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Reconstructing the Primitive Magmas Fueling Voluminous Silicic Volcanism Using Olivine-Hosted Melt Inclusions Simon J

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Reconstructing the Primitive Magmas Fueling Voluminous Silicic Volcanism Using Olivine-Hosted Melt Inclusions Simon J https://doi.org/10.1130/G47422.1 Manuscript received 22 July 2019 Revised manuscript received 21 January 2020 Manuscript accepted 23 January 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 27 February 2020 What lies beneath? Reconstructing the primitive magmas fueling voluminous silicic volcanism using olivine-hosted melt inclusions Simon J. Barker1*, Michael C. Rowe2, Colin J.N. Wilson1, John A. Gamble1,3, Shane M. Rooyakkers1,4, Richard J. Wysoczanski5, Finnigan Illsley-Kemp1 and Charles C. Kenworthy2 1 School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand 2 School of Environment, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand 3 School of Biological, Earth and Environmental Sciences, University College Cork, Cork T12 YN60, Ireland 4 Department of Earth and Planetary Sciences, McGill University, Montreal, Quebec H3A 0E8, Canada 5 National Institute of Water and Atmospheric Research (NIWA), Private Bag 14901, Kilbirnie, Wellington 6021, New Zealand ABSTRACT 1994). Most evidence for mafic-silicic magma Understanding the origins of the mantle melts that drive voluminous silicic volcanism is interactions therefore comes from mingled mag- challenging because primitive magmas are generally trapped at depth. The central Taupō mas, foreign crystal populations or zoned crys- Volcanic Zone (TVZ; New Zealand) hosts an extraordinarily productive region of rhyolitic tals, or co-erupted mafic enclaves (Bacon, 1986; caldera volcanism. Accompanying and interspersed with the rhyolitic products, there are Pritchard et al., 2013; Barker et al., 2016). traces of basalt to andesite preserved as enclaves or pyroclasts in caldera eruption products The central Taupō Volcanic Zone (TVZ; and occurring as small monogenetic eruptive centers between calderas. These mafic materi- Fig. 1), New Zealand, is a frequently active and als contain MgO-rich olivines (Fo79–86) that host melt inclusions capturing the most primitive exceptionally productive region of Quaternary basaltic melts fueling the central TVZ. Olivine-hosted melt inclusion compositions associated silicic volcanism, ultimately fueled by a basalt with the caldera volcanoes (intracaldera samples) contrast with those from the nearby, mafic flux from the mantle that is unusually high for its intercaldera monogenetic centers. Intracaldera melt inclusions from the modern caldera vol- continental arc setting (Wilson et al., 2009). The canoes of Taupō and Okataina have lower abundances of incompatible elements, reflecting mantle processes driving this extreme flux are distinct mantle melts. There is a direct link showing that caldera-related silicic volcanism challenging to study because unmodified man- is fueled by basaltic magmas that have resulted from higher degrees of partial melting of a tle-derived basalts are rarely erupted through the more depleted mantle source, along with distinct subduction signatures. The locations and crustal silicic reservoirs. Over the past ∼60 k.y., vigor of Taupō and Okataina are fundamentally related to the degree of melting and flux of a volume of >780 km3 magma (>99% silicic) basalt from the mantle, and intercaldera mafic eruptive products are thus not representative has erupted from the central TVZ, almost en- of the feeder magmas for the caldera volcanoes. Inherited olivines and their melt inclusions tirely from two caldera volcanoes: Okataina and provide a unique “window” into the mantle dynamics that drive the active TVZ silicic mag- Taupō (Fig. 1; Wilson et al., 2009). Between matic systems and may present a useful approach at other volcanoes that show evidence for Taupō and Okataina, volcanic activity since ca. mafic recharge. 200 ka also includes scattered intercaldera maf- ic (basaltic to basaltic andesite), small-volume INTRODUCTION mentally driven from below by mantle-derived (collectively ∼1 km3) eruptive centers that are The magmatic systems that underpin large- basaltic magmas. Therefore, the question aris- typically aligned along northeast-southwest– scale silicic volcanism encompass large portions es: Are the basalts parental to the generation of trending faults (Gamble et al., 1993; Table DR1 of the crust, with partially molten mushy res- large silicic volcanic eruptions derived from a in the GSA Data Repository1). ervoirs that can be thousands of cubic kilome- different source compared to surrounding re- Here, we investigated the compositions of ters in volume (Bachmann and Huber, 2016). gional volcanism, or do they just represent local- primitive melts feeding young volcanism in the Although dominated by evolved compositions ly enhanced (spatially and temporally) magma central TVZ to see if there were any differences at upper-crustal levels, these systems are funda- fluxes? This question is challenging to address between the caldera centers and the less active because ascending primitive magmas are gener- areas in between. We used the novel approach of ally intercepted by large silicic reservoirs and analyzing olivine-hosted melt inclusions (MIs) *E-mail: [email protected] are rarely erupted in unmodified form (Wiebe, contained within juvenile mafic materials that 1GSA Data Repository item 2020145, geochemical data tables, primary melt–corrected trace-element figure, and trace-element models, is available online at http:// www.geosociety.org/datarepository/2020/, or on request from [email protected]. CITATION: Barker, S.J., et al., 2020, What lies beneath? Reconstructing the primitive magmas fueling voluminous silicic volcanism using olivine-hosted melt inclusions: Geology, v. 48, p. 504–508, https://doi.org/10.1130/G47422.1 504 www.gsapubs.org | Volume 48 | Number 5 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/5/504/4980635/504.pdf by guest on 29 September 2021 o E176o E176.5o A Rhyolitic Mafic S38 Fig. 4 glass enclave 0 20 km Rt profile New HT KA Ok Zealand Kp 6 Oh TVZ 5 Rp o 4 Wk 3 S38.5 Olivine crystals 1 2 TVZ young Inter-caldera outline (<350 ka) centres sampled Tp Andesite lavas Other basalt vents Silicic lavas Inferred active o (dacite to rhyolite) young calderas S39 Axial ranges 5 mm Lakes and ocean Extinct calderas <350 ka Tongariro B Fo85 Olivine Figure 1. Map showing setting of Taupo¯ Volcanic Zone (TVZ) in New Zealand (inset) with sample xenocryst locations. Outlines of calderas and young TVZ (≤350 ka) boundary are from Wilson et al. (2009), and locations and compositions of young lavas are from Leonard et al. (2010). Location of Ker- madec arc (KA) northeast of New Zealand is shown by black triangles, and Havre Trough (HT) back-arc basin is denoted by black dashed line. Intercaldera samples discussed here: 1—Kin- loch, 2—Punatekahi, 3—Tatua, 4—Kakuki, 5—Ongaroto, 6—Harry Johnson Road. The two most recently active caldera volcanoes Tp (Taupo¯) and Ok (Okataina) are sources for intracaldera samples discussed here. Other caldera outlines: Kp—Kapenga, Oh—Ohakuri, Rp—Reporoa, Rt—Rotorua, and Wk—Whakamaru. See Table DR3 (see footnote 1) for further sampling details. Trapped Black dashed line shows approximate line of schematic cross section in Figure 4. pockets of melt (melt inclusions) were erupted during rhyolitic events at the cal- tions to reach the surface in this area over the dera volcanoes and compared these with their past ∼200 k.y. (Gamble et al., 1993; Table DR1). counterparts from the interspersed intercaldera Olivines in the sampled units overlap in compo- 100 µm mafic centers (Fig. 2A; Fig. DR2). sition (Fo78–90) with the caldera-related olivines (Table DR3), but their MIs are less common OLIVINE IN CENTRAL TVZ ERUPTIVE and tend to be smaller, and all have experienced Figure 2. Images of representative mafic PRODUCTS some postentrapment crystallization. samples from Taupo¯ volcano (New Zealand) highlighting the context of materials analyzed We studied olivine crystals from mafic en- MIs were homogenized through standard in this study. (A) Juvenile mafic enclave from claves in deposits of the 25.5 ka Oruanui and 1 atm heating experiments to remove posten- the 25.4 ka Oruanui eruption (P560) hosting 3.5 ka Waimihia eruptions from Taupō, and the trapment crystallization (Danyushevsky et al., sampled olivine crystals (photo inset). Note 1314 Kaharoa eruption from Okataina (Fig. 2A; 2002; Rowe et al., 2015). Rehomogenized MIs the crenulated margin to the enclave and adhering and ingested rhyolitic pumiceous Fig. DR2). The enclaves are interpreted as ju- and olivine hosts were analyzed for major el- glass, taken to indicate the molten nature of venile because they have crenulated chilled ements by electron microprobe (Table DR3), the enclave upon entrainment (e.g., Rooyak- margins and adhering rhyolitic glass, and they and then MIs >35 µm across were analyzed for kers et al., 2018). (B) High-Fo olivine hosting host rhyolite-derived crystals ingested during trace-element concentrations by laser-ablation multiple large, but partially crystalline, melt syneruptive interactions (Leonard et al., 2002; inductively coupled plasma–mass spectrome- inclusions. See Figure DR2 (see footnote 1) for more images and Table DR3 for details of Rooyakkers et al., 2018). Olivines in these en- try (Table DR4). Following analysis, measured melt inclusion rehomogenization and analyti- claves are Mg-rich (Fo80–86) [Fo = molar Mg/ glass compositions were corrected for over/un- cal techniques. (Mg
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