Glass chemistry, paleomagnetism, and correlation of middle Pleistocene tuffs in southern North Island, New Zealand, and Western Pacific
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Abstract New Zealand middle Pleistocene (Castlecliffian) rhyolitic tuffs older than 0.5 Ma are characterised on the basis of glass chemistry and paleomagnetism for the purpose of correlation. Of the tuffs examined, Rewa Pumice (c. 0.74 Ma), Potaka Pumice (c. 0.64 Ma), and Lower Te Muna Tephra (c. 0.75 Ma) are particularly widespread marker horizons. Their proven proximity to the Brunhes‐Matuyama paleomagnetic reversal provides a useful framework for correlation and absolute age control between regions in southern North Island— Wanganui, Wairarapa, and Hawke's Bay—and with deep‐sea cores from the Western Pacific. The tuffs allow correlation between a diverse range of sedimentary facies including marine, freshwater, and terrestrial, and provide absolute age control for thick sequences of unfossiliferous strata in Wairarapa and Hawke's Bay. The characterisation and correlation of the tuffs has a number of implications for Castlecliffian paleoclimatic, tectonic, and stratigraphic studies. The Rewa Pumice is contained in periglacial facies, and we assign it to oxygen isotope stage 20. Some tuffs in Hawke's Bay contain charcoal and are considered to be derived from ignimbrites. Their presence implies that the main axial ranges in Hawke's Bay were uplifted after 0.6 Ma. Previous lithostratigraphic correlations from the Castlecliffian stratotype along the Wanganui coast to sections across the Wanganui Basin, which contain radiometrically dated tuffs, are not sustainable, suggesting greater facies variations than hitherto assumed. The sequence of strata in the Rangitikei section containing the Rewa and Potaka Pumices is not a correlative of Kaimatira Pumice Sand at the coast section. Previous tuff correlations were made on the basis of age and mineralogy. Only the integrated use of glass chemistry and paleomagnetism allows correlation of tuffs in this time interval.Keywords:
Pumice
Tephrochronology
Volcanic glass
Radiometric dating
Explosive volcanic eruptions generate plumes of hot gas and quenched molten rock that has been fragmented by the expansion of gas as the magma exits the vent. These fragments are called pyroclasts . The tephra layers are comprised of volcanic glass, crystals, and lithic material. Given that tephra is dispersed over wide areas and forms a geologically instantaneous layer, these tephra layers can be particularly useful for chronology – providing a relative chronology between sites and age if the eruption has been dated using radiometric methods. Correlating volcanic ash layers between sites and to specific eruption deposits preserved at their source volcanoes can be achieved using the composition of the volcanic glass shards. The major and trace element glass compositions remain the same for a specific eruption deposit irrespective of the distance from the vent, and they constitute the chemical fingerprint of the tephra. Volcanic deposits can be dated using many commonly employed radiometric dating methods.
Tephrochronology
Chronology
Volcanic ash
Peléan eruption
Volcanic glass
Radiometric dating
Volcanic hazards
Phreatomagmatic eruption
Vulcanian eruption
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Tephrochronology (the dating of sedimentary sequences using volcanic ash layers) is an important tool for the dating and correlation of sedimentary sequences containing archives and proxies of past environmental change. In addition, tephra layers provide valuable information on the frequency and nature of ash fallout from volcanic activity. Successful tephrochronology is usually reliant on the correct geochemical identification of the tephra which has, until now, been based primarily on the analysis of major element oxide composition of glass shards using electron probe microanalysis (EPMA). However, it is often impossible to differentiate key tephra layers using EPMA alone. For example, the Hekla AD 1947 and 1510 tephras (which are found as visible layers in Iceland and also as ‘crypto-tephra’ microscopic layers in NW Europe) are currently indistinguishable using EPMA. Therefore, other stratigraphic or chronological information is needed for their reliable identification. Raman spectroscopy is commonly used in chemistry, since vibrational information is specific to the chemical bonds and symmetry of molecules, and can provide a fingerprint by which these can be identified. Here, we demonstrate how Raman spectroscopy can be used for the successful discrimination of mineral species in tephra through the analysis of individual glass shards. In this study, we obtained spectra from minerals within the glass shards – we analysed the microlites and intratelluric mineral phases that can definitely be attributed to the tephra shards and the glass itself. Phenocrysts were not analysed as they could be sourced locally from near-site erosion. Raman spectroscopy can therefore be considered a valuable tool for both proximal and distal tephrochronology because of its non-destructive nature and can be used to discriminate Hekla 1510 from Hekla 1947.
Tephrochronology
Phenocryst
Volcanic glass
Volcanology
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This paper presents a detailed record of volcanism extending back to ∼80 kyr BP for southern South America using the sediments of Laguna Potrok Aike (ICDP expedition 5022; Potrok Aike Maar Lake Sediment Archive Drilling Project - PASADO). Our analysis of tephra includes the morphology of glass, the mineral componentry, the abundance of glass-shards, lithics and minerals, and the composition of glass-shards in relation to the stratigraphy. Firstly, a reference database of glass compositions of known eruptions in the region was created to enable robust tephra correlations. This includes data published elsewhere, in addition to new glass-shard analyses of proximal tephra deposits from Hudson (eruption units H1 and H2), Aguilera (A1), Reclus (R1, R2-3), Mt Burney (MB1, MB2, MBx, MB1910) and historical Lautaro/Viedma deposits. The analysis of the ninety-four tephra layers observed in the Laguna Potrok Aike sedimentary sequence reveals that twenty-five tephra deposits in the record are the result of primary fallout and are sourced from at least three different volcanoes in the Austral Andean Volcanic Zone (Mt Burney, Reclus, Lautaro/Viedma) and one in the southernmost Southern Volcanic Zone (Hudson). One new correlation to the widespread H1 eruption from Hudson volcano at 8.7 (8.6–9.0) cal ka BP during the Quaternary is identified. The identification of sixty-five discrete deposits that were predominantly volcanic ashes (glass and minerals) with subtle characteristics of reworking (in addition to three likely reworked tephra, and one unknown layer) indicates that care must be taken in the analysis of both visible and invisible tephra layers to decipher their emplacement mechanisms.
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Maar
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A continuous ∼5280 calendar (cal.) yr long cryptotephrostratigraphic record of a peat core from northern New Zealand demonstrates that cryptotephra studies can enhance conventional tephra records by extending the known distribution of ash fall and enabling re-assessment of volcanic hazards. A systematic sampling strategy was used to locate peaks in glass-shard concentrations and to determine loci of individual geochemical populations, and a palynological method involving spiking samples with Lycopodium spores was adapted to facilitate accurate counting of glass-shard concentrations. Using glass shard major element compositions, and a core chronology based on eight AMS 14 C ages and two visible macroscopic tephra layers, Taupo Tephra (Unit Y) (1688-1748 cal. BP) and Tuhua Tephra (6800-7230 cal. BP) (2cr-age ranges), four cryptotephras were correlated with known eruptions: Whakaipo (Unit V) (2743-2782 cal. BP), Stent (Unit Q) (4240-4510 cal. BP), and Unit K (4970-5290 cal. BP), erupted from Taupo Volcanic Centre, and Whakatane Tephra (5470-5600 cal. BP) erupted from Okataina Volcanic Centre. Mixed glass populations were found in the core, most likely an artefact of post-depositional remobilization of shards vertically (both up and down) in the peat or on its surface by wind, or a result of closely spaced eruption events, or a combination of these. A secondary glass population identified within the macroscopic Taupo Tephra was tentatively attributed to either an earlier phase within that eruption or to mixing with a slightly older Taupo-derived eruptive or (less likely) a currently unknown Okataina-derived eruptive. These results indicate that, in the absence of continuous cryptotephrostratigraphic analysis, a peak in shard concentrations may not in itself be indicative of the ‘true’ stratigraphic (ie, isochronous) level of a tephra layer. For cryptotephra studies of peat cores, we recommend (1) using a detailed sampling strategy for the analysis of distal tephra-derived glass to detect and account for any mixed populations and possible vertical spread of glass shards through the peat, and (2) analysing more shards from larger samples to help ‘capture’ sparsely represented cryptic andesitic tephra deposits.
Tephrochronology
Volcanic glass
Chronology
Volcanic hazards
Peléan eruption
Lapilli
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Tephrochronology
Microprobe
Volcanic glass
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Cryptotephrochronology, the use of hidden, diminutive volcanic ash layers to date sediments, has rarely been applied outside western Europe but has the potential to improve the tephrochronology of other regions of the world. Here we present the first comprehensive cryptotephra study in Alaska. Cores were extracted from five peatland sites, with cryptotephras located by ashing and microscopy and their glass geochemistry examined using electron probe microanalysis. Glass geochemical data from nine tephras were compared between sites and with data from previous Alaskan tephra studies. One tephra present in all the cores is believed to represent a previously unidentified eruption of Mt. Churchill and is named here as the ‘Lena tephra’. A mid-Holocene tephra in one site is very similar to Aniakchak tephra and most likely represents a previously unidentified Aniakchak eruption, ca. 5300–5030 cal yr BP. Other tephras are from the late Holocene White River eruption, a mid-Holocene Mt. Churchill eruption, and possibly eruptions of Redoubt and Augustine volcanoes. These results show the potential of cryptotephras to expand the geographic limits of tephrochronology and demonstrate that Mt. Churchill has been more active in the Holocene than previously appreciated. This finding may necessitate reassessment of volcanic hazards in the region.
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Volcanic glass
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Abstract A core drilled in a swamp at the northern end of Lake Tutira, northern Hawke's Bay, New Zealand, as part of a programme to determine the erosion history of the Lake Tutira catchment, contains 14 tephra layers (10 rhyolitic and 4 andesitic). The mineralogy of each layer was determined, together with electron microprobe chemistry of glass shards, and of hornblendes from two of the andesitic tephras. The rhyolitic layers are identified from their stratigraphic position, mineralogy, and glass/mineral chemistry as: Taupo (1850 conventional radiocarbon years old), Mapara (2160 yr), Whakaipo (2685 yr), Waimihia (3280 yr), Hinemaiaia (4510 yr), Whakatane (4830 yr), and Motutere (5430 yr), but three additional rhyolitic layers (aged c. 3700, 4100, and 4300 yr) could not be correlated with known tephras. Two are possibly reworked layers, but one (c. 4300 yr) may represent a new tephra which has not been previously recognised. The mineralogy of three of the andesitic tephras (aged c. 3100, 4900, and 6000 yr) indicates that they were probably derived from Egmont Volcanic Centre, and tentative correlations are suggested for them. The fourth andesitic tephra (c. 5400 yr) is probably derived from Tongariro Volcanic Centre. Nine radiocarbon ages were obtained from the core and two of these, 6530 ± 110 yr B.P and 6330 ± 70 yr B.P located near the base of the core, indicate that Lake Tutira formed c. 6500 years ago.
Tephrochronology
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Volcanic glass
Volcanic ash
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Volcanic glass
Tephrochronology
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A rhyolitic ash 4 to 8 cm thick is well preserved within a thick loess unit in a coastal section 2 km long near Teviotdale, Canterbury district, South Island, New Zealand. The ash (informally named Tiromoana ash) contains fresh glass shards which give a fission-track age of 20,300 ± 7100 yr B.P. The only possible source for such a tephra with this age range is from Taupo Volcanic Zone (TVZ), North Island, some 550 km north of Teviotdale. Within the time span ca. 15,000 to 42,000 yr B.P. five widespread and voluminous rhyolitic tephras (viz. Rerewhakaaitu Ash, Rotoehu Ash, Kawakawa Tephra, Omataroa Tephra, and Mangaone Tephra) were erupted from TVZ. On the basis of the fission-track age, ferromagnesian mineralogy, and electron-microprobe analyses of glass shards and titanomagnetites from Tiromoana ash and the five possible correlatives listed above, Tiromoana ash is correlated with Kawakawa Tephra (dated by 14 C at ca. 20,000 yr B.P.). This is the only known occurrence to date of Kawakawa Tephra in the South Island. Its preservation is attributed to special site conditions (low precipitation and minimal sheet erosion) leeward of a prominent terrace. The identification of the ash at Teviotdale as Kawakawa Tephra supports recently revised age assignments for the upper loess sheet in Canterbury. Moreover, it implies that loess enclosing Kawakawa Tephra in nonglaciated districts of southern North Island and Taupo Volcanic Zone is a correlative.
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Large volcanic eruptions from Iceland can produce significant volumes of glass-rich rhyolitic tephra, which are then deposited across NW Europe and the North Atlantic-Arctic region, forming time-parallel marker horizons useful to palaeoenvironmental studies. Here we investigate new ways of improving the tephrochronological record of Iceland using (thermo)luminescence analysis of rhyolitic volcanic glass shards that dominate airfall ash deposits of the Þórsmörk Ignimbrite (ÞIG), tephra from the Askja 1875 AD, Öræfi 1362 AD eruptions, and the Óþoli tephra from NW Iceland. Following screening experiments, which showed that pure volcanic glass samples retained age-related TL signals, we undertook glass-phase TL dating of the ÞIG and Óþoli tephra. Our TL age estimate of c. 40 ± 10 ka for the ÞIG supports the phenocryst-based radiometric age of c. 50 ka rather than older age estimates of c. 200 ka. Results from the Óþoli tephra were consistent with the fission track age established at c. 2 Ma age, but further investigations of high dose sensitivity changes and longer-term stability factors such as athermal fading are required for quantitative dating of volcanic glass deposits >100 ka. However, as thermoluminescence signals from purified glass fractions of Icelandic tephra can be obtained over 100–1,000,000-year time scales, luminescence characterisation of glass shards can be used alongside geochemical and morphological analysis to distinguish between distal tephras with similar geochemical signatures, and assist with tephrochronological investigations beyond the limits of radiocarbon dating.
Volcanic glass
Tephrochronology
Phenocryst
Thermoluminescence dating
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Volcanic ash
Lapilli
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