Ash layers, hyaloclastite, and alteration of basaltic glass at DSDP Leg 65 Holes

Three types of tephra deposits were recovered on Leg 65 of the Deep Sea Drilling Project (DSDP) from three drill sites at the mouth of the Gulf of California: (1) a series of white ash layers at Sites 483, 484, and 485; (2) a layer of plagioclase- phyric sideromelane shards at Site 483; and (3) an indurated, cross-bedded hyaloclastite in Hole 483B. The ash layers in (1) are composed of colorless, fresh rhyolitic glass shards with minor dacitic and rare basaltic shards. These are thought to be derived from explosive volcanoes on the Mexican mainland. Most of the shards in (2) are fresh, but some show marginal to complete alteration to palagonite. The composition of the glass is that of a MORB-type tholeiite, low in Fe and moderately high in Ti, and possibly erupted from off-axis seamounts. Basaltic glass shards occurring in silt about 45 meters above the basement at Site 484 A in the Tamayo Fracture Zone show a distinctly alkalic composition similar to that of the single basement basalt specimen drilled at this site. The hyaloclastite in (3) is made up chiefly of angular sideromelane shards altered to smectite and zeolites (mainly phillipsite) and minor admixtures of terrigenous silt. A very high K and Ba content indicates significant uptake of at least these elements from seawater. Nevertheless, the unusual chemical composition of the underlying massive basalt flow is believed to be reflected in that of the hyaloclastite. This is a powerful argument for interpreting the massive basalt as a surface flow rather than an intrusion. Glass alteration is different in the glassy margins of flows than in thicker glassy pillow rinds. Also, it appears to proceed faster in coarse- than fine-grained sediments.

Fluid compositions and mineralogy of precipitates from Mid Atlantic Ridge hydrothermal vents at 4°48'S

The effect of volcanic activity on submarine hydrothermal systems has been well documented along fast- and intermediate-spreading centers but not from slow-spreading ridges. Indeed, volcanic eruptions are expected to be rare on slow-spreading axes. Here we report the presence of hydrothermal venting associated with extremely fresh lava flows at an elevated, apparently magmatically robust segment center on the slow-spreading southern Mid-Atlantic Ridge near 5°S. Three high-temperature vent fields have been recognized so far over a strike length of less than 2 km with two fields venting phase-separated, vapor-type fluids. Exit temperatures at one of the fields reach up to 407°C, at conditions of the critical point of seawater, the highest temperatures ever recorded from the seafloor. Fluid and vent field characteristics show a large variability between the vent fields, a variation that is not expected within such a limited area. We conclude from mineralogical investigations of hydrothermal precipitates that vent-fluid compositions have evolved recently from relatively oxidizing to more reducing conditions, a shift that could also be related to renewed magmatic activity in the area. Current high exit temperatures, reducing conditions, low silica contents, and high hydrogen contents in the fluids of two vent sites are consistent with a shallow magmatic source, probably related to a young volcanic eruption event nearby, in which basaltic magma is actively crystallizing. This is the first reported evidence for direct magmatic-hydrothermal interaction on a slow-spreading mid-ocean ridge.

(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.

Data on mineral composition of volcanic glass and radiocarbon age of Upper Quaternary sediments from the Sea of Okhotsk

Lithological horizons have been distinguished in sediments cores from different parts of the Sea of Okhotsk based on primary descriptions of sediments and smear slides, and analyses of contents of both calcium carbonate and organic carbon, and opal. Sediment lithology has been correlated with oxygen isotope records and the standard isotope scale and radiocarbon data by AMS method for three cores studied in detail. This allowed to determine in detail periods of carbonaceous and diatomaceous ooze accumulation in the Sea of Okhotsk. Changes in magnetic susceptibility and grain size composition of sediments have been also compared with oxygen-isotope curves and radiocarbon datings. Obtained results confirm that variations in magnetic susceptibility are related with oxygen-isotope stages and influenced by climatic changes. Tephra interlayers K0, TR, K2, K3 have been identified by mineralogical analyses in all studied cores. Stratigraphic location of these tephra interlayers in detailed studied cores and their radiocarbon ages (8.1, 8.05, 26.8, and about 60 ka, respectively) provided base correlation between the interlayers and volcanic eruptions on the Kamchatka Peninsula and the Kuril Islands. This allows to use the former ones as time markers for deep-sea sediments of the Sea of Okhotsk. New lithostratigraphic and tephrochronologic data obtained allowed to correlate Upper Quaternary sediments from the Sea of Okhotsk.

(Table T1) Ages of white mica clasts in core samples of ODP Hole 210-1276A

Fifty-seven white mica clasts were separated from five samples taken from near the bases of turbidites ranging in age from early Albian to middle Eocene. Twenty two (39%) of the micas have ages between 260 and 340 Ma and five (9%) have older ages (~400-600 Ma). The former age range is characteristic of the North American Alleghenian orogeny and the Iberian Variscan orogeny. The latter range is characteristic of the North American Acadian orogeny and older basement rocks in the Grand Banks and Newfoundland areas. Both age ranges are present in the middle Eocene sample, but only the younger range occurs in the middle Albian sample. This difference could be a sampling artifact. If this is not the case, then the most likely explanation is that the Acadian-aged micas within the Meguma Zone underlying the Grand Banks were totally reset by Alleghenian reactivation of the zone, a feature which occurs extensively in Nova Scotia. The addition of Acadian-aged micas in the middle Eocene sample may reflect a change in sediment provenance as drainage systems unrelated to rift topography developed. With the exception of one clast dated at 186 Ma, the 12 other micas obtained from the upper Paleocene sample yielded ages between 55 and 74 Ma, with 7 falling within ±2 m.y. of the 57-Ma age of the sample indicated by the biostratigraphic age-depth plot for Site 1276. This, together with the volcaniclastic content of the sample, indicates an input from near-contemporaneous volcanism. The nearest known occurrences of near-contemporaneous late Paleocene volcanism that could have produced white micas are in Greenland and Portugal, some 2000 and 1500 km distant, respectively, from Site 1276 during the Paleocene. However, ages of volcanism in these areas indicate that they could probably not be sources of micas younger than 60 m.y., which suggests some as-yet unknown volcanic source in the North Atlantic area. Accumulation in the Grand Banks area of airborne-transported volcaniclastic material from eruptions of slightly different ages, followed by a single resedimentation event, could account for the spread of dates obtained from the sample. White micas from the lowermost Albian sample show a spread of ages between 37 and 284 Ma that is completely different from the age distribution pattern of the middle Albian and middle Eocene samples. The sample location is between, and at least 25 m above and below, two igneous sills dated at 98 and 105 Ma. The sills have narrow thermal aureoles and ages older than the youngest detrital micas in the sample. It is unlikely, therefore, that the spread of mica ages in the sample is due to partial resetting of ages caused by thermal effects associated with the intrusion of the sills. The resetting may have been associated with a longer lived thermal event.

Chemistry of volcanic ash layers from the southern Atlantic

Petrographic and geochemical investigations were carried out on 21 ash layers from four sites of ODP Legs 113 and 114 in the southern Atlantic Ocean. With the help of geochemical data and petrographic characterization three rock series can be distinguished for stratigraphically different ash layers from Site 701 (Leg 114) located east of the South Sandwich Island Arc, whereas the Leg 113 tephras from the southern slope of the South Orkney Microcontinent belong to another magmatic series. Geochemical correlation of the Leg 113 tephras with possible source areas indicates that they were probably erupted from the Antarctic Peninsula. The Miocene ashes from Site 701 are probably derived from the now-extinct Discovery Arc, the precursor of the South Sandwich Islands. The Pliocene ashes from the site show some affinity with the South Shetland Islands, although the available data do not permit a clear correlation. The Quaternary ashes from Site 701 display a chemistry typical of island-arc tholeiites and are therefore most probably derived from eruptions on the South Sandwich Islands. Because of their distant position the southern Andes seem to be rather improbable as a potential source region for the tephra layers investigated.

(Table 2) Picked maximum velocities from 3 radars (RAY, SHK, and SUM), showing 10 explosions at Erebus volcano, Antarctica, and their respective directivity vectors

We used a novel system of three continuous wave Doppler radars to successfully record the directivity of i) Strombolian explosions from the active lava lake of Erebus volcano, Antarctica, ii) eruptions at Stromboli volcano, Italy, and iii) a man-made explosion in a quarry. Erebus volcano contains a convecting phonolite lava lake, presumably connected to a magma chamber at depth. It is one of the few open vent volcanoes that allow a direct observation of source processes during explosions. Its lava lake is the source of frequent violent Strombolian explosions, caused by large gas bubbles bursting at the lake surface. The exact mechanism of these bubble bursts is unclear, as is the mechanism of the creation of the infrasound signal accompanying the explosions. We use the Doppler radar data to calculate the directivity of Strombolian eruptions at Erebus. This allows us to derive information about the expected type of infrasound source pattern (i.e. the role of a dipole in addition to the monopole signature) and the physical structure of the volcano. We recorded 10 large explosions simultaneously with three radars, enabling us to calculate time series of 3D directivity vectors (i.e. effectively 4D), which describe the direction of preferred expansion of the gas bubble during an explosion. Such directivity information allows a comparison to dipole infrasound radiation patterns recorded during similar explosions only a few weeks later. Video observations of explosions support our interpretation of the measurements. We conclude that at Erebus, the directivity of explosions is mainly controlled by random processes. Since the geometry of the uppermost conduit is assumed to have a large effect on the directivity of explosions, the results suggest a largely symmetrical uppermost conduit with a vertical axis of symmetry. For infrasound recordings, a significant dipole signature can be expected in addition to the predominant monopole signature.

Observation of manganese deposits made during the 1936 transect of C/S Lord Kelvin through the Northern Atlantic Ocean

In May and June 1936 Dr. C. S. Piggot of the Geophysical Laboratory, Carnegie Institution of Washington, took a series of 11 deep-sea cores in the North Atlantic Ocean between the Newfoundland banks and the banks off the Irish coast. These cores were taken from the Western Union Telegraph Co.'s cable ship Lord Kelvin with the explosive type of sounding device which Dr. Piggot designed. All but two of these cores (Nos. 8 and 11) are more than 2.43 meters (8 feet) long, and all contain ample material for study. Of the two short cores, No. 8 was taken from the top of the Faraday Hills, as that part of the mid-Atlantic ridge is known, where the material is closely packed and more sandy and consequently more resistant; No. 11 came from a locality where the apparatus apparently landed on volcanic rock that may be part of a submarine lava flow.

Pleistocene rhyolithic tephra of ODP Leg 181 sites

Taupo Volcanic Zone (TVZ), in the North Island, New Zealand, is arguably the most active Quaternary rhyolitic system in the world. Numerous and widespread rhyolitic tephra layers, sourced from the TVZ, form valuable chronostratigraphic markers in onshore and offshore sedimentary sequences. In deep-sea cores from Ocean Drilling Program (ODP) Leg 181 Sites 1125, 1124, 1123 and 1122, located east of New Zealand, ca 100 tephra beds are recognised post-dating the Plio-Pleistocene boundary at 1.81 Ma. These tephras have been dated by a combination of magnetostratigraphy, orbitally tuned stable-isotope data and isothermal plateau fission track ages. The widespread occurrence of ash offshore to the east of New Zealand is favoured by the small size of New Zealand, the explosivity of the mainly plinian and ignimbritic eruptions and the prevailing westerly wind field. Although some tephras can be directly attributed to known TVZ eruptions, there are many more tephras represented within ODP-cores that have yet to be recognised in near-source on-land sequences. This is due to proximal source area erosion and/or deep burial as well as the adverse effect of vapour phase alteration and devitrification within near-source welded ignimbrites. Despite these difficulties, a number of key deep-sea tephras can be reliably correlated to equivalent-aged tephra exposed in uplifted marine back-arc successions of Wanganui Basin where an excellent chronology has been developed based on magnetostratigraphy, orbitally calibrated sedimentary cycles and isothermal plateau fission track ages on tephra. Significant Pleistocene tephra markers include: the Kawakawa, Omataroa, Rangitawa/Onepuhi, Kaukatea, Kidnappers-B, Potaka, Unit D/Ahuroa, Ongatiti, Rewa, Sub-Rewa, Pakihikura, Ototoka and Table Flat Tephras. Six other tephra layers are correlated between ODP-core sites but have yet to be recognised within onshore records. The identification of Pleistocene TVZ-sourced tephras within the ODP record, and their correlation to Wanganui Basin and other onshore sites is a significant advance as it provides: (1) an even more detailed history of the TVZ than can be currently achieved from the near-source record, (2) a high-resolution tephrochronologic framework for future onshore-offshore paleoenvironmental reconstructions, and (3) well-dated tephra beds correlated from the offshore ODP sites with astronomically tuned timescales provide an opportunity to critically evaluate the chronostratigraphic framework for onshore Plio-Pleistocene sedimentary sequences (e.g. Wanganui Basin, cf. Naish et al. (1998, doi:10.1016/S0277-3791(97)00075-9).

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.