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1.
 Canonical discriminant function analysis was employed to discriminate between electron microprobe-determined titanomagnetite and hornblende analyses from Egmont volcano and Tongariro Volcanic Centre. Data sets of 436 titanomagnetite and 206 hornblende analyses from the two sources were used for the study. Titanomagnetite chemistry provided the best discrimination between these two sources with classification efficiencies of 99% for sample averages and 95% for individual analyses. The difference between sources for hornblende chemistry was less marked, but classification efficiencies of 100% for sample averages and 87% for individual analyses were achieved. Using the same methods a preliminary discrimination of individual Egmont volcano-sourced tephras was attempted. Titanomagnetite chemistry enabled the discrimination of several individual tephras or at least pairs of tephra units, but hornblende chemistry provided little discrimination. This technique provides an improvement on previous methods for chemically distinguishing distal tephra from the two sources as well as potentially identifying individual tephras from a particular source. A major advantage over previous discrimination techniques is that individual analyses can be classified with a known probability of group membership (with groups such as volcano source or an individual tephra unit). Tephras in a depositional environment where mixing is common such as within soil, loess and marine sequences, can be sourced or identified more easily with classification of individual grains. Received: 19 July 1995 / Accepted: 13 February 1996  相似文献   

2.
Acquiring detailed eruption frequency datasets for a volcano system is essential for realistic eruption forecasts. However, accurate datasets are inherently difficult to compile, even if one or more well-dated eruption records are available. A single record typically under-represents the eruption frequency, while combining two or more records may result in an overrepresentation. Although glass compositions have proven to be successful in tephrochronological studies of dominantly rhyolitic tephras; microlitic growth and thin glass shards inhibit their application to andesitic tephras. A method consisting of a combination of two techniques for correlating syn-eruptive deposits is demonstrated on data from the typical andesitic stratovolcano of Mt. Taranaki, New Zealand. Firstly, tentative matches are identified using the radiocarbon age and associated error of each event. Secondly, the compositions of titanomagnetite micro-phenocrysts are used as an independent check, and shown to be a useful correlation tool where age data is available. Using two lake-core records containing tephra layers in an overlapping time-frame, the radiocarbon age-correlation procedure suggested 31 tephra matches. Geochemistry data were available for 15 of these pairs. In three of these cases, the titanomagnetite compositions did not match. Hence, these “paired” tephras were from compositionally distinct magmas and therefore likely represent separate events. An additional three matches were reassigned within the temporal uncertainty limits of the dating procedure, based on better geochemical pairing. The final combined dataset suggests that there have been at least 138 separate ash fall-producing eruptions between 96 and 10 150 years B.P. from Taranaki. Using the combined dataset the mixture of Weibulls renewal model forecasts a probability of 0.52 for an eruption occurring in the next 50 years at this volcano. The present annual eruption probability is estimated at 1.6%. This likelihood is almost double that obtained when relying on a single stratigraphic record.  相似文献   

3.
Macro- and crypto-tephra layers deposited in European climate archives during the Last Glacial-Interglacial Transition (LGIT ca. 16–8 ka) have become increasingly important as a means to robustly correlate palaeoclimate records, and to test the spatial and temporal synchronicity of climatic transitions. However, correlations between climate archives are currently limited by the number of tephra-linkages that can be made. This disparity in the observed distributions of tephras may lie with methodological limitations relating to the resolution of cryptotephra refinement within palaeoclimate records. Here we present new data from Quoyloo Meadow, Orkney Mainland, Scotland, where nine tephra horizons and ten chemically distinct tephra populations have been identified and correlated to known eruptions during the LGIT. Three of the tephras; the Hässeldalen, Hovsdalur and the Fosen are characterised and placed into a reliable tephrostratigraphy for the first time in the British Isles. The detection of new tephra layers in this case is thought to reflect modifications to the sampling approach applied here. The resulting tephrostratigraphy is used to produce an age model with centennial-scale precision, providing new age estimates for three poorly dated tephras. The chronology rivals the output of more traditionally dated radiocarbon chronologies, and illustrates the potential for tephra to develop robust age-depth models for carbonate sequences.  相似文献   

4.
 Tufa Trig Formation comprises a sequence of at least 19 andesitic tephras erupted from Mt. Ruapehu (Tongariro Volcanic Centre, New Zealand). Tephras of Tufa Trig Formation are the most recent eruptives from Ruapehu, dated between ca. 1850 years B.P. and the present. Members of the Formation show restricted dispersals, principally to the east of Mt. Ruapehu. Volumes calculated for the most widespread members are all less than 0.1 km3. Compared with other Mt. Ruapehu eruptives, Tufa Trig Formation tephras represent small eruptions that have contributed little tephra to the ring plain. They do, however, show a greater frequency of eruption with one event occurring on average every 100 years. Tufa Trig Formation members Tf3–Tf18 are black to dark grey, vitric, coarse-ash and lapilli-grade tephras which mantle the relief. They contain juvenile vitric particles which exhibit varying degrees of vesicularity, together with free crystals of pyroxene and feldspar, and few lithic fragments. Several morphological types of vitric pyroclasts are recognised in these tephras, the dominant type being of equant blocky morphology with fracture-bound surfaces (type-1 morphology). Field characteristics, tephra distributions, and the morphologies and textures of constituent pyroclasts suggest that these members (Tf3–Tf18) are the products of small-volume hydrovolcanic eruptions resulting from the interaction of fresh magma and meteoric water. We propose that a source of this water was an ancestral crater lake which formed within the late Holocene ca. 3000 years B.P. The morphological, compositional, and chemical (major-element) characteristics of three Tufa Trig Formation Tephras are compared with those of two new tephras erupted from Ruapehu Volcano during the October 1995 eruptions which comprise part of a newly defined member (Tf19) of Tufa Trig Formation. The comparisons support our interpretation that the majority of the Tufa Trig Formation tephras are primarily the products of hydrovolcanic eruptions. Other members of the Formation (Tf1 and Tf2) are coarse-grained scoriaceous tephras and are interpreted to be the products of strombolian events. Received: 14 September 1996 / Accepted: 6 June 1997  相似文献   

5.
We investigated a late Quaternary terrestrial sedimentary sequence (Uwa Formation) in core IC2, from a site adjacent to that of the reported core IC on NW Shikoku Island, SW Japan, and developed its tephra and pollen stratigraphy to refine the age model of the formation. First, we identified 19 horizons with high glass shard concentrations in the IC2 core sediments as possible tephras or cryptotephras, and correlated them with reported tephras on the basis of the major- and trace-element compositions of their glass shards. All correlated widespread tephras and cryptotephras were products of volcanoes in the Kyushu volcanic zone (Aso, Kakuto, Aira, Ata, and Kikai calderas). Second, we confirmed the presence in core IC2 of two pollen zones dominated by Quercus subgen. Cyclobalanopsis, which is an indicator of very warm interglacial vegetation. In the Japanese Islands, these two vegetation zones have usually been considered to characterize marine isotopic stages (MISs) 1 and 11. A previous study of the Uwa Formation correlated the upper pollen zone to MIS 1, but the lower zone was not correlated to MIS 11; rather, it was inferred to be older than MIS 12 because it was stratigraphically below the “Oda” tephra (equivalent to a distal Kasamori 5 [Ks5] tephra [MIS 12]). In this study, however, noting that the Naruohama-IV tephra (Nh-IV; MIS 10d) and Ks5 cannot be distinguished by their shard chemistries, we inferred that the suggested “Oda” tephra actually correlates to Nh-IV, rather than to the Ks5 tephra. By re-assigning the “Oda” tephra to Nh-IV, we could correlate the underlying Quercus subgen. Cyclobalanopsis-abundant zone to MIS 11 and, consequently, a pair of pollen zones indicating cool and warm conditions below the MIS 11 pollen zone to MISs 12 and 13, respectively. The resulting age model whereby tephra and pollen constraints are integrated showed a roughly constant sedimentation rate from MIS 13, without any long-term gaps; further, our MIS 13 horizon in core IC2 corresponds to the reported 1 Ma tephra horizon in core IC. Therefore, these findings represent a dramatic change in the Uwa Formation age model and validate the Uwa Formation as one of the most useful terrestrial archives of Quaternary tephrostratigraphy and paleoclimatic fluctuation in SW Japan.  相似文献   

6.
Apatite is a common accessory mineral in intermediate and felsic igneous rocks. Because apatite has a wide range of trace-element compositions as well as strong resistance to diagenetic alteration, the trace-element composition of apatite has been used for tracing petrogenetic processes of plutonic bodies as well as the tephrochronology of Paleozoic tephras. However, it has been little used in Quaternary volcaniclastic deposits. Our studies of apatite trace-element compositions in representative Quaternary ignimbrites and their corresponding co-ignimbrite ashes in Japan demonstrated that such compositions are not affected by welding and can be successfully used for discrimination of, and correlation between, tephras and ignimbrites. We also could distinguish different tephra deposits and ignimbrites from the same caldera. Although most apatite trace-element compositions did not change throughout the succession of ejecta from a single eruption, the Fe contents in apatite varied across stratigraphic horizons. Consequently, we were able to correlate the Fe contents of apatite phenocrysts in specific ignimbrite horizons and their corresponding co-ignimbrite ashes and thus identify the phase of the eruption during which the major part of a co-ignimbrite ash was released.  相似文献   

7.
The Ko-g and Ma-f~j tephras are two key isochronous marker layers in northern Japan, which are from the largest Plinian eruptions of Komagatake volcano (VEI = 5) and Mashu caldera (VEI = 6), respectively. Despite extensive radiocarbon studies associated with the two tephras, individual calibrated results show considerable variations and thus accurate ages of these important eruptions remain controversial. Bayesian statistical approaches to calibrating radiocarbon determinations have proven successful in increasing accuracy and sometimes precision for dating tephras, which is achieved through the incorporation of additional stratigraphic information and the combination of evidence from multiple records. Here we use Bayesian approaches to analyse the proximal and distal information associated with the two tephra markers. Through establishing phase and deposition models, we have taken into account all of the currently available stratigraphic and chronological information. The cross-referencing of phase models with the deposition model allows the refinement of eruption ages and the deposition model itself. Using this we are able to provide the most robust current age estimates for the two tephra layers. The Ko-g and Ma-f~j tephras are hereby dated to 6657-6505 (95.4%; 6586±40, μ±σ) cal yr BP, and 7670-7395 (95.4%; 7532±72, μ±σ) cal yr BP, respectively. These updated age determinations underpin the reported East Asian Holocene tephrostratigraphic framework, and allow sites where the tephra layers are present to be dated more precisely and accurately. Our results encourage further applications of Bayesian modelling techniques in the volcanically active East Asian region.  相似文献   

8.
The role of tephrochronology, as a dating and stratigraphic tool, in precise palaeoclimate and environmental reconstruction, has expanded significantly in recent years. The power of tephrochronology rests on the fact that a tephra layer can stratigraphically link records at the resolution of as little as a few years, and that the most precise age for a particular tephra can be imported into any site where it is found. In order to maximise the potential of tephras for this purpose it is necessary to have the most precise and robustly tested age estimate possible available for key tephras. Given the varying number and quality of dates associated with different tephras it is important to be able to build age models to test competing tephra dates. Recent advances in Bayesian age modelling of dates in sequence have radically extended our ability to build such stratigraphic age models. As an example of the potential here we use Bayesian methods, now widely applied, to examine the dating of some key Late Quaternary tephras from Italy. These are: the Agnano Monte Spina Tephra (AMST), the Neapolitan Yellow Tuff (NYT) and the Agnano Pomici Principali (APP), and all of them have multiple estimates of their true age. Further, we use the Bayesian approaches to generate a revised mixed radiocarbon/varve chronology for the important Lateglacial section of the Lago Grande Monticchio record, as a further illustration of what can be achieved by a Bayesian approach. With all three tephras we were able to produce viable model ages for the tephra, validate the proposed 40Ar/39Ar age ranges for these tephras, and provide relatively high precision age models. The results of the Bayesian integration of dating and stratigraphic information, suggest that the current best 95% confidence calendar age estimates for the AMST are 4690–4300 cal BP, the NYT 14320–13900 cal BP, and the APP 12380–12140 cal BP.  相似文献   

9.
This paper presents results of a study of titanomagnetite from specimens of ten different rock units. The rock units comprise trachytes, basalts, ignimbrites, nephelinites, olivine melanephelinites and welded tuffs from Kenya, with ages ranging from recent to Precambrian. The correlations of the stability index with opaque petrology and with thermomagnetic analyses of the titanomagnetite and titanomaghemite bearing specimens appear to be significant. Electron microprobe analyses of the titanomagnetite grains indicate a high percentage of “impurities” in the low stability index specimens and elevation of the Curie point in the high stability index specimens.  相似文献   

10.
The paper reviews the existing data on the Y-3 tephra layer, first recognised in the Ionian Sea (Mediterranean basin). The collection and collation of old and new data on distal tephra occurrences in terrestrial, marine and lacustrine successions indicate that the Y-3 layer is dispersed over a wide area of the central Mediterranean basin. The peculiar homogeneous chemical composition of this layer makes its recognition rather straightforward and permits it being distinguished from other stratigraphically adjacent tephras. The best age estimate for the Y-3 layer of ca 30–31 cal ka BP, its peculiar stratigraphic position close to the Marine Isotope Stage 3/2 transition or Heinrich Event 3 onset, as well as its wide dispersion makes this layer an important marker to link and date late Pleistocene terrestrial and marine archives of the central Mediterranean basin.  相似文献   

11.
 The postglacial eruption rate for the Mount Adams volcanic field is ∼0.1 km3/k.y., four to seven times smaller than the average rate for the past 520 k.y. Ten vents have been active since the last main deglaciation ∼15 ka. Seven high flank vents (at 2100–2600 m) and the central summit vent of the 3742-m stratocone produced varied andesites, and two peripheral vents (at 2100 and 1200 m) produced mildly alkalic basalt. Eruptive ages of most of these units are bracketed with respect to regional tephra layers from Mount Mazama and Mount St. Helens. The basaltic lavas and scoria cones north and south of Mount Adams and a 13-km-long andesitic lava flow on its east flank are of early postglacial age. The three most extensive andesitic lava-flow complexes were emplaced in the mid-Holocene (7–4 ka). Ages of three smaller Holocene andesite units are less well constrained. A phreatomagmatic ejecta cone and associated andesite lavas that together cap the summit may be of latest Pleistocene age, but a thin layer of mid-Holocene tephra appears to have erupted there as well. An alpine-meadow section on the southeast flank contains 24 locally derived Holocene andesitic ash layers intercalated with several silicic tephras from Mazama and St. Helens. Microprobe analyses of phenocrysts from the ash layers and postglacial lavas suggest a few correlations and refine some age constraints. Approximately 6 ka, a 0.07-km3 debris avalanche from the southwest face of Mount Adams generated a clay-rich debris flow that devastated >30 km2 south of the volcano. A gravitationally metastable 2-to 3-km3 reservoir of hydrothermally altered fragmental andesite remains on the ice-capped summit and, towering 3 km above the surrounding lowlands, represents a greater hazard than an eruptive recurrence in the style of the last 15 k.y. Received: 24 June 1996 / Accepted: 6 December 1996  相似文献   

12.
Mayor Island is a peralkaline rhyolitic caldera volcano characterised by numerous, sector-confined pyroclastic deposits, together with lavas forming at least five composite shields. Correlation of sequences between sectors is difficult because of the scarcity of island-wide marker beds. However, eight distal calc-alkaline fall tephras (ca. 7.3 14C ka to 64 ka) from Okataina and Taupo volcanic centres in the nearby Taupo Volcanic Zone (TVZ) have been identified on the island. These “foreign” TVZ tephras provide marker planes to correlate activity in different sectors of Mayor Island volcano, and refine an eruptive chronology. At least seventeen pyroclastic eruptions and fourteen lava-producing events (including multiple, shield-forming events) have occurred in the past ca. 64 ka. Age controls provided by the calc-alkaline tephras confirm the extremely local dispersal characteristics of many of the Mayor Island eruptives and show that K/Ar ages as young as 25–33 ka on obsidians with 4.2–4.4% K2O are reliable.  相似文献   

13.
Refractive indices of incompletely hydrated tephra glasses vary widely. Thus, glass refractive index is not a practical indicator for identifying Holocene tephras. Hydration near the surface of tephra glass shards can be removed by either hydrofluoric acid treatment or annealing for 12 h at 400°C. The annealing procedure is a particularly reliable and simple way to dehydrate tephra glasses. Standard deviations (s) of refractive indices for glasses after 12 h annealing are small (s=0.0014–0.0018), in contrast with untreated glasses (s=0.0034–0.0405). The refractive indices of dehydrated tephra glasses are 0.006–0.014 lower than those of the untreated (hydrated) glasses. Using the 400°C 12-h annealing procedure, values for the refractive indices of eight Holocene tephras in Hokkaido were obtained. These refractive indices are useful indicators for identification and correlation of Holocene tephras.  相似文献   

14.
Water, F, and Cl contents of melt inclusions in phenocrysts from the 2-ka-old Taupo and Hatepe plinian tephras, and the 22-ka-old Okaia tephra from the Taupo volcanic center, New Zealand, were measured by electron and ion microprobe. Major and trace element chemistry of the inclusions is similar to that of bulk rock, supporting our assumption that volatile contents of inclusions are representative of the magma in which the crystals grew. Inclusions in the 2-ka Taupo plinian tephra contain a mean of 4.3 wt% H2O, 450 ppm F, and 1700 ppm Cl; from the Hatepe plinian tephra 4.3 wt% H2O, 430 ppm F, and 1700 ppm Cl; and from the Okaia tephra 5.9 wt% H2O, 470 ppm F, and 2100 ppm Cl. Sulfur was below the detection limit of 200 ppm. The constant H2O, F and Cl from a number of stratigraphic horizons in the tephra deposits suggest that the Taupo and Hatepe plinian tephras (>8.2 km3 magma volume) were derived from a magma body that did not contain a strong volatile gradient. By inference, there is no pre-eruptive volatile difference between these plinian eruptions and a phrea-toplinian eruption which occurred between the two. Virtually no major element zonation is seen in this eruptive sequence. Although the Okaia tephra was also erupted from the Taupo volcanic center, probably from a similar vent area, its higher volatile contents and distinct composition as compared to the Taupo tephras show that it was derived from a different, and possibly deeper, magma body.  相似文献   

15.
Tephra layers occur in deep-sea sediments of the northeastern Indian Ocean, adjacent to western Indonesian are. The layers range in age from Recent to Late Miocene. Relative abundance of light and heavy mineral species in all tephra layers have been determined, and pure glass shards from representative samples have been analyzed chemically for major oxides. On the basis of the chemical data, three distinct provinces can be recognized: (1) an extensive province of rhyolitic tephra layers, ranging in age back to Late Miocene, is found adjacent to Sumatra; (2) a more restricted province of dacitic layers, adjacent to Sunda Strait and western Java; and (3) a province of andesitic layers, found adjacent to eastern Java and the Lesser Sunda Islands. Chemical composition of tephra layers in each province remains constant with time. As an example, tephra layers from the rhyolitic province are characterized by a high and restricted range of SiO2 (75–77%) when expressed on an H2O-free basis.Tephra layers recovered from the study area were examined for chemical evidence of secondary alteration. The analyses revealed that H2O is the only major oxide in the glass shards which increases progressively with the age of the tephra layers regardless of the bulk composition. H2O, however, reaches a “saturation point” of 4–5% in the layers 250–400 thousands of years old and remains constant to the oldest recovered tephra layer (7.5 m.y. old).The decrease in silica content in deep-sea tephra layers eastward along the Indonesian volcanic arc coincides with a similar eastward decrease in average silica content in Indonesian lavas. A relatively high silica content in lavas from Sumatra, with associated ignimbrites and their deep-sea ash-fall equivalents is closely linked to thick pre-Cenozoic crust. In the portion of the arc to the east of Sumatra, the crust is Cenozoic and thin. Difference in silica content of both the lavas and deep-sea tephras along the Indonesian arc is considered in regard to the hypothesis of “magma filtering” which is based on the contrasting density gradients of ascending magma and the upper crust.  相似文献   

16.
Shirouma-Oike volcano, a Quaternary composite volcano in central Japan, consists mostly of calc-alkaline andesitic lavas and pyroclastic rocks. Products of the earlier stage of the volcano (older group) are augite-hypersthene andesite. Hornblende crystallized during the later stage of this older group, whereas biotite and quartz crystallized in the younger group.Assemblages of phenocrysts in disequilibrium, such as magnesian olivine(Fo30)/quartz, iron-rich hypersthene(En55)/iron-poor augite(Wo43.5, En42.5, Fs14.0), and two different types of zoning on the rim of clinopyroxene are found in a number of rocks. Detailed microprobe analyses of coexisting minerals reveal that phenocrysts belong to two distinctly different groups; one group includes magnesian olivine + augite which crystallized from a relatively high-temperature (above 1000°C) basaltic magma; the second group, which crystallized from relatively low temperature (about 800°C) dacitic to andesitic magma, includes hypersthene + hornblende + biotite + quartz + plagioclase + titanomagnetite ± ilmenite (in the younger group) and hypersthene + augite + plagioclase + titanomagnetite ± hornblende (in the older group). The temperature difference between the two magmas is clarified by Mg/Fe partition between clinopyroxene and olivine, and Fe-Ti oxides geothermometer. The compositional zoning of minerals, such as normal zoning of olivine and magnesian clinopyroxene, and reverse zoning of orthopyroxene, indicate that the basaltic and dacitic-andesitic magmas were probably mixed in a magma reservoir immediately before eruption. It is suggested that the basaltic magma was supplied intermittently from a deeper part to the shallower magma reservoir, in in which dacitic-andesitic magma had been fractionating.  相似文献   

17.
We present results from a cryptotephra investigation performed at a high resolution (0.5 cm) on sediments from Körslättamossen in southernmost Sweden. Six peak concentration levels were detected and extracted for geochemical analysis by electron probe microanalyser. Five of these levels were successfully analysed and we propose correlations to the Hässeldalen Tephra, the Vedde Ash, and the Laacher See Tephra (adding new analysis results for the first geochemically confirmed finding of the latter in Sweden), as well as an undetermined Borrobol-type tephra. The tephra identifications were combined with radiocarbon dated macrofossils in order to create an age model for the sampled sediments based on Bayesian methods. Stratigraphical and chronological results were found to concur with a previous study of the site and our results form the basis for discussion concerning the issues surrounding Lateglacial Borrobol-type tephras, of which we suggest further review in order to unlock these tephras’ full potential for Quaternary studies.  相似文献   

18.
A 5 mm thick tephra layer has been identified in the lacustrine sediments of Moon Lake in the Arxan-Chaihe volcanic field (ACVF) in Greater Khingan Mountains (NE China). The visible tephra layer is clearly revealed as a distinct peak in magnetic susceptibility measurements. The tephra layer consists mainly of brown vesicular glass shards and minor amounts of plagioclase, olivine and clinopyroxene. Major and minor element analysis has been carried out on the glass shards and plagioclase minerals. Glass shards show low concentrations of K2O, similar to the eruptive products derived from post-Miocene volcanoes of the ACVF. The plagioclase phenocrysts in both lava and tephra from ACVF, and in the tephra recorded in Moon Lake are labradorites. During the Late Pleistocene to Holocene, there were also extensive explosive eruptions in the nearby Nuominhe volcanic field (NVF). Volcanic rocks from the ACVF are easily distinguished from those derived from the NVF, having distinctly different K2O concentrations. This compositional variation is likely the result of different magmatic processes operating in the ACVF and NVF. Radiocarbon dating on organic materials from the lacustrine sediments dates the tephra layer to ca. 14,200 cal yrs BP, which implies that it was generated by a previously unknown Late Pleistocene explosive eruption in the ACVF. These results, for the first time, give a direct tephra record in this area, and suggest that identification of further tephra and/or cryptotephra in local sedimentary basins such as crater lakes of scoria cones and maars will be significant for dating the Late Pleistocene to Holocene volcanic eruptions and will help to establish a detailed record of the volcanic activity in the ACVF. The newly discovered tephra layer also provides a dated tephrochronological marker layer, which will in future studies provide a means to synchronise local sedimentary records of the climatically variable Late Glacial.  相似文献   

19.
We have documented 80 tephra beds dating from ca. 9.5 to >50 ka, contained within continuously deposited palaeolake sediments from Onepoto Basin, a volcanic explosion crater in Auckland, New Zealand. The known sources for distal (>190 km from vent) tephra include the rhyolitic Taupo Volcanic Centre (4) and Okataina Volcanic Centre (14), and the andesitic Taranaki volcano (40) and Tongariro Volcanic Centre (3). The record provides evidence for four new events between ca. 50 and 28 ka (Mangaone Subgroup) suggesting Okataina was more active than previously known. The tephra record also greatly extends the known northern dispersal of other Mangaone Subgroup tephra. Ten rhyolitic tephra pre-date the Rotoehu eruption (>ca. 50 ka), and some are chemically dissimilar to post-50 ka rhyolites. Some of these older tephra were produced by large-magnitude events; however, their source remains uncertain. Eight tephra from the local basaltic Auckland Volcanic Field (AVF) are also identified. Interpolation of sedimentation rates allow us to estimate the timing of 12 major explosive eruptions from Taranaki volcano in the 27.5-9.5-ka period. In addition, 28 older events are recognised. The tephra are trachytic to rhyolitic in composition. All have high K2O contents (>3 wt%), and there are no temporal trends. This contrasts with the proximal lava record that shows a trend of increasing K2O with time. By combining the Onepoto tephra record with that of the previously documented Pukaki crater, 15 AVF basaltic fall events are constrained at: 34.6, 30.9, 29.6, 29.6, 25.7, 25.2, 24.2, 23.8, 19.4, 19.4, 15.8 and 14.5 ka, and three pre-50 ka events. This provides some of the best age constraints for the AVF, and the only reliable data for hazard recurrence calculations. The minimum event frequency of both distal and local fall events can be estimated, and demonstrates the Auckland City region is frequently impacted by ash fall from many volcanoes.  相似文献   

20.
Zircon U–Pb dating using LA-ICP-MS was applied to six Quaternary tephras in Boso Peninsula, central Japan: J1, Ks4, Ks5, Ks10, Ks11, and Ch2 in descending order. Accurate age determination of these tephras is of critical importance because they are widespread tephras in Japan and also relevant to a candidate site for the global boundary stratotype section and point of the early–middle Pleistocene boundary. Twenty grains were dated for each tephra and the following results were obtained. The J1 tephra had only 5 grains that yielded <2 Ma. The obtained age was ∼0.2 m.y. older than the stratigraphic age. No Quaternary ages were obtained from the Ks4 tephra. The Ks5 and Ks10 tephras had 10–12 grains that were ∼0.1–0.3 m.y. older than the stratigraphic age. The Ks11 tephra had 14 grains that yielded a weighted mean age of 0.52 ± 0.04 Ma (error reported as 95% confidence level), which was in agreement with the stratigraphic age. The Ch2 tephra had 16 grains that yielded a weighted mean age of 0.61 ± 0.02 Ma, which was also in agreement with the stratigraphic age. The good agreement between zircon U–Pb ages and the stratigraphy for Ks11 and Ch2 tephras validates the reliability of the established stratigraphy and our dating approach. The other tephras that yielded ∼0.1–0.3 m.y. older ages than the stratigraphy may indicate that the analyzed zircons were antecrysts that crystallized before eruption or they were detrital zircons incorporated during deposition.  相似文献   

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