Tracing mantle source variation through xenocrystic olivine in the Taupo Volcanic Zone, New Zealand: A role for lithospheric mantle in the shift from andesitic to rhyolitic compositions

Storage, evolution, and mixing in basaltic eruptions from around the Okataina Volcanic Centre, Taupō Volcanic Zone, Aotearoa New Zealand

“…Olivine composition is variable between eruptions, with a narrow range in forsterite content at Terrace Rd (78.3 ± 0.4) and Tarawera (81.9 ± 0.6) and a wide range (72-82) at Rotomakariri and Rotokawau (Figure 3a-e). Based on minor element chemistry (NiO and CaO) and Fo, Law et al (2021) defined olivines from Terrace Rd, Tarawera, and Rotokawa as group 1 and those from Rotomakariri as group 2, which is supported by this data. Groundmass olivine from Tarawera analysed by Rowe et al (2021) has a lower forsterite content than the macrocrystals (Figure 3a).…”

“…Terrace Rd, Rotokawau, and Rotomakariri contain abundant macrocrysts, mostly as glomerocrysts, whereas Tarawera is almost macrocryst-and glomerocryst-free (see also Law et al, 2021). All eruptions have a similar mineralogy of olivine (~10-15 area%), plagioclase (~50 area%), and clinopyroxene (~20-25 area%), with Rotomakariri additionally containing abundant orthopyroxene (~20 area%).…”

“…Basalt scoria cones and tuffs are volumetrically minor surface features, being one and two-tothree orders of magnitude less voluminous than andesites/dacites and rhyolites, respectively (Wilson et al, 1995). Basalts of the TVZ are classified as high-alumina and are generated by a combination of rift-induced decompression melting and fluid-induced flux melting (Hiess et al, 2007;Law et al, 2021). Active calderas have high inputs of basalt from the mantle wedge, which is caused by fluid-fluxed melting of fertile mantle: i.e., mantle that has not undergone much previous melting (Barker et al, 2020;Zellmer et al, 2020).…”

“…Active calderas have high inputs of basalt from the mantle wedge, which is caused by fluid-fluxed melting of fertile mantle: i.e., mantle that has not undergone much previous melting (Barker et al, 2020;Zellmer et al, 2020). The mantle source is lherzolitic as the sub-continental lithospheric mantle found further south in the TVZ has been removed by rifting and crustal thinning, causing the shift to rhyolitic rather than andesitic volcanism (Law et al, 2021). Regions without active calderas have lower inputs of basalt due to either a subdued influence from fluid-fluxing or a more depleted mantle source (Barker et al, 2020;Zellmer et al, 2020).…”

“…All OVC basalts (including blebs in rhyolitic eruptions) contain olivine, clinopyroxene, and plagioclase crystals (sometimes in aggregates) within a glassy (e.g., Matahi) to highly microcrystalline groundmass (e.g., Cole, 1970b;Law et al, 2021;Nairn, 2002Nairn, , 1992Rowe et al, 2021;Sable et al, 2009;Schmitz and Smith, 2004;Shane et al, 2008a). Additionally, all basalts contain xenocrystic quartz and rhyolitic material entrained during ascent (Beanland, 1989;Cole, 1973a;Nairn, 2002;Schmitz and Smith, 2004).…”