Small Volume Andesite Magmas and Melt–Mush Interactions at Ruapehu, New Zealand: Evidence from Melt Inclusions

Contrib Mineral Petrol DOI 10.1007/s00410-013-0880-7

ORIGINAL PAPER

Small volume andesite magmas and melt–mush interactions at Ruapehu, New Zealand: evidence from melt inclusions

Geoff Kilgour • Jon Blundy • Kathy Cashman • Heidy M. Mader

Received: 1 November 2012 / Accepted: 12 April 2013 Ó Springer-Verlag Berlin Heidelberg 2013

Abstract Historical eruptions from Mt. Ruapehu (New From these data, we infer that individual melt batches rise Zealand) have been small (\0.001 km3 of juvenile magma) through, and interact with, crystal mush zones formed by and have often occurred without significant warning. antecedent magmas. From this perspective, we envision the Developing better modelling tools requires an improved magmatic system at Ruapehu as frequently recharged by understanding of the magma storage and transport system small magma inputs that, in turn, cool and crystallise to beneath the volcano. Towards that end, we have analysed varying degrees. Melts that are able to erupt through this the volatile content and major element chemistry of network of crystal mush entrain (to a greater or lesser groundmass glass and phenocryst-hosted melt inclusions in extent) exotic crystals. In the extreme case (such as the 1996 erupted samples from 1945 to 1996. We find that during this eruption), the resulting scoria contain melt inclusion-bear- time period, magma has been stored at depths of *2–9 km, ing crystals that are exotic to the transporting magma. consistent with inferences from geophysical data. Our data Finally, we suggest that complex interactions between also show that Ruapehu magmas are relatively H2O-poor recharge and antecedent magmas are probably common, but (\2 wt%) and CO2-rich (B1,000 ppm) compared to typical that the small volumes and short time scales of recharge at arc andesites. Surprisingly, we find that melt inclusions are Ruapehu provide a unique window into these processes. often more evolved than their transporting melt (as inferred from groundmass glass compositions). Furthermore, even Keywords Andesite Á Volatile Á Melt inclusions Á eruptions that are separated by less than 2 years exhibit Ruapehu Á Crystal mush Á Antecryst Á H2O Á CO2 Á distinct major element chemistry, which suggests that each Magma mixing eruption involved magma with a unique ascent history.

Introduction Communicated by G. Moore.

Electronic supplementary material The online version of this Magma erupted from andesitic volcanoes often records a article (doi:10.1007/s00410-013-0880-7) contains supplementary complex history, and interactions between recharge and material, which is available to authorized users. antecedent magmas and/or crystal mush zones are common G. Kilgour Á J. Blundy Á K. Cashman Á H. M. Mader (e.g., Nakamura 1995; Murphy et al. 1998; Nakagawa et al. School of Earth Sciences, University of Bristol, Wills Memorial 1999, 2002; Devine et al. 1998). Most studies that have Building, Bristol BS8 1RJ, UK highlighted this complexity, however, have focussed on moderate to large volume andesitic eruptions, where subtle, G. Kilgour (&) Wairakei Research Centre, GNS Science, Taupo 3330, fine-scale interactions may be obscured by large recharge New Zealand volumes. Mt. Ruapehu, New Zealand, a frequently active e-mail: [email protected] andesite volcano that has historically erupted very small magma volumes thus provides an interesting test case for K. Cashman Department of Geological Sciences, 1272 University of Oregon, imaging the complexity of subvolcanic magmatic pro- Eugene, OR 97403, USA cesses. In addition, eruptions from Ruapehu have been 123 Contrib Mineral Petrol extremely difficult to predict (Sherburn et al. 1999), due to involved a series of overlapping craters (Hackett and both the small volumes of magma involved in individual Houghton 1989), with the most recent activity confined to eruptions and the presence of an active hydrothermal sys- the southernmost crater, currently occupied by the warm tem, which generates a rather noisy background seismic (15–40 °C) and acidic (0–1 pH) Crater Lake (Hackett, signal (Hurst 1998). Therefore, a secondary goal of our 1985). study is to improve our knowledge of both the magma Historical activity has consisted primarily of frequent plumbing and magma transport systems beneath Ruapehu. small phreatic (e.g., Kilgour et al. 2007, 2010) and phre- Pre-eruptive conditions of magma storage and recharge atomagmatic eruptions through Crater Lake (Healy et al. can be determined by analysing the compositions of co- 1978; Nairn et al. 1979; Houghton et al. 1987). Larger existing crystals (phenocrysts and microlites) and glasses magmatic events occurred in 1945 (Oliver 1945; Reed (groundmass and crystal-hosted melt inclusions). Together 1945; Beck 1950; Gregg 1960), 1969 (Healy et al. 1978), these data provide information on storage temperature and 1975 (Houghton et al. 1987), and 1995–96 (Houghton et al. pressure, magma–magma interactions, and the mixing of 1996; Bryan and Sherburn 1999; Nakagawa et al. 1999, magma and crystal mush zones (e.g., Roedder 1984; 2002; Johnston et al. 2000). Eruptive activity typically Blundy and Cashman 2005; Liu et al. 2006). When com- involves surtseyan jets of lake water and steam accompa- bined with airborne gas chemistry monitoring (e.g., nied by base surges and ballistic fall-out up to 2 km from Christenson et al. 2010), the volatile content of the glass the vent. These events are usually confined to the summit phases also provides information on the fate of the main area (Houghton et al. 1987; Kilgour et al. 2010), but rare volatile components during degassing. strombolian activity and more widespread sub-plinian to We present volatile and major element chemistry of plinian ash falls also occur (Donoghue 1991; Pardo et al. Ruapehu melt inclusions and groundmass glass from more 2011). than 50 years of eruptions (1945–1996). We use samples of scoria and lava from every historical magmatic eruption Historical eruption narrative during this period to track changes in magma composition through time. Importantly, we provide new evidence that Volcanic activity at Ruapehu has been observed and recent eruptions have been driven by small batches of recorded since c. 1850 (Gregg 1960; Hackett and Houghton recharge magma that have mingled with, entrained crystals 1989; Reed 1945). More than 40 eruptions have been from, and remobilised regions of shallow-stored, anteced- reported since 1945, covering a range of eruptive styles and ent magma. sizes (B. Scott pers. comm.). Our samples derive from We analysed representative samples (from GNS Science, magmatic eruptions that ejected juvenile scoria and ash New Zealand rock archives) from six magmatic eruptions at onto the summit plateau and beyond. Ruapehu: 1945, 1969, 1971, 1977, 1995, and 1996. In this Eruptive activity at Ruapehu between March and July paper, we identify eruptions by year, not by the specific 1945 (Reed 1945) occurred prior to the installation of eruption date. This is particularly relevant for the volcanic monitoring systems. The 1945 eruption initiated 1995–1996 eruptive episode, as we were not able to dis- with explosive magmatic activity that produced high steam tinguish between eruptions in September and October 1995 plumes and dispersed ash to c. 200 km from the vent and the June and July 1996 eruptions (c.f. Nakagawa et al. (Johnston et al. 2000). A series of lava domes were then 1999, 2002). Similarly, we were unable to identify a unique constructed and partially destroyed, presumably through eruption date for the 1945 and 1971 samples. Thus, our mass wasting and sector collapse (Reed 1945; Beck 1950). samples are from eruptions (1) occurring between March The total volume of erupted magma is estimated at and December 1945 [1945]; (2) on 22 June 1969 [1969]; (3) *0.1 km3 (Johnston et al. 2000). from April to July 1971 [1971]; (4) on 2 November 1977 Between 1945 and the next magmatic eruption in 1969, [1977]; (5) from September to October 1995 [1995]; and (6) a limited seismic network was installed and Crater Lake from June to July 1996 [1996] (Table 1). temperature measurements and chemical sampling were initiated. A moderately large eruption (0.9 9 106 m3)on Geological background 22 June 1969 ejected older lava lithics, lake sediments, and *5 vol % juvenile scoria (Healy et al. 1978). We have At 2,797 mASL, Mt. Ruapehu is the largest and most active calculated the bulk volume of juvenile magma to be andesitic stratovolcano in New Zealand. It is located at the 4.5 9 104 m3, with a dense rock equivalent (DRE) of southern end of the Taupo Volcanic Zone (TVZ) and the 1.7 9 104 m3. This event was preceded by only limited summit area is covered by small permanent glaciers and seismic precursors and a regular Crater Lake heating– snowfields (Fig. 1). Ruapehu has been active for at least cooling cycle considered to reflect normal activity (Healy 200 ka (Gamble et al. 2003). Holocene activity has et al. 1978). 123 Contrib Mineral Petrol

Table 1 Bulk XRF data of scoria from selected historical eruptions from Ruapehu Eruption year 1945 1969 1971 1977 1977 1995 1995 1995 1995 1996a Sample number 1945A 22/5/69-1 1971A 1977A 1977-8 31195B 31195A 71195-04 161195-34 130896

Major elements (wt %)

SiO2 60.16 61.10 58.50 60.44 58.88 57.68 57.90 58.02 61.54 57.57

Al2O3 16.87 15.79 16.24 16.43 16.09 16.30 16.33 16.24 16.58 16.40

Fe2O3 5.80 6.22 6.94 7.65 7.17 7.36 7.33 7.25 6.36 7.43 MnO 0.09 0.09 0.10 0.07 0.10 0.11 0.11 0.11 0.08 0.12 MgO 3.55 4.00 5.04 3.62 4.73 5.45 5.42 5.31 3.61 5.37 CaO 6.02 6.10 7.31 7.08 7.30 7.75 7.74 7.67 6.30 7.87

Na2O 3.47 3.36 3.30 2.55 3.11 3.25 3.25 3.22 2.88 3.30

K2O 1.67 2.15 1.54 1.39 1.48 1.36 1.38 1.43 1.64 1.32

TiO2 0.66 0.65 0.62 0.66 0.64 0.62 0.62 0.62 0.67 0.64

P2O5 0.14 0.16 0.14 0.14 0.14 0.13 0.14 0.14 0.14 0.10 LOI 1.49 0.20 0.05 -0.12 0.20 -0.04 -0.07 -0.08 0.00 0.00 Total 99.92 99.81 99.77 99.92 99.83 99.98 100.15 99.93 99.80 100.00 Trace elements (ppm) As46 3221 2 3 3 4 Ba 384 422 358 353 346 307 320 330 390 352 Ce 27 30 25 39 30 26 21 28 23 22 Cr 48 78 117 97 115 105 113 95 92 114 Cu 64 59 68 86 62 66 68 66 72 72 Ga 20 17 17 18 17 17 16 16 17 17 La 18 14 19 22 \5 \511\5 \510 Nb \17 \1 \1 \1 \1 \1 \1 \14 Ni 22 41 57 50 57 65 62 57 45 59 Pb 11 13 12 12 13 13 12 10 14 10 Rb 60 83 54 51 53 48 48 50 56 47 Sc 19 19 23 22 24 24 28 21 21 26 Sr 280 243 261 252 258 262 263 259 269 264 Th79 5555 4 4 4 4 U 23 3222 \12 2 1 V 152 153 176 184 178 182 189 175 173 196 Y 20212019181817191719 Zn 65 61 65 64 66 67 68 68 67 68 Zr 134 154 113 121 115 104 103 105 126 106 a The 1996 sample is taken from Gamble et al. (1999)

During the 3 months prior to the 1971 eruption, Crater Crater Lake increased from c. 19 to 30 °C, yet without any Lake temperatures rose from ca. 25 to 55 °C, and the coincident volcanic earthquakes (Sherburn et al. 1999). The abundance and amplitude of volcanic earthquakes (signal- absence of volcanic earthquakes could reflect the small ling magma movement) increased (Sherburn et al. 1999). amount of fresh magma injected to shallow levels, as well as There also appears to have been an increase in volcanic the rather sparse seismometer network installed at the time. earthquakes over a few weeks before first eruption on 3 April Although limited field data exist for either eruption, they 1971. The 1971 eruption was a small, phreatomagmatic appear to have been similar in size to a well-characterised event that was confined to the summit plateau. A phreato- eruption in 2007 (based on photographs of the summit area magmatic eruption on 2 November 1977 was small com- after each eruption). Juvenile scoria was erupted during pared to eruptions of 1945, 1969, and 1975 (samples of the those events, and we have again assigned a value of 5 wt% 1975 scoria were not available for this study). The 1977 of the bulk deposit as juvenile material. Therefore, if we take eruption occurred one to 2 weeks after the temperature of a similar bulk volume to that of the 2007 event (Kilgour et al.

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a b

Fig. 1 a Location map of Mt Ruapehu at the southern end of the North with Ngauruhoe in the background (GNS Science archive Taupo Volcanic Zone. b Photograph of Ruapehu’s summit plateau image). Approximate locations for the two main vents (North and and Crater Lake. This photograph is taken from the South towards the Central) beneath the lake (Christenson et al. 2010) are marked

2010), the amount of magma erupted in each eruption in of deformation determined from theodolite levelling sur- 1971 and 1977 is of the order 1 9 104 m3 (DRE). veys (B. Scott 2011 pers. comm.), indicated that the A period of relative quiescence occurred from 1988 to amount of magma driving the Mg/Cl ratio changes was 1994. However, volcanic tremor (c. 2 Hz) at Ruapehu very small. On 18 September 1995, a small phreato- rose to, and was maintained at, high levels starting in the magmatic eruption occurred with few seismic precursors early 1990s. During this time period, Crater Lake heating (Bryan and Sherburn 1999); following this event, a large cycles were often punctuated by steam explosions, phreatomagmatic eruption developed. A lull in activity although there were no signs of signiﬁcant (magmatic) from November 1995 to June 1996 allowed direct mea- eruptive activity (Sherburn et al. 1999). A period of rapid surements of the main fumaroles within the inner crater heating of Crater Lake occurred in November 1994. (Christenson 2000). Tremor increased to pre-September During this time, the lake temperature increased from c. 1995 levels on 15–16 June 1996 and on 17 June, pul- 19 to 50 °C in 1 month, and numerous phreatic eruptions sating, phreatomagmatic eruptions graded into a more occurred within Crater Lake. However, this activity was continuous eruption, with plumes reaching 8.5 km asl not interpreted as indicative of magmatic injection to (Prata and Grant 2001). The 1995–1996 eruptions were shallow levels (Christenson 2000). The lake heating cycle approximately two orders of magnitude larger than the appeared to be over by April 1995, but was immediately eruptions of the 1960s and 1970s. Tephra dispersal followed by another heating event. More phreatic erup- mapping suggests DRE volumes for the 11–14 October tions occurred from April to early July. By this time, a 1995 and the 17–18 June 1996 eruptions of 3 9 107 m3 steady increase in the Mg/Cl ratio of the lake water and 6 9 106 m3, respectively (Cronin et al. 1998; Fig. 2). suggested that magma was being injected to shallow Between 1996 and 2007, Ruapehu remained relatively levels and interacting with the hydrothermal system quiet except for one small phreatic event on 4 October (Christenson 2000). Moderate levels of seismicity and 2006 (Kilgour et al. 2007; Mordret et al. 2010). Then on 25 Crater Lake temperatures, combined with and the absence September 2007, after *9 min of precursory seismic

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Fig. 2 Variations in eruption volume, magmatic temperature, and circles) geothermometers of Putirka (2008), and Fe–Ti oxides mineral chemistry between historical eruptions at Ruapehu. Dense (crosses) using LePage (2003) for 1969 only. Plagioclase core, rim, rock equivalent (DRE) volumes for 1945 (Johnston et al. 2000), 1969 and microlite (Plag C, R, M, respectively) anorthite content (An %), (Healy et al. 1978), 1971 and 1977 (assuming a similar volume and clinopyroxene (cpx) and orthopyroxene (opx) magnesian number erupted in 2007 from Kilgour et al. (2010)), and 1995–1996 (Cronin (Mg#) are also shown. Plagioclase rim MgO content is expressed as a et al. 1998). Magmatic temperatures were calculated using the range (open rectangles). Average values for plagioclase (An) and plagioclase-liquid (open triangles) and clinopyroxene-liquid (ﬁlled pyroxene (Mg#) are denoted by an ‘‘x’’ signals (Jolly et al. 2010), a short-lived phreatic eruption The synthesis of data from historical eruptions presented from Crater Lake created a northerly directed blast that above shows that recent eruptions of Ruapehu are small but deposited ballistic blocks and surtseyan jets (Kilgour et al. frequent, and may occur without warning. The volumes of 2010). Mapping of the deposits within the summit plateau magma involved in priming the magmatic system for yielded a total volume of *3 9 105 m3. The presence of eruption also appear to be small. Here, we use detailed juvenile magma in the 2007 eruption is equivocal and so compositional analyses of samples from these eruptions, in we have not included these samples in our study. the context of this eruptive narrative, to improve our

123 Contrib Mineral Petrol understanding of the physical conditions of magma storage, to reduce the effects of alkali migration (e.g., Humphreys recharge, and eruption at Ruapehu. et al. 2006). Plagioclase and pyroxene crystals were analysed using a 20-kV accelerating voltage, 10 nA beam current and a focussed beam. Calibration used a selection Methods of mineral and oxide standards. Data were reduced using the ZAF procedure. All samples were analysed for bulk rock chemistry by X-ray fluorescence (XRF) for major and trace elements at SpectraChem, Wellington (NZ). Samples were then pre- Results pared for the analysis of phase compositions by lightly crushing individual scoria clasts for each eruption (we Petrography of Ruapehu samples combined two scoria clasts from 1995) and hand-picking phenocrysts for mounting in epoxy resin onto glass slides. Samples available for this study include a lava sample from The preparation of crystal-separate thin sections, rather 1945 and scoria from 1969, 1971, 1977, 1995, and 1996. than grain mounts, minimised the carbon background from The scoriae are vesicular, porphyritic, and microlite-rich excess epoxy during secondary ion mass spectrometry (with the exception of 1969), similar to pre-historic Ru- (SIMS) analysis of volatiles. Each crystal separate was apehu scoria (Hackett 1985; Graham and Hackett 1987; polished to ca. 100 lm thickness to expose melt inclusions. Gamble et al. 1999). Here, we use the term phenocryst to We obtained back-scattered electron (BSE) images using a signify a crystal that is significantly larger (*1–2 mm) Hitachi S-3500 N SEM at the University of Bristol at than microlites present in the groundmass, independent of 15 kV and at a working distance of *15 mm. These its origin. We use antecryst to mean a crystal that images were used to map melt inclusions trapped in pla- demonstrably grew within a different magma than its cur- gioclase, orthopyroxene, and clinopyroxene phenocrysts. rent host (i.e., exotic), and cognate to mean a crystal that Approximately, 70 % of the melt inclusions were larger grew from and erupted with its host magma. than 25 lm, the minimum spot size of SIMS analyses. Phenocrysts of plagioclase dominate the mineral SIMS analyses of the volatiles dissolved in melt inclu- assemblage with lesser amounts of clinopyroxene and sions were carried out on Au-coated grain mounts using a orthopyroxene (Table 2). However, there is no clear crys- Cameca IMS-4f ion microprobe at the University of tallisation sequence; pyroxene is often found within pla- Edinburgh. We constructed working curves 1H/30Si versus gioclase phenocrysts, while plagioclase inclusions are also 12 30 H2O and C/ Si versus CO2 (e.g., Blundy and Cashman seen in pyroxene crystals. The implication is that these 2008) using a total of nine rhyolitic glass standards that three phases precipitated cotectically. Of the minor phases, range from 0.15 to 4.1 wt% H2O and 0–2,860 ppm CO2. magnetite is present in all samples as small blocky grains Standards were run at three intervals during each day to up to 30 lm across. Ilmenite is absent in all samples except account for drift in the analyses. 1H was analysed at low in 1969 scoria, in agreement with previous observations mass resolution. For 12C, interference from 24Mg2? is (Nakagawa et al. 1999; Price et al. 2012). Hornblende is significant at the relatively high MgO contents (2.5 wt%) absent in all samples; this is also the case for all but one of Ruapehu melt inclusions. Separation of the Mg and CO2 lava exposed on the edifice of the volcano (Hackett 1985). spectra thus required us to conduct our SIMS measure- Microlites (crystals \100 lm across) of plagioclase, ments at high mass resolution, and first analyse melt clinopyroxene, orthopyroxene, magnetite, and rare ilmenite inclusions for CO2 followed by H2O. We did not analyse the 1945 lava sample via SIMS due to concerns over H2O diffusion out of the melt inclusions in such slowly cooled Table 2 Representative crystallinity of Ruapehu scoria samples (e.g., Hauri 2002). a Electron probe micro-analysis (EPMA) can damage Eruption date 1945 1969 1971 1977 1995 1996 Sample number 1945F 1969A 1971A 1977A 031195B 57536 hydrous silicate glass. For this reason, we measured the major element composition of the inclusions by EPMA Phenocrysts (%) 40 29 33 31 26 36 after SIMS analysis was completed (e.g., Blundy et al. Groundmass (%) 59 42 50 49 54 39 2010). EPMA was conducted using a CAMECA SX-100 Vesicle (%) 1 29 17 20 21 25 five-spectrometer wavelength dispersive spectrometry Phenocrysts (WDS) instrument at the University of Bristol. Melt Plagioclase 29 20 23 22 18 19 inclusions and groundmass glasses were analysed using a Clinopyroxene 6 4 5 4 3 8 15-kV accelerating voltage, 4 nA beam current with a Orthopyroxene 5 5 5 4 5 9 defocused 10 lm beam, with K and Na being analysed first a The 1996 sample is taken from Gamble et al. (1999) 123 Contrib Mineral Petrol

are present in the groundmass of most samples (c.f. Nak- compositional range (with a mean of Wo*42;En*43; agawa et al. 1999). Fs*15; Fig. 2). Oscillatory zoning of clinopyroxenes is The bulk compositions of scoria and lava samples from evident in all Ruapehu scoria (e.g., Nakagawa et al. 1999,

1945 to 1996 have been reported by Gamble et al. (1999) 2002). Normal zoning (Mg#53–77) is most common, and Nakagawa et al. (1999) and pre-historical compositions although many clinopyroxene crystals (both phenocrysts by Price et al. (2012). We have conducted further bulk rock and microlites) preserve a very thin (2–5 lm) Mg-rich XRF analyses on samples from historical eruptions, which outermost rim (Fig. 3). Melt inclusions are common as agree well with data from Gamble et al. (1999) (Table 1). both small (\10 lm), glassy inclusions forming concen- There are no discernible systematic changes in bulk trically to the growth pattern and large ([50 lm), isolated chemistry, which ranges from *57 to 64 wt% SiO2, inclusions near the core of the crystal. mineral content or abundance, or isotopic composition Orthopyroxene phenocrysts are euhedral to subhedral, through time (Gamble et al. 1999). This chemical monot- relatively unzoned, and B4 mm in length, with a compo- ony attests to a genetic link among all Ruapehu magmas. sition of Wo*3;En*60;Fs*37—enstatite (Fig. 2). Rare Interestingly, scoria erupted during the 1995–1996 erup- zoned (both normal and reverse; Mg#36–50) orthopyroxene tions span the entire range of the historical record crystals are found in all historical samples. Orthopyroxene-

(*58–62 wt% SiO2). hosted melt inclusions are less common than in clinopyroxene and are present as small (\ 10 lm) inclusions on the margins of the crystal. Larger ([30 lm) melt inclusions Mineralogy are uncommon in phenocrysts from all eruptions. Plagioclase phenocrysts are up to 4 mm across and are generally zoned from a calcic core (An55–82) to a sodic rim Major element chemistry of groundmass glass (An52–65) (Fig. 2). The anorthite (An) content of phenocryst rims is similar to that of plagioclase microlites The groundmass glasses of Ruapehu scoria span a wide

(An50–65). Rare plagioclase phenocrysts are reversely compositional range from 58 to 78 wt% SiO2 (Table 3; zoned from *An52 to * An60. The MgO content of pla- Figs. 4a, 5, Supplementary Table 1). Glass compositions gioclase is commonly used to monitor maﬁc inputs to the from individual Ruapehu eruptions can be distinguished magmatic system (e.g. Hattori and Sato 1996). There is not from each other by means of major element binary plots. much variation in MgO content among our samples, except The least evolved glasses are found in 1995 and 1996 for 1971 and 1995 samples, which show elevated MgO in scoria (Fig. 5). Groundmass glasses from 1945 lava and some plagioclase phenocryst rims (up to 0.15 wt%; see 1969 scoria are signiﬁcantly more evolved than all other also Nakagawa et al. 1999). historical samples; in the case of 1945, this is possibly due Clinopyroxene phenocrysts are subhedral to euhedral to slower cooling of the lava sample. Conversely, glasses and B3 mm in length. All analysed samples show the same from the 1971 and 1977 eruptions extend to lower SiO2

a b

Fig. 3 a BSE image of a clinopyroxene phenocryst from the 1995 b Dashed black lines denote the boundary between relatively diffuse eruption of Ruapehu. Oscillatory zoning is common throughout all Fe- and Mg-rich zones. Arrow points to the *3 lm wide, dark grey eruptions. The greyscale images highlight zones of relatively higher rim (Mg-rich) on the outermost margin of the crystal, elsewhere Fe (light grey) and Mg (dark grey) zones. Note the more diffuse interpreted as late-stage mixing (e.g., Saunders et al. 2012). Ground- boundary at the core compared to the sharp boundary at the rim. mass glass (gl) is shown in (b) 123 Contrib Mineral Petrol

Table 3 Average major element composition of groundmass glass from historical Ruapehu eruptions Major Element (wt %) 1945 1969 1971 1977 1995 1996

SiO2 77.28 (0.91) 72.07 (1.24) 68.88 (2.08) 67.93 (1.07) 63.2 (1.24) 62.06 (1.23)

Al2O3 12.41 (0.7) 13.32 (0.91) 14.36 (1.34) 13.95 (0.43) 15.34 (0.59) 15.68 (1.2) FeO 1.82 (0.35) 3.2 (0.52) 5.09 (0.89) 6.07 (0.39) 6.47 (0.55) 4.61 (3.24) MnO 0.02 (0.01) 0.05 (0.02) 0.07 (0.02) 0.09 (0.02) 0.11 (0.04) 2.48 (3.37) MgO 0.11 (0.13) 0.49 (0.32) 0.82 (0.53) 1.22 (0.17) 2.5 (0.61) 2.45 (0.48) CaO 1.03 (0.37) 2.06 (0.51) 3.45 (0.69) 3.9 (0.3) 5.58 (0.6) 5.85 (0.66)

Na2O 1.97 (0.42) 3.83 (0.31) 2.88 (0.84) 2.33 (0.67) 3.18 (0.61) 3.36 (0.65)

K2O 4.47 (0.45) 4.15 (0.3) 3.01 (0.31) 3.05 (0.16) 2.45 (0.26) 2.36 (0.45)

TiO2 0.88 (0.23) 0.68 (0.08) 1.17 (0.18) 1.19 (0.05) 0.99 (0.08) 1.01 (0.11)

P2O5 0.19 (0.05) 0.13 (0.05) 0.25 (0.06) 0.25 (0.04) 0.17 (0.08) 0.2 (0.04) Number of analyses 18 36 17 31 28 45 Standard deviations are given in parentheses. The full dataset can be found in Supplementary Table 1 with little overlap in major element composition with the evolved inclusions are from the 1995–1996 eruptions

1969 glasses. This offset is most clearly seen in Na2O (Fig. 5). (Fig. 4a). Therefore, although some of the eruptions analysed here were less than 2 years apart (1969 and 1971) Volatile content of melt inclusions their groundmass glass composition is distinct (Fig. 4a–d).

A further striking feature of the groundmass glasses is Our SIMS analyses (CO2,H2O, Li, Be, B, F, Cl) of melt the wide range in SiO2 content (4–8 wt%) within an indi- inclusions from historical eruptions at Ruapehu (Table 4; vidual eruption, attesting to the heterogeneous nature of Fig. 6) are the ﬁrst direct measurements of the volatile groundmass glasses from Ruapehu (Fig. 4a–d). For content of Ruapehu magmas. Most Ruapehu melt inclu- instance, the SiO2 content of groundmass glasses erupted in sions have H2O contents of 1–1.5 wt%, with a small 1969 ranges from 69 to 74 wt%, 1971 from 65 to 73 wt%, number of inclusions exceeding 2.5 wt% (Fig. 7), which is 1977 from 66 to 70 wt%, and 1995–96 from 59 to 66 wt%. relatively dry compared to similar andesitic systems in arc The latter are less evolved, however, than the range of settings (e.g., Blundy et al. 2010; Devine et al. 1998;

62–70 wt% SiO2 reported for the 1995–1996 eruptions by Portnyagin et al. 2007; Wallace 2005). According to the Nakagawa et al. (1999); we are not able to explain this literature, the only arc andesite magma with a similarly low discrepancy. For all eruptions, the groundmass glass H2O content is from the 1994 to 1998 eruption of Popo- exhibits a linear trend towards the bulk rock composition in cate´petl (Mexico), with a range of 0.8–3.02 wt% H2O most binary major element plots (Fig. 4a–d). A notable (Atlas et al. 2006). The low H2O contents have implica- exception is Na2O (Fig. 4a), where linear trends of nega- tions for magma evolution, phase relations, and transport tive slope do not extrapolate to the bulk rock composition. properties.

Ruapehu melt inclusions range in CO2 from c. 25 to Major element chemistry of melt inclusions 1,059 ppm, with most inclusions having less than c.

600 ppm CO2 (Table 4; Fig. 7). These CO2 values are The major element chemistry of some pyroxene- and significantly higher than most intermediate subduction plagioclase-hosted melt inclusions is presented in Table 4 zone magmas (Wallace 2005) but are again similar to those (for the full dataset, refer to Supplementary Table 2) and from Popocate´petl, where melt inclusions preserve plotted in Fig. 4e–h. The major element composition of B1,458 ppm CO2 (Atlas et al. 2006). pyroxene- and plagioclase-hosted inclusions is similar in Melt inclusions preserve elevated halogen contents with all samples and covers the same range as the groundmass up to 2,069 ppm F and 1,342 ppm Cl (Table 4; Fig. 6d, g, glass (i.e., 60 to 73 wt% SiO2). Unlike the groundmass h). Average halogen values in melt inclusions are c. glass compositions, however, plagioclase- and pyroxene- 955 ± 175 ppm F and 659 ± 128 ppm Cl; average hosted melt inclusions from individual eruptions do not groundmass glass values are 838 ± 138 ppm F and form distinct clusters in major element plots, but instead 515 ± 88 ppm Cl. These data suggest only limited exhibit significant overlap between eruptions (Fig. 4e–h). degassing at very low pressures, consistent with experi- Overall, however, the most evolved inclusions are from ments on basaltic bulk compositions that show that the the earliest eruption analysed (1945), and the least significant Cl loss requires very low pressures, after most

123 Contrib Mineral Petrol

Fig. 4 Major element plots of groundmass glass (a–d) and pheno- eruption is clear in the groundmass glass, yet the melt inclusions are cryst-hosted melt inclusion compositions (e–h) from 1945 to 1996. relatively tightly clustered with signiﬁcant overlap. Samples are from Bulk rock XRF data are shown as the open black ellipses. All data are the same suite of samples as Fig. 2 and Table 1 re-calculated to anhydrous values. Note the distinction between each

of the initial H2O, SO2, and CO2 has already exsolved groundmass glasses tend to increase with increasing H2O (Lesne et al. 2012). (Fig. 6a), suggesting that Li partitions modestly into the Li concentrations in melt inclusions vary between 22 and vapour phase during degassing. Be concentrations are low,

80 ppm, with most of the data conﬁned to between 40 and from 1 to 5 ppm, and show no correlation with H2O 70 ppm for all eruptions (Table 4; Fig. 6). Matrix glasses (Fig. 6b). Be concentrations in the matrix glass are similar to contain 26–50 ppm Li. Li contents of melt inclusions and the lowest value in melt inclusions of *1 ppm. B contents

123 Contrib Mineral Petrol

Fig. 5 Plot of K2O versus SiO2 showing the evolution of groundmass glasses and melt inclusions to less-evolved compositions with time. All data have been re-calculated to anhydrous values. Symbols are the same as in Fig. 4

of most melt inclusions range from *30 to *80 ppm and recorded in groundmass glass from the 1969 eruption. Evi- are not correlated with H2O (Fig. 6c). Matrix glasses range dently, the behaviour of B and H2O is decoupled during from *30 to *50 ppm B, with some higher values magmatic degassing.

123 oti iea Petrol Mineral Contrib Table 4 Volatile content (SIMS) and major element chemistry (EPMA) of phenocryst-hosted melt inclusions from historical Ruapehu scoria. Major elements are recalculated to anhydrous values

Melt inclusions Secondary ion mass spectrometry (SIMS) data Electron probe micro-analysis (EPMA) data (wt%) Pressure XH2O Host (bars) mineral Sample number H2O CO2 Li Be B F Cl SiO2 TiO2 Na2OAl2O3 FeO K2O CaO MgO Cr2O3 MnO Total (wt%) ppm ppm ppm ppm ppm ppm

1969py-07 inc 03 1.52 766 43 2 61 803 538 70.13 0.87 3.74 14.39 3.66 3.60 2.86 0.69 0.00 0.04 1002159 0.19 Pyroxene 1969py-07 inc 04 1.32 484 41 2 63 876 692 70.16 0.82 3.75 14.11 3.62 4.05 2.75 0.68 0.00 0.05 1001686 0.19 Pyroxene 1969py-07 inc 06 1.58 1059 37 2 53 840 607 69.44 0.78 3.78 14.47 4.03 3.52 3.04 0.83 0.00 0.07 1002682 0.18 Pyroxene 1969py-07 inc 07 1.59 176 43 2 62 883 767 70.01 0.86 3.63 14.53 3.56 3.94 2.76 0.71 0.00 0.00 100 878 0.41 Pyroxene 1969py-08 inc 01 0.97 84 46 4 70 673 304 73.18 0.49 3.57 12.70 3.55 4.46 1.54 0.41 0.00 0.10 100 529 0.29 Pyroxene 1969py-09 inc 01 1.27 250 38 2 58 733 846 69.86 0.75 3.23 14.59 3.72 4.58 2.56 0.64 0.01 0.04 1001085 0.26 Pyroxene 1969py-09 inc 02 1.41 286 44 2 66 810 1037 68.64 0.95 3.19 14.66 3.96 4.52 3.10 0.85 0.02 0.08 1001235 0.28 Pyroxene 1969py-09 inc 04 1.68 503 59 2 48 2005 1131 68.84 0.75 3.88 14.64 4.41 3.19 3.20 0.95 0.00 0.08 1001730 0.26 Pyroxene 1969py-09 inc 05 0.73 33 44 2 66 839 422 72.69 0.67 3.74 12.82 3.31 4.51 1.67 0.43 0.00 0.16 100 303 0.33 Pyroxene 1969py-09 inc 06 1.54 360 44 2 58 705 824 68.33 0.94 3.31 14.49 4.57 4.44 3.03 0.72 0.03 0.08 1001547 0.26 Pyroxene 1969py-12 inc 04 1.90 79 79 4 79 1046 603 68.71 0.98 4.66 14.46 3.95 3.87 2.54 0.79 0.01 0.01 100 729 0.55 Pyroxene 1969py-12 inc 05 2.19 104 43 4 58 1213 614 68.21 0.70 4.29 15.88 3.49 3.41 2.90 1.04 0.00 0.07 100 789 0.67 Pyroxene 1969py-12 inc 06 2.15 185 44 4 61 1281 670 68.07 0.63 4.54 15.65 3.35 3.68 2.94 1.03 0.02 0.08 1001013 0.53 Pyroxene 1969pl-05 inc 01 0.96 176 51 2 67 1001 685 68.64 1.57 3.12 12.71 5.72 3.75 3.01 1.33 0.00 0.11 1001336 0.16 Plagioclase 1969pl-05 inc 02 0.93 343 57 2 74 1507 997 67.16 1.98 3.21 12.81 6.16 3.56 3.44 1.49 0.00 0.12 1001750 0.13 Plagioclase 1969pl-05 inc 03 0.94 39 57 2 78 1404 999 66.71 1.85 3.57 12.64 6.45 3.64 3.42 1.53 0.02 0.11 100 806 0.22 Plagioclase 1969pl-10 inc 01 0.85 247 54 2 63 1306 821 68.17 1.57 3.87 13.00 5.33 3.73 2.63 1.55 0.02 0.09 1001522 0.12 Plagioclase 1969pl-11 inc 01 0.99 66 57 2 63 844 597 70.65 0.90 3.92 13.32 4.12 4.19 2.12 0.66 0.00 0.10 100 537 0.30 Plagioclase 1969pl-11 inc 02 0.96 303 51 2 53 924 553 70.40 0.87 4.06 13.52 4.04 4.14 2.22 0.65 0.00 0.06 1001449 0.14 Plagioclase 1969pl-11 inc 03 0.98 292 53 2 47 1076 594 69.90 1.05 4.22 13.60 3.85 4.30 2.30 0.66 0.02 0.04 1001446 0.14 Plagioclase 1969pl-11 inc 04 0.98 279 52 2 49 1015 515 70.69 0.73 4.05 13.39 4.13 4.17 2.13 0.62 0.00 0.09 1001411 0.14 Plagioclase 1969pl-11 inc 05 0.71 216 43 2 53 978 421 70.33 0.94 3.84 13.36 4.28 4.04 2.31 0.73 0.00 0.15 1001112 0.11 Plagioclase 1969pl-11 inc 06 0.95 216 50 2 66 1001 434 70.33 0.94 3.84 13.36 4.28 4.04 2.31 0.73 0.00 0.15 1001268 0.15 Plagioclase 1969pl-11a inc 06 0.83 309 47 2 59 999 586 70.17 0.90 3.88 13.44 4.12 4.12 2.49 0.81 0.00 0.02 1001454 0.11 Plagioclase 1969pl-12 inc 01 0.77 350 43 2 57 1095 599 69.59 1.11 3.06 12.64 5.43 4.12 2.65 1.28 0.00 0.07 1001669 0.09 Plagioclase 1971py-02 inc 01 1.36 169 42 4 57 737 584 68.34 1.07 3.87 14.43 5.10 3.92 2.61 0.65 0.00 0.00 1001358 0.23 Pyroxene 1971py-02 inc 03 1.33 155 39 2 53 816 515 67.71 1.13 3.91 14.30 5.67 3.61 2.72 0.80 0.00 0.11 1001054 0.28 Pyroxene 1971py-03 inc 01 1.20 530 35 2 56 1411 699 68.47 0.83 3.97 14.81 4.55 4.53 2.14 0.57 0.01 0.05 1001996 0.15 Pyroxene 1971py-03 inc 02 1.29 850 32 2 52 1382 599 68.66 0.70 4.17 14.33 4.39 5.01 2.05 0.60 0.01 0.00 1002758 0.12 Pyroxene 1971py-07 inc 01 1.00 90 24 2 47 717 371 69.96 1.16 3.59 14.24 4.28 3.69 2.44 0.48 0.03 0.11 100 567 0.34 Pyroxene 123 1971py-07 inc 02 1.96 321 36 3 52 963 491 69.38 1.05 3.16 14.73 4.43 3.12 3.24 0.77 0.00 0.10 1001273 0.43 Pyroxene 1971py-07 inc 03 2.18 587 38 4 49 908 458 68.95 1.01 3.61 15.03 3.99 2.91 3.50 0.92 0.00 0.05 1001781 0.38 Pyroxene 1971py-08 inc 01 1.75 530 55 3 63 1287 885 68.51 1.27 3.41 13.97 4.52 4.40 3.00 0.78 0.00 0.05 1001953 0.26 Pyroxene 123 Table 4 continued

1971py-08 inc 02 2.03 360 54 2 60 910 687 69.04 1.10 3.85 14.56 4.09 3.84 2.71 0.70 0.00 0.06 1001503 0.38 Pyroxene 1971py-08 inc 03 1.72 380 43 2 57 957 670 69.63 0.76 3.11 13.79 4.59 5.02 2.33 0.68 0.00 0.07 1001654 0.27 Pyroxene 1971py-08 inc 04 1.66 436 46 2 62 901 734 68.68 1.18 3.08 13.72 5.09 4.93 2.41 0.70 0.00 0.04 1001876 0.24 Pyroxene 1971py-09 inc 01 1.36 538 41 3 62 752 510 70.48 0.76 3.49 15.37 3.01 3.51 2.48 0.83 0.02 0.00 1001446 0.25 Pyroxene 1977py-01 inc 02 1.41 314 42 2 65 1010 617 69.03 1.02 3.63 13.80 4.64 3.05 3.51 1.23 0.02 0.06 1001274 0.27 Pyroxene 1977py-01 inc 03 1.47 106 41 2 63 837 465 69.06 1.03 3.74 13.53 4.85 2.98 3.53 1.22 0.00 0.02 100 785 0.41 Pyroxene 1977py-01 inc 04 1.58 271 41 3 57 1272 954 65.84 1.01 4.41 15.18 4.76 3.40 3.63 1.53 0.02 0.07 1001298 0.31 Pyroxene 1977py-04 inc 01 2.48 486 71 4 34 825 675 66.20 0.95 3.99 15.95 4.16 3.19 3.69 1.31 0.02 0.05 1001741 0.46 Pyroxene 1977py-05 inc 02 1.74 377 47 2 44 1218 571 63.76 0.74 4.09 14.97 5.29 2.78 5.53 2.73 0.01 0.05 1001513 0.31 Pyroxene 1977py-06 inc 01 1.45 65 44 2 60 1198 807 67.76 1.02 3.75 14.12 5.07 3.58 3.40 1.19 0.00 0.09 100 715 0.43 Pyroxene 1977py-06 inc 02 1.42 62 43 4 63 970 849 66.34 1.04 3.54 12.94 5.77 3.21 4.74 2.25 0.02 0.15 100 844 0.37 Pyroxene 1977py-06 inc 03 1.43 242 42 3 60 1013 766 64.20 0.92 3.25 11.18 6.37 2.94 6.79 4.24 0.02 0.08 1001628 0.22 Pyroxene 1977py-06 inc 04 1.42 90 43 4 61 948 785 65.98 0.94 3.43 12.74 5.65 3.24 5.12 2.72 0.00 0.16 100 984 0.33 Pyroxene 1977py-07 inc 01 1.56 501 37 4 53 1221 809 67.13 0.85 3.75 13.37 5.67 3.26 4.08 1.75 0.01 0.07 1001911 0.22 Pyroxene 1977py-08 inc 03 1.53 831 43 2 62 954 844 66.95 1.00 3.67 14.40 5.14 3.85 3.66 1.27 0.01 0.00 1002477 0.18 Pyroxene 1995py-02 inc 01 1.30 292 29 2 56 1094 649 64.63 1.22 4.00 14.29 6.65 3.06 4.23 1.71 0.02 0.12 1001504 0.22 Pyroxene 1995py-03 inc 02 1.52 282 35 1 45 951 614 66.14 1.00 4.26 14.99 5.87 2.60 3.75 1.29 0.00 0.05 1001326 0.30 Pyroxene 1995py-05 inc 04 1.39 729 39 3 50 725 753 64.14 1.16 3.79 13.78 8.01 3.11 4.19 1.54 0.00 0.18 1002538 0.16 Pyroxene 1995py-05 inc 05 1.56 63 40 1 41 832 640 63.32 0.96 4.00 15.25 6.67 2.21 5.14 2.32 0.00 0.05 100 691 0.53 Pyroxene 1995py-06 inc 05 1.63 190 36 1 46 1394 882 63.09 0.80 3.88 15.58 6.51 2.85 5.03 2.10 0.00 0.10 1001133 0.37 Pyroxene 1995py-07 inc 06 0.89 1043 41 2 38 1036 882 60.14 0.94 4.19 15.99 7.46 2.28 6.26 2.55 0.00 0.13 1002599 0.1 Pyroxene 1995py-08 inc 02 1.28 546 36 1 64 1103 727 60.57 0.87 6.50 14.43 5.89 4.34 5.25 2.07 0.00 0.05 1002330 0.11 Pyroxene 1995py-09 inc 01 1.29 404 40 2 51 971 1092 65.65 0.72 3.72 15.10 5.36 3.12 4.43 1.68 0.00 0.18 1001481 0.23 Pyroxene 1995py-09 inc 02 1.42 646 41 3 53 943 1207 62.80 1.44 3.68 13.50 8.57 2.81 5.00 1.93 0.00 0.20 1002406 0.17 Pyroxene 1995py-09 inc 03 1.33 376 39 2 51 910 995 64.94 0.85 3.84 14.90 6.12 3.07 4.51 1.55 0.01 0.16 1001553 0.22 Pyroxene 1995py-10 inc 01 1.42 201 35 2 63 1032 888 68.32 0.92 3.53 14.18 4.75 3.84 3.24 1.08 0.00 0.07 1001029 0.33 Pyroxene 1995py-10 inc 02 1.49 358 35 5 68 1217 1043 67.63 0.88 3.57 14.43 4.63 4.10 3.44 1.21 0.00 0.04 1001451 0.27 Pyroxene

1995py-10 inc 04 1.13 133 26 4 43 1065 556 68.32 0.92 3.53 14.18 4.75 3.84 3.24 1.08 0.00 0.07 100 782 0.31 Pyroxene Petrol Mineral Contrib 1996py-01 inc 01 0.45 115 30 3 37 800 479 64.86 1.18 3.20 13.99 6.28 3.34 4.85 2.17 0.01 0.06 100 821 0.11 Pyroxene 1996py-02 inc 01 1.25 471 42 4 55 1313 872 66.26 0.80 3.89 14.57 4.88 3.86 3.90 1.68 0.00 0.09 1001736 0.19 Pyroxene 1996py-02 inc 04 1.89 274 38 1 52 1435 1216 64.90 0.80 2.90 14.49 5.12 4.68 4.73 2.24 0.00 0.07 1001354 0.39 Pyroxene 1996py-03 inc 01 1.64 379 34 4 54 1038 686 67.23 0.94 3.26 14.05 5.46 3.78 3.87 1.29 0.01 0.08 1001561 0.29 Pyroxene 1996py-03 inc 02 1.77 626 33 3 57 1133 679 66.70 0.95 3.54 13.85 5.75 3.73 3.93 1.36 0.00 0.12 1002131 0.25 Pyroxene 1996py-04 inc 01 1.45 897 40 4 57 935 821 65.78 1.12 3.69 14.46 5.79 3.61 4.05 1.34 0.00 0.10 1002551 0.17 Pyroxene oti iea Petrol Mineral Contrib Table 4 continued

1996py-04 inc 02 1.41 603 40 4 54 1163 703 64.77 1.07 3.59 14.90 6.47 3.65 4.10 1.39 0.00 0.02 1002132 0.19 Pyroxene 1996py-06 inc 01 1.38 755 26 3 55 710 688 67.97 0.86 3.49 13.66 5.16 4.23 3.35 1.22 0.00 0.06 1002359 0.17 Pyroxene 1996py-08 inc 01 1.65 279 74 1 66 2069 1342 66.26 0.63 2.27 14.25 6.23 4.99 3.77 1.44 0.02 0.11 1001444 0.32 Pyroxene 1996py-09 inc 01 1.50 181 37 4 49 942 538 66.15 1.21 3.49 14.23 5.96 3.01 4.17 1.62 0.01 0.09 1001070 0.36 Pyroxene 1996py-09 inc 02 1.39 259 35 4 53 944 584 66.51 1.13 3.91 13.84 5.98 3.15 3.83 1.49 0.04 0.12 1001351 0.26 Pyroxene 1996py-09 inc 03 1.48 167 36 4 52 1061 588 65.73 1.14 4.17 13.87 6.30 3.17 3.90 1.51 0.00 0.16 1001137 0.31 Pyroxene 1996py-09 inc 04 1.53 87 35 4 50 1089 528 65.97 1.02 4.09 14.67 5.87 2.87 3.87 1.49 0.00 0.13 100 759 0.46 Pyroxene Groundmass glass 1969-09 gl01 0.07 245 26 1 21 233 129 71.64 0.67 3.77 12.93 3.81 4.10 1.98 0.89 0.04 0.02 100 1969-10 gl01 0.34 0 33 2 37 514 294 71.23 0.74 4.38 12.71 3.74 4.37 1.95 0.65 0.00 0.06 100 1969-10 gl02 0.21 382 46 2 67 549 321 73.29 0.59 3.92 12.52 3.04 4.46 1.62 0.43 0.01 0.02 100 1969-07 gl01 0.40 0 50 2 69 466 270 72.62 0.68 4.06 12.53 3.19 4.52 1.78 0.47 0.00 0.05 100 1969-12 gl01 0.83 45 42 2.0 63 852 537 72.89 0.77 3.45 13.25 3.18 4.00 1.91 0.39 0.04 0.04 100 1969-04 gl01 0.61 7 44 2.1 73 502 239 72.82 0.73 3.85 12.68 3.35 4.17 1.72 0.43 0.00 0.05 100 1969-12 gl08 0.44 22 41 4.5 57 707 351 74.15 0.52 3.35 13.03 2.64 4.40 1.48 0.34 0.00 0.04 100 1969-14 gl06 0.17 10 32 3.5 43 431 197 72.67 0.70 3.32 13.44 3.06 3.95 2.22 0.46 0.00 0.04 100 1977-07 gl03 0.31 1005 38 1.7 48 595 414 66.80 1.21 3.24 13.91 6.30 3.04 4.00 1.17 0.00 0.11 100 1977-08 gl05 0.88 79 40 1.8 54 951 590 68.10 1.24 2.09 13.59 6.29 3.13 3.91 1.22 0.02 0.09 100 1995-06 gl01 0.23 177 33 1.6 41 727 648 63.89 1.02 2.16 15.34 6.99 2.73 5.17 2.40 0.02 0.07 100 1995-08 gl01 0.17 419 34 1 42 816 581 62.54 0.97 3.90 15.30 6.74 2.09 5.88 2.30 0.01 0.09 100 1995-08 gl02 0.13 917 32 2 41 812 479 63.79 1.08 3.63 15.03 6.40 1.91 5.83 1.99 0.00 0.09 100 1995-07 gl01 0.15 154 31 1 42 918 538 62.64 0.92 3.49 15.89 6.48 2.36 5.51 2.39 0.01 0.09 100 1995-07 gl02 0.36 199 32 1 42 860 641 63.58 1.01 2.10 15.65 6.55 2.53 5.70 2.50 0.01 0.16 100 1996-06 gl01 0.42 2 32 1 40 930 667 61.15 1.03 2.42 15.87 7.42 2.89 6.07 2.84 0.01 0.10 100 1996-10 gl01 0.21 442 31 1 36 1081 473 60.79 1.04 3.37 15.40 7.92 2.90 5.34 2.85 0.00 0.16 100 123 Contrib Mineral Petrol

Fig. 6 Trace element (Li, Be, e B, Cl) variation with H2O a measured by SIMS. a Li increases with increasing H2O. Be b and B c show no correlation with H2O. d A weak, positive correlation exists between Cl and H2O. (e–h) Trace element (Li, B, F, Cl) content versus pressure plots. Cl degasses at low pressure, while the other trace elements show f no change with pressure b

c g

d h

Discussion and geophysical interpretations (Ingham et al. 2009; Row- lands et al. 2005). While these methods yield different Magmatic storage conditions absolute temperatures, the trend in the data from one eruption to another is consistent (Fig. 2). All historical We have determined magmatic temperatures for the his- eruptions plot between 910 and 1,080 °C, with the bulk of torical Ruapehu eruptions using several different geother- the data clustering between 950 and 1,050 °C. There is a mometers. The absence of touching Fe–Ti oxide pairs in all general increase in magmatic temperature from the earliest samples (except for disparate pairs in the 1969 sample) sample analysed (1945) to the latest scoria sample (1996) precludes us from using the method of Lindsley and (Fig. 2). The relatively low temperature for 1945 may Anderson (1983); for this reason, we have used the pla- reﬂect slow cooling and re-equilibration of this lava sample. gioclase-liquid (Putirka 2008), clinopyroxene-liquid (Put- Temperature estimates for the 1995–1996 eruptions irka 2008), orthopyroxene-liquid (Putirka 2008), and two- occupy a limited range between 1,000 and 1,080 °C with pyroxene (Lindsley and Frost 1992) geothermometers no apparent clustering towards the high or low estimates.

(assuming a H2O content of *1.5 wt%). In all geother- These data contrast with those of Nakagawa et al. (1999), mometer calculations, we have assumed a pressure of who found two separate populations of clinopyroxene– 250 MPa, which is based on the volatile data (see below) orthopyroxene pairs, one that yielded temperatures of

123 Contrib Mineral Petrol

Fig. 7 Volatile contents as measured by SIMS against modelled the coherence of all melt inclusions between closed-system degassing degassing histories for pyroxene-hosted melt inclusions from Ru- paths 1 and 3. Significant H2O-loss or CO2-fluxing would result in the apehu. a H2O versus CO2 content of melt inclusions plotted with inclusion population following a systematic decrease in XH2O, with a calculated isobars (dashed grey lines) and vapour isopleths in mol % slight decrease in saturation pressure (arrowed line). Rare inclusions H2O(dashed black lines). Isopleths and isobars were calculated using that fall below degassing curve 1 in (b) probably did lose significant Papale et al. (2006). Illustrative closed-system degassing curves H2O. Closed-system degassing of distinct magmas with a similar (curved black lines) are plotted from three starting compositions volatile content can explain the observed variations in H2O versus (Curve 1–1.28 wt% H2O and 1,060 ppm CO2, Curve 2–1.75 wt% CO2 and XH2O versus pressure space. Average propagated errors in H2O and 800 ppm CO2, and Curve 3–2.18 wt% H2O and 700 ppm the SIMS analyses are shown CO2). b Plot of calculated XH2O versus saturation pressure showing

*1,000 °C and another with temperatures of 1,000– B1,000 ppm CO2 and *1.5 wt% H2O, which suggests a 1,200 °C. The high temperatures, which were calculated minimum (volatile saturation) trapping pressure of using the method of Lindsley and Anderson (1983), appear *50–270 MPa (at between 920 and 1,030 °C using the unreasonably high for the andesite composition of the calculation of Papale et al. 2006). This is in agreement with Ruapehu ejecta. For instance, the liquidus temperature of estimates from the phenocryst melt and two-pyroxene the 1995–1996 magma is approximately 1,150 °C (using geobarometers of Putirka (2008). If we assume a crustal Danyushevsky and Plechov, 2011) which constrains the density of 2,600 kg/m3 and volatile saturation, this pressure maximum phenocryst temperature. Also, the updated geo- range suggests that the magma storage region beneath thermometers of Putirka (2008) produce a more homoge- Ruapehu extends from *2 to 9 km below the volcano. neous temperature than the Lindsley and Anderson (1983) Our data show that Ruapehu magmas are relatively dry method. Therefore, for the purposes of this paper, we have compared to other arc andesites. While the phase equilibria used the Putirka (2008) geothermometers throughout. of Moore and Carmichael (1998) appear consistent with a The volatile contents of melt inclusions are similar for dry, andesitic magma, it has been proposed that pheno- all analysed eruptions (Figs. 6, 7). They contain cryst-hosted melt inclusions are able to rapidly hydrate or

123 Contrib Mineral Petrol de-hydrate due to H? diffusion (e.g., Gaetani et al. 2012) pairs. However, as we were unable to ﬁnd touching pairs in and rapid equilibration with the surrounding melt. Lloyd the 1969 sample, we tested all possible combinations of et al. (2013) also suggest that melt inclusions within large disparate ilmenite and magnetite compositions for equi- scoria or lapilli clasts are prone to signiﬁcant dehydration. librium using the method of Bacon and Hirschmann

Clearly, the H2O content of our melt inclusions will have a (1988). Only equilibrium pairs were used to determine the significant effect on the pressure determinations. In order to fugacity and temperature of the 1969 magma using ILMAT assess our melt inclusion measurements, we plotted H2O (LePage 2003). We calculated an oxygen fugacity of log versus CO2 (Fig. 7a) and the calculated XH2O (mol frac- fO2 -11.20, equivalent to the Ni-NiO oxygen buffer (NNO) tion H2O of the vapour) against saturation pressure and temperature c. 939 °C (O’Neill and Pownceby 1993; (Fig. 7b). We then compared our data to modelled open or Frost 1991), which is *20 °C higher than the average of closed-system degassing profiles (Papale et al. 2006). Our the plagioclase-liquid and pyroxene-liquid geothermome- data are weakly scattered around a mean of 1.5 wt% H2O ters. The remarkably similar mineralogy and phenocryst and range from 50 to 1,000 ppm CO2. While these data compositions suggest that the oxygen fugacity of all his- could record a complex interplay between CO2-fluxing, torical Ruapehu magmas lies close to NNO, similar to pre- crystallisation, and H2O loss (e.g., Blundy and Cashman, historical magmas (Price et al. 2012). 2008, Spilliaert et al. 2006), we consider a simpler inter- To further assess the consistency of our pressure–tem- pretation. In the XH2Ovapor versus pressure diagram, all the perature estimates, we compare our data to high tempera- data appear to follow a relatively simple closed-system ture and pressure melting experiments on andesites. The degassing profile wherein XH2O increases with decreasing bulk composition of Ruapehu magmas is similar to that of pressure (Fig. 7b). There is some scatter in the data, which Volcań Colima, Mexico (Moore and Carmichael 1998), may reflect different initial volatile contents or degassing thus the starting compositions for hydrous phase equilibria trajectories. However, the data are not consistent with experiments from that volcano provide a reasonable com- significant diffusive loss of H2O from the melt inclusions. parison to natural Ruapehu samples, except that the Colima If the inclusions had dehydrated significantly, we would experiments were run under H2O-saturated conditions, that expect a large number (if not all) of the melt inclusions to is PH2O = Ptot. Because Ruapehu magmas are H2O-poor record low XH2Ovapor and anomalously low pressures, (PH2O \ Ptot), a pressure correction must be applied to the inconsistent with other independent data. To illustrate the experimental data. This correction is relatively straight- effect of diffusive H2O loss, we took one melt inclusion forward because of the negligible effects of CO2 on phase composition and progressively reduced its H2O content by equilibria in silicate systems at low pressures. If we take 0.5 wt%, from 2.5 wt%. At each point, we calculated the the most volatile-rich melt inclusion of *1,000 ppm CO2 XH2O and saturation pressure. The result provides a vector and 1.5 wt% H2O, we calculate a saturation pressure of for which melt inclusions would trend towards given sig- *270 MPa, with an XH2Oof*12–16 % at a temperature nificant H2O loss (Fig. 7b). From this, we can see that range of between 915 and 1,030 °C (Papale et al. 2006). Ruapehu melt inclusions do not exhibit significant H2O This equates to a PH2Oof*32–43 MPa. If we assume that loss. the addition of CO2 simply increases the Ptot, without To explain the H2O and CO2 data, we first must consider effecting phase relations, then we can use this value of that the bulk chemistry of Ruapehu magmas is broadly PH2O to match our data to the experiments. Using this similar (Table 1), with relatively small variations between correction, Ruapehu magmas plot near the 2 wt% H2O eruptions. This indicates that all eruptions are derived from isopleth, in a region mostly outside of the hornblende a similar parental magmatic system (e.g., Gamble et al. stability field, but with plagioclase, orthopyroxene, clino- 1999). Therefore, we considered the degassing trajectories pyroxene, and magnetite stable, in accord with the of three magmas with broadly similar major element observed phenocryst populations. The experimental equi- chemistry and variable initial H2O and CO2. Most of the librium plagioclase composition is *An60–65, consistent data plot between the two bounding closed-system degas- with the measured composition of plagioclase rims sing curves (curve 1–1.28 wt% H2O and 1,060 ppm CO2; (Fig. 8). From this comparison, we conclude that the curve 3–2.18 wt% H2O and 700 ppm CO2). Based on these mineralogy, volatile content, and temperature of Ruapehu data, we can conclude that Ruapehu magmas have a H2O magma are consistent with the andesite phase equilibria of content of up to 2.18 wt% and a CO2 content of at least Moore and Carmichael (1998)atPH2Oof*40 MPa. 700 ppm. As stated above, ilmenite is only observed within scoria Interaction between magma and crystal mush from 1969 (magnetite is noted in all scoria). In order to calculate equilibrium magmatic temperature and oxygen A compositional comparison of melt inclusions, ground- fugacity, it is best to analyse touching ilmenite–magnetite mass glass, and bulk rock can be used to determine the 123 Contrib Mineral Petrol

extent to which melt inclusions and their phenocryst hosts separately MgO versus Al2O3 (Fig. 9) for plagioclase- and are in chemical equilibrium with the host magma. How- pyroxene-hosted inclusions, as these elements are differ- ever, we must first consider whether any of the melt ently compatible in pyroxene and plagioclase crystals. For inclusions have been modified by post-entrapment crys- example, pyroxene-hosted melt inclusions that crystallise tallisation. Daughter minerals are absent from all melt on the host would result in a displacement to very low inclusions analysed. To evaluate the extent of melt inclu- MgO content with little change in Al2O3, whereas crys- sion crystallisation onto the host crystal, we plotted tallisation of plagioclase would lead to a decrease in Al2O3 and slight increase in MgO. These different trends are shown as vectors corresponding to 5 wt% crystallisation in Fig. 9. In general, the compositional overlap between plagioclase- and pyroxene-hosted melt inclusions suggests that post-entrapment crystallisation was limited. Specifi- cally, compositional variations in the melt inclusions follow cotectic crystallisation trends; that is, the trends do not follow the vectors anticipated for post-entrapment crystallisation of the host mineral (Fig. 9). As melt inclusions do not appear to have experienced significant post-entrapment crystallisation, they can be used to examine variations in crystallisation (driven by

cooling, decompression, and/or H2O loss) and magma mingling/mixing in the small magma batches produced by Ruapehu eruptions. It is useful to discuss these data in two groups: (1) 1945–1977 and (2) 1995–1996. We use the

incompatible elements K2O and TiO2 as plotting parame- ters as these two components best illustrate the compositional differences between eruptions. Fig. 8 Phase diagram of an andesite of similar bulk composition to Ruapehu (Volcan Colima), in terms of PH2O versus temperature, 1945 to 1977 redrawn from Moore and Carmichael (1998). Grey box represents the calculated (using Papale et al. 2006)PH2O conditions of H2O-poor, The major element chemistry of plagioclase- and pyrox- CO2-rich historical Ruapehu magmas. Magmatic temperatures were determined by crystal-melt and Fe–Ti oxide geothermometry. ene-hosted melt inclusions from 1945 plots along the same Ruapehu magmas occupy a PH2O-temperature space where the fractional crystallisation trend as the bulk rocks (Fig. 10). equilibrium phase assemblage consists of plagioclase (Plag), ortho- The melt inclusion compositions occupy a wider range than pyroxene (Opx), clinopyroxene (Aug), and magnetite (Mt). Horn- the groundmass glass. Under equilibrium conditions, the blende (Hbl) is absent from Ruapehu due to the relatively low H2O- saturated pressure and high-temperature conditions of Ruapehu compositions of melt inclusions should lie between the magmas. Sub-horizontal dashed lines are H2O concentration isopleths bulk rock and the groundmass glass on a plot of two

Fig. 9 MgO v Al2O3 plot of plagioclase- and pyroxene- hosted melt inclusions from the 1995 eruption of Ruapehu. These data show that the chemistry of each inclusion is largely independent of the host mineral. Plagioclase- and pyroxene-hosted melt inclusions that had crystallised after being trapped would trend away from their respective host crystal along the vectors shown as black arrows. The length of arrows approximates 5 % crystallisation of plagioclase (Plag) and pyroxene (Pyrox)

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nominally incompatible major elements, in this case K2O The 1969 melt inclusions and groundmass glasses plot and TiO2 (e.g., Faure and Schiano 2005). Melt inclusions along a fractional crystallisation trend from the bulk rock that fall outside of the equilibrium line may indicate that composition (Fig. 10). Most inclusions are less evolved the crystals are exotic to the host magma, as appears to be than the groundmass glass, which is to be expected if the case for the 1945 data. The most likely source for crystallisation continues in the melt after inclusions exotic crystals at Ruapehu is crystal mush, such as that become trapped within a crystal. This suggests that the invoked by Nakagawa et al. (1999), because of both 1969 eruptions were driven by a small volume magma that numerous crystal clots within scoria clasts and the presence was isolated from the larger magma storage region. In of a high- and low-temperature signature from two- contrast, most melt inclusions from the 1971 eruption pyroxene geothermometry. Together, these data suggest describe a fractionation trend that is different from the that the 1945 magma probably intersected, and interacted groundmass glass. The shift of exotic inclusions to high with, at least one crystal mush zone during ascent. K2O at constant TiO2 requires that they crystallised from a

Fig. 10 Plots of TiO2 versus K2O (two incompatible elements) compositions. Most of the 1971 and 1977 inclusions are considered showing the composition of groundmass glass, melt inclusion and exotic to the host melt. The 1995 melt inclusions have a larger bulk rock XRF data for each Ruapehu eruption analysed. Cognate compositional spread than the groundmass glass. Many inclusions are melt inclusions should fall on the same line (grey dashed line) as the of a similar composition to the melt (cognate), while the more maﬁc bulk rock and groundmass glass (e.g., Faure and Schiano, 2005). The and silicic end members are possibly exotic. The 1996 groundmass 1945 melt inclusions span a similar range in K2O to the groundmass glass is less evolved than the melt inclusions. This implies that all of glass, while the melt inclusions are displaced to lower TiO2. The the 1996 melt inclusions and hence the entire population of inclusion- crystals are therefore equivocally exotic. The 1969 melt inclusions are bearing phenocrysts are exotic. The groundmass glass and melt mostly less evolved than the groundmass glass, appear on a similar inclusions are chemically distinct, but appear on the same mixing chemical trend to the whole rock, and we conclude that most melt line; therefore, both melts must have had a similar parent composition inclusions (and hence the crystals) are cognate with the groundmass and mineralogy. In all plots, black triangles represent the bulk rock glass. The 1971 and 1977 melt inclusions are more evolved than the composition for scoria from 1945 to 1996 (Gamble et al. 1999; this groundmass and are displaced either side of their respective work) equilibrium mixing lines from the groundmass glass and whole rock 123 Contrib Mineral Petrol melt with a bulk composition that was distinct from the As also seen in 1971, a small number of 1977 melt inclu- transporting magma (represented by the groundmass glass). sions have a composition that is similar to that of the bulk Importantly, both exotic and cognate melt inclusions can rock. This suggests that less-evolved magma was intro- be found within the same crystal, and without an obvious duced into the base of the andesitic magma storage region spatial distribution. However, there is evidence for some shortly before each eruption. Moreover, although the crystallisation within the transporting magma in the small groundmass glass compositions of 1971 and 1977 scoria number of 1971 inclusions that are chemically similar to are similar, they are not identical (Fig. 4), and thus suggest the groundmass glass and could thus be genetically related that the two events involved two distinct magmas despite to the host magma (i.e., cognate). The lack of overlap their extremely small volumes (e.g., Houghton et al. 1987). between either the melt inclusion populations or the groundmass glass compositions in 1971 and 1969 magmas 1995 to 1996 is signiﬁcant given the limited time between eruptions. These data suggest that prior to the eruption, the 1971 The groundmass glass and melt inclusions of the 1995 magma entrained crystals from a mush zone that was dis- magma span a similar range in major element chemistry to tinct from the 1969 magma. The exact date or size of earlier eruptions (Figs. 4, 5). The bulk of the melt inclu- eruptions that provided the antecrysts cannot be con- sions from 1995 are more evolved than the groundmass strained, but we can discount crystals generated in 1945 glass, but appear to lie on a similar liquid line of descent, based on their different melt inclusion compositions indicating that some of the crystals are probably cognate. (Figs. 4, 5). The groundmass glass and melt inclusion compositions As seen in the 1971 scoria, most of the phenocryst- from 1995 plot near the bulk rock composition (Fig. 10), hosted melt inclusions in the 1977 scoria are displaced to a which suggests minimal crystallisation prior to eruption. higher K2O content than the host glass and are therefore Pyroxene- and plagioclase-hosted inclusions are both considered exotic (Fig. 10). Moreover, exotic melt inclu- more- and less-evolved than the groundmass glass. Those sions from the 1971 and 1977 eruptions are not chemically that are more evolved are likely to be hosted by antecrysts similar, which suggests that the 1971 and 1977 magmas derived from a crystal mush that is genetically linked to the ascended through different parts of the subvolcanic system. historical eruptions. In contrast, a striking aspect of the

Fig. 11 Conceptual model for the magmatic system at Ruapehu. (Ingham et al. 2009). Small volume sills and dykes are distinct but Small volume andesitic melts are residing as sills and dykes from\2 closely spaced. When magma ascends, it interacts with a crystal mush to *9 km depth, based on the volatile content of phenocryst-hosted zone/s (1 and 2), entraining some of those exotic crystals into the melt inclusions (using Papale et al. 2006), combined with magneto- melt. On ascent, small defects in individual crystals allow for the telluric soundings (Ingham et al. 2009) and seismic tomography ingress and then trapping of cognate melt inclusions (3). These (Rowlands et al. 2005). A hydrothermally altered boundary zone cognate melt inclusions record the magmatic conditions of the new (grey diffuse boundary) exists on the margins of the dyke system melt alongside those that of previous melt/s 123 Contrib Mineral Petrol

1996 scoria is that most of the melt inclusions are more (Fig. 11). We suggest that prior to eruption, high-angle (c. evolved than the hosting glass, which is similar in com- 80˚) dykes (calculated from geophysical data) transport position to that of the 1995 eruption (Fig. 10). For this magma to the active vent beneath Crater Lake. These dykes reason, we consider that most, possibly all, of the melt probably pass through and interact with partial melt zones inclusions analysed from the 1996 eruption are exotic to that may take the form of small sills (crystal mush zones) the transporting melt. More-evolved melt inclusion com- (Fig. 11). The sill-like nature of these bodies enables dif- positions may indicate that the mush from which antecrysts ferent eruptions to interact with mushes that were chemi- were entrained had cooled/crystallised significantly prior to cally and physically isolated from one another, as evinced interaction. by the melt inclusion data discussed above. There is substantial overlap in the compositions of The small volumes of these magma bodies beneath plagioclase- and pyroxene-hosted inclusions from the 1995 Ruapehu are unlikely to be readily imaged by geophysical and 1996 scoria; because some of the 1995 crystals are techniques such as MT or seismic tomography. In fact, considered cognate, it seems likely that some of the Ingham et al. (2009) use their MT data to suggest that it is phenocrysts from 1996 are antecrysts that originally grew unlikely that large volume magma bodies are able to in the 1995 magma (Fig. 10). That the groundmass glass accumulate in the shallow crust beneath Ruapehu. from 1996 is distinctly more mafic than the melt inclusions Although physically isolated at shallow depths, the suggests that the 1996 eruption involved a very crystal- various Ruapehu magmas are likely to be genetically poor, relatively mafic magma that entrained crystals from linked at depth, and furthermore, it is possible that a tem- the partially crystalline 1995 magma and possibly from poral trend can be drawn based on our groundmass glass other parts of the magma storage region. and major element chemistry data, whereby eruptions have become more mafic with time (since 1945). However, Magma volumes and storage architecture given that there are a number of eruptions for which we do not have samples or analyses (including 1975), we can only The distinctive chemical signatures of groundmass glasses speculate that a temporal-chemical trend exists. from individual Ruapehu eruptions suggest that each eruption tapped a slightly different magma. We know from field investigations and measurements of the eruptive Conclusions deposits that the eruptive volumes were very small (between 1945 and 1996). The total volume of magma Historical eruptions at Ruapehu (1945, 1969, 1971, 1977, erupted between 1945 and 1996 is approximately 1995, and 1996) are characterised by very small volume 3.6 9 107 m3. This total magma volume estimate is dom- magmas, each with a unique chemical composition and inated by the 1995–1996 eruptions and is very small in history. Volatile contents of melt inclusions and crystal- comparison with a single, moderately sized andesitic melt barometry have constrained the depth at which these eruption (e.g., Bezymianny; Belousov et al. 2002, Colima; magmas originated to be *2to*9 km, which corre- Saucedo et al. 2010). sponds well to geophysical data. These small volume melts Based on the saturation pressure calculated from the probably resided as distinct and closely spaced sills or

H2O and CO2 content of phenocryst-hosted melt inclu- dykes from 2 to 9 km. Before an eruption, magma was sions, we determined a magma storage depth of *2–9 km. injected into the sill/dyke system leading to common This compares well to magnetotelluric (MT) data (Ingham magma-mush and magma–magma interaction. Most mag- et al. 2009) and seismic tomography (Rowlands et al. 2005) mas interacted with crystal mush zones (at\*3 km depth) from Ruapehu. Ingham et al. (2009) observed a diffuse and formed from antecedent magmas during ascent and even- weak low-resistivity anomaly that extends to *6 km, tually eruption. Due to their small volumes, Ruapehu which they interpreted to be a dyke system. From *6to magmas since 1945 show sensitivity to interaction between more than 10 km and slightly east of the cone, a more magmas and crystal mush zones that would be difficult to intense low-resistivity anomaly (melt-bearing zone) was determine in larger magmatic systems. Therefore, data identified using both 2-D and 3-D inversions. The seismic from Ruapehu offer a unique insight into the small-scale tomography data also show a low velocity zone to the east interactions that magmas experience on their ascent to of the cone from *3to*9 km depth, although these data eruption. have been interpreted as a combination of crustal down- We have shown that the chemical composition of phe- warping and the presence of thick volcaniclastic sediments nocryst-hosted melt inclusions is often distinct from the (Rowlands et al. 2005). Our data are consistent with a groundmass glass. This implies that a significant proportion magma storage region (possibly in the form of discrete, yet of the crystals are antecrysts; in some cases, antecrysts closely spaced sills and dykes) down to 9 or 10 km have incorporated rare melt inclusions from the new melt. 123 Contrib Mineral Petrol

In that respect, it is clear that the interpretation of mag- leading to phreatic eruption events: insights from the 25 matic processes at depth can only be achieved from cog- September 2007 eruption through Crater Lake, Ruapehu. New Zealand. J Volcanol Geoth Res 191(1–2):15–32 nate, rather than exotic, melt inclusions. Cronin SJ, Hedley MJ, Neall VE, Smith RG (1998) Agronomic Ruapehu magmas are low in H2O but CO2-rich com- impact of tephra fallout from the 1995 and 1996 Ruapehu pared to intermediate magmas from subduction settings Volcano eruptions, New Zealand. Environ Geol 34:21–30 elsewhere. This relatively low concentration of volatiles Danyushevsky LV, Plechov P (2011) Petrolog3: integrated software for modelling crystallization processes. Geochem Geophys and the very small volumes of magma combine to account Geosys 12:Q07021. doi:10.1029/2011GC003516 for the low explosivity and short duration of most eruptions Devine JD, Murphy MD, Rutherford MJ, Barclay J, Sparks RSJ, at Ruapehu. While the largest eruptive episode (in Carroll MR, Young SR, Gardner JE (1998) Petrologic evidence 1995–1996) produced plumes to 20 km, the volatile con- for pre-eruptive pressure–temperature conditions, and recent re- heating, of andesite magma at Soufrie`re Hills Volcano, Mont- tent is similar to the smallest episode analysed (in 1971). serrat, W.I. Geophys Res Let 25:3669–3672 Therefore, the controls on the size of eruptions at Ruapehu Donoghue SL (1991) The Tufa Trig Formation: recent (0–1800 years are not determined by the volatile content of magma alone. B.P.) eruptives from Mount Ruapehu. Geological Society of New Zealand miscellaneous publication 59A:54 p Acknowledgments This work was funded by the New Zealand Faure F, Schiano P (2005) Experimental investigation of equilibration Ministry of Science and Innovation (MSI) Geological Hazards Pro- conditions during forsterite growth and melt inclusion formation. gramme (GHZ) in the form of a PhD studentship to GK at the Uni- Earth Planet Sci Lett 236:882–898 versity of Bristol. Holly Goddard and Neville Orr are thanked for their Frost BR (1991) Introduction to oxygen fugacity and its petrologic assistance with sample preparation. We gratefully acknowledge sup- importance. Rev Mineral Geochem 25:1–9 port from NERC for access to the SIMS facility, Edinburgh, where Gaetani GA, O’Leary JA, Shimizu N, Bucholz CE, Newville M Cees-Jan de Hoog provided expert guidance and patience. 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