oxygen isotope studies of carbonaceous chondrites abstract the carbonaceous chondrites display the widest range of oxygen isotopic composition of any meteorite group as a consequence of the interaction of primordial isotopic reservoirs in the solar nebula these isotopic variations can be used to identify the reservoirs and to determine conditions and loci of their interactions we present a comprehensive set of whole rock analyses of cv co ck cm cr ch and ci chondrites as well as selected components of some of these meteorites a simple model is developed which describes the isotopic behavior during parent body aqueous alteration processes the process of thermal dehydration also produces a recognizable effect in the oxygen isotopic composition

40ar 39ar geochronology of the eocene green river formation wyoming the deposits of eocene lake gosiute that constitute the green river formation of wyoming contain numerous tuff beds that represent isochronous correlatable stratigraphic markers tuff beds selected for 40ar 39ar analysis occur within laminated mudstone are matrix supported and lack evidence of reworking these tuffs contain 2 15 euhedral phenocrysts of quartz plagioclase sanidine biotite and minor amphibole pyroxene and zircon encased in a matrix of altered glassy ash air abrasion and handpicking under refractive index oils were required to obtain clean unaltered phenocrysts of sanidine 40ar 39ar age determinations from single crystal and 1 mg multigrain aliquots of sanidine and biotite allowed the identification and exclusion of xenocrystic contamination laser fusion experiments on phenocrysts from the rife firehole c bed grey main sixth and analcite tuff beds from the tipton wilkins peak and laney members yielded weighted mean ages 2 analytical uncertainties of 51 25 0 31 ma 50 70 0 14 ma 50 56 0 26 ma 50 39 0 13 ma 49 96 0 08 ma 49 70 0 10 ma and 48 94 0 12 ma respectively ages for sanidine and biotite from the main tuff are indistinguishable when presumably xenocrystic contaminants are excluded from the age calculation moreover the 40ar 39ar ages are consistent with the stratigraphic order of the tuff beds and with provenance in the absaroka and challis volcanic fields our 40ar 39ar based age model indicates that sediment accumulated three times more rapidly 327 86 m yr during the evaporative wilkins peak phase than the freshwater to saline tipton 88 34 m yr and laney 104 18 m yr phases the much lower accumulation rates for the tipton and laney members are permissive of an annual origin for 1 mm thick laminae and precessional forcing of 1 3 m thick depositional cycles in these units however previously described cycles in the wilkins peak member have average durations that are significantly shorter than the 19 23 k y precessional modes the green river formation encompasses an 5 m y period between ca 53 5 and 48 5 ma spanning magnetic chrons 24n through 21r the green river formation was therefore deposited during the warmest period of the cenozoic corresponding to the early eocene climatic optimum as defined by the global marine o isotope record deposition of bedded evaporites trona of the wilkins peak member began at ca 51 ma or 2 m y after the onset of the highest inferred temperatures of the climatic optimum the bridgerian wasatchian faunal turnover occurred subsequently during wilkins peak time at ca 50 6 ma thus our 40ar 39ar ages strongly suggest that wilkins peak evaporite deposition and the turnover from wasatchian to bridgerian fauna were not directly caused by the initiation of maximum greenhouse conditions

terminal infill of eocene lake gosiute wyoming u s a abstract deposition of lacustrine sediments in the greater green river basin was progressively terminated in the middle eocene as volcaniclastic detritus prograded across the basin the lacustrine sediment of the laney member was replaced by deposition of the deltaic and fluvial sediments of the volcaniclastic bridger and washakie formations and the sand butte bed of the laney member the transition from the deposition of lacustrine to alluvial sediment also records a reversal in the direction of drainage across the basin that occurred between 49 5 ma and 48 9 ma prior to this transition drainage entered the basin from the east and the arkosic detritus of the cathedral bluffs member of the wasatch formation was deposited in the washakie and great divide subbasins at the same time evaporitic sediments of the wilkins peak member of the green river formation accumulated simultaneously in the bridger basin indicating that the clastic sediment was baffled east of the rock springs arch an intrabasinal structural high the subsequent transition to deposition of the laney member is traditionally interpreted as an isochronous expansion of lake gosiute across the greater green river basin alternatively the transition may have been diachronous with continued accumulation of the wilkins peak member in the bridger basin coinciding with the earliest deposition of the laney member in the washakie basin by 48 9 ma the dominant drainage was clearly from the west which reflects the capture of an external drainage network through the northwest corner of the basin that delivered extrabasinal volcaniclastic detritus to the greater green river basin more broadly these results illustrate that the geomorphic evolution of the landscape influenced the character of sedimentary fill in the nonmarine greater green river basin

modeling aqueous alteration of cm carbonaceous chondrites results from an inorganic geochemical modeling study support a scenario in which low temperature aqueous alteration of an anhydrous cm asteroidal parent body and melt water from h20 and co ices produces the altered assemblage observed in cm carbonaceous chondrites chrysotile greenalite tochilinite cronstedtite and minor calcite and magnetite we consider a range of possible precursor mineral assemblages varying with respect to the fe oxidation state of the initial anhydrous phases the aqueous solutions produced by this alteration are generally strongly basic and reducing and a large quantity of h2 and possible ch4 gas can be released during aqueous alteration

direct observation of ferrimagnetic ferroelastic domain interactions in magnetite below the verwey transition abstract the magnetic behaviour of magnetite at low temperatures is dominated by its transformation to a monoclinic crystal structure that is simultaneously ferrimagnetic ferroelastic and ferroelectric below 125 k the verwey transition here we use electron microscopy to reveal the relationship between ferrimagnetic and ferroelastic domain structures in monoclinic magnetite we present dynamic observations of magnetite during heating and cooling across the verwey transition revealing a diversity of unexpected interaction phenomena between crystallographic twins and magnetic domain walls this study provides a new understanding of the low temperature magnetic properties of magnetite that will affect a broad range of rock magnetic studies from the interpretation of magnetic remanence in terrestrial rocks and sediments to the search for biogenic magnetite in extraterrestrial materials

changes in remanence coercivity and domain state at low temperature in magnetite abstract submicron magnetite crystals with mean sizes of 0 037 0 10 and 0 22 m undergo major changes in hysteresis properties and domain states in crossing the verwey transition t v 120 k the 0 037 m crystals are single domain sd both in the cubic phase at room temperature t 0 and in the monoclinic phase below t v the 0 10 and 0 22 m crystals have a mixture of sd and two domain 2d states at room temperature t 0 but mainly sd structures below t v in agreement with micromagnetic calculations coercive force h c increases on cooling through t v by a factor 3 5 in the submicron magnetites and 40 in a 1 3 mm single crystal because of the high crystalline anisotropy and magnetostriction of monoclinic magnetite as a result domain walls and sd moments are so effectively pinned below t v that all remanence variations in warming or cooling are reversible however between 100 k and t 0 remanence behavior is variable saturation remanence sirm produced in monoclinic magnetite at 5 k drops by 70 100 in warming across t v with minor recovery in cooling back through t v ultimate levels at 5 k of 23 37 for the submicron crystals and 3 for the 1 3 mm crystal in contrast sirm produced in the cubic phase at 300 k decreases 5 35 submicron or 95 1 3 mm during cooling from 300 to 120 k due to continuous re equilibration of domain walls but there is little further change in cooling through t v itself however the submicron magnetites lose a further 5 15 of their remanence when reheated through t v these irreversible changes in cycling across t v and the amounts of the changes have potential value in determining submicron magnetite grain sizes the irreversibility is mainly caused by 2d sd transformations on cooling through t v which preserve or enhance remanence while sd 2d transformations on warming through t v cause remanence to demagnetize

the fall recovery orbit and composition of the tagish lake meteorite a new type of carbonaceous chondrite the preatmospheric mass of the tagish lake meteoroid was about 200 000 kilograms its calculated orbit indicates affinity to the apollo asteroids with a semimajor axis in the middle of the asteroid belt consistent with a linkage to low albedo c d and p type asteroids the mineralogy oxygen isotope and bulk chemical composition of recovered samples of the tagish lake meteorite are intermediate between cm and ci meteorites these data suggest that the tagish lake meteorite may be one of the most primitive solar system materials yet studied

trm arm and isr of two natural magnetites of md and psd grain size summary trm arm and isr stabilities with respect to alternating fields and high and low temperature treatment are compared for a set of artificial rock specimens the specimens contain grain size fractions of a homogeneous natural magnetite and a natural magnetite showing exsolution lamellae the grain size fractions are in the psd md range and vary between 5 and 250 m for large md grains the coercivity spectra of arm are similar to those of isr but differ from those of trm particularly in the lower part of the spectrum for small 5 m grains the coercivity spectrum of arm is similar to that of trm and both spectra are different from the coercivity spectrum of isr the use of arm instead of trm to determine palaeo intensities is discussed in relation to the results obtained during high and low temperature treatment arm behaves similarly to isr but unlike trm for all grain sizes studied the results of this study are discussed in terms of a remanence composed of psd surface moments and md and psd bulk moments

interstellar hydroxyl in meteoritic chondrules implications for the origin of water in the inner solar system abstract ion probe determinations of water concentrations and d h ratios have been performed on different phases pyroxene olivine and mesostasis of chondrules from the bishunpur and semarkona ll3 chondrites mean water concentrations average 1290 ppm in olivine 500 to 2100 ppm 2400 ppm in pyroxene 400 to 9800 ppm and up to 3950 ppm in the mesostasis 1000 to 16500 ppm in one chondrule an iron oxide phase exhibits a mean water concentration of 1 70 wt 1 21 to 2 15 wt hydrogen isotopic measurements reveal the presence of two water sources with high and low d h ratios pyroxenes range from 74 to 479 10 6 with a mean value of 186 10 6 n 54 the mesostasis from 124 to 203 10 6 with a mean value of 152 10 6 n 8 olivines from 63 to 209 10 6 with a mean value of 130 10 6 n 13 the iron oxide phase from 157 to 360 10 6 with a mean value of 219 10 6 n 7 hydroxyls in pyroxenes have been identified as submicroscopic poorly crystallized amphibole lamellae through high resolution transmission electron microscopy considering the broad range of d h ratios in pyroxene chondrules alteration in a unique meteoritic parent body seems to be excluded an early hydroxylation of the pyroxene occurring during chondrule formation due to the presence of water bearing minerals among chondrule precursors seems the only possible interpretation of the data accordingly the distribution of d h ratios in chondrules reflects that of their precursors such a distribution is quite similar to that found in matrix minerals as recorded by water concentration and d h ratio the mesostasis has been subjected to hydrothermal alteration after the incorporation of the chondrules into the meteoritic parent body this alteration took place with water having a mean d h ratio reflecting the mixing of the d depleted and d rich sources the d rich component is interpreted as a preserved interstellar source while the low d h ratio reflects a water component that underwent isotopic exchange with protosolar molecular hydrogen

hydrogen isotopic composition of water from fossil micrometeorites in howardites we have measured the hydrogen isotopic composition d h ratios of the water from 13 carbonaceous chondritic microclasts ccms size 1 mm trapped in two howardites kapoeta and yamato 793497 early in the evolution of solar system the division into tochilinite rich magnetite rich olivine poor magnetite rich olivine rich ccm types is corroborated by the hydrogen isotopic compositions both mineralogy and hydrogen isotopic compositions demonstrate that tochilinite rich ccms represent cm2 chondritic matter in contrast there is no good match between the isotopic and mineralogical properties of the magnetite rich ccms and the known groups of carbonaceous chondrites suggesting that magnetite rich ccms represent a new kind of chondritic matter not yet sampled in meteorite collections this demonstrates that the view of the asteroid belt revealed by the collection of meteorites is incomplete the study of micro clasts offers a unique opportunity to better decipher the nature and relative abundance of asteroids the average hydrogen isotopic composition of water belonging to ccms d h 152 0 4 8 x 10 6 1 sigma m is similar to that of antarctic micrometeorites amms d h 161 2 3 8 x 10 6 1 sigma m the similarity in terms of mineralogy and hydrogen isotopic composition between ccms and amms demonstrates that the composition of the micrometeorites has not been modified over the whole history of the solar system it indicates that the composition of the micrometeorite flux onto earth has been and is dominated by a mixture of cm2 like magnetite rich olivine poor magnetite rich olivine rich carbonaceous chondritic matter exemplified by ccms found in howardites because ccms have not suffered atmospheric entry they provide an abundant source of pristine micrometeorites the average d h ratio of the whole population of ccms is identical within errors to that of the earth 149 3 x 10 6 the match between the ccms d h ratio and that of the earth is especially remarkable because 1 three different populations of ccms are needed to make the d h ratio of the earth 2 there is no single carbonaceous chondrite group for which a similar match exists this observation suggests that ccms population might be representative of the late veneer agent s that delivered water to the earth

nature and degree of aqueous alteration in cm and ci carbonaceous chondrites we investigated the petrologic geochemical and spectral parameters that relate to the type and degree of aqueous alteration in nine cm chondrites and one ci ivuna carbonaceous chondrite our underlying hypothesis is that the position and shape of the 3 m band is diagnostic of phyllosilicate mineralogy we measured reflectance spectra of the chondrites under dry conditions elevated temperatures and vacuum 10 8 to 10 7 torr to minimize adsorbed water and mimic the space environment for subsequent comparison with reflectance spectra of asteroids we have identified three spectral cm groups in addition to ivuna group 1 the least altered group as determined from various alteration indices is characterized by 3 m band centers at longer wavelengths and is consistent with cronstedtite fe serpentine group 3 the most altered group is characterized by 3 m band centers at shorter wavelengths and is consistent with antigorite serpentine group 2 is an intermediate group between group 1 and 3 ivuna exhibits a unique spectrum that is distinct from the cm meteorites and is consistent with lizardite and chrysotile serpentine the petrologic and geochemical parameters which were determined using electron microprobe analyses and microscopic observations are found to be consistent with the three spectral groups these results indicate that the distinct parent body aqueous alteration environments experienced by these carbonaceous chondrites can be distinguished using reflectance spectroscopy high quality ground based telescopic observations of main belt asteroids can be expected to reveal not just whether an asteroid is hydrated but also details of the alteration state

magnetic hysteresis of pseudo single domain and multidomain magnetite below the verwey transition abstract hysteresis loops have been measured as a function of temperature between 10 k and room temperature for two size fractions of crushed magnetite 1 5 m pseudo single domain psd and 100 150 m multidomain md the material under study appears to be fairly close to stoichiometric magnetite with a verwey transition temperature of 115 k for the md sample and 118 k for the psd sample for both samples hysteresis properties below the verwey transition depend critically on whether or not the samples were cooled through the transition in the magnetic field for the psd sample an even more complex behavior has been observed temperature dependences of coercive force obtained after zero field cooling differ significantly for the demagnetized versus magnetized starting magnetic state

direct imaging of nanoscale magnetic interactions in minerals the magnetic microstructure of a natural finely exsolved intergrowth of submicron magnetite blocks in an ulvospinel matrix is characterized by using off axis electron holography in the transmission electron microscope single domain and vortex states in individual blocks as well as magnetostatic interaction fields between them are imaged at a spatial resolution approaching the nanometer scale the images reveal an extremely complicated magnetic structure dominated by the shapes of the blocks and magnetostatic interactions magnetic superstates in which clusters of magnetite blocks act collectively to form vortex and multidomain states that have zero net magnetization are observed directly

a comparative study of the arm and isr of some natural magnetites of md and psd grain size summary arm and isr stability with respect to alternating field and low temperature treatment were measured for a set of artificial rock specimens each of the specimens contained a known amount of a natural magnetic mineral of a known grain size the following minerals were used a homogeneous natural magnetite a natural titanomagnetite with submicroscopic fe rich inclusions and a natural titanomagnetite and a natural titano maghemite that both show exsolution phenomena the grain size of the fractions varied from 5 to 250 m it is found that the stability of arm and irs of multi domain grains with respect to alternating fields and low temperature treatment is similar whereas for the smallest grains studied 5 m arm is more stable than isr multi domain arm data from this study are in agreement with current multi domain arm theory low temperature behaviour of arm and isr can be explained either in terms of a model in which stable moments in the bulk of the grains are screened by the surrounding matrix or in terms of a model in which regeneration of the bulk remanence plays a role and in which stable moments are situated on the surfaces of the grains

origin of lacustrine rocks of wilkins peak member wyoming wilkins peak member of the green river formation with its interbeds of trona oil shale and dolomitic mudstone contrasts markedly with the predominantly oil shale and calcitic mudstone sequences of the remainder of the succession much discussion has centered on the origin of these lacustrine rocks a perennial meromictic stratified lake model has been replaced in recent years by a playa lake model the subsurface stratigraphy however was not well known when the models were developed paleogeographic reconstruction indicates that the region was subjected to variations in hydrographic conditions mainly in response to climatic fluctuations during humid subtropical periods widespread during the deposition of the tipton shale member and laney member the lakes were p edominantly open meromictic systems with rivers entering the basin from the north these environmental conditions were modified during deposition of the wilkins peak member as the climate became more arid and along the southern margin of the basin shallow saline alkaline playa lakes formed in response to these changes the widespread occurrence in the subsurface of fluvial clastic sequences of the wasatch formation associated with the wilkins peak member would indicate that the playa lakes were frequently flooded and freshened by southern drainage systems therefore the lakes were continuously undergoing modifications under the influence of climatic and possibly tectonic processes a composite model is supported that combines aspects of both the meromictic and playa lake models beca se of these complexities it is unlikely that one modern lake can serve as an analog for ancient lake gosiute

sedimentary sulfur geochemistry of the paleogene green river formation western usa implications for interpreting depositional and diagenetic processes in saline alkaline lakes abstract the sulfur geochemistry of the lacustrine paleogene green river formation colorado utah and wyoming usa is unlike that of most marine and other lacustrine rocks distinctive chemical isotopic and mineralogical characteristics of the formation are pyrrhotite and marcasite high contents of iron mineral sulfides strikingly enriched in 34 s cyclical trends in sulfur abundance and 34 s values and long term evolutionary trends in 34 s values analyses that identified and quantified these characteristics include carbonate free abundance of organic carbon 0 13 47 wt total iron 0 31 13 wt reactive iron 70 of total iron total sulfur 0 02 16 wt acid volatile monosulfide s av disulfide s di 70 of total sulfur sulfate s so 4 and organosulfur s org isotopic composition of separated sulfur phases 34 s di av up to 49 and mineralogy morphology and paragenesis of sulfide minerals mineralogy morphology 34 s di av and 34 s org have a distinctive relation reflecting variable and unique depositional and early diagenetic conditions in the green river lakes when the lakes were brackish dissimilatory sulfate reducing bacteria in the sediment produced h 2 s which initially reacted with labile iron to form pyrite framboids and more gradually with organic matter to form organosulfur compounds during a long lived stage of saline lake water the amount of sulfate supplied by inflow decreased and alkalinity and ph of lake waters increased substantially extensive bacterial sulfate reduction in the water column kept lake waters undersaturated with sulfate minerals a very high h 2 s so 4 ratio developed in stagnant bottom water aided by the high ph that kinetically inhibited iron sulfidization progressive removal of h 2 s by coeval formation of iron sulfides and organosulfur compounds caused the isotopic composition of the entire dissolved sulfur reservoir to evolve to 34 s values much greater than that of inflow sulfate which is estimated to have been 20 a six million year interval within lake uinta cores records this evolution as well as smaller systematic changes in 34 s interpreted to reflect 100 000 year lake level cycles when porewater was exceptionally reducing unstable fes phases eventually recrystallized to pyrrhotite during diagenesis a much later reaction related to weathering altered pyrrhotite to marcasite

hosts of hydrogen in allan hills 84001 evidence for hydrous martian salts in the oldest martian meteorite the martian meteorite allan hills alh 84001 contains d rich hydrogen of plausible martian origin leshin et al 1996 the phase identity of the host s of this hydrogen are not well known and could include organic matter mckay et al 1996 phlogopite brearley 2000 glass mittlefehldt 1994 and or other unidentified components of this rock previous ion microprobe studies indicate that much of the hydrogen in alh 84001 as texturally associated with concretions of nominally anhydrous carbonates glass and oxides boctor et al 1998 sugiura and hoshino 2000 we examined the physical and chemical properties of the host s of this hydrogen by stepped pyrolysis of variously pre treated subsamples a continuous flow method of water reduction and mass spectrometry eiler and kitchen 2001 was used to permit detailed study of the small amounts of this hydrogen poor sample available for study we find that the host s of d rich hydrogen released from alh 84001 at relatively low temperatures 500 c is soluble in orthophosphoric and dilute hydrochloric acids and undergoes near complete isotopic exchange with water within hours at temperatures of 200 to 300 c these characteristics are most consistent with the carrier phase s being a hydrous salt e g carbonate sulfate or halide the thermal stability of this material is inconsistent with many examples of such minerals e g gypsum and instead suggests one or more relatively refractory hydrous carbonates e g hydromagnesite hydrous salts particularly hydrous carbonates are common on the earth only in evaporite sabkha and hydrocryogenic weathering environments we suggest that much if not all of the martian hydrogen in alh 84001 was introduced in analogous environments on or near the martian surface rather than through biological activity or hydrothermal alteration of silicates in the crust

oxygen isotopic homogeneity of the earth new evidence abstract the two oxygen isotope ratios 18 o 16 o and 17 o 16 o were analyzed in precambrian 3 5 gy and modern cherts and in mantle derived lavas all samples exhibit oxygen isotopic compositions consistent with mass dependent isotopic fractionation from a single reservoir and thus suggest that 3 5 gy ago the earth was already a well mixed body these results would therefore not support models of ocean accretion by cometary impact later than 3 7 0 1 gy new measurements on lunar rocks confirm oxygen isotopic homogeneity of the earth moon system and thus suggests that the internal mixing of the two bodies pre dates their accretion the recent giant impact model used to explain how the moon was formed is also compatible with our results

isothermal magnetization of samples with stable preisach function a survey of hysteresis remanence and rock magnetic parameters isothermal magnetization curves like hysteresis loops initial curves back field curves acquisition curves and demagnetizing curves of isothermal remanent magnetization are commonly used for rock magnetic purposes in this study we investigate the relations among these curves and other useful magnetization curves saturation initial curve and induced and remanent hysteretic magnetization curves in order to compare coercivity and domain state parameters which can be derived from them most natural samples especially sediments are weakly magnetic and possess relatively stable preisach functions their magnetization states can therefore be described by classical preisach theory this approach verifies well known rules and establishes some formerly unreported relations between isothermal magnetization curves and parameters it is possible to point out sets of mathematically independent isothermal magnetization curves and to state theoretical interrelations between dependent curves by simply inspecting preisach diagrams furthermore we define six elemental isothermal magnetization curves from a general partition scheme of the preisach diagram they can be easily obtained from common measurements and generate all above mentioned curves the experimental applicability of our results is demonstrated for three single domain pseudo single domain multidomain marine sediment samples a physical rationale of the elemental curves reveals favorable properties for the investigation of interaction and domain state as a spin off from the general results a new hysteresis based procedure for the measurement of hcr is presented we also propose an apparently more robust hysteresis derived domain state parameter and a generalized version of the well known r parameter all presented methods can be applied without actually measuring preisach functions

controls on sr isotopic evolution in lacustrine systems eocene green river formation wyoming abstract strontium isotopes from lacustrine carbonates record detailed weathering histories of exposed bedrocks and thus are potentially useful for understanding interactions between tectonics and climate that may be driven by local or regional factors in basin scale hydrologic systems we combine extensive 87 sr 86 sr and 18 o datasets from the green river formation in order to identify environmental factors driving the evolution of sr concentration and isotopic composition in lacustrine systems through the use of sr mass balance modeling two models are developed the first tests the effect of drainage capture that drove a constant volume lake from balance filled to overfilled conditions and is applied to the laney member of the green river formation the second model focuses on lacustrine evolution in response to changing the concentration composition and mass of water influx in a variable size sr reservoir lake this model is applied to the wilkins peak member of the green river formation 87 sr 86 sr ratios are correlated with 18 o values only during the laney member when both are readily explained by a common forcing mechanism drainage capture modeling of sr isotope compositions and concentrations indicates a short residence time for sr 10 3 10 4 years or less in both balance overfilled and underfilled phases of the green river formation lacustrine system this in turn suggests that paleo lacustrine sediments in most lakes can preserve sr isotope records with high resolution 10 2 years timescales rates of sr sequestration in carbonate are shown to have a strong influence on lacustrine sr concentration and high sr concentrations of lacustrine carbonate consistent with high salinity are observed in the underfilled wilkins peak member and the balance filled tipton and lower laney members in the laney member 87 sr 86 sr mass balance modeling results provide additional support for previous interpretations that the introduction of a large drainage system produced an isotopic shift across the lower laclede upper laclede boundary major drainage reorganization is not required to drive high variability in 87 sr 86 sr ratios however modeling shows that variability in 87 sr 86 sr ratios of 0 004 observed in the wilkins peak member can be explained by change in the characteristics of intrabasinal water sources during highstand vs lowstand conditions

low temperature magnetic behavior of titanomagnetites titanium substitution has a significant effect on the temperature dependence 5 300 k of low temperature saturation remanence and of initial susceptibility in synthetic and natural titanomagnetite fe3 xtixo4 0 x 0 4 and electronic and lattice relaxation phenomena over the entire titanomagnetite compositional range these features produce diagnostic magnetic behavior useful for titanomagnetite identification in natural samples moreover the apparent similarities between the thermal decay of low temperature remanence of titanomagnetite with pyrrhotite pure magnetite and superparamagnetic phases can lead to complications in interpreting low temperature remanence data when titanomagnetite is present

deuterium enrichments in chondritic macromolecular material implications for the origin and evolution of organics water and asteroids here we report the elemental and isotopic compositions of the insoluble organic material iom isolated from several previously unanalyzed meteorites as well as the reanalyses of h isotopic compositions of some previously measured samples alexander et al 2007 the iom in ordinary chondrites ocs has very large d enrichments that increase with increasing metamorphism and decreasing h c the most extreme d value measured being almost 12 000 we propose that such large isotopic fractionations could be produced in the oc parent bodies through the loss of isotopically very light h2 generated when fe was oxidized by water at low temperatures 200 c we suggest that similar isotopic fractionations were not generated in the iom of cv and co chondrites with similar metamorphic grades and iom h c ratios because proportionately less water was consumed during metamorphism and the remaining water buffered the h isotopic composition of the iom even a h was being lost from it hydrogen would also have been generated during the alteration of ci cm and cr carbonaceous chondrites the iom in these meteorites exhibit a considerable range in isotopic compositions but all are enriched in d as well as 15n relative to terrestrial values we explore whether these enrichments could also have been produced by the loss of h2 but conclude that the most isotopically anomalous iom compositions in meteorites from these groups are probably closest to their primordial values the less isotopically anomalous iom has probably been modified by parent body processes the response of iom to these processes was complex and varied presumably reflecting differences in conditions within and between parent bodies the d enrichments associated with h2 generation along with exchange between d rich iom and water in the parent bodies means that it is unlikely that any chondrites retain the primordial h isotopic composition of the water ice that they accreted the h isotopic compositions of the most water rich chondrites the cms and cis are probably the least modified and their compositions d 25 suggest that their water did not form at large radial distances from the sun where ice is predicted to be very d rich yet models to explain the o isotopic composition of inner solar system bodies require that large amounts of ice were transported from the outer to the inner solar system

isotopic variations of the fixed nitrogen in metamorphic rocks the content and isotopic composition of fixed nitrogen nh 4 n in metasedimentary rocks phyllites mica schists and gneisses from prograde metamorphic zones in the erzgebirge saxon ore mountains g d r were measured and show that the nitrogen content decreases and 15n increases with increasing metamorphic grade the geochemical and isotopic behaviour of nitrogen during contact metamorphism was investigated in a mica schist profile through the contact aureole of a granitic pluton the nitrogen concentrations of the wall rocks decrease towards the contact whereas the 15n values become higher various model experiments were carried out in the laboratory to explain the changes observed in the rock samples the nitrogen isotope fractionation factor was determined for the isotopic exchange in the system ammonium sulphate ammonia at 250 c 1 0143 0 0012 and 350 c 1 0128 0 0006 in accordance with the model experiments the conclusion seems warranted that the change in nitrogen content and its isotopic composition in rocks during metamorphic processes depend mainly on the metamorphic temperature

correlated alteration effects in cm carbonaceous chondrites three parameters are proposed to determine the relative extent of alteration in cm chondrites the mineralogic alteration index monitors the relative progress of coupled substitutions in the progressive alteration of cronstedtite to mg serpentine and increases with increasing alteration to calculate values of this index an algorithm has been developed to estimate the average matrix phyllosilicate composition in individual cm chondrites the second parameter is the volume percent of isolated matrix silicates which decreases with progressive alteration due to mineral hydration finally the volume percent of chondrule alteration monitors the extent of chondrule phyllosilicate production and increases as alteration proceeds these parameters define the first cm alteration scale that relies on multiple indicators of progressive alteration the following relative order of increasing alteration is established by this model murchison bells pollen murray mighei nogoya cold bokkeveld the relative degree of aqueous processing cochabamba and boriskino experienced is less precisely constrained although both fall near the middle of this sequence a comparison between the mineralogic alteration index and literature values for the whole rock chemistry of cm chondrites reveals several correlations a positive nearly linear correlation between bulk h content and progressive cm alteration suggests an approximately constant production rate of new phyllosilicates relative to the mineralogical transition from cronstedtite to mg serpentine the abundance of trapped planetary 36ar decreases systematically in progressively altered cm chondrites suggesting the wholesale destruction of primary noble gas carrier phase s by aqueous reactions because low temperature fluid rock reactions are generally associated with large isotopic mass fractionation factors we also compared our model predictions with 18o values for bulk cm samples although some of these data are poorly resolved the order of increasing 18o values approximates the order of increasing alteration predicted by our model parameters multiple correlations between diverse alteration parameters strongly suggest that a different cm chondrites experienced similar kinds of processes and conditions and b cm materials experienced in situ alteration on the cm parent body or bodies

basin scale cyclostratigraphy of the green river formation wyoming the fl uviolacustrine wilkins peak member of the eocene green river formation preserves repetitive sedimentary facies that have been interpreted as an orbitally induced climate signal however previous quantitative studies of cyclicity in this member have used oil yield data derived from single locations here macrostratigraphy is used to quantitatively describe the spatiotemporal patterns of three different lithofacies associations from 8 to 12 localities that span much of the basin macrostratigraphic time series demonstrate that there is a reciprocal basinscale relationship between carbonate rich lacustrine facies and siliciclastic rich alluvial facies spectral analyses identify statistically signifi cant periods 90 confi dence level in basin scale sedimentation that are consistent with milankovitch predicted orbital periodicities with a particularly strong 100 k y cycle expressed in all lithofacies associations numerous non milankovitch periods are also recognized indicating complex depositional responses to orbital forcing autocyclic controls on sedimentation or harmonic artifacts although fl uctuations in lake gosiute water level did affect basin scale patterns of sedimentation they are not directly related to the 100 k y short eccentricity cycle as previously supposed instead 100 k y cycles are principally recorded by the recurrence of alluvial environments which exerted a dominant control on basin scale patterns of sedimentation generally thus the hydrologic controls on lake level that have been classically linked to short eccentricity actually occurred at finer temporal scales 100 k y understanding the complex links between orbital forcing and sedimentation in the wilkins peak member is facilitated by analysis of time series that refl ect spatial as well as temporal variability in stratigraphic data macrostratigraphy is therefore promising as an analytical tool for basin scale cyclostratigraphy

stepwise and continuous low temperature demagnetization 1 magnetites with sizes from 1 m to 135 m were cooled in zero field and their magnetizations m t measured continuously m t changed reversibly in cooling from t0 300 k to 200 k and in subsidiary warming cooling cycles ti t0 ti for any ti changes in m t in cooling from 200 k to 130 k were largely irreversible due to decreasing magnetocrystalline anisotropy which promotes wall unpinning and domain nucleation low temperature demagnetization ltd is almost complete by 130 k in 20 135 m magnetites but in 1 14 m magnetites further ltd occurs on cooling to 120 k as magnetocrystalline easy axes change and domains reorganize at the verwey transition the observed irreversible changes are the basis of stepwise ltd as a method of paleomagnetic cleaning decrements m in remanence due to cooling are most accurately measured at t0 requiring a set of warming cooling cycles ti t0 ti a less accurate method continuous ltd measures decrements m ti m ti 1 from the main cooling curve below 200 k without intermediate warming cooling cycles this requires remanence measurements at ti t0 stepwise or continuous ltd curves m ti discriminate among remanence types and grain sizes the signal of finer psd grains is enhanced compared to coarser md grains analogous to the lowrie and fuller 1971 test the inverse thermoremanence itrm of 1 14 m grains is harder to stepwise ltd than saturation remanence sirm while anhysteretic remanence arm is harder than either for 20 135 m multidomain grains itrm is softer than sirm

petrology of two tertiary oil shale deposits from queensland australia sediments from two queensland tertiary lacustrine oil shale deposits the lowmead and duaringa basins have been petrologically characterized from standard x ray powder diffraction analyses backscattered electron microscopy and electron microprobe data the results presented illustrate the potential of backscattered electron imagery for the study of diagenesis in fine grained organic rich sediments the lowmead formation consists of lamosites claystones and brown coals deposited in freshwater lacustrine and fringing peat swamp environements their dominant constituents are kaolinite and quartz with opaline silica and siderite important as eogenetic phases despite its similar geological setting the duaringa formation contains a more varied lithological assemblage with less organic matter being preserved freshwater lacustrine claystones and lamosites are interbedded with carbonate sediments deposited during periods of saline alkaline conditions in the lake within these carbonate sediments major growth of eogenetic mineral phases occurred principally dolomite and analcite as in the lowmead basin siderite is an important eogenetic phase in the duaringa formation two types of sideritic ironstone band occur throughout both basins sphaerosiderites within claystone coal sediments and fine grained bands of siderite within lamosites the bulk mineralogy of the siliciclastic detritus shows no significant trends with depth within either basin

low temperature magnetic properties of magnetite using first order reversal curve analysis implications for the pseudo single domain state 1 low temperature magnetic hysteresis properties of nearly stoichiometric magnetite were studied using the first order reversal curve forc technique forc distributions were measured at 20 k after cooling in zero magnetic field or cooling in the presence of a strong 1 5 t magnetic field five polycrystalline samples ranged in mean grain size and shape representing mostly single domain sd pseudo single domain psd and multidomain md magnetic hysteresis behavior the forc distributions measured from psd magnetite after zero field cooling zfc manifest an unusual behavior characterized by two distinct maxima located almost symmetrically with respect to the zero interaction field hu 0 axis on a forc diagram in contrast the low temperature forc distributions for sd and md samples have a single maximum located at the hu 0 axis the appearance of two maxima on low temperature forc distributions is interpreted as a manifestation of a perminvar like behavior due to stabilization of the magnetic domain structure by irregularities of crystalline lattice induced by twinning in monoclinic magnetite the experimental data suggest that the effect of monoclinic twinning on remagnetization is only pronounced within a certain grain size range the double maxima zfc forc distribution appears to be a distinctive property of psd hysteresis behavior which cannot be reproduced by mixing sd and md magnetites

theory and application of the day plot mrs ms versus hcr hc 2 application to data for rocks sediments and soils 1 new theoretical curves relating the hysteresis parameters mrs ms and hcr hc for single domain sd superparamagnetic sp pseudo single domain psd and multidomain md grains and their mixtures are applied to published data for natural materials the day plot of mrs ms versus hcr hc has been used to crudely classify samples into box like sd psd and md or sometimes incorrectly md sp regions with arbitrary boundaries new type curves for md psd sd md and sd sp grains and mixtures permit more subtle and precise modeling the predicted md trend and its junction with the psd trend are observed in two data sets for magnetite spherules from carbonate rocks and for temperature varying hysteresis results spanning the verwey transition the latter data are the basis of a suggested new method for pinpointing the psd md threshold size selected data for pottery clays soils and paleosols generally follow sd md type curves and indicate intermediate size psd magnetite with narrow to broad size distributions a lake sediment section with known grain size progression tracks in the predicted sense along the sd md trend selected data for glaciomarine and pelagic sediments are also generally compatible with sd md trends examples of remagnetized carbonate rocks submarine basaltic glasses and glassy rims of pillow basalts all follow predicted sp sd or sp psd mixing curves with a large range in volume fraction of sp grains 0 75 but a narrow range of sp particle sizes 10 2 nm larger sp grains spanning the range to sd size 25 30 nm are absent for unknown reasons oceanic dolerites gabbros and serpentinized peridotites in some cases fall in a novel region of the day plot parallel to but below magnetite sd md mixing curves

magnetic hysteresis of 0 6 110 μm magnetites across the verwey transition we report saturation magnetization ms saturation remanence mrs coercive force hc and remanence coercivity hcr as a function of grain size d and temperature t for 0 6 135 m magnetites

low temperature remanence in stable single domain magnetite 1 a numerical model is developed in order to simulate the loss of low temperature remanence while warming through the verwey transition we concentrate on the amount of remanence lost fc and zfc during field cooled and zero field cooled low temperature demagnetization curves for stable single domain ssd particles of 1 inorganic magnetite and 2 biogenic magnetite in the form of magnetosome chains the model predicts that delta ratios fc zfc for inorganic ssd magnetite increase from 1 to 1 6 with increasing aspect ratio and that magnetosome chains have elevated delta ratios of at least 2 0 disaggregated magnetosomes are predicted to behave similarly to inorganic acicular ssd magnetite the numerical results agree with experimental results

low temperature behavior of single domain through multidomain magnetite abstract we have investigated the low temperature behavior of a suite of grown synthetic and natural magnetites that span single domain sd and multidomain md behavior synthetic samples had been grown in the laboratory either in an aqueous medium or in glass natural samples included sd magnetites occurring in plagioclase and truly md magnetites in the form of large octahedra in all experiments a sample was first given a saturation remanence at room temperature next moment was measured continuously during cooling and warming between 230 k and 60 k similar to results reported earlier by other workers magnetic memory is large in sd samples whereas truly md samples are almost completely demagnetized by cycling between room temperature and 60 k pseudo single domain samples exhibit behavior that is intermediate with respect to that of the sd and truly md states when data from this study are combined with data obtained by hartstra 10 from sized natural magnetites it is found that the percentage of total remanence that survives cycling between room temperature and 60 k decreases linearly with the logarithm of grain size and thus with increasing number of domains this relation suggests that memory can provide a reasonable estimate of grain size in those magnetite bearing rocks for which these samples provide good analogues remarkably some of the large natural octahedra provide a magnified view of md response to low temperatures and thus reveal two surprising and intriguing types of behavior first below approximately 180 k these octahedra demagnetize through a series of large barkhausen jumps second near 117 k these same octahedra exhibit a wild zone where magnetic moment executes large random excursions we interpret these two phenomena as direct evidence for the unpinning and irreversible displacement of domain walls in response to the drop in coercivity and possibly the broadening of domain walls as temperatures drop toward the isotropic point one implication of this behavior is that cooling to progressively lower temperatures could provide an effective method for stepwise removal of paleomagnetic components carried by md grains even without passage through the isotropic point of magnetite

synoptic reconstruction of a major ancient lake system eocene green river formation western united states numerous 40ar 39ar experiments on sanidine and biotite from 22 ash beds and 3 volcaniclastic sand beds from the greater green river piceance creek and uinta basins of wyoming colorado and utah constrain 8 m y of the eocene epoch multiple analyses were conducted per sample using laser fusion and incremental heating techniques to differentiate inheritance 40ar loss and 39ar recoil when considered in conjunction with existing radioisotopic ages and lithostratigraphy biostratigraphy and magnetostratigraphy these new age determinations facilitate temporal correlation of linked eocene lake basins in the laramide rocky mountain region at a significantly increased level of precision to compare our results to the geomagnetic polarity time scale and the regional volcanic record the ages of eocene magnetic anomalies c24 through c20 were recalibrated using seven 40ar 39ar ages overall the ages obtained for this study are consistent with the isochroneity of north american land mammal ages throughout the study area and provide precise radioisotopic constraints on several important biostratigraphic boundaries applying these new ages average sediment accumulation rates in the greater green river basin wyoming were approximately three times faster at the center of the basin versus its ramp like northern margin during deposition of the underfilled wilkins peak member in contrast sediment accumulation occurred faster at the edge of the basin during deposition of the balanced filled to overfilled tipton and laney members sediment accumulation patterns thus reflect basin center focused accumulation rates when the basin was underfilled and supply limited accumulation when the basin was balanced filled to overfilled sediment accumulation in the uinta basin at indian canyon utah was relatively constant at 150 mm k y during deposition of over 5 m y of both evaporative and fluctuating profundal facies which likely reflects the basin margin position of the measured section the most rapid sediment accumulation for the entire system 1 m k y occurred between 49 0 and 47 5 ma when volcaniclastic materials from the absaroka and or challis volcanic fields entered the green river formation lakes from the north our new ages combined with existing paleomagnetic and biostratigraphic control permit the first detailed synoptic comparison of lacustrine depositional environments in all the green river formation basins coupled with previously published paleocurrent observations our detailed correlations show that relatively freshwater lakes commonly drained into more saline downstream lakes the overall character of eocene lake deposits was therefore governed in part by the geomorphic evolution of drainage patterns in the surrounding laramide landscape freshwater overfilled lakes were initially dominant 53 5 52 0 ma possibly related to high erosion rates of remnant cretaceous strata on adjacent uplifts expansion of balanced fill lakes first occurred in all green river formation basins at 52 0 51 3 ma and again between 49 6 and 48 5 ma evaporative underfilled lakes occurred in various basins between 51 3 and 45 1 ma coincident with the end of the early eocene climatic optima and subsequent onset of global cooling defined from marine record however evaporite intervals in the different depocenters were deposited at different times rather than being confined to a single episode of arid climate evaporative terminal sinks were initially located in the greater green river and piceance creek basins 51 3 48 9 ma then gradually migrated southward to the uinta basin 47 1 45 2 ma this history is likely related to progressive southward construction of the absaroka volcanic province which constituted a major topographic and thermal anomaly that contributed to a regional north to south hydrologic gradient the greater green river and piceance creek basins were eventually filled from north to south with absarokaderived detritus at sedimentation rates 1 2 orders of magnitude greater than the underlying lake deposits

chronostratigraphic correlation of lacustrine deposits using 87sr 86sr ratios eocene green river formation wyoming u s a abstract the reconstruction of detailed basin scale depositional histories from sedimentary rocks fundamentally depends on the availability of reliable time markers unlike marine strata lacustrine strata typically lack rapidly evolving cosmopolitan fauna or flora that might serve this purpose depending on their geologic context lacustrine strata may also lack tephras that could provide isochronous markers or radioisotopic age variations in 87 sr 86 sr ratios could potentially provide an alternative means of chronostratigraphic correlation for carbonate rich lake deposits based on the hypothesis that sr isotopes are well mixed in a lake and do not experience significant fractionation to test this hypothesis we measured 87 sr 86 sr ratios in 114 samples from two drill cores of the upper wilkins peak member from the green river formation that are located 23 km apart these cores can be independently correlated using distinctive tephras and organic carbon rich mudstone horizons measured 87 sr 86 sr ratios range from 0 71154 to 0 71504 and vary inversely with lake water depth as interpreted by sedimentary lithofacies characteristics lower ratios of 87 sr 86 sr are found in lithofacies deposited during lake highstands which are marked by laminated dark gray mudstone and elevated organic carbon enrichment as measured by fischer assay analysis higher 87 sr 86 sr ratios occur in lithofacies deposited during lake lowstands which are marked by organic lean gray green mudstone 87 sr 86 sr in approximately time equivalent samples from the two cores show a strong positive correlation r 0 68 despite the likely presence of small temporal mismatches between approximately correlative samples we conclude that lake water was consistently well mixed with respect to sr across distances of at least 23 km these results suggest that 87 sr 86 sr can serve as a powerful tool to aid high resolution chronostratigraphic correlation of lake deposits

hydrogen isotope evidence for the origin and evolution of the carbonaceous chondrites we present new hydrogen isotope data for separated matrix hydrated chondrules and other hydrated coarse silicate fragments from nine carbonaceous chondrites these data were generated using a micro analytical method involving stepped combustion of tens to hundreds of micrograms of hydrous solids we also re evaluate hydrogen isotope data from previous conventional stepped combustion experiments on these and other carbonaceous chondrites hydrogen isotope compositions of matrix and whole rock samples of cm chondrites are correlated with oxygen isotope indices major and minor element abundances and abundance and isotope ratios of other highly volatile elements these correlations include a monotonic decrease in d with increasing extent of aqueous alteration and decreasing abundances of highly volatile elements including c n and ar between extremes of 0 least altered most volatile rich and 200 most altered least volatile rich in plots involving only abundances and or isotope ratios of highly volatile elements ci chondrites fall on the high d volatile rich end of the trends defined by cm chondrites i e ci chondrites resemble the least altered cm chondrites in these respects these trends suggest the protoliths of the cm chondrites i e before aqueous alteration contained an assemblage of volatiles having many things in common with those in the ci chondrites if so then the volatile element inventory of the ci chondrites was a more widespread component of early solar system objects than suggested by the scarcity of recognized ci meteorites differences in volatile element chemistry between the ci and average cm chondrites can be attributed to aqueous alteration of the latter previous models of carbonaceous chondrite aqueous alteration have suggested 1 the protoliths of the cm chondrites are volatile poor objects like the co or cv chondrites and 2 the ci chondrites are more altered products of the same process producing the cm chondrites both suggestions appear to be inconsistent with hydrogen isotope data and other aspects of the volatile element geochemistry of these rocks we present a model for aqueous alteration of the cm chondrites that reconciles these inconsistencies and suggests revised relationships among the major subtypes of carbonaceous chondrites our model requires among other things that the water infiltrating cm chondrites had a d value of 158 consistent with initial accretion of cm parent bodies at 4 au

low temperature cycling of isothermal and anhysteretic remanence microcoercivity and magnetic memory abstract this paper reports low temperature cycling ltc through the verwey transition of anhysteretic remanence arm partial arms and partially demagnetised saturation isothermal remanence sirm induced at room temperature in pseudo single domain and multidomain md magnetite the remanences were cooled in zero field to 50 k and then heated back to room temperature by inducing partial arms over different field ranges and by partially alternating field demagnetising sirms it was possible to isolate both low coercive force and high coercive force fractions of remanence on cooling through the verwey transition a sharp increase in the remanence was observed the relative size of the jump increased as the high coercive force fraction was increasingly isolated this behaviour is interpreted as being due to both an increase in the single domain multidomain threshold size on cooling through the verwey transition and to the reduction or elimination of closure domains in the low temperature phase in addition the memory ratio i e the fraction of remanence remaining after ltc divided by the initial remanence was found to be higher for the high coercive force fraction than the low coercive force fraction in our interpretation the high coercivity fraction behaviour is associated with reversible domain re organisation effects whilst the low coercive force fraction s behaviour is associated with irreversible domain re organisation and de nucleation processes due to the decrease in magnetocrystalline anisotropy on cooling to the verwey transition the high coercive force fraction is likely to be magnetoelastically controlled thus a rock displaying high coercive force behaviour is likely to carry a palaeomagnetically meaningful remanence with high unblocking temperatures in addition ltc analysis can be used to identify the domain state dominating the natural remanence in magnetite bearing rocks

evidence for magnetostrictive control of intrinsic susceptibility and coercive force of multidomain magnetite in rocks abstract it is shown that intrinsic susceptibility xi susceptibility in the absence of self demagnetizing fields as well as coercive force hc of multidomain magnetite in rocks are likely to be commonly magnetostrictively controlled presumably through internal stresses opposing domain wall motion we use observations of the thermal variation of observed susceptibility x per cm3 of magnetite in the sample and coercive force hc on cooling to 130 k for seven basaltic rocks and for 0 25 0 21 mm magnetite grains dispersed in cement as expected theoretically with magnetostrictively controlled xi a plot of 1 x versus j s 2 for each sample where is the polycrystalline magnetostriction coefficient and js is saturation magnetization gives a reasonably straight line whose 1 x intercept agrees with n average self demagnetizing factor of magnetite grains in the sample given by h c j r n h c j r h c j ri where jr is saturation remanence and ri is jr when n 0 as expected theoretically with magnetostrictively controlled hc a plot of hc versus j s for each sample reveals an approximate proportionality this also implies magnetostrictive control of xi because a plot of 1 x versus h c j s for each sample yields a straight line of 1 x intercept agreeing with n from h c j r n h c j r h c j ri as expected if hc and xi are both magnetorestrictively controlled

lake type controls on petroleum source rock potential in nonmarine basins based on numerous empirical observations of lacustrine basin strata we propose a three fold classification of lacustrine facies associations that accounts for the most important features of lacustrine petroleum source rocks and provides a predictive framework for exploration in nonmarine basins where lacustrine facies are incompletely delineated 1 the fluvial lacustrine facies association is characterized by freshwater lacustrine mudstones interbedded with fluvial deltaic deposits commonly including coal shoreline progradation dominates basin fill resulting in the stacking of indistinctly expressed cycles up to 10 m thick in map view the deposits may be regionally widespread but laterally discontinuous and contain strong facies contrasts transported terrestrial organic matter contributes to mixed type i iii kerogens that generate waxy oil type i kerogen is hydrogen rich and oil prone type iii kerogen is hydrogen poor and mainly gas prone the luman tongue of the green river formation wyoming and the honyanchi formation junggar basin china provide examples of this facies association which is also present in the songliao basin of northeastern china the central sumatra basin and the cretaceous doba doseo basins in west central africa 2 the fluctuating profundal facies association represents a combination of progradational and aggradational basin fill and includes some of the world s richest source rocks deposits are regionally extensive in map view having relatively homogenous source facies containing oil prone type i kerogen examples include the laney member of the green river formation wyoming the lucaogou formation junggar basin china the bucomazi formation offshore west africa and the lagoa feia formation campos basin brazil 3 the evaporative facies association represents dominantly aggradational fill related to desiccation cycles in saline to hypersaline lakes and may include evaporite and eolianite deposits sublittoral organic rich mudstone facies are relatively thin but may be begin page 1034 quite rich and widespread the highest organic enrichment coincides with the deepest lake stages low input of land plant organic matter results in minimal lateral contrasts in organic content in some cases a distinctive type i s sulfur rich kerogen may generate oil at thermal maturities as low as 0 45 vitrinite reflectance equivalent examples include the wilkins peak member of the green river formation wyoming the jingjingzigou formation junggar basin china the jianghan and qaidam basins china and the blanca lila formation argentina

inverse thermoremanent magnetization 1 inverse thermoremanent magnetization itrm is reversed to the thermoremanent magnetization trm process itrm results from warming from low temperature t in a magnetic field while trm results from field cooling from high t the development of itrm was studied in magnetites of grain sizes from submicron to 135 m in pyrrhotites and in hematite crystals all three minerals acquired itrm after warming through their magnetic transitions 35 k for pyrrhotite 120 and 130 k for magnetite 250 k for hematite however when an impacting meteorite s cold interior warms to ambient t in the geomagnetic field magnetite is the most likely candidate for acquiring itrm the magnetite itrm blocking temperature distribution was determined from 12 neighboring partial itrms in nested field on warming plus field off cooling cycles 300 20 k the largest partial itrms are produced in t intervals around magnetite s verwey transition tv 110 120 k and isotropic point tk 130 k both transitions involve large changes in crystalline anisotropy and renucleation of magnetic domains itrm is blocked when initially broad domain walls narrow and are pinned by dislocations itrm has contrasting properties to trm which is mainly due to blocked single domain moments itrm is strongest for 3 to 20 m grains whereas trm peaks for submicron magnetites only 10 20 of itrm survives low temperature demagnetization ltd at 77 k or af demagnetization to 10 15 mt compared to 30 90 for trm itrm decreases quasi linearly with t in thermal demagnetization the median unblocking temperature tub is 300 c and 20 25 survives at 550 c the low tub part of itrm could mimic extraterrestrial nrm of low tub cited as evidence of negligible heating of meteorites in their transfer to earth the high tub itrm would contaminate paleointensity determinations up to the highest t steps the best cure for itrm contamination is af or ltd pretreatment

review of the low temperature magnetic properties of magnetite from a rock magnetic perspective the low temperature magnetic properties of magnetite are reviewed and implications for rock magnetism considered the behaviour of fundamental properties of magnetite at low temperatures near the verwey transition tv are documented and attention is given to various verwey transition theories the low temperature behaviour of the magnetic energies that control domain structure is reviewed in detail for the first time in rock magnetic literature the low temperature anomaly in spontaneous magnetization ms is documented and the differences between the saturation magnetization and ms near the verwey transition are discussed it is argued that the low temperature behaviour of the magnetocrystalline anisotropy and in particular the anomaly at tv is most likely to affect multidomain remanence during low temperature cycling for multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through tv is likely to reduce the stability of closure domains

some additional parameters to estimate domain state from isothermal magnetization measurements abstract domain state and mixing analysis based on isothermal magnetization measurements is often restricted to the use of the day plot in order to make more information from simple rock magnetic measurements accessible for the quantitative analysis of domain state four additional parameters are proposed two of them a hysteresis shape parameter and a coercivity ratio can be directly calculated from standard measurements of the hysteresis loop and backfield curve the other two a transient energy dissipation ratio and the viscosity of isothermal remanent magnetization require additional measurements which however do not increase measurement time noticeably the theoretical background of the above parameters is discussed their ability to discern domain state is shown for a small set of synthetic magnetite and titanomagnetite samples and in a case study of a transect from the rim to the center of a dredged mid ocean ridge basalt

effect of the magnetic field applied during cooling on magnetic hysteresis in the low temperature phase of magnetite first order reversal curve forc analysis the strength of a magnetic field applied during cooling through the verwey transition can control the low temperature lt magnetic hysteresis properties of magnetite this effect is investigated using the first order reversal curve forc technique lt forc distributions were measured at 20 k from four polycrystalline samples after cooling from 300 k in zero magnetic field or in the presence of a magnetic field hfc from a 0 01 to 1 5 t range the samples represented nearly single domain sd pseudo single domain psd and multidomain md magnetic hysteresis behavior the lt forc distributions measured from psd magnetite after zero field cooling are characterized by two distinct maxima located almost symmetrically with respect to the zero interaction field hu 0 axis on a forc diagram application of an intermediate magnetic field 10 mt to 50 mt during cooling results in a shift of the peaks to near symmetry with respect to the hu hfc line the manifestation of the field controlled offset of forc distribution strongly depends on the direction of the field hfc the effect is best expressed when a forc diagram is measured parallel or antiparallel to the cooling field direction for hfc 100 mt the upper peak disappears and the lt forc distribution becomes single peaked and symmetrical with respect to the hu 0 axis in contrast no correlation between hfc and features of forc distributions is observed for sd and md magnetite these experimental results provide further support to a phenomenological model which suggests that magnetic hysteresis behavior of monoclinic magnetite is controlled by an intrinsic interplay between the magnetic and crystallographic twin domain structures

physical interpretation of hysteresis loops micromagnetic modeling of fine particle magnetite 1 hysteresis measurements have become an important part of characterizing magnetic behavior of rocks in paleomagnetic studies theoretical interpretation is often difficult owing to the complexity of mineral magnetism and published data sets demonstrate remanence and coercivity behavior that is currently unexplained in the last decade numerical micromagnetic modeling has been used to simulate magnetic particles such simulations reveal the existence of nonuniform remanent states between single and multidomain known as the flower and vortex configurations these suggest plausible explanations for many hysteresis measurements yet fall short of explaining high saturation remanence high coercivity data such as those commonly observed in fine grained submarine basalts in this paper we review the theoretical and experimental progress to date in understanding hysteresis of geological materials we extend numerical simulations to a greater variety of shapes and sizes including random assemblages of particles and shapes more complex than simple rods and cubes our simulations provide plausible explanations for a wide range of hysteresis behavior

isotopic composition of carbonaceous chondrite kerogen evidence for an interstellar origin of organic matter in meteorites abstract stepwise combustion has revealed systematic patterns of isotopic heterogeneity for c h and n in the insoluble organic fraction m kerogen from the orgueil and murray carbonaceous chondrites those patterns are essentially identical for both meteorites indicating a common source of m kerogen the data cannot be reconciled with a single mass fractionation process acting upon a single precursor composition this indicates either a multi path history of mass dependent processing or a significant nucleogenetic contribution or both if mass fractionation were the dominant process the magnitude of the observed isotopic variability strongly suggests that ion molecule reactions at very low temperatures probably in interstellar clouds were responsible in any case an interstellar rather than solar nebular origin for at least some of the meteoritic organic matter is indicated this has interesting implications for the origin of prebiotic molecules temperatures in the early solar system and the isotopic compositions of volatiles accreted by the terrestrial planets