wall rock alteration and metallogenic chronology of jinchang gold deposit in dongning county heilongjiang province the jinchang gold deposit located geographically in eastern heilongjiang province and tectonically at the easternmost portion of the central asian orogenic belt is one of the important gold districts in eastern jilin heilongjiang provinces mesozoic granitic intrusions and hydrothermal rocks related to gold mineralization are widespread in this region although numerous researches have been undertaken on this deposit its genetic type age and tectonic setting remain controversial mainly because of the lack of precise and reliable geochronological data vertical and lateral hydrothermal alteration zoning of potassic phyllic propylitic and argillic minerals can be observed within and around the porphyry granite these hydrothermal characteristics are similar to things of the typical porphyritic deposits sericites seperated from the pyrite phyllic altered granite with early mesozoic u pb ages and porphyry granite yield rb sr ages of ca 110 ma or so and zircons from the porphyry granite yield a u pb age of 113 2 ma 2 therefore the timing of the main gold mineralization in the jinchang gold deposit is constrained around 110 ma and the porphyry granite is spatially and temporally associated with gold mineralization the denudation of the ore bodies indicates that the porphyry granite has not been exposed at the surface and the prospecting potential at depth is great

the rock elm meteorite impact structure wisconsin geology and shock metamorphic effects in quartz the rock elm structure in southwest wisconsin is an anomalous circular area of highly deformed rocks 6 5 km in diameter located in a region of virtually horizontal undeformed sedimentary rocks shock produced planar microstructures pms have been identified in quartz grains in several lithologies associated with the structure sandstones quartzite pebbles and breccia two distinct types of pms are pres ent p1 features which appear identical to planar fractures pfs or cleavage and p2 features which are interpreted as possible incipient planar deformation features pdfs the latter are uniquely produced by the shock waves associated with meteorite impact events both types of pms are oriented parallel to specific crystallographic planes in the quartz most commonly to c 0001 11 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 2 end document 2 and r z 10 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document overline 1 end document 1 the association of unusual structurally deformed strata with distinct shock produced microdeformation features in their quartz bearing rocks establishes rock elm as a meteorite impact structure and supports the view that the presence of multiple parallel cleavages in quartz may be used independently as a criterion for meteorite impact preliminary paleontological studies indicate a minimum age of middle ordovician for the rock elm structure a similar age estimate 450 400 ma is obtained independently by combining the results of studies of the general morphology of complex impact structures with estimated rates of sedimentation for the region such methods may be applicable to dating other old and deeply eroded impact structures formed in sedimentary target rocks

magmatism geological setting alteration and metallogenic potential of donghua area dehua county fujian province southeast china insights from porphyry zircon u pb and pyrite rb sr geochronology geochemistry and remote sensing abstract located in the southern section of the wuyi mountain metallogenic belt the dehua youxi yongtai dyy ore district is one of the potential areas of gold deposits in the fujian province southeast china gold mineralization is widely distributed in this district including qiucun shuangqishan ancun and dongyang gold deposits however the dyy ore district is still at a low degree of exploration and research this paper focuses on the donghua area in the northwest of the dyy ore district as its typical and integrated volcano magma mechanism revealed in this study combined approaches including zircon shrimp u pb geochronology rb sr isochron dating and mineral chemistry of pyrite and hyperspectral remote sensing are employed aiming to identify the magmatism alteration geological setting and metallogenic potential of the donghua area the zircon shrimp u pb ages of the granite porphyritic dykes in donghua are 154 6 1 1 ma and 153 2 1 2 ma respectively indicating that the dykes were formed in the late jurassic the rb sr geochronological age of hydrothermal epithermal pyrite in donghua is 153 7 2 4 ma which is almost coeval with that of the granite porphyritic dykes within error and consistent with the regional volcanic magmatic activity and the metallogenic age of the regional metallogenic event the granitic dykes in donghua exhibit the characteristics of both a type and i type granitoids thus indicating the presence of transitional geochemical characteristics as a result of the subduction of the pacific plate towards the eurasia the ca 154 153 ma magmatic event in the donghua area occurred at the beginning of large scale delamination of the continental crust the zoning distribution of the mineral alteration of chlorite dickite kaolinite and gypsum the interpretation of volcanic activity from the annular structure of hyperspectral remote sensing and the inferred deep seated intrusion emplacing from northwest to southeast are in agreement thus displaying the volcano magma alteration mechanism developed in the donghua area the magmatism dykes faults and neoproterozoic metamorphic rock provide favorable advantages for the mineralization in donghua suggesting metallogenic potential

sedimentary signatures and processes during marine bolide impacts a review abstract studies of submarine impact craters resulting from impacts of comets or asteroids demonstrate that the presence of water and the physical properties of target rocks have a major influence on sedimentary processes associated with meteorite impacts this results in difference in sedimentary signature of bolide impacts in marine environments compared to subaerial impact craters in subaerial impacts the targets are commonly hard rocks frequently of igneous and or metamorphic origin whereas in submarine impacts the targets are mostly unconsolidated or poorly lithified sediments or sedimentary rocks with high volumes of pore water such differences result in variability in crater morphology and in sedimentary processes inside and outside the impact area impacts in shallow water marine neritic environments produced craters with low or absent rims and wide and shallow brims as characterize by both the montagnais on the scotian shelf the mjolnir in the barents sea 45 and 40 km in diameter respectively and the chesapeake bay 90 km in diameter lack of elevated rims is thought to be the result of current reworking and resurge of the water back into the excavated cavity as the water in the crater is vaporized during this process resurge gullies can be cut across the rim while mass and debris flows turbidites and other gravity deposits are produced as results of tsunami and crater wall and central high collapse during and after the crater excavation stage such deposits are found both within and outside the crater structure the only difference between gravity deposits triggered by an impact or other rare events such as earthquakes is the admixture of various melt particles and possible enrichments in iridium in the former impacts near the shelf edge may cause partial collapse of the continental margin as shown by the montagnais and chicxulub impacts some of the gravity and debris flows generated by margin collapse may be channelized with final deposits up to several hundred meters thick extending for hundreds of kilometers from the impact site other impact features such as shatter cones tektites spherules and ni spinels shocked quartz isotropication and partial melts are common to both submarine and subaerial impacts theoretical calculations of the destructive forces of mega tsunami waves triggered by meteorite impact in the ocean greatly exceed those based on geologic evidence a dearth of gigantic tsunami evidence for the chicxulub impact outside of the gulf of mexico where from theoretical modelling the maximum near bottom orbital velocity of water flow crossing the deep north atlantic basin should have been 1 m s could be the result of the mitigating effect of the bathymetry of surrounding area causing wave diffraction and interference calculated maximum horizontal orbital velocity near the seafloor at the shelf for the montagnais impact is 22 m s for a 200 m high wave 5 5 m s for a 50 m high wave at 500 km decreasing to 0 5 m s at 1000 km strong enough to scour the deep ocean bottom and produce distinct erosion surfaces and disconformities in marine sedimentary record however lack of cores across impact horizon prevents confirmation of occurrence of such bottom water flows

the age geological setting and types of gold deposits in the yanbian and adjacent areas ne china abstract the eastern part of the xing an mongolian orogenic belt xmob in jilin and heilongjiang provinces ne china is highly prospective for gold mineralization and more than ten gold and gold copper deposits have been found in the yanbian and dongning areas in the last decade these deposits are classified based on geological characteristics and mineralogical and geochemical information into orogenic intrusion related gold rich porphyry and epithermal deposits the latter are further divided into high sulfidation hs and low sulfidation ls deposit types we present the results of zircon laser ablation u pb dating of some representative gold deposits and associated magmatic rocks within the eastern xmob these are the orogenic wudaogou deposit the intrusion related naozhi deposit and the epithermal jiusangou gold deposit the weighted mean age of the granodiorite that hosts mineralization at wudaogou is 253 1 0 3 ma whereas zircons from a porphyritic dacite dike that was intruded at the same time as the formation of the naozhi deposit yielded two groups of weighted mean ages 198 9 3 6 and 126 3 1 7 ma zircons from a porphyritic quartz diorite within the breccia that hosts the jiusangou deposit yielded a weighted mean age of 108 1 1 4 ma these data combined with data from previous studies enable the identification of three periods of phanerozoic gold mineralization within the yanbian area and adjacent regions these are ca 270 240 ca 130 120 and ca 115 100 ma the late permian to early triassic 270 240 ma events formed the orogenic gold deposits at yangjingou and wudaogou both are located near the xar moron changchun suture zone between the north china craton and the xmob the early cretaceous 130 120 ma events produced intrusion related gold mineralization such as the naozhi and miantian deposits which are associated with jurassic granodiorite plutons or volcanic rocks almost all of the gold rich porphyry and epithermal gold deposits in this area formed during the late early cretaceous 115 110 ma we propose that the orogenic gold mineralization in the yanbian area was closely related to the collision between the north china craton and the eastern xmob whereas the other gold deposit types formed at early cretaceous lithospheric extensions associated with post subduction tectonics

high resolution oi 630 nm image measurements of f region depletion drifts during the guara campaign a high performance all sky imaging system has provided data on the evolution and drift motions of f region depletions above the magnetic dip equator at alc ntara brazil 2 3 s 44 5 w monochromatic images of depletions in the oi 630 nm nightglow were recorded on eight nights during 1 16 october 1994 as part of the guara campaign the drift motions of the depletions were typically 80 100 m s eastward prior to local midnight and reduced to a minimum of 30 50 m s in the morning hours in accord with previous observations however on october 2 3 and 12 13 the depletions were observed to reverse direction for 60 90 min achieving westward speeds of 30 m s before the motion reverted to eastward around 0100 lt and accelerated to 35 45 m s near dawn magnetic activity and other evidence suggests that these reversals in the motion of the airglow depletions probably result from reversals in the f region dynamo rather than from shifts in the altitude of the shear in the nighttime f region plasma drift

detailed structural analysis of the rim of a large complex impact crater bosumtwi crater ghana the 1 ma bosumtwi crater in ghana is an 11 km diameter presumably complex well preserved impact structure that is associated with the ivory coast tektite strewnfield detailed structural geologic studies along a complete traverse through the northwestern rim section indicated four zones characterized by distinct deformation styles from just outside of the crater rim to near the crater floor zone 1 is dominated by thick deposits of lithic impact breccia intercalated in places with products of local mass wasting zone 2 contains inward dipping thrust planes conjugate radial fractures isoclinal folding and overturned stratigraphic sequences zone 3 represents a megabreccia zone in which block size decreases upward and outward toward the rim crest the innermost zone 4 is dominated by intense thrust faulting of multiple orientations resulting in complex duplex and lens shaped bodies these deformation styles generally correspond to those previously reported from the rims of simple bowl shaped meteorite impact craters and appear to be characteristic of impact structures in general

drill core lb 08a bosumtwi impact structure ghana petrographic and shock metamorphic studies of material from the central uplift during a recent drilling project sponsored by the international continental scientific drilling progam icdp two boreholes lb 07a and lb 08a were drilled into the crater fill of the bosumtwi impact structure and the underlying basement into the deep crater moat and the outer flank of the central uplift respectively the bosumtwi impact structure in ghana west africa which is 10 5 km in diameter and 1 07 myr old is largely filled by lake bosumtwi here we present the lithostratigraphy of drill core lb 08a recovered between 235 6 and 451 33 m depth below lake level as well as the first mineralogical and petrographic observations of samples from this core this drill core consists of approximately 25 m of polymict clast supported lithic breccia intercalated with suevite which overlies fractured brecciated metasediment that displays a large variation in lithology and grain size the lithologies present in the central uplift are metasediments composed dominantly of fine grained to gritty meta graywacke phyllite and slate as well as suevite and polymict lithic impact breccia the suevites principally present between 235 6 and 240 5 m and between 257 6 and 262 2 m display a fine grained fragmental matrix about 39 to 45 vol and a variety of lithic and mineral clasts that include meta graywacke phyllite slate quartzite carbon rich organic shale and calcite as well as melt particles fractured quartz unshocked quartz unshocked feldspar quartz with planar deformation features pdfs diaplectic quartz glass mica epidote sphene and opaque minerals the crater fill suevite contains calcite clasts but no granite clasts in contrast to suevite from outside the northern crater rim the presence of melt particles in suevite samples from the uppermost 25 meters of the core and in suevite dikelets in the basement is an indicator of shock pressures exceeding 45 gpa quartz grains present in suevite and polymict lithic impact breccia abundantly display 1 to rarely 4 sets of pdfs per grain the shock pressures recorded by the pdfs in quartz grains in the polymict impact breccia range from 10 to 30 gpa we also observed a decrease of the abundance of shocked quartz grains in the brecciated basement with increasing depth meta graywacke samples from the basement are heterogeneously shocked with shock pressures locally ranging up to 2530 gpa suevites from this borehole show a lower proportion of melt particles and diaplectic quartz glass than suevites from outside the northern crater rim fallback impact breccia as well as a lack of ballen quartz which is present in the external breccias similar variations of melt particle abundance and shockmetamorphic grade between impact breccia deposits within the crater and fallout impact breccia outside the crater have been observed at the ries impact structure germany

large scale porphyry type mineralization in the central asian metallogenic domain a review abstract a series of porphyry type cu mo au deposits including some giant world class deposits such as kounrad aktogai kal makyr oyu tolgoi and chalukou occur in the central asian metallogenic domain camd the outline of the camd almost overlaps with that of the central asian orogenic belt caob which is located between the east europe siberia tarim and north china cratons the camd comprises sparsely exposed tectonic slices of precambrian high grade metamorphic basement rocks that are sandwiched between phanerozoic accretionary complexes and arc rocks according to geological metallogenic geochronological and tectonic setting characteristics the camd comprises three metallogenic provinces 1 the kazakhstan cu au mo 2 the mongolia cu au and 3 the northeast china mo cu metallogenic provinces early paleozoic 490 to 440 ma and late paleozoic 330 to 295 ma large scale porphyry type mineralization in the kazakhstan metallogenic province is intimately associated with the evolution of the central tianshan yili northern tianshan kokchetav chingiz paleozoic arc system which was terminated by the late permian closure of the western segment of the paleo asian ocean in contrast the 370 ma and 240 ma porphyry type mineralization of the mongolia metallogenic province is related to the tuva mongolia arc collage system that was formed during the evolution of the paleo asian ocean in the paleozoic and the mongol okhotsk ocean in the early mesozoic furthermore the northeast china metallogenic province was affected by three distinct geodynamic processes that were associated with the paleo asian mongol okhotsk and paleo pacific oceans during the paleozoic 250 ma the early mesozoic 200 ma and the late mesozoic 200 90 ma respectively except for the early paleozoic cu mineralization at duobaoshan 480 ma and bainaimiao 440 ma the porphyry type mineralization of this metallogenic province is predominantly characterized by widespread mesozoic mo mineralization in general porphyry type deposits of the camd have geological structural metallogenic alteration and fluid characteristics which are consistent either with those related to the oceanic subductional arc settings of the circum pacific metallogenic domain or with those of the continental collisional environment in the tethyan metallogenic domain the involvement of highly oxidized intermediate to felsic magmas is fundamental for the formation of porphyry deposits even though the prevalent mash and slab melting models can be employed to interpret the genesis of some porphyry cu deposits in the camd a here newly suggested model which is based on pre enriched mafic lower crust and the subduction of a relictic mid oceanic ridge is thought to explain the large scale copper mineralization in the balkhash west junggar metallogenic belt of the kazakhstan metallogenic province nevertheless most of the porphyritic suites associated with the mo deposits in the northeast china metallogenic province have relatively depleted mantle like neodymium and hafnium isotopic compositions suggesting that the ore forming granitic magma may have predominantly been derived from juvenile lower crust upwelling of asthenosphere may have triggered the melting of the lower crust during post collisional extension subsequent to the closure of the paleo asian ocean in the triassic the subduction of the paleo pacific ocean in the jurassic and the thinning of the lithosphere caused by rollback of the paleo pacific plate in the cretaceous ancient crustal components were only locally found to have been involved in the magma source of several medium large mo deposits

ridge segmentation and the magnetic structure of the southwest indian ridge at 50 30 e 55 30 e and 66 20 e implications for magmatic processes at ultraslow spreading centers 1 the aim of this paper is to investigate the relationships between the segmentation and the magnetic structure of the ultraslow spreading southwest indian ridge contrary to faster spreading ridges magnetization usually decreases from high values along the neovolcanic axis to low values in the nontransform discontinuities there is a direct correlation between the deepening of the axial valley and the decrease of the magnetization from the neovolcanic axis toward the deepest parts of the axial discontinuities we suggest that less frequent eruptions as the distance from the segment center and the length of these discontinuities increase result in thinner extrusive lavas and thus control the along axis magnetization variations by thinning the magnetic source layer a unique segment centered at 50 28 e shows a marked low magnetization anomaly at its center similarly to the segments of the slow spreading mid atlantic ridge we suggest that in this segment both the mantle temperature and the magmatic activity are high enough for the lavas not to be highly fractionated a higher rate of melt production to the west of gallieni transform fault may have created some form of reservoir where mixing of melts occurs and where crystalline fractionation is low producing low magnetization lavas to the east magma chambers may be smaller with cooler mantle temperatures resulting in restricted mixing and significant fractionation which may lead to relatively high intensity magnetization lavas finally we propose that serpentinization of peridotites has no significant contribution to the variation of the magnetization along the axial valley off axis in thin crust areas upper mantle rocks may become progressively more altered as distance from the axis increases the strong faulting and alteration of a thin basaltic cap and underlying upper mantle rocks can produce the disappearance of the magnetic reversal pattern and the increase of the magnetization which is observed along the traces of the largest amagmatic discontinuities by contrast in thicker crust areas the upper mantle rocks are shielded from the alteration and the serpentinization process may be delayed resulting as on the mid atlantic ridge in slightly more positive magnetization values along the traces of axial discontinuities regardless of polarity

first detection of the supersonic upward plasma flow structures in the early morning sector we present the first observations of the supersonic updrafting plasma drifts in the predawn sector two dmsp satellites quasi simultaneously detected two fast speed events one of 385 km spatial extension and with the maximum upward velocity of 1683 m s appeared at 3 lt and the other of 1500 km large with maximum speed of 1770 m s occurred at 5 lt both supersonic structures were observed above the eastern pacific region separated by 35 of longitude in space and by 45 min in time the events occurred at the recovery phase of the geomagnetic storm of 19 february 2014 during rapid oscillations of the interplanetary magnetic field b z and the interplanetary electric field e y components which increased the eastward electric field in the equatorial nighttime ionosphere and triggered the generation of plasma irregularities the storm time penetration electric fields seem to be the principal driver of the observed supersonic events

red wing creek structure north dakota petrographical and geochemical studies and confirmation of impact origin the 9 km diameter red wing creek structure north dakota is located within the oil rich williston basin at 47 36 n and 103 33 w earlier geophysical studies indicated that this subsurface structure has a central uplift surrounded by an annular crater moat and a raised rim breccias were encountered during drilling between 2000 and 2800 m depth in the central uplift area and the presence of shatter cone fragments in drill core samples was suggested to indicate an impact origin of the red wing creek structure we studied the petrographic and geochemical characteristics of samples of well cuttings from two boreholes at the center of the structure the true oil 22 27 burlington northern and true oil 11 27 burlington northern wells we found planar deformation features pdfs in quartz with up to three sets of different crystallographic orientations in sandstone and siltstone dominated samples from the true oil 11 27 borehole u stage measurements of the crystallographic orientations of the pdfs showed the occurrence of the shock characteristic 0001 and orientations with a dominance of 0001 and orientations the relative frequencies of the orientations indicate a shock pressure of at least 12 20 gpa these results provide unambiguous evidence for shock metamorphism at red wing creek and confirm that the structure was formed by impact

geochemistry and age of ivory coast tektites and microtektites abstract ivory coast tektites were first reported in 1934 from a geographically restricted area at ivory coast west africa although some additional specimens have been found later the total number remains small a few hundred the bosumtwi impact crater in ghana is most likely the source crater for the ivory coast tektites based on the finding that the tektites and the crater have the same age as well as similar isotopic and chemical compositions in addition to tektites on land microtektites were found in so far eleven deep sea cores off the west african coast between about 9 n and 11 s and 0 and 23 w defining the extent of the ivory coast tektite strewn field in this study we analyzed eleven ivory coast tektites for their major and trace element composition studied their petrographical characteristics provided major element data for 111 microtektites and major and trace element data for four microtektites we determined the 40ar 39ar step heating age of five ivory coast tektites and four microtektites and obtained fission track dates for ten tektites and one bosumtwi impact glass the tektites have very small intersample and intrasample variations of their major and trace element composition 111 ivory coast microtektites from eleven cores were analyzed for their major element compositional range their compositional range is significantly wider than that of the ivory coast tektites but the majority of all microtektites have compositions very similar to those of the tektites within a factor of 1 2 trace element compositions of the tektites also show little variation between samples the samples do not show any distinct eu anomaly in the ree patterns this characteristic as well as the high absolute ree abundances and la nybn ratios of about 8 indicate that archean rocks are plausible source rocks the major and trace element contents of four individually analyzed ivory coast microtektites show compositions that are very similar to those of the ivory coast tektites however the microtektites contain 20 rel higher abundances of some of the lithophile and siderophile trace elements such as sc cr co ni sr zr ba hf ta th and the rees these differences are probably due to incorporation of a higher abundance of accessory trace minerals with the microtektite forming melt the ivory coast microtektites also have a uniform internal composition duplicate 40ar 39ar step heating age analyses were performed on five tektites the best age estimate for the formation age of the tektites was calculated by taking a weighted average of the ages from the plateau portions of the runs resulting in an age of 1 1 0 05 ma we also tried to date four microtektites by 40ar 39ar age analyses but their young age and small sample size makes it impossible to assign a reliable age to the microtektites one run yielded satisfactory results that were similar to the tektite age in addition we determined the fission track ages for ten individual ivory coast tektite samples and for one impact glass sample from the bosumtwi crater the track size corrected ages for the ivory coast tektites ranged from 0 91 to 1 18 ma resulting in an average fission track age of 1 05 0 11 ma this age is within errors identical to that of the bosumtwi impact glass at 1 03 0 11 ma and to the 40ar 39ar age of 1 1 0 05 ma the preferred age of the ivory coast tektite event is 1 07 ma

petrography geochemistry and alteration of country rocks from the bosumtwi impact structure ghana samples of the country rocks that likely constituted the target rocks at the 1 07 myr old bosumtwi impact structure in ghana west africa collected outside of the crater rim in the northern and southern parts of the structure were studied for their petrographic characteristics and analyzed for their major and trace element compositions the country rocks mainly meta graywacke shale and phyllite of the early proterozoic birimian supergroup and some granites of similar age are characterized by two generations of alteration a pre impact hydrothermal alteration often along shear zones is characterized by new growth of secondary minerals such as chlorite sericite sulfides and quartz or replacement of some primary minerals such as plagioclase and biotite by secondary sericite and chlorite a late argillic alteration mostly associated with the suevites is characterized by alteration of the melt glass clasts in the groundmass of suevites to phyllosilicates suevite which occurs in restricted locations to the north and to the south southwest of the crater rim contains melt fragments diaplectic quartz glass ballen quartz and clasts derived from the full variety of target rocks no planar deformation features pdfs in quartz were found in the country rock samples and only a few quartz grains in the suevite samples show pdfs and in rare cases two sets of pdfs based on a total alkali element silica tas plot the bosumtwi granites have tonalitic to quartz dioritic compositions the nb versus y and ta versus yb discrimination plots show that these granites are of volcanic arc tectonic provenance provenance studies of the metasedimentary rocks at the bosumtwi crater have also indicated that the metasediments are volcanic arc related compared to the average siderophile element contents of the upper continental crust both country rocks and impact breccias of the bosumtwi structure show elevated siderophile element contents this however does not indicate the presence of an extraterrestrial component in bosumtwi suevite because the birimian country rocks also have elevated siderophile element contents which is thought to result from regional hydrothermal alteration that is also related to widespread sulfide and gold mineralization

when a cosmic impact strikes the sea bed most bolides that collide with the earth hit the sea limited knowledge about marine target impacts hampers predictions about their perilousness this study presents geological features that are particular to craters formed at sea the features are most likely a result of the influence of the target on the cratering process marine target craters form only if the target sea is shallow enough to admit sufficient kinetic energy into the sea bed when the crater diameter is large compared to the water depth the crater resembles its counterparts that are formed on land craters formed in deeper water are concentric and often lack melt sheets and rim walls but have deposits and radial gullies formed by the resurge of the sea impacts on the deep shelf are probably much more energetic than is suggested by the dimensions of the preserved crater

c nofs observations of deep plasma depletions at dawn 1 the communication navigation outage forecasting system c nofs satellite was launched in 2008 during solar minimum conditions an unexpected feature in the c nofs plasma density data is the presence of deep plasma depletions observed at sunrise at all satellite altitudes ionospheric irregularities are often embedded within these dawn depletions their frequencies strongly depend on longitude and season dawn depletions are also observed in coincident satellite passes such as dmsp and champ in one example the depletion extended 50 14 in the n s and e w directions respectively these depletions are caused by upward plasma drifts observed in c nofs and ground based measurements the reason for these upward drifts is still unresolved we discuss the roles of dynamo electric fields over shielding and tidal effects as sources for the reported depletions

equatorial ionospheric plasma bubble irregularity occurrence and zonal velocities under quiet and disturbed conditions from polarimeter observations amplitude scintillation and faraday rotation angle fluctuations of satellite vhf beacons registered by electronic polarimeters operated at two stations spaced 110 km apart in the magnetic east west direction namely cachoeira paulista 22 41 s 45 w dip angle of 26 and sao jose dos campos 23 12 s 45 51 w dip angle of 26 in brazil are analyzed for equinoctial and summer months of 1982 1983 to determine the irregularity patch occurrence characteristics and their zonal velocities under varying levels of magnetic disturbances the scintillation patch occurrences and total electron content fluctuations representing plasma depletion structures decrease during premidnight hours and are enhanced in the postmidnight period the occurrences often extend to morning hours with increasing magnetic activity index while the night events are associated with eastward irregularity patch motion the morning events have westward velocities the reversal occurring around 0400 lt reflecting the morning velocity reversal in the f region bulk plasma motion reported earlier from jicamarca radar measurements the westward plasma bubble motions reported here represent the first observation of this kind another important finding from the present investigation is a clear tendency for the zonal plasma motion namely the equatorial meridional electric field to become enhanced mainly in the premidnight hours immediately following strong magnetic activity

hydrothermal alteration in the bosumtwi impact structure evidence from 2m1 muscovite alteration veins and fracture fillings drill core samples from the bosumtwi impact structure 1 07 myr old and 10 5 km in diameter in ghana exhibit mineralogical evidence for post impact hydrothermal alteration nine samples of drill core obtained through the 2004 international continental scientific drilling project icdp were studied including an uppermost fallback layer overlying impactite breccias and partly deformed massive meta graywacke bedrock the petrographic study revealed alteration veins containing secondary sericitic muscovite comparable to 2m1 muscovite crosscutting original bedding in meta graywacke and forming a matrix between clasts in impactite breccias x ray diffraction xrd shows that these impactite samples are rich in 2m1 muscovite consistent with post impact fluid deposition and alteration optical analysis indicates the presence of a pre impact stratiform chlorite in meta graywacke samples and a secondary alteration chlorite occurring in all samples secondary illite was detected in upper impactites of drill core lb 08a and samples containing accretionary lapilli the lower temperature constraint for the hydrothermal event is given by 2m1 muscovite secondary chlorite and illite all of which form at temperatures greater than 280 c an absence of recrystallization of quartz and feldspar indicates an upper temperature constraint below 900 c the presence of alteration materials associated with fractures and veins in the uppermost impactites of drill cores lb 07a and lb 08a indicates that a post impact hydrothermal system was present in and adjacent to the central uplift portion of the bosumtwi impact structure a sample containing accretionary lapilli obtained from drill core lb 05a exhibits limited evidence that hydrothermal processes were more widespread within the impactites on the crater floor

shock metamorphic petrography and microraman spectroscopy of quartz in upper impactite interval icdp drill core lb 07a bosumtwi impact crater ghana standard and universal stage optical microscope and microraman spectroscopic examination of quartz from the upper impactite interval of the international continental scientific drilling program icdp lake bosumtwi crater drill core lb 07a demonstrates widespread but heterogeneous evidence of shock metamorphism in the upper impactite which comprises interbedded polymict lithic breccia and suevite from a drilling depth of 333 4 415 7 m quartz occurs as a major component within metasedimentary lithic clasts and as abundant isolated single crystal grains within matrix the noted quartz shock metamorphic features include phenomena related to a deformation such as abundant planar microstructures grain mosaicism and reduced birefringence b phase transformations such as rare diaplectic quartz glass and very rare coesite c melting such as isolated colorless to dark glassy and devitrified vesicular melt grains and d secondary post shock features such as abundant variable decoration of planar microstructures and patchy grain toasting common to abundant planar deformation features pdfs in quartz are dominated by 1013 equivalent crystallographic planes although significant percentages of 1012 and other higher index orientations also occur notably c 0001 planes are rare significantly the quartz pdf orientations match most closely those reported elsewhere from strongly shocked crystalline target impactites barometry estimates based on quartz alteration in the upper impactite indicate that shock pressures in excess of 20 gpa were widely reached pressures exceeding 40 45 gpa were more rare the relatively high abundances of decorated planar microstructures and grain toasting in shocked quartz together with the nature and distribution of melt within suevite suggest a water or volatile rich target for the bosumtwi impact event

possible reasons of shock melt deficiency in the bosumtwi drill cores pre drilling numerical modeling of the bosumtwi impact event predicted a 200 m thick coherent melt layer as well as abundant highly shocked target material within the central part of the crater structure however these predictions are in disagreement with data from drill core obtained in 2004 2005 here i provide a brief overview of previous results and discuss possible reasons behind melt deficiency such as specific impact scenarios low impact velocity and or low impact angle and specific target properties different composition high porosity high content of volatiles i conclude that the most likely explanation is the dispersion of impactites due to the vaporization of pore water which was not included in the original numerical model

early morning irregularities detected with spaceborne gps measurements in the topside ionosphere a multisatellite case study we present observations of the equatorial plasma bubbles epb in the topside ionosphere at early morning hours 05 08 lt in the recovery phase of the 18 19 february 2014 geomagnetic storm this rare type of irregularities was detected in the pacific sector using gps measurements on board several low earth orbit leo satellites we use a multisatellite constellation consisted of the three swarm and one terrasar x satellites that on 19 february flew in the same region and at similar altitudes 500 km the epb occurrence in the leo gps data was observed for several consecutive orbits from 11 ut to 16 17 ut on 19 february 2014 which suggests the following 1 rather long duration hours of favorable conditions for epb generation 2 formation and evolution of epb over wide longitude range of the pacific ocean and 3 possible movement of the epb region in the westward direction with dawn registration of the early morning epb in leo gps data was supported by concurrent in situ swarm and dmsp defense meteorological satellite program and ground based ionosonde and gps measurements leo based gps technique is found to be essential and promising data source to study the topside epb over regions with lack of the ground based facilities in addition we use the prompt penetration model and the thermosphere ionosphere electrodynamics global circulation model tie gcm to identify the possible mechanisms responsible for the observed phenomenon the model simulation results indicate the occurrence of the zone with the enhanced vertical plasma drift 40 45 m s owing to the disturbance dynamo action in the predawn dawn sector during 09 17 ut

regional metallogeny of mo bearing deposits in northeastern china with new re os dates of porphyry mo deposits in the northern xilamulun district with many new discoveries of giant mo deposits in the past decade northeastern china has become the largest molybdenum mineralization region in china the xilamulun district is in the southern part of northeastern china and contains 25 mo deposits and prospects dominantly of porphyry type these deposits are mostly along the e striking xilamulun fault re os dating of hydrothermal molybdenite from four deposits in the northern xilamulun district revealed mineralization ages of 137 to 129 ma 129 4 3 4 ma for aolunhua 135 3 2 6 ma for shabutai 136 4 0 8 ma for haisugou and 136 1 6 6 ma for banlashan in general agreement with the crystallization ages of their host granitoids the compilation of existing data on mo bearing deposits in northeastern china including the new data of this study shows that mesozoic mo deposits in this region have a wide range of ages from 250 to 90 ma we propose that they are linked to three tectonic magmatic events 1 triassic mo deposits 250 220 ma are mainly distributed along the e trending xilamulun fault and are related to postcollisional crustal extension following the final closure of the paleo asian ocean 2 jurassic mo mineralization 200 140 ma displays a clear younging trend from southeast to northwest coincides with the regional magmatism trend and is interpreted to be related to the northwestward flat slab subduction of the paleo pacific plate beneath the eurasian continent that started in the early jurassic ca 200 ma 3 cretaceous mo mineralization 140 90 ma shows a reversed migration trend from northwest to southeast and can be explained by the coastward migration of slab rollback related to lower crust delamination asthenospheric upwelling and lithospheric thinning in eastern china the spatial temporal distribution of mesozoic mo mineralization is important for regional metallogeny and exploration recently numerous epithermal au cu mo deposits have been recognized in the southeast margin of northeastern china according to this study we predict that hidden porphyry skarn systems could be located beneath these epithermal deposits

erratum microscopic planar features in quartz from scollard canyon alberta and the cretaceous tertiary boundary event quartz grains separated from the cretaceous tertiary k t boundary clay at scollard canyon alberta have prominent microscopic planar features these occur in 15 30 of the grains from the lower

origin of toasted quartz in terrestrial impact structures quartz present in the target rocks at impact sites and as clasts within impact melt rocks and some pseudotachylytes commonly exhibits a brown nonpleochroic grainy appearance in plane and cross polarized light known as toasted quartz this variety of quartz also possesses a higher albedo in hand sample than colorless examples despite the intense coloration no compositional origin for the browning is evident a high proportion of exceptionally small typically submicron scale fluid inclusions which are typically too small to be observed with the petrographic microscope is considered to be the cause these fluid inclusions are predominantly located along decorated planar deformation features and they enhance scattering of transmitted light and reflect larger quantities of incident light statistically lower sio 2 and total oxide contents present in toasted quartz relative to untoasted quartz are consistent with increased surface electron scattering during microprobe analysis by fluid inclusions intersecting the surface

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

shocked quartz in the cretaceous tertiary boundary clays evidence for a global distribution shocked quartz grains displaying planar features were isolated from cretaceous tertiary boundary clays at five sites in europe a core from the north central pacific ocean and a site in new zealand at all of these sites the planar features in the shocked quartz can be indexed to rational crystallographic planes of the quartz lattice the grains display streaking indicative of shock in x ray diffraction photographs and also show reduced refractive indices these characteristic features of shocked quartz at several sites worldwide confirm that an impact event at the cretaceous tertiary boundary distributed ejecta products in an earth girdling dust cloud as postulated by the alvarez impact hypothesis

impact origin of the chesapeake bay structure and the source of the north american tektites seismic profiles drill core samples and gravity data suggest that a complex impact crater 35 5 million years old and 90 kilometers in diameter is buried beneath the lower chesapeake bay the breccia that fills the structure contains evidence of shock metamorphism including impact melt breccias and multiple sets of planar deformation features shock lamellae in quartz and feldspar the age of the crater and the composition of some breccia clasts are consistent with the chesapeake bay impact structure being the source of the north american tektites

continuous penetration of the interplanetary electric field to the equatorial ionosphere over eight hours during intense geomagnetic storms 1 the interplanetary electric field penetrates to the low latitude ionosphere and has significant influence on the ionospheric electrodynamics during intense geomagnetic storms it is critical to understand how much of the interplanetary electric field can penetrate to the ionosphere and how long penetration electric fields can exist in this study we present the defense meteorological satellite program dmsp satellite measurements of the equatorial ionospheric ion vertical velocity in the dusk sector during two intense geomagnetic storms we find that the equatorial ionospheric electric field at dusk is approximately proportional to the interplanetary electric field for 8 10 hours during the storm main phase the penetration efficiency is relatively constant over the entire interval and is 5 at dusk in one storm case the long lasting storm time penetration electric field causes significant redistribution of the ionospheric plasma this study reveals that penetration electric fields can last for 8 10 hours without effective shielding when the magnetic storm activity is strengthening during continuous southward interplanetary magnetic field the result implies that the interplanetary electric field is linearly coupled with the dusk equatorial ionosphere during the entire main phase of intense magnetic storms which is very important for understanding the storm time ionospheric electric field and electrodynamics

pronounced central uplift identified in the bosumtwi impact structure ghana using multichannel seismic reflection data the lake bosumtwi impact structure is the youngest and best preserved complex terrestrial impact crater and serves as an important reference site for the study of cratering processes because the impacting body struck continental crystalline target rocks and not a submerged sedimentary platform no significant backwash processes have modified the crater morphology not only may bosumtwi contain the best preserved central uplift structure on earth but it is the most accessible relatively large young crater in the solar system generated in a large gravity field there is a well established link between the lake bosumtwi impact structure and the ivory coast tektite field and the lacustrine sediments within the crater contain a unique 1 m y record of paleoclimate in the continental tropics south of the sahel eight profiles of marine type multichannel seismic reflection mcs data were acquired from the 8 km diameter similar to75 m deep lake that fills much of the crater these were augmented by wide angle seismic data acquired with ocean bottom hydrophones mcs data reveal a well defined central uplift near the northwest central part of the lake and a maximum postimpact lacustrine sediment thickness of similar to310 m the central uplift structure has a diameter of 1 9 km and a maximum height of 130 m above the annular moat inside the crater an intermediate velocity layer 3200 m s beneath the lacustrine sediment is interpreted as fallback breccia or a breccia melt horizon the measured apparent depth of the crater d a is 500 m implying a slightly higher aspect ratio for the structure than predicted from published empirical relationships the bosumtwi structure is a small complex crater that deviates slightly from trends predicted from classical scaling laws perhaps because of the effects of a large gravity field

the newporte impact structure north dakota usa abstract the 3 km diameter newporte structure is located close to the usa canada border in north dakota in the williston basin the structure is currently covered by about 3 km of various supracrustal rock formations and was first studied in the course of hydrocarbon exploration in the late 1970s the structure is situated in precambrian crystalline basement which is highly fractured and brecciated detailed petrographic and geochemical studies of rock samples from the duerre 43 5 the mott 14 34 and the wisdahl 23 10 drill cores show the presence of three types of breccias a mainly granite derived breccia a predominantly meta sediment derived breccia and a mixed breccia type quartz grains as well as a few rare feldspar grains from all three breccia types show planar deformation features pdfs with up to five sets per grain measurements of the crystallographic orientations of the pdfs show predominantly 10 1 3 and 10 1 2 orientations which are characteristic of shock metamorphism and indicate peak shock pressures in excess of 12 gpa the major and trace element composition of the target rocks granitoid and sedimentary rocks and the fragmental impact breccias was measured all three rock types sediments granitoids and breccias show a wide compositional range mixing calculations were performed and yield results that are in general agreement with the petrographic observations but the compositional similarity of the target rock components and their wide range in chemical composition make unambiguous mixing calculations difficult the results of our study confirm the impact origin of the newporte structure

simultaneous radar observations of meter scale f region irregularities at and off the magnetic equator over india 1 simultaneous observations of equatorial spread f irregularities made with an 18 mhz radar from trivandrum located at the geomagnetic equator and a 53 mhz radar from gadanki located at a magnetic latitude of 6 5 n corresponding to nearly the same longitude zone are presented the observations correspond to 8 3 and 2 8 m irregularities respectively the spread f irregularities at both the locations are found to occur nearly at the same time but are observed for longer duration at gadanki than at trivandrum the spread f structures as observed in the intensity maps corresponding to gadanki are characterized by multiple periodic plumes in contrast to a limited number of plumes observed over trivandrum the doppler velocities associated with these irregularities corresponding to trivandrum are in the range of 100 150 m s 1 whereas they are in the range of 100 250 m s 1 at gadanki further the fluctuating velocity fields are much stronger in the gadanki observations than in the trivandrum observations remarkably the spectral widths are 100 m s 1 in trivandrum observations in contrast to those observed at gadanki with values as high as 300 m s 1 the observations are compared with those made elsewhere and are discussed in the light of current understanding of the meter scale irregularities responsible for the radar backscatter

petrographic studies of fallout suevite from outside the bosumtwi impact structure ghana field studies and a shallow drilling program carried out in 1999 provided information about the thickness and distribution of suevite to the north of the bosumtwi crater rim suevite occurrence there is known from an 1 5 km 2 area its thickness is 15 m the present suevite distribution is likely the result of differential erosion and does not reflect the initial areal extent of continuous bosumtwi ejecta deposits here we discuss the petrographic characteristics of drill core samples of melt rich suevite macroscopic constituents of the suevites are melt bodies and crystalline and metasedimentary rock granite graywacke phyllite shale schist and possibly slate clasts up to about 40 cm in size shock metamorphic effects in the clasts include multiple sets of planar deformation features pdfs diaplectic quartz and feldspar glasses lechatelierite and ballen quartz besides biotite with kink bands basement rock clasts in the suevite represent all stages of shock metamorphism ranging from samples without shock effects to completely shock melted material that is indicative of shock pressures up to 60 gpa

uppermost impact fallback layer in the bosumtwi crater ghana mineralogy geochemistry and comparison with ivory coast tektites in 2004 an international continental scientific drilling program icdp drilling project at the bosumtwi impact crater ghana 10 5 km in diameter 1 07 myr old was performed to study the sediments that fill the lake as well as the underlying impactites in one lb 05 of 16 cores drilled into the lake sediments the zone between the impact breccias and the post impact sediments was penetrated preserving the final fine grained impact fallback layer this 30 cm thick layer contains in the top 10 cm accretionary lapilli microtektite like glass spherules and shocked quartz grains glass particlesmostly of splash form less than 1 mm size make up the bulk of the grains 70 78 by number in the coarser size fraction 125 m of the top of the fallback layer about one third of all quartz grains in the uppermost part of the layer are shocked with planar deformation features pdfs almost half of these grains are highly shocked with 3 or more sets of pdfs k feldspar grains also occur and some show shock deformation the abundance of shocked quartz grains and the average shock level as indicated by the number of sets of pdfs for both quartz and k feldspar decrease with depth into the layer the well preserved glass spherules and fragments are chemically rather homogeneous within each particle and also show relatively small variations between the various particles on average the composition of the fallback spherules from core lb 5b is very similar to the composition of ivory coast tektites and microtektites with the exception of cao contents which are about 1 5 to 2 times higher in the fallback spherules this is a rare case in which the uppermost fallback layer and the transition to the post impact sediments has been preserved in an impact structure its presence indicates that the impactite sequence at bosumtwi is complete and that bosumtwi is a very well preserved impact crater

petrology and geochemistry of target rocks from the bosumtwi impact structure ghana and comparison with ivory coast tektites the 10 5 km diameter bosumtwi crater in ghana west africa is the most likely source crater for the ivory coast tektites as the tektites and the crater have the same age 1 07 ma and there are close similarities between the isotopic and chemical compositions of the tektites and crater rocks the crater is excavated in 2 1 2 2 ga old metasediments and metavolcanics of the birimian supergroup here we present the first integrated petrographic and geochemical study of rocks from the bosumtwi impact crater a variety of target rocks from the bosumtwi impact structure were selected to represent the major rock types that have been described before resulting in four groups shale phyllite graywacke and two different types of granites from dispersed dikes and from the so called pepiakese intrusion at the northeastern side of the crater these rocks were analyzed for their major and trace element composition and their petrographic characteristics in addition representative samples were also analyzed for their o sr and nd isotopic compositions the target rocks do not show any unambiguous evidence of shock metamorphism i e planar deformation features pdfs distinct impact characteristic shock effects pdfs were identified only in clasts within suevite derived melt fragments the compositional range of the target rocks is significantly wider than that of the ivory coast tektites but overlap the tektite compositions a best fit line for the bosumtwi crater rocks in a rb sr isotope evolution diagram yields an age of 1 98 ga and an initial 87sr 86sr ratio of 0 701 which is close to results previously obtained for granitoid intrusions in the birimian of ghana our nd isotopic data yield depleted mantle model ages ranging from 2 16 to 2 64 ga and nd values of 17 2 to 25 9 harmonic least squares hmx mixing calculations were able to reproduce the composition of ivory coast tektites from a mixture of bosumtwi country rocks that include about 70 phyllite graywacke 16 granite dike and 14 pepiakese granite the oxygen isotopic composition of the metasedimentary rocks and granite dikes 18o 11 3 13 6 and the tektites 18o 11 7 12 9 also this work agree fairly well while the pepiakese granites have lower values 18o 8 6 9 0 indicating that only minor amounts of these rocks were incorporated in the formation of the ivory coast tektites the large variation in sr and nd isotopic compositions of the target rocks do not allow the unambiguous identification of distinct endmember compositions but in both a 87sr 86sr vs 1 sr plot and an sr vs nd diagram the tektites plot within the area occupied by the metasedimentary and granitic bosumtwi crater rocks thus our data support the interpretation that the composition of the ivory coast tektites is similar to that of rocks exposed at the bosumtwi impact structure indicating formation during the same impact event

topside ionospheric irregularities as seen from multisatellite observations we use in situ data from champ and dmsp satellites along with data of gps receiver onboard champ satellite and ground based gps receivers to study the occurrence and global distribution of ionospheric irregularities during the main phase of the geomagnetic storm of 29 31 august 2004 minimum dst excursion of a128 nt using the champ gps measurements we created maps of gps phase fluctuation activity and found two specific zones of the most intense irregularities 1 the region of the auroral oval at high latitudes of both hemispheres and 2 the low latitudes equatorial region between africa and south america at high latitudes the topside ionospheric irregularities appeared to be more intensive in the southern hemisphere which is most likely due to seasonal variations in the interhemispheric field aligned currents system an analysis of multi instrumental observations reveals reinforcement of the equatorial ionization anomaly after sunset in atlantic sector on 30 august and formation of the significant plasma depletions and irregularities over a large longitudinal range equatorial irregularities were also found in the morning sector at the recovery phase of the storm in addition to low earth orbit leo gps measurements we analyze the leo in situ measurements and we show that these two techniques cannot be interchangeable in all cases because of the altitudinal extent of plasma irregularities overall we demonstrate that the leo gps technique can serve a useful tool for detection of the topside ionospheric irregularities during space weather events and may essentially contribute to other methods based on various instruments

chemical analyses of bosumtwi crater target rocks compared with the ivory coast tektites abstract the bosumtwi crater ghana was excavated in phyllites and greywackes with subordinate microgranite dykes and quartz veins of the 2000 ma old lower birimian system with a granodiorite intrusion at pepiakese on its northeastern side new major and trace element analyses are presented for 7 phyllites 5 greywackes 2 microgranites 3 pepiakese intrusion rocks and 1 suevite using xrf and inna means and standard deviations were calculated using all available modern analyses for each element in the bosumtwi target rocks bosumtwi suevite glasses and ivory coast tektites good agreements between the means were found for the three groups with the suevite glasses and tektites having more limited compositional ranges than the target rocks least squares mixing between target rock types shows that the best fits to the tektite and suevite glass compositions require components of about a third or a quarter from the pepiakese intrusion and some extra silica derived from quartz veins as well as the metasediments the new data provide evidence for vapour phase fractionation of p2o5 and na2o in the tektites in addition to the previously reported pb and rb evidence for a meteoritic component in the tektites was found to be equivocal since the target rocks are probably a sufficient source of the meteorite indicator elements ni and ir

effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread f we use radar observations from the jicamarca observatory from 1968 to 1992 to study the effects of the f region vertical plasma drift velocity on the generation and evolution of equatorial spread f the dependence of these irregularities on season solar cycle and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts in the early night sector the bottomside of the f layer is almost always unstable the evolution of the unstable layer is controlled by the history of the vertical drift velocity when the drift velocities are large enough the necessary seeding mechanisms for the generation of strong spread f always appear to be present the threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux the geomagnetic control on the generation of spread f is season solar cycle and longitude dependent these effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields the occurrence of early night spread f decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset the generation of late night spread f requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour these irregularities occur most often at 0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes the occurrence of late night spread f is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux

investigating the lake bosumtwi impact structure insight from numerical modeling 1 the current topography of the lake bosumtwi crater and some of its structural dimensions have been determined by geophysical methods we combine these data with sophisticated numerical models to evaluate the cratering process itself for example melt and tektite generation as well as to test the modeling code the geophysical maps show some asymmetry in plan view with the main anomaly north of the crater center the simulations of the early stage show asymmetric patterns only in ejecta and tektite distributions while the late stage is modeled for the vertical impact without any asymmetry we estimate the projectile size from scaling laws and then varying material properties reproduce a crater which is similar to the bosumtwi but too deep bulking allows us to reconcile differences between the model results and the observed topography shock melt estimates are in good agreement with the bosumtwi magnetic signature modeled distribution of tektites assumes an impact angle of 30 45 and an impact direction from the n ne the combination of numerical models and field evidence not only provides necessary information for upcoming scientific drilling of the structure but also suggests interesting and well suited drill sites besides the central uplift and the annular moat with a suggested thick breccia cover drilling at the location of the geophysical anomalies and comparison of downrange and transversal locations will provide new insight into preimpact and impact induced asymmetries

the gardnos impact structure norway petrology and geochemistry of target rocks and impactites abstract the gardnos structure norway is an approximately circular area of anomalously fractured and brecciated rock about 4 5 km in diameter emplaced in a metamorphic terrane composed chiefly of granitic gneisses with minor amphibolite and quartzite the original recognition of gardnos as a deeply eroded impact structure between 900 and 400 ma old has been followed up by detailed petrographic and chemical studies of approximately thirty samples of target rocks and various types of shocked rocks impactites deep erosion of the structure has erased the original rim removed much of the crater fill deposits and exposed large areas at or near the original crater floor however a wide variety of distinctive impactites fractured and blackened quartzites in the sub crater basement rocks lithic breccias and melt bearing breccias are still preserved these impactites show petrographic and chemical characteristics that confirm an impact origin distinctive planar deformation features pdfs in quartz and feldspar incipient melting of feldspar clasts in the melt bearing breccias close matches between the chemical composition of the breccias and mixtures of the target lithologies and the detection of an extraterrestrial component a minor extraterrestrial component 0 15 was detected in the melt bearing breccias based on significantly elevated ir and os contents and lower 187os 188os ratios compared to those in the target rocks the gardnos impactites are significantly enriched in c 5 10x over the exposed target rocks this may reflect the presence of a c rich shale overlying the metamorphic basement at the time of impact this idea is supported by 13c values of 28 1 to 31 5 measured in the impactites mixing calculations show that the chemical compositions of the impactites can be reproduced by mixtures of target rocks ranging from approximately 60 90 wt granite gneiss 0 30 wt amphibolite 0 12 wt quartzite and 3 19 wt of a c rich shale component the deeply eroded state of the structure and the preliminary state of detailed geologic mapping make crater reconstruction difficult one possible scenario involves the impact of a 300 m diameter stony meteorite that released 1019 j of energy and formed a transient cavity 3 km in diameter that evolved to a complex crater 5 km in diameter with a central uplift of about 350 m the original crater was filled with at least 0 3 km3 of allochthonous melt matrix breccias containing about 0 06 km3 of impact melt the structure underwent low grade greenschist metamorphism in caledonian time about 400 ma ago and was subsequently eroded to its present appearance the value of the gardnos structure for further cratering studies lies in its easy access to large areas of the original crater floor zone in the preservation of a possibly complete sequence of crater fill breccias beneath a cap of elastic sediments and in the unusual carbon enrichment of its impactites

woodleigh impact structure australia shock petrography and geochemical studies the large complex woodleigh structure in the carnarvon basin of western australia has recently been added to the terrestrial impact crater record many aspects of this structure are however still uncertain this work provides a detailed petrographic assessment of a suite of representative drill core samples from the borehole woodleigh 1 that penetrated uplifted basement rocks of the central part of this structure fundamental rock and mineral deformation data and high precision chemical data including results of pge and oxygen isotopic analysis are presented the sampled interval displays likely impact produced macrodeformation in the form of fracturing and breccia veining at the microscopic scale contrary to earlier reports that these breccias represent pseudotachylite friction melt or even shock shear produced pseudotachylitic melt breccia cannot be confirmed due to pervasive post impact alteration abundant planar deformation features pdfs in quartz in addition to diaplectic glass and partial isotropization are the main shock deformation effects observed confirming that woodleigh is of impact origin over the investigated depth interval the statistics of quartz grains with a variable number of sets of pdfs does not change significantly and the patterns of crystallographic orientations of pdfs in randomly selected quartz grains does not indicate a change in absolute shock pressure with depth either the value of oxygen isotopes for the recognition of meteoritic contamination as proposed by earlier woodleigh workers is critically assessed neither ina nor pge analyses of our samples support the presence of a meteoritic component within this basement section as had been claimed in earlier work

zircon u pb and molybdenite re os geochronology and geochemistry of jinchang porphyry gold copper deposit ne china two phase mineralization and the tectonic setting abstract whether multiple mineralization styles in porphyry deposits are the result of one or multiple pulses of magmatism is unknown in this paper we present new age and geochemical data on the jinchang porphyry gold copper au cu deposit an important deposit in ne china and in the eastern central asian orogenic belt caob field geological study suggests that both the mineralization and ore causative magmatism at jinchang are multiphase la icp ms zircon u pb dating of the monzogranite ore host yielded 197 1 ma which is considerably older than the ore causative monzonite 115 3 ma and granodiorite 107 2 ma molybdenite re os dating on the breccia pipe hosted au cu ores yielded a well defined isochron age of 114 3 ma coeval to the monzonite emplacement the au cu mineralization was followed by another phase of crypto explosive breccia pipe hosted disseminated vein type au mineralization this au only mineralization phase was auriferous pyrite re os dated to be 103 101 ma coeval to the granodiorite emplacement geochemically the monzonite and granodiorite are enriched in liles and lrees but depleted in hfses and hrees the ore causative rocks are i type and display continental arc geochemical affinities we suggest that the monzonite and granodiorite were generated in an intracontinental extensional setting probably caused by the early cretaceous subduction roll back of the paleo pacific plate

mid latitude ionospheric irregularities persisting into late morning during the magnetic storm on 19 march 2001 1 post sunrise intense ionospheric irregularities at low to mid latitudes are reported for the first time a method based on spaced receiver technique is proposed to estimate the apparent drift velocity of the irregularities by using the correlation of the slant total electron content tec from 3 gps receivers the irregularities were detected at 23 45 n by the gps earth observation network geonet of japan from 0455 lt to 1051 lt on 21 march 2001 during the recovery phase of the magnetic storm at the corresponding time spread f was observed with 3 ionosondes located meridionally at okinawa 127 8 e 26 3 n 20 7 n magnetic latitude kokubunji 139 5 e 35 7 n 30 0 n magnetic latitude and wakkanai 141 7 e 45 4 n 40 5 n magnetic latitude while an intense negative ionospheric storm was processing and propagating from mid to lower latitudes the irregularities were seen as plasma bubbles by the defense meteorological satellite program dmsp in situ measurements westward drift with the speed of 100 m s was estimated by spaced receivers from geonet and confirmed by dmsp during the evolution of the irregularities the plasma bubbles at mid latitude sustained 1 5 hour after sunrise and at the lower latitudes they survived more than 4 hours after sunrise the observations suggested that the irregularities were generated by an eastward electric field associated with the disturbance dynamo and the long time existence of the irregularities after sunrise seemed to be related to the daytime intense negative ionospheric storm on 21 march

lithostratigraphic and petrographic analysis of icdp drill core lb 07a bosumtwi impact structure ghana lithostratigraphic and petrographic studies of drill core samples from the 545 08 m deep international continental scientific drilling program icdp borehole lb 07a in the bosumtwi impact structure revealed two sequences of impactites below the post impact crater sediments and above coherent basement rock the upper impactites 333 38 415 67 m depth comprise an alternating sequence of suevite and lithic impact breccias the lower impactite sequence 415 67 470 55 m depth consists essentially of monomict impact breccia formed from meta graywacke with minor shale as well as two narrow injections of suevite which differ from the suevites of the upper impactites in color and intensity of shock metamorphism of the clasts the basement rock 470 55 545 08 m depth is composed of lower greenschist facies metapelites shale schist and minor phyllite meta graywacke and minor meta sandstone as well as interlaminated quartzite and calcite layers the basement also contains a number of suevite dikelets that are interpreted as injection veins as well as a single occurrence of granophyric textured rock tentatively interpreted as a hydrothermally altered granitic intrusion likely related to the regional pre impact granitoid complexes impact melt fragments are not as prevalent in lb 07a suevite as in the fallout suevite facies around the northern crater rim on average 3 6 vol of melt fragments is seen in the upper suevites and up to 18 vol in the lower suevite occurrences shock deformation features observed in the suevites and polymict lithic breccias include planar deformation features in quartz 1 to 3 sets rare diaplectic quartz glass and very rare diaplectic feldspar glass notably no ballen quartz which is abundant in the fallout suevites has been found in the within crater impact breccias an overall slight increase in the degree of shock metamorphism occurs with depth in the impactites but considerably lower shock degrees are seen in the suevites of the basement rocks which show similar features to each other the bulk of the suevite in lb 07a appears to have been derived from the 35 gpa shock zone of the transient crater

storm time plasma irregularities in the pre dawn hours observed by the low latitude rocsat 1 satellite at 600 km altitude large scale ion density depletions were detected in the nighttime sector by rocsat 1 for over 10 hours during the 22 october 1999 geomagnetic storm prominent depletion structures bubbles that are characterized by large amplitude density decrease n no 1 with rapid horizontal ion drift 600 800 m s are found to cluster in the 03 00 04 30 local time sector and at magnetic latitudes 14 20 s when the storm was in its early recovery phase these presunrise bubbles are positively correlated to the enhanced eastward electric fields of greater than 1 2 mv m which were in response to the storm time disturbances resulting from the in phase contributions of the prompt penetration magnetospheric and the long lasting ionospheric disturbance dynamo electric fields further analyses of the field aligned and cross field ion drifts within the depletions reveal that bubble plasma were driven by the eastward polarization electric fields to move upward but these upward velocities were compensated by large downward field aligned diffusive motions these features confirm that the disturbance electric fields produced during a great magnetic storm can significantly affect the occurrence timing and spatial extent of severe plasma irregularities in low latitude ionosphere the spatial dimensions of the pre sunrise irregularities may exceed the large region observed by the 35 inclined circular orbiting rocsat 1

on postmidnight low latitude ionospheric irregularities during solar minimum 1 equatorial atmosphere radar and gps tec observations in indonesia 1 using the 47 mhz equatorial atmosphere radar ear in west sumatra indonesia 10 36 s dip latitude it is shown that postmidnight irregularities during solar minimum are morphologically different from those detected during solar maximum and are quite similar to those observed with the middle and upper atmosphere mu radar in midlatitudes 29 3 n dip latitude utilizing the rapid beam steering capability of the ear the spatial structure of the postmidnight irregularities is clearly presented for the first time it is found that they usually propagate westward and can be categorized into two types one shows sharp upwelling plumes near local midnight which should not be a mere passage of fossil plasma bubbles the other has successive tilted structures which have the same orientation as medium scale traveling ionospheric disturbances typically observed at midlatitudes we suggest that the convergence of the equatorward thermospheric wind which is believed to be responsible for the midnight temperature maximum may be an important factor to produce a preferable condition for the upwelling plumes in the postmidnight sector the displacement between geographic and magnetic equators may also be important for seasonal longitudinal variation of the postmidnight irregularities

geology and geochemistry of shallow drill cores from the bosumtwi impact structure ghana the 1 07 ma well preserved bosumtwi impact structure in ghana 10 5 km in diameter formed in 2 ga old metamorphosed and crystalline rocks of the birimian system the interior of the structure is largely filled by the 8 km diameter lake bosumtwi and the crater rim and region in the environs of the crater is covered by tropical rainforest making geological studies rather difficult and restricted to road cuts and streams in early 1999 we undertook a shallow drilling program to the north of the crater rim to determine the extent of the ejecta blanket around the crater and to obtain subsurface core samples for mineralogical petrological and geochemical studies of ejecta of the bosumtwi impact structure a variety of impactite lithologies are present consisting of impact glass rich suevite and several types of breccia lithic breccia of single rock type often grading into unbrecciated rock with the rocks being shattered more or less in situ without much relative displacement autochthonous and lithic polymict breccia that apparently do not contain any glassy material allochtonous the suevite cores show that melt inclusions are present throughout the whole length of the cores in the form of vesicular glasses with no significant change of abundance with depth twenty samples from the 7 drill cores and 4 samples from recent road cuts in the structure were studied for their geochemical characteristics to accumulate a database for impact lithologies and their erosion products present at the bosumtwi crater major and trace element analyses yielded compositions similar to those of the target rocks in the area graywacke phyllite shale and granite graywacke phyllite and granite dikes seem to be important contributors to the compositions of the suevite and the road cut samples fragmentary matrix with a minor contribution of pepiakese granite the results also provide information about the thickness of the fallout suevite in the northern part of the bosumtwi structure which was determined to be 15 m and to occupy an area of 1 5 km2 present suevite distribution is likely to be caused by differential erosion and does not reflect the initial areal extent of the continuous bosumtwi ejecta deposits our studies allow a comparison with the extent of the suevite at the ries another well preserved impact structure

geodynamic setting of mesozoic magmatism in ne china and surrounding regions perspectives from spatio temporal distribution patterns of ore deposits abstract north eastern china and surrounding regions host some of the best examples of phanerozoic juvenile crust on the globe however the mesozoic tectonic setting and geodynamic processes in this region remain debated here we attempt a systematic analysis of the spatio temporal distribution patterns of ore deposits in ne china and surrounding regions to constrain the geodynamic milieu from an evaluation of the available geochronological data we identify five distinct stages of ore formation 240 205 ma 190 165 ma 155 145 ma 140 120 ma and 115 100 ma the triassic 240 205 ma magmatism and associated mineralisation occurred during in a post collisional tectonic setting involving the closure of the paleo asian ocean the early mid jurassic 190 165 ma events are related to the subduction of the paleo pacific ocean in the eastern asian continental margin whereas in the erguna block these are associated with the subduction of the mongol okhotsk ocean from 155 to 120 ma large scale continental extension occurred in ne china and surrounding regions however the late jurassic magmatism and mineralisation events in these areas evolved in a post orogenic extensional environment of the mongol okhotsk ocean subduction system the early stage of the early cretaceous events occurred under the combined effects of the closure of the mongol okhotsk ocean and the subduction of the paleo pacific ocean the widespread extension ceased during the late phase of early cretaceous 115 100 ma following the rapid tectonic changes resulting from the paleo pacific oceanic plate reconfiguration

the bosumtwi meteorite impact structure ghana a magnetic model abstract a magnetic model is proposed for the bosumtwi meteorite impact structure in ghana africa this relatively young 1 07 ma structure with a diameter of 10 5 km is exposed within early proterozoic birimian tarkwaian rocks the central part of the structure is buried under postimpact lake sediments and because of lack of drill cores geophysics is the only way to reveal its internal structure to study the structure below and beyond the lake a high resolution low altitude 70 m airborne geophysical survey across the structure was conducted which included measurements of the total magnetic field electromagnetic data and gamma radiation the magnetic data show a circumferential magnetic halo outside the lakeshore 12 km in diameter the central north part of the lake reveals a central negative magnetic anomaly with smaller positive side anomalies north and south of it which is typical for magnetized bodies at shallow latitudes a few weaker negative magnetic anomalies exist in the eastern and western part of the lake together with the northern one they seem to encircle a central uplift our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact melt rock or melt rich suevite bodies petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites suevites have a higher magnetization and have low densities and high porosities compared to the target rocks in suevites the remanent magnetization dominates over induced magnetization koenigsberger ratio 3 preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the jaramillo normal polarity epoch this interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake the reverse polarity remanence component superimposed on the normal component is probably acquired during subsequent reverse polarity events

an international and multidisciplinary drilling project into a young complex impact structure the 2004 icdp bosumtwi crater drilling project an overview the bosumtwi impact crater in ghana arguably the best preserved complex young impact structure known on earth displays a pronounced rim and is almost completely filled by lake bosumtwi a hydrologically closed basin it is the source crater of the ivory coast tektites the structure was excavated in 2 1 2 2 gyr old metasediments and metavolcanics of the birimian supergroup a drilling project was conceived that would combine two major scientific interests in this crater 1 to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago at a near equatorial location in africa for which very few data are available so far and 2 to obtain a complete record of impactites at the central uplift and in the crater moat for ground truthing and comparison with other structures within the framework of an international and multidisciplinary drilling project led by the international continental scientific drilling program icdp 16 drill cores were obtained from june to october 2004 at six locations within lake bosumtwi which is 8 5 km in diameter the 14 sediment cores are currently being investigated for paleoenvironmental indicators the two impactite cores lb 07a and lb 08a were drilled into the deepest section of the annular moat 540 m and the flank of the central uplift 450 m respectively they are the main subject of this special issue of meteoritics planetary science which represents the first detailed presentations of results from the deep drilling into the bosumtwi impactite sequence drilling progressed in both cases through the impact breccia layer into fractured bedrock lb 07a comprises lithic in the uppermost part and suevitic impact breccias with appreciable amounts of impact melt fragments the lithic clast content is dominated by graywacke besides various metapelites quartzite and a carbonate target component shock deformation in the form of quartz grains with planar microdeformations is abundant first chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and mg but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values core lb 08a comprises suevitic breccia in the uppermost part followed with depth by a thick sequence of graywacke dominated metasediment with suevite and a few granitoid dike intercalations it is assumed that the metasediment package represents bedrock intersected in the

microscopic lamellar deformation features in quartz discriminative characteristics of shock generated varieties the appearance and orientation of microscopic lamellar deformation features in quartz have been examined in samples from different geologic environments lamellar deformation features from known shock and or impact environments are sharp parallel closely spaced and tend to extend in a continuous manner across entire grains they have specific well documented orientations and occur in multiple sets per grain lamellar deformation features from other geologic environments including tectonic and explosive volcanic only superficially resemble shock features they are generally less well defined slightly curved wider spaced and tend to extend in a discontinuous manner across parts of grains they have relatively random orientations only some of which correspond to known shock orientations and generally occur as single sets per grain on the basis of these observations we conclude that there is a characteristic set of microscopic deformation features produced in quartz by shock these so called planar features differ from microscopic lamellar deformation features produced by other dynamic processes therefore the case for the formation of characteristic planar features in quartz by natural processes other than impact is still not proven