TephraNZ: a major- and trace-element reference dataset for glass-shard analyses from prominent Quaternary rhyolitic tephras in New Zealand and implications for correlation
Authors: Hopkins Jenni L. , Bidmead Janine E. , Lowe David J. , Wysoczanski Richard J. , Pillans Bradley J. , Ashworth Luisa , Rees Andrew B. H. , Tuckett Fiona 
Published: September 2021

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.