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Geochronology Advances in geochronological science
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https://doi.org/10.5194/gchron-2020-34
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gchron-2020-34
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  09 Nov 2020

09 Nov 2020

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This preprint is currently under review for the journal GChron.

TephraNZ: a major and trace element reference dataset for prominent Quaternary rhyolitic tephras in New Zealand and implications for correlation

Jenni L. Hopkins1, Janine E. Bidmead1, David J. Lowe2, Richard J. Wysoczanski3, Bradley J. Pillans4, Luisa Ashworth1, Andrew B. H. Rees1, and Fiona Tuckett1 Jenni L. Hopkins et al.
  • 1School of Geography Environment and Earth Science, Victoria University of Wellington, Wellington, PO Box 600, New Zealand
  • 2School of Science (Earth Sciences), University of Waikato, Hamilton, Private Bag 3105, New Zealand 3240
  • 3National Institute of Water and Atmospheric Research, Wellington, Private Bag 14901, New Zealand
  • 4Research School of Earth Science, Australian National University

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 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 twenty-three proximal and twenty-seven distal tephra samples characterising 45 eruptive episodes ranging from Kaharoa (636 ± 12 cal. yrs 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 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, FeOt (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, Okaia, and Unit L (of the Mangaone subgroup eruption sequence). Other characteristics can be used to separate and distinguish all of these otherwise-similar eruptives 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 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 geochemically fingerprint.

Jenni L. Hopkins et al.

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Short summary
Here we present the foundation dataset for “TephraNZ”, a formal, comprehensive, open access, reference dataset of glass-shard compositions for New Zealand tephras. We geochemically characterise 45 eruptive episodes ranging from Kaharoa (c. 636 cal. yrs BP) to the Hikuroa Pumice member (c. 2.0 Ma) from six or more caldera sources, most from the central Taupō Volcanic Zone.
Here we present the foundation dataset for “TephraNZ”, a formal, comprehensive, open access,...
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