transmission electron microscope analyses of alteration phases in martian meteorite mil 090032 abstract the nakhlite group of martian meteorites found in the antarctic contain varying abundances of both martian and terrestrial secondary alteration phases the aim of this study was to use transmission electron microscopy tem to compare martian and terrestrial alteration embodied within a single nakhlite martian meteorite find mil 090032 martian alteration veins in mil 090032 are composed of poorly ordered fe smectite phyllosilicate this poorly ordered smectite appears to be equivalent to the nanocrystalline phyllosilicate hydrated amorphous gel phase previously described in the martian alteration veins of other nakhlites chemical differences in this nanocrystalline phyllosilicate between different nakhlites imply localised alteration which occurred close to the martian surface in mil 090032 both structurally and compositionally the nakhlite nanocrystalline phyllosilicate shows similarities to the amorphous poorly ordered phase recently discovered in martian soil by the mars curiosity rover at rocknest gale crater terrestrially derived alteration phases in mil 090032 include jarosite and gypsum amorphous silicates and fe oxides and hydroxides similarities between the mineralogy and chemistry of the mil 090032 terrestrial and martian alteration phases suggest the alteration conditions on mars were similar to those in the antarctic at both sites a small amount of fluid at low temperatures infiltrated the rock and became acidic as a result of the conversion of fe2 to fe3 under oxidising conditions

aqueous alteration in the northwest africa 817 nwa 817 martian meteorite samples of a new martian meteorite of the nakhlite family nwa 817 contain traces of an iron rich alteration product textural arguments indicate that this alteration product has been formed on the parent body of the meteorite mars the chemical composition and structural data x ray diffraction transmission electron microscopy and vibrational spectroscopy show that the alteration mineral is a hydrous phase from the smectite family major elements and rare earth elements suggest that the formation of the alteration phase is related to the circulation of an aqueous fluid which composition is controlled by the dissolution of feldspars to account for a positive eu anomaly olivine and possibly apatite hydrogen isotope data display negative deltad values ranging from 60 to 280parts per thousand in olivine and pyroxenes and from 140 to 181parts per thousand in the alteration phase the values of deltad for the alteration product show a small scatter with a mean value of 170 14parts per thousand these values are lower than those previously obtained on other martian meteorites which give mainly positive deltad values these positive values have been interpreted as resulting from the interaction of the martian meteorites with water from the martian atmosphere ruling out the effect of terrestrial alteration it is suggested that alteration in the nwa 817 meteorite was likely produced on mars by the circulation of an aqueous fluid originating from a chemical reservoir such as the martian mantle which has not equilibrated with a fractionated martian atmosphere c 2002 elsevier science b v all rights reserved

glacier meltwater stream chemistry in wright and taylor valleys antarctica significant roles of drift dust and biological processes in chemical weathering in a polar climate article this study examines stream chemistry within glacial drainages in wright and taylor valley antarctica in con junction with geochemical modeling and sediment leach experiments in order to assess the source of chem ical weathering fluxes within polar drainages the targeted catchments are underlain by granitoid basement with streams flowing through glacial drifts of variable composition analyses of meltwater from clark glacier in wright valley and howard glacier delta stream in taylor valley show increases in solute concentrations as a function of distance from the glaciers surface area normalized weathering rate estimates based on si and k fluxes within the streams confirm that significant chemical weathering is occurring in these systems at rates comparable to field and laboratory rates of silicate weathering under much warmer conditions down stream increases in several cations ca na k and mg occur within both drainages however solute concen trations between the two streams differ significantly clark glacier stream contains na ca whereas howard glacier stream contains ca na distally geochemical reaction path modeling for clark glacier predicts in creasing concentrations of ca na and mg over the reaction progress paralleling the results of field data from clark stream modeling of howard glacier stream also shows an increase in several cations however na concentrations exceed those for ca which differs from the observed field data and suggests that the dif ferences in stream chemistry are not due to differing bedrock mineralogy instead ion leaching experiments conducted on drift sediments demonstrate that 1 glacial deposits within wright valley and taylor valley may significantly affect stream chemistry through direct water rock interactions within the stream channel and or 2 the water chemistries are influenced by eolian additions of locally sourced dust to glacier surfaces differences in fe concentrations between the two stream systems indicate that biological activity may also significantly influence iron concentrations within howard glacier stream

constancy of silicate mineral weathering rate ratios between natural and experimental weathering implications for hydrologic control of differences in absolute rates abstract inverse models which apportion watershed effluxes geochemical mass balance over estimated mineral surface area give weathering rates for individual silicate minerals e g feldspar 1 3 orders of magnitude slower than laboratory rates comparisons of rates calculated from several recent watershed geochemical mass balance studies with experimentally determined rates reveal that the ratio of mineral weathering rates for a given silicate mineral pair determined in an internally consistent field study equals the ratio of rates for the same minerals in any internally consistent set of laboratory experimental data thus the coefficient which corrects the weathering rate for any mineral from a laboratory data set to a field setting is identical to the corresponding coefficient for any other mineral in the same data sets the correction factor linking experimental and natural rates must be independent of composition physical e g hydrologic controls rather than compositional or chemical controls cause the difference between weathering rates in nature and the laboratory because flow in natural weathering profiles is spatially heterogeneous not all of the potentially available surface in natural systems actually participates in reactions with pore fluids

concentrations of siderophile elements in nonmagnetic fractions of antarctic h and l chondrites a quantitative approach on weathering effects the nonmagnetic separates of antarctic h and l chondrite finds from the lewis cliff stranding area are enriched in fe and siderophile elements such as ni co cu as ir and au relative to those of non antarctic falls the siderophile enrichments are caused by the oxidation of metallic fe ni to nonmagnetic hydrous fe ni oxides the concentrations of fe ni and co in each sample were used to calculate the amount of oxidized metal which serves as a quantitative measure of the degree of weathering the amount of oxidized metal is generally between 1 10 wt and shows a rough correlation with the qualitative a b c weathering classification for increasing rustiness no clear correlation was found between weathering and terrestrial age although meteorites older than 0 2 ma contain on the average 50 more oxidized metal also no correlation was found between the degree of weathering and the natural thermoluminescence tl level which is dominated by the thermal history in space rather than by the thermal history in or on the antarctic ice the only significant parameter for the degree of weathering seems to be the location of find with the most weathered specimens being found in a part of the stranding area where melt water ponds are known to occur this observation confirms that the occasional presence of liquid water plays an important role in the antarctic weathering process and explains the poor correlation of the degree of weathering of antarctic meteorites with terrestrial age and surface exposure age

basalt and olivine dissolution under cold salty and acidic conditions what can we learn about recent aqueous weathering on mars 1 to test which variables may be important for weathering on mars the effects of temperature 22 c 6 c and 19 c high ionic strength and oxygen concentrations were investigated in batch dissolution experiments containing forsterite fayalite and basalt glass cacl2 nacl h2o brine can remain liquid to temperatures of 55 c and thus may be liquid in the cold dry climate that currently characterizes mars to understand weathering under such conditions dissolution rates were measured in experiments in distilled water with and without cacl2 and nacl as observed by others dissolution rates increased with temperature and only fayalite dissolution was significantly affected by the presence or absence of oxygen enhanced fayalite dissolution under anoxic conditions suggests that fe rich olivine would dissolve more rapidly than mg rich olivine on mars dissolution in the two most dilute experimental solutions deionized water and cacl2 nacl h2o solution of ionic strength 0 7 m were the same within uncertainty but apparent dissolution rate constants in cacl2 nacl h2o brines were significantly slower steady silica concentrations are decreased in the brines consistent with other work and precipitation rates of silica decrease with decreasing temperatures these results suggest that enhanced silica precipitation could be an indicator of high ionic strength solutions on mars consistent with these observations weathering of basalt has been observed to sometimes be accompanied by precipitated layers of silica in cold dry environments on earth if dissolution on mars occurs or occurred under conditions similar to our experiments cation leaching would be expected to be accompanied by silica precipitates on weathering surfaces

alteration assemblages in the nakhlites variation with depth on mars secondary mineral assemblages in the nakhlite meteorites lafayette governador valadares gv nakhla yamato y 000593 y 000749 have been studied using scanning electron microscopy transmission electron microscopy and electron probe micro analysis the different nakhlites have distinctive secondary assemblages in their olivine grains and mesostases showing compositional fractionation correlated with their relative depths below the martian surface fracture filled veins in lafayette at the bottom of the pile consist of a siderite phyllosilicate fe oxide hydrated silicate gel assemblage corresponding veins in nakhla and gv further up the pile are predominantly a siderite gel assemblage with additional evaporites including gypsum y 000593 y 000749 veins are dominated by gel the gel s mg mg fe ratio decreases from lafayette 0 37 to gv 0 32 nakhla 0 24 and y 000593 0 15 we suggest that hydrothermal fluid flowed up this depth profile initiated by melting of buried h2o co2 ice our results show a complex mix of fe rich phyllosilicate within the veins and mesostasis of lafayette with d spacings of 0 7 1 1 nm suggesting a mixture of smectite and serpentine the phyllosilicate formed at close to neutral ph 150 c we also suggest that water rock ratios w r of 1 10 occurred in lafayette with smaller values for the other nakhlites this is reflected in the volume of alteration minerals 10 of olivine in lafayette to 3 in nakhla textural evidence of rapid cooling together with the w r and likely fluid velocities suggest that the secondary assemblages formed quickly e g within months a model is proposed in which the secondary assemblages formed in an impact induced hydrothermal system terminated by precipitation of the gel and evaporation of soluble salts

the nakhlite hydrothermal brine on mars abstract water is the basis of habitability and formation conditions of hydrous alteration minerals are key to temperature and chemical conditions of that water using new detailed observations on nakhlite martian meteorite alteration composition temperature and redox conditions of the water that formed the observed hydrous alteration can be determined through thermochemical modelling we show that the nakhlite parent rocks on mars encountered a co2 rich hydrothermal fluid at 150 t 200 c ph 6 8 with a water rock ratio w r 300 under these conditions fe rich carbonate was precipitated within brittle fractures as the fluid cooled to 50 c at ph 9 and w r of 6 fe rich phyllosilicate precipitated followed in turn by rapid precipitation of an amorphous gel it was enriched in the most soluble species e g k na of alkaline ph and similar to terrestrial i e not seawater influenced dilute brines in basaltic terrains on earth our results show that environments associated with this type of fluid were habitable unlike those associated with acid sulphate fluids considering the timing of the nakhlite alteration the most likely cause is impact generated hydrothermal alteration of the nakhlite pile at the margins of an impact crater the martian subsurface fluid forming phyllosilicates provided habitable temperatures and many of the nutrients required for life

laihunite and jarosite in the yamato 00 nakhlites alteration products on mars 1 nakhlites martian clinopyroxenites are believed to have experienced aqueous alteration on the martian surface on the basis of the presence of reddish brown iddingsite on the rim and along cracks of large olivine crystals amorphous to poorly crystalline phyllosilicates smectites and carbonates are major products of aqueous alteration of the yamato 00 nakhlites the yamato 000593 nakhlite and its pair yamato 000749 contain large olivine crystals having opaque black material intimately associated with iddingsite on their rims and along cracks synchrotron radiation x ray diffraction micro raman spectroscopy and transmission electron microscopy revealed that the opaque material is the nonstoichiometric olivine type mineral laihunite 0 42 mg2 fe2 0 79fe3 0 79si1 00o4 typically formed as a high temperature oxidation product under nonequilibrium conditions because laihunite has been decomposed into a mixture of magnetite and amorphous silicate near the fusion crust it is preterrestrial in origin this is the first observation of large olivine crystals in martian meteorites being replaced by laihunite although its presence in olivine in the mesostasis of the miller range 03346 nakhlite has been suggested by micro raman spectroscopy because laihunite in the nakhlites does not form nanometer sized intergrowth with iron oxides fluid assisted high temperature oxidation may have played an important role in transporting away free fe2 cations liberated from olivine during laihunite formation although the yamato 00 nakhlites also contain jarosite we could not confirm the presence of preterrestrial jarosite based only on its occurrences

modal abundances of pyroxene olivine and mesostasis in nakhlites heterogeneity variation and implications for nakhlite emplacement nakhlites clinopyroxenite meteorites from mars share common crystallization and ejection ages suggesting that they might have been ejected from the same place on mars by the same ejection event impact and are different samples of the same thick volcanic flow unit or shallow sill mean modal abundances and abundance ranges of pyroxene olivine and mesostasis vary widely among different thin sections of an individual nakhlite lithologic heterogeneity is the main factor contributing to the observed modal abundance variations measured in thin sections prepared from different fragments of the same stone two groups of nakhlites are distinguished from one another by which major constituent varies the least and the abundance of that constituent the group consisting of nakhla lafayette governador valadares and the yamato nakhlite pairing group is characterized by low modal mesostasis and pyroxene olivine covariance whereas the group consisting of the miller range nakhlite pairing group and northwest africa 5790 is characterized by low modal olivine and pyroxene mesostasis covariance these two groups sample the slowest cooled interior portion and the chilled margin respectively of the nakhlite emplacement body as presently understood and appear to be also related to recently proposed nakhlite groups independently established using compositional rather than petrographic observations phenocryst modal abundances vary with inferred depth in the nakhlite igneous body in a manner consistent with solidification of the nakhlite stack from dynamically sorted phenocryst rich magmatic crystal liquid mush

noble gases in iddingsite from the lafayette meteorite evidence for liquid water on mars in the last few hundred million years we analyzed noble gases from 18 samples of weathering products iddingsite from the lafayette meteorite potassium argon ages of 12 samples range from near zero to 670 91 ma these ages confirm the martian origin of the iddingsite but it is not clear whether any or all of the ages represent iddingsite formation as opposed to later alteration or incorporation of martian atmospheric 40ar in any case because iddingsite formation requires liquid water this data requires the presence of liquid water near the surface of mars at least as recently as 1300 ma ago and probably as recently as 650 ma ago krypton and xe analysis of a single 34 g sample indicates the presence of fractionated martian atmosphere within the iddingsite this also confirms the martian origin of the iddingsite the mechanism of incorporation could either be through interaction with liquid water during iddingsite formation or a result of shock implantation of adsorbed atmospheric gas our strongest conclusion is that the iddingsite in lafayette formed on mars in agreement with the microstratigraphic arguments of gooding et al 1991 and treiman et al 1993 a preterrestrial origin of the iddingsite is required both by the many non zero k ar ages and by the presence of xe that is isotopically distinct from any terrestrial xe the xe is accompanied by kr but the kr and xe have been fractionated if they are derived from the present martian atmosphere this is presumably the result of either incorporation via interaction with liquid water drake et al 1994 bogard and garrison 1998 or by adsorption from the martian atmosphere perhaps accompanied by shock see also gilmour et al 1998 1999 although the iddingsite is enriched in kr and xe compared to whole rock analyses it is not clear whether iddingsite is the dominant carrier of the atmospheric derived gas drake et al 1994 or merely a minor carrier gilmour et al 1999 our 40ar 39ar experiment was disappointing in that it mostly served to confirm that the iddingsite which contains fine grained clays is susceptible to recoil loss of 39ar during irradiation only one sample of five gave a clear signal of radiogenic or extraterrestrial 40ar and that was only by 3 potassium argon ages of the second set of samples were more successful ranging from near 0 to 670 91 ma it is not clear whether any or all of the ages represent iddingsite formation as opposed to later alteration the fact that a rb sr experiment shih et al 1998 gave an apparent age for iddingsite of 679 66 ma 2a suggests that perhaps formation of iddingsite occurred or began 650 ma ago and that some samples either formed or were thermally altered later the ages could be even younger than 650 ma if the samples have incorporated martian atmospheric 40ar this means that liquid water was certainly present on mars in the last 1300 ma the formation age of lafayette and probably within the last 650 ma

back calculation of the in den arelen avalanche with ramms interpretation of model results two and three dimensional avalanche dynamics models are being increasingly used in hazard mitigation studies these models can provide improved and more accurate results for hazard mapping than the simple one dimensional models presently used in practice however two and three dimensional models generate an extensive amount of output data making the interpretation of simulation results more difficult to perform a simulation in three dimensional terrain numerical models require a digital elevation model specification of avalanche release areas spatial extent and volume selection of solution methods finding an adequate calculation resolution and finally the choice of friction parameters in this paper the importance and difficulty of correctly setting up and analysing the results of a numerical avalanche dynamics simulation is discussed we apply the two dimensional simulation program ramms to the 1968 extreme avalanche event in den arelen w e show the effect of model input variations on simulation results and the dangers and complexities in their interpretation

terrestrial weathering of antarctic stone meteorites formation of mg carbonates on ordinary chondrites abstract white efflorescences of weathering origin occur superposed on fusion crusts or along fractures in the interiors of approximately 5 of all meteorites in the us antarctic collection efflorescences from equilibrated ordinary chondrites consist of the hydrous mg carbonates nesquehonite hydromagnesite x ray diffraction and scanning electron microscope studies of efflorescences from lew 85320 h5 show abundant elongate prismatic crystals of nesquehonite idiomorphic not pseudomorphous after lansfordite with minor local encrustations of hydromagnesite abundances of na k ca and rb in efflorescences from lew 85320 suggest that the observed contents of these elements would require only modest fractionation of chondritic composition whereas extensive fractionation would be required to derive the observed cation ratios from terrestrial sea salts therefore cations in evaporite minerals on antarctic meteorites are most likely not products of contamination by terrestrial marine salts the mg in the efflorescences probably originated from weathering of meteoritic olivine other cations in the efflorescences are also of meteoritic provenance thermodynamic analysis of the reaction forsterite water carbon dioxide nesquehonite silica at antarctic temperatures and pco 2 indicates spontaneity for all water activities greater than 0 65 compatible with the presence of liquid water as brines and or thin films

basalt weathering rates on earth and the duration of liquid water on the plains of gusev crater mars where martian rocks have been exposed to liquid water chemistry versus depth profi les could elucidate both martian climate history and potential for life the persistence of primary minerals in weathered profi les constrains the exposure time to liquid water on earth mineral persistence times range from 10 k y olivine to 250 k y glass to 1 m y pyroxene to 5 m y plagioclase such persistence times suggest mineral persistence minima on mars however martian solutions may have been more acidic than on earth relative mineral weathering rates observed for basalt in svalbard norway and costa rica demonstrate that laboratory ph trends can be used to estimate exposure to liquid water both qualitatively mineral absence or presence and quantitatively using reactive transport models qualitatively if the martian solution ph 2 glass should persist longer than olivine therefore persistence of glass may be a ph indicator with evidence for the ph of weathering the reactive transport code crunchflow can quantitatively calculate the minimum duration of exposure to liquid water consistent with a chemical profi le for the profi le measured on the surface of the exposed martian rock known as humphrey in gusev crater the calculated exposure time is 22 k y which is a minimum due to physical erosion if correct this estimate is consistent with short term episodic alteration accompanied by ongoing surface erosion more of these depth profi les should be measured to illuminate the weathering history of mars

40ar 39ar studies of whole rock nakhlites evidence for the timing of formation and aqueous alteration on mars 20 25 mg whole rock samples of the nakhlites lafayette and nakhla have been analyzed via the 40ar 39ar technique in part to verify their formation ages but primarily in an attempt to determine the timing of aqueous alteration in these martian meteorites as in previous studies plateaus in apparent age are observed at about 1300 ma 1322 10 for lafayette 1332 10 and 1323 11 for nakhla presumably corresponding to crystallization ages the plateaus are not entirely flat perhaps reflecting the effects of recoil during creation of 39ar in the nuclear irradiation the first 5 20 of the k derived ar released from all three samples give apparent ages 1300 ma coupled with the fact that chronometric isotopic studies of nakhlites typically show some disturbance we believe the low temperature pattern represents more recent than 1300 ma formation of martian aqueous alteration products such as iddingsite no low temperature plateaus are observed this is consistent with petrographic evidence for multiple formation events although the lack of low temperature plateaus is far from conclusive on the other hand if there was a single time of alteration we believe that it will be difficult if not impossible to determine it using the k ar system

forsterite dissolution rates in mg sulfate rich mars analog brines and implications of the aqueous history of mars high salinity brines although rare on earth s surface may have been important in the geologic history of mars increasing evidence suggests the importance of liquid brines in multiple locations on mars in order to interpret the effect of high ionic strength brines on olivine dissolution which is widely present on mars 47 new batch reactor experiments combined with 35 results from a previous study conducted at 25 c from 1 ph 4 in magnesium sulfate sodium sulfate magnesium nitrate and potassium nitrate solutions with ionic strengths as high as 12 m show that very high ionic strength brines have an inhibitory effect of forsterite dissolution rates multiple linear regression analysis of the data suggests that the inhibition in dissolution rates is due to decreased water activity at high ionic strengths regression models also show that mmg up to 4 m and mso4 up to 3 m have no effect on forsterite dissolution rates the effect of decreasing dissolution rates with decreasing ah2o is consistent with the idea that water acts as a ligand that participates in the dissolution process less available water to participate in the dissolution reaction results in a slower dissolution rate multiple linear regression analysis of the data produces the rate equation logr 6 81 0 52ph 3 26logah2o forsterite in dilute solutions with a water activity of one dissolves twice as fast as those in brines with a water activity of 0 8

dissolution of primary basaltic minerals in natural waters saturation state and kinetics abstract the state of saturation of olivine orthopyroxene and plagioclase of variable composition has been assessed in various types of natural waters in iceland including river waters groundwater and geothermal waters with temperatures up to 250 c the stability of olivine and orthopyroxene decreases with increasing mg content similarly the stability of plagioclase decreases with increasing anorthite content the river waters which are representative of the weathering environment are always undersaturated with both olivine and orthopyroxene the degree of undersaturation being dominated by the water ph all river waters tend to dissolve olivine of the composition encountered in basalts according to the linear rate law and orthopyroxene when the ph is 200 c are undersaturated with olivine orthopyroxene and plagioclase the first two particularly when fe rich and sufficient to cause olivine to dissolve according to the linear rate law at intermediate temperatures 50 150 c the geothermal waters are close to equilibrium with these minerals except for mg rich olivine it seems likely that dissolution of glass from basalt which is very reactive will favor stability of the igneous minerals

age of allan hills 82102 a meteorite found inside the ice the recovery of thousands of meteorites in the antarctic since 1969 has not only greatly increased knowledge of the meteorites themselves but has also provided a new tool for glaciology most antarctic meteorites are found on blue ice areas where old ice is continuously ablated a measurement of the age of this ice helps us understand meteorite accumulation mechanisms and the dynamics of ice movement the terrestrial age of a meteorite the time period since the date of meteorite fall can be determined from the reduction in concentration of cosmogenic radionuclides during the time the meteorite has been shielded by the earth s atmosphere here we report the terrestrial age of a meteorite that was recovered from below the surface of the ice and argue that this represents a measurement of the age of the ice itself we measured the cosmogenic radionuclides 10be half life 1 5 myr 14c 5 730 yr 26a1 0 71 myr 36c1 0 30 myr and 53mn 3 7 myr in the meteorite and 10be and 36c1 in the ice we obtained a terrestrial age of 11 000 years for the meteorite which suggests that the snow accumulation area where it fell was only a few tens of kilometres away

formation of iddingsite veins in the martian crust by centripetal replacement of olivine evidence from the nakhlite meteorite lafayette abstract the lafayette meteorite is an olivine clinopyroxenite that crystallized on mars 1300 million years ago within a lava flow or shallow sill liquid water entered this igneous rock 700 million years later to produce a suite of secondary minerals collectively called iddingsite that occur as veins within grains of augite and olivine the deuterium hydrogen ratio of water within these secondary minerals shows that the aqueous solutions were sourced from one or more near surface reservoirs several petrographically distinct types of veins can be recognised by differences in their width shape and crystallographic orientation augite and olivine both contain veins of a very fine grained hydrous fe and mg rich silicate that are 1 2 m in width and lack any preferred crystallographic orientation these narrow veins formed by cementation of pore spaces that had been opened by fracturing and probably in response to shock the subset of olivine hosted veins whose axes lie parallel to 0 0 1 have serrated walls and formed by widening of the narrow veins by interface coupled dissolution precipitation widening started by replacement of the walls of the narrow precursor veins by fe mg silicate and a crystallographic control on the trajectory of the dissolution precipitation front created micrometre scale 1 1 1 serrations the walls of many of the finely serrated veins were subsequently replaced by siderite and the solutions responsible for carbonation of olivine also partially recrystallized the fe mg silicate smectite was the last mineral to form and grew by replacement of siderite this mineralization sequence shows that lafayette was exposed to two discrete pulses of aqueous solutions the first of which formed the fe mg silicate and the second mediated replacement of vein walls by siderite and smectite the similarity in size shape and crystallographic orientation of iddingsite veins in the lafayette meteorite and in terrestrial basalts demonstrates a common microstructural control on water mineral interaction between mars and earth and indicates that prior shock deformation was not a prerequisite for aqueous alteration of the martian crust

element abundances patterns and mobility in nakhlite miller range 03346 and implications for aqueous alteration abstract nakhlite miller range mil 03346 contains many secondary phases resulting from aqueous processes including formation of poorly crystalline iddingsite like veins in olivine the precipitation of ca sulfates and fe k sulfates from evaporating fluids alteration of titanomagnetite to secondary fe oxides and the dissolution of magmatic ca phosphates and residual glass in the mesostasis a surprising variety of alteration products occur in association with olivine in mil 03346 including patches of incipiently altered olivine large si enriched olivine hosted veins up to 10 m across some of which are complex in morphology and are composed of several phases small fe s k rich veinlets that crosscut the si enriched veins ca sulfates filling cracks in olivine and secondary ca phosphates elemental abundances and distributions in these alteration products are consistent with the mobilization of elements from readily dissolved phases in the mesostasis such as phosphates and residual glass under favorable weathering conditions these phases dissolve more readily than pyroxenes plagioclase and even olivine at low ph the occurrence crosscut and devolatilized by the fusion crust and composition of si enriched alteration veins in olivine are consistent with their formation on mars si enriched poorly crystalline alteration products and secondary ca sulfates commonly occur in nakhlites but the habit and composition of these alteration products differ between meteorites elemental distributions in these secondary phases suggest at least two episodes of alteration have affected mil 03346 and subtle differences in secondary minerals and chemistry indicate that each nakhlite experienced its own unique alteration history either on mars earth or both the variable al content and range of morphologies of the olivine hosted si enriched veins suggest variable alteration conditions consistent with a water limited regime if the secondary phases in mil 03346 can be shown to have formed on mars their chemistry will provide important clues to the aqueous environments and processes at the time of their formation however elevated s and rees ce anomalies and association of secondary minerals with post impact cracks and voids indicate that terrestrial weathering has significantly affected mil 03346 this work highlights the difficulty in distinguishing pre terrestrial aqueous alteration from later chemical weathering of susceptible mineral phases even in meteorites with limited terrestrial modification

opal a in the nakhla meteorite a tracer of ephemeral liquid water in the amazonian crust of mars the nakhlite meteorites are clinopyroxenites that are derived from a 1300 million year old sill or lava flow on mars most members of the group contain veins of iddingsite whose main component is a fine grained and hydrous fe and mg rich silicate siderite is present in the majority of veins where it straddles or cross cuts the fe mg silicate this carbonate also contains patches of ferric oxy hydroxide despite 40 years of investigation the mineralogy and origins of the fe mg silicate is poorly understood as is the paragenesis of the iddingsite veins nanometer scale analysis of fe mg silicate in the nakhla meteorite by electron and x ray imaging and spectroscopy reveals that its principal constituents are nanoparticles of opal a this hydrous and amorphous phase precipitated from acidic solutions that had become supersaturated with respect to silica by dissolution of olivine each opal a nanoparticle is enclosed within a ferrihydrite shell that formed by oxidation of iron that had also been liberated from the olivine siderite crystallized subsequently and from solutions that were alkaline and reducing and replaced both the nanoparticles and olivine the fluids that formed both the opal a ferrihydrite and the siderite were sourced from one or more reservoirs in contact with the martian atmosphere the last event recorded by the veins was alteration of the carbonate to a ferric oxy hydroxide that probably took place on mars although a terrestrial origin remains possible these results support findings from orbiter and rover based spectroscopy that opaline silica was a common product of aqueous alteration of the martian crust

dissolution of primary minerals in natural waters abstract the saturation state of olivine plagioclase clinopyroxene orthopyroxene fe ti oxides and apatite of variable composition has been assessed in natural waters in iceland with temperature ranging from 0 c to 300 c and in situ ph from below 5 to above 10 cold waters are undersaturated with respect to olivine orthopyroxene clinopyroxene and plagioclases indicating that all these minerals tend to dissolve under weathering conditions with increasing ph and temperature the waters approach saturation with pyroxene olivine and plagioclases also the degree of undersaturation of olivine and orthopyroxene decreases with increasing fe content of the minerals and fe rich olivine and orthopyroxene are stable between 50 c and 150 c whereas mg rich ones tend to dissolve natural waters in iceland are saturated with respect to pure albite when above 50 c they are on the other hand undersaturated with ca rich plagioclase up to 250 c where the waters reach saturation pure magnetite and hematite are stable at all temperatures with increasing titanium content the minerals become unstable and the waters are undersaturated with respect to pure ulvospinel at temperatures up to 300 c pure ilmenite is however close to saturation under weathering conditions but undersaturated above 200 c f apatite is close to saturation at all temperatures on the other hand undersaturation with respect to oh apatite is observed at all temperatures the weathering susceptibilities of primary minerals of basalt in icelandic waters in increasing order are mg olivine fe olivine ti rich magnetite ca plagioclase mg orthopyroxene fe orthopyroxene clinopyroxene na plagioclase f apatite ti rich ilmenite ti poor magnetite ti poor hematite

formation of a martian pyroxenite a comparative study of the nakhlite meteorites and theo s flow the unusual composition of the nakhlites a group of pyroxenitic martian meteorites with young ages presents an opportunity to learn about nonbasaltic magmatic activity on another planet however the limited number of these meteorites makes unraveling their history difficult a promising terrestrial analog for the formation of the nakhlites is theo s flow in ontario canada this atypical 120 m thick flow differentiated in place forming distinct layered lithologies of peridotite pyroxenite and gabbro theo s pyroxenite and the nakhlites share strikingly similar petrographies with concentrated euhedral to subhedral augite grains set in a plagioclase rich matrix these two suites of rocks also share specific petrologic features mineral and whole rock compositional features and size and spatial distributions of cumulus grains the numerous similarities suggest that the nakhlites formed by a similar mechanism in a surface lava flow or shallow intrusion their formation could have involved settling of crystals in a phenocryst laden flow or in situ nucleation and growth of pyroxenes in an ultramafic lava flow the latter case is more likely and requires steady state nucleation and growth of clusters of pyroxene grains and olivine in the nakhlites circulating in a strongly convecting melt pool followed by settling and continued growth in a thickening cumulate pile trapped pockets of intercumulus liquid in the pile gradually evolved finally growing fe enriched rims on cumulus grains with sufficient evolution the melt reached plagioclase supersaturation causing rapid growth of plagioclase sprays and late stage mesostasis growth

the rocks of mars from far and near award of the meteoritical society 2001 h y mcsween jr the leonard medal address the rocks of mars from far and near

a combined electron microprobe emp and raman spectroscopic study of the alteration products in martian meteorite mil 03346 1 we examine the secondary alteration products in mil 03346 using raman spectroscopic and electron microprobe traverses discussion focuses on the single olivine in 177 supplemented with observations from 168 and 169 traverses start at the rim and progress into the interior dark brown nearly opaque laihunite fe2 fe3 2 sio4 2 is present as overgrowths and 20 50 m veins of reddish brown stilpnomelane k na ca 4 ti0 1 al2 3 fe3 35 5 mn0 8 mg9 3 si63al9 o oh 206 n h2o occur inside the olivine stilpnomelane crosscuts and postdates the laihunite veins are in sharp contact with the host olivine but lined by 5 m films of jarosite kfe3 3 so4 2 oh 6 from a later generation of alteration an interstitial laihunite also hosts stilpnomelane the most recent secondary phases are gypsum and bassanite in our x ray maps of 168 and 169 ca sulfates were not observed in x ray maps of 177 but were detected in our raman point count all sulfates are believed to be martian the groundmass of mil indicates rapid cooling from elevated temperatures with fo2 near qfm reports of laihunite synthesis by olivine oxidation at elevated temperatures 100 800 c suggest the overgrowths formed during consolidation in terrestrial rocks stilpnomelane is a product of late diagenesis to garnet grade metamorphism in mil stilpnomelane appears to be a secondary phase formed at the lower end of this stability range at conditions akin to diagenesis raman spectra indicate that the stilpnomelane jarosite and ca sulfates are hydrated the stilpnomelane contains cl and was followed by jarosite a product of acid alteration and the deposition of ca sulfates and halide salts from more neutral chloride solutions

clay mineraloid weathering products in antarctic meteorites abstract clay mineraloids are associated with gypsum k fe sulfates possibly jarosite cryptocrystalline rust and possibly with zeolites and or secondary alkali feldspars as terrestrial weathering products in stony meteorites from the allan hills alh and elephant moraine eet collection areas in victoria land antarctica weathering formed fracture fillings in fusion crusts of eucrites eeta79004 and eeta79005 diogenites alha77256 and eeta79002 shergottite eeta79001 and h5 chondrite alh82102 and decomposition products of glass and plagioclase in interior samples of achondrites clay mineraloids displayed massive to incipient vermicular forms with elemental compositions that suggest smectite and mica like phases deposits were too small to yield pure material for phase identification although x ray diffractometry and differential scanning calorimetry of bulk samples suggested that the clay mineraloids were not well crystallized a typical structural formula calculated on a smectite basis follows k 0 26 ca 0 04 al 0 97 ti 0 02 fe 3 0 77 mg 0 03 si 4 00 o 10 oh 2 n 2 o net charge 0 28 excessions 0 58 si 0 02 cl 0 09 so 4 0 02 po 4 hydrocryogenic diagenesis can initiate clay mineral formation on timescales of 10 4 10 6 y even with only limited supplies of liquid water cosmochemists must take care not to confuse possible weathering induced elemental and isotopic fractionations in antarctic meteorite samples with signatures of pre terrestrial cosmic processes given proper sample selection and testing though occurrence of clay mineraloid and other weathering products should probably not impede progress in studies of antarctic meteorites

ejection of martian meteorites we investigated the transfer of meteorites from mars to earth with a combined mineralogical and numerical approach we used quantitative shock pressure barometry and thermodynamic calculations of post shock temperatures to constrain the pressure temperature conditions for the ejection of martian meteorites the results show that shock pressures allowing the ejection of martian meteorites range from 5 to 55 gpa with corresponding post shock temperature elevations of 10 to about 1000 c with respect to shock pressures and post shock temperatures an ejection of potentially viable organisms in martian surface rocks seems possible a calculation of the cooling time in space for the most highly shocked martian meteorite allan hills alh 77005 was performed and yielded a best fit for a post shock temperature of 1000 c and a meteoroid size of 0 4 to 0 6 m the final burial depths of the sub volcanic to volcanic martian rocks as indicated by textures and mineral compositions of meteorites are in good agreement with the postulated size of the potential source region for martian meteorites during the impact of a small projectile 200 m as defined by numerical modeling artemieva and ivanov 2004 a comparison of shock pressures and ejection and terrestrial ages indicates that on average highly shocked fragments reach earth crossing orbits faster than weakly shocked fragments if climatic changes on mars have a significant influence on the atmospheric pressure they could account for the increase of recorded ejection events of martian meteorites in the last 5 ma

halloysite as a kinetically controlled end product of arid zone basalt weathering abstract mineralogical and isotope results paired with field observations suggest that halloysite is the favored albeit metastable aluminosilicate end product of arid zone basalt weathering on kohala hawai i and that the formation of smectite has been inhibited by kinetic factors soils sampled on constructional lava flows having ages ranging from 4 to 350 ka provide a chronosequence that has had minimal physical disturbance thus allowing us to interpret chemical and mineralogical changes as the result of a time dependent process the halloysite content of the soil increases with increasing soil age its growth is at the expense of allophane which in turn forms at the expense of primary minerals these mineral relationships suggest that halloysite has been forming continuously throughout the lifetime of the soil smectite the thermodynamically stable phyllosilicate phase predicted by soil solution composition is not found in these arid soils we determined that our soil system is controlled by kinetic factors and that therefore thermodynamic predictions do not reflect reality the main factors favoring kinetic control of halloysite formation are intense but short wet periods followed by prolonged extremely dry seasons and microenvironmental conditions leading to immediate uptake of released al by the halloysite precursor mineral allophane the 18o relationship between present day soil water and halloysite formed over the last 170 ka was used as a tracer of past soil conditions results from a reconstruction of paleo climate and soil conditions combined with 18o data observed mineral relationships along the 350 ka chronosequence and field evidence for long term aridity rule out the possibility that the arid side of kohala was affected by prolonged periods of higher rainfall that could have produced more dilute soil waters and therefore altered mineralogical stabilities we conclude that pedogenic halloysite has been continually forming from the early stage of soil formation and that it has consistently formed under isotopic equilibrium conditions indicating that halloysite 18o compositions imply paleoclimatic conditions over the time of its formation that are similar to today s

transmission electron microscopy of subsolidus oxidation and weathering of olivine olivine crystals in basaltic andesites which crop out in the abert rim south central oregon have been studied by high resolution and analytical transmission electron microscopy the observations reveal three distinct assemblages of alteration products that seem to correspond to three episodes of olivine oxidation the olivine crystals contain rare dense arrays of coherently intergrown ti free magnetite and inclusions of a phase inferred to be amorphous silica we interpret this first assemblage to be the product of an early subsolidus oxidation event in the lava the second olivine alteration assemblage contains complex ordered intergrowths on 001 of forsterite rich olivine and laihunite distorted olivine structure with fe3 charge balanced by vacancies based on experimental results for laihunite synthesis kondoh et al 1985 these intergrowths probably formed by olivine oxidation between 400 and 800 c the third episode of alteration involves the destruction of olivine by low temperature hydrothermal alteration and weathering elongate etch pits and channels in the margins of fresh olivine crystals contain semi oriented bands of smectite olivine weathers to smectite and hematite and subsequently to arrays of oriented hematite crystals the textures resemble those reported by eggleton 1984 and smith et al 1987 we find no evidence for a metastable phase intermediate between olivine and smectite m eggleton 1984 the presence of laihunite exerts a strong control on the geometry of olivine weathering single laihunite layers and laihunite forsteritic olivine intergrowths increase the resistance of crystals to weathering preferential development of channels between laihunite layers occurs where growth of laihunite produced compositional variations in olivine rather than where coherency strain is associated with laihunite olivine interfaces

40ar 39ar and cosmic ray exposure ages of nakhlites nakhla lafayette governador valadares and chassigny we present 40ar 39ar dating results of handpicked mineral separates and wholerock samples of nakhla lafayette and chassigny our data on nakhla and lafayette and recently reported ages for some nakhlites and chassigny misawa et al 2006 park et al 2009 point to formation ages of approximately 1 4 ga rather than 1 3 ga that is consistent with previous suggestions of close in time formation of nakhlites and chassigny in lafayette mesostasis we detected a secondary degassing event at approximately 1 1 ga which is not related to iddingsite formation it may have been caused by a medium grade thermal event resetting the mesostasis age but not influencing the k ar system of magmatic inclusions and the original igneous texture of this rock cosmic ray exposure ages for these meteorites and for governador valadares were calculated from bulk rock concentrations of cosmogenic nuclides 3he 21ne and 38ar individual results are similar to literature data the considerable scatter of t3 t21 and t38 ages is due to systematic uncertainties related to bulk rock and target element chemistry production rates and shielding effects this hampers efforts to better constrain the hypothesis of a single ejection event for all nakhlites and chassigny from a confined martian surface terrain eugster 2003 garrison and bogard 2005 cosmic ray exposure ages from stepwise release age spectra using 38ar and neutron induced 37ar from ca in irradiated samples can eliminate errors induced by bulk chemistry on production rates although not from shielding conditions

the geochemistry of antarctic streams and their role in the evolution of four lakes of the mcmurdo dry valleys abstract major ion concentrations are reported for nineteen meltwater streams in the mcmurdo dry valleys antarctica and for the four lakes fed by these streams the most important solute acquisition processes for these flowing waters are dissolution of marine derived salts dissolution of calcite coatings derived from chemical weathering of parent rock and direct weathering of silicates although lakes joyce hoare and fryxell have undergone significant evaporative concentration the only simple salt which is predicted to form at present is calcite the shallow waters of all four lakes were supersaturated with respect to this mineral but were undersaturated with respect to other simple salts geochemical budgets however indicate that so4 k and mg in addition to ca and hco3 have been removed from some of these lakes over recent geological time bacterial sulfate reduction followed by precipitation of metal sulfides is likely to be a sink for sulfate sinks for mg and k are more problematic but may include formation of mgcalcites and in the case of k reverse weathering reactions the ages of the contemporary water columns as indicated by chloride budgets are relatively young and these lakes could have evolved their chemical compositions over a few thousand years of present stream flows it is not necessary to invoke trapped seawater or other salt sources to explain their evolution

aqueous corrosion of olivine in the mars meteorite miller range mil 03346 during antarctic weathering implications for water on mars abstract several nakhlites clinopyroxenite meteorites from mars contain olivine phenocrysts with corrosion features identical in size shape and distribution to the smaller etch pits of well characterized weathered terrestrial olivine miller range mil 03346 is an antarctic nakhlite find recovered after long exposure to antarctic conditions the distribution of discrete olivine etch pits almost exclusively within a few hundred microns of allocation mil 03346 171 s documentably exposed surface suggests that they formed by terrestrial weathering in antarctica the small size of olivine etch pits in mil 03346 171 relative to commonly much larger etch pits in even incipiently weathered terrestrial examples suggests that the duration of its exposure to weathering conditions was short or the weathering conditions to which it was exposed did not favor olivine corrosion in the form of etch pit formation or both time scales for the formation of etch pits estimated from experimentally determined dissolution rates of olivine over a range of phs are comparable to the measured terrestrial age of the meteorite and short relative to the time available for possible similar corrosion on mars etch pits of the observed size on mil 03346 olivine phenocrysts would be relatively easy to form supraglacially under brief episodic acidic antarctic conditions but the terrestrial age of mil 03346 is long enough that its olivine might have been weathered to the observed state by englacial films of alkaline antarctic water the paucity of similar etch pits in olivine from the interior of mil 03346 suggests that olivine in this mars meteorite was exposed to even less aqueous alteration after iddingsitization during its 1 3 billion years on mars than its exterior was subjected to during its pleistocene holocene exposure to antarctic weathering conditions

petrogenesis of the nakhlite meteorites evidence from cumulate mineral zoning abstract a simple igneous petrogenesis for the meteorite nakhla has previously been called into question because mg fe ratios in olivine indicate substantial disequilibrium between the predominant cumulus minerals olivine and augite comparative analyses of simulated diffusive zoning and the observed cumulus mineral zoning for all three nakhlites nakhla governador valadares and lafayette show that their current compositions do not necessarily reflect parental magma compositions instead diffusion has altered primary cumulus compositions to varying degrees nakhla being the least affected and lafayette being almost completely re equilibrated only the cores of augite grains in nakhla and governador valadares appear to have preserved their original compositions mineral zoning in each meteorite is strongly concentric around mesostasis areas suggesting that reaction with intercumulus liquid has controlled the observed zoning the presence of pigeonite and orthopyroxene overgrowths in nakhla and governador valadares and of poikilitic orthopyroxene enclosing olivine relicts in lafayette indicate substantial late magmatic reactions two pyroxene geothermometry for lafayette indicates temperatures around 950 c suggesting subsolidus equilibration as well the nakhlites appear to be a series of relatively simple cumulate rocks which have undergone various amounts of late magmatic and subsolidus diffusion possibly reflecting their relative positions in a cooling cumulate pile

petrology of the miller range 03346 nakhlite in comparison with the yamato 000593 nakhlite we petrologically examined the miller range mil 03346 nakhlite the main phase modal abundances are 67 7 vol augite 0 8 vol olivine and 31 5 vol mesostasis among all known nakhlites mil 03346 s modal abundance of olivine is the smallest and of mesostasis is the largest augite occurs as cumulus phenocrysts having a homogeneous core composition en36 38fs24 22wo40 which is identical with other nakhlites they accompany thin ferroan rims divided into inner and outer rims with a compositional gap at the boundary between the two rims olivine grains have magnesian cores fa 55 and show normal zoning toward ferroan rims fa 84 mesostasis consists mostly of glass 26 0 vol with minor skeletal fayalites skeletal titanomagnetites acicular phosphate massive cristobalite and sulfides we conclude that mil 03346 is the most rapidly cooled nakhlite among all known nakhlites based on the petrography we obtain the intercumulus melt composition for mil 03346 from the mass balance calculation using the modal abundances and discuss the crystallization sequence of mil 03346 in comparison with that of yamato y 000593 although magnesian olivines of y 000593 are phenocrystic magnesian olivine grains of mil 03346 seem to have texturally crystallized from the intercumulus melt after the mil 03346 magma intruded upward to the martian surficial zone the magnesian olivine crystallized and then the ferroan inner rim formed on phenocrystic core augite the outer rim of phenocrystic augites formed after the crystallization of skeletal fayalites and skeletal titanomagnetites resulting in a compositional gap between the inner and outer rims finally glassy mesostasis formed from the residual melt this crystallization sequence of mil 03346 is different from those of other nakhlites including y 000593

ferric saponite and serpentine in the nakhlite martian meteorites abstract transmission electron microscopy and fe k x ray absorption spectroscopy have been used to determine structure and ferric content of the secondary phase mineral assemblages in the nakhlite martian meteorites nwa 998 lafayette nakhla gv y 000593 y 000749 mil 03346 nwa 817 and nwa 5790 the secondary phases are a rapidly cooled metastable assemblage that has preserved mg and ca fractionation related to distance from the fluid source for most of the nakhlites though one nwa 5790 appears not to have experienced a fluid pathway all nine nakhlite samples have also been analysed with scanning electron microscopy electron probe micro analysis bright field high resolution transmission electron microscopy hrtem and selected area electron diffraction by measuring the energy position of the fe k xanes 1 s 3 d pre edge transition centroid we calculate the ferric content of the minerals within the nakhlite meteorites the crystalline phyllosilicates and amorphous silicate of the hydrothermal deposits filling the olivine fractures are found to have variable fe 3 fe values ranging from 0 4 to 0 9 in lafayette the central silicate gel parts of the veins are more ferric than the phyllosilicates around it showing that the fluid became increasingly oxidised the mesostasis of lafayette and nwa 817 also have phyllosilicate which have a higher ferric content than the olivine fracture deposits with fe 3 fe values of up to 1 0 further study via tem analyses reveal the lafayette and nwa 817 olivine phyllosilicates to have 2 1 t o t lattice structure with a the d 001 spacing of 0 96 nm whereas the lafayette mesostasis phyllosilicates have 1 1 t o structure with d 001 spacings of 0 7 nm based on our analyses the phyllosilicate found within the lafayette olivine fractures is trioctahedral ferric saponite ca 0 2 k 0 1 0 3 mg 2 6 fe 2 1 3 fe 3 1 7 mn 0 1 5 7 si 6 7 al iv 0 9 fe 3 0 4 8 0 o 20 oh 4 n h 2 o and that found in the mesostasis fractures is an fe serpentine ca 0 1 mg 0 7 fe 3 1 0 al vi 0 4 2 2 si 2 o 5 oh 4 with a ferric gel of similar composition in lafayette and found as fracture fills throughout the other nakhlites

preliminary quantification of a shape model for etch pits formed during natural weathering of olivine abstract many etch pits on olivine grains occur as a pair of cone shaped pits sharing a base which consequently appear as diamond shaped etch pits in cross section quantitative image analysis of back scattered electron images establishes empirical dimensions of olivine etch pits in naturally weathered samples from hawaii and north carolina images of naturally etched olivine were acquired from polished thin sections by scanning electron microscopy an average cone radius to height ratio r h of 1 78 was determined for diamond shaped cross sections of etch pits occurring in naturally weathered olivine grains largely consistent with previous qualitative results olivine etch pit shape as represented by r h varies from slightly more than half the average value to slightly more than twice the average etch pit shape does not appear to vary systematically with etch pit size

martian subsurface fluid pathways and 3d mineralogy of the nakhla meteorite the three dimensional structure of the nakhla meteorite has been investigated in order to provide a detailed picture of the fluid pathways in the volcanic subsurface of mars a combination of computed tomography and electron microscopy have been used to identify characterise and interpret the distribution size interconnectivity and secondary mineralisation of fractures through which water flowed secondary minerals which formed during aqueous alteration are found as fracture filling silicates in olivine grains and a range of carbonates sulfates and halite are found throughout olivine pyroxene and mesostasis the fractures which acted as fluid pathways are highly interconnected branching and reconnecting at multiple locations leading to a pervasive and homogeneous distribution of secondary minerals throughout the sample miniature topographic basins have been identified and provide a unique method of identifying the true orientation of the meteorite while on its host planetary surface halite and sulfate are found to be related to one another likely formed in the same episode of fluid flow and are found to postdate both carbonate and olivine hosted silicate alteration products later episodes of fluid flow may have interacted with and potentially eroded earlier generations of secondary minerals nakhla and by implication the other nakhlite meteorites therefore preserves a complex record of multiple episodes of fluid flow on mars in the past billion years

a review of meteorite evidence for the timing of magmatism and of surface or near surface liquid water on mars 1 there is widespread photogeological evidence for ubiquitous water flowing on the surface of mars however the age of surface and near surface water cannot be deduced with high precision from photogeology while there is clear evidence for old and young fluvial features in the photogeologic record the uncertainty in the absolute calibration of the martian crater flux results in uncertainties of 1 5 gyr in the middle period of martian geologic history aqueous alteration of primary igneous minerals produces secondary minerals in martian meteorites here we use the ages of secondary alteration minerals in martian meteorites to obtain absolute ages when liquid water was at or near the surface of mars aqueous alteration events in martian meteorites occurred at 3929 37 ma carbonates in alh84001 633 23 ma iddingsite in nakhlites and 0 170 ma salts in shergottites furthermore these events appear to be of short duration suggesting episodic rather than continuous aqueous alteration of the meteorites the martian meteorites appear to be contaminated by martian surface pb characterized by a 207pb 206pb ratio near 1 lead of this composition could be produced by water based alteration on the martian surface the high 129xe 132xe ratio in the martian atmosphere compared to martian meteorites indicates fractionation of i from xe within 100 myr after nucleosynthesis of 129i such fractionation is difficult to achieve through magmatic processes however water very efficiently fractionates i from xe raising the intriguing possibility that mars had a liquid water ocean within its first 100 myr 1

paired nakhlites mil 090030 090032 090136 and 03346 insights into the miller range parent meteorite abstract miller range mil 03346 is the most oxidized and least equilibrated nakhlite known and displays the highest amount of intercumulus phase the discovery of three new nakhlites mil 090030 mil 090032 and mil 090136 in the miller range antarctica geographically close to the location of mil 03346 suggests that they may come from the same parent meteorite in this study we investigate the mineralogy and texture of cumulus and intercumulus phases in situ major and trace element compositions for the cumulus phases as well as pyroxene crystal size distribution patterns and spatial distribution patterns of mil 090030 090032 and 090136 using these combined results we conclude that the three nakhlites studied here are paired with mil 03346 however modal mineral abundances of mil 090030 090032 090136 and 03346 exhibit variations indicating that a single sample of a miller range nakhlite is not modally representative of the parent meteorite and that analyses of multiple samples for a single nakhlite may be necessary to obtain representative modal data for placement in the cumulate pile our calculated parental melt composition based on all the paired samples confirms a previous study suggesting that the nakhlite parent melt crystallized as a closed system

acid sulfate weathering of synthetic martian basalt the acid fog model revisited the acid fog model has received considerable attention as a model of soil formation on mars previous evaluations of this model have focused on experimental weathering of terrestrial basalt samples however these samples differ significantly from what now is thought to be typical of martian basalt the acid fog model is tested here using synthetic basaltic analogs derived from mars pathfinder soil and rock compositions reaction of synthetic basalt with various acidic solutions and subsequent evaporation has led to the formation of several putative secondary mineral phases many of these phases were not produced in prior experimental studies aimed at aqueous interactions on mars of these alteration phases mg fe ca and al sulfates were identified in addition secondary ferric oxide phases formed via rapid fe oxidation under relatively high ph levels buffered by basalt dissolution amorphous silica is a ubiquitous product in these experiments and has formed by precipitation from solution and by the dissolution of minerals and glasses leaving behind leached surface layers composed of residual silica the secondary products formed in these experiments demonstrate the importance of primary mineralogy when testing models of aqueous interactions on mars new constraints are placed on both the reactivity of primary basalt and the secondary mineralogy present at the martian surface copyright 2004 by the american geophysical union

evidence for silicate dissolution on mars from the nakhla meteorite veins containing carbonates hydrous silicates and sulfates that occur within and between grains of augite and olivine in the nakhla meteorite are good evidence for the former presence of liquid water in the martian crust aqueous solutions gained access to grain interiors via narrow fractures and those fractures within olivine whose walls were oriented close to 001 were preferentially widened by etching along 001 this orientation selective dissolution may have been due to the presence within olivine of shock formed 001 100 and 001 110 screw dislocations the duration of etching is likely to have been brief possibly less than a year and the solutions responsible were sufficiently cool and reducing that laihunite did not form and fe liberated from the olivine was not immediately oxidized the pores within olivine were mineralized in sequence by siderite nanocrystalline smectite a fe mg phyllosilicate and then gypsum whereas only the smectite occurs within augite the nanocrystalline smectite was deposited as submicrometer thick layers on etched vein walls and solution compositions varied substantially between and sometimes during precipitation of each layer together with microcrystalline gypsum the fe mg phyllosilicate crystallized as water briefly returned to some of the veins following desiccation fracturing of the smectite these results show that etching of olivine enhanced the porosity and permeability of the nakhlite parent rock and that dissolution and secondary mineralization took place within the same near static aqueous system

chemical divides and evaporite assemblages on mars assemblages of evaporite minerals record detailed physical and chemical characteristics of ancient surficial environments accordingly newly discovered regions of saline minerals on mars are high priority targets for exploration the chemical divide concept of evaporite mineral formation is used successfully to predict evaporite mineralogy and brine evolution on earth however basaltic weathering largely controls fluid compositions on mars and the robust predictive capabilities of terrestrial chemical divides cannot be used to interpret martian evaporites here we present a new chemical divide system that predicts evaporite assemblages identified in snc type meteorites ancient evaporites discovered on meridiani planum by the opportunity rover and mars express omega data we suggest that a common fluid type that has been buffered to different ph levels by basaltic weathering controls the variability among martian evaporite assemblages and that evaporite mineralogy and brine evolution is essentially established by the initial composition of the dilute evaporating fluid b 2005 elsevier b v all rights reserved

trace element mobility during spheroidal weathering of basalts and andesites in hawaii and guatemala abstract trace element mobility has been studied in spheroidally weathered corestone shell complexes developed on late tertiary and quaternary volcanic rocks the parent materials are basalts and andesites related to 1 a subduction zone tecuamburro and moyuta volcanoes guatemala and 2 a hot spot hawaiian islands of oahu maui and hawai i the initial major element compositions are largely preserved in the corestones each corestone sample shows major element abundances characteristic of the different magmatic processes through which the lavas were generated however the changes in trace element compositions due to weathering show similar patterns in the corestone shell complexes independent of the tectonic setting and age element mobility within the different corestone shell complex was determined using an isovolumetric reference frame in this method bulk density is chosen as an independent measure of the extent of weathering based on volumetric concentrations the notion of element immobility cannot be applied to all the corestone shell complexes elements commonly assumed to be immobile e g zr ti and al vary in volumetric concentration for some corestone shell complex despite petrographic evidence of only minor weathering two corestones oahu and tecuamburro have become enriched in ree 3 relative to other lava flows from the same volcanoes and display negative ce anomalies the exfoliated shells from these complexes tend to have lower concentrations of ree 3 and have positive ce anomalies the fractionation of ce from the other rare earth elements ree is inferred to be a manifestation of changes in the valence state of ce from trivalent to tetravalent under oxidizing conditions in settings where the development of corestone shell complex is advanced an acidic solution can mobilize trivalent ree leach them out of more extensively weathered materials and transport them to less acidic portions of the regolith undergoing incipient weathering the precipitation of the ree 3 in the rinds and centers of the corestones are likely due to increases of the ph of the weathering solution systematic weathering induced ree behavior enrichment of most ree with lesser ce enrichment resulting in a negative ce anomaly can be discerned even in corestone samples that exhibit major element abundances and petrographic features otherwise similar to unweathered basalts and andesites

quantification of the dry history of the martian soil inferred from in situ microscopy 1 the particle size distribution psd of a martian soil sample a useful indicator of the underlying soil formation processes has been determined using optical and atomic force microscopy data acquired by the phoenix mars lander in particular the presence and fraction of clay sized particles in the psd reflects the extent of aqueous interaction with the soil two size populations have been identified for the martian sample larger mostly rounded grains and small reddish fines notably with a very low mass proportion in the clay size range below 2 m these fines reflect the smallest scale formation processes and indicate a single method of production for the particles up to 11 m a much larger value than that expected for the aqueous interaction of clay formation this suggests the fines are predominantly the product of global aeolian weathering under very dry conditions the proportion of clay sized soils can be used to estimate that there has been much less than 5 000 years exposure to liquid water over the history of the soil from the perspective of the psd lunar regolith rather than terrestrial soil is the best analog to martian soil a globally homogenous soil with such a psd would be an unlikely habitat for the propagation of life on mars

dissolution of olivine during natural weathering abstract naturally weathered olivine occurring as phenocrysts in hawai ian volcanic rocks from several volcanic centers and regolith outcrop settings and as tectonized olivines from several metadunite bodies in the southern appalachian blue ridge are all similarly corroded by natural weathering conical funnel shaped etch pits occur as individual pits base to base pairs of cone shaped pits or en echelon arrays etch pit shapes and orientations in the smallest etch pit arrays visible in conventional scanning electron microscopy resemble even smaller features previously reported from transmission electron microscope investigations of olivine weathering etch pits occur in samples with chemical and or mineralogical evidence of weathering and or are associated with or proximal or directly connected to fractures or exposed outcrop surface and therefore are formed by weathering and not inherited from pre weathering aqueous alteration e g serpentinization iddingsitization of these parent rocks many etch pits are devoid of weathering products natural weathering of olivine is surface reaction limited similarity of corrosion forms from naturally weathered olivine from multiple igneous and metamorphic parent rock bodies suggests that olivine weathers in the same manner regardless of its specific crystallization recrystallization history eruption weathering exposure ages of the olivine s host rock and the local regolith history

hydrogen generation from mantle source rocks in oman hydrogen gas associated with ca2 oh rich alkaline groundwaters ph 10 12 is at present emanating from ultramafic rocks of the oman ophiolite isotopic and chemical evidence indicates that hydrogen is formed by low temperature redox reactions in a closed groundwater environment this is normally a cryptic process in the hydrosphere but is fortuitously revealed by the unusual hydrogeological conditions in oman where atmospheric oxygen is totally assimilated it is suggested that hydrogen generation in mantle source rocks at depth and in the early archaean surface environment may be more widespread than has hitherto been realised

martian volatiles in shergottite eeta 79001 new evidence from oxidized sulfur and sulfur rich aluminosilicates abstract high vacuum pyrolysis of individual petrologically documented chips of lithology a pyroxenemaskelynite host rock and lithology c glass inclusion from the same interior portion of eeta 79001 produced sulfur in substantially different proportions of oxidized so2 and reduced h2s s2 species sulfur pyrolyzed from lith a possessed a bulk atomic ratio of s ox s red 0 21 whereas sulfur from lith c displayed a bulk ratio of 0 54 and included a component with a ratio of 1 83 that was released at 950 c most of the oxidized sulfur in lith c was probably dissolved or occluded in glass although some of the sulfate probably existed in sulfur and chlorine rich aluminosilicate mineraloids that occurred as trace quantities of 10 m sized grains parallel sem eds and pyrolysis analyses of control samples including weathered exterior chips of eeta 79001 and of eucrite eeta 79004 produced evidence against antarctic weathering as the source of oxidized sulfur and sulfur rich aluminosilicates in lith c oxidized sulfur evolved from control samples including lith a was accompanied by substantial amounts of h2o and co2 although virtually none of the latter species were found in lith c gypsum was abundant in the weathered eucrite and in weathered lith a but was not found in lith c because lith c contains trapped noble gases and nitrogen of martian atmospheric affinity a logical explanation for its oxidized sulfur would be rock atmosphere reactions that occurred on mars prior to or during the melting event that produced lith c bulk elemental compositions of two varieties of sulfur rich aluminosilicates in lith c can be arithmetically mixed to produce a composition that agrees remarkably well with that of surface fines at the viking landing sites suggesting that lith c might actually contain relict grains of martian weathering or alteration products

meteoric water basalt interactions ii a field study in n e iceland the compositions of rain snow melt spring and geothermal waters from the rift zone of n e iceland can be explained by seaspray addition chemical fractionation at the seawater air interface burning of fossil fuel farming activities purification by partial melting of snow and ice dissolution of basalts and buffering by alteration minerals the dissolution of the rocks appears to be incongruent during solute acquisition spring compositions move through the stability fields of kaolinite and smectite to the laumontite and illite fields all but four of the springs are undersaturated with respect to calcite silica concentrations are compatible with the solubility of basaltic glass the reactions reflected in the spring waters appear to have taken place sealed off from atmospheric co2 after initial saturation the geothermal waters which are recharged by the spring waters are depleted in mg and ca but enriched in carbon and sulfur with respect to dissolution of primary rocks expressions are derived relating dissolution rates of rocks age of groundwaters physical properties of groundwaters and mass transfer the characteristic rock particle radii in the cold water aquifers range from 0 2 to 2 cm and the characteristic crack openings are of the order 0 04 to 0 4 cm using laboratory studies on the icelandic lavas as a guide the residence times of the cold waters in the aquifers can be estimated at 60 days to 4 years the average active surface area of the aquifers enclosing 1000 g of spring water is of the order of 0 6 to 6 m2 and these 1000 g of water have reacted with 0 1 to 1 g of basaltic rocks the same mass of thermal water has interacted with 100 to 300 g of unaltered rocks

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

martian cratering 8 isochron refinement and the chronology of mars abstract this paper reviews and refines the technique of dating martian surfaces by using impact crater isochrons defined as size distributions of impact craters on undisturbed martian surfaces of specified ages in the 1970s this system identified not only abundant ancient martian volcanic surfaces but also extensive lava plains with ages of a few 108 y old this dating was initially controversial but confirmed in the 1980s and 90s by martian meteorites the present update utilizes updated estimates of the mars moon cratering ratio the most important calibration factor improves treatment of gravity and impact velocity scaling effects combines aspects of the crater size distribution data from earlier work by both neukum and hartmann and for the first time applies a correction for loss of small meteoroids in the martian atmosphere from popova et al 2003 meteorit planet sci 38 905 925 the updated isochrons are not radically different from the previous 2002 iteration but fit observed data better and give somewhat older model ages for features dated from small craters diameter d 100 m crater counts from young lava flows in various areas give good fits to the new isochrons over as much as 3 orders of magnitude in d confirming the general isochron shape and giving crater retention ages in the range of some 106 to some 108 y interpreted as lava flow ages more complex older units are also discussed uncertainties are greatest if only small craters d 100 m are used suggestions by other workers of gross uncertainties due to local secondary craters and deposition exhumation are discussed they do not refute our conclusions of significant volcanic fluvial and other geologic activity in the last few percent of martian geologic time or the importance of cratering as a tool for studying processes such as exhumation indeed crater count data suggest certain very recent episodes of deposition exhumation and ice flow possibly associated with obliquity cycles of 10 7 y timescale evidence from ancient surfaces suggests higher rates of volcanism fluvial activity glaciation and other processes in noachian hesperian time than in amazonian time

comparisons of the four miller range nakhlites mil 03346 090030 090032 and 090136 textural and compositional observations of primary and secondary mineral assemblages abstract petrological and geochemical analyses of miller range mil 03346 indicate that this meteorite originated from the same augitic cumulate layer s as the nakhlite martian meteorites but underwent rapid cooling prior to complete crystallization as with the other nakhlites mil 03346 contains a secondary alteration assemblage in this case consisting of iddingsite like alteration veins in olivine phenocrysts fe oxide alteration veins associated with the mesostasis and ca and k fe sulfate veins we compared the textural and mineralogical compositions of mil 090030 090032 and 090136 with mil 03346 focusing on the composition and raman spectra of the alteration assemblages these observations indicate that the meteorites are paired and that the preterrestrial olivine bound alteration assemblages were produced by weakly acidic brine although these alteration assemblages resemble similar assemblages in nakhla the absence of siderite and halite in the miller range nakhlites indicates that the parental alteration brine was comparatively hco3 depleted and less concentrated than that which altered nakhla this indicates that the miller range nakhlite alteration brine experienced a separate evolutionary pathway to that which altered nakhla and therefore represents a separate branch of the lafayette nakhla evaporation sequence thin sections cut from the internal portions of these meteorites away from any fusion crust or terrestrially exposed edge contain little ca sulfate identified as gypsum and no jarosite whereas thin sections with terrestrially exposed edges have much higher sulfate abundances these observations suggest that at least the majority of sulfate within the miller range nakhlites is terrestrially derived

a 3 5 ga record of water limited acidic weathering conditions on mars abstract the secondary mineral budget on earth is dominated by clay minerals al hydroxides and fe oxides which are formed under the moderate ph high water to rock ratio conditions typical of earth s near surface environment in contrast geochemical analyses of rocks and soils from landed missions to mars indicate that secondary mineralogy is dominated by mg fe ca sulfates and fe oxides this discrepancy can be explained as resulting from differences in the chemical weathering environment of earth and mars we suggest that chemical weathering processes on mars are dominated by 1 a low ph sulfuric acid rich environment in which the stoichiometric dissolution of labile mineral phases such as olivine and apatite fe ti oxides is promoted and 2 relatively low water to rock ratio such that other silicate phases with slower dissolution rates e g plagioclase pyroxene do not contribute substantially to the secondary mineral budget at the martian surface under these conditions al mobilization is limited and the formation of significant al bearing secondary phases e g clays al hydroxides al sulfates is inhibited the antiquity of rock samples analyzed in situ on mars suggest that water limited acidic weathering conditions have more than likely been the defining characteristic of the martian aqueous environment for billions of years

the legacy of crystal plastic deformation in olivine high diffusivity pathways during serpentinization crystal plastic olivine deformation to produce subgrain boundaries composed of edge dislocations is an inevitable consequence of asthenospheric mantle flow although crystal plastic deformation and serpentinization are spatio temporally decoupled we identified compositional readjustments expressed on the micrometric level as a striped fe enriched bar x text fe 0 24 0 02 zones 0 12 0 02 bulk or fe depleted bar x text fe 0 10 0 01 zones 0 13 0 01 bulk zoning in partly serpentinized olivine grains from two upper mantle sections in norway focused ion beam sample preparation combined with transmission electron microscopy tem and aberration corrected scanning tem enabling atomic level resolved electron energy loss spectroscopic line profiling reveals that every zone is immediately associated with a subgrain boundary we infer that the zonings are a result of the environmental fe2 mg 1 exchange potential during antigorite serpentinization of olivine and the drive toward element exchange equilibrium this is facilitated by enhanced solid state diffusion along subgrain boundaries in a system which otherwise re equilibrates via dissolution reprecipitation fe enrichment or depletion is controlled by the silica activity imposed on the system by the local olivine orthopyroxene mass ratio temperature and the effect of magnetite stability the fe mg exchange coefficients k text d text atg text ol between both types of zoning and antigorite display coalescence toward exchange equilibrium with both types of zoning mn is enriched and ni depleted compared with the unaffected bulk composition nanometer sized heterogeneously distributed antigorite precipitates along olivine subgrain boundaries suggest that water was able to ingress along them crystallographic orientation relationships gained via electron backscatter diffraction between olivine grain domains and different serpentine vein generations support the hypothesis that serpentinization was initiated along olivine subgrain boundaries

serpentine mineral replacements of natural olivine and their seismic implications oceanic lizardite versus subduction related antigorite we report on microstructural data obtained by optical microscopy and transmission electron microscopy concerning the crystallographic relationships of serpentine minerals with their host olivine in two contrasting situations in the first case mesh textured lizardite liz is developed in a standard 60 serpentinized oceanic harzburgite from the oman ophiolite where olivine converts to columnar lizardite the joined columns are perpendicular to the basal plane 001 liz corresponding to the pseudofibres observed optically the plane 001 liz is locally parallel to the narrow boundary ol liz thus column orientations register the interface of serpentinization the ol liz relationships are not strictly topotactic but reflect preferred cracking orientations in olivine parallel to 010 ol in the second case antigorite atg develops in a rare sample of antigorite schist in a kimberlite from moses rock colorado plateau representative of a suprasubduction zone mantle wedge high resolution transmission electron microscopy hrtem images along 010 atg show domains of very regular modulation with a 43 center dot 5 a wavelength m 17 where m is the number of silicate tetrahedra along the wave with few defects indicative of hp ht antigorite and also heavily kinked regions as fingerprints of strong tectonic shear tem imaging and electron diffraction patterns reveal two topotactic relationships between antigorite and olivine 100 atg 010 ol and atg ol the planes in contact are 001 atg 100 ol and 001 atg 010 ol respectively the 010 atg 001 ol and antigorite lamellae are parallel to the forsterite b axis in both cases the topography of olivine serpentine interfaces is controlled by open fluid pathways along microcracks oriented according to the anisotropy of the olivine aggregate in the cases studied the serpentine aggregate exhibits a preferred orientation inherited from that of the peridotite these results have some relevance to the seismic anisotropy of serpentinized mantle anisotropy of propagation of seismic waves as a result of the olivine fabric is maintained and reinforced with the development of lizardite conversely the development of antigorite produces a trench parallel fast s wave polarization and an anisotropy that is lowered at low degrees of serpentinization and then increased with increasing serpentinization

using a mineral lifetime diagram to evaluate the persistence of olivine on mars we present a diagram that shows the effect of ph temperature grain size composition hydrodynamics and the laboratory field discrepancy on the lifetime of olivine grains in weathering environments because the persistence of olivine grains on mars can be used to constrain the duration of liquid water we can use this diagram to predict a range of possible maximum contact times for olivine grains with liquid water before they dissolve away completely depending upon the physicochemical conditions this contact time could range between a few thousand and several million years

hydrogen isotope analyses of alteration phases in the nakhlite martian meteorites abstract secondary alteration phases such as carbonates smectite clays and fe oxides are found within the martian meteorites if these meteorites were seen to fall the assumption can be reasonably made that the secondary phases have a martian origin however for martian meteorite finds this is not the case deuterium hydrogen d h ratio analysis can be used to distinguish between terrestrial and martian secondary alteration phases the martian atmosphere is currently five times enriched in deuterium compared to terrestrial seawater producing a deuterium enrichment in the martian alteration phases large enough to be detected with modern ion microprobe techniques we aimed to measure the d h ratio of the iddingsite like alteration veins in the nakhlite martian meteorites nakhla mil 03346 mil 090030 and y 000593 to help confirm the martian origin of this material and determine the relative amounts of terrestrial contamination in each meteorite as an observed fall nakhla is nominally uncontaminated by terrestrial alteration and shows elevated d values in its alteration veins however even the highest of these values is much below that of the martian atmosphere indicating that terrestrial contamination probably combined with fractionation effects between the martian atmosphere and the alteration material have reduced the d values of these veins the speed of hydrogen isotope exchange in the nakhlite iddingsite like alteration veins is demonstrated by the purely terrestrial d values in the veins of nakhla 110 a thin section exposed to the terrestrial atmosphere since its preparation in 1998 mineralogical heterogeneity also affects the hydrogen isotope exchange rate in these veins the wide variation in d within the veins of nakhla and y 000593 appears to be due to the heterogeneous nature of the vein material where some phases exchange hydrogen with the atmosphere at a faster rate than others

temperature dependence of silicate weathering in nature how strong a negative feedback on long term accumulation of atmospheric co2 and global greenhouse warming estimation of the temperature dependence of natural feldspar weathering in two catchments at different elevations yields an apparent arrhenius activation energy of 18 4 kcal mol 77 0 kj mol much higher than most laboratory values this finding supports recent suggestions that hydrolytic weathering of silicate minerals may consume carbonic acid and thereby remove atmospheric carbon dioxide more rapidly with increasing temperature than previously thought this result provides a stronger negative feedback on long term greenhouse warming than has been assumed in most models of global carbon cycling the present estimate was determined from the ratio of feldspar weathering rates determined by geochemical mass balance in the southern blue ridge mountains of north carolina united states temperature a function of elevation is the only factor that differs between the two catchments parent rock type aspect hillslope hydrology and vegetation type and successional stage are the same in both

crustal thickness variations along the southeast indian ridge 100 116 e from 2 d body wave tomography 1 axial morphology along the southeast indian ridge seir systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100 116 e west of the australian antarctic discordance aad basalt geochemistry has a consistent indian mid ocean ridge basalt morb type isotopic signature so changes in axial topography are attributed to variations in both mantle temperature and melt supply wide angle seismic refraction lines were shot to four ocean bottom hydrophones within seir segments p1 p2 s1 and t where each segment is characterized by a different morphology we constructed 2 d crustal velocity models by jointly inverting hand picked p wave refraction pg and moho reflection pmp traveltime data using a top down minimum structure methodology the results show a 1 5 km eastward decrease in crustal thickness across the study area with segment averages ranging from 6 1 km at p1 to 4 6 km at t melt generation models require a 30 c decrease in mantle temperature toward the aad to account for the crustal thickness trend significant changes in axial morphology accompany small scale variations in crustal thickness consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature melt supply and cooling from hydrothermal circulation layer 3 thins by 3 0 km as layer 2 thickens by 1 4 km between segments p1 and t reflecting the eastward decrease in melt supply and increase in melt lens depth the trade off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material

a halite siderite anhydrite chlorapatite assemblage in nakhla mineralogical evidence for evaporites on mars we report the results of a study of a halite siderite anhydrite chlorapatite assemblage in the shergotty nakhla chassigny snc martian meteorite nakhla these minerals are found associated with each other in interstitial areas with halite often being adjacent to or enclosing siderite we suggest the halite and other minerals are martian in origin because 1 the conditions of fall preclude significant amounts of terrestrial contamination or weathering having taken place 2 textures indicate that the minerals within this assemblage crystallized at the same stage as some silicate and oxide minerals within the nakhla parent and 3 the association with siderite which previous studies have shown has c and o isotopic compositions incompatible with an origin on the earth siderite has the range of compositions caco3 0 1 5 7 mgco3 2 0 40 9 feco3 23 2 87 0 mnco3 1 0 39 9 mol there are two compositional groupings high mnco3 30 mol and low mnco3 high feco3 with a gap identified between the two this may be a miscibility gap or alternatively the two compositional groupings may mark separate generations of carbonate we have not found any textural evidence for the latter explanation but acceptance of the presence of a miscibility gap would require independent work on fe mn carbonates to verify its existence trace element abundances have been determined by ion microprobe analysis on three siderite and one anhydrite grains siderite has light rare earth elements 2 2 7 3 c1 greater than heavy rare earth elements 0 32 0 79 c1 without ce or eu anomalies and the anhydrite has a similar pattern these abundances reflect the source composition rather than partitioning or complexing controls they are not typical of hydrothermal signatures which generally do not have such smooth ree abundance patterns the nature of the mineral assemblage suggests that its source rocks on mars were evaporites these may be common in the craters and flood plains of the martian southern highlands two models are suggested in this paper to explain the incorporation of evaporitic material into the nakhla igneous parent it may have happened as a low temperature process 200 c by crystallisation from an aqueous fluid an origin at low temperature is compatible with the available experimental data on siderite stability alternatively we suggest evaporitic material may have been incorporated into the nakhla parent while melt was still present and crystallized 800 c the latter model can more readily explain the trace element abundances and also the siderite textures that imply intergrowth with residual interstitial melt both high and low temperature models are consistent with the presence of evaporite sediments on mars