An atomic level study of rhenium and radiogenic osmium in molybdenite

Local atomic structures of Re and radiogenic Os in molybdenite from the Onganja mine, Namibia, were examined using X-ray absorption fine structure (XAFS). Rhenium L_III-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) show that the oxidation state of Re, the interatomic distances between Re and the neighboring atoms, and the coordination number of Re to S are very similar to those of Mo in molybdenite. The results confirm that Re is present as Re(IV) in the Mo site in molybdenite.Measurement of L_III-edge XANES and EXAFS of a minor concentration (8.55 ppm) of radiogenic Os was accomplished in fluorescence mode by removing the interfering X-rays from Re and other elements using a crystal analyzer system. The data indicate that the oxidation state of radiogenic Os is Os(III) and Os(IV) and clearly different from Os(II) in natural sulfide minerals, such as OsS₂ (erlichmanite). XANES data also suggest that radiogenic Os does not form a secondary Os phase, such as OsS₂ or Os metal, in molybdenite.EXAFS of radiogenic Os was successfully simulated assuming that Os is present in the Mo site in molybdenite. The data are consistent with the XANES data; Os does not form Os phases in molybdenite. The EXAFS simulation showed that the interatomic distance between Os and S is 2.27 Å, which is 0.12 Å smaller than the distances of Re-S and Mo-S (2.39 Å) in molybdenite. Similar valences and ionic sizes of Re and Mo in molybdenite support the fact that large amounts of Re can be incorporated into the Mo site as has been observed in previous studies, whereas the different properties of Os compared to Mo and Re suggested here support much lower abundance of common Os in molybdenite. This makes molybdenite an ideal mineral for the Re-Os geochronometer as shown in many studies. However, the shorter distance between radiogenic Os and S compared to those of Re-S and Mo-S in molybdenite suggests that the radiogenic Os has a smaller ionic size than Re(IV) and Mo(IV). Furthermore, Os may be partly present as Os(III). Smaller and lower charge Os can diffuse faster than larger and higher charge Re in molybdenite at a given set of conditions. Hence, our study provides an atomic-level explanation for the high mobility of Os compared to Re, which has been suggested by earlier workers using laser ablation ICP-MS.     

Occurrence of arsenic (V) in forearc mantle serpentinites based on X-ray absorption spectroscopy study

We examined As concentration, mineralogical site, and oxidation state in the serpentinites associated with the Tso Morari eclogitic rocks in the Indus suture zone, northwest Himalaya, to examine how highly fluid-soluble elements like As are transferred from slabs to arc magmas in subduction zones. The serpentinite samples represent hydrated peridotites at the base of the mantle wedge beneath Eurasia, and were exhumed from a depth of ∼100 km during subduction of the Indian continental margin. The bulk serpentinite samples contain total As concentrations ranging from 6 to 275 ppm. Arsenic in the samples is most likely present with magnetite and antigorite, since electron probe analysis yielded up to 90 ppm As in these minerals. X-ray absorption near-edge structure spectra indicate that As in serpentinites is mostly As(V) and that the neighboring atoms of As are O, although there are minute grains of sulfides and arsenides. The ratio of As(V) to total As is greater in samples with higher As contents, suggesting that the major source of As was oxidized As(V), introduced to the mantle wedge. Arsenic(V), originally adsorbed on Fe-oxides in slabs and overlying sediments, was most likely liberated during their subduction and incorporated subsequently in the overlying mantle wedge. Our data constrain the introduction of As into the mantle wedge at relatively shallow levels, much shallower than 25 km. Arsenic incorporated in the serpentinites was transported to deeper levels by mantle flow downward along the subduction zone, to be subsequently exhumed together with eclogitic rocks.     

Fate of carbonates within oceanic plates subducted to the lower mantle, and a possible mechanism of diamond formation

We report on high-pressure and high-temperature experiments involving carbonates and silicates at 30–80 GPa and 1,600–3,200 K, corresponding to depths within the Earth of approximately 800–2,200 km. The experiments are intended to represent the decomposition process of carbonates contained within oceanic plates subducted into the lower mantle. In basaltic composition, CaCO₃ (calcite and aragonite), the major carbonate phase in marine sediments, is altered into MgCO₃ (magnesite) via reactions with Mg-bearing silicates under conditions that are 200–300°C colder than the mantle geotherm. With increasing temperature and pressure, the magnesite decomposes into an assemblage of CO₂ + perovskite via reactions with SiO₂. Magnesite is not the only host phase for subducted carbon—solid CO₂ also carries carbon in the lower mantle. Furthermore, CO₂ itself breaks down to diamond and oxygen under geotherm conditions over 70 GPa, which might imply a possible mechanism for diamond formation in the lower mantle.     

Spatial and Temporal Variability of Satellite Primary Production in the Japan Sea from 1998 to 2002

The spatial and seasonal variability of primary production in the Japan Sea from 1998 to 2002 was estimated using a satellite primary production model. A size-fractionated primary production model was validated by in situ primary production data measured in the Japan Sea. Estimated primary production and in situ primary production showed a good positive correlation. Estimated primary production showed spatial variability. Annual primary production levels were 170, 161, 191 and 222 gC m⁻²year⁻¹ at the Russian coast, in the middle of the Japan Basin, the southeastern area and the southwestern area, respectively. It was higher to the south around 40°N than to the north, and higher in the western area than in the eastern one. Peaks of primary production appeared twice, in spring and fall, in the southern area, while a single peak appeared in the northern area. Primary production along the Russian coast was higher than in other areas during summer. The spring bloom contributed 42% to the annual primary production in these four areas. Furthermore, estimated primary production showed an interannual variability that was largest in spring. Primary production in fall also showed interannual variability, especially in the middle of the Japan Basin and the southwestern area. This corresponded mainly to the size of the phytoplankton bloom in each year. Winter convection by wind and the depth of nutrient-rich, cold subsurface water underlying the Tsushima Current may contribute to the nutrient supply to upper layer and interannual variations of primary production in spring.     

Seagrasses from the Nansei Islands, Southern Japanese Archipelago: species composition, distribution and biogeography

The Nansei Islands in the southern Japanese Archipelago have 15 taxa of seagrasses from seven genera within three families. Seagrasses in this region grow on coral sands or coral debris in shallow reefs and on sandy or muddy substrata in the shallow areas of bays and inlets. Certain Halophila species grow in deep water off some islands. Enhalus acoroides only reaches to Ishigaki I. with winter sea water temperature (WST) at 23 °C, while Okinawa I. (WST at 21.6 °C) is the northern biogeographic limit for Halophila decipiens, H. okinawensis, H. major and H. gaudichaudii. Amami‐oshima I. (WST at 20.7 °C) is the northern border for Thalassia hemprichii, H. minor, H. ovalis, Cymodocea serrulata, Cymodocea rotundata, Syringodium isoetifolium, Halodule uninervis and Halodule pinifolia. Halophila mikii the sole seagrass collected from Yakushima I. (WST at 19.3 °C), is of volcanic origin. The distribution of tropical seagrasses in the Nansei Islands is clearly associated with the warm Kuroshio Current, WST and habitat availability. Zostera japonica is the only temperate species occurring in the region. Meadows of Z. japonica, H. ovalis and Halodule pinifolia have disappeared from certain localities in the Archipelago, due probably to human activities and natural siltation.     

Seasonal and interannual variability of sea surface chlorophyll a concentration in the Japan/East Sea (JES)

Seasonal and interannual variability of chlorophyll a concentration in the Japan/East Sea (JES) was detected spatially by ocean color satellite remote sensing. Start timing of the spring bloom was different spatially. The spring bloom started at the subpolar front and southward of it in March, northward of subpolar front, along the Primorye coast and off Hokkaido in April and in the middle of the Japan Basin in May. The start of the spring bloom showed interannual variability that corresponded with the wind speed in the area. The spring bloom in 1998 and 2002 appeared about four weeks earlier than in 1997, 1999 and 2001, and it corresponded with weak winds that can lead to an early development of the thermocline. The bloom was late in 1999 and 2001 in the Japan Basin and along the Primorye coast, and in the southern area in 2000. It corresponded with stronger wind stress that delayed seasonal thermocline formation. The bloom along the Primorye coast appeared later in 1999, and it corresponded with stronger wind stress, and at the same time, it seemed to be related with the delay of melting of sea ice in Mamiya Strait. The fall bloom appeared from early October to early December, and it did not have a clear temporal transition. The area where chlorophyll a concentration exceeded 0.8 μg l⁻¹ was wider in the western area than in the eastern area every year. The magnitude of the fall bloom was different between years, but it did not show a correlation with average wind speed in fall. Those results indicated that the timing of the seasonal bloom in the JES is largely affected by the variability of global climate such as ENSO events.     

A model of tuned liquid damper for suppressing pitching motions of structures

A tuned liquid damper (TLD), which consists of rigid tanks partially filled by liquid, is a type of passive control device relying upon liquid sloshing forces or moments to change the dynamical properties and to dissipate vibrational energy of a structure. An analytical non-linear model is proposed for a TLD using rectangular tanks filled with shallow liquid under pitching vibration, utilizing a shallow water wave theory. The model includes the linear damping of the sloshing liquid, which is an important parameter in the study of a TLD as it affects the efficiency of the TLD. Shaking table experiments were conducted for verification; good agreement between the analytical simulations and the experimental results was observed in a small excitation amplitude range. The simulations of TLD-structure interaction by using the proposed model show that the TLD can efficiently suppress resonant pitching vibration of a structure. It is also found that the effectiveness of a TLD for suppressing the pitching vibration depends not only on the mass of liquid in the TLD but also on the configuration of the liquid as well as upon the position where the TLD is located. If the configuration of the liquid, i.e. the liquid depth and the TLD tank size, is designed suitably, the TLD can have a large suppressing moment and can be very effective even with a small mass of liquid.     

Plastic deformation of garnets: systematics and implications for the rheology of the mantle transition zone

Plastic properties of materials with garnet structure have been studied under wide temperature conditions, ranging from room temperature to 95% of the melting temperatures, using uniaxial compression and hot microhardness tests. Garnets studied include single crystals of oxide garnets (Y₃Al₅O₁₂, Gd₃Ga₅O₁₂ and Y₃Fe₅O₁₂) and silicate garnets (various solid solutions, including grossular, almandine, andradite, pyrope, spessartine and uvarovite). Both uniaxial compression and hot hardness tests indicate that there is a general trend in the plasticity of garnets when the data are compared at normalized conditions (T/T_m andσ/μ), and that the resistance to plastic deformation in garnets is significantly higher than most of the other minerals in the Earth's mantle. Based on both stress-dip tests and microstructural observations, it is proposed that the creep strength of garnet is largely controlled by the resistance to dislocation glide rather than by recovery processes. This conclusion is consistent with the high Peierls stress inferred from the hot hardness tests. The high Peierls stress in garnets is, presumably, due to the large unit cell (i.e., long Burgers vectors) and/or the bcc packing, which are common to all garnets. We postulate, therefore, that the present results can be applied to the strength of high-pressure garnet (majorite) and suggest that garnet-rich layers in the Earth, such as subducted oceanic crust in the transition zone or a possibly garnet-rich (bottom part of the) transition zone, will be considerably stronger than surrounding regions.     

Titanochondrodite and titanoclinohumite derived from the upper mantle in the Buell Park Kimberlite,Arizona, USA

Kimberlite from Buell Park, Arizona, which was intruded into Permian sediments about 30 m.y. ago, is characterized by the hydrous silicates titanochondrodite and titanoclinohumite. Titanochondrodite is the first finding in kimberlites. Optical properties, chemical compositions and cell dimensions of these two minerals are determined.Titanochondrodite and titanoclinohumite are considered to crystallize from kimberlite magma at a depth of about 100 km and at 1,000° C, on the basis of kimberlite mineralogy, petrography and high pressure experimental work. Although there is no direct evidence, the importance of these two minerals in peridotite wedges above sinking lithosphere at the continental margins is also discussed.