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.
Highlights
Tephrochronology is the method by which volcanic ash deposits are used as stratigraphic isochronous marker horizons for correlating, dating, and synchronising deposits and events in geologic, paleoenvironmental, and archaeological records (Sarna-Wojcicki, 2000; Shane, 2000; Dugmore et al, 2004; Lowe, 2011; Alloway et al, 2013)
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
In this study we present TephraNZ as a foundation reference dataset of internally consistent, openaccess data for major- and trace-element compositions of glass shards from a selection of the most pervasive Quaternary tephra deposits in New Zealand (Table 1)
Summary
Tephrochronology is the method by which volcanic ash (tephra) deposits are used as stratigraphic isochronous marker horizons (isochrons) for correlating, dating, and synchronising deposits and events in geologic, paleoenvironmental, and archaeological records (Sarna-Wojcicki, 2000; Shane, 2000; Dugmore et al, 2004; Lowe, 2011; Alloway et al, 2013). A number of key trace elements have been identified as important for the correlation of rhyolitic tephras, including the high field strength elements (HFSEs) Zr and Nb; the large ion lithophile elements (LILEs) Rb, Sr, and Ba; the heavy rare-earth elements (HREEs) Gd, Yb, Sc, and Y; and the light rare-earth elements (LREEs) La and Nd. Trace-element ratios are identified as important, including (1) HFSE/HREE – for example Zr/Y, Nb/Y, Hf/Y; (2) LILE/HFSE – for example Ba/Th; (3) LREE/HFSE – for example Ce/Th, La/Nb; (4) LREE/HREE – for example La/Yb, Ce/Yb; and (5) HFSE/HFSE – for example Zr/Nb, Zr/Th. Some studies have shown that trace elements and trace-element ratios can distinguish between tephra beds that have indistinguishable glass-shard major-element signatures and are a robust way of providing accurate correlations Some studies have shown that trace elements and trace-element ratios can distinguish between tephra beds that have indistinguishable glass-shard major-element signatures and are a robust way of providing accurate correlations (e.g. Westgate et al, 1994; Pearce et al, 1996, 2002, 2004; Allan et al, 2008; Hopkins et al, 2017)
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