Chemistry of particles from the Toba ash layer

Two cores from the southern South China Sea contain discrete ash layers that mainly consist of rhyolithic glass shards. On the basis of the SPECMAP time scale, the ash layers were dated to ca. 74 ka, the age of the youngest Toba eruption in northern Sumatra. This link is supported by the chemical composition of the glass, which is distinct from volcanic glass supplied from the Philippines and the northern South China Sea, but is almost identical with the chemistry of the Toba ash. The youngest Toba ash layers in the South China Sea expand the previously known ash-fall zone over more than 1800 km to the east. The dispersal of ashes from Sumatra in both western and eastern directions indicates two contrasting wind directions and suggests that (1) the Toba eruption probably happened during the Southeast Asian summer monsoon season, and (2) the volume of erupted magma was larger than previously interpreted.

(Table 1) Argon dating of ash layers from DSDP Hole 18-178

Ash layers from Deep Sea Drilling Project site 178 in the northeast Pacific Ocean have been dated by the 40Ar-39Ar stepwise heating technique to resolve published discrepancies concerning the length of time explosive volcanism has affected the eastern Aleutian arc and Alaskan Peninsula. The results of the investigation indicate that the record of ash-fall deposition at site 178 extends back at least 6.5 m.y. Assuming that 6.5 m.y. ago marks the onset of renewed calc-alkalic volcanism of the volcanic arc, proposed models of continuous and discontinuous motion between the Pacific and North American lithospheric plates can be evaluated. If appreciable time elapsed between the onset of subduction and the onset of arc volcanism, the 6.5-m.y. record of ash-fall deposition in the north-east Pacific is most compatible with models of continuous plate motion throughout late Cenozoic time.

Chemical composition of volcanic ash layers from ODP Leg 134 sites

Volcanic ash layers (1-3 cm thick) are abundant in the North Aoba Basin drill sites but less common at forearc sites. Ash deposited on the forearc slopes is liable to be redistributed as turbidites. In addition, the westerly upper winds also minimize ash-fall on the western (forearc) side of the New Hebrides Island Arc. Crystalline components in the ashes are primarily plagioclase (An90-An44), clinopyroxene (Ca46Mg49Fe5-Ca43Mg33Fe24), olivine (Fo87-Fo62), and titanomagnetite. There are also small amounts of orthopyroxene, magnetite, apatite, and quartz. Glass shards occur in most of the ashes and range in composition from basalt to rhyolite. There is often a variety of glass compositions within a single ash layer. One explanation for this is that the rate of accumulation of ash from several different eruptions or eruptive phases exceeded the background sedimentation rate: there may also have been a certain amount of reworking. The high-K and low-K trends previously recognized in volcanic rocks from the New Hebrides Island Arc are clearly represented in the Leg 134 glasses. All of the ashes investigated here are thought to have originated from the Central Chain volcanoes. The source of the high-K group was probably the Central Basin volcanoes of Santa Maria, Aoba, and Ambrym. The lower-K series includes a distinctive group of dacites and is likely to have originated from the Epi-Tongoa-Tongariki sector of the arc where major pyroclastic eruptions, associated with caldera collapse, have occurred during the Holocene, perhaps as recently as 400 yr ago.

Major and trace element analyses from DSDP Hole 26-253 at the Ninetyeast Ridge in the Indian Ocean

A basaltic sequence of Eocene submarine-erupted pyroclastic sediments totals at least 388 m at DSDP Site 253 on the Ninetyeast Ridge. These fossiliferous hyaloclastic sediments have been erupted and fragmented by explosive volcanism (hydroexplosions) in shallow water. The occurrence of interbedded basaltic ash-fall tuffs within the younger horizons of the hyaloclastic sequence marks the emergence of some Ninetyeast Ridge volcanic vents above sea level. Considerable textural variation allows subdivision of the sequence into six informal lithostratigraphic units. Hydrothermal and diagenetic alteration has caused the complete replacement of all original glass by smectites, and the introduction of abundant zeolite and calcite cements. The major and trace element contents of the hyaloclastites vary due to the alteration, and the admixture of biogenous calcite. On a calcium carbonate-free basis systematic variations are recognisable. Mg, Ni, Cr and Cu are enriched, and Li and Zn depleted in the three older units relative to the younger three. The chemical variability is reflected by the development of saponite in the older part of the sequence and montmorillonite in the younger; and by the presence of a quartz-normative basalt flow occurring in Unit II, in contrast to the Mg-rich highly olivine-normative basalt at the base of the sequence. The younger and older parts of the sequence therefore appear to have been derived from magmas of different chemistry. The sequence, like other basaltic rocks recovered from the Ninetyeast Ridge, is enriched in the light relative to the heavy rare earth elements (REE) although the REE contents vary unsystematically with depth, probably because of the high-temperature subaqueous alteration and the presence of biogenous calcite. This REE data indicates that the Ninetyeast Ridge volcanism was different from that which produces mid-ocean ridge basalts.

Chemistry of smectite and volcanic glass of ODP Site 159-959

The Ocean Drilling Program (ODP) Site 959 was drilled in the northern border of the Côte d'Ivoire-Ghana Ridge at a water depth of 2100 m. Pleistocene total thickness does not exceed 20 m. Winnowing processes resulted in a low accumulation rate and notable stratigraphic hiatuses. During the Late Pleistocene, bottom circulation was very active and controlled laminae deposition (contourites) which increased the concentration of glauconitic infillings of foraminifera, and of volcanic glass and blue-green grains more rarely, with one or several subordinate ferromagnesian silicates. Volcanic glass generally was X-ray amorphous and schematically classified as basic to intermediate (44-60% SiO2). Opal-A or opal-CT suggested the beginning of the palagonitisation process, and previous smectitic deposits may have been eroded mechanically. The blue-green grains presented two main types of mineralogic composition: (1) neoformed K, Fe-smectite associated with zeolite (like phillipsite) and unequal amounts of quartz and anorthite; (2) feldspathic grains dominated by albite but including quartz, volcanic glass and smectites as accessory components. They were more or less associated with the volcanic glass. On the basis of their chemical composition, the genetic relationship between the blue-green grains and the volcanic glass seemed to be obvious although some heterogeneous grains seemed to be primary ignimbrite and not the result of glass weathering. The most reasonable origin of these pyroclastic ejecta would be explosive events from the Cameroon Volcanic Ridge, especially from the Sao Thome and Principe Islands and Mount Cameroon area. This is supported both by grain geochemistry and the time of volcanic activity, i.e. Pleistocene. After westward wind transport (some 1200 km) and ash fall-out, the subsequent winnowing by bottom currents controlled the concentration of the volcanic grains previously disseminated inside the hemipelagic sediment. Palagonitisation, and especially phillipsite formation, may result from a relatively rapid reaction during burial diagenesis (<1 m.y.), in deep-sea deposits at relatively low sedimentation rate. However, it cannot be excluded that the weathering had begun widely on the Cameroon Ridge before the explosive event.

Geochemistry from the S1 tephra of core GeoTü SL112 and samples from Erciyes Dag eruptions Dikkartin, Karagüllü and Perikartin

A hitherto unknown distal volcanic ash layer has been detected in a sediment core recovered from the southeastern Levantine Sea (Eastern Mediterranean Sea). Radiometric, stratigraphical and sedimentological data show that the tephra, here termed as S1 tephra, was deposited between 8,970 and 8,690 cal yr BP. The high-silica rhyolitic composition excludes an origin from any known eruptions of the Italian, Aegean or Arabian volcanic provinces but suggests a prevailing Central Anatolian provenance. We compare the S1 tephra with proximal to medial-distal tephra deposits from well-known Mediterranean ash layers and ash fall deposits from the Central Anatolian volcanic field using electron probe microanalyses on volcanic glass shards and morphological analyses on ash particles. We postulate a correlation with the Early Holocene 'Dikkartin' dome eruption of Erciyes Dag volcano (Cappadocia, Turkey). So far, no tephra of the Central Anatolian volcanic province has been detected in marine sediment archives in the Eastern Mediterranean region. The occurrence of the S1 tephra in the south-eastern part of the Levantine Sea indicates a wide dispersal of pyroclastic material from Erciyes Dag more than 600 km to the south and is therefore an important tephrostratigraphical marker in sediments of the easternmost Mediterranean Sea and the adjacent hinterland.

Benthic foraminifera and ostracoda in ODP Hole 107-650A

Twenty-three sediment intervals from top of Site 650 down to 510 m below seafloor have been studied. Their thicknesses vary between 0.25 m and about 40 m. The studied deposits are turbidites or parts of them except one which is interpreted as an ash-fall layer. The composition of the turbidites signalizes sources from shallow water/coastal areas as well as from deep water levels. Repeated mobilization and displacement seems to have been common. Volcaniclastic material is the dominant component of the whole studied part of Site 650 sedimentary sequence. Ashfall deposits as well as normal open marine sediments are rare.

Diatom biostratigraphy, Argon ages and chronology of ODP Hole 183-1138A

A thick Neogene section was recovered in the upper ~300 m of Ocean Drilling Program Hole 1138A, drilled on the Central Kerguelen Plateau in the Indian sector of the Southern Ocean. Sediment lithologies consist primarily of mixed carbonate and biosiliceous clays and oozes, with several thin (1-3 cm) tephra horizons. The tephras are glass rich, well sorted, and dominantly trachytic to rhyolitic in composition. Volcaniclastic material in these horizons is interpreted to have originated from Heard Island, 180 km northwest of Site 1138, and was likely emplaced through both primary ash fall and turbiditic, submarine flows. A Neogene age-depth model for Hole 1138A is constructed primarily from 36 diatom biostratigraphic datums. Nannofossil and planktonic foraminifer biostratigraphy provides supporting age information. Additionally, four high-precision 40Ar-39Ar ages are derived from ash and tephra horizons, and these radiometric ages are in close agreement with the biostratigraphic ages. The integrated age-depth model reveals a reasonably complete lower Miocene to upper Pleistocene section in Hole 1138A, with the exception of a ~1-m.y. hiatus at the Miocene/Pliocene boundary. Another possible hiatus is also identified at the Oligocene/Miocene boundary. High Neogene sedimentation rates and the presence of both calcareous and siliceous microfossils, combined with datable tephra horizons, establish Site 1138 as a suitable target for future drilling legs with paleoceanographic objectives. This report also proposes two new diatom species, Fragilariopsis heardensis and Azpeitia harwoodii, from Pliocene strata of Hole 1138A.

(Table 1) Occurrences of volcanic ash at DSDP Leg 78A Holes

Air-fall volcanic ash recovered at Deep Sea Drilling Project Sites 541, 542, and 543 on and east of the toe of the Barbados Ridge delineate middle and late Miocene, early Pliocene, and Pleistocene-Quaternary pulses of explosive volcanism in the Lesser Antilles arc. The ash beds at Site 541 allow precise correlation of intervals repeated by a probable reverse fault at this convergent margin.

Age and petrographic characterization of volcanic ash from ODP Leg 198 and Leg 126 sites

In this study of volcanic ash retrieved from Shatsky Rise during Ocean Drilling Program Leg 198, the texture and composition of the volcanic components (glass and crystals) were used to fingerprint ash layers for detailed correlation. Correlations among ash layers in holes drilled at the same site as well as between sites, including sites on different parts (highs) of the rise, were tested. Although high-to-high correlations failed, intrahigh correlations were more successful. Our data suggest a significantly different source for some pyroclastic debris, especially at Site 1208, possibly associated with pumice rafts carried northward from the Izu-Bonin arc by the Kuroshio Current. Other ashes are consistent with rhyolitic to dacitic air fall ash from Asian arc volcanoes. We were not able to texturally distinguish between air fall ash and pumice-raft fallout but suspect that the latter is associated with higher percentages of vesiculated ash components, as we demonstrate occur in more proximal Izu-Bonin pyroclastic deposits.

Geochemistry of ash from Santorin, Greece

Santorin, am südlichsten Punkt des Kykladenbogens gelegen, ist der einzige noch tätige Vulkan in der südlichen Ägäis. Der Vulkanismus begann vor ca. 1.6 Mio. Jahren. Santorin besteht aus 5 Inseln, die nahezu vollständig aus vulkanischen Gesteinen aufgebaut sind, die im Laufe der Vulkangeschichte aus verschiedenen Eruptionszentren gefördert wurden. Abgesehen von den Laven im N der Hauptinsel Thera, wird der Zentral- und Südteil der Insel in der Hauptsache von den pyroklastischen Förderprodukten des sog. Thera-Vulkans aufgebaut. In der vorliegenden Arbeit waren diese pyroklastischen Serien Ziel der Untersuchungen. Die Ergebnisse daraus können folgendermaßen zusammengefaßt werden: - Die Aufnahme von 14 detaillierten Profilen und deren Korrelierung erbrachte die Einteilung der pyroklastischen Schichten in 5 Haupt-Folgen: T5/1 - Untere Bimsstein-Folge (Bu), T5/2-Mittlere Bimsstein-Folge (Bm), - T5/3, Die Obere Bimsstein-Folge (Bo) wurde dabei nicht weiter berücksichtigt, da sie bereits in zahlreichen Arbeiten untersucht worden ist. - Die als T5/1-3 bezeichneten Serien bestehen aus Aschen, Schlacken, wenigen Bimsstein-Horizonten und untergeordnet Ignimbriten, 'pyroclastic' und 'ash flow'-Ablagerungen, sowie Laharen. Umlagerungen und Bodenhorizonte zeigen die Unterbrechung in der vulkanischen Tätigkeit an. - In den Tg-Folgen konnten jeweils einer oder mehrere Leit-Horizonte gefunden werden, die es ermöglichen die drei Tg-Serien zu unterscheiden und zu korrelieren. - Die Untere Bimsstein-Folge (Bu) wurde in sechs Einheiten unterteilt, die eine Wechselfolge von 'pumice fall' und 'pumice flow'-Ablagerungen bilden. - Mineralogische Untersuchungen zeigen für die 5 Haupt-Folgen nur geringe Unterschiede. Die Bimssteine und Schlacken bestehen überwiegend aus Glas und haben nur wenige Phänokristalle (3-12 Vol.*), wobei der Plagioklas (Andesin-Labradorit) überwiegt; Orthopyroxen (Hypersthen) und Klinopyroxen (diops. Augit) stellen ca. 30% der Einsprengunge. An Akzessorien sind vorhanden: Apatit, Magnetit, Hämatit und sehr selten Hornblende. - Ein Versuch zur Unterscheidung der Gesteine in den einzelnen Schicht-Einheiten war die Bestimmung der Lichtbrechung und der Dichte. Es zeigte sich, daß die Dichte weniger geeignet ist, die Lichtbrechung aber eine schwache Differenzierung widerspiegelt und somit für die Bimssteine und Schlacken, aber auch für die Bimssteine innerhalb des Bu unterschiedliche Werte gefunden wurden - Aus den Korngrößen-Analysen des Asche-Leithorizontes der T5/3-Folge ergaben sich die Lage des Eruptionszentrums und die damals vorherrschende Windrichtung. - Die Oberen Ignimbrite (Ign., im Hangenden der T5/3-Folge) konnten erstmals in 5 Einheiten unterteilt werden. - Die Seltenen Erden-Analysen erbrachten für die einzelnen Folgen in etwa die gleichen SEE-Spektren. - Die geochemischen Untersuchungen von ca. 120 Proben sind in verschiedenen Diagrammen dargestellt. Daraus wird deutlich, daß die T5-Folgen die basischsten Glieder sind. Es handelt sich überwiegend um Quarz-Andesite und Quarz-Latiandesite, während die Bimsstein-Serien (Bu u. Bm) eine quarz-latiandesitische bis rhyodacitische Zusammensetzung haben. Es sind aber alles kalkalkaline Gesteine, die in den Bereich der Kontinentalrand-Andesite gehören. - Es wird angenommen, daß die Gesteine des Thera-Vulkans aus einer Magmakammer stammen. Während Zeiten ± kontinuierlicher, explosiver Tätigkeit wurden die Aschen und Schlacken der T5-Serien gefördert, die gegenüber den Bimsstein-Folgen relativ basisch sind. Während längerer Ruheperioden (Bodenhorizonte am Top der Tc-Folgen) differenzierte die Schmelze in der Magmakammer, vorwiegend durch Kristallfraktionierung. In den paroxysmalen Ausbrüchen wurden dann die sauren, gasreichen Bimssteine des Bu, Bm und Bo gefördert. - Anhand der lithologischen und geochemischen Untersuchungen ließen sich die etwaigen Eruptionszentren, die Ausbruchsmechanismen und der Ablagerungstyp der Schichten herleiten und daraus die Vulkangeschichte rekonstruieren, wie sie in vier Tabellen übersichtlich zusammengefaßt sind. - Schließlich sollen paläomorphologische Karten die einzelnen Stadien des Thera-Vulkans veranschaulichen.

Geochemistry of ash and pumice of ODP Hole 132-810C

Air-fall volcanic ash and pumice were recovered from 22 intervals in upper Miocene-Pleistocene nannofossil oozes cored in Hole 810C on Shatsky Rise, northwest Pacific. Shatsky Rise is near the eastern limit of ash falls produced by explosive volcanism in arc systems in northern Japan and the Kuriles, more than 1600 km away. Electron probe analyses establish that the ash beds and pumice pebbles are andesitic to rhyolitic in composition, and belong to both tholeiitic and high-alumina lineages similar to tephra from Japanese volcanoes. High-speed winds in the polar-front and subtropical jets are evidently what propelled the ash for such a distance. The pumice arrived by flotation, driven from the same directions by winds, waves, and currents. It is not ice-rafted debris from the north. One thick pumice bed probably was deposited when a large pumice mat passed over Shatsky Rise. Far more abundant ash occurs in sediments cored at DSDP Sites 578 through 580, about 500 km west of Shatsky Rise. Most of the ash and pumice at Shatsky Rise can be correlated with specific ash beds at 1, 2, or all 3 of these sites by interpolating to precisely determined magnetic reversal sequences in the cores. Most of the correlations are to thick ash layers (5.7 +/- 3.0 cm) at one or more sites. These must represent extremely large eruptions that spread ash over very wide areas. Whereas several of the thicker correlative ashes fell from elongate east-trending plumes directed from central Japan, the majority of them - dating from about 2 Ma - came from the North Honshu and Kurile arc systems to the northwest. This direction probably was in response to both long-term and seasonal fluctuations in the location and velocity of the polar-front jet, and to more vigorous winter storm fronts originating over glaciated Siberia.