global vector and scalar magsat magnetic anomaly maps empirical and analytical techniques for modeling ionospheric fields in magsat data have been developed that facilitate ionospheric field removal from uncorrected anomalies to obtain better estimates of regional lithospheric anomalies this task has been accomplished for equatorial x z and b component and polar z and b component measurements the techniques for modeling ionospheric fields have been integrated into a processing sequence that incorporates some of the important data processing techniques developed during the last decade data processing techniques include retention of common signal in a correlation analysis of adjacent passes analysis of field differences between dawn and dusk data at points where their orbits cross and retention of common signal in a covariant spherical harmonic analysis procedure results suggest that implementation of the above processing scheme leads to the mapping of the most robust magnetic anomalies of the lithosphere vector components as well as scalar

thermal stresses and the bimodal distribution of elastic thickness estimates of the oceanic lithosphere observations of plate flexure beneath seamounts and at deep sea trenches suggest that the mechanical strength of the oceanic lithosphere increases with age the mechanical thickness h seems to follow the depth to a particular isotherm in the cooling plate model a detailed review of available estimates of elastic thickness te shows that the strength deduced from studies at seamounts is significantly different from the strength inferred at trenches and fracture zones whereas te beneath seamounts approximately follows the 400 c isotherm h at trenches follows the depth to the 700 800 c isotherms the cause of this bimodal distribution is not fully understood thermal bending stresses due to cooling of the lithosphere have been suggested to play an important role in the flexural deformation at transform faults and fracture zones and their distribution with depth is consistent with intraplate seismicity this paper evaluates the effects of thermal stresses on deformation beneath seamounts and at trenches and how their interaction with flexural stresses may modify plate strength the results indicate that thermal bending stresses will bias the moment curvature relationship such that a plate of a certain strength will appear relatively weaker beneath a seamount positive curvature but relatively stronger at a trench negative curvature this systematic bias appears to be in qualitative agreement with published te estimates for old lithosphere the theory predicts a diminishing amount of bias the general lack of sufficient te determinations from old ocean basins and the uncertainties associated with modeling of trenches prohibit a detailed analysis of this part of the te versus age relationship i conclude that the scatter in the elastic thickness estimates appears to be caused by a combination of lithospheric reheating during seamount emplacement bias introduced by thermal stresses and variable amount of yielding of the plate beneath seamounts in proportion to seamount size and lithospheric age at the time of loading

thermoremanent magnetization of the martian lithosphere 1 this paper investigates the thermoremanent magnetization trm of the martian lithosphere assuming that the upper part of the lithosphere acquired trm in the early history of the planet and in the presence of the core field the primary magnetization whereas the lower part has been gradually magnetized by the magnetic field of the upper part as it has cooled below the curie temperature secondary magnetization the secondary magnetization is relatively weak if the magnetic properties of the lower lithosphere are taken to be similar to that of the extrusive basalt near the oceanic ridge axes on earth however if the martian lower lithosphere is more magnetic e g by an order of magnitude the secondary magnetization can be significant although still weaker than the primary magnetization of the upper part such a highly magnetic lower lithosphere does not seem plausible by terrestrial standards and it remains to be confirmed whether it is geologically plausible for mars i also examine a lithospheric model with a strongly magnetic layer beneath the crust 25 times more magnetic than the extrusive basalt representing a possible ilmenite rich layer with a considerable amount of hematite ilmenite lamellae i assume that the layer was not magnetized by the core field because it was hotter than the curie temperature of the lamellae when the core dynamo was active it acquired secondary magnetization later in the presence of the magnetic field of the crust despite adopting an extraordinarily high magnetic layer which seems implausible by terrestrial standards the secondary magnetization of the layer has minor contributions to the observed magnetic anomalies in all models considered the primary magnetization is 20 30 a m and has the dominant contribution to the observed magnetic anomalies it is worth noting that the radial components of the primary and secondary magnetization are parallel whereas the tangential components are in opposite directions because the strong magnetic anomalies in the south likely arise from the tangential magnetization considering the secondary magnetization requires even stronger primary magnetization

simple kinetic models of petroleum formation part ii oil gas cracking kinetic schemes describing oil gas cracking are a required component of models which predict the masses compositions and phases of petroleum expelled from source rocks an additional application is in predicting the temperature zone over which reservoired oil deposits will be degraded to gas this simple first order kinetic model calibrated on closed system laboratory pyrolysis of 16 different source rock samples describes the bulk conversion of oil c6 molecular range to gas c1 5 molecular range cracking rates vary markedly between samples depending on the saturate to aromatic ratio of the generated oil the initial hydrogen index hl0 of the sample taken as an indirect measure of the saturate to aromatic ratio of the oil it generates correlates strongly with optimized rate constants for oil cracking in individual samples thus cracking rates are predictable using a simple routinely performed geochemical screening measurement source rocks with high hl0 tend to generate saturate rich oils with a high mean activation energy and a tight distribution of bond energies which crack relatively slowly over a high but relatively narrow temperature range source rocks with low hl0 generate aromatic rich oils with a low mean activation energy and a broad distribution of bond energies which crack relatively rapidly over a lower but relatively wide temperature range this is an important contributing factor in the observed gas proneness of type iii source rocks with low hl0 the projected cracking window defined limits at 10 90 of initial oil degraded reference heating rate 2 c ma 1 varies from 155 to 205 c in very high quality source rocks hl0 600 mg g c 1 to 115 205 c in very poor quality source rocks hl0 50 mg g c 1 an order of magnitude increase decrease in heating rate elevates depresses these temperature windows by ca 15 c the extrapolations to the geological subsurface are subject to a confidence limit no better than 7 c extrapolation to the reservoir environment requires caution because kerogen an important potential catalyst and hydrogen donor allowing 100 cracking efficiency in the source rock is absent furthermore fractionation during expulsion causes saturate enrichment of expelled ultimately reservoired oils thus in reservoir cracking should be at least as slow as cracking in good quality source rocks and coke or pyrobitumen formation is needed to conserve the hydrogen balance a case history based on deep petroleum pools of the uk central north sea confirms these projections demonstrating no positive evidence of significant in reservoir cracking at temperatures as least as high as 174 c and perhaps even 195 c previous studies have overestimated the importance of in reservoir cracking future studies aiming to predict petroleum composition in traps should concentrate on understanding the charge composition expelled from the kitchen

the role of burial diagenesis in hydrocarbon destruction and h2s accumulation upper jurassic smackover formation black creek field mississippi organic inorganic interactions during burial of the smackover formation at black creek field mississippi have resulted in nearly complete destruction of hydrocarbons the formation has been buried to a depth of 6 km has experienced temperatures of over 200 degrees c and presently contains 78 h 2 s 20 co 2 and 2 ch 4 three distinct stages of burial diagenesis correspond to three phases of organic matter maturation pre oil window diagenesis was dominated by precipitation of prebitumen calcite cement diagenesis in the oil window was characterized by precipitation of saddle dolomite and anhydrite in water filled layers and by formation of solid bitumen in the oil column diagenesis in the gas window was dominated by thermochemical sulfate reduction tsr resulting in hydrocarbon destruction anhydrite dissolution large amounts of h 2 s co 2 and s generation and postbitumen calcite cementation during tsr anhydrite reacted with h 2 s to produce s which in turn reacted with ch 4 to generate more h 2 s in a self reinforcing cycle the lack of metal cations to stabilize h 2 s as metal sulfides availability of sufficient sulfate to generate h 2 s and a closed system to prevent h 2 s from escaping resulted in the continuation of the tsr cycle until nearly all hydrocarbons were consumed in mississippi concentration of h 2 s is nearly zero in smackover hydrocarbon reservoirs that have experienced temperatures of 120 degrees c for more than 50 m y suggesting that tsr is not a kinetic time dependent process high h 2 s concentrations initiate at temperatures above 140 degrees c and increase with temperature indicating that tsr is a thermodynamic phenomenon reported high h 2 s concentrations at low temperatures 80 120 degrees c from other locations may be explained by the following processes 1 migration of h 2 s into these reservoirs 2 high geothermal gradients or local thermal perturbations in the past 3 a biochemical origin for the h 2 s or 4 exposure of these reservoirs to temperatures greater than 150 degrees c and a rapid uplift in black creek field burial cementation and pressure solution resulted in total destruction of porosity and permeability in limestone reservoirs but not in dolomite reservoirs which still possess up to 20 porosity and 100 md permeability secondary porosity was not created as a result of hydrocarbon migration abundant co 2 derived during hydrocarbon destruction resulted in calcite cementation rather than carbonate dissolution late secondary porosity development in carbonates may be related to acids generated by metal sulfide precipitation

mapping and modeling of magnetic anomalies in the northern polar region of mars vector crustal magnetic field maps of the northern polar zone 60 n to 90 n are constructed from selected mars global surveyor magnetometer data obtained during the period from may 28 to september 13 1998 two medium anomalies amplitudes 50 nt at 170 km altitude are mapped in locations consistent with earlier studies no visible surface features correlate with the anomalies suggesting that the sources lie beneath the visible veneer of polar deposits and volcanic lava flows if so then they formed prior to the immediate end of the heavy bombardment upper hesperian period modeling of anomaly vector field components combined with independent constraints on the depth to the curie isotherm yields lower limits on bulk magnetization intensities 0 4 0 9 a m that are significantly greater than those measured for martian snc meteorite samples rocks that contain substantially more titanomagnetite than snc meteorites or that contain magnetic phases in addition to titanomagnetite possibly resulting from hydrothermal alteration are therefore suggested alternatively remanence acquisition in a field of earthlike intensity 50 t rather than in the relatively weak inferred paleointensities for snc meteorites 1 10 t would also help to explain the relatively strong inferred remanent magnetizations the approximate south paleomagnetic pole positions corresponding to these two anomaly sources are located in a region between olympus mons and the present north rotational pole this region is adjacent to the approximate location predicted by melosh 1980 for the paleopole prior to the formation of the tharsis gravity anomaly

a 50 degree spherical harmonic model of the magnetic field of mars this paper presents a 50 degree and order spherical harmonic model of the magnetic field of mars derived at 120 km altitude using the three orthogonal components of the vector magnetic field data acquired by the mars global surveyor within the 80 to 200 km altitude range the downward continued vector magnetic field components to the surface of mars delineate details of the martian magnetic field the strong magnetic anomalies in the southern hemisphere do not show compelling evidence for magnetic lineations similar to those associated with the seafloor spreading on earth the anomalies are more or less equidimensional the magnetic signatures surrounding the impact basins hellas argyre and isidis show that the prominent magnetic anomalies are older than the impact events an average magnetization of 2 a m is estimated for the upper crust surrounding hellas basin four isolated magnetic anomalies are modeled by vertical prisms with circular cross sections of 170 to 190 km radius and 20 to 30 km thickness the magnetization of the model prisms is 5 a m more than that of the surroundings

strength of martian lithosphere beneath large volcanoes this paper provides an estimate of the thermal state of martian lithosphere established since the formation of the shield volcanoes comparison of the spherical harmonic models of martian topography derived using harmonics truncated at degrees 10 11 12 and 14 shows that the volcanoes have almost no contribution from harmonic coefficients of the topography less than degree 11 therefore intermediate scale surface topography and gravity anomalies of mars specified by harmonics of degree 11 50 are considered in this study these harmonics are dominated by shield volcanoes alba patera and isidis basin the central part of valles marineris is also well defined by these harmonics the strong correlation of the topography and gravity anomalies over these harmonics and the positive bouguer anomalies of the volcanoes suggest significant excess masses beneath these surface features in addition to their topographic masses this indicates that the martian lithosphere has been strong enough to support both the topography and the internal excess masses we determine the rigidity of the lithosphere beneath a given surface feature the lithosphere is modeled by a thin elastic spherical shell that overlies an inviscid interior of higher density and is subjected to both surface and internal loads the thickness of the shell and the magnitude of the internal load are calculated such that the deformed structure resulting from the flexure of the shell under both surface and internal loads gives rise to the observed topography and gravity anomaly it is shown that the lithosphere beneath elysium mons alba patera and the central part of valles marineris can be modeled by elastic shells subjected to topographic loads alone whereas the lithosphere beneath olympus and tharsis montes cannot internal excess masses of at least 40 and more likely 50 of the topographic masses are required and the lithosphere beneath must have an elastic core of 100 80 km to support these volcanoes the mean thermal gradient beneath this region of tharsis is estimated to be 7 10 k km on the basis of the strength envelopes calculated using diabase rheology for the crust of 50 km thickness and olivine rheology for the underlying mantle

thermal maturity limit for primary thermogenic gas generation from humic coals as determined by hydrous pyrolysis hydrous pyrolysis experiments at 360 c 680 f for 72 h were conducted on 53 humic coals representing ranks from lignite through anthracite to determine the upper maturity limit for hydrocarbon gas generation from their kerogen and associated bitumen i e primary gas generation these experimental conditions are below those needed for oil cracking to ensure that generated gas was not derived from the decomposition of expelled oil generated from some of the coals i e secondary gas generation experimental results showed that generation of hydrocarbon gas ends before a vitrinite reflectance of 2 0 this reflectance is equivalent to rock eval maximum yield temperature and hydrogen indices his of 555 c 1031 f and 35 mg g total organic carbon toc respectively at these maturity levels essentially no soluble bitumen is present in the coals before or after hydrous pyrolysis the equivalent kerogen atomic h c ratio is 0 50 at the primary gas generation limit and indicates that no alkyl moieties are remaining to source hydrocarbon gases the convergence of atomic h c ratios of type ii and i kerogen to this same value at a reflectance of indicates that the primary gas generation limits for humic coal and type iii kerogen also apply to oil prone kerogen although gas generation from source rocks does not exceed vitrinite reflectance values greater than trapped hydrocarbon gases can remain stable at higher reflectance values distinguishing trapped gas from generated gas in hydrous pyrolysis experiments is readily determined by of the hydrocarbon gases when a depleted water is used in the experiments water serves as a source of hydrogen in hydrous pyrolysis and as a result the use of depleted water is reflected in the generated gases but not pre existing trapped gases

variations in magnetization intensity and low temperature titanomagnetite oxidation of ocean floor basalts the remanent magnetization of the oceanic crust exhibits a systematic long term variation which correlates with the amplitudes of marine magnetic anomalies after a sharp initial decrease of natural remanent magnetization intensity a minimum is reached at 20myr followed by a gradual increase up to ages of 120 myr the progressive sea floor alteration of the magnetic minerals carrying the crustal magnetism is proposed as a cause for this behaviour

rigidity of the atlantic oceanic lithosphere beneath new england seamounts abstract investigation of the flexural response of the oceanic plate subject to loading of new england seamounts provides an insight into the rigidity of the lithosphere fourier domain analysis of the bathymetry and gravity anomalies over new england seamounts suggests that internal density perturbations exist in the crust and or uppermost mantle the loads due to the topography of the seamounts and density perturbations must have been supported by the lithosphere we group the seamounts into three representative ages and adopt a 3 d thin elastic plate model over an invicid mantle to assess the rigidity of the lithosphere under these loads the rigidity values are about 3 7 10 23 n m for lithospheric ages ranging from 30 to 60 my the base of the elastic part of the lithosphere follows an isotherm in the range of 750 850 c suggesting a strong lithosphere that makes the north atlantic ocean difficult to initiate subduction however the thermal blanketing by the thick sediments at scotian basin reduces the rigidity by one to two orders of magnitude and may facilitate initiation of subduction of the north atlantic oceanic lithosphere at scotian margin

thermoviscous remanent magnetization of oceanic lithosphere inferred from its thermal evolution remanent magnetization patterns of the oceanic lithosphere determined on the basis of the thermal evolution of the lithosphere while taking into account both thermal and viscous magnetizations give rise to magnetic anomalies similar to those observed by magsat over the cretaceous quiet zones of the atlantic ocean and by marine magnetic surveys over anomaly 33 viscous magnetization acquired during a given core field polarity period is not removed completely by that acquired in the next period of opposite polarity because of cooling of the lithosphere and the strong temperature dependence of viscous magnetization the magnetization pattern of layer 2a resembles that of the standard seafloor spreading magnetic model consisting of normally and reversely magnetized zones separated by vertical boundaries and produces symmetric magnetic anomalies the magnetization pattern of the middle crust is similar to that of layer 2a for high magnetic blocking temperature models but it is slightly skewed with respect to layer 2a for low magnetic blocking temperature models the magnetization pattern of the lower crust is strongly skewed relative to that of layer 2a and has diffuse boundaries with pronounced slopes between normally and reversely magnetized zones it produces magnetic anomalies with appreciable skewness suggesting that observed skewness of marine magnetic anomaly 33 arises from the magnetization of this layer the magnetization pattern of the upper mantle is appreciably skewed and almost devoid of small scale variations it produces strongly skewed magnetic anomalies at satellite altitudes which may explain the observed skewness of magsat anomalies over the cretaceous quiet zones of the atlantic ocean

stability of natural gas in the deep subsurface thermodynamic calculation of equilibrium compositions the deepest hole in a sedimentary section is currently 31 441 ft 9 583 m but the deepest production is only 26 536 ft 8 088 m the depth gap between deepest hole and deepest producer is the largest in the history of the petroleum industry this prompts a critical reevaluation of methane stability in deep potential reservoirs we developed a computer program to calculate the equilibrium composition of gases in deep reservoirs of various lithologies the program establishes the assemblage with minimum free energy for specified conditions of temperature and pressure corresponding to conditions down to 40 000 ft 12 192 m and can handle up to 70 components 25 elements and 20 phases it does not assume ideal gas behavior but does assume ideal mixing calculations have been made for average high and low geothermal gradients hydrostatic and lithostatic pressures and with and without graphite calculated equilibrium compositions show that methane alone in an inert reservoir has considerable stability and 99 4 survives to 40 000 ft 12 192 m even in the geologically more realistic system with water 99 1 survives the full capability of the program is demonstrated for a sandstone reservoir with graphite calcite cement and a range of minor minerals there methane shows considerable stability for average geothermal gradients with both normal and abnormal pressures for high geothermal gradients part of the methane is lost by reaction and significant amounts of carbon dioxide are added to give a gas composition at 40 000 ft 12 192 m that contains only 9 methane the program shows that in general temperature is much more important than pressure in controlling gas composition crude oil is not thermodynamically stable at any depth and a few percent of hydrogen is frequently present in deep gases

formation of seep carbonates along the makran convergent margin northern arabian sea and a molecular and isotopic approach to constrain the carbon isotopic composition of parent methane abstract authigenic carbonate deposits have been sampled with the remotely operated vehicle marum quest 4000 m from five methane seeps between 731 and 1823 m water depth along the convergent makran continental margin offshore pakistan northern arabian sea two seeps on the upper slope are located within the oxygen minimum zone omz ca 100 to 1100 m water depth the other sites are situated in oxygenated water below the omz below 1100 m water depth the carbonate deposits vary with regard to their spatial extent sedimentary fabrics and associated seep fauna within the omz carbonates are spatially restricted and associated with microbial mats whereas in the oxygenated zone below the omz extensive carbonate crusts are exposed on the seafloor with abundant metazoans bathymodiolin mussels tube worms galatheid crabs aragonite and mg calcite are the dominant carbonate minerals forming common early diagenetic microcrystalline cement and clotted to radial fibrous cement the 18 o carbonate values range from 1 3 to 4 2 v pdb indicating carbonate precipitation at ambient bottom water temperature in shallow sediment depth extremely low 13 c carbonate values as low 54 6 v pdb point to anaerobic oxidation of methane aom as trigger for carbonate precipitation with biogenic methane as dominant carbon source prevalence of biogenic methane in the seepage gas is corroborated by 13 c methane values ranging from 70 3 to 66 7 v pdb and also by back calculations considering 13 c methane values of carbonate and incorporated lipid biomarkers these calculations 13 c methane carbonate 13 c anme methane 13 c mox methane prove to be useful to assess the carbon stable isotope composition of seeping methane if this has not been determined in the first place such an approach represents a useful tool to reconstruct fluid composition of ancient seeps aom is also revealed by lipid biomarkers of anaerobic methane oxidizing archaea such as crocetane pentamethylicosane pmi and sn2 hydroxyarchaeol strongly depleted in 13 c 13 c values as low as 127 v pdb biomarkers of sulphate reducing bacteria are also abundant showing slightly less negative 13 c values but still significantly 13 c depleted average values as low as 101 other bacterial biomarkers such as bacteriohopanepolyols bhps hopanols and hopanoic acids are detected in most carbonates but are particularly common in seep carbonates from the non omz sites the bhp patterns of these carbonates and their low 13 c values resemble patterns of aerobic methanotrophic bacteria in the shallower omz sites bhps revealed much lower contents and varying compositions most likely reflecting other sources than aerobic methanotrophic bacteria 230 th u carbonate ages indicate that aom induced carbonate precipitation at the deeper non omz seeps occurred mainly during the late pleistocene holocene transition i e between 19 and 15 ka before present when the global sea level was lower than today

state of the art of research in kinetic modelling of oil formation and expulsion abstract the last ten years have seen a rapid development in the understanding of petroleum generation and expulsion by a combined use of natural series and experimental simulation this knowledge is now used to develop mathematical models which reproduce the kinetics of petroleum generation and migration in the purpose of basin evaluation by a combination of observation of homogeneous geochemical series experimental simulation and mathematical modelling it has been now established that parameters governing the quantity and composition of petroleum are temperature type of organic matter time or heating rate catalysis by clay minerals which would produce important amounts of isoalkanes is not necessary to produce petroleum the role of minerals although still controversial is rather to retain the heavy fraction of petroleum pressure seems to have a limited influence within the range for effective source rocks kerogen is a tremendously complex macromolecule which is now insufficiently characterized to develop fundamental predictive models of thermal cracking i e involving intermediates such as radicals and olefins such models presently apply only to the cracking of simple molecules hexane hexadecane at limited degrees of cracking for this reason kinetic models are still empirically calibrated on pyrolysis experiments and or on natural maturation trends the most popular models involve a series or a continuous distribution of parallel reactions to describe primary cracking i e direct formation of mobile compounds from kerogen these are generally calibrated against open system pyrolysis with various temperature programs consideration of natural maturation trends i e decrease in petroleum potential may be used also for calibration but discrepancies of organic matter properties availability of deep samples and uncertainties on thermal history limit the effectiveness of such methods whatever the method main activation energies are found in the range 45 60 kcal mol secondary cracking which refers generally to cracking of mobile compounds cannot be modelled from open system pyrolysis nor from natural series unknown residence times but only from closed system pyrolysis although models of primary and secondary cracking still need experimental improvements and more realistic kinetic schemes their ability to predict quantities and composition of petroleum depends primarily on migration which is poorly known indeed migration controls the residence time of petroleum in hot zones of sedimentary basins and thus determines the extent of oil conversion into gas as reproduced by recent numerical models preliminary studies documenting the interactions between thermal cracking and expulsion are shown from haltenbanken area and paris basin in the last few years a kind of agreement has been reached that primary migration was most likely occuring in a separate phase from water and that a sufficient concentration of petroleum must be generated for expulsion to start indeed high expulsion efficiencies over 50 are systematically found for rich and mature source rocks a further understanding of primary migration requires observations of natural series petrophysical studies of source rocks porosities under 10 permeabilities in the nanodardy range thermodynamic investigation of unconventional multiphase organic systems kerogen liquid s gas and experimental simulation under stress this on going research should enable models to track correctly migration paths and thermal history of petroleum fluids as necessary to model their composition

generation accumulation and resource potential of biogenic gas biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms more than 20 of the world s discovered gas reserves are of biogenic origin a higher percentage of gases of predominantly biogenic origin will be discovered in the future biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread large quantities at shallow depths in rapidly accumulating marine sediments a succession of microbial ecosystems leads to the generation of biogenic gas after oxygen is consumed by aerobic respiration sulfate reduction becomes the dominant form of respiration methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted the most important mechanism of methane generation in marine sediments is the reduction of co2 by hydrogen electrons produced by the anaerobic oxidation of organic matter co2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures the factors that control the level of methane production after sediment burial are anoxic environment sulfate deficient environment low temperatu e availability of organic matter and sufficient space the timing of these factors is such that most biogenic gas is generated prior to burial depths of 1 000 m in marine sediments most of the biogenic gas formed can be retained in solution in the interstitial pore waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column under certain conditions of high pressures and or low temperatures biogenic methane combines with water to form gas hydrates biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses the hydrocarbon fraction of biogenic gas consists predominantly of methane the presence of as much as 2 of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low temperature degradation of organic matter the amounts of hydrocarbon components other than methane generally are proportional to temperature age and organic matter content of the sediments biogenic methane is enriched in the light isotope 12c dgr13c1 lighter than 55 ppt owing to kinetic isotope fractionation by methanogens the variations in isotopic composition of biogenic methane are controlled primarily by dgr13c of the original co2 substrate which reflects the net isotopic effect of both addition and removal of co2 the methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas the possible complicating factors require that geologic chemical and isotopic evidence be considered in attempts to interpret the origin of gas accumulations accumulations of biogenic gas have been discovered in canada germany italy japan trinidad the united states and ussr in cretaceous and younger rocks at less than 3 350 m of burial and in marine and nonmarine rocks other gas accumulations of biogenic origin have undoubtedly been discovered however data that permit their recognition are not available

growth of the continental crust a planetary mantle perspective abstract this paper addresses the net growth through time of the total continental crust cc not to be confused with sporadic growth of individual continental masses e g by accretion of island arcs a process that does not necessarily require net growth geochemists tend to emphasize differences between cc and oceanic crust oc but from a whole planet perspective cc and oc are remarkably similar in composition and density the cc oc distinction is canonically defined in terms of elevation however the earth s bimodal elevation pattern clearly reflects a genetic distinction the oc participates directly in mantle convection whereas the cc is the remaining relatively stable portion of the crust the earth s advective cooling mechanism requires massive subduction of lithosphere which due to the inherent buoyancy of crust tends to require a low crust total lithosphere thickness ratio given relatively constant lithosphere thickness except near thermally buoyant mid ocean ridges the earth responds by heaping most of its crust into a few thick continents allowing subduction to freely operate over a large area of relatively thin crust this crustal thickness bimodality engenders the bimodal elevation pattern prevailing wisdom tends to hold that the rate of cc growth has been slow and ongoing typical models show cc volume roughly proportional to 1 h where h is the total heat generation by the earth s k th and u however such models are generally vague as to the mechanism that supposedly holds back growth even after 4 5 ga the strong t dependence of mantle rheology probably ensured that the mantle took only a few million years to cool within a few hundred degrees of its modern t gradual compositional evolution of the cc from a plausible initial composition e g basaltic anorthositic might have slightly lowered the mean cc density but this effect is of second order importance in comparison to the general increase in crust total lithosphere thickness ratio for any given crustal thickness engendered by temporal cooling of the mantle intense cratering occurring coevally with early intense crust mantle recycling probably reconstituted virtually all of the cc extant prior to 3 9 ga but cc was probably not recycled into the mantle any faster than it was regenerated lunar samples and meteorites provide strong circumstantial evidence that in primordial times the outer few hundred kilometers of all terrestrial planets were largely molten outcomes of primordial magmatism must have been strongly dependent on the pressure regimes within the planetary interiors in general the far greater dt dp of silicate melting curves vs adiabats probably tended to foster a layered magmasphere with a partially molten or paracumulate layer constrained by dt dp relationships to a p range of roughly 3 gpa surmounted by fully molten magma ocean layer also limited in thickness by efficient convective cooling in the earth p stabilized garnet and al rich pyroxene were key factors limiting ultimate yield of crustal feldspar this effect has been examined with quantitative models of high p fractional crystallization including in some models periodic replenishment of the magma ocean from below results indicate that a plausibly deep 300 400 km magma ocean would produce a crust comparable in both composition and thickness to that of the modern earth mildly worrisome discrepancies involve cao for which high p pyroxene compositions are poorly constrained and sio 2 the crust probably evolved to its present sio 2 rich composition through differentiation catalyzed by h 2 o which presumably was not as abundant in the magma ocean as in the crust soon afterward nominal model results for mean crustal thickness are typically 1 2 1 3 the modern global mean but for this parameter the model is designed to give conservatively high results so the discrepancy is probably not significant after the chaotic first few million years of solar system history i suggest that planetary size and composition for more than time govern crustal volume

constraints on the depth of magnetized crust on mars from impact craters large diameter greater than 500 km martian impact basins are associated with observed magnetic fields which are statistically distinct from and smaller than fields associated with smaller craters we suggest that this effect arises because impacts cause shock heating and excavation reducing the magnetization of previously magnetized crust for a simple uniformly magnetized model the magnetic field at 100 km altitude is reduced by 50 when a crater shaped demagnetization zone reaches the base of the magnetized layer by analogy with terrestrial data we assume that in martian craters the zone of demagnetization extends to a depth of 0 04 0 15 crater diameters on the basis of this assumption the data suggest that the depth to the base of the magnetized layer on mars if uniform is 35 km with lower and upper bounds of 10 and 100 km respectively these bounds imply magnetizations of 5 40 a m 1 and are consistent with likely mars geotherms at 4 gyr b p

magnetization of the martian crust 1 this paper presents the lateral variations of the vertically averaged magnetization model of the martian crust determined from the degree 50 spherical harmonic model of the magnetic potential the model potential is derived using the mars global surveyor low altitude magnetic data acquired within elevations of 85 200 km the potential model is first verified by comparing the three orthogonal components of the corresponding vector field with the observed high altitude 380 km data using the paleopole positions calculated through modeling 10 isolated small magnetic anomalies the model potential is inverted to determine the vertically averaged magnetization of a nominal 50 km thick crust the crust is highly magnetic with a magnetization contrast peak to trough as high as 35 a m the lack of distinct magnetic signatures for the giant impact basins hellas argyre and isidis and the absence of a magnetic anomaly associated with the north south topographic dichotomy indicate that the upper parts of the crust are low magnetic and that the strong magnetic source bodies are in the lower parts

the composition and thickness of the crust of mars estimated from rare earth elements and neodymium isotopic compositions of martian meteorites isotopic and trace element compositions of martian meteorites show that early differentiation of mars produced complementary crustal and mantle reservoirs that were sampled by later magmatic events this paper describes a mass balance model that estimates the rare earth element ree content and thickness of the crust of mars from the compositions of shergottites the diverse ree and nd isotopic compositions of shergottites are most easily explained by variable addition of light rare earth element lree enriched crust to basaltic magmas derived from lree depleted mantle source regions antarctic shergottites eet 79001 alh 77005 lew 88516 and que 94201 all have strongly lree depleted patterns and positive initial 143nd isotopic compositions which is consistent with the generation of these magmas from depleted mantle sources and little or no interaction with enriched crust in contrast shergotty and zagami have negative initial 143nd isotopic compositions and less pronounced depletions of the lree which have been explained by incorporation of enriched crustal components into mantle derived magmas jones 1989 longhi 1991 borg et al 1997 the mass balance model presented here derives the ree composition of the crustal component in shergotty by assuming it represents a mixture between a mantle derived magma similar in composition to eet 79001a and a lree enriched crustal component the amount of crust in shergotty is constrained by mixing relations based on nd isotopic compositions which allows the ree pattern of the crustal component to be calculated by mass balance the effectiveness of this model is demonstrated by the successful recovery of important characteristics of the earth s continental crust from terrestrial columbia river basalts self consistent results for nd isotopic compositions and ree abundances are obtained if shergotty contains 10 30 of lree enriched crust with 10 ppm nd this crustal component would have moderately enriched lree sm nd 0 25 0 27 147sm 144nd 0 15 0 17 la yb 2 7 3 8 relatively unfractionated heavy rare earth elements hree and no eu anomaly crust with these characteristics can be produced from a primitive lherzolitic martian mantle by modest amounts 2 8 of partial melting and it would have a globally averaged thickness of 45 km which is consistent with geophysical estimates mars may serve as a laboratory to investigate planetary differentiation by extraction of a primary basaltic crust

mineralogy of the sources for magnetic anomalies on mars abstract recent discovery of intense magnetic anomalies on mars which are due to remanent magnetization requires some explanation for the possible minerals responsible for the anomalous signature thermoremanent magnetization trm in single domain sd and multidomain md sized magnetite hematite and pyrrhotite all potential minerals are considered the intensity of trm in 0 05 mt is in descending order sd sized magnetite sd sized pyrrhotite md sized hematite md sized pyrrhotite md sized magnetite sd sized hematite the trm intensity is 50 of the sirm each of these minerals and estimated concentrations of magnetic remanence carriers assumed to be titanomagnetite in the shergotty nakhlaxhassigny martian meteorites are used in a thin sheet approximation model to reveal the concentration of each mineral required for the generation of an observed magnetic anomaly 1 500 nt at 100 km altitude assuming trm acquisition in a 0 05 mt magnetic field