Extraterrestrial chromite in latest Maastrichtian and Paleocene pelagic limestone at Gubbio,Italy: The flux of unmelted ordinary chondrites     

Anders CRONHOLM  Birger SCHMITZ 《Meteoritics & planetary science》2007,42(12):2099-2109

Abstract— –The distribution of sediment‐dispersed extraterrestrial (ordinary chondritic) chromite (EC) grains (>63 μm) has been studied across the latest Maastrichtian and Paleocene in the Bottaccione Gorge section at Gubbio, Italy. This section is ideal for determining the accumulation rate of EC because of its condensed nature and well‐constrained sedimentation rates. In a total of 210 kg of limestone representing eight samples of 14–28 kg distributed across 24 m of the Bottaccione section, only 6 EC grains were found (an average of 0.03 EC grains kg^?1). In addition, one probable pallasitic chromite grain was found. No EC grains could be found in two samples at the Cretaceous‐Tertiary (K‐T) boundary, which is consistent with the K‐T boundary impactor being a carbonaceous chondrite or comet low in chromite. The average influx of EC to Earth is calculated to ～～0.26 grain m^?2 kyr^?1. This corresponds to a total flux of ～～200 tons of extraterrestrial matter per year, compared to ～～30,000 tons per year, as estimated from Os isotopes in deep‐sea sediments. The difference is explained by the EC grains representing only unmelted ordinary chondritic matter, predominantly in the size range from ～～0.1 mm to a few centimeters in diameter. Sedimentary EC grains can thus give important information on the extent to which micrometeorites and small meteorites survive the passage through the atmosphere. The average of 0.03 EC grain kg^?1 in the Gubbio limestone contrasts with the up to ～～3 EC grains kg^?1 in mid‐Ordovician limestone that formed after the disruption of the L chondrite parent body in the asteroid belt at ～～470 Ma. The two types of limestone were deposited at about the same rate, and the difference in EC abundance gives support for an increase by two orders of magnitude in the flux of chondritic matter directly after the asteroid breakup.  相似文献   

X-ray, Lyα and Hα emission from simulated disk galaxies     

Jesper Sommer-Larsen  Sune Toft  Jesper Rasmussen  Kristian Pedersen  Martin Götz  Laura Portinari 《Astrophysics and Space Science》2003,284(2):693-696

The X-ray properties of the haloes of disk galaxies formed in fully cosmological, hydro/gravity simulations are discussed. The results are found to be consistent with observational X-ray detections and upper limits. Disk galaxy haloes are predicted to be about an order of magnitude brighter at z ∼ 1 than at z=0. The Lyα and Hα surface brightness of an edge-on, Milky Way like model galaxy has been determined. The emission is found to be quite extended, with a scale height of about 600 pc, neglecting dust corrections. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

New analysis of the Mössbauer spectra of olivine basalt rocks from Gusev crater on Mars     

H.P. Gunnlaugsson  H. Rasmussen  M.B. Madsen  P. Nørnberg 《Planetary and Space Science》2009,57(5-6):640-645

The Mars Exploration Rover, Spirit, landed on 4 January 2004, in a lava field in Gusev crater on Mars. Samples interpreted as olivine basalt have been investigated with Mössbauer spectroscopy and chemically with Alpha-particle-X-ray spectrometry (APXS).In this contribution we present the results of a new analysis of the Mössbauer spectra of selected rock targets in Gusev crater. The results show that the rock surfaces investigated are inhomogeneous, and show strong enhancement of olivine in the surface layer. By subtraction of the surface signal to obtain the spectrum of the true interior of the rock samples, the measurements show the usual correlation between olivine and iron oxides of olivine basalt.It is argued that the compositional changes observed are related to high temperature oxidation of the rocks, probably during solidification, a process known to lead to anomalously magnetic rocks. The rock Mazatzal is discussed in some detail, and it is suggested that the surface is covered with deposits rich in ferric iron rather than these ferric phases being due to oxidation of the rock. The fact that all the surfaces in this investigation show this same pattern, suggests that the dominating erosion of the surface layer of basaltic rocks at Gusev crater has been mechanical rather than chemical.  相似文献   

Relict silicate inclusions in extraterrestrial chromite and their use in the classification of fossil chondritic material     

Carl Alwmark  Birger Schmitz 《Geochimica et cosmochimica acta》2009,73(5):1472-155

Chromite is the only common meteoritic mineral surviving long-term exposure on Earth, however, the present study of relict chromite from numerous Ordovician (470 Ma) fossil meteorites and micrometeorites from Sweden, reveals that when encapsulated in chromite, other minerals can survive for hundreds of millions of years maintaining their primary composition. The most common minerals identified, in the form of small (<1-10 μm) anhedral inclusions, are olivine and pyroxene. In addition, sporadic merrillite and plagioclase were found.Analyses of recent meteorites, holding both inclusions in chromite and corresponding matrix minerals, show that for olivine and pyroxene inclusions, sub-solidus re-equilibration between inclusion and host chromite during entrapment has led to an increase in chromium in the former. In the case of olivine, the re-equilibration has also affected the fayalite (Fa) content, lowering it with an average of 14% in inclusions. For Ca-poor pyroxene the ferrosilite (Fs) content is more or less identical in inclusions and matrix. By these studies an analogue to the commonly applied classification system for ordinary chondritic matrix, based on Fa in olivine and Fs in Ca-poor pyroxene, can be established also for inclusions in chromite. All olivine and Ca-poor pyroxene inclusions (>1.5 μm) in chromite from the Ordovician fossil chondritic material plot within the L-chondrite field, which is in accordance with previous classifications. The concordance in classification together with the fact that inclusions are relatively common makes them an accurate and useful tool in the classification of extraterrestrial material that lacks matrix silicates, such as fossil meteorites and sediment-dispersed chromite grains originating primarily from decomposed micrometeorites but also from larger impacts.  相似文献   

The history of environmental change and adaptation in eastern Saloum–Senegal—Driving forces and perceptions     

Cheikh Mbow  Ole Mertz  Awa Diouf  Kjeld Rasmussen  AnetteReenberg   《Global and Planetary Change》2008,64(3-4):210

Environmental change in the Sahel–Sudan zone of West Africa has been a major issue in development debates over the last decades. Using remote sensing based land cover change analysis, archival data, national and international statistical data, and household interviews, we analyze the drivers of environmental change in Eastern Saloum in Central East Senegal as well as the local perceptions of these changes and adaptation. Being part of the ground nut basin, Eastern Saloum has witnessed rapid environmental degradation caused by the conversion of forest and savanna areas to agricultural land during the last 20–30 years and by a combination of decline in precipitation, soil degradation, a diversity of policies with little concern for the environment, fluctuating markets and population pressure. Farmers perceive the environmental change mainly as land degradation and poor soil fertility, though recent extensification of agriculture counters this effect and has led to increased vegetation cover in marginal areas. They identified erratic climate, agricultural policies, insufficient food production and desire to increase income as the main drivers of change in the area. We conclude that while climate variability has influenced environmental change in the area, various types of State interventions in agriculture and global market fluctuations appear to have been the main underlying causes of environmental degradation.  相似文献   

Linking gold mineralization to regional-scale drivers of mineral systems using in situ U–Pb geochronology and pyrite LA-ICP-MS element mapping     

Imogen O.H. Fielding  Simon P. Johnson  Sebastien Meffre  Jianwei Zi  Stephen Sheppard  Ross R. Large  Birger Rasmussen 《地学前缘(英文版)》2019,10(1):89-105

Proterozoic orogens commonly host a range of hydrothermal ores that form in diverse tectonic settings at different times. However, the link between mineralization and the regional-scale tectonothermal evolution of orogens is usually not well understood, especially in areas subject to multiple hydrothermal events.Regional-scale drivers for mineral systems vary between the different classes of hydrothermal ore, but all involve an energy source and a fluid pathway to focus mineralizing fluids into the upper crust. The Mount Olympus gold deposit in the Proterozoic Capricorn Orogen of Western Australia, was regarded as an orogenic gold deposit that formed at ca. 1738 Ma during the assembly of Proterozoic Australia. However,the trace element chemistry of the pyrite crystals closely resembles those of the Carlin deposits of Nevada,with rims that display solid solution gold accompanied by elevated As, Cu, Sb, Hg, and Tl, surrounding gold-poor cores. New SHRIMP UeP b dating of xenotime intergrown with auriferous pyrite and ore-stage alteration minerals provided a weighted mean~(207) Pb*/~(206) Pb* date of 1769 ± 5 Ma, interpreted as the age of gold mineralization. This was followed by two discrete episodes of hydrothermal alteration at 1727 ± 7 Ma and 1673 ± 8 Ma. The three ages are linked to multiple reactivation of the crustal-scale Nanjilgardy Fault during repeated episodes of intracratonic reworking. The regional-scale drivers for Carlin-like gold mineralization at Mount Olympus are related to a change in tectonic regime during the final stages of the intracratonic 1820 -1770 Ma Capricorn Orogeny. Our results suggest that substantial sized Carlin-like gold deposits can form in an intracratonic setting during regional-scale crustal reworking.  相似文献   

Holocene relative sea level history and Storegga tsunami run‐up in Lyngen,northern Norway          下载免费PDF全文

Henrik Rasmussen  Stein Bondevik  Geoffrey D. Corner 《第四纪科学杂志》2018,33(4):393-408

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Annual Mass Balance of Blue Glacier, USA: 1955–97     

H. Conway  L.A. Rasmussen  & H.-P. Marshall 《Geografiska Annaler: Series A, Physical Geography》1999,81(4):509-520

Mass changes of Blue Glacier, USA are calculated from topographic maps made from vertical aerial photography in late summer of 1939, 1952, 1957, and 1987, along with laser altimetry flown in June 1996. Changes in elevation between maps were adjusted for seasonal variations in the snow cover, and to account for the ablation between the date of photography and 1 October. Topography obtained from the laser altimetry was adjusted for snow thickness and glacier motion to estimate topography of 1 October 1995. The mass of Blue Glacier has changed less than 7 m (water equivalent) during this 56 year period which is minor compared with other glaciers in the region and elsewhere in the world. Glacier-average annual mass balances, beginning in 1956, have been calculated either from stake measurements and probing of late-season snow, or from a regression analysis using late-season measurements of the equilibrium line altitude. A comparison with the changes derived from surface maps shows values obtained from field measurements are too positive by about 0.4 m a^?1 , indicating that considerable caution is needed when interpreting time series of mass balance. Two alternative time series of mass balance consistent with the long-term mass changes are created by making simple adjustments: (1) a single constant is subtracted from each value so that the series is consistent with the 1957–95 mass change; (2) one constant is subtracted from each value over 1957–87 and another is subtracted from each value over 1987-95 so that the series is consistent with both the 1957–87 and 1987–95 mass changes. The mass balance of Blue Glacier was generally positive until the mid-1970s and negative since. The fluctuations of mass balance closely resemble those of snowfall on the glacier as estimated from the joint distribution of temperature and precipitation. The climate in western Washington was cooler and wetter during the decade before the mid-1970s, but the trend since has been towards warmer and drier conditions.  相似文献   

Gas stripping in galaxy groups – the case of the starburst spiral NGC 2276     

Jesper Rasmussen  Trevor J. Ponman  John S. Mulchaey 《Monthly notices of the Royal Astronomical Society》2006,370(1):453-467

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Deep massive sulphide exploration using 2D and 3D geoelectrical and induced polarization data in Skellefte mining district,northern Sweden          下载免费PDF全文

Saman Tavakoli  Tobias E. Bauer  Thorkild M. Rasmussen  Pär Weihed  Sten‐Åke Elming 《Geophysical Prospecting》2016,64(6):1602-1619

Geoelectrical and induced polarization data from measurements along three profiles and from one 3D survey are acquired and processed in the central Skellefte District, northern Sweden. The data were collected during two field campaigns in 2009 and 2010 in order to delineate the structures related to volcanogenic massive sulphide deposits and to model lithological contacts down to a maximum depth of 1.5 km. The 2009 data were inverted previously, and their joint interpretation with potential field data indicated several anomalous zones. The 2010 data not only provide additional information from greater depths compared with the 2009 data but also cover a larger surface area. Several high‐chargeability low‐resistivity zones, interpreted as possible massive sulphide mineralization and associated hydrothermal alteration, are revealed. The 3D survey data provide a detailed high‐resolution image of the top ～450 m of the upper crust around the Maurliden East, North, and Central deposits. Several anomalies are interpreted as new potential prospects in the Maurliden area, which are mainly concentrated in the central conductive zone. In addition, the contact relationship between the major geological units, e.g., the contact between the Skellefte Group and the Jörn Intrusive Complex, is better understood with the help of 2010 deep‐resistivity/chargeability data. The bottommost part of the Vargfors basin is imaged using the 2010 geoelectrical and induced polarization data down to ～1‐km depth.  相似文献