siderophile trace elements in alh84001 other snc meteorites and eucrites evidence of heterogeneity possibly time linked in the mantle of mars we report neutron activation analyses including radiochemical determination of trace siderophile elements au ge ir ni os and re for three snc martian meteorites and os and re results for numerous eucrites ratios such as ga al in the snc orthopyroxenite alh84001 confirm its martian affinity its many distinctive characteristics most notably its near primordial age notwithstanding to the list of alh84001 s idiosyncrasies can now be added extraordinarily low concentrations of au ni and especially re 17 pg g for a martian meteorite we consider several possible origins for the anomalously low re content in alh84001 including metasomatism or alteration the pyroxene cumulate nature of this rock probably does not account for its low re content other snc meteorites are also cumulates an examination of re nd variations among terrestrial basalts and komatiites suggests that re is compatible with mantle minerals in general and only incompatible with olivine however olivine dominates the mantle residuum especially during komatiite genesis our preferred model is that the alh84001 parent melt formed in a mantle source region that was far more re depleted and or at a substantially lower oxygen fugacity than the sources of the young snc meteorites such a contrast is consistent with models that replenish siderophile elements in planetary mantles by gradual admixture of late accreting matter and similarly derive most planetary water which serves as an oxidant very late in accretion according to this model alh84001 formed before the siderophile rich matter and water had been mixed well into the martian interior possibly the martian mantle never became generally as re rich and or oxidized as the source region s of the younger sncs

growth patterns and implications of complex dendrites in calcite travertines from lýsuholl snaefellsnes iceland calcite dendrite crystals are important but poorly understood components of calcite travertine that forms around many hot springs the l suholl hot spring deposits located in western iceland are formed primarily of siliceous sinters that were precipitated around numerous springs that are now inactive calcite travertine formed around the vent and on the discharge apron of one of the springs at the northern edge of the area the travertine is formed largely of two types i and ii of complex calcite dendrite crystals up to 1 cm high that grew through the gradual addition of trilete sub crystals the morphology of the dendrite crystals was controlled by flow direction and the competition for growth space with neighbouring crystals densely crowded dendrites with limited branching characterize the rimstone dams whereas widely spaced dendrites with open branching are found in the pools many dendrite bushes in the pools nucleated around plant stems growth of the dendrite crystals was seasonal and incremental calcite precipitation was driven by rapid co2 degassing of co2 rich spring waters during the spring and summer during winter when snow covered the ground and temperatures were low opal a precipitated on the exposed surfaces of the dendrites segmentation of dendrite branches by discontinuities coated with opal a and overgrowth development around sub crystals resulted from this seasonal growth cycle the calcite dendrite crystals in the l suholl travertine differ in morphology from those at other hot springs such as those at lake bogoria kenya and waikite in new zealand comparison with the calcite dendrite crystals found at those sites shows that dendrite morphology is site specific and probably controlled by carbonate saturation levels that in turn are controlled by the rate of co2 degassing and location in the spring outflow system

biomimetic properties of minerals and the search for life in the martian meteorite alh84001 the existence of extraterrestrial life was heralded by controversial claims made in 1996 that the martian meteorite alh84001 harbored relics of ancient microorganisms we review here the accumulated evidence for and against past extraterrestrial life in this martian meteorite the main pro life arguments the presence of polycyclic aromatic hydrocarbons magnetite crystals carbonate globules and structures resembling terrestrial life forms known as nanobacteria can be deemed ambiguous at best although these criteria are compatible with living processes each one of them can be explained by nonliving chemical processes by undergoing amorphous to crystalline transformations and binding to multiple substrates including other ions and simple organic compounds minerals especially those containing carbonate have been shown to display biomimetic properties producing forms that resemble bacteria this simple and down to earth explanation can account fully for the existence of mineral entities resembling put

effects of pre existing discontinuities on the residual strength of rock mass insight from a discrete element method simulation abstract when rock failure is unavoidable the designer of engineering structures must know and account for the residual strength of the rock mass this is particularly relevant in underground coal mine openings pre existing discontinuities play an important role in the mechanical behavior of rock masses and thus it is important to understand the effects of such pre existing discontinuities on the residual strength for this purpose the present study demonstrates a numerical analysis using a discrete element method simulation the numerical results indicate that fracture intensity has no significant influence on the residual strength of jointed rock masses independent of confining conditions as confining pressures increase both peak and residual strengths increase with residual strength increasing at a faster rate the finding was further demonstrated by analyzing documented laboratory compressive test data from a variety of rocks along with field data from coal pillars a comprehensive interpretation of the finding was conducted using a cohesion weakening friction strengthening cwfs model the effect of rock bolts on rock mass strength was also evaluated by using a discrete element method model which suggested that rock bolts can significantly increases residual strength but have limited effect on increasing the peak strength of rock masses

formation and relative ages of maskelynite and carbonate in alh84001 the morphology and stoichiometry of feldspathic glass in the martian meteorite alh84001 indicates it is maskelynite a diaplectic glass rather than a flowed glass although this glass was heterogeneously affected by a tertiary set of processes an impact event with shock pressures in excess of 31 gpa was needed to convert the original plagioclase an36ab60or4 to maskelynite carbonate is intimately associated with the maskelynite and the carbonate s radiating crystalline fabric and globular forms suggest it was produced after plagioclase was converted to maskelynite textures also suggest carbonate was produced at the expense of maskelynite in a dissolution precipitation reaction that involved a carbonic fluid this fluid system is tentatively estimated to have been active for at least a few years at temperatures 300 c based on dissolution rates of plagioclase in mildly to strongly alkaline hydrothermal systems which is the only analogue currently available this reaction does not need to be mitigated by microbial life neither are bacteria needed to produce the radiating textures and globular forms of carbonate which may instead reflect kinetic phenomena associated with crystal nucleation and growth because the carbonate was produced at the expense of maskelynite it is younger than maskelynite which has previously been shown to have last degassed 3 92 0 04 ga turner et al 1997 however the specific age of the carbonate and the carbonic fluid system remains unknown

stress transfer during three sequential moderate earthquakes along the central calaveras fault california seismic and geodetic information on three sequential moderate earthquakes the 1979 m5 9 coyote lake the 1984 m6 2 morgan hill and the 1988 m5 1 alum rock along the central calaveras fault is used to examine how tractions on fault segments are transferred from one fault segment to the other geodetic data obtained before and after each earthquake are inverted for slip at depth the perturbed stress field induced by each earthquake is calculated using 3 d elastic dislocation theory although only long wavelength features of the slip distributions are well resolved the model suggests that the distribution of slip is nonuniform the largest slips tend to occur in regions having relatively few aftershocks or near the hypocenter according to the model the coyote lake earthquake induced 0 1 1 bar 0 01 0 1 mpa shear stress across the northern part of the morgan hill segment and only about 0 1 bar at the hypocenter of the morgan hill mainshock a shear stress of 0 5 bar was induced by the morgan hill earthquake at the hypocenter of the alum rock mainshock the average stress drops for the coyote lake morgan hill and alum rock earthquakes are estimated to be 30 25 and 15 bars respectively thus the earthquake induced shear stress of 0 1 0 5 bar at the hypocenter of the next earthquake is only a few percent of the total stress drop and cannot be solely responsible for the occurrence of the sequential earthquakes however the induced shear stress can trigger the adjacent earthquakes in the sense that adjacent earthquakes would have occurred at a later time if there was no such coseismic stress transfer it is inferred that fault geometry may influence the local stress field thereby initiation and termination of the rupture but not the regional stress accumulation

stress heterogeneity and its impact on seismicity pattern along the equatorial bifurcation zone of the great sumatran fault indonesia abstract a stress transfer modeling revealed the stress heterogeneity and its correlation with the seismicity pattern in the bifurcation zone of the great sumatran fault gsf where the gsf is divided into two separate sub parallel segments up to 35 km apart i e angkola and barumun forming a strike slip duplex at around equator the analysis was conducted using a seismic catalog compiled from five institutions i e the indonesian agency for meteorology climatology and geophysics bmkg united states geological survey usgs harvard and global centroid moment tensor gcmt international seismological center isc and geoforschungzentrum gfz germany in the period of 1970 2017 our frequency magnitude distribution analysis indicates that this zone has a high stress regime especially at the tip of the bifurcation zone in which 20 clockwise rotation of principal stress and 20 increase in stress ratio are observed from the stress inversion consequently events with magnitude larger than 6 0 are concentrated in this area in addition our stress transfer modeling reveals the mechanism of the stress heterogeneity at the bifurcation zone and a possible interaction between segment ruptures two main internal structures perpendicular to the strike slip duplex are also observed from the displacement vector due to its high stress concentration and its close distance to populous cities of padang sidempuan bukittinggi solok and padang panjang in western sumatra the tip of gsf bifurcation poses a big potential of seismic hazard the information of the principal stress rotation and stress ratio obtained in this study can be used further to assess the potential of great ruptures and estimate the possible damage in the study area

the role of vaterite and aragonite in the formation of pseudo biogenic carbonate structures implications for martian exobiology abstract a simple synthesis of various forms of calcium carbonate with spherical and floral morphologies is reported vaterite formation occurs at 25 c aragonite at 70 c and calcite at about 80 c these are produced when co 2 is reacted with an aqueous solution of calcium chloride in the presence of ammonia these conditions may have existed at the surface of mars in the past leading us to conclude that such mineral formations may be common there although the initial phases are modified over time with changing temperature and pressure conditions they still influence the final morphology of the carbonates observed a comparison of these structures with those found in the martian meteorite alh84001 suggests but does not confirm a non biogenic origin for the alh84001 carbonates

how similar were the 1934 and 1966 parkfield earthquakes bakun and mcevilly 1984 showed that surface waves from the 1934 and 1966 parkfield earthquakes are remarkably similar this observation together with other seismic and geologic data led to the forecast of a repeat of a characteristic parkfield earthquake by 1993 in this paper we model near field geodetic measurements collected before and after the 1934 and 1966 parkfield earthquakes the slip in 1934 on the northwestern half of the rupture is poorly constrained due to the limited station coverage in this part of the rupture near the hypocenter the geodetic measurements permit the two earthquakes to have the same slip distribution on the other hand the data clearly show that the 1966 earthquake ruptured through the right step in the surface trace of the fault while the 1934 earthquake stopped northwest of the step if we assume only that the slip distribution is to some degree smooth we find that the centroid of the 1934 slip distribution could be as much as 10 km northwest of the centroid of the 1966 slip the data are consistent with but do not demand the largest slip in 1934 occurring in a region of low slip in 1966 the geodetically determined moments of the two earthquakes from smooth inversions are indistinguishable m0 4 4 0 4 1018 n m we also examine inversions in which we assume that the two earthquakes start identically and estimate how far down the fault the two earthquakes could have remained the same we find that the 1934 event could have had the same slip as in 1966 up to the fault step the 1934 earthquake must have stopped at the step whereas the 1966 event slipped southeast of the step in this quasi characteristic model m0 of the 1934 quake is only 60 of that in 1966 the difference presumably occurring as afterslip in 1966 if the 1934 geodetic moment was indeed less than in 1966 and the loading rate was constant this observation could explain the early occurrence of the 1934 earthquake

petrography and bulk chemistry of martian orthopyroxenite alh84001 implications for the origin of secondary carbonates abstract new petrologic and bulk geochemical data for the snc related martian meteorite alh84001 suggest a relatively simple igneous history overprinted by complex shock and hydrothermal processes alh84001 is an igneous orthopyroxene cumulate containing penetrative shock deformation textures and a few percent secondary extraterrestrial carbonates rare earth element ree patterns for several splits of the meteorite reveal substantial heterogeneity in ree abundances and significant fractionation of the rees between crushed and uncrushed domains within the meteorite complex zoning in carbonates indicates nonequilibrium processes were involved in their formation suggesting that co2 rich fluids of variable composition infiltrated the rock while on mars we interpret petrographic textures to be consistent with an inorganic origin for the carbonate involving dissolution replacement reactions between co2 charged fluids and feldspathic glass in the meteorite carbonate formation clearly postdated processes that last redistributed the ree in the meteorite

mechanical properties and damage constitutive model of coal in coal rock combined body abstract coal and rock mass bear loads together in underground engineering such as coal mining and roadway support resulting in different mechanical properties from those of pure coal and rock mass accurately obtaining the mechanical properties of coal in coal rock combined body can help to properly understand the deformation and failure characteristics of the underground coal mass first an experimental method to obtain the stress and strain of coal in coal rock combined samples using strain gauges was proposed and validated then tests on thirteen types of coal rock combined samples were conducted the results show that the rock experienced a strain recovery and played a loading effect on the failure of coal when coal rock samples failed the compressive strength of coal in the sample increased with the rock strength and decreased with the coal rock height ratio furthermore two damage constitutive models of coal were established by serially connecting one damage body with one or two newton body s to reveal the influences of rock s on the mechanical behavior of coal in coal rock samples finally test results were fitted using the two models it was showed that the two models can accurately describe the stress strain curves of coal in coal rock samples and reflect the influences of rock s coal rock combination form and coal rock height ratio the fitting constants of the two models have clear physical meanings and can be easily obtained which are expected to be widely used in underground engineering

bacterial mineralization patterns in basaltic aquifers implications for possible life in martian meteorite alh84001 to explore the formation and preservation of biogenic features in igneous rocks we have examined the organisms in experimental basaltic microcosms using scanning and transmission electron microscopy four types of microorganisms were recognized on the basis of size morphology and chemical composition some of the organisms mineralized rapidly whereas others show no evidence of mineralization many mineralized cells are hollow and do not contain evidence of microstructure filaments either attached or no longer attached to organisms are common unattached filaments are mineralized and are most likely bacterial appendages e g prosthecae features similar in size and morphology to unattached mineralized filaments are recognized in martian meteorite alh84001

bacterial extracellular polymeric substances eps mediate caco3 morphology and polymorphism abstract the effect of bacterial extracellular polymers eps on calcium carbonate morphology and polymorphism was investigated by means of free drift mineralisation experiments conducted using cells of the eps producing thermotolerant bacillus licheniformis s 86 the cells either had the eps layer intact native cells or had the eps layer removed eps free cells experiments were also undertaken in the absence of cells but with the addition of the extracted eps solution it was found that the abiotic control and eps free cell experiments contained vaterite and calcite when sampled after 12 h and 48 h whereas the native cell and eps solution experiments contained only calcite these results suggest that the presence of eps inhibits vaterite precipitation it is proposed that dissolved organic carbon doc released from the eps complexes ca 2 ions in solution reducing the calcium carbonate saturation and favouring calcite precipitation over vaterite after 7 days all experiments contained only calcite however distinct morphological differences between the calcite crystals precipitated in the four experiments were preserved after a period of 7 days suggesting that crystal morphology may indicate the biological conditions under which calcite precipitated

naturally precipitating vaterite μ caco3 spheres unusual carbonates formed in an extreme environment vaterite a rare hexagonal caco3 polymorph was identified in precipitates forming at a supra glacial sulfur spring in borup fiord pass northern ellesmere island canadian high arctic vaterite occurs in a precipitate mound along with calcite gypsum and native sulfur the unusual conditions of the site including an extremely cold climate supersaturated alkaline waters and the presence of gypsum mimics conditions used to grow vaterite in laboratory experiments stable isotope data suggest that vaterite may preferentially form during colder periods of the year vaterite found at the site is characterized by 2 to 10 m rounded to spherical shaped particles comprising smaller 0 5 to 2 m spheres as both individuals and in chainlike structures the spherical habit of vaterite resembles carbonate structures that have been interpreted to be organic however 13 c values are indicative of an inorganic origin the thick permafrost and the extreme cold and dry environment make this site a good terrestrial analog for carbonate precipitates that might be expected at potential deposits associated with water discharge on other planetary bodies copyright 2003 elsevier science ltd

the possible role of nannobacteria dwarf bacteria in clay mineral diagenesis and the importance of careful sample preparation in high magnification sem study abstract bacterial textures are present on clay minerals in oligocene frio formation sandstones from the subsurface of the corpus christi area texas in shallower samples beads 0 05 0 1 m in diameter rim the clay flakes at greater depths these beads become more abundant and eventually are perched on the ends of clay filaments of the same diameter we believe that the beads are nannobacteria dwarf forms that have precipitated or transformed the clay minerals during burial of the sediments rosettes of chlorite also contain after hcl etching rows of 0 1 m bodies in contrast kaolinite shows no evidence of bacterial precipitation we review other examples of bacterially precipitated clay minerals a danger present in interpretation of our earlier work and much work of other is the development of nannobacteria looking artifacts caused by gold coating times in excess of one minute we strongly recommend a 30 second coating time bacterial growth of clay minerals may be a very important process both in the surface and subsurface

state of stress from focal mechanisms before and after the 1992 landers earthquake sequence the state of stress in the eastern california shear zone ecsz changed significantly because of the occurrence of the 1992 m w 6 1 joshua tree and the m w 7 3 landers earthquakes to quantify this change focal mechanisms from the 1975 galway lake sequence the 1979 homestead valley sequence background seismicity from 1981 to 1991 and the 1992 landers sequence are inverted for the state of stress in all cases the intermediate principal stress axis s2 remained vertical and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes s 1 and s 3 and small variations in the value of the relative stress magnitudes in general the stress state in the ecsz has s 1 trending east of north and 0 43 to 0 65 suggesting that the ecsz is a moderate stress refractor and the style of faulting is transtensional south of the pinto mountain fault in the region of the 1992 joshua tree earthquake the stress state determined from the 1981 to 1991 background seismicity changed on 23 april and again on 28 june 1992 in the central zone s 1 rotated from n14 5 e to n28 5 e on 23 april and back again to n16 5 e on 28 june thus the landers mainshock in effect recharged some of the shear stress in the region of the m w 6 1 joshua tree earthquake comparison of the state of stress before and after 28 june 1992 along the landers mainshock rupture zone showed that the mainshock changed the stress orientation the s1 trend rotated 7 to 20 clockwise and became progressively more fault normal from south to north along the emerson camp rock faults the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor the complex distribution of p and t axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock the san bernardino mountains region of the m w 6 2 big bear earthquake has a distinctively different state of stress as compared to the landers region with s 1 trending n3 5 w this region did not show any significant change in the state of stress following the 1992 m w 6 2 big bear sequence

critical stress difference fault orientation and slip direction in anisotropic rocks under non andersonian stress systems abstract the coulomb navier failure criterion is applied to geological faulting in the general three dimensional case of rocks containing arbitrarily oriented strength anisotropies and subject to non andersonian stress systems i e with none of the principal stresses acting in a vertical direction general expressions for the critical stress difference necessary to cause failure as a function of depth are given in terms of material parameters pore fluid pressure orientation of the stress field and orientation of the strength anisotropy the range of angles between a plane of anisotropy and the maximum principal stress direction for which slip occurs along the pre existing anisotropy rather than along a new fault is calculated as a function of depth for different stress regimes when the stress field is non andersonian and or strength anisotropies not containing the intermediate stress direction occur in the rock faulting generally is oblique slip a kinematic classification of faulting is given on the basis of the angle between the strike direction and the slip direction on the fault plane triangular diagrams analogous to those used in petrology are introduced to describe i faulting in isotropic rock subject to arbitrarily oriented stress fields and ii faulting in anisotropic rock when one principal stress direction is vertical the type of faulting as a function of stress field and anisotropy orientation can be read off directly from these diagrams

stress rotation during the coalinga aftershock sequence this study considers spatial and temporal changes of the stress regime during the 1983 coalinga aftershock sequence in both cases the observed changes manifest themselves as rotations of the most compressive stress axis inversion of 165 m 3 aftershocks shows that in the southern aftershock zone the azimuth of the most compressional stress axis is oriented 24 e of north while in the northern and central zones it is oriented 51 e of north statistical analysis by use of non parametric bootstrap resampling shows that this rotation is significant at above the 95 confidence level inverting the focal mechanisms of 122 m 3 aftershocks in the central and northern aftershock zones reveals a rotation of the principal stress axes during the first 500 days after the main shock the compressional axis rotates from an azimuth of 62 e of north to 47 e of north statistical analysis by use of nonparametric bootstrap resampling shows that this rotation is just significantly different from zero at the 95 confidence level subject to the assumption that 80 of the fault planes can be correctly picked from the two nodal planes as the number of fault planes picked correctly varies from 50 random choices to 100 the significance of the rotation varies from 87 to 98 further investigation fails to find the signature of the stress changes predicted by dislocation theory in the focal mechanisms of the aftershocks active folds in the area suggest that the pre main shock stress regime had a compressional axis with a ne sw orientation thus the rotation during the aftershock sequence is thought to be a rebound from a stress anomaly induced by the main shock this stress anomaly may be causally related to the occurrence of the aftershocks

local stress perturbations associated with the 2008 wenchuan m 8 0 earthquake near the longmenshan fault zone in the eastern margin of the tibetan plateau abstract the 2008 wenchuan m 8 0 earthquake induced local stress perturbations near the longmenshan lms fault zone using in situ measurements and stress tensor inversions from focal mechanisms we investigated the post earthquake perturbed stress field at shallow depths 1 2 km in the upper crust above a basal attachment 5 18 km as well as in the middle crust beneath a basal attachment 19 32 km at shallow depths in the beichuan jiangyou section the maximum principal stress 1 orientations changed from a pre earthquake orientation of nww to a post earthquake orientation of nee however near the baoxing an xian and qingchuan mianxian sections the pre earthquake 1 orientation of nw nww was largely unaltered before the mainshock in the upper crust above the basal detachment the entire lms fault zone exhibited a consistent 1 orientation of nw nww however after the earthquake some sections such as the beichuan nanba section achieved 1 orientations of nee that were distinctly different from the pre seismic stress field in the middle crust beneath the basal detachment the stress field was mostly unchanged the crust beneath the basal detachment along the eastern margin of the tibetan plateau is dominated by nww oriented maximum principle stress and strain directions and a thrust faulting stress regime along the entire lms fault zone these observations are representative of the southeastward motion of the bayan har block and its ongoing collision with the south china block along the lms thrust belt the spatially heterogeneous stress field that occurred in the upper crust 18 km after the mainshock was most likely caused by the co seismic static stress changes associated with the wenchuan earthquake prior to the onset of seismic activity the deviatoric stress magnitudes were estimated at less than 22 mpa and 4 mpa at depths of 10 km and 20 km respectively suggesting that the mainshock fault zone has a relatively low fault strength to overcome in order to initiate significant co seismic rupture and aftershocks especially in the crust beneath the basal detachment these smaller deviatoric magnitudes are indicative of the weaker fault materials that resides in the lms region

coseismic stress changes induced by the 1989 loma prieta california earthquake earthquake focal mechanisms from before and after the 1989 loma prieta california earthquake are used to infer the coseismic stress change before the main shock most earthquakes correspond to right lateral slip on planes sub parallel to the san andreas fault and imply a generally n s most compressional stress axis and a vertical intermediate stress axis aftershocks within the main shock rupture zone however display almost every style and orientation of faulting implying an extremely heterogeneous stress field this suggests that the main shock relieved most if not all of the shear stress acting on its fault plane aftershocks that lie on the perimeter of the rupture agree with spatially uniform stress states but only when considered in three groups north south and above the main shock rupture area in each of these areas the stress state may reflect stress transfer by the main shock

polycyclic aromatic hydrocarbons pahs in antarctic martian meteorites carbonaceous chondrites and polar ice recent analyses of the carbonate globules present in the martian meteorite alh84001 have detected polycyclic aromatic hydrocarbons pahs at the ppm level mckay et al 1996 the distribution of pahs observed in alh84001 was interpreted as being inconsistent with a terrestrial origin and were claimed to be indigenous to the meteorite perhaps derived from an ancient martian biota we have examined pahs in the antarctic shergottite eeta79001 which is also considered to be from mars as well as several antarctic carbonaceous chondrites we have found that many of the same pahs detected in the alh84001 carbonate globules are present in antarctic carbonaceous chondrites and in both the matrix and carbonate druse component of eeta79001 we also investigated pahs in polar ice and found that carbonate is an effective scavenger of pahs in ice meltwater moreover the distribution of pahs in the carbonate extract of antarctic allan hills ice is remarkably similar to that found in both eeta79001 and alh84001 the reported presence of l amino acids of apparent terrestrial origin in the eeta79001 druse material mcdonald and bada 1995 suggests that this meteorite is contaminated with terrestrial organics probably derived from antarctic ice meltwater that had percolated through the meteorite our data suggests that the pahs observed in both alh84001 and eeta79001 are derived from either the exogenous delivery of organics to mars or extraterrestrial and terrestrial pahs present in the ice meltwater or more likely from a mixture of these sources it would appear that pahs are not useful biomarkers in the search for extinct or extant life on mars

experimental hydrothermal alteration of a martian analog basalt implications for martian meteorites a number of martian meteorite samples contain secondary alteration minerals such as ca mg fe carbonates fe oxides and clay minerals these mineral assemblages hint at hydrothermal processes occurring in the martian crust but the alteration conditions are poorly constrained this study presents the results of experiments that examined the alteration of a high fe basalt by co2 saturated aqueous fluids at 23 and 75 c and by mixed h2o co2 vapors at 200 and 400 c and water rock ratios of 1 1 and 1 10 results indicate that observable alteration of the basalt takes place after runs of only seven days this alteration includes mobilization of silica into phases such as opal ct and quartz as well as the formation of carbonates oxides and at some conditions zeolites and hydrous silicates the degree of alteration increases with run temperature and in high temperature vapor experiments with increasing water content of the vapor the degree of alteration and the mineralogy observed in the martian meteorites suggests that none of these samples were exposed to aqueous fluids for long periods of time nakhla and lafayette probably interacted with water for relatively brief periods of time if so silica may have been leached from the parent rocks by the altering fluids allan hills 84001 shows possible evidence for very limited interaction with an aqueous fluid but the overall slight degree of alteration described for this meteorite strongly suggests that it never interacted extensively or at high temperature with any water bearing fluid elephant moraine a79001 may not have been altered by aqueous fluids at all the results of this study best support models wherein the meteorite parent rocks were wetted intermittently or for brief periods of time rather than models that invoke long term reaction with large volumes of water our experiments studied alteration of a high fe basalt by dilute co2 saturated aqueous solutions at 23 and 75 c and by mixed h2o co2 vapors at 200 and 400 c the results suggest that alteration of the parent rock takes place even after very short reaction times of seven days all experiments produced carbonate minerals including calcite and in some cases magnesite siderite and ankerite a free silica phase either opal quartz or hydrated silica formed in most experiments more altered experiments also contained minerals such as zeolites and hydrous phyllosilicates clay minerals were not observed to form in any experiments in aqueous fluids higher temperature corresponded with a higher degree of alteration whereas changing fluid composition had no observable effect in high temperature vapors the degree of alteration was controlled by temperature and the proportion of h2o to co2 with water rock ratio also playing a role in transport of silica application of these results to martian meteorites that contain secondary alteration minerals suggests that none of the martian rocks underwent extensive interaction with aqueous fluids nakhla and lafayette contain clay minerals which suggests that they interacted with water to some extent possibly at elevated temperatures although alh84001 shows possible evidence of very limited interaction with aqueous fluids eeta79001 does not these results support models for the alteration of these meteorites that do not invoke long term interaction with water or reaction with large volumes of water except for some models for alteration of alh84001 this conclusion agrees with most of the literature on alteration of martian meteorites

the july 1986 oceanside ml 5 3 earthquake sequence in the continental borderland southern california an earthquake of m l 5 3 occurred at 32 58 7 n 117 51 5 w southwest of oceanside in san diego county at 13 47 13 july 1986 ut this main shock was followed by an extensive aftershock sequence with 55 events of m l 3 0 during july 1986 the epicenters of the main shock and aftershocks are located at the northern end of the san diego trough bahia soledad fault zone sdt bs where it changes strike from northwest to a more westerly direction through a left offset or a bend in the fault the northwest striking sdt bs is one of three strike slip fault systems that constitute the offshore agua blanca fault system the spatial distribution of the aftershocks indicates a unilateral 7 to 9 km long rupture to the east southeast away from the epicenter of the main shock the focal mechanism of the main shock also has an east southeast striking and south dipping plane with mostly reverse movement on it focal mechanisms of the m l 3 0 aftershocks show both reverse and strike slip movement the reverse focal mechanisms indicate that this sequence may have occurred on a thrust fault that provides for a left stepping offset or a bend in the san diego trough fault as movement is transferred to the west along the santa cruz catalina island escarpment some of the aftershocks that are located to the southeast of the main shock and have strike slip focal mechanisms suggest activation of the northwest trending san diego trough fault a stress inversion of the focal mechanism data shows that the maximum principal stress determined from the focal mechanisms of the main shock and 22 aftershocks that occurred within 36 hours of the main shock has an azimuth of s30 w plunging 18 the maximum principal stress determined from 30 aftershocks that occurred from 15 july to 2 october 1986 has an azimuth of s20 w plunging 18 the values the measure of the relative sizes of the principal stresses are approximately 0 07 and 0 1 respectively indicating that the intermediate and minimum principal stress are of similar magnitude the results of the stress inversion and the focal mechanisms that showed reverse faulting suggest that the inner continental borderland offshore from oceanside is not currently a pure strike slip tectonic regime but rather a strike slip mixed with reverse faulting regime when the 52 aftershock focal mechanisms are divided into four groups and the stress inversion is repeated the change in stress can be described as a progressive counter clockwise rotation of 14 of the orientation of the maximum principal stress to a more southerly direction with the greatest change in stress orientation observed shortly after the main shock the abundance of aftershocks may be related to the large temporal variation in stress orientation that in turn may have resulted from the small stress drop of the main shock

transmission electron microscopy of minerals in the martian meteorite allan hills 84001 we have studied carbonate and associated oxides and glasses in a demountable section of allan hills 84001 alh 84001 using optical scanning and transmission electron microscopy tem to elucidate their origins and the shock history of the rock massive fracture zone and fracture filling carbonates in typical locations were characterized by tem x ray microanalysis and electron diffraction in a comprehensive study that preserved textural and spatial relationships orthopyroxene is highly deformed fractured partially comminuted and essentially unrecovered lamellae of diaplectic glass and other features indicate shock pressures 30 gpa bridging acicular crystals and foamy glass at contacts of orthopyroxene fragments indicate localized melting and vaporization of orthopyroxene carbonate crystals are 5 m in size untwinned and very largely exhibit the r3c calcite structure evidence of plastic deformation is generally found mildly only in fracture zone and fracture filling carbonates even adjacent to highly deformed orthopyroxene and appears to have been caused by low stress effects including differential shrinkage high dislocation densities like those observed in moderately shocked calcite are absent carbonate contains impact derived glasses of plagioclase silica and orthopyroxene composition indicating brief localized impact heating stringers and lenses of orthopyroxene glass in fracture filling carbonate imply flow of carbonates and crystallization during an impact periclase mgo occurs in magnesite as 30 50 nm crystals adjacent to voids and negative crystals and as 1 m patches of 3 nm crystals showing weak preferred orientation consistent with 111 mgo 0001 carb as observed in the thermal decomposition of caco3 to cao magnetite crystals that are epitaxially oriented at voids negative crystals and microfractures clearly formed in situ fully embedded faceted magnetites are topotactically oriented in general with 111 mag 0001 carb so that their oxygen layers are aligned in optically opaque rims magnetites are more irregularly shaped and except for the smallest crystals poorly aligned all magnetite and periclase crystals probably formed by exsolution from slightly non stoichiometric co2 poor carbonate following impact induced thermal decomposition any magnetites that existed in the rock before shock heating could not have preserved evidence for biogenic activity

precipitation and growth morphology of calcium carbonate induced by myxococcus xanthus implications for recognition of bacterial carbonates abstract it is thought that morphologies of bacterial carbonates can be used to identify microbial fossils and or precipitates in sediments and rocks this study shows that calcite and vaterite formed in a gel medium in the presence of myxococcus xanthus display a range of morphologies that depend on whether the bacteria are live or dead metabolic activity of the bacteria induced 1 aggregates of calcified bacteria formed at maximum supersaturation 2 vaterite spheres final growth stage of dumbbell fibrous radiated aggregates and 3 dipyramid and disphenoid like calcite crystals combination of 011 and 0001 forms morphologies 2 and 3 developed at a lower supersaturation and are typically found in gel like media dipyrimidal like calcite crystals were also obtained abiotically in gel medium dead m xanthus cells induced heterogeneous precipitation of calcite with rhombohedral morphologies at low supersaturation a growth mechanism resulting from self assembly of calcium carbonate nanocrystals may account for the observed morphologies crystal microstructure and crystallite size measurements all of the above mentioned morphologies of bacterial carbonate have been observed in other laboratory experiments and in continental and marine environments however all of them have also been produced abiotically with the exception of calcified bacterial cells this may make it more difficult to identify bacterial activity in the rock record nonetheless bacterially induced alkalinization appears to be a prerequisite for the development of spherulitic and dipyramid or disphenoid like forms in natural mucilaginous biofilms and microbial mats the morphologies reported here may facilitate the recognition of early and recent marine and continental microcrystalline bacterial carbonates and cements

mechanism diversity of the loma prieta aftershocks and the mechanics of mainshock aftershock interaction the diverse aftershock sequence of the 1989 loma prieta earthquake is inconsistent with conventional models of mainshock aftershock interaction because the aftershocks do not accommodate mainshock induced stress changes instead the sense of slip of the aftershocks is consistent with failure in response to a nearly uniaxial stress field in which the maximum principal stress acts almost normal to the mainshock fault plane this orientation implies that i stress drop in the mainshock was nearly complete ii mainshock induced decreases of fault strength helped were important in controlling the occurrence of after shocks and iii mainshock rupture was limited to those sections of the fault with preexisting shear stress available to drive fault slip

cyclic ductile and brittle deformation related to coseismic thrust fault propagation structural record at the base of a basement nappe preveli crete 1 the structural record at the base of a basement nappe preveli nappe crete greece thrust upon sedimentary rocks is investigated aimed on understanding mechanisms which result in decoupling of the thrust sheet from its original substratum we identify several superimposed deformation stages each with characteristic structural style and indications of episodic deformation at initially high differential stress the final stage involves formation of a matrix supported breccia transected by pseudotachylytes comprising the lowermost 30 m of the nappe brecciation and pseudotachylyte formation occurred in a single event and structures were not modified afterward complete induration of breccia and composition of phengite crystallized during devitrification of pseudotachylytes place the sequence of events into the middle crust we propose a model relating episodic deformation and cyclic stress history to propagation of a thrust fault in a limited number of seismic events terminal brecciation and frictional fusion record passage of the fault front beneath the site of observation and decoupling of the thrust sheet absence of discernible further deformation is consistent with negligible basal friction during transport as a nappe brecciation and pseudotachylyte formation mark the switch from a history of repeated coseismic loading and postseismic stress relaxation in the plastosphere driven by seismic events on the approaching thrust fault to passive transport with deformation localized in a weak thrust plane for a sequence of superimposed ductile to brittle structures our model provides an alternative to progressive cooling and exhumation concomitant with deformation over millions of years

rotation of the principal stress directions due to earthquake faulting and its seismological implications earthquake faulting results in stress drop over the rupture area because the stress drop is only in the shear stress and there is no or little stress drop in the normal stress on the fault the principal stress directions must rotate to adapt such a change of the state of stress using two constraints i e the normal stress on the fault and the vertical stress the overburden pressure which do not change before and after the earthquake we derive simple expressions for the rotation angle in the 1 axis for a dip slip earthquake the rotation angle is only a function of the stress drop ratio defined as the ratio of the stress drop to the initial shear stress and the angle between the 1 axis and the fault plane but for a strike slip earthquake the rotation angle is also a function of the stress ratio depending on the faulting regimes the 1 axis can either rotate toward the direction of fault normal or rotate away from the direction of fault normal the rotation of the stress field has several important seismological implications it may play a significant role in the generation of heterogeneous stresses and in the occurrence and distribution of aftershocks the rotation angle can be used to estimate the stress drop ratio which has been a long lasting topic of debate in seismology

carbonates in martian meteorite alh84001 a planetary perspective on formation temperature we have re examined the formation temperature of carbonates in alh84001 accounting for the prior evolution of the stable isotopes used in the analysis oxygen and carbon in the atmosphere likely have become isotopically fractionated due to various nonthermal loss mechanisms operating throughout geologic time taking this previous evolution into account we derive a formation temperature from oxygen isotope thermometry of 40 250 c this range of temperatures represents a lower limit an increase in groundwater salinity could further raise the 18o of the groundwater as seen in terrestrial hydrothermal systems allowing for an even higher temperature of formation derived from the isotopic data

calcium carbonate and sulfate of possible extraterrestrial origin in the eeta 79001 meteorite abstract two varieties of ca carbonate were found in a total of three interior 2 cm depth samples of glass inclusions from the shergottite meteorite elephant moraine antarctica a79001 two of the samples including the largest deposit around a vug near the center of the meteorite 8 cm depth contained veins of granular calcite with significant magnesium avg atomic mg mg ca 0 14 0 15 and phosphorus avg atomic p ca 0 04 either as mg calcite with dissolved p or as calcite with very finely intergrown mg bearing phosphate the second variety which occurred in a third sample with a previously documented high concentration of trapped gases consisted of disseminated 10 20 m anhedral grains of nearly pure caco3 and was intimately associated with laths and needles of ca sulfate possibly gypsum the coexisting carbonate and sulfate appeared to be partially decrepitated relict grains that were trapped during rapid solidification of quench textured pyroxene and glass for at least the latter occurrence textural relationships clearly indicate a pre terrestrial origin for the salts all evidence considered it is probable that both varieties of cacarbonate and the ca sulfate formed on a planetary body probably mars before the meteorite fell on earth

the tatahouine diogenite mineralogical and chemical effects of sixty three years of terrestrial residence comparison between clasts of the tatahouine diogenite collected the day of the fall in 1931 and 63 years later in 1994 allows the evaluation of mineralogical and chemical effects of terrestrial residence on meteorites secondary minerals are found in the 1994 samples iron stains and carbonate rosettes major and trace element abundances have been determined on fallen and found clasts no significant differences have been observed for most elements with the exception of rb sr and in a single case the light rare earth elements lree in this case the ree pattern of a 1994 clast displays a weak positive ce anomaly probably linked to the presence of iron hydroxide the contents of rb and sr are significantly higher in the 1994 samples than in the 1931 clasts and reflect the formation of calcite inside some of the clast fractures these results demonstrate that weathering processes may change the chemistry of meteorites in a very short time

experimental investigation of the effect of change in control modes on the post failure behavior of coal and coal measures rock abstract this paper presents post failure tests on rock and coal specimens obtained from various coal mines in west virginia and hiawatha berea sandstone was used for comparing post failure behavior of coal measures rock two feedback control modes axial strain control and lateral strain control modes were used to study the post failure behaviors of the rock and coal specimens the results obtained from the experiments show that coal measure rocks show significant change in post failure behavior when the control modes were changed from axial strain to lateral strain it was found that when the lateral strain control mode was used in the post failure region coal measures rock exhibited class ii behavior also coal specimen under lateral strain control showed class ii behavior however it was observed that when the same coal specimens are tested under confining pressure the class ii behavior is replaced by class i behavior

mars a water rich planet abstract mars had outgassed at least 0 5 to 1 km of water 10 to 20 bar of co2 and 0 1 to 0 3 bar of n2 the volatiles that have been retained are mostly in the cratered uplands terrain softening fretted channels debris flows and closed depressions indicate that at least the upper 2 km of the cratered uplands at high latitudes 30 contain ice in amounts that exceed the porosity estimated to be 10 20 theoretical studies and lack of these features in the cratered uplands at low latitudes suggest that the upper 1 km of the uplands at low latitudes is ice poor however valley networks indicate that water was present near the surface early in the planet s history although in amounts smaller than at high latitudes the entire upper 1 km planetwide is estimated to have contained 75 125 m of water at the end of heavy bombardment the largest sink for water is the megaregolith below 1 km episodic eruption of water from the deep megaregolith cut many of the large outflow channels from the volume of water needed to cut the circum chryse channels and assuming uniform planetwide distribution of water the deep megaregolith is estimated to have contained at least 350 m of water at the end of heavy bombardment thereby giving a total minimum inventory of 424 475 m planetwide most of the water lost from the low latitude uplands by diffusion and in cutting the valley networks is now believed to be in the polar layered terrains most of the water involved in cutting the outflow channels is in the low lying northern plains where a variety of features that have been attributed to ground ice is present a large fraction of the planet s surface has been overplated with water poor volcanics of which we have samples in the snc meteorites the younger volcanics have reacted extensively with the old volatile rich basement some of the co2 and n2 outgassed was lost during heavy bombardment by impact erosion of the atmosphere and other processes the remaining was fixed carbonates and folded deep into the megaregolith during heavy bombardment

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

ar ar chronology of the martian meteorite alh84001 evidence for the timing of the early bombardment of mars abstract alh84001 a cataclastic cumulate orthopyroxenite meteorite from mars has been dated by ar ar stepped heating and laser probe methods both methods give ages close to 3 900 ma the age calculated is dependent on assumptions made about 39ar recoil effects and on whether significant quantities of 40ar from the martian atmosphere are trapped in the meteorite if as suggested by xenon and nitrogen isotope studies martian atmospheric argon is present then it must reside predominantly in the k rich phase maskelynite independently determined 129xe abundances in the maskelynite can be used to place limits on the concentration of the atmospheric 40ar these indicate a reduction of around 80 ma to ages calculated on the assumption that no martian atmosphere is present after this correction the nominal ages obtained are 3940 50 3870 80 and 3970 100 ma by stepped heating and 3900 90 ma by laser probe 1 statistical errors giving a weighted mean value of 3 920 ma ambiguities in the interpretation of 39ar recoil effects and in the contribution of martian atmospheric 40ar lead to uncertainties in the ar ar age which are difficult to quantify but we suggest that the true value lies somewhere between 4 050 and 3 800 ma this age probably dates a period of annealing of the meteorite subsequent to the shock event which gave it its cataclastic texture the experiments provide the first evidence of an event occurring on mars coincident with the time of the late heavy bombardment of the moon and may reflect a similar period of bombardment in the southern highlands of mars whether the age determined bears any relationship to the time of carbonate deposition in alh84001 is not known such a link depends on whether the temperature associated with the metasomatic activity was sufficient to cause argon loss from the maskelynite and or whether the metasomatism and metamorphism were linked in time through a common heat source

travertines depositional morphology and the bacterially constructed constituents abstract investigation of travertine accumulations throughout central italy and the west central u s has shown that the carbonate is precipitated in response to both inorganic and organic processes individual deposits range up to 85 m thick and hundreds of square kilometers in areal extent all of the carbonate is low magnesian calcite water chemistry temperature and morphology of the accumulation greatly influence the constituents comprising these deposits harsh environmental situations favor inorganic deposits while increasingly more moderate conditions result in the formation of a greater abundance of organically precipitated material morphological variations of travertine deposits recognized include 1 waterfall or cascade 2 lake fill 3 sloping mound fan or cone 4 terraced ound and 5 fissure ridge gross morphology internal stratification and constituents comprising these deposits vary systematically depending upon the type of accumulation and chemistry of the waters bacterially precipitated calcite forms a large percentage of the carbonate in many travertine accumulations exceeding 90 of the framework grains comprising some of the lake fill deposits bacteria are among the first taxa to inhabit and reproduce in harsh spring environments and produce a variety of fascinating constituents the bacteria are primarily rods generally 0 2 m in diameter and less than 1 0 m in length the rods readily decay resulting in calcite crystals loaded with micropores the basic building block of bacterially constructed travertine is a clump of bacteria averaging 20 m in diameter enclosed in a single crystal of calcite aggregates of these crystals produce a variety of deposits including 1 crudely laminated carbonate mud 2 finely laminat d layers of mud 3 intraclasts 4 foam rock and 5 shrubs the shrubs are most striking commonly forming layers 1 3 cm thick but also producing bacterial pisoids at some locales bacterial stromatolites composed of layers of shrubs alternating with finely laminated layers of bacterial mud comprise essentially the entire deposit the shrub layers are the result of flourishing summer growth of bacteria and furthermore show remarkable daily laminae 0 1 0 5 mm thick the importance of bacteria in the formation of travertine and their universally recognized abundance in modern sediments provides an impetus for a reexamination of the role of bacteria in the origin of other types of ancient deposits

the age of the carbonates in martian meteorite alh84001 the age of secondary carbonate mineralization in the martian meteorite alh84001 was determined to be 3 90 0 04 billion years by rubidium strontium rb sr dating and 4 04 0 10 billion years by lead lead pb pb dating the rb sr and pb pb isochrons are defined by leachates of a mixture of high graded carbonate visually estimated as approximately 5 percent whitlockite trace and orthopyroxene approximately 95 percent the carbonate formation age is contemporaneous with a period in martian history when the surface is thought to have had flowing water but also was undergoing heavy bombardment by meteorites therefore this age does not distinguish between aqueous and impact origins for the carbonates

localization patterns in sandbox scale numerical experiments above a normal fault in basement abstract the finite element program elfen is used to study the effect of basement fault dip on the evolution of shear band patterns in unconsolidated sand the material properties and boundary conditions of the model were chosen to correspond to generic sandbox experiments model results reproduce the range of structural styles found in corresponding sandbox experiments with a basement fault dip of 60 and lower a graben structure is formed composed of a synthetic shear band followed by one or more antithetic shear bands with a basement fault dip of 70 and steeper a reverse precursor shear band forms first followed by a synthetic normal shear band that accommodates all further displacement the dip of the synthetic shear band is close to the basement fault dip for basement fault dips between 60 and 70 we observe a transition in localization patterns an analysis of the stress fields and velocity vectors in the model explains the first order aspects of the relationships observed we consider the observed precursor dominated and graben dominated structural domains to be important components of normal fault systems in which the first order structural style and deformation patterns are only weakly dependent on the details of the rheology of the model materials and explore the interesting problem of the change in structural style from precursor dominated to graben dominated structural domains above a normal fault in basement we find similar structural domains in sandbox experiments for the same set of boundary conditions but with slightly different material properties suggesting that the modeled patterns are robust within these two structural domains i e will occur over a range of similar material properties and boundary conditions the results of this study contribute to our ability to validate numerical models against experiments in order to finally better simulate natural systems

bulk and stable isotopic compositions of carbonate minerals in martian meteorite allan hills 84001 no proof of high formation temperature understanding the origin of carbonate minerals in the martian meteorite allan hills alh 84001 is crucial to evaluating the hypothesis that they contain traces of ancient martian life using arguments based on chemical equilibria among carbonates and fluids an origin at greater than 650 c inimical to life has been proposed however the bulk and stable isotopic compositions of the carbonate minerals are open to multiple interpretations and so lend no particular support to a high temperature origin other methods possibly less direct will have to be used to determine the formation temperature of the carbonates in alh 84001

martian microfossils in lunar meteorites one of the five lines of evidence used by mckay et al 1996 for relic life in the martian meteorite allan hills alh 84001 was the presence of objects thought to be microfossils these ovoid and elongated forms are similar to structures found in terrestrial rocks and described as nanobacteria folk 1993 mcbride et al 1994 using the same procedures and apparatus as mckay et al 1996 we have found structures on internal fracture surfaces of lunar meteorites that cannot be distinguished from the objects described on similar surfaces in alh 84001 the lunar surface is currently a sterile environment and probably always has been however the lunar and martian meteorites share a common terrestrial history which includes many thousands of years of exposure to antarctic weathering although we do not know the origin of these ovoid and elongated forms we suggest that their presence on lunar meteorites indicates that the objects described by mckay et al 1996 are not of martian biological origin