We have developed a high-resolution scanning superconducting quantum interference device (SQUID) microscope for imaging the magnetic field of geological samples at room temperature. In this paper, we provide details about the scanning SQUID microscope system, including the magnetically shielded box (MSB), the XYZ stage, data acquisition by the system, and initial evaluation of the system. The background noise in a two-layered PC permalloy MSB is approximately 40-50 pT. The long-term drift of the system is approximately ≥1 nT, which can be reduced by drift correction for each measurement line. The stroke of the XYZ stage is 100 mm × 100 mm with an accuracy of ~10 µm, which was confirmed by laser interferometry. A SQUID chip has a pick-up area of 200 μm × 200 μm with an inner hole of 30 μm × 30 μm. The sensitivity is 722.6 nT/V. The flux-locked loop has four gains, i.e., ×1, ×10, ×100, and ×500. An analog-to-digital converter allows analog voltage input in the range of about ±7.5 V in 0.6-mV steps. The maximum dynamic range is approximately ±5400 nT, and the minimum digitizable magnetic field is ~0.9 pT. The sensor-to-sample distance is measured with a precision line current, which gives the minimum of ~200 µm. Considering the size of pick-up coil, sensor-to-sample distance, and the accuracy of XYZ stage, spacial resolution of the system is ~200 µm. We developed the software used to measure the sensor-to-sample distance with line scan data, and the software to acquire data and control the XYZ stage for scanning. We also demonstrate the registration of the magnetic image relative to the optical image by using a pair of point sources placed on the corners of a sample holder outside of a thin section placed in the middle of the sample holder. Considering the minimum noise estimate of the current system, the theoretical detection limit of a single magnetic dipole is ~1 × 10 −14 Am 2 . The new instrument is a powerful tool that could be used in various applications in paleomagnetism such as ultrafine-scale magnetostratigraphy and single-crystal paleomagnetism.

12Hydrogenetic ferromanganese crusts are iron-manganese oxide chemical 13 precipitates on the seafloor that grow over periods of tens of millions of years. Their 14 secular records of chemical, mineralogical, and textural variations are archives of deep-15 sea environmental changes. However, environmental reconstruction requires reliable 16 high-resolution age dating. Earlier chronological methods using radiochemical and stable 17 isotopes provided age models for ferromanganese crusts, but have limitations on the 18 millimeter scale. For example, the reliability of 10 Be/ 9 Be chronometry, commonly 19 considered the most reliable technique, depends on the assumption that the production 20 and preservation of 10 Be are constant, and requires accurate knowledge of the 10 Be half-21 life. To overcome these limitations, we applied an alternative chronometric technique, 22 2 magnetostratigraphy, to a 50-mm-thick hydrogenetic ferromanganese crust (D96-m4) 23 from the northwest Pacific. Submillimeter-scale magnetic stripes originating from 24 approximately oppositely magnetized regions oriented parallel to bedding were clearly 25 recognized on thin sections of the crust using a high-resolution magnetometry technique 26 called scanning SQUID (superconducting quantum interference device) microscopy. By 27 correlating the boundaries of the magnetic stripes with known geomagnetic reversals, we 28 determined an average growth rate of 5.1 ± 0.2 mm/m.y., which is within 16% of that 29 deduced from 10 Be/ 9 Be method (6.0 ± 0.2 mm/m.y.). This is the finest-scale 30 magnetostratigraphic study of a geologic sample to date. Ultrafine-scale 31 magnetostratigraphy using SQUID microscopy is a powerful new chronological tool for 32 estimating ages and growth rates for hydrogenetic ferromanganese crusts. It provides 33 chronological constraints with the accuracy promised by the astronomically calibrated 34 magnetostratigraphic time scale (1-40 k.y.). 35 INTRODUCTION 36

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We estimated time scales of magma-mixing processes just prior to the 2011 sub-Plinian eruptions of Shinmoedake volcano to investigate the mechanisms of the triggering processes of these eruptions. The sequence of these eruptions serves as an ideal example to investigate eruption mechanisms because the available geophysical and petrological observations can be combined for interpretation of magmatic processes. The eruptive products were mainly phenocryst-rich (28 vol%) andesitic pumice (SiO 2 57 wt%) with a small amount of more silicic pumice (SiO 2 62-63 wt%) and banded pumice. These pumices were formed by mixing of low-temperature mushy silicic magma (dacite) and high-temperature mafic magma (basalt or basaltic andesite). We calculated the time scales on the basis of zoning analysis of magnetite phenocrysts and diffusion calculations, and we compared the derived time scales with those of volcanic inflation/deflation observations. The magnetite data revealed that a significant mixing process (mixing I) occurred 0.4 to 3 days before the eruptions (preeruptive mixing) and likely triggered the eruptions. This mixing process was not accompanied by significant crustal deformation, indicating that the process was not accompanied by a significant change in volume of the magma chamber. We propose magmatic overturn or melt accumulation within the magma chamber as a possible process. A subordinate mixing process (mixing II) also occurred only several hours before the eruptions, likely during magma ascent (syn-eruptive mixing). However, we interpret mafic injection to have begun more than several tens of days prior to mixing I, likely occurring with the beginning of the inflation (December 2009). The injection did not instantaneously cause an eruption but could have resulted in stable stratified magma layers to form a hybrid andesitic magma (mobile layer). This hybrid andesite then formed the main eruptive component of the 2011 eruptions of Shinmoedake.