New Perspectives on the Chronostratigraphy and Magmatic Evolution of the Auckland Volcanic Field, New Zealand

<p>Understanding the eruptive history of a volcanically active region is critical in assessing the hazard and risk posed by future eruptions. In regions where surface deposits are poorly preserved, and ambiguously sourced, tephrostratigraphy is a powerful tool to assess the characteristics of past eruptions. The city of Auckland, New Zealand’s largest urban centre and home to ca. 1.4 million people, is built on top of the active Auckland Volcanic Field (AVF). The AVF is an intraplate monogenetic basaltic volcanic field, with ca. 53 eruptive centres located in an area of ca. 360 km2. Little is known however, about the evolution of the field because the numerical and relative ages of the eruptions are only loosely constrained, and therefore the precise order of many eruptions is unknown. Here I apply tephrostratigraphic and geochemical techniques to investigate the chronology and magmatic evolution of the AVF eruptions. First, I present an improved methodology for in-situ analysis of lacustrine maar cores from the AVF by employing magnetic susceptibility and X-ray density scanning on intact cores. These techniques are coupled with geochemical microanalysis of the tephra-derived glass shards to reveal details of reworking within the cores. These details not only allow assessment of the deposit relationships within cores (e.g. primary vs. reworked horizons), but also to correlate tephra horizons between cores. Through the correlation of tephra units across cores from a variety of locations across the field, an improved regional tephrostratigraphic framework for the AVF deposits has been established. Following on from this, I detail the methods developed in this study to correlate tephra horizons within the maar cores back to their eruptive source. This technique uses geochemical fingerprinting to link the glass analyses from tephra samples to whole rock compositions. Such an approach has not been previously attempted due to the complications caused by fractional crystallisation, which affects concentrations of certain key elements in whole rock analyses. My method resolves these issues by using incompatible trace elements, which are preferentially retained in melt over crystals, and therefore retain comparable concentrations and concentration ratios between these two types of sample. Because of the primitive nature of the AVF magmas, their trace element signature is largely controlled by the involvement of several distinct mantle sources. This leads to significant variability between the volcanic centres that thus can be used for individually fingerprinting, and correlating tephra to whole rocks. Nevertheless, in some cases geochemistry cannot provide an unambiguous correlation, and a multifaceted approach is required to allow the correlation of the tephra horizons to source. The other criteria used to correlate tephra deposits to their source centre include, Ar-Ar ages of the centres, modelled and calculated ages of the tephra deposits, the scale of eruption, and the deposit locations and thicknesses. The results of this research outline the methodology for assessing occurrence and characteristics of basaltic tephra horizons within lacustrine maar cores, and the methodology for correlating these horizons to their eruptive source. In doing this the relative eruption order of the AVF is accurately determined for the first time. Temporal trends suggest acceleration of eruption repose periods to 21 ka followed by deceleration to present. Although no spatial evolution is observed, coupling of some centres is seen when spatial and temporal evolution are combined. The geochemical signature of the magmas appears to evolve in a cyclic manner with time, incorporating increasing amount of a shallow source. This evolution is seen both during a single eruption sequence and throughout the lifespan of the AVF. Finally, pre-eruptive processes are assessed as part of the study of the magmatic evolution of the AVF. The effects of contamination from the crust and lithosphere through which the magma ascends are evaluated using the Re-Os isotope system. The results show there are variable inputs from crustal sources, which have previously not been identified by traditional isotope systems (e.g. Pb-Sr-Nd isotopes). Two sources of contamination are identified based on their Os systematics relating to two terranes beneath the AVF: the metasedimentary crust and the Dun Mountain Ophiolite Belt. The identification of this process suggests there is interaction of ascending melt with the crust, contrary to what previous studies have concluded. This body of research has provided a detailed reconstruction of the chronostratigraphy and magmatic evolution of the AVF to aid accurate and detailed risk assessment of the threat posed by a future eruption from the Auckland Volcanic Field.</p>

New Perspectives on the Chronostratigraphy and Magmatic Evolution of the Auckland Volcanic Field, New Zealand

<p>Understanding the eruptive history of a volcanically active region is critical in assessing the hazard and risk posed by future eruptions. In regions where surface deposits are poorly preserved, and ambiguously sourced, tephrostratigraphy is a powerful tool to assess the characteristics of past eruptions. The city of Auckland, New Zealand’s largest urban centre and home to ca. 1.4 million people, is built on top of the active Auckland Volcanic Field (AVF). The AVF is an intraplate monogenetic basaltic volcanic field, with ca. 53 eruptive centres located in an area of ca. 360 km2. Little is known however, about the evolution of the field because the numerical and relative ages of the eruptions are only loosely constrained, and therefore the precise order of many eruptions is unknown. Here I apply tephrostratigraphic and geochemical techniques to investigate the chronology and magmatic evolution of the AVF eruptions. First, I present an improved methodology for in-situ analysis of lacustrine maar cores from the AVF by employing magnetic susceptibility and X-ray density scanning on intact cores. These techniques are coupled with geochemical microanalysis of the tephra-derived glass shards to reveal details of reworking within the cores. These details not only allow assessment of the deposit relationships within cores (e.g. primary vs. reworked horizons), but also to correlate tephra horizons between cores. Through the correlation of tephra units across cores from a variety of locations across the field, an improved regional tephrostratigraphic framework for the AVF deposits has been established. Following on from this, I detail the methods developed in this study to correlate tephra horizons within the maar cores back to their eruptive source. This technique uses geochemical fingerprinting to link the glass analyses from tephra samples to whole rock compositions. Such an approach has not been previously attempted due to the complications caused by fractional crystallisation, which affects concentrations of certain key elements in whole rock analyses. My method resolves these issues by using incompatible trace elements, which are preferentially retained in melt over crystals, and therefore retain comparable concentrations and concentration ratios between these two types of sample. Because of the primitive nature of the AVF magmas, their trace element signature is largely controlled by the involvement of several distinct mantle sources. This leads to significant variability between the volcanic centres that thus can be used for individually fingerprinting, and correlating tephra to whole rocks. Nevertheless, in some cases geochemistry cannot provide an unambiguous correlation, and a multifaceted approach is required to allow the correlation of the tephra horizons to source. The other criteria used to correlate tephra deposits to their source centre include, Ar-Ar ages of the centres, modelled and calculated ages of the tephra deposits, the scale of eruption, and the deposit locations and thicknesses. The results of this research outline the methodology for assessing occurrence and characteristics of basaltic tephra horizons within lacustrine maar cores, and the methodology for correlating these horizons to their eruptive source. In doing this the relative eruption order of the AVF is accurately determined for the first time. Temporal trends suggest acceleration of eruption repose periods to 21 ka followed by deceleration to present. Although no spatial evolution is observed, coupling of some centres is seen when spatial and temporal evolution are combined. The geochemical signature of the magmas appears to evolve in a cyclic manner with time, incorporating increasing amount of a shallow source. This evolution is seen both during a single eruption sequence and throughout the lifespan of the AVF. Finally, pre-eruptive processes are assessed as part of the study of the magmatic evolution of the AVF. The effects of contamination from the crust and lithosphere through which the magma ascends are evaluated using the Re-Os isotope system. The results show there are variable inputs from crustal sources, which have previously not been identified by traditional isotope systems (e.g. Pb-Sr-Nd isotopes). Two sources of contamination are identified based on their Os systematics relating to two terranes beneath the AVF: the metasedimentary crust and the Dun Mountain Ophiolite Belt. The identification of this process suggests there is interaction of ascending melt with the crust, contrary to what previous studies have concluded. This body of research has provided a detailed reconstruction of the chronostratigraphy and magmatic evolution of the AVF to aid accurate and detailed risk assessment of the threat posed by a future eruption from the Auckland Volcanic Field.</p>

Some Studies of New Zealand Quaternary Pyroclastic Rocks

<p>The development of volcanic "ash" studies in New Zealand can be traced through three broad periods (Jeune 1970). During the late 19th century the extensive pumice deposits surrounding Lake Taupo received considerable comment (Crawford 1875, Smith 1876 and Cussen 1887). Thomas (1887) recognised a covering of younger andesitic ash from Mts. Tongariro and Ruapehu overlying the pumice from Taupo and in his 1888 report on the eruption of Mt. Tarawera in 1886, Thomas provided a valuable description of the eruption and the deposits resulting from it. Tephra deposits received only cursory attention during the following years until soil surveys initiated as part of the research effort into bush sickness demonstrated a relationship between incidence of the disease and soil derived from tephra (Aston 1926). Extended soil surveys followed (Granger 1929, 1931, 1937, Taylor 1930, 1933, Grange and Taylor 1931, 1932) during the course of which many important soil forming tephras were named, described and mapped. On the basis of minerals studies, contributions were recognised from four recently active volcanic centres; Taupo, Rotorua, Tongariro National Park and Mt. Egmont.</p>

Some Studies of New Zealand Quaternary Pyroclastic Rocks

<p>The development of volcanic "ash" studies in New Zealand can be traced through three broad periods (Jeune 1970). During the late 19th century the extensive pumice deposits surrounding Lake Taupo received considerable comment (Crawford 1875, Smith 1876 and Cussen 1887). Thomas (1887) recognised a covering of younger andesitic ash from Mts. Tongariro and Ruapehu overlying the pumice from Taupo and in his 1888 report on the eruption of Mt. Tarawera in 1886, Thomas provided a valuable description of the eruption and the deposits resulting from it. Tephra deposits received only cursory attention during the following years until soil surveys initiated as part of the research effort into bush sickness demonstrated a relationship between incidence of the disease and soil derived from tephra (Aston 1926). Extended soil surveys followed (Granger 1929, 1931, 1937, Taylor 1930, 1933, Grange and Taylor 1931, 1932) during the course of which many important soil forming tephras were named, described and mapped. On the basis of minerals studies, contributions were recognised from four recently active volcanic centres; Taupo, Rotorua, Tongariro National Park and Mt. Egmont.</p>

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

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.

Variable preservation of tephra from the 1991 eruption of Volcán Hudson in small lakes: implications for reconstructing past eruption parameters

&lt;p&gt;Volcanic ash (tephra) deposits are used to reconstruct past eruption parameters. The ways in which tephra deposits are modified between deposition and their long-term preservation in the stratigraphic archive are poorly understood. In particular, we don&amp;#8217;t know if tephra layers preserved in lake sediments from small lakes accurately reflect the initial tephra fallout. We address this by re-surveying tephra deposits from the 1991 eruption of Volc&amp;#225;n Hudson, Chile. We measured tephra thickness, mass-loading and grain-size distribution of tephra from multiple cores in six small (&lt;0.2 km&lt;sup&gt;2&lt;/sup&gt;) lakes at locations 76-110 km from the volcano and in areas of contrasting land cover and climate. We also measured tephra preservation in terrestrial sites within each lake catchment. These data were compared with measurements taken shortly (days to weeks) after the eruption to determine how the tephra deposits have changed in the 29 years since the eruption. Preservation is variable within and between lakes, and also varies with the vegetation cover at terrestrial sites adjacent to the lakes. Tephra thicknesses are broadly comparable to the original fallout, but the degree of similarity varied notably and is sensitive to preservation environment. These findings have implications for reconstructing eruption parameters from tephra deposits in small lakes, and where the fallout area crosses large environmental gradients and contrasting vegetation regimes.&lt;/p&gt;