Miocene and Pliocene Silicic Coromandel Volcanic Zone Tephras from ODP Site 1124-C: Petrogenetic Applications and Temporal Evolution

Volcanic Zone ◽

Arc Magmas ◽

Glass Shard ◽

Tephra Layers ◽

The Coromandel Volcanic Zone (CVZ) was the longest-lived area of volcanism in New Zealand hosting the commencement of large explosive rhyolitic and ignimbrite forming eruptions. The NW trending Coromandel Peninsula is the subaerial remnant of the Miocene-Pliocene CVZ, which is regarded as a tectonic precursor to the Taupo Volcanic Zone (TVZ), currently the most dynamic and voluminous rhyolitic volcanic centre on Earth. This study presents new single glass shard major and trace element geochemical analyses for 72 high-silica volcanic tephra layers recovered from well-dated deep-sea sediments of the SW Pacific Ocean by the Ocean Drilling Program (ODP) Leg 181. ODP Site 1124, ~720 km south and east from the CVZ, penetrated sediments of the Rekohu Drift yielding an unprecedented record of major explosive volcanic eruptions owing to the favourable location and preservation characteristics at this site. This record extends onshore eruptive sequences of CVZ explosive volcanism that are obscured by poor exposure, alteration, and erosion and burial by younger volcanic deposits. Tephra layers recovered from Site 1124 are well-dated through a combination of biostratigraphic and palaeomagnetic methods allowing the temporal geochemical evolution of the CVZ to be reconstructed in relation to changes in the petrogenesis of CVZ arc magmas from ~ 10 to 2 Ma. This thesis establishes major and trace element geochemical "fingerprints" for all Site 1124-C tephras using well-established (wavelength dispersive electron probe microanalysis) and new (laser ablation inductively coupled plasma mass spectrometry) in situ single glass shard microanalytical techniques. Trace element analysis of Site 1124-C glass shards (as small as 20 um) demonstrate that trace element signatures offer a more specific, unequivocal characterisation for distinguishing (and potentially correlating) between tephras with nearly identical major element compositions. The Site 1124-C core contains 72 unaltered Miocene-Pliocene volcanic glass-shard-bearing laminae > 1 cm thick that correspond to 83 or 84 geochemical eruptive units. Revised eruptive frequencies based on the number of geochemical eruptive units identified represent at least one eruption every 99 kyr for the late Miocene and one per 74 kyr for the Pliocene. The frequency of tephra deposition throughout the history of the CVZ has not been constant, rather reflecting pulses of major explosive eruptions resulting in closely clustered groups of tephra separated by periods of reduced activity, relative volcanic quiescence or non-tephra deposition. As more regular activity became prevalent in the Pliocene, it was accompanied by more silicic magma compositions. Rhyolitic volcanic glass shards are characterised by predominantly calc-alkaline and minor high-K enriched major element compositions. Major element compositional variability of the tephras deposited between 10 Ma and 2 Ma reveals magma batches with pre-eruptive compositional gradients implying a broad control by fractional crystallisation. Trace element characterisation of glass shards reveals the role of magmatic processes that are not readily apparent in the relatively homogeneous major element compositions. Multi-element diagrams show prominent negative Sr and Ti anomalies against primitive mantle likely caused by various degrees of plagioclase and titanomagnetite fractional crystallisation in shallow magma chambers. Relative Nb depletion, characteristic of arc volcanism, is moderate in CVZ tephras. HFSEs (e.g. Nb, Zr, Ti) and HREEs (e.g. Yb, Lu) remain immobile during slab fluid flux suggesting they are derived from the mantle wedge. LILE (e.g. Rb, Cs, Ba, Sr) and LREE (e.g. La, Ce) enrichments are consistent with slab fluid contribution. B/La and Li/Y ratios can be used as a proxy for the flux of subducting material to the mantle wedge, they suggest there is a strong influence from this component in the generation of CVZ arc magmas, potentially inducing melting. CVZ tephra show long-term coherent variability in trace element geochemistry. Post ~ 4 Ma tephras display a more consistent, less variable, chemical fingerprint that persists up to and across the CVZ/TVZ transition at ~ 2 Ma. Initiation of TVZ volcanism may have occurred earlier than is presently considered, or CVZ to TVZ volcanism may have occurred without significant changes in magma generation processes.

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

10.5194/gchron-2020-34 ◽

2020 ◽

Author(s):

Jenni L. Hopkins ◽

Janine E. Bidmead ◽

David J. Lowe ◽

Richard J. Wysoczanski ◽

Bradley J. Pillans ◽

...

Keyword(s):

New Zealand ◽

Trace Element ◽

Major Element ◽

Chemical Compositions ◽

Volcanic Centre ◽

Reference Dataset ◽

Future Studies ◽

Glass Shard ◽

Tephra Deposits ◽

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 &pm; 12 cal. yrs BP) to the Hikuroa Pumice member (2.0 &pm; 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.

AMS 14C Dates and Major Element Composition of Glass Shards of Late Pleistocene Tephras on Tanegashima Island, Southern Japan

Radiocarbon ◽

10.1017/s0033822200047135 ◽

2012 ◽

Vol 54 (3-4) ◽

pp. 351-358 ◽

Cited By ~ 4

Author(s):

Mitsuru Okuno ◽

Masayuki Torii ◽

Hideto Naruo ◽

Yoko Saito-Kokubu ◽

Tetsuo Kobayashi

Keyword(s):

Late Pleistocene ◽

Major Element ◽

Volcanic Glass ◽

Archaeological Site ◽

Major Element Composition ◽

Ams Radiocarbon Dating ◽

Southern Japan ◽

Tephra Layers ◽

C 32 ◽

Four late Pleistocene tephra layers—Tane I (Tn1), II (Tn2), III (Tn3), and IV (Tn4) in ascending order—are intercalated between widespread tephras, Kikai-Tozurahara (K-Tz: 95 ka) and Aira-Tn (AT: 30 cal kBP), on Tanegashima Island, in southern Japan. Paleolithic ruins such as the Yokomine C and Tatikiri archaeological sites were excavated from the loam layer between the Tn4 and Tn3 tephras. To refine the chronological framework on the island, we conducted accelerator mass spectrometry (AMS) radiocarbon dating for 2 paleosol and 6 charcoal samples related with the late Pleistocene tephras and the Yokomine C archaeological site. The obtained 14C dates are consistent with the stratigraphy in calendar years, 33 cal kBP for Tn4, 40 cal kBP for Tn3, and >50 cal kBP for Tn2 and Tn1. The charcoal dates from Yokomine C, 32–38 cal kBP, not only constrain the age of Tn4 and Tn3 ashes, but also serve as a possible date for the site. We also measured the major element compositions of volcanic glass shards with EDS-EPMA to characterize these tephras. Although we could not find a possible correlative for Tn3 and Tn4 ashes using major element oxides of the glass shards, i.e. 75–76 wt% in SiO2, the glass chemistry obtained in this study will be valuable in correlating these tephras with their source volcanoes in the near future.

An Elemental and Isotopic Investigation of Quaternary Silicic Taupo Volcanic Zone Tephras from ODP Site 1123: Chronostratigraphic and Petrogenetic Applications

10.26686/wgtn.16959538.v1 ◽

2021 ◽

Author(s):

◽

Aidan Stuart Robert Allan

Keyword(s):

Trace Element ◽

Sediment Cores ◽

Deep Ocean ◽

Taupo Volcanic Zone ◽

Volcanic Zone ◽

Age Model ◽

Tephra Layers ◽

Mafic Magmas ◽

Silicic Magmas

This thesis presents a chemical and isotopic investigation of well-dated silicic tephra layers sourced from the Taupo Volcanic Zone (TVZ), central North Island, New Zealand, that were recovered from deep ocean sediment cores at Ocean Drilling Program Site 1123 (41 degrees 47.16' S, 171 degrees 29.94' W; 3290 m water depth), located approximately 1000 km east of the TVZ. The relative quiescence of the deep ocean sedimentary setting, the continuous supply of biogenic and terrigenous sediment and the favourable location of Site 1123 close to the main TVZ ash dispersal path have resulted in an extensive TVZ tephra record (70 Quaternary tephra layers preserved in 3 sediment cores) at Site 1123. This record extends and compliments the onshore record of silicic TVZ volcanism which has been obscured by erosion of non-consolidated volcanic material and burial of older units by younger volcanic deposits. The Site 1123 cores comprise an important paleo-oceanographic record for the Southwest Pacific Ocean and as a result of previous paleo-environmental studies, the Site 1123 tephras have been assigned orbitally tuned stable isotope ages that are more precise than is currently possible by any radiometric dating techniques. These features of the Site 1123 tephra record highlight its potential to be established as a type section for Quaternary tephrochronological studies in the New Zealand region. In addition, the continuous stratigraphy and precise age control of these tephras enables the Site 1123 record to be used as a petrogenetic archive to investigate changes in chemical and isotopic composition of these tephras that may be related to changes in the petrogenesis of TVZ silicic magmas during the last ~ 1.65 Ma. This thesis establishes major and trace element chemical 'fingerprints' for the Site 1123 tephras using traditional (electron probe microanalysis) and novel (laser ablation inductively coupled plasma mass spectrometry) in situ geochemical techniques. Trace element fingerprints are demonstrated to provide a more precise means of correlating and distinguishing between tephras with essentially identical major element chemistries. These fingerprints are used to refine the original Site 1123 composite stratigraphy and age model and identify a section of repeated sediments in the Site 1123 cores that have introduced a significant error into the original composite stratigraphy and age model for the interval ~1.1 to 1.4 Ma. Correlation of the tephra layers between the 3 sediment cores (1123A, B and C) establishes that ~37-38 individual tephra units are recorded with ages ranging from 1.655 Ma to 27.1 ka. Approximately 50% of the eruptive units and cumulative tephra thickness at the site were recorded during the first ~ 150 ka of silicic TVZ volcanism (1.65 to 1.50 Ma). The fragmentary onshore record does not preserve clear evidence for this early period of hyperactivity. Four broad silicic melt types are identified on the basis of chemistry and eruptive age. Trace element indices of fractional crystallisation suggests the origin of the four melt types is primarily due to differential degrees of fractional crystallisation of accessory zircon, hydrous mineral phases and Fe-Ti oxides. Sr-Nd-Pb isotopic compositions of 13 representative Site 1123 tephras cannot be generated using traditional models in which Torlesse meta-sedimentary rocks are the sole contaminant of mafic magmas. Instead the data support a model in which ascending TVZ basalts assimilate crustal rocks of both meta-greywacke terranes: firstly up to 15% of Waipapa crust is assimilated at depth, followed by assimilation of between 20 and 45% Torlesse crust at shallower levels. In this model the majority of Site 1123 tephras indicate a remarkably uniform amount of crust (~ 35%) with the most evolved sample requiring 45% crustal contribution. However, extensive fractional crystallisation (55-85%) is required to have accompanied crustal assimilation in order to drive the relatively low SiO2 compositions of these contaminated mafic magmas (SiO2 = 53-58 wt% after crustal contamination) to the high SiO2 rhyolite (74-78 wt%) compositions of the Site 1123 tephras. The large crustal contributions to TVZ silicic magmas (35-45%) implied by these data are high compared to large volume silicic magmas from different settings (e.g. Yemen-Ethiopia; Long Valley, USA), a feature that likely reflects the thin crust and high thermal flux into the continental crust beneath the TVZ.

Micro-Analytical Studies of the Petrogenesis of Silicic Arc Magmas in the Taupo Volcanic Zone and Southern Kermadec Arc, New Zealand

10.26686/wgtn.16959340.v1 ◽

2021 ◽

Author(s):

◽

Katharine Emma Saunders

Keyword(s):

Trace Element ◽

Melt Inclusions ◽

Taupo Volcanic Zone ◽

Continental Lithosphere ◽

Crystal Mush ◽

Volcanic Zone ◽

Element Modelling ◽

Magma Genesis ◽

Arc Magmas

The petrogenesis of silicic arc magmas is controversial with end-member models of fractional crystallisation and crustal anatexis having been invoked. A prime example of this is the archetypical continental Taupo Volcanic Zone and the adjacent oceanic Kermadec Arc. Insights into the genesis and timescales of magmatic processes of four continental rhyolitic magmas (Whakamaru, Oruanui, Taupo and Rotorua eruptives) and an oceanic (Healy seamount) rhyodacitic magma are documented through micro-analytical chemical studies of melt inclusions and crystal zonation of plagioclase and quartz. Electron probe microanalysis, laser ablation inductively coupled plasma mass spectrometry and Fourier transform infrared spectroscopy have been used to measure major, trace and volatile element concentrations, respectively, of melt inclusions and crystals. Melt inclusions are high silica (e.g. 74 - 79 wt%) irrespective of arc setting and display a wide range of trace element compositions (e.g. Sr = 17 - 180 ppm). Taupo Volcanic Zone melt inclusions exhibit higher K2O and Ce/Yb relative to Healy melt inclusions reflecting the assimilation of continental lithosphere. Quantitative trace element modelling of melt inclusion compositions: (a) demonstrates that magma genesis occurred through 62 - 76% fractional crystallisation at Healy whereas assimilation of continental lithosphere (greywacke) in addition to 60 - 80% fractional crystallisation is required for the Taupo Volcanic Zone magmas; and (b) suggests the presence of crystal mush bodies beneath silicic magma chambers in both continental and oceanic arc environments. Water concentrations of melt inclusions ranged between 1.4 - 5.1 wt% for the Whakamaru, Taupo and Healy samples. However, the inconsistency in the measured molecular water to hydroxyl concentrations of melt inclusions relative to those determined experimentally for groundmass rhyolitic glasses provide evidence for the degassing of inclusions prior to quenching, by diffusion of hydroxyl groups through the crystal host. Thus, partial pressures of water estimated from the inclusions and inferred depths of the crystallising magma bodies are underestimated. Chemical profiles of mineral zonation, however, indicate a more complex origin of silicic melts than simple fractionation and assimilation. For example, trace element modelling of Whakamaru plagioclase suggests that the three distinct textural plagioclase populations present in Whakamaru samples crystallised from four physiochemically discrete silicic melts. This modelling indicates a strong petrogenetic link between andesitic and silicic magmas from the chemical variation of selected Whakamaru plagioclase crystals possessing high anorthite (45-60 mol %) cores and low anorthite (~ 30 mol %) rim compositions and the interaction of greywacke partial melts. Furthermore, Sr diffusion modelling of core-rim interfaces of the same plagioclase crystals indicate the amalgamation of the magma chamber occurred continuously over the 15,000 years preceding the climactic eruption. Conversely, the major element zonation of Taupo plagioclases implies magma genesis occurred solely through assimilation and fractional crystallisation without the incorporation of evolved crystal mush magmas, indicating a spectrum of magmatic processes are occurring beneath the Taupo Volcanic Zone with each eruption providing only a snapshot of the petrogenesis of the Taupo Volcanic Zone.

An Elemental and Isotopic Investigation of Quaternary Silicic Taupo Volcanic Zone Tephras from ODP Site 1123: Chronostratigraphic and Petrogenetic Applications

10.26686/wgtn.16959538 ◽

2021 ◽

Author(s):

◽

Aidan Stuart Robert Allan

Keyword(s):

Trace Element ◽

Sediment Cores ◽

Deep Ocean ◽

Taupo Volcanic Zone ◽

Volcanic Zone ◽

Age Model ◽

Tephra Layers ◽

Mafic Magmas ◽

Silicic Magmas

This thesis presents a chemical and isotopic investigation of well-dated silicic tephra layers sourced from the Taupo Volcanic Zone (TVZ), central North Island, New Zealand, that were recovered from deep ocean sediment cores at Ocean Drilling Program Site 1123 (41 degrees 47.16' S, 171 degrees 29.94' W; 3290 m water depth), located approximately 1000 km east of the TVZ. The relative quiescence of the deep ocean sedimentary setting, the continuous supply of biogenic and terrigenous sediment and the favourable location of Site 1123 close to the main TVZ ash dispersal path have resulted in an extensive TVZ tephra record (70 Quaternary tephra layers preserved in 3 sediment cores) at Site 1123. This record extends and compliments the onshore record of silicic TVZ volcanism which has been obscured by erosion of non-consolidated volcanic material and burial of older units by younger volcanic deposits. The Site 1123 cores comprise an important paleo-oceanographic record for the Southwest Pacific Ocean and as a result of previous paleo-environmental studies, the Site 1123 tephras have been assigned orbitally tuned stable isotope ages that are more precise than is currently possible by any radiometric dating techniques. These features of the Site 1123 tephra record highlight its potential to be established as a type section for Quaternary tephrochronological studies in the New Zealand region. In addition, the continuous stratigraphy and precise age control of these tephras enables the Site 1123 record to be used as a petrogenetic archive to investigate changes in chemical and isotopic composition of these tephras that may be related to changes in the petrogenesis of TVZ silicic magmas during the last ~ 1.65 Ma. This thesis establishes major and trace element chemical 'fingerprints' for the Site 1123 tephras using traditional (electron probe microanalysis) and novel (laser ablation inductively coupled plasma mass spectrometry) in situ geochemical techniques. Trace element fingerprints are demonstrated to provide a more precise means of correlating and distinguishing between tephras with essentially identical major element chemistries. These fingerprints are used to refine the original Site 1123 composite stratigraphy and age model and identify a section of repeated sediments in the Site 1123 cores that have introduced a significant error into the original composite stratigraphy and age model for the interval ~1.1 to 1.4 Ma. Correlation of the tephra layers between the 3 sediment cores (1123A, B and C) establishes that ~37-38 individual tephra units are recorded with ages ranging from 1.655 Ma to 27.1 ka. Approximately 50% of the eruptive units and cumulative tephra thickness at the site were recorded during the first ~ 150 ka of silicic TVZ volcanism (1.65 to 1.50 Ma). The fragmentary onshore record does not preserve clear evidence for this early period of hyperactivity. Four broad silicic melt types are identified on the basis of chemistry and eruptive age. Trace element indices of fractional crystallisation suggests the origin of the four melt types is primarily due to differential degrees of fractional crystallisation of accessory zircon, hydrous mineral phases and Fe-Ti oxides. Sr-Nd-Pb isotopic compositions of 13 representative Site 1123 tephras cannot be generated using traditional models in which Torlesse meta-sedimentary rocks are the sole contaminant of mafic magmas. Instead the data support a model in which ascending TVZ basalts assimilate crustal rocks of both meta-greywacke terranes: firstly up to 15% of Waipapa crust is assimilated at depth, followed by assimilation of between 20 and 45% Torlesse crust at shallower levels. In this model the majority of Site 1123 tephras indicate a remarkably uniform amount of crust (~ 35%) with the most evolved sample requiring 45% crustal contribution. However, extensive fractional crystallisation (55-85%) is required to have accompanied crustal assimilation in order to drive the relatively low SiO2 compositions of these contaminated mafic magmas (SiO2 = 53-58 wt% after crustal contamination) to the high SiO2 rhyolite (74-78 wt%) compositions of the Site 1123 tephras. The large crustal contributions to TVZ silicic magmas (35-45%) implied by these data are high compared to large volume silicic magmas from different settings (e.g. Yemen-Ethiopia; Long Valley, USA), a feature that likely reflects the thin crust and high thermal flux into the continental crust beneath the TVZ.

Micro-Analytical Studies of the Petrogenesis of Silicic Arc Magmas in the Taupo Volcanic Zone and Southern Kermadec Arc, New Zealand

10.26686/wgtn.16959340 ◽

2021 ◽

Author(s):

◽

Katharine Emma Saunders

Keyword(s):

Trace Element ◽

Melt Inclusions ◽

Taupo Volcanic Zone ◽

Continental Lithosphere ◽

Crystal Mush ◽

Volcanic Zone ◽

Element Modelling ◽

Magma Genesis ◽

Arc Magmas

The petrogenesis of silicic arc magmas is controversial with end-member models of fractional crystallisation and crustal anatexis having been invoked. A prime example of this is the archetypical continental Taupo Volcanic Zone and the adjacent oceanic Kermadec Arc. Insights into the genesis and timescales of magmatic processes of four continental rhyolitic magmas (Whakamaru, Oruanui, Taupo and Rotorua eruptives) and an oceanic (Healy seamount) rhyodacitic magma are documented through micro-analytical chemical studies of melt inclusions and crystal zonation of plagioclase and quartz. Electron probe microanalysis, laser ablation inductively coupled plasma mass spectrometry and Fourier transform infrared spectroscopy have been used to measure major, trace and volatile element concentrations, respectively, of melt inclusions and crystals. Melt inclusions are high silica (e.g. 74 - 79 wt%) irrespective of arc setting and display a wide range of trace element compositions (e.g. Sr = 17 - 180 ppm). Taupo Volcanic Zone melt inclusions exhibit higher K2O and Ce/Yb relative to Healy melt inclusions reflecting the assimilation of continental lithosphere. Quantitative trace element modelling of melt inclusion compositions: (a) demonstrates that magma genesis occurred through 62 - 76% fractional crystallisation at Healy whereas assimilation of continental lithosphere (greywacke) in addition to 60 - 80% fractional crystallisation is required for the Taupo Volcanic Zone magmas; and (b) suggests the presence of crystal mush bodies beneath silicic magma chambers in both continental and oceanic arc environments. Water concentrations of melt inclusions ranged between 1.4 - 5.1 wt% for the Whakamaru, Taupo and Healy samples. However, the inconsistency in the measured molecular water to hydroxyl concentrations of melt inclusions relative to those determined experimentally for groundmass rhyolitic glasses provide evidence for the degassing of inclusions prior to quenching, by diffusion of hydroxyl groups through the crystal host. Thus, partial pressures of water estimated from the inclusions and inferred depths of the crystallising magma bodies are underestimated. Chemical profiles of mineral zonation, however, indicate a more complex origin of silicic melts than simple fractionation and assimilation. For example, trace element modelling of Whakamaru plagioclase suggests that the three distinct textural plagioclase populations present in Whakamaru samples crystallised from four physiochemically discrete silicic melts. This modelling indicates a strong petrogenetic link between andesitic and silicic magmas from the chemical variation of selected Whakamaru plagioclase crystals possessing high anorthite (45-60 mol %) cores and low anorthite (~ 30 mol %) rim compositions and the interaction of greywacke partial melts. Furthermore, Sr diffusion modelling of core-rim interfaces of the same plagioclase crystals indicate the amalgamation of the magma chamber occurred continuously over the 15,000 years preceding the climactic eruption. Conversely, the major element zonation of Taupo plagioclases implies magma genesis occurred solely through assimilation and fractional crystallisation without the incorporation of evolved crystal mush magmas, indicating a spectrum of magmatic processes are occurring beneath the Taupo Volcanic Zone with each eruption providing only a snapshot of the petrogenesis of the Taupo Volcanic Zone.

Cryptotephra from Lipari Volcano in the eastern Gulf of Taranto (Italy) as a time marker for paleoclimatic studies

Quaternary Research ◽

10.1017/qua.2018.2 ◽

2018 ◽

Vol 89 (2) ◽

pp. 520-532

Author(s):

Valerie Menke ◽

Steffen Kutterolf ◽

Carina Sievers ◽

Julie Christin Schindlbeck ◽

Gerhard Schmiedl

Keyword(s):

High Resolution ◽

Large Scale ◽

Sediment Cores ◽

Volcanic Glass ◽

Geochemical Fingerprinting ◽

Glass Shard ◽

Shelf Sediments ◽

Age Model ◽

Tephra Layers ◽

AbstractWe present the first tephroanalysis based on geochemical fingerprinting of volcanic glass shards from eastern Apulian shelf sediments in the Gulf of Taranto (Italy). High sedimentation rates in the gulf are ideal for high-resolution paleoclimate studies, which rely on accurate age models. Cryptotephrostratigraphy is a novel tool for the age assessment of marine sediment cores in the absence of discrete tephra layers. High-resolution quantitative analysis of glass shard abundance in the uppermost 45 cm of a gravity core identified two cryptotephras. Microprobe analysis of glass shards supported by an accelerator mass spectrometry 14C–based age model identified the pronounced primary cryptotephra at 36 cm bsf (below sea floor) as the felsic AD 776 Monte Pilato Eruption on the island of Lipari, whereas the thinner, mafic tephra layer at 1.5 cm bsf is associated with the AD 1944 eruption of Somma-Vesuvius. Identifying these tephra layers provides an additional, 14C-independent, stratigraphic framework for further paleoclimatic studies allowing us to link Mediterranean climate and hydrology to orbital variation and large-scale atmospheric processes. Our results underline the importance of qualitative tephrostratigraphy in a highly geodynamic region, where solely quantitative approaches have demonstrated to bear a high potential for false correlations between tephra layers and eruptions.

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

Geochronology ◽

10.5194/gchron-3-465-2021 ◽

2021 ◽

Vol 3 (2) ◽

pp. 465-504

Author(s):

Jenni L. Hopkins ◽

Janine E. Bidmead ◽

David J. Lowe ◽

Richard J. Wysoczanski ◽

Bradley J. Pillans ◽

...

Keyword(s):

New Zealand ◽

Trace Element ◽

Major Element ◽

Chemical Compositions ◽

Volcanic Centre ◽

Reference Dataset ◽

Future Studies ◽

Glass Shard ◽

Tephra Deposits ◽

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.

Philosophical Transactions of the Royal Society of London Series A Physical and Engineering Sciences ◽

Element fluxes associated with subduction related magmatism

10.1098/rsta.1991.0054 ◽

1991 ◽

Vol 335 (1638) ◽

pp. 393-405 ◽

Cited By ~ 161

Keyword(s):

Trace Element ◽

Incompatible Element ◽

Mantle Wedge ◽

Isotope Ratios ◽

Noticeable Effect ◽

Arc Magmas ◽

Mid Ocean Ridge ◽

Ocean Ridge ◽

Arc Magma ◽

Subducted Sediment

Destructive plate margin magmas may be subdivided into two groups on the basis of their rare earth element (REE) ratios. Most island arc suites have low Ce/Yb, and remarkably restricted isotope ratios of 87 Sr/ 86 Sr = 0.7033, 143 Nd/ 144 Nd = 0.51302, 206 Pb/ 204 Pb = 18.76 , 207 Pb/ 204 Pb = 15.57, and 208 Pb/ 204 Pb = 38.4. However, they also have Rb/Sr (0.03), Th/U (2.2) and Ce/Yb (8.5) ratios which are significantly less than accepted estimates for the bulk continental crust. The high Ce/Yb suites have higher incompatible element contents, more restricted heavy REE, and much more variable isotope ratios. Such rocks are found in the Aeolian Islands, Grenada, Indonesia and Philippines, and their isotope and trace element features have been attributed both to contributions from subducted sediment, and/or old trace element enriched material in the mantle wedge. It is argued that for isotope and trace element models the slab component can usefully be taken to consist of subducted sediment and altered mid-ocean ridge basalts, since these may contain ca. 80% of the water in the subducted slab, and the distinctive trace element features of arc magmas are generally attributed to the movement of material in hydrous fluids. The isotope data indicate that not more than 15% of the Sr and Th in an average arc magma were derived from subducted material, and that the rest were derived from the mantle wedge. The fluxes of elements which cannot be characterized isotopically are more difficult to constrain, but for most minor and trace elements the slab derived contribution in arc magmas is too small to have a noticeable effect on the residual slab.