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