mineralogic and isotopic constraints on the origin of strontium rich cap rock tatum dome mississippi u s a abstract the limestone portion of the salt dome cap rock at tatum dome mississippi is composed of an upper massive zone and a lower banded zone the upper zone consists of equigranular fine crystalline calcite with veinlets and disseminations of carbonaceous matter associated with minor amounts of detrital and authigenic quartz sulfides and sr minerals the lower zone is composed of alternating light and dark calcite bands the dark bands are composed of fine grained and peloidal calcite quartz bitumen and disseminated sulfide minerals the lighter bands consist of variable proportions of generally coarse crystalline euhedral calcite celestite and strontianite resulting in sr contents of up to 30 locally solubility data for celestite strontianite calcite and anhydrite suggest that a decrease in temperature favors the replacement of ca minerals by sr minerals which is consistent with the observed mineral textures and paragenesis however the source of cap rock sr is difficult to determine anhydrite at tatum dome contains 800 ppm sr but the abundance of sr minerals in the limestone cap rock and the 87 sr 86 sr ratios of limestone cap rock minerals require a sr source other than local anhydrite sr released by the replacement of anhydrite by calcite is capable of producing a molar ratio of celestite to calcite of only 0 001 yet locally this ratio is 3 a likely source of additional sr is oilfield brines such as those in central mississippi that contain up to 3000 ppm sr along with significant pb and zn episodic introduction of brines into the cap rock in combination with the action of sulfate reducing bacteria probably caused the sequential production of the dark bands the lighter bands probably reflect introduction of a later sr enriched fluid or evolution of the original fluid in combination with changes in the chemical parameters controlling mineral precipitation calcite was replaced by sr minerals during this later sr rich event

multiple fluid components of salt diapirs and salt dome cap rocks gulf coast u s a abstract salt diapirs contain a few percent of anhydrite that accumulated as residue to form anhydrite cap rocks during salt dissolutions reported 87sr 86sr ratios of these salt hosted and cap rock anhydrites in the gulf coast u s a indicate their derivation from middle jurassic seawater however a much wider range of 87sr 86sr ratios incorporating a highly radiogenic component in addition to the middle jurassic component has been found in several gulf coast salt domes this wide range of 87sr 86sr ratios of anhydrite within the salt stocks records sr contributions from both marine water and formation water that has equilibrated with siliciclastics during cap rock formation this anhydrite either recrystallized in the presence of or was cemented by a low sr fluid with a late cretaceous seawter type sr isotope ratio or simply lost sr during recrystallization later the cap rock was invaded by warm saline brines with high sr isotope ratios from which barite and metal sulfides were precipitated subsequently low salinity water hydrated part of the anhydrite bringing to six the total number of fluids that interacted througout the history of salt dome and cap rock growth the progenitor of these salt diapirs the louann formation is generally thought to have formed from marine water evaporated to halite and rarely higher evaporite facies salt domes in the east texas north louisiana and mississippi salt basins have 87sr 86sr and 34s values that corroborate a mid jurassic age for the mother salt however salt domes in the houston and rio grande embayments of the gulf coast basin have 87sr 86sr ration ranging to values higher than both middle jurassic seawater and all rb free marine phanerozoic rocks these anomalous 87sr 86sr ratios are probably derived from radiogenic sr bearing fluids that equilibrated with siliciclastic rocks and invaded the salt either prior to or during diapirism potential sources of the radiogenic 87sr component include clay and or feldspar located either in older units beneath the louann formation or younger units flanking the salt diapirs and k salts within the louann evaporites because partial sr exchange in anhydrite had to take place in a fluid medium admittance of radiogenic 87sr bearing fluids into the salt may have led to diapirism by lowering the shear strength of the crystalline salt the slight number of anomalous 87sr 86sr values in the interior basins indicates that anomalous values are related to areally discrete structural or stratigraphic controls that affected only the gulf coast basin

chemical studies of h chondrites i mobile trace elements and gas retention ages abstract trends for 16 trace elements ag as au bi cd co cs ga in k rb sb se te tl and zn chosen to span a broad geochemical and thermal response range in 44 h4 6 chondrites differ widely from those in l4 6 chondrites in particular h chondrites classified as heavily shocked petrologically do not necessarily exhibit 40 ar loss and vice versa the clear cut causal relationship between siderophile and mobile element loss with increasing late shock seen in l chondrites is not generally evident in the h group h chondrite parent material experienced an early high temperature genetic episode that mobilized a substantial proportion of these trace elements so that later thermal episodes resulted in more subtle collateral fraction ations mildly shocked l chondrites escaped this early high temperature event indicating that the two most numerous meteorite groups differ fundamentally in genetic history

metallic sulfide deposits in the winnfield salt dome louisiana evidence for episodic introduction of metalliferous brines during cap rock formation abstract winnfield dome is a shallow piercement salt structure which penetrates late jurassic through early tertiary siliciclastic and carbonate strata of the north louisiana basin quarrying operations in the calcite and anhydrite portions of the cap rock have exposed zones of metallic sulfides and barite within the anhydrite portion of the cap rock sulfides form laminar zones parallel to the tightly intergrown layered anhydrite sulfides occur either as cement between anhydrite grains or as coarse euhedral crystals in open space the sulfide layers are up to several centimeters thick and extend for tens of meters these layers locally thicken into laminated flat bottomed concave downward massive sulfide mounds monoclinic pyrrhotite is the dominant sulfide sphalerite galena or barite may also be present in weathered samples hydrating to gypsum marcasite is common and appears to be an alteration product of pyrrhotite a roughly laminated massive sulfide lens is exposed at the calcite anhydrite transition zone this sulfide body has a maximum thickness of about 5 m 16 ft and extends for about 45 m 150 ft the edges of the lens taper to a layer about 30 cm 1 ft thick which separates the overlying calcite from the anhydrite the massive sulfide lens is comprised of marcasite and pyrite complexly intergrown with calcite gypsum and anhydrite colloform sphalerite galena marcasite pyrite and barite line vugs and fractures pyrrhotite is found locally within the sulfide lens examination of rotary drill samples suggests that massive sulfide concentrations similar to the exposed example are common along the calcite anhydrite transition zone these sulfide concentrations are believed to have originated from the interaction of metalliferous basinal brines with reduced sulfur trapped within the cap rock textural relationships and variations in chemical compositions between the sulfide layers in the anhydrite portion of the cap rock suggest that distinct pulses of metalliferous brines were responsible for the sulfide concentrations anhydrite grains that are completely surrounded by sulfides are euhedral and undeformed similar to the anhydrite disseminated throughout the salt mass anhydrite grains outside the mineralized areas are deformed and tightly intergrown these textures suggest that mineralizing fluids were introduced episodically along the salt anhydrite interface at the zone of salt dissolution before that portion of the anhydrite zone was compressed and accreted to overlying anhydrite cap rock therefore the earliest formed sulfides originating by this mechanism occur at the top of the anhydrite cap rock zone while the last sulfides to form are found at the base extensive sulfide concentrations along the anhydrite calcite contact suggest that this contact also acted as a permeable zone allowing metalliferous brines into the cap rock textural and compositional relationships suggest that sulfides that formed along the anhydrite calcite contact are locally superimposed on sulfides that formed at the salt anhydrite contact

36cl and53mn in antarctic meteorites and10be 36cl dating of antarctic ice cosmic ray produced53mn t1 2 3 7 106years has been measured in twenty antarctic meteorites by neutron activation analysis 36cl t1 2 3 0 105years has been measured in fourteen of these objects by tandem accelerator mass spectrometry cosmic ray exposure ages and terrestrial ages of the meteorites are calculated from these results and from rare gases 14c t1 2 5740years and26al t1 2 7 2 105years data the terrestrial ages range from 3 104 to 5 105 years many of the l3 allan hills chrondrites seem to be a single fall based on these results in addition 10be t1 2 1 6 106years and36cl have been measured in six antarctic ice samples the first measurements of10be 36cl ratios in the ice core samples demonstrate a new dating method for ice

the breakup of a meteorite parent body and the delivery of meteorites to earth whether many of the 10 000 meteorites collected in the antarctic are unlike those failing elsewhere is contentious the antarctic h chondrites one of the major classes of stony meteorites include a number of individuals with higher induced thermoluminescence peak temperatures than observed among non antarctic h chondrites the proportion of such individuals decreases with the mean terrestrial age of the meteorites at the various ice fields these h chondrites have cosmic ray exposure ages of about 8 million years experienced little cosmic ray shielding and suffered rapid postmetamorphic cooling breakup of the h chondrite parent body 8 million years ago may have produced two types of material with different size distributions and thermal histories the smaller objects reached earth more rapidly through more rapid orbital evolution

composite methodology for interpreting sediment transport pathways from spatial trends in grain size a case study of the lithuanian coast to achieve a more robust interpretation of sediment conditions and transport based on grain size this study combines the perspectives of two different interpretation techniques that have been developed separately and applied in the literature i grain size trend analysis interprets changes between interrelated sampling sites assuming that sediment in the direction of transport should become either coarser better sorted and more positively skewed or finer better sorted and more negatively skewed and ii the population anomalies method which estimates the balance between erosion and accumulation processes at each sampling site based on individual site deviations compared with average values for grain size parameters in the area of interest the simultaneous use of both approaches enables sedimentological conditions and transport alternatives to be distinguished this improvement is necessary because site specific changes and general textural trends provide information that complements the strengths and weaknesses of each perspective the methods are implemented here using two dimensional geographic information system tools and illustrated for the lithuanian coast grain size characteristics of 712 surface samples of sandy sediments are used to interpret sediment transport pathways and predominant sedimentological conditions in the lithuanian coastal zone in general coast parallel sediment transport dominates the entire investigation area although wave induced movement perpendicular to the coastline is inferred in the shallow nearshore zone the deepest areas offshore are characterized by sea floor erosion in the north whereas an accumulation zone occurs in the relatively deep central part of the study area

origin of exotic minerals in mississippi salt dome cap rocks results of reaction path modeling abstract shallow salt domes in central mississippi pierce mesozoic formations hosting regionally extensive metal rich brines this association coupled with the multiple fluid migration pathways around salt domes suggests that these brines were the source of fe and base metal sulfide sr and ba minerals in salt dome cap rocks fluid inclusions in barite and celestite indicate that the brines had a relatively constant average composition of approximately 200 000 mg i total dissolved solids which is similar to present day oil field brines from central mississippi geochemical reaction path modeling indicates that the suite of observed cap rock minerals can be produced by alternating periods of biogenic so4 reduction with inorganic anhydrite dissolution or by mixing with seawater sulfide rich portions of cap rock at hazlehurst salt dome have molar fe pb and fe zn ratios similar to present day oil field brines indicating that mixing of brines with abundant amounts of h2s produced by biogenic so4 reduction led to rapid precipitation of virtually all dissolved metals as relatively insoluble sulfide minerals

chemical studies of l chondrites iii mobile trace elements and ar 40 ar 39 ages abstract we report data for trace elements ag au bi cd co cs ga in rb se te tl and zn determined by radiochemical neutron activation analysis in l4 6 chondrites with undisturbed 40 ar release patterns or with patterns showing some disturbance in the 4 4 4 6 gyr plateau indicating shock induced loss mean concentrations are lower many significantly so in 16 chondrites with disturbed patterns than in 4 with undisturbed ones consistent with shock induced mobilization similar trends were noted earlier in l4 6 chondrites having mineralogically observable shock indicators mean concentrations are lower in strongly shocked i e 22 gpa than in mildly shocked 40 ar release patterns are mildly shocked but chondrites with disturbed patterns are more strongly heated on average than those of shock facies d f i e 22 to 57 gpa pooling these populations significantly lower mean concentrations of nearly all trace elements in 26 strongly shocked l4 6 chondrites than in 14 mildly shocked ones indicate loss in shock formed fes fe eutectic and or by vaporization during cooling of shock heated collisional debris two element correlations and the pattern of them i e correlation profiles are also consistent with this picture trace elements can act as thermometers for collisional episodes in l4 6 chondrites but not for earlier thermal fractionations unless compensation can be made for late shock heating

the induced thermoluminescence and thermal history of plagioclase feldspars feldspars are a common component in igneous and metamorphic rocks most feldspars exhibit luminescence and this has proved useful in a number of mineralogical applications in this paper we concentrate on the thermoluminescence tl properties of feldspar or the luminescence produced when a sample of feldspar is heated we determined the induced tl properties of four feldspars of various compositions in their natural states and after heating and we compared the tl data with structural changes as determined by x ray diffraction the major tl peak at 120 240 c in the tl glow curve a plot of light intensity against temperature varies significantly among feldspar bearing samples meteorites and lunar samples with slow cooling histories of 10 c my as determined by independent methods have induced tl peak temperatures of 120 c while samples with fast cooling histories 100 c my have induced tl peak temperatures of 220 c this variation in tl peak temperature can be reproduced by heating the present feldspar samples meteorites and lunar samples prior to the tl measurement most of the present samples in their natural state had tl peak temperatures of 120 c heating below 750 c in the laboratory caused no change in tl peak temperatures or the structural disorder of the feldspar while heating 750 c caused tl peak temperatures to move to 220 c and disordered the feldspar structure we suggest that induced tl peak temperature in feldspar is influenced by the degree of al si ordering in the feldspar thus induced tl peak temperature can be used as an indicator of cooling rate for igneous and metamorphic rocks

chemical and physical studies of type 3 chondrites i metamorphism related studies of antarctic and other type 3 ordinary chondrites abstract thermoluminescence sensitivity measurements have been made on 18 unequilibrated ordinary chondrites 12 finds from antarctica 5 non antarctic finds and 1 fall the tl sensitivities of these meteorites normalized to dhajala range from 0 034 st mary s county to 2 3 allan hills a78084 and based primarily on these data petrologic type assignments range from 3 3 st mary s county to 3 9 allan hills a78084 although the very low levels of metamorphism experienced by types 3 0 to 3 4 evidently cause large changes in tl sensitivity the new data demonstrate that they are unable to cause any appreciable homogenization of silicate compositions we have therefore slightly revised the silicate heterogeneity ranges corresponding to the lower petrologic types we have discovered that the temperature of maximum tl emission and the broadness of the major tl peak vary systematically with tl sensitivity as tl increases these parameters first decrease and then increase several mechanisms which could account partially or completely for the relationship between tl sensitivity and metamorphism are discussed those which involve the formation of feldspar the tl phosphor in equilibrated meteorites seem to be consistent with the trends in peak temperature and peak width since experiments on terrestrial albite show that the tl peak broadens and moves to higher temperatures as the stable form changes from the low ordered state to the high disordered state the post metamorphism equilibration temperature of type 3 5 meteorites would then correspond to the transformation temperature for the high to low form of meteorite feldspar other factors which may be involved are obscuration of the tl by carbonaceous material changes in the composition of the phosphor and changes in the identity of the phosphor

recent hydrocarbon alteration sulfate reduction and formation of elemental sulfur and metal sulfides in salt dome cap rock abstract geochemical analysis of hydrocarbons and scanning electron microscopy sem provide insight to recent alteration of crude oil microbial sulfate reduction and mineral deposition in carbonate cap rock at damon mound brazoria county texas u s a previous work showed that intense aerobic biodegradation of hydrocarbons occurs in shallow cap rock where s 0 and metal sulfides are not abundant the present study focuses on some deeper carbonate cap rock characterized by more limited oxygen availability the c 1 c 4 hydrocarbons in deeper cap rock are a biodegraded residue of hydrocarbons derived from crude oil alteration of higher molecular weight crude oil components is best explained by the effects of water washing accompanied by some biodegradation sem of cavities in carbonate cap rock shows microbes in association with globular masses of solid crude oil and sulfate minerals late forming minerals in cavities include s 0 pyrrhotite pyrite sphalerite and aragonite the presence of s 0 and sulfide minerals indicates that microbial sulfate reduction is associated with limited hydrocarbon biodegradation in this deeper cap rock environment similar processes could explain the origin of some s 0 and metal sulfides at damon mound in the geologic past

differences between antarctic and non antarctic meteorites an assessment abstract the discovery of a statistically significant number of meteorites in antarctica over the past 20 years has posed many questions one of the most intriguing suggestions that came up during the study of the antarctic samples was that there might be a difference between the parent populations of antarctic and non antarctic meteorites this interpretation was put forward after the detection of a significant difference in the abundances of volatile and mobile trace elements in h l and c chondrites and achondrites other major differences include the occurrence of previously rare or unknown meteorites different meteorite type frequencies petrographic characteristics oxygen isotopic compositions and smaller average masses in addition antarctic meteorites have greater terrestrial residence ages and have collected on the ice shield over several 10 5 years the reality of numerous such differences has now been established beyond doubt however the main question regarding the cause of these differences remains it seems that they have a wide variety of origins ranging from pre terrestrial traits to collection recovery effects and terrestrial weathering studies of terrestrial weathering have shown that over the long time the meteorites spend in and on the ice even subtle processes can produce substantial effects for future investigations it will be important to pay more attention to the weathering status of samples and to develop a more reliable and quantitative weathering index e g based on infrared reflectance spectrometry or differential scanning calorimetry not all differences between the antarctic and non antarctic meteorite populations can be explained by weathering pairing or different collection procedures variable trace element abundances and distinct differences in the thermal history and thermoluminescence characteristics have to be interpreted as being pre terrestrial in origin such differences imply the existence of meteoroid streams whose existence poses problems in the framework of our current knowledge of celestial mechanics however several independent studies support the existence of such meteoroid streams thus being consistent with the suggestion of a time variable influx of extraterrestrial material to earth the generally smaller average size of antarctic meteorites may be the cause for the different meteorite type frequency and the higher abundance of rare samples because smaller meteorites may come from a slightly different parent population in this paper we summarize the contributions in this series and provide a review of the current state of the question for the reality and cause of differences between antarctic and non antarctic meteorites

x ray computed tomography imaging a not so nondestructive technique x ray computed tomography has become a popular means for examining the interiors of meteorites and has been advocated for routine curation and for the examination of samples returned by missions here we report the results of a blind test that indicate that ct imaging deposits a considerable radiation dose in a meteorite and seriously compromises its natural radiation record ten vials of the bruderheim l6 chondrite were placed in ct imager and exposed to radiation levels typical for meteorite studies half were retained as controls their thermoluminescence tl properties were then measured in a blind test five of the samples had tl data unaltered from their original 10 cps while five had very strong signals 20 000 cps it was therefore very clear which samples had been in the ct scanner for comparison the natural tl signal from antarctic meteorites is 5000 50 000 cps using the methods developed for antarctic meteorites the apparent dose absorbed by the five test samples was calculated to be 83 5 krad comparable with the highest doses observed in antarctic meteorites and freshly fallen meteorites while these results do not preclude the use of ct scanners when scientifically justified it should be remembered that the record of radiation exposure to ionizing radiations for the sample will be destroyed and that tl or the related optically stimulated luminescence are the primary modern techniques for radiation dosimetry this is particularly important with irreplaceable samples such as meteorite main masses returned samples and samples destined for archive

chemical and physical studies of type 3 chondrites iii chondrules from the dhajala h3 8 chondrite abstract fifty eight chondrules were separated from the dhajala h3 8 chondrite and their thermoluminescence properties were measured chips from 30 of the chondrules were examined petrographically and with electron microprobe techniques the bulk compositions of 30 chondrules were determined by the fused bead technique porphyritic chondrules especially 5 which have particularly high contents of mesostasis tend to have higher tl mass normalized than non porphyritic chondrules significant correlations between log tl and the bulk cao al 2 o 3 and mno content of the chondrules and between log tl and the cao al 2 o 3 sio 2 and normative anorthite content of the chondrule glass indicate an association between tl and the abundance and composition of mesostasis unequilibrated chondrules i e those whose olivine is compositionally heterogeneous and high in ca have low tl whereas equilibrated chondrules have a wide range of tl depending on their chemical and petrographic properties we suggest that the tl level in a given chondrule is governed by its bulk composition which largely determined the abundance and composition of constituent glass and by metamorphism which devitrfied the glass in those chondrules with high ca glass to produce the tl phosphor we also suggest that one reason why certain chondrules in type 3 ordinary chondrites are unequilibrated while others are equilibrated is that the mesostasis of the unequilibrated chondrules resisted the devitrification this devitrification is necessary for the diffusive communication between chondrule grains and matrix that enables equilibration

studies of an artificially shock loaded h group chondrite five samples of the naturally unshocked kernouve h6 meteorite were artificially shock loaded to pressures of 70 165 270 and 390 kbar and the silicates and metal examined optically by scanning and transmission electron microscopy and by thermoluminescence tl olivine deformation is closely comparable to that in naturally shocked meteorites producing dislocations with burgers vector 001 at pressures of 165 kbar these are formed in well defined slip planes at 270 kbar olivine develops optical mosaicism has high dislocation densities throughout and is also highly fractured recovery due to heating is minimal in orthopyroxene the deformation mechanism changes from the clino inversion to unit dislocation slip between 70 and 165 kbar in diopside 001 and 100 twinning was produced plagioclase is inferred to have been progressively converted to maskelynite but some is still present in 270 kbar sample the microhardness of the kamacite in the samples increases with shock pressure the transformation pressure in the kamacite is 30 40 kbar higher than observed for iron meteorites annealed kamacite displays incipient polycrystallinity and martensite and taenite sometimes contains slip lines troilite acquired cracks undulose extinction twins polycrystallinity and finally melted as the shock pressure increased at pressures over 200 kbar there was a systematic decrease in the natural tl and the tl sensitivity detailed considerations of changes in the natural tltl sensitivity ratio for various regions of the tl glow curve suggest that two processes were effective during shock thermal drainage of electron traps and a reduction in the effective trap density it is suggested that the latter process associated with the vitrification of feldspar the tl phosphor an additional sample was subjected to a shock pulse which was spiked instead of square very distinctive changes were apparent thermal effects are conspicuous and with widespread annealing 600 800 c of metal and sulfide glassy opaque veins were produced which are analogous to the black veins in shock lithified gas rich meteorites anomalous low temperature tl was induced suggesting that a new or modified phase or mineral has become the dominant tl phosphor

chemical studies of l chondrites ii shock induced trace element mobilization abstract previous studies of chondrites heated in the laboratory for extended periods under conditions approximating those in shock heated collisional debris indicate that au co se ga rb cs te bi in ag zn tl and cd progress in mobility we report data for these 13 trace elements in 14 l4 6 chondrites of established shock history and discuss these and 13 additional chondrites studied earlier trace element contents vary with petrologic type s fe sub group and shock history the last dominating strongly absolute abundances and interelement relationships for the 6 or 7 most mobile elements vary with degree of shock loading i e residual temperatures established from mineralogic petrologic study a tertiary process shock heating previously known to have affected radiogenic 40 ar and or 4 he in meteorites but not other elements apparently was at least as effective as other open system processes secondary parent body and primary nebular and or accretionary episodes in establishing mobile trace element contents of l chondrites and probably others if conditions during early genetic episodes are to be deduced from compositional information shocked meteorites should be avoided or effects of later processes should be compensated for

deposition of magnetic pyrrhotite during alteration of crude oil and reduction of sulfate abstract crude oil seepage is widespread in carbonate cap rock and in overlying sediments of the damon mound salt dome brazoria county texas scanning electron microscopy of calcite lined cavities provides insight to the latest stage of mineral deposition in this salt dome environment solid crude oil residues and microbes occur in association with surface minerals such as gypsum and barite deposition of pyrrhotite pyrite marcasite sphalerite and elemental sulfur is attributed to microbial sulfate reduction and deposition of aragonite and calcite is related to microbial oxidation of crude oil hydrocarbons the c1 c4 hydrocarbons in pyrrhotite bearing cap rock samples are a biodegraded residue derived from crude oil higher molecular weight crude oil components in cap rock are altered by water washing and to a lesser extent by biodegradation hydrocarbon compositions could be explained by limited aerobic biodegradation but a component of anaerobic biodegradation cannot be excluded some damon mound samples are highly magnetic because of abundant ferrimagnetic pyrrhotite this study provides new evidence of a link between crude oil migration microbial activity and diagenetic magnetic anomalies in rocks

chemical and physical studies of type 3 chondrites 2 thermoluminescence of sixteen type 3 ordinary chondrites and relationships with oxygen isotopes thermoluminescence tl sensitivity values for sixteen type 3 ordinary chondrites fourteen of them from antarctica have been measured the values obtained normalized to the tl sensitivity of the dhajala meteorite range from 1 6 allan hills a77216 to 0 010 allan hills a77176 and include two reckling peak a80207 and allan hills a77176 that are particularly low they fill a hiatus in the tl distribution that previously existed between st mary s county and bishunpur the latter being a meteorite with one of the lowest tl sensitivities known the histogram of tl sensitivity values now shows a single distribution with higher values preferred it resembles the histogram for l chondrites occupying the petrologic types 3 4 5 and 6 there is a tendency for the tl sensitivity of meteorites to decrease as 18o increases equilibrated meteorites have tl sensitivities of 5 50 and 18o 4 5 to 5 0 while the most primitive meteorites with tl 0 01 have 18o 5 7 to 6 2 meteorites with intermediate tl have intermediate 18o theoretically it is possible that the range of 18o values observed may reflect progressive loss of o in the form of co at very low temperatures but very restrictive physical conditions and a complex history seem to be required

origin of an anhydrite sheath encircling a salt diapir and implications for the seismic imaging of steep sided salt structures egersund basin northern north sea the ingress of meteoric fluids into the crests of salt structures typically results in the formation of anhydrite dominated caprock the migration of connate fluids up the margins of salt structures has also been documented although the products related to this are rarely sampled in the deep subsurface we use 3d seismic reflection and borehole data to document the geometry and lithology of a salt diapir seismic data suggest that a weld is developed along the diapir stem although borehole data indicate that the stem consists of an inner c 1500 m thick halite dominated zone and an outer c 250 m thick anhydrite dominated sheath the anhydrite may have formed early in the basin history as a depositional anhydrite or crestal caprock an alternative interpretation is that the anhydrite represents lateral caprock which formed late in the basin history in response to the migration of nacl poor fluid up the margins of the diapir and dissolution of halite the possibility that lateral caprock may form adjacent to salt structures has implications for understanding the patterns and vigour of groundwater flow in sedimentary basins furthermore our study shows the margins of steep sided salt structures may be misidentified by several hundred metres if time migrated seismic reflection data are used

evolution of metal ratios and δ34s composition of sulfide mineralization during anhydrite cap rock formation hockley dome texas u s a metal zonation and sulfur isotope patterns of cap rock hosted sulfides reveal a complex evolution of the sulfide depositing system at the hockley salt dome the general metal trend observed is an overall increase of zn relative to pb downward in the anhydrite cap rock in the upper cap rock section the pb pb zn ratios commonly range from 0 25 to 0 40 ratios of 0 20 and commonly 0 10 are typical of the lower cap rock section the concentration of ag also appears to increase in the lower part of the anhydrite cap rock textural evidence indicates that some of the metal concentrations represent stratiform sulfides that were precipitated at the salt cap rock contact as the anhydrite zone accumulated by sequential underplating the metal ratio data suggest that the sulfide precipitating fluid became more zn and possibly ag rich during anhydrite cap rock accumulation 34s values of sulfide minerals within the hockley dome cap rock range from 30 to 4 cdt 59 analyses a profile of 34s values from a single drill hole indicates that sulfide minerals become progressively heavier with depth to the approximate middle of the cap rock where the isotopic trend reverses the inflection point corresponds with a local highly mineralized biogenic calcite zone a mixing model involving variable contributions of isotopically heavy and light sulfur end members appears to best account for the observed isotopic trends fault leaked h2s from mature oil and gas reservoirs is a possible extrinsic source of heavy sulfur but local thermochemical sulfate reduction of cap rock sulfate cannot be eliminated by existing data bacterial reduction of cap rock sulfate is believed to be the source of light h2s the relative abundance of deep sourced metalliferous brines appears to have increased with time during the accretion of the upper part of the anhydrite cap rock biogenic h2s subsequently began to dominate the mixed sulfur reservoir resulting in a 15 shift and relatively light 34s values of pyrite to the present cap rock salt contact

the thermoluminescence sensitivity metamorphism relationship in ordinary chondrites experimental data on the mechanism and implications for terrestrial systems hydrothermal annealing experiments have been performed on samples of the sharps meteorite in order to investigate the mechanism responsible for the metamorphism related 105 fold range in the thermoluminescence tl sensitivity in ordinary chondrites duplicate 50 mg samples of meteorite were annealed under the following conditions a 168 h at 785 c and 1 kbar b the same time temperature and pressure but with 2 wt water c 174 h at 855 c and 0 77 kbar with 2 wt water and 2 molal sodium disilicate nadisi d the same time temperature and pressure as the preceding samples but with 10 wt h2o and 2 molal nadisi samples annealed under the first three sets of conditions showed little or no change in their tl sensitivities however the samples annealed with 10 wt water and 2 molal nadisi showed a 3 to 10 fold increase in tl sensitivity and the temperature of the tl peak was suggestive of feldspar in the high temperature form we suggest that these data are consistent with the tl sensitivity metamorphism relationship in ordinary chondrites being due to the formation of the tl phosphor feldspar by the crystallization of chondrule glass

mossbauer and thermoluminescence studies of meteorites from frontier mountain antarctica mossbauer and thermoluminescence tl data have elucidated the terrestrial histories of five equilibrated chondrites of petrologic types 5 and 6 collected on blue ice fields near frontier mountain in antarctica mossbauer spectroscopy substantially reflects the destruction of iron bearing species with formation of finely divided aggregates of iron oxides or oxyhydroxides tl sensitivity reflects the possible coating of the feldspar grains due to the iron oxide mobility during progressive weathering

antarctic and non antarctic meteorites form different populations compared with non antarctic falls antarctic meteorites include many of hitherto rare unique or unknown type1 such as lunar and putative martian samples2 4 attention has not previously been paid to the more common sorts of antarctic meteorites and here we focus on the most frequently encountered h5 chondrites we find that compositional differences between antarctic and non antarctic specimens of this class especially those involving thermally sensitive trace and ultra trace elements are so substantial that it seems extremely doubtful that the two sample populations are derived from the same parent population this implies that the contemporary falls less than 200 years old and the substantially older on average 300 000 years for victoria land antarctic meteorites either differ in genetic history or sample different extraterrestrial parent populations or both

petrography and geochemistry of water rock interactions in richton dome cap rock southeastern mississippi u s a abstract cap rock at richton dome consists of gypsum veined anhydrite overlain by dark and light banded calcite the anhydrite accumulated directly during dissolution of the salt stock without forming gypsum indicating temperatures 60 c and nacl salinity 4 m sr and 87 sr 86 sr were inherited directly from salt stock the anhydrite is a simple dissolution residue textures and 13c indicate that dark colored calcite formed as a replacement of precursor anhydrite cap rock in the presence of methane oxidizing bacteria coprecipitated light colored calcite is more enriched in 13c indicating influx of carbonate bearing groundwater as well during calcite cap rock formation all calcite appears to be enriched in trace metals relative to the anhydrite precursor sr is also enriched but 87 sr 86 sr is similar to jurassic seawater the calcite cap rock geochemistry is most easily explained if meteoric groundwaters at the top of the salt diapir were mixed with sr rich metal rich brines brought up the sides of the dome during calcite cap rock formation petrographic analysis suggests that gypsum veins in the anhydrite cap rock formed by simple hydration of the wall rock anhydrite along fractures d and 18o of the gypsum crystal water indicate that the incoming water was meteoric and that the water rock ratio was very low modern hydrologic communication through the cap rock is suggested by dissolution enhancement of fractures in both anhydrite and calcite cap rock particularly along the calcite anhydrite interface

chemical and physical studies of type 3 chondrites vii annealing studies of the dhajala h3 8 chondrite and the thermal history of chondrules and chondrites abstract samples of the dhajala h3 8 chondrite have been annealed for 10 hours at 600 700 800 900 and 1000 c and at 1000 c for 1 2 20 and 100 h and their thermoluminescence tl properties measured the tl sensitivity decreased by a factor of 2 after annealing at but at higher temperatures fell by an order of magnitude an abrupt increase in the temperature of the tl peak from 172 9 cto231 8 c and a steady increase in the width of the peak from 169 7 cto212 5 c were caused by the annealing treatment the tl phosphor in dhajala is thought to be feldspar predominantly in the high temperature disordered form but the present data indicate that a contribution from the low temperature form is also present and that this low temperature component is converted to the high form by the annealing treatment the low temperature feldspar is located in a few of the chondrules 20 of those separated from the meteorite which are also noteworthy for having high tl sensitivities these chondrules must have suffered greater crystallization of their mesostasis than the other chondrules and equilibrated to lower temperatures it is argued that for compositional reasons their mesostasis constituted less of a barrier to diffusion and therefore equilibration presumably the post metamorphic cooling rate of the meteorite through the stability field of the low form was slow enough to permit some crystallization and the width and temperature of the tl peaks for petrologic types 3 5 3 9 are somehow related to cooling rate based on tl there is no indication of a correlation between petrologic type and cooling rate for types 3 5 3 9 this is not consistent with a simple single internally heated meteorite parent body

chemical studies of h chondrites ii weathering effects in the victoria land antarctic population and comparison of two antarctic populations with non antarctic falls abstract we report rnaa data for 14 siderophile lithophile and chalcophile volatile mobile trace elements ag au bi cd co cs ga in sb se te tl u zn in interior portions of 45 different h4 6 chondrites 49 samples from victoria land antarctica and 5 h5 chondrites from the yamato mts antarctica relative to h5 chondrites of weathering types a and b all elements are depleted 10 at statistically significant levels in extensively weathered types b c and c samples probably by leaching on the ice sheet surface contents of 8 elements in extensively weathered samples may provide a qualitative ranking for terrestrial surface residence chondrites of weathering types a and b seem compositionally uncompromised and as useful as contemporary falls for trace element studies when data distributions for these 14 trace elements in non antarctic h chondrite falls and unpaired samples from victoria land and from the yamato mts queen maud land are compared statistically numerous significant differences are apparent concentrations of 8 elements differ significantly in the victoria land and non antarctic h5 chondrite populations essentially the same compositional differences and 53 mn content and shock history differences appear when h4 6 populations are compared contents of 8 elements differ when queen maud land and victoria land populations are compared and 5 for the queen maud land non antarctic comparison these and other differences give ample cause to doubt that the various sample populations derive from the same parent population the observed differences do not reflect weathering chance or other trivial causes a preterrestrial source must be responsible the least unlikely of these involves a temporal variation in the source regions from which meteoroids derive

chemical and physical studies of type 3 chondrites iv annealing studies of a type 3 4 ordinary chondrite and the metamorphic history of meteorites abstract samples of a type 3 4 chondrite have been annealed at 400 1000 c for 1 200 hours their thermoluminescence properties determined and analyzed for k na mn sc and ca by instrumental neutron activation analysis after annealing at 900 c the samples showed a 50 decrease in tl sensitivity while after annealing at 1000 c it fell to 0 1 0 01 times its unannealed value and loss of na and k occurred the tl and compositional changes resemble those observed for the equilibrated kernouve chondrite after similar annealing treatments except that the sharp tl decrease and element loss occurred at 1100 c this difference is presumably due to petrographic differences in the feldspar of the two meteorites the temperature and the width of the tl peak showed a discontinuous increase after annealing at 800 c peak temperature jumped from 130 to 200 c and peak width increased from 90 to 150 c the activation energies for these tl changes are 7 10 kcal mole similar increases in the tl peak temperature have been reported in tl studies of amelia va albite where they were associated with the low to high temperature transformation however the activation energy for the transformation is 80 kcal mole these changes in tl emission characteristics resemble trends observed in type 3 ordinary chondrites and it is suggested that type 3 3 3 5 chondrites have a low feldspar as tl phosphor and 3 5 have high feldspar as the phosphor thermoluminescence therefore provides a means of palaeothermometry for type 3 ordinary chondrites

a petrographic and geochemical model for the origin of calcite cap rock at damon mound salt dome texas u s a abstract calcite cap rock at damon mound texas consists of five calcite generations recognizable through standard petrography and cathodoluminescence stage ia calcite formed by dissolution of anhydrite followed by calcite precipitation and was partly replaced by stage ib calcite stage ii formed above stage ia and ib substrates either filling open voids or cementing breccia fragments stage iiia and iiib calcites fill late fractures that crosscut all previously formed generations microprobe analysis indicates that each calcite generation is geochemically distinct sr mg ratios indicate that calcite precipitated from variable proportions of formation water meteoric water and or seawater wide variation in calcite fluid inclusion compositions 1 0 9 4 eq wt nacl indicates mixing of high salinity water and low salinity meteoric water the single phase nature of most fluid inclusions and the homogenization temperatures indicate that calcite precipitated at temperatures of calcite 13 c values range from 31 1 to 14 4 pdb generally decrease with depth and are most similar to crude oil 13 c values calcite 18 o values range from 7 6 to 4 8 pdb generally increase with depth and are compatible with derivation from local meteoric water or formation water at high temperature late stage calcite can be either isotopically heavier or lighter than adjacent early stage calcite with regard to both carbon and oxygen and because isotopic values vary with depth it appears that fluid mixing is the mechanism most responsible for isotopic variations although 13 c and 18 o values of stage i calcite are similar to adjacent stage ii and iii calcite 87 sr 86 sr ratios of adjacent calcite pairs are generally different indicating that each formed from different fluids or a fluid whose 87 sr 86 sr composition varied through time due to mixing isotopic modeling indicates that calcite cap rock formed from either of two fluid mixtures 1 formation water and meteoric water between 40 and 80 c the source of light carbon being either thermogenic ch 4 or liquid hydrocarbons or 2 formation water meteoric water and seawater between 50 and 70 c with thermogenic ch 4 being the light carbon source sr was contributed by both anhydrite and fluid overall it appears that calcite cap rock formed from top to base from mixtures of formation water containing isotopically light carbon and heavy oxygen and seawater and or meteoric water containing isotopically heavy carbon and light oxygen during this progression the contributions of formation water increased at the expense of seawater and or meteoric water the correlation between 18 o and 13 c probably indicates that separate fluid pulses had different carbon abundances and or slightly different 13 c values

fluid rock interactions in the salt dome environment an introduction and review abstract the salt dome environment in the gulf of mexico coast u s a provides one of the most diverse and long lived records of fluid rock interactions in an active sedimentary basin dominated by saline formation waters the geochemical record most of which is isotopic appears to span a temperature range between earth surface temperatures and lower greenschist facies most mineral forming processes in the salt dome cap rocks appear to involve mixtures of warm saline formation fluids from deep basin sources and cool dilute meteoric waters papers in this issue of chemical geology document some of the current research on fluid rock interactions in the salt dome environment these papers highlight the diversity of opinion about salt dome processes offering clues for further research the present paper reviews some of the key and more current literature on salt dome geochemistry fluid convection around salt domes and fluid rock interactions within salt and within the salt dome environment

thermoluminescence and the shock and reheating history of meteorites iv the induced tl properties of type 4 6 ordinary chondrites abstract the induced thermoluminescence tl properties of 121 equilibrated ordinary chondrites have been measured the samples were 74 h and 47 l chondrites of which 33 h and 32 l were from antarctica the distribution of tl sensitivities for non antarctic l chondrites differs from that of non antarctic h chondrites consistent with a greater proportion of the former class suffering post metamorphic shock data on the effect of laboratory annealing on tl sensitivity and step wise ar release measurements enable the meteorites to be sorted into three shock related temperature groups

seismic interpretation and characterization of anhydrite caprocks in the tromso basin sw barents sea abstract the origin of anhydrite caprocks found in many evaporite basins is currently under debate existing models in the literature for anhydrite formation include rotated stratigraphic layering rotated crestal caprock and diagenetic anhydrite in this work we interpret high quality 2d seismic reflection data to understand the processes associated with anhydrite caprock formation our study focuses on anhydrite caprock formation in the tromso basin which represents one of several underexplored salt basins in the arctic we use well log data and seismic stratigraphic as well as seismic attribute analyses to characterize the anhydrite layers associated with salt diapirs our results reveal that the anhydrite layers are high amplitude reflections at the crests which are often found close to the seabed and lateral caprocks on the flanks of salt diapirs a third class of anhydrite layers is folded faulted and boudinaged intrasalt stringers encased by the salt diapirs lateral caprocks in the tromso basin are formed in response to rotated crestal caprock failure the crestal caprocks are secondary in origin occur at shallow depths and are formed as insoluble residues from the dissolution of halite by the ingress of basinal fluids into the tops of diapirs anhydrite stringers or floaters originate from tectonic related multi dimensional salt flow salt diapirs in the tromso basin are associated with varied anhydrite caprocks their formation cannot be fully explained using a single model our work demonstrates from seismic reflection data the occurrence of caprock events that formed progressively on the flanks of salt diapirs due to local folding

diagenesis of the carrizo sandstone at butler salt dome east texas basin u s a evidence for fluid sediment interaction near halokinetic structures abstract the eocene carrizo sandstone adjacent to the butler salt dome in the east texas basin was cemented locally as the result of processes similar to those that have created salt dome calcite cap rocks throughout the gulf coast the geometry of this anomalous cemented zone was established using carrizo exposures in a large quarry and cores in the contiguous area the cemented zone is delimited by a steeply dipping normal fault that is radial to the dome the cements extend southeast of the fault for about 750 m the carrizo is a fine to medium grained quartzarenite to sublitharenite with 42 to 50 cement principally poikilotopic calcite with local pyrite pyrite concretions are found on both northwest and southeast sides of the fault as well as along the fault plane whereas the calcite cement is present only to the southeast 13c data for calcite 19 to 37 suggest that bicarbonate was supplied mainly by bacterial oxidation of methane whereas 18o data for calcite 6 to 9 indicate that meteoric water was the principal diagenetic fluid 87sr 86sr values for calcite 0 7073 are within the range for middle jurassic seawater suggesting that dissolution of anhydrite from the louann salt provided the ca pyrite is more 34s rich 34s range of 10 to 16 than typical sulfides in gulf coast salt dome cap rocks and are interpreted to represent h2s supply from deep sour gas reservoirs petrographic and isotope evidence suggests that early h2s supply from deep sour gas reservoirs was channeled upward along the flank of the dome and outward along the radial fault resulting in early pyrite precipitation along the fault zone and in adjacent sediments early pyrite cementation along the fault may have formed a barrier to subsequent fluid circulation blocking the northwestward movement of both ca rich waters from the dome and deep natural gas moving along the fault thus confining calcite precipitation to the southeast bacterial oxidation of this thermogenic gas resulted in calcite precipitation although mixing with shallow ground water bicarbonate may account for the range in 13c values in the calcite cement we propose that this type of cementation occurs in similar settings associated with hydrocarbon leakage to shallow levels including adjacent to many salt domes in the gulf of mexico basin and may represent a previously unrecognized self sealing type of hydrocarbon trap

thermoluminescence and the shock and reheating history of meteorites ii annealing studies of the kernouve meteorite abstract samples of the unshocked equilibrated chondrite kernouve h6 have been annealed for 1 100 hours at 500 1200 c their thermoluminescence sensitivity measured and na k mn ca and sc determined by instrumental neutron activation analysis the tl sensitivity decreased with temperature until by 1000 c it had fallen by 40 the process responsible has an activation energy of 8 kcal mole and probably involves diffusion samples annealed 1000 1200 c had tl sensitivities 10 2 times the unannealed values most of the decrease occurring 1100 c this process has an activation energy of 100 kcal mole and is probably related to the melting of the tl phosphor feldspar with some decomposition and loss of cs na and k meteorites whose petrography indicates healing 1100 c by natural shock heating events shock facies d f have tl sensitivities similar to samples annealed 1100 c our own and literature compositional data indicate that tl is more stable to annealing than ag in tl bi zn and te and less stable than na k mn ca se and co while the tl decrease resembles very closely the pattern of cs loss on annealing

biogenic carbonates are they a criterion for underlying hydrocarbon accumulations an example from the gulf of suez region egypt the gulf of suez petroliferous basin is characterized by the occurrence of a middle upper miocene evaporite cap field and subsurface investigations in the southwest gulf of suez region indicate that the evaporites usually contain two types of biogenic carbonates in addition to sulfur the first type of carbonate is biogenic syngenetic in origin and was formed in low temperature euxinic aquatic environments by the reduction of dissolved sulfates and the oxidation of organic matter by sulfate reducing bacteria the resulting carbonate and sulfur are stratigraphically restricted within the evaporite sequence and occur as interlaminated gypsum calcite sulfur or as sulfur ooids sulfur globules and stromatolitic sulfur that are enclosed in gypsum or calcite the biogenic syngenetic carbonate and sulfur did not originate from petroleum and have a significance of euxinic aquatic settings the second type of carbonate is a biogenic epigenetic carbonate that formed under low temperatures by biochemical reactions between seeping hydrocarbons and the overlying miocene evaporites the carbonate is vuggy is included within or caps the evaporites and usually shows hydrocarbon stains or emits a hydrocarbon smell the sulfur is usually yellow but may be brown to black due to bitumen staining carbonates replace gypsum or anhydrite and occur as aragonite calcite or dolomite the d13c mean values for carbonate minerals range from 7 8 to 27 2 pdb being relatively heavier than the d13c values of the nearby crude oil 28 1 to 29 3 pdb carbon isotope values indicate that these carbonates were formed from the heavier organic carbon dioxide under reducing conditions where the lighter methane is stable the occurrence of biogenic epigenetic carbonates and sulfur in the evaporite deposits of the gulf of suez petroliferous basin is similar to that recorded in other basins such as the gulf coast basins of texas louisiana and southern mexico the fergana and amudarya depression of central asia and the mesopotamian depression in iraq the biogenic epigenetic carbonates or sulfur in some regions is considered as a criterion for petroleum microseepage that may indicate a potential hydrocarbon bearing reservoir at depth

a thermoluminescence study of experimentally shock loaded oligoclase and bytownite the thermoluminescence tl of samples of terrestrial oligoclase and bytownite shock loaded to 10 5 45 gpa have been measured the tl sensitivities of both feldspars decreased 10 fold after shock loading to 25 32 gpa and for the oligoclase 25 fold after shock loading to greater pressures the shock induced changes for oligoclase are very similar to those of the kernouve meteorite in which the tl phosphor is oligoclase after similar shock loading the data indicate that ordinary chondrites of shock facies d f have suffered shock pressures over 25 gpa consistent with petrographic estimates petrographic studies on the shocked samples suggest that the lowering of tl sensitivity is caused by maskelynization and at the highest pressures melting the oligoclase samples shocked above 34 gpa showed changes in their glow curve shapes similar to those produced by annealing amelia albite and a type 3 4 ordinary chondrite above 700 c the dominant peak moves from 132 8 c to 230 12 c changes in the glow carve shape for bytownite resembled those produced by annealing certain shergottites in addition to a major peak at 138 3 c a peak appeared at 178 8 c for albite the changes were shown to be associated with small degrees of disordering and we suggest that shock induced disordering or some process closely associated with the onset of disordering is occurring in the trace amounts of feldspar that survive maskelynization after shock loading to pressures 34 gpa