the platinum group mineralogy of the merensky reef at the impala platinum mines bophuthatswana the platinum group mineralogy of the merensky reef at the impala platinum mines south africa has been investigated polished sections were prepared from pyroxenitic and pegmatoidal reef samples collected underground microscope studies were followed up by the identification and analysis of platinum group minerals by electron microprobe the most abundant base metal sulfides occurring in the merensky reef in this area are pyrrhotite 41 pentlandite 37 chalcopyrite 18 and pyrite 4 the core of a typical base metal sulfide bleb consists of pyrrhotite surrounded by a mosaic of pentlandite grains which in turn may be enclosed by a rim of chalcopyrite the relative volumetric abundances of the platinum group minerals were determined to be as follows coopetite 44 laurite 21 moncheite 17 and braggite 12 thirteen additional platinum group minerals comprised the remaining 6 percent cooperite and laurite occur preferentially in the chromitite layers whereas moncheite was most commonly found in the silicate part of the merensky reef observed textures include the marked tendency of platinum group minerals to occur with chalcopyrite the exsolution of laurite and moncheite from pyrrhotite and stringers of mon cheite and sperrylite cutting across silicate and base metal sulfide grain boundaries pt fe alloy commonly forms the core with cooperite as a rim of compound zoned platinum group mineral grains the textural features observed indicate that the platinum group minerals formed at a relatively late stage in the crystallization history of the merensky reef the preferential association of platinum group minerals with chalcopyrite is important chalcopyrite formed from the cu rich sulfide liquid of the original monosulfide solid solution which separated from the silicate magma it is suggested that the platinum group elements concentrated in this liquid and that the platinum group minerals crystallized only after the chalcopyrite as they occur mostly on the chalcopyrite margins a part of the platinum group element fraction remained in the fe ni monosulfide liquid and crystallized as platinum group minerals associated with pyrrhotite and pentlandite the remaining platinum group element fraction especially ru and pd remained in solid solution in the fe ni monosulfide the ru exsolved at lower temperatures as laurite but the pd remained in solid solution in the pentlandite this is concluded from the work of other authors ross and keays 1979 watkinson and dunnington 1979 and by the paucity of pd minerals although pd is the second most abundant platinum group element in the merensky reef

models for midcontinent tectonism an update during the past decade significant progress has been made in defining the nature of midcontinent tectonism as a result of an improved geophysical geologic data base which permits an integration of seismicity data crustal structures and current stress directions critically placed microseismic networks have shown that many earthquakes with common focal mechanisms occur in specific spatial patterns the direction of the maximum horizontal compressive stress is generally directed east northeast which supports an origin associated with ridge push forces related to movement of the north american plate crustal structures have been mapped locally by seismic studies and regionally by extensive gravity and magnetic anomaly data these data have been used to relate current seismicity and associated tectonism to one of two models increasing evidence from across the midcontinent supports reactivation of preexisting zones of crustal weakness appropriately oriented with respect to the prevailing stress field the zone of weakness model as the dominant control on contemporary tectonism and the local basement inhomogeneity model as a mechanism for minor low energy release earthquake activity

atmospheric volcanic loading derived from bipolar ice cores accounting for the spatial distribution of volcanic deposition 1 previous studies have used small numbers of ice core records of past volcanism to represent hemispheric or global radiative forcing from volcanic stratospheric aerosols with the largest ever assembly of volcanic ice core records and state of the art climate model simulations of volcanic deposition we now have a unique opportunity to investigate the effects of spatial variations on sulfate deposition and on estimates of atmospheric loading we have combined 44 ice core records 25 from the arctic and 19 from antarctica and goddard institute for space studies modele simulations to study the spatial distribution of volcanic sulfate aerosols in the polar ice sheets we extracted volcanic deposition signals by applying a high pass loess filter to the time series and examining peaks that exceed twice the 31 year running median absolute deviation our results suggest that the distribution of volcanic sulfate aerosol follows the general precipitation pattern in both regions indicating the important role precipitation has played in affecting the deposition pattern of volcanic aerosols we found a similar distribution pattern for sulfate aerosols from the 1783 1784 laki and 1815 tambora eruptions as well as for the total b activity after the 1952 1954 low latitude northern hemisphere and 1961 1962 high latitude northern hemisphere atmospheric nuclear weapon tests this confirms the previous assumption that the transport and deposition of nuclear bomb test debris resemble those of volcanic aerosols we compare three techniques for estimating stratospheric aerosol loading from ice core data radioactive deposition from nuclear bomb tests pinatubo sulfate deposition in eight antarctic ice cores and climate model simulations of volcanic sulfate transport and deposition following the 1783 laki 1815 tambora 1912 katmai and 1991 pinatubo eruptions by applying the above calibration factors to the 44 ice core records we have estimated the stratospheric sulfate aerosol loadings for the largest volcanic eruptions during the last millennium these loadings agree fairly well with estimates based on radiation petrology and model simulations we also estimate the relative magnitude of sulfate deposition compared with the mean for greenland and antarctica for each ice core record which provides a guideline to evaluate the stratospheric volcanic sulfate aerosol loading calculated from a single or a few ice core records

variations in mid continent rift magma volumes consistent with microplate evolution 1 modeling of gravity data along the 1 1 ga failed mid continent rift system shows systematic patterns in magma volume between and along the rift s two arms the volume of magma increases towards the lake superior region consistent with magma flowing away from a hotspot source there the west arm experienced significantly more magmatism these patterns are consistent with a model in which the two rift arms acted as boundaries of a microplate the volume of magma along the west arm increases with distance from the euler pole indicating that it acted essentially as a spreading ridge whereas the much smaller magma volumes along the east arm are consistent with its acting as a leaky transform this view of the rift system s evolution is compatible with the rift being part of an evolving plate boundary system rather than an isolated episode of midplate volcanism

a continuous 770 year record of volcanic activity from east antarctica a 100 m ice core from site g 15 accumulation rate 0 1 m water yr 1 mean annual temperature 38 c on the mizuho plateau dronning maud land east antarctica has been analysed using the dielectric profiling dep technique the capacitance and conductance of the core were measured at ac frequencies 20 hz 300 khz the high frequency conductivity profile shows variations that are primarily related to the strong acids derived from volcanic activity the tambora 1815 eruption can be identified with the aid of an approximate chronology based on the firn densification rate other historic eruptions can then be recognised beyond about 300 years historical observations are very few however if a constant overall accumulation rate is assumed a well known eruption of 1259 a d can be found near the bottom of the core other peaks in the conductivity profile can then be assigned dates accurate to within a few years using the conductivity profile it is possible to estimate the relative acid deposition fluxes produced by the main eruptions with reasonable accuracy the estimated acid deposisition fluxes realtive to the tambora 1815 eruption of agung 1963 is 27 krakatoa 1883 25 the signal of 1601 28 and that of 1259 53

grenville basement structure associated with the eastern tennessee seismic zone southeastern usa the eastern tennessee seismic zone extends more than 300 km from as far north as southeastern kentucky southward into alabama southeastern united states we propose that a large scale shear zone which originated as a continental transform fault during the grenville orogeny and assembly of supercontinent rodinia constitutes the framework for earthquake activity in the eastern tennessee seismic zone this new seismotectonic model is based on a diverse set of geophysical and geological observations including paleomagnetic and isotopic constraints on the growth of southeastern laurentia during the grenville orogeny

making it and breaking it in the midwest continental assembly and rifting from modeling of earthscope magnetotelluric data abstract a three dimensional lithospheric scale resistivity model of the north american mid continent has been estimated based upon earthscope magnetotelluric data details of the resistivity model are discussed in relation to lithospheric sutures defined primarily from aeromagnetic and geochronologic data which record the southward growth of the laurentian margin in the proterozoic the resistivity signature of the 1 1 ga mid continent rift system is examined in detail in particular as relates to rift geometry extent and segmentation an unrecognized expanse of concealed proterozoic deltaic deposits in kansas is identified and speculated to result from axial drainage along the southwest rift arm akin to the rio grande delta which drains multiple rift basins a prominent conductor traces out cambrian rifting in arkansas missouri tennessee and kentucky this linear conductor has not been imaged before and suggests that the cambrian rift system may have been more extensive than previously thought the highest conductivity within the mid continent is imaged in minnesota michigan and wisconsin where it is coincident with paleoproterozoic metasedimentary rocks the high conductivity is attributed to metallic sulfides and in some cases graphite the former is a potential source of sulfur for multiple mineral deposits types occurrences of which are found throughout the region finally the imprint left within the mantle following the 1 1 ga rifting event is examined variations in lithospheric mantle conductivity are observed and are interpreted to reflect variations in water content depleted versus metasomatized mantle imprinted upon the mantle by the keweenawan mantle plume

crustal velocity structure associated with the eastern tennessee seismic zone vp and vs images based upon local earthquake tomography we present three dimensional p and s wave velocity models for the active eastern tennessee seismic zone etsz using arrival time data from more than 1000 local earthquakes a nonlinear tomography method is used that involves sequential inversion for model and hypocenter parameters we image several velocity anomalies that persist through most of the inversion volume some anomalies support the presence of known features such as an ancient rift zone in northern tennessee other anomalies reveal the presence of basement features that can be correlated with regional gravity and magnetic anomalies we image a narrow ne sw trending steeply dipping zone of low velocities that extends to a depth of at least 24 km and is associated with the vertical projection of the prominent new york alabama magnetic lineament the low velocity zone may have an apparent dip to the se at depths exceeding 15 km the low velocity zone is interpreted as a major basement fault juxtaposing granite rhyolite basement to the nw from grenville southern appalachian basement to the se relocated hypocenters align in near vertical segments suggesting reactivation of a distributed zone of deformation associated with a major strike slip fault we suggest that the etsz represents reactivation of an ancient shear zone established during formation of the super continent rodinia

spatio temporal variability in volcanic sulphate deposition over the past 2 kyr in snow pits and firn cores from amundsenisen antarctica in the framework of the european project for ice coring in antarctica epica a comprehensive glaciological pre site survey has been carried out on amund senisen dronning maud land east antarctica in the past decade within this survey four intermediate depth ice cores and 13 snow pits were analyzed for their ionic compos ition and interpreted with respect to the spatial and temporal variability of volcanic sul phate deposition the comparison of the non sea salt nss sulphate peaks that are related to the well known eruptions of pinatubo and cerro hudson in ad1991 revealed sulphate depositions of comparable size 15 8 3 4 kg km 2 in 11 snow pits there is a tendency to higher annual concentrations for smaller snow accumulation rates the combination of seasonal sodium and annually resolved nss sulphate records allowed the establishment of a time scale derived by annual layer counting over the last 2000 years and thus a detailed chronology of annual volcanic sulphate deposition using a robust outlier detection algo rithm 49 volcanic eruptions were identified between ad165 and 1997 the dating uncer tainty is 3 years between ad1997 and 1601 around 5 years between ad1601 and 1257 and increasing to 24 years at ad165 improving the accuracy of the volcanic chronology during the penultimate millennium considerably abstract in the framework of the european project for ice coring in antarctica epica a comprehensive glaciological pre site survey has been carried out on amund senisen dronning maud land east antarctica in the past decade within this survey four intermediate depth ice cores and 13 snow pits were analyzed for their ionic compos ition and interpreted with respect to the spatial and temporal variability of volcanic sul phate deposition the comparison of the non sea salt nss sulphate peaks that are related to the well known eruptions of pinatubo and cerro hudson in ad1991 revealed sulphate depositions of comparable size 15 8 3 4 kg km 2 in 11 snow pits there is a tendency to higher annual concentrations for smaller snow accumulation rates the combination of seasonal sodium and annually resolved nss sulphate records allowed the establishment of a time scale derived by annual layer counting over the last 2000 years and thus a detailed chronology of annual volcanic sulphate deposition using a robust outlier detection algo rithm 49 volcanic eruptions were identified between ad165 and 1997 the dating uncer tainty is 3 years between ad1997 and 1601 around 5 years between ad1601 and 1257 and increasing to 24 years at ad165 improving the accuracy of the volcanic chronology during the penultimate millennium considerably

replacement of base metal sulfides by actinolite epidote calcite and magnetite in the ug2 and merensky reef of the bushveld complex south africa interaction of base metal sulfides pyrrhotite pentlandite and chalcopyrite with aqueous fluids in the ug2 chromitite layer and the merensky reef in the western bushveld complex has produced four common replacements 1 replacement of sulfides and orthopyroxene by actinolite or tremolite 2 replacement of sulfides and plagioclase by epidote 3 replacement of sulfides by calcite and 4 replacement of sulfides by magnetite the first three replacements are directly related to the presence of sulfides in the rocks and occur at the grain scale silicate minerals that are not in direct contact with sulfides are usually not affected the replacement of sulfides by magnetite accompanies regional serpentinization superimposed on the union section platinum group minerals including ru pt and pd sulfides pt and pd tellurides and pd arsenides occur in replacement aureoles around sulfide aggregates as well as within the aggregates suggesting that the platinum group minerals are more resistant to hydrothermal alteration than the associated base metal sulfides redistribution of chalcopyrite at least at the millimeter scale variable losses of s and ni and perhaps decoupling of pt and pd at the centimeter scale took place during secondary hydrothermal alteration these effects should be considered before models of primary mineralization processes can be meaningfully applied

a 220 year continuous record of volcanic h2so4 in the antarctic ice sheet continuous h2so4 profiles observed in snow from several antarctic locations reveal four major volcanic events of the past two centuries agung krakatoa tambora and another large scale event not recorded historically acid deposition and interhemispheric distribution mechanisms are quantified and then used to obtain an order of magnitude estimate for the h2so4 emissions from these eruptions

collision induced rifting the grenville orogeny and the keweenawan rift of north america abstract we propose that the keweenawan rift formed as a result of convergence related to the synchronous grenville orogeny this proposal is based on 1 1 isotopic ages in the keweenawan rift and the grenville province are similar according to the best u pb and rb sr dates the grenville orogeny existed from 1150 to 1100 ma and keweenawan rifting occurred 1120 1100 ma 2 2 a similar modern orogenic belt and rift relationship in the middle east serves as an actualistic model the red sea gulf of aden rift system has formed as a continuous series of coalesced pull apart basins and is opening parallel to the orogenic front of the zagros and bitlis mountains rifting began in the early miocene while the collision began in the eocene suggesting that rifting is related to the collision 3 3 many other ancient rift collision pairs have a similar age relationship and geometrical configuration based on comparisons of these tectonic regimes we have derived a model whereby these rifts formed as a result of strike slip faults forming in either the foreland or the hinterland of orogenic zones

greenland ice core signal characteristics an expanded view of climate change the last millenium of earth history is of particular interest because it documents the environmental complexities of both natural variability and anthropogenic activity we have analyzed the major ions contained in the greenland ice sheet project 2 gisp 2 ice core from the present to 674 a d to yield an environmental reconstruction for this period that includes a description of nitrogen and sulfur cycling volcanic emissions sea salt and terrestrial influences we have adapted and extended mathematical procedures for extracting sporadic e g volcanic events secular trends and periodicities found in the data sets finally by not assuming that periodic components signals were stationary and by utilizing evolutionary spectral analysis we were able to reveal periodic processes in the climate system which change in frequency turn on and turn off with other climate transitions such as that between the little ice age and the medieval warm period

crustal and upper mantle velocity structure in the vicinity of the eastern tennessee seismic zone based upon radial p wave transfer functions teleseismic transfer function analysis is used to investigate crust and upper mantle velocity structure in the vicinity of the active eastern tennessee seismic zone etsz the etsz is associated with the new york alabama ny al magnetic lineament a prominent aeromagnetic anomaly indicative of grenville age basement structure radial component p wave transfer functions for 10 short period stations operated by the center for earthquake research and information are inverted for velocity structure velocity profiles are also determined for three broadband stations by converting the instrument response to that of an s 13 short period seismometer distinct differences in the velocity profiles are found for stations located on either side of the ny al magnetic lineament velocities west of the lineament are lower than velocities to the east of the lineament in the upper 10 km and in the depth range 30 to 50 km a gradational moho boundary is found beneath several stations located in the valley and ridge province a moho boundary is absent at four valley and ridge stations located east of the magnetic lineament and south of 35 5 n

cooperite and braggite from the bushveld complex implications for the miscibility gap and identification analyses of the pt pd ni sulphides cooperite braggite and vysotskite reported from worldwide occurrences seem to imply a continuum of compositions between vysotskite and cooperite with no obvious miscibility gap this is contrary to the experimentally confirmed miscibility gap between cooperite and braggite and the established compositional gap between co existing cooperite and braggite from the merensky reef although the only unambiguous way of distinguishing between cooperite and braggite is to obtain structural information through x ray diffraction or equivalent techniques most identification of pt pd ni sulphides is based on microanalytical techniques due to the small grain size of most platinum group minerals ni contents also have to be considered because a classification based on the pt pd ratio alone can be very misleading naming of pt pd ni sulphide compositions with high pt contents based on qualitative or semi quantitative analyses should be avoided natural pt pd ni sulphides which project into the compositional gap established by experimental work cannot be named without supporting structural information compositions of grains which plot inside the gap are considered to be metastable and to result from the loss of pd through interaction with hydrothermal fluids

modeling the distribution of the volcanic aerosol cloud from the 1783 1784 laki eruption 1 we conducted simulations of the atmospheric transformation and transport of the emissions of the 1783 1784 laki basaltic flood lava eruption 64 10 n 17 15 w using the nasa goddard institute for space studies modele climate model coupled to a sulfur cycle chemistry model the model simulations successfully reproduced the aerosol clouds of the 1912 katmai and 1991 mount pinatubo eruptions giving us confidence in the laki simulations simulations of the laki eruption produce peak zonal mean sulfate so4 concentrations of over 70 ppbv during august and into september 1783 in the upper troposphere and lower stratosphere at high latitudes while the majority of the sulfate aerosol was removed during the fall and early winter a significant aerosol perturbation remained into 1784 the peak so2 gas loading was just over 37 megatons mt in late june with the sulfate loading peaking in late august 1783 at 60 mt over the average of 3 runs this yielded a peak sulfate aerosol 75 h2so4 25 h2o loading of over 80 mt with the total aerosol produced during the entire eruption being about 165 mt the resulting sulfate deposition compares well with ice cores taken across greenland the top of atmosphere net radiative forcing peaks at 27 w m2 over the high latitudes during late summer 1783 and produces a global mean forcing of 4 w m2 the model results confirm that northern hemisphere high latitude volcanic eruptions produce aerosols that remain mostly confined north of 30 n latitude

coal rank trends in the central appalachian coalfield virginia west virginia and kentucky abstract coal rank determined from vitrinite maximum reflectance and from chemical rank parameters increases from high volatile c bituminous in portions of the eastern kentucky coalfield to low volatile bituminous in buchanan county virginia and mcdowell county west virginia and then decreases significantly to the dry fork anticline 20 km to the southeast the latter rank decrease as noted when rank data from individual coals is used across the fold 400 500 m structural relief can be accounted for by post tectonic coalification at a paleogeothermal gradient of about 40 45 c km reflectance gradients decrease from about 0 09 rmax 100 m in central buchanan county virginia to 0 06 rmax 100 min southeastern pike county kentucky to 0 03 max 100 m and less among the high volatile a and b coals to the nortwest of pike county the reflectance gradients are likely an indication of a decrease in paleogeothermal gradient to the northwest depth of burial may have also decreased to the northwest northward trending rank anomalies superimposed on the regional trend in kentucky may have been caused by subtle changes in the geothermal gradients associated with basement discontinuities or alternately by a depth of burial not accounted for at present perhaps under now eroded thrust sheets

joint hypocenter velocity inversion for the eastern tennessee seismic zone a joint hypocenter velocity inversion for the eastern tennessee seismic zone etsz has resolved velocity features in basement rock below detached appalachian thrust sheets p and s wave arrival times from 492 earthquakes have been inverted for one 1 d and three dimensional 3 d velocity models to midcrustal depths the 3 d p and s wave velocity solutions are computed independly and are very similar in relation to the 1 d model velocity anomalies range from 8 to 16 in the first layer upper 5 km and between 7 in deeper layers prominent velocity anomalies parallel the seismic zone and are consistent from layer to layer the most persistent anomaly is a low velocity region that borders the seismic zone to the northwest and is flanked on either side by regions of anomalously high velocity the new york alabama ny al magnetic lineament coincides with or lies close to the southeast boundary of the prominent velocity low in both the p and s wave velocity images the spatial coincidence between velocity gravity and magnetic gradients suggests that major discontinuities are present in the basement relocation in the 3 d velocity model reduced the number of very deep earthquakes below 20 km and further accentuated differences in seismogenic properties on either side of the ny al lineament after relocation most earthquakes occur in a vertically bounded region roughly 30 km wide extending from 4 to 22 km in depth most earthquakes occur in regions characterized by either average velocity or small velocity anomalies

origin of phlogopite orthopyroxene inclusions in chromites from the merensky reef of the bushveld complex south africa about 30 of the chromite grains of variable sizes in a chromitite seam at the base of the merensky reef of the bushveld complex on the farm vlakfontein contain abundant composite mineral inclusions the inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite they vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains na phlogopite and orthopyroxene are most abundant in the inclusions edenitic hornblende k phlogopite oligoclase and quartz are less abundant cl rich apatite rutile zircon and chalcopyrite are present at trace levels na phlogopite is unique to the inclusions it has not been found elsewhere in the bushveld complex other minerals in the inclusions are also present in the matrix of the chromitite seam but their compositions are different the mg mg fe2 ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix k phlogopite in the inclusions contains more na than in the matrix the average compositions of the inclusions are characterized by high mgo 26 wt na2o 2 4 wt and h2o 2 6 wt and low cao 1 1 wt and feo 4 4 wt the 18o value of the trapped melt estimated by analysis of inclusion rich and inclusion poor chromites is 7 this value is consistent with the previous estimates for the bushveld magma and with the 18o values of silicate minerals throughout the reef the textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatile rich melts during chromite crystallization the volatile rich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and na k rich fluids expelled from the underlying crystal pile the addition of fluid to the magma is thought to have caused dissolution of orthpyroxene leaving the system saturated only in chromite both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution most of the cr and al in the inclusions was contributed through wall dissolution of the host chromite dissolution of minor rutile trapped along with orthopyroxene provided most of the ti in the inclusions the na and k rich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatile rich melts

reviewing the configuration and extent of the midcontinent rift system abstract uncertainty exists in the configuration and extent of the midcontinent rift system mrs because of deficiencies in geophysical data and limited information from outcrops and basement drill holes additional ambiguity is caused by misunderstanding the definition of continental rifts six principal problematic regions in mapping the mrs are described gravity and magnetic data supported by drill hole and seismic reflection data show that the eastern lake superior rift segment of the mrs continues south from lake superior and connects to a much narrower rift in northern lake michigan the eastern margin of this transition is ill defined because of the lack of definitive anomalies and supporting seismic and drill hole data but is interpreted to occur near the u s canada border the rift segment in southeastern michigan intersects the grenville front and likely continues eastwards in modified form to near the boundary with canada cross cutting gravity and magnetic signatures may reflect grenvillian overthrusts near the terminus of the mrs in michigan the proposed southerly extensions of both branches of the rift system into oklahoma and ohio are based primarily on positive gravity anomalies but neither postulated extension appears to be associated with rifted troughs rather the gravity anomalies of the western branch are related to intrusive mafic rocks and those of the eastern branch are most likely related to deep crustal metamorphic rocks thrust into juxtaposition with less dense crust by grenville orogenesis recent paleomagnetic investigations in conjunction with high resolution radiometric dating imply that the mrs developed during the rapid southward movement of laurentia during a quiescent period along its eastern continental margin massive magmatic activity accompanying the rifting was likely due to rising mantle material that was displaced by subducted lithosphere along the southern margin the heated crust was made more ductile fostering rifting due to extensional stresses the nipigon embayment remains as a possible candidate for an early third branch of the mrs but current evidence is insufficient to include the fort wayne rift as part of the mrs future studies of the mrs would be well served by new age dating and high resolution seismic studies of the lithosphere

intraplate stresses and seismicity the role of topography and density heterogeneities abstract the stress induced by the topography and by density heterogeneities in the lithosphere has been computed for two different tectonic regions of the north american plate the lithosphere was assumed to consist of a two dimensional layered elastic slab over an inviscid fluid the calculations show that these stresses are locally comparable in magnitude to the regional tectonic stress 20 50 mpa two areas have been analyzed in greater detail the southeastern united states and the rio grande rift the stress in the southeastern united states is computed along a cross section perpendicular to the strike of the appalachian mountains the computed stress appears largest beneath the blue ridge and valley and ridge provinces where the appalachian seismicity is located the stress in the western united states is computed along a cross section perpendicular to the rio grande rift large stress is shown beneath the rift and the adjacent regions the stress induced by surface and internal loading is extensional in the rio grande rift and compressional in the colorado plateau and in the great plains

antarctic volcanic flux ratios from law dome ice cores 3 climatestudiescenter institutefor quaternarystudies universityofmaine orono me 04469 5717 u s a abstract explosivevolcanic eruptions can inject largequantitiesof sulphur dioxide intothe stratosphere theaerosolsthat result fromoxidationof thesulphur dioxidecanpro duce significant cooling of the troposphere by reflecting or absorbing solar radiation it is possibleto obtainanestimateoftherelativestratosphericsulphuraerosolconcentrationpro duced by different volcanoes by comparing sulphuric acid fluxes determined by analysis of polaricecores here weuseanon sea saltsulphatetimeseriesderivedfromthreewell dated law dome ice cores to investigate sulphuric acid flux ratios for major eruptions over the period ad1301 1995 weuseadditionaldata fromothercorestoinvestigatesystematicspa tial variability in the ratios only for the kuwae eruption law dome ice date ad1459 5 wasthe h2so4 flux largerthanthatdepositedbytambora law domeicedate ad1816 7

annually resolved southern hemisphere volcanic history from two antarctic ice cores the continuous sulfate analysis of two antarctic ice cores one from the antarctic peninsula region and one from west antarctica provides an annually resolved proxy history of southern semisphere volcanism since early in the 15th century the dating is accurate within 3 years due to the high rate of snow accumulation at both core sites and the small sample sizes used for analysis the two sulfate records are consistent with each other a systematic and objective method of separating outstanding sulfate events from the background sulfate flux is proposed and used to identify all volcanic signals the resulting volcanic chronology covering 1417 1989 a d resolves temporal ambiguities about several recently discovered events a number of previously unknown moderate eruptions during late 1600s are uncovered in this chronology the eruption of tambora 1815 and the recently discovered eruption of kuwae 1453 in the tropical south pacific injected the greatest amount of sulfur dioxide into the southern hemisphere stratosphere during the last half millennium a technique for comparing the magnitude of volcanic events preserved within different ice cores is developed using normalized sulfate flux for the same eruptions the variability of the volcanic sulfate flux between the cores is within 20 of the sulfate flux from the tambora eruption

sulphate record from a northeast greenland ice core over the last 1200 years based on continuous flow analysis a 150 m deep ice core from the low accumulation area of northeast greenland was analyzed for sulphate calcium sodium and electrolytical meltwater conductivity at a depth resolution of approximately 1 cm by continuous flow analysis cfa the calcium and sodium profiles are used to establish a relatively precise ice core chronology by annual layer counting back to ad 830 inspection of the novel cfa method for sulphate revealed relative errors typically around 15 but at least 20 ng g 1 for concentrations 5130 ng g 1 and a current detection limit for routine ice core analyses of 40 ng g 1 annual sulphate peaks are shown to occur over almost the entire core with only a small shift in seasonality between the modern and pre industrial sections inspection of volcanic horizons allowed more accurate timing of these peaks and clear identification of calcium rich events disregarding clear volcanic peaks significant long term changes of sulphate are only seen over the industrial period however a higher frequency of important volcanic inputs was identified around ad 1200

horizontal gravity gradient an aid to the definition of crustal structure in north america a map of the magnitude of the horizontal bouguer gravity gradient over the north american continent is used to delineate lateral discontinuities in upper crustal density and or thickness associated with such processes as suturing and rifting the usefulness of gradient trends in mapping major structural boundaries which are sometimes poorly exposed or completely buried is demonstrated by examples such as the buried southward extension of the grenville front and buried boundaries of the superior province gradient trends also draw attention to poorly known structures which may have major tectonic significance and to a continent wide structural fabric which may provide a record of the tectonic growth of the north american continent

a possible southeastern extension of the midcontinent rift system located in ohio in 1988 a stratigraphic test core drilled in southwestern ohio penetrated a previously unknown structure and sedimentologic unit core analyses disclosed a sequence of lithic arenite and siltstone of probable pre phanerozoic age deposited within an alluvial fluvial environment a seismic reflection profile across the core site shows a sequence of strong horizontal paleozoic reflectors unconformably overlying eastward dipping layered units of poor reflectivity beyond the core hole total depth a regime of excellent reflectors is seen the pre mount simon upper cambrian sedimentary sequence encountered has been defined as the type section of the middle run formation point counter analyses of typical middle run formation samples indicate that this sequence is similar in composition to middle proterozoic units associated with the midcontinent rift system in the lake superior region the newly discovered structure in ohio is also similar to half graben structures composing the lake superior basin portion of the midcontinent rift on the basis of similarities in lithology stratigraphy structure and proximity to regional gravity anomalies the middle run formation and its apparent half graben basin is proposed as evidence for the probable extension of the midcontinent rift system southward from its normally accepted termination in southeastern michigan into southwestern ohio

some comparisons of the structure and evolution of the southern appalachian ouachita orogen and portions of the trans european suture zone region abstract many aspects of the tectonic history of the appalachian ouachita orogen and the trans european suture region are similar in a tectonic sense they are geographically linked because at the end of the paleozoic one could travel from texas to poland following a continuous orogenic belt much of each of these orogenic belts is however buried by younger strata and thus geophysical studies are a key element of efforts to better understand them many more geophysical data are needed to provide a comprehensive picture of the deep structure but in several areas new models of the lithospheric structure are emerging in this paper we review some of these models and introduce two new ones for the southern appalachians in addition we explore the implications of these models for the evolution of these orogenic belts drawing attention to similarities and differences in their structure and evolution the appalachian and caledonian orogens represent temporally parallel development along irregular late proterozoic continental margins but the crustal structure is different because of different accretionary elements and complications in the geometry of the original margins lateral transport via thrusting can confuse efforts to tie geologic structures identified on the surface with deep structures for example in the appalachian ouachita orogen the thrust related transport distance for major features such as the blue ridge piedmont sheet and the benton uplift is at least 200 km on a crustal scale deformation in the appalachian ouachita orogen varies greatly in style and intensity in the ouachita orogen the late proterozoic continental margin seems to be preserved while in the northern appalachians crustal scale deformation is intense along the us appalachians the major crustal structure variation is the thin dense crust of avalonia abutting the relatively thick crust of laurentia along the ouachita orogen several large crustal blocks can be identified that are unlike anything seen along the appalachians

the role of rifting in the tectonic development of the midcontinent u s a abstract recent studies have proposed the existence of several major ancient rift zones in the midcontinent region of north america although the dating of some of these rifts and even the rift interpretations are subject to question an analysis of these paleo rifts reveals three major episodes of rifting keweenawan 1 1 b y b p eocambrian 600 m y b p and early mesozoic 200 m y b p the extent of these events documents that rifting has played a major role in the tectonic development of the midcontinent region this role goes well beyond the initial rifting event because these features display a strong correlation with paleozoic basins and a strong propensity for reactivation for example the eocambrian reelfoot rift was reactivated in the mesozoic to form the mississippi embayment and is the site of modern seismicity which suggests reactivation in a contemporary stress field of ene compression even though the importance of rifting can be established recognition of rifts and delineation of their complexities remain a major problem which requires more study

glaciological investigations in the crete area central greenland a search for a new deep drilling site the results of the 1984 85 post gisp campaigns in central greenland are presented eight ice cores were obtained some spanning up to 360 years we present i geographical positions and elevations at the drill sites

a 200 year sub annual record of sulfate in west antarctica from 16 ice cores sixteen high resolution ice core records from west antarctica and south pole are used to examine the spatial and temporal distribution of sulfate for the last 200 years the preservation of seasonal layers throughout the length of each record results in a dating accuracy of better than 1 year based on known global scale volcanic events a dual transport source for west antarctic sea salt ss so4 2 and excess xs so4 2 is observed lower tropospheric for areas below 1000 m elevation and mid upper tropospheric stratospheric for areas located above 1000 m our xsso4 2 records with volcanic peaks removed do not display any evidence of an anthropogenic impact on west antarctic so4 2 concentrations but do reveal that a major climate transition takes place over west antarctica at 1940 global scale volcanic eruptions appear as significant peaks in the robust spline residual xsso 4 2 records from sites located above 1000 m elevation but do not appear in the residual records from sites located below 1000 m

rift structure of the northern mississippi embayment from the analysis of gravity and magnetic data gravity and magnetic data have been simultaneously inverted to derive a crustal model of the northern mississippi embayment which is compatible with geologic and seismic refraction data the results indicate that this region is a site of a late precambrian early paleozoic rift system an anomalously dense layer is present at the base of the crust and thickens beneath a broad northeast trending graben that formed during the initial phases of rifting the thickest part of anomalous crust occurs beneath the region of greatest seismic activity which lies within the geographic limits of the graben it is suggested from this correlation that strain accumulates within the anomalous crust layer which acts as an inhomogeneity in a relatively homogeneous lithosphere based on the results of this study and previous geophysical studies the complex tectonic evolution of the upper mississippi embayment since late precambrian time is rift formation possibly along a pre existing shear zone graben development in cambrian time reactivations in permian and cretaceous time and subsequent formation of the present mississippi embayment and fault zones with large influences from rift structures

historic eruptions of tambora 1815 krakatau 1883 and agung 1963 their stratospheric aerosols and climatic impact abstract decreases in surface temperatures after the eruptions of tambora 1815 krakatau 1883 and agung 1963 were of similar magnitude even though the amount of material dust and volatiles injected into the stratosphere by these three events differed greatly large amounts of fine ash and volatiles were dispersed into the upper atmosphere by krakatau and tambora the agung eruption in 1963 was a much smaller vulcanian type eruption which injected dust and volatiles into the stratospheric aerosol layer more directly analyses of magmatic volatiles indicate that the agung eruption was proportionately richer in so 2 and cl than either tambora or krakatau relative amounts of fine ash produced by the tambora krakatau and agung eruptions are estimated at about 150 20 1 whereas the masses of atmospheric sulfate aerosols produced were on the order of 7 5 3 1 decreases in surface temperature of a few tenths of a degree c for several years following volcanic eruptions are primarily a result of the sulfate aerosols rather than of the silicate dust the similarity in the atmospheric response after these three eruptions supports the idea of limiting mechanisms on volcanic stratospheric aerosol loading which is suggested by microphysical processes of aerosol particles fluctuations in stratospheric aerosol optical depth seem to be controlled to a large degree by high intensity sulfur rich eruptions e g agung 1963 which may however be relatively small in total ejecta volume such eruptions leave little geologic record but appear as acidity peaks in polar ice cores

magnetization models for the source of the kentucky anomaly observed by magsat abstract both the aeromagnetic data and magnetic anomaly data obtained by magsat indicate the presence of a very magnetic source region within the crust beneath kentucky and tennessee a source model was previously developed to fit surface gravity and long wavelength aeromagnetic data using limited seismic constraint for the present study the model was further developed and it is demonstrated that the source region for the satellite anomaly is considerably more extensive than the kentucky body sensu stricto the extended source region is modeled using both prismatic model sources and dipole array sources magnetization directions for the source region found by inversion of various combinations of scalar and vector data are found to be close to the main field direction implying the lack of a strong remanent component it is shown by simulation that in a case such as this where the geometry of the source is known if a strong remanent component is present its direction is determinable but by scalar data as readily as vector data magnetization magnitude for the extended source region is about 4 a m if the vertical extent of the source includes the whole of the crust

velocity structure and seismicity of southeastern tennessee the seismic zone in southeastern tennessee is at the confluence of major crustal features which have been interpreted largely from potential data and their relation to seismicity could help us understand why major earthquakes sometimes occur in the eastern united states in this paper we solve for the previously unknown velocity structure of the upper crust by an inversion of travel time residuals from relocated earthquakes the gravity anomalies are included by using a linear relation between average anomalous density and average anomalous velocity the velocity model demonstrates that the seismicity is concentrated in areas of average to below average velocity and does not appear to be associated with one of the previously identified major crustal features the high velocity zones mark areas that are generally lacking in seismicity the association of earthquake hypocenters with regions of low velocity crustal rocks is consistent with other intraplate seismic zones and this association supports the conjecture that intraplate earthquakes occur in crust that may have been weakened the velocity anomalies at midcrustal depths do not support the new york alabama ny al lineament as a linear feature extending through southeastern tennessee and parallel to contours in gravity anomalies as originally proposed a continuation of the ny al lineament to the southwest requires either a 15 degree southwestward change in direction or a displacement to be consistent with the velocity anomalies the seismically active areas in southeastern tennessee do not appear to be constrained by the major crustal features but instead the seismicity is characterized by the distribution of hypocenters and their association with low velocity regions at midcrustal depths

ice cores as an index of global volcanism from 1850 to the present to evaluate an important cause of past climate change climate researchers need reliable estimates of volcanic aerosol loading in the atmosphere previous indices the dust veil index volcanic explosivity index and those by mitchell 1970 sato et al 1993 and khmelevtsov et al unpublished manuscript 1993 all have drawbacks ice core acidity and sulfate records because they contain physical evidence of atmospheric loading are a promising source of information on past volcanic aerosols but these records contain large nonvolcanic signals as well we have compared northern hemisphere nh and southern hemisphere sh annual average versions of five indices with eight nh and six sh ice core records for the period 1850 to the present in an attempt to identify the volcanic signal common to all records the indices are all highly correlated with each other for the nh although the individual ice core records are in general not well correlated with each other or with any of the indices a composite derived from averaging the cores the ice core volcano index ivi shows promise as a new index of volcanic aerosol loading this new index correlates well with the existing non ice core volcanic indices and with high frequency temperature records for the sh the individual ice cores and indices are better correlated the shivi is again highly correlated with all indices and individual ice cores but not with high frequency temperature records for both hemispheres the southern oscillation index shows no significant correlation with the volcanic indices ice cores or the ivi thus providing no evidence for the impact of volcanic eruptions on el nino southern oscillation events

a 10 century comparison of prominent bipolar volcanic events in ice cores measurements of key chemical and physical parameters made along continuous and selected long sections of polar ice cores provide reliable past snow accumulation rates and other environmental records the prime accumulation indicators include variations found in the stable isotopes ionic constituents and acidity concentration levels and physical changes in the strata and structure cross correlations of the time series curves resulting from multiparameter analyses of ice cores from antarctica nby89 sp78 bs68 and greenland cr74 d3 81 d3 18c have been made the results permit construction of a bipolar chemical stratigraphy chronology of volcanic events that is coherent with the 18 o and electrical conductivity method data sets and consistent back in time with historically recorded volcanic activity a bihemispheric comparison of high non sea salt so 4 2 peaks shows strong correlation at 1885 1836 1816 1810 1459 1346 1287 1278 1259 1228 and 1168 a d at time of snow deposit the prominent 1259 a d volcanic signal appears to be a significant bipolar ice core index horizon

a comparison of the volcanic records over the past 4000 years from the greenland ice core project and dye 3 greenland ice cores since 1980 the electrical conductivity method ecm has been used to infer volcanic acid signals in greenland ice cores the method reveals the great majority of major volcanic eruptions including several known from historic records subsequent ion chromatographic analyses of the acid volcanic layers show the chemical composition i e the concentration of the volcanic acids h2so4 hci and hf plus e g the nonvolcanically derived hno3 while ecm data are available from a large number of shallow depth greenland ice cores covering the past 500 1500 years only the greenland ice core project grip greenland ice sheet project 2 gisp2 and dye 3 deep ice cores exist for a detailed comparative study of volcanic signals in greenland ice cores representing several thousand years comparison of the volcanic signals registered in the grip and gisp2 cores will be presented elsewhere the latter cores were augered 30 km apart and essentially represent the same atmospheric conditions such as temperature snow accumulation and chemical composition of the air here we present a comparison between the major volcanic signals over the past 4000 years in the grip core from central greenland and the dye 3 core from se greenland in order to investigate the depositional differences many of the major signals are detected in both cores but some of the differences in the records can be used to infer the latitudinal band of some eruption sites furthermore the influence of the amount of annual precipitation and glaciological postdepositional processes on the volcanic signals is discussed

evidence for a major late precambrian tectonic event rifting in the eastern midcontinent region united states recently acquired gravity and aeromagnetic data delineate a large linear gravity anomaly which extends through eastern kentucky and tennessee and coincides with a zone of complex high amplitude magnetic anomalies basement lithologies in the area can be interpreted as a bimodal volcanic suite which is locally peralkaline in nature these volcanics appear to be metamorphosed where they lie east of the grenville front suggesting they predate the grenville metamorphic event the available gravity aeromagnetic seismic refraction and petrologic data along with regional correlations suggest that the best tectonic interpretation of these data is that a keweenawan rift zone extended through the area this rift can be roughly outlined by the gravity high which is locally offset suggesting the presence of transform faults the boundaries of this rift have been locally reactivated and in fact a recent earthquake was located along its western boundary in northern kentucky

a new isostatic residual gravity map of the conterminous united states with a discussion on the significance of isostatic residual anomalies to display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust a new colored isostatic residual gravity map of the conterminous united states has been prepared using the gravity data set compiled for the gravity anomaly map of the united states society of exploration geophysicists 1982 the new isostatic residual gravity map is based on an airy heiskanen model of local compensation in which the surface load requiring compensation is defined by 5 min topographic and bathymetric data sets a colored first vertical derivative map of isostatic residual gravity further enhances the short wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field for the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features anomalies on isostatic residual gravity maps should not be casually interpreted in terms of undercompensation or overcompensation because large amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies we present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment in general highs correlate with intruded or accreted mafic material or with upthrusted crustal sections whereas lows occur over low density sedimentary or volcanic sections felsic intrusive bodies or down warped crustal sections although some longer wavelength anomalies such as the broad gravity high centered over montana could be manifestations of density contrasts deep in the mantle many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers so that their source bodies need not be deep the fact that certain of these broader anomalies have well defined boundaries which correlate with near surface geologic features increases the likelihood that their sources lie entirely within the lithosphere if so then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions we have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the united states these fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories

modelling massive sulphate aerosol pollution following the large 1783 laki basaltic eruption abstract the climatic impact of volcanic eruptions depends mainly on the amount of sulphur rich gases released into the stratosphere these are rapidly converted to sulphate aerosols which result in cooling of the lower troposphere and warming of the lower stratosphere these gases are retained in the stratosphere and then concentrated by atmospheric processes forming acid fog near the earth s surface with potentially dramatic environmental consequences on plants animals and humans we have modelled such emissions in the case of the unusually large 1783 1784 basaltic fissure eruption of laki iceland using an atmospheric general circulation model results show good agreement with historical observations such as typical time taken by individual pulses to reach continental europe or geographical extent of the deadly haze that covered much of the northern hemisphere the model could be adjusted to predict the climatic consequences of very large eruptions up to the scale of massive flood basalts and test the proposal that these are the main agent in most mass extinctions of life on earth

climate of the last 500 years high resolution ice core records precipitation accumulating on the earth s ice sheets and ice caps records a variety of physical and chemical information about the atmosphere and in some cases provides unique insight to both the history and the mechanics of the earth s environmental system high resolution well dated dust records from both polar ice sheets suggest a linkage between increased atmospheric dust and cooler temperatures over antarctica but a similar relationship is not observed in ice cores from either greenland or china net accumulation histories for the last 490 years reveal no discemible global pattern although interesting regional differences and similarities exist excess non sea salt sulfate profiles provide

au ir ni and co in some chromitites of the eastern bushveld complex south africa abstract instrumental neutron activation analyses of the lower group middle group and upper group chromitite layers of the eastern bushveld complex are reported the elements analysed were ir 300 ppb au 240 ppb ni 540 to 2240 ppm co 101 to 370 ppm ir and au are higher in the upper critical zone compared to the lower critical zone the relationship of the element abundance to the amount and composition of the chromite in the sample and the possible controls on the distribution of the elements are discussed at low levels of concentration ir is most likely controlled by discrete primary ir bearing platinum group minerals and au is related to small amounts of base metal sulphide both being sympathetic with the volume percent chromite in the sample higher ir and au levels are possibly related to sulphides present in even greater amounts

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

was the midcontinent rift part of a successful seafloor spreading episode the 1 1 ga midcontinent rift mcr the 3000 km long largely buried feature causing the largest gravity and magnetic anomaly within the north american craton is traditionally considered a failed rift formed by isolated midplate volcanism and extension we propose instead that the mcr formed as part of the rifting of amazonia precambrian northeast south america from laurentia precambrian north america and became inactive once seafloor spreading was established a cusp in laurentia s apparent polar wander path near the onset of mcr volcanism recorded by the mcr s volcanic rocks likely reflects the rifting this scenario is suggested by analogy with younger rifts elsewhere and consistent with the mcr s extension to northwest alabama along the east continent gravity high southern appalachian rocks having amazonian affinities and recent identification of contemporaneous large igneous provinces in amazonia

possible extension of the midcontinent rift in west texas and eastern new mexico the midcontinent rift system forms one of the most prominent gravity features in north america the recognized geophysical anomaly extends in an arc from southern oklahoma to lake superior and then into southern michigan the midcontinent rift system was active between 1185 1010 ma as indicated in the lake superior region by age determinations on intrusive igneous rocks we suggest that the period of formation of the midcontinent rift was also a time of extensive igneous activity in texas and new mexico this activity is represented by intrusions beneath the central basin platform texas and new mexico intrusions which crop out at the pajarito mountain in the sacramento mountains new mexico a basaltic debris flow in the franklin mountains texas basalt flows at van horn texas and the crosbyton geophysical anomaly east of lubbock texas these bodies and other bodies located by geophysical anomalies and wells drilled into mafic precambrian rocks may be related to the midcontinent rift system

petrogenesis of the merensky reef in the rustenburg section of the bushveld complex a petrogenetic model for the merensky reef in the rustenburg section of the bushveld complex has been developed based on detailed field and petrographic observations and electron microprobe data the model maintains that the reef formed by reaction of hydrous melt and a partially molten cumulate assemblage the model is devised to account for several key observations 1 although the dominant rock type in the rusterburg sections is pegmatoidal feldspathic pyroxenite there is a continous range of reef lithology from pyroxenite to pegmatoidal harzburgite and dunite and small amounts of olivine are present in nearly all pegmatoids 2 the pegmatoid is usually bounded above and below by chromitite seams and the basal chromitite separated from underlying norite by a centimeter thick layer of anorthosite the thicknesses of the two layers exhibit a well defined positive correlation 3 inclusion of pyroxenite identical to the hanging wall and of leuconorite identical to the footwall are present in the pegmatoid the leuconorite inclusions are surrounded by thin anorthosite and chromitite layers in the same sequence as that at the base of the reef 4 chromite in seams adjacent to plagioclase rich rocks is characterized by higher mg mg fe and al r3 and lower cr r3 than that in seams adjacent to pyroxene rich rocks similar variations in mineral compositions are observed across individual chromitite seams where the underlying and overlying rock types differ the chromite compositional variations cannot be rationalized in terms of either fractional crystallization or reequilibration with surrounding silicates it is proposed that the present reef was originally a melt rich horizon in norite immediately overlain by relatively crystallized pyroxenite magmatic vapor generated by crystallization of intercumulus melt migrated upward through fractures in the cumulate pile below the protoreef the melt rich protoreef became hydrated because fractures were unable to propagate through it and because the melt itself was water undersaturated hydration of the intercumulus melt was accompanied by melting and the hydration melting front migrated downward into the footwall and upward into the hanging wall in the footwall melting resulted first in the dissolution of orthopyroxene and then of plagioclase with continued hydration chromite was stabilized as melt alumina content increased the regular variations in chromite compositions reflect the original gradients in melt composition at the hydration front the stratigraphic sequence downward through the base of the reef or pegmatoid melt chromitite anorthosite norite represents the sequence of stable mineral assemblages across the hydration melting front the sequence is shown to be consistent with knowledge gained from experiments on melting of hydrous mafic systems at crustal pressures with cooling the hydrated mixture from partial melting of norite footwall and more mafic hanging wall crystallized in the sequence chromite olivine pyroxene plagioclase with peritectic loss of some olivine calculations of mass balance indicate that a significant proportion of the melt was lost from the melt rich horizon variations in the development of the pegmatoid and associated lithologies and amount of modal olivine in the pegmatoids along the strike of the merensky reef resulted because the processes of hydration melting and melt loss operated to varying extents

eight centuries of volcanic signal and climate change at talos dome east antarctica 1 during the 1996 programma nazionale di ricerche in antartide international trans antarctic scientific expedition traverse two firn cores were retrieved from the talos dome area east antarctica at elevations of 2316 m td 89 m long and 2246 m st556 19 m long cores were dated by using seasonal variations in non sea salt nss so42 concentrations coupled with the recognition of tritium marker level 1965 1966 and nss so42 spikes due to the most important volcanic events in the past pinatubo 1991 agung 1963 krakatoa 1883 tambora 1815 kuwae 1452 unknown 1259 the number of annual layers recognized in the td and st556 cores was 779 and 97 respectively the d record obtained from the td core has been compared with other east antarctic isotope ice core records dome c epica south pole taylor dome these records suggest cooler climate conditions between the middle of 16th and the beginning of 19th centuries which might be related to the little ice age lia cold period because of the high degree of geographical variability the strongest lia cooling was not temporally synchronous over east antarctica and the analyzed records do not provide a coherent picture for east antarctica the accumulation rate record presented for the td core shows a decrease during part of the lia followed by an increment of about 11 in accumulation during the 20th century at the st556 site the accumulation rate observed during the 20th century was quite stable

an inter hemispheric volcanic time marker in ice cores from greenland and antarctica a strong volcanic acid signal is clearly registered using an acidity measuring technique in the a d 1259 ice layer in four different greenland ice cores camp century milcent crete and dye 3 this signal is similar in amplitude to the laki iceland a d 1783 volcanic event as recorded in the central and south greenland ice cores measurement of ice layers from corresponding age levels in antarctic ice cores byrd station south pole and j 9 on the ross ice shelf provides similar strong acid signals there is no historical record of a significant volcanic eruption for the period around a d 1260 in the northern hemisphere subsequent chemical analyses of all a d 1259 ice layers show similar compositions we suggest that the a d 1259 signals registered in both greenland and antarctica were caused by the same volcanic disturbance and that its epicenter was located at the earth s equatorial zone which enabled global distribution of the acid gases these results indicate that inter hemispheric dating of ice sheets is possible by the chemical identification of major eruptive volcanic events in the equatorial zone

the pinatubo eruption in south pole snow and its potential value to ice core paleovolcanic records snow samples collected in the 1996 austral summer at south pole show that sulfate concentrations in snow and by inference sulfur aerosol concentrations in the antarctic atmosphere were elevated from the end of 1991 to mid 1994 over a stable non volcanic background the new data support earlier findings that the june 1991 pinatubo eruption and the hudson eruption in the same year deposited volcanic sulfate and tephra in south pole snow and provide strong evidence of the global distribution of volcanic materials from the pinatubo eruption in this study snow samples were taken in six snow pits spatially distributed around the south pole station in order to evaluate the local spatial variability of volcanic signals due to glaciological variables such as snow accumulation rates and snow redistribution by wind after initial deposition the results indicate that pinatubo sulfate flux varies by as much as 20 throughout a 400 km2 area centered around the south pole station this glaciological variability probably represents the likely range of volcanic signals due to variations in snow deposition and post depositional changes the pinatubo eruption provides an unprecedented opportunity to estimate aerosol mass loadings by explosive volcanic eruptions found in antarctic ice cores via a quantitative relationship between aerosol mass loadings and sulfate flux in antarctic snow here the satellite estimated pinatubo so2 emission and the measured volcanic sulfate flux in snow with an assumed linearly quantitative relationship are used to calculate so2 loadings for several well known volcanic eruptions in the past 300 years covered by a shallow 42 m south pole firn core drilled in 1996 the errors for the calculated mass loadings are estimated by means of the glaciological variability associated with pinatubo volcanic flux

a 4100 year record of explosive volcanism from an east antarctica ice core extensive archives of volcanic history are available from ice cores recovered from the antarctic and greenland ice sheets that receive and preserve sulfuric acid fallout from explosive volcanic eruptions the continuous detailed average 1 2 samples per year sulfate measurements of a 200 m ice core from a remote east antarctica site plateau remote provide a record of southern hemisphere volcanism over the last 4100 years this extends the volcanic record beyond the last 1000 years covered by previous antarctic ice cores an average of 1 3 eruptions per century is recorded in east antarctic snow during the last 4100 years the record shows that on average eruptions have been more frequent and more explosive during the most recent 2000 years than in the previous 2100 years intervals up to 500 years are observed in which few explosive volcanic signals are detected these periods include 2000 1500 b c no eruptions 500 1 b c two eruptions and 700 1200 a d two eruptions this new plateau remote volcanic record is compared with those from previous antarctic ice cores covering the last 1000 years in terms of dates for volcanic events the new record is in excellent agreement with the earlier records however significant discrepancies are found between these records in relative signal magnitude volcanic flux of several well known events the discrepancies among the records may be explained by the differences in the glaciology at the ice core sites analytical techniques used for sulfate and sulfuric acid measurement and the selection of detection thresholds for volcanic signals comparison with greenland ice core volcanic records indicates that during the last millennium nine large low latitude eruptions contributed significant amounts of volcanic aerosols to the atmosphere of both hemispheres potentially affecting global climate in contrast only one or possibly two such eruptions are found in the first millennium a d

the 1452 or 1453 a d kuwae eruption signal derived from multiple ice core records greatest volcanic sulfate event of the past 700 years 1 we combined 33 ice core records 13 from the northern hemisphere and 20 from the southern hemisphere to determine the timing and magnitude of the great kuwae eruption in the mid 15th century we extracted volcanic deposition signals by applying a high pass loess filter to the time series and examining peaks that exceed twice the 31 year running median absolute deviation by accounting for the dating uncertainties associated with each record these ice core records together reveal a large volcanogenic acid deposition event during 1453 1457 a d the results suggest only one major stratospheric injection from the kuwae eruption and confirm previous findings that the kuwae eruption took place in late 1452 or early 1453 which may serve as a reference to evaluate and improve the dating of ice core records the average total sulfate deposition from the kuwae eruption was 93 kg so4 km 2 in antarctica and 25 kg so4 km 2 in greenland the deposition in greenland was probably underestimated since it was the average value of only two northern greenland sites with very low accumulation rates after taking the spatial variation into consideration the average kuwae deposition in greenland was estimated to be 45 kg so4 km 2 by applying the same technique to the other major eruptions of the past 700 years our result suggests that the kuwae eruption was the largest stratospheric sulfate event of that period probably surpassing the total sulfate deposition of the tambora eruption of 1815 which produced 59 kg so4 km 2 in antarctica and 50 kg so4 km 2 in greenland

the midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting