Arcuate band texture in a dark inclusion from the Vigarano CV3 chondrite: Possible evidence for early sedimentary processes

Abstract— A dark inclusion in the Vigarano CV3 carbonaceous chondrite consists almost exclusively of small (<5 μm in diameter) grains of Fe-rich olivine and is devoid of chondrules, Ca-Al-rich inclusions (CAIs) and their pseudomorphs. In backscattered electron images, this dark inclusion shows an unusual texture comprising a network of arcuate bands. Two or more bands occur roughly parallel, forming a set of succesive parallel bands, some crosscutting one another. The bands contain slightly higher amounts of relatively small (<1 μm) olivine grains and so are more densely packed than other areas. The olivine grains in the bands are slightly more Fe-rich than those in other areas. The bands commonly show gradation on the concave side due to a decrease in the abundance of the small Fe-rich olivine grains. Texturally, the arcuate bands closely resemble “dish structures” that are commonly observed in siltstones and sandstones on Earth. Dish structures are characterized by thin, dark-colored, subhorizontal to concave-upward laminations that are rich in relatively fine-grained material. On Earth, dish structures form during compaction and dewatering of unconsolidated fine-grained sediments; they are one of the characteristic sedimentary structures formed through fluidization of fine grains. The dark inclusion in Vigarano, therefore, provides the first evidence that sedimentary processes due to water migration may have taken place within planetesimals and further suggests that fluidization may have played a significant role in the formation of the carbonaceous chondrites.     

Three‐dimensional shapes and Fe contents of Stardust impact tracks: A track formation model and estimation of comet Wild 2 coma dust particle densities

Abstract– We investigated three‐dimensional structures of comet Wild 2 coma particle impact tracks using synchrotron radiation (SR) X‐ray microtomography at SPring‐8 to elucidate the nature of comet Wild 2 coma dust particles captured in aerogel by understanding the capture process. All tracks have a similar entrance morphology, indicating a common track formation process near the entrance by impact shock propagation irrespective of impactor materials. Distributions of elements along the tracks were simultaneously measured using SR‐XRF. Iron is distributed throughout the tracks, but it tends to concentrate in the terminal grains and at the bottoms of bulbs. Based on these results, we propose an impact track formation process. We estimate the densities of cometary dust particles based on the hypothesis that the kinetic energy of impacting dust particles is proportional to the track volume. The density of 148 cometary dust particles we investigated ranges from 0.80 to 5.96 g cm^?3 with an average of 1.01 (±0.25) g cm^?3. Moreover, we suggest that less fragile crystalline particles account for approximately 5 vol% (20 wt%) of impacting particles. This value of crystalline particles corresponds to that of chondrules and CAIs, which were transported from the inner region of the solar system to the outer comet‐forming region. Our results also suggest the presence of volatile components, such as organic material and perhaps ice, in some bulbous tracks (type‐C).     

青藏高原花石峡冻土站高寒湿地CH4排放研究

利用静态箱技术对青藏高原花石峡冻土站附近湿地生态系统ＣＨ４排放的初步调查表明,各个植物群落内部和不同群落之间的ＣＨ４排放量变化都很大．花石峡地区高寒湿地基本可分为潮湿高寒草甸、沼泽化草甸、杉叶藻沼泽和毛柄水毛茛沼泽,其群落夏季ＣＨ４平均排放量分别为４４３１,１００５,４５９４和－０２８ｍｇ·ｍ－２ｄ－１．花石峡融化季节ＣＨ４排放量为４０８ｇ·ｍ－２ａ－１．简单外推表明,青藏高原高寒湿地ＣＨ４年排放量约为１Ｔｇ·ａ－１．     

Multiple nitrogen isotopic components coexisting in ureilites

Abstract— Nitrogen and noble gas isotopic compositions and C abundance of ureilites were analyzed using a stepwise combustion technique. Four Antarctic ureilites, ALHA77257, Asuka 881931, Yamato 791538 and Yamato 790981 were analyzed. Multiple N isotopic components were observed in these ureilites. The δ¹⁵N values of these N components ranged from +160 to ?120%. The minimum δ¹⁵N values of typically ?120% were observed at combustion temperatures at 700–900 °C where large amounts of C were released. A heavy N component was observed in only two ureilites, ALHA77257 and Asuka 881931. Silicate-enriched fractions and C-concentrated fractions were prepared for these two ureilites. We conclude that both the light N and the heavy N are trapped in the carbonaceous vein minerals. The lack of correction between the N/C ratio and the ³⁶Ar/C ratio suggests that the primary carrier phase of the light N does not correspond to that of the planetary noble gases. We consider that the isotopically heavy N, which was observed in this study, is related to the heavy N observed among polymict ureilites. Small amounts (<0.5 ppm) of light N with the minimum δ¹⁵N value of ?120% were observed among the silicate fractions at the highest combustion temperature of 1200 °C, although the exact carrier phase of this light N is not known. We consider that the currently observed ureilites were produced by injection of several volatile-rich objects into volatile-poor ureilitic silicates.     

Hydrothermal Alteration of Limestone and Mineral Exploration of Zn-Pb Skarn Deposits in the Sako-nishi Area of the Kamioka Mine, Central Japan

Abstract: Crystalline limestone of the Sako-nishi area in the Kamioka Zn-Pb mine, central Japan, is depleted in ¹⁸O and ¹³C toward the center of mineralization due to interaction with hydrothermal fluids with a dominant meteoric water component. The relationship between isotopic composition and mineral assemblage, texture, the chemical composition of the minerals, and the bulk chemical composition in the limestone was examined. A decrease in the δ¹⁸OSMOW value correlated with: (1) increase of fine-grained calcite which is enriched in Mn and exhibits a bright cathodoluminescence, (2) progressive hy-drothermal alteration of clinopyroxene in the original limestone into tremolite within the weakly-altered zone, and into chlorite and actinolite within the strongly-altered zone, (3) dominance of hydrothermal chlorite in altered limestone having δ¹⁸O values of less than 10%. This chlorite was enriched in Fe compared to mafic minerals in the unaltered limestone. The enrichment of Fe and Mn was more conspicuous in calcite and chlorite in skarn deposits. The occurrence and chemical composition of hydrothermal minerals in the limestone, skarn, and ore indicate that the ¹⁸O–depleted zones were formed in the later stage from fluids, which were responsible for mineralization and skarnization, and for Fe and Mn enrichment. The Al, Mn, and Fe contents, and the ratios of Mg/(Mg+Mn+Fe), Al/Mg, and Mn/Sr in the hydrochloric acid leachate of limestone varied with decreasing δ¹⁸O and δ¹³C values, reflecting increases in high-Mn calcite and high-Fe chlorite. These indexes were useful for the identification of hydrothermally altered limestone. Furthermore, the potential score weighted by each index was more effective and accurate means of detecting promising mineralization zones. An anomalous potential score due to the presence of hydrothermal minerals in the outcropping limestone occurred along the Atotsu–1GO fault. This structure indicates that the skarn deposits of the Sako-nishi area belong to Mozumi-type Zn–Pb skarn deposits, in which fissures and faults served as major passages for the hydrothermal fluid. High-Mn carbonate and high-Fe chlorite widely occur in base-metal vein deposits and Zn-Pb type skarn deposits. Leaching of altered rock with hydrochloric acid in addition to stable isotope composition and cathodoluminescence imaging is effective for geochemical exploration for hydrothermal deposits because it makes possible the detection of the elemental composition of hydrothermal minerals such as chlorite and carbonate and because of the rapidity and convenience of analysis.     

PDF orientations in shocked quartz grains around the Chicxulub crater

Abstract— We measured 852 sets of planar deformation features (PDFs) in shocked quartz grains in impactite samples of the Yaxcopoil (YAX‐1) core and from 4 Cretaceous/Tertiary (K/T) boundary deposits: the Monaca, the Cacarajícara, and the Peñalver formations in Cuba, and DSDP site 536, within 800 km of the Chicxulub crater, in order to investigate variations of PDF orientations in the proximity of the crater. Orientations of PDFs show a broad distribution with peaks at ω {101¯3}, π {101¯2}, and ω {111¯2}, plus r, z {1011¯} orientations with minor c(0001), s{112¯1}, t{224¯1} plus x{516¯1}, and m{101¯0} plus a{112¯0} orientations. Planar deformation features with c(0001) orientation are relatively more abundant in the proximity of the Chicxulub crater than in distal sites such as North America, the Pacific Ocean, and Europe. This feature indicates that in the proximity of the crater, part of the shocked quartz grains in the K/T boundary deposits were derived from the low shock pressure zones. Moreover, the orientations of PDFs with ω {112¯2} plus r, z {101¯1} are high in our studied sites, and frequencies of these orientations decrease with increasing distance from the crater. On the other hand, absence of c(0001) and the rare occurrence of PDFs with ?ω {112¯2} plus r, z {101¯1} orientations in the sample from the YAX‐1 core that was taken at the top of the impactite layer of the Chicxulub crater suggests that the sampling horizon that reflects a certain cratering stage is also an important factor for variations in shocked quartz.     

Triassic eclogite from northern Vietnam: inferences and geological significance

T he first finding of low‐temperature eclogites from the Indochina region is reported. The eclogites occur along the Song Ma Suture zone in northern Vietnam, which is widely regarded as the boundary between the South China and Indochina cratons. The major lithology of the area is pelitic schist that contains garnet and phengite with or without biotite, chloritoid, staurolite and kyanite, and which encloses blocks and lenses of eclogite and amphibolite. The eclogites commonly consist of garnet, omphacite, phengite, rutile, quartz and/or epidote with secondary barroisite. Omphacite is commonly surrounded by a symplectite of Na‐poor omphacite and Na‐rich plagioclase. In highly retrograded domains, diopside + tremolite + plagioclase symplectites replace the primary phases. Estimated peak‐pressure metamorphic conditions based on isochemical phase diagrams for the eclogites are 2.1–2.2 GPa and 600–620 °C, even though thermobarometric results yield higher pressure and temperature conditions (2.6–2.8 GPa and 620–680 °C). The eclogites underwent a clockwise P–T trajectory with a post‐peak‐pressure increase of temperature to a maximum of >750 °C at 1.7 GPa and a subsequent cooling during decompression to 650 °C and 1.3 GPa, which was followed by additional cooling before close‐to‐isothermal decompression to ∼530 °C at 0.5 GPa. The surrounding pelitic schist (garnet–chloritoid–phengite) records similar metamorphic conditions (580–600 °C at 1.9–2.3 GPa) and a monazite chemical age of 243 ± 4 Ma. A few monazite inclusions within garnet and the cores of some zoned monazite in garnet–phengite schist record an older thermal event (424 ± 15 Ma). The present results indicate that the Indochina craton was deeply (>70 km) subducted beneath the South China craton in the Triassic. The Silurian cores of monazite grains may relate to an older non‐collisional event in the Indochina craton.