a new martian meteorite from oman mineralogy petrology and shock metamorphism of olivine phyric basaltic shergottite sayh al uhaymir 150 the sayh al uhaymir sau 150 meteorite was found on a gravel plateau 43 3 km south of ghaba oman on october 8 2002 oxygen isotope 17o 2 78 18o 4 74 cre age 1 3 ma and noble gas studies confirm its martian origin sau 150 is classified as an olivine phyric basalt having a porphyritic texture with olivine macrocrysts set in a finer grained matrix of pigeonite and interstitial maskelynite with minor augite spinel ilmenite merrillite pyrrhotite pentlandite and secondary terrestrial calcite and iron oxides the bulk rock composition in particular mg 68 molar mg mg fe 100 fe mn 37 9 and na al 0 22 are characteristic of martian meteorites based on mineral compositions cooling rates determined from crystal morphology and crystal size distribution it is deduced that the parent magma formed in a steady state growth regime magma chamber that cooled at 70 365 c sau 150 is essentially identical to sau 005 094 all representing samples of the same fall that are similar to but distinct from the dag shergottites

multiphase u pb geochronology of sintered breccias from the steen river impact structure canada mixed target considerations for a jurassic cretaceous boundary event abstract in situ u pb geochronology has been conducted using laser ablation inductively coupled mass spectrometry la icp ms on shocked and thermally metamorphosed apatite titanite and zircon grains from the steen river impact structure canada the dated relict mineral phases occur within impact melt bearing breccias that underwent post deposition sintering at 450 c t

frictional melting processes and the generation of shock veins in terrestrial impact structures evidence from the steen river impact structure alberta canada abstract shock produced melt within crystalline basement rocks of the steen river impact structure sris are observed as thin 1 510 m wide interlocking networks of dark veins which cut across and displace host rock minerals solid state phase transformations such as ferro pargasite to an almandine andradite majorite garnet and amorphization of quartz and feldspar are observed in zones adjacent to comparatively wider 50 500 m sections of the shock veins shock pressure estimates based on the coupled substitution of na ti4 and si4 for divalent cations al3 and cr3 in garnet 14 19 gpa and the pressure required for plagioclase ab62 83 amorphization at elevated temperature 14 20 gpa are not appreciably different from those recorded by deformation effects observed in non veined regions of the bulk rock 14 20 gpa this spatial distribution is the result of an elevated temperature gradient experienced by host rock minerals in contact with larger volumes of impact generated melt and large deviatoric stresses experienced by minerals along vein margins micrometer size equant crystals of almandine pyrope majorite garnet define the shock vein matrix consistent with rapid quench 100 200 ms at 7 5 10 gpa crystallization of the vein occurred during a 0 1 0 15 s shock pressure pulse majoritic garnet formed during shock compression by solid state transformation of pargasite along shock vein margins is observed in tem bright field images as nanometer size gouge particles produced at strain rates in the supersonic field 106 108 these crystals are embedded in vesiculated glass and this texture is interpreted as continued movement and heating along slip planes during pressure release the deformation of high pressure minerals formed during shock compression may be the first evidence of oscillatory slip in natural shock veins which accounts for the production of friction melt via shear when little or no appreciable displacement is observed our observations of the mineralogy chemistry and microtextures of shock veins within crystalline rocks of the sris allow us to propose a model for shock vein formation by shear induced friction melting during shock compression

frictional melting processes in planetary materials from hypervelocity impact to earthquakes frictional melting is the result of the conversion of mechanical deformation to heat under adiabatic conditions of slip within planetary materials which are mainly natural ceramics frictional melting occurs at high strain rates typically 10 2 s 1 and at slip velocities greater than 0 1 m s 1 t he pathway to friction melting is controlled by the mechanical properties of a rock s constituent minerals especially fracture toughness minerals with the lowest fracture toughnesses and breakdown temperatures are preferentially comminuted and fused to form the melt the product is a polyphase suspension comprising mineral and rock fragments enclosed in a liquid matrix this cools to form the rock type known as pseudotachylyte and at even higher strain rates it forms shock veins in meteorites and in impact craters which may contain high pressure mineral polymorphs the generation of melt on sliding surfaces can lubricate earthquake faults facilitate the post shock modification of impact craters and make landslides more hazardous

evidence of impact melting and post impact decomposition of sedimentary target rocks from the steen river impact structure alberta canada abstract hypervelocity bolide impacts deliver vast amounts of energy to the earth s near surface this crustal process almost universally includes sedimentary target rocks however their response to impact is poorly understood in part because of complexities due to layering pore space and the presence of volatiles that are difficult to model the response of carbonates to bolide impact remains contentious yet whether they melt or decompose and liberate gases by the reaction caco3 s cao s co2 g has significant implications for post impact climatic effects we report on previously unknown carbonate impact melts at the steen river impact structure canada and the first description of naturally shocked barite baso4 carbonate melts are preserved as groundmass supported calcite rich clasts sampled from an up to 164 m thick continuous sequence of crater fill polymict breccias electron microscopy reveals fluidal and ocellar textured calcite and barite intimately associated with silicate melt consistent with these phases being in the liquid state at the same time raman spectroscopy and electron backscatter diffraction ebsd mapping confirm the presence of high pressure phases reidite and coesite within some steen river carbonate melt bearing breccias these minerals attest to the strong shock provenance of the breccia and provide constraints on their shock history preservation of reidite lamellae in zircon indicates a shock pressure 30 gpa

early silurian glaciation along the western margin of gondwana peru bolivia and northern argentina palaeogeographic and geodynamic setting the record of the ordovician silurian transition within the southern peru bolivia palaeozoic basin extending across peru bolivia and northern argentina is characterized by a diamictite bearing unit which overlies several different ordovician units and underlies mid silurian shales we review the lithostratigraphy sedimentology and biostratigraphy of this unit revising stratigraphic relationships and several ideas and interpretations previously proposed analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and environments involved during its deposition turbidite and shale interbeds within the diamictites indicate a deep marine environment with interbedded mud flows debris flows slumps and large displaced slabs providing evidence for sediment instability and resedimentation chitinozoan biostratigraphy indicates a llandovery age in contrast with previous proposals of a hirnantian age and suggests the need for more detailed sedimentologic and biostratigraphic studies and a reassessment of this diamictite unit in relation to adjacent regions in south america the resedimented character of the deposit explains some of the ordovician fauna previously described which must be considered as recycled from underlying units in situ fauna needs to be reassessed as it may be endemic diachronic and or indicative of migrations within gondwana glacially faceted and striated clasts as well as large granitoid boulders within the resedimented materials provide evidence for glaciation of the source area and are interpreted as recycled from former glacigenic deposits the evidence found in the central andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late llandovery telychian the precise age of glaciation in this part of gondwana cannot be confirmed do to a lack of true tillites tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western gondwana during the late ordovician and early silurian the new data imply that the ordovician silurian boundary was not preserved in some areas of the basin and that an erosional hiatus disconformity with variable regional significance is present between different ordovician units and the diamictite unit late ordovician late caradoc early ashgill euxinic black shales locally present in the central altiplano and eastern cordillera of bolivia may represent the youngest ordovician unit in the region and more research should be focused on them paleogeographic reconstruction of the early silurian peru bolivia basin depicts the extension of the diamictite unit southwards into northern argentina and western paraguay connecting with the parana basin through the asuncion arch to the north it continued into northern peru and ecuador connecting with the silurian record in colombia and venezuela and to the northeast into brazil solimoes and amazonas basins the sedimentary record of glaciation in the peru bolivia basin was due to the development of local ice fields to the west of the basin related with tectonism and resulting reliefs along the active margin of gondwana and probably did not coincide with the development of the north african late ashgillian ice cap

mechanisms of ringwoodite formation in shocked meteorites evidence from l5 chondrite dhofar 1970 the formation of the high pressure compositional equivalents of olivine and pyroxene has been well documented within and surrounding shock induced veins in chondritic meteorites formed by crystallization from a liquid or solid state phase transformation typically polycrystalline ringwoodite grains have a narrow range of compositions that overlap with those of their olivine precursors whereas the formation of iron enriched ringwoodite has been documented from only a handful of meteorites here we report backscattered electron images quantitative wavelength dispersive spectrometry wds analyses qualitative wds elemental x ray maps and micro raman spectra that reveal the presence of fe rich ringwoodite fa44 63 as fine grained 500 nm polycrystalline rims on olivine fa24 25 wall rock and as clasts engulfed by shock melt in a previously unstudied l5 chondrite dhofar 1970 crystallization of majorite magnesiowustite in the vein interior and metastable mineral assemblages within 35 m of the vein margin attest to rapid crystallization of a superheated shock melt 2300 k from 20 25 gpa to ambient pressure and temperature the texture and composition of bright polycrystalline ringwoodite rims fa44 63 mno 0 01 0 08 wt surrounding dark polycrystalline olivine fa8 14 mno 0 56 0 65 wt implies a solid state transformation mechanism in which fe was preferentially partitioned to ringwoodite the spatial association between ringwoodite and shock melt suggests that the rapidly fluctuating thermal regimes experienced by chondritic minerals in contact with shock melt are necessary to both drive phase transformation but also to prevent back transformation

fracture surface energy of the punchbowl fault san andreas system the energetics of earthquake rupture slip weakening and rupture surface formation are poorly understood by measuring the energy associated with fracture damage along the punchbowl fault in the devil s punchbowl los angeles county park part of the san andreas system chester et al show that the fracture surface energy for rupture along a mature fault of this type accounts for only a small fraction of the energy released during large earthquakes as determined by seismologists this suggests that mechanisms such as melt lubrication and thermal pressurization are responsible for slip weakening fracture energy is a form of latent heat required to create an earthquake rupture surface and is related to parameters governing rupture propagation and processes of slip weakening1 2 3 fracture energy has been estimated from seismological and experimental rock deformation data4 5 6 7 8 yet its magnitude mechanisms of rupture surface formation and processes leading to slip weakening are not well defined8 9 10 here we quantify structural observations of the punchbowl fault a large displacement exhumed fault11 12 in the san andreas fault system and show that the energy required to create the fracture surface area in the fault is about 300 times greater than seismological estimates would predict for a single large earthquake if fracture energy is attributed entirely to the production of fracture surfaces then all of the fracture surface area in the punchbowl fault could have been produced by earthquake displacements totalling 1 km but this would only account for a small fraction of the total energy budget and therefore additional processes probably contributed to slip weakening during earthquake rupture

hirnantian ordovician through wenlock silurian conodont biostratigraphy bioevents and integration with graptolite biozones cape phillips formation slope facies cornwallis island canadian arctic islands cornwallis island in the canadian arctic archipelago provides one of the world s best areas for establishing an integrated graptolite conodont biozonation for the late ordovician silurian given the well exposed interfingering relationship of the basinal shale and carbonate platform facies building on earlier graptolite work 180 samples were collected from nine sections of the cape phillips formation of which 118 yielded approximately 7600 conodont elements representing 54 species in 25 genera including one new genus and species mayrodus melchini the conodonts are well preserved with minimal thermal alteration colour alteration index cai 1 nine conodont zones are recognized and defined for this region namely the amorphognathus ordovicicus distomodus kentuckyensis aspelundia expansa as fluegeli and pterospathodus celloni interval zones pt a amorphognathoides taxon range zone pt pennatus procerus and kockelella ranuliformis highest occurrence zones and k ortus absidata taxon range zone an integration of graptolite and conodont zones is documented earlier paleontological and isotope geochemical studies have demonstrated the dynamic nature of the silurian ocean climate system and identified major faunal turnovers or events five of these are recognized in the cornwallis fauna and related to oceanographic and climate changes in part referring to recent oxygen isotope data from conodonts from the cornwallis collections hirnantian extinction event sandvika event late aeronian llandovery snipklint primo episode early telychian llandovery ireviken event late telychian llandovery sheinwoodian wenlock and mulde event early homerian wenlock

high pressure phases in a shock induced melt vein of the tenham l6 chondrite constraints on shock pressure and duration abstract the microtexture and mineralogy of a 580 m wide melt vein in the tenham l6 chondrite were investigated using field emission scanning electron microscopy and transmission electron microscopy to better understand the shock conditions the melt vein consists of a matrix of silicate plus metal sulfide grains that crystallized from immiscible melts and sub rounded fragments of the host chondrite that have been entrained in the melt and transformed to polycrystalline high pressure silicates the melt vein matrix contains two distinct textures and mineral assemblages corresponding to the vein edge and interior the 30 m wide vein edge consists of vitrified silicate perovskite ringwoodite akimotoite majorite with minor metal sulfide the 520 m wide vein interior consists of majorite magnesiowustite with irregular metal sulfide blebs although these mineral assemblages are distinctly different the pressure stabilities of both assemblages are consistent with crystallization from similar pressure conditions the melt vein edge crystallized at about 23 25 gpa and the vein interior crystallized at about 21 25 gpa this relatively narrow pressure range suggests that the melt vein either crystallized at a constant equilibrium shock pressure of 25 gpa or during a relatively slow pressure release using a finite element heat transfer program to model the thermal history of this melt vein during shock we estimate that the time required to quench this 580 m wide vein was 40 ms because the entire vein contains high pressure minerals that crystallized from the melt the shock pressure duration was at least 40 ms using a synthetic hugoniot for tenham and assuming that the sample experienced a peak shock pressure of 25 gpa near the impact site we estimate that the tenham parent body experienced an impact with collision velocity 2 km s based on a one dimensional planar impact model we estimate that the projectile size was 150 m in thickness

stability of pargasite during ultrahigh temperature metamorphism a consequence of titanium and ree partitioning orthopyroxene clinopyroxene plagioclase needles and symplectite along the cleavage planes and grain boundaries of fluorine bearing titanian ferroan pargasite from the highland complex sri lanka are interpreted as evidence for dehydration melting at ultrahigh temperature conditions high ti up to 0 4 pfu and f x f up to 0 56 content in pargasite extends its stability to higher temperatures and the composition indicates the dehydration melting reaction may take place at ultrahigh temperatures 950 c at a pressure around 10 kbar close to peak metamorphic conditions the increase of ti content close to the grain boundaries and cleavage planes in pargasite indicates titanium partitioning from the melt during dehydration melting enhanced the stability of the mineral toward ultrahigh temperature conditions the ree content in the pargasite shows a similar behavior to that of titanium the cores with no breakdown assemblage consist of low and flat ree concentrations with respect to the high and eu depleted rim clinopyroxene in symplectite and needle shaped lamellae within the pargasite porphyroblasts have similar ree patterns with slightly low concentrations relative to that of pargasite in the breakdown assemblage lrees are partitioned mainly into plagioclase while the hrees are partitioned into orthopyroxene the ree enrichment in the pargasite rims signals their relative partitioning between pargasite rims and melt modeling of the partitioning of ti and rees associated with pargasite breakdown demonstrates that its stability is greatly enhanced at uht conditions this investigation implies that the stability of hydrous minerals such as amphibole can be extended to uht conditions and expands our knowledge of metamorphism in the lower crust

lower middle jurassic nonmarine source rocks and petroleum systems of the northern qaidam basin northwest china the qaidam basin is a nonmarine petroliferous basin on the northeastern margin of the tibet plateau potential source rocks are reported from tertiary saline lacustrine deposits and jurassic freshwater lacustrine or terrestrial strata the jurassic source rock is similar in terms of depositional system lithology age and organic geochemistry to nonmarine jurassic rocks that are thought to be potentially significant hydrocarbon sources in many basins of central asia including the tarim junggar and other important petroleum producing basins these jurassic source rocks were deposited in freshwater lacustrine systems in intracontinental foreland style basins during the early and middle jurassic these lacustrine strata are associated with coals and coaly mudstones with possible secondary importance as hydrocarbon source rocks oils from the northern qaidam basin can be divided into two groups based on molecular indicators of depositional environment and age diagnostic biomarkers those derived from this jurassic freshwater lacustrine source rock and those derived from a tertiary hypersaline lacustrine source rock the correlation of some qaidam oils to a jurassic source establishes a petroleum system involving these nonmarine jurassic source rocks oils of this petroleum system have been produced from tertiary and mesozoic reservoirs in anticlinal and thrust related traps in northeastern qaidam maturation modeling of the jurassic source rock indicates that these oils were generated and expelled during the miocene pliocene in much of northern qaidam the geological context of the basin suggests that drainage within the basin primarily was vertical probably mainly along faults until evaporite or overpressured shale seals were encountered in the overlying cenozoic section documentation of this petroleum system suggests other exploration targets remain underevaluated in northern and southwestern qaidam furthermore the northern qaidam petroleum system is a useful analog to interpret the role of similar jurassic source rocks in other sparsely documented basins of central asia

phase relation and physical properties of an al depleted komatiite to 23 gpa abstract knowledge of the mineralogical constituents of the earth s mantle depends heavily on high p experiments of pyrolite composition in order to understand the planetary interior better phase relations and mineral physics properties of compositions other than pyrolite may also be important high p and high t experiments were conducted on an al depleted komatiite which can be regarded as a kind of piclogite composition fourteen experiments were carried out along a model geotherm between 5 and 23 gpa and 1100 and 1600 c with a multi anvil apparatus analyses of quenched run products showed that in the studied komatiite the fractions of olivine and its high p polymorphs wadsleyite and ringwoodite are 40 50 vol pyroxene garnet phase transition occurs at 13 17 gpa while the respective values for pyrolite are 60 vol and 12 15 gpa the density and seismic velocity profiles of komatiite mantle were calculated using thermoelastic parameters of related minerals and they showed quite good agreements with those of seismic observations around the transition zone as well as those for pyrolite

ries crater and suevite revisited observations and modeling part ii modeling we present the results of numerical modeling of the formation of the ries crater utilizing the two hydrocodes sova and isale these standard models allow us to reproduce crater shape size and morphology and composition and extension of the continuous ejecta blanket some of these results cannot however be readily reconciled with observations the impact plume above the crater consists mainly of molten and vaporized sedimentary rocks containing very little material in comparison with the ejecta curtain at the end of the modification stage the crater floor is covered by a thick layer of impact melt with a total volume of 6 11 km3 the thickness of true fallback material from the plume inside the crater does not exceed a couple of meters ejecta from all stratigraphic units of the target are transported ballistically no separation of sedimentary and crystalline rocks as observed between suevites and bunte breccia at ries is noted we also present numerical results quantifying the existing geological hypotheses of ries ejecta emplacement from an impact plume by melt flow or by a pyroclastic density current the results show that none of these mechanisms is consistent with physical constraints and or observations finally we suggest a new hypothesis of suevite formation and emplacement by postimpact interaction of hot impact melt with water or volatile rich sedimentary rocks

integrated graptolite conodont biostratigraphy and organic carbon chemostratigraphy of the llandovery of kallholn quarry dalarna sweden abstract a revised graptolite and conodont biostratigraphy together with new organic carbon isotope data for the middle aeronian to lower telychian llandovery lower silurian of kallholn quarry central sweden are presented the base of the section is marked by an unconformity between the ordovician boda limestone and aeronian shales with limestone nodules belonging to the pribylograptus leptotheca biozone these are overlain by strata assigned to the lower lituigraptus convolutus biozone the stimulograptus sedgwickii biozone previously recorded from the section is absent and marked by an unconformity which is overlain by radiolarian rich graptolitic shales belonging to the stimulograptus halli biozone the overlying telychian strata are assigned to the spirograptus guerichi spirograptus turriculatus and streptograptus crispus biozones conodonts indicate that the boundary between the distomodus staurognathoides biozone and pterospathodus eopennatus superzone lies within the sp turriculatus biozone as it does also in the ohesaare core estonia the 13corg record shows mostly minor fluctuations but with a distinct twin peaked positive excursion in the upper sp turriculatus biozone close to the boundary with the overlying streptograptus crispus biozone the excursion in the kallholn section may correlate with the earliest positive shift in 13c values at the onset of the excursion associated with the valgu event or it may represent a new excursion provisionally named the kallholn excursion preceding this many more bentonites 36 occur in the section than previously recorded geochemical studies are needed to confirm whether the osmundsberg bentonite which may or may not be equivalent to the o bed in estonia and latvia is present

ca rich majorite derived from high temperature melt and thermally stressed hornblende in shock veins of crustal rocks from the ries impact crater germany shock veins up to 1 1 mm thick were found within non porous lithic clasts from suevite breccia of the nordlinger ries impact structure these veins were studied by optical microscopy in transmitted and reflected light and by scanning electron microscopy in shocked amphibolites two types of ca rich majorite occur within and adjacent to the veins the first type crystallized from shock induced melts within the veins si contents of these majorites suggest dynamic pressure of 15 17 gpa implying minimum temperatures in the range of 2 150 2 230 c the second type of majorite was formed adjacent to the shock veins within pargasitic hornblende this majorite contains significant amounts of h2o 0 7 0 9 wt based on the textural setting the shrinkage cracks and the chemical compositions of both phases a solid state mechanism is deduced for the hornblende to majorite phase transition both genetic types of ca rich majorite are described for the first time from a terrestrial impact crater along with stishovite majorite constitutes the second silicate mineral displaying sixfold coordination of si at ries using micro raman spectroscopy jadeite coesite and jadeite grossular were identified within local melt glasses of alkali feldspar and plagioclase composition respectively stishovite aggregates produced by solid state reaction along with shock induced high pressure melt glasses of almandine composition were also detected in shock veins of a garnet cordierite sillimanite restite the quenched homogeneous almandine glasses point to melting temperatures of more than 2 500 c for the veins our findings demonstrate that terrestrial shock veins can give valuable information on shock induced mineral transformations and transient high pressures of host rocks during a natural impact event

majoritic garnet grains within shock induced melt veins in amphibolites from the ries impact crater suggest ultrahigh crystallization pressures between 18 and 9 gpa shock induced melt veins in amphibolites from the nordlinger ries often have chemical compositions that are similar to bulk rock i e basaltic but there are other veins that are confined to chlorite rich cracks that formed before the impact and these are poor in ca and na majoritic garnets within the shock veins show a broad chemical variation between three endmembers 1 text viii text m 2 3 text vi text al 2 text iv text sio 4 3 normal garnet grt 2 text viii text m 2 3 text vi text m 2 text si ti text iv text sio 4 3 majorite maj and 3 text viii text na text m 2 2 text vi text si ti text al text iv text sio 4 3 na majorite50grt50 whereby m2 mg2 fe2 mn2 ca2 in particular we observed a broad variation in vi si ti which ranges from 0 12 to 0 58 cations per formula unit cpfu all these majoritic garnets crystallized during shock pressure release at different ultrahigh pressures those with high vi si ti 0 36 0 58 cpfu formed at high pressures and temperatures from amphibole rich melts while majoritic garnets with lower vi si ti of 0 12 0 27 cpfu formed at lower pressures and temperatures from chlorite rich melts furthermore majoritic garnets with intermediate values of vi si ti 0 24 0 39 crystallized from melts with intermediate contents of ca and na to the best of our knowledge the morb type ca na rich majoritic garnets with maximum contents of 2 99 wt na2o and calculated crystallisation pressures of 16 18 gpa are the most extreme representatives ever found in terrestrial shocked materials at the ries the duration of the initial contact and compression stage at the central location of impact is estimated to only 0 1 s we used a 200 m thick shock induced vein in a moderately shocked amphibolite to model its pressure temperature time p t t path the graphic model manifests a peak temperature of 2600 c for the vein continuum pressure lasting for 0 02 s a quench duration of 0 02 s and a shock pulse of 0 038 s the small difference between the continuum pressure and the pressure of majoritic garnet crystallization underlines the usefulness of applying crystallisation pressures of majoritic garnets from metabasites for calculation of dynamic shock pressures of host rocks majoritic garnets of chlorite provenance however are not suitable for the determination of continuum pressure since they crystallized relatively late during shock release an extraordinary glass and majorite bearing amphibole fragment in a shock vein of one amphibolite documents the whole unloading path

characteristics and petroleum origin of the carboniferous volcanic rock reservoirs in the shixi bulge of junggar basin western china abstract shixi bulge of the central junggar basin in western china is a unique region that provides insight into the geological and geochemical characteristics of large scale petroleum reservoirs in volcanic rocks of the western central asian orogenic belt carboniferous volcanic rocks in the shixi bulge mainly consist of striped lava and agglomerate as well as breccia lava and tight tuff volcanic rocks differ in porosity and permeability striped lava exhibits the highest porosity average 14 2 but the lowest permeability average 0 67 10 15 m among the rock types primary gas pores are widely developed and mostly filled secondary dissolution pores and fractures are two major reservoir storage spaces capillary pressure curves suggest the existence of four pore structure types of reservoir rocks several factors namely lithology pore structure and various diagenesis govern the physical properties of volcanic rocks the oil is characterized by a high concentration of tricyclic terpane a terpane distribution of c 23 21 c 20 and sterane distributions of c 27 28 29 and c 27 c 28 29 oil and gas geochemistry revealed that the oil is a mixture derived primarily from p 2 w source rock and secondarily from p 1 j source rock in the sag west of pen 1 well the gases are likely gas mixtures of humic and sapropelic organic origins with the sapropelic gas type dominant in the mixture the gas mixture is most likely cracked from kerogen rather than oils the carboniferous volcanic reservoirs in shixi bulge share some unique characteristics that may provide useful insights into the various roles of different volcanic reservoir types in old volcanic provinces the presence of these reservoirs will undoubtedly encourage future petroleum exploration in volcanic rocks up to the deep parts of sedimentary basins

a record of late ordovician to silurian oceanographic events on the margin of baltica based on new carbon isotope data elemental geochemistry and biostratigraphy from two boreholes in central poland abstract new stable isotope data from organic carbon 13corg and inorganic elemental geochemistry data have been generated from upper ordovician to silurian strata in two boreholes in the lublin basin of poland grabowiec 6 and zwierzyniec 1 they have been integrated here with biostratigraphical data from graptolites acritarchs and chitinozoans faunal assemblages from grabowiec 6 indicate that it spans from the katian clingani graptolite biozone to the gorstian scanicus graptolite biozone 13corg values from this section record the sheinwoodian ireviken excursion the homerian mulde excursion and a minor positive shift associated with the lower ludfordian leintwardinensis biozone the second section zwierzyniec 1 spans the sandbian through gorstian nilssoni biozone 13corg values record the hirnantian carbon isotope excursion hice and the ireviken excursion as well elemental geochemistry data is used to recognize subtle changes in provenance and lithology significant increases in the abundance of v and mo are recognized in strata deposited above the ireviken excursion the enrichment of these redox sensitive elements suggests that persistent regional anoxia and euxinia may be associated with the aftermath of these oceanographic disturbances some of these same trace elements along with fe2o3 and pb are also abundant in strata coeval with or just below the ireviken and hice excursions this may have a causal link with malformed palynomorphs observed at these intervals by some workers which are thought to reflect toxic levels of dissolved heavy metals in the world s oceans

stratigraphy and ages of four early silurian through late devonian early and middle mississippian glaciation events in the parnaiba basin and adjacent areas ne brazil abstract turbidity currents tectonism debris flows glaciation and other geologic processes are considered to be responsible for the generation of silurian mississippian diamictites in northern brazil processes that best account for the features observed in these strata are glaciation deglaciation and sedimentary deformation four main glacial ages are identified to have occurred in the parnaiba basin during the paleozoic glacially influenced deposits are found in the lower silurian upper devonian lower mississippian and middle mississippian strata the parnaiba basin is characterized by outcrops providing robust lithological evidence for three glacial ages identified by lithological features and dated by palynological studies a fourth glaciation as old as tournaisian was recently recognized only in the subsurface of the basin the glacial features were illustrated and the rocks accurately dated with palynomorphs based on very accurate palynological studies additional palynological information derives from outcrops and shallow boreholes drilled by a private company themag and the national state mineral production department departamento nacional de producao mineral dnpm and the brazilian geological survey company companhia de pesquisa de recursos minerais cprm the purpose of this reexamination including many new illustrations interpretations and conclusions is to evaluate primary sedimentological and paleontological data related to the above mentioned glaciations some authors remain skeptical about the record of early to early late paleozoic glaciations in western gondwana especially in brazil mainly because evidence provided in the literature is relatively scanty the parnaiba basin provides special conditions for the study of these glaciations although evidence for each of them varies considerably as discussed in the text

early silurian sea level changes global sea level fluctuated markedly during the early silurian probably as a result of the waxing and waning of ice sheets in the south american portion of gondwana the highest sea levels of the silurian are recorded by the telychian upper crispus lower griestoniensis and spiralis lower lapworthi biozones other highstands occurred in the early aeronian during the convolutus zone mid aeronian guerichi zone and late turriculatus zone early telychian and early sheinwoodian low sea levels characterized much of the argenteus and sedgwickii zones aeronian the utilis subzone late guerichi early turriculatus zones early telychian the late telychian commencing in the mid lapworthi zone and after a period of apparently only small amplitude sea level fluctuations in the late sheinwoodian and earliest homerian the mid late homerian in particular the early nassa zone facies and faunal changes in the lower silurian do not support the p and s model of jeppsson and others but are consistent with the sea level changes proposed herein mid telychian marine red beds appear to have been deposited during a minor sea level fall immediately after a period of very high sea levels rather than during a transgressive episode as previously suggested comparison of the sea level curve presented herein with those constructed in the past is hampered by the lack of precision currently possible in the correlation of early silurian deep water graptolitic and shallow water shelly sequences improving the precision of this correlation should be a priority for future research

pressure induced amorphization of albitic plagioclase in an externally heated diamond anvil cell 1 high pressure and high temperature experiments of albitic plagioclase up to 41 gpa and 270 c were carried out using an externally heated diamond anvil cell raman spectroscopy and transmission electron microscopy of the recovered samples revealed that the amorphization of albite was complete at 37 gpa and room temperature the amorphization pressure at 170 c was nearly the same as that at room temperature in contrast the pressure largely decreased to 31 gpa at 270 c in comparison with the amorphization pressure of albite in laboratory shock experiments that in the present static compression experiments is significantly lower 10 gpa even at room temperature this suggests that shorter pressure duration results in a lower degree of amorphization of plagioclase the formation of maskelynite in shocked meteorites does not necessarily require the very high shock pressure 30 90 gpa that was previously estimated on the basis of shock recovery experiments

hyperspectral imaging of drill core from the steen river impact structure canada implications for hydrothermal activity and formation of suevite like breccias

variability in the δ13c of lower palaeozoic palynomorphs implications for the interpretation of ancient marine sediments abstract we have analyzed the stable carbon isotope ratios of various organic walled microfossils preserved in marine sediments of silurian and devonian age the aim of this study is to determine the amplitude of isotopic fractionations among the different extinct palynomorph groups twenty six samples of marine palynomorphs have 13c values in the range 30 0 to 26 3 whereas two samples likely of terrestrial origin tracheids and woody tissue have the highest 13c values 24 8 and 23 3 among six sediments that only contain marine palynomorphs three show 13c variations of 0 8 1 0 and 2 5 respectively whereas the others do not show significant difference in the presence of fragments of woody tissue mixed with the marine plankton these internal isotopic variations may reach about 4 the bulk 13c values of organic matter in these ancient marine sediments are interpreted as average values which integrate in addition to chemical characteristics of surface waters e g co2 aq source and species dependent isotopic compositions the possible occurrence of such 13c heterogeneities at the scale of a sediment sample could result at least from three kinds of material fluxes 1 autotroph and heterotroph elements from the water column 2 oceanic current and river inputs and 3 selective molecular preservation related to specific biochemical structures

ries crater and suevite revisited observations and modeling part i observations we report results of an interdisciplinary project devoted to the 26 km diameter ries crater and to the genesis of suevite recent laboratory analyses of crater suevite occurring within the central crater basin and of outer suevite on top of the continuous ejecta blanket as well as data accumulated during the past 50 years are interpreted within the boundary conditions imposed by a comprehensive new effort to model the crater formation and its ejecta deposits by computer code calculations artemieva et al 2013 the properties of suevite are considered on all scales from megascopic to submicroscopic in the context of its geological setting in a new approach we reconstruct the minimum maximum volumes of all allochthonous impact formations 108 116 km3 of suevite 14 22 km3 and the total volume of impact melt 4 9 8 0 km3 produced by the ries impact event prior to erosion these volumes are reasonably compatible with corresponding values obtained by numerical modeling taking all data on modal composition texture chemistry and shock metamorphism of suevite and the results of modeling into account we arrive at a new empirical model implying five main consecutive phases of crater formation and ejecta emplacement numerical modeling indicates that only a very small fraction of suevite can be derived from the primary ejecta plume which is possibly represented by the fine grained basal layer of outer suevite the main mass of suevite was deposited from a secondary plume induced by an explosive reaction fuel coolant interaction of impact melt with water and volatile rich sedimentary rocks within a clast laden temporary melt pool both melt pool and plume appear to be heterogeneous in space and time outer suevite appears to be derived from an early formed melt rich and clast poor plume region rich in strongly shocked components melt clasts and originating from an upper more marginal zone of the melt pool crater suevite is obviously deposited from later formed clast rich and melt poor plumes dominated by unshocked and weakly shocked clasts and derived from a deeper central zone of the melt pool genetically we distinguish between primary suevite which includes dike suevite the lower sublayer of crater suevite and possibly a basal layer of outer suevite and secondary suevite represented by the massive upper sublayer of crater suevite and the main mass of outer suevite

carbon isotope fractionation during natural gas generation from kerogen abstract in petroleum exploration it is important to be able to determine the origin of any gas which is found this paper describes a new method of estimating the source type and maturity of a gas based on a rayleigh fractionation model kerogen is divided conceptually into a labile dominantly oil generating fraction and a refractory gas prone component 13 c of methane from either kerogen type and ethane propane and butane for gases from labile kerogen can be defined as a function of 13 c of the gas precursor groups in kerogen a kinetic isotope fractionation factor k and the extent of gas generation the isotopic ratio of the methane precursors relative to bulk kerogen determined from laboratory pyrolysis are 17 5 for labile kerogen and 1 4 for refractory kerogen values for ethane propane and butane from labile kerogen based on field correlations are 4 9 2 2 and 1 6 respectively the corresponding fractionation factors are 0 9892 0 9919 0 9947 and 0 9975 for methane ethane propane and butane respectively from labile kerogen and 0 9984 for methane from refractory kerogen using these parameters summary diagrams are constructed which allow differentiation of these sources from each other and from biogenic gases and cracked oil and recognition of gases of mixed origin if an independent estimate of 13 c for the source kerogen is possible then 13 c of the gas components can be used to estimate maturity in terms of the gas generation index the fraction of gas potential which has been realized

early silurian δ13corg excursions in the foreland basin of baltica both familiar and surprising the sommerodde 1 core from bornholm denmark provides a nearly continuous sedimentary archive from the upper ordovician through to the wenlock series lower silurian as constrained by graptolite biostratigraphy the cored mudstones represent a deep marine depositional setting in the foreland basin fringing baltica and we present high resolution data on the isotopic composition of the section s organic carbon 13 c org this chemostratigraphical record is correlated with previously recognized 13 c excursions in the upper ordovician lower silurian including the hirnantian positive isotope carbon excursion hice the early aeronian positive carbon isotope excursion eacie and the early sheinwoodian positive carbon isotope excursion escie a new positive excursion of high magnitude 4 is discovered in the telychian oktavites spiralis biozone lower silurian and we name it the sommerodde carbon isotope excursion socie the socie appears discernible in 13 c carb data from latvian and estonian cores but it is not yet widely recognized however the magnitude of the excursion within the deep marine depositional setting represented by the sommerodde 1 core suggests that the socie reflects a significant event in addition the chemostratigraphical record of the sommerodde 1 core reveals the negative excursion at the transition from the aeronian to telychian stages the rumba low and suggests that the commencement of the eacie at the base of the demirastrites triangulatus biozone potentially is a useful chemostratigraphical marker for the base of the aeronian stage

numerical simulation of temperature effects at fissures due to shock loading the localized appearance of specific shock features in target rocks and meteorites such as melt veins and high pressure polymorphs suggests that regions with a local increase in pressure and temperature exist as a shock wave propagates through an inhomogeneous rock in this paper we investigate the effect of planar fissures on the local temperature distribution using numerical simulations time dependent parameters such as temperature pressure and displacement are evaluated the simulation model is based on a shock equation of state for the involved materials dunite and quartzite and simulates geometries that were also used in shock loading experiments an artificial gap between the materials simulates an open fissure at the interface a strong temperature increase occurs at a gap size of 0 1 mm which potentially can cause melting in a thin layer at the interfaces the temperature decreases with decreasing gap size temperature and pressure excursions at the interface are induced by the closure of the gap which causes a second shock wave to superpose the primary wave open fissures and fractures which occur ubiquitously in shallow buried target rocks and projectiles thus act as local pressure and temperature amplyfiers and may be responsible for thin melt vein formation in shocked rocks

graptolites in british stratigraphy 697 taxa of planktonic graptolites are recorded and their stratigraphical ranges are given through 60 biozones and subzones in the ordovician and silurian strata of england wales and scotland in the first such stratigraphical compilation for great britain since the synthesis of elles wood 1901 1918

discovery of bridgmanite the most abundant mineral in earth in a shocked meteorite meteorites exposed to high pressures and temperatures during impact induced shock often contain minerals whose occurrence and stability normally confine them to the deeper portions of earth s mantle one exception has been mgsio 3 in the perovskite structure which is the most abundant solid phase in earth here we report the discovery of this important phase as a mineral in the tenham l6 chondrite and approved by the international mineralogical association specimen ima 2014 017 mgsio 3 perovskite is now called bridgmanite the associated phase assemblage constrains peak shock conditions to 24 gigapascals and 2300 kelvin the discovery concludes a half century of efforts to find identify and characterize a natural specimen of this important mineral

shock veins in the central uplift of the manicouagan impact structure context and genesis we describe the development of shock veins that penetrate the anorthositic central uplift of the manicouagan impact structure they occur as thin 2 5 mm wide linear micro fault systems that can be traced for several meters in length and which predominantly trend radially from the point of impact the shock veins are distinguished by the development of maskelynite along vein margins and stishovite in vein matrices which are otherwise absent in non veined regions these phases define a shock excursion of up to 30 gpa in contrast to bulk shock effects of 12 gpa defined by development of shatter cones planar fractures and planar deformation features in various minerals the shock veins at manicouagan share many similarities with vein systems developed in meteorites they also provide an in situ context with which to better understand meteoroid source and lofting conditions in addition to containing high pressure phases the shock veins exhibit evidence for high temperature partial melting of host silicate clasts with the generation of flow textured fragments and glasses the formation of microcrystallites and dendrites from some melts indicates rapid cooling we propose a two stage generation mechanism comprising an initial high pressure shock excursion estimated to last 0 5 s based on projectile size considerations followed by a longer high temperature pulse of a few seconds duration we suggest that the shock excursion is initiated by target heterogeneities that cause distortions in the hemispherically propagating shock front this results in radially oriented tearing and vein formation with shock amplification occurring via intra vein shock reverberation high speed displacement along the veins is driven by stress release on rarefaction which results in frictional melting via adiabatic heating

periodicity in the early palaeozoic marine realm evidence from the british isles suggests that during 3 intervals in the early palaeozoic conditions approximating to those of a non uniformitarian greenhouse g state earth were realized middle early upper cambrian st davids merioneth ages 30 ma mid ordovician llandeilo caradoc ages 32 ma and early silurian llandovery early wenlock ages 12 ma during these times sea levels were high and ocean waters below the zone of surface overturning poorly aerated and at times euxinic pelagic biotas graptoloids were diverse and in aerated portions of the highly productive shelf seas benthic biotas brachiopods flourished opposing conditions marking a well oxygenated o state earth occurred to varying degrees in the intervening intervals early cambrian comley age 20 ma late cambrian mid ordovician tremadoc late llandeilo ages 46 ma late ordovician ashgill age 4 ma and mid to late silurian post early wenlock age 15 ma within the constraints of the present data perturbations in the early palaeozoic marine realm do not appear to follow the same regular periodicity as in the mesozoic and caenozoic among a number of likely causes for this the greater tendency for the early palaeozoic seas to become anoxic seems the most important

glaucophanic amphibole in a monian shear zone on the mainland of n wales glaucophanic amphibole occurs within recrystallized basic volcanic rocks derived from the gwna group at penrhyn nefyn in the lleyn peninsula the glaucophanic rocks are restricted to the penmynydd zone of metamorphism which at nefyn is interpreted as a narrow blastomylonite belt produced by the shearing of the gwna group against a body of tonalite the blue amphibole coexists with blue green amphibole and grades into actinolite greenschist away from the focus of shearing fragments of gwna group dated as lower cambrian and blastomylonitic penmynydd lithologies lie in the cambro ordovician sediments of the welsh basin therefore the metamorphism of parts of the gwna group during the penmynydd subduction zone shearing is interpreted as having occurred in cambrian times

graptolite biostratigraphy of the lower silurian llandovery shelf deposits of the western iberian cordillera spain eight lower silurian graptolite biozones triangulatus convolutus linnaei turriculatus cris pus griestoniensis tullbergi and spiralis and three subzones runcinatus gemmatus palmeus and hispani cus have been recognized in the sierra menera nevera and tremedal massifs in the castilian branch of the iberian cordillera western iberian cordillera early silurian rhuddanian low diversity normalograp tid faunas are also present and these together with the rich graptolite faunas of the aeronian triangulatus and convolutus biozones come from black shaly intercalations within the quartzose sandstones of the upper part of the los puertos quartzite telychian graptolite biozones have been recognized in the succeeding black shale sequence of the badenas formation diachroneity of the transition from sandstones to black shales is dated by graptolites it ranges from about the base to at least the top of the linnaei biozone we sug gest that the lower silurian black shales of the western iberian cordillera were deposited in a shallow shelf environment not much deeper than that of presumably storm influenced sandstones of the los puertos quartzite

composition of inland waters with marine like fauna and inferences for a miocene lake in spain abstract knowledge of the ionic composition of athalassic non marine waters where marine like organisms currently live allows to evaluate their ionic and total dissolved solid tds tolerance ranges together with other ecological data these chemical parameters can be used to interpret hydrological and hydrochemical conditions in ancient lake basins where analogous faunas have been recorded an example of such a utilization of the biological record is presented for miocene clastic dominated lacustrine deposits of the tertiary ebro basin of ne spain which contain marine like fossils comparison of the deduced chemical parameters for the miocene lake waters with the chemical composition of recent groundwaters and saline springs in the same area allows us to infer a similar mode of solute acquisition this implies that there were important cl so 4 saline inflows to the miocene lakes

generation of fragment rich pseudotachylite bodies during central uplift formation in the vredefort impact structure south africa abstract target rocks underlying the central portions of large terrestrial impact structures are characterized by the pervasive presence of fragment rich pseudotachylite bodies debates regarding the formation of these bodies include the origin of pseudotachylitic melts i e friction versus shock induced melting melt mobility causes of target rock fragmentation and timing of fragmentation and melt emplacement with respect to stages of cratering comprehensive structural analysis of pre impact mineral fabrics and properties of fragment rich pseudotachylite in the vredefort dome suggests that melt is allochthonous and was emplaced at an advanced stage of cratering into tensional fracture zones within the crater floor both concentration of bending strains imparted on target rocks during central uplift formation and thermal stresses induced by the emplacement of allochthonous melt led to fragmentation of target rock tensional fracture zones formed in an overall dilational strain field towards the end of cratering likely during collapse of the central uplift and constitute low pressure zones into which melt was forcefully drawn melt may have been drained from the overlying impact melt sheet or from sites within the crater floor and transported into fragment rich dilation zones our field based analysis failed to identify the presence of bona fide shear faults that could potentially have generated in situ frictional melts and fragments rather target rock fragmentation and melt generation that resulted in fragment rich pseudotachylite bodies are processes separated in space and time during cratering

early silurian glaciations in brazil abstract the opening of the brazilian paleozoic basins during the late ordovician was followed by a tectonic stabilization in the silurian at the time of the early silurian glaciations deposition of continental sandstones prevailed in the basins offshore environments were present in the amazonas basin and to a smaller extent in the parana basin the ice sheets advanced over fluviatile and littoral sediments except for the south margin of the amazonas basin where the glacial diamictites immediately overlie marine facies the ice produced much meltwater even when advancing and this was probably climatically controlled the ice movements were from the northwest at the northern margin of the amazonas basin and towards the northeast at the southern margin in the parnaiba basin and its outliers the jatoba basin and cariri valley the ice came from southeast and probably also from east further to the south in the parana basin it came from the north and northeast four glaciations have been dated to the early silurian the oldest is dated as latest ashgill and or earliest llandovery in the parana basin as evidenced from tillites in the upper rio ivai and lower vila maria formations including the former iapo formation it is contemporaneous with a glaciation in the cape basin south africa the next glaciation is dated as early middle llandovery gregarius graptolite zone and is present in the amazonas basin and the andean belt and probably also in the parnaiba basin and its outliers the two youngest tillites were widespread in the northern brazilian basins amazonas parnaiba and jatoba basins and cariri valley as well as in the andean belt and are dated as early late llandovery possibly starting already in the late middle llandovery and latest llandovery to earliest wenlock the latter tillite is the most widespread one assuming these glaciations are related to polar positions the apparent migration of the south pole ice cap must have been quite rapid from central north africa in late ashgill towards central brazil in the middle late llandovery the glaciation in the parana basin took place up to 45 from the late ashgillian south pole the cool climate continued during the silurian but no new glaciations are known before latest devonian

experimental approach to generate shock veins in single crystal olivine by shear melting a shock experiment has been devised to produce large shear in a single crystal sample of olivine the recovered sample exhibits macroscopic shear faults resembling shock veins in ordinary chondrites examination with transmission electron microscopy reveals a high density of dislocations in the bulk olivine the shear faults appear as thin veins containing small grains of olivine and pockets of glass the microstructure and composition of the material in the veins point to fractional crystallization of a melt an order of magnitude calculation is consistent with the idea that the veins were produced by shear melting these results support the view that shock veins in meteorites are the result of shear heating rather than of pressure heterogeneities

a lifetime of the variscan orogenic plateau from uplift to collapse as recorded by the prague basin bohemian massif the ordovician to middle devonian prague basin bohemian massif represents the shallowest crust of the variscan orogen corresponding to c 1 4 km palaeodepth the basin was inverted and multiply deformed during the late devonian to early carboniferous variscan orogeny and its structural inventory provides an intriguing record of complex geodynamic processes that led to growth and collapse of a tibetan type orogenic plateau the northeastern part of the prague basin is a simple syncline cross cut by reverse thrust faults and represents a doubly vergent compressional fan accommodating c 10 19 nw se shortening only minor syncline axis parallel extension and significant crustal thickening the compressional structures were locally overprinted by vertical shortening kinematically compatible with ductile normal shear zones that exhumed deep crust in the orogen s interior at c 346 337 ma on a larger scale the deformation history of the prague syncline is consistent with building significant palaeoelevation during variscan plate convergence based on a synthesis of finite deformation parameters observed across the upper crust in the centre of the bohemian massif we argue for a differentiated within plateau palaeotopography consisting of domains of local thickening alternating with topographic depressions over lateral extrusion zones the plateau growth involving such complex three dimensional internal deformations was terminated by its collapse driven by multiple interlinked processes including gravity voluminous magma emplacement and thermal softening in the hinterland and far field plate boundary forces resulting from the relative dextral motion of gondwana and laurussia

a previously unrecognized high temperature impactite from the steen river impact structure alberta canada here we report a previously unrecognized impactite from the steen river impact structure in alberta canada which was intersected by continuous diamond drill core into the allochthonous proximal deposits of this buried 25 km diameter complex crater a suite of high temperature minerals defines the matrix formed by grain growth in a solid state by static recrystallization of an originally clastic matrix deposited at temperatures 800 c this rock type is predominantly a result of the recrystallization of target material driven by the acceleration of hot gasses from volatilized sedimentary cover mixed with variably shocked crystalline basement approximately one third of terrestrial impact structures occur in mixed target rocks therefore this type of impactite may be more common than previously realized contact metamorphism between entrained sedimentary target rocks and the juxtaposed hot matrix resulted in carbonate decomposition to form a rare spinel group mineral magnesioferrite in crater environments magnesioferrite has been found in the distal chicxulub mexico ejecta and may prove a novel indicator mineral for impact into carbonate bearing target rocks

the history of allan hills 84001 revised multiple shock events the geologic history of martian meteorite allan hills alh 84001 is more complex than previously recognized with evidence for four or five crater forming impacts onto mars this history of repeated deformation and shock metamorphism appears to weaken some arguments that have been offered for and against the hypothesis of ancient martian life in alh 84001 allan hills 84001 formed originally from basaltic magma its first impact event i1 is inferred from the deformation d1 that produced the granular textured bands crush zones that transect the original igneous fabric deformation d1 is characterized by intense shear and may represent excavation or rebound flow of rock beneath a large impact crater an intense thermal metamorphism followed d1 and may be related to it the next impact i2 produced fractures fr2 in which carbonate pancakes were deposited and produced feldspathic glass from some of the igneous feldspars and silica after i2 carbonate pancakes and globules were deposited in fr2 fractures and replaced feldspathic glass and possibly crystalline silicates next feldspars feldspathic glass and possibly some carbonates were mobilized and melted in the third impact i3 microfaulting intense fracturing and shear are also associated with i3 in the fourth impact i4 the rock was fractured and deformed without significant heating which permitted remnant magnetization directions to vary across fracture surfaces finally alh 84001 was ejected from mars in event i5 which could be identical to i4 this history of multiple impacts is consistent with the photogeology of the martian highlands and may help resolve some apparent contradictions among recent results on alh 84001 for example the submicron rounded magnetite grains in the carbonate globules could be contemporaneous with carbonate deposition whereas the elongate magnetite grains epitaxial on carbonates could be ascribed to vapor phase deposition during i3

an experimental investigation of the pyroxene garnet transformation in a pyrolite composition and its bearing on the constitution of the mantle abstract the pyroxene garnet transformation in a model pyrolite composition has been investigated over the pressure interval 4 0 to 17 5 gpa at temperatures of 1200 1400 c the transformation is largely completed in the pressure interval 14 16 gpa and a single phase aluminum deficient garnet majorite becomes stable above 16 gpa the mineral assemblages and the density and seismic velocity distributions for pyrolite are calculated as a function of depth on the basis of this and other recent experimental data density and seismic velocity profiles calculated for pyrolite are consistent with models based on seismic observations to a depth of 550 km if the existence of a high density velocity gradient between 550 km and 650 km is confirmed the presence of further phase transformations in majorite and or some degree of chemical heterogeneity in this depth interval would be indicated

late wenlock ludlow silurian graptolite extinction evolution and biostratigraphy perspectives from arctic canada late wenlock early ludlow was a period of profound change in the graptolite world and two crises are evident the most important changeover occurred within the late wenlock homerian stage during which time cyrtograptus became extinct monograptids while severely reduced introduced new thecal and and sicular types and plectograptines underwent a striking diversification the second lesser change coincided with the wenlock ludlow boundary at which time monograptids reemerged as the dominant taxon group and plectograptines declined and shortly thereafter became extinct two species pristiograptus praedeubeli jaeger and lobograptus cornuatus n sp recovered in isolated and uncompressed form are described and illustrated the presence of the former species permits the subdivision of the upper homerian of the canadian arctic into two biozones the lower pristiograptus praedeubeli biozone and the upper pristiograptus ludensis pristiograptus sherrardae biozone both species exemplif

late ordovician mass extinction a new perspective from stratigraphic sections in central nevada integrated sequence stratigraphic biostratigraphic and chemostratigraphic analyses of three stratigraphic sections in central nevada indicate that late ordovician glaciation induced sea level fall produced diachronous stepwise faunal turnover in graptolites conodonts chitinozoans and radiolarians and also triggered a strong but transient positive 13c excursion this pattern is very different from that described for most mass extinction events

the nature distribution and genesis of the coesite and stishovite associated with the pseudotachylite of the vredefort dome south africa abstract the vredefort dome represents the uplift centre of a well known 2 00 ga old impact structure of unusually large magnitude shock features like shatter cones planar features and high pressure silica polymorphs are common the present study deals with the description mode of occurrence field distribution and post shock metamorphic alteration of coesite and stishovite which were poorly documented up to now these minerals occur as unusually large crystals in the quartzite of the wall rock in contact with very thin pseudotachylite veins in the pseudotachylite itself which can be interpreted as a friction and or a shock recovery melt fine needles of kyanite are ubiquitous it is proposed that these thin pseudotachylites a type formed during the transit of the shock wave they preceded the development of thick pseudotachylite and microbreccia veins b type which formed during the tensional period which immediately followed from comparison with the model of formation of high pressure polymorphs in porous sandstone it is suggested that higher pressure and stress was concentrated along the a type veins at the arrival of the shock front at this time the quartz was transformed into a high pressure phase which was probably poorly crystalline behind the shock front that is during the rarefaction the pressure was progressively released and the polymorphs crystallized from this initial high pressure phase stishovite crystallized first followed by coesite the high pressure conditions may have lasted an unusually long time due to the magnitude of the vredefort impact this long time probably about one second may account for the large size of the coesite and stishovite crystals most of the high pressure silica polymorphs are corroded to a variable degree by secondary quartz and preserved only in a restricted area of the impact structure this alteration is attributed to post shock metamorphism due to the temperature of the rock before impact plus the heat left after shock recovery some of the evidence put forward by authors against the impact model is discussed according to the facts arising from this investigation it is concluded that an origin by impact remains the simplest and most realistic model to explain the origin of the vredefort dome

shock conditions recorded in nwa 8159 martian augite basalt with implications for the impact cratering history on mars nwa 8159 is an augite rich martian basalt formed by cooling of a relatively evolved ca rich ti poor and lree depleted lava under relatively oxidizing conditions during the early amazonian in addition to its distinct igneous petrogenesis and high fo 2 nwa 8159 is also set apart from most martian shergottites with respect to the low degree of shock metamorphism required to preserve crystalline igneous plagioclase an 50 65 in this study mineral transformations within and adjacent to shock veins in nwa 8159 were investigated using scanning electron microscopy raman spectroscopy and transmission electron microscopy to better constrain the unusal shock history of this meteorite the transformation of olivine to ahrensite fe ringwoodite along shock vein margins and tissintite and coesiteformed from igneous mineral labradorite and silica grains entrained as clasts within shock veins has been documented in this study we report on a previously unidentified mineral assemblage of ca na majoritic garnet sodic clinopyroxene and stishovite crystallized from shock melt this mineral assemblage indicates a crystallization pressure of approximately 16 gpa which is within the range of previous shock pressure estimates for this meteorite 15 23 gpa the presence of a majoritic garnet bearing assemblage throughout veins up to 0 6 mm wide indicates that the sample remained at high pressure throughout the melt vein quench based on thermal models the sample must have remained at high pressure for 100 ms this shock duration is an order of magnitude longer than those experienced by more highly shocked shergottites such as tissint or zagami 30 gpa 10 20 ms and would seem to imply a relatively large impact event recent numerical models demonstrate that a range of shock pressures and durations are realized by rocks within the ejected spall zone of a hypervelocity impact the shock conditions experienced by nwa 8159 therefore do not require an impact event distinct from other shergottites rather our findings suggest that this meteorite originated from near the martian surface at the edge of the impact site the shock history of nwa 8159 provides a picture of mars consistent with that derived from remote observation that of a random cratering process that samples a geologically long lived and complex planet

experimental generation of shock induced pseudotachylites along lithological interfaces to understand the mechanism of formation of shock induced pseudotachylites and particularly the role that rock heterogeneities and interfaces play in their formation shock recovery experiments were carried out on samples consisting of two distinct lithologies dunite and quartzite it was possible to generate melt veins of 1 6 m width along lithological interfaces at moderate shock pressures 6 to 34 gpa the magnitudes of displacement along the interface strain rate and the kinetic heat production indicate that friction is an important heat source that largely contributes to the energy budget of the melt veins the experimentally produced veins resemble natural s type pseudotachylites the geometry of the veins depends on the orientation of the interface with respect to the shock front and includes strong variations in thickness formation of melt pockets and injection veins sudden changes in vein orientation and sharp vein margins two types of melt were observed vesicle free and vesicular melts dense vesicle free melt rock is likely to represent high pressure melts vesicular melts were also generated during shock compression but they remained in a molten state during pressure release and continued shearing intermingling of comminuted olivine and melt suggests that ultracataclasis of olivine induced by a dynamic tensile failure is a precursor stage to frictional melting shock wave interferences at the lithological interface provide the necessary stress conditions to start dynamic failure of olivine the composition of the frictional melts ranges from olivine normative to enstatite normative and is thus largely determined by olivine melting the validity of the sequence of friction melting susceptibilities of rock forming minerals inferred from tectonically produced pseudotachylites is confirmed and can now be applied to ultra high strain rates during shock compression

initial plant diversification and dispersal event in upper silurian of the prague basin abstract a relatively rich association of six species of embryophyte plants is known from the upper silurian of the prague basin bohemian massif czech republic all stratigraphically controlled specimens come from the neocolonograptus parultimus neocolonograptus ultimus zone of the p idoli a new genus and species tichavekia grandis psenicka sakala et kraft is established the new combination aberlemnia bohemica schweitzer sakala psenicka et kraft comb nov illustrates its lycophytina affinity the distribution and taphonomy of these plants in the prague basin indicates the proximity of exposed land interpreted to be islands of volcanic origin two local associations show primary differences in vegetated areas of the islands and apparent ecological responses of the primitive vascular plants to their habitats suitable environments in the coastal zones of volcanic islands in the prague basin which was situated at the outer periphery of a broad gondwanan shelf likely represent significant transfer points for an initial dispersion and a first expansion of land plants repeated vegetation of the islands is inferred from the discrete distribution of land plants at different stratigraphic levels these changes are explained partly in context of local changes of the island relief and partly as responses to global climatic and sea level fluctuations and fit environmental dynamics of islands of volcanic origin in general this initial plant diversification and dispersal event ipdde is of considerable significance in the evolution of plants documented from the bohemian massif

plagioclase breakdown as an indicator for shock conditions of meteorites the collision history of asteroids in the early solar system is difficult to reconstruct a study of plagioclase breakdown using x ray diffraction measurements under increasing pressures and temperatures suggests that peak pressures in collisions that formed shocked meteorites have been overestimated shocked meteorites that were formed when their parent body underwent shock metamorphism often contain the mineral plagioclase either in an amorphous form or in its high pressure phase the peak pressures in shock metamorphism can be constrained by shock recovery experiments that determine the amorphization pressures of plagioclase1 2 3 however in these experiments temperatures have been unrealistically low and timescales much shorter than those in natural events here we present in situ x ray diffraction measurements of two kinds of plagioclase feldspar in conditions of increasing pressures and temperatures we find that the amorphization pressure of plagioclase decreases with increasing temperature suggesting that previous studies overestimated this parameter1 2 3 we also found that the mineral jadeite forms first from amorphous plagioclase whereas the nucleation of other minerals such as stishovite or garnet is significantly delayed the occurrence of jadeite in shocked meteorites that do not contain stishovite4 5 6 7 8 can therefore be explained as a result of the crystallization kinetics we conclude that the study of plagioclase breakdown can constrain the pressure temperature time history of shock events and thus help to reconstruct the collisional history of asteroids in the early solar system

the late aeronian graptolite sedgwickii event associated positive carbon isotope excursion and facies changes in the prague synform barrandian area bohemia study of the lower silurian black shale succession of the prague synform has enabled detailed insight into graptolite faunal dynamics and diversity trends from the mid aeronian diversity maximum through to the late aeronian crisis graptolite diversity decreased from 33 taxa in the lituigraptus convolutus biozone to 17 taxa in the upper part of the stimulograptus sedgwickii biozone and newly erected lituigraptus rastrum biozone the graptolite assemblages of the latter biozones exhibit low species richness along with high dominance many graptolite species that became extinct in the early part of the sedgwickii zone were promptly replaced by new forms in the later part of the sedgwickii zone however replacement of extinct species by new forms considerably decelerated the increased rate of graptolite extinction recorded in the convolutus sedgwickii biozone boundary beds coincided with subtle changes in black shale lithologies and a positive shift in 13 c org of 2 2 compared to baseline values sea level drawdown can be inferred from siltstones and or temporary nondeposition in the middle sedgwickii zone this level also sees total organic carbon toc fluctuations and a strong positive 13 c org excursion with a peak shift of at least 7 the sedgwickii event exhibits substantial reorganization of the graptolite fauna its taxonomic impoverishment and concomitant increase in species dominance rather than a sudden collapse of the pre extinction assemblage associated changes in lithology toc and the pronounced 13 c org excursion suggest a relatively extended and probably multi phase period of stressed conditions that affected the pelagic realm inhabited by graptolites in the course of the late aeronian interval

heterogeneous mineral assemblages in martian meteorite tissint as a result of a recent small impact event on mars the microtexture and mineralogy of shock melts in the tissint martian meteorite were investigated using scanning electron microscopy raman spectroscopy transmission electron microscopy and synchrotron micro x ray diffraction to understand shock conditions and duration distinct mineral assemblages occur within and adjacent to the shock melts as a function of the thickness and hence cooling history the matrix of thin veins and pockets of shock melt consists of clinopyroxene ringwoo dite stishovite embedded in glass with minor fe sulfide the margins of host rock olivine in contact with the melt as well as entrained olivine fragments are now amorphosed silicate perovskite magnesiowustite or clinopyroxene magnesiowustite the pressure stabilities of these mineral assemblages are 15 gpa and 19 gpa respectively the 200 lm wide margin of a thicker mm size up to 1 4 mm shock melt vein contains clinopyroxene olivine with central regions comprising glass vesicles fe sulfide spheres fragments of host rock within the melt are polycrystalline olivine after olivine and tis sintite glass after plagioclase from these mineral assemblages the crystallization pressure at the vein edge was as high as 14 gpa the interior crystallized at ambient pressure the shock melts in tissint quench crystallized during and after release from the peak shock pressure crystallization pressures and those determined from olivine dissociation therefore represent the minimum shock loading shock deformation in host rock minerals and complete transformation of plagioclase to maskelynite suggest the peak shock pressure experienced by tissint p 29 30 gpa these pressure estimates support our assessment that the peak shock pressure in tissint was significantly higher than the minimum 19 gpa required to transform olivine to silicate perovskite plus magnesiowustite small volumes of shock melt 100 lm quench rapidly 0 01 s whereas thermal equilibration will occur within 1 2 s in larger volumes of melt 1 mm 2 the apparent variation in shock pressure recorded by variable mineral assemblages within and around shock melts in tissint is consistent with a shock pulse on the order of 10 20 ms combined with a longer duration of post shock cooling and complex thermal history this implies that the impact on mars that shocked and ejected tissint at 1 ma was not exceptionally large

eclogitic minerals in a shocked basaltic meteorite analytical transmission electron microscopy atem was used to characterize mineral parageneses occurring in melt veins of shock origin in the martian meteorite zagami we discovered a new ultra high pressure eclogite facies mineral assemblage consisting of omphacite stishovite and kalsi3o8 hollandite these tiny 200 300 nm high pressure minerals are embedded in silicate glass which forms the groundmass of the shock veins the microtexture of the veins and zoning of the clinopyroxene indicate the formation of minerals by crystallization from a high pressure melt using calibration data from shock and multi anvil experiments we infer that frictional melting occurred in veins at minimum temperatures of 2400 2500 c and at a pressure of ca 30 gpa subsequently the high pressure phases crystallized during decompression in the pressure interval from 25 5 gpa

graptolite biozone correlation charts charts are presented showing the correlation of graptolite biozonations for the lower ordovician to lower devonian i e for the entire stratigraphical range of the planktonic graptoloid graptolites regions chosen are those for which the most detailed biostratigraphical studies have been undertaken for baltica average graptolite zone chron duration appears to vary from c 300000 years ludlow epoch to 2 4 ma lochkovian epoch

exogenic and endogenic breccias a discussion of major problematics abstract pseudotachylitic breccias impact melt rock ultra cataclasites and even ultramylonites may macroscopically be of very similar appearance even detailed optical microscopic work may not suffice to identify the actual type of melt rock studied the difficulties to distinguish different tectonically produced breccias fault rocks and impact breccias for which no uniformly accepted nomenclature exists are examined and the various formational processes friction melting or shock melting at the macro and micro scales are discussed special emphasis is placed on the occurrence of different and multiple breccia types within impact structures where not only impact induced friction melt and impact melt may occur but where tectonically produced fault rocks that are not related to the impact event may further complicate the breccia situation melting on shatter cone surfaces and its likely formation the proposed origin as distal impact breccia of certain diamictites and problems with carbonate breccias and caused by metamorphic overprint on breccias are discussed it is cautioned not too easily to reach generalized conclusions for example with regard to the usage of the a and b type pseudotachylite terminology areas in need of further breccia studies if possible in close collaboration between impact and tectonic researchers are emphasized