active strike slip faulting in el salvador central america several major earthquakes have affected el salvador central america during the past 100 yr as a consequence of oblique subduction of the cocos plate under the caribbean plate which is partitioned between trench orthogonal compression and strike slip deformation parallel to the volcanic arc focal mechanisms and the distribution of the most destructive earthquakes together with geomorphologic evidence suggest that this transcurrent component of motion may be accommodated by a major strike slip fault el salvador fault zone we present field geological structural and geomorphological data collected in central el salvador that allow the constraint of the kinematics and the quaternary activity of this major seismogenic strike slip fault system data suggest that the el salvador fault zone consists of at least two main e w fault segments san vicente and berlin segments with associated secondary synthetic wnw ese and antithetic nnw sse riedel shears and nw se tensional structures the two main fault segments overlap in a dextral en echelon style with the formation of an intervening pull apart basin our original geological and geomorphologic data suggest a late pleistocene holocene slip rate of 11 mm yr along the berlin segment in contrast with low historical seismicity the kinematics and rates of deformation suggested by our new data are consistent with models involving slip partitioning during oblique subduction and support the notion that a trench parallel component of motion between the caribbean and cocos plates is concentrated along e w dextral strike slip faults parallel to the volcanic arc

the structural inventory of a small complex impact crater jebel waqf as suwwan jordan the investigation of terrestrial impact structures is crucial to gain an in depth understanding of impact cratering processes in the solar system here we use the impact structure jebel waqf as suwwan jordan as a representative for crater formation into a layered sedimentary target with contrasting rheology the complex crater is moderately eroded 300 420 m with an apparent diameter of 6 1 km and an original rim fault diameter of 7 km based on extensive field work ikonos imagery and geophysical surveying we present a novel geological map of the entire crater structure that provides the basis for structural analysis parametric scaling indicates that the structural uplift 250 350 m and the depth of the ring syncline 200 m are anomalously low the very shallow relief of the crater along with a ne vergence of the asymmetric central uplift and the enhanced deformations in the up range and down range sectors of the annular moat and crater rim suggest that the impact was most likely a very oblique one 20 one of the major consequences of the presence of the rheologically anisotropic target was that extensive strata buckling occurred during impact cratering both on the decameter as well as on the hundred meter scale the crater rim is defined by a circumferential normal fault dipping mostly toward the crater footwall strata beneath the rim fault are bent up in the down range sector but appear unaffected in the up range sector the hanging wall displays various synthetic and antithetic rotations in the down range sector but always shows antithetic block rotation in the up range sector at greater depth reverse faulting or folding is indicated at the rim indicating that the rim fault was already formed during the excavation stage

tsunami generation and propagation from the mjolnir asteroid impact in the late jurassic period about 142 million years ago an asteroid hit the shallow paleo barents sea north of present day norway the geological structure resulting from the impact is today known as the mjlnir crater the present work attempts to model the generation and the propagation of the tsunami from the mjolnir impact a multi material hydrocode sova is used to model the impact and the early stages of tsunami generation while models based on shallow water theories are used to study the subsequent wave propagation in the paleo barents sea we apply several wave models of varying computational complexity this includes both three dimensional and radially symmetric weakly dispersive and nonlinear boussinesq equations as well as equations based on nonlinear ray theory these tsunami models require a reconstruction of the bathymetry of the paleo barents sea the mjolnir tsunami is characteristic of large bolides impacting in shallow sea in this case the asteroid was about 1 6 km in diameter and the water depth was around 400 m contrary to earthquake and slide generated tsunamis this tsunami featured crucial dispersive and nonlinear effects a few minutes after the impact the ocean surface was formed into an undular bore which developed further into a train of solitary waves our simulations indicate wave amplitudes above 200 m and during shoaling the waves break far from the coastlines in rather deep water the tsunami induced strong bottom currents in the range of 30 90 km h which presumably caused a strong reworking of bottom sediments with dramatic consequences for the marine environment

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

high resolution sequence stratigraphy of lower paleozoic sheet sandstones in central north america the role of special conditions of cratonic interiors in development of stratal architecture well known difficulties in applying sequence stratigraphic concepts to deposits that accumulated across slowly subsiding cratonic interior regions have limited our ability to interpret the history of continental scale tectonism oceanographic dynamics of epeiric seas and eustasy we used a multi disciplinary approach to construct a high resolution stratigraphic framework for lower paleozoic strata in the cratonic interior of north america within this framework these strata proved readily amenable to modern sequence stratigraphic techniques that were formulated based on successions along passive margins and in foreland basins settings markedly different from the cratonic interior parasequences parasequence stacking patterns systems tracts maximum flooding intervals and sequence bounding unconformities can be confidently recognized in the cratonic interior using mostly standard criteria for identification the similarity of cratonic interior and foreland basin successions in size geometry constituent facies and local stacking patterns of nearshore parasequences is especially striking this similarity indicates that the fundamental processes that establish shoreface morphology and determine the stratal expression of retreat and progradation were likewise generally the same despite marked differences in tectonism physiography and bathymetry between the two settings our results do not support the widespread perception that paleozoic cratonic interior successions are so anomalous in stratal geometries and constitute such a poor record of time that they are poorly suited for modern sequence stratigraphic analyses the particular arrangement of stratal elements in the cratonic interior succession we studied is no more anomalous or enigmatic than the variability in architecture that sets all sedimentary successions apart from one another thus paleozoic strata of the cratonic interior are most appropriately considered as a package that belongs in a continuum of variable stratigraphic packages reflecting variable controls such as subsidence and shelf physiography special conditions of exceptionally slow subsidence rate shallow bathymetry and nearly flat regional shelf gradient are manifest mostly by the presence of individual systems tracts of relatively long duration that extend for much greater distances across depositional strike than those that characterize successions deposited in more dynamic tectonic and physiographic settings these results suggest that if other cratonic interior successions are as anomalous as reported a low sediment supply may have played a primary role in development of their apparently condensed stratal architecture the results also lead us to suggest that a nonvegetated lower paleozoic landscape played a relatively insignificant role in the development of what are commonly perceived to be enigmatic stratigraphic features of sheet sandstones particularly their widespread yet thin geometry and a scarcity of shale and siltstone

resolving volatile sources along the western sunda arc indonesia we present the chemical and isotope hecn characterization of active fumaroles and hydrothermal gases and waters from the summits and flanks of 19 volcanic centers along the western sunda arc indonesia samples were collected over two field expeditions 1991 and 2010 and cover 13 volcanic centers in sumatra 5 in java and one in bali in addition we present data from three geothermal sites in sumatra associated with active fault systems in between volcanic centers ibvc the overall aim is to resolve volatiles associated with the sub arc mantle subducting slab and mantle wedge from inputs derived from the over riding arc crust the western sunda arc is a prime locality to assess controls on volatile provenance at subduction zones due to changes in composition and thickness of over riding crust and variations in sediment input rates along the strike of the arc the dry gas chemistry of the majority of volcanic samples is dominated by co2 with inert gas variations hearn2 typical of subduction zone gases however there is a strong crustal control on the heco2 isotope and relative abundance systematics on a number of volcanic centers this effect is most clearly observed at flank localities and in water phase samples filtering the entire database for modifications due to air contamination degassing induced fractionation c isotopes and co2 3he ratios and crustal contamination associated with volatile transport within shallow level hydrothermal systems allows recognition of the magmatic volatile signature of individual volcanoes along the arc helium isotopes at all but two volcanic centers talang and dempo on sumatra range from 5 3 ra to 8 1 ra ra air 3he 4he pointing to the mantle wedge as the principal source of he but with a small input of crustal radiogenic he at some localities samples from java and bali span an even more limited range 6 6 to 7 9 ra implying a relatively smaller input of crustal he carbon isotope and co2 3he ratios vary from 1 4 to 6 4 and 4 38 to 150 109 respectively with higher and more variable values to the north of sumatra this latitudinal effect is not apparent in air corrected n isotope values 15nc 3 91 to 5 92 or various elemental ratios such as n2 ar and n2 he the three ibvc sites all located in sumatra have significantly lower 3he 4he ratios 3 6 ra with co2 3he values both higher and lower than volcanic centers their 13c 15nc and gas ratios overlap with the volcanic centers the elemental and isotopic characteristics of carbon and helium have been modified at ibvc sites due to crustal processes however based upon relationships between ch4 and 3he as well as co variations between hecn isotopes the over riding crust and associated sediments add minimal volatiles to volcanic centers throughout the western sunda arc in turn subducted sediment particularly the nicobar fan which supplies himalayan derived sediment to the slab off sumatra exerts a strong control on the magmatic co2 characteristics although it is less influential for n2 in spite of large sections of incoming sedimentary material being off scraped during subduction our data suggest that a significant fraction must enter the trench enhancing fluid melt production in the mantle wedge we propose that subduction related source contamination plays the dominant role over thick old crustal basement in supplying the major volatile output budget of the western sunda arc

geomaterials in cultural heritage this volume gives a broad view of the application of geoscience techniques to the study of monuments and objects from excavations and museums including their origin technique of manufacture age and conservation it reaffirms the important contribution of geosciences in the interdisciplinary approach to the study of complex materials such as minerals rocks glass metals mortar plaster slags and pottery the papers in this book cover three topics the study of pottery glass stone and mortar the application of raman spectroscopy to a wide variety of objects and the future of archaeometry interdisciplinary studies including field geology geophysics microscopy textural analysis physical methods and geochemistry are used to unlock information from the ancient materials such as the provenance of the raw materials the firing technology the ancient recipes and the alteration pathways

nitrogen sources and recycling at subduction zones insights from the izu bonin mariana arc 1 we report new chemical and nitrogen isotopic data from 29 volcanic and hydrothermal gas samples covering eight centers in the izu bonin mariana ibm arc to investigate the sources flux and mass balance of nitrogen at a cool convergent margin the majority of samples have high n2 he 1217 17 300 and low co2 n2 exc 78 937 implying addition of nitrogen from the subducting slab this inference is supported by positive i e sediment like 15n values up to 5 5 in most samples the exception to these trends is agrigan in the mariana arc with low n2 he 200 high co2 n2 exc 1500 and negative 15n mixing calculations suggest an average of 34 of the nitrogen in our samples is derived from subducted sediment or 75 after correction for atmospheric contamination sediment derived n2 fluxes estimated by three different methods range from 0 25 108 to 1 11 108 mol yr 1 n2 representing 4 17 of the total nitrogen input flux or 11 51 of the sedimentary nitrogen input flux the altered oceanic crust is identified as an important contributor to the arc nitrogen budget and the 15n of the residual nitrogen subducted into the mantle is estimated at approximately 1 9 despite similarities in gas chemistry and 15n values our conclusions regarding nitrogen recycling for ibm are markedly different than those for the nicaraguan segment of the central american arc and we suggest that thermal regime is the major control on nitrogen recycling within subduction zones the global nitrogen cycle is estimated to be in steady state suggesting either that subducted sediments are an unlikely source for heavy nitrogen in plume related rocks or secular variation in the isotopic composition of subducted sediments better constraints on nitrogen recycling at other arcs are required to test these conclusions

unchanged nitrogen isotopic composition of organic matter in the south china sea during the last climatic cycle global implications the 15n of surface and down core sediments spanning the last 20 200 kyr from the entire south china sea scs ranges only from 3 0 to 6 5 with no correlation with discernible paleoclimatic oceanographic changes detailed profiles of the uppermost sediment column including fluff samples indicate a minor diagenetic overprint of 0 3 1 2 at the sediment water interface the absence of any correlation with reconstructed glacial interglacial changes in primary production terrigenous input and or sea level related basin configuration is attributed to a complete consumption of nitrate during primary production in this marginal basin during at least the last 140 000 years this in turn implies that the 15n of the nitrate used during primary production remained approximately constant during the last climatic cycle the proposed scenario infers an unchanged nitrogen isotopic composition of the western pacific subsurface nitrate between glacial and interglacial stages as well as during terminations and thus constrains proposed changes in the oceanic n inventory

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

fluid geochemistry and geothermometry in the unexploited geothermal field of the vicano cimino volcanic district central italy abstract the vicano cimino volcanic district vcvd is related to the post orogenic magmatic activity of the peri tyrrhenian sector of central italy the chemical and isotopic compositions of 333 water discharges and 25 gas emissions indicate the occurrence of two main sources 1 cold ca hco 3 to ca na k hco 3 type waters from relatively shallow aquifers hosted in volcanic and sedimentary formations and 2 thermal ca so 4 hco 3 type waters located in a deep co 2 pressurized reservoir hosted in carbonate evaporite rocks and separated from the shallow aquifers by thick sequences of low permeability formations carbon dioxide is mainly produced by thermal metamorphic decarbonation within the deepest and hottest parts of the carbonate evaporite reservoir 13 c co 2 from 3 1 to 2 2 vs vpdb likely affected by a mantle rooted co 2 release of co 2 rich gases from the deep aquifer into the overlying shallow aquifers produces high co 2 springs and bubbling pools the spatial distribution of thermal waters and co 2 rich cold discharges is strongly controlled by fractures and faults located in correspondence with buried structural highs stable isotopes d and 18 o suggest that meteoric water feeds both the shallow and deep reservoirs the relatively low r r a values 0 27 1 19 indicate that he is mainly deriving from a crustal source with minor component from the mantle affected by crustal contamination related to the subduction of the adriatic plate consistently relatively high n 2 ar and n 2 3 he ratios and positive 15 n n 2 values from 0 91 to 5 7 vs air characterize the vcvd gas discharges suggesting the occurrence of a significant excess nitrogen isotopic compositions of ch 4 13 c ch 4 and d ch 4 values from 28 9 to 22 1 vs vpdb and from 176 to 138 vs vsmow respectively and composition of light alkanes are indicative of prevalent thermogenic ch 4 although the occurrence of abiogenic ch 4 production cannot be excluded the 34 s h 2 s values from 9 3 to 11 4 vs vcdt are consistent with the hypothesis of h 2 s production from thermogenic reduction of triassic anhydrites gas geothermometry in the h 2 o h 2 ar h 2 s system suggests that the vcvd gases equilibrated in a liquid phase at redox conditions controlled by interactions of fluids with the local mineral assemblage at temperatures lower 2000 m geothermal wells this confirms that secondary processes i e steam condensation gas dissolution in shallow aquifers re equilibration at lower temperature and microbial activity significantly affect the chemistry of the uprising fluids thermal water chemistry supports the occurrence in this area of an anomalous heat flow that coupled with the recent demographic growth makes this site suitable for direct and indirect exploitation of the geothermal resource in agreement with the preliminary surveys carried out in the 1970 s 1990 s for geothermal exploration purposes

noble gas and carbon isotopes in mariana trough basalt glasses abstract noble gas elemental and isotopic compositions have been measured as well as the abundance of c and its isotopic ratios in 11 glasses from submarine pillow basalts collected from the mariana trough the 3he 4he ratios of 8 22 and 8 51 ratm of samples dredged from the central mariana trough 18 n agree well with that of the mid ocean ridge basalt morb glasses 8 4 0 3 ratm whereas a mean ratio of 8 06 0 35 ratm in samples from the northern mariana trough 20 n is slightly lower than those of morb one sample shows apparent excess of 20ne and 21ne relative to atmospheric ne suggesting incorporation of solar type ne in the magma source there is a positive correlation between 3he 4he and 40ar 36ar ratios which may be explained by mixing between morb type and atmospheric noble gases excess 129xe is observed in the sample which also shows 20ne and 21ne excesses observed 13c values of 20 n samples vary from 3 76 to 2 80 and appear higher than those of morb and the corresponding co2 3he ratios are higher than those of mara samples at 18 n suggesting c contribution from the subducted slab

temporal variations in fumarole gas chemistry at poas volcano costa rica we report the chemical and isotopic composition of fumarole gas discharges collected at poas volcano costa rica from 2001 to 2014 covering a period during which the volcano experienced a series of phreatic eruptions march 2006 to october 2014 the relative abundances of poas c s h o gas species are controlled by reactions involving the so2 h2s and so so2 gas buffers indicating magmatic temperatures of up to 800 c although fumarole outlet temperatures are 120 c for most samples so2 is the dominant sulfur gas and hcl contents are relatively high gas compositional changes within the magma lake hydrothermal system likely result from a combination of several processes including 1 the injection of new and undegassed magma in late 2000 early 2001 2 the heating of the hydrothermal system accompanied by gas pressure build up and 3 hydrofracturing through 2006 these processes culminated in the phreatic eruptions of 2006 and 2008 since 2005 the lake level has declined and is now january 2014 at the lowest level 10 m since the last periods that it dried out completely april 1984 and april 1994 the most recent data of 2014 show high level of degassing from the dome fumaroles and the release of hcl rich and co2 poor gases implies that the magma injected in late 2000 continues to supply volatiles our data show that time series sampling of fumarole gases provides important insights to better understand magmatic and hydrothermal processes at active volcanoes and also to potentially forecast phreatic eruptions

intimate link between ammonium loss of phengite and the deep earth s water cycle abstract phengite a high pressure white mica is stable down to 300 km depth in subduction environment and represents an important carrier of nitrogen and hydrogen to the deep earth although the determination of nitrogen and hydrogen recycling efficiency to subduction zones is crucial for building global geodynamic models of volatile elements no experimental data have so far evaluated the rate of ammonium release from phengite and its potential effect on phengite dehydration in a descending slab here we investigated the kinetics of ammonium loss and dehydration of ammonium bearing and ammonium free phengite samples using ftir spectroscopy single phengite grains were analyzed after annealing at high temperatures from 720 to 850 c for different periods of time the results show that ammonium loss is one order of magnitude faster than dehydration at the same temperature for instance with diffusion coefficients of 1 5 10 15 m2 s and 1 0 10 16 m2 s at 800 c for ammonium loss and dehydration respectively moreover dehydration of ammonium bearing phengite is one order of magnitude faster than that of ammonium free phengite with diffusion coefficients of 1 0 10 16 m2 s and 2 0 10 17 m2 s at 800 c for dehydration in ammonium bearing and ammonium free phengite respectively the activation energies of ammonium loss dehydration in ammonium bearing phengite and dehydration in ammonium free phengite are around 150 535 548 kj mol respectively these results imply that ammonium release from phengite is not only easier than dehydration but also can promote dehydration the geochemical behavior of ammonium in phengite is consistent with previous observations on natural samples from subduction zones of different geothermal gradients furthermore since ammonium release can trigger dehydration water may be released at depth locus shallower than theoretical phengite breakdown i e

effect of the volcanic front migration on helium nitrogen argon and carbon geochemistry of hydrothermal magmatic fluids from hokkaido volcanoes japan abstract active volcanoes from hokkaido are distributed along the sw ne trending volcanic front where the subducting pacific plate changes direction from n s to wsw ese one exception is the tokachi volcano which is located approximately 50 km to the north west of the current volcanic front the tokachi oki slab cracking zone represents the transitional zone between the honshu and kurile trenches this change is responsible for the rapid lateral variation of the subduction angle and slab shape we investigate the consequence of this change on the chemical and isotopic compositions from hydrothermal magmatic fluids and mineral separates collected from three volcanoes of hokkaido northern japan namely tarumae tokachi and meakan our data show that these volcanoes have a distinct range of he n and c isotopic compositions meakan fumaroles have a 3he 4he ratio that ranges from 4 46ra to 6 88ra 15n from 2 2 to 3 5 and 13c co2 from 8 47 to 9 81 consistent with subducted sediment contribution tarumae fumaroles have a particularly low 3he 4he ratio 3 08ra in comparison with their hot springs up to 5 24ra and are associated with high co2 3he ratios 2 2 5 3 1011 hydrothermal crustal interactions from the dome or from a deeper hydrothermal system are the main mechanisms that control the 3he 4he and co2 3he ratios in tarumae fumaroles the 15n and 13c co2 values for tarumae volcano range from 0 6 to 3 1 and from 6 34 to 6 43 respectively tokachi fumaroles are characterized by high 3he 4he up to 7 38ra low 15n 2 4 0 1 and low 13c co2 up to 5 32 consistent with mantle contribution and minor subducted sediment contribution the limestone sediment mantle ratio shows a decreasing contribution from subducted sediments starting from tokachi 1 9 to meakan 3 4 and finally tarumae 5 3 this change of isotopic compositions from the sw ne of hokkaido is the result of the tokachi oki slab cracking zone slab dehydration under tokachi occurs at a deeper level 150 200 km than in other hokkaido volcanoes 100 km which produces the migration of the volcanic front therefore we argue that the isotopic signature of tokachi is the result of i a lower sediment contribution from the slab ii a deeper dehydration of the slab and iii a lower degree of partial melting

chemical and isotopic compositions of thermal springs fumaroles and bubbling gases at tacana volcano mexico guatemala implications for volcanic surveillance this study presents baseline data for future geochemical monitoring of the active tacana volcano hydrothermal system mexico guatemala seven groups of thermal springs related to a nw se oriented fault scarp cutting the summit area 4 100m a s l discharge at the northwest foot of the volcano 1 500 2 000m a s l another one on the southern ends of tacana la calera the near neutral ph from 5 8 to 6 9 thermal t from 25 7 c to 63 0 c hco3 so4 waters are thought to have formed by the absorption of a h2s so2 co2 enriched steam into a cl rich geothermal aquifer afterwards mixed by na hco3 enriched meteoric waters originating from the higher elevations of the volcano as stated by the isotopic composition d and 18o of meteoric and spring waters boiling temperature fumaroles 89 c at 3 600m a s l nw of the summit formed after the may 1986 phreatic explosion emit isotopically light vapour d and 18o as low as 128 and 19 9 respectively resulting from steam separation from the summit aquifer fumarolic as well as bubbling gases at five springs are co2 dominated the 13cco2 for all gases show typical magmatic values of 3 6 1 3 vs v pdb the large range in 3he 4he ratios for bubbling dissolved and fumarolic gases from 1 3 to 6 9 atmospheric 3he 4he ratio r a is ascribed to a different degree of near surface boiling processes inside a heterogeneous aquifer at the contact between the volcanic edifice and the crystalline basement 4he source tacana volcano offers a unique opportunity to give insight into shallow hydrothermal and deep magmatic processes affecting the co2 3he ratio of gases bubbling springs with lower gas water ratios show higher 3he 4he ratios and consequently lower co2 3he ratios e g zarco spring typical central american co2 3he and 3he 4he ratios are found for the fumarolic agua caliente and zarco gases 3 1 1 6 1010 and 6 0 0 9 r a respectively the l s 5 9 0 5 and l s m ratios 9 2 0 7 for the same gases are almost identical to the ones calculated for gases in el salvador suggesting an enhanced slab contribution as far as the northern extreme of the central american volcanic arc tacana

difference in n2 ar ratio of magmatic gases from northeast and southwest japan new evidence for different states of plate subduction two different plates are subductiog beneath the japanese island arcs the pacific plate in the north and the philippine sea plate in the south geophysical evidence indicates that the philippine sea plate subducts without extending beneath many of the volcanoes in the southwest japan arc the n 2 ar ratio 424 of a common magmatic gas from geothermal and volcanic areas in southwestern japan is an order of magnitude lower than the value 4250 in northeastern japan because the high n 2 ar values reflect input from subducted sediments the large difference in the n 2 ar ratios of magmatic gases indicates that the contribution of subducted sediment to the source of magma in southwestern japan is much lower than that in northeastern japan the n 2 ar ratios off magmatic gases from island arcs elsewhere may also yield information regarding the degree of contribution of subducted sediments to arc magma sources and differences in the states of plate subduction

structural geology of impact craters abstract the formation of impact craters is a highly dynamic and complex process that subjects the impacted target rocks to numerous types of deformation mechanisms understanding and interpreting these styles of micro meso and macroscale deformation has proved itself challenging for the field of structural geology in this paper we give an overview of the structural inventory found in craters of all size ranges on earth and look into the structures of craters on other planetary bodies structural features are discussed here that are caused by i extremely high pressures and temperatures that occur during the initial passage of the shock wave through the target rock and projectile ii the resulting flow field in the target that excavates and ejects rock materials and iii the gravitationally induced modification of the crater cavity into the final crater form a special focus is put on the effects that low angle impacting bodies have on crater formation we hope that this review will help both planetary scientists and structural geologists understand the deformation processes and resulting structures generated by meteorite impact

kinetic nitrogen isotope fractionation associated with thermal decomposition of nh3 experimental results and potential applications to trace the origin of n2 in natural gas and hydrothermal systems abstract ammonia nh3 is the major intermediate phase in the pathway of nitrogen n transfer from the fixed n phases e g in crustal material to free n2 e g in natural gas reservoirs and volcanic gases yet the n isotopic behavior during these n cycling processes remains poorly known in an attempt to contribute to the understanding of n cycling using n isotopes we carried out laboratory experiments to investigate the n isotopic effect associated with thermal decomposition of ammonia 2nh3 n2 3h2 pure nh3 with initial 15 n nh 3 of 2 relative to air standard was sealed into quartz tubes and thermally decomposed at 600 700 or 800 c from 2 hours to 500 days with the progress of the reaction the 15n of the remaining nh3 and the accumulated n2 increased from 2 to 35 and from 20 to 2 respectively the differences of the n isotope fractionations at the three temperatures are not significant modeling using the rayleigh distillation model yielded similar kinetic n isotope fractionation factors n 2 nh 3 of 0 983 0 002 for 600 700 and 800 c applied to geological settings this significant isotope discrimination 17 associated with partial decomposition of nh3 nh4 from crustal sources 15naverage 6 3 can produce mantle like i e 5 or even lower 15n values of n2 this may explain the large variation of 15n 20 to 30 of n2 in natural gas reservoirs it can also possibly explain the extreme 15n depletion of n2 in some volcanic gases this possibility has to be carefully considered when using n isotopes to trace geological n cycling across subduction zones by analysis of volcanic n2

the chicxulub asteroid impact and mass extinction at the cretaceous paleogene boundary the cretaceous paleogene boundary similar to 65 5 million years ago marks one of the three largest mass extinctions in the past 500 million years the extinction event coincided with a large asteroid impact at chicxulub mexico and occurred within the time of deccan flood basalt volcanism in india here we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction notably a single ejecta rich deposit compositionally linked to the chicxulub impact is globally distributed at the cretaceous paleogene boundary the temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations for example darkness and cooling lead us to conclude that the chicxulub impact triggered the mass extinction

fluid geochemistry of montserrat island west indies two geochemical surveys carried out in march 1991 and september 1992 revealed the existence of a hydrothermal system in the southern portion of montserrat island below soufriere hills volcano this conclusion is supported by the presence of a the thermal springs of plymouth which are fed by deep na cl waters cl concentration 25 000 mg kg temperature ca 250 c mixed with shallow steam heated waters b the four fumarolic fields of galway s soufriere gages upper soufriere gages lower soufriere and tar river soufriere where acid to neutral steam heated waters are present together with several fumarolic vents discharging vapors formed through boiling of hydrothermal aqueous solutions involvement of magmatic fluids in the recharge of the hydrothermal aquifers is suggested by a the high 3he 4he ratios of fumarolic fluids i e 8 2 ra at galway s soufriere and 5 9 ra at gages lower soufriere b the d and 18o values of na cl thermal springs and steam condensates indicating the involvement of arc type magmatic water in the formation of deep geothermal liquids and c the ch4 co2 ratios of fumarolic fluids which are lower than expected for equilibrium with the feo feo1 5 hydrothermal rock buffer but being shifted towards the so2 h2s magmatic gas buffer

boron geochemistry of the central american volcanic arc constraints on the genesis of subduction related magmas boron contents were measured in representative quaternary lavas from the central american volcanic arc to evaluate along strike variations in subduction processes despite the significant range in b concentrations 2 37 ppm in the mafic lavas b la ratios vary in a systematic fashion along the arc higher values 1 are typical between guatemala and northern costa rica whereas low values most 0 5 typify central costa rica and western panama b la is highly correlated with 10be 9be r2 0 94 excluding one sample and appears to be a useful indicator of subduction contributions to the magma sources if enrichments of both b and 10be are proportional to the flux of subducted sediment along strike variations in b la suggest at least a twofold variation in this flux with maximum values below western nicaragua and minimum values below costa rica and western panama where the cocos ridge is being subducted these data may also reflect significant differences in thermal state of the descending slab which in turn differentially affects release patterns of fluids and fluid mobile trace elements and possibly melting processes beneath different parts of the arc the following scenario is suggested to explain the geochemical results beneath the northwestern part of the arc steep subduction of older relatively cold slab favors more efficient subduction of fluid components to depths beneath the volcanic front the released fluids carry fluid mobile elements to the overlying mantle which upon melting produces calcalkalic magmas shallow subduction of warmer slab beneath the southeastern part of the arc favors shallow release of fluids and limits fluid related metasomatism of sub arc mantle beneath the volcanic front under such conditions b la and ba la ratios in the sub arc mantle vary little from values seen in oceanic island basalts magmas in this part of the arc nevertheless display the highest la yb and lowest ba la and b la ratios which are consistent with prior light rare earth element enrichment in the source significantly lower degrees of melting or a combination thereof because some of the largest volcanoes occur in costa rica and magma flux there is nearly an order of magnitude higher than elsewhere in the arc source enrichment is considered to be the more plausible explanation it is proposed that quaternary magma production below costa rica involved lithospheric sources containing trapped or stored melt components but this enrichment process is unlikely to have involved typical arc magmas or subduction fluids because we see no b enrichment

tracing magma sources in an arc arc collision zone helium and carbon isotope and relative abundance systematics of the sangihe arc indonesia 1 the sangihe arc is presently colliding with the halmahera arc in northeastern indonesia forming the world s only extant example of an arc arc collision zone we report the first helium and carbon isotopic and relative abundance data from the sangihe arc volcanoes as a means to trace magma origins in this complicated tectonic region results of this study define a north south trend in 3he 4he co2 3he and 13c suggesting that there are variations in primary magma source characteristics along the strike of the arc the northernmost volcanoes awu and karangetang have higher co2 3he and 13c up to 179 109 and 0 4 respectively and lower 3he 4he 5 4 ra than the southernmost volcanoes ruang lokon and mahawu resolving the arc co2 into component structures mantle derived plus slab derived organic and carbonate co2 the northern volcanoes contain an unusually high 90 contribution of co2 derived from isotopically heavy carbonate associated with the subducting slab sediment and altered oceanic basement furthermore the overall slab contribution co2 of carbonate and organic origin relative to carbon of mantle wedge origin is significantly enhanced in the northern segment of the arc these observations may be caused by greater volumes of sediment subduction in the northern arc along strike variability in subducted sediment composition or enhanced slab derived fluid melt production resulting from the superheating of the slab as collision progresses southward

asteroid breakup linked to the great ordovician biodiversification event the great ordovician biodiversification event coincides 470 million years ago with the break up of a large asteroid and the resultant frequent bombardment of earth with asteroid fragments

extraterrestrial impact events the record in the rocks and the stratigraphic column abstract the known terrestrial impact record is a biased sample of a much larger population of impact events the biases are due to the modifying effects of terrestrial geologic processes coupled with incomplete searches for impact structures and impact related materials terrestrial impact structures have the same basic forms as impact craters on the other planets of the inner solar system but because of post impact modification by terrestrial geologic process are recognised by the occurrence of shock metamorphic effects in some cases siderophile anomalies have been identified in impact lithologies and have been used to estimate the composition of the impacting body similar shock metamorphic effects and a siderophile anomaly in k t boundary materials are indicative of a major impact event which has been correlated with the formation of the chicxulub structure mexico evidence of a small number of other impacts occur in the stratigraphic record most commonly as tektite or microtektite horizons in some cases they are known to be accompanied by geochemical anomalies in other cases a number of ir anomalies have been reported in the stratigraphic record but there is no confirmatory evidence that they are due to impact the majority of known impact events in the stratigraphic record are from relatively recent geologic time logic dictates however that many more impacts must be recorded in terrestrial sediments and model calculations indicate that relatively small impacts d 20 km have the potential to cause atmospheric blow out and thus global dispersion of some of the impact products geochemical detection however of such events may not be easy in some cases because of relatively small absolute signals against the background of the daily infall of cosmic material in addition non chondritic bodies may result in no appreciable geochemical anomaly in view of this any claim to a geochemical signature of impact in the stratigraphic record should be accompanied by a physical search for impact materials although in the case of impacts into oceanic crust this too will be difficult given the k t experience however and the fact that large scale impact on earth is a natural consequence of the character of the solar system the potential of impacts to provide local and global marker horizons can not be ignored similarly the fact that impacts may have the potential to result in short term biologic or climatic excursions can not be dismissed arbitrarily when considering the causes of such phenomena as stable isotope anomalies in the stratigraphic record

feather features a low shock pressure indicator in quartz 1 feather features are a recently discovered type of microstructure in shocked quartz they occur as short parallel to subparallel lamellae with spacing similar to that of planar deformation features pdfs these lamellae are always found in combination with a planar fracture pf feather features are crystallographically controlled 0001 101 and 112 orientations are common while 103 and 102 orientations are lacking samples from the nordlinger ries and the matt wilson impact structures are compared with plane wave shock recovery experiments 16 gpa microscopic analysis of quartz grains with feather features show that their formation is linked to shearing along the associated pfs during shock deformation the generation of shear fractures with feather features in plane wave shock recovery experiments suggests a pressure range of 7 10 gpa although further constraints are needed for an upper and lower pressure limit the orientation of feather features in rock samples is suggested to be controlled by the direction of the principal axis of stress with sheared pfs at 45 angles to the axis and most feather feature lamellae aligned parallel to the axis 20 the majority of feather feature lamellae emanate from the pfs in the same direction and in experiments point in the direction from which the shock wave came on the basis of their appearance in shocked quartz grains from 26 impact craters and the current lack of reports of these features in endogenically deformed crustal rocks their uniqueness as a new type of impact induced planar microstructure alongside pdfs and pfs is proposed

a global rain of micrometeorites following breakup of the l chondrite parent body evidence from solar wind implanted ne in fossil extraterrestrial chromite grains from china previous studies of limestone beds of mid ordovician age from both sweden and china show that the earth saw an at least two orders of magnitude increase in the influx of extraterrestrial material approximately 470 ma following the disruption of an l chondrite parent body in the asteroid belt recovered extraterrestrial material consists of fossil meteorites and sediment dispersed extraterrestrial chromite sec grains both with l chondritic origin ne isotope analysis of sec grains from one of the swedish limestone sections revealed that the vast majority of the grains were delivered to earth as micrometeorites in this study we extend the previous work both in time and geographically by measuring concentrations and isotopic ratios of ne in individual sec grains 60120 mu m in diameter from three different beds from a contemporary middle ordovician limestone section in china all of the chinese sec grains 44 in total contain surface implanted ne of fractionated solar wind composition implying that these grains were as in the case of the swedish sec grains delivered to earth as micrometeorites this gives further compelling evidence that the two to three orders of magnitude increase in the influx of micrometeoritic material following the breakup of the l chondrite parent body was indeed a global event the rain of micrometeorites prevailed for at least 2 myr the estimated time of the deposition of the topmost chinese bed after the breakup event less

glass production in late antiquity and the early islamic period a geochemical perspective first millennium ad glass production was divided between a relatively small number of workshops that made raw glass and a large number of secondary workshops that fabricated vessels glass compositions reflect the primary glassmaking source for most of the period egyptian mineral soda was fused with lime bearing siliceous sand to produce soda lime silica glass the location of the belus glassmaking sand which is known from the classical literature is located on that part of the levantine coast where iron contents are lowest sr 87 sr 86 of primary glass from workshops in the levantine region is close to that of modern seawater and confirms the use of beach sand which contained shell heavy mineral assemblages of levantine beach sands are dominated by hornblende hence the primary glasses are characterized by very similar trace element signatures glasses believed on archaeological grounds to have been made in other regions for example in inland egypt may have higher sr 87 sr 86 reflecting terrigenous sources of lime and have different trace element signatures compositional data for glasses from as far away as britain suggest origins of the glass material in the eastern mediterranean recycling of old glass may be recognized by the presence of elevated transition metals the use of plant ash as a flux became dominant practice in the ninth century and preliminary data for plant ash glasses from the early islamic world indicate that primary production centres may be separated using strontium and oxygen isotopes as well as by major and trace elements

the first archaeometric data on polychrome iron age glass from sites located in northern italy abstract a large sample set of transparent and opaque glass artefacts recovered from etruscan contexts in northern italy bologna and spina fe necropoleis and dated to a period between the 6th and 4th century bc are analysed in this paper samples of highly decorated beads spindle whorls and vessels of the mediterranean group i alabastron oinochoes amphoriskos were selected in order to determine whether these different glass artefacts were produced at the same manufacturing site while the vessels almost certainly originate from greece the beads could derive from a more ancient local production ascertained at the site of frattesina rovigo italy and dated to the bronze age the aims of this study are i to characterise a set of iron age glass artefacts thus providing new quantitative chemical data and ii to identify possible differences between finds recovered from different localities and between glass vessels and beads in order to establish whether different manufactures were active contemporaneously the chemical analyses of major and minor elements were performed with an electron microprobe the nature of the opacifying and colouring agents was determined by x ray diffraction the sample set is relatively homogeneous and most of the samples can be classified as low magnesium glass the yellow opaque decorations are rich in pb and sb while only sb is present in the white and light blue samples the xrd analyses confirmed the presence of lead antimonates in the yellow decorations and of calcium antimonates in the white and light blue decorations

l chondrite asteroid breakup tied to ordovician meteorite shower by multiple isochron 40ar 39ar dating radiochronometry of l chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 myr ago fossil meteorites from sweden indicate a highly increased influx of extraterrestrial matter in the middle ordovician 480 myr ago an association with the l chondrite parent body event was suggested but a definite link is precluded by the lack of more precise radiometric ages suggested ages range between 450 30 myr and 520 60 myr and can neither convincingly prove a single breakup event nor constrain the delivery times of meteorites from the asteroid belt to earth here we report the discovery of multiple 40ar 39ar isochrons in shocked l chondrites particularly the regolith breccia ghubara that allow the separation of radiogenic argon from multiple excess argon components this approach applied to several l chondrites yields an improved age value that indicates a single asteroid breakup event at 470 6 myr fully consistent with a refined age estimate of the middle ordovician meteorite shower at 467 3 1 6 myr according to a geologic time scale 2004 our results link these fossil meteorites directly to the l chondrite asteroid destruction rapidly transferred from the asteroid belt the increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate which implies severe environmental stress

extraterrestrial chromite in the resurge deposits of the early late ordovician lockne crater central sweden the distribution of extraterrestrial chromite grains 63 mu m has been studied in the resurge deposit of the early late ordovician 458 ma lockne impact structure in central sweden a 1 kg sample of resurge sediment from the rim of the crater is extremely rich in extraterrestrial chromite in total the sample contains 125 chromium spinel grains kg 1 most grains being chromite due to post depositional alterations the origin of several of the grains is dubious although it is clear that the major part of the chromites is of extraterrestrial origin 75 grains kg 1 most likely derived from the impactor the alterations are primarily due to the hydrothermal system induced by the impactor with zinc enrichment in the chromites as the most common characteristic the element chemistry of the least altered chromite grains indicates that the impactor was an ordinary l chondrite this concurs with a suggested increase in the flux of l chondritic asteroids to earth following the disruption of the l chondrite parent body at ca 470 ma more than 170 impact craters are known from around the earth but only few of the impactors have been identified mainly due to the low survival rate of projectile material further studies of impact craters will reveal if survival of extraterrestrial chromite is unique for the lockne structure c 2006 elsevier b v all rights reserved less

a new lagerstatte from the middle ordovician st peter formation in northeast iowa usa a new fossil fauna has been discovered from a recently recognized shale unit within the middle ordovician st peter formation in northeast iowa it contains a variety of invertebrates and vertebrates including soft body tissues impressions and 3 dimensionalpreservations the exceptional preservation reveals a new konservat lagerstatte the winneshiek lagerstatte and opens a unique window into the community that inhabited the margins of the laurentian cratonic seaway during middle ordovician transgression among the fossils several conodont assemblages including the apparatus of enigmatic coleodontids are preserved some conodont assemblages associated with soft body tissues are particularly noteworthy

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

formation of resurge gullies at impacts at sea the lockne crater sweden the lockne crater in sweden is a marine target crater formed in a shelf sea approximately 460 ma ago the crater structure consists of an inner crater surrounded by an outer inclined surface that extends to almost 12 km from the center marine craters differ in several respects from craters formed on land one special feature is the formation of resurge gullies excavated by the erosional force of the resurging sea water after the impact the formation of these gullies strongly depends on the ratio crater rim height to water depth as well as on the size of the impact structure such gullies are known from very few marine target craters at the lockne impact site four gullies are identified each of which cuts radially through the rim of the outer crater the rapid collapse of that part of the crater cavity which formed in the seawater resulted in forceful flooding of the crater the resurging seawater not only contained fallback ejecta on its way towards the cavity on the sea bottom it incorporated fractured lithologies from the sea bottom as well this entrained material disintegrated during transport and constitutes today the dominantly monomict lower part of the resurge sequence the resurge flood was highly turbulent highly erosive and developed to a probable hyperconcentrated flow or a possible water flood the erosion in the gullies proceeded as headward erosion down to the transition zone between the brecciated and the less disintegrated crystalline basement

what lies beneath geophysical mapping of a concealed precambrian intrusive complex along the iowa minnesota border large amplitude gravity and magnetic highs over northeast iowa are interpreted to reflect a buried intrusive complex composed of mafic ultramafic rocks the northeast iowa intrusive complex neiic intruding yavapai province 1 8 1 72 ga rocks the age of the complex is unproven although it has been considered to be keweenawan 1 1 ga because only four boreholes reach the complex which is covered by 200 700 m of paleozoic sedimentary rocks geophysical methods are critical to developing a better understanding of the nature and mineral resource potential of the neiic lithologic and cross cutting relations interpreted from high resolution aeromagnetic and airborne gravity gradient data are presented in the form of a preliminary geologic map of the basement precambrian rocks numerous magnetic anomalies are coincident with airborne gravity gradient agg highs indicating widespread strongly magnetized and dense rocks of likely mafic ultramafic composition a yavapai age metagabbro unit is interprete

analyzing nitrogen in natural and synthetic silicate glasses by secondary ion mass spectrometry abstract volatile releases through volcanic eruptions are one of the major processes contributing to the global nitrogen cycle past studies have often estimated the magnitude of this flux using volcanic emission measurements which are limited to currently active systems and sensitive to atmospheric contamination another possible approach is the measurement of nitrogen in melt inclusions which are parcels of magmatic melt trapped prior to eruption this methodology requires appropriate analytical parameters for nitrogen analysis in silicate glasses by secondary ion mass spectrometry sims which have not yet been established to this end calibrations for nitrogen were obtained using ion implanted basaltic and rhyolitic glasses we demonstrate that water content significantly affects the ion yields of 14n and 14n16o as well as the background intensity of 14n and 12c application of implant derived calibrations to natural samples provide the first reported concentrations of nitrogen in melt inclusions these measurements were made on samples from the bishop tuff in california the huckleberry ridge tuff of the yellowstone volcanic center and material from the okaia and oruanui eruptions in the taupo volcanic center in studied material we often find maximum nitrogen contents of less than 45 ppm and that nitrogen concentration varies positively with co2 concentration which reflects a partial degassing trend using the maximum measured nitrogen contents for each eruption we find that the bishop released 3 4 1013 the huckleberry ridge released 1 4 1014 the okaia released 1 0 1011 and the oruanui released 4 5 1013 g of nitrogen simple calculations suggest that with concentrations such as these rhyolitic eruptions may ephemerally increase the nitrogen flux to the atmosphere but are insignificant compared to the 4 1021 g of nitrogen stored in the atmosphere

the chemistry of fumarolic vapor and thermal spring discharges from the nevado del ruiz volcanic magmatic hydrothermal system colombia on the basis of three sets of gas samples taken during the three months preceding the november 13 1985 eruption of nevado del ruiz from fumaroles on the crater floor the composition of the vapors discharged following minor eruptive activity in september corresponded to that of an essentially magmatic gas during the subsequent quiet period in october the proportion of a hydrothermal component in the vapor discharge increased steadily up until the major eruption the gases were found to be exceptionally high in total sulfur and are super saturated with respect to elemental sulfur their thermodynamic equilibration temperatures range from 200 to 600 c gases and waters from fumaroles and thermal springs on the flanks of the volcano are likely to be derived from a two phase vapor brine envelope surrounding the magmatic system the chemistry of the fumarole discharges is compatible with the assumption of an eruptive mechanism involving the build up of pressure in an extensive vapor zone by gas released from an essentially stagnant already extensively degassed body of magmatic material at depth production of such a residual gas over extended periods may account for the large amount of so2 discharged from the volcano in relation to the small amount of ejected solids

the number of impact craters on earth any room for further discoveries abstract only 128 impact craters exposed at earth s surface have been found so far while new craters are discovered occasionally taking into account the permanent consumption of craters by erosion we present the first estimate on the number of impact craters that should be present at earth s surface our study yields no evidence for any systematic incompleteness in the available inventory of the craters larger than about 6 km in diameter exposed at the surface in contrast more than 90 craters in the diameter range from 1 km to 6 km should still be waiting to be discovered and even more than 250 between 0 25 km and 1 km diameter the transition from a probably complete inventory above 6 km to a strongly incomplete record at smaller sizes may be related to the difference between simple and complex craters beyond these results on the terrestrial crater record our findings tentatively suggest that erosion rates on the 10 to 100 million year scale may be closer to present day erosion rates than previously assumed

role of water in the formation of the late cretaceous wetumpka impact structure inner gulf coastal plain of alabama usa the effect of shallow marine water 30 100 m deep in the late excavation and early modification stages of a marine target crater 5 km in diameter as exemplified by the late cretaceous wetumpka impact structure in alabama usa is manifest in the early collapse of a weak part of the rim excavation flow and connate marine water are interpreted to be factors in this collapse this partial rim collapse catastrophically emplaced an upper structure filling unit of broken and redistributed sedimentary target formations which presently mantles the deeper fallback breccia deposits within the structure furthermore rim collapse flow facilitated the formation of a structurally modified extrastructure terrain which is located outside and adjacent to the collapsed rim segment this extrastructure terrain appears to be the product of extensive slumping of poorly consolidated target sedimentary formations

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

in situ multiphase u pb geochronology and shock analysis of apatite titanite and zircon from the lac la moinerie impact structure canada in situ u pb geochronology has been performed via laser ablation inductively coupled mass spectrometry on shocked apatite titanite and zircon from the 8 km diameter lac la moinerie impact structure canada the three analyzed phases are inherited from target rocks that were transformed to impact melt bearing breccias and clast laden impact melts apatite yields an array of u pb ratios populating a region between a younger regression with a refined age of 453 5 ma and an older regression with a lower intercept age of 1708 10 ma titanite defines a younger regression with an age of 444 15 ma and an older array with a lower intercept at 1844 24 ma zircon yields an upper intercept age of 1810 13 ma and a lower intercept age of 433 21 ma within error of apatite and titanite the oldest ages from all three phases define paleoproterozoic igneous metamorphic events in granitoid target rocks of the de pas suite isotopic resetting in apatite and titanite with relatively high pb diffusion rates was controlled by a combination of proximity to impact generated superheated melt and the formation of fast diffusion pathways facilitated by shock effects and dynamic recrystallization zircon resetting was controlled by shock effects and metamictization which resulted in significant pb loss we outline an approach for discriminating true impact ages from metamict zircons with high common pb that have experienced post impact pb loss lower intercept ages of all phases constrain the age of formation for the lac la moinerie impact structure with apatite yielding the most precise age of 453 5 ma with an age of 453 ma lac la moinerie joins a growing cluster of more than 12 terrestrial impact structures formed in the middle to late ordovician with potential connection to the l chondrite parent body breakup event and a period of enhanced bombardment on earth

a rain of ordinary chondritic meteorites in the early ordovician forty fossil meteorites with a total original mass of v7 7 kg have been recovered in the first systematic search for fossil meteorites pursued in an active quarry in lower ordovician 480 ma marine limestone in southern sweden the meteorites represent at least 12 different falls over a seafloor area of v6000 m 2 during 91 75 myr making the quarry one of the most meteorite dense areas known in the world geochemical analyses of relict chromite grains indicate that all or most of the meteorites are ordinary chondrites and probably l chondrites mechanisms for meteorite delivery from the asteroid belt to earth were the same 480 ma as today however the flux was one to two orders of magnitude higher most likely reflecting the disruption of the l chondrite parent body at about that time this is a major event in late solar system history which may also have led to an enhanced flux of asteroids to earth during v30 myr fl 2001 elsevier science b v all rights reserved

tracing nitrogen in volcanic and geothermal volatiles from the nicaraguan volcanic front we report new chemical and isotopic data from 26 volcanic and geothermal gases vapor condensates and thermal water samples collected along the nicaraguan volcanic front the samples were analyzed for chemical abundances and stable isotope compositions with a focus on nitrogen abundances and isotope ratios these data are used to evaluate samples for volatile contributions from magma air air saturated water and the crust samples devoid of crustal contamination based upon he isotope composition but slightly contaminated by air or air saturated water are corrected using n2 ar ratios in order to obtain primary magmatic values composed of contributions from upper mantle and subducted hemipelagic sediment on the down going plate using a mantle endmember with d 15 n 5 and n2 he 100 and a subducted sediment component with d 15 n 7 and n2 he 10 500 the average sediment contribution to nicaraguan volcanic and geothermal gases was determined to be 71 most of the gases were dominated by sediment de

investigation of iron age north eastern scottish glass beads using element analysis with la icp ms abstract class 13 and 14 iron age scottish glass beads are a group of highly decorated beads of british origin or design dating indicatively to the 1st and 2nd century ad and typically found in aberdeenshire and moray district guido 1978 85 9 their distinctive stylistic characteristics and geographical segregation renders them ideal for the investigation of whether the glasses employed in their manufacture were imported rather than produced locally and for the assessment of the technology used in the production of the deep colours studies performed in the 1980s on different specimens pertaining to the same classes henderson 1982 showed compositional characteristics differing from iron age southern british beads suggesting a different source of glass for their manufacture here a set of 19 beads which was never investigated before was analysed for 32 major minor and trace elements using laser ablation inductively coupled plasma mass spectrometry la icp ms the sample set shows good homogeneity in major and minor element composition indicating the use of imported natron glass with standardized composition typical of roman glass of the period also reflected in the recipes used for colouration evidence for the use of cullet and waste glass was found which along with the particularity of the design suggests a local origin of the beads and possible production by native glassworkers

the terrestrial impact cratering record approximately 130 terrestrial hypervelocity impact craters are currently known due to variations in preservation and in geologic knowledge this sample is biased towards young x 20 km craters on the cratons of australia europe including the former u s s r and north america the rate of discovery of new craters is 3 5 craters per year although modified by erosion terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies such as the moon terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters which can be used to constrain interpretations of lunar samples and models of crater formation due to erosion and its effects on form terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects these include shatter cones microscopic planar deformation features solid state and fusion glasses high pressure polymorphs and whole rock melting and vaporization shock recovery experiments indicate that these features occur over shock pressures of 5 gpa to x 100 gpa terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact induced fracturing they include gravity and magnetic lows and reductions in seismic velocity the gravity anomalies are seldom greater than 30 mgal due to the limiting effects of lithostatic pressure on fracturing at large complex craters the gravity signature may include a central relative gravity high due to uplift and short wavelength central magnetic anomalies due to a variety of processes much current work is focused on the effects of impact on earth evolution previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from cretaceous tertiary boundary sites as resulting from a major impact suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record by analogy with the lunar record and modelling of the effects of very large impacts it has been proposed that biological and atmospheric evolution of the earth could not stabilize before the end of the late heavy bombardment 3 8 ga ago the present terrestrial cratering rate is 5 4 2 7 10 15 km 2a 1 for a diameter 20 km this represents a local threat on historic time scales on a global scale a major impact sufficient to cripple human civilization severely will occur on time scales of 106 a

nitrogen isotopic composition of the morb mantle a reevaluation 1 nitrogen in the mantle source of mid oceanic ridge basalts morbs is commonly believed to have 15n 5 this is based on a selective statistical average drawn from nitrogen isotopic data obtained simultaneously with argon from morbs however on critical evaluation using the accompanying 40ar 36ar data we show this approach to be somewhat ambiguous considering the fact that morbs can have a number of geochemical components in them as has been shown by several recent studies the approach of statistical averaging may actually point to a mixture rather than the true mantle end member a more reasonable approach also used for deriving the mantle noble gas signatures suggests nitrogen in the morb mantle to be much lighter 15 in its isotopic composition such a signature is consistent with nitrogen data from morbs as well as from other mantle derived samples that are believed to have sampled volatiles from the morb mantle

chlorine isotope variations along the central american volcanic front and back arc volcanic outputs ash tephra and lava samples of 23 volcanic centers from the volcanic front secondary front and back arc in central america were analyzed for their 37cl values with the goal of using chlorine isotopes as a tracer of fluids sourced from the subducting cocos slab 37cl values range from 2 6 to 3 0 with systematic variations along the length of the front values from the northernmost guatemala and el salvador and southernmost costa rica ends of the front have mantle like signatures in contrast 37cl values are both positive and negative in the center of the front nicaragua implying a sediment and or serpentinite derived component geophysical observations are consistent with extensive hydration of the cocos plate offshore of nicaragua in support of a serpentinite derived fluid source fluids from dehydrating serpentinites may also incorporate cl from overlying sediments resulting in a multiple source chlorine signature

stable isotopes evidence of recycled subduction fluids in the hydrothermal volcanic activity across nicaragua and costa rica abstract the central america volcanic front provides a unique opportunity to study hydrothermal inputs and their interaction and mixing with modern meteoric waters the objectives of this study were to a characterize the isotopic composition 18 o 2 h d excess and lc excess of hydrothermal volcanic systems b analyze the influence of kinetic fractionation and meteoric water inputs in the isotopic composition of hydrothermal waters and c estimate the andesitic water contribution recycled subduction fluids within the volcanic front of nicaragua and costa rica hydrothermal evaporation lines are described as 2 h 4 7 18 o 13 0 costa rica and 2 h 2 7 18 o 31 6 nicaragua these regressions are significantly p 2 h 7 6 18 o 7 4 costa rica and 2 h 7 4 18 o 5 2 nicaragua the greater rainfall inputs in costa rica with respect to nicaragua resulted in the attenuation of the evaporative effect as observed in the strong bimodal distribution of the hydrothermal waters which can be divided in fluids a isotopically close to meteoric conditions and b isotopically altered by the interaction with recycled subduction fluids and kinetic fractionation the latter is clearly depicted in the significantly p d excess and lc excess median values between costa rica 5 10 5 25 and nicaragua 2 42 10 65 respectively poor correlations between 18 o 2 h and the elevation gradient emphasize that the contribution of recycled subduction fluids and subsequent surface kinetic fractionation are the main drivers of the isotopic departure from the orographic distillation trend captured in the rainfall isoscapes end member mixing calculations resulted in a significant difference p 2 he contributions across the volcanic front of nicaragua and costa rica

nitrogen geochemistry as a tracer of fluid flow in a hydrothermal vent complex in the karoo basin south africa abstract we have investigated the n geochemistry of minerals and rocks from contact metamorphic aureoles and hydrothermal vent complexes hvc in the karoo basin in south africa the hvc formed during phreatic eruptions associated with rapid devolatilization and pressure build up in contact aureoles around early jurassic sill intrusions by combining outcrop data from a hvc and core data from contact aureoles we investigate the relationship between light element release during metamorphism and vertical fluid migration sandstone and breccia from the hvc contain early diagenetic ammonium nh 4 bearing feldspar buddingtonite and illite ammonium occupies up to 95 of the a site in feldspar corresponding to concentrations up to 5 2 wt n bulk rock n isotope data for rocks from inside and outside the hydrothermal vent complex fall into two distinct groups background samples have 15nair between 1 5 and 4 9 whereas minerals from the vent complex have 15n in the range 7 5 to 10 6 the n geochemistry of contact metamorphic shale from the lower stratigraphic units of the karoo basin shows that the vitrinite reflectance and 15n values are positively correlated shale with reflectivity values 4 ro are enriched in 15n with 15n values between 6 and 14 implying the release of isotopically light n into metamorphic fluids probably as n2 we suggest that the relatively high 15n values of the early diagenetic buddingtonite in the hvc reflect exchange of buddingtonite with n bearing fluids ascending from greater depth after their release during contact metamorphism and dehydration we present a qualitative model whereby hydrothermal vent complexes represent fluid flow structures after their formation focusing n bearing metamorphic fluids sourced in deeper levels of the basin the release of organic n from sediments at depth in volcanic basins could play a role in the geochemical cycle of n becoming particularly important during periods of intense volcanic activity

trace element discriminants between egyptian and mesopotamian late bronze age glasses the origins of late bronze age glass artefacts found throughout the mediterranean and near and middle east remain controversial previous analyses of major and minor elements in both egyptian and mesopotamian glasses have proven equivocal revealing no significant compositional differences between glasses from these two regions here we present new laser ablation icpms analyses of 32 trace elements in 54 samples of both egyptian and mesopotamian blue and colourless glasses our results show that there are consistent differences between these two suites which are not related to the colorant and clearly indicate the use of different raw materials and or manufacturing processes compositional variations are related to geological controls hence this method holds promise for the development of a minimally destructive technique for discriminating between archaeological glasses of different provenance which will be essential in the interpretation of ancient trade patterns and contacts

understanding oblique impacts from experiments observations and modeling natural impacts in which the projectile strikes the target vertically are virtually nonexistent nevertheless our inherent drive to simplify nature often causes us to suppose most impacts are nearly vertical recent theoretical observational and experimental work is improving this situation but even with the current wealth of studies on impact cratering the effect of impact angle on the final crater is not well understood although craters rims may appear circular down to low impact angles the distribution of ejecta around the crater is more sensitive to the angle of impact and currently serves as the best guide to obliquity of impacts experimental studies established that crater dimensions depend only on the vertical component of the impact velocity the shock wave generated by the impact weakens with decreasing impact angle as a result melting and vaporization depend on impact angle however these processes do not seem to depend on the vertical component of the velocity alone finally obliquity influences the fate of the projectile in particular the amount and velocity of ricochet are a strong function of impact angle

u pb geochronology of apatite and zircon from the brent impact structure canada a late ordovician sandbian katian boundary event associated with l chondrite parent body disruption in situ la icp ms u pb geochronology has been performed on apatite and zircon within thermally recrystallized clast laden and clast poor impact melt rocks from the brent impact structure a total of 377 laser analyses on 120 impact melt grown n 9 and impact recrystallized zircon grains n 111 were obtained from which a concordia age of 452 8 2 7 ma mswd 0 57 n 11 and a weighted average mean 206pb 238u age of 453 2 2 9 ma mswd 0 60 n 11 are calculated a total of 300 laser analyses from 100 relict apatite grains were obtained with an unanchored regression through all data yielding a lower intercept age of 453 2 6 0 ma mswd 5 8 n 300 that overlaps within error of zircon 207pb 206pb ratios obtained from feldspar clasts within clast laden impact melt retain the same initial pb composition as the target rocks from which they are derived while feldspars that crystallized from impact melt have 207pb 206pb ratios indicative of isotopic re equilibration between basement lithologies of two different ages a similar variability in 207pb 206pb is recorded by apatite this provides evidence for the involvement of neoproterozoic lake nipissing alkaline suite as well as mesoproterozoic grenville gneisses in the production of impact melt at brent recrystallized apatite grains exhibit enrichments in light rare earth elements lrees along neoblast grain boundaries indicative of trace element substitution and phase precipitation during impact induced recrystallization an age of 452 8 2 7 ma from zircon and 453 2 6 0 ma from apatite places the impact event in the late ordovician at or near the sandbian katian boundary confirming brent s involvement in the middle to late ordovician crater cluster event a period of enhanced impactor flux to earth related to the l chondrite parent body disruption

crater taphonomy and bombardment rates in the phanerozoic impact structures can be catalogued according to the age of the rocks in which they are now found country rocks the observed frequency distribution of craters by age of country rock is shown to be statistically indistinguishable from the predictions of a simple model in which it is assumed that the survival time for craters is the same as that for their target rocks other models are considered but do not match the data a lower limit of the rate of bombardment through the phanerozoic based only on documented craters is 0 13 pm 0 09 events ma 10 8 km 2 for craters with diameters geq 10 km and 0 09 pm 0 08 events ma 10 8 km 2 for craters with diameters geq 20 km the data allow but do not demand an increase in meteorite flux over the phanerozoic but do not allow any significant decrease we estimate that only about 6 of the existing terrestrial impact structures of diameter greater than 10 km have been discovered to date and only 16 of those with diameter greater th

a preliminary study of 15n14n in octahedral growth form diamonds abstract the 13 c 15 n and nitrogen concentrations have been determined in 43 octahedral diamonds from southern africa australia and north america the sample set included locations such as finsch south africa a mine which produces diamonds having 13c values close to 5 and argyle australia a mine which produces 13c depleted diamonds the nitrogen within high 13c diamonds 13 c 6 4 to 2 9 was generally depleted in 15n relative to atmospheric nitrogen 15n 12 3 to 5 9 mode 4 similar to the results which have been obtained previously from fibrous diamond it is believed that the volatiles from which these diamonds grew are primitive being derived from a source located beneath the continental lithosphere which may have changed little in terms of 13c and 15n since the mid archean the low 13c diamonds 13c 19 4 to 9 5 contained nitrogen generally enriched in 15n relative to air 15n 3 0 to 16 4 mode 5 it is suggested that the major carbon isotope heterogeneity within the mantle as represented by diamonds such as these is located in deep mechanically unstable regions of the continental lithosphere the isotopic compositions of both c and n are consistent with this heterogeneity resulting from the subduction of crustal material however isotope fractionation related directly to diamond growth cannot be ruled out

terrestrial impact structures impact structures are the dominant landform on planets that have retained portions of their earliest crust the present surface of the earth however has comparatively few recognized impact structures this is due to its relative youthfulness and the dynamic nature of the terrestrial geosphere both of which serve to obscure and remove the impact record although not generally viewed as an important terrestrial as opposed to planetary geologic process the role of impact in earth evolution is now receiving mounting consideration for example large scale impact events may have been responsible for such phenomena as the formation of the earth s moon and certain mass extinctions in the biologic record the importance of the terrestrial impact record is greater than the relatively small number of known structures would indicate impact is a highly transient high energy event it is inherently difficult to study through experimentation because of the problem of scale in addition sophisticated finite element code calculations of impact cratering are gen erally limited to relatively early time phenomena as a result of high com putational costs terrestrial impact structures provide the only ground truth against which computational and experimental results can be com pared these structures provide information on aspects of the third dimen sion the preand postimpact distribution of target lithologies and the nature of the lithologic and mineralogic changes produced by the passage of a shock wave they also provide data for cratering rate estimates and in some cases on the nature of the impacting body in this review emphasis is placed on the nature of terrestrial impact structures the criteria for their identification and their contribution to

isotopic and chemical composition of water and steam discharges from volcanic magmatic hydrothermal systems of the guanacaste geothermal province costa rica the guanacaste geothermal province encompasses three major geothermal systems each centered on its respective volcanic structure rincon de la vieja to the nw miravalles in the center and tenorioto the se each shows corresponding sets of surface manifestations vapor discharges from fumaroles and steam heated pools at altitudes 500 m lower temperature so4cl springs on the lower slopes of the respective volcano and cooler neutral cl springs to the s of the volcanic chain at altitudes 500 m the production of hco3 rich waters is limited to a narrow belt stretching to the s of miravalles volcano chemical and isotopic evidence suggests that the neutral cl waters also discharged from deep wells are derived from a more primitive clso4 water formed by transfer of readily mobilised originally magmatic constituents to deeply circulating groundwater isotopic evidence suggests that this groundwater is derived largely from areas to the north of the volcanic chain the presence of immature waters within drillable depth is indicated by the incursion of an acid so4 water into a well at miravalles after deepening to 2000 m as the cl waters rise they start to boil the separated vapors feed the fumaroles and steam heated features at the surface the residual water travels underground to be discharged some 5 25 km to the s the uniformity in chemical and isotopic compositions of the neutral cl waters is compatible with the assumption that the individual thermal systems are linked at depth to form large contiguous geothermal reservoirs beneath the guanacaste geothermal province

a record of ancient cataclysm in modern sand shock microstructures in detrital minerals from the vaal river vredefort dome south africa the record of terrestrial meteorite impacts is fragmentary because most impact structures and ejecta are removed by erosion or buried discovery of the missing impact record from hadean to present may be advanced through identifi cation of residual shocked detritus to evaluate which shocked minerals survive erosion and sedimentary transport we investigated modern sands from the vaal river in south africa where it crosses the 2 02 ga vredefort dome the largest terrestrial impact structure known to date shocked minerals were identifi ed in all sediment samples including from the vaal channel and tributaries within the structure in transmitted light detrital quartz preserves discontinuous decorated planar features previously identifi ed as brazil twins which are readily visible as bright continuous features in cathodoluminescence images detrital zircons preserve fi ve orientations of planar fractures pfs which can produce dramatically offset growth zoning and apparent rotation of subgrains other zircons contain fi lled fractures that may represent a new shock microstructure detrital monazite preserves four orientations of pfs and many grains contain oscillatoryzoned shocked zircon inclusions which thus represent shocked inclusions within shocked accessory grains zircon and monazite with granular texture were also identifi ed this study is proof of the concept that shocked minerals can be identifi ed in sediments up to 2 billion years after an impact event and it demonstrates their potential for preserving evidence of ancient impacts the recognition of a new geological repository for impact evi dence provides a means for identifying distal shocked detritus from eroded structures of any age and may be particularly relevant to early earth studies

chemical and mineralogical trends during acid sulfate alteration of pyroclastic basalt at cerro negro volcano and implications for early mars 1 acid sulfate alteration of pyroclastic basalts in active fumaroles at cerro negro volcano nicaragua was studied as a means to infer the mineralogical and chemical consequences of basalt alteration in analogous environments on early mars at this site recently erupted basaltic cinders are undergoing alteration by so2 bearing steam during alteration silicate phenocrysts including plagioclase olivine and augite react much more rapidly than basaltic glass secondary mineralogy is dominated by a very limited number of phases that include amorphous silica gypsum fe bearing natroalunite and fe oxides oxyhydroxides including hematite and magnetite the major element chemistry of the deposits is controlled by two processes gradual depletion of the major cations other than si as the basalt components decompose and elements are mobilized out of the deposits and enrichment in ca and s from precipitation of gypsum with ca apparently supplied from sources below the surface reaction path models constrained by these observations but extrapolated to martian conditions predict that alteration of pyroclastic deposits in similar environments on mars should produce a secondary mineral assemblage that includes amorphous silica fe bearing natroalunite anhydrite kieserite and hematite iron bearing natroalunite was found to produce a mossbauer signal similar to that of jarosite suggesting that this phase should be considered as an alternative to the jarosite component identified at meridiani planum spheroidal hematite formed in close association with natroalunite suggests a pathway for formation of hematite deposits on mars

origin of a classic cratonic sheet sandstone stratigraphy across the sauk ii sauk iii boundary in the upper mississippi valley the origin of cratonic sheet sandstones of proterozoic and early paleozoic age has been a long standing problem for sedimentologists lower paleozoic strata in the upper mississippi valley are best known for several such sandstone bodies the regional depositional histories of which are poorly understood we have combined outcrop and subsurface data from six states to place the upper cambrian wonewoc ironton and galesville sandstone in a well constrained stratigraphic framework across thousands of square kilometers this framework makes it possible for the first time to construct a regional scale depositional model that explains the origin of this and other cratonic sheet sandstones the wonewoc sandstone although mapped as a single contiguous sheet is a stratigraphically complex unit that was deposited during three distinct conditions of relative sea level that span parts of four trilobite zones during a relative highstand of sea level in crepicephalus zone time quartzose sandstone lithofacies aggraded more or less vertically in nearshore marine and terrestrial environments across much of the present day outcrop belt around the wisconsin arch at the same time finer grained feldspathic sandstone siltstone and shale aggraded in deeper water immediately seaward of the quartzose sand and shale and carbonate sediment accumulated in the most distal areas during aphelaspis and dunderbergia zones time a relative fall in sea level led to the dispersal of quartzose sand into a basinward tapering sheet like body across much of the upper mississippi valley during early elvinia zone time a major transgression led to deposition of a second sheet sandstone that is generally similar to the underlying regressive sheet the results of this investigation also demonstrate how subtle sequence bounding unconformities may be recognized in mature cratonic siliciclastics we place the sauk ii sauk iii subsequence boundary at the base of the coarsest bed in the wonewoc sandstone a lag developed through erosion that occurred during the regional regressive transgressive event that spanned aphelaspis to early elvinia zones time such sequence bounding unconformities are difficult to recognize where they are contained within coarse siliciclastics of the upper mississippi valley because they separate strata that are texturally and mineralogically similar and because erosion occurred on a loose sandy substrate along a low uniform gradient and in a nonvegetated terrestrial environment furthermore the ultramature mineral composition of the exposed substrate is resistant to the development of a recognizable weathering profile the well known sheet geometry of the wonewoc and other units of lower paleozoic sandstone of this area is not dependent on atypical terrestrial depositional conditions conducive to the widespread distribution of sand as commonly believed sand was spread into a sheet dominantly within the marine realm in a manner similar to that inferred for many better known sandstone bodies deposited in the north american cretaceous western interior seaway and tertiary gulf of mexico the laterally extensive thin character of the upper mississippi valley sandstone bodies compared to these other sandstone bodies simply reflects deposition of a continuously abundant supply of sand on a relatively stable nearly flat basin of slow uniform subsidence during changes in sea level the dearth of shale in this and other cratonic sandstones can be indirectly attributed to the same controls which led to an uncommonly low preservation potential for fairweather deposits on the shoreface

gas geochemistry of the magmatic hydrothermal fluid reservoir in the copahue caviahue volcanic complex argentina abstract copahue volcano is part of the caviahue copahue volcanic complex ccvc which is located in the southwestern sector of the caviahue volcano tectonic depression argentina chile this depression is a pull apart basin accommodating stresses between the southern liquine ofqui strike slip and the northern copahue antinir compressive fault systems in a back arc setting with respect to the southern andean volcanic zone in this study we present chemical inorganic and organic and isotope compositions 13c co2 15n 3he 4he 40ar 36ar 13c ch4 d ch4 and d h2o and 18o h2o of fumaroles and bubbling gases of thermal springs located at the foot of copahue volcano sampled in 2006 2007 and 2012 helium isotope ratios the highest observed for a southern american volcano r ra up to 7 94 indicate a non classic arc like setting but rather an extensional regime subdued to asthenospheric thinning 13c co2 values from 8 8 to 6 8 vs v pdb 15n values 5 3 to 5 5 vs air and co2 3he ratios from 1 4 to 8 8 109 suggest that the magmatic source is significantly affected by contamination of subducted sediments gases discharged from the northern sector of the ccvc show contribution of 3he poor fluids likely permeating through local fault systems despite the clear mantle isotope signature in the ccvc gases the acidic gas species have suffered scrubbing processes by a hydrothermal system mainly recharged by meteoric water gas geothermometry in the h2o co2 ch4 co h2 system suggests that co and h2 re equilibrate in a separated vapor phase at 200 220 c on the contrary rock fluid interactions controlling co2 ch4 production from sabatier reaction and c3h8 dehydrogenation seem to occur within the hydrothermal reservoir at temperatures ranging from 250 to 300 c fumarole gases sampled in 2006 2007 show relatively low n2 he and n2 ar ratios and high r ra values with respect to those measured in 2012 such compositional and isotope variations were likely related to injection of mafic magma that likely triggered the 2000 eruption therefore changes affecting the magmatic system had a delayed effect on the chemistry of the ccvc gases due to the presence of the hydrothermal reservoir however geochemical monitoring activities mainly focused on the behavior of inert gas compounds n2 and he should be increased to investigate the mechanism at the origin of the unrest started in 2011

pathways for nitrogen cycling in earth s crust and upper mantle a review and new results for microporous beryl and cordierite figure 1 earth s atmosphere contains 27 30 of the planet s nitrogen and recent estimates are that about one half that amount 11 16 is located in the continental and oceanic crust combined the percentage of n in the mantle is more difficult to estimate but it is thought to be near 60 at very low concentrations knowledge of the behavior of n in various fluid melt rock settings is key to understanding pathways for its transfer among the major solid earth reservoirs nitrogen initially bound into various organic materials is transferred into silicate minerals during burial and metamorphism often as nh4 substituting for k in layer silicates clays and micas and feldspars low grade metamorphic rocks appear to retain much of this initial organic n signature in both concentrations and isotopic compositions thus in some cases providing a relatively un or little modified record of ancient biogeochemical cycling devolatilization can release significant fractions of the n initially fixed in crustal rocks through organic diagenesis during progressive metamorphism at temperatures of 350 550 c depending on pressure loss of fractionated n during devolatilization can impart an appreciable isotopic signature on the residual rocks producing shifts in 15n values mostly in the range of 2 to 5 these rocks then retain large fractions of the remaining n largely as nh4 despite further heating and ultimately partial melting with little additional change in 15n this retention leads to the storage of relatively large amounts of n largely as nh4 in the continental crust nitrogen can serve as a tracer of the mobility of organic sedimentary components into and within the upper mantle this contribution focuses on our growing but still fragmentary knowledge of the n pathways into shallow to deep continental crustal settings and the upper mantle we discuss the factors controlling the return of deeply subducted n to shallower reservoirs including the atmosphere via metamorphic devolatilization and arc magmatism we discuss observations from natural rock suites providing tests of calculated mineral fluid fractionation factors for n building on our discussion of n behavior in continental crust we present new measurements on the n concentrations and isotopic compositions of microporous beryl and cordierite from medium and high grade metamorphic rocks and pegmatites both phases containing molecular n2 and nh4 bearing micas coexisting with them we suggest some avenues of investigation that could be particularly fruitful toward obtaining a better understanding of the key n reservoirs and the more important pathways for n cycling in the solid earth 1 pending yes

the lawn hill annulus an ordovician meteorite impact into water saturated dolomite the lawn hill impact structure lhis is located 250 km n of mt isa in nw queensland australia and is marked by a highly deformed dolomite annulus with an outer diameter of similar to 18 km overlying low metamorphic grade siltstone sandstone and shale along the ne margin of the georgina basin this study provides detailed field observations from sections of the lawn hill annulus and adjacent areas that demonstrate a clear link between the deformation of the dolomite and the lawn hill impact ar 40 ar 39 dating of impact related melt particles provides a time of impact in the ordovician 472 8 ma when the georgina basin was an active depocenter the timing and stratigraphic thickness of the dolomite sequence in the annulus suggest that there was possibly up to 300 m of additional sedimentary rocks on top of the currently exposed thorntonia limestone at the time of impact the exposed annulus is remarkably well preserved with preservation attributed to postimpact sedimentation the lhis has an atypical crater morphology with no central uplift the heterogeneous target materials at lawn hill were probably low strength porous and water saturated with all three properties affecting the crater morphology the water saturated nature of the carbonate unit at the time of impact is thought to have influenced the highly brecciated nature of the annulus and restricted melt production the impact timing raises the possibility that the lawn hill structure may be a member of a group of impacts resulting from an asteroid breakup that occurred in the mid ordovician 470 6 ma

estimates of yet to find impact crater population on earth abstract one hundred and seventy six craters on earth are currently accepted as resulting from bolide impact this list is growing at a rate of approximately two craters per year because of new discoveries and revised interpretations here we take the subset of craters most relevant to sedimentary basin exploration and utilization phanerozoic kilometre scale craters and tackle the question of how many remain to be found in phanerozoic strata on earth a surprising range in predictions of crater numbers is obtained from published crater population functions reaching two orders of magnitude variation for craters in the range 1 5 km diameter this range in predictions is largely due to variations in treatment of impactor breakup in the atmosphere also required is quantification of the total area and timespan of phanerozoic sediment currently preserved on earth an estimate of 8 87 10 15 km 2 a is derived here together with a relatively conservative crater population function this yields a prediction of 845 craters subtracting the 131 known phanerozoic craters larger than 1 km leaves a predicted population yet to find of 714 of which 228 are larger than 2 5 km uncertainties subsets of this population and spatial distribution are also considered

the degassing character of a young volcanic center cerro negro nicaragua cerro negro volcano is a young basaltic cinder cone which is part of the nicaraguan volcanic arc eruptive activity at cerro negro is characterized by explosive strombolian to subplinian eruptions driven by volatile rich basaltic magma ascending rapidly from various crustal depths 15 to 6 km resulting in the onset of precursory activity only 30 min before an eruption in this paper we present a comprehensive degassing characterization of the volcano over a 4 year period aimed at improving our understanding of the magmatic plumbing network and its relationship with regional tectonics a total of 124 individual soil gas samples were collected between 2010 and 2013 and analyzed for stable carbon isotopes 13c from co2 high temperature fumaroles were sampled for 18o d and 3he 4he isotope analysis and major degassing zones were mapped using soil co2 flux measurements gases at cerro negro are characterized by a strong 3he 4he mantle signature 6 3 to 7 3 ra magmatic 13c ratios 2 3 to 3 0 meteoric 18o and d ratios and stable co2 fluxes 31 t d 1 the lack of 13c fractionation and an increase in the relative mantle component from 2002 to 2012 suggest that the volatile flux at cerro negro originates from the mantle and ascends to the surface via a series of crustal fractures that act as permeable conduits despite the lack of new eruptions the hydrothermal system of cerro negro continues to evolve due to seasonal inputs of meteoric water slope failures that expose and bury sites of active degassing and bursts of regional seismicity that have the potential to open up new conduits for gas release as well as magma continuing geophysical and geochemical monitoring of the main edifice and the recently formed south zone is essential as the volcano remains overdue to erupt

nitrogen isotope compositions of metasedimentary rocks in the catalina schist california implications for metamorphic devolatilization history abstract in the catalina schist subduction zone metamorphic complex california metasedimentary rocks show a decrease in n concentration and an increase in 15nair with increasing metamorphic grade lowest grade lawsonite albite rocks contain 632 185 ppm n with 15 n 1 9 0 6 whereas highgrade amphibolite equivalents contain 138 76 ppm n with 15 n 4 3 0 8 loss of n accompanied devolatilization reactions that evolved h2o rich c o h s n fluids through consumption of chlorite and phengitic white mica and production of mineral assemblages containing muscovite biotite garnet and kyanite whole rock n concentrations of up to 200 ppm in veins and pegmatites produced during high p t metamorphism reflect the redistribution of n during devolatilization and partial melting of the metasedimentary rocks bulk fluid rock n isotope fractionations 15n 15nfluid 15nrock of 1 5 1 were calculated with the rayleigh distillation equation taking into account variability in rock composition by comparison of samples with similar k2o concentrations these fractionations are similar to but slightly lower than published calculated fractionations for n2 nh4 exchange at the temperature range of 350 600 c over which most of the devolatilization occurred in the catalina schist approximately 3 4 to 2 25 the n systematics appear to be explained by n2 nh4 exchange and a devolatilization process intermediate in behavior to batch volatilization and rayleigh distillation the observed shifts in n concentration and 15n cannot be explained by nh3 nh4 exchange at these temperatures using the equilibrium models the distillation devolatilization process implicated in this study may govern the behavior of other trace elements partitioned into hydrous fluids during devolatilization e g b cs u similarity of the calculated fluid 15n 1 5 to 5 5 with compositions of natural gases inferred to be derived from metasedimentary sources indicates the possibility of using n as a tracer of large scale volatile transport

assessment of environmental controls on acid sulfate alteration at active volcanoes in nicaragua applications to relic hydrothermal systems on mars 1 a variety of secondary mineralogies has been detected on mars from both orbiters and landers indicating widespread aqueous alteration of the crust many of these locales exhibit sulfates which in some cases imply acidic fluids at present there are few constraints on the paleoenvironmental conditions that existed during formation of the widespread and diverse classes of secondary minerals on mars we investigated hydrothermal systems at three active acidic volcanic systems in nicaragua including cerro negro momotombo and telica the recently erupted materials are similar in composition to the martian crust and are undergoing extensive acid sulfate alteration predominately in gas dominated settings fumaroles we characterized the secondary mineralogy and local variables including temperature ph rock and gas composition and fluid rock ratio we find that these environmental parameters exhibit strong controls on the alteration mineralogy the environments studied include ph that ranged from 1 to 6 temperatures from ambient to hundreds of degrees celsius and fumaroles to hot springs the hottest and most acidic systems contained sulfur silica and minor gypsum while moderately acidic and cooler fumaroles included abundant silica gypsum and other hydrated sulfates and phyllosilicates a setting with a higher fluid rock ratio but similar temperature and acidity was dominated by phyllosilicates and to a lesser degree sulfates the characterization of aqueous alteration of basalts under a variety of environmental conditions provides a conceptual framework for interpretation of similar relic environments on mars finally while identification of phyllosilicates on mars is generally thought to require neutral to alkaline fluids we documented significant formation of these minerals in the acidic volcanic systems

the effect of the oceans on the terrestrial crater size frequency distribution insight from numerical modeling on earth oceanic impacts are twice as likely to occur as continental impacts yet the effect of the oceans has not been previously considered when estimating the terrestrial crater size frequency distribution despite recent progress in understanding the qualitative and quantitative effect of a water layer on the impact process through novel laboratory experiments detailed numerical modeling and interpretation of geological and geophysical data no definitive relationship between impactor properties water depth and final crater diameter exists in this paper we determine the relationship between final and transient crater diameter and the ratio of water depth to impactor diameter using the results of numerical impact models this relationship applies for normal incidence impacts of stoney asteroids into water covered crystalline oceanic crust at a velocity of 15 km s 1 we use these relationships to construct the first estimates of terrestrial crater size frequency distributions over the last 100 million years that take into account the depth area distribution of oceans on earth we find that the oceans reduce the number of craters smaller than 1 km in diameter by about two thirds the number of craters 30 km in diameter by about one third and that for craters larger than 100 km in diameter the oceans have little effect above a diameter of 12 km more craters occur on the ocean floor than on land below this diameter more craters form on land than in the oceans we also estimate that there have been in the region of 150 impact events in the last 100 million years that formed an impact related resurge feature or disturbance on the seafloor instead of a crater

the importance of being cratered the new role of meteorite impact as a normal geological process this paper is a personal and in many ways incomplete view of the past development of impact geology and of the newly recognized importance of impact events in terrestrial geological history it also identifies some exciting scientific challenges for future investigators to determine the full range of impact effects preserved on the earth to apply the knowledge obtained from impact phenomena to more general geological problems and to continue the merger of the once exotic field of impact geology with mainstream geosciences since the recognition of an impact event at the cretaceous tertiary k t boundary much current activity in impact geology has been promoted by traditionally trained geoscientists who have unexpectedly encountered impact effects in the course of their work their studies have involved 1 the recognition of additional major impact effects in the geological record the chesapeake bay crater the alamo breccia and multiple layers of impact spherules in precambrian rocks and 2 the use of impact structures as laboratories to study general geological processes e g igneous petrogenesis at sudbury canada and archean crustal evolution at vredefort south africa other research areas in which impact studies could contribute to major geoscience problems in the future include 1 comparative studies between low level 7 gpa shock deformation of quartz and the production of quartz cleavage in both impact and tectonic environments and 2 the nature origin and significance of bulk organic carbon kerogen and other carbon species in some impact structures gardnos norway and sudbury canada

geochemistry of the volcano hydrothermal system of el chichon volcano chiapas mexico the 1982 eruption of el chichon volcano ejected more than 1 km3 of anhydrite bearing trachyandesite pyroclastic material to form a new 1 km wide and 300 m deep crater and uncovered the upper 500 m of an active volcano hydrothermal system instead of the weak boiling point temperature fumaroles of the former lava dome a vigorously boiling crater spring now discharges 20 kg s of cl rich 15 000 mg kg and sulphur poor 200 mg kg of so4 almost neutral ph up to 6 7 water with an isotopic composition close to that of subduction type magmatic water d 15 18o 6 5 this spring as well as numerous cl free boiling springs discharging a mixture of meteoric water with fumarolic condensates feed the crater lake which compared with values in 1983 is now much more diluted 3000 mg kg of cl vs 24 030 mg kg less acidic ph 2 6 vs 0 56 and contains much lower amounts of s 200 mg kg of so4 vs 3550 mg kg with 34s 0 5 4 2 17 in 1983 agua caliente thermal waters on the southeast slope of the volcano have an outflow rate of approximately 100 kg s of 71 c na ca cl water and are five times more concentrated than before the eruption b r molina unpublished data relative n2 ar and he gas concentrations suggest extensional tectonics for the el chichon volcanic centre the 3he 4he and 4he 20ne ratios in gases from the crater fumaroles 7 3ra 2560 and agua caliente hot springs 5 3ra 44 indicate a strong magmatic contribution however relative concentrations of reactive species are typical of equilibrium in a two phase boiling aquifer sulphur and c isotopic data indicate highly reducing conditions within the system probably associated with the presence of buried vegetation resulting from the 1982 eruption all cl rich waters at el chichon have a common source this water has the appearence of a partially matured magmatic fluid condensed magmatic vapour neutralized by interaction with fresh volcaniclastic deposits and depleted in s due to anhydrite precipitation shallow ground waters emerging around the volcano from the thick cover of fresh pumice deposits red waters are ca so4 rich and have a negative oxygen isotopic shift probably due to ongoing formation of clay at low temperatures

fluid immiscibility in volcanic environments volatile components enter the atmosphere oceans and other surface waters through pre syn and post eruptive volcanic processes that involve a variety of fluids aluminosilicate poor fluids include aqueous or carbonic to sulfide sulfate chloride fluoride carbonate and phosphate rich compositions in volcanic environments but other more complex combinations of these constituents may be involved giggenbach 1977 roedder 1984 1992 lowenstern 1995 other rare gases and dissolved constituents e g h2 n2 he ar h3bo3 hg ch4 and other hydrocarbon compounds metals and metalloids are also present but in general they are not sufficiently concentrated to form their own phases or control bulk fluid composition and volcanic processes as a result of their diverse compositions volcanic fluids ranging from vapor to liquid exhibit widely different densities and show strong distinctions in heat capacity dielectric behavior viscosity and other chemical and physical characteristics geiger et al 2006a the density of a saline liquid in the system nacl h2o at 50 mpa and 800 c for example is more than 7 times that of the coexisting aqueous vapor e g 0 2 gm cm3 henley and mcnabb 1978 multiple fluids move mix and or unmix in magma chambers and volcanic conduits in the root zone of fumaroles and in the convective hydrothermal systems that are ubiquitous to volcanic environments hydrothermal systems are typically centered on magmatic intrusions and may include crater lakes at the top of eruptive conduits hydrothermal processes involving two or more fluid phases operate through a variety of geologically relevant shallow crustal pressure temperature composition conditions volcanic fluids occur at supersolidus to subsolidus conditions meaning that the dense aluminosilicate melt that is common to volcanism may or may not be present this chapter addresses the evidence for and occurrence of two or more low density aluminosilicate poor fluids

the winneshiek biota exceptionally well preserved fossils in a middle ordovician impact crater the winneshiek shale middle ordovician darriwilian was deposited in a meteorite crater the decorah impact structure in ne iowa this crater is 5 6 km in diameter and penetrates cambrian and ordovician cratonic strata it was probably situated close to land in an embayment connected to the epicontinental sea typical shelly marine taxa are absent the konservat lagerstatte within the winneshiek shale is important because it represents an interval when exceptional preservation is rare the biota includes the earliest eurypterid a giant form as well as a new basal chelicerate and the earliest ceratiocarid phyllocarid conodonts some of giant size occur as bedding plane assemblages bromalites and rarer elements including a linguloid brachiopod and a probable jawless fish are also present similar fossils occur in the coeval ames impact structure in oklahoma demonstrating that meteorite craters represent a novel and under recognized setting for konservat lagerstatten

symmetrical and segmented variation of physical and geochemical characteristics of the central american volcanic front abstract the regional variation of physical and geochemical characteristics of central american volcanoes occurs in two fundamentally different patterns the first pattern is symmetrical about nicaragua crustal thickness silica contents of mafic lavas and volcanic edifice heights are lowest in nicaragua and increase smoothly toward costa rica to the south and guatemala to the north magma density is maximum in nicaragua and decreases smoothly outward the regional variation in crustal thickness is just enough so that magma densities calculated at appropriate moho pressures are the same at the base of the crust throughout the region this is consistent with magma ponding at the base of the crust the bulk compositions of central american basalts show the same symmetrical variation suites of nicaraguan basalts plotted in pseudo ternary cmas projections indicate large olivine and plagioclase primary phase volumes toward costa rica and guatemala the olivine and plagioclase fields inferred from suites of basaltic lavas are smaller which is consistent with fractionation at increasing depth the second pattern is the segmentation of the volcanic front and the plate margin in general the segmentation strongly affects the spacing and size of volcanic centers at segment boundaries volcanic centers are generally small and unusually widely spaced toward segment interiors volcano spacing and size increase systematically the lil element contents of lavas strongly reflect this pattern for lavas with similar silica contents the larger the volcano the higher the lil element contents the relationships between segmentation volcano spacing and volcano size are compatible with diapiric rise of magma accumulated in narrow ribbons near the upper surface of the underthrust slab the relationship between volcano volume and lil element content is qualitatively in agreement with an open system fractionation model

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