Chicxulub impact ejecta from Albion Island, Belize

Impact ejecta from the Albion Formation are exposed in northern Belize. The ejecta come from the outer portion of the continuous ejecta blanket of the Chicxulub crater, which is located 360 km to the northwest. The basal unit of the Albion Formation is a 1-m-thick clay and dolomite spheroid bed composed of up to four discrete flows. The clay spheroids are altered impact glass, and the dolomite spheroids are accretionary lapilli. The upper unit is a 15-m-thick coarse diamictite bed containing altered glass, large accretionary blocks, striated, polished, and impacted cobbles, and rare shocked quartz. The abundance of accretionary clasts, evidence for atmospheric drag sorting, and the presence of multiple flows in the Albion Formation indicate that atmospheres play an important role in the formation of the outer portions of continuous ejecta blankets of large craters.     

The Crater Mountain Deposit,Papua New Guinea: Porphyry‐related Au–Te System

Ore mineralization and wall rock alteration of Crater Mountain gold deposit, Papua New Guinea, were investigated using ore and host rock samples from drill holes for ore and alteration mineralogical study. The host rocks of the deposit are quartz‐feldspar porphyry, feldspar‐hornblende porphyry, andesitic volcanics and pyroclastics, and basaltic‐andesitic tuff. The main ore minerals are pyrite, sphalerite, galena, chalcopyrite and moderate amounts of tetrahedrite, tennantite, pyrrhotite, bornite and enargite. Small amounts of enargite, tetradymite, altaite, heyrovskyite, bismuthinite, bornite, idaite, cubanite, native gold, CuPbS₂, an unidentified Bi‐Te‐S mineral and argentopyrite occur as inclusions mainly in pyrite veins and grains. Native gold occurs significantly in the As‐rich pyrite veins in volcanic units, and coexists with Bi‐Te‐S mineral species and rarely with chalcopyrite and cubanite relics. Four mineralization stages were recognized based on the observations of ore textures. Stage I is characterized by quartz‐sericite‐calcite alteration with trace pyrite and chalcopyrite in the monomict diatreme breccias; Stage II is defined by the crystallization of pyrite and by weak quartz‐chlorite‐sericite‐calcite alteration; Stage III is a major ore formation episode where sulfides deposited as disseminated grains and veins that host native gold, and is divided into three sub‐stages; Stage IV is characterized by predominant carbonitization. Gold mineralization occurred in the sub‐stages 2 and 3 in Stage III. The fS₂ is considered to have decreased from ～10^?2 to 10^?14 atm with decreasing temperature of fluid.     

The transition from complex crater to peak-ring basin on Mercury: New observations from MESSENGER flyby data and constraints on basin formation models

The study of peak-ring basins and other impact crater morphologies transitional between complex craters and multi-ring basins is important to our understanding of the mechanisms for basin formation on the terrestrial planets. Mercury has the largest population, and the largest population per area, of peak-ring basins and protobasins in the inner solar system and thus provides important data for examining questions surrounding peak-ring basin formation. New flyby images from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft have more than doubled the area of Mercury viewed at close range, providing nearly complete global coverage of the planet's surface when combined with flyby data from Mariner 10. We use this new near-global dataset to compile a catalog of peak-ring basins and protobasins on Mercury, including measurements of the diameters of the basin rim crest, interior ring, and central peak (if present). Our catalog increases the population of peak-ring basins by ∼150% and protobasins by ∼100% over previous catalogs, including 44 newly identified peak-ring basins (total=74) and 17 newly identified protobasins (total=32). A newly defined transitional basin type, the ringed peak-cluster basin (total=9), is also described. The new basin catalog confirms that Mercury has the largest population of peak-ring basins of the terrestrial planets and also places the onset rim-crest diameter for peak-ring basins at , which is intermediate between the onset diameter for peak-ring basins on the Moon and those for the other terrestrial planets. The ratios of ring diameter to rim-crest diameter further emphasize that protobasins and peak-ring basins are parts of a continuum of basin morphologies relating to their processes of formation, in contrast to previous views that these forms are distinct. Comparisons of the predictions of peak-ring basin-formation models with the characteristics of the basin catalog for Mercury suggest that formation and modification of an interior melt cavity and nonlinear scaling of impact melt volume with crater diameter provide important controls on the development of peak rings. The relationship between impact-melt production and peak-ring formation is strengthened further by agreement between power laws fit to ratios of ring diameter to rim-crest diameter for peak-ring basins and protobasins and the power-law relation between the dimension of a melt cavity and the crater diameter. More detailed examination of Mercury's peak-ring basins awaits the planned insertion of the MESSENGER spacecraft into orbit about Mercury in 2011.     

Motion of blocks on the surface of Phobos: New constraints for the formation of grooves

Although grooves are common on asteroids, on Phobos the grooves have been the subject of various formation hypotheses for two reasons: the fact that Phobos is orbiting around Mars close to the Roche limit, implies an unusual gravity environment as well as the presence of the huge crater impact, Stickney, which seems to be related at least to a family of grooves. Among many hypotheses, it has been suggested that the grooves could have been dug by rolling Stickney ejecta, but this hypothesis was questioned using two main arguments: no block was observed at the end of the grooves, nor do they run downslope. Thus, the study of surface and near surface dynamics on Phobos can clarify the soundness of these controversial arguments.The present study explores this idea by computing the trajectory of a test mass gliding on the surface of Phobos for any initial position and velocity. An ellipsoidal model of Phobos is used for both the gravity and surface geometry, and several orbiting distances from Mars are considered. It is shown that, due to the Phobos rotation, the trajectories generally do not run downslope even for velocities as low as ∼1 m/s. In consequence the corresponding argument against the rolling blocks hypothesis is not applicable. This hypothesis, however, is clearly ruled out for the sets of parallel grooves in the polar regions because of the curvature of the computed trajectories. On the contrary the trajectories issued from the rim of Stickney close to the equator, with velocities of a few m/s, show similitude with the corresponding groove patterns, in particular for the east–west asymmetry. In some conditions the trajectories leave the surface, which may account for regions free of grooves and for trails without blocks at the end. Consequently, the rolling block hypothesis cannot be rejected outright until further analysis is carried out. Damping of the motion on the regolith is simulated introducing a solid damping coefficient in the equations. A relatively small but not unrealistic coefficient of ∼0.1 accounts for the length of the long main western hypothetic block trails. The simulations with damping confirm that the trajectories turn downslope only at the very end for velocities ?1 m/s, but valuable comparison with the observations requires a better model.     

Reconstructing the Wolfe Creek meteorite impact: deep structure of the crater and effects on target rock

The Wolfe Creek Meteorite Crater is an impact structure 880 m in diameter, located in the Tanami Desert near Halls Creek, Western Australia. The crater formed?<?300 000 years ago, and is the second largest crater from which fragments of the impacting meteorite (a medium octahedrite) have been recovered. We present the results of new ground-based geophysical (magnetics and gravity) surveys conducted over the structure in July?–?August 2003. The results highlight the simple structure of the crater under the infilling sediments, and forward modelling is consistent with the true crater floor being 120 m beneath the present surface. The variations in the dip of the foliations around the crater rim confirm that the meteorite approached from the east-northeast, as is also deduced from the ejecta distribution. Crater scaling arguments suggest a projectile diameter of?>?12.0 m, a crater formation time of 3.34 s, and an energy of impact of ～0.235 Mt of TNT. We also use the distribution of shocked quartz in the target rock (Devonian sandstones) to reconstruct the shock loading conditions of the impact. The estimated maximum pressures at the crater rim were between 5.59 and 5.81 GPa. We also use a Simplified Arbitrary Langrangian–Eulerian hydrocode (SALE 2) to simulate the propagation of shock waves through a material described by a Tillotson equation of state. Using the deformational and PT constraints of the Wolfe Creek crater, we estimate the maximum pressures, and the shock-wave attenuation, of this medium-sized impact.     

山东临朐柳山地区发现早白垩世冰碛砾岩及其意义

20世纪90年代在山东临朐县柳山镇地区的早白垩世莱阳群城山后组底部砾岩中发现具压坑、冰川擦痕、压裂张裂隙、压磨蚀平面的石英岩质、石灰岩质砾石和塑性变形形成的灯盏石、马鞍石石灰岩质砾石,经研究认为其成因属冰碛砾石,组成了冰碛砾岩。并探讨分析该冰川事件形成的古地理和古气候条件,指出为大陆上升局部形成的高山寒冷气候小冰期,形成了大陆型高原山岳山谷冰川。依据古生物和同位素地质年龄推断该小冰期发生在距今128~130Ma早白垩世热河期早期,建议称为临朐柳山小冰期。推断在我国东部地区其分布具一定区域性,应引起广大工作者的关注和研究。     

Leakage of Active Crater lake brine through the north flank at Rincón de la Vieja volcano, northwest Costa Rica, and implications for crater collapse

The Active Crater at Rincón de la Vieja volcano, Costa Rica, reaches an elevation of 1750 m and contains a warm, hyper-acidic crater lake that probably formed soon after the eruption of the Rio Blanco tephra deposit approximately 3500 years before present. The Active Crater is buttressed by volcanic ridges and older craters on all sides except the north, which dips steeply toward the Caribbean coastal plains. Acidic, above-ambient-temperature streams are found along the Active Crater's north flank at elevations between 800 and 1000 m. A geochemical survey of thermal and non-thermal waters at Rincón de la Vieja was done in 1989 to determine whether hyper-acidic fluids are leaking from the Active Crater through the north flank, affecting the composition of north-flank streams.Results of the water-chemistry survey reveal that three distinct thermal waters are found on the flanks of Rincón de la Vieja volcano: acid chloride–sulfate (ACS), acid sulfate (AS), and neutral chloride (NC) waters. The most extreme ACS water was collected from the crater lake that fills the Active Crater. Chemical analyses of the lake water reveal a hyper-acidic (pH0) chloride–sulfate brine with elevated concentrations of calcium, magnesium, aluminum, iron, manganese, copper, zinc, fluorine, and boron. The composition of the brine reflects the combined effects of magmatic degassing from a shallow magma body beneath the Active Crater, dissolution of andesitic volcanic rock, and evaporative concentration of dissolved constituents at above-ambient temperatures. Similar cation and anion enrichments are found in the above-ambient-temperature streams draining the north flank of the Active Crater. The pH of north-flank thermal waters range from 3.6 to 4.1 and chloride:sulfate ratios (1.2–1.4) that are a factor of two greater than that of the lake brine (0.60). The waters have an ACS composition that is quite different from the AS and NC thermal waters that occur along the southern flank of Rincón de la Vieja.The distribution of thermal water types at Rincón de la Vieja strongly indicates that formation of the north-flank ACS waters is not due to mixing of shallow, steam-heated AS water with deep-seated NC water. More likely, hyper-acidic brines formed in the Active Crater area are migrating through permeable zones in the volcanic strata that make up the Active Crater's north flank. Dissolution and shallow subsurface alteration of north-flank volcanoclastic material by interaction with acidic lake brine, particularly in the more permeable tephra units, could weaken the already oversteepened north flank of the Active Crater. Sector collapse of the Active Crater, with or without a volcanic eruption, represents a potential threat to human lives, property, and ecosystems at Rincón de la Vieja volcano.     

Hickman Crater,Ophthalmia Range,Western Australia: evidence supporting a meteorite impact origin ∗

A newly discovered, morphologically well-preserved crater with a mean diameter of 260 m is reported from the Ophthalmia Range, Western Australia. The crater is located in hilly terrain ～36 km north of Newman, and is situated in the Paleoproterozoic Woongarra Rhyolite and the overlying Boolgeeda Iron Formation. The morphometry of the crater is consistent with features characteristic of small meteorite impact craters. The rhyolite of the crater's rim exhibits widespread shatter features injected by veins of goethite bound by sharply defined zones of hydrous alteration. The alteration zones contain micro-fractures injected by goethite, which also fills cavities in the rhyolite. The goethite veins are interpreted in terms of forceful injection of aqueous iron-rich solutions, probably reflecting high-pressure hydrothermal activity by heated iron-rich ground water. None of these features are present in the Woongarra Rhyolite outside the immediate area of the crater. Petrography of the rhyolite indicates possible incipient intracrystalline dislocations in quartz. The Boolgeeda Iron Formation, which crops out only on the southern rim of the crater, displays brecciation and mega-brecciation superposed on fold structures typical of the banded iron-formations in the region. Geochemical analysis of two goethite veins discloses no siderophile element (Ni and PGE) anomalies, negating any contribution of material from an exploding meteorite. Instead, the strong iron-enrichment of the fractured rhyolite is attributed to a hydrothermal system affecting both the Boolgeeda Iron Formation and the Woongarra Rhyolite, and localised to the area of the crater. An absence of young fragmental volcanic material younger than the Woongarra Rhyolite is inconsistent with an explosive diatreme, leading us to a preferred interpretation in terms of an original impact crater about 80 m deep excavated by a ～10 m-diameter projectile and accompanied by hydrothermal activity. A minor north–south asymmetry of the crater, and an abundance of ejecta north, up to about 300 m northwest and northeast of the crater, suggest high-angle impact from the south. A youthful age of the structure, probably Late Pleistocene (10⁴–10⁵ years old), is indicated by damming of the drainage of a south-southeast-flowing creek by the southern crater rim.     

雨海地区晚雨海世-爱拉托逊纪月海玄武岩充填过程研究

月海玄武岩是月球四大岩类之一,主要充填于月球大型撞击盆地之中。月海玄武岩充填过程的研究,对于了解撞击盆地充填过程和月海玄武岩充填规律及活动规模,揭示月球的热演化历史具有重要意义。本文综合利用嫦娥一号LAM数据、CCD影像数据和Clementine UVVIS数据,对雨海地区的地形地貌、岩石化学组成进行了提取和分析,对雨海地区月海玄武岩进行了单元划分,并运用撞击坑尺寸-频率分布法对各月海玄武岩单元进行了表面年龄的估算。结果表明,雨海地区月海玄武岩随着时代变新钛和铁元素更加富集,总体上从晚雨海世至爱拉托逊纪由低钛低铁玄武岩向高钛高铁方向演化;月海玄武岩充填活动具有多期次性,每期月海玄武岩的充填流动大体上保持由南向北方向,并且活动规模逐步减小,相对年轻月海玄武岩对早期月海玄武岩的覆盖范围不断减小。正是这种玄武岩流动与覆盖关系和充填过程造成了雨海地区从南向北地势的逐渐降低,以及较老月海玄武岩在较北部地区出露。最后,根据雨海地区月海玄武岩单元在地形地貌、岩石化学组成与表面年龄上的相关性,我们提出雨海地区月海玄武岩经历了多期次逐层填充过程,且每期由南向北流动、规模逐步减小。     

月球雨海北部陆地区域构造及其含义

月球雨海北部陆地是雨海多环盆地的第二层,平均高程约-1 km。DEM图像显示,大量来自虹湾与柏拉图月坑的掘积物使本地区高程变得非常不均一。统计了研究区内的月坑,并根据其深度与宽度之比(深宽比)将它们划分为4组。深宽比较小而扁率较大的月坑被认为是较古老的月坑。这些古老月坑分布于比较接近月海的位置。对研究区内线性构造的制图研究揭示了3个优选方位,分别是E—W、NEE—SWW和NW—SE向。这种分布样式与月球格子构造系统大致匹配,因而它们很可能形成于雨海事件之前。这些线性构造,包括断裂与月溪,在月海玄武岩泛滥时期为玄武质岩浆的侵入提供了大量通道。在研究区内一些地形较低的地点,玄武岩上侵并出露在月表,它们的FeO平均含量接近但是略低于月海玄武岩。总结了本地区的地质构造演化历史,并且推论月球上的确存在类月海的陆地。