a new cretaceous tertiary boundary site at flaxbourne river new zealand biostratigraphy and geochemistry abstract an exceptionally complete rock sequence across the cretaceous tertiary k t boundary has been discovered near the flaxboume river marlborough province south island new zealand the boundary is marked by a large ir anomaly 21 ng g on a decalcified basis with an integrated abundance of 134 ng cm2 after correction for background above the boundary there is a 30 cm transition zone in which a few cretaceous foraminiferal taxa such as hedbergella monmouthensis and guembelitria cretacea survived though with reduced abundance and size only ca 1 2 to 1 4 of normal apparently reflecting environmental stresses inaa and icp analyses show that in addition to ir the boundary clay is also enriched in cr and ni mainly from meteoritic material and as co cu sb and zn from terrestrial sources volcanic sources even when scaled to the 10 7 km 3 volume of the deccan basalts fail by three orders of magnitude to account for the ir and as at the k t boundary and by even larger factors for sb zn cu etc comparison of our data with those from six other k t boundary sites shows that the zn sb as sb and zn as ratios generally fall between crustal and oceanic values suggesting contributions from both sources mass balance calculations show that as and sb could be derived from only 300 500 m of ocean water or also from modest amounts 20 36 g cm 2 of average crustal rock copper and zn on the other hand can only be derived from crustal or mantle rock 5 15 g cm 2 presumably impact ejecta such an amount of ejecta is fairly close to the global fallout of boundary clay 2 5 g cm 2

contact diagenesis the effect of an intrusion on reservoir quality in the triassic sherwood sandstone group northern ireland abstract intrusion of a dolerite sill into clay bearing arkosic triassic sherwood group sandstones with moderate porosity 22 and high but anisotropic permeability mode 500 md led to the progressive transformation of precursor diagenetic minerals in the host rock locally pore filling mg rich smectite saponite quartz and dolomite existing in the rock following earlier burial diagenesis reacted to produce grain coating flakes of talc at the lowest contact temperatures 130 180 c based on equilibrium reaction path and thermal history calculations at higher temperatures 200 230 c talc reacted with calcite to produce an actinolitic amphibole close to the intrusion at temperatures of 250 c bundles of acicular needles of actinolite protruding into open pores were produced by a second reaction involving saponite hematite and calcite framework feldspar minerals mainly k feldspars played little or no part in the reactions because there were no k al rich minerals produced by contact diagenesis and because the detrital feldspar grains are essentially unaltered thus while equilibrium may have been approached on the intrapore scale 10 m it was not achieved between grains e g detrital feldspars pore water and clays in the pores total porosity has been unaffected by the intrusion and consequent mineral reactions because reactions involved recrystallization of original pore filling minerals despite the contact diagenetic changes in the rock the permeability of the sandstone remained largely unaffected by intrusion

a geochemical clock in earliest paleogene pelagic carbonates based on the impact induced os isotope excursion at the cretaceous paleogene boundary 1 an impact induced osmium os isotope excursion provides a unique means of assessing the completeness of marine cretaceous paleogene k pg boundary sections and surmounting challenges associated with constraining the time scale of the earth system recovery from this extreme perturbation a model of the recovery of seawater187os 188os following the impact event allows independent estimates of the time elapsed since the impact which can be directly compared to time estimates derived from biostratigraphy magnetostratigraphy and cyclostratigraphy this approach is tested using data from three deep ocean sites cored by the ocean drilling program odp data from odp 1262b south atlantic and odp 690c southern ocean display the expected 187os 188os minimum very close to the biostratigraphically defined k pg boundary and yield os based accumulation rate estimates similar to those obtained from magnetostratigraphy and orbital tuning in contrast the187os 188os minimum in odp 1209c western pacific occurs 9 cm below the k pg boundary low os concentrations throughout the boundary interval and an implausibly rapid recovery to higher pre impact187os 188os ratios provide strong evidence for a previously unrecognized gap in the k pg interval of site 1209 results presented here provide strong empirical evidence that os isotope data are uniquely valuable in assessing the completeness and accumulation rates of earliest paleogene sediments from the deep sea they are of broad interest because they have implications for astronomical tuning of the geologic time scale and illustrate that whole ocean geochemical perturbations can provide an alternative to biostratigraphy for correlation and timekeeping during abrupt biotic events

inversion of the horizontal to vertical spectral ratio in presence of strong lateral heterogeneity the horizontal to vertical spectral ratio hvsr method has recently gained a resurgence of interest because of its new interpretation through the diffuse field approach according to this theory the hvsr measurement is linked to the green s function retrieval while the hvsr method is traditionally used to evaluate simple and shallow velocity model we demonstrate that complex earth structure that includes composite geological layering can be assessed by choosing appropriate data processing and inversion scheme in particular we explore the effects of a strong lateral heterogeneity on the hvsr by using measurements along a vertical crater wall however wave propagation computation with lateral heterogeneity is inefficient to conduct an inversion then to accelerate the forward calculation we identified a simplified model consisting of an unbounded 2 d multilayer medium that allows describing the observed measurements with the use of a correction factor the 2 d character comes from a nonstandard noise illumination related to the wind blowing perpendicularly to the crater walls it is supported by several theoretical and experimental arguments we also show that the h and v components can be retrieved independently which may improve the sensitivity of the inversion at high frequency to reduce the non uniqueness of the problem we consider the joint inversion of the h and hvsr s at several positions where the thicknesses of the layers were evaluated thanks to a large geological outcrop the good agreement between the recovered velocity structure and the geology shows that the method is not limited to unbounded simple shallow structures which opens the domains of application of the method to exploration geophysics or to hazard assessment

proximal cretaceous tertiary boundary impact deposits in the caribbean trace element isotopic and mineralogic studies indicate that the proposed impact at the cretaceous tertiary k t boundary occurred in an ocean basin although a minor component of continental material is required the size and abundance of shocked minerals and the restricted geographic occurrence of the ejecta layer and impact wave deposits suggest an impact between the americas coarse boundary sediments at sites 151 and 153 in the colombian basin and 5 to 450 meter thick boundary sediments in cuba may be deposits of a giant wave produced by a nearby oceanic impact on the southern peninsula of haiti a approximately 50 centimeter thick ejecta layer occurs at the k t boundary this ejecta layer is approximately 25 times as thick as that at any known k t site and suggests an impact site within approximately 1000 kilometers seismic reflection profiles suggest that a buried approximately 300 km diameter candidate structure occurs in the colombian basin

using microearthquakes to track repeated magma intrusions beneath the eyjafjallajokull stratovolcano iceland 1 we have mapped microearthquakes caused by magma migration preceding and during the flank and summit eruptions in march may 2010 of eyjafjallajokull stratovolcano in iceland using a coalescence microseismic mapping technique spatial and temporal clustering of 5 000 microearthquakes under the eastern flank of the volcano illuminates several northeast southwest striking sub vertical dikes at 2 6 km b s l emplaced before the fimmvor uhals flank eruption in march this intense precursory seismicity had a lateral extent of 6 km east west and 3 km north south a sequence of 386 microearthquakes during the summit eruption refined by double difference relative relocation defines a sub linear trend inclined 5 10 from vertical extending from the upper mantle at 30 km depth to the summit crater this sequence includes two major clusters at 19 km and 24 km b s l each containing 100 earthquakes all microearthquakes display characteristics of brittle fracture with several subsets of events exhibiting closely similar waveforms within clusters this suggests similar repetitive source processes the deeper clusters may be caused by fracturing solidified magma plugs that form constrictions in an otherwise aseismic melt conduit or they may occur at exit points from melt pockets in which case they indicate positions of magma storage at depth the seismicity deeper than 10 km only starts three weeks after the onset of the summit eruption after which the largest clusters occur at progressively greater depths this temporal pattern may result from pressure release at shallow levels in the magmatic plumbing system progressively feeding down to mobilize deeper melt pockets

magma fingers and host rock fluidization in the emplacement of sills the golden valley sill in south africa like many similar structures imaged in three dimensional seismic data is saucer shaped with a transgressive rim this rim exhibits magma fingers tens to hundreds of meters wide radiating from a central axis and forming the outer dipping rim localized fluidization of the host rock is observed close to the margins where the fingers occur suggesting a causal link between fluidization of the country rocks and finger formation we relate the large scale formation of fingers to host rock fluidization caused by flash boiling of pore fluids following the tensional failure of the roof of the inner dish at a maximum radius once fluidization has occurred by this method the mechanical heterogeneity controlling the propagation of a concordant sill is broken and the intrusion can transgress this work emphasizes the importance of understanding the nature of magma intrusion on host rocks depending on their lithology and in particular the ability of host rocks to behave in either a brittle or nonbrittle manner during magma intrusion

deflection of dykes into sills at discontinuities and magma chamber formation abstract many rift zones contain composite volcanoes central volcanoes most of which are supplied with magma from shallow crustal magma chambers volcanotectonic studies of eroded rift zones such as in iceland as well as geophysical studies indicate that many shallow magma chambers evolve from sills here i provide field description of sills and extinct magma chambers in the quaternary and tertiary palaeorift zones of iceland focusing on the felsic pluton of slaufrudalur southeast iceland as a fine example of a magma chamber developed through the injection of sills in this paper i review analyse and develop further two related mechanisms for the deflection of dykes into sills and thus the potential initiation of a magma chamber namely a cook gordon debounding delamination and b stress barriers and propose a third mechanism c favourable material toughness ratios due to elastic mismatch difference in young s moduli or stiffnesses of layers in contact in the cook gordon mechanism a weak contact opens up as a result of dyke induced tensile stress this mechanism is likely to operate primarily at shallow depths as is supported by field observations and numerical models a stress barrier is a layer where the local stress is unfavourable to a particular type of rock fracture here a dyke field observations and numerical models show that on meeting a stress barrier a dyke either changes into a sill or becomes arrested the material toughness mechanism indicates that when the upper layer at a contact has the same or less stiffness than the lower layer hosting the dyke there is little tendency for the dyke to become deflected into the contact however when the upper layer is stiffer such as when a stiff basaltic lava flow is on the top of a soft pyroclastic layer the dyke tends to become deflected into the contact to form a sill field results suggest that all these mechanisms may cause dyke deflection into sills and may operate together particularly where the rock consists of alternating soft and stiff layers since the quaternary lava pile in iceland has many more soft hyaloclastite layers than the tertiary lava pile these mechanisms were probably very efficient in generating sills and by implication shallow magma chambers during the quaternary this may be one reason why in comparison with the tertiary lava pile the quaternary lava pile contains so many sills and extinct plutons and active shallow magma chambers

geology of the snap lake kimberlite intrusion northwest territories canada field observations and their interpretation the cambrian 523 ma snap lake hypabyssal kimberlite intrusion northwest territories canada is a complex segmented diamond bearing ore body detailed geological investigations suggest that the kimberlite is a multi phase intrusion with at least four magmatic lithofacies in particular olivine rich ork and olivine poor opk varieties of hypabyssal kimberlite have been identified key observations are that the olivine rich lithofacies has a strong tendency to be located where the intrusion is thickest and that there is a good correlation between intrusion thickness olivine crystal size and crystal content heterogeneities in the lithofacies are attributed to variations in intrusion thickness and structural complexities the geometry and distribution of lithofacies points to magmatic co intrusion and flow segregation driven by fundamental rheological differences between the two phases we envisage that the low viscosity opk magma acted as a lubricant for the highly viscous ork magma the presence of such low viscosity crystal poor magmas may explain how crystal laden kimberlite magmas 60 vol are able to reach the surface during kimberlite eruptions we also document the absence of crystal settling and the development of an unusual subvertical fabric of elongate olivine crystals which are explained by rapid degassing induced quench crystallization of the magmas during and after intrusion supplementary material additional figures are available at http www geolsoc org uk sup18503

buoyancy driven fracture ascent experiments in layered gelatine laboratory experiments on air filled fracture propagation in solidified homogeneous and layered gelatine have been carried out providing an analogue model for magma filled dikes ascending in the crust the effects of layering on fracture velocity and shape have been analyzed in detail the free surface is found to accelerate approaching fractures layering accelerates or decelerates fractures approaching discontinuities of the elastic parameters depending on the value of the rigidity contrast the shape of fractures are strongly influenced as they pass from one layer to another the observed cross sectional shape when crossing a layer interface and the acceleration with decreasing rigidity can be explained with theoretical models our experiments also reproduce the arrest of fractures in proximity of joints and the formation of sills in the layer below the interface these findings could help in the interpretation of accelerated seismicity and deformation rates observed in volcanic areas

sill morphology and comparison of brittle and non brittle emplacement mechanisms magmatic sheet intrusions contribute significantly to the upper crustal magma transport network the emplacement mechanism of the magmatic sheets controls the final geometry of the intrusions and the characteristics of host rock deformation previous observations have highlighted the preponderance of brittle structures associated with shallow level sheet intrusions however recent studies have suggested that non brittle host rock behaviour also occurs particularly related to the formation of magma fingers during shallow level sill intrusion here we examine both brittle and non brittle intrusion mechanisms and expand upon them with field observations from a series of widespread and variable magmatic systems non brittle emplacement appears primarily associated with viscous flow of the host rock during intrusion and is therefore intimately linked to the contemporaneous host rock rheology as well as magma dynamics purely brittle and non brittle emplacement processes are found to be end members with many intrusions containing evidence of both behaviours deriving the host rock characteristics is therefore important for discerning potential diagnostic intrusion indicators and intrusion geometries both within the field and in modelling incorporation of variable host material behaviours in numerical and analogue modelling tuned using direct field observations may consequently further our understanding of the controls on shallow level intrusion

mechanisms of dike propagation in layered rocks and in massive porous sedimentary rocks the mechanisms of dike propagation are analyzed through detailed examination of small structures associated with the dike intrusion exposures in the ramon area israel offer an opportunity to evaluate theoretical models of tensile fracture propagation and to better understand the factors governing the dike geometry dike related structures are studied in two jurassic units the ardon formation a well stratified sequence of shales and dolomites and the inmar formation which is composed of porous massive sandstones with a few shale lenses it is shown that for the different host rocks the propagation mechanisms and structures differ significantly dikes in the ardon formation are dominated by 1 20 m wide segments confined to distinct layers the mechanisms of segment containment are determined by calculating the stress intensity factor for an extension crack approaching a mechanical interface it is apparent that dike segmentation and containment is controlled mainly by in situ stress and shear moduli differences between adjacent layers and partly by bedding plane slippage dikes in the inmar formation are dominated by smaller segments and by 1 10 cm wide and 10 100 cm long fingers with intermittent smooth patches and slickensides opposite dike walls display exact mirror images of these features dikes propagate in alternating stages of viscous flow and brittle deformation the viscous stages include fluidization of the sandstone by the dike related fluids and increase of the pore pressure in the rock a viscous instability develops between the highly viscous fluidized sandstone and the less viscous dike related fluids forming a zone of viscous fingering in front of the dike stages of brittle fracturing occur when the pore pressure is released and the flux of fluids is insufficient to fluidize the rock

fracture dominated flow in the borrowdale volcanic group at sellafield nw england the identification of potential flowing features development of conceptual models and derivation of effective parameters

time resolved construction of a bimodal laccolith torres del paine patagonia abstract granitic and mafic magma pulses were sequentially accreted in the spectacularly exposed shallow crustal torres del paine laccolith in southern patagonia this 12 5 ma pluton forms a composite intrusion with a subvertical feeding system in the west and a laccolith in the east a key unknown in the formation of sill complexes is how individual magma pulses are assembled over time and the geometry and localization of their feeding system high resolution zircon ca id tims u pb dating shows that the laccolith grew first by under accretion of granitic sills over 90 30 ka linked to a sheet like feeding system followed by underplating of mafic sills after 20 ka of quiescence in the mafic sills complex individual sills were injected by over accretion during 41 11 ka our data show that successive granitic and mafic magmas emplacement generated a volume of 88 km3 in 162 11 ka

terrestrial carbon and nitrogen isotopic ratios from cretaceous tertiary boundary nanodiamonds one hypothesis for the origin of the nanometer size diamonds found at the cretaceous tertiary k t boundary is that they are relict interstellar diamond grains carried by a postulated asteroid the 13 c 12 c and 15 n 14 n ratios of the diamonds from two sites in north america however show that the diamonds are two component mixtures differing in carbon and nitrogen isotopic composition and nitrogen abundance samples from a site from italy show no evidence for either diamond component all the isotopic signatures obtained from the k t boundary are material well distinguished from known meteoritic diamonds particularly the fine grain interstellar diamonds that are abundant in primitive chondrites the k t diamonds were most likely produced during the impact of the asteroid with earth or in a plasma resulting from the associated fireball

rhodium distribution at the cretaceous tertiary boundary analysed by ultrasensitive laser photoionization the extraordinarily high concentrations of indium and other siderophiles found in cretaceous tertiary k t boundary deposits have been interpreted1 as a result of a large extraterrestrial body falling upon the earth causing mass extinction of the biota dominant in the cretaceous the existing alternative models of the cretaceous terminal event try to explain the anomaly of siderophile elements by their being concentrated through sedimentation or volcanic eruptions2 3 to understand the nature of the k t event it is important to establish the proportions of siderophiles and their isotopes in boundary deposits of that age especially the proportions of platinum group elements their concentrations in extraterrestrial material being high and occurring in ratios differing from those typical of terrestrial material here we present the first data on rhodium concentrations at the k t boundary in the sumbar sm 4 section turkmen ssr obtained by the ultrasensitive laser photoionization spectroscopy lapis technique4 5 the maximum rh concentration in the samples studied is 24 2 ng g 1the rh ir ratio is 0 34 0 06 which is close to the cosmic ratio of these elements the lapis technique is efficient and can be used for determining the concentrations of all siderophile elements and their isotopes in geological materials

geometric difference between non feeder and feeder dikes feeder dikes bring magma to the surface non feeder dikes become arrested and never reach the surface the differences if any between these dike types remain largely unexplored because in the field it is normally unknown if a particular dike is a feeder or non feeder here we present measurements of feeder and non feeder dikes exposed from depths of 200 m to the surface in the walls of the a d 2000 caldera collapse of the miyakejima volcano japan a typical feeder thickness reaches a maximum of 2 4 m at the surface decreases rapidly to 1 m at a depth of 20 40 m and then remains constant to the bottom of the exposure by contrast a typical non feeder thickness reaches a maximum of 1 5 2 m at 15 45 m below the tip and then decreases slowly with depth to 0 5 1 m at the bottom of the exposure we propose that free surface effects and magmatic overpressure driving pressure changes during the eruption cause the overall shape of a feeder to differ from that of a non feeder

form and dimensions of dykes in eastern iceland abstract a study was made of about 700 dykes in eastern iceland the majority of these belong to three swarms about 73 dip within 10 of the vertical most strike between 10 and 40 ne the strike of the dykes within the southernmost swarm alftafjordur changes along its trace from almost n at the north end to nne ssw southward along the swarm the average thickness of the dykes is about 4 1 m and the thickness does not change notably along the alftafjordur swarm the thinner dykes tend to have smaller grains than the thicker dykes of five dykes followed laterally the longest is over 22 km the thickness of individual dykes changes irregularly along their length and the dyke is often offset where its thickness changes many dykes appear to be completely discontinuous but some parts are connected by veins where the dykes end in a vertical section most of them simply taper away only about 10 5 of the dykes occupy faults the mechanical and thermal effects of the dykes on the country rock are small many of the dykes appear to be non feeders i e dykes that never reached the surface to feed lava flows using the length width ratio the depth of origin of three dykes has been estimated the maximum depth of origin of these three dykes is 7 5 km 9 km and 10 km below the original surface

analytical model of surface uplift above axisymmetric flat lying magma intrusions implications for sill emplacement and geodesy abstract in this paper we develop a new axisymmetric analytic model of surface uplift upon sills and laccoliths based on the formulation of a thin bending plate lying on an elastic foundation in contrast to most former models also based on thin bending plate formulation our model accounts for i axi symmetrical uplift ii both upon and outside the intrusion the model accounts for shallow intrusions i e the ratio a h 5 where a and h are the radius and depth of the intrusion respectively the main parameter of the model is the elastic length l which is a function of the elastic properties of the bending plate and of the elastic foundation the model exhibits two regimes depending on the ratio a l when a l a l 5 the uplift is mostly restricted upon the intrusion when the elastic foundation is very stiff our model converges towards that of a clamped plate we provide as supplementary material a matlab function that calculates the surface uplift from the set of system and control parameters we discuss three possible applications of our model i the model can be used to describe sill propagation by introducing a propagation criterion for realistic values our model reproduces well the behavior of horizontal intrusions simulated in experiments ii the model can also be used to compute the critical size of saucer shaped sills it shows for instance that a soft elastic foundation favors the horizontal spreading of sills before they form inclined sheets iii we show that the classical mogi point source model cannot be used to constrain sill properties from the surface uplift we thus propose that our model can be used as a valuable alternative to both simple analytical models like mogi s and more complex numerical models used to analyze ground deformation resulting from sill intrusions in active volcanoes

mechanically disrupted and chemically weakened zones in segmented dike systems cause vent localization evidence from kimberlite volcanic systems deformation and alteration zones along kimberlite dikes hold clues as to how point source volcanic vents can localize along sheet like intrusions brittle deformation zones occur in host rock adjacent to kimberlite intrusions of the swartruggens kimberlite dike swarm south africa deformation includes local fracturing and brecciation and is associated with relay zones between offset dike segments breccia zones indicate dilation and hydraulic fracturing and some zones along the dikes were affected by chemical corrosion forming fresh cores surrounded by onion skin concentric shells of altered rock the alteration was caused by either exsolved magmatic volatiles moving in advance of the magma through the fracture or by hydrothermal fluids consideration of the time scales needed for chemical corrosion of host rock requires intrusions to stall at depth prior to transport to higher crustal levels highly disrupted offsets could be preferred locations for explosive activity and initial vent formation as dikes approach the surface a kimberlite pipe forms after magma breaks through to the surface the altered zones are reamed out and fresh cores in spheroidally altered rock are incorporated into the pipe fill along with more angular country rock material as observed in layered volcanic breccias in kimberlite pipes at the venetia mine south africa this model may have wider implications for the localization of conduits along dikes in other volcanic systems dike segmentation provides weak zones where hydrothermal fluids and magmatic volatiles can be preferentially channeled chemical corrosion can further weaken these zones which may then become the locus for initial phreatic and phreatomagmatic explosions creating shallow vents that can then channel magma to the surface during eruption

insights of dyke emplacement mechanics from detailed 3d dyke thickness datasets abstract dyke thickness datasets offer new insights into the detailed 3d geometry of dyke swarms and an exceptional opportunity to evaluate theoretical emplacement models the swartruggens kimberlite dyke swarm extends over 7 km along strike and intrudes a dolerite quartzite shale and andesitic lava succession the star kimberlite dykes cut shales and sandstones intersect a large dolerite sill and extend 15 km along strike both dyke swarms comprise anastomosing en echelon segments each several hundred metres long in total 1532 swartruggens dyke thickness measurements were taken to 750 m below the surface over a 250 m depth range and 3354 star dyke thickness measurements were taken over a 520 m depth range the swartruggens dyke thicknesses are 0 05 1 95 m mean 0 64 m whereas the star kimberlites range from

elemental anomalies at the cretaceous tertiary boundary woodside creek new zealand iridium and 26 other elements were determined in shale from the cretaceous tertiary boundary at the locus classicus for iridium anomalies at woodside creek new zealand iridium gold copper cobalt chromium nickel arsenic molybdenum and iron were enriched in the basal 2 millimeters of the 8 millimeter shale parting as compared with the rest of the stratigraphic column no other shale partings in the column had anomalous concentrations of any element when the data were expressed on a carbonate free basis the boundary material showed striking compositional similarities with the stevns klint danish boundary shale elemental concentrations were in general much higher in the new zealand material than in nonboundary shales from elsewhere in the world the high concentration of iridium 153 nanograms per gram in the basal layer of the boundary together with the enrichment of other siderophile elements supports the idea of an extraterrestrial source for much of the material the iridium gold ratio of 2 1 is also in accordance with such a source the iridium content of the basal layer is higher than for any other marine boundary shale obtained on land the integrated iridium value is 187 nanograms per square centimeter of boundary surface

new method for the measurement of osmium isotopes applied to a new zealand cretaceous tertiary boundary shale the determination of osmium content and isotopic abundances in geological materials has received increasing attention in recent years following the proposal of alvarez et al 1 that mass extinctions at the end of the cretaceous period were caused by the impact of a large 10km meteorite which left anomalously high iridium levels as a geochemical signature in the boundary shales here we report a new and simple method for measuring osmium in geological materials involving fusion of the sample with sodium peroxide distillation of the osmium as the tetroxide using perchloric acid extraction into chloroform and absorption of the chloroform extract onto graphite powder before instrumental neutron activation analysis in a variant of this technique the chloroform extract is back extracted into an aqueous phase and the osmium isotopes are determined by plasma source mass spectrometry icpms we have used this method on the woodside creek new zealand cretaceous tertiary boundary clay and have obtained the first osmium content 6g ng g 1 for this material the 187os 186os ratio is 1 12 0 16 showing a typical non crustal signature this combined distillation extraction icpms method will prove to be useful for measuring osmium isotopes in other geological materials

cretaceous tertiary boundary event evidence for a short time scale abstract three non meteoritic trace elements sb as and zn are strongly enriched at eleven k t boundary sites along with mainly or partly meteoritic elements ir ni cr fe and co the proportions as sb zn ir are remarkably constant over a 100 fold range in concentration this correlation persists in sub layers of boundary clay and even extends to soot from burned land biomass apparently all the components despite their diverse origins became associated in a single global component prior to deposition no wholly satisfactory source is available for as sb and zn the trace element pattern in volcanic gases does not match that in k t boundary clay with ratios to ir falling short by 1 to 2 orders of magnitude terrestrial rocks do not reach high enough concentrations and modern ocean water contains too little zn but on balance the latter source seems preferable perhaps augmented by volatiles from the impact crater apparently ir bearing ejecta and soot from forest fires coagulated in the stratosphere and then fell out together sweeping out oceanic biomass and anoxically precipiting as sb and zn significantly the amounts of marine and land biomass at the k t boundary correspond to about the steady state global inventory 1 generation and the amounts of as sb and zn are also within a factor of 5 of the global inventory this is expected in a catastrophic but not a gradualist scenario

mechanisms and controls on the formation of sand intrusions sandstone intrusions are found in all sedimentary environments but have been reported most commonly from deep water settings they also appear to be more frequently developed in tectonically active settings where applied tectonic stresses facilitate development of high fluid pressures within the sediments a variety of mechanisms have been cited as triggers for clastic intrusions these include seismicity induced liquefaction application of tectonic stresses excess pore fluid pressures generated by deposition related processes and the influx of an overpressured fluid from deeper within the basin into a shallow sand body the formation of sandstone dykes and sills is investigated here by considering them as natural hydraulic fractures when the seal on an unconsolidated overpressured sand body fails the resulting steep hydraulic gradient may cause the sand to fluidize the fluidized slurry can then inject along pre existing or new fractures to form clastic intrusions the scale and the geometry of an intrusive complex is governed by the stress state depth and pre existing joints or faults within the sedimentary succession as well as the nature of the host sediments for the simplest tectonic setting where the maximum stress in a basin is vertical gravitational loading small irregular intrusions commonly result in the formation of sills at shallow depths within a few metres of the surface whereas at greater depth dykes and sills forming clastic intrusion networks are more typical a simple relationship is derived to calculate the maximum burial depth at which a dyke sill complex forms as a function of the source bed to sill height the bulk density of the surrounding sediments and the ratio of the vertical to horizontal effective stresses k 0 when applied to three examples of large scale dyke sill complexes developed within paleocene and eocene deep water reservoir sand bodies of the north sea maximum burial depths in a range of 375 to c 500 m 450 700 m and 550 850 m are estimated for intrusion of each of the three complexes

modeling the growth of laccoliths and large mafic sills role of magma body forces 1 the use of elastic plate theory to model the emplacement of laccoliths and large mafic sills has been debated for nearly 40 years these intrusions typically attain a horizontal width that is large relative to the emplacement depth provided that large scale plasticity and or heterogeneity is not observed in the overlying host rock it should then be valid to approximate its deformation based on analysis of a thin elastic plate with effective properties that are the consequence of interaction among heterogeneities that are small relative to the size of the intrusion but the predictions that are usually cited from elastic plate theory are characterized by bell shaped geometry in contrast to the flat topped steep sided geometry typical of many laccoliths or the nearly uniform thickness typical of large mafic sills this fact has motivated several alternate explanations of laccolith and large mafic sill emplacement nonetheless elastic plate theory should be revisited in light of the fact that previous elastic plate based predictions have in general not taken into account an appropriate fracture propagation condition fluid flow in the growing intrusion and importantly the influence of the weight of the magma on intrusion growth we present a model for the growth of circular intrusions that accounts for all of these factors the model predicts the appropriate geometry for both laccoliths and large mafic sills the predicted thickness to length relationships are also consistent with field data hence while it may sometimes be appropriate there is in general no fundamental need to appeal to large scale rock plasticity in order to explain observed intrusion geometries and it may in fact be appropriate to understand the growth of laccoliths and large sills in light of a single underlying mechanical model

the shapes of dikes evidence for the influence of cooling and inelastic deformation we document the shape of dikes from well exposed field locations in the isle of rum scotland 14 and helam mine south africa the basaltic rum dikes crop out on a smaller scale than the 15 helam kimberlite dikes and have a smaller length to thickness ratio 100 1 isle of rum 16 1000 1 helam mine we compare dike thickness field measurements with the geometry 17 predicted by elastic theory finding best fit models to estimate magma overpressure and regional 18 stress gradients at the time of dike emplacement most of the dike shapes fit poorly with elastic 19 theory being too thick at the dike ends and too narrow in the middle our calculated 20 overpressures and stress gradients are much larger than independent estimates based on rock 21 strength dike shape can be explained by a combination of host rock inelastic deformation and 22 magma chilling at the dike s tapering edges preventing its closure as magma pressure declines 23 during emplacement the permanent wedging of the dike edges due to chilling has implications 24 for crustal magma transport and strain response in the crust due to dike emplacement

dike propagation through layered rocks 1 dike penetration through a succession of upper crustal layers with different densities is studied with a new numerical code for an individual layer to significantly affect dike ascent its thickness must be of order 1 when scaled to the characteristic length scale for the inflated nose region that develops below the dike tip this characteristic length is l q 1 6 g 1 1 2 g 2 3 where and are the viscosity and buoyancy of magma g and are elastic moduli for the encasing rocks q is the magma flow rate and g gravity for basaltic dikes l is 1 km which is of the same order of magnitude as the typical thickness of sedimentary strata and volcanic deposits in such conditions dike ascent proceeds irregularly with large changes of velocity and width at an interface scaling laws for the ascent rate and dike width are derived penetration through low density layers is determined by a local buoyancy balance in the inflated nose region of the dike independently of the total buoyancy of the magma column between source and tip in such conditions a dike develops an internal overpressure that may be large enough to generate a horizontally propagating sill for this to occur the thickness of the low density layers must exceed a threshold value which depends only on the rock strength and on the average negative buoyancy of magma for basaltic melt we estimate that this threshold thickness cannot be less than about 700 m and is 2 km on average

osmium isotopes in the aerosols of the mantle volcano mauna loa aerosols and reactive gases from the spring 1984 eruption of mauna loa volcano on hawaii were collected and analyzed for osmium and its isotopic composition the measured187os 186os ratio of1 14 0 03 is close to the ratio in matter with solar systemre os abundance this result shows that the aerosols from mauna loa originated in the mantle and that their composition was not or only slightly influenced by their contact with the crust

saucer shaped sill with lobate morphology revealed by 3d seismic data implications for resolving a shallow level sill emplacement mechanism we use 3d seismic data to describe the 3d geometry of a large igneous intrusion the solsikke compound sill and address a number of issues related to sill emplacement the solsikke compound sill formed by amalgamation of a number of sills and exhibits a complex internal morphology dominated by saucer shaped depressions and linear discontinuities one of the saucer shaped sub elements of the compound sill the solsikke lobate sill has a previously unrecognized morphology it has a basal feeder and consists of a bifurcating network of interlinked lobe shaped sill segments we propose two models for the development of this intrusive style based on analogues from igneous systems and hydrofracturing experiments the lobate pattern indicates that the solsikke lobate sill was fed at its deepest point and adopted its geometry through outwards and upwards propagation the feeder location is coincident with a fault intersection suggesting that magma transport from the underlying source exploited the zone surrounding the intersection

fracture mode analysis and related surface deformation during dyke intrusion results from 2d experimental modelling surface deformation analysis in volcanic edifices in response to shallow magma intrusion is crucial for assessing volcanic hazards in this paper we discuss the effect of dyke propagation mode on surface deformation through 2d laboratory models our experimental setup consists of a hele shaw cell in which a model magma is injected into a cohesive model crust using an optical image correlation technique particle imaging velocimetry we measured the surface deformation the displacements and the strain field induced by magma emplacement within the country rock we identify two types of intrusion morphologies types a and b which exhibit two evolutional stages during the first stage both types resulted in a vertical dyke at depth its propagation was controlled by both shear deformation and tensile opening the model surface lifted up to form a smooth symmetrical dome resulting in tensile cracks during the second stage types a and b experiments differ when the dyke reaches a critical depth in type a the intrusion gradually rotates forming an inclined sheet dipping between 451 and 651 this rotation results in asymmetrical surface uplift and shear failure upon the tip of the dyke in type b the dyke tip interacts with tensile cracks formed during the first stage this fracture controls the subsequent propagation of the dyke toward the surface in both types of experiments intrusions result in surface uplift which can be accommodated by reverse faults our study suggests that dykes propagate as viscous indenters rather than linear elastic fracturing

geometry and growth of sill complexes insights using 3d seismic from the north rockall trough doleritic sill complexes which are an important component of volcanic continental margins can be imaged using 3d seismic reflection data this allows unprecedented access to the complete 3d geometry of the bodies and an opportunity to test classic sill emplacement models the doleritic sills associated with basaltic volcanism in the north rockall trough occur in two forms radially symmetrical sill complexes consist of a saucer like inner sill at the base with an arcuate inclined sheet connecting it to a gently inclined commonly ragged outer rim bilaterally symmetrical sill complexes are sourced by magma diverted from a magma conduit feeding an overlying volcano with an elongate concave upwards trough like geometry bilaterally symmetrical sills climb away from the magma source from which they originate both sill complex types can appear as isolated bodies but commonly occur in close proximity and consequently merge producing hybrid sill complexes radial sill complexes consist of a series of radiating primary flow units with dimensions up to 3 km each primary flow unit rises from the inner saucer and is fed by primary magma tube primary flow units contain secondary flow units with dimensions up to 2 km each being fed by a secondary magma tube branching from the primary magma tube secondary flow units in turn are composed of 100 m scale tertiary flow units a similar branching hierarchy of flow units can also be seen in bilaterally symmetrical sill complexes with their internal architecture resembling an enlarged version of a primary flow unit from a radial sill complex this branching flow pattern as well as the interaction between flow units of varying orders provides new insights into the origin of the structures commonly seen within sill complexes and the hybrid sill bodies produced by their merger the data demonstrate that each radially symmetrical sill complex is independently fed from a source located beneath the centre of the inner saucer grows by climbing from the centre outwards and that peripheral dyking from the upper surface is a common feature these features suggest a laccolith emplacement style involving peripheral fracturing and dyking during inner saucer growth and thickening the branching hierarchy of flow units within bilaterally symmetrical sill complexes is broadly similar to that of primary flow units within a radially symmetrical sill complex suggesting that the general features of the laccolith emplacement model also apply

multiple factors in the origin of the cretaceous tertiary boundary the role of environmental stress and deccan trap volcanism a review of the scenarios for the cretaceous tertiary k t boundary event is presented and a coherent hypothesis for the origin of the event is formulated many scientists now accept that the event was caused by a meteorite impact at chicxulub in the yucatan peninsula mexico our investigations show that the oceans were already stressed by the end of the late cretaceous as a result of the long term drop in atmospheric co2 the long term drop in sea level and the frequent development of oceanic anoxia extinction of some marine species was already occurring several million years prior to the k t boundary the biota were therefore susceptible to change the eruption of the deccan traps which began at 66 2 ma coincides with the k t boundary events it erupted huge quantities of h2so4 hcl co2 dust and soot into the atmosphere and led to a significant drop in sea level and marked changes in ocean temperature the result was a major reduction in oceanic productivity and the creation of an almost dead ocean the volcanism lasted almost 0 7 m y extinction of biological species was graded and appeared to correlate with the main eruptive events elements such as ir were incorporated into the volcanic ash possibly on soot particles this horizon accumulated under anoxic conditions in local depressions and became the marker horizon for the k t boundary an oxidation front penetrated this horizon leading to the redistribution of elements the eruption of the deccan traps is the largest volcanic event since the permian triassic event at 245 ma it followed a period of 36 m y in which the earth s magnetic field failed to reverse instabilities in the mantle are thought to be responsible for this eruption and therefore for the k t event we therefore believe that the k t event can be explained in terms of the effects of the deccan volcanism on an already stressed biosphere the meteorite impact at chicxulub took place after the onset of deccan volcanism it probably played a regional rather than a global role in the k t extinctions

rock interface strength influences fluid filled fracture propagation pathways in the crust abstract rock interface strengths have often been assumed to be zero in numerical and analogue models of fracture propagation and magma intrusion in the crust rock strength tests were performed to explore the role that rock interfaces have on the geometry and propagation dynamics of fluid filled fractures in the crust we used a 1 kn test machine to study 5 mm thick cuboidal specimens cut from a sandstone siltstone rock core where the strata were known to host magma intrusions and the rock interface between the units was intact by measuring the load required to grow a crack running along the lithological contact between the layers we calculate its fracture toughness k c the siltstone had an average k c of 0 56 0 03 mpa m 1 2 compared to the sandstone at 0 42 0 02 mpa m 1 2 the rock interface had intermediate average fracture toughness to the parent units at 0 45 0 03 mpa m 1 2 these results have important implications on fracture propagation pathways through rocks as well as for the geometry and propagation dynamics of magma intrusions in the crust

determining magma flow in sills dykes and laccoliths and their implications for sill emplacement mechanisms three dimensional seismic data from the faeroe shetland basin provides detailed information on the relationships between sills dykes laccoliths and contemporaneous volcanic activity the data shows that sills are predominantly concave upwards being complete or partial versions of radially or bilaterally symmetrical forms that possess flat inner saucers connected to a flat outer rim by a steeply inclined sheet such morphologies are only partially modified by pre existing faults sills can be sourced from dykes or the steep climbing portions of deeper sills both sills and dykes can provide magma to overlying volcanic fissures and sills can be shown to feed shallow laccoliths magma flow patterns as revealed by opacity rendering suggest that sills propagate upwards and outwards away from the magma feeder as an individual sill can consist of several leaves emplaced at different stratigraphic levels and as a sill or dyke can provide magma to volcanic fissures other sills and laccoliths the data suggests that neutral buoyancy concepts may not provide a complete explanation for the mechanism and level of sill emplacement instead the data suggests that the presence of lithological contrasts particularly ductile horizons such as overpressured shales may permit sill formation at any level below the neutrally buoyant level

correction to modeling the growth of laccoliths and large mafic sills role of magma body forces in the paper modeling the growth of laccoliths andlarge mafic sills role of magma body forces by andrew p bunger and alexander r cruden journal of geophysicalresearch 116 b02203 doi 10 1029 2010jb007648 thetwo sentences preceding equation 14 should be replacedwith finally equation 10 shows that the quantity p

physical controls and depth of emplacement of igneous bodies a review the formation and growth of magma bodies are now recognised as involving the amalgamation of successive discrete pulses such as sills sills would thus represent the building blocks of larger plutons sensu lato mechanical and thermal considerations on the incremental development of these plutons raise the issue of the crustal levels at which magma can stall and accumulate as sills reviewing the mechanisms that could a priori explain sill formation it is shown that principal physical controls include rigidity contrast where sills form at the interface between soft strata overlaid by comparatively stiffer strata rheology anisotropy where sills form within the weakest ductile zones and rotation of deviatoric stress where sills form when the minimum compressive stress becomes vertical comparatively the concept of neutral buoyancy is unlikely to play a leading control in the emplacement of sills although it could assist their formation these different controls on sill formation however do not necessarily operate on the same length scale the length scale associated with the presence of interfaces separating upper stiffer layers from lower softer ones determines the depth at which rigiditycontrolled sills will form on another hand the emplacement depths for rheology controlled sills is likely to be determined by the distribution of the weakest ductile zones whereas the emplacement depth of stress controlled sills is determined by a balance between the horizontal maximum compressive stress which favours sill formation and the buoyancy of their feeder dykes which drives magma vertically ultimately the depth at which a sill forms depends on whether crustal anisotropy or stress rotation is the dominant control i e which of these processes operates at the smallest length scale using dimensional analysis it is shown that sill formation controlled by remote stress rotation would occur on length scales of hundreds of meters or greater this therefore suggests that crustal heterogeneities and their associated anisotropy are likely to play a larger role than remote stress rotation in controlling sill emplacement unless these heterogeneities are several hundred meters or more apart this also reinforces the role of local stress barriers owing to interactions between deviatoric stress and crustal heterogeneities in the formation of sills

control of pore fluid pressure on depth of emplacement of magmatic sills an experimental approach article in sedimentary basins the emplacement of magmatic sills tends to occur within rock of low mechanical competence and permeability such as shale this often contains pore fluids at abnormally high pressure we first theoretically show that in anisotropic media the higher the pore fluid pressure the deeper the sill emplacement then we introduce a new technique for analogue modelling of such intrusive bodies under conditions of fluid overpressure greater than hydrostatic which corroborate the theoretical analysis as an analogue of brittle sediment we use a diatomite powder this material is a dry fine grained frictional cohesive and permeable material as an analogue for magma we take a silicone putty rtv silicone which is at first newtonian but then solidifies at room temperature we use compressed air as a pore fluid under these experimental conditions we investigate the intrusion of magma into the host powder under various fluid overpressures in homogeneous diatomite powder having uniform cohesion intrusive bodies are segmented dykes these become feeders to sills if the fluid overpressure exceeds the weight of overburden where the sedimentary column has a horizontal discontinuity in strength the transition from dyke to sill occurs at a smaller overpressure hydrostaticb exb1 as a possible illustration of these results we consider sills within source rocks of the neuquen basin and of the parana basin both in south america where overpressure may have resulted from maturation of organic material

koolau dike complex oahu intensity and origin of a sheeted dike complex high in a hawaiian volcanic edifice the dike complex in the eroded koolau tholeiitic shield volcano is unique among described dike swarms for its coherence high dike injection intensity relatively uniform and non gaussian 50 to 65 intensity level and lack of dike divergence parts are like a sheeted dike complex though high in an intraplate volcanic edifice a model is proposed in which highly mobile tholeiitic magma having considerable freedom to move through the strongly disjointed volcano seeks zones in which it is gravitationally most stable these zones are marginal parts of the dike complex separating lavas having a lower than magmatic bulk density from a 50 dike complex having a higher than magmatic bulk density this model also explains the strong propensity for the magma of kilauea volcano to remain underground in a gravitationally stable zone magma erupts only when it vesiculates sufficiently often when it has degassed some of it promptly plunges back toward the stable zone again

osmium tungsten and chromium isotopes in sediments and in ni rich spinel at the k t boundary signature of a chondritic impactor it is now established that a large extraterrestrial object hit the earth at the end of the cretaceous period about 65 ma ago we have investigated re os hf w and mn cr isotope systems in sediments from the cretaceous and the paleogene in order to characterize the type of impactor within the cretaceous tertiary k t boundary layer extraterrestrial material is mixed with terrestrial material causing a dilution of the extraterrestrial isotope signature that is difficult to quantify a phase essentially composed of ni rich spinel formed in the atmosphere mainly from melted projectile material is likely to contain the extraterrestrial isotopic signature of the impactor we show that the analysis of spinel is indeed the best approach to determine the initial isotope composition of the impactor and that w and cr isotopes confirm that the projectile was a carbonaceous chondrite

dynamics of magmatic intrusions in the upper crust theory and applications to laccoliths on earth and the moon 1 to understand the dynamics of shallow magmatic intrusions i propose a theoretical model of magma spreading laterally below an elastic crust nondimensionalization of the flow equation leads to the identification of characteristic scales for the intrusion while the characteristic intrusion length is controlled by the elastic response of the crust its characteristic thickness primarily depends on magma properties and injection rate three spreading regimes are identified and characterized by different morphologies as well as by scaling laws for thickness versus length and thickness versus time the first spreading regime is controlled by the elastic response of the crust and the shape of the flow is self similar when the time length and thickness become larger than an elastic time length and thickness scale the edges of the flow become steeper and the system transitions to a gravity current regime when the intrusion is thick enough to accommodate the pressure head the flow enters a regime of lateral propagation and keeps a constant thickness the intrusion shape in the elastic regime fits the observed shape of terrestrial laccoliths the elastic scaling law for intrusion thickness versus length fits observations of laccoliths at elba island italy and provides for a physical explanation for the observed relationship between length and thickness on terrestrial laccoliths laccoliths are predicted to form over short time scales depending on magma viscosity that vary between approximately a month to several years for felsic magmas on earth on the moon several elongated low slope domes have recently been identified as possibly formed by laccolith intrusions at depth although they are much larger than terrestrial laccoliths because the moon has a smaller gravity than on the earth a deeper magma source and a more mafic magma composition than for terrestrial laccoliths implying smaller pressure gradient and dike width lunar intrusions have a larger characteristic length and a smaller characteristic thickness after nondimensionalization the morphologies length versus thickness of terrestrial and inferred lunar laccoliths follow the same curve and are well fitted by the elastic scaling law this model thus explains the size discrepancy between terrestrial laccoliths and lunar low slope domes therefore low slope domes identified on the moon are good candidates for laccolith type intrusions at depth

a multi isotopic and trace element investigation of the cretaceous tertiary boundary layer at stevns klint denmark inferences for the origin and nature of siderophile and lithophile element geochemical anomalies abstract os sr nd and pb isotope data were collected from a profile across the cretaceous tertiary k t boundary layer at stevns klint denmark nd t 65 ma values from within the boundary layer fish clay are lower by 1 unit than those of the underlying maastrichtian limestone and the overlying danian chalk sequences systematic profile upward changes of pb sr and os isotopic compositions and concentrations in the boundary layer cannot be accounted for by in situ growth of daughter products since the sedimentation of the fish clay while os nd and pb isotopes indicate the admixing of less radiogenic components to the fish clay sr isotopes show elevated radiogenic values in the boundary layer relative to the carbonate sequences beneath and above it the sudden change in lithophile e g sr pb and nd isotope compositions at the base of the fish clay and profile upward trends of 87sr 86sr and 206pb 204pb ratios towards those of the overlying danian chalk are interpreted to reflect recovery from enhanced acid rain induced continental local weathering input to the seawater however a continental crustal source is invalid for the siderophile element os in the light of evidence from chromium isotopes for a cosmic origin of the platinum group elements pges and certain moderately siderophile elements cr ni co v in k t boundary sediments including stevns klint shukolyukov and lugmair science 282 1998 927 929 and supported by the finding of projectile debris bauluz et al earth planet sci lett 182 2000 127 136 and the occurrence of abundant ni rich spinel at many k t sites robin et al nature 363 1993 615 617 kyte nature 396 1998 237 239 we favor to explain the sudden drop of 187os 188os ratios from 0 210 to 0 160 at the k t boundary to derive from global fall out of extraterrestrial matter the present 186os 188os ratio of 0 119836 0 000004 measured in the basal layer of the fish clay is within the uncertainty a chondritic value we therefore exclude the possibility of a major contribution of pges to the sediment from iron meteorites chondrite normalized ru ir n ratios of 0 95 0 14 and os ir n ratios of 0 93 0 14 in the fish clay cannot distinguish between abundance ratios of different types of chondrites and strongly sub chondritic pt ir n ratios of 0 62 0 09 2 suggest differential pge remobilization through the sedimentary column and consequently the alteration of inter element ratios pges and the moderately siderophile elements cr ni v and co form an elemental association with systematically upward decreasing concentrations in the fish clay low co ni ratios of 0 12 in the fish clay relative to values of 0 35 in the over and underlying carbonate sequences support mixing of meteorite derived co ni 0 05 and terrestrial upper mantle crustal co ni 0 3 sources while lithophile element isotope data indicate an increased continental crustal input to the fish clay at the k t transition the uncertainty with respect to possible post depositional alteration of abundance patterns of siderophile and moderately siderophile elements though not affecting the chondritic isotopic composition of os does not allow confirmation of indications from chromium isotopes for a carbonaceous cv type meteorite as the preferred k t impactor type by shukolyukov and lugmair science 282 1998 927 929

experimental modelling of shallow magma emplacement application to saucer shaped intrusions abstract saucer shaped dolerite and sandstone intrusions are common in sedimentary basins world wide we have conducted a series of scaled experiments simulating the process of magma emplacement in sedimentary basins with particular attention on the formation of saucer shaped sills the model materials were 1 cohesive fine grained silica flour representing brittle crust and 2 molten low viscosity oil representing magma the experiments were performed in both homogeneous and layered models in all the experiments oil injection resulted in doming of the surface in the homogeneous models the injected oil formed cone sheets and sub vertical dykes cone sheets formed for shallow injection 1 3 cm and vertical dykes formed for deeper injection 4 5 cm in layered models the injected oil always formed saucer shaped intrusions our experimental results show that 1 sill intrusion results in the formation of a dome with melt erupting at the rim 2 layering controls the formation of sills and saucer shaped sills 3 saucer shaped sills are fed from the bottom and the fluid flows upward and outward and 4 the diameter of saucer shaped sills increase with increasing emplacement depth the systematic relation between domes and sills and the depth dependence of sill diameters show that saucer shaped intrusions result from the interaction between a growing flat lying shallow sill and doming of the free surface we conclude that saucer shaped intrusions represent fundamental geometries formed by shallow magma intrusion in stratified basins

an experimental investigation of sill formation and propagation in layered elastic media a series of experiments are described where dyed water a magma analogue was intruded into solid gelatine a crustal analogue to investigate the formation of sills we considered a layered gelatine system with contrasting adjacent layers by varying the density and rigidity of the gelatine we found that experimental sills form when the upper layer is more rigid than the lower layer with intrusion occurring in a plane directly below the interface experimental dykes were observed to propagate to the surface when the young s modulus ratio of upper to lower gelatine layers was less than one experimental dyke arrest occurred when the upper layer was more rigid and the interface was strong two varieties of experimental sill formed when the upper layer was more rigid than the lower layer and the interface was sufficiently weak the form of the intrusion depends on the balance of driving pressures and the young s modulus ratio of contrasting adjacent layers when the rigidity ratio is high and there is a large driving pressure the experimental feeder dyke completely converts to propagate as a sill however when the rigidity ratio and driving pressure are both close to one a dyke sill hybrid forms under these conditions the experimental sill formation is accompanied by contemporaneous dyke intrusion into the overlying more rigid layer during sill propagation deformation structures such as faults and en echelon fractures are formed into the lower layer experimental sill propagation dynamics are controlled by viscous dissipation along the length of the sill causing acceleration with increasing length our study suggests that rigidity contrasts may play a major role in the location of sills and development of igneous complexes in ancient cratonic areas the moho is a suitable site for the preferential formation of sills with higher rigidity continental crust overlying weaker mantle mantle plumes impacting ancient continents provide a situation in which large sills can form to fractionate prior to eruption of flood basalts the boundary between the upper and lower crust conrad discontinuity may provide a preferential focus for the emplacement of sheets of silicic magma at continental arcs where the lower crust is weakened by prolonged heating and possible hydration

a complete os excursion across a terrestrial cretaceous paleogene boundary at the west bijou site colorado with evidence for recent open system behavior abstract the few previously reported values of 187os 188os ratios from non marine k pg boundary sections are distinctly higher than the range of 187os 188os ratios measured in chondrites and the range of ratios predicted by models of physical mixing between chondrites and upper crust here re os data from the west bijou continental k pg boundary site located within the denver basin are used to better constrain the os isotopic composition of fallout from the chicxulub ejecta plume for the first time full vertical profiles of 187os 188os ratios and re and os concentrations across a continental k pg boundary section are reported within this section of lignite the lowest measured 187os 188os ratio 0 182 coincides with the k pg boundary interval as previously determined by palynology and the distribution of shocked quartz in a nearby outcrop however sediments with elevated os concentrations and measured 187os 188os below 0 23 extend over a 30 cm interval from 5 cm above the clay rich boundary interval 5 cm thick to 25 cm below maximum ir and os concentrations occur 3 cm and 10 cm below the k pg boundary respectively demonstrating greater diagenetic mobility of os relative to ir low 187os 188os above the k pg boundary suggests that accumulation of impact derived os at this site persisted after the impact event perhaps due to redistribution of impact debris in the surrounding area importantly calculated initial 187os 188os ratios throughout most of the section are impossibly low and require open system behavior likely in the form of re addition within the last 10 million years within the boundary interval re concentrations are among the lowest measured at bowring pit and calculated initial 187os 188os ratios are very similar to those previously reported from the western interior of the united states k pg boundary material 0 14 suggesting super chondritic 187os 188os is characteristic of the chicxulub ejecta plume beyond implications for our understanding of os as an impact tracer this study emphasizes the dramatic effect diagenesis can have on the re os system in organic rich environments especially through post depositional addition of re

the marine187os 186os record of the past 80 million years we report new187os 186os data and re and os concentrations in metalliferous sediments from the pacific to construct a composite os isotope seawater evolution curve over the past 80 m y analyses of four samples of upper cretaceous age yield187os 186os values of between 3 and 6 5 and187re 186os values below 55 mass balance calculations indicate that the pronounced minimum of about 2 in the os isotope ratio of seawater at the k t boundary probably reflects the enormous input of cosmogenic material into the oceans by the k t impactor s following a rapid recovery to187os 186os of 3 5 at 63 ma data for the early and middle part of the cenozoic show an increase in187os 186os to about 6 at 15 ma variations in the isotopic composition of leachable os from slowly accumulating metalliferous sediments show large fluctuations over short time spans in contrast analyses of rapidly accumulating metalliferous carbonates do not exhibit the large oscillations observed in the pelagic clay leach data these results together with sediment leaching experiments indicate that dissolution of non hydrogenous os can occur during the hydrogen peroxide leach and demonstrate that os data from pelagic clay leachates do not always reflect the os isotopic composition of seawater new data for the late cenozoic further substantiate the rapid increase in the187os 186os of seawater during the past 15 ma we interpret the correlation between the marine sr and os isotope records during this time period as evidence that weathering within the drainage basin of the ganges brahmaputra river system is responsible for driving seawater sr and os toward more radiogenic isotopic compositions the positive correlation between87sr 86sr and u concentration the covariation of u and re concentrations and the high dissolved re u and sr concentrations found in the ganges brahmaputra river waters supports this interpretation accelerating uplift of many orogens worldwide over the past 15 ma especially during the last 5 ma could have contributed to the rapid increase in187os 186os from 6 to 8 5 over the past 15 ma prior to 15 ma the marine sr and os record are not tightly coupled the heterogeneous distribution of different lithologies within eroding terrains may play an important role in decoupling the supplies of radiogenic os and sr to the oceans and account for the periods of decoupling of the marine sr and os isotope records

saucer shaped intrusions occurrences emplacement and implications abstract magmatic intrusions and fluidized injected sands represent the two main types of sheet intrusions in the earth s crust in this paper we show that both intrusion types often display a saucer like geometry as revealed by 3d seismic imaging and field observations saucer shaped intrusions are fairly common in sedimentary basins as for example offshore in the norwegian and north sea basins and onshore in the karoo basin of south africa the formation of the saucer geometry is controlled by the low viscosity of the injected fluid and by the interaction between a growing shallow hydraulic fracture and overburden deformations statistics gathered from observations and modelling show a linear relationship between the depth of emplacement and the size of the saucer shaped intrusions we anticipate future cross disciplinary studies aiming to discover other occurrences of saucer shaped intrusions and to identify the physical processes controlling the development of this fundamental geometry

constraints on an intrusive system beneath the soufriere hills volcano montserrat from finite difference modeling of a controlled source seismic experiment 1 the sea calipso wide angle seismic experiment revealed a high velocity region beneath the island of montserrat field recordings show a decrease in the amplitude of seismic signals crossing this high velocity region we constrain the geometry and nature of this attenuating body by forward modeling of the seismic wave field with a viscoelastic finite difference method we interpret the attenuation observed as caused by a scattering region which we model as a stochastic perturbation of the velocity field with characteristic length scale of 0 4 km the scattering region approximately coincides with the top part of the high velocity region and is estimated to have a volume of about 800 km3 we argue that the scattering is caused by geological heterogeneities corresponding to a system of dikes and sills and to the complex structure of the volcanic edifice

controls of conduit geometry and wallrock elasticity on lava dome eruptions abstract many lava dome building eruptions show periodic to complex non periodic pulsatory activity typical time scales associated with this activity range from hours to decades previous studies modelled the ascent of magma using a set of transient 1 d transport equations accounting for degassing induced crystallization kinetics gas exsolution and viscosity increase due to crystal growth these models assumed flow in a cylindrical conduit with a fixed cross section area since several observations suggest that extrusions are mainly fed by dykes with cylindrical geometries developing only at shallow levels here we generalised the model to the flow geometry represented by an elliptical dyke with major and minor semi axes changing with depth quasi static elastic deformation of the dyke is accounted by an analytical solution that couples cross section area with the magmatic overpressure the effects of the main dyke geometrical parameters and boundary conditions on the eruption dynamics were investigated the presence of a deformable dyke can lead to a more complex periodic behaviour with a wider range of time scales and cyclicity patterns with respect to a uniform cylindrical conduit there is a regime where the period of pulsations is controlled by the elasticity of the dyke and a regime where the period is controlled by the volume of the magma chamber intermediate regimes are possible periodic variations in discharge rate are also possible for both fixed pressure in dyke source region and fixed influx rate into the dyke our study emphasizes the strong non linearities and complex behaviours of lava dome eruptions from a forecasting and hazard perspective intrinsic uncertainties in governing parameters may make volcanic systems in some circumstances unpredictable on the other hand lava dome systems may also develop episodic and systematic behaviours so that behaviour becomes predictable for a while

rethinking the emplacement and evolution of zoned plutons geochronologic evidence for incremental assembly of the tuolumne intrusive suite california new u pb geochronologic data indicate that the tuolumne intrusive suite california was assembled over a period of at least 10 m y between 95 and 85 ma and that the half dome granodiorite intruded over a period approaching 4 m y simple thermal considerations preclude the possibility that a magma chamber the size of the half dome pluton could have existed as a liquid at shallow crustal depths for that long rather field evidence for sheeting along the margins of the suite the range of ages and the regular decrease of ages toward the center of the suite and within individual plutons suggest incremental assembly geochronologic evidence for incremental assembly is consistent with the failure of geophysical methods to detect large magma chambers with more than 20 melt even in active volcanic areas because it is unlikely that the individual plutons composing the tuolumne ever coexisted as liquid rich magmas the chemical evolution of the suite cannot be the result of simple fractionation and or mixing between exposed units but instead must reflect processes occurring during magma generation

187os 186os in marine manganese nodules and the constraints on the crustal geochemistries of rhenium and osmium during their formation marine manganese or more properly ferromanganese oxide nodules include and commonly concentrate elements dissolved in seawater this attribute makes them especially valuable recorders of the isotopic composition of elements supplied to the oceans having radiogenic isotopes for these can be used in exploring crustal composition and evolution1 5 from measurements of 187os 186os ratios in marine manganese nodules and comparison with the 143nd 144nd ratios of the same nodules we have calculated the os isotopic composition and the osmium and rhenium concentrations of the weatherable portion of the continental crust together with the proportions of ultra mafic and granitic rock that constitute this crustal mass