 12.2 1.86 0.44El Hoyo 1528 532,955.77 m E 1381187.97 m N 50.9 0.878 16.6 11.8 0.186 5.94 11.0 2.45 0.80El Hoyo 1954 533,012.40 m E 1381204.41 m N 49.9 0.703 17.5 10.9 0.177 6.05 12.2 2.08 0.60

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subalkaline basalts (Table 1). Groundmass values are narrow for both ElHoyo samples and thus sufficiently represent the evolved, differentiatedmelt: 52–55 wt% SiO2, 10.4–12.7 wt% FeO*, 1.16–1.58 wt% K2O and0.02–0.07 wt% H2O. The main distinction between the 1528 and 1954groundmass is FeO* such that the 1954 El Hoyo late-stage melts aremuch more iron rich (FeO* N 12 wt%) than the 1528 groundmass. ElHoyo olivines range from Fo71–Fo84 with many exhibiting zonationwith core Fo77–Fo84 and rim Fo75–Fo71. Melt inclusions trapped withinthe primitive core plot within the field of Cerro Negro melt inclusions(Fig. 2).

El Hoyo pyroxenes from the 1528 sample also have preserved zona-tion with a wide range of Mg# from 69 to 87, suggesting crystallisationoccurred over a wide temperature range. Additionally, some pyroxenecores appear to be as primitive as olivines while others are moreevolved implying coeval crystallisation of olivine and pyroxene withinthe El Hoyo magma reservoir (Fig. 3).

Fig. 2. Cerro Negro and El Hoyo samples. A) Thin section of the 1999 Cerro Negro sample illustrmicrolites in the groundmass are dominantly plagioclase and pyroxene. B) Cerro Negro olivinsection of the 1528 El Hoyo sample including subhedral and glomeroporphyritic phenocrysts.

Using the groundmass compositions, olivines with a composition ofFo65 and pyroxenes with Mg# 72 are in equilibrium with the late stagemelt. Pyroxenes with Mg# 79 are in equilibrium with the whole rocksample of 1528 whereas olivines with a forsterite composition of Fo77are in equilibrium with the 1954 El Hoyo whole rock composition.

In terms of El Hoyo olivine-hosted melt inclusions, there is a largeoxide range: 47.1–54.4 wt% SiO2, 8.78–12.9 wt% FeO*, 4.91–6.36 wt%MgO and 0.35–1.23 wt% K2O (Table 1; Fig. 3). The high variability ofmajor oxides is consistentwith thepresence of zoned olivines. Similarly,melts hosted in pyroxene are compositionally diverse: 48.4–58.4 wt%SiO2, 6.21–12.3 wt% FeO*, 2.37–7.39 wt% MgO and 0.35–2.20 wt% K2O.El Hoyo differentiation (Fig. 3) begins with primitive melts containinglow SiO2, K2O and high MgO trapped in Cerro Negro-like olivinesthat differentiate towards an evolved composition represented bymelts trapped in pyroxene rims with high SiO2, K2O and low MgOcontents.

ating the subhedral plagioclase (plag), olivine (olv) and pyroxene (pyx) phenocrysts. Thee-hosted melt inclusion (CN-D; Fo 82) with glass and shrinkage bubble outlined. C) ThinD) El Hoyo pyroxene-hosted melt inclusion (LP-E; Mg# 71).

Table 2Corrected trace element concentrations for Cerro Negro and El Hoyo MI's. Values given in ppm. Those indicated in bold (V to Nd) have better than 15% error.

Sample V51 Co59 Ni60 Rb85 Sr88 Y89 Zr90 Nb93 Ba137 La139 Ce140 Pr141 Nd146 Sm147 Eu153 Gd157 Tb159 Dy163 Ho165 Er166 Yb172 Lu175 Hf178 Ta181 Pb208 Th232 U238

Cerro Negro olivine-hosted MI'sCN-1 99E MI1 – 52.6 38.0 3.51 441 11.8 20.3 1.12 249 2.15 5.25 0.88 4.58 1.40 0.62 2.06 0.318 2.11 0.470 1.31 1.26 0.202 0.620 – 1.66 0.096 0.098CN-1 99E MI2 – 35.5 11.5 3.45 434 11.1 19.6 1.11 248 2.17 5.24 0.87 4.59 1.42 0.64 1.83 0.305 2.00 0.443 1.29 1.22 0.174 0.658 0.058 1.56 0.102 0.096CN-1 99D MI – 49.9 77.0 2.39 390 9.52 23.6 1.75 214 2.73 6.43 0.96 4.05 1.33 0.61 1.89 0.232 1.79 0.458 0.91 1.14 0.180 1.10 0.111 1.21 0.185 0.129CN-1 99B MI-2 – 72.9 112 5.09 369 10.6 31.1 1.57 355 3.18 7.65 1.13 5.36 1.58 0.71 1.11 0.352 2.04 0.476 1.30 1.56 0.125 1.15 – 2.02 0.281 0.270CN-1 99O MI1 – 22.7 11.0 3.44 367 9.40 22.4 2.28 211 2.68 6.21 1.14 5.09 1.26 0.69 1.99 0.230 1.70 0.381 0.80 0.912 0.129 0.392 – 1.59 0.129 –CN-1 99C MI-1 298 30.7 8.96 5.25 416 10.2 29.5 1.92 322 3.66 8.44 1.27 6.76 2.03 0.69 2.09 0.308 2.07 0.403 1.14 1.06 0.164 0.852 0.106 1.98 0.237 0.202CN-1 99F MI-1 357 41.2 28.1 4.09 456 10.9 25.4 1.72 291 3.28 8.03 1.16 5.85 1.71 0.76 1.63 0.321 2.12 0.423 1.17 0.766 0.160 0.562 0.141 2.06 0.214 0.117CN-1 99G MI-1 372 29.8 9.40 4.18 466 13.6 26.3 1.58 299 3.11 7.48 1.33 7.19 1.74 0.85 2.62 0.404 2.63 0.554 1.39 1.71 0.211 0.815 – – 0.167 0.133CN2 MI99J 359 28.9 8.87 4.08 471 12.5 21.5 1.28 292 2.68 6.45 1.00 5.43 1.80 0.67 2.05 0.334 2.30 0.485 1.40 1.42 0.216 0.582 – 1.99 0.118 0.090CN2 MI99J-2 379 23.1 2.87 4.40 498 13.4 22.5 1.44 312 2.92 6.69 1.04 5.92 1.70 0.82 2.03 0.331 2.52 0.529 1.61 1.45 0.243 0.666 0.091 2.01 0.135 0.112CN2 MI99N 326 32.9 8.56 8.35 438 10.7 30.1 2.70 308 3.33 8.24 1.23 5.76 2.15 0.68 1.86 0.384 2.18 0.395 1.09 1.35 0.122 0.739 0.595 4.63 0.369 0.144

El Hoyo olivine-hosted MI'sLP40 MI1 – 67.2 102 9.22 530 14.9 40.5 1.16 305 5.21 12.1 1.98 9.33 2.44 0.89 2.74 0.426 2.72 0.575 1.62 1.55 0.239 1.13 0.049 1.93 0.466 0.390LP2a 15 – 34.8 24.9 7.09 555 14.4 35.5 0.59 203 4.85 11.2 1.74 8.57 2.20 0.77 2.26 0.541 2.46 0.611 1.50 1.41 0.174 1.36 – 1.46 0.392 0.171LP51 MI1 373 38.3 12.8 20.5 438 22.4 81.0 2.81 572 8.2 18.9 2.83 14.0 4.04 1.04 4.70 0.599 3.75 0.824 2.3 2.61 0.354 1.98 0.149 3.67 0.883 0.809LP51 MI1B 371 36.5 11.5 20.4 447 23.3 82.5 2.95 583 8.41 19.7 2.86 14.5 3.92 1.12 3.99 0.630 4.23 0.874 2.51 2.37 0.347 2.16 0.186 3.62 0.947 0.757LP50 mi1 442 40.7 23.5 7.63 549 13.9 32.8 0.67 193 4.48 10.9 1.63 8.51 2.21 0.89 2.32 0.412 2.56 0.525 1.56 1.49 0.229 0.910 – 1.68 0.292 0.179LP52 mi1 372 47.5 26.6 23.9 441 24.1 89.8 3.02 628 9.15 20.8 3.09 15.5 3.94 1.21 3.95 0.666 4.29 0.871 2.76 2.38 0.364 2.50 0.175 3.73 1.01 0.837LP54 mi1 358 42.7 15.5 9.38 596 26.6 43.2 1.20 511 4.17 10.3 1.81 9.43 3.72 1.18 4.07 0.817 5.16 1.12 2.91 2.87 0.406 1.11 – 2.5 0.263 0.235LP55 mi2 444 41.6 12.7 20.1 471 24.9 78.0 2.90 561 7.96 18.8 2.79 13.7 3.48 1.11 3.75 0.634 4.29 0.932 2.61 2.67 0.487 2.19 0.263 3.73 0.774 0.677LP55 mi1 371 34.7 11.9 21.6 458 25.5 87.5 3.07 608 8.78 20.6 3.15 15.6 4.20 1.13 4.47 0.700 4.51 1.00 2.72 2.80 0.391 2.36 0.172 3.42 1.00 0.815LP56 mi1 418 39.6 14.1 22.0 468 23.6 79.4 2.81 576 8.57 20.0 3.00 14.8 4.10 1.23 4.11 0.637 4.48 0.921 2.65 2.64 0.384 2.20 0.175 3.59 0.923 0.762LP30 MI 351 36.0 23.7 25.6 448 22.3 83.5 3.28 670 8.49 19.8 2.89 14.7 3.98 1.13 3.91 0.625 4.00 0.809 2.33 2.25 0.327 2.18 0.197 3.79 0.962 0.838LP34MI 361 38.6 – 20.9 460 23.1 81.1 2.91 613 8.50 19.8 2.95 14.5 3.81 1.25 4.26 0.644 4.45 0.872 2.37 2.60 0.314 2.19 0.185 4.08 0.905 0.780LP35 MI 341 40.0 30.8 19.2 434 22.3 86.5 2.95 596 8.92 20.2 3.24 15.1 3.44 1.33 4.15 0.567 4.21 0.758 2.35 1.95 0.346 2.63 0.174 3.52 0.951 0.796LP38 MI1 370 40.6 14.2 23.8 483 26.1 88.0 3.08 637 9.21 21.4 3.21 14.9 4.67 1.41 4.08 0.751 4.95 0.995 2.71 2.75 0.356 2.32 0.221 4.11 1.04 0.876LP39 MI1 371 38.7 15.4 19.0 439 23.5 83.3 3.12 601 8.54 20.0 2.73 14.9 3.68 1.08 3.81 0.581 4.15 0.845 3.05 2.80 0.436 2.07 0.231 3.6 0.984 0.894

El Hoyo pyroxene-hosted MI'sLP5 MI1 – 36.2 38.6 6.06 412 11.9 33.9 1.19 157 3.77 8.88 1.37 7.12 2.07 0.63 2.18 0.309 2.27 0.514 1.26 1.32 0.156 0.926 0.085 1.17 0.388 0.233LP2 MI2 – 33.5 47.1 6.29 353 9.30 28.5 0.98 154 3.22 7.83 1.23 5.88 1.50 0.55 1.97 0.257 1.68 0.356 1.05 0.940 0.122 0.904 0.089 1.14 0.259 0.198LPE MI-1 177 26.8 4.55 48.8 328 37.4 174 8.24 1341 14.0 31.2 4.41 21.9 5.77 1.55 6.50 0.969 6.73 1.41 4.02 4.07 0.625 4.64 0.520 7.21 2.19 1.914LPH MI-1 253 34.2 4.75 29.4 352 48.3 145 5.57 772 10.9 26.8 4.27 23.1 7.14 1.77 8.15 1.312 8.97 1.83 5.39 5.18 0.771 4.06 0.363 4.68 1.54 1.228LP AA MI2 252 29.6 3.30 26.3 313 47.1 126 5.05 879 10.9 26.6 4.29 22.5 6.73 1.50 7.61 1.238 8.94 1.70 5.21 4.78 0.769 3.12 0.316 4.74 1.40 1.059LP 2a 8 MI1 240 28.4 4.68 29.8 317 29.1 115 4.99 943 9.72 22.0 3.35 16.1 4.59 1.27 4.53 0.797 5.21 1.09 3.20 3.29 0.412 3.16 0.310 5.39 1.41 1.146LP8 MI3 310 34.2 21.6 11.2 430 20.6 75.1 2.68 423 6.45 15.1 2.45 13.1 2.38 1.15 3.68 0.410 3.82 1.12 2.76 3.14 0.430 2.63 0.290 2.12 0.770 0.660LPC MI-1 191 23.2 26.3 3.70 142 11.4 24.7 0.73 119 2.01 5.29 0.81 4.78 1.92 0.54 1.85 0.283 2.30 0.478 1.26 1.04 0.163 0.853 0.067 0.72 0.154 0.220LPA MI-1 420 44.5 11.9 19.8 560 30.8 109 4.87 752 11.4 25.7 3.63 21.3 5.82 1.44 6.95 0.810 5.66 1.25 3.99 3.65 0.360 3.47 0.290 4.53 1.27 1.12

Values indicated in bold (V to Nd) have better than 15 % error.

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Fig. 3. Major oxide diagrams for Cerro Negro and El Hoyo melt inclusions (MI). Overall, Cerro Negro olivine-hosted melts dominate the primitive endmember while El Hoyo pyroxene-hosted melts represent the evolved endmember. All values given in wt%. Error bars are 2σ. See Table 1 for values.

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Volatile contents for El Hoyo olivine-hostedmelt inclusions are 266–1288 ppm S, 638–1534 ppm Cl, 0.24–2.09 wt% H2O and 42–1239 ppmCO2 (Fig. 4). Sulphur contents are much lower than Cerro Negromelts whereas chlorine is enriched. Water and CO2 contentswere measured in 10 inclusions (Fo71–Fo77). Volatile contents forpyroxene-hosted melt inclusions show a wider sulphur range, similarchlorine contents: 96–1273 ppm S and 651–1769 ppm Cl. Waterand CO2 contents were analyzed for 7 inclusions hosted in pyroxenesMg# 69–79 (Fig. 4). Overall, pyroxene-hosted inclusions containthe lowest H2O and CO2 contents measured: 0.47–1.17 wt% H2O, and64–486 ppm CO2.

In terms of trace elements, El Hoyo olivine- and pyroxene-hostedmelt inclusions display a wider variability than Cerro Negro melts(Fig. 5). Elements presented in Fig. 5 are those indicated in bold in

Table 2. When normalised to primitive mantle values (Sun et al.,1989), compositions range from depleted to enriched but maintain thetypical arc-like pattern with elevated large ion lithophile element con-tents (LILE). Though similar patterns of depleted LREE and elevated con-centrations of fluid-mobile elements exist, the degree of enrichment/depletion is not as strong as the Cerro Negro samples. There is a cleardiscrepancy between primitive and evolved hosts such that primitivehosts are much more depleted in all elements relative to evolved hosts.

Interestingly, sample LP-E consistently defines the maxima of thenormalised trace element spectrum for both compatible and incompat-ible elements. This evolved melt is hosted in a pyroxene with Mg# 71and contains the highest content of all fluid mobile elements (Rb to Pbon Fig. 5; Table 2).

Fig. 4. Volatile contents relative to K2O contents of melt inclusions. Sulphur, water and CO2 degas with increasing differentiation while chlorine remains incompatible and enriches in themelt. Errors bars are 2σ. See Table 1 for values.

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4.3. Post entrapment modifications

Following entrapment, melt inclusions can undergo modificationsthat are mainly controlled by temperature. Modifications include bub-ble formation, host crystallisation, Fe and Mg diffusion, formation ofdaughter minerals and volatile leakage. All prepared melt inclusionsare primary, lack daughter minerals, and are not fractured.

Once the melt inclusion is trapped and begins to cool, the firstcrystallisation event is the host phase in the form of an overgrowthrim (Schiano, 2003). Rim crystallisation will continue until the trappedmelt is saturated with respect to the host. This rim must be accountedfor in order to derive the original melt composition. Using the well-known Fe-Mg distribution coefficient between olivine and melt(KD = 0.30 ± 0.03; Roeder and Emslie, 1970; Toplis, 2005), correctingolivine-hosted inclusions to their original composition is wellestablished. Correcting pyroxene-hosted inclusions, on the other hand,is not as straightforward. Recent studies have shown that the Fe-Mg dis-tribution coefficient between pyroxene and melt can be approximatedto KD = 0.28 ± 0.08 (Putirka, 2008), but this value is associated withsignificant uncertainty. Therefore, we cannot confidently correct thesemelt inclusion compositions for pyroxene host crystallisation since it in-duces high degrees of error on the resulting melt compositions.

For Cerro Negro olivine-hosted melt inclusions, the KD values (KD Ol-

Melt = [XFe-Olv/XMg-Olv] / [XFe-Melt/XMg-Melt]) are scattered from 0.20 to0.35 with 6 melt inclusions generating high KD values (KD N 0.35). ElHoyo olivine-hosted melts have KD values scattered between 0.23 and0.35, with 11 melt inclusions yielding KD N 0.35. According to themodel by Toplis (2005), KD values higher than 0.35 nevertheless corre-spond to equilibrium values (0.30–0.40) and could be elevated due tosilica and alkali effects. We have corrected the major oxides contentsof olivine-hosted melt inclusions using Petrolog3 software(Danyushevsky & Plechov, 2011) and the Toplis (2005) model. Equilib-rium values between melt and host suggest the melt inclusions suffi-ciently represent the melt at the time of entrapment with little to nopost entrapment modifications (PEC values between 0 and 10.2%;

Table 1). Corrected values for major and trace elements are presentedin Tables 1 and 2, respectively.

Based on the method by Putirka (2008), the calculated KD forclinopyroxenes in this study is 0.24–0.28, which is consistent with theliterature value of 0.28 (Putirka, 2008). This similarity suggests therewas minimal host crystallisation of pyroxene within the melt inclusion.

Furthermodifications include the diffusion of Fe from themelt to theovergrowth rim and diffusion of Mg from the rim to the melt creating acompositional gradient along the rim (Danyushevsky et al., 2002). Evi-dence of diffusion is in the form of anomalously low iron content ofmelt inclusions and abnormally high KD values. Since our melts are suf-ficiently rich in iron with acceptable KD values, there has likely been noFe-Mg diffusion in our samples.

5. Discussion

5.1. Magmatic storage conditions

Using melt inclusion compositions, magmatic conditions such astemperature, oxygen fugacity, and pressure can be estimated. The ad-vantage of using both olivines and pyroxenes is that changes in storageconditions can be tracked throughout the crystallisation sequence. Asummary of calculated values is provided as Supplementary material.

Crystallisation temperatures were estimated from olivine-liquidthermometry and clinopyroxene-liquid thermometry using methodsfrom Putirka (2008) and Putirka et al. (2003), respectively. CerroNegro melts are 1083–1174 °C (±60 °C), El Hoyo olivine-hosted meltsare 1089–1151 °C (±60 °C) and the temperature range for El Hoyoclinopyroxenes is 1017–1183 °C (±60 °C). Interestingly, the pyroxenetemperature range encompasses both Cerro Negro and El Hoyoolivine-hosted melts, which is consistent with the wide range ofclinopyroxene host Mg#.

Knowledge of melt temperature allows for the estimation of the ox-ygen fugacity. Two separate calculations were performed: the first isbased on Jugo et al. (2005) and considers the equilibrium S6+ contentof themelt inclusion. This can only be applied over narrowΔFMQvalues

Fig. 5. Primitive-mantle normalised trace elements for Cerro Negro (in red) and ElHoyo olivine- and pyroxene-hostedmelts (in blue and grey). The depletion in Th and enrichment in Ba istypical for Cerro Negro. Cerro Negro occupies the lower end member while LP-E (in black) defines the maxima. There is a clear trend of melts trapped in primitive hosts (red and blue)towards those trapped in evolved hosts (grey). See Table 2 for concentrations. (For interpretation of the references to color in thisfigure legend, the reader is referred to theweb version ofthis article.)

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that cover sulphide stability (0.75 b ΔFMQ b 1.9). The second approach,developed byWallace and Carmichael (1994), is an expression to derivethe sulphur speciation from the oxidation state. It is based on S6+/ST, SKα wavelength shifts, temperature and coefficients obtained from lin-ear regression. Both approaches yield similar fugacity values of NNO toNNO + 2 for Cerro Negro and El Hoyo olivine-hosted melts whereasEl Hoyo pyroxene-hostedmelts plot closer to the NNO buffer. These rel-atively oxidising conditions are reflected in the elevated Fe3+/FeT valuesobtained for the two volcanoes (0.30 for Cerro Negro and 0.21–0.25 forEl Hoyo; Supplementary material).

Pressure estimates were calculated using three independent sourcesand equations. Papale et al. (2006) developed a barometric model thatconsiders thermodynamic equilibrium between gaseous and liquid vol-atile components in silicate melts. Based on this approach, Cerro Negropressure estimates are 1.2–2.4 kbar (±0.001 kbar), which equates to3.5–7.3 km depth. For El Hoyo olivine- and pyroxene-hosted melts,pressure values are 0.3–2.6 kbar ± 0.001 kbar (0.7–7.9 km) and 0.2–0.8 kbar± 0.001 kbar (0.6–2.6 km), respectively. These values correlatewell with the second approach, which is a clinopyroxene barometer de-veloped by Putirka et al. (2003) and the third method, which comparesthe H2O and CO2 contents with isobaric solubility curves usingVolatileCalc (Newman & Lowenstern, 2002; Supplementary material).

To summarize, the Cerro Negro melt is 1174 °C, NNO + 1.4 andhosted at 7.0 km depth. These conditions are similar to the CerroNegro-like olivine-hosted melts from El Hoyo: 1151 °C, NNO + 1 andtrapped at 8.0 km depth. Using differentiated El Hoyo olivine hosts(Fo72–Fo74), the evolved El Hoyo melts are 1129 °C, NNO + 1 andhosted in a reservoir at 4.0 km depth.

These conditions are consistent with findings from previous geo-physical work by MacQueen (2013) who revealed an interconnectedpositive density anomaly at 2 km depth. This storage region followsthe aforementioned shallow reservoir. Additionally, our results are inaccordancewith the study by Roggensack et al. (1997)who investigatedthe variable nature of the explosive 1992 and effusive 1995 eruptions atCerro Negro. The more energetic eruption in 1992 was due to the rapidascent of primitive and volatile-rich magma from a 6 km storage region(Roggensack et al., 1997), which is consistentwith the 7 kmCerroNegroreservoir calculated in this work. Conversely, the 1995 eruption was a

result of volatile-rich magma ascending, stalling and extensivelydegasing within a 2 km reservoir prior to eruption.

However, some variability exists between our dataset and the workby previous authors. These discrepancies are primarily due to the vola-tile content of themelt inclusions analyzed and to the plumbing systeminherent to Cerro Negro. Roggensack (2001) studied relatively magne-sian (Fo73–Fo83) olivine-hosted melt inclusions from the 1867 CerroNegro eruption and found high, yet variable, volatile contents. Thesedata correlate to deep saturation pressures (N4.2 kbar) and representthe primitive source region for Cerro Negro, calculated to reside at~14 km depth. Based on the variable volatile saturation pressuresfound in almost all Cerro Negro melt inclusion studies, along with thepetrologic work by Walker and Carr (1986) and Carr and Walker(1987), it was proposed that Cerro Negro magmas represent fraction-ation and ascent within ephemeral dykes and reservoirs rather thanlong-lived magma chambers (Roggensack, 2001).

Similarly, Portnyagin et al. (2012) studied olivine-hostedmelt inclu-sions from the 1971 and 1992 eruptions and found variable saturationpressures as well as evidence for decompression-driven fractionationwithin dykes sourced from 14 km depth. These melts show muchmore variability than our samples and this is likely due to the mecha-nism of phenocryst re-distribution as proposed by Portnyagin et al.(2012) and Carr and Walker (1987). Essentially, the minor composi-tional variability between eruptions at Cerro Negro can be best ex-plained by the degree of plagioclase, olivine and pyroxene separationin ascending magma at 7 km depth (Portnyagin et al., 2012; Carr andWalker, 1987). Since the 1971 and 1992 eruptions contain a higher pro-portion of plagioclase phenocrysts (63% by volume) than the 1999 sam-ples (up to 20%), the trapped melt inclusions and resulting whole rockstherefore shift to slightly more evolved compositions compared to oursamples.

5.2. Magmatic processes

Major oxides and volatiles between Cerro Negro and El Hoyo meltscan reveal a first order evolutionary pathway for ascending magma.Fig. 3 illustrates a clear differentiation trend where Cerro Negro meltsdefine the primitive, slightly degassed endmember and El Hoyopyroxene-hosted melts define the evolved, degassed endmember. The

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compositional continuum evident in Figs. 3 and 4 support a genetic linkbetween Cerro Negro and El Hoyo magmas.

Though major element and volatile concentrations can reveal im-portant information regarding processes, they are less sensitive thantrace elements in distinguishing between fractional crystallisation andmagma mixing. It is therefore imperative to consider trace elements astheir concentrations can strongly delineate the processes occurring inthe melt during evolution.

The incompatible elements Th, La, Rb and Ce are chosen because oftheir low partition coefficients between olivine, clinopyroxene andmelt. For olivine-melt partitioning, DTh, DLa, DRb and DCe are 0.03, 0.01,0.04 and 0.01, respectively. For clinopyroxene-melt partitioning DTh,DLa, DRb and DCe are 0.04, 0.031, 0.13 and 0.26 (Villemant et al., 1981;Fugimaki et al., 1984; Paster et al., 1974). All values have 10% error.

Compatible elements Co and Ni are chosen due to their strong affin-ity for olivine and clinopyroxene in basaltic systems: DCo for olivine andclinopyroxene-melt partitioning is 5.21± 1.50 and 2.84± 0.65, respec-tively. DNi for olivine-melt is 22.3 ± 9.12 and for clinopyroxene-melt is0.95 ± 0.11 (Laubier et al., 2014).

A linear correlation exists when comparing Th and La (Fig. 6a),which represents either fractional crystallisation or magma mixing.Due to error propagation, it is uncertain whether the best-fit lines passthrough the origin making it difficult to determine the dominant pro-cess. However, when comparing Th/Ce versus Th, Cerro Negro magmasyield a linear correlation, which represents fractional crystallisationwhereas the clear hyperbolic relation that defines El Hoyo suggestsmagma mixing (Fig. 6b; Schiano et al., 2010). To mitigate any uncer-tainty, incompatible versus compatible elements are compared to betterpinpoint the major processes occurring (Fig. 6c–d). When comparingthe compatible elements Co and Ni against the incompatible elementsLa and Rb, the hyperbolic trend for CerroNegro provides further supportfor fractional crystallisation (Fig. 6c). Interestingly, Fig. 6c & d both show

Fig. 6. Comparison of incompatible elements to delineate themajor processes. A) A linear correlNegro melts to yield El Hoyo melts. However, this requires extrapolation through the origin. ErHowever, themixing curve dominantly represents the El Hoyomagmawhile fractional crystallilinear relation between 1/Co and Rb/Co (C) and 1/Ni and La/Ni (D). From C & D, it appears that tcan be approximated by LP-E, the pyroxene-hosted El Hoyo sample with the highest Th/Ce an

a distinct linear mixing trend for El Hoyo magmas with Cerro Negro asone end member. In addition to these plots, further support for mixingcomes from El Hoyo zoned olivine phenocrysts. Mixing of magmas canlead to zonation where the primitive core is evidence of one end mem-ber and the evolved rim is the signature of the second, more differenti-ated end member. Melts trapped within the primitive core of El Hoyozoned olivines are essentially the same composition as Cerro Negromelt inclusions whereas the melts trapped within the rims are muchmore evolved and deviate away from the Cerro Negro composition(Figs. 3, 4 & 5). As it is more likely to have a combination of processesrather than one single event, it is plausible that Cerro Negro melts frac-tionally crystallise while El Hoyo magmas are products of mixing.

To further investigate themagmatic processes, we use two softwareprograms to model the melt inclusion compositions: Petrolog3(Danyushevsky and Plechov, 2011) and alphaMELTS (Smith andAsimow, 2005; Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998).When using the most primitive Cerro Negro sample, CN-D (Fo82),Petrolog3 predicts 20% fractional crystallisation of olivine, plagioclaseand clinopyroxene and yields a resulting melt composition that is com-parable to the melt inclusions from the 1999 eruption. In terms of ElHoyo melts, Petrolog3 fractionation modelling cannot reproduce themajor oxide and trace element patterns. Therefore, the lack of consis-tency between a fractional crystallisation model and analytical resultsprompts the investigation of a mixing trend.

When considering magma mixing, it is clear that the primitive endmember is Cerro Negro (Fig. 6c & d). Cerro Negro melts have a uniquetrace element composition due to the dissection of the thinNicaraguan crust by extensional faults and fractures that facilitatesrapid upwelling of tholeiitic magma (Carr, 1984). El Hoyo magmas, onthe other hand, show an even more distinct pattern with enrichmentin large ion lithophile elements (LILE) and light rare earth elements(LREE; Fig. 5). Enrichment in these elements is generally due to the

ation between Th and La suggests that themain process is fractional crystallisation of Cerroror bars are 2σ. B) When comparing Th/Ce and Th, a clear binary mixing curve is evident.sation controls Cerro Negromelt compositions. Magmamixing is further supported by thehe primitive endmember of mixing is Cerro Negromagmawhile the evolved endmemberd Rb/K ratios measured.

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influence offluids supplied by the subducting slab (Carr, 1984).We pro-pose that the evolved end member is best represented by the melt in-clusion that consistently defines the maxima of the trace elementspectrum in Figs. 5 and 6. Sample LP-E is the most evolved melt ana-lyzed and contains the highest content of incompatible elements(Table 2). Additionally, El Hoyo melt inclusions show anomalouslyhigh CO2 contents (~1230 ppm; Fig. 4), which suggests the evolvedend member is further enriched in a CO2 fluid phase.

Based on the calculations presented in Section 5.1 and the suggestedend members, magma mixing is modelled with alphaMELTS underisenthalpic conditions between 1.2–2 kbars and 1050–1200 °C by incre-mentally assimilating the evolved El Hoyomelt (LP-E) with additions ofprimitive Cerro Negro melt (CN-D). This successfully reproduces the ElHoyo melt inclusion trend in terms of both major and trace elementcompositions (Fig. 7). Modelling with alphaMELTS also predicts thatcrystallisation occurs simultaneously with each addition of CerroNegro melt. The predicted phenocryst assemblage is consistent with

Fig. 7. Comparing alphaMELTS output (light blue line) with Cerro Negro and El Hoyo melt(represented by sample CN-D, Fo 82) to the evolved end member composition (represented bFurthermore, alphaMELTS also predicts simultaneous crystallisation of pyroxene and plagfractionation, as proposed by Portnyagin et al. (2012).

our whole rock samples. However, it is important to note that olivineis a product of decompression-driven crystallisation (Portnyagin et al.,2012), which is difficult to reproduce with modelling.

The evolved end member, sample LP-E, likely represents residualmagma within the El Hoyo complex that has differentiated due to inac-tivity. In fact, the mixing and fractionation process described abovecould be a variant of the MASH (melting, assimilating, storage and ho-mogenisation) process originally proposed by Hildreth and Moorbath(1988) where partial melts assimilate wall rock and create a hybridmagma. In the case of Cerro Negro and El Hoyo, primitive Cerro Negromagma originally sourced from the 14 km storage region first fraction-ates olivine in the 7–8 km magma storage region. From here, olivine-bearing primitive melts are injected into the El Hoyo region around4 km depth where they progressively assimilate the residual evolvedmagma and produce a hybrid magma containing olivine, clinopyroxeneand plagioclase phenocrysts.

inclusion data from this study. By incrementally mixing primitive Cerro Negro magmay sample LP-E Mg# 71), the El Hoyo melt inclusion samples are successfully reproduced.ioclase. We estimate that olivine crystallisation is a product of decompression-driven

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5.3. Conceptual model

To envision the array of processes occurring beneath Cerro Negroand El Hoyo,we present a conceptualmodel of the subsurfacemagmaticplumbing system (Fig. 8). This model combines ideas from previousstudies as well as those proposed here.

Themain features of the subsurfacemodel include a source region at14 km (Roggensack, 2001), Cerro Negro reservoirs at 7–8 km, an ElHoyo reservoir at 3–4 km and an interconnected region at 2 km(MacQueen, 2013). In addition to these reservoirs, there are likely nu-merous, active and inactive cross cutting dykes and shallow intrusivecomplexes present within the plumbing system.

Geilert et al. (2012) compared whole rocks from El Hoyo and thenearby La Paz Centro-Malpaisillo lineament and showed a systematicvariation in trace element concentrations over a small distance(20 km). From this they concluded that magma beneath the lineament,

Fig. 8. The proposedmodel for the plumbing system beneath Cerro Negro and the El Hoyo Comregion (Roggensack, 2001), Cerro Negro reservoirs at 2 and 7–8 km (Roggensack et al., 1997; Mstudy). Bluefilled circles represent primitive Cerro Negromagmas and pink circles are evolved aof hybrid El Hoyo magmas and the large black dashed ellipse indicates the several reservoirs bcutting dykes. Thick black lines represent dykes.

El Hoyo, and by extension Cerro Negro, follows predominantly verticalpathways from source to edifice. This idea is similar to the concept ofdyke capture whereby ascending magma travels along faults as amore mechanically favourable path (Connor and Conway, 2000;Gaffney et al., 2007). As this follows the hypothesis of ephemeraldykes beneath Cerro Negro (Roggensack, 2001), we employ vertical,structurally controlled dykes in our subsurface model.

In fact, due to the structural control of the Maribios Range and theextensional tectonic regime created by oblique subduction, the rela-tively large primitive source at 14 kmdepthwould preferentially supplymagmas from the edges of the reservoir feeding faults and fractures.From here, primitive, Mg- and volatile-rich melts are injected upwardsthrough dykes and undergo differentiation and fractional crystallisationwithin a 7–8 km transient reservoir beneath Cerro Negro (Fig. 8). Fromhere, there would be a series of dykes with various destinations: onedyke would transport melt directly to the Cerro Negro edifice, as was

plex. This illustration is a combination of previous studies, notably: 14 km primitive sourceacQueen, 2013; this study) and an interconnected region at 2 km (MacQueen, 2013; thisnd unerupted El Hoyomagmas. The gradation (mixing) between the two is representativeelonging to the El Hoyo massif. The dashed green circle represents a dense area of cross-

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the case in the 1992 eruption of primitive, volatile-rich magma whileanother dyke would feed a 2 km magma storage region where meltsdegas prior to erupting (Roggensack et al., 1997; Fig. 8). One (ormany) dyke(s) would supply Cerro Negro magma, containing primitiveolivines, to the El Hoyo reservoir, which contains evolved residualmagma. This 4 km reservoir would be the location of mixing betweenthe primitive Cerro Negro magma and the long-lived, El Hoyo magma(Fig. 8). Mixing of magmas would then produce the outer rim ofzoned olivines and the observed trace element differences of El Hoyomagmas. From the 2 km Cerro Negro storage region and the 4 km ElHoyo mixing reservoir, there would be a series of cross cutting dykesand intrusive complexes that connect the reservoir to their respectivemain vent (Fig. 8). Themost active dykes are likely formed via dyke cap-ture andwould parallel theNNW trending normal faults in theMaribiosRange (La Femina et al., 2002). In the study byMacQueen (2013), it wasnoted that the 2 km connection imaged by gravity surveys could be anarea of numerous dyke intersections. Additionally, the tendency of theEl Hoyo gravity anomaly to extend deeper than the Cerro Negro anom-aly is explained by the 4 km reservoir.

As amodern analogue, the Lemptégy cinder cones in theMassif Cen-tral, France are considered as they are believed to be similar to CerroNegro in terms of eruptive style and, potentially, intrusive structures.Fieldwork on these monogenetic cones has revealed alternating epi-sodes of converging and diverging dyke intrusions triggered by slightshifts in the local stress regime (Petronis et al., 2013; Delcamp et al.,2012). Our model seems increasingly conceivable as Lemptégy showsthat both the internal structure of Cerro Negro and the upper crustcould be composed of branching dykes, especially since the local stressregime in Nicaragua has a significant impact on volcanic activity. There-fore, it is plausible for new episodes of converging and diverging dykesto either promote the longevity of Cerro Negro volcano or create newvolcanic centres within the El Hoyo complex, respectively.

6. Implications

The main implication of this model is that Cerro Negro and El Hoyoare elements of the same complex and as such, Cerro Negro should beconsidered as the newest active conduit within the El Hoyo Complex.Eruptions at Cerro Negro will likely intensify over time and lead to thedevelopment of stratovolcanic features. Moreover, with respect to the1952 and 1954 eruptions, tectonic relief of stress within magma reser-voirs can reoccur and cause El Hoyo to erupt once again. Though it ispossible that the dyke connecting the 8 km and 4 km reservoirs is nolonger active, it does not rule out the possibility of new dyke formationvia divergence.

It is therefore critical to continue monitoring this entire complex asincreasingly intensive eruptions at Cerro Negro coupled with the possi-bility of future eruptions at El Hoyo increases the risks posed to nearbytowns and villages. Monitoring efforts should focus on seismic activitydue to its control on dyke formation. Active monitoring should beginon El Hoyo: based on the anomalously high number of eruptions andseismic activity in Nicaragua over the past few years, reactivation ElHoyo is not impossible.

7. Conclusions

Analyses of olivine- and pyroxene-hostedmelt inclusions fromCerroNegro and the El Hoyo Complex reveal a compositional continuum be-tween these two systems. Major oxide contents suggest this trend rep-resents fractional crystallisation. However, the linear and hyperboliccorrelations of trace elements imply that Cerro Negromagmas fraction-ally crystallise while El Hoyo magmas are products of magma mixing.Incompatible-compatible element relationships indicate Cerro Negrois one endmember of mixing while the other is a more evolved compo-sition. This end member is well represented by sample LP-E, which isthemost evolvedmelt inclusion analyzedwith thehighest incompatible

element contents.We propose that the evolved endmember is residualmagma within the El Hoyo Complex that has formed by differentiationin this part of the system due to inactivity.

Combining the results from this study with all available literaturedata, a conceivable subsurface model, illustrating the mechanisms ofmagma mixing, is created in order to explain the diverse compositionaltrends (Fig. 8). An important consequence of this model is that CerroNegro should be considered as the newest ventwithin the El Hoyo Com-plex. Similar to the Lemptégy cinder cones in France, diverging and con-verging dykes are vital to the internal structure of Cerro Negro and theshallow upper crust. Since these dykes respond to shifts in the localstress regime, the likelihood of dykes diverging towards El Hoyomeans future eruptions around this vent are possible, imposing signifi-cant risk for surrounding towns and cities.

In addition tomonitoring efforts, futureworkwithin this area shouldfocus on El Hoyo melt inclusions to determine whether they representthe evolved end member. Since volcanic lineaments are common inNicaragua, future work should also focus on melt inclusions from Rotavolcano, Lake Asososca and Cerro LaMula, which all lie on the same vol-canic lineament as Cerro Negro and Las Pilas. If these systems have com-positional similarities then it is possible for the subsurface plumbingsystem to be more expansive than previously imagined.

Acknowledgments

We thank Jean-Luc Devidal (LMV) for assistance with the electronmicroprobe and LA-ICP-MS and Nicolas Cluzel (LMV) for his assistancewith melt inclusion preparation. We thank Etienne Deloule for hishelp with the ion probe at CRPG Nancy. Jeffrey Zurek and PatriciaMacQueen are thanked for their help in the field. We greatly appreciatethe valuable input and discussion from Muriel Laubier, Mark Jellinek,Nathalie Vigouroux, and Benjamin van Wyk de Vries. This work waspartially supported by a NSERC Discovery grant to G. Williams-Jones.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jvolgeores.2016.06.001.

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