mineralogical characterization of a unique material having heavy oxygen isotope anomaly in matrix of the primitive carbonaceous chondrite acfer 094 abstract we report the mineral compositions and micro texture of the isotopically anomalous 17 18osmow 180 fe s ni o material recently discovered in matrix of the primitive carbonaceous chondrite acfer 094 sakamoto n seto y itoh s kuramoto k fujino k nagashima k krot a n and yurimoto h 2007 oxygen isotope evidence for remnants of the early solar system primordial water science 317 231 233 synchrotron radiation x ray diffraction and transmission electron microscopy studies indicate that this material consists of the symplectitically intergrown magnetite fe3o4 and pentlandite fe5 7ni3 3s8 with magnetite pentlandite volume ratio of 2 3 magnetite forms column shaped grains 10 30 nm in diameter and 100 200 nm in length pentlandite occurs as worm shaped grains or aggregates of grains 100 300 nm in size between magnetite crystals although both the x ray diffraction and electron energy loss spectra support identification of iron oxide as magnetite the electron diffraction patterns show that magnetite has a weak 3 fold superstructure possibly due to ordering of vacancies we infer that the isotopically anomalous symplectite formed by sulfurization and oxidization of metal grains either in the solar nebula or on an icy planetesimal the intersite cation distribution of pentlandite suggests that timescale of oxidation was no longer than 1000 years

fluid evolution in cm carbonaceous chondrites tracked through the oxygen isotopic compositions of carbonates the oxygen isotopic compositions of calcite grains in four cm carbonaceous chondrites have been determined by nanosims and results reveal that aqueous solutions evolved in a similar manner between parent body regions with different intensities of aqueous alteration two types of calcite were identified in murchison mighei cold bokkeveld and lapaz icefield 031166 by differences in their petrographic properties and oxygen isotope values type 1 calcite occurs as small equant grains that formed by filling of pore spaces in meteorite matrices during the earliest stages of alteration on average the type 1 grains have a 18o of 32 36 vsmow and 17o of between 2 and 1 most grains of type 2 calcite precipitated after type 1 they contain micropores and inclusions and have replaced ferromagnesian silicate minerals type 2 calcite has an average 18o of 21 24 vsmow and a 17o of between 1 and 3 such consistent isotopic differences between the two calcite types show that they formed in discrete episodes and from solutions whose 18o and 17o values had changed by reaction with parent body silicates as predicted by the closed system model for aqueous alteration temperatures are likely to have increased over the timespan of calcite precipitation possibly owing to exothermic serpentinisation the most highly altered cm chondrites commonly contain dolomite in addition to calcite dolomite grains in two previously studied cm chondrites have a narrow range in 18o 25 29 vsmow with 17o 1 to 3 these grains are likely to have precipitated between types 1 and 2 calcite and in response to a transient heating event and or a brief increase in fluid magnesium calcium ratios in spite of this evidence for localised excursions in temperature and or solution chemistry the carbonate oxygen isotope record shows that fluid evolution was comparable between many parent body regions the cm carbonaceous chondrites studied here therefore sample either several parent bodies with a very similar initial composition and evolution or more probably a single c type asteroid

aragonite breunnerite calcite and dolomite in the cm carbonaceous chondrites high fidelity recorders of progressive parent body aqueous alteration abstract carbonate minerals in cm carbonaceous chondrite meteorites along with the silicates and sulphides with which they are intergrown provide a detailed record of the nature and evolution of parent body porosity and permeability and the chemical composition temperature and longevity of aqueous solutions fourteen meteorites were studied that range in petrologic subtype from mildly aqueously altered cm2 5 to completely hydrated cm2 0 all of them contain calcite whereas aragonite occurs only in the cm2 5 cm2 2 meteorites and dolomite in the cm2 2 cm2 0 all of the aragonite crystals and most of the calcite and dolomite grains formed during early stages of parent body aqueous alteration by cementation of pores produced by the melting of tens of micrometre size particles of h2o rich ice aragonite was the first carbonate to precipitate in the cm2 5 to cm2 2 meteorites and grew from magnesium rich solutions in the least altered of these meteorites the aragonite crystals formed in clusters owing to physical restriction of aqueous fluids within the low permeability matrix the strong correlation between the petrologic subtype of a meteorite the abundance of its aragonite crystals and the proportion of them that have preserved crystal faces is because aragonite was dissolved in the more altered meteorites on account of their higher permeability and or greater longevity of the aqueous solutions dolomite and breunnerite formed instead of aragonite in some of the cm2 1 and cm2 2 meteorites owing to higher parent body temperatures the pore spaces that remained after precipitation of aragonite dolomite and breunnerite cements were occluded by calcite following completion of cementation the carbonates were partially replaced by phyllosilicates and sulphides calcite in the cm2 5 cm2 2 meteorites was replaced by fe rich serpentine and tochilinite followed by mg rich serpentine in the cm2 1 and cm2 0 meteorites dolomite breunnerite and calcite were replaced by fe rich serpentine and fe ni sulphide again followed by mg rich serpentine the difference between meteorites in the mineralogy of their replacive sulphides may again reflect greater temperatures in the parent body regions from where the more highly altered cms were derived this transition from fe rich to mg rich carbonate replacement products mirrors the chemical evolution of parent body solutions in response to consumption of fe rich primary minerals followed by the more resistant mg rich anhydrous silicates almost all of the cms examined contain a second generation of calcite that formed after the sulphides and phyllosilicates and by replacement of remaining anhydrous silicates and dolomite dedolomitization the ca and co2 required for this replacive calcite is likely to have been sourced by dissolution of earlier formed carbonates and ions may have been transported over metre plus distances through high permeability conduits that were created by impact fracturing

metamorphosed gabbroic dikes related to opening of iapetus ocean at the st lawrence promontory blair river inlier nova scotia canada abstract metamorphosed mafic dikes in the laurentian blair river inlier of northern cape breton island nova scotia are related to the opening of the iapetus ocean during the breakup of the supercontinent rodinia the dikes were metamorphosed during a paleozoic likely silurian thermal event related to appalachian orogenesis but were not deformed significantly two dikes yielded u pb zircon ages of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 581 pm 6 end document and documentclass aastex usepackage amsbsy usepackag

carbonates in cm2 chondrites constraints on alteration conditions from oxygen isotopic compositions and petrographic observations high precision measurements of the oxygen isotopic compositions of carbonates calcite and dolomite from five cm2 chondrites are presented and put into context of the previously determined mineralogic alteration index mai which places these meteorites into an alteration sequence the carbonate oxygen isotopic compositions range from 20 0 to 35 7 for 18o 8 0 to 17 7 for 17o and 0 7 to 2 7 for 17o carbonate 17o values are inversely correlated with mai and track the evolution of fluid composition from higher to lower 17o values with increasing alteration on the cm parent body similar 17o values for calcite and dolomite fractions from the same splits of the same meteorites indicate that calcite and dolomite in each split precipitated from a single fluid reservoir however reversed calcite dolomite fractionations 18odol 18occ indicate that the fluid was subject to processes such as freeze thaw or evaporation that fractionated isotopes in a mass dependent way consideration of the carbonate isotopic data in the context of previously proposed models for aqueous alteration of carbonaceous chondrites has provided important insights into both the evolving alteration conditions and the utility of the models themselves the data as a whole indicate that the isotopic evolution of the fluid was similar to that predicted by the closed system two reservoir models but that a slightly larger matrix water fractionation factor may apply in the context of this model more altered samples largely reflect greater reaction progress and thus probably indicate more extended times of fluid exposure petrographic observations of carbonates reveal a trend of variable carbonate morphology correlated with alteration that is also consistent with changes in the duration of fluid rock interaction the data can also be reconciled with fluid flow models in a restricted region of the parent body which is consistent with assertions that the different types of carbonaceous chondrites derive from different regions of their parent bodies in this case the model results for a 9 km radius body and our data place the location of the cm chondrite formation in a 100 m thick zone 1 km from the surface the size of this zone could be increased if the model parameters were adjusted

widespread oxidized and hydrated amorphous silicates in cr chondrites matrices implications for alteration conditions and h2 degassing of asteroids abstract the cr chondrites carry one of the most pristine records of the solar nebula materials that accreted to form planetesimals they have experienced very variable degrees of aqueous alteration ranging from incipient alteration in their matrices to the complete hydration of all of their components in order to constrain their chemical alteration pathways and the conditions of alteration we have investigated the mineralogy and fe oxidation state of silicates in the matrices of 8 cr chondrites from type 3 to type 1 fe l edge x ray absorption near edge structure xanes was performed on matrix fib sections using synchrotron based scanning transmission x ray microscopy stxm the fe 3 fe ratio of submicron silicate particles was obtained and coordinated with tem observations in all the least altered cr chondrites que 99177 eet 87770 eet 92042 lap 02342 gra 95229 and renazzo we find that the matrices consist of abundant submicron fe rich hydrated amorphous silicate grains mixed with nanometer sized phyllosilicates the fe 3 fe ratios of both amorphous and nanocrystalline regions are very high with values ranging from 68 to 78 in the most altered samples al rais and gro 95577 fine grained phyllosilicates also have a high fe 3 fe ratio around 70 whereas the coarse micrometer sized phyllosilicates are less oxidized down to 55 and have a lower iron content these observations suggest the following sequence submicron fe2 amorphous silicate particles were the building blocks of cr matrices after accretion they were quickly hydrated and oxidized leading to a metastable amorphous gel like phase nucleation and growth of crystalline phyllosilicates was kinetically limited in most type 3 and 2 crs but increased as alteration became more extensive in al rais and gro 95577 the decreasing fe 3 fe ratio is interpreted as a result of the transfer of fe3 from silicates to oxides during growth while aqueous alteration progressed higher temperature longer duration change of fluid composition in a fully closed system equilibrium thermodynamics suggest that the water to rock ratios typically assumed to be low fe 3 fe ratio such a high fe 3 fe value could be accounted for however if the system was partially open at least with respect to h2 and other gases as well rapid degassing of the fluid would have favored more oxidizing fluid conditions recently proposed scenarios involving some degree of water d h increase through rayleigh isotopic fractionation are supported by these results

oxygen isotope studies of carbonaceous chondrites abstract the carbonaceous chondrites display the widest range of oxygen isotopic composition of any meteorite group as a consequence of the interaction of primordial isotopic reservoirs in the solar nebula these isotopic variations can be used to identify the reservoirs and to determine conditions and loci of their interactions we present a comprehensive set of whole rock analyses of cv co ck cm cr ch and ci chondrites as well as selected components of some of these meteorites a simple model is developed which describes the isotopic behavior during parent body aqueous alteration processes the process of thermal dehydration also produces a recognizable effect in the oxygen isotopic composition

temperatures of aqueous alteration and evidence for methane generation on the parent bodies of the cm chondrites aqueous alteration of primitive meteorites was among the earliest geological processes during the evolution of our solar system clumped isotope thermometry of carbonates in the cm chondrites cold bokkeveld murray and murchison demonstrates that they underwent aqueous alteration at 20 71 c from a fluid with 18 o vsmow of 2 0 to 8 1 and 17 o vsmow of 0 1 to 3 0 the 13 c vpdb values of these carbonates exhibit a negative correlation with the 18 o vsmow of their formation waters consistent with formation and escape of 13 c depleted ch 4 during aqueous alteration methane generation under these conditions implies that the alteration fluid was characterized by an eh 0 67 and ph 12 5 or lower at the highest alteration temperatures our findings suggest that methane generation may have been a widespread consequence of planetesimal and planetary aqueous alteration perhaps explaining the occurrence of methane on titan triton pluto and other kuiper belt objects

the oxygen isotope record in murchison and other carbonaceous chondrites the ranges of 18o and 17o in components of the murchison c2 chondrite exceed those in all other meteorites analyzed previous authors have proposed that c2 chondrites are the products of aqueous alteration of anhydrous silicates a model is presented to determine whether the isotopic variations can be understood in terms of such alteration processes the minimum number two of initial isotopic reservoirs is assumed two major stages of reservoir interaction are required one at high temperature to produce the16o mixing line observed for the anhydrous minerals and another at low temperature to produce the matrix minerals the isotopic compositions severely constrain the conditions of the low temperature process requiring temperatures 44 extension of the model to the data on c1 chondrites requires aqueous alteration in a warmer wetter environment

murchisite cr5s6 a new mineral from the murchison meteorite murchisite ima 2010 003 cr 5s 6 is a new chromium sulfide mineral discovered in the murchison cm2 meteorite the type material occurs as one subhedral crystal 1 3 4 m in size in contact with low ni iron kamacite martensitic iron schreibersite and a ca al rich glass all of which are included in an isolated forsteritic olivine grain in the meteorite s matrix the mean chemical composition determined by electron microprobe analysis of the type material is wt cr 53 32 s 42 87 v 1 44 fe 1 14 p 0 10 ni 0 10 sum 98 97 the empirical formula calculated on the basis of 6 s atoms is cr 4 60 v 0 13 fe 0 09 ni 0 01 4 83 s 6 00 p 0 01 6 01 murchisite was also identified in another isolated olivine grain from the same meteorite these crystals are subhedral to round in shape 300 nm to 1 m in size and occur in association with tochilinite and serpentine within which it is included chromite and eskolaite its electron backscatter diffraction patterns are an excellent match to that of synthetic cr 5s 6 with the p31c structure showing a 5 982 c 11 509 a v 356 67 a 3 and z 2 based on previously published data from synthetic material murchisite is named for the locality the murchison meteorite it is a low temperature phase 327 c in the cr s system probably formed from higher temperature cr 1 x s exsolved or expelled from a cr s bearing metal rich spherule included in forsteritic olivine grains that were probably derived from chondrule fragments the formation of this phase reflects sulfur fugacities intermediate between those that would have led to troilite and those that would have led to no sulfide at all the high temperature cr 1 x s phase equilibrated with coexisting alloys to 600 c murchisite formed from this precursor at low temperatures 300 c through ordering of s vacancies

paleomagnetism of the cambrian moore hollow group texas evidence for a primary magnetization carried by detrital magnetite petrographic and paleomagnetic studies of the cambrian moore hollow group texas establish the presence of three magnetic minerals and several components of magnetization hematite is the dominant carrier of the magnetic remanence in most samples and is primarily a diagenetic replacement of preexisting fe bearing phases hematite dominant samples have characteristic directions which range from shallow easterly to shallow southerly this distribution suggests multiple components of magnetization acquired at various times in the paleozoic the paleopole obtained for an unmineralized breccia 135 7 e 15 4 n represents either a chemical remanent magnetization of middle ordovician to early silurian age or a vector addition of two or more cambrian through silurian components mineralized breccias at the silver creek mississippi valley type deposit yield a permian direction 117 0 e 51 6 n because hematite in this deposit predates mineralization the permian age represents a maximum age for the mineralization at silver creek many samples contain goethite as a replacement of fe dolomite glauconite and pyrite the direction of magnetization carried by goethite is near the present geomagnetic field or in a minority of samples antipodal to it rounded ti magnetite was identified by sem in magnetic extracts from siliciclastic poor facies irm acquisition and thermal demagnetization confirms the presence of magnetite in these samples some samples contain an apparent modern viscous remanent magnetization the dominant component 72 samples 10 sites has a site mean declination of 93 8 and inclination of 14 5 with an 95 8 5 corresponding to a pole position at 163 0 e 0 6 s the presence of detrital magnetite polarity changes lack of evidence for prolonged burial and the proximity of this pole position to other late precambrian cambrian poles suggest that this component of magnetization indeed reflects the late cambrian paleopole for cratonic north america

geology geochemistry and paleomagnetism of the cheneaux metagabbro helikian of southern quebec and eastern ontario abstract the cheneaux metagabbro helikian is a slightly to moderately metamorphosed intrusive body in the southern grenville province the paleomagnetic study undertaken on it yielded 3 main components of magnetization the first component w d 249 i 45 with a corresponding paleopole of 44 e 6 s may represent the time of emplacement or an early phase of mechanical deformation the second component w d 258 i 34 with a corresponding paleopole of 8 e 21 n represents a main metasomatic event related to the initial phase of the grenville orogeny the third component se d 135 i 30 with a corresponding paleopole of 32 w 16 s is related to the final phase of the grenville orogeny the estimated age of acquisition of magnetisation for these 3 components are 1300 1075 and 950 ma respectively the significance of the new set of data in relation to the established grenville apparent polar wandering curve is discussed finally a fourth component ne d 37 i 41 with a corresponding paleopole of 37 e 53 n represents a much younger event its pole may be located on the late hadrynian track 750 ma

the hydrology of carbonaceous chondrite parent bodies and the evolution of planet progenitors oxygen isotope ratio data from carbonaceous chondrites are explained if aqueous fluids flowed from high to low temperatures within the parent objects of these primitive meteorites recognition of ancient hydrological activity on primitive asteroid precursors affords a new class of constraints on the heat sources ages and sizes of planet precursors in the early solar system based on the geological evolution of planetesimals

collisional facilitation of aqueous alteration of cm and cv carbonaceous chondrites abstract cm chondrites exhibit a strong correlation between the degree of alteration and the extent of particle alignment i e the strength of the petrofabric it seems likely that the s1 shock stage of essentially every cm and the high matrix abundance 70 vol of these samples ensured that the shock waves that produced cm petrofabrics by collapsing matrix pores and squeezing chondrules into pore spaces were significantly attenuated and were too weak to damage olivine crystal lattices random collisions on the cm body produced petrofabrics and created fractures in the target rocks subsequent impact mobilization of water caused hydrated phases to form preferentially in the more fractured regions those with the strongest petrofabrics the less deformed less fractured cm regions experienced lower degrees of aqueous alteration many cv3 chondrites also have petrofabrics roughly half are from the oxidized bali like subgroup cv3 oxb roughly half are from the reduced subgroup cv3 r and none is from the oxidized allende like subgroup cv3 oxa which is less altered than cv3 oxb nearly all cvs with petrofabrics are s3 s4 and nearly all cvs that lack petrofabrics are s1 oxidized cvs have much higher porosities typically 20 28 than reduced cvs 0 6 8 facilitating more extensive aqueous alteration the cv3 r chondrites formed from low porosity material that inhibited oxidation during alteration the oxidized cv subgroups formed from higher porosity materials the cv3 oxb samples were shocked became extensively fractured and developed petrofabrics the cv3 oxa samples were not shocked and never developed petrofabrics when water was mobilized both sets of porous cv chondrites became oxidized the more fractured cv3 oxb subgroup was more severely altered

modeling aqueous alteration of cm carbonaceous chondrites results from an inorganic geochemical modeling study support a scenario in which low temperature aqueous alteration of an anhydrous cm asteroidal parent body and melt water from h20 and co ices produces the altered assemblage observed in cm carbonaceous chondrites chrysotile greenalite tochilinite cronstedtite and minor calcite and magnetite we consider a range of possible precursor mineral assemblages varying with respect to the fe oxidation state of the initial anhydrous phases the aqueous solutions produced by this alteration are generally strongly basic and reducing and a large quantity of h2 and possible ch4 gas can be released during aqueous alteration

middle cambrian paleomagnetism of the avalon terrane in cape breton island nova scotia interbedded volcanics and sediments of the bourinot group mid cambrian in central cape breton island were studied as part of a paleomagnetic investigation to quantify the geologically plausible displacements of the avalon terrane with respect to the north american craton both volcanics and sediments reveal prefolding magnetizations with steep to intermediate inclinations dual polarity magnetizations in different volcanic flows are probably primary and they yield a mean direction of d 293 i 66 n 9 k 41 8 95 8 1 the sediments reveal upon detailed thermal demagnetization complex multivectorial remanences however a few samples show characteristic components and a large number of samples 38 have intersecting great circle remagnetization trajectories when combined this yields a mean direction of d 267 i 65 k 16 mean radius of error ellipse is 8 5 the mean directions for the bourinot rocks correspond to a paleolatitude of 49 s 11 which is different from the range of possible values for the predicted mid cambrian paleolatitudes for nova scotia 29 s to 18 n inferred from the north american apparent polar wander path this discrepancy between predicted paleolatitudes and the observed paleolatitude implies that nova scotia was not part of north america during the mid cambrian but rather they were separated by an ocean with a minimum width of 1000 km 20 10 this interpretation of the relative positions of nova scotia and north america based on paleomagnetism is consistent with the geological record in the northern appalachians and supports previous interpretations of the dissimilar early paleozoic faunas in avalon and north america it is paleomagnetically quite plausible that the avalon terrane formed part of a larger continent comprising africa gondwana and armorica during the cambrian

minor and trace element concentrations in carbonates of carbonaceous chondrites and implications for the compositions of coexisting fluids abstract abundances of fe mg mn sr na b and ba in meteoritic calcites and dolomites have been determined using the ion microprobe the compositions of these carbonates are consistent with their precipitation from or recrystallization in equilibrium with aqueous solutions at low temperatures coexisting calcites and dolomites in cm chondrites are not in equilibrium and presumably formed in distinct events calculated molar element ca ratios for fluids in equilibrium with these carbonates suggest that dolomites in cm and cl chondrites formed from compositionally similar brines calcite solutions were also saline but lower in mg fe and mn calculations suggest that these solutions became more like dolomite solutions with increasing alteration of the host cm meteorite reflecting progressive evolution of solution compositions asteroid brines may have formed by membrane nitration after the formation of compacted phyllosilicates lowered permeability by addition of components to fluids during phyllosilicate forming reactions or through low pressure boiling occurrences of vein filling calcite followed by dolomite and later by sulfates in carbonaceous chondrites represent the final stages of a protracted aqueous alteration sequence

abundances of presolar silicon carbide grains in primitive meteorites determined by nanosims abstract it has been suggested that the matrices of all chondrites are dominated by a common material with ivuna like ci abundances of volatiles presolar grains and insoluble organic matter iom e g alexander 2005 however matrix normalized abundances of presolar silicon carbide sic grains estimated from their noble gas components show significant variations in even the most primitive chondrites huss and lewis 1995 huss et al 2003 in contradiction to there being a common chondrite matrix material here we report presolar sic abundances determined by nanosims raster ion imaging of iom extracted from primitive members of different meteorite groups we show that presolar sic abundance determinations are comparable between nanosims instruments located at three different institutes between residues prepared by different demineralization techniques and between microtomed and non microtomed samples our derived sic abundances in cr chondrites are comparable to those found in the ci chondrites 30 ppm and are much higher than previously determined by noble gas analyses the revised higher cr sic abundances are consistent with the crs being amongst the most primitive chondrites in terms of the isotopic compositions and disordered nature of their organic matter similar abundances between cr1 cr2 and cr3 chondrites indicate aqueous alteration on the cr chondrite parent body has not progressively destroyed sic grains in them a low sic abundance for the reduced cv3 rbt 04133 can be explained by parent body thermal metamorphism at an estimated temperature of 440 c minor differences between primitive members of other meteorite classes which did not experience such high temperatures may be explained by prolonged oxidation at lower temperatures under which sic grains formed outer layers of sio2 that were not thermodynamically stable leading to progressive degassing destruction of sic

sr nd and pb isotope geochemistry of the oslo rift igneous province southeast norway abstract sr nd and pb isotopic compositions are presented for a series of 290 to 250 ma old basaltic to granitic rocks from the oslo rift southeast norway a large group of basalts larvikites and rhomb porphyry lavas cluster within a restricted range of initial isotope ratios tsr 3 to 16 tnd 3 3 to 4 2 206 pb 204 pb i 18 9 to 19 3 207 pb 204 pb i 15 59 to 15 66 208 pb 204 pb i 38 6 to 39 1 the remainder of the rocks trend towards higher initial sr and lower initial nd and pb isotopic ratios data are tested against mantle crust mixing models the oslo rift magmatic rocks originated from a somewhat heterogeneous mildly depleted source located in the subcrustal lithosphere the isotopic character of this source has been inherited by the rocks showing uniform isotopic ratios which have thus not suffered significant crustal contamination the rest of the mafic to intermediate rocks and the syenites were contaminated in the intermediate to lower crust which increased slightly their sr but decreased their nd and pb isotopic ratios as much as 40 50 contamination is suggested for some of the samples strong increases in radiogenic sr seen primarily in the granites are attributed to contamination in the upper crust the mildly depleted mantle proposed as the source for the oslo rift mafic rocks has an isotopic character which can only be explained in terms of a multistage history which included periods of relatively high u pb and th pb ratios resulting from a relatively short depletion history in the subcontinental lithosphere and metasomatic enrichment of the mantle in selected elements

effect of anisotropy on the paleomagnetic contact test for a grenville dike we report a detailed paleomagnetic contact test for a mafic dike and its tonalitic gneiss host rock in the precambrian grenville province of canada directions of natural remanent magnetization nrm in the country rock were corrected for assumed deflection by the strong magnetic anisotropy due to the rock fabric using the measured anisotropy of laboratory induced thermoremanent magnetization trm the trm ellipsoids have the same principal axes as the anisotropy of magnetic susceptibility ams ellipsoids but the anisotropy of remanence is more pronounced than the ams anisotropy correction moves nrm directions in the unbaked zone almost 20 bringing them close to typical grenvillian paleofield directions the anisotropy seems to be controlled by multidomain rather than single domain magnetic minerals nrm directions from the baked gneiss near the dike contact after anisotropy correction agree well with the nrm directions from the chilled margin of the early cambrian 570 3 ma dike this positive baked contact test is augmented by the presence of a hybrid zone in which country rock nrms are increasingly overprinted by the dike nrm as one moves from the unbaked toward the baked zone the dike nrm is therefore primary however the virtual geomagnetic pole vgp for the dike nrm 51 n 131 w dp 10 dm 12 n 24 samples or 41 n 153 w dp 13 dm 18 n 4 chilled margin samples does not fall near most established cambrian paleopoles post emplacement tectonic movement of the dike is unlikely it may be that the dike cooled and acquired trm during a geomagnetic field excursion because of its geologically short cooling blocking period of a few tens of years

geochemistry and u pb zircon age of comenditic metafelsites of the tibbit hill formation quebec appalachians metafelsites in waterloo area quebec represent the only known silicic volcanic rocks in the predominantly basaltic tibbit hill formation low grade metamorphism accompanied by hydration and albitization has converted the felsic volcanic rocks mainly to muscovite quartz albite schists the volcanic parent of these metafelsites was formed partly as lava flows and partly as tuffs the principal compositional type was a comendite a component of intermediate rocks is also present but its extent is undetermined and probably minor u pb zircon studies of the metafelsites have yielded a reliable age of this early cambrian age is probably representative of the age of the tibbit hill formation as a whole the tibbit hill formation accumulated at one of the clearest examples of a rrr rift rift rift triple junction the sutton mountains triple junction of the continental rift system formed as a prelude to the opening of the iapetus ocean its volcanic rocks are products of the youngest major episode of rift rela

paleomagnetism of lower cambrian volcanics and a cross cutting cambro ordovician diabase dyke from buckingham quebec the buckingham area quebec is characterized by cambrian volcanic flows 573 32 ma cut by a cambro ordovician diabase dyke which is dated at 497 14 ma by k ar method the volcanics yield multicomponent magnetizations a hard characteristic magnetization and a relatively soft one these components could be separated easily and the intensity decay curves of the different components could also be obtained separately using zijderveld s method three different groups of site mean characteristic magnetizations can be distinguished each representing a different stratigraphic level the paleopoles derived from these magnetizations are from oldest to youngest 186 e 10 s 173 e 01 n and 154 e 06 n this is interpreted as a rapid motion of the paleopole for the north american craton during the early cambrian the paleopole derived from the hard characteristic dyke direction is 123 e 03 s this magnetization direction was also found at one of the sites in the volcanics this means that the contact test

gravity and magnetic anomalies of the sutton mountains region quebec and vermont expressions of rift volcanics related to the opening of iapetus prominent nearly coincident positive gravity and magnetic anomalies occur in the sutton mountains region centered about 100 km east of montreal quebec several lines of evidence indicate that t

oroclinal bending in the caledonides of western ireland new palaeomagnetic data from the caledonides of western ireland indicate that the silurian rocks of south mayo underwent oroclinal bending following folding in siluro devonian time bending was accommodated on faults cutting the silurian sequence and was driven by strike slip motion across the antrim galway line a recently recognized major curvilinear lineament we propose that the silurian rocks in the east of the region suffered up to 30 clockwise rotation increasing towards the west to approximately 70 this newly inferred differential rotation of the silurian rocks about a sub vertical axis implies that the underlying dalradian rocks of the connemara massif to the south must also have undergone clockwise rotation in late silurian early devonian time this is consistent with published palaeomagnetic data from the connemara gabbro and accounts for the swing in strike of the caledonian orogen in this part of western ireland from this we deduce that the swing in strike of the caledonian rocks elsewhere on the west coast of ireland might have a similar explanation

the bakersville dike swarm geochronology and petrogenesis of late proterozoic basaltic magmatism in the southern appalachian blue ridge

determining the δ47 acid fractionation in dolomites abstract this paper reports the fractionation of 47 during the digestion of dolomite in phosphoric acid between 25 c and 90 c using five different samples including three of pliocene age from the bahamas one from the jurassic in the middle east and one obtained from the national institute of standards nist 88b the composition of the dolomites analyzed varied from ca0 56mg0 44co3 to ca0 50mg0 50co3 fractionation values were also compared between the common acid bath and sealed vessel techniques at various temperatures no statistically significant differences were observed either between these two methods or as a function of the dolomite s stoichiometry these data produce a difference in fractionation of 0 153 0 011 for dolomite samples digested at 90 c compared to those reacted at 25 c a value higher and statistically different to previously published values utilization of this value in a study of dolomites from a core drilled on the island of san salvador in the bahamas yielded temperatures and 18ofluid values which agree with previous interpretations on the formation of these dolomites application of this value to other published studies produces lower estimates of temperature and 18ofluid values for the dolomitization process that are more consistent with the geologic models suggested for these studies

progressive aqueous alteration of cm carbonaceous chondrites abstract cm chondrites are aqueously altered rocks that contain 9 wt h 2 o i e indigenous water bound in phyllosilicates also present are clumps of serpentine tochilinite intergrowths previously called poorly characterized phases or pcp pentlandite and ni bearing pyrrhotite we studied 11 cm chondrites that span the known range from least altered to most altered we used various petrologic properties many previously identified that provide information regarding the degree of aqueous alteration there are no known unaltered or slightly altered cm chondrites e g rocks containing numerous chondrules with primary igneous glass some cm properties result from processes associated with early and intermediate stages of the alteration sequence i e hydration of matrix alteration of chondrule glass and production of large pcp clumps other petrologic properties reflect processes active throughout the alteration sequence these include oxidation of metallic fe ni alteration of chondrule phenocrysts changes in pcp composition reflecting an increase in the phyllosilicate sulfide ratio and changes in carbonate mineralogy reflecting the development of dolomite and complex carbonates at the expense of ca carbonate on the basis of these parameters we propose a numerical alteration sequence for cm chondrites because there are no known cm samples that display only incipient alteration the least altered sample was arbitrarily assigned to subtype 2 6 the most altered cm chondrites currently classified cm1 are assigned to subtype 2 0 these highly altered rocks have essentially no mafic silicates they contain chondrule pseudomorphs composed mainly of phyllosilicate however their bulk compositions are cm like and they are closer in texture to other c2 chondrites than to ci1 chondrites which lack chondrule pseudomorphs using several diagnostic criteria we assigned petrologic subtypes 0 1 to every cm chondrite in our study que 97990 cm2 6 murchison cm2 5 kivesvaara cm2 5 murray cm2 4 2 5 y 791198 cm2 4 que 99355 cm2 3 nogoya cm2 2 cold bokkeveld cm2 2 que 93005 cm2 1 lap 02277 cm2 0 met 01070 cm2 0 the proposed cm numerical alteration sequence improves upon the existing scheme of browning et al 1996 in that it does not require a complicated algorithm applied to electron microprobe data to determine the average matrix phyllosilicate composition the new sequence is more comprehensive and employs petrologic subtypes that are easier to use and remember than mineralogic alteration index values new neutron activation analyses of que 97990 que 93005 met 01070 murchison and crescent together with literature data confirm the compositional uniformity of the cm group different degrees of alteration among cm chondrites do not lead to resolvable bulk compositional differences this suggests that the textural differences among individual cm chondrites reflect progressive alteration of similar hypothetical cm3 0 starting materials in different regions of the same parent body with minimal aqueous transport of materials over appreciable e g meters distances

p o rich sulfide phase in cm chondrites constraints on its origin on the cm parent body cm chondrites are a group of primitive meteorites that have recorded the alteration history of the early solar system we report the occurrence chemistry and oxygen isotopic compositions of p o rich sulfide phase in two cm chondrites grove mountains grv 021536 and murchison this p o rich sulfide is a polycrystalline aggregate of nanometer size grains it occurs as isolated particles or aggregates in both cm chondrites these grains in the matrix and in type i chondrules from murchison were partially altered into tochilinite however grains enclosed by ca carbonate are much less altered this p o rich sulfide in murchison is closely associated with magnetite feni phosphide brezinaite cr3s4 and eskolaite cr2o3 in addition to sulfur as the major component this sulfide contains 6 3 wt o 5 4 wt p and minor amounts of hydrogen analyses of oxygen isotopes by sims resulted in an average 18o value of 22 5 and an average 17o value of 0 2 9 2 2 limited variations in both chemical compositions and electron diffraction patterns imply that the p o rich sulfide may be a single phase rather than a polyphase mixture several features indicate that this p o rich sulfide phase formed at low temperature on the parent body most likely through the alteration of feni metal a close association with other low temperature alteration products b the presence of hydrogen c high 17o values and the presence in altered mesostasis of type i chondrules and absence in type ii chondrules the textural relations of the p o rich sulfide and other low temperature minerals reveal at least three episodic alteration events on the parent body of cm chondrites 1 formation of p o rich sulfide during sulfur rich aqueous alteration of p rich feni metal 2 formation of ca carbonate during local carbonation and 3 alteration of p o rich sulfide and formation of tochilinite during a period of late stage intensive aqueous alteration

the carbon and oxygen isotopic composition of meteoritic carbonates the 13c12c and 18o16o isotopic ratios of carbonates from carbonaceous and ordinary chondrites have been measured on co2 released by the action of h3po4 on whole rock samples carbonates from ci cm and cr carbonaceous chondrites exhibit a range in 18o of ca 15 20 5 to 35 1 relative to smow limited data from co2 water equilibration experiments suggest that meteoritic carbonates do not possess grossly anomalous 17o isotopic compositions therefore they are truly enriched in 13c with 13c between 23 7 and 80 7 relative to pdb large internal variations in 13c and 18o were found in individual meteorites and suggest that two or more isotopically distinct carbonates of different origin may be present the abundance 13c and 18o of carbonate in cm2 chondrites may be related to the extent of aqueous alteration of the meteorites carbonates in ci and cr chondrites have a median 13c ca 50 to 60 whereas 13c of cm meteorites lie in the range 40 to 50 although exceptions exist in both sets of samples cv3 and co3 carbonaceous chondrites and unequilibrated ordinary chondrites release small amounts of co2 on acid treatment which might be from carbonate dissolution but which is not enriched in 13c exhibiting 13c values ca 0 10 the exception to this is bishunpur with 13c ca 23 5 the difference in 13c of the ci cm and cr vs cv co and ordinary chondrite carbonates may be a result of the progressive enrichment in 13c of percolating fluids brought about by increasing solubilization of exotic 13c enriched grains

carbonates in cm chondrites complex formational histories and comparison to carbonates in ci chondrites cm chondrites are primitive solar system materials that have undergone high degrees of aqueous alteration resulting in the formation of secondary minerals including carbonates two different carbonate minerals calcite aragonite and dolomite together constitute 1 4 2 8 vol of cm chondrites in contrast ci chondrites contain four different carbonate minerals calcite aragonite dolomite breunnerite and siderite ci chondrites have abundant dolomite a mineral that seems to be absent in the most aqueously altered cm chondrites in this study carbonates in seven cm chondrites y 791198 lapaz icefield 04796 cold bokkeveld nogoya queen alexandra range 93005 allan hills 83100 and meteorite hills 01070 were studied petrographically and by electron microprobe the results indicate that carbonate formation in cm chondrites differs from that in ci chondrites and is more complex than previously recognized our studies of cm chondrites indicate that 1 carbonates formed on the parent asteroid in an aqueous environment that gradually changed in composition 2 at some stage ca and mg activities in the environment were high enough to form metastable dolomite and 3 dolomites disappeared in the most aqueously altered cm chondrites

nd isotope evidence for crust mantle interaction in the generation of a type granitoid suites in labrador canada early proterozoic a type granite suites in labrador northeastern canada have initial nd isotopic compositions that lie between those of a depleted mantle reservoir and the local basement rocks one of these a type granite suites consists of a linear array of discrete plutons that crosses the boundary between an archean craton and an adjoining early proterozoic crustal block the nd of these granites at formation t 1720 ma changes from 8 to 4 across this boundary reflecting changes in the nature and antiquity of the continental crust however in both domains the granites cannot be derived entirely by anatexis of the local basement rocks and apparently require significant amounts of juvenile mantle derived material a type granites have been widely interpreted as anatectic derivatives of lower crustal rocks that were depleted by previous melt extraction the nd isotopic data suggest that these labrador examples were instead mixtures of mantle derived magmas and overlying lower crustal rocks and that new material may have been the volumetrically dominant component 50 90 in most cases as an alternative to the prevalent residual source model it is proposed that the distinctive geochemical characteristics of these a type granites are a function of the high liquidus temperatures and primitive mg rich character of subcrustal magma fluxes in post orogenic and anorogenic environments where such magmas are emplaced into stabilized continental crust further nd isotope studies of other a type granite suites are required to ascertain if models invoking crust mantle interaction are more widely applicable

age of the grenville dyke swarm ontario quebec implications for the timing of lapetan rifting u pb baddeleyite and zircon ages for three diabase dykes from widely spaced localities within the grenville dyke swarm indicate a single age of emplacement at ma the 700 km long grenville dyke swarm located in the southeastern part of the canadian shield was emplaced syntectonically with the development of the ottawa graben this graben may represent a plume generated lapetan failed arm that developed at the onset of the breakup of laurentia other precisely dated lapetan rift related units such as the callander alkaline complex and the tibbit hill formation volcanic rocks indicate a protracted 36 ma period of rifting and magmatism prior to volcanism along this segment of the lapetan margin the age of the grenville dykes is the youngest in a progression of precisely dated mafic magmatic events from the 723 ma franklin dykes and sills to the 615 ma long range dykes along the northern and northeastern margins of laurentia respectively thus the age for these dykes represents a key time marker for

paleomagnetism of the catoctin volcanic province a new vendian cambrian apparent polar wander path for north america the vendian cambrian segment of the lauretian apparent polar wander path apwp has been poorly constrained and the subject of some controversy the catoctin volcanic province in central virginia is well dated at 570 35 ma rb sr and 597 18 ma u pb and therefore presented an excellent paleomagnetic target for resolving the laurentian vendian cambrian apwp a total of 206 samples from 32 sites were collected from the catoctin basalts feeder dikes and sills the study revealed three ancient directions of magnetization the youngest c component fails the fold test and yielded a characteristic in situ direction of d 147 i 44 k 21 95 9 the corresponding paleopole falls along the middle ordovician segment of the laurentian apwp and we consider this component to be the result of a taconic remagnetization the second component the b component is carried by hematite exhibits dual polarities and passes a fold test the tilt corrected b component characteristic direction is d 92 i 17 k 16 95 13 the corresponding paleopole at 4 s 193 e falls near a well established late cambrian 505 ma pole for laurentia and we consider this component to be a remagnetization during a late cambrian tectonic event in the central appalachians the third component isolated in the catoctin basalts the a component yields a tilt corrected mean of d 68 i 84 k 59 95 9 this component passes a fold and reversal test a suite of samples was collected from two catoctin feeder dikes and surrounding country rocks that yield a positive baked contact test the a pole at 43 s 128 e falls significantly away from previously proposed vendian poles for laurentia a reevaluation of previous paleomagnetic studies from coeval rock units reveals similarly steep directions and leads us to propose a new apwp this new apw track indicates that laurentia was located near the pole during the interval 615 580 ma and drifted rapidly 16 cm yr 1 toward its late cambrian equatorial position

organic acid metastability in sedimentary basins thermodynamic calculations indicate that co sub 2 in sedimentary basins may be in stable redox equilibrium with organic acids found in basinal brines but that ch sub 4 is not these calculations also show that the high concentrations of carboxylic acids are from equilibrium with decarboxylation reactions a compilation of recent analyses of carboxylic acids from aqueous solutions associated with petroleum reservoirs shows that there is no trend in acid concentrations with temperature however inspection of these data at approximately 100 c indicates that acetic and propanoic acids are in homogeneous metastable equilibrium oxygen fugacities indicated by this metastable assemblage are consistently lower than those set by iron oxide assemblages this is not surprising because both hematite and magnetite are unstable relative to siderite in the presence of detectable concentrations of acetic acid

low temperature aqueous alteration on the cr chondrite parent body implications from in situ oxygen isotope analyses the presence of hydrated minerals in chondrites indicates that water played an important role in the geologic evolution of the early solar system however the process of aqueous alteration is still poorly understood renazzo like carbonaceous cr chondrites are particularly well suited for the study of aqueous alteration samples range from being nearly anhydrous to fully altered essentially representing snapshots of the alteration process through time we studied oxygen isotopes in secondary minerals from six cr chondrites of varying hydration states to determine how aqueous fluid conditions including composition and temperature evolved on the parent body secondary minerals analyzed included calcite dolomite and magnetite the o isotope composition of calcites ranged from 18o 9 to 35 dolomites from 18o 23 to 27 and magnetites from 18o 18 to 5 calcite in less altered samples showed more evidence of fluid evolution compared to heavily altered samples likely reflecting lower water rock ratios most magnetite plotted on a single trend with the exception of grains from the extensively hydrated chondrite mil 090292 the mil 090292 magnetite diverges from this trend possibly indicating an anomalous origin for the meteorite if magnetite and calcite formed in equilibrium then the relative 18o fractionation between them can be used to extract the temperature of co precipitation isotopic fractionation in al rais carbonate magnetite assemblages revealed low precipitation temperatures 60 c assuming that the cr parent body experienced closed system alteration a similar exercise for parallel calcite and magnetite o isotope arrays yields global alteration temperatures of 55 to 88 c these secondary mineral arrays indicate that the o isotopic composition of the altering fluid evolved upon progressive alteration beginning near the al rais water composition of 17o 1 and 18o 10 and becoming increasingly 16o enriched toward a final fluid composition of 17o 1 2 and 18o 15

an oxygen isotope dichotomy in cm2 chondritic carbonates a sims approach abstract we present petrologic and secondary ion mass spectrometry sims oxygen isotope analyses of ca carbonate within a group of paired antarctic cm2 chondrites the carbonates can be grouped into two isotopically and morphologically distinct populations type 1 grains small matrix grains possess average 18 o of 33 7 2 3 1 and average 17 o of 0 81 0 90 1 type 2 grains calcite aggregates possess distinct oxygen isotopic compositions average 18 o of 19 4 1 5 1 and average 17 o of 1 98 0 9 1 these differences are interpreted to indicate that the two populations of calcite formed under different conditions at different times the carbonates have textural features that suggest an extraterrestrial origin the data presented here fall within error of a previously measured array for carbonates from cm falls benedix et al 2003 the presence of two generations of carbonate suggests carbonate formation in two discrete events on the parent body of these meteorites the oxygen isotopic data presented here deviate from prior bulk carbonate measurements undertaken for these meteorites most likely this deviation is because bulk carbonate analyses included vein carbonate which formed during terrestrial weathering

extended chronologies of aqueous alteration in the cm2 carbonaceous chondrites evidence from carbonates in queen alexandra range 93005 abstract the antarctic cm2 carbonaceous chondrite que 93005 contains four compositionally distinct carbonates namely breunnerite calcite dolomite and a ca poor dolomite these carbonates can form monomineralic grains or may be intergrown as bimineralic grains consisting of dolomite plus breunnerite and dolomite plus calcite or polymineralic grains containing an intergrowth of breunnerite ca poor dolomite and calcite carbonates in all grain types have inclusions of fe ni sulphides and or mg fe phyllosilicates in the bimineralic grains dolomite crystallised first to be overgrown by breunnerite or partially replaced by calcite polymineralic grains are concentrically layered with breunnerite crystallising first on pore margins to be later etched then overgrown and partially replaced by ca poor dolomite that was itself partly dissolved prior to being overgrown by calcite calcite and dolomite have also cemented fractures that cross cut the fine grained rims to aqueously altered chondrules and were formed by expansion of the chondrules during their hydration overall the sequence of mineralisation in que 93005 was 1 dolomite 2 breunnerite 3 ca poor dolomite then 4 calcite this secular change in carbonate composition and mineralogy reflects changing solution composition and probably also provenance mg fe phyllosilicates replaced dolomite breunnerite and ca poor dolomite prior to calcite crystallisation and most or all of the sulphides formed after both the phyllosilicates and calcite following sulphide crystallisation the edges of carbonate grains were abraded either by impact gardening or as a consequence of fluidisation of the matrix during rapid loss of gas or vapour determination of the crystallisation age of dolomite via the mn cr system indicates that aqueous alteration of que 93005 began on or before 3 93 0 23 ma after the formation of the oldest solar system solids overall the water rock ratio and fo2 during alteration of que 93005 was similar to that of the cm1s and cr1s but the lower degree of alteration of que 93005 overall suggests that alteration timescales were shorter possibly due to loss of intergranular liquid water during fluidisation

the cm carbonaceous chondrite regolith diepenveen a carbonaceous chondrite was recovered immediately after the fall near the village of diepenveen in the netherlands on october 27 1873 but came to light only in 2012 analysis of sodium and poly aromatic hydrocarbon content suggests little contamination from handling diepenveen is a regolith breccia with an overall petrology consistent with a cm classification unlike most other cm chondrites the bulk oxygen isotopes are extremely 16o rich apparently dominated by the signature of anhydrous minerals distributed on a steep slope pointing to the domain of intrinsic cm water a small subset plots closer to the normal cm regime on a parallel line 2 lower in 17o different lithologies in diepenveen experienced varying levels of aqueous alteration processing being less aqueously altered at places rather than more heated the presence of an agglutinate grain and the properties of methanol soluble organic compounds point to active impact processing of some of the clasts diepenveen belongs to a cm clan with 5 ma cre age longer than most other cm chondrites and has a relatively young k ar resetting age of 1 5 ga as a cm chondrite diepenveen may be representative of samples soon to be returned from the surface of asteroid 162173 ryugu by the hayabusa2 spacecraft

low temperature volatile production at the lost city hydrothermal field evidence from a hydrogen stable isotope geothermometer although commonly utilized in continental geothermal work the water hydrogen and methane hydrogen isotope geothermometers have been neglected in hydrothermal studies here we report d ch4 and d h2 values from high temperature black smoker type hydrothermal vents and low temperature carbonate hosted samples from the recently discovered lost city hydrothermal field methane deuterium content is uniform across the dataset at 120 12 hydrogen d values vary from 420 to 330 at high temperature vents to 700 to 600 at lost city the application of several geothermometer equations to a suite of hydrothermal vent volatile samples reveals that predicted temperatures are similar to measured vent temperatures at high temperature vents and 20 60 c higher than those measured at the lost city vents we conclude that the overestimation of temperature at lost city reflects 1 that methane and hydrogen are produced by serpentinization at n110 c and 2 that isotopic equilibrium at temperatures b70 c is mediated by microbial sulfate reduction the successful application of hydrogen isotope geothermometers to low temperature lost city hydrothermal samples encourages its employment with lowtemperature diffuse hydrothermal fluids 2005 elsevier b v all rights reserved

deuterium enrichments in chondritic macromolecular material implications for the origin and evolution of organics water and asteroids here we report the elemental and isotopic compositions of the insoluble organic material iom isolated from several previously unanalyzed meteorites as well as the reanalyses of h isotopic compositions of some previously measured samples alexander et al 2007 the iom in ordinary chondrites ocs has very large d enrichments that increase with increasing metamorphism and decreasing h c the most extreme d value measured being almost 12 000 we propose that such large isotopic fractionations could be produced in the oc parent bodies through the loss of isotopically very light h2 generated when fe was oxidized by water at low temperatures 200 c we suggest that similar isotopic fractionations were not generated in the iom of cv and co chondrites with similar metamorphic grades and iom h c ratios because proportionately less water was consumed during metamorphism and the remaining water buffered the h isotopic composition of the iom even a h was being lost from it hydrogen would also have been generated during the alteration of ci cm and cr carbonaceous chondrites the iom in these meteorites exhibit a considerable range in isotopic compositions but all are enriched in d as well as 15n relative to terrestrial values we explore whether these enrichments could also have been produced by the loss of h2 but conclude that the most isotopically anomalous iom compositions in meteorites from these groups are probably closest to their primordial values the less isotopically anomalous iom has probably been modified by parent body processes the response of iom to these processes was complex and varied presumably reflecting differences in conditions within and between parent bodies the d enrichments associated with h2 generation along with exchange between d rich iom and water in the parent bodies means that it is unlikely that any chondrites retain the primordial h isotopic composition of the water ice that they accreted the h isotopic compositions of the most water rich chondrites the cms and cis are probably the least modified and their compositions d 25 suggest that their water did not form at large radial distances from the sun where ice is predicted to be very d rich yet models to explain the o isotopic composition of inner solar system bodies require that large amounts of ice were transported from the outer to the inner solar system

comparison of clumped isotope signatures of dolomite cements to fluid inclusion thermometry in the temperature range of 73 176 c abstract widespread application of the novel clumped isotope paleothermometer 47 using dolomite samples from shallow crustal settings has been hindered by a lack of adequate constraints on clumped isotope systematics in dolomites that formed at temperatures greater than 50 c consequently many high temperature applications involving diagenetic dolomites have required an assumption that the relationship between temperature and 47 in diagenetic dolomite resembles the theoretical temperature dependence for calcite here we present 47 results from dolomite cements for which precipitation temperatures were determined independently using fluid inclusion microthermometry we compare a rock based calibration for samples from the temperature range of 73 to 176 c to previously published laboratory derived calibrations for synthetic calcites this novel combination of approaches yields results that are broadly consistent with results reported from controlled laboratory experiments providing an important confirmation of the utility of clumped isotopes in real world systems our results suggest that the 47 of dolomite cements may provide key information in the reconstruction of burial and thermal histories and also in the recognition of potential petroleum reservoirs

clumped isotope geochemistry the study of naturally occurring multiply substituted isotopologues clumped isotope geochemistry is concerned with the state of ordering of rare isotopes in natural materials that is it examines the extent to which rare isotopes d 13 c 15 n 18 o etc bond with or near each other rather than with the sea of light isotopes in which they swim abundances of isotopic clumps in natural materials are influenced by a wide variety of factors in most cases their concentrations approach within ca 1 relative the amount expected for a random distribution of isotopes deviations from this stochastic distribution result from enhanced thermodynamic stability of heavy isotope clumps slower kinetics of reactions requiring the breakage of bonds between heavy isotopes the mass dependence of diffusive and thermo gravitational fractionations mixing between components that differ from one another in bulk isotopic composition biochemical and photochemical fractionations that may reflect combinations of these simpler physical mechanisms and in some cases other processes we do not yet understand although clumped isotope geochemistry is a young field several seemingly promising applications have already emerged most importantly it appears that proportions of 13 c 18 o bonds in carbonate minerals are sensitive to their growth temperatures independent of bulk isotopic composition thus clumped isotope analysis of ancient carbonates can be used as a quantitative paleothermometer that requires no assumptions about the 18 o of waters from which carbonates grew this approach has been used to reconstruct marine temperatures across the phanerozoic reaching back to the silurian terrestrial ground temperatures across the cenozoic thermal histories of aqueously altered meteorites among other applications clumped isotope geochemistry is also placing new constraints on the atmospheric budget and stratospheric photochemistry of co 2 and should be capable of placing analogous new constraints on the budgets of other atmospheric gases finally this field could be extended to encompass sulfates volatile hydrocarbons organic moieties and other materials

correlated alteration effects in cm carbonaceous chondrites three parameters are proposed to determine the relative extent of alteration in cm chondrites the mineralogic alteration index monitors the relative progress of coupled substitutions in the progressive alteration of cronstedtite to mg serpentine and increases with increasing alteration to calculate values of this index an algorithm has been developed to estimate the average matrix phyllosilicate composition in individual cm chondrites the second parameter is the volume percent of isolated matrix silicates which decreases with progressive alteration due to mineral hydration finally the volume percent of chondrule alteration monitors the extent of chondrule phyllosilicate production and increases as alteration proceeds these parameters define the first cm alteration scale that relies on multiple indicators of progressive alteration the following relative order of increasing alteration is established by this model murchison bells pollen murray mighei nogoya cold bokkeveld the relative degree of aqueous processing cochabamba and boriskino experienced is less precisely constrained although both fall near the middle of this sequence a comparison between the mineralogic alteration index and literature values for the whole rock chemistry of cm chondrites reveals several correlations a positive nearly linear correlation between bulk h content and progressive cm alteration suggests an approximately constant production rate of new phyllosilicates relative to the mineralogical transition from cronstedtite to mg serpentine the abundance of trapped planetary 36ar decreases systematically in progressively altered cm chondrites suggesting the wholesale destruction of primary noble gas carrier phase s by aqueous reactions because low temperature fluid rock reactions are generally associated with large isotopic mass fractionation factors we also compared our model predictions with 18o values for bulk cm samples although some of these data are poorly resolved the order of increasing 18o values approximates the order of increasing alteration predicted by our model parameters multiple correlations between diverse alteration parameters strongly suggest that a different cm chondrites experienced similar kinds of processes and conditions and b cm materials experienced in situ alteration on the cm parent body or bodies

early cambrian continental reconstructions climatically related sedimentary facies and faunal distributions have been combined with palaeomagnetic data to make provisional reconstructions of the early cambrian world laurentia baltica and siberia appear to have formed a continental group which rifted apart from each other prior to 600 ma the consolidation of much of gondwana probably occurred in the pan african orogeny well before the cambrian though the assembly of several east asian terranes is still speculative the archaeocyathan reefs of siberia southern europe and morocco with their bigotinid trilobite fauna suggest that these areas were adjacent to each other and at low latitudes avalonia had close faunal links with western gondwana but lacked bigotinids and archaeocyathans and may have been situated off west africa and florida during the early cambrian there was a northward migration of laurentia towards the equator as its separation from baltica and siberia increased and a southward movement of gondwana

have olivine will gas serpentinization and the abiogenic production of methane on mars 1 spatial variability of methane ch4 on mars suggests the presence of localized subsurface sources here we show that olivine hydration in the martian regolith and crust may be a major ch4 source which contributed significantly to the warming of early mars methane production is kinetically and thermodynamically favored during low t aqueous alteration of olivine rich rocks sustained release of ch4 on present day mars may come through the breakdown of ancient ch4 hydrates and from springs driven by geothermal heat

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

an early solar system magnetic field recorded in cm chondrites abstract we present a paleomagnetic study of seven cm carbonaceous chondrites cm chondrites are believed to be some of the most chemically primitive materials available in our solar system and may sample the continuum of transitional objects between asteroids and comets formed in the outer solar system as such cm chondrites can help us to understand primordial aspects of the history of the early solar system including protoplanetary disk and planetesimal magnetism the ferromagnetic assemblage of cm chondrites is composed of a mixture of primary metallic iron pyrrhotite and magnetite the remanent properties are usually dominated by secondary pyrrhotite paleomagnetic analyses using thermal and alternating field demagnetization identified a stable origin trending component of magnetization in the seven studied cm chondrites in each meteorite this component is homogeneous in direction at least at the cm scale and is therefore post accretional we interpret this stable component as a pre terrestrial chemical remanent magnetization acquired during crystallization of magnetite and pyrrhotite during parent body aqueous alteration in a field of at least a few t 2 1 5 t considering the timescale and intensities of primordial magnetic fields both internally generated fields from a putative dynamo and external fields generated in the protoplanetary disk may have been recorded by cm chondrites it is presently difficult to discriminate between the two hypotheses regardless cm chondrites likely contain the oldest paleomagnetic record yet identified

paleoelevation reconstruction using pedogenic carbonates paleoelevation reconstruction using stable isotopes although a relatively new science is making a significant contribution to our understanding of the recent growth of the world s major orogens in this review we examine the use of both light stable isotopes of oxygen and the new clumped isotope 47 carbonate thermometer in carbonates from soils globally the oxygen isotopic composition 18 o of rainfall decreases on average by about 2 8 km of elevation gain this effect of elevation will in turn be archived in the 18 o value of soil carbonates and paleoelevation can be reconstructed provided 1 temperature of formation can be estimated 2 the effects of evaporation are small 3 the effects of climate change can be accounted for and 4 the isotopic composition of the carbonate is not diagenetically altered we review data from modern soils to evaluate some of these issues and find that evaporation commonly elevates 18 o values of carbonates in deserts an effect that would lead to underestimates of paleoelevation some assessment of paleoaridity using qualitative indicators or carbon isotopes from soil carbonate is therefore useful in evaluating the oxygen isotope based estimates of paleoelevation sampling from deep 50 cm in paleosols helps reduce the uncertainties arising from seasonal temperature fluctuations and from evaporation the new clumped isotope 47 carbonate thermometer expressed as 47 offers an independent and potentially very powerful approach to paleoelevation reconstruction in contrast to the use of 18 o values nothing need be known about the isotopic composition of water from which carbonate grew in order to estimate of temperature of carbonate formation from 47 values using assumed temperature lapse rates with elevation paleoelevations can thereby be reconstructed case studies from the andes and tibet show how these methods can be used alone or in combination to estimate paleoelevation in both cases the potential for diagenetic alteration of primary carbonate values first has to be assessed clear examples of both preservation and alteration of primary isotopic values are available from deposits of varying ages and burial histories 47 values constitute a relatively straightforward test since any temperature in excess of reasonable surface temperatures points to diagenetic alteration for 18 o values preservation of isotopic heterogeneity between different carbonate phases offers a check on diagenesis results of these case studies show that one area of south central tibet attained elevations comparable to today by the late oligocene whereas 2 7 0 4 km of uplift occurred in the bolivian altiplano during the late miocene

53mn 53cr dating of aqueously formed carbonates in the cm2 lithology of the sutter s mill carbonaceous chondrite radiometric dating of secondary minerals can be used to constrain the timing of aqueous alteration on meteoritic parent bodies dolomite is a well documented secondary mineral in cm chondrites and is thought to have formed by precipitation from an aqueous fluid on the cm parent body within several million years of accretion the petrographic context of crosscutting dolomite veins indicates that aqueous alteration occurred in situ rather than in the nebular setting here we present 53mn 53cr systematics for dolomite grains in sutter s mill section sm51 1 the mn cr isotope data show well resolved excesses of 53cr correlated with 55mn 52cr ratio which we interpret as evidence for the in situ decay of radioactive 53mn after correcting for the relative sensitivities of mn and cr using a synthetic mn and cr bearing calcite standard the data yield an isochron with slope corresponding to an initial 53mn 55mn ratio of 3 42 0 86 10 6 the reported error includes systematic uncertainty from the relative sensitivity factor when calculated relative to the u corrected pb pb absolute age of the d orbigny angrite sutter s mill dolomites give a formation age between 4564 8 and 4562 2 ma 2 4 5 0 myr after the birth of the solar system this age is contemporaneous with previously reported ages for secondary carbonates in cm and ci chondrites consistent carbonate precipitation ages between the carbonaceous chondrite groups suggest that aqueous alteration was a common process during the early stages of parent body formation probably occurring via heating from internal 26al decay the high precision isochron for sutter s mill dolomite indicates that late stage processing did not reach temperatures that were high enough to further disturb the mn cr isochron

paleoclimate reconstruction using carbonate clumped isotope thermometry carbonate clumped isotope thermometry is a relatively new paleotemperature proxy based on measurements of the degree of ordering of 13 c and 18 o into bonds with each other making the 13 c 18 o 16 o 2 2 ion group in lattices of carbonate minerals this technique has several unusual properties that complement existing methods of paleoclimate reconstruction most importantly it is based on a homogeneous isotope exchange equilibrium and thus constrains temperature independent of the isotopic composition of waters from which carbonates grew this method also appears to be generally insensitive to vital effects that compromise many other paleothermometers based on the chemical properties of biominerals or organic matter at least for those organisms that have been subjected to systematic study to date corals and foraminifera however discrepancies among some calibrations particularly at low temperatures may point toward the existence of vital effects in mollusks and other organisms this review discusses the principles and calibrations of the technique its uses in combination with conventional stable isotope measurements to constrain the 18 o of past waters preservation of paleotemperatures in ancient materials as well as current problems in our understanding of calibrations and interlaboratory data comparisons

sm nd geochemistry and u pb geochronology of the mont rigaud stock quebec canada a late magmatic event associated with the formation of the iapetus rift abstract the mont rigaud intrusion is a syeno granitic stock located in the st lawrence river ottawa river graben and surrounded by sedimentary rocks of the st lawrence lowlands the intrusion is circular and divided into three units an outer ring of hornblende syenite an inner ring of feldspar hornblende syenite and an inner core of quartz syenite and granite geochemical u pb and nd isotopic data gathered from the stock indicate that all three units are comagmatic and were intruded within a short period of time rock samples from the intrusion have a geochemical profile typical of within plate a type granites the pluton is enriched in high field strength elements and rare earth elements and is characterized by negative eu anomalies in the more fractionated units the end values from the two syenitic units range from 1 5 to 2 2 while the granite yields values of about 0 5 the nd isotopic data lie below depleted mantle values but above those of the underlying grenville crust 2 5 thus the

a lower ordovician paleomagnetic pole from the oneota dolomite upper mississippi river valley of 149 oriented specimens of the lower ordovician oneota dolomite subjected to stepwise thermal or alternating field demagnetization 50 contained a characteristic shallow east southeasterly or west northwesterly component of magnetization identifiable on orthogonal vector projections for 48 other specimens the demagnetization trajectories defined greats circle paths which intersect close to the characteristic direction the site mean directions for the characteristic component were determined by using both the characteristic directions and great circle data and these yield a formation mean direction obtained from 10 sites with a declination of 105 an inclination of 2 and a precision parameter k of 17 6 95 11 9 the mean of the site virtual poles falls at 10 4 n 166 4 e k 48 0 a95 7 0 which is close to previously published late cambrian paleopoles both polarity groups have predominantly positive inclinations suggesting the presence of an uncleaned downward component whose unblocking temperature and coercivity spectra overlap very strongly with those of the characteristic magnetization we interpret this downward component as a drilling induced remanent magnetization and we hope to have removed its contribution by averaging the dual polarity results multicomponent magnetizations were also present in samples collected from blocks of oneota dolomite incorporated into the overlying new richmond sandstone lower ordovician but only six out of eight samples yielded well defined greats circle paths the best fit intersection of these defines a northerly and steep recent component while the directions of the higher stability components are different for each block due to the small number of samples and imperfect resolution of magnetic directions the conglomerate test is not conclusive nevertheless it suggests that the stable remanence in the oneota dolomite was acquired prior to the deposition of the new richmond sandstone

palaeozoic palaeogeography of laurentia and its margins a reassessment of palaeomagnetic data summary the recognition of wide spread remagnetization events during the palaeozoic has greatly added to the problems of reconstructing the palaeozoic drift history of laurentia and in recent years increasing emphasis has been placed on evaluating the relative reliability of palaeomagnetic data in an attempt to overcome some of these difficulties in the light of these developments a revised apparent polar wander apw path has been constructed following a rigorous evaluation of all published palaeozoic palaeomagnetic data from laurentia and its palaeozoic margins i e north america northern britain northern ireland and greenland the use of data from the laurentian margins when corrected for the opening of the north atlantic has resulted in a revision of previously published palaeozoic palaeogeographic reconstructions and these new palaeogeographic reconstructions for laurentia are in good agreement with faunal and lithological data a comparison of this new apw path with other apw paths from avalonia and baltica reveals a very close agreement between all three paths from mid silurian times onwards indicating that the closure of the iapetus ocean and the tornquist sea was essentially complete by this time laurentia occupied equatorial latitudes from cambrian to early silurian times and its drift history was dominated by anti clockwise rotation up to 0 7 ma 1 and low latitudinal drift rates up to 3 cm yr 1 late silurian and early devonian times were characterized by increased rates of anti clockwise rotation up to 1 3 ma 1 and southerly directed latitudinal drift rates up to 13 cm yr 1 of the continent which place laurentia in moderate southerly latitudes 30 40 s by the early devonian these palaeolatitudes are comparable with those from avalonia and baltica for this time indicating that the amalgamation of eurmerica had already taken place by late devonian time laurentia had begun to drift north rates up to 5 cm yr 1 towards equatorial latitudes by mid carboniferous time

mineralogy of iron sulfides in cm1 and ci1 lithologies of the kaidun breccia records of extreme to intense hydrothermal alteration the polymict kaidun microbreccia contains lithologies of c type chondrites with euhedral iron sulfide crystals of hydrothermal origin our fib tem study reveals that acicular sulfides in a cm1 lithology are composed of fe rich pyrrhotite with nonintegral vacancy superstructures nc pyrrhotite troilite and pentlandite all showing distinct exsolution textures based on phase relations in the fe ni s system we constrain the temperature of formation of the originally homogeneous monosulfide solid solution to the range of 100 300 c in some crystals the exsolution of pentlandite and the microtextural equilibration was incomplete probably due to rapid cooling we use thermodynamic modeling to constrain the physicochemical conditions of the extreme hydrothermal alteration in this lithology unless the cm1 lithology was sourced from a large depth in the parent body internal pressure 85 bar or the temperatures were in the lower range of the interval determined the water was likely present as vapor previously described light 34s compositions of sulfides in kaidun s cm1 lithology are likely due to the loss of 34s enriched h2s during boiling platy sulfide crystals in an adjacent intensely altered ci1 lithology are composed of fe poor monoclinic 4c pyrrhotite and nc pyrrhotite and probably formed at lower temperatures and higher fs2 relative to the cm1 lithology however a better understanding of the stability of fe poor pyrrhotites at temperatures below 300 c is required to better constrain these conditions

opaque minerals in chondrules and fine grained chondrule rims in the bishunpur ll3 1 chondrite we present a detailed petrographic and electron microprobe study of metal grains and related opaque minerals in the chondrule interiors and rims of the bishunpur ll3 1 ordinary chondrite there are distinct differences between metal grains that are completely encased in chondrule interiors and those that have some portion of their surface exposed outside of the chondrule boundary even though the two types of metal grains can be separated by only a few microns metal grains in chondrule interiors exhibit minor alteration in the form of oxidized p cr and si bearing minerals metal grains at chondrule boundaries and in chondrule rims are extensively altered into troilite and fayalite the results of this study suggest that many metal grains in bishunpur reacted with a type i chondrule melt and incorporated significant amounts of p cr and si as the system cooled some metal oxidation occurred in the chondrule interior producing metal associated phosphate chromite and silica metal that migrated to chondrule boundaries experienced extensive corrosion as a result of exposure to the external atmosphere present during chondrule formation it appears that chondrule derived metal and its corrosion products were incorporated into the fine grained rims that surround many type i chondrules contributing to their fe rich compositions we propose that these fine grained rims formed by a combination of corrosion of metal expelled from the chondrule interior and accretion of fine grained mineral fragments and microchondrules

the age of the sept iles layered mafic intrusion canada implications for the late neoproterozoic cambrian history of southeastern canada the large sept iles layered mafic intrusion of eastern canada has been precisely dated by u pb zircon methods at 565 4 ma consideration of other magmatic events at that time in southeastern canada reveals three different phases of magmatism the first phase was composed of two tholeiitic dike swarms at 615 ma and 590 ma the second including the sept iles intrusion comprised a widespread series of mostly alkaline intrusions emplaced around 575 565 ma along the rift faults of the st lawrence graben the last phase was composed predominantly of alkali and transitional basalts now exposed in the appalachians erupted during the latest neoproterozoic and early cambrian the second and third phases are best accounted for by the arrival of a major mantle plume near the town of sept iles 565 million years ago the widespread but volumetrically minor alkali magmatism that peaked 10 m y earlier is typical of other plumes elsewhere the plume model is reinforced by sedimentological evidence for a crusta

carbonate abundances and isotopic compositions in chondrites we report the bulk c abundances and c and o isotopic compositions of carbonates in 64 cm chondrites 14 cr chondrites 2 ci chondrites lew 85332 c2 kaba cv3 and semarkona ll3 0 for the unheated cms the total ranges of carbonate isotopic compositions are 13c 25 75 and 18o 15 35 and bulk carbonate c contents range from 0 03 to 0 60 wt there is no simple correlation between carbonate abundance and isotopic composition or between either of these parameters and the extent of alteration unless accretion was very heterogeneous the uncorrelated variations in extent of alteration and carbonate abundance suggests that there was a period of open system behavior in the cm parent body probably prior to or at the start of aqueous alteration most of the ranges in cm carbonate isotopic compositions can be explained by their formation at different temperatures 0 130 c from a single fluid in which the carbonate o isotopes were controlled by equilibrium with water 18o 5 and the c isotopes were controlled by equilibrium with co and or ch4 13c 33 or 20 for co or ch4 dominated systems respectively however carbonate formation would have to have been inefficient otherwise carbonate compositions would have resembled those of the starting fluid a quite similar fluid composition 18o 5 5 and 13c 31 or 17 for co or ch4 dominated systems respectively can explain the carbonate compositions of the cis although the formation temperatures would have been lower 10 40 c and the relative abundances of calcite and dolomite may play a more important role in determining bulk carbonate compositions than in the cms the cr carbonates exhibit a similar range of o isotopes but an almost bimodal distribution of c isotopes between more 13c 65 80 and less altered samples 13c 30 40 this bimodality can still be explained by precipitation from fluids with the same isotopic composition 18o 9 25 and 13c 21 or 8 for co or ch4 dominated systems respectively if the less altered crs had higher mole fractions of co2 in their fluids semarkona and kaba carbonates have some of the lightest c isotopic compositions of the meteorites studied here probably because they formed at higher temperatures and or from more co2 rich fluids the fluids responsible for the alteration of chondrites and from which the carbonates formed were almost certainly accreted as ices by analogy with cometary ices co2 and or co would have dominated the trapped volatile species in the ices the chondrites studied are too oxidized for co dominated fluids to have formed in their parent bodies if ch4 was the dominant c species in the fluids during carbonate formation it would have to have been generated in the parent bodies from co and or co2 when oxidation of metal by water created high partial pressures of h2 the fact that the chondrite carbonate c h2o mole ratios are of the order predicted for co co2 h2o ices that experienced temperatures of 50 100 k suggests that the chondrites formed at radial distances of 4 15 au

uplift of the central andes of nw argentina associated with upper crustal shortening revealed by multiproxy isotopic analyses this study contributes to the uplift history of the andes which has received increasing attention in recent years because of its implications for geodynamic models and climate feedbacks shortening resulting in crustal thickening and removal of gravitationally unstable mantle lithosphere has been proposed to control deformation and uplift of cordillera type orogenic systems such as the puna plateau of the central andes and its eastern margin the eastern cordillera we present new clumped isotope 47 18o and 2 data from carbonate nodules marlstone spring deposits and volcanic ashes from the puna plateau and eastern cordillera of nw argentina when combined with other geological evidence our data indicate that the puna plateau was near its present elevation since at least 10 ma whereas the eastern cordillera rose 1 5 km between 14 and 7 ma this history of uplift correlates with active shortening in the eastern cordillera and with incorporation of a regional foreland into the propagating orogenic wedge our study suggests that the elevation of the puna plateau changed little during the miocene pliocene whereas the margin experienced significant uplift associated with active deformation and crustal thickening

the oxygen isotopic composition of water from tagish lake its relationship to low temperature phases and to other carbonaceous chondrites the fall and recovery of the tagish lake meteorite in british columbia in january 2000 provided a unique opportunity to study relatively pristine samples of carbonaceous chondrite material measurements of the oxygen isotopic composition of water extracted under stepped pyrolysis from a bulk sample of this meteorite have allowed us to make comparisons with similar data obtained from ci and cm chondrites and so further investigate any relationships that may exist between these meteorites the much lower yield of water bearing a terrestrial signature in tagish lake is indicative of the pristine nature of the meteorite the relationship between the isotopic composition of this water and reported isotopic values for carbonates bulk matrix and whole rock have been used to infer the extent and conditions under which parent body aqueous alteration occurred in tagish lake the difference in 17o isotopic composition between the water and other phases is greater than that found in either cms or cis suggesting that reaction and isotopic exchange between components was more limited this in turn suggests that in the case of tagish lake conditions during the processes of aqueous alteration on the parent body which ultimately controlled the formation of new minerals were distinct from those on both ci and cm parent bodies

constraints on the anhydrous precursor mineralogy of fine grained materials in cm carbonaceous chondrites mass balance calculations were performed to constrain the precursor mineralogy of fine grained aqueously altered materials in cm carbonaceous chondrites the bulk composition of unaltered fine grained cm materials was estimated and then used to calculate phase proportions for several different initial assemblages all starting assemblages contain relic unaltered fe poor phases observed in cm chondrites plus iron sulfides the original sources of fe are uncertain because most primary fe rich phases were aqueously altered four endmember assemblages are considered by adding fe metal fa50 fa100 or feo rich amorphous materials to the fe poor phases these represent the fe bearing phases in cm and or other chondritic classes results indicate that the precursor cm assemblage may have contained a maximum of either 10 vol fe metal 57 vol fa50 28 vol fa100 or 37 0 vol feo rich amorphous materials additional calculations were performed in which fe metal was added to the various feo bearing assemblages these reveal a strong positive correlation between the forsterite forsterite enstatite ratio and the amount of feo bearing phases that coexist with metal if forsterite was more abundant than low ca pyroxene in the accreted cm materials then these materials must have also contained significant amounts of feo rich phases e g at least 36 vol fa50 10 vol fa100 or 17 vol feo bearing glasses calculated mineral proportions suggest that intact calcium aluminum rich inclusions cais represent only about one half of the original cai budget which is consistent with previous hypotheses that the initial cai contents of cm and co chondrites were similar some similarities exist between the primary cm assemblages calculated here and the mineralogies of other chondrite classes but the initial cm materials do not appear to be represented in our meteorite inventory

hydrogen isotope evidence for the origin and evolution of the carbonaceous chondrites we present new hydrogen isotope data for separated matrix hydrated chondrules and other hydrated coarse silicate fragments from nine carbonaceous chondrites these data were generated using a micro analytical method involving stepped combustion of tens to hundreds of micrograms of hydrous solids we also re evaluate hydrogen isotope data from previous conventional stepped combustion experiments on these and other carbonaceous chondrites hydrogen isotope compositions of matrix and whole rock samples of cm chondrites are correlated with oxygen isotope indices major and minor element abundances and abundance and isotope ratios of other highly volatile elements these correlations include a monotonic decrease in d with increasing extent of aqueous alteration and decreasing abundances of highly volatile elements including c n and ar between extremes of 0 least altered most volatile rich and 200 most altered least volatile rich in plots involving only abundances and or isotope ratios of highly volatile elements ci chondrites fall on the high d volatile rich end of the trends defined by cm chondrites i e ci chondrites resemble the least altered cm chondrites in these respects these trends suggest the protoliths of the cm chondrites i e before aqueous alteration contained an assemblage of volatiles having many things in common with those in the ci chondrites if so then the volatile element inventory of the ci chondrites was a more widespread component of early solar system objects than suggested by the scarcity of recognized ci meteorites differences in volatile element chemistry between the ci and average cm chondrites can be attributed to aqueous alteration of the latter previous models of carbonaceous chondrite aqueous alteration have suggested 1 the protoliths of the cm chondrites are volatile poor objects like the co or cv chondrites and 2 the ci chondrites are more altered products of the same process producing the cm chondrites both suggestions appear to be inconsistent with hydrogen isotope data and other aspects of the volatile element geochemistry of these rocks we present a model for aqueous alteration of the cm chondrites that reconciles these inconsistencies and suggests revised relationships among the major subtypes of carbonaceous chondrites our model requires among other things that the water infiltrating cm chondrites had a d value of 158 consistent with initial accretion of cm parent bodies at 4 au

calibration of the carbonate clumped isotope paleothermometer for otoliths paleothermometry is an essential tool for understanding past changes in climate the carbonate clumped isotope thermometer is a temperature proxy related to ordering of 13 c and 18 o in the carbonate lattice based on measurements of 13 c 18 o 16 o in co 2 produced by acid digestion of carbonate this thermometer has been previously calibrated for inorganic calcite and aragonitic corals ghosh p adkins j affek h balta b guo w f schauble e a schrag d and eiler j m 2006 c 13 o 18 bonds in carbonate minerals a new kind of paleothermometer geochim cosmochim acta70 6 1439 1456 here we determine the relationship between growth temperatures of aragonitic fish otoliths and abundances of 13 c 18 o 16 o produced by acid digestion of those otoliths our calibration is based on analyses of otoliths from six species from four genera of modern fish sampled from a latitudinal transect of the atlantic ocean between 54 s and 65 n plus one species from the tropical western pacific the temperatures at which fish otoliths precipitated were estimated by the mean temperature in the waters in which they lived averaged over their estimated lifetimes estimated growth temperatures of our samples vary between 2 and 25 c our results show that the abundance of 13 c 18 o 16 o in co 2 produced by acid digestion of fish otolith aragonite is a function of growth temperature following the relationship 47 0 0568x10 6 t 2 where 47 is the enrichment in per mil of 13 c 18 o 16 o in co 2 relative to the amount expected for a stochastic random distribution of isotopes among all co 2 isotopologues and t is the temperature in kelvin this relationship closely approaches that previously documented for inorganic calcite and aragonitic coral ghosh et al 2006

replacement of olivine by serpentine in the carbonaceous chondrite nogoya cm2 abstract coarse chondrule and isolated olivine in some cm chondrites is replaced by serpentine in both centripetal and meshwork replacement textures locally preserved textures formed by partial replacement of coarse olivine by serpentine in the carbonaceous chondrite nogoya cm2 establish unique associations between each individual mass of serpentine and the specific olivine from which that serpentine formed electron probe microanalyses show that the composition of serpentine replacing coarse olivine is uniform throughout all analyzed volumes of nogoya and is independent of the composition of the olivine being replaced if as previously proposed late stage alteration fluids were mg rich because fe source minerals were depleted in earlier stages then the uniform mg rich composition of the serpentine replacing large silicate grains during advanced stages of alteration may indicate diffusional homogenization of the aqueous solutions over progressively larger spatial scales enabled by long timescales and previously proposed stagnant or slow moving fluids the range of olivine compositions replaced in nogoya is even larger than previously reported from alh 81002 cm2 this militates against hypotheses of strong primary mineral control on the compositions of alteration products at least at advanced stages of alteration the serpentine formed by olivine replacement in nogoya is more magnesian than the counterpart serpentine replacing all anhydrous primary silicates in alh 81002 this intermeteorite heterogeneity of replacement serpentine composition between alh 81002 and nogoya indicates that the aqueous solutions in which the olivine serpentine replacement reactions occurred were of different compositions in the two different cm parent body volumes sampled by alh 81002 and nogoya the more magnesian character of serpentines in nogoya than in alh 81002 indicates that the nogoya aqueous alteration environment was even more highly evolved toward mg rich solutions than the environment indicated by the composition of the serpentine in alh 81002 persistence of primary silicate remnants within centripetal and meshwork serpentine indicates that either the aqueous alteration episodes in the parent body volumes represented by individual meteorites were too short to allow complete replacement of olivine by serpentine or one or more reactants most likely water were completely consumed before the coarse primary silicate was completely replaced seemingly incompatible arguments for and against primary mineral control of serpentine composition during cm chondrite alteration may be reconciled by considering the different grain sizes and reaction timescales that likely existed in different textural settings

terrestrial alteration of carbonate in a suite of antarctic cm chondrites evidence from oxygen and carbon isotopes the oxygen 18o 17o and carbon 13c fmod14c the fraction of modern 14c isotopic compositions of carbonate were measured for a set of paired antarctic cm chondrites eet 96006 eet 96016 eet 96017 and eet 96019 while the oxygen isotopic compositions do not plot on the terrestrial fractionation line and indicate that a component of the carbonate minerals has an extraterrestrial origin they also do not fall on the array defined for carbonates by cm falls and are thus consistent with the presence of a terrestrial carbonate component the 13c and fmod14c measurements of carbonate suggest the presence of at least two carbon sources carbonate derived from atmospheric co2 that is inferred to have been produced as a result of silicate weathering reactions and carbonate derived from another carbon source that is either old or non atmospheric the relationships between oxygen and carbon isotope data provide additional constraints on the weathering process and allow the possibility that rock dominated weathering of the meteorite caused the oxygen isotopic composition of antarctic water added to the meteorite to evolve away from the terrestrial mass fractionation array leading to formation of low temperature terrestrial alteration products that do not lie on the terrestrial fractionation line

circum iapetus paleogeography of the precambrian cambrian transition with a new paleomagnetic constraint from laurentia abstract the paleogeography of the precambrian cambrian transition is still poorly known but is fundamental for understanding late neoproterozoic climate extremes and the spatial associations of the rapidly evolving ediacaran and cambrian fauna this period has also been proposed to host unusual geodynamic events such as rapid plate motions and or the bulk tumbling of the earth with respect to its spin axis called true polar wander tpw new paleomagnetic and 40ar 39ar geochronologic results have been obtained from a pair of shallowly emplaced syenitic intrusions in western quebec the mont rigaud and the chatham grenville stocks are related to a failed rift arm of the iapetus ocean both intrusions bear hornblendes which provide overlapping 40ar 39ar plateau ages of 533 2 1 1 ma mont rigaud and 531 4 3 4 ma chatham grenville corresponding to early cambrian nemakit daldynian time characteristic paleomagnetic reversed directions from both intrusions are easterly and shallow d 89 3 i 30 1 95 6 2 k 38 8 n 15 sites and the mont rigaud intrusion also carries antipodal normal polarity directions the combined result yields a north paleopole at location 11 9 s 184 6 e dp 3 8 dm 6 9 placing the montreal region at 16 4 s paleolatitude in the early cambrian in good agreement with late ediacaran and middle to late cambrian paleomagnetic results from laurentia laurentia likely resided at low southerly paleolatitudes throughout the cambrian ruling out a major inertial interchange tpw event for early to middle cambrian time a comparison of late ediacaran to middle cambrian paleomagnetic results from laurentia baltica siberia and elements of west gondwana implies that by early cambrian time a large paleolatitudinal gap existed between laurentia and its presumed conjugate rift margins thus placing a wide iapetus ocean between them the position of baltica within the circum iapetus paleogeography is contentious due to differing interpretations of the few paleomagnetic results during ediacaran time laurentia underwent large apparent polar wander that is manifested as motion from low latitude at 615 ma to the south polar region at 590 575 ma and then rapidly back to low latitude by 565 550 ma the large and perhaps rapid apparent polar wander of laurentia and other continents in the ediacaran remains enigmatic and controversial

paleomagnetism of the cambro ordovician mcclure mountain alkalic complex colorado the middle to late cambrian loop in the north american apparent polar wander path apwp has been variously attributed to tectonic rotations remagnetizations and primary magnetizations although no primary thermal remanent magnetizations or primary detrital remanent magnetizations have as yet been demonstrated the temporally self consistent nature of the loop has been used as an argument for primary magnetizations we have studied 535 5 ma nepheline syenites and syenites of the mcclure mountain alkalic complex as well as 495 10 ma red trachyte dikes which intruded the complex in an effort to find a primary trm because zijderveld analysis yielded consistent results for only one trachyte dike remagnetization great circle analysis was employed giving a pole for the trachyte dikes at the tip of the loop 43 n l14 e while the syenites and nepheline synenites gave a pole at the base of the loop 18 n 142 e the magnetic carrier in the trachytes is hematite which apparently formed during a pervasive hydrothermal alteration k ar whole rock dating of the trachytes suggests a pennsylvanian age for the alteration and thus a late paleozoic remagnetization of the trachytes thus the low latitude cambrian pole is confirmed but we find no evidence in this study to support the primary nature of the cambrian apwp

isotope geochemistry of caliche developed on basalt enormous variations in oxygen and carbon isotopes occur in caliche developed on 3 ma basalts in 3 volcanic fields in arizona significantly extending the range of 18o and 13c observed in terrestrial caliche within each volcanic field 18o is broadly co variant with 13c and increases as 13c increases the most 18o and 13c enriched samples are for subaerial calcite developed on pinnacles knobs and flow lobes that protrude above tephra and soil the most 18o and 13c depleted samples are for pedogenic carbonate developed in soil atmospheres the pedogenic caliche has 18o fixed by normal precipitation in local meteoric waters at ambient temperatures and has low 13c characteristic of microbial soil co2 subaerial caliche has formed from 18o rich evapoconcentrated meteoric waters that dried out on surfaces after local rains the associated 13c enrichment is due either to removal of 12c by photosynthesizers in the evaporating drops or to kinetic isotope effects associated with evaporation caliche on basalt lava flows thus initially forms with the isotopic signature of evaporation and is subsequently over layered during burial by calcite carrying the isotopic signature of the soil environment the large change in carbon isotope composition in subsequent soil calcite defines an isotopic biosignature that should have developed in martian examples if mars had a warm wet early period and photosynthesizing microbes were present in the early soils the approach can be similarly applied to terrestrial precambrian paleocaliche in the search for the earliest record of life on land large variations reported for 18o of carbonate in martian meteorite alh84001 do not necessarily require high temperatures playa lakes or flood runoff if the carbonate is an example of altered martian caliche

oxygen isotopic ratios of primordial water in carbonaceous chondrites abstract in this work i estimate the 18 o and 17 o values of primordial water in cm chondrites to be 55 13 and 35 9 respectively based on whole rock o and h data also i found that the o and or h data of antarctic meteorites are biased which is attributed to terrestrial weathering this characteristic o isotopic ratio of water together with corresponding water abundances in cm chondrites are consistent with the origin of water as ice processed by photochemical reactions at the outer regions of the solar nebula where mass independent o isotopic fractionation and water destruction may have occurred another possible mechanism to produce the inferred o isotopic ratio of water would be o isotopic fractionation between water vapor and ice which likely occurred near the condensation front of h2o snow line in the solar nebula the inferred o isotopic ratio of water suggests that carbonate in cm chondrites formed at low temperatures of

mineralogy and oxygen isotope systematics of magnetite grains and a magnetite dolomite assemblage in hydrated fine grained antarctic micrometeorites

paleomagnetism of latest cambrian early ordovician and latest cretaceous early tertiary rocks of the florida mountains southwest new mexico paleomagnetic investigations have been carried out on latest cambrian to early ordovician alkaline intrusions their contact intermediate to mafic composition metaigneous rocks and overlying lower paleozoic and early tertiary strata in the florida mountains southwestern new mexico a well defined predominantly normal polarity magnetization of high median destructive induction and high usually discrete laboratory unblocking temperatures is isolated from sites in metaigneous rocks in contact with syenites exposed on the northwestern flank of this late cenozoic uplift combinations of stable endpoint and planar data are used in data analysis at the site level d 262 i 30 95 10 k 12 n 22 site means in addition to the presence of reverse polarity magnetizations at two sites the dispersion of site mean directions in these rocks vgp angular standard deviation of 19 6 suggests that considerable variation of the geomagnetic field has been averaged after correction for a combination of early tertiary laramide deformation and late tertiary basin range block tilting this magnetization gives a paleomagnetic pole at 6 s 169 e and is interpreted to have been acquired soon after intrusion of alkaline plutons at 503 10 ma based on published u pb zircon isotopic age determinations a lack of well defined coherent magnetizations in the plutonic rocks may reflect the complex subsolidus alteration history of these rocks as interpreted with rb sr isotopic data hematitic sandstones of the earliest ordovician bliss formation which directly overlie the crystalline rocks in portions of the range give dual polarity magnetizations whose directions suggest that these rocks have probably been remagnetized this remagnetization possibly occurred during deformation in late cretaceous tertiary time ordovician carbonate rocks of the el paso and montoya formations give only a holocene magnetization paleocene to earliest eocene hematite cemented clastic rocks of the lobo formation possess dual polarity magnetizations the origin of their magnetization is difficult to assess with the available data in total however paleomagnetic data from the bliss and lobo formations argue against any significant vertical axis rotation of the area since laramide deformation ending in early tertiary time the intermediate to mafic composition metaigneous rocks from the florida mountains provide a latest cambrian early ordovician paleomagnetic pole that compares favorably with other north american paleopoles satisfying several acceptance criteria the florida mountains data further support recent constructions of apparent polar wander paths for north america and thus the hypothesis that significant counterclockwise rotation of north america occurred during the ordovician