Kokichi Iizasa

Submarine silicic caldera at the front of the Izu-Bonin arc, Japan: Voluminous seafloor eruptions of rhyolite pumice

Myojin Knoll caldera, a submarine rhyolitic center 400 km south of Tokyo, is one of nine silicic calderas along the northern 600 km of the Izu-Bonin(-Ogasawara) arc and the first anywhere to receive detailed, submersible-based study. The caldera, slightly smaller than the Crater Lake structure in Oregon, is 6 × 7 km in diameter; its inner walls are 500–900 m high, and it has a remarkably flat floor at 1400 m below sea level (mbsl). The caldera collapse volume is ∼18 km 3 , suggesting that more than 40 km 3 of pumiceous tephra may have been erupted at the time the caldera formed. Precaldera seafloor eruptions built a broad volcanic edifice consisting of overlapping composite volcanoes made of rhyolitic lavas, shallow intrusions, and a variety of volcaniclastic deposits—including thick accumulations of rhyolitic pumice erupted at 900– 500 mbsl. The caldera-forming eruption produced a 150–200 m deposit of nonwelded, fines-depleted pumice that resembles a colossal layer of popcorn at the top of the caldera wall. Freshly erupted pumice behaved as “sinkers” or “floaters,” depending on the environment in which it cooled. The pumice clasts deposited proximally and exposed in the caldera wall were likely quenched in eruption columns that remained below sea level. This pumice ingested seawater and sank as gases filling its vesicles cooled, particularly as steam in its vesicles condensed to liquid water. Some eruption columns may have broken through the sea surface and entered the air, especially during vigorous phases of the caldera-forming eruption. These pumices had the opportunity to ingest air as they cooled, becoming floaters as they fell back to the sea; these could have been carried distally on the sea surface by the combined effects of ocean currents and wind. The age of the caldera is unknown, but it may be as young as several thousand years. Its magmatic system at depth retains sufficient heat to sustain an actively growing intracaldera Kuroko- type polymetallic sulfide deposit, rich in gold and silver and topped by chimneys emitting fluids as hot as 278 °C. Sufficient time has elapsed, however, for a 250-m-high postcaldera dome to grow on the caldera floor and for the caldera rim to be deeply scalloped by slumping.

Submarine hydrothermal mineralization on the Izu-Bonin Arc, South of Japan: An overview

Abstract Considerable effort has been expended in studying the Izu–Bonin Arc over the past 15 years. In particular, 43 dives of the Shinkai 2000 have been undertaken there to discover and evaluate the extent of submarine hydrothermal activity and mineralization. Most effort has been focused on Myojin Knoll (23 dives), Suiyo Seamount (6 dives), and Kaikata Caldera (10 dives). The Izu–Bonin Arc is divided in two by the Sofugan Tectonic Line. Eight submarine caldera are located north of this line but only one is south of it. The physiography of the northern sector of the arc is quite different from that of the southern sector. Volcanic rocks from the northern sector are more acidic than those from the southern sector. Evidence for submarine hydrothermal mineralization has been observed at four seamounts along the Izu–Bonin Arc (Myojin Knoll, Myojinsho, Suiyo Seamount, and Kaikata Caldera), and submarine hydrothermal activity is evident at another three seamounts along the arc (Kurose Hole, Mokuyo Seamount, and Doyo Seamount). The most extensive submarine hydrothermal mineral deposit so far located on the Izu–Bonin Arc is the Sunrise deposit at Myojin Knoll. This deposit, at least 400 m in diameter and 30 m high, is associated with black smoker venting, inactive sulfide chimneys, massive sulfides, hydrothermal Mn crusts, and a hydrothermal vent fauna. The maximum recorded temperature of the hydrothermal vents there was 278°C. Some of the sulfide chimneys contained as much as 49 μg/g Au and 3,400 μg/g Ag. The sunrise deposit is one of the largest submarine volcanic massive sulfide deposits so far discovered in midocean ridge, backarc, or arc settings and has an estimated mass of 9 × 106 t. This deposit may be of the Kuroko-type. The discovery of the Sunrise deposit in 1997 gives hope that other, similarly large, sulfide deposits may be found in other caldera along the Izu–Bonin Arc. The geological variability along the arc, the high seismicity, the occurrence of active volcanism and submarine hydrothermal venting, and a proven submarine hydrothermal mineral potential coupled with the proximity of the region to Japan suggest that the Izu–Bonin Arc could profitably serve as a natural laboratory for the long-term monitoring of the seafloor.

Development of a hybrid gravimeter system onboard an underwater vehicle

We are developing a hybrid gravity survey system using a gravimeter and a gravity gradiometer to survey sub-seafloor density structure. This paper describes the development of a gravimeter with forced gimbals for this purpose. The aimed precision is about 0.1 mgal, one order better than sea surface gravimetry. After a successful feasibility study by using a Scintrex CG-3M gravimeter under a condition of pitching and rolling as well as heaving, we are developing a new underwater gravimeter system with a gravity sensor by Micro-G LaCoste. The design, performance, and future tasks are described.

Submersible observations of manganese nodule and crust deposits on the Tenpo Seamount, Northwestern Pacific

Pavements of manganese nodules and crusts and outcrops of Miocene limestones were observed on the flanks and flat top of the Tenpo Seamount during three Shinkai 2000 dives. The pre‐Miocene volcano supplied nuclei of volcanic rocks and hydrothermal manganese deposits, and subsequent slow or no sedimentation promoted deposition of abundant hydrogenetic nodules and crusts, mainly on the upper flank of the seamount. Nodule pavements generally cover calcareous sand surface sediments, while crusts cover hard outcrops composed probably of volcanic rocks. The fields of crusts and nodules are sparsely distributed with each other on scales of meters to tens of meters. The on‐site observation suggests the deposits have encountered tectonic and/or mass movements that resulted in unusual occurrences of densely stacked nodules and occasionally the nodules resting directly on crusts or hard substrates. Mineralogical and chemical compositions reveal that for nodules and crusts the encrusting manganese layers of around 1 ...

Gravity gradiometer implemented in AUV for detection of seafloor massive sulfides

Gravity surveys are useful for profiling underground density structures. We propose a hybrid gravity survey method using gravimeters and gravity gradiometers to detect submarine ore deposits, such as massive sulfides buried below the seafloor. This paper describes a gravity gradiometer, which can be implemented in an AUV (Autonomous Underwater Vehicle) for this purpose. We developed a new gravity gradiometer comprising of two vertically separated accelerometers with astatic reference pendulums, whose motions are precisely detected by optical sensors. Two identical pendulums are installed in a forced gimbal to reduce the rotational motion of the underwater vehicle. The Laboratory measurements show that the noise level of the gravity gradiometer is estimated to be 7 E (1 E = 1 × 10-9 /s2), with which typical submarine ore deposits can be detected using the gravity gradiometer implemented in an AUV. The gravity gradiometer is housed in a pressure-proof vessel made of titanium alloy, 500 mm in diameter and 700 mm in height, and the vessel is implemented in an AUV. We plan a sea trial in September, 2012, in Sagami Bay, off the coast of Shizuoka Prefecture in Japan.

A gravity gradiometer to search for submarine ore deposits

Gravity surveys are useful in profiling the underground density structure. We propose a hybrid gravity survey method using gravimeters and gravity gradiometers to detect submarine ore deposits. This paper describes the development of a gravity gradiometer for this purpose. The required resolution is estimated to be finer than approximately 10 E (= 1 × 10−8 /s2), considering typical dimensions of submarine ore deposits and survey altitude from the seafloor. To attain the required resolution, we newly developed a gravity gradiometer comprising two vertically-separated accelerometers with astatic reference pendulums. Laboratory measurements showed that the gravity gradiometer attained the required resolution. Its design, performance, and future tasks are described.

Geochemical assessment on hydrothermal contribution to floor sediments from the Kita‐Bayonnaise submarine caldera, Izu‐Ogasawara arc, northwestern pacific

A geochemical study of sediment and rock samples was conducted in the Kita‐Bayonnaise submarine caldera, Shichito‐Iwojima Ridge, Izu‐Ogasawara Arc, northwestern Pacific, in order to assess hydrothermal contributions to caldera sediments. The caldera is covered mainly with ash to cobble‐sized pumice, and its wall consists of pumice and rhyolitic lava flows. The amounts of Au, Ag, Zn, Pb, Cu, Ba, Sb, As, and Cd in the caldera floor sediments are extremely high relative to those in the volcanic rocks that comprise the caldera wall The sediments enriched in these elements usually contain varying amounts of sulfides, fahlerz, and barite. R‐mode factor analysis of element abundances reveals three major factors that explain most of the chemical variation: sulfides and barite, Mn‐oxyhydroxides, and detrital Fe‐bearing minerals. The high factor scores of the eastern floor samples represent sulfides‐barite mineralization with Au and Ag enrichments, while those of the western floor samples signify both Mn‐oxyhydroxi...

Volcanic-hosted sulfide-barite deposit from the Kita-Bayonnaise submarine caldera, Izu-Ogasawara Arc, northwestern Pacific

Volcanic‐hosted sulfide and barite mineralization with high Au content (19.3 ppm) was discovered by submersible study (Shinkai 2000) on the eastern floor of the Kita‐Bayonnaise submarine caldera on the Shichito‐Iwojima Ridge, Izu‐Ogasawara Arc, northwestern Pacific. The hydrothermally mineralized rhyolitic tuff is partly coated by Mn oxyhydrox‐ide, and associated with hydrothermal Mn crusts and debris of mussel shells in the vicinity of the hydrothermally altered tuff‐breccias. The minearlized rhyolitic tuff occurs as a fragment of a large block (about 2 ×1 ×1 m) on talus, while the crusts occur on the caldera floor as a few thin patches several meters across or smaller. Ore‐mineral composition of the mineralized rhyolitic tuff consists mainly of sulfides and barite. Randomly interstratified seriate and montmorillonite is the only hydrothermal clay mineral from glass shards in the mineralized rhyolitic tuff. The mineralized tuff is significantly enriched in Zn, Fe, Cu, Ba, Au, Ag, and As, and has high Au ...

Development of an underwater gravimeter and the first observation by using autonomous underwater vehicle

We developed an underwater gravimeter for exploration of a seafloor hydrothermal deposit. Our hybrid gravimeter system consists of an underwater gravimeter and an underwater gravity gradiometer. We adopted Micro-g LaCoste S-174 as a gravity sensor. The sensor is mounted on a gimbal mechanism with a fiber gyroscope. A titanium sphere contains the sensor system. Maximum depth rating is 4,200 m. The data are sent to a recording system housed in another cylinder-shape capsule. The whole system is controlled and monitored via acoustic link of the AUV. In September 2012, the first practical measurement in marine area was carried out by using JAMSTECs AUV URASHIMA to evaluate performance of the system. The gravimeter and gravity gradiometer were simultaneously mounted on the URASHIMA and the first measurement was performed in Sagami-Bay. From the survey, we obtained the gravity data and supplemental data for compensation of the gravity data with good quality. According to preliminary analyses, the resolution of the gravity data from the first practical measurement is estimated to reach 0.1 mgal.