Nobutatsu Mochizuki

K-Ar ages of the Auckland geomagnetic excursions

K-Ar age determinations were made on two monogenetic volcanoes in the Auckland volcanic field, New Zealand, which have recorded the Auckland geomagnetic excursions. For the Wiri volcano with the north-down intermediate paleomagnetic direction, five samples gave a weighted mean age of 27 ± 5 (1σ) ka. For the Hampton Park volcano with the west-up intermediate direction, three samples gave a weighted mean of 55 ± 5 (1σ) ka. Since these two K-Ar ages are distinguished at 2σ level, it is inferred that at least two geomagnetic excursions can be recognized in Auckland. The age of the Hampton Park is barely distinguished from the established age range of the Laschamp excursion (39–45 ka) at 2σ level. The age of the Wiri coincides with the age of c. 30 ka in which excursions have been found from sedimentary and volcanic records. The reported excursions from volcanic rocks show a VGP cluster in the central to northern Pacific region which is distinct from the VGP paths or clusters during polarity reversals.

Further K-Ar dating and paleomagnetic study of the Auckland geomagnetic excursions

Three different excursional paleomagnetic directions were reported from eight volcanoes of the Auckland volcanic field in New Zealand: north-down (ND) directions obtained from five volcanoes, west-up (WU) from two volcanoes, and south-up (SU) from one volcano. K-Ar ages have been reported for two of these volcanoes: 27±5 (1σ) ka for the Wiri volcano of the ND group and 55±5 ka for the Hampton Park volcano of the WU group. In the present study, we have carried out further K-Ar age determinations on three other volcanoes and obtained reliable ages for two of them: 30±5 ka for the Puketutu volcano of the ND group and 50±6 ka for the McLennan Hills volcano of the SU group. The age of Puketutu agrees well with that of Wiri, and these two ages give a weighted mean age of 29±3 (1σ) ka for the ND group. The age of the ND group is distinguishable from those of the SU and WU groups at the 2σ level, confirming that excursions occurred at two different times separated by a few tens of thousands of years. The age of the SU group is indistinguishable from that of the WU group, and a weighted mean age of 53±4 ka can be calculated for this combined group (SU-WU group). The age of the ND group and that of the SU-WU group are distinguishable from the latest age estimate of the Laschamp excursion. Overall, these age data from volcanic rocks show that at least three excursions occurred between approximately 30 and 60 ka. These three excursions are likely to be confined in the weak dipole interval of 20–70 ka, and all of these excursions yield particularly low virtual dipole moments (VDMs) of 2×1022 A m2 or less. Since it is suggested that the larger virtual geomagnetic pole (VGP) deviations from the geographic pole are related to the lower VDMs, the excursional fields possibly have resulted from a significantly reduced dipole field and comparable non-dipole components.

High‐resolution magnetic signature of active hydrothermal systems in the back‐arc spreading region of the southern Mariana Trough

High-resolution vector magnetic measurements were performed on five hydrothermal vent fields of the back-arc spreading region of the southern Mariana Trough using Shinkai 6500, a deep-sea manned submersible. A new 3-D forward scheme was applied that exploits the surrounding bathymetry and varying altitudes of the submersible to estimate absolute crustal magnetization. The results revealed that magnetic-anomaly-derived absolute magnetizations show a reasonable correlation with natural remanent magnetizations of rock samples collected from the seafloor of the same region. The distribution of magnetic-anomaly-derived absolute magnetization suggests that all five andesite-hosted hydrothermal fields are associated with a lack of magnetization, as is generally observed at basalt-hosted hydrothermal sites. Furthermore, both the Pika and Urashima sites were found to have their own distinct low-magnetization zones, which could not be distinguished in magnetic anomaly data collected at higher altitudes by autonomous underwater vehicle due to their limited extension. The spatial extent of the resulting low magnetization is approximately 10 times wider at off-axis sites than at on-axis sites, possibly reflecting larger accumulations of nonmagnetic sulfides, stockwork zones, and/or alteration zones at the off-axis sites.

Developing near-bottom magnetic measurements using a 3D forward modeling technique: Application to hydrothermal vent fields

We developed a method for mapping the magnetic structure of absolute magnetization on the seafloor using a three-dimensional forward modeling technique and near-bottom vector magnetic data. We constrained the spatial scale of hydrothermal alteration zones in a back-arc spreading axis of the Southern Mariana Trough. The magnetic signatures of hydrothermal alteration zones are characterized by low magnetization similar to that in mid-ocean ridge vent sites.

Gamma Ray Doses in Water Around Sea Floor Hydrothermal Area in the Southern Mariana Trough

The dose rates in sea water in the Southern Mariana Trough sea floor hydrothermal area were investigated. The dose rate in sea water was less than detection limit (about 0.1 mGy/year) near the sea surface while it was around 2 mGy/year at the sea floor of hydrothermal area, where those levels of dose rates will be negligible for ESR (electron spin resonance) dating of barite of sulfide deposits. During the dive by a submarine vehicle, there were three locations at which the level of dose rate increases more than ten times as much as the usual level. One was very close to a hydrothermal vent, but no hydrothermal activities were found near the other locations.

Examination of Volcanic Activity: AUV and Submersible Observations of Fine-Scale Lava Flow Distributions Along the Southern Mariana Trough Spreading Axis

A high-resolution acoustic investigation using the AUV Urashima has revealed detailed volcanic and tectonic features along the neo-volcanic zone of the intermediate-rate spreading Southern Mariana Trough, where the high magma flux forms fast-spreading type axial high morphology. Side-scan sonar imagery suggests that the survey area mainly consists of two types of terrain: high-backscattering lumpy terrain occupies the majority of the neo-volcanic zone, and low-backscattering terrain is scattered over the entire area to form various bathymetric features. Visual observations by the submersible Shinkai 6500 show that the former corresponds to bulbous pillow lava and the latter to jumbled or wrinkled sheet lavas. The estimated proportion of sheet lava with respect to study area is approximately 10 %. Pillow lavas are flatly distributed and do not form the pillow mounds that are common in the slow-spreading Mid-Atlantic Ridge. Furthermore, we did not observe any pillars, collapse features, or axial summit troughs, all of which are frequently reported in the fast-spreading East Pacific Rise.

Composition law of oblique anhysteretic remanent magnetization and its relation to the magnetostatic interaction

The basic properties of oblique anhysteretic remanent magnetization (OARM) acquired in a weak and steady magnetic field with an arbitrary angle to the alternating field direction were studied. OARM and rock-magnetic experiments were conducted on samples of basalt, granite, and sediment containing non-interacting single-domain (SD), interacting SD, pseudo-single-domain, and multidomain low-Ti titanomagnetites. The intensity of OARM (MOARM) systematically increased or decreased with increasing angle between alternating and steady field directions (θSF), while the angle between alternating field and OARM directions (θOARM) increased with increasing θSF for all samples. During stepwise alternating field demagnetization, the OARM vector shows a single component parallel to the steady field direction for θSF = 0° (ARM||) and 90° (ARM⊥). The median destructive field of ARM⊥ is larger than that of ARM||. For intermediate angles (θSF = 30°, 45°, and 60°), the OARM vector was not parallel to the applied steady field; instead, it gradually increased with coercivity. These experiments indicate that the OARM vector is approximately given by the sum of two orthogonal magnetizations coinciding with ARM|| and ARM⊥, respectively. Thus, the OARM vector can be determined by acquisition efficiencies of ARM|| and ARM⊥ in an individual sample. Based on these experiments and associated rock-magnetic measurements, non-interacting SD samples show lower ARM⊥/ARM|| ratios, compared to other samples. This result suggests that OARM can be used as a conventional tool to detect non-interacting SD particles in the paleomagnetic samples.

Large‐volume lateral magma transport from the Mull volcano: An insight to magma chamber processes

Long-distance lateral magma transport within the crust has been inferred for various magmatic systems including oceanic island volcanoes, mid-oceanic ridges, and large igneous provinces. However, studying the physical and chemical properties of active fissure systems is difficult. Hence, this study investigates the movement of magma away from the Mull volcano in the North Atlantic Igneous Province, where erosion has exposed its upper crustal dike networks. Magmatic lineations within dikes indicate that the magma flow in the Mull dike suite changed from near vertical to horizontal within 30 km of the volcanic center. This implies that distal dikes were fed by lateral magma transport from Mull. Geochemical characteristics indicate that many <50 km long dikes have deep crustal signatures, reflecting storage and assimilation in Lewisian basement. Following crystallization and assimilation in the lower crust, magma fed an upper crustal reservoir, where further fractionation and incorporation of Moinian rocks generated felsic compositions. Distal dikes are andesitic and reflect events in which large volumes of mafic and felsic magma were combined by mixing between lower and upper crustal reservoirs to generate the 30–80 km3 required to supply the long-distance dikes. Once propagated, compositions along dikes were not significantly affected by assimilation and crystallization. Supplying the distal dikes with magma would have required a large-scale evacuation of the crustal reservoirs that acted as a potential trigger for explosive volcanism and the caldera formation recorded in Mull central complex.

Brief Report of Side-Scan Sonar Imagery Observations of the Archaean, Pika, and Urashima Hydrothermal Sites

A high-resolution acoustic investigation using AUV-Urashima reveals the geological features of three off-axis hydrothermal sites at the Southern Mariana Trough. They are developed ~5 km in distance from backarc spreading axis. The Archaean site is developed at the foot of axial horst and forms 60 m-high mound. Hydrothermal chimneys in the site are arrayed along the ridge of the mound. Our acoustic observations detect small structures along the ridge which could be interpreted as the chimneys. To the south of the Archaean mound, the seafloor is characterized by rough and elongated fabrics approximately trending in NE–SW direction, which coincides with the strike of background seafloor slope. Visual observation indicates that the area consists of lava tube slightly covered by sediment. The Pika and the Urashima sites are developed on top and at foot of a ~1,800 m-high off-axis knoll. Unprocessed sidescan sonar imagery above the Pika and the Urashima sites shows anomalous backscattering signatures in water column. A series of hills with convex shape develops on the southwestern slope of the off-axis knoll. It shows unique facies that is rough surface with high-backscattering intensity on the sonar imagery. Tube lavas are recognized in corresponding seafloor by visual observation.

Discovery and characterization of a new hydrothermal vent based on magnetic and acoustic surveys

A new hydrothermal vent site in the southern Mariana Trough has been discovered using acoustic and magnetic surveys. Two stage surveys by the autonomous underwater vehicle (AUV) Urashima (Japan Agency for Marine-Earth Science and Technology, JAMSTEC) and the manned submersible Shinkai 6500 (JAMSTEC) were conducted in 2009 and 2010, respectively. In the first stage, we detected a clear magnetization low that extends from a previously known hydrothermal vent site, the Pika site, by using a vector magnetometer attached to the AUV. The acoustic signals suggest the presence of hydrothermal plumes emanating from the seafloor within the water column of the 120kHz side scan sonogram; 10 m scale chimney like structures in the 400-kHz multibeam bathymetry data were also detected in the area of low magnetization. These observations strongly suggest the presence of an unseen hydrothermal vent. The subsequent submersible dive discovered a new vent site, which was named the Urashima site [1]. The calculated absolute magnetization using the submersible-attached magnetometer succeeded in estimating the extent of hydrothermally altered zone for both the Pika and Urashima sites. It should be emphasized that the search for a hydrothermal vent site was a direct consequence of the geophysical surveys undertaken in the first stage. Our method can bridge the gap between conventional hydrothermal plume surveys (resolution on a scale of 103 m) and visual/photographic surveys (resolution on a scale of 100 m) and can precisely delineate the hydrothermally altered zone.