Internal deformation of Sikhote Alin volcanic belt, far eastern Russia: Paleocene paleomagnetic results

We present paleomagnetic results of Paleocene welded tuffs of the 53–50 Ma Bogopol Group from the northern region (46°N, 137°E) of the Sikhote Alin volcanic belt. Characteristic paleomagnetic directions with high unblocking temperature components above 560 °C were isolated from all the sites. A tilt-corrected mean paleomagnetic direction from the northern region is D=345.8°, I=49.9°, α₉₅=14.6° (N=9). The reliability of the magnetization is ascertained through the presence of normal and reversed polarities. The mean paleomagnetic direction from the northern region of the Sikhote Alin volcanic belt reflects a counterclockwise rotation of 29° from the Paleocene mean paleomagnetic direction expected from its southern region. The counterclockwise rotation of 25° is suggested from the paleomagnetic data of the Kisin Group that underlies the Bogopol Group. These results establish that internal tectonic deformation occurred within the Sikhote Alin volcanic belt over the past 50 Ma. The northern region from 44.6° to 46.0°N in the Sikhote Alin volcanic belt was subjected to counterclockwise rotational motion through 29±17° with respect to the southern region. The tectonic rotation of the northern region is ascribable to relative motion between the Zhuravlevka terrane and the Olginsk–Taukhinsk terranes that compose the basements of the Sikhote Alin volcanic belt.     

Paleomagnetic evidence for the clockwise rotation of Southwest Japan

Over 500 oriented samples of felsic rocks of Cretaceous to Middle Miocene age were collected along the Go¯River in the central part of Southwest Japan, in an attempt to detect the process of tectonic rotation of Southwest Japan from the paleomagnetic view point. Thermal demagnetization was successful in isolating characteristic directions from the remanent magnetization of samples. Reliability of the paleomagnetic direction is ascertained through the agreement of directions from different kinds of rocks as well as the presence of both normal and reversed polarities. The paleomagnetic results establish that Southwest Japan began to rotate clockwise through58 ± 14° later than 28 Ma and ceased its motion by about 12 Ma. Southwest Japan has undergone no detectable north-south translation since 28 Ma. These results imply that southwest Japan was rotated about the pivot around 34°N, 129°E between 28 Ma and 12 Ma in association with the opening of the Japan Sea.     

Characteristics and implication of clay minerals in the northern and southern parts of the Chelung-pu fault, Taiwan

In this study, we report the characteristics of clay minerals present in the Chelung-pu fault in Taiwan. In the 1999 Chi-Chi earthquake, different slip motions were recorded along the Chelung-pu fault in the northern and southern parts of the fault. The characteristics of clay minerals present in the fault zones can be attributed to the differences in motion. We analyzed the shallow drill core samples obtained from the northern and southern sites penetrating the fault. The clay minerals identified in most of the samples from both the sites are smectite, illite, and chlorite. There are illite–smectite mixed layers with a high illite content and no chlorite–smectite mixed layer. In some samples, no smectite is detected. We also examined the iron content and symmetry of iron and magnesium in the silicate and hydroxide layers in chlorite. At the northern site, the total iron content in chlorite of gouge is larger than that of the host rocks. On the other hand, at the southern site, the total iron content varies widely. It is hypothesized that the smectite consumption and differences in the characteristics of chlorite may be controlled by the differences in the lithology, fluid chemistry, fluid temperature, or fault activities (heating or breakage) between the northern and southern sites.     

Thermal alteration of hydrated minerals during hypervelocity capture to silica aerogel at the flyby speed of Stardust

Abstract— Outside the Earth's atmosphere, silica aerogel is one of the best materials to capture finegrained extraterrestrial particles in impacts at hypervelocities. Because silica aerogel is a superior insulator, captured grains are inevitably influenced by frictional heat. Therefore, we performed laboratory simulations of hypervelocity capture by using light‐gas guns to impact into aerogels finegrained powders of serpentine, cronstedtite, and Murchison CM2 meteorite. The samples were shot at >6 km s^?1 similar to the flyby speed at comet P/Wild‐2 in the Stardust mission. We investigated mineralogical changes of each captured particle by using synchrotron radiation X‐ray diffraction (SR‐XRD), transmission electron microscope (TEM), and field emission scanning electron microscope (FE‐SEM). SR‐XRD of each grain showed that the majority of the bulk grains keep their original mineralogy. In particular, SR‐XRD and TEM investigations clearly exemplified the presence of tochilinite whose decomposition temperature is about 300 °C in the interior of the captured Murchison powder. However, TEM study of these grains also revealed that all the samples experienced melting and vesiculation on the surface. The cronstedtite and the Murchison meteorite powder show remarkable fracturing, disaggregation, melting, and vesiculation. Steep thermal gradients, about 2500 °C/μm were estimated near the surface of the grains (<2 μm thick) by TEM observation. Our data suggests that the interior of >4 μm across residual grains containing abundant materials that inhibit temperature rise would have not experienced >300 °C at the center.     

Infrared spectroscopic taxonomy for carbonaceous chondrites from speciation of hydrous components

Abstract— Mid‐infrared absorption spectra for all types of carbonaceous chondrites were obtained in this study to establish a versatile method for spectroscopic classification of carbonaceous chondrites. Infrared spectra were measured using a conventional KBr pellet method and diamond press method. Spectra of hydrous carbonaceous chondrites exhibit intense O‐H stretching vibrations. CI chondrites are identifiable by a characteristic sharp absorption band appearing at 3685 cm^?1, which is mainly attributable to serpentine. X‐ray diffraction analysis showed the presence of serpentine. However, Yamato (Y‐) 82162 (C1) does not have the band at 3685 cm^?1 because of its thermal metamorphism. CM and CR chondrites have an intense absorption band at approximately 3600 cm^?1. This absorption tends to appear in CM chondrites more strongly than CR chondrites because the intensity ratios of an OH stretching mode at 3520 cm^?1 compared to 3400 cm^?1 for CM chondrites are in the range of 0.95–1.04, which is systematically higher than those of CR chondrites (0.86–0.88). Therefore, the two types of chondrites are distinguishable by their respective infrared spectra. The spectrum feature of the Tagish Lake meteorite is attributable to neither CI nor CM chondrites. CO chondrites are characterized by weak and broad absorption at 3400 cm^?1. CV chondrites have weak or negligible absorption of water. CK chondrites also have no water‐induced absorption. CH and CB chondrites have a sharp absorption at 3692 cm^?1 indicating the presence of chrysotile, which is also supported by observations of X‐ray diffraction and TEM. The combination of spectroscopic classification and the diamond press method allows classification of carbonaceous chondrites of very valuable samples with small quantities. As one example, carbonaceous chondrite clasts in brecciated meteorites were classified using our technique. Infrared spectra for a fragment of carbonaceous clasts (<1 μg) separated from Willard (b) and Tsukuba were measured. The 3685 cm^?1 band found in CI chondrites was clearly detected in the clasts, indicating that they are CI‐like clasts.     

Mass dispersal and angular momentum transfer during collisions between rubble-pile asteroids. II. Effects of initial rotation and spin-down through disruptive collisions

Expanding on our previous N-body simulation of impacts between initially non-rotating rubble-pile objects [Takeda, T., Ohtsuki, K., 2007. Icarus 189, 256-273], we examine effects of initial rotation of targets on mass dispersal and change of spin rates. Numerical results show that the collisional energy needed to disrupt a rubble-pile object is not sensitive to initial rotation of the target, in most of the parameter range studied in our simulations. We find that initial rotation of targets is slowed down through disruptive impacts for a wide range of parameters. The spin-down is caused by escape of high-velocity ejecta and asymmetric re-accumulation of fragments. When these effects are significant, rotation is slowed down even when the angular momentum added by an impactor is in the same direction as the initial rotation of the target. Spin-down is most efficient when the impact occurs in the equatorial plane of the target, because in this case most of the ejected fragments originate from the equatorial region of the target and a significant amount of angular momentum can be easily removed. In the case of impacts from directions inclined relative to the target's equatorial plane, spin-down still occurs with reduced degree, unless impacts occur onto the pole region from the vertical direction. Our results suggest that such spin-down through disruptive impacts may have played an important role in spin evolution of asteroids through collisions in the gravity-dominated regime.     

Thermal structure and paleo-heat flow in the Shimanto accretionary prism, Southwest Japan

Abstract Thermal structural analysis and paleo-heat flow estimation provide clues to understanding the thermal evolution of the accretionary complex. The thermal structure and heat flow in the Jurassic Chichibu and Cretaceous to Tertiary Shimanto accretionary complex, Southwest Japan, have been investigated by vitrinite reflectance measurement and fluid inclusion analysis. As a result, the local and multistage metamorphisms were recognized as follows. First, the Tertiary complex around the Miocene Ashizuri granite underwent exposure to extra-high temperatures. Second, the Okitsu Melange underwent exposure to higher temperatures than the surrounding strata and was formed concurrently with the Kula-Pacific ridge subduction beneath the Japanese Islands in the Eocene. Finally, the thermal structure of most of the Cretaceous and southern Jurassic complexes is independent of the geologic structure, indicating that these areas suffered thermal overprint. Regional radiometric dating studies show that most of the Cretaceous Shimanto complex was heated in the Eocene; the thermal overprint might have occurred as a result of ridge subduction. The heat flow during peak heating was estimated to be 95–120 mW/m² except for the Cretaceous Okitsu melange and the Cretaceous Nonokawa formation, north of the Okitsu Melange; a much higher value of heat flow of ~200 mW/m² was estimated in the Okitsu Melange. An estimation of heat flow failed for the non-okawa formation because thermal equilibrium between the fluid and rocks has not yet been reached. It is probable that the southern strata underwent a higher heat flow. Such a trenchward increase in heat flow resembles the present situation of the Nankai Trough, although the heat flow in the Eocene was much higher.     

Two extraterrestrial dust horizons found in the Dome Fuji ice core,East Antarctica

Two silicate-rich dust layers were found in the Dome Fuji ice core in East Antarctica, at Marine Isotope Stages 12 and 13. Morphologies, textures, and chemical compositions of constituent particles reveal that they are high-temperature melting products and are of extraterrestrial origin. Because similar layers were found ～ 2000 km east of Dome Fuji, at EPICA (European Project for Ice Coring in Antarctica)-Dome C, particles must have rained down over a wide area 434 and 481 ka. The strewn fields occurred over an area of at least 3 × 10⁶ km². Chemical compositions of constituent phases and oxygen isotopic composition of olivines suggest that the upper dust layer was produced by a high-temperature interaction between silicate-rich melt and water vapor due to an impact explosion or an aerial burst of a chondritic meteoroid on the inland East Antarctic ice sheet. An estimated total mass of the impactor, on the basis of particle flux and distribution area, is at least 3 × 10⁹ kg. A possible parent material of the lower dust layer is a fragment of friable primitive asteroid or comet. A hypervelocity impact of asteroidal/cometary material on the upper atmosphere and an explosion might have produced aggregates of sub-μm to μm-sized spherules. Total mass of the parent material of the lower layer must exceed 1 × 10⁹ kg. The two extraterrestrial horizons, each a few millimeters in thickness, represent regional or global meteoritic events not identified previously in the Southern Hemisphere.     

A 60-year record of rainfall from the sediments of Jinheung Pond,Jeongeup, Korea

This paper describes mean grain-size data from the ¹³⁷Cs- and ²¹⁰Pb-dated sediment core BS-3 (33-cm long) recovered from Jinheung Pond, located in the southwestern part of the Korean Peninsula. Grain-size analysis of the Jinheung Pond sediments shows a clear signal for changes in annual precipitation over the past 60 years. Instrumental records of annual precipitation (AP) and the annual summation of the precipitation of >50 mm per day (AP50), which reflects the energy available for sediment transport, correlate well with the mean grain-size distributions measured in the core. The most plausible mechanism for this response in mean grain size is variations in the annual amount and intensity of precipitation. Heavy precipitation enhances soil erosion over the catchment area and increases the transport capacity of streams and rivers. Thus, coarser mean grain size should reflect higher precipitation, and smaller mean grain size should reflect lower rainfall. In the data from core BS-3, however, grain-size peaks attributed to increased annual precipitation are not prominent. This is because a dam prevents removal of fine particles from the pond via the outflow. Therefore, the mean grain-size value represents somewhat larger sediments together with fine clays. The results of this study show that sediments of dammed lakes and ponds are well suited for high-resolution environmental investigations, especially for records of changes in precipitation over time.     

Experimental study on prevention of acid mine drainage by silica coating of pyrite waste rocks with amorphous silica solution

Acid mine drainage (AMD) is a widespread environmental problem associated with working and abandoned mining operations. It results from the microbial oxidation of pyrite in the presence of water and air, affording an acidic solution that contains toxic metal ions. Pyrite microencapsulation, utilizing silica coating, is a novel approach for controlling AMD that has been shown to be very effective in controlling pyrite oxidation. The roles of the solution pH and silica concentration in the formation mechanism for the AMD-preventing coating were investigated. A silica coating can be formed from silica solution at pH 7, at which the amount of Fe eluted from pyrite into the solution is small. No coating was formed at other pH values, and the amounts of eluted Fe were larger than at pH 7, especially at pH 11. The silica coating forms from 2,500 to 5,000 mg/L silica solutions, but not from 0 or 1,000 mg/L silica solutions. The coating formation rate was slower in the 2,500 mg/L silica solution than in the 5,000 mg/L silica solution. The formation of silica coating on pyrite surfaces depends on three main steps: formation of Fe(OH)₃ on the surface of pyrite, reaction between Fe(OH)₃ and silicate in the solution on the pyrite surface, and growth of the silica layer on the first layer of silica. The best pH condition to enable these steps was around 7, and the silica coating formation rate can be controlled by the concentration of silica.