Shock barometry of the Siljan impact structure,Sweden     

Sanna HOLM  Carl ALWMARK  Walter ALVAREZ  Birger SCHMITZ 《Meteoritics & planetary science》2011,46(12):1888-1909

Abstract– The Siljan impact structure in Sweden is the largest confirmed impact structure in Western Europe. Despite this, the structure has been poorly studied in the past, and detailed studies of shock metamorphic features in the target lithologies are missing. Here, we present the results of a detailed systematic search for shock metamorphic features in quartz grains from 73 sampled localities at Siljan. At 21 localities from an area approximately 20 km in diameter located centrally in the structure, the orientations of 2851 planar deformation feature sets in 1179 quartz grains were measured. Observations of shatter cones outside of the zone with shocked quartz extend the total shocked area to approximately 30 km in diameter. The most strongly shocked samples, recording pressures of up to 20 GPa, occur at the very central part of the structure, and locally in these samples, higher pressures causing melting conditions in the affected rocks were reached. Pressures recorded in the studied samples decrease outwards from the center of the structure, forming roughly circular envelopes around the proposed shock center. Based on the distribution pattern of shocked quartz at Siljan, the original transient cavity can be estimated at approximately 32–38 km in diameter. After correcting for erosion, we conclude that the original rim to rim diameter of the Siljan crater was somewhere in the size range 50–90 km.  相似文献   

A 3‐D study of mineral inclusions in chromite from ordinary chondrites using synchrotron radiation X‐ray tomographic microscopy—Method and applications     

Carl ALWMARK  Birger SCHMITZ  Sanna HOLM  Federica MARONE  Marco STAMPANONI 《Meteoritics & planetary science》2011,46(8):1071-1081

Abstract– A method is described for imaging in 3‐D the interiors of meteoritic chromite grains and their inclusions using synchrotron radiation X‐ray tomographic microscopy. In ordinary chondrites, chromite is the only common mineral that survives long‐term weathering on Earth. Information about the silicate matrix of the original meteorite, however, can be derived from mineral inclusions preserved in the protecting chromite. The inclusions are crucial in the classification of fossil meteorites as well as sediment‐dispersed chromite grains from decomposed meteorites and larger impacts, as these are used for characterizing the past influx of material to Earth, but have previously been difficult to locate. The method is non‐destructive and time efficient for locating inclusions. The method allowed quantitative and morphological studies of both host chromite grains and inclusions in three dimensions. The study of 385 chromite grains from eight chondrites (H4–6, L4–6, LL4, LL6) reveals that inclusions are abundant and equally common in all samples. Almost two‐thirds of all chromite grains contain inclusions, regardless of group and type. The study also shows that the size of the inclusions and the host chromite grains, as well as the number of inclusions, within the host chromite grains vary with petrographic type. Thus, the petrographic type of the host of a suite of chromite grains can be determined based solely on inclusion content. The study also revealed that the amount of fractures in the host chromite can be correlated to previously assigned shock stages for the various chondrites. The study has thus shown that the features and inclusions of fossil chromite grains can give similar information about a former host meteorite as do studies of an unweathered whole meteorite, meaning that this technique is essential in the studies of ancient meteorite flux to Earth.  相似文献   

Sampling the Cape Verde Mantle Plume: Evolution of Melt Compositions on Santo Antao, Cape Verde Islands   总被引：1，自引：0，他引：1  

HOLM  P. M.; WILSON  J. R.; CHRISTENSEN  B. P.; HANSEN  L.; HANSEN  S. L.; HEIN  K. M.; MORTENSEN  A. K.; PEDERSEN  R.; PLESNER  S.; RUNGE  M. K. 《Journal of Petrology》2006,47(1):145-189

The volcanic history of Santo Antão, NW Cape Verde Islands,includes the eruption of basanite–phonolite series magmasbetween 7·5 and 0·3 Ma and (melilite) nephelinite–phonoliteseries magmas from 0·7 to 0·1 Ma. The most primitivevolcanic rocks are olivine ± clinopyroxene-phyric, whereasthe more evolved rocks have phenocrysts of clinopyroxene ±Fe–Tioxide ± kaersutite ± haüyne ± titanite± sanidine; plagioclase occurs in some intermediate rocks.The analysed samples span a range of 19–0·03% MgO;the most primitive have 37–46% SiO₂, 2·5–7%TiO₂ and are enriched 50–200 x primitive mantle in highlyincompatible elements; the basanitic series is less enrichedthan the nephelinitic series. Geochemical trends in each seriescan be modelled by fractional crystallization of phenocrystassemblages from basanitic and nephelinitic parental magmas.There is little evidence for mineral–melt disequilibrium,and thus magma mixing is not of major importance in controllingbulk-rock compositions. Mantle melting processes are modelledusing fractionation-corrected magma compositions; the modelssuggest 1–4% partial melting of a heterogeneous mantleperidotite source at depths of 90–125 km. Incompatibleelement enrichment among the most primitive magma types is typicalof HIMU OIB. The Sr, Nd and Pb isotopic compositions of theSanto Antão volcanic sequence and geochemical characterchange systematically with time. The older volcanic rocks (7·5–2Ma) vary between two main mantle source components, one of whichis a young HIMU type with ²⁰⁶Pb/²⁰⁴Pb = 19·88, 7/4 =–5, 8/4 0, ⁸⁷Sr/⁸⁶Sr = 0·7033 and ¹⁴³Nd/¹⁴⁴Nd= 0·51288, whereas the other has somewhat less radiogenicSr and Pb and more radiogenic Nd. The intermediate age volcanicrocks (2–0·3 Ma) show a change of sources to two-componentmixing between a carbonatite-related young HIMU-type source(²⁰⁶Pb/²⁰⁴Pb = 19·93, 7/4 = –5, 8/4 = –38,⁸⁷Sr/⁸⁶Sr = 0·70304) and a DM-like source. A more incompatibleelement-enriched component with 7/4 > 0 (old HIMU type) isprominent in the young volcanic rocks (0·3–0·1Ma). The EM1 component that is important in the southern CapeVerde Islands appears to have played no role in the petrogenesisof the Santo Antão magmas. The primary magmas are arguedto be derived by partial melting in the Cape Verde mantle plume;temporal changes in composition are suggested to reflect layeringin the plume conduit. KEY WORDS: radiogenic isotopes; geochemistry; mantle melting; Cape Verde  相似文献   

Pedoiogisk kortlægning og jordbundsbonitering i et geestrandområde ved Tønder     

BJARNE HOLM JAKOBSEN M.Sc. 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(1):17-32

Jakobsen, Bjarne Holm: Pedoiogisk kortlægning og jordbundsbonitering i et geestrandområde ved Tønder. Geografisk Tidsskrift 81: 17–32. København, Juni 1, 1981.

The result is presented of a detailed pedological survey of the geest at the margin of the Tønder Marsh-Area. Maps showing the parents sediments, the pedology and the drainage class have been constructed. Furthermore a soil capability map is shown using the land valuation principles of the Federal Republic of Germany.  相似文献   

THE KNUD RASMUSSEN MEMORIAL EXPEDITION Introduction     

dr. phil. N. KINGO JACOBSEN  CHR. BAY  B. FREDSKILD  B. GRøNNOW  B. HOLM JAKOBSEN  M. MELDGAARD 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(1):29-44

Jacobsen, N. Kingo, et.al. Report of activities at Tugtuligssuaq, Melville Bugt, 1978–79. Geografisk Tidsskrift 80: 29–44. Copenhagen, June 1980.

A joint cooperative Greenland/Danish expedition to the Melville Bugt area, Tugtuligssuaq/Kap Seddon to throw light on the Eskimos' immigration routes to Greenland seen in relation to climatic fluctuations. Studies of geography, botany, zoology, archaeology, and ethnohistory have been carried out.  相似文献   

Petrogenesis of Silicic Rocks from the Krksfjrdur Central Volcano, NW Iceland     

JNASSON  K.; HOLM  P. M.; PEDERSEN  A. K. 《Journal of Petrology》1992,33(6):1345-1369

Calc-alkaline dacites are found in the Tertiary Kroksfjordurcentral volcano besides the more typical tholeiitic dacitesand rhyolites characteristic of Icelandic rift zones. It isclear that these calc-alkaline dacites are not subduction related,but their chemistry and petrography are definitely calc-alkaline.They are much lower in Fe and higher in Ca and Al than otherrocks from Iceland with comparable silica percentages. Theyare significantly depleted in the high field strength elements(HFSE, e.g., Nb, Zr, Y, and heavy rare earth elements), andsome of them contain hydrous phases or their relics. The anhydrousphase assemblage and relatively high Fe content of the tholeiiticsilicic rocks indicate generation at shallow depths (P_H2o <1kb). The high Ca and Al contents and the depletion of HFSE inthe calc-alkaline dacites indicate generation by partial meltingof amphibolite facies rocks at 5–6 km depth. The generationof the tholeiitic silicic rocks requires a shallow magma chamber,where they could be formed by fractionation of basaltic magma,or by partial melting of country rock heated by basaltic magma.The calc-alkaline dacites require a different volcanotectonicenvironment for their generation. The geothermal gradient requiredis comparable with that of regional geothermal gradients closeto the active rift zones. They were probably formed when thecentral volcano was drifting away from the rift axis and itsactivity was waning. Intrusion of basaltic magma, probably relatedto another volcanic center, mobilized the dacite magma.  相似文献