Kozo Uto

The characteristics and chronology of the earliest Acheulean at Konso, Ethiopia

The Acheulean technological tradition, characterized by a large (>10 cm) flake-based component, represents a significant technological advance over the Oldowan. Although stone tool assemblages attributed to the Acheulean have been reported from as early as circa 1.6–1.75 Ma, the characteristics of these earliest occurrences and comparisons with later assemblages have not been reported in detail. Here, we provide a newly established chronometric calibration for the Acheulean assemblages of the Konso Formation, southern Ethiopia, which span the time period ∼1.75 to <1.0 Ma. The earliest Konso Acheulean is chronologically indistinguishable from the assemblage recently published as the world’s earliest with an age of ∼1.75 Ma at Kokiselei, west of Lake Turkana, Kenya. This Konso assemblage is characterized by a combination of large picks and crude bifaces/unifaces made predominantly on large flake blanks. An increase in the number of flake scars was observed within the Konso Formation handaxe assemblages through time, but this was less so with picks. The Konso evidence suggests that both picks and handaxes were essential components of the Acheulean from its initial stages and that the two probably differed in function. The temporal refinement seen, especially in the handaxe forms at Konso, implies enhanced function through time, perhaps in processing carcasses with long and stable cutting edges. The documentation of the earliest Acheulean at ∼1.75 Ma in both northern Kenya and southern Ethiopia suggests that behavioral novelties were being established in a regional scale at that time, paralleling the emergence of Homo erectus-like hominid morphology.

Gigantic SO2 emission from Miyakejima volcano, Japan, caused by caldera collapse

An extremely large amount of volcanic gas has been released since mid-August 2000 from the volcanic island of Miyakejima, Japan, after formation of a summit caldera of 1.6 km diameter. The volcanic gas emission was continuous with very little extrusive magma activity. Variation of the SO 2 emission rate was monitored by repeated measurements with an airborne correlation spectrometer. In December 2000, the SO 2 emission rate averaged for the month peaked at 54 kt/d, which is twice the global SO 2 emission rate from nonerupting volcanoes evaluated before this activity. The SO 2 emission rate gradually decreased, almost linearly when plotted on a log scale, to 7 kt/d by the end of 2002, and then remained constant until at least December 2003. The total SO 2 emission amounts to 18 Mt, comparable to the emission of a large explosive eruption such as Pinatubo in 1991. A theoretical evaluation, based on the model of magma convection in a conduit, suggests that extremely large volcanic gas emissions can be caused by formation of a magma pathway with a slightly larger diameter than exists in common systems, because the magma-transport rate is proportional to the fourth power of the conduit radius. Because volcanic gas emissions were initiated by formation of a summit collapse caldera of 1.6 km diameter, the creation of a large magma-conduit system through fractures formed during caldera collapse is likely the underlying cause of the extremely large volcanic gas emissions from the volcano.

Geology and eruptive history of Unzen volcano, Shimabara Peninsula, Kyushu, SW Japan

Abstract During the past 500 thousand years, Unzen volcano, an active composite volcano in the Southwest Japan Arc, has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben. The volcano can be divided into the Older and the Younger Unzen volcanoes. The exposed rocks of the Older Unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200–300 ka. Drill cores recovered from the basal part of the Older Unzen volcano are dated at 400–500 ka. The volcanic rocks of the Older Unzen exceed 120 km3 in volume. The Younger Unzen volcano is composed of lava domes and pyroclastic deposits, mostly younger than 100 ka. This younger volcanic edifice comprises Nodake, Myokendake, Fugendake, and Mayuyama volcanoes. Nodake, Myokendake and Fugendake volcanoes are 100–70 ka, 30–20 ka, and

Noble gas study of the Reunion hotspot: Evidence for distinct

We present an extensive He, Ne and Ar isotope data set from the Reunion hotspot that demonstrates the presence of a homogeneous plume source that has unique isotopic characteristics. 3He/4He ratios of the two volcanoes on Reunion (Piton de la Fournaise, <0.53 Ma; Piton des Neiges, 2–0.44 Ma) are uniform 12–13.5 Ra regardless of 4He concentration and sample age. The shield-building Older Series of Mauritius (5–8 Ma) has a constant 3He/4He ratio around 11.5 Ra. The similarity of 3He/4He and Sr–Nd isotope ratios among them demonstrate that the volcanoes have had a common homogeneous source related to the mantle plume activity over a period of 8 Myr. 20Ne/22Ne and 21Ne/22Ne of these volcanoes define a linear trend on a Ne three-isotope diagram with a slope between the Loihi and MORB correlation lines. There is a clear correlation between 20Ne/22Ne and 40Ar/36Ar. In contrast, Intermediate (2–3 Ma) and Younger Series (<1 Ma) of Mauritius and Rodrigues (1.5 Ma) have 3He/4He ratios similar to MORB and Sr and Nd isotope ratios closer to MORB than lavas from Reunion and Older Series of Mauritius. These Intermediate and Younger Series lavas therefore record a late stage thermal rejuvenation beneath Mauritius derived from a source that is unrelated to the mantle plume. The isotopic characteristics of the source of the Reunion magmatism are relatively low 3He/4He (13 Ra), an intermediate slope in a Ne three-isotope diagram and relatively radiogenic Sr isotope ratios. These source characteristics cannot be explained by either crustal contamination or MORB source mixing with Loihi-type primitive mantle. Thus He–Ne–Ar–Sr–Nd isotopes demonstrate that this plume source is distinct from the source of other large plumes (Loihi and Iceland), clearly showing that the mantle contains several relatively less-degassed reservoirs and not a single primitive source. Two possible models can account for the different isotopic signature of Reunion and Hawaii hotspots; (1) the Reunion source contains more recycled material than Loihi source and (2) the Reunion source experienced stronger degassing/differentiation than the Loihi source in the early stage of mantle evolution. In both cases a convection mode in the mantle is required that isolates and preserves several less-degassed reservoirs in the convectively stirred lower mantle.

Evidence of porphyry copper-type hydrothermal activity from a submerged remnant back-arc volcano of the Izu-Bonin arc: Implications for the volcanotectonic history of back-arc seamounts

The first example of a porphyry copper-type hydrothermal activity associated with the back-arc volcanism in the Izu-Bonin arc is presented. Hydrothermally-altered rocks exposed at the flat summit of the Manji Seamount show strong similarities to those associated with porphyry copper-type deposits, i.e. (1) alteration mineral assemblages corresponding to potassic and propylitic alteration zone, (2) occurrence of stockwork of quartz–magnetite veinlets, (3) copper mineralization, (4) occurrence of fluid inclusions trapping high temperature (ca. 600°C) and hyper-saline (ca. 63 wt% NaCl equivalent) fluid, (5) occurrence of acidic plutonic rocks (e.g. tonalite porphyry), which are often genetically associated with porphyry copper deposits. Volcanism of Manji Seamount occurred at around 7 Ma and shares common chemical characteristics with other back-arc seamounts in the Izu-Bonin arc, i.e. more calc-alkaline affinity, enrichment in large ion lithophile elements (LILE) and high-field strength elements (HFSE) compared to those of volcanic front. The plutonic rocks are almost contemporaneous with the volcanic rocks and are regarded as comagmatic. The age of the volcanism is within the age range of other back-arc seamounts (17–3 Ma). The porphyry copper-type hydrothermal activity is presumed to have been driven by the volcanism in the back-arc during the period between the cessation of the back-arc spreading of the Shikoku Basin and the initiation of currently active rifting. The seamount was exposed above sea level when active, but was later deeply eroded down to the interior of the volcano. Subsequent subsidence and erosion produced a wave-cut flat summit and consequently the porphyry copper system outcrops at 700 m water depth. Some characteristics of the hydrothermal activity at Manji Seamount are shared by the porphyry copper deposits in the oceanic arcs in the southwestern Pacific, e.g. petrography and chemical characteristics of related igneous rocks. However, the hydrothermal activity at Manji Seamount is located further away from the volcanic front than the deposits in the southwestern Pacific, and regional back-arc volcanism is extensive and voluminous in the Izu-Bonin arc. The porphyry copper hydrothermal system reported here is a well-preserved example associated with voluminous back-arc volcanism (i.e. arc volcanism behind the volcanic front which is not associated with back-arc rifting) in an oceanic arc setting. The discovery of porphyry copper-type hydrothermal activity associated with extensive back-arc volcanism in an intra-oceanic arc implies that back-arc regions of ancient oceanic arcs which are exposed on land might host more porphyry copper deposits.

Volcanism in the earliest stage of back-arc rifting in the Izu-Bonin arc revealed by laser-heating 40Ar/39Ar dating

Abstract The back-arc region of the Izu-Bonin arc has complex bathymetric and structural features, which, due to repeated back-arc rifting and resumption of arc volcanism, have prevented us from understanding the volcano-tectonic history of the arc after 15 Ma. The laser-heating 40Ar/39Ar dating technique combined with high density sampling of volcanic rocks from the back-arc region of this arc successfully revealed the detailed temporal variation of volcanism related to the back-arc rifting. Based on the new 40Ar/39Ar dating results: (1) Back-arc rifting initiated at around 2.8 Ma in the middle part of the Izu-Bonin arc (30°30′N–32°30′N). Volcanism at the earliest stage of rifting is characterized by the basaltic volcanism from north–south-trending fissures and/or lines of vents. (2) Following this earliest stage of volcanism, at ca. 2.5 Ma, compositionally bimodal volcanism occurred and formed small cones in the wide area. This volcanism and rifting continued until about 1 Ma in the region west of the currently active rift zone. (3) After 1 Ma, active volcanism ceased in the area west of the currently active rift zone, and volcanism and rifting were confined to the currently active rift zone. The volcano-tectonic history of the back-arc region of the Izu-Bonin arc is an example of the earliest stage of back-arc rifting in the oceanic island arc. Age data on volcanics clearly indicate that volcanism changed its mode of activity, composition and locus along with a progress of rifting.

Geochemistry of Cenozoic basalts in the Fukuoka district (northern Kyushu, Japan): implications for asthenosphere and lithospheric mantle interaction

Abstract Fukuoka volcanic field in northern Kyushu (Japan) is comprised of scattered small, monogenetic volcanoes with ages ranging from 1.1 to 4.4 Ma. A set of samples from the area, together with some from nearby localities, was collected and analyzed for major and trace element abundances and Sr, Nd and Pb isotope compositions. The basalts, unlike lavas from other nearby centers in northern Kyushu, show the highest FeO*, TiO2 and lowest SiO2 characteristics, which are interpreted to reflect high melting temperature and pressure; whereas high Sr, Sm and high-field-strength elements (HFSE) such as Zr and Nb, high light rare earth element (LREE), relatively low Ba, Rb, and broadly oceanic island basalt (OIB)-like primitive mantle normalized incompatible trace element patterns are interpreted to reflect source characteristics. In addition to lead isotopic compositions that are the most radiogenic yet analyzed from northern Kyushu and the Sea of Japan, strontium and neodymium isotopic compositions of Fukuoka lavas free from crustal contamination are among the highest and lowest, respectively (average, 0.7052 and 0.5126), in the region. The samples show the signature of enriched mantle type 2 (EM2), differing from most of the other Dupal anomaly bearing lavas reported from the Sea of Japan and elsewhere in northern Kyushu. The EM2-like characteristics and relatively low concentrations of large ionic lithophile elements (LILE), low LILE/LREE and LREE/high-field-strength elements (HFSE), and mid-ocean ridge basalt (MORB)-like Rb/Sr and Nb/Zr ratios in the Fukuoka lavas are explained by melts from an asthenospheric source that experienced previous melt extraction. Because the chemical characteristics of Fukuoka basalts are strictly, geographically localized, we suggest that, while the mantle beneath most of northern Kyushu is very much similar to that of the Sea of Japan, represented by a spectrum of depleted MORB-EM1 (Dupal-like) hybrids, the Fukuoka EM2-rich component may have been added from shallower levels.

Volcanic history of the back-arc region of the Izu-Bonin (Ogasawara) arc

Abstract The laser-heating 40Ar/39Ar dating method was applied to volcanic rocks systematically collected from the back-arc region of the central part of the Izu-Bonin arc. Dating results combined with whole-rock chemistry and other geological information reveal the volcanic history of the back-arc region of the Izu-Bonin arc. In the back-arc seamount chains area, andesitic-basaltic volcanism initiated at c. 17 Ma, slightly before the Shikoku Basin ceased spreading, and continued until c. 3 Ma. Relatively old volcanism (>8 Ma) has been found only from the western part of the seamount chains, and younger volcanism mainly occurs in the eastern part of the chains, indicating the western margin of the active volcanic zone of the Izu-Bonin arc has migrated eastward with time. At around 2.8 Ma, volcanism initiated in the western part of the back-arc knolls zone. This volcanism is characterized by eruption of clinopyroxene-olivine basalt. In the first stage of rifting, this type of basalt erupted from N-S-trending fissures and/or vents aligned in this direction and formed N-S-trending ridges. Between 2.5 and 1 Ma, many small knolls were formed by eruption of basalt and minor felsic rocks. Volcanism younger than 1 Ma occurred only in the currently active rift zone and its adjacent area. The active volcanic zone in the back-arc seamount chains area converged to the volcanic front with time from 17 to 3 Ma. Active rifting and rifting-related volcanism also migrated or converged eastward after 1 Ma. The observed temporal variation of locus of volcanism may be explained by rapid retreat of the Philippine Sea Plate relative to the Pacific Plate and resulting steepening of the subducting slab.

The 1989 submarine eruption off eastern Izu Peninsula, Japan: ejecta and eruption mechanisms

The submarine eruption of a new small knoll, which was named “Teishi knoll”, off eastern Izu Peninsula behind the Izu-Mariana arc occurred in the evening of 13 July 1989. This is the first historic eruption of the Higashi-Izu monogenetic volcano group. The eruption of 13 July followed an earthquake swarm near Ito city starting on 30 June. There were subsequent volcanic tremors on 11 and 12 July, and the formation of the Teishi knoll on the 100 m deep insular shelf 4 km northeast of Ito city. There were five submarine explosions, which were characterized by intermittent domelike bulges of water and black tephra-jets, which occurred within 10 min on 13 July. Ejecta of the eruption was small in volume and composed of highly crystalline basalt scoria, highly vesiculated “pumice”, and lithic material. Petrographical features suggest that the “pumice” was produced by vesiculation of reheated wet felsic tuff of an older formation. The Teishi knoll, before the eruption, was a circular dome, 450 m across and 25 m high, with steep sides and a flat summit. Considerations of submarine topographic change indicate the knoll was raised by sill-like intrusion of 106 m3 of magma beneath a 30 m thick sediment blanket. This shallow intrusion is assumed to have started on 11 July when volcanic tremors were observed for the first time, but there was no indications of violent interaction between wet host sediments and intruding magma. The submarine eruption of 13 July appears to have been Friggered by a major lowering of the magma-column. The basalt scoria, having crystal-contents of more than 60%, is assumed to be derived from the cooled plastic margin of the shallow intrusive body. However, glassy scoria, which would indicate the interaction between hot fluidal magma and external water, was not observed. A scenario for the 1989 submarine eruption is as follows. When rapid subsidence of the hot interior of the intrusive magma occurred, reduced pressure caused the implosion of cooled plastic magma, adjacent pressurized, hot host material, and wet sediment. The mixing of these materials triggered the vigorous vapor explosions.

Tectono-magmatic processes investigated at deep-water flanks of Hawaiian volcanoes

Hawaiian volcanoes are exceptional examples of intraplate hotspot volcanism. Hotspot volcanoes, which frequently host large eruptions and related earthquakes, flank-failure landslides, and associated tsunamis, can present severe hazards to populated regions. Many studies have focused on subaerial parts of Hawaiian volcanoes, but the deep-water flanks of the edifices, which can reach 5700 m below sea level, remain poorly understood because they are so inaccessible. In 1998 a collaborative program between Japan and the United States was initiated to explore the evolution of Hawaiian volcanoes, including their growth and degradation.