TephraNZ: a major- and trace-element reference dataset for glass-shard analyses from prominent Quaternary rhyolitic tephras in New Zealand and implications for correlation

Abstract

Abstract. Although analyses of tephra-derived glass shards have been undertaken in New Zealand for nearly four decades (pioneered by Paul Froggatt), our study is the first to systematically develop a formal, comprehensive, open-access reference dataset of glass-shard compositions for New Zealand tephras. These data will provide an important reference tool for future studies to identify and correlate tephra deposits and for associated petrological and magma-related studies within New Zealand and beyond. Here we present the foundation dataset for TephraNZ, an open-access reference dataset for selected tephra deposits in New Zealand. Prominent, rhyolitic, tephra deposits from the Quaternary were identified, with sample collection targeting original type sites or reference locations where the tephra's identification is unequivocally known based on independent dating and/or mineralogical techniques. Glass shards were extracted from the tephra deposits, and major- and trace-element geochemical compositions were determined. We discuss in detail the data reduction process used to obtain the results and propose that future studies follow a similar protocol in order to gain comparable data. The dataset contains analyses of glass shards from 23 proximal and 27 distal tephra samples characterising 45 eruptive episodes ranging from Kaharoa (636 ± 12 cal yr BP) to the Hikuroa Pumice member (2.0 ± 0.6 Ma) from six or more caldera sources, most from the central Taupō Volcanic Zone. We report 1385 major-element analyses obtained by electron microprobe (EMPA), and 590 trace-element analyses obtained by laser ablation (LA)-ICP-MS, on individual glass shards. Using principal component analysis (PCA), Euclidean similarity coefficients, and geochemical investigation, we show that chemical compositions of glass shards from individual eruptions are commonly distinguished by major elements, especially CaO, TiO2, K2O, and FeOtt (Na2O+K2O and SiO2/K2O), but not always. For those tephras with similar glass major-element signatures, some can be distinguished using trace elements (e.g. HFSEs: Zr, Hf, Nb; LILE: Ba, Rb; REE: Eu, Tm, Dy, Y, Tb, Gd, Er, Ho, Yb, Sm) and trace-element ratios (e.g. LILE/HFSE: Ba/Th, Ba/Zr, Rb/Zr; HFSE/HREE: Zr/Y, Zr/Yb, Hf/Y; LREE/HREE: La/Yb, Ce/Yb). Geochemistry alone cannot be used to distinguish between glass shards from the following tephra groups: Taupō (Unit Y in the post-Ōruanui eruption sequence of Taupō volcano) and Waimihia (Unit S); Poronui (Unit C) and Karapiti (Unit B); Rotorua and Rerewhakaaitu; and Kawakawa/Ōruanui, and Okaia. Other characteristics, including stratigraphic relationships and age, can be used to separate and distinguish all of these otherwise-similar tephra deposits except Poronui and Karapiti. Bimodality caused by K2O variability is newly identified in Poihipi and Tahuna tephras. Using glass-shard compositions, tephra sourced from Taupō Volcanic Centre (TVC) and Mangakino Volcanic Centre (MgVC) can be separated using bivariate plots of SiO2/K2O vs. Na2O+K2O. Glass shards from tephras derived from Kapenga Volcanic Centre, Rotorua Volcanic Centre, and Whakamaru Volcanic Centre have similar major- and trace-element chemical compositions to those from the MgVC, but they can overlap with glass analyses from tephras from Taupō and Okataina volcanic centres. Specific trace elements and trace-element ratios have lower variability than the heterogeneous major-element and bimodal signatures, making them easier to fingerprint geochemically.

Geochemistry, Magmatic Processes and Timescales of Recent Rhyolitic Eruptives of the Ōkataina Volcanic Centre, Taupō Volcanic Zone, Aotearoa/New Zealand

“…Whole-rock major-and trace-element data, and glass major-element data, are available for all Rotorua Subgroup eruptive units at Ōkataina (e.g. Smith et al 2005;Hopkins et al 2021), and whole-rock Sr isotopic data are available for several (Bowyer 2001;Rocco 2020). However, such data are often patchy, especially for glasses (melts), rare earth element concentrations (REEs), and Pb and Nd isotopic data.…”

“…Ōkataina was first mapped several decades ago (Nairn 1981(Nairn , 2002 and geochemical work on many eruptive units has been carried out since (e.g. Davis 1985;Nairn 1992;Wright 2000;Nairn et al 2004;Schmitz and Smith 2004;Smith et al 2004Smith et al , 2005Smith et al , 2006Shane et al 2007Shane et al , 2008aRocco 2020;Hopkins et al 2021). However, most geophysical studies conducted in and around the Ōkataina area are much more recent.…”

“…Many studies also do not mention the magma types analysed, despite it being known that the Ōkataina magmas show wide geochemical variation (e.g. Johnson et al 2011Johnson et al , 2013Rocco 2020;Hopkins et al 2021).…”

“…(2005) presented average major element compositions of glass shards for each magma type, the majority of recent studies of the Te Rere (e.g. Rocco 2020;Hopkins et al 2021) do not identify the magma type. This can be problematic as Ōkataina tephras are often heterogeneous due to comprising multiple magma types (e.g.…”

“…This can be problematic as Ōkataina tephras are often heterogeneous due to comprising multiple magma types (e.g. Shane et al 2008c;Hopkins et al 2021). In these cases, reference was made to the sampling localities recorded in these studies, and comparisons made with data collected for this thesis.…”