The first samples from Almahata Sitta showing contacts between ureilitic and chondritic lithologies: Implications for the structure and composition of asteroid 2008 TC3

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC₃). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008 TC₃ was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC₃. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondrules (~130–600 μm) and chondrule fragments. The C1 material consists of fine‐grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28‐42), an unidentified Ca‐rich silicate phase, Fe,Ni sulfides, and minor Ca‐phosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ¹⁸O = 13.53‰, δ¹⁷O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of several CC‐like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75‐88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal‐sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and/or CC, based on mineral and oxygen isotope compositions. Some of the metal‐sulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04–0.05) and relatively featureless in VNIR, and have an ~2.7 μm absorption band due to OH^? in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the F‐type spectrum of the asteroid, suggests that 2008 TC₃ consisted mainly of ureilitic and AhS 91A‐like materials, with as much as 40–70% of the latter, and <10% of OC, EC, and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g cm^?3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7–2.2 g cm^?3). Its porosity (36%) is near the low end of estimates for the asteroid (33–50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 TC₃ in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when a CC‐like body impacted into already well‐gardened ureilitic + impactor‐derived debris. AhS 91A bulk samples do not show a solar wind component, so they represent subsurface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5–9 Ma) than previously studied AhS stones (11–22 Ma). The spread in CRE ages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands.     

Park Forest (L5) and the asteroidal source of shocked L chondrites

The Park Forest (L5) meteorite fell in a suburb of Chicago, Illinois (USA) on March 26, 2003. It is one of the currently 25 meteorites for which photographic documentation of the fireball enabled the reconstruction of the meteoroid orbit. The combination of orbits with pre‐atmospheric sizes, cosmic‐ray exposure (CRE), and radiogenic gas retention ages (“cosmic histories”) is significant because they can be used to constrain the meteoroid's “birth region,” and test models of meteoroid delivery. Using He, Ne, Ar, ¹⁰Be, and ²⁶Al, as well as a dynamical model, we show that the Park Forest meteoroid had a pre‐atmospheric size close to 180 g cm^?2, 0–40% porosity, and a pre‐atmospheric mass range of ~2–6 tons. It has a CRE age of 14 ± 2 Ma, and (U, Th)‐He and K‐Ar ages of 430 ± 90 and 490 ± 70 Ma, respectively. Of the meteorites with photographic orbits, Park Forest is the second (after Novato) that was shocked during the L chondrite parent body (LCPB) break‐up event approximately 470 Ma ago. The suggested association of this event with the formation of the Gefion family of asteroids has recently been challenged and we suggest the Ino family as a potential alternative source for the shocked L chondrites. The location of the LCPB break‐up event close to the 5:2 resonance also allows us to put some constraints on the possible orbital migration paths of the Park Forest meteoroid.     

The Vicência meteorite fall: A new unshocked (S1) weakly metamorphosed (3.2) LL chondrite

The Vicência meteorite, a stone of 1.547 kg, fell on September 21, 2013, at the village Borracha, near the city of Vicência, Pernambuco, Brazil. It was recovered immediately after the fall, and our consortium study showed it to be an unshocked (S1) LL3.2 ordinary chondrite. The LL group classification is based on the bulk density (3.13 g cm^?3); the chondrule mean apparent diameter (0.9 mm); the bulk oxygen isotopic composition (δ¹⁷O = 3.768 ± 0.042‰, δ¹⁸O = 5.359 ± 0.042‰, Δ¹⁷O = 0.981 ± 0.020‰); the content of metallic Fe,Ni (1.8 vol%); the Co content of kamacite (1.73 wt%); the bulk contents of the siderophile elements Ir and Co versus Au; and the ratios of metallic Fe⁰/total iron (0.105) versus total Fe/Mg (1.164), and of Ni/Mg (0.057) versus total Fe/Mg. The petrologic type 3.2 classification is indicated by the beautifully developed chondritic texture, the standard deviation (~0.09) versus mean Cr₂O₃ content (~0.14 wt%) of ferroan olivine, the TL sensitivity and the peak temperature and peak width at half maximum, the cathodoluminescence properties of chondrules, the content of trapped ¹³²Xe_tr (0.317 × 10^?8cm³STP g^?1), and the Raman spectra for organic material in the matrix. The cosmic ray exposure age is ~72 Ma, which is at the upper end of the age distribution of LL group chondrites. The meteorite is unusual in that it contains relatively large, up to nearly 100 μm in size, secondary fayalite grains, defined as olivine with Fa_>75, large enough to allow in situ measurement of oxygen and Mn‐Cr isotope systematics with SIMS. Its oxygen isotopes plot along a mass‐dependent fractionation line with a slope of ~0.5 and Δ¹⁷O of 4.0 ± 0.3‰, and are similar to those of secondary fayalite and magnetite in the unequilibrated chondrites EET 90161, MET 96503, and Ngawi. These data suggest that secondary fayalite in Vicência was in equilibrium with a fluid with a Δ¹⁷O of ~4‰, consistent with the composition of the fluid in equilibrium with secondary magnetite and fayalite in other unequilibrated ordinary chondrites. Secondary fayalite and the chondrule olivine phenocrysts in Vicência are not in isotopic equilibrium, consistent with low‐temperature formation of fayalite during aqueous alteration on the LL parent body. That alteration, as dated by the ⁵³Mn‐⁵³Cr chronology age of secondary fayalite, took place $4.0{}_{?1.1}^{+1.4}$ Ma after formation of CV CAIs when anchored to the quenched angrite D'Orbigny.     

International Pellet Watch: Global monitoring of persistent organic pollutants (POPs) in coastal waters. 1. Initial phase data on PCBs, DDTs, and HCHs

Samples of polyethylene pellets were collected at 30 beaches from 17 countries and analyzed for organochlorine compounds. PCB concentrations in the pellets were highest on US coasts, followed by western Europe and Japan, and were lower in tropical Asia, southern Africa and Australia. This spatial pattern reflected regional differences in the usage of PCBs and was positively correlated with data from Mussel Watch, another monitoring approach. DDTs showed high concentrations on the US west coast and in Vietnam. In Vietnam, DDT was predominant over its metabolites (DDE and DDD), suggesting the principal source may be current usage of the pesticide for malaria control. High concentrations of pesticide HCHs were detected in the pellets from southern Africa, suggesting current usage of the pesticides in southern Africa. This study demonstrates the utility and feasibility of the International Pellet Watch approach to monitor POPs at a global scale.     

High Temperature Strain Structures and Quartz C-Axis Fabrics from Mylonitic Rocks in the Ailaoshan-Red River Shear Zone, Yunnan, and Their Tectonic Implication

<正>The Ailaoshan-Red River(ASRR) shear zone is one of the major Southeast Asian tectonic discontinuities that have figured the present tectonic framework of the eastern Tibet.Several metamorphic massifs are distributed linearly along the shear zone,e.g.Xuelongshan,Diancangshan, Ailaoshan and Day Nui Con Voi from north to south.They bear a lot of lines of evidence for the tectonic evolution of the eastern Tibetan at different crustal levels in different tectonic stages.Controversy still exists on the deformation structures,microstructures and their relationship with metamorphisms along the ASRR.In this paper detailed microstructural and EBSD(Electron Backscattered Diffraction) fabric analysis of some highly sheared granitic rocks from different massifs along the ASRR are conducted.High temperature structures and microstructures are preserved in unsheared gneisses,in weakly sheared xenoliths or in some parts of the highly sheared rocks(mylonites).Several types of high temperature quartz c-axis fabrics show symmetrical patterns or transitions from symmetrical to asymmetrical patterns.The former are attributed to coaxial deformation during regional shortening in an early stage of the Indian-Eurasian tectonic interaction and the latter are related to the transitions from coaxial compression to noncoaxial shearing during the post-collisional ASRR left lateral shearing.     

Paucity of sulfide in a large slab of Esquel: New perspectives on pallasite formation

Abstract— The 0.7 ton Esquel meteorite, found in Patagonia before 1951, is a typical main-group pallasite in most respects. We examined petrographically a slab ～1 m long having an area of 3000 cm² that shows the typical pallasitic texture with fragmental olivine. Phase abundances (in vol%) are olivine (66%), metal (32%), schreibersite (0.76%), troilite (0.46%) and chromite (0.31%). Esquel can be divided into four lithologies: (1) “pallasitic” matrix consisting of olivine fragments embedded in metal (81%); (2) large (>5 cm) olivine nodules having low metal contents (18%); (3) massive metal (0.3%); and (4) zones dominated by FeS and fine olivine (0.7%). Main-group pallasites appear to have formed by the intrusion of a highly evolved (low Ir, high Ni, Au and S) metallic magma into fragmented olivine. This model implies that FeS should be abundant in main-group pallasites, and we had speculated that examination of an exceptionally large slab might reveal a high troilite content. We found instead an exceptionally low FeS content. New compositional data confirm that Esquel has a lower Au content than other main-group pallasites having similar Ir contents. Literature data (based, however, on relatively small sections) suggest that high-Au pallasites have higher S contents than Esquel but have lower S contents than expected from a trapped-melt model. We conclude that a relatively complex model is required to explain the origin of main-group pallasites. After intrusion, the degree of crystallization of the metallic magma varied from location to location but, in almost all cases, an FeS-rich liquid either escaped or formed FeS-rich pallasitic rocks that are underrepresented in the meteorite inventory.     

大兴安岭北段三道湾子碲化物型金矿床的发现及意义

大兴安岭北段三道湾子金矿床属于中一低温热液型金矿床。含金石英脉产于上侏罗统塔木兰沟组粗面安山岩和粗面安山质角砾岩中。通过光学显微镜、扫描电子显微镜观察和能谱分析。在矿脉中发现大量金、银碲化物。该矿物组合中Au、Ag、Te的含量统计分别为：Au30．20wt％、Ag34．35wt％和Te35．45wt％。该矿床可以与国际上典型的浅成低温热液Au-Ag-Te矿床对比。     

Evidence that earthquakes have been triggered by reservoir in the Song Tranh 2 region,Vietnam

In the Song Tranh 2 (ST2) hydropower reservoir located in the Quang Nam province, Central Vietnam, earthquakes started occurring soon after impoundment of the reservoir in late 2010. Earthquakes continue to occur in the region, and two earthquakes of M 4.6 and 4.7 on October 22, 2012 and November 15, 2012, respectively, have been reported (Trieu et al. 2014; Giang et al. 2015) in the vicinity of the reservoir. In the present study, b-value has been estimated, and focal mechanism solutions have been computed for the first time using moment tensor inversion approach. Also, the influence of impoundment of reservoir on the occurrence of earthquakes has been computed for the ST2 region based on Coulomb stress. A quality data set of 595 earthquakes recorded for the period of October 2012 to April 2014 at ten stations of the seismic network operated by the Institute of Geophysics (IGP) has been used to calculate b-values for the northern and southern seismicity clusters of the region. In general, the b-values associated with reservoir-triggered seismicity (RTS) are found to be higher than the regional b-values in the frequency-magnitude relation of earthquakes. For the ST2 region, it is found that the b-values for the northern and southern clusters are 0.94 ± 0.04 and 0.90 ± 0.04, respectively. Focal mechanism solutions obtained for the two earthquakes close to the reservoir have a right-lateral strike-slip mechanism, with the preferred planes trending NW-SE. These results are concurrent with the orientation of the nearby local surface faults, which we confirm as the active faults in this region. Influence of the stresses due to reservoir water load on the local seismicity is computed based on the obtained focal mechanism by using the concept of fault stability. It is found that most of the earthquakes occur in the positive Coulomb stress region, which shows the influence of reservoir impoundment on earthquake occurrence in the vicinity. Our results suggest that the local earthquakes are triggered by the impoundment of the ST2 reservoir.     

越南东北部佐田铅锌矿床硫化物S、Pb同位素特征及其地质意义

越南东北部佐田(Cho Dien)铅锌矿床分布在北太省(Bac thai)西部则屯西北,属于华南褶皱系越北隆起区。为了探讨该矿床的成矿物质来源和成矿机制,对该矿床的矿石样品进行了S、Pb同位素组成分析。结果表明,5件样品的11种金属硫化物(闪锌矿、磁黄铁矿、方铅矿)的δ34S值为4.1‰~6.8‰,平均值为5.51‰,极差为2.7‰,显示S同位素组成比较稳定,经计算热液的δ34S值约为7‰。11种金属硫化物样品的206Pb/204Pb、207Pb/204Pb和208Pb/204Pb变化范围分别为18.451~18.682、15.685~15.878、38.909~39.636,平均值分别为18.59855、15.78227和39.30773,矿石矿物μ值为9.36~9.98,平均值为9.80,ω值为39.04~42.63,平均值为40.79。Th/U值为3.92~4.13,平均值为4.03。结果表明,佐田铅锌矿床的硫主要来自沉积围岩及三叠纪的Phia Bioc杂岩,矿石铅主要来自上地壳物质,也有地幔物质的加入。综合上述成果及已有数据认为,佐田铅锌矿是在特提斯成矿域早三叠世碰撞环境下形成的,板块碰撞过程中形成的Phia Bioc花岗岩体为矿床的形成提供了一定的物质基础,属热液型矿床。