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Rapid Assembly of High-Mg Andesites and Dacites by Magma Mixing at a Continental Arc Stratovolcano

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Rapid Assembly of High-Mg Andesites and Dacites by Magma Mixing at a Continental Arc Stratovolcano Rapid assembly of high-Mg andesites and dacites by magma mixing at a continental arc stratovolcano Item Type Article Authors Conway, Chris; Chamberlain, Katy J.; Harigane, Yumiko; Morgan, Daniel; Wilson, Colin Citation Chamberlain, K. (2020). 'Rapid assembly of high-Mg andesites and dacites by magma mixing at a continental arc stratovolcano'. Geology, pp. 1-5. DOI 10.1130/G47614.1 Publisher The Geological Society of America Journal Geology Download date 30/09/2021 15:25:06 Link to Item http://hdl.handle.net/10545/624941 https://doi.org/10.1130/G47614.1 Manuscript received 13 March 2020 Revised manuscript received 21 May 2020 Manuscript accepted 27 May 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Rapid assembly of high-Mg andesites and dacites by magma mixing at a continental arc stratovolcano Chris E. Conway1,2, Katy J. Chamberlain3, Yumiko Harigane4, Daniel J. Morgan5 and Colin J.N. Wilson6 1 Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan 2 Department of Geology and Paleontology, National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, Ibaraki 305-0005, Japan 3 School of Environmental Science, University of Derby, Kedleston Road, Derby DE22 1GB, UK 4 Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan 5 School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK 6 School of Geography, Environment and Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand ABSTRACT rock Mg# values of 60–69 (andesites) and 53–57 Studies of pre-eruptive processes at active volcanoes require precise petrochronological (dacites), plus elevated Ni (∼30–150 ppm) and constraints if they are to contribute to hazard assessment during future eruption events. We Cr (∼100–470 ppm) at equivalent SiO2 contents present petrological and geochemical data and orthopyroxene diffusion time scales for samples when compared with all other lavas from Rua- from Late Pleistocene high-Mg andesite-dacite lavas (Mg# 53–69) at Ruapehu volcano, New pehu (Conway et al., 2018; Fig. S1 in the Supple- Zealand, as a case study of rapid magma genesis and eruption at a continental arc stratovol- mental Material1). Samples used for this study cano. Assembly of Ruapehu high-Mg magmas involved the mixing of primitive magmas plus represent ranges in eruption age, composition, entrained mantle-equilibrated olivines with mid-crustal felsic mush bodies, yielding hybridized geographic distribution, and cooling rate (Table magmas with ubiquitous pyroxene reverse-zoning patterns. Orthopyroxene Fe-Mg interdiffu- S2 in the Supplemental Material). The lavas are sion time scales linked to quantitative crystal orientation data show that most lavas erupted <10 non- to moderately vesicular with 15%–35% days after resumption of crystal growth following magma mixing events. The eruption of lavas macrocrystals (>100 µm). Backscattered electron within days of mixing events implies that pre-eruptive warnings may be correspondingly short. (BSE) images and major-element data for miner- als were collected by electron probe microanaly- INTRODUCTION ant et al., 2011), petrographic studies highlight sis. Crystal axis orientations for orthopyroxenes Arc stratovolcanoes are active foci for the role of mafic-felsic magma mixing accom- used for diffusion modeling were determined by research on the generation of intermediate-com- panied by olivine accumulation in dispropor- electron backscatter diffraction (EBSD). See the position magmas and their frequent, commonly tionately raising the bulk-rock Mg abundance in Supplemental Material for details of analytical hazardous eruptions. Magma mixing is a process other cases (e.g., Streck et al., 2007). Examples methods and supplementary data (Table S3). of relevance for both topics because the hybrid- of the latter type are valuable for understanding ization of mafic and felsic materials may act as the types and time lines of crustal-level magma MINERAL CHARACTERISTICS AND an eruption trigger (e.g., Kent et al., 2010). Min- hybridization processes. Here we present miner- ORIGINS eral zonation patterns that record mixing dynam- alogical characteristics and orthopyroxene Fe-Mg Pyroxene ics can be used to measure pre-eruptive time interdiffusion time scales for a suite of HMAD Sharply reverse-zoned orthopyroxene and scales (e.g., Kahl et al., 2011), which are useful lavas from Ruapehu volcano, New Zealand, a clinopyroxene are ubiquitous in Ruapehu parameters for hazard mitigation efforts during large andesite-dacite stratovolcano of the Taupo HMAD lavas (type 1 crystals; Figs. 2A and 2B). modern-day eruption scenarios. Mixing is com- Volcanic Zone with a 200 k.y. history of effusive Type 1 orthopyroxene and clinopyroxene cores monly described for arc volcanic rocks, including eruptions (Fig. 1; Conway et al., 2016). vary over Mg# 67–80, but individual cores are some examples of high-Mg andesite and dacite generally unzoned, slightly resorbed, and over- (HMAD). These are defined as having whole- SAMPLES AND ANALYTICAL grown by 8–15-µm-thick rims of Mg# 82–89, rock Mg# values [Mg# = 100 × Mg / (Mg + Fe)] METHODS with outermost higher-FeO reaction rinds. of ≥45, higher than otherwise expected over the Samples were collected from HMAD lavas Type 1 pyroxenes occur as single euhedral or SiO2 range of 54–65 wt% (Kelemen et al., 2003). that were erupted from 48 to 37 ka onto the broken grains and as crystal clusters that can While some examples of HMAD volcanism are southern and western flanks of Ruapehu volcano include ortho- and clinopyroxene (Fig. 2A). attributed to a primary mantle origin (e.g., Bry- (Conway et al., 2016; Fig. 1). They have whole- Type 2 pyroxenes are unzoned, with Mg# values 1Supplemental Material. Details of analytical methods, and supplemental data and figures. Please visithttps://doi .org/10.1130/GEOL.S.12475946 to access the supplemental material, and contact [email protected] with any questions. CITATION: Conway, C.E., et al., 2020, Rapid assembly of high-Mg andesites and dacites by magma mixing at a continental arc stratovolcano: Geology, v. 48, p. XXX–XXX, https://doi.org/10.1130/G47614.1 Geological Society of America | GEOLOGY | Volume XX | Number XX | www.gsapubs.org 1 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G47614.1/5077922/g47614.pdf by guest on 29 June 2020 10.5–19.0 wt% Al2O3, indicative of a depleted A arc mantle source (Fig. S4). Using overgrowth rates from Coombs and Gardner (2004) and Zellmer et al. (2016), the dimensions of orthopy- roxene coronae on type 4 olivines are consis- tent with their mixing into felsic magma over periods of weeks to months. We infer that the olivines were carried by mantle-derived basalts (∼11 wt% MgO; Barker et al., 2020) that induced the reverse-zoned growth of type 1 pyroxene rims on mixing with felsic mush in the crust. The Figure 1. (A) Location of high-Mg andesite and large, anhedral and sometimes glomerocrystic dacite (HMAD) eruptive nature of the olivines suggests that they were not centers in the Taupo Vol- cogenetic with the mafic magma but were instead canic Zone (TVZ), New incorporated from a preexisting cumulate body. Zealand, and distribu- tion of HMAD lavas at Olivines with Fo74–78 cores that show resorp- Ruapehu volcano (after tion and diffuse normal or no zoning and have Conway et al., 2016). Sam- extensive >100 µm orthopyroxene rinds are ples used in this study classed as type 5 crystals (Fig. 2F). The over- are marked (circles), with growth orthopyroxene crystals are mostly type ages (rounded, in ka) shown for dated samples. 3, but type 1 crystals are also present (Fig. 2F). (B) Mg# versus SiO2 plot Type 5 olivines are interpreted to represent type of Ruapehu HMAD lavas 4 olivines that experienced prolonged crustal (purple and blue dia- storage resulting in extended orthopyroxene B monds) compared with other eruptives from Rua- overgrowths and diffusive reequilibration of pehu (gray diamonds) and their cores. The presence of type 1 orthopyrox- TVZ (squares). See Figure ene as overgrowths indicates that type 5 oliv- S1 (see footnote 1) for ines experienced a final mixing event, consistent data sources. with there having been recurring mafic magma recharge pulses into the felsic magma bodies. Plagioclase and Hornblende Plagioclase is ubiquitous in the samples, with typical lengths of 300–700 µm, and maxi- mum lengths of 5 mm. Two prominent types show sharply defined reverse (type 6) or normal (type 7) zonation of their rims (outer ∼100 µm; Figs. 2G and 2H). Hornblende (type 8 crystals) occurs only in Te Kohatu member dacite lavas (Conway et al., 2018), reflecting more hydrous equivalent to those of type 1 crystal cores, and turbed, and that outer zones (and type 3 grains) source magmas compared with the andesites commonly occur as clusters (Fig. 2C). Type 3 grew from more primitive melts. Broken grains (Putirka, 2016). They are 200–500-µm-long orthopyroxene crystals have high Mg# cores that lack a complete higher-Mg# rim (Fig. 2B) grains, commonly with adhering plagioclase or (like type 1 crystal rims) and display normal were likely derived from crystal clusters that pyroxene, and are partially to fully opacitic, with zoning toward their rims (Fig. 2D). were disaggregated during magma hybridization symplectite assemblages of orthopyroxene, pla- Several features imply that type 1 and 2 events shortly prior to eruption. gioclase, and Fe-Ti oxides replacing their outer pyroxene cores were derived from a felsic 20–100 µm rims (Fig. 2I). These crystals yielded source. First, average orthopyroxene (opx) Olivine and Spinel an average pressure of 3.5 kbar (∼13 km depth core values of Mg# 75 are in equilibrium with Type 4 crystals (mostly ∼600 µm, rarely to in continental crust) using the Al-in-hornblende relatively evolved melt compositions (Mg# 45), 4 mm) and glomerocrysts are high-forsterite barometer of Ridofli et al.
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