oxygen isotopic compositions of allende type c cais evidence for isotopic exchange during nebular melting and asteroidal metamorphism abstract in situ oxygen isotopic measurements of primary and secondary minerals in type c cais from the allende cv3 chondrite reveal that the pattern of relative enrichments and depletions of 16 o in the primary minerals within each individual cai are similar to the patterns observed in types a and b cais from the same meteorite spinel is consistently the most 16 o rich 17 o 25 to 15 followed by al ti dioside 17 o 20 to 5 and anorthite 17 o 15 to 0 melilite is the most 16 o depleted primary mineral 17 o 5 to 3 we conclude that the original melting event that formed type c cais occurred in a 16 o rich 17 o 20 nebular gas and they subsequently experienced oxygen isotopic exchange in a 16 o poor reservoir at least three of these abc ts26f1 and 93 experienced remelting at the time and place where chondrules were forming trapping and partially assimilating 16 o poor chondrule fragments the observation that the pyroxene is 16 o rich relative to the feldspar even though the feldspar preceded it in the igneous crystallization sequence disproves the class of cai isotopic exchange models in which partial melting of a 16 o rich solid in a 16 o poor gas is followed by slow crystallization in that gas for the typical not associated with chondrule materials type c cais as well for as the types a and b cais the exchange that produced internal isotopic heterogeneity within each cai must have occurred largely in the solid state the secondary phases grossular monticellite and forsterite commonly have similar oxygen isotopic compositions to the melilite and anorthite they replace but in one case cai 160 grossular is 16 o enriched 17 o 10 to 6 relative to melilite 17 o 5 to 3 meaning that the melilite and anorthite must have exchanged its oxygen subsequent to secondary alteration this isotopic exchange in melilite and anorthite likely occurred on the cv parent asteroid possibly during fluid assisted thermal metamorphism

origin of zoned metal grains in the que94411 chondrite laser ablation inductively coupled plasma mass spectrometry la icp ms was used to measure distributions of the siderophile elements p v cr fe co ni mo ru rh pd w re os ir and pt in metal grains in the metal rich chondrite que94411 with a spatial resolution of similar to 30 mum the platinum group elements pges except pd exhibit radial zoning in these grains that mimics that previously observed in ni and co the concentrations of these elements decreases from the cores to the rims of the grains the pge distributions support a condensation origin for the enhanced refractory element abundances in the zoned grains the lack of zoning in pd refutes an origin by a redox controlled process and none of the pge ni relationships support an origin by fractional crystallization from a metallic melt several models of grain formation were explored including equilibrium fractional condensation which failed to yield the correct radial zoning the zoning may be the product of a nonequilibrium fractional condensation process in which the refractory siderophiles remained supersaturated in the cooling solar nebula or of diffusion between refractory enhanced fe ni cores and other fe ni metal that may have been deposited later from the solar nebula copyright c 2001 elsevier science ltd

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

26al in plagioclase rich chondrules in carbonaceous chondrites evidence for an extended duration of chondrule formation abstract the 26al 26mg isotope systematics in 33 petrographically and mineralogically characterized plagioclase rich chondrules prcs from 13 carbonaceous chondrites ccs one ungrouped acfer 094 six cr five cv and one co reveal large variations in the initial 26al 27al ratio 26al 27al 0 well resolved 26mg excesses 26mg from the in situ decay of the short lived nuclide 26al t1 2 0 72 ma were found in nine chondrules two from acfer 094 five from the cv chondrites allende and efremovka and one each from the paired cr chondrites eet 92147 and eet 92042 with 26al 27al 0 values ranging from 3 10 6 to 1 5 10 5 data for seven additional chondrules from three cv and two cr chondrites show evidence suggestive of the presence of 26al but do not yield well defined values for 26al 27al 0 while the remaining chondrules do not contain excess radiogenic 26mg and yield corresponding upper limits of 11 2 10 6 for 26al 27al 0 the observed range of 26al 27al 0 in prcs from ccs is similar to the range seen in chondrules from unequilibrated ordinary chondrites uocs of low metamorphic grade 3 0 3 4 however unlike the uoc chondrules there is no clear trend between the 26al 27al 0 values in prcs from ccs and the degree of thermal metamorphism experienced by the host meteorites high and low values of 26al 27al 0 are found equally in prcs from both ccs lacking evidence for thermal metamorphism e g crs and ccs where such evidence is abundant e g cvs the lower 26al 27al 0 values in prcs from ccs relative to most cais are consistent with a model in which 26al was distributed uniformly in the nebula when chondrule formation began approximately a million years after the formation of the majority of cais the observed range of 26al 27al 0 values in prcs from ccs is most plausibly explained in terms of an extended duration of 2 3 ma for the formation of cc chondrules this interval is in sharp contrast to most cais from ccs whose formation appears to be restricted to a narrow time interval of less than 105 years the active solar nebula appears to have persisted for a period approaching 4 ma encompassing the formation of both cais and chondrules present in ccs and raising important issues related to the storage assimilation and mixing of chondrules and cais in the early solar system

magnitude of cenozoic erosion from mean sonic transit time offshore taiwan abstract mean sonic transit times from eight stratigraphic units in forty four wells in western and northern offshore taiwan were plotted against depth apparent erosion displacement on the depth axis from the normal compaction was determined for each unit in each well cretaceous and eocene units show greater apparent erosion than miocene and pliocene units in the same well which in turn show more erosion than pleistocene units apparent erosions derived from two different units are plotted against each other for correlation analysis based on the analysis the main periods of erosion were during the oligocene and plio pleistocene the timing of erosion inferred from sonic interval transit is consistent with recognized unconformities and its areal distribution is consistent with the observed structural evolution of the region the maximum true erosion corrected for post erosion burial at the oligocene unconformity was 3500 m on the penghu platform approximately 2500 m of true erosion occurred on the kuanin ulift and the pengchiahsu platform due to moderate to minor erosion and a great amount of post erosion burial true magnitude of erosion is masked at the miocene unconformity in most of the area erosion of up to 500 m on the eastern side of the penghu island at the pleistocene unconformity defines the shape of the present day penghu waterway in the northern offshore area up to 1000 m of plio pleistocene true erosion occurred in the taiwan sinzi folded zone where miocene folds are observed the magnitude of true erosion decreased west of the taiwan sinzi folded zone to 500 m in the pengchiahsu basin a maximum of 500 m of true erosion is also observed in the west offshore area along hsing chu to miaoli coast

early solar system events and timescale some recent information on the mn cr and al mg systems is reviewed this information is used to derive constraints on the timing of processes and events which took place in the early solar system using reasonable assumptions a timeline is constructed where the estimated age of the solar system is 4571 ma this age is taken to mark the time when most calcium aluminum rich inclusions cais were starting to form a process that may have lasted for several 105 years almost contemporaneously small planetesimals have accreted that served to store these cais for later dispersal among larger planetesimals by the time large numbers of planetesimals of several tens of kilometers in size had formed the interior of these objects started to melt through the decay of 26al collisional disruption of these planetesimals allowed gases dust and melt to escape into the surrounding space the fine droplets of melt reacted with gas and dust to form chondrules which after rapid cooling were partially re accreted onto the residual rubble pile this process of primary chondrule formation in most cases involving several generations of planetesimals most plausibly lasted only for 2 ma towards the end of this period and during the following 3 to 4 ma planetary objects of several hundred kilometers in size were formed they still stored enough energy to continue melting from the inside to finally differentiate into chemically stratified layers with basaltic volcanism occurring within a few million years

an interlaboratory comparison of vitrinite reflectance measurement a vitrinite reflectance vr round robin analysis was undertaken by the society for organic petrology tsop research committee fourteen laboratories from five countries participated in the study which involved samples of one shale and three coals ranging in rank maturity from lignite to high volatile a bituminous the main objective was to evaluate interlaboratory comparability and accuracy of vr measurements in a context of practical data interpretation e g maturity determination of petroleum source rocks the between laboratory standard deviations x of the vr data are 0 033 0 048 for mean random and 0 037 0 049 for mean maximum measurements the results suggest that at a moderate level of confidence 68 assuming normally distributed populations a vr differential of 0 1 is significant in indicating different thermal maturities within the rank range studied a differential of 0 2 vr is required if a high level 95 of confidence is desired at the 5 significance level between laboratory consistency tests h statistic show that the random vr data of one out of fourteen laboratories for each sample are found to be inconsistent with the main population whereas the other thirteen laboratories are consistent with each other the maximum vr data are shown to be consistent at the same significance level of 5 except for one data point the within laboratory standard deviations are relatively high from 0 023 to 0 110 with an average ranging from 0 037 to 0 070 the relatively high is likely related to natural variations in the vitrinite macreal which can result in the inconsistent selection of vitrinite particles for measurements by the operators maximum vr measurements are found to give rise consistently to slightly higher interlaboratory and average within laboratory standard deviations compared to those of corresponding random vr results this may be related to experimental errors introduced during maximum vr analyses e g imperfect focusing of the sample and centering of the stage however this observation by no means suggests that random analysis is a more precise measurement

thermal modelling of the melut basin sudan and south sudan implications for hydrocarbon generation and migration abstract the thermal history of the melut rift basin was studied in a number of wells and along a northeast southwest cross sectionacross the basin using 1 d and 2 d basin models modelling was conducted using petromod software to test subsidence thermal history and their implications for hydrocarbon generation migration and accumulations geotherms were found ranging between 24 and 44 c km and average 32 c km and present day heat flow was found to vary between 53 and 65 averaging 59 mw m 2 a geologicaly realistic paleoheat flow model with higher heat flow peaks of 75 70 and 65 mw m 2 during the first second and third rifting phases respectively calibrated against vitrinite reflectance data was employed the model was found sensitive to cenozoic erosions and a 600 m of exhumation did support the vitrinite data in providing a reasonable fit in most locations using the easy ro vitrinite kinetic model for maturity the average present day oil window was found between 1565 and 4050 m in the wells studied and between 1705 and 4200 m along the cross section using type ii kerogen for the aptian albian source rocks the hydrocarbon generation and expulsion were modelled and revealed that the generation started as early as 95 ma in some areas but significant generation rates were during the second rifting phase and expulsion started at around 85 ma calculations of the probable expelled amounts of hydrocarbon from the northern sub basin are high and up to 2 2 10 11 bbls of oil and 2 3 10 14 ft 3 of gas migration of oils from cretaceous source rocks kitchen to the cenozoic traps was probably through faults porous carrier beds and breaching of traps cap rocks the filling of these traps might be periodic during the rifting episodes and hydrocarbons remigrated from deeper traps to shallow ones it is possible that thinner and siltier cap rocks along the hydrocarbon migration path may have allowed gas to scape but holded the oil back it is most probable that part of the hydrocarbons might have been generated from shallower source rocks areas and a 2 d hybrid darcy flow path model have demonstrated this and predicted a number of accumulations which are similar to the known large discoveries such as palogue fal and adar yale oil fields

fission track lengths in the apatite annealing zone and the interpretation of mixed ages abstract measurements of confined fission tracks in apatites from deep boreholes show that their mean length is reduced and the length distribution becomes progressively broader through the fission track annealing zone at subsurface temperatures of around 100 c and above the length distribution characteristically becomes very broad without a pronounced peak the lengths of the longest tracks remain essentially constant at all stages of natural annealing observed in the boreholes this pattern is similar to that found in laboratory annealing of spontaneous fission tracks in apatite from an outcrop sample of the same formation these observations show that confined track lengths can be used as important indicators of the type of thermal history that a sample has experienced in the temperature zone of increasing track stability the length distributions reported here provide a basis for interpretation of fission track ages that might otherwise be ambiguous an apatite age which results from a uniform slow cooling history will have a broad negatively skewed length distribution while a bimodal distribution provides clear evidence of a two stage history involving partial annealing by a discrete thermal event the apparent fission track age associated with a bimodal length distribution will be a mixed age intermediate between the original age and that of the later event

heat flow and thermal history of the anadarko basin and the western oklahoma platform abstract the average geothermal gradient in the anadarko basin and the oklahoma platform estimated from 856 corrected bottom hole temperatures bhts is 21 c km analysis of previously published thermal maturation data indicates that the anadarko basin has undergone from 1 to 3 km of erosion starting about 40 to 50 ma the average thermal gradient at the time of maximum burial was in the range of 22 to 25 c km there is no evidence that the thermal state of the anadarko basin has changed significantly since the late paleozoic 250 ma to estimate heat flow in the anadarko basin and the oklahoma platform we made 652 thermal conductivity measurements on drill cuttings from 9 oil and gas wells all measurements were corrected for the effects of anisotropy temperature and porosity matrix conductivities perpendicular to bedding at 22 c range from 1 7 w m 1 k 1 for permian age sandstone and shale to 2 6 w m 1 k 1 for devonian silurian age limestone and shale in situ thermal conductivities vary from 1 3 w m 1 k 1 to 1 9 w m 1 k 1 estimated heat flow 20 ranges from 30 mw m2 to 42 mw m2 with a mean of 36 mw m2 we found no evidence for heat flow to increase significantly from the anadarko basin in the south to the oklahoma platform to the north these results contradict a previous study which found heat flow increases from south to north with heat flow in the oklahoma platform as much as 50 greater than heat flow in the anadarko basin at the present time we do not know if these differences are related to spatial variations or reflect methodological errors if our results are averaged with those reported in previous work mean heat flows in the anadarko basin and oklahoma platform are 39 mw m2 and 51 mw m2 respectively

a short duration of chondrule formation in the solar nebula evidence from 26al in semarkona ferromagnesian chondrules abstract the 26 al 26 mg systems of five ferromagnesian chondrules from the least metamorphosed ordinary chondrite semarkona ll3 0 were studied using a secondary ion mass spectrometer their glass or plagioclase portions contain excesses of 26 mg and in two chondrules the 26 mg excesses are well correlated with 27 al 24 mg which demonstrates the in situ decay of 26 al the initial 26 al 27 al ratios in these chondrules obtained from the slope of isochrons show a narrow range of between 6 10 6 and 9 10 6 indicating their short formation duration of less than 1 my if the solar nebula was initially homogeneous in al isotopes the chondrule formation ages are 2 my younger than those of cais our results based on the study of the least metamorphosed uoc are consistent with the previous studies on al rich chondrules that the chondrule formation started at least 2 my after cais alternatively the older records before 2 my were erased by chondrule recycling process it further suggests that the young apparent ages 3 to 5 my after cais for chondrules in type 3 4 uocs are due to the disturbance of the 26 al 26 mg system possibly during parent body metamorphism the result is not consistent with the extended nebular time scale of 5 my and the chondrule formation by planetary processes the ni isotopic analysis of the feo rich olivines in a type ii chondrule in semarkona did not show any detectable excess 60 ni in spite of their high fe ni ratios the upper limit of the initial 60 fe 56 fe ratio of the solar system was estimated to be 3 4 10 7 which is consistent with the previous estimate 0 2 1 9 10 7 from eucrites this result confirms that the 60 ni excess previously observed from cais was not due to the decay of the short lived nuclide 60 fe but a ni isotopic anomaly of nucleosynthetic origin

plagioclase rich chondrules in the reduced cv chondrites evidence for complex formation history and genetic links between calcium aluminum rich inclusions and ferromagnesian chondrules plagioclase rich chondrules prcs in the reduced cv chondrites efremovka leoville vigarano and grosvenor mountains gro 94329 consist of magnesian low ca pyroxene al ti cr rich pigeonite and augite forsterite anorthitic plagioclase feni metal sulfide nodules and crystalline mesostasis composed of silica anorthitic plagioclase and al ti cr rich augite the silica grains in the mesostases of the cv prcs are typically replaced by hedenbergitic pyroxenes whereas anorthitic plagioclase is replaced by feldspathoids nepheline and minor sodalite some of the prcs contain regions that are texturally and mineralogically similar to type i chondrules and consist of forsterite low ca pyroxene and abundant feni metal nodules several prcs are surrounded by igneous rims or form independent compound objects twelve prcs contain relic calcium aluminum rich inclusions cais composed of anorthite spinel high ca pyroxene forsterite and al rich low ca pyroxene anorthite of these cais is generally more heavily replaced by feldspathoids than anorthitic plagioclase of the host chondrules this suggests that either the alteration predated formation of the prcs or that anorthite of the relic cais was more susceptible to the alteration than anorthitic plagioclase of the host chondrules these observations and the presence of igneous rims around prcs and independent compound prcs suggest that the cv prcs may have had a complex multistage formation history compared to a more simple formation history of the cr prcs relatively high abundances of moderately volatile elements such as cr mn and si in the prcs suggests that these chondrules could not have been produced by volatilization of ferromagnesian chondrule precursors or by melting of refractory materials only we infer instead that prcs in carbonaceous chondrites formed by melting of the reduced chondrule precursors magnesian olivine and pyroxene feni metal mixed with refractory materials relic cais composed of anorthite spinel high ca pyroxene and forsterite the mineralogical chemical and textural similarities of the prcs in several carbonaceous chondrite groups cv co ch cr and common presence of relic cais in these chondrules suggest that prcs may have formed in the region s intermediate between the regions where cais and ferromagnesian chondrules originated

redox conditions in the solar nebula observational experimental and theoretical constraints crystallization experiments on liquids with compositions similar to those of compact type a type b1 and type b2 refractory inclusions were conducted under controlled temperature and fo2 conditions application of the results to the compositions of coexisting ti 3 bearing fassaitic clinopyroxene melilite pairs in natural inclusions shows that if they crystallized at 1509 k they did so at log fo2 19 8 0 9 only slightly below the equilibrium log fo2 of a partially condensed system of solar composition at the same temperature

evidence for the presence of 53mn in the early solar system chromium isotopic anomalies appear to be widespread in the allende inclusions clear anomalies are shown in the 53cr 52cr isotopic ratio deviations from the terrestrial value are correlated with the mn cr ratios indicating that 53mn half life t 3 7 106y existed at the time of formation of the meteorite inclusions the initial 53cr 52cr in the inclusions is lower than the terrestrial value recent results have shown that chromium isotopic anomalies are frequent on the 54cr isotope birck and allegre 1984 at the same time our results suggested the possibility of some anomalies on the 53cr isotope which can be produced by the decay of 53mn the present article presents improved precision measurements on the same inclusions of the allende meteorite and an extension of the measurements to mineral separates these results are the first evidence for live 53mn in the early solar system noted 53mn in the following

diagenesis and organic maturation of sedimentary rocks under volcanic strata oregon 1 in the pacific northwest diagenesis of cretaceous strata mantled by extrusive volcanic rocks occurred at temperatures of 150 200 degrees c due to heat conducted downward from a perched confined geothermal aquifer this aquifer produced a homogeneous temperature field within the underlying sedimentary rock authigenic minerals such as sphene chlorite illite and quartz developed throughout a vertical section composed of andesites basalts and both silicic and mafic sedimentary rocks the geologically rapid heating also resulted in nearly constant organic maturation profiles within the sedimentary rock and produced unknown quantities of hydrocarbons the contemporaneous diagenesis and maturation took place before extrusion of the columbia river basalt group and was p rhaps associated with earlier intrusive activity such hydrothermal maturation accounts for both local diagenesis and other organic maturation anomalies in the pacific northwest the mechanism has ramifications for the petroleum potential of geologically active basins because it provides a means by which volcanism and associated geothermal activity can increase maturation and generate hydrocarbons

paleo heat flow evolution of the tabei uplift in tarim basin northwest china abstract the paleo heat flow evolution of the tabei uplift in the tarim basin is investigated based on burial and thermal history reconstruction of 14 wells and using basin modeling numerous geological parameters such as temperature data and missing sediment thickness by erosion were used in the modeling the basin model was calibrated using 460 measured vitrinite reflectance vr o and vitrinite like maceral reflectance vlmr o values to constrain the validity of the maturity model the heat flow history of the tabei uplift tarim basin shows the following characteristics 1 the highest paleo heat flow was predicted to have occurred in the early ordovician as 65 5 mw m 2 and gradually decreased to 55 5 mw m 2 during the late carboniferous 2 a thermal kick was modeled to have occurred in the permian as suggested by an abrupt rise in the heat flow 3 the heat flow gradually decreased since the triassic 4 the present day heat flow was predicted to be as low as 38 mw m 2 this heat flow history honors the geologic and tectonic evolution history of the tabei uplift and is suggested as the best case heat flow model

boron in chondrules isotopic compositions and abundances of boron were measured in sixteen chondrules from seven chondrites by ion microprobe mass spectrometry the chondrules are of the porphyritic barred and radial type and host meteorites include carbonaceous ordinary and enstatite chondrites boron abundances are generally low with average boron concentrations of between 80 and 500 ppb these abundances are lower than those of bulk chondrites 0 35 to 1 2 ppm zhai et al 1996 confirming earlier suggestions that boron is mostly contained in the matrix no significant variation in the 11b 10b ratio is observed among these chondrules outside our experimental error limits of several permil and b isotopic compositions agree with those reported for bulk chondrites the lack of a significant isotope fractionation between chondrules and matrix implies that the low boron abundances are not the result of a rayleigh fractionation during chondrule formation isotopic heterogeneities within individual chondrules are constrained to be 20 0 at 95 confidence level at a spatial scale of 20 30 m significantly lower than the value of about 40 0 previously reported for chondrules from carbonaceous and ordinary chondrites chaussidon and robert 1995 1998 the observed b isotopic homogeneity does not conflict with the presence of decay products from extinct 10be with 10be 9be 0 10 3 as was inferred for calcium aluminum rich inclusions extinct 10be in chondrules would shift the abundance ratio 11b 10b at best by several permil because of their commonly observed low be b ratios 2 the results show that potential b isotopic heterogeneities in the solar nebula due to the presence of components with different b isotopic signatures such as boron produced by high energy galactic cosmic rays 11b 10b 2 5 or by the hypothetical low energy particle irradiation 11b 10b 3 5 11 or boron from type ii supernovae 11b 10b 1 did not survive the chondrule formation processes to a measurable extent

refractory calcium aluminum rich inclusions and aluminum diopside rich chondrules in the metal rich chondrites hammadah al hamra 237 and queen alexandra range 94411 the metal rich chondrites hammadah al hamra hh 237 and queen alexandra range que 94411 paired with que 94627 contain relatively rare 1 vol calcium aluminum rich inclusions cais and al diopside rich chondrules forty cais and cai fragments and seven al diopside rich chondrules were identified in hh 237 and que 94411 94627 the cais 50 400 mum in apparent diameter include a 22 56 pyroxene spinel melilite forsterite rim b 11 28 forsterite bearing pyroxene spinel melilite anorthite forsterite rim c 2 5 grossite rich spinel melilite pyroxene rim d 2 5 hibonite melilite spinel pyroxene forsterite rim e 1 2 hibonite bearing spinel perovskite melilite pyroxene rim f 1 2 spinel melilite pyroxene anorthite and g 1 2 amoeboid olivine aggregate each type of cai is known to exist in other chondrite groups but the high abundance of pyroxene spinel melilite cais with igneous textures and surrounded by a forsterite rim are unique features of hh 237 and que 94411 94627 additionally oxygen isotopes consistently show relatively heavy compositions with delta o 17 ranging from 6 parts per thousand to 10 parts per thousand 1 sigma 1 3 parts per thousand for all analyzed cai minerals grossite hibonite melilite pyroxene spinel this suggests that the cais formed in a reservoir isotopically distinct from the reservoir s where normal o 16 rich delta o 17 20 cais in most other chondritic meteorites formed

anorthite rich chondrules in cr and ch carbonaceous chondrites genetic link between ca al rich inclusions and ferromagnesian chondrules anorthite rich chondrules in cr and ch carbonaceous chondrites consist of magnesian low ca pyroxene and forsterite phenocrysts feni metal nodules interstitial anorthite al ti cr rich low ca and high ca pyroxenes and crystalline mesostasis composed of silica anorthite and high ca pyroxene three anorthite rich chondrules contain relic ca al rich inclusions composed of anorthite spinel al diopside and forsterite a few chondrules contain regions which are texturally and mineralogically similar to magnesian type i chondrules and consist of forsterite low ca pyroxene and abundant feni metal nodules anorthite rich chondrules in cr and ch chondrites are mineralogically similar to those in cv and co carbonaceous chondrites but contain no secondary nepheline sodalite or ferrosilite relatively high abundances of moderately volatile elements such as cr mn and si in the anorthite rich chondrules suggest that these chondrules could not have been produced by volatilization of the ferromagnesian chondrule precursors or by melting of the refractory materials only we infer instead that anorthite rich chondrules in carbonaceous chondrites formed by melting of the reduced chondrule precursors olivine pyroxenes feni metal mixed with the refractory materials including relic cais composed of anorthite spinel high ca pyroxene and forsterite the observed mineralogical and textural similarities of the anorthite rich chondrules in several carbonaceous chondrite groups cv co ch cr may indicate that these chondrules formed in the region s intermediate between the regions where cais and ferromagnesian chondrules originated this may explain the relative enrichment of anorthite rich chondrules in 16o compared to typical ferromagnesian chondrules russell et al 2000

interpretation of discontinuous vitrinite reflectance profiles time temperature modeling was used to examine the effects of reburial on vitrinite reflectance profiles associated with stratigraphic uncomformities offsets in reflectance data for shallow unconformities can in many cases be explained by erosion and may provide a minimum estimate of the amount of missing section as reburial progresses the offset in reflectance values at the unconformity decreases in accordance with the thermal energy required to activate reaction pathways in strata above and below the unconformity until no statistically significant difference exists in vitrinite reflectance across the stratigraphic boundary the reflectance profile is thus annealed when observed at great depths significant offsets in reflectance values typically require geologic lly unreasonable amounts of erosion alternate explanations for offsets at great burial depths include igneous and metamorphic events and complications associated with editing and interpreting vitrinite reflectance data

thermal maturation and hydrocarbon migration within la popa basin northeastern mexico with implications for other salt structures some outcrops adjacent to the 25 km 15 mi long salt weld in the la popa basin northeastern mexico are visibly stained and contain enrichment of organic carbon their presence which is known only along the northwestern part of the weld and along the southwestern side of the weld provides evidence for hydrocarbon migration along the weld this distribution is presumed to be partially controlled by the restricted distribution of the source rock that generated the hydrocarbons within the salt withdrawal basin southwest of the weld but may have been in part controlled by a previously existing salt wall or the weld that formed as a result of salt evacuation acting as a barrier to lateral migration and serving as a vertical conduit for fluid flow upturned halokinetic strata adjacent to the weld provided a conduit for migration along bedding planes although no flowing seeps were seen during this study we documented discoloration and remineralization as well as the presence of waxy appearing soil that smelled like crude oil at some sites near the weld organic geochemical analyses confirm the presence of nonbiodegraded hydrocarbons along the salt sediment interface and within adjacent upturned lithologies up to 5 m 16 ft southwest of the weld the results document for the first time the definitive presence of actively migrating hydrocarbons associated with a subaerial salt weld an oil source rock correlation is inferred with the upper mudstone member of the potrerillos formation based on biomarker data thermal and subsidence modeling of the basin indicate that all of the strata in the basin are mature enough to have generated hydrocarbons the lower part of the stratigraphic section entered the gas generation window and much of the cretaceous section and all of the tertiary entered the oil window prior to uplift of the basin significant unroofing as much as 7 km 4 3 mi of sediment may have been removed occurred sometime after the eocene as suggested by previous studies

petrology of feo poor porphyritic pyroxene chondrules in the semarkona chondrite abstract the mineralogy and petrology of feo poor porphyritic pyroxene and olivine rich chondrules in the semarkona ll3 0 chondrite are described in detail in an extension of the textural classification scheme these chondrules are designated types iab and ib in type iab chondrules the proportion of olivine phenocrysts is between 20 80 and in type ib chondrules olivine constitutes

thermal maturation of organic matter in the middle devonian to tertiary section fort norman area central mackenzie plain the fort norman area is marked by a complex structural and depositional history with temporally and spatially variable temperature and maturity gradients maturity of the middle devonian canol hare indian unit indicates two terrains of markedly different paleotemperatures and thermal gradients roughly coinciding with the palaeozoic keele arch north and root basin south with a sharp transition lateral discontinuity between them maturity of the unconformably overlying cretaceous to early tertiary section is lower and laterally continuous across the entire study area showing no relation to the sub cretaceous structures the vertical maturity profile in the southern terrain is unique in that two discontinuous segments record markedly different thermal histories neither of which is related to present burial conditions maturity of the devonian formations was established some time prior to the sub cretaceous erosion whereas that in the cretaceous to tertiary section postdates palaeocene but predat

comparative optical properties of macerals and statistical evaluation of mis identification of vitrinite and solid bitumen from early mature middle devonian lower mississippian new albany shale implications for thermal maturity assessment abstract vitrinite reflectance vro in is used routinely to quantify thermal maturity in sedimentary basins reflectance and fluorescence of other macerals such as solid bitumen and amorphous organic matter aom can provide an independent means to assess thermal maturity and hydrocarbon generation potential however similarity in petrographic characteristics of these macerals solid bitumen and vitrinite in particular often causes difficulties with their identification and consequently reflectance can be measured on misidentified particles making reported vro values unreliable the purpose of this study is to compare reflectance values of various macerals in early mature shales and to evaluate the implications of misidentifying solid bitumen and vitrinite for assessing thermal maturity to address this issue 15 organic matter rich samples from the middle devonian lower mississippian new albany shale from a corehole in daviess county indiana were selected these samples were chosen because they had distinct and easily identified aom solid bitumen vitrinite and inertinite particles allowing for statistically relevant comparisons vro values 0 57 to 0 65 of the studied sample suite cover the early mature stage and expressed no trend with depth in comparison reflectance values of solid bitumen bro and aom aomro from the same samples are lower and range from 0 44 to 0 52 and 0 27 to 0 31 respectively these differences are accompanied by corresponding differences in chemistry of macerals as demonstrated by the micro ftir technique specifically compared to vitrinite and inertinite solid bitumen shows lower aromaticity and compared to aom and alginite it exhibits shorter aliphatic chains reflectance was observed to vary systematically samples having higher vro also feature elevated solid bitumen and aom reflectance values the relationship between vitrinite and solid bitumen can be expressed by the following equation vitrinite reflectance equivalent vroe in 0 83 bro 0 22 whereas for vitrinite and aom vroe 0 84 aomro 0 38 statistical evaluation of the differences in reflectance values caused by maceral misidentification indicates that in extreme cases when a petrographer cannot distinguish between vitrinite and solid bitumen the reflectance can be shifted by 0 06 0 09 for this set of samples such a difference could shift maturity assessment from early mature to immature in more common cases when the analyst can distinguish between the macerals but has difficulties with their overlapping reflectance interval the reflectance difference that results from misidentification is only within a 0 0 0 02 range therefore if the level of uncertainty in maceral identification can be determined the calculation of vroe values from measured bro can reduce inaccuracy in vro values for interpreting the thermal histories of sedimentary basins which in turn is essential for the assessment of oil and gas resources and for building a successful exploration model

26 al 26 mg and 207 pb 206 pb systematics of allende cais canonical solar initial 26 al 27 al ratio reinstated abstract the precise knowledge of the initial 26 al 27 al ratio 26 al 27 al 0 is crucial if we are to use the very first solid objects formed in our solar system calcium aluminum rich inclusions cais as the time zero age anchor and guide future work with other short lived radio chronometers in the early solar system as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation new high precision multi collector inductively coupled plasma mass spectrometry mc icp ms measurements of 27 al 24 mg ratios and mg isotopic compositions of nine whole rock cais six mineralogically characterized fragments and three micro drilled inclusions from the cv carbonaceous chondrite allende yield a well defined 26 al 26 mg fossil isochron with an 26 al 27 al 0 of 5 23 0 13 10 5 internal mineral isochrons obtained for three of these cais a44a ajef and a43 are consistent with the whole rock cai isochron the mineral isochron of ajef with 26 al 27 al 0 4 96 0 25 10 5 anchored to our precisely determined absolute 207 pb 206 pb age of 4567 60 0 36 ma for the same mineral separates reinstate the canonical 26 al 27 al 0 of 5 10 5 for the early solar system the uncertainty in 26 al 27 al 0 corresponds to a maximum time span of 20 ka thousand years suggesting that the allende cai formation events were culminated within this time span although all allende cais studied experienced multistage formation history including melting and evaporation in the solar nebula and post crystallization alteration likely on the asteroidal parent body the 26 al 26 mg and u pb isotopic systematics of the mineral separates and bulk cais behaved largely as closed system since their formation our data do not support the supra canonical 26 al 27 al ratio of individual minerals or their mixtures in cv cais suggesting that the supra canonical 26 al 27 al ratio in the cv cais may have resulted from post crystallization inter mineral redistribution of mg isotopes within an individual inclusion this redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk cai and bulk mineral data obtained by mc icp ms the radiogenic 208 pb 206 pb ratio obtained as a by product from the pb pb age dating is used to estimate time integrated 232 th 238 u ratio value of cais limited variations among the minerals within a single cai contrasted by much larger variations among the bulk cais suggest th u fractionation occurred prior to crystallization of igneous cais if interpreted as primordial heterogeneity the value can be used to calculate the mean age of the interstellar dust from which the cais condensed

titanium isotopic anomalies in meteorites high precision analyses of ti are reported for samples from a variety of meteorite classes the expanded data base for allende inclusions still shows ti isotopic anomalies in every inclusion all the coarse grained inclusions give quite similar patterns but fine grained inclusions show more variable and sometimes larger anomalies one inclusion 3675a was analyzed because others identified it as a possible fun inclusion due to its mass fractionated mg this designation is supported by the significantly more complex ti isotopic pattern for 3675a compared to all our other allende inclusions available data fail to suggest that any particular allende mineral phase including a chromite carbon fraction from an acid residue is especially rich in anomalous ti we also find anomalous ti in a bulk sample of a cl chondrite and in matrix separates from c2 chondrites the excesses of 50ti are smaller than for allende inclusions and subtle differences in ti isotopic patterns tentatively suggest that parent materials for c1 c2 matrix and allende inclusions are not directly related analyses of chondrules from unequilibrated ordinary chondrites did not yield clear evidence for anomalous ti but some larger than usual deficits at 5046 give encouragement for future work in this direction comparing the magnitude of isotopic shifts at 50ti and 16o for all these meteorite samples indicates that they are not correlated but the data do not preclude a correlation between concentrations of exotic 50ti and 16o atoms whether or not allende fun inclusions are considered at least 4 distinct isotopic components of ti are required to account for the observed isotopic variations the ti data cannot be plausibly explained in terms of an early solar system particle irradiation instead neutron rich hydrostatic burning within a star is probably responsible for the dominant 50ti anomalies while s process mechanisms are viable sources for some of the more subtle ti variations we suggest that the ti anomalies are linked to a diversity of nucleosynthetic sources and the highly refractory behavior of ti therefore some form of chemical memory from the ism rather than late stage supernova injection is most likely responsible for the preservation of observed isotopic heterogeneities

origin and chronology of chondritic components a review abstract mineralogical observations chemical and oxygen isotope compositions absolute 207pb 206pb ages and short lived isotope systematics 7be 7li 10be 10b 26al 26mg 36cl 36s 41ca 41k 53mn 53cr 60fe 60ni 182hf 182w of refractory inclusions ca al rich inclusions cais and amoeboid olivine aggregates aoas chondrules and matrices from primitive unmetamorphosed chondrites are reviewed in an attempt to test i the x wind model vs the shock wave model of the origin of chondritic components and ii irradiation vs stellar origin of short lived radionuclides the data reviewed are consistent with an external stellar origin for most short lived radionuclides 7be 10be and 36cl are important exceptions and a shock wave model for chondrule formation and provide a sound basis for early solar system chronology they are inconsistent with the x wind model for the origin of chondritic components and a local irradiation origin of 26al 41ca and 53mn 10be is heterogeneously distributed among cais indicating its formation by local irradiation and precluding its use for the early solar system chronology 41ca 41k and 60fe 60ni systematics are important for understanding the astrophysical setting of solar system formation and origin of short lived radionuclides but so far have limited implications for the chronology of chondritic components the chronological significance of oxygen isotope compositions of chondritic components is limited the following general picture of formation of chondritic components is inferred cais and aoas were the first solids formed in the solar nebula 4567 4568 myr ago possibly within a period of 10 and have 26al 27al 0 similar to those in chondrules 5 nebular gas the majority of chondrules within a chondrite group may have formed over a much shorter period of time

application of vitrinite reflectance in reconstruction of tectonic features in anambra basin nigeria implication for petroleum potential quantitative analysis of vitrinite reflectance data from the anambra basin nigeria has revealed for the first time tectonism and unconformities corresponding to the campanian maastrichtian and paleocene eocene boundaries this analysis also afforded definite subsurface evidence for the cenomanian as well as the santonian deformations the estimated thickness of each rock section that was removed by erosion to give the cenomanian santonian and upper campanian unconformities is approximately 373 m 512 m and 217 m respectively the estimates for the cenomanian and santonian erosion also represent the height of the corresponding uplifts in the lower benue trough the thicknesses of the rock sections eroded upon cessation of subsidence in the anambra basin in the early ertiary to produce the present topography vary systematically from 73 to 3063 m therefore the reconstructed depth corresponding to the top of the oil window in the early tertiary ranges from 1521 to 5127 m a temperature range of 81 to 191 c because the top of the oil window was located at that time within the nkporo shale or younger units the eze aku shale awgu shale nkalagu formation and in many cases the nkporo shale were originally placed at temperatures 150 c that favored mainly generation of gaseous hydrocarbons in many parts of the basin this could have played at least a supportive role in producing the predominance of gas plays in the anambra basin hence reconstruction of the maximum burial depth and paleotemperature of viable source beds is recommended as necessary input into a successful exploration strategy for the basin

chondrules matrix and coarse grained chondrule rims in the allende meteorite origin interrelationships and possible precursor components bulk compositions of 20 chondrules 13 coarse grained chondrule rims and two composite samples of matrix material from cv3 allende were determined by instrumental neutron activation analysis three rare nonporphyritic cv chondrules from allende leoville and vigarano were analyzed by broad beam electron microprobe analysis chondrule precursor components deduced from chondrule compositions are characterized by 1 refractory and semi refractory lithophiles with low feo 2 common siderophiles and chalcophiles 3 common to slightly volatile lithophiles si cr mn and modal pyroxene with moderately high feo feo mgo 4 moderately volatile siderophiles and 5 alkalies these precursor components can be explained by plausible processes occurring in the solar nebula the precursor components of coarsegrained rims are apparently the same as those of chondrules these components differ from those inferred previously for oc eh or cm co chondrules in general the compositions of coarse grained rims are more similar to the mean chondrule composition than to those of the chondrules they enclose rim siderophile contents are generally 2 5x greater than those in the chondrules and 0 5 to 2x those in bulk allende rims as a group are more homogeneous than chondrules suggesting that rims formed from finer grained precursors several chondrules and rims have fractionated rare earth patterns and may have been affected by metamorphism the enclosure of some coarse grained rims by fine grained matrix like rims that are almost certainly of nebular origin indicates that coarse grained rims were also formed in the nebula two matrix samples are similar in composition to some coarse grained rims suggesting that matrix was derived from similar compositional reservoirs of nebular dust chondrules with coarse grained rims were reheated in space following the accretion of dusty coatings this restricts plausible chondrule forming mechanisms to those that allow multiple nebular heating events

role of gas melt interaction during chondrule formation abstract it is generally believed that most chondrules formed in the protoplanetary disk by melting of fine grained solid precursor materials and that during these melting events chondrules behaved as closed systems both chemically and isotopically i e the observed range in chondrule bulk chemistry and modal mineralogy was entirely inherited from solid precursors that experienced melting to varying degrees followed by igneous crystallization accompanied by very limited if any evaporation recondensation our mineralogical and chemical study of magnesium rich porphyritic type i chondrules in primitive ordinary and carbonaceous chondrites revealed that chondrule glasses are not located on the subtraction lines of olivine or low ca pyroxene in appropriate phase diagrams indicating that type i chondrules do not obey closed system crystallization we suggest that gas melt interaction played a major role in evolution of mineralogy bulk chemical and isotopic compositions of type i chondrules chondrules are thus complex objects composed of an inherited solid component and a melt component that equilibrated with nebular gas suggesting that chondrules formed in regions with high ambient gas partial pressures as a result chondrule glasses melts are therefore thermochemical and isotopic sensors of the surrounding nebular gas in the chondrule forming regions of the protoplanetary disk and can be used for evaluating its physico chemical conditions

mesozoic cenozoic denudation and thermal history in the central uplift of the south yellow sea basin and the implications for hydrocarbon systems constraints from the csdp 2 borehole abstract this study focuses on the mesozoic cenozoic tectono thermal events in the central uplift of the south yellow sea basin sysb based on the csdp 2 borehole csdp 2 is the first borehole to penetrate the integrated paleozoic mesozoic sedimentary strata and recovered approximately 2736 m of core the drilling results confirm a significant stratigraphic hiatus i e indosinian unconformity between the neogene quaternary and the underlying lower triassic the eroded thickness estimated from sonic log and vitrinite reflectance data range from 1200 to 1400 m multiple paleo thermometer data from the csdp 2 cores including vitrinite reflectance apatite zircon fission track aft zft and fluid inclusions were used to reconstruct the mesozoic cenozoic thermal history in the study area there was a continuous heating process during the early mesozoic the peak paleo temperature may have reached up to 160 c base permian during the late jurassic with the highest paleo geothermal gradient of approximately 58 8 c km 1 the tectono thermal evolution was dominated by the mesozoic intracontinental collision between the yangtze craton and the sino korean platform the heating process was mainly caused by rapid burial on the passive continental margin and igneous activities the paleo temperature significantly decreased following the late jurassic two rapid cooling processes ca 100 65 ma and ca 35 ma are suggested by the aft modeling results cooling history indicates that the tectonic uplift and denudation likely occurred from the early cretaceous to the miocene the reconstruction results of thermal evolution confirm that the mesozoic paleozoic source rocks are mature or even highly mature and hydrocarbon generation migration and charge undoubtedly occurred in the central uplift showing substantial hydrocarbon potential moreover the reconstruction of the tectono thermal history provides important constraints for regional tectonic studies

tectonic history from vitrinite reflectance summary the degree of organic metamorphism of carbonaceous matter is expressed in terms of the reflectance of vitrinite vitrinite reflectance is a function of temperature and time a model relating vitrinite reflectance to temperature and time is proposed from published vitrinite reflectance temperature time nomograms a plot of the logarithm of vitrinite reflectance versus depth gives a linear curve for vitrinite reflectance greater than 0 3 per cent in a sedimentary basin which has had a constant geothermal gradient throughout its tectonic history the geothermal gradient c km 1 is equal to the slope of the curve multiplied by 194 8 the model is also applied to sedimentary basins with decreasing geothermal gradient with time i e cooling continental margin basins the vitrinite reflectance model is demonstrated to be consistent with observed reflectance data and tectonic information from the sydney bowen surat and gippsland basins in eastern australia

oxygen isotopic constraints on the origin of magnesian chondrules and on the gaseous reservoirs in the early solar system abstract we report in situ ion microprobe analyses of the oxygen isotopic composition of the major silicate phases olivine low ca pyroxene silica and mesostasis of 37 magnesian porphyritic type i chondrules from cv vigarano usnm 477 2 vigarano uh5 mokoia and efremovka and cr eet 92042 eet 92147 eet 87770 el djouf 001 mac 87320 and gra 95229 carbonaceous chondrites in spite of significant variations of the modal proportions of major mineral phases in cr and cv chondrules the same isotopic characteristics are observed i olivines are isotopically homogeneous at the level within a chondrule although they may vary significantly from one chondrule to another ii low ca pyroxenes are also isotopically homogeneous but systematically 16o depleted relative to olivines of the same chondrule and iii all chondrule minerals analyzed show 16o enrichments relative to the terrestrial mass fractionation line enrichments that decrease from olivine spinel to low ca pyroxene and to silica and mesostasis the observation that in most of the type i chondrules studied the coexisting olivine and pyroxene crystals and glassy mesostasis have different oxygen isotopic compositions implies that the olivine and pyroxene grains are not co magmatic and that the glassy mesostasis is not the parent liquid of the olivine the 18o and 17o values of pyroxene and olivine appear to be strongly correlated for all the studied cr and cv chondrules according to 18 o pyroxene 0 72 0 16 0 13 18 o olivine 1 21 0 54 0 79 17 o pyroxene 0 81 0 15 0 13 17 o olivine 0 60 1 00 0 95 these correlations are consistent with mass balance of oxygen isotopes calculated in the case of chondrule formation according to the following gas melt mineral reactions mg 2 sio 4 olivine sio 2 melt mg 2 si 2 o 6 pyroxene sio 2 melt sio gas 1 2 o 2 gas these equations imply that 2 3 of the oxygen incorporated by low ca pyroxene during its crystallization in the melt is derived from the dissolution of precursor olivine and 1 3 from the addition of chemical components such as sio from the nebular gas the composition of the nebular gaseous reservoir in which type i cr and cv chondrules formed is determined from the present data set to be 18o gas 3 6 1 and 17o gas 1 8 1 this isotopic composition is in excellent agreement with that of the gaseous reservoir previously inferred from the oxygen isotope systematics of bulk chondrules from various chondrite groups i e cv cr unequilibrated ordinary and eh thus the oxygen isotopic systematic strongly supports the idea that most of the olivines in type i chondrules are relict grains and that gas melt interaction played a major role in the formation of type i chondrules

multi proxy approach to screen the hydrocarbon potential of the jurassic succession in the matruh basin north western desert egypt abstract this paper discusses the hydrocarbon potential of the jurassic succession in a mature basin in egypt as an important element of a larger petroleum system study prior to any organic petrographic and organic geochemical analyses a palynological age dating of the originally undifferentiated jurassic sequence was carried out to identify different formations under investigation this was based on key bioevents of some recovered dinoflagellate cysts and the identified lithology palynological toc rock eval pyrolysis including modified rock eval methods and vitrinite reflectance vro data from a total of 14 samples taken from the uppermost lower upper jurassic sequence represented by the wadi natrun toarcian aalenian khatatba late bathonian callovian and masajid oxfordian formations in the abu tunis 1 well are presented in addition two samples from the abu tunis 1 well and the proximal 32 km to the east siqueifa 1 well having the highest remaining hydrocarbon potential s2 yields were analyzed and their results were compared using modified pyrolysis programs although the data showed a good correlation between the tai of the palynomorph assemblage vitrinite reflectance and tmax from rock eval pyrolysis in the shallower intervals the correlation between vro and tmax was poor in the lower half of the studied succession middle khatatba and wadi natrun formations the very low tmax values indicate immature om while vro and tai indicated middle stage of oil window to past peak oil generation the reason for this discrepancy is that the deeper samples are reservoir rocks not source rocks and the majority of the organic matter is not composed of reactive kerogen but consists of migrated hydrocarbons and nso compounds contamination due to oil based mud obm was eliminated because the abu tunis 1 well was drilled in 1969 prior to the extensive use of obm in drilling this study showed that a multi proxy approach is the best way to screen the hydrocarbon potential in a thick succession that contains interbedded source and reservoir rocks

hibonite bearing microspherules a new type of refractory inclusions with large isotopic anomalies reported are petrographic descriptions major and trace element chemistry and mg ca and ti isotopic compositions of a new class of refractory inclusions that consist of spherules composed of hibonite and a silicate glass the distinctive features of these inclusions are excesses in 48ca and 50ti in both glass and hibonite and 26mg depletions relative to terrestrial isotopic compositions three spherules have been examined and analyzed one from the lance co3 meteorite and two from the murchison cm2 meteorite lance 3413 1 31 la3413 1 31 and murchison 7 228 mur7 228 have euhedral to subhedral hibonite crystals enclosed within glass murchison 7 753 mur7 753 has a rounded hibonite core with several small inclusions of perovskite a small fragment of glass is attached to the hibonite and an fe silicate rim is imperfectly preserved around the grain la3413 1 31 has a group ii ree pattern mur7 228 a refractory pattern with depletions in the relatively volatile elements sr ba nb v and eu and mur7 753 a pattern characterized by the prior removal of an ultrarefractory component and overall fractionation of all rees the partitioning of the lrees between hibonite and glass in mur7 228 is consistent with equilibrium hibonite liquid partition coefficients previously determined la3413 1 31 shows much less partitioning while mur7 753 shows no evidence for partitioning and preserves an unequilibrated refractory component highly enriched in gd all spherules have initial magnesium depleted in 26mg by around 3 relative to terrestrial mg but only mur7 228 shows evidence for in situ decay of 26al with an initial 26al27al of 1 7 0 7 10 5 both hibonite and glass in all three spherules show excesses of 48ca and 50ti ranging up to 40 and 20 respectively relative to terrestrial ca and ti spherules such as these are rare and the only other occurrence is in the unique chondrite alh85085 the hibonite in the spherules shows similarities to isotopically anomalous hibonite crystal fragments placs but it is unlikely that the spherules formed by remelting of placs the precursors include isotopically anomalous ca ti carriers but also isotopically normal refractory components that probably formed as condensates the spherules formed by melting of these precursors under disequilibrium conditions and rapid cooling after hibonite crystallization these inclusions must have formed early prior to the dilution of isotopic anomalies by mixing processes and in an area characterized by excesses of 48ca and 50ti depletions of 26mg and lack of 26al

aluminum 26 in calcium aluminum rich inclusions and chondrules from unequilibrated ordinary chondrites in order to investigate the distribution of 26 a1 in chondrites we measured aluminum magnesium systematics in four calcium aluminum rich inclusions cais and eleven aluminum rich chondrules from unequilibrated ordinary chondrites uocs all four cais were found to contain radiogenic 26 mg 26 mg from the decay of 26 a1 the inferred initial 26 al 27 al ratios for these objects 26 al 27 al 0 5 10 5 are indistinguishable from the 26 al 27 al 0 ratios found in most cais from carbonaceous chondrites these observations together with the similarities in mineralogy and oxygen isotopic compositions of the two sets of cais imply that cais in uocs and carbonaceous chondrites formed by similar processes from similar or the same isotopic reservoirs or perhaps in a single location in the solar system we also found 26 mg in two of eleven aluminum rich chondrules the 26 al 27 al 0 ratio inferred for both of these chondrules is 1 10 5 clearly distinct from most cais but consistent with the values found in chondrules from type 3 0 3 1 uocs and for aluminum rich chondrules from lightly metamorphosed carbonaceous chondrites 0 5 10 5 to 2 10 5 the consistency of the 26 al 27 al 0 ratios for cais and chondrules in primitive chondrites independent of meteorite class implies broad scale nebular homogeneity with respect to 26 al and indicates that the differences in initial ratios can be interpreted in terms of formation time a timeline based on 26 al indicates that chondrules began to form 1 to 2 ma after most cais formed that accretion of meteorite parent bodies was essentially complete by 4 ma after cais and that metamorphism was essentially over in type 4 chondrite parent bodies by 5 to 6 ma after cais formed type 6 chondrites apparently did not cool until more than 7 ma after cais formed this timeline is consistent with 26 al as a principal heat source for melting and metamorphism

thermal maturity and petroleum kitchen areas of liassic black shales lower jurassic in the central upper rhine graben germany in the central upper rhine graben urg several major oil fields have been sourced by liassic black shales in particular the posidonia shale lias e lower toarcian acts as excellent and most prominent source rock in the central urg this study is the first comprehensive synthesis of liassic maturity data in the urg area and sw germany the thermal maturity of the liassic black shales has been analysed by vitrinite reflectance vrr measurements which have been verified with t max and spore coloration index sci data in outcrops and shallow wells 600 m the liassic black shales reached maturities equivalent to the very early or early oil window ca 0 50 0 60 vrr this maturity is found in liassic outcrops and shallow wells in the entire urg area and surrounding swabian jura mountains maximum temperatures of the posidonia shale before graben formation are in the order of 80 90 c these values were likely reached during late cretaceous times due to significant upper jurassic and minor cretaceous deposition and influenced by higher heat flows of the beginning rift event at about 70 ma in this regard the consistent regional maturity data vrr t max sci of 0 5 0 6 vrr for the posidonia shale close to surface suggest a major burial controlled maturation before graben formation these consistent maturity data for liassic outcrops and shallow wells imply no significant oil generation and expulsion from the posidonia shale before formation of the urg a detailed vrr map has been created using vrr values of 31 wells and outcrops with a structure map of the posidonia shale as reference map for a depth dependent gridding operation highest maturity levels occur in the area of the rastatt trough ca 1 5 vrr and along the graben axis with partly very high vrr gradients e g well scheibenhardt 2 in these deep graben areas the maximum temperatures which were reached during upper oligocene to miocene times greatly exceed those during the cretaceous

petroleum generation and accumulation in the berkine basin eastern algeria the berkine ghadames basin is regarded as an important hydrocarbon play in eastern algeria an evaluation of organic source facies the application of petroleum geochemistry and an extensive basin modeling study were carried out to improve the understanding of the petroleum charge of the basin geochemical data indicate that the llandoverian wenlockian and the frasnian hot shales are the most important oil prone source rocks for the known hydrocarbon accumulations in the basin maturity data reflect the late cretaceous tertiary burial and are consistent with a 60 63 mw m2 heat flow in the vicinity of the anticlines and 50 mw m2 heat flow farther away from the folded area a two dimensional basin modeling study revealed the importance of poorly constrained parameters such as the amount and rate of hercynian erosion on hydrocarbon generation and trapping efficiencies the model shows that a cretaceous tertiary petroleum generation and emplacement for both source rocks can better explain the different gas and oil fields than can an earlier hercynian event the faults related to the triassic jurassic rifting are most certainly permeable and can be considered as major migration pathways for petroleum in contrast the overpressure that developed along the el biod hassi messaoud axis in the ordovician reservoirs was probably caused by the low permeability of the faults bounding the anticlines that therefore prohibited any lateral or vertical fluid movements the model also indicates two petroleum systems that have triassic reservoirs 1 a frasnian sourced system that is confined to the central part of the basin and 2 a llandoverian wenlockian sourced system in the western part of the basin migration into paleozoic reservoirs was followed by dismigration into overlying triassic traps after the cretaceous compressive movements

diagenesis and anchimetamorphism in an overthrust belt external domain of the taconian orogen southern canadian applachians ii paleogeothermal gradients derived from maturation of different types of organic matter abstract 3hermal maturation patterns have been established for the external domain of taconian orogen of the southern quebec appalachians between quebec city and the u s border reflectance of three types of organic matter om measured in the same sample shows that graptolite reflectance r g is higher than pyrobitumen reflectance r b which in turn is higher than reflectance of amorphous organic matter r a r g r b and r a are converted to vitrinite equivalent reflectance from which maximum paleotemperatures were calculated inverted diagenesis is preserved in northwest part of the chaudiere nappe to the southeast original burial diagenetic grade was upgraded by later synorogenic heating in the st lawrence lowlands paleogeothermal gradients at maximum burial were 33 37 c km a gradual southwestward increase in maturation level suggests increasing post taconian burial depth from 4 6 to 6 7 km indicating an increasing thickness of eroded rocks from northeast to southwest along logan s line one of the most important contributions of this study is the proof that the acadian orogeny has affected the successor basin of the taconian orogeny in the st lawrence lowlands the thermal effect of the mid cretaceous intrusion of the monteregian hills on their country rocks is limited to one diameter or less of the intrusive body as shown by the narrow

geology of norton basin and continental shelf beneath northwestern bering sea alaska the rocks that floor the norton basin and the northwestern bering sea are most likely of precambrian and paleozoic age like those rocks that crop out around the basin a maximum of 6 5 km of mainly cenozoic strata lie over basement in the basin on the basis of the geometry of reflections in seismic data we believe alluvial fans to be present deep in the basin and to border major basement fault blocks these fans are the lowest units of the basin fill in many areas and consist of uppermost cretaceous or lower paleogene possibly coal and volcanic rich rocks mainly clastic nonmarine sedimentary rocks overlie the fan deposits the neogene and quaternary basin rocks apparently were deposited in a marine environment the basement of the northwestern bering sea west of norton basin is 1 km below sea level for the most part but local structural lows are filled with at most 2 km of strata of probable late neogene and quaternary age the continental shelf in this area is separated structurally from the seward peninsula by a zone of small basement antiforms over which relatively high gravity values have been measured the norton basin comprises two structural areas that are separated by a major northwest striking horst the first structural area lies west of this horst where major normal faults strike northwest to form local areas where the basin is as deep as 5 km the second area east of the horst includes major normal faults that strike east and northeast the deepest part of the norton basin 6 5 km lies there during the late jurassic and early cretaceous basement rocks now beneath the norton basin were affected by the orogeny that formed the brooks range these basement rocks were metamorphosed and thrust and then eroded deeply nearly 50 m y before the basin began to form in the middle late cretaceous the area of the eastern seward peninsula and eastern norton sound was subjected to east west compression and consequent eastward thrusting the strike slip kaltag fault probably formed at this time if norton basin formed as a pull apart basin along this fault then the basin may be as old as middle late cretaceous however during the latest cretaceous and early paleogene the compression gave way to regional extension that formed northeast trending grabens onshore east of norton basin we believe that the extensional norton basin most likely formed in response to this regional extension initial deepening of the basin was controlled by major normal faults and occurred rapidly whereas subsidence was slower and more regional in scale

three common source rock evaluation errors made by geologists during prospect or play appraisals geologists are frequently called on to evaluate the source rocks associated with their exploration prospects or plays the three most common questions asked and answered about the source rock during project reviews are what s the total organic carbon toc what kerogen type does rock eval indicate and what maturity level does the vitrinite reflectance data point to the answers to these seemingly innocuous questions may in fact be providing a false sense of security about the source rock in question understanding how this line of questioning can lead you astray and make you the victim of the toc myth if i have high toc i have a good source rock the rock eval fallacy the rock eval data tell me what kind of kerogen is in my source rock and the vitrinite reflectance deficiency vitrinite reflectance will tell me if my source rock is generating is important some of the solutions to these problems include fully integrating toc and rock eval data supplementing rock eval data with pyrolysis gas chromatography and using burial history diagrams to help interpret vitrinite reflectance harry dembicki jr is a senior geological advisor for geochemistry in the geological technology group at anadarko petroleum corporation where he provides technical support to both exploration and development teams he holds a b s degree 1973 in geology from the state university college of new york at new paltz and a ph d 1977 in geology with emphasis in organic geochemistry from indiana university he previously worked as an organic geochemist for conoco and marathon

pb isotopic age of the allende chondrules we have studied pb isotope systematics of chondrules from the oxidized cv3 carbonaceous chondrite allende the chondrules contain variably radiogenic pb with a 206pb 204pb ratio between 19 5 268 pb pb isochron regression for eight most radiogenic analyses yielded the date of 4566 2 2 5 ma internal residue leachate isochrons for eight chondrule fractions yielded consistent dates with a weighted average of 4566 6 1 0 ma our best estimate for an average age of allende chondrule formation this pb pb age is consistent with the range of model 26al 26mg ages of bulk allende chondrules reported by bizzarro et al 2004 and is indistinguishable from pb pb ages of ca al rich inclusions cais from cv chondrites 4567 2 0 6 ma amelin et al 2002 and the oldest basaltic meteorites we infer that chondrule formation started contemporaneously with or shortly after formation of cv cais and overlapped in time with formation of the basaltic crust and iron cores of differentiated asteroids the entire period of chondrule formation lasted from 4566 6 1 0 ma allende to 4564 7 0 6 ma cr chondrite acfer 059 to 4562 7 0 5 ma cb chondrite gujba and was either continuous or consisted of at least three discrete episodes since chondrules in cb chondrites appear to have formed from a vapor melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated krot et al 2005 there were possibly a variety of processes in the early solar system occurring over at least 4 5 myr that we now combine under the umbrella name of chondrule formation

statistical analysis of vitrinite reflectance data a new approach abstract the reflectance of vitrinite collotelinite particles is a widely used parameter as a geothermometer for the estimation of the thermal maturity of organic matter enclosed in rocks however several problems have occurred during the last decades which can be traced back to basically three causes human mistakes technical problems and problems associated with the structural and compositional inhomogeneity of organic matter whilst in most cases the first two types of uncertainties can be handled by standardization the third can cause significant problems during interpretation due to its generally inestimable character the suppression of vitrinite reflectance and statistical problems originated from small sample size and outliers belong to this latter type international standards such as the astm and the iso define the vitrinite reflectance parameter as a statistical average of measured data disregarding the fact that the average is an unresisting and unrobust statistical parameter in other words the average is very sensitive to outliers and distribution the aim of this research was to find and test a better more resistant and robust statistical parameter used by traditional parametric and nonparametric statistics which can be applied in practice instead of the average three categories of statistical problems were studied on coal and disperse organic matter dom samples the distribution of measured values the effect of data number and the effect of outliers on statistical parameters the statistical experiments carried out on numerous original and generated sample sets show that the median med and the most frequent value mn a special weighted average are better parameters to estimate the thermal maturity of organic matter especially above 1 reflectance value

manganese chromium isotope systematics of carbonaceous chondrites abstract in this article we present the results of cr isotope investigations of different types of carbonaceous chondrites and of the pallasite eagle station the 53 cr 52 cr ratios in the bulk samples of carbonaceous chondrites are correlated with 55 mn 52 cr ratios the slope of the correlation line yields a 53 mn 55 mn ratio of 8 5 1 5 10 6 at the time of mn cr fractionation mapping this ratio onto an absolute time scale yields a time for this event of 4568 1 0 8 1 1 ma ago this time is very similar to the formation age of efremovka cais of 4567 2 0 6 ma y amelin a n krot i d hutcheon a a ulyanov lead isotopic ages of chondrules and calcium aluminum rich inclusions science 297 2002 1678 1683 to a time of the chondrule formation of 4568 1 ma ago l e nyquist d lindstrom d mittlefehldt c y shih h wiesmann s wentworth r martinez manganese chromium formation intervals for chondrules from the bishunpur and chainpur meteorites meteorit planet sci 36 2001 911 938 which most likely constrains early global high temperature mn cr fractionation in a nebular setting the bulk samples of carbonaceous chondrites exhibit clear 54 cr excesses 54 cr that are correlated with the 53 cr excesses 53 cr and also with mn cr ratios one possible explanation of this correlation is that 54 cr is also radiogenic like 53 cr and was formed by the decay of the short lived parent radionuclide 54 mn the very short half life of 54 mn of 312 days would require that both short lived radionuclides 53 mn and 54 mn were generated locally in spallation reactions during the early period of an active sun the alternative and possibly more plausible explanation is the heterogeneous addition of presolar material the presolar component enriched in 54 cr is mostly contained in the matrix of carbonaceous chondrites the relative amount of matrix decreases in the sequence ci cm co cv a large proportion of mn is associated with the matrix while cr preferentially resides in the chondrules thus the mn cr ratio also follows the sequence ci cm co cv and is correlated with 54 cr the acid resistant residues of carbonaceous chondrites are characterized by relatively large excesses of 54 cr and moderate deficits of 53 cr the magnitude of these excesses and deficits decreases in the sequence ci cm cv and may imply that cr in the cm and cv residues is increasingly more equilibrated with cr from the rest of the meteorites the 53 cr 52 cr and 54 cr 52 cr ratios in the residues are anti correlated indicating that there are at least two cr components of possibly presolar origin all residues have large excesses of the most neutron rich ti isotope 50 ti the pattern of 50 ti does not exactly follow that observed for 54 cr but it is generally similar the cr isotope systematic of the pallasite eagle station indicates that the precursor of this meteorite was a cv type material the 53 mn 53 cr system indicates that the cr isotopes equilibrated in this meteorite 4557 5 0 6 ma ago

primitive feni metal grains in ch carbonaceous chondrites formed by condensation from a gas of solar composition some feni metal grains similar to 150 mu m in apparent diameter in ch carbonaceous chondrites are concentrically zoned in ni similar to 5 10 wt co 0 2 0 4 wt and cr 0 3 0 8 wt silicon is present at the similar to 0 1 wt level these observations are consistent with predicted gas solid condensation from a gas of solar composition at temperatures of similar to 1370 1270 k and total pressure of similar to 10 4 bar estimates of feni metal grain growth and cooling rates in this temperature range are consistent with brief and localized thermal episodes in the solar nebula compositionally similar feni metal grains have also been reported in cr and bencubbin like chondrites because feni metal is highly susceptible to secondary alteration i e metamorphism melting oxidation the observed feni metal condensates in ch bencubbin like and cr chondrites indicate that these meteorites experienced no thermal processing after their lithification and thus are among the most primitive meteorites in our collections

determination of the petrologic type of cv3 chondrites by raman spectroscopy of included organic matter abstract this paper reports the first reliable quantitative determination of the thermal metamorphism grade of a series of nine cv3 chondrites allende axtell bali mokoia grosnaja efremovka vigarano leoville and kaba the maturity of the organic matter in matrix determined by raman spectroscopy has been used as a powerful metamorphic tracer independent of the mineralogical context and extent of aqueous alteration this tracer has been used along with other metamorphic tracers such as fe zoning in type i chondrules of olivine phenocrysts presolar grain abundance and noble gas abundance bulk and p3 component the study shows that the petrologic types determined earlier by induced thermoluminescence were underestimated and suggests the following values pt allende axtell 3 6 pt bali mokoia grosnaja 3 6 pt efremovka leoville vigarano 3 1 3 4 pt kaba 3 1 the most commonly studied cv3 allende is also the most metamorphosed bali is a breccia containing clasts of different petrologic types the attribution suggested by this study is that of clasts of the highest petrologic types as pointed out by iom maturity and noble gas bulk abundance cv3 chondrites have complex asteroidal backgrounds with various degrees of aqueous alteration and or thermal metamorphism leading to complex mineralogical and petrologic patterns fe mg chemical zoning in olivine phenocrysts on the borders of type i chondrules of porphyritic olivine and pyroxene rich textural types has been found to correlate with the metamorphism grade this suggests that chemical zoning in some chondrules often interpreted as exchanges between chondrules and nebular gas may well have an asteroidal origin furthermore the compositional range of olivine matrix is controlled both by thermal metamorphism and aqueous alteration this does not support evidence of a nebular origin and does not necessarily mirror the metamorphism grade through fe mg equilibration on the other hand it may provide clues on the degree of aqueous alteration vs thermal metamorphism and on the timing of both processes in particular mokoia experienced significant aqueous alteration after the metamorphism peak whereas grosnaja which has similar metamorphism grade did not

the association between tectonic deformation inorganic composition and coal rank in the bituminous coals from the south wales coalfield united kingdom analysis of vertical distribution of coal rank in three collieries and one opencast mine in the se of the south wales coalfield has revealed deviations from hilt s law in both proximate analyses and in vitrinite reflectance the variations in rank parameters studied are closely correlated with variations in the intensity of tectonic deformation measured using a numerical deformation index ndi three seams in the sequence were also analysed for major and trace element geochemistry and rock eval pyrolysis variations in these parameters are related to thrust inclusion of floor and or roof mudrocks into the seams a detailed conceptual model of a major thrust and associated in seam thrust detachments and lag faults is developed to explain the vertical rank profile by post coalification faulting it is concluded that although the model can explain the vertical pattern of rank the complexity of the required faults render the model improbable the alternative mechanisms for enhancement of the rank parameters associated with tectonic deformation including shear stress control frictional heating and localised fluid flow are discussed and are thought more likely to be responsible for the deviations from hilt s law in the vertical rank profile rank suppression shown by three seams two of which are overlain by marine bands is thought to be related to biochemical activity in the peat environment resulting from the influx of saline waters

crystallization sequences of ca al rich inclusions from allende an experimental study the equilibrium crystallization sequence at 1 atmosphere in air of a melt corresponding in composition to the average composition of type b ca al rich inclusions from the allende meteorite is spinel 1550 c melilite 1400 c ak22 anorthite 1260 c ti al rich clinopyroxene 1230 c ti fassaite the melilite becomes increasingly akermanitic with decreasing temperature the pyroxene is similar in composition to fassaites from type b inclusions preliminary results suggest that the crystallization sequence is similar at oxygen fugacities near the iron wustite buffer the results of these experiments have been integrated with available phase equilibrium data in the system cao mgo al 2o 3 sio 2tio 2 and a phase diagram for predicting the crystallization sequences of liquids with compositions of coarse grained ca al rich inclusions has been developed available bulk compositions of coarse grained inclusions form a well defined trend in terms of major elements extending from type a and bl inclusions near the spinel melilite join to more pyroxene rich type b2 inclusions the trend deviates from the expected sequence of solid condensates from a nebular gas at p 10 3 atm if pure diopside is assumed to be the clinopyroxene that condenses the type a b1 end of the trend is similar in composition to calculated equilibrium condensates at 1202 1227 c and the trend as a whole parallels the sequence of condensates expected from diopside condensation at 1170 c the trend is consistent to first order with the condensation of solid ti rich fassaite in place of pure diopside at higher temperatures than those at which pure diopside is predicted to condense partially molten condensates may be likely in this case or if the nebular pressure is higher than 10 3 atm

an improved hydrocarbon generation potential method for quantifying hydrocarbon generation and expulsion characteristics with application example of paleogene shahejie formation nanpu sag bohai bay basin abstract to evaluate petroleum resources it is important to quantify the hydrocarbon generation and expulsion characteristics of source rock the hydrocarbon generation potential method is an efficient model for calculating the characteristics of source rocks and evaluating the resource potential the previous hydrocarbon generation potential model was mainly established by measured rock eval pyrolysis data and due to inappropriate preservation measures an important component volatile hydrocarbon content of source rock may be significantly reduced prior methods may therefore not objectively reflect the properties of a source rock and cause unacceptable errors in the resource assessment this study proposes an improved hydrocarbon generation potential method the new method considers the initial hydrogen index hi0 and transformation ratio tr during the evolution of source rocks which can more realistically reflect the hydrocarbon generation and expulsion history of source rocks therefore the hydrocarbon evaporative loss qloss is calibrated and the hydrocarbon generation and expulsion features of source rocks are more accurately evaluated the proposed method is used to examine the effectiveness of the third member of the upper paleogene shahejie formation es3 in nanpu sag by utilizing the improved hydrocarbon generation potential model the hydrocarbon expulsion threshold of the es3 source rock is positioned at ro 0 875 the maximum hydrocarbon generation and expulsion potential qg and qe are 593 78 and 323 78 mg hc g toc respectively and the maximum hydrocarbon expulsion efficiency is 54 the maximum hydrocarbon generation and expulsion intensities are 500 104 t km2 and 250 104 t km2 respectively which locate source rock at the laoyemiao and gaoshangpu structural belts the amounts of generated and expelled hydrocarbon are 13 7 108 t and 6 9 108 t respectively and the resource potential of the tight oil of the es3 in the nanpu sag is 0 69 104 t compared with the previous method the improved hydrocarbon generation potential method limits the maximum qg and calibrates the qloss therefore the new method more objectively reflects the hydrocarbon generation and expulsion characteristics and resource prospect of the source rock

direct surface thermodynamic observations within the rear flank downdrafts of nontornadic and tornadic supercells abstract despite the long surmised importance of the hook echo and rear flank downdraft rfd in tornadogenesis only a paucity of direct observations have been obtained at the surface within hook echoes and rfds in this paper in situ surface observations within hook echoes and rfds are analyzed these mobile mesonet data have unprecedented horizontal spatial resolution and were obtained from the verifications of the origins of rotation in tornadoes experiment vortex and additional field experiments conducted since the conclusion of vortex the surface thermodynamic characteristics of hook echoes and rfds associated with tornadic and nontornadic supercells are investigated to address whether certain types of hook echoes and rfds are favorable or unfavorable for tornadogenesis tornadogenesis is more likely and tornado intensity and longevity increase as the surface buoyancy potential buoyancy as measured by the convective available potential energy and equivalent potential temperature in the r

thermal maturity patterns in alaska implications for tectonic evolution and hydrocarbon potential nearly 10 000 vitrinite reflectance and conodont color alteration index determinations from sedimentary rocks in alaska were used to produce a thermal maturity map of rocks exposed at the surface and to evaluate subsurface thermal maturity relations in the colville and cook inlet basins rocks exposed at the surface of the tertiary interior basins and in the aleutian forearc and backarc basins uniformly are of very low thermal maturity indicating that these basins are at or near maximum burial have seen little uplift and exhumation and are probably thermally immature with respect to hydrocarbon generation in contrast many sedimentary basins show elevated levels of thermal maturity at the surface with the highest values at basin margins this geometry suggests a patt rn of greater uplift along basin margins possibly reflecting isostatic readjustments as crustal loads are removed by erosion we investigated thermal maturity relations in three sedimentary basins colville cook inlet and kandik in more detail thermal maturity patterns in the colville basin are broadly asymmetric indicating systematic differential uplift ranging from a minimum of no uplift in the north point thomson area to 9 13 km of uplift and exhumation in the central brooks range even greater uplift further to the south is indicated by the presence of greenschist facies and higher grade metamorphic rocks this pattern may reflect the deflexing of the lithosphere subsequent to the principal episode s of crustal convergence and thickening these patterns further suggest a similar thermal history for the proximal colville basin and the northern foothills belt suggesting the possibility of hydrocar on accumulations in the foothills thermal maturity isograds within the brooks range cut major thrust faults indicating that maximum burial postdated the principal phases of thrusting in contrast isograds in the foothills belt to the north are warped broadly by local structure indicating continued north south shortening subsequent to maximum burial such deformation could have remobilized hydrocarbons in early traps a broad southward extension of thermally immature rocks in the central portions of the foothills belt suggests relatively young east west shortening parallel to the strike of the orogene a feature that to date has not been included in regional tectonic syntheses alternatively the thermal maturity pattern could be explained by tectonically unrelated episodes of uplif in the eastern and western parts of the brooks range in the cook inlet basin vitrinite reflectance isograds also are indicative of relatively greater uplift at the basin margins than at the basin center which appears to be presently at its maximum burial depth uplift in the cook inlet basin may reflect compression along the faults bounding the basin relatively high thermal maturity along the western margin of the basin also may reflect magmatic heat sources from the alaska peninsula aleutian volcanic arc the seldovia arch a major structural feature of the end page 1874 basin does not appear to deform vitrinite reflectance isograds implying that deformation on that structure ceased prior to maximum burial in the kandik basin a thermal maturity anomaly thermally mature younger rocks in fault contact with thermally immature older rocks provides clues to the nature and timing of east west thrusting mesozoic foreland basin deposits associated with thrusting buried paleozoic rocks of the easternmost part of this fold and thrust belt to relatively shallow depths driving potential hydrocarbon source rocks into the oil generation window the western foreland basin deposits were overridden by advancing thrusts and tectonically buried to as deep as 10 km these disparate thermal domains are juxtaposed along the glenn creek fault which may represent a terrane boundary in east central alaska

thermal maturity evaluation by sonic log and seismic velocity analysis in parts of upper assam basin india vitrinite reflectance is an important index used in assessment of thermal maturity of petroleum source rocks vitrinite reflectance typically increases with depth of burial as a function of time and temperature and is accompanied by a decrease in log derived interval transit time the relationship between these parameters has been investigated in the upper assam tertiary basin india an equation that relates sonic log transit time to vitrinite reflectance has been derived thus interpretation of the sonic log enables a quantitative estimation of thermal maturity that is not affected by the problems frequently encountered in measurement and interpretation of vitrinite reflectance data further the approach has been extended to the determination of thermal maturity from seismic velocity analysis the results suggest that seismic data can be used as an additional maturity evaluation tool which can provide preliminary estimates of maturity in undrilled areas of the basin

u pb ages of angrites abstract precise u pb ages determined with double spike 202 pb 205 pb thermal ionization m1ass spectrometry are reported for angrites angra dos reis ador lewis cliff 86010 lew and d orbigny nineteen of 23 acid washed pyroxene fractions from these meteorites and whole rock fractions from d orbigny contain between 0 5 and 1 3 pg of total common pb indistinguishable from analytical blank measured 206 pb 204 pb ratios in these fractions are between 6300 and 14 100 for ador 1160 4500 for lew and 608 8500 for d orbigny blank corrected 206 pb 204 pb ratios for all three meteorites vary from 2160 to over 100 000 these fractions yielded precise and reproducible 207 pb 206 pb dates with the average values of 4557 65 0 13 ma for ador 4558 55 0 15 ma for lew and 4564 42 0 12 ma for d orbigny pb pb isochrons including data with slightly elevated common pb and u pb upper concordia intercepts yield similar dates the implications of these new pb isotopic ages of angrites are threefold first they demonstrate that ador and lew are not coeval and the group of slowly cooled angrites is therefore genetically diverse second the new age of lew suggests an upward revision of 53 mn 53 cr absolute ages by 0 7 ma third a precise age of d orbigny allows consistent linking of the 53 mn 53 cr and 26 al 26 mg extinct nuclide chronometers to the absolute lime scale

formation of metal and silicate globules in gujba a new bencubbin like meteorite fall gujba is a coarse grained meteorite fall composed of 41 vol large kamacite globules 20 vol large light colored silicate globules with cryptocrystalline barred pyroxene and barred olivine textures 39 vol dark colored silicate rich matrix and rare refractory inclusions gujba resembles bencubbin and weatherford in texture oxygen isotopic composition and in having high bulk 15n values 685 the 3he cosmic ray exposure age of gujba 26 7 ma is essentially identical to that of bencubbin suggesting that they were both reduced to meter size fragments in the same parent body collision the gujba metal globules exhibit metal troilite quench textures and vary in their abundances of troilite and volatile siderophile elements we suggest that the metal globules formed as liquid droplets either via condensation in an impact generated vapor plume or by evaporation of preexisting metal particles in a plume the lower the abundance of volatile elements in the metal globules the higher the globule quench temperature we infer that the large silicate globules also formed from completely molten droplets their low volatile element abundances indicate that they also formed at high temperatures probably by processes analogous to those that formed the metal globules the coarse grained bencubbin weatherford gujba meteorites may represent a depositional component from the vapor cloud enriched in coarse and dense particles a second class of bencubbin like meteorites represented by hammadah al hamra 237 and que 94411 may be a finer fraction derived from the same vapor cloud

the art of maturity modeling part 2 alternative models and sensitivity analysis the sensitivity of exploration decisions to variations in several input parameters for maturity modeling was examined for the miti rumoi well hokkaido japan decisions were almost completely insensitive to uncertainties about formation age and erosional removal across some unconformities but were more sensitive to changes in removal during unconformities which occurred near maximum paleotemperatures exploration decisions were not very sensitive to the choice of a particular kinetic model for hydrocarbon generation uncertainties in kerogen type and the kinetics of different kerogen types are more serious than differences among the various kinetic models results of modeling using the tti method were unsatisfactory thermal history and timing and amount of hydrocarbon generation estimated or calculated using the tti method were greatly different from those obtained using a purely kinetic model we strongly recommend use of the kinetic r o method instead of the tti method if we had lacked measured r o data subsurface temperature data or both our confidence in the modeling results would have been sharply reduced conceptual models for predicting heat flow and thermal conductivity are simply too weak at present to allow us to carry out highly meaningful modeling unless our input is constrained by measured data maturity modeling therefore requires the use of more not fewer measured temperature and maturity data the use of sensitivity analysis in maturity modeling is very important for understanding the geologic system for knowing what level of confidence to place on the results and for determining what new types of data would be most necessary to improve our confidence sensitivity analysis can be carried out easily using a rapid interactive maturity modeling program