silicification and dolomitization of anhydrite nodules in argillaceous terrestrial deposits an example of meteoric dominated diagenesis from the triassic of central spain cauliflower shaped nodules are widespread in a single red mudstone bed in the buntsandstein triassic facies of the iberian range they consist mostly of quartz dolomite and calcite but other minerals such as barite kaolinite and iron oxyhydroxides are also present the nodules are spherical ovoid or elongate in shape and range from 1 to 8 cm across the surface of the nodules is irregular and some show a pedogenic coating of microspar the sedimentological and petrographic data suggest that the initial anhydrite nodules formed through a progressive increase in the porewater concentration of ca2 and so4 2 in a vadose environment occasionally under the influence of pedogenic processes partial replacement of the anhydrite by megaquartz occurred under more dilute conditions in the same type of setting as indicated by the presence of organic filaments on the quartz crystal surfaces in type a nodules the dissolution of the innermost anhydrite was complete and different types of quartz cement filled the porosity fracturing and meteoric cementation by calcite and minor amounts of kaolinite were the latest processes affecting these nodules in type b nodules the dissolution of the anhydrite was incomplete inhibiting quartz cementation and enabling later dolomitization of the anhydrite dolomitization appears to have been driven by sulphate reduction as indicated by the presence of bacterial bodies within the dolomite crystals dedolomitization and precipitation of barite kaolinite and calcite spar cements occurred later under the influence of meteoric solutions the nodules may mark the former locations of the water table and provide evidence for an episode of highly evaporitic conditions throughout wide areas of the basin their occurrence reveals not only a complex diagenetic history but is also indicative of palaeogeographic and palaeoclimatic conditions

the terrestrial alteration of saharan shergottites dar al gani 476 and 489 a case study of weathering in a hot desert environment abstract rare earth element ree and other selected trace and minor element concentrations were determined in various phases of dar al gani 476 and 489 two basaltic shergottites recently found in the libyan sahara whereas merrillite and feldspathic glass compositions do not seem to have been altered since the fall of these meteorites olivine and pyroxenes commonly show light ree enrichments sometimes accompanied by ce anomalies that are attributed to terrestrial alteration sr and ba enrichments are also observed the calcium carbonate that resides in most cracks is essentially devoid of ree but a fine grained amorphous silicate material that fills some veins and pockets has striking light ree enrichments and elevated p sr and ba it is argued that this material likely to be impact melt as well as the notoriously light ree poor olivine and pyroxenes were preferentially affected by a crustal light ree rich contaminant caution is urged when using whole rock or even mineral separate compositional and isotopic data for hot desert meteorites

magnesium isotopic composition of the deep continental crust to constrain the behavior of mg isotopes during deep crustal processes and the mg isotopic composition of the middle and lower continental crust 30 composite samples from high grade metamorphic terranes and 18 granulite xenoliths were investigated the composites derive from eight different high grade metamorphic terranes in the two largest archean cratons of china including 13 ttg gneisses 5 amphibolites 4 felsic 4 intermediate and 4 mafic granulites they have variable bulk compositions with sio 2 ranging from 45 7 to 72 5 representative of the middle crust beneath eastern china the 26 mg values of these samples vary from 0 40 to 0 12 reflecting heterogeneity of their protoliths which could involve upper crustal sediments the granulite xenoliths from the cenozoic hannuoba basalts also have a diversity of compositions with mgo ranging from 2 95 to 20 2 these xenoliths equilibrated under high temperatures of 800 950 c corresponding to depths of the lower continental crust 30 km they yield a large 26 mg variation of 0 76 to 0 24 the light mg isotopic compositions likely result from interactions with isotopically light metamorphic fluids probably carbonate fluids together with previously reported data the average 26 mg values of the middle and lower continental crusts are estimated to be 0 21 0 07 and 0 26 0 06 respectively the bulk continental crust is estimated to have an average 26 mg of 0 24 0 07 which is similar to the average of the mantle the large mg isotopic variation in the continental crust reflects the combination of several processes such as continental weathering involvement of supracrustal materials in the deep crust and fluid metasomatism

secular change in the precambrian silica cycle insights from chert petrology chert deposits preserve a record of secular change in the oceanic silica cycle the evolutionary radiation of silica secreting organisms resulted in a transition from abiological silica deposition characteristic of the archean and proterozoic eons to the predominantly biologically controlled silica deposition of the phanerozoic comparative petrography of phanerozoic and precambrian cherts indicates that an earlier change in chert deposition occurred toward the end of the paleoproterozoic era ca 1 8 ga in neoproterozoic and mesoproterozoic strata early diagenetic chertification is largely restricted to peritidal environments these early diagenetic cherts typically occur as nodules or discontinuous beds within carbonate deposits that have similar depositional textures the cherts formed primarily by carbonate replacement with subsidiary direct silica precipitation some of the paleoproterozoic cherts associated with iron formations however are distinctly different from younger cherts and appear to have formed largely by direct silica precipitation at or just below the seabed these primary cherts lack ghosts or inclusions of carbonate precursors have fine scale grain fracturing possibly from syneresis exhibit low grain packing densities and are not associated with unsilicified carbonate deposits of similar depositional composition cherts in some paleoproterozoic iron formations e g the gunflint formation northwestern lake superior region are composed of silica types similar to those in phanerozoic sinters e g the devonian rhynie and windyfield cherts scotland such cherts may provide evidence that basinal and perhaps global oceanic silica concentrations were higher during the paleoproterozoic era than at later times

interplay between depositional facies diagenesis and early fractures in the early cretaceous habshan formation jebel madar oman abstract diagenesis and fracturing can significantly alter petrophysical properties of subsurface carbonate reservoirs but the impacts of these processes at the inter well scale are hard to predict however the initial distribution of sedimentary facies is easier to predict and could template future diagenetic transformation and potentially influence fracturing as well this paper provides a case study investigating the mutual influence of depositional facies on diagenesis and interpreted early fractures in the berriasian valanginian habshan formation in jebel madar oman the habshan formation mostly comprises ooidal limestones but dolomitized beds a few metres in thickness and laterally correlatable over at least 5 km are present discordant vertical dolomite streaks are present in the limestone but are rare the various sedimentary and diagenetic processes were investigated using fieldwork xrd analysis petrography and geochemistry major and trace elements stable carbon and oxygen isotopes and strontium isotopes the lithologies consist of mud lean ooidal grainstones mud rich ooidal packstones and bioturbated limestones the changes in the amount of fine grained sediment suggest changing energy conditions at a backshoal position of a carbonate platform the mud lean versus mud rich depositional textures impacted on early diagenesis by promoting abundant calcite cementation in the mud lean ooidal grainstones and dolomitization in the mud rich packstones the discordant bodies interpreted as sediment filled early fractures are dolomitized in the same way as the mud rich ooidal packstones dolomitization occurred probably during the valanginian hauterivian in a shallow burial setting and was most likely caused by seepage reflux salt doming post dated dolomitization of the habshan formation the results of this study document the inter relationship between sedimentary facies early diagenesis and early fractures

magnesium isotopic composition of the moon abstract the mg isotopic compositions of 47 well characterized lunar samples including mare basalts highland rocks regolith breccias and mare and highland soils were measured to address the behavior of mg isotopes during lunar magmatic differentiation constrain the mg isotopic composition of the moon and evaluate the degree of mg isotopic fractionation between planetary bodies the 26 mg values range from 0 61 0 03 to 0 02 0 06 in 22 mare basalts from 0 34 0 04 to 0 18 0 06 in 3 highland rocks from 0 33 0 05 to 0 14 0 08 in 7 regolith breccias from 0 23 0 05 to 0 14 0 07 in 6 highland soils and from 0 41 0 05 to 0 20 0 09 in 9 mare soils the limited mg isotopic variation among bulk mare and highland soils and regolith breccias indicates negligible mg isotope fractionation by lunar surface processes by contrast the large mg isotopic fractionation between low ti and high ti basalts suggests the source heterogeneity produced during fractional crystallization of the lunar magma ocean with ilmenite having lighter mg isotopic compositions than olivine and pyroxene overall the moon has a weighted average mg isotopic composition 26 mg 0 26 0 16 indistinguishable from the earth 26 mg 0 25 0 07 and chondrites 26 mg 0 28 0 06 suggesting homogeneous mg isotopic distribution in the solar system and the lack of mg isotope fractionation during the moon forming giant impact

chert appearance in the cueva bedon carbonate platform upper cretaceous northern spain nodular chert from the cueva bedon carbonate platform late turonian earliest coniacian occurs above the type 1 sequence boundary and in association with a transgressive systems tract deposit it formed by selective replacement of grainstones packstones and thalassinoides burrow fillings other minor types of selective silicification include the replacement of anhydrite nodules forming quartz geodes and the partial or total replacement of oysters and inoceramids the chert nodules consist predominantly of microquartz with volumetrically minor fibrous quartz length fast chalcedony and megaquartz the silica has a mainly biogenic source siliceous sponge spicules and radiolarians lepisphere textures that are indicative of an intermediate opal ct precursor are evident in chert nodules which indicates the chert formed by a maturation process from opala to opal ct to quartz the 18 o values of microquartz suggest a precipitation from marine fluids the fabric of quartz geodes consists of irregular bands of heterometric megaquartz quartzine and lutecite spherulites and a well developed band of petaloidal megaquartz that contains small anhydrite inclusions bivalve shells are replaced mainly by fibrousquartz quartzine lutecite which forms extensive beekite rings the silica source of the replaced fossils is also biogenic but silicification couldhave occurred by direct quartz replacement without an opal ct precursor field and petrographic evidence indicate that silicification was an early diagenetic process that affected both uncompacted and compacted sediments andwas inhibited when the dolomitization began there seems to exist a syncrony in age late turonian earliest coniacian between the nodular chert in the cueva bedon carbonate platform and the bedded and nodular chert ofthe plentzia carbonate turbidites in the deeper basque arc

raman microspectroscopy bitumen reflectance and illite crystallinity scale comparison of different geothermometry methods on fossiliferous proterozoic sedimentary basins dr congo mauritania and australia abstract sedimentary rocks containing microfossils are crucial archives to reconstitute early life evolution on earth however the preservation of microfossils within rocks depends on several physico chemical factors among these factors the thermal evolution of the host rocks can be decisive here we investigated carbonaceous shale samples containing exquisitely preserved organic walled microfossils assemblages from three proterozoic shallow marine sedimentary sequences the mbuji mayi supergroup democratic republic of congo congo basin the atar el mreiti group mauritania taoudeni basin and the kanpa formation australia officer basin thermal maturity of these rock samples is evaluated with raman geothermometry raman reflectance solid bitumen reflectance illite crystallinity and thermal alteration index the comparison of results coming from these different techniques validates the use of raman reflectance on proterozoic carbonaceous material and especially for poorly ordered carbonaceous material we show that extracted kerogen microfossils and amorphous organic material is more accurate to estimate the thermal maturity of low grade temperature proterozoic sequences than kerogen in thin section all techniques provide consistent range of temperatures except for raman geothermometry giving slightly higher estimates raman reflectance appears to be a fast robust and non destructive tool to evaluate the thermal maturity of poorly organized carbonaceous material from proterozoic rocks

unusual sedimentary geodes filled by gold bearing hematite laths unusual mineralized and silicified carbonate rich geodes were found hosted in lower triassic red mudstones in central spain from their borders to their centres the geodes display a a millimetric carbonate crust b a quartz rim c massive dolomite d gold bearing iron rich infills hematite laths and goethite with up to 7000 ppm of gold and e calcite cements that sometimes seal the central hollow textural evidence indicates that the geodes were originally anhydrite which was subsequently replaced by quartz and dolomite the resultant porosity from this replacement or by later dissolution has been filled by epithermal gold bearing iron oxide hydroxides romanechite and calcite 13c values indicate the participation of meteoric waters in an environment which was characterized by both a sub desert climate and a temperate tropical climate oxygen signatures reflect very variable temperatures for all minerals with the exception of calcite which appears to have precipitated at 38 c iron oxide temperature values can reach up to 85 c epithermal stage the mineralogical assemblages of these unusual geodes denote early diagenetic replacement followed by the epithermal activity this mineralization is linked to the late hercynian calc alkaline volcanism of central spain the hiendelaencina mining district

a paleoproterozoic drowned carbonate platform on the southeastern margin of the wyoming craton a record of the kenorland breakup abstract the nash fork formation in the upper part of the early paleoproterozoic snowy pass supergroup medicine bow mountains of wyoming was deposited on a mature passive margin along the southern flank of the wyoming craton and straddles the end of the ca 2 2 2 1 ga carbon isotope excursion two drowning events marked by black shales subdivide the carbonate platform into three parts the lower nash fork formation consists of outer shelf to supratidal deposits represented by massive and stromatolitic dolomites heterolithic siliciclastics carbonates large silicified domal digitate stromatolites nodular dolomites and stromatolitic dolomites molds after evaporite crystals are pervasive in the heterolithic siliciclastics carbonates large silicified domal digitate stromatolites formed biostromes and bioherms following flooding events the middle nash fork formation comprises two intervals of black shale separated by inner shelf heterolithic siliciclastics nodular carbonates black shales are organic and pyrite rich contain turbidites and developed in response to drowning of the platform overlying massive dolomite of the upper nash fork formation was deposited in an unprotected intertidal setting and displays an upward shallowing trend terminated by a prominent karstic surface in the middle of the unit the nash fork formation is open marine with no evidence for restricted circulation on the carbonate platform the two drowning events on the carbonate platform are likely related to dissection of the mature passive margin associated with the breakup of kenorland the younger drowning event is associated with the end of the carbon isotope excursion the main building elements of the lower and middle nash fork carbonate platform are dolomitic mudstones and stromatolites macroscopic seafloor precipitates are volumetrically negligible with the exception of tufa deposits and domes in the massive and stromatolitic dolomites and possibly digitate stromatolites within domal digitate stromatolites the upper nash fork formation comprises dolomitic mudstones relatively rare stromatolites and inorganic precipitates that are more common than in the underlying carbonates styles of carbonate deposition on this early paleoproterozoic platform differ from those documented on late archean carbonate platforms there are fewer macroscopic seafloor precipitates and more dolomitic mudstones this pattern is considered to be related to a rise of the atmospheric oxygen level that led to a decrease in bicarbonate saturation in the ocean

host rock dolomitization and secondary porosity development in the upper devonian cairn formation of the fairholme carbonate complex south west alberta canadian rockies diagenesis and geochemical modelling the upper devonian carbonate reefs in west central alberta are important petroleum reservoirs that are well known for their extensive secondary porosity an outcrop analogue study indicates that an early matrix selective dolomitization event occurred which is characterized by a major late devonian sea water component with increased salinity because of evaporation it is interpreted that the matrix replacive dolomite formed during the famennian as the result of a combination of both seepage and latent reflux dolomitization although an additional type or overprinting of later immediate burial dolomitization cannot be excluded formation of the moulds is attributed mainly to the dissolution of undolomitized fossil cores most typically stromatoporoids geochemical modelling indicates that carboxylic acid fluids have the highest potential for dissolving residual calcite in this case geochemical models consistent with this analysis and interpretations can reproduce the secondary porosite and suggest a viable dolomitization process for the localities studied

dolomite calcitization and cement zonation related to uplift of the raisby formation zechstein carbonate northeast england abstract the raisby formation is the basal zechstein carbonate of northeast england it has undergone a complex diagenetic history during burial including multiphase dolomitization and replacement by sulphates upon uplift meteoric derived fluids have dissolved this replacive sulphate and precipitated calcite cements in the resultant porosity study of these cements by cathodoluminescence and trace element geochemistry shows that they are zoned and different cathodoluminescence zones correlate well with iron and manganese geochemical changes demonstrating a gradual increase in oxygenation of the precipitating fluids through time major diagenetic processes during cement growth include precipitation of anhydrite fluorite and barite calcitization of dolomite and internal sedimentation relating these processes to cement zonation allows this complex diagenesis to be explained in terms of uplift under the influence of meteoric derived groundwaters of constantly changing geochemical and electrochemical characteristics

mechanism and controls of silicification of fossils in limestones silicification of calcitic fossils occurs along thin solution films at which calcite dissolves and silica precipitates the mechanism is replacement controlled by force of crystallization whereby the growth of the silica phase exerts pressure across the silica carbonate contact increasing the gibbs free energy and hence solubility of shell calcite physical evidence for force of crystallization controlled replacement includes restriction of carbonate dissolution to the silica carbonate contact and euhedral quartz terminations at some quartz calcite contacts correlations among some replacement quartz types and fossil taxa indicate that shell microstructure and bulk pore water silica concentration control replacement quartz type

sedimentation in an actively tilting half graben sedimentology and stratigraphy of late tournaisian visean mississippian lower carboniferous carbonate rocks in south county wexford ireland the wexford basin south eastern ireland is a ne sw trending sedimentary basin containing carbonates and evaporites deposited during the late tournaisian and visean two separate depositional areas are defined on the basis of facies and facies associations sediments were deposited in inner ramp lagoonal and peritidal environments near rosslare and in a more open marine shallow to moderately deep water mid to outer ramp environment in the western area around duncormick thick breccia deposits that occur in the wexford basin formed as a result of i fault movement that produced syn sedimentary debris flows in the late chadian breccia type i ii dissolution of anhydrite gypsum and subsequent collapse of sedimentary strata breccia type ii and iii fracturing and brecciation of porous rock caused by the movement of high temperature late diagenetic fluids along fault planes breccia type iii the ne sw facies polarity displayed by both sedimentary successions was the result of nw se extension and the reactivation of the ne sw trending wexford boundary fault during the chadian extension at the se margin of the basin with downthrow to the nnw gave the basin a half graben character thickening of the debris flow deposits to the sw suggests that while the half graben was being tilted it also underwent a ne sw block rotation due to an axial component of that normal fault

experimental petrology crystallization history and parental magma characteristics of olivine phyric shergottite nwa 1068 implications for the petrogenesis of enriched olivine phyric shergottites abstract northwest africa nwa 1068 is one of the few olivine phyric shergottites e g nwa 1068 larkman nunatak lar 06319 and roberts massif rbt 04262 that is not depleted in light rare earth elements lree its ree pattern is similar to that of the basaltic shergottite shergotty suggesting a possible connection between the olivine phyric and the basaltic shergottites to test this possible link we have investigated the high pressure near liquidus phase equilibria for the nwa 1068 meteorite bulk composition our results show that the nwa 1068 bulk composition does not represent an unmodified mantle derived melt the olivine and pyroxene in our near liquidus experiments are more magnesian than in the rock itself which suggests that nwa 1068 contains cumulate minerals extra olivine we have then used these experimental results combined with the pyroxene compositions in nwa 1068 to constrain the possible high pressure crystallization history of the parental magma these results suggest that nwa 1068 had a complex polybaric history finally we have calculated a model parental magma composition for the nwa 1068 meteorite the calculated parental magma is an evolved basaltic composition which is too ferroan to be a primitive melt directly derived from the mantle we suggest that it ponded and crystallized at approximately the base of the crust this provided an opportunity for the magma to become contaminated by an enriched crustal component prior to crystallization the results and modeling from these experiments are applicable not only to the nwa 1068 meteorite but also to lar 06319 and possibly any other enriched olivine phyric shergottite

investigating the behaviour of mg isotopes during the formation of clay minerals abstract we present elemental and isotopic data detailing how the mg isotope system behaves in natural and experimentally synthesized clay minerals we show that the bulk mg isotopic composition 26 mg of a set of natural illite montmorillonite and kaolinite spans a 2 range and that their isotopic composition depends strongly on a balance between the relative proportions of structural and exchangeable mg after acid leaching these natural clays become relatively enriched in isotopically heavy mg by between 0 2 and 1 6 results of exchange experiments indicate that the mg that has adsorbed to interlayer spaces and surface charged sites is relatively enriched in isotopically light mg compared to the residual clay the isotopic composition of this exchangeable mg 1 49 to 2 03 is characteristic of the isotopic composition of mg found in many natural waters further experiments with an isotopically characterized mgcl 2 solution shows that the clay minerals adsorb this exchangeable mg with little or no isotopic fractionation although we cannot discount the possibility that the uptake of exchangeable mg does so with a slight preference for 24 mg to characterize the behaviour of mg isotopes during clay mineral formation we synthesized brucite mg oh 2 which we consider to be a good analogue for the incorporation of mg into the octahedral sheet of mg rich clay minerals or into the brucitic layer of clays such as chlorite in our experiment the brucite mineral becomes enriched in the heavy isotopes of mg while the corresponding solution is always relatively enriched in isotopically light mg the system reaches a steady state after 10 days with a final fractionation factor solid solution of 1 0005 at near neutral ph this result is consistent with the general consensus that secondary clay minerals preferentially take up isotopically heavy mg during their formation however our results also show that exchangeable mg is an important component within bulk clay minerals and can have an important influence over the bulk clay 26 mg value modeling shows that in certain soils or sediments where the percentage of exchangeable mg is 30 and the isotopic composition of the exchangeable mg is around 2 the generation of bulk 26 mg values of 26 mg values that are closer in composition to the ucc despite this results of modeling together with experimental observation suggests that the uptake of exchangeable mg into these clay rich sediments could cause a decrease in the bulk 26 mg value by up to 0 3 0 4 this should be accounted for when assessing the 26 mg value of sediments on a crustal scale

high temperature inter mineral magnesium isotope fractionation in eclogite from the dabie orogen china abstract to investigate the magnitude and mechanism of inter mineral mg isotope fractionation at high temperatures we report high precision analyses of mg isotopes for 10 whole rocks and 13 mineral separates for a set of eclogites from bixiling in the dabie orogen china magnesium isotopic compositions of whole rocks 26mg of 0 44 to 0 26 are similar to the estimated 26mg values of the mantle suggesting mg isotopic inheritance from a gabbroic protolith with limited mg isotope fractionation during eclogite facies metamorphism by contrast mineral separates are highly heterogeneous with 26mg values ranging from 0 30 to 0 60 in phengite from 0 16 to 0 40 in omphacite and from 0 95 to 0 74 in garnet phengite and omphacite are 1 heavier in 26mg than coexisting garnet indicating large high temperature inter mineral mg isotope fractionations the constant 26mgomphacite garnet 26mgomphacite 26mggarnet value 1 14 0 04 together with homogeneous mineral chemistry and equilibrium oxygen isotopic partitioning between omphacite and garnet suggests an equilibrium mg isotope fractionation controlled by the difference in coordination number of mg between omphacite six and garnet eight the 1 14 fractionation is the largest high temperature equilibrium inter mineral mg isotope fractionation observed so far and makes the omphacite garnet mg isotope fractionation a potential geothermometer by contrast mg isotope fractionations between phengite and garnet and between phengite and omphacite vary from 1 25 to 1 47 and from 0 14 to 0 32 respectively this implies mg isotopic disequilibria between phengite and garnet omphacite which might result from the mg isotopic variation in phengites due to mg isotope exchange between phengites and retrograde fluids in bixiling eclogites

quartz geodes early diagenetic silicified anhydrite nodules related to dolomitization abstract quartz geodes interpreted to be silicified anhydrite nodules are common in the mississippian ramp creek formation and harrodsburg limestone sanders group of southern indiana the abundance of a stenohaline fauna in the sanders group and the absence of sedimentologic features indicative of supratidal or hypersaline conditions such as stromatolites desiccation crocks and birdseye vugs suggest a subtidal normal marine depositional environment geodes in the ramp creek and harrodsburg formations as well as other geode bearing strata are almost invariably associated with dolomite seepage reflux of magnesium rich brines produced during the deposition of the lower st louis limestone is a likely mechanism for dolomitization of the sanders group replacement of calcite an aragonite by dolomite in hypersaline pore water may have increased the activity of calcium so that supersaturation with respect to anhydrite or possibly gypsum was reached and nodules formed in the still unlithified sediments

silicification of sulphate evaporites and their carbonate replacements in eocene marine sediments tunisia two diagenetic trends samples of chert nodules diagenetic carbonates and evaporites gypsum anhydrite collected from the gypsiferous limestones of the kef eddour member ypressian priabonian near metlaoui and sehib tunisia show selective silicification with great variety in the silicified by products based on d 13 c values which support an organic origin for the carbon carbonates replaced evaporites microbially through bacterial sulphate reduction observations and results suggest two scenarios for chert formation that are related to the rate and timing of diagenetic carbonate replacement of the evaporites anhydrite gypsum in the absence of early diagenetic carbonate phases silica with d 18 o values from 25 to 28ae6 standard mean ocean water smow replaced the outer parts of anhydrite nodules at ph 9 in the innermost parts of the anhydrite nodules prevented silica precipitation the record of this chemical barrier is preserved in the microquartz rims and geode features that formed in the inner parts of the nodules after dissolution of the anhydrite nucleus the microbial diagenetic replacement of evaporites bacterial sulphate reduction by carbonates calcite aragonite and dolomite favoured silica replacement of carbonates rather than evaporites silica with d 18 o signature of 21 to 26 smow replaced carbonates on a volume for volume basis yielding a more siliceous groundmass and accounting for 90 95 of the nodules the relatively higher d 18 o values of quartz replacing anhydrite can be explained by a diagenetic fluid in equilibrium with mixed meteoric marine to marine water the lower d 18 o values of the quartz that replaced the diagenetic carbonates are ascribed to flushing by meteoric water in a later diagenetic stage the silica supply for chert formation could be derived from the reworked bio siliceous deposits diatomites to the west of the basin vestiges of an opal ct precursor undetectable by x ray diffraction xrd were revealed by d 29 si magic angle spinning nuclear magnetic resonance investigations diagenesis of the extraformational and overlying clay rich beds the host limestones are clay poor as shown by xrd measurements and minor volcanogenic and hydrothermal contributions during early diagenetic stages

non chondritic magnesium and the origins of the inner terrestrial planets abstract high precision magnesium mg isotope data obtained using a large geometry high resolution mc icpms are reported for 9 carbonaceous and ordinary chondrites 9 eucrites and diogenites generally considered to originate from asteroid 4 vesta together with 4 martian meteorites and a variety of terrestrial and lunar materials the variation in mg isotopic composition found for mafic and ultramafic rocks mafic minerals and chondrites is smaller than reported previously the range of 26mg and 25mg of 0 6 and 0 3 defines a single mass dependent fractionation line consistent with a homogeneous mix of nucleosynthetic components data for the earth mars and vesta display no systematic mg isotopic differences despite large variations in the level of depletion in moderately volatile elements lunar mare basalts exhibit a significant range in 26mg 0 53 to 0 05 and 25mg 0 27 to 0 02 attributable to magmatic differentiation in the lunar magma ocean lmo lunar basalts derived from early segregated cumulates and thought to be the most primitive are on average slightly 0 17 26mg heavy relative to basalts from the earth mars and vesta however the difference is not well resolved more strikingly all differentiated planets and planetesimals as sampled have mg that is on average isotopically heavy compared with most chondrites analyzed thus far chondrules and cais also are generally heavy in terms of mg so this might reflect sorting of material in the proto planetary disk such an explanation would be similar to one previously proposed hewins r h and herzberg c t 1996 earth planet sci lett 144 1 7 to explain the non chondritic si mg of the earth in this model chondrule like objects separate from volatile rich planetary dust by accumulation in stagnant regions between eddies in the solar nebula small 1 km planetesimals formed by accumulation of such molten material then develop into planetary embryos and thence to larger terrestrial planets by combinations of runaway growth and collisions as such accumulation of molten chondrule like droplets provides an explanation that obviates some of the dynamic difficulties associated with the onset of planetary accretion the non chondritic mg isotopic composition of the earth is consistent with data for li and has important implications for earth s bulk composition and putative hidden reservoirs

the chemistry of eolian quartz dust and the origin of chert abstract among the numerous models that have been suggested for the primary and predominant source of silica for chert we suggest that eolian dust is worthy of further considerations such considerations are supported by the common association of phanerozoic chert with evaporites limestones dolomites or other strata that were deposited within or near arid paleoclimates this association suggests a direct or indirect causal link between aridity and chert formation in addition eolian processes export large quantities of quartz rich dust from arid climate ergs or loess lands the chemistry of abraded quartz particles derived therefrom is consistent with chert formation abrasion of quartz particles produces an amorphous surface layer and underlying lattice disorder an inverse relation between particle size distribution 64 m and the enthalpy of solution of abraded quartz is indicative of the degree of lattice disorder as particle size decreases the enthalpy of solution increases the amorphous surface layer and underlying lattice disorder enhance both the rate and amount of dissolution of quartz dust particles the solubilized silica can then be reprecipitated and diagenetically altered to the various silica polymorphs that occur in chert an eolian supply and the chemistry of eolian abraded quartz particles may account for chert formation in disparate depositional environments that encompass deep sea chert chert in epicontinental seas including shallow shelves and chert that formed in continental eolinites lakes and soils arid climate silcretes

magnesium isotope systematics in martian meteorites abstract magnesium isotope compositions are reported for a suite of martian meteorites that span the range of petrological and geochemical types recognized to date for mars including crustal breccia northwest africa nwa 7034 the 26mg values per mil units relative to dsm 3 reference material range from 0 32 to 0 11 basaltic shergottites and nakhlites lie to the heavier end of the mg isotope range whereas olivine phyric olivine orthopyroxene phyric and lherzolitic shergottites and chassignites have slightly lighter mg isotope compositions attesting to modest correlation of mg isotopes and petrology of the samples slightly heavier mg isotope compositions found for surface related materials nwa 7034 black glass fraction of the tissint shergottite fall 26mg 0 17 indicate measurable mg isotope difference between the martian mantle and crust but the true extent of mg isotope fractionation for martian surface materials remains unconstrained the range of 26mg values from 0 19 to 0 11 in nakhlites is most likely due to accumulation of clinopyroxene during petrogenesis rather than garnet fractionation in the source or assimilation of surface material modified at low temperatures the rather restricted range in mg isotope compositions between spatially and temporally distinct mantle derived samples supports the idea of inefficient absent major tectonic cycles on mars which would include plate tectonics and large scale recycling of isotopically fractionated surface materials back into the martian mantle the cumulative 26mg value of martian samples which are not influenced by late stage alteration processes and or crust mantle interactions is 0 271 0 040 2sd and is considered to reflect 26mg value of the bulk silicate mars this value is robust taking into account the range of lithologies involved in this estimate it also attests to the lack of the mg isotope variability reported for the inner solar system bodies at current analytical precision also noted for several other major elements

a proposed mechanism for the growth of chalcedony the structural disparities that distinguish chalcedony from macrocrystalline quartz suggest that different crystallization mechanisms are operative during the growth of these two forms of silica although the paragenesis of chalcedony has provoked marked disagreement among researchers a review of previous studies supports the idea that chalcedony can precipitate from slightly saturated aqueous solutions at relatively low temperatures 100 c these conditions for deposition suggest a model for chalcedony crystallization that involves the assembly of short chain linear polymers via bridging silica monomers this assembly occurs through a spiral growth mechanism activated by a screw dislocation withb n 2 110 wheren is an integer the proposed model can account for a number of peculiarities that have been observed in chalcedony at the microstructural scale such as 1 the direction of fiber elongation along 110 rather than 001 2 the periodic twisting of chalcedony fibers about 110 3 the high density of brazil twin composition planes 4 the common intergrowth of moganite within chalcedony

early martian mantle overturn inferred from isotopic composition of nakhlite meteorites following the crystallization of a magma ocean the martian mantle probably underwent an overturning event but its initiation timing and geochemical consequences are poorly constrained isotopic data for martian meteorites and numerical simulations provide strong evidence for early overturning in the martian mantle