Progressive alteration in CV3 chondrites: More evidence for asteroidal alteration

Abstract— The oxidized CV3 chondrites can be divided into two major subgroups or lithologies, Bali-like (CV3_oxB) and Allende-like (CV3_oxA), in which chondrules, calcium-aluminum-rich inclusions (CAIs) and matrices show characteristic alteration features (Weisberg et al, 1997; Krot et al, 1997d; Kimura and Ikeda, 1997). The CV3_oxB lithology is present in Bali, Kaba, parts of the Mokoia breccia and, possibly, in Grosnaja and Allan Hills (ALH) 85006. It is characterized by the presence of the secondary low-Ca phyllosilicates (saponite and sodium phlogopite), magnetite, Ni-rich sulfides, fayalite (Fa>₉₀), Ca-Fe-rich pyroxenes (Fs_10–50Wo_45–50) and andradite. Phyllosilicates replace primary Ca-rich minerals in chondrules and CAIs, which suggests mobilization of Ca during aqueous alteration. Magnetite nodules are replaced to various degrees by fayalite, Ca-Fe-rich pyroxenes and minor andradite. Fayalite veins crosscut fine-grained rims around chondrules and extend into the matrix. Thermodynamic analysis of the observed reactions indicates that they could have occurred at relatively low temperatures (<300 °C) in the presence of aqueous solutions. Oxygen isotopic compositions of the coexisting magnetite and fayalite plot close to the terrestrial fractionation line with large Δ¹⁸O_{fayalite-magnetite} fractionation (～20%). We infer that phyllosilicates, magnetite, fayalite, Ca-Fe-rich pyroxenes and andradite formed at relatively low temperatures (<300 °C) by fluid-rock interaction in an asteroidal environment. Secondary fayalite and phyllosilicates are virtually absent in chondrules and CAIs in the CV3_oxA lithology, which is present in Allende and its dark inclusions, Axtell, ALHA81258, ALH 84028, Lewis Cliff (LEW) 86006, and parts of the Mokoia and Vigarano breccias. Instead secondary nepheline, sodalite, and fayalitic olivine are common. Fayalitic olivine in chondrules replaces low-Ca pyroxenes and rims and veins forsterite grains; it also forms coarse lath-shaped grains in matrix. Secondary Ca-Fe-rich pyroxenes are abundant. We infer that the CV3_oxA lithology experienced alteration at higher temperatures than the CV3_oxB lithology. The presence of the reduced and CV3_oxA lithologies in the Vigarano breccia and CV3_oxA and CV3_oXB lithologies in the Mokoia breccia indicates that all CV3 chondrites came from one heterogeneously altered asteroid. The metamorphosed clasts in Mokoia (Krot and Hutcheon, 1997) may be rare samples of the hotter interior of the CV asteroid. We conclude that the alteration features observed in the oxidized CV3 chondrites resulted from the fluid-rock interaction in an asteroid during progressive metamorphism of a heterogeneous mixture of ices and anhydrous materials mineralogically similar to the reduced CV3 chondrites.     

黄海及其邻区深部结构特点与地质演化

根据黄海及其周边地区的布格重力资料，通过多种方法处理，得到有关断裂的信息并求取了研究区的地壳厚度分布. 经过与地震层析成像结果、地质资料的对比和综合分析，认为朝鲜半岛西缘断裂带和济州岛南缘断裂带均为深大断裂，断裂带的两侧速度结构存在较大差异. 推断朝鲜半岛和南黄海分别属于不同的地质单元. 根据对岩石层结构的综合分析，认为中朝与扬子块体在黄海海域的接触关系是扬子块体推覆于中朝块体之上. 从目前的地震层析成像、重力异常、地壳厚度分布等结果来看，还不足以判断扬子与华南块体结合带在黄海海域中的准确位置.     

Oxygen isotopes in magnetite and fayalite in CV chondrites Kaba and Mokoia

Abstract— We report in situ measurements of O‐isotopic compositions of magnetite and primary and secondary olivine in the highly unequilibrated oxidized CV chondrites Kaba and Mokoia. In both meteorites, the magnetite and the secondary olivine (fayalite, Fa_90–100) have O‐isotopic compositions near the terrestrial fractionation (TF) line; the mean Δ¹⁷O (= δ¹⁷O‐0.52 × δ¹⁸O) value is about ?1%‰. In contrast, the compositions of nearby primary (chondrule), low‐FeO olivines (Fa_1–2) are well below the TF line; Δ¹⁷O values range from ?3 to ?9%‰. Krot et al. (1998) summarized evidence indicating that the secondary phases in these chondrites formed by aqueous alteration in an asteroidal setting. The compositions of magnetite and fayalite in Kaba and Mokoia imply that the O‐isotopic composition of the oxidant was near or somewhat above the TF line. In Mokoia the fayalite and magnetite differ in δ¹⁸O by ～20%‰, whereas these same materials in Kaba have virtually identical compositions. The difference between Mokoia magnetite and fayalite may indicate formation in isotopic equilibrium in a water‐rich environment at low temperatures, ～300 K. In contrast, the similar compositions of these phases in Kaba may indicate formation of the fayalite by replacement of preexisting magnetite in dry environment, with the O coming entirely from the precursor magnetite and silica. The Δ¹⁷O of the oxidant incorporated into the CV parent body (as phyllosilicates or H₂O) appears to have been much (7–8%‰) lower than that in that incorporated into the LL parent body (Choi et al, 1998), which suggests that the O‐isotopic composition of the nebular gas was spatially or temporally variable.     

Effect of ENSO on landfalling tropical cyclones over the Korean peninsula

The effect of ENSO on landfalling tropical cyclones (TCs) over the Korean Peninsula is examined. It is found that although the landfalling frequency does not show any statistically significant difference among ENSO phases, the landfalling tracks are shifted northward in response to the decrease in Niño-3.4 index. In the neutral ENSO phase, many TCs pass through mainland China before landfalling over the Korean Peninsula due to the westward expansion of the western North Pacific subtropical high. Therefore, the landfalling TC intensity over the Korean Peninsula in the neutral phase is similar to that in the La Niña phase because more than half of those TCs made landfall over mainland China. However, it is found that the preceding winter ENSO phases are not related to the landfalling TC activity over the Korean Peninsula during summer.     

Fault tectonic analysis of Kii peninsula,Southwest Japan: Preliminary approach to Neogene paleostress sequence near the Nankai subduction zone

The southern part of the Outer Zone of Southwest Japan including the Kii peninsula belongs to the tectonic ‘shadow zone’, where fewer conspicuous active faults and less Quaternary sediments develop than in the Nankai subduction zone and Inner Zone of Southwest Japan. In order to study the paleostress sequence of the Kii peninsula, we analyzed fault‐slip data and tension gashes at pilot sites of Early–Middle Miocene forearc sediments and Late Cretaceous accretionary complex. According to the results, six faulting events are reconstructed in sequence: (i) east–west extension (normal faulting); (ii) east–west compression and north–south extension (strike‐slip faulting); (iii) NNW–SSE compression and ENE–WSW extension (strike‐slip faulting); (iv) northeast–southwest compression and northwest–southeast extension (strike‐slip faulting); (v) WNW–ESE compression (strike‐slip or reverse faulting); and (vi) NNE–SSW extension. The north–south to NNW–SSE trending dyke swarm of Middle Miocene age in the Kii peninsula is thought to be related to Event 3, implying that Event 3 was active at least during the Middle Miocene. Because Event 6 is recognized solely at a site, the overall latest faulting event seems to be Event 5. Assuming that the compression results from the motion of the crust or plate, the compression direction of Event 5 is in good accordance with the present‐day WNW crustal velocity vectors of the Kii peninsula. The stress trajectory map of Southeast Korea and Southwest Japan reveals that the current compression directions of the Kii peninsula correspond to the combinatory stress fields of the Himalayan and Philippine Sea tectonic domains.     

Exploratory Data Analysis Applied in Mapping Multi-element Soil Geochemical Anomalies for Drill Target Definition: A Case Study from the Unpha Layered Non-magmatic Hydrothermal Pb-Zn Deposit,DPR Korea

A factor analysis was applied to soil geochemical data to define anomalies related to buried Pb-Zn mineralization. A favorable main factor with a strong association of the elements Zn, Cu and Pb, related to mineralization, was selected for interpretation. The median + 2MAD (median absolute deviation) method of exploratory data analysis (EDA) and C-A (concentration-area) fractal modeling were then applied to the Mahalanobis distance, as defined by Zn, Cu and Pb from the factor analysis to set the thresholds for defining multi-element anomalies. As a result, the median + 2MAD method more successfully identified the Pb-Zn mineralization than the C-A fractal model. The soil anomaly identified by the median + 2MAD method on the Mahalanobis distances defined by three principal elements (Zn, Cu and Pb) rather than thirteen elements (Co, Zn, Cu, V, Mo, Ni, Cr, Mn, Pb, Ba, Sr, Zr and Ti) was the more favorable reflection of the ore body. The identified soil geochemical anomalies were compared with the in situ economic Pb-Zn ore bodies for validation. The results showed that the median + 2MAD approach is capable of mapping both strong and weak geochemical anomalies related to buried Pb-Zn mineralization, which is therefore useful at the reconnaissance drilling stage.     

Changma onset definition in Korea using the available water resources index and its relation to the Antarctic oscillation

This study defines the Changma onset using the available water resources index (AWRI) for 25?years (1985–2009) and verifies the validity of this definition. The three conditions for defining the Changma onset are established as follows: (i) The first day exceeding the June AWRI (threshold) averaged over the 25-year period. (ii) The continuation of the value over the threshold for at least 1?week after the onset. (iii) After the continuation of more than 1?week, the non-continuation of the value under the threshold for at least 1?week. The 25-year average Changma onset date is 24 June with a standard deviation of 9?days. The defined Changma onset is verified through the analysis on the relationship with the Antarctic oscillation (AAO). AAO in June shows a high correlation with not only the Changma onset but also the June precipitation (AWRI) in Korea. These three variables are influenced by Mascarene and Australian (positive AAO pattern) highs from in the preceding March. When these two pressure systems develop, the cold cross-equatorial flow in the direction from the region around Australia to the equator is intensified, which in turn, forces a western North Pacific high (WNPH) to develop northward; this eventually drives the rain belt north. As a result, the Changma begins early in the positive AAO phase, and the June precipitation increases in Korea. In addition, a WNPH that develops more northward increases the landfalling frequency of tropical cyclones in Korea, which plays an important role in increasing the June precipitation.     

Probabilistic Assessment of Wave Overtopping of Nampo Dike in the West Coast of Korea

The Nampo dike, which is located at the west coast of Korea, was destroyed by wave overtopping during the storms on 30 August and 17 September in 1959. In this paper, is performed the probabilistic assessment of wave overtopping of Nampo dike by use of Owen model, Van der Meer ＆ Janssen model and Hedges ＆ Reis model for wave overtopping of seawall. Based on the available tidal and wave data for storm surges in 1989, the risk assessment of wave overtopping of the Nampo dike has been carried out by both Level Ⅱ and Level Ⅲ reliability methods. The calculated resuhs show the general agreement of failure probability between the two methods. By utilizing the rehabilitated cross section of Nampo dike, the failure probability of wave overtopping for the Nampo dike after rehabilitation will be rapidly reduced to that of initial design at crest level of 9.0 m with the improved slope from 1 ： 2 to 1 ： 4 at seaside. Since the sea level may only rise 1.0 m in the next few decades, the failure probability of Nampo dike will be still in the safe range.     

Wave-tide-surge coupled model simulation for Typhoon Maemi

1 IntroductionThe main reason for coupling the tide and surgehydrodynamic model with a surface wave model canbe found in the physical interactions taking place inthe surface and bottom boundary layers. During thesevere storm conditions such interactions a…