magmatic effects of the lunar late heavy bombardment abstract large volumes of mare basalts are present on the surface of the moon located preferentially in large impact basins mechanisms relating impact basins and mare basalt eruptions have previously been suggested lunar impacts removed low density material that may have inhibited eruption and created cracks for fluid flow icarus 139 1999 246 and lunar basins have long been described as catchments for magma e g rev geophys space phys 18 1980 107 and references therein we present a new model for melt creation under near side lunar basins that is triggered by the impacts themselves magma can be produced in two stages first crater excavation depressurizes underlying material such that it may melt in situ second the cratered lithosphere rises isostatically warping isotherms at the lithosphere asthenosphere boundary which may initiate convection in which adiabatic melting can occur the first stage produces by far the largest volume of melt but convective melting can continue for up to 350 ma we propose that giant impacts account for a large portion of the volume and longevity of mare basalt volcanism as well as for several compositional groups including high alumina high titanium kreep rich and picritic magmas

extraterrestrial influences on mantle plume activity abstract we use time series analysis to compare the impact histories of the earth and moon with the record of mantle plume activity we use events with errors in their ages of 150 ma the terrestrial and lunar impact records when smoothed at a 45 ma interval correlate at a 97 confidence level this high confidence level suggests that we have an adequate sampling of most of the major impact events on the earth we then test the idea that existing mantle plumes may be strengthened by impacts when smoothed at a 45 ma interval strong plumes correlate with the terrestrial impact record at better than a 99 confidence level no time lag is discernible between the data sets which is expected given their present error level when the time series are smoothed at a 30 ma interval there are 10 major peaks in impact activity nine out of ten of these peaks have a counterpart in either or both of the strong mantle plume or the mantle plume time series as a result the strong mantle plume and the impact time series correlate at the 97 confidence level the mantle plume and the impact time series correlate at the 90 confidence level finally the deccan plume showed greatly increased activity immediately after the chixculub impact the results of our analysis suggest that large meteorite and cometary impacts may well increase the amount of volcanism from already active mantle plumes

geochronology and stratigraphic relationships of the sulphur springs group and strelley granite a temporally distinct igneous province in the archaean pilbara craton australia abstract rocks from the sulphur springs group a newly identified stratigraphic succession hosting volcanogenic massive sulphide vms base metal deposits in the soanesville belt pilbara craton northwestern australia have been analyzed by sensitive high resolution ion microprobe shrimp u pb zircon mass spectrometry to determine their age and tectonostratigraphic relationships most volcanism occurred between 3238 and 3235 ma distinctly different from the time interval spanned by the wyman formation type section is 3315 ma other dates 3325 ma with which it was previously correlated outlying volcanic rocks with similar geochemistry at the bernts and obelix prospects also produce dates from 3240 to 3235 ma and are therefore best interpreted as structurally detached fragments of the sulphur springs group the strelley granite an intrusion which shares geochemical characteristics with the overlying volcanic rocks also yields similar ages thus extending the period of temporally linked extrusive intrusive silicic magmatism in the evolution of the pilbara craton model pb pb dating of galena from the zn cu mineralization at the various vms prospects has yielded ages consistent with the zircon u pb ages of the host rocks indicating that the hydrothermal mineralizing systems were contemporaneous with and apparently initiated by igneous activity an epicontinental tectonic setting for the sulphur springs group during vms mineralization is implied by the broad range of old zircon ages obtained from a tuff at the base of the volcanic pile as the igneous episode coincides with the peak regional metamorphic event recorded in other east pilbara supracrustal rocks and with voluminous intrusive activity in nearby granitoid complexes it marks a major event in the crustal evolution of the eastern pilbara craton

geological and geochemical record of 3400 million year old terrestrial meteorite impacts beds of sand sized spherules in the 3400 million year old fig tree group barberton greenstone belt south africa formed by the fall of quenched liquid silicate droplets into a range of shallow to deep water depositional environments the regional extent of the layers their compositional complexity and lack of included volcanic debris suggest that they are not products of volcanic activity the layers are greatly enriched in iridium and other platinum group elements in roughly chondritic proportions geochemical modeling based on immobile element abundances suggests that the original average spherule composition can be approximated by a mixture of fractionated tholeiitic basalt komatiite and ci carbonaceous chondrite the spherules are thought to be the products of large meteorite impacts on the archean earth

asteroid impact ejecta units overlain by iron rich sediments in 3 5 2 4 ga terrains pilbara and kaapvaal cratons accidental or cause effect relationships abstract the significance of temporal and spatial associations between asteroid comet impact ejecta units and overlying iron rich sediments including banded iron formation bif jaspilite and ferruginous shale observed in the pilbara craton western australia and the barberton greenstone belt bgb kaapvaal craton south africa is considered such associations include 1 3470 1 1 9 ma impact spherule units and tsunami type breccia overlain by jaspilite in the antarctic chert member central pilbara craton 2 3258 3 ma impact spherule unit s2 in the bgb overlain by bif jaspilite and ferruginous shale 3 3243 4 ma impact unit s3 in the bgb overlain by iron rich sediments ulundi formation 4 two formations of bif and ferruginous shale at the base nimmingarra iron formation and lower part paddy market formation of the gorge creek group central pilbara craton which overlie 3235 3 ma felsic volcanics along a boundary correlated with the bgb s3 unit 5 2629 5 ma impact spherule unit and tsunami type deposit overlain by bif and ferruginous shale marra mamba iron formation central pilbara craton 5 2481 4 ma spherule unit intercalated at the lower part of the dales gorge iron member of the brockman iron formation hamersley basin and possible equivalents in the kuruman formation western transvaal no significant thicknesses of iron rich sediments are known to overlie carbonate hosted impact ejecta units including the bee gorge member bgm of the wittenoom formation 2561 8 ma carawine dolomite eastern hamersley basin pilbara craton monteville formation and reivilo formation griqualand west basin transvaal group or the graensco vallen spherule occurrence southwest greenland the juxtaposition between impact ejecta units and overlying bif and jaspilite may be accidental or purely related to preservation of these units in below wave base environments alternatively this association may hint at enrichment of seawater in soluble iron under low oxygen atmosphere hydrosphere conditions due to enhanced denudation of mafic volcanic terrains uplifted exposed through impact induced tectonic movements and or impact triggered mafic volcanic and hydrothermal activity the general absence of iron rich sediments above carbonate hosted impact ejecta units may be attributed to lack of fe enrichment or to ph and eh conditions unsuitable for the precipitation of iron oxides the near absence of bif in high energy shallow water environments overlying impact ejecta units may reflect retardation of colloidal chemical sedimentation in such environments a systematic association between impact ejecta and iron rich sediments if confirmed may yield useful stratigraphic tracers in the search for impact signatures in early precambrian terrains

gas reservoir potential of the lower ordovician beekmantown group quebec lowlands canada the beekmantown group in the quebec lowlands was deposited as part of an extensive early ordovician coastal and shallow marine complex on the eastern margin of the north american craton the beekmantown is stratigraphically equivalent to the beekmantown knox arbuckle and ellenburger rocks of the united states and is subdivided into two formations the sandstone rich theresa formation and the overlying dolomite rich beauharnois dolomites of the beekmantown provide an important exploration target in both the autochthon and the overlying thrust sheets of the canadian and u s appalachians the reservoir potential of the autochthonous beekmantown group in the quebec lowlands can be determined from seismic data well logs cuttings and petrographic analyses of depositional and diagenetic textures deposition of the beekmantown occurred along the western passive margin of the iapetus ocean by the late ordovician the passive margin had been transformed into a foreland basin faulting locally positioned upper ordovician utica source rocks against the beekmantown and contributed to forming hydrocarbon reservoirs the largest beekmantown reservoir found to date is the st flavien field with 7 75 bcf of original gas methane in place in fractured and possibly karst influenced allochthonous dolomites within a thrust fault anticline the beekmantown below the thrust sheets forms a northward thinning wedge of peritidal and subtidal deposits seven major depositional units can be distinguished in cuttings and correlated with wireline logs most of these units form northward thinning sediment wedges and were deposited on a gently dipping ramp quartz sandstones dominate updip whereas shallow subtidal pelletal to skeletal limestones dominate downdip a widespread blanket of shaly dolomite is the uppermost unit of the beekmantown but is of poor reservoir quality dolomites in the beekmantown contain vuggy moldic intercrystalline and fracture porosity early porosity formed at the top of the major depositional units in peritidal dolomites however much of this porosity was later filled by late stage calcite cement after hydrocarbon migration thus a key to finding gas reservoirs in the autochthonous beekmantown is to define ordovician paleostructures in which early and continuous entrapment of hydrocarbons prevented later cementation

a multielement geochronologic study of the great dyke zimbabwe significance of the robust and reset ages abstract new sm nd u pb and pb pb age determinations indicate that the great dyke of zimbabwe an elongate intrusion of mafic and ultramafic rocks some 550 km long and between 3 and 10 km wide is over 100 ma older than previously believed based on rb sr ages the intrusion was emplaced as a series of subchambers with similar stratigraphy comprising a lower ultramafic sequence with cyclic layering of dunite or harzburgite grading upwards into bronzitite the top sections of which include pt enriched sulfide zones and an upper mafic sequence of pyroxenites capped by olivine gabbro and gabbronorite the sm nd method has yielded a combined mineral whole rock isochron of 2586 16 ma and e nd t of 1 1 for samples from the darwendale sebakwe and wedza subchambers as well as the satellite east dyke this isochron age is in excellent agreement with the u pb age for three concordant rutile fractions extracted from a feldspathic pyroxenite of the selukwe subchamber with an error weighted mean at 2587 8 ma two zircon fractions from the same feldspathic pyroxenite sample as the rutile are discordant and although not well constrained suggest pb loss from the zircons at ca 830 ma this may be related to the onset of the widespread and diachronous pan african tectonothermal event in southern africa whole rock samples and clinopyroxene and plagioclase separates from a darwendale subchamber drill core yielded a 207 pb 204 pb vs 206 pb 204 pb isochron age of 2596 14 ma which is in agreement with the sm nd isochron and the rutile u pb crystallization age this new age information shows that emplacement of the great dyke and its satellite dikes closely followed the amalgamation of the kaapvaal and zimbabwe cratons and was contemporaneous with emplacement of the youngest of the trondhjemite tonalite granodiorite granitoid suite in the zimbabwe craton assuming that amalgamation of the kaapvaal and zimbabwe cratons was largely by nnw directed convergence it follows that the source of the great dyke was asthenospheric mantle hydrated and enriched in incompatible elements by subduction processes isochrons of 206 pb 204 pb vs 238 u 204 pb and 207 pb 204 pb vs 235 u 204 pb yield ages with large errors but well constrained initial pb ratios 206 pb 204 pb 14 15 0 30 and 207 pb 204 pb 15 04 0 06 assuming a two stage model for common lead evolution this result yields a value of 9 5 along with the calculated initial sr and nd isotopic compositions these data are consistent with derivation of the great dyke magmas by large volume melting of a mantle that has been hydrated and enriched by subduction while a small amount of crustal contamination of magma derived from depleted mantle could produce the composition of the great dyke the uniformity of initial ratios between subchambers supports the notion of enrichment in incompatible elements being an intrinsic characteristic of the mantle source

spinel from archean impact spherules abstract large meteorite impacts are recorded in distal fall deposits preserved in two early archean greenstone belt sequences 3 2 3 5 ga mapping stratigraphie and geochemistry studies have been used to identify at least four impact layers in the barberton greenstone belt of southern africa and one in the pilbara greenstone belt of western australia spherules that represent impact melt droplets distinguish these impact layers from associated tuffs spinel with unusual compositional traits occurs within spherules in two of these impact layers spinel is the only preserved primary phase and is found within up to 10 of the spherules in the best preserved layers some spherules contain up to 40 spinel which is often the only primary phase that crystallized within the spherules spinel morphologies include dendrites and skeletal octahedra typical of those found in high temperature melts spinel compositions are similar to those of cenozoic impacts including very high ni and ferric iron contents but the archean spinel is distinctive in being composed dominantly of the chalcophile elements fe ni cr and v this spinel is not metamorphic or metasomatic in origin closely associated komatiites in the underlying onverwacht group both fresh and highly altered have unaltered spinel with igneous compositions and morphologies quite distinct from the impact spinel characteristics of spinel preserved in these archean impact spherules suggest a rather complex series of steps following a large bolide impact initial high temperature reduction produced melt droplets some of which were immiscible silicate metal sulfide density differences allowed separation of some of these immiscible melts which subsequently evolved independently and formed spherules with highly variable chalcophile element contents oxidation during atmospheric reentry increased ni fe and fe 3 fe 2 in the chalcophile siderophile melts archean impact spinel has higher ni fe than cenozoic impact spinel though distinctly lower fe 3 fe 2 which probably reflects a substantially less oxidizing atmosphere during the archean the archean atmospheric oxidation may in fact be a transient phenomenon associated with a large impact into the terrestrial ocean resulting in very high atmospheric h 2 o h 2 bolide size is estimated to be 24 km in diameter based on the average diameter of 0 85 mm for the spinel bearing spherules this estimate is consistent with the observed ir fluence in spinel bearing impact layers of approximately 5 3 mg cm2 which would be produced by a 30 km diameter chondritic bolide

genetic modelling for banded iron formation of the hamersley group pilbara craton western australia abstract the banded iron formation bif of the hamersley group pilbara craton western australia particularly from the well studied dales gorge member is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60 000 km 2 enabling reasonable estimates for the annual input of components to the depository in the model of this paper varying supply of materials for the medley of mesoband types particularly of iron and silica in the oxide bif can be accommodated by the interaction of two major oceanic supply systems 1 surface currents and 2 convective upwelling from mid oceanic ridge mor or hot spot activity both modified by varied input of pyrochastic material 1 the surface currents were saturated in silica and carried minimal iron due to photic precipitation but were periodically recharged by storm mixing precipitation from them gave rise to the banded chert rich horizons including the varves whose regular and finely laminated iron silica distribution resulted from seasonal meteorological influences 2 precipitation from convection driven upwelling of high iron solution from mor or hot spot activity periodically overwhelmed the delicate seasonal patterns of 1 to produce the iron dominated mesobands a wide range of intermediate mesoband types resulted where the deep water supply was modified by varied mor activity or by partial blocking of upwelling waters by surface currents such as by the present el nino during these periods of oxide dominated bif silica was deposited from saturated solution mainly by evaporative concentration and iron by oxidation due to photolysis and photosynthetically produced oxygen superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources changing the meteorological and depositional conditions occasional input of extremely fi ash during bif precipitation produced mesoband cm scale variations involving increased carbonate silicate precipitation sustained volcanic periods resulted in s macroband deposition chert carbonate silicate bif with shale gradually returning to the dominant hematite magnetite chert bif as the volcanic input waned during volcanic periods the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron and by photosynthetic production of oxygen was modified by turbidity in the atmosphere aerosols and dust and in the water colloids from reactive ash s surface precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone augmenting the iron content of the zone precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub photic zone exceeded their respective solubilities during volcanism the increased availability of nutrients particularly phosphorus to surface waters increased the organic contribution despite lower light values leading to an almost total absence of ferric iron oxides in the s macrobands i e no magnetite or hematite cooling of warm silica saturated sea water during these periods of olcanic winter increased the ratio of precipitation of silica to iron which however was still controlled by seasonal conditions intermediate concentrations of organic matter insufficient to totally convert the ferric compounds either during precipitation or diagenesis resulted in overgrowths of magnetite on hematite and eventually in the substantial conversion of hematite to magnetite where higher temperatures were achieved during low grade regional metamorphism changes in sea level to explain facies changes in bif are not required in this model but are not excluded the preferred conditions are for a very low oxygen to anoxic atmosphere a much higher level of mor activity than at present the presence of photosynthetic plankton the absence of si silica secreting organisms and a deep sea water temperature higher than 20 c however none of these conditions is essential to the model a narrow carbonate bank is postulated for part of the fortescue river valley area during marra mamba iron formation times basal hamersley group with bif precipitation on either side the reef is postulated to have grown northward becoming a major shallow water carbonate platform on the pilbara continent during upper marra mamba iron formation and wittenoom dolomite times but ceased to play an important role in subsequent periods

could bacteria have formed the precambrian banded iron formations banded iron formations bifs are prominent sedimentary deposits of the precambrian but despite a century of endeavor the mechanisms of their deposition are still unresolved interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy on the basis of chemical analyses from bif units of the 2 5 ga hamersley group western australia we show here that even during periods of maximum iron precipitation most if not all of the iron in bifs could be precipitated by iron oxidizing bacteria in cell densities considerably less than those found in modern fe rich aqueous environments those ancient microorganisms would also have been easily supported by the concentrations of nutrients p and trace metals v mn co zn and mo found within the same iron rich bands these calculations highlight the potential importance of early microbial activity on ancient metal cycling

u pb zircon age constraints on the hamersley spherule beds evidence for a single 2 63 ga jeerinah carawine impact ejecta layer seven spherule beds interpreted to be distal impact ejecta were recently found in late neoarchean and early paleoproterozoic successions of northwestern australia and south africa inadequate age constraints have led to uncertainty about the number of impacts they represent and their correlation here we report two new isotopic ages from the pilbara region of western australia zircons from a tuff in the carawine dolomite 30 m below the carawine spherule layer give a 2 0 7 pb 2 0 6 pb age of 2630 6 ma suggesting that the carawine and ca 2 63 ga jeerinah spherule layers formed during a single impact significantly increasing the known area of the strewn field to 17 000 km 2 this date also requires that the carawine dolomite was probably time equivalent to the uppermost jeerinah formation and lowermost marra mamba iron formation negating previous lithostratigraphic correlations and suggesting that the deposition of banded iron formation was synchronous with carbonate precipitation in a shallow water platform a tuff in the mount mcrae shale yielded a zircon 2 0 7 pb 2 0 6 pb age of 2504 5 ma providing new age constraints on the overlying wittenoom and dales gorge spherule layers the new dates show that the hamersley spherule layers represent three impacts that occurred ca 2 63 ga 2 56 2 50 ga and 2 50 2 48 ga the three hamersley spherule beds broadly correlate with three beds in south africa highlighting the possible global extent of the impact ejecta fallout the layers potentially provide three geologically instantaneous markers spanning 150 m y across the archean proterozoic boundary possibly underpinning a chronostratigraphic framework across two continents and constraining late neoarchean paleogeography

the sedimentary record of extraterrestrial impacts in deep shelf environments evidence from the early precambrian abstract impact ejecta layers in four formations in the hamersley basin western australia and in one formation from the transvaal supergroup south africa show striking evidence for impact related reworking each layer contains sand sized spherules of a former silicate melt that resemble those found in well documented impact layers given available isotopic age dates of associated strata and uncertainties in stratigraphic correlation these layers represent a minimum of three and a maximum of five impacts between ca 2 49 and 2 63 ga all of these layers were deposited below wave base in deep shelf environments yet they show a common suite of sedimentary features indicating deposition and reworking under high energy conditions these features occur in a consistent order 1 extensive erosion including the transport of meter scale rip up clasts 2 reworking by waves 3 synwave to postwave offshore directed bottom return flow and 4 later reworking by sediment gravity flows we interpret the con

field evidence of eros scale asteroids and impact forcing of precambrian geodynamic episodes kaapvaal south africa and pilbara western australia cratons abstract the role of asteroid and comet impacts as triggers of mantle crust processes poses one of the fundamental questions in earth science i present direct field evidence for close associations between impact ejecta fallout units major unconformities and lithostratigraphic boundaries in archaean and early proterozoic terrains including abrupt changes in the composition of volcanic and sedimentary assemblages across stratigraphic impact boundaries with implications for the nature and composition of their provenance terrains as originally observed by d r lowe and g r byerly in the barberton greenstone belt eastern kaapvaal craton south africa 3 26 3 24 ga asteroid mega impact units are closely associated with the abrupt break between an underlying simatic mafic ultramafic volcanic crust and an overlying association of turbidites banded iron formations felsic tuff and conglomerates of continental affinities contemporaneous stratigraphic relationships are identified in the pilbara craton western australia evidence for enrichment of seawater in ferrous iron in the wake of major asteroid impacts reflects emergence of new source terrains likely dominated by mafic compositions attributed to impact triggered oceanic volcanic activity relationships between impact and volcanic activity are supported by the onset of major mafic dyke systems associated with 2 48 ga and possibly the 2 56 ga mega impact events

impacts do not initiate volcanic eruptions eruptions close to the crater many papers on meteorite impact suggest that large impacts can induce volcanic eruptions through decompression melting of the underlying rocks we perform numerical simulations of the impact of an asteroid with a diameter of 20 km striking at 15 km s 1 into a target with a near surface temperature gradient of 13 k km 1 cold case or 30 k km 1 hot case the impact creates a 250 300 km diameter crater with 10 000 km3 of impact melt however the crater collapses almost flat and the pressure field returns almost to the initial lithostat even an impact this large cannot raise mantle material above the peridotite solidus by decompression statistical considerations also suggest that impacts cannot be the common initiator of large igneous provinces any time in post heavy bombardment earth history

impact induced melting and the development of large igneous provinces abstract we use hydrodynamic modelling combined with known data on mantle melting behaviour to examine the potential for decompression melting of lithosphere beneath a large terrestrial impact crater this mechanism may generate sufficient quantity of melt to auto obliterate the crater melting would initiate almost instantaneously but the effects of such massive mantle melting may trigger long lived mantle up welling that could potentially resemble a mantle hotspot decompression melting is well understood it is the main method advocated by geophysicists for melting on earth whether caused by thinned lithosphere or hot rising mantle plumes the energy released is largely derived from gravitational energy and is outside but additive to the conventional calculations of impact modelling where energy is derived solely from the kinetic energy of the impacting projectile be it comet or asteroid the empirical correlation between total melt volume and crater size will no longer apply but instead there will be a discontinuity above some threshold size depending primarily on the thermal structure of the lithosphere we estimate that the volume of melt produced by a 20 km diameter iron impactor travelling at 10 km s may be comparable to the volume of melt characteristic of terrestrial large igneous provinces 106 km3 similar melting of the mantle beneath an oceanic impact was also modelled by roddy et al int j impact eng 5 1987 525 the mantle melts will have plume like geochemical signatures and rapid mixing of melts from sub horizontal sub crater reservoirs is likely direct coupling between impacts and volcanism is therefore a real possibility that should be considered with respect to global stratigraphic events in the geological record we suggest that the end permian siberian traps should be reconsidered as the result of a major impact at 250 ma auto obliteration by volcanism of all craters larger than 200 km would explain their anomalous absence on earth compared with other terrestrial planets in the solar system

early archean spherule beds chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type three early archean spherule beds from barberton south africa have anomalous cr isotope compositions in addition to large ir anomalies confirming the presence of meteoritic material with a composition similar to that in carbonaceous chondrites the extra terrestrial components in beds s2 s3 and s4 are estimated to be approx l 50 60 and 15 30 respectively these beds are probably the distal and possibly global ejecta from major large body impacts these impacts were probably much larger than the cretaceous tertiary event and all occurred over an interval of approx 20 m y implying an impactor flux at 3 2 ga that was more than an order of magnitude greater than the present flux

impact origin for the greater ontong java plateau abstract the 120 ma ontong java plateau and neighboring contemporaneous nauru east mariana and probably pigafetta basin flood basalts in the western equatorial pacific ocean comprise the earth s largest flood basalt province geophysical geochemical and geodynamic evidence from the province are difficult to reconcile with mantle plume models absence of an obvious hotspot source or track minor crustal uplift associated with emplacement minor total subsidence compared with normal oceanic crust or other oceanic plateaus and submarine ridges high degrees of melting at shallow upper mantle depths low water contents of basalts enrichment of platinum group elements in basalts and a 300 km deep seismically slow mantle root are more consistent with the consequences of an impacting bolide an object 20 km in diameter impacting relatively young 20 myr pacific lithosphere and penetrating into the uppermost asthenosphere would have initiated massive decompression melting in the upper mantle and may have resulted in emplacement of the greater ontong java plateau