corumbella an ediacaran grade organism from the late neoproterozoic of brazil abstract the morphology and phylogenetic affinities of corumbella werneri hahn g hahn r leonardos o h pflug h d and walde d h g 1982 korperlich erhaltene scyphozoen reste aus dem jungprakambrium brasiliens geologica et paleontologica 16 1 18 type species of the genus is reinterpreted based on new material from the tamengo formation corumba group upper neoproterozoic of western mato grosso do sul brazil corumbella secreted a narrow elongate tetraradially symmetrical tube reinterpretation of tube morphology and new evidence of reproduction by means of budding indicate close affinities with the present day coronate scyphozoan cnidarian stephanoscyphus and possibly the conulariids corumbella species were distributed between the amazon craton and laurentia corumbella is interpreted as a sessile predator and provides evidence of the importance of predation in animal evolution during the late neoproterozoic

implications of the carbonaceous chondrite mn cr isochron for the formation of early refractory planetesimals and chondrules abstract an excellent 53 mn 53 cr isochron for bulk ci cm co cv cb and ungrouped c3 chondrites seems to suggest that each carbonaceous chondrite group acquired its mn cr ratio 4568 1 myr ago this age is indistinguishable from the age of ca al rich inclusions cais which is considered to be the start of the solar system t 0 however carbonaceous chondrites were not assembled until at least 1 5 5 myr after t 0 to judge by the 207 pb 206 pb and 26 al 26 mg ages of the chondrules within them and by the fact that they were not melted by heat from the decay of 26 al presumably therefore these meteorites inherited their bulk mn cr isochron from precursor materials which experienced mn cr fractionation at t 0 as a possible physical mechanism for how the isochron was established initially and later inherited by the carbonaceous chondrites we propose the rapid formation at t 0 of planetesimals that were variably depleted in moderately volatile elements and hence had variably low mn cr the planetesimals and the undepleted high mn cr primitive dust from which they were made shared the same initial e 53 cr and therefore evolved on an isochron we suggest that later impact disruption of the planetesimals produced dusty debris which became mixed in various proportions with unprocessed high mn cr dust before accreting to the carbonaceous chondrite parent bodies with mixing in a closed system the isochron was unchanged we infer that some debris rich material was converted to chondrules prior to accretion the chondrules could have been formed by flash melting of the mixed dust or could instead have been made directly by the impact splashing of molten planetesimals or by condensation from impact generated vapor plumes

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

early solar system events and timescale some recent information on the mn cr and al mg systems is reviewed this information is used to derive constraints on the timing of processes and events which took place in the early solar system using reasonable assumptions a timeline is constructed where the estimated age of the solar system is 4571 ma this age is taken to mark the time when most calcium aluminum rich inclusions cais were starting to form a process that may have lasted for several 105 years almost contemporaneously small planetesimals have accreted that served to store these cais for later dispersal among larger planetesimals by the time large numbers of planetesimals of several tens of kilometers in size had formed the interior of these objects started to melt through the decay of 26al collisional disruption of these planetesimals allowed gases dust and melt to escape into the surrounding space the fine droplets of melt reacted with gas and dust to form chondrules which after rapid cooling were partially re accreted onto the residual rubble pile this process of primary chondrule formation in most cases involving several generations of planetesimals most plausibly lasted only for 2 ma towards the end of this period and during the following 3 to 4 ma planetary objects of several hundred kilometers in size were formed they still stored enough energy to continue melting from the inside to finally differentiate into chemically stratified layers with basaltic volcanism occurring within a few million years

chromium isotope anomaly in an impactite sample from the el gygytgyn structure russia evidence for a ureilite projectile the 3 6 ma 18 km diameter el gygytgyn impact structure arctic russia is unique among the currently known terrestrial impact craters in that it is the only one that was formed in acid volcanic rocks previous analyses of impactites from el gygytgyn showed minor enrichments of the siderophile elements including ir which together with distinct cr enrichments gave rise to speculation that an achondritic projectile was involved we studied the major and trace element composition in samples from the new icdp drill core obtained near the center of the structure as well as the chromium isotopic composition of an impact glass sample collected on the surface several suevitic breccias from the upper part of the suevite sequence in the drill core show higher cr and ni contents compared with felsic volcanic rocks in the lower part of the core and from surface samples however it is difficult to unambiguously establish a meteoritic component from trace element data as input from rare mafic target rocks is a possibility in contrast the cr isotopic composition of the impact glass sample yielded a nonterrestrial e 54 cr value of 0 72 0 31 2 std err this negative e 54 cr is different from known carbonaceous chondrite values e 54 cr of 0 95 to 1 65 but is nearly identical to reported values for ureilites approximately 0 77 the value is however also within analytical error of eucrites approximately 0 38 and ordinary chondrites approximately 0 42 given the chemical signatures found in previous analyses of el gytgytgyn impactites and the similarity of our cr isotopic data to ureilites we suggest that the impacting asteroid could have been an f type asteroid of mixed composition similar to the recent almahata sitta fall in sudan

eh3 matrix mineralogy with major and trace element composition compared to chondrules abstract we investigated the matrix mineralogy in primitive eh3 chondrites sahara 97072 alh 84170 and lar 06252 with transmission electron microscopy measured the traceand major element compositions of sahara 97072 matrix and ferromagnesian chondruleswith laser ablation inductively coupled plasma mass spectrometry la icpms andanalyzed the bulk composition of sahara 97072 with la icpms solution icpms andinductively coupled plasma atomic emission spectroscopy the ne grained matrix of eh3chondrites is unlike that in other chondrite groups consisting primarily of enstatite cristobalite troilite and kamacite with a notable absence of olivine matrix and pyroxene rich chondrule compositions differ from one another and are distinct from the bulkmeteorite refractory lithophile elements are enriched by a factor of 1 5 3 in chondrulesrelative to matrix whereas the matrix is enriched in moderately volatile elements thecompositional relation between the chondrules and matrix is reminiscent of the differencebetween eh3 pyroxene rich chondrules and eh3 si rich highly sul dized chondrules similar refractory element ratios between the matrix and the pyroxene rich chondrulessuggest the ne grained material primarily consists of the shattered sul dized remains of theformerly pyroxene rich chondrules with the minor addition of metal clasts the matrix chondrule and metal sul de nodule compositions are probably complementary suggestingall the components of the eh3 chondrites came from the same nebular reservoir introductionthe origin and nature of chondrite matrix materialthat is amorphous or with grains 5 lm in diameter are debated scott et al 1988 suggestions include thatit consists mainly of chondrule fragments alexanderet al 1989 brearley 1996 ci like material with anorigin distinct from chondrules alexander 2005 reprocessed material from the parent molecular cloud ofthe solar system buseck and hua 1993 zolensky et al 1993 huss et al 2005 and products of nebularcondensation larimer and anders 1967 the enstatite chondrites ec were estimated tocontain between 2 and 15 vol matrix scott and krot2005 rubin et al 2009 material that has not beenwidely studied ec are thought to have accreted in theinner solar system shukolyukov and lugmair 2004 they are linked to earth by nearly identical isotopiccompositions of several major and minor elements javoy et al 2010 jacobsen et al 2013 kaminski andjavoy 2013 and they have been proposed as potentialsource material for earth and the moon clayton et al 1984 javoy 1995 javoy et al 2010 therefore like theec chondrules the matrix in highly unequilibrated eh3chondrites potentially provides insight into events andmaterials from the early inner solar system preliminary work showed that the eh matrixconsists primarily of enstatite cristobalite troilite andkamacite lehner and buseck 2010 lehner et al 2012b the abundant cristobalite suggests a possiblelink with sul dized chondrules which are a subset ofeh3 chondrules in which forsterite and enstatite were

highly siderophile element and osmium isotope evidence for postcore formation magmatic and impact processes on the aubrite parent body abstract aubrites exhibit a wide range of highly siderophile element hse re os ir ru rh pt pd au concentrations and 187os 188os compositions their hse concentrations are one to three orders of magnitude less than chondrites with the exception of the shallowater and mt egerton samples while most aubrites show chondritic hse abundance ratios significant enrichments of pd and re relative to os ir and ru are observed in 12 of 16 samples present day 187os 188os ratios range from subchondritic values of 0 1174 to superchondritic values of up to 0 2263 half of the samples have 187os 188os ratios of 0 127 to 0 130 which is in the range of enstatite chondrites along with the brecciated nature of aubrites the hse and re os isotope systematics support a history of extensive postaccretion processing including core formation late addition of chondritic material and or core material and potential breakup and reassembly highly siderophile element signatures for some aubrites are consistent with a mixing of hse rich chondritic fragments with a hse free aubrite matrix the enrichments in incompatible hse such as pd and re observed in some aubrites reminiscent of terrestrial basalts suggest an extensive magmatic and impact history which is supported by both the 187re 187os isotope system and silicate hosted isotope systems rb sr k ar yielding young formation ages of 1 3 3 9 ga for a subset of samples compared with other differentiated achondrites derived from small planetary bodies aubrites show a wide range in hse concentrations and 187os 188os most similar to angrites while similarities exist between the diverse groups of achondrites formed early in solar system history the aubrite parent body ies clearly underwent a distinct evolution different from angrites brachinites ureilites howardites eucrites and diogenites

manganese chromium systematics in sulfides of unequilibrated enstatite chondrites we measured with a secondary ion mass spectrometer mn cr ratios and cr isotopes in individual grains of mn bearing sulfides i e sphalerites zns alabandites mns and niningerites mgs in nine unequilibrated enstatite chondrites uecs the goals were to determine whether live 53mn half life 3 7 ma was incorporated in these objects at the time of their isotopic closure and to establish whether mn cr systematics in sulfides in uecs can be used as a high resolution chronometer to constrain formation time differences between these meteorites sulfide grains analysed in four of these uecs mac 88136 el3 mac 88184 el3 mac 88180 el3 and indarch eh4 have clear 53cr excesses these 53cr excesses can be very large 53cr 52cr ranges up to 18 400 the largest 53cr excess measured so far and in some grains are well correlated with the mn cr ratios thus they were most likely produced by the in situ decay of 53mn in the meteorite samples in the remaining five meteorites no detectable excesses of 53cr were found and only upper limits on the initial 53mn 55mn ratios could be established the four meteorites with 53cr excesses show variations in the inferred 53mn 55mn ratios in various sulfide grains of the same meteorite the mn cr systematics in these sulfides were disturbed during and or after the decay of 53mn by varying degrees of reequilibration provided 53mn was homogeneously distributed in the region of the early solar system where these objects formed the data suggest that the time of the last isotopic equilibration of sulfides in el chondrites occurred at least 3 ma after a similar episode in eh chondrites

isotope stratigraphy of neoproterozoic cap carbonates in the araras group brazil abstract the neoproterozoic carbonate sequence on the southeastern border of the amazon craton is divided into three lithostratigraphic units a basal cap dolomite an intermediate limestone limestone mudstone unit and an upper dolarenite dolorudite unit sections of the cap carbonate were measured from the inner shelf to the outer shelf carbon isotope ratios relative to pdb vary between 10 5 and 1 7 in cap dolomite and between 5 4 and 0 1 in laminated limestone and mud limestone limestones and mud limestones exhibit 87sr 86sr ratios ranging from 0 70740 to 0 70780 a comparative isotope stratigraphy between the inner shelf and the middle shelf basin shows differences in carbon isotope ratios the cap dolomite and limestones have lower 13c ratios on the border of the basin inner shelf than in the middle shelf of the basin these lower values can be related to shallower environmental conditions and to a stronger influence of the continental border the 87sr 86sr ratios are the same in both areas and are consistent with seawater composition at around 600 ma

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

the copper gold and gold deposits of the neoproterozoic mara rosa magmatic arc central brazil mineral exploration in the neoproterozoic goias magmatic arc central brazil dates back to the beginning of the 1970s the goias magmatic arc extends for more than 1000 km in the western and northern parts of goias into tocantins and disappears under the phanerozoic parnaiba basin two main areas of neoproterozoic juvenile crust the arenopolis and the mara rosa arcs are identified they lie in the southern and northern sectors of the goias arc respectively and are relatively well studied the goias magmatic arc dominantly comprises tonalitic dioritic orthogneisses and narrow nne striking volcano sedimentary belts recent u pb zircon data indicate crystallization of the tonalite protoliths in two main episodes the older between ca 890 and 790 ma and the younger at 670 600 ma nd isotopic data indicate the very primitive nature of the original magmas with tdm model ages mostly within the interval between 0 9 and 1 0 ga and end t values between 3 0 and 4 6 in the chapada mara rosa area the supracrustal rocks form three individual nne belts known as the eastern central and western belts separated from each other by metatonalites metadiorites gold and cu au deposits of the mara rosa area occur in four main associations i au ag ba e g zacarias which are interpreted as stratiform disseminated volcanogenic deposits ii cu au e g chapada which has been interpreted either as volcanogenic or as a porphyry type deposit iii au only deposits e g posse interpreted as an epigenetic disseminated deposit controlled by a mesozonal shear zone and iv au cu bi e g the mundinho occurrence which are considered as vein type deposits controlled by magnetite rich diorites the gold and cu au deposits located within the goias magmatic arc can be spatially and temporally related to the magmatic evolution of a collisional belt or in other words to an orogenic gold deposit model these models are based on the continuous evolution of collisional plates which can be subdivided into four stages with distinct magmatic characteristics i subduction stage ii syntectonic collisional magmatism stage iii post tectonic collisional magmatism stage and iv post orogenic extension stage

response of the ganges dispersal system to climate change a source to sink view since the last interstade the forcing of sediment dispersal systems by climate is widely accepted but there remains disagreement on the general responses to climate change such as how resulting signals propagate downstream and ultimately how they are preserved in the stratigraphic record towards a better understanding of these issues this paper presents a comprehensive overview of ganges dispersal system behavior since the last interstade which reveals major coeval responses to changes in the south asian monsoon at 3000 km from its mountain headwaters to deep sea fan the ganges dispersal system is immense and yet shows tight coupling between source area catchment basin and coastal and marine depocenters furthermore system wide responses and downstream transfer of the signal appear to occur at least as fast as current temporal resolution of 1 2 ky this tight linkage of source to sink components is considered a function of the southwest summer monsoon s overwhelming control on regional hydrology about 80 of the ganges discharge and 95 of its sediment load are delivered to the margin during only 4 months making the system extremely sensitive to this seasonal forcing in addition the regional scale and distribution of the monsoon weather system means that changes in atmospheric circulation affect the entire drainage basin rather than local subcomponents finally despite its present intensity strength of the summer monsoon has varied significantly over the past 150 ky under orbitally driven changes in insolation and global glacial boundary conditions the resulting changes in precipitation both well above and below modern values have forced system wide responses ranging from glacier advance retreat to sediment fluxes to the margin and deep sea fan the overall conclusions are 1 that this immense dispersal system responds to multimillennial scale 104 years climate change in a system wide and largely contemporaneous manner and 2 that major sedimentary signals can be transferred rapidly from source to sink with little apparent attenuation furthermore these acute responses to climate change have produced sedimentary stratigraphic features that diverge from traditional sequence models in their nature and timing

c o sr isotopic stratigraphy of cap carbonates overlying marinoan age glacial diamictites in the paraguay belt brazil abstract the neoproterozoic paraguay belt located along the southeastern border of the amazon craton and eastern border of the rio apa block comprises a thick passive margin succession of glaciomarine turbidites carbonates and siliciclastic sedimentary rocks that were deformed during the brasiliano pan african orogeny the carbonate sequence comprises a 1300 m thick succession of platformal carbonate rocks araras formation which directly overlie marinoan glacial diamictites puga formation the late neoproterozoic age of this carbonate sequence is indicated by the presence of cloudina lucianoi and corumbela werneri found in the upper part of the corumba group southern paraguay belt carbon and sr isotope data were obtained on marine carbonate samples across three different sections of the araras formation 13 c pdb values of the carbonate from the border of the basin range between 10 5 and 2 7 including a short stratigraphic interval of cap dolomite 22 m directly overlying diamictites the isotopic profile across the basal 200 m of deep shelf laminated microcrystalline limestone and clay limestone cap carbonate that overlie glacial diamictites also show negative 13 c pdb values 5 3 and 0 6 in contrast with the lower portion of the carbonate sequence a 800 m thick unit of shallow water dolostones forming the upper part of the carbonate sequence present uniform positive 13 c pdb values ranging from 1 9 to 2 4 dolostone and arenaceous dolostone of the upper parts of the araras formation have high 13 c pdb values 4 1 to 9 6 which are followed by an abrupt decrease in 13 c values down to 1 0 the carbon isotope profile of the paraguay belt is quite different from the bambui group an other important neoproterozoic sequences in central brazil which also overlie glacial sediments negative 13 c pdb values are found only within the lowermost meters of these sequence and are followed by an important and extensive positive carbon isotope excursion values ranging from 6 9 to 16 0 on the other hand the 13 c pdb profile observed across the araras formation exhibits an approximately 200 m thick section of carbonates with low 13 c pdb values carbonates of the araras formation have 87 sr 86 sr ratios ranging from 0 70753 to 0 70803 comparable to cap carbonates that overly marinoan glacial deposits from other neoproterozoic sequences

chromium isotopic studies of terrestrial impact craters identification of meteoritic components at bosumtwi clearwater east lappajarvi and rochechouart chromium isotopic compositions and elemental abundances in impact melt rock and impact glass samples from four terrestrial impact craters were measured to verify the presence of an extraterrestrial component and to identify the meteorite type of the impactor all meteorite classes have cr isotopic signatures that are different from those of terrestrial rocks thus precise measurements of cr isotopic abundances can unequivocally distinguish terrestrial from extraterrestrial materials for all four studied craters bosumtwi ghana clearwater east canada lappajarvi finland and rochechouart france we found positive 53cr excesses that eliminate carbonaceous chondrite projectiles because those would show apparent negative excesses and enstatite chondrites because of the magnitude of the excess in all four cases ordinary chondrites have been identified as the best fit for the data in the case of lappajarvi interelement correlations together with the cr isotope data make an h chondrite the most likely projectile whereas in the case of clearwater east both l or h chondrites are possible for bosumtwi and rochechouart the high indigenous contents of the siderophile elements and disturbances of the elemental abundances by weathering and hydrothermal alteration respectively do not allow further constraints to be placed on the type of ordinary chondrite involved in the impact

microscale trace element distribution across the cretaceous palaeogene ejecta layer at the agost section constraining the recovery of pre impact conditions abstract a high resolution sub mm scale analysis of the distribution of major and trace elements across the ejecta layer marking the cretaceous palaeogene boundary kpgb at the agost section se spain was performed using laser ablation inductivity coupled plasma mass spectrometry la icp ms a kpgb interval 17 mm thick has been selected for this study it includes gray calcareous marl from the uppermost maastrichtian cretaceous 2 02 mm thick red clay the ejecta layer and blackish gray clay boundary clay layer from the lowermost danian palaeogene the unconsolidated sediments were resin embedded under o2 free conditions and analyzed by la icp ms line continuous scan measurements at 20 m increments and a laser beam of 120 m these micron scale analyses show that the anomalous contents of trace and major elements in this boundary are restricted to the ejecta layer which displays a relatively uniform distribution over its 2 mm thickness trace and major elemental ratios such as ca al ti al cr al fe al ni al cu al zn al as al sb al and pb al have similar values below and above the ejecta layer consistent with similar palaeoenvironmental conditions prior to and after the impact event such element distribution points to anomalies exclusively restricted to the ejecta thus a nearly instantaneous re establishment of pre impact conditions right after the impact event is observed

trapped nitrogen in individual chondrules nature of chondrule precursors and clues to formation mechanisms 1 nitrogen amount and isotopic composition have been measured in individual chondrules of six ordinary chondrites oc and two each of carbonaceous cc and enstatite chondrites ec the n ppm 15nt corrected for cosmogenic contribution for the chondrules have ranges of 0 8 to 61 97 to 164 19 to 140 21 to 31 and 2 to 77 27 to 116 among oc ec and cc respectively for oc while n contents of chondrules fall in the range of bulk oc 15n spans a wide range in contrast to the narrow range 25 to 14 for bulk for cc chondrules n contents are lower while 15n is different and heavier than bulk in contrast ec chondrules and bulk have comparable n and 15n values neither n nor 15nt of oc and cc chondrules show any size expected for nebular exchange or chemical mineralogical dependence ruling out the dominance of secondary effects of nebular or parent body origin the range of 15nt among oc and cc chondrules represents 15nt heterogeneity of chondrule precursors and their survival after a high temperature chondrule forming event on the basis of 15n of the chondrules we propose the presence of at least three n components carried by phase q presolar diamonds hl and insoluble organic matter iom the precursors of chondrules from oc and cc are different than the bulk oc and cc which represents the different environment for their formation compared to the bulk oc and cc however for ec chondrules precursors as well as the formation environment are the same as those of enstatite chondrites

26al 26mg deficit dating ultramafic meteorites and silicate planetesimal differentiation in the early solar system abstract meteorites with significantly sub chondritic al mg that formed in the first 2 million years of the solar system should be characterised by deficits in the abundance of 26mg 26mg due to the absence of in growth of 26mg from the decay of short lived 26al t1 2 0 73 myr however these 26mg deficits will be small 26mg 0 037 even for material that formed at the same time as the solar system s oldest solids calcium aluminium rich inclusions and thus measurement of these deficits is analytically challenging here we report on a search for 26mg deficits in three types of ultramafic meteorites pallasites ureilites and aubrites by multiple collector inductively coupled plasma mass spectrometry a range of analytical tests were carried out including analysis of 1 a range of synthetic mg solution standards 2 mg gravimetrically doped with a high purity 26mg spike 3 mg cuts collected sequentially from cation exchange separation columns with fractionated stable mg isotope compositions 4 mg separated from samples that was bracketed by analyses of both dsm 3 and mg separated from a natural olivine sample subjected to the same chemical processing as the samples these tests confirm it is possible to resolve differences in 26mg from the terrestrial materials that are 0 005 however if mg yields from chemical separation are low or an inappropriate equilibrium isotopically fractionated standard is used this will generate analytical artefacts on 26mg when this is calculated with the kinetic exponential mass fractionation law as is the case when correcting for instrumental mass bias during mass spectrometric analysis olivine from four different main group pallasites and four bulk ureilites have small deficits in the abundance of 26mg with 26 mg dsm 3 0 0120 0 0018 and 26 mg dsm 3 0 0062 0 0023 respectively relative to terrestrial olivine 26 mg dsm 3 0 0029 0 0028 six aubrites have 26 mg dsm 3 0 0015 0 0020 which is identical to terrestrial olivine model ages from these deficits can be calculated by assuming that 26al was homogeneously distributed in the planetesimal forming regions of the proto planetary disc at the same level as calcium aluminium rich inclusions cais the absence of 26mg deficits in aubrites means these can only be constrained to have formed relatively late 2 9 myr after cai formation model ages calculated from pallasite olivine deficits would suggest that pallasite olivine crystallised and was diffusively isolated on its parent body 1 24 0 28 0 40 myr after formation of cais similarly ureilites would have experienced silicate partial melting and lowering of their bulk al mg ratios 1 9 0 7 2 2 myr after cai formation the model ages for silicate differentiation on the main group pallasite parent body are intermediate between those for metal silicate fractionation for core formation obtained from magmatic iron meteorites and those for asteroidal silicate magmatism obtained from basaltic meteorites

composition and origin of clasts and inclusions in the abee enstatite chondrite breccia abstract the concentrations of 25 major minor and trace elements have been determined in four clasts a metal rich inclusion and two dark metal poor inclusions from the abee enstatite chondrite the clasts are heterogeneous displaying 2 fold enrichments or depletions in some elements the data suggest that there are two generations of metal one with low the other with high concentrations of refractory siderophiles the other elemental patterns can be understood in terms of variations in the abundance of major minerals we infer that sc and mn are located largely in the niningerite fe mg s v in the troilite fes and rare earth elements in the oldhamite cas heterogeneities among the clasts are probably primary resulting from the accretion agglomeration process although shock processes in a regolithic setting remain a possibility provided that they were followed by a period of metamorphism sufficient to erase petrologic evidence in the dark inclusions the concentrations of the rare earths eu excepted are 4 higher than mean eh levels this infers enhanced amounts of cas the dark inclusions are low in siderophiles sc mn k na and al implying low amounts of metal niningerite and feldspar the origin of the dark inclusions is unclear they do not appear to be the result of a simple single stage process

glacially influenced sedimentation in the later proterozoic of the paraguay belt mato grosso brazil abstract late proterozoic probably early vendian 610 590 ma glaciogenic and turbiditic sediments and metasediments are present along the transitional zone between the amazonian craton to the northwest and the paraguay belt to the southeast in the cuiaba region mato grosso brazil a few exposures of diamictites are associated with conglomerates and graded pebbly sandstones on the cratonic fringe and pass southeastwards into fine grained sediments deposited in the deeper part of the probably marine basin in the paraguay belt these rocks have been affected by the brasiliano orogeny possibly around 570 ma they show increasing deformation and metamorphism from the craton towards the fold belt the sedimentation scheme proposed for the western border of the paraguay belt and the eastern edge of the amazonian craton involves three probably marine main depositional systems to the west on the craton the platform domain can be subdivided into a western inner shelf and an eastern outer shelf the deposits on the inner shelf subdomain show an alternation of dominant massive diamictites sandstones and fine grained sediments with few dropstones this diamictite association is tentatively interpreted as glaciomarine in origin in the outer shelf subdomain the massive diamictites are progressively replaced southeastwards by an association of massive diamictites stratified diamictites and fine grained sediments this diamictite association is interpreted as a whole as resedimented glacial debris deposited by subaqueous debris flows the slope domain located at the transition between craton and fold belt is characterized by strong reworking of the glaciomarine sediments by gravity flows responsible for a type of submarine fan deposition the coarse clastic facies which result from a reworking by rapid downslope transport of cohesive debris flows and gravity flows with high viscosity fill feeder channels in the upper fan eastward inside the belt a progressive sorting in the deeper portions of the fan is shown by the transition from diamictites massive gravels sandstone intercalations with occasional inverse and or normal grading debris flow turbidity current to normal graded finer sediments turbidity current sandstone and siltstone intercalations represent interchannel deposits formed by turbidity currents east of the slope well bedded fine grained sediments were generated by low density turbidity currents direct glacial influence is reduced to the presence of a few isolated clasts or dropstones two distinct upper proterozoic glaciogenic units are known in brazil the older and more widespread one dated at about 950 ma is located on the sao francisco craton and in the adjacent brasiliano fold belts the younger one 600 ma is restricted to the paraguay belt and the adjacent border of the amazonian craton in southern western gondwana late proterozoic glaciation is of mountain type and developed at latitudes of 60 on highs probably generated by pan african brasiliano rifting at the same time in its northern part a widespread ice sheet covered the west african craton

age and isotopic relationships among the angrites lewis cliff 86010 and angra dos reis results of a wide ranging isotopic investigation of the unique antarctican angrite lew 86010 lew are presented together with a reassessment of the type angrite angra dos reis ador the principal objectives of this study are to obtain precise radiometric ages initial sr isotopic compositions and to search for the erstwhile presence of the short lived nuclei 146sm and 26al via their daughter products the isotopic compositions of sm u ca and ti were also measured this allows a detailed appraisal to be made of the relations between and the geneology of these two angrites lew proves to be severely contaminated with modern terrestrial pb which is shown to result from terrestrial weathering nevertheless concordant pb pb model ages of pyroxene separates were obtained 2 4 55784 52 ga for lew and 4 55780 42 ga for ador uranium isotopic compositions are normal within error the inferred initial pb isotopic composition is within error of primordial pb as defined by canon diablo troilite because of the extreme upb ratios of both angrites this places an upper limit of 2 ma on the time between volatile element loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates initial 87sr86sr ratios were found to be indistinguishable in lew ador and the cumulate eucrite moore county the derived initial 87sr86sr for all three meteorites is 0 698970 15 2 a 147sm 143nd isochron for lew was obtained yielding an age of 4 553 34 ga with an initial 143nd144nd of 0 506682 49 2 146sm142nd isotope systematics were also measured anomalies on 142nd arising from extinct 146sm were clearly resolved resulting in an initial 146sm144sm of 0 0071 17 with nd142 2 57 0 62 at the time of isotopic closure both of these sm nd methods imply derivation of lew from a reservoir with chondritic smnd ratio they are also quite consistent with the previously reported systematics for ador overall the age and isotopic similarities between lew and ador are striking it suggests almost simultaneous production on the same asteroid even though recent experimental studies imply that the two are not comagmatic calcium and titanium do not exhibit enrichments in the n rich isotopes in contrast to cais making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula no evidence was found for live 26al in lew when combined with the pb pb age and initial 87sr86sr data this firmly excludes 26al being an important heat source in the early solar system heat supplied for for example 1 the differentiation of the angrite and eucrite parent bodies and 2 metamorphism of the chondrite parent bodies must come from some other source the debate over live vs extinct 26al in the earliest solar system remains unresolved by this new data from lew however both the angrite and eucrite parent bodies have extremely low rbsr and high upb ratios compared to the solar nebula therefore the common initial 87sr86sr of 0 698970 can be firmly associated with the u pb angrite age above and the formation of both bodies this enables an absolute chronology of the early solar system to be established based upon 87sr86sr differences the oldest cais are a minimum of 11 4 ma older than the angrites i e 4 569 5 ga the age and isotopic constraints are discussed with respect to current collapse condensation and accretion timescales calculated for the solar nebula

the compositional classification of chondrites ii the enstatite chondrite groups we present new data from a neutron activation analysis of four enstatite chondrites including the taxonomically important st sauveur and discuss the classification of enstatite chondrites the enstatite chondrites can be divided into two compositionally distinct sets in one set abundances of nonrefractory siderophiles and moderately volatile chalcophiles and alkalis are 1 5 2 0 higher than in the other a well resolved compositional hiatus separates these two sets the differences in composition are as great as those between the groups of ordinary chondrites and therefore it appears best to treat these sets as separate groups by analogy with the symbols used for ordinary chondrites we propose to designate the high fe high siderophile group eh and the low fe low siderophile group el known members of the eh group belong to petrologic types 4 and 5 whereas all el members are petrologic type 6 within the eh group no correlation is observed between petrologic type and abundance of nonrefractory siderophiles or moderately volatiles or alkalis two physical properties show only modest overlap between the eh and el groups cosmic ray ages for eh chondrites are 0 5 7 ma while those for el chondrites are 4 18 ma relative to bjurbole i xe formation intervals are 1 3 0 6 ma for eh chondrites and 2 9 0 5 ma for el chondrites the weight of the chemical and physical evidence indicates that the eh and el groups formed separate bodies at similar distances from the sun the available evidence for shallowater and happy canyon two strongly recrystallized silicate rich meteorites containing 40 mg g fe ni indicates that the former is an enstatite clan chondrite altered by loss of sulfide and plagioclase rich melts whereas the latter is intermediate in composition between el chondrites and the chondritic silicates in the pine river iab anomalous meteorite

verification and interpretation of the i xe chronometer abstract comparisons of i xe and pb pb relative ages are made for phosphate and feldspar separates from 12 different meteorites in all cases where i xe ages can be measured in phosphate i xe and pb pb chronometers agree within experimental uncertainty no discordant phosphate samples are observed with the exception of allegan i xe ages for feldspar separates generally agree with pb pb ages for the corresponding phosphate the general concordancy observed between i xe and pb pb chronometers suggests that the i xe system is a reliable and interpretable chronometer when applied to single minerals systems allegan feldspar differs from the other feldspar separates predating its phosphate by 17 ma with an i xe age indistinguishable from those of orgueil and murchison magnetites among the oldest observed

contributors to chromium isotope variation of meteorites we report the results of a comprehensive high precision survey of the cr isotopic compositions of primitive chondrites along with some differentiated meteorites to ensure complete dissolution of our samples they were first fused with lithium borate tetraborate at 1050 1000 c relative to the nist cr standard srm 3112a carbonaceous chondrites exhibit excesses in 54cr 52cr from 0 4 to 1 6e 1e 1 part in 10 000 and ordinary chondrites display a common 54cr 52cr deficit of 0 4e analyses of acid digestion residues of chondrites show that carbonaceous and ordinary chondrites share a common 54cr enriched carrier which is characterized by a large excess in 54cr 52cr up to 200e associated with a very small deficit in 53cr 52cr 2e we did not find 54cr anomalies in either bulk enstatite chondrites or in leachates of their acid digestion residues this either requires that the enstatite chondrite parent bodies did not incorporate the 54cr anomaly carrier phase during their accretion or the phase was destroyed by parent body metamorphism chromium in the terrestrial rocks and lunar samples analyzed here show no deviation from the nist srm 3112a cr standard the eucrite and martian meteorites studied exhibit small deficits in 54cr 52cr the 54cr 52cr variations among different meteorite classes suggest that there was a spatial and or temporal heterogeneity in the distribution of a 54cr rich component in the inner solar system we confirm the correlated excesses in 54cr 52cr and 53cr 52cr for bulk carbonaceous chondrites but the new data yield a steeper slope 6 6 than that reported in shukolyukov and lugmair 2006 the correlated excesses may affect the use of the mn cr chronometer in carbonaceous chondrites we could not confirm the bulk carbonaceous chondrite mn cr isochron reported by shukolyukov and lugmair 2006 and moynier et al 2007 mostly because we find much smaller total variations in e53cr 0 2 all bulk chondrites have small e53cr excesses up to 0 3 relative to the earth most likely reflecting the sub chondritic mn cr ratio of the earth the e53cr variations in chondrites do seem to grossly correlate with mn cr and yield an initial solar system 53mn 55mn value of 5 4 2 4 10 6 corresponding to an absolute age of 4566 4 2 2 ma nuclear interactions with cosmic rays result in coupled excesses in e54cr and e53cr with a 4 1 ratio in phases with high fe cr these are most dramatically demonstrated in the iron meteorite carbo showing excesses in e54cr of up to 140e these new results show that the mn cr chronometer should be used with caution in samples minerals with high fe cr and long cosmic ray exposure ages

main stages of the development of the sedimentary basins of south america and their relationship with the tectonics of supercontinents abstract twelve main stages of sedimentary basins of the south american platform have been identified as follows one neoarchean seven proterozoic and four phanerozoic basin forming tectonics are related to the accretion fusion and fission of the major continental landmasses of the earth s history such as atlantica nena rodinia gondwana pannotia and pangea the first proposed stage of basin development precedes the formation of these major landmasses and was developed on mesoarchean microcontinents the last stage succeeds the fission of pangea and is still in progress

compositional continuity of enstatite chondrites and implications for heterogeneous accretion of the enstatite chondrite parent body abstract chemical compositions of eleven enstatite chondrites 1 eh5 4 eh3 3 el3 i el5 and 2 el6 and their metal and nonmagnetic fractions determined by instrumental neutron activation analysis are reported the abundances of nonvolatile lithophile elements such as al sc and mg increase from eh to el while those of siderophile and moderately volatile elements decrease in the sequence of eh5 to eh3 el3 and el5 6 the continuity in compositions of enstatite chondrites and in particular the inverse variations of moderately volatile element abundances with petrographic type between eh and el groups demonstrate that enstatite chondrites have been derived from a common parent body the enstatite chondrite parent body formed by heterogeneous accretion of materials available in the accretional region metal was effectively accreted into the core where eh5 chondrites were derived and the abundances of metal decreased as accretion proceeded in complement silicate abundances gradually increased in the layers that accreted later the lack of correlation between volatile element abundances and metamorphic degree demonstrates that losses of moderately volatile elements from enstatite chondrites cannot have resulted from parent body processes furthermore it is expected that during accretion various components would mix and thus erase any early fractionation of moderately volatile elements of a single parent body variations of ambient gas temperatures during accretion are also impossible to produce the volatile element pattern of enstatite chondrites it is suggested that moderately volatile elements in the enstatite chondrites were lost during local heating the chondrule formation process formation of chondrules appears to have proceeded during the accretion of the enstatite chondrite parent body and lasted for a certain period the mineralogical and textural features of enstatite chondrites can be explained in terms of two stages of metamorphism the metamorphic trend from eh3 to eh5 resulted from internal heating whereas the trend from el3 to el6 was due to an external heat source it seems very likely that the eh chondrites were metamorphosed during accretion thus cooled rather rapidly activities of the early sun could serve as an energy source for the external heating

timing and role of the maranhao river thrust in the evolution of the neoproterozoic brasilia belt and tocantins province central brazil abstract a low angle thrust fault places high pt granulites hangingwall of the internal zone of the neoproterozoic brasilia belt tocantins province central brazil in contact with a lower grade footwall external zone comprised of nappes of distal passive margin and back arc basin related supracrustals the footwall units were emplaced at 750 ma onto proximal sedimentary rocks paranoa group of the sao francisco paleo continent passive margin the high pt belt is comprised of 645 630 ma granulite facies paragneiss and orthogneiss and mafic ultramafic complexes that include three major layered intrusions and metavolcanic rocks granulitized at 750 ma these complexes occur within lower grade metasedimentary rocks in the hangingwall of the maranhao river thrust which forms the internal zone external zone boundary fault to the north of the pirineus zone of high strain detailed lithostructural studies carried out in maranhao river thrust hangingwall and footwall metasedimentary rocks between the niquel ndia and barro alto complexes and also to the east of these indicate the same lithotypes and sm nd isotopic signatures and the same d1 d2 progressive deformation and greenschist facies metamorphism additionally footwall metasedimentary rocks exclusively display a post d2 deformation indicating that the maranhao river thrust propagated through upper crustal rocks of the paranoa group relatively late during the tectonic evolution of the belt fault propagation was a consequence of intraplate underthrusting during granulite exhumation the results allow for a better tectonic understanding of the brasilia belt and the tocantins province as well as explaining the presence of the pirineus zone of high strain

the puncoviscana trough a large basin of late precambrian to early cambrian age on the pacific edge of the brazilian shield during the late precambrian to early cambrian a large basin with a homogeneous psammitic pelitic sediment fill existed in the area of the nw argentine andes it is now exposed in different tectonic levels this basin of meridional elongation was situated on a stable continental margin at the western edge of the brazilian shield it was underlain by segmented older continental crust according to the modal and chemical composition of the greywackes and subgreywackes a predominant metasedimentary source may be supposed four clastic facies types indicate a transport by gravity currents within a submarine fan system while massive red pelites represent cut off periods and rare carbonates deposition on rises the gravity currents derived from easterly directions the sedimentation age is proved by a rich ichnofauna and some medusoid impressions

petrographic and geochemical characterization of the granitic rocks of the araguainha impact crater brazil petrographic and geochemical data obtained on the araguainha impact crater goias mato grosso states brazil indicate the existence of several molten products that originated during impact induced congruent melting of an alkali granite exposed in the inner part of the central uplift of the structure although previous studies have described these melts to some extent there is no detailed discussion on the petrographic and geochemical variability in the granite and its impactogenic derivatives and therefore little is known about the geochemical behavior and mobility of trace elements during its fusion in the central part of the araguainha crater this paper demonstrates that the preserved granitoid exposed in the core of the structure is a magnesium rich granite similar to postcollisional a type granites also found in terrains outside the araguainha crater in the brasilia orogenic belt the molten products are texturally distinct and different from the original rock but have very similar geochemical composition making it difficult to separate these lithotypes based on concentrations of major and minor elements this also applies for trace and rare earth elements ree thus indicating a high degree of homogenization during impact induced congruent melting under high pressure and postshock temperature conditions petrographic observations along with geochemical data indicate that melting occurs selectively where some of the elements are transported with the melt simultaneously there is an effective dissolution of the rock granite which leads to entrainment of the most resistant solid phases intact or partially molten minerals into the melt minerals more resistant to melting such as quartz and oxides contribute substantially to a chemical balance between the preserved granite and the fusion products generated during the meteoritic impact

manganese chromium isotope systematics of enstatite meteorites abstract we present results of a study of the 53 mn 53 cr isotope systematics in the enstatite chondrites and achondrites aubrites the goal of this study was to explore the capabilities of this isotope system to obtain chronological information on these important classes of meteorites and to investigate the original distribution in the inner solar system of the short lived radionuclide 53 mn our earlier work lugmair and shukolyukov 1998 shukolyukov and lugmair 2000a has shown that the asteroid belt bodies are characterized by essentially the same initial 53 mn abundance however we have found the presence of a gradient in the abundance of the radiogenic 53 cr between the earth moon system mars and the asteroid vesta if this gradient is considered as a function of the heliocentric distance a linear radial dependence is indicated this can be explained either by an early volatility controlled mn cr fractionation in the nebula or by an original radially heterogeneous distribution of 53 mn the enstatite chondrites are suggested to form in the inner zones of the solar nebula much closer to the sun than the ordinary chondrites therefore their investigation may be an important test on the hypothesis on a radial heterogeneity in the initial 53 mn we have studied the bulk samples of the eh4 chondrites indarch and abee and the el6 chondrite khairpur although these meteorites have essentially the same mn cr ratio as the ordinary chondrites the relative abundance of the radiogenic 53 cr is three times smaller than in the ordinary chondrites because these meteorites are primitive undifferentiated and no mn cr fractionation had occurred within their parent bodies this difference is a strong argument in favor of an initially heterogeneous distribution of 53 mn in the early inner solar system this finding is also consistent with formation of the enstatite chondrites in the inner zones of the solar nebula using the characteristic 53 cr excess of the enstatite chondrites and the observed gradient their place of origin falls at about 1 4 au or somewhat closer to the sun i e 1 0 1 4 au we also present chronological results for the enstatite chondrites and achondrites the absolute 53 mn 53 cr ages of the eh4 chondrites are old 4565 ma the el6 chondrite khairpur is 4 5 ma younger which is in good agreement with the 129 i 129 xe data from the literature the age of the aubrite pena blanca spring appears to be similar to those of the enstatite chondrites while that of the aubrite bishopville is at least 10 ma younger which is also in agreement with the 129 i 129 xe data the results from bulk samples of aubrites indicate that the last mn cr fractionation in their parent body occurred 4563 ma ago and imply an evolution of the mn cr isotope system in an environment with an higher than chondritic mn cr ratio for several millions of years

the redox state feo content and origin of sulfur rich magmas on mercury 1 mercury surface space environment geochemistry and ranging messenger orbital observations of mercury have revealed elevated s abundances ca s and mg s correlations and a low upper limit for ferrous iron in surface silicates these data indicate the presence of ca and or mg sulfides in volcanic rocks and a low oxygen fugacity fo2 in their parental magmas we have evaluated coupled fo2 and fs2 values and feo contents in mercury s magmas from silicate sulfide equilibria and empirical models for silicate melts and metallurgical slags the evaluated fo2 at 1700 1800 k is 4 5 to 7 3 log10 units below the iron wustite buffer these values correspond to 0 028 0 79 wt feo implying that fe must be also present in sulfides and metal and are also consistent with the composition of the partial melt of an enstatite chondrite this derived upper limit for feo is substantially lower than the limits obtained from reflectance measurements of mercury s surface materials the low fo2 and feo values provide new constraints for igneous processes on mercury as well as the formation evolution and internal structure of the innermost planet

post brasiliano pan african high k granitic magmatism in central brazil the role of late precambrian early palaeozoic extension abstract in western goias brazil the emplacement of large high k postorogenic granites and associated small gabbro dioritic intrusions followed immediately after the last deformational events of the brasiliano pan african orogeny at 600 ma well fitted whole rock rb sr isochrons indicate ages which suggest two discrete intrusive events the older between 588 and 560 ma and the younger between 508 and 485 ma the older granites display general petrographic and geochemical characteristics of highly differentiated calc alkaline i type granitoids whereas the younger intrusions are more alkaline similar to a type granites initial 87 sr 86 sr ratios vary from 0 703 to 0 710 and initial nd isotope ratios yield nd t values in the range between 4 0 and 3 0 there are no major differences in initial isotopic compositions between the two granite groups suggesting that the parental magmas for both groups of rocks mostly originated by refusion of crustal sources isotopically similar to the 940 640 ma basement arc type metatonalites metagranodiorites and associated arc metavolcanics the major and trace element compositional differences between the two granite groups is explained in terms of modifications in the melting conditions within the crust with younger melts being produced by the refusion of anhydrous depleted crustal sources left after the extraction of previous batches of more hydrated calc alkaline magmas the heat input required to promote extensive remelting of the continental crust was most likely provided by mantle derived mafic magmas that invaded and probably underplated the crust during uplift and extension the two intrusive events are bimodal in nature and are interpreted as shallow level extension related events associated with regional uplift and denudation occurring just after two orogenic pulses at the end of the proterozoic and early palaeozoic the older at 600 ma and the younger between 550 and 510 ma the onset of the granite magmatism at 590 ma shortly post dating the brasiliano orogeny 600 ma is broadly coeval with the initial stages of sedimentation of the terrigenous and carbonatic rocks of the ensialic paraguay belt in brazil and its correlative in bolivia the tucavaca belt which probably correspond to rift deposits related to the break up of laurentia from gondwana at the end of the proterozoic and beginning of the palaeozoic granites of the younger group cut the deformational structures of the paraguay belt metasediments and pre date by between 40 and 20 ma the initial stages of subsidence and sedimentation of the parana basin

isotopic evidence for the cretaceous tertiary impactor and its type high precision mass spectrometric analysis of chromium in sediment samples from the cretaceous tertiary k t boundary coincident with the extinction of numerous organisms on earth confirms the cosmic origin of the k t phenomenon the isotopic composition of chromium in k t boundary samples from stevns klint denmark and caravaca spain is different from that of earth and indicates its extraterrestrial source the chromium isotopic signature is consistent with a carbonaceous chondrite type impactor the observed differences in the chromium isotopic composition among various meteorite classes can serve as a diagnostic tool for deciphering the nature of impactors that have collided with earth during its history

geochemistry of 2 63 2 49 ga impact spherule layers and implications for stratigraphic correlations and impact processes thin layers rich in spherules formed during impacts by large extraterrestrial objects have the potential to shed new light on impact processes and aid in the long distance stratigraphic correlation of precambrian successions seven formations in western australia and south africa clustered around the archean proterozoic boundary each contain a single spherule layer all of which were deposited between ca 2 49 and 2 63 ga analyses of 25 samples from 6 of the 7 spherule layers and 23 samples from closely associated strata free of spherules revealed an extraterrestrial component etc in all six layers based on pge ratios and or cr isotopic composition the amount of etc varies from marginally detectable to clear and obvious it generally amounts to ca a few percent both pge ratios and cr isotopic anomalies indicate most if not all of the impactors were ordinary chondritic in composition in contrast all spherule layers older than ca 3 0 ga that have been analyzed were produced by carbonaceous chondritic impactors normalized rare earth element patterns suggest most target rocks were basaltic in composition and variability in pge ratios favors ballistic emplacement of melt droplets rather than spherule formation via vapor condensation the geochemical data also provide a means to test proposed intra and intercontinental stratigraphic correlations cr isotopic compositions are consistent with the formation of the oldest layers on both continents by a single impact event about 2 63 ga in contrast clear geochemical differences between the middle layers both deposited ca 2 54 ga suggest they were not produced by the same impact event the youngest banded iron formation hosted layers on both continents have also been correlated but no geochemical data are available on the african layer to test this at present

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

zircon and rutile upb geochronology of the niquelândia layered mafic and ultramafic intrusion brazil constraints for the timing of magmatism and high grade metamorphism abstract the niquel ndia intrusion is among the largest layered intrusions in the world it is part of a 350 km long belt of large mafic ultramafic complexes in central brazil both intrusion and country rocks were subjected to highgrade metamorphism ranging from amphibolite to granulite facies and associated ductile deformation the first u pb zircon and rutile ages for this region are presented zircon fractions from a meta anorthosite from the upper gabbro anorthositic unit are variably discordant 0 28 with regard to the lower intercept along a mixing line from 777 10 ma lower intercept and 1583 25 ma upper intercept zircon fractions from a later quartz diorite within the central mafic unit are variably discordant 0 14 with regard to the lower intercept along a mixing line from 794 6 4 lower intercept and 1565 22 upper intercept the upper intercepts for both rocks interpreted to be the magmatic age overlap and suggest a mesoproterozoic age for the intrusion the lower intercept is interpreted to be the high grade metamorphic event zircons from two quartz rich blastomylonites representing different metamorphic domains and cutting through the layered intrusions were analyzed a single zircon from the amphibolite facies domain blastomylonite gives a concordant 0 3 207 pb 206 pb age of 777 3 ma and a single zircon from the granulite facies blastomylonite gives a 207 pb 206 pb age of 786 5 ma 0 8 discordance both results are consistent with the lower intercept ages and set the 770 795 ma interval for the high grade metamorphic event rutile ages 650 ma are considerably younger than zircon metamorphic ages and suggest a milder regional tectonomagmatic episode or a complex and long lasting cooling history for the region the u pb data provide evidence for an important mesoproterozoic extensional tectonic event in central brazil it also sets the 770 795 ma interval for the high grade amphibolite to granulite facies prograde metamorphism interpreted to represent the timing of a continental collision event

manganese chromium formation intervals for chondrules from the bishunpur and chainpur meteorites whole chondrule mn cr isochrons are presented for chondrules separated from the chainpur ll3 4 and bishunpur ll3 1 meteorites the chondrules were initially surveyed by instrumental neutron activation analysis ll chondrite normalized mn cr mn fe and sc fe served to identify chondrules with unusually high or low mn cr ratios and to correlate the abundances of other elements to sc the most refractory element measured a subset of chondrules from each chondrite was chosen for analysis by a scanning electron microscope equipped with an energy dispersive x ray spectrometer prior to high precision cr isotopic analyses 53cr 52cr correlates with 55mn 52cr to give initial 53mn 55mn i 9 4 1 7 10 6 for chainpur chondrules and 53mn 55mn i 9 5 3 1 10 6 for bishunpur chondrules the corresponding chondrule formation intervals are respectively tlew 10 1 ma for chainpur and 10 2 ma for bishunpur relative to the time of igneous crystallization of the lewis cliff lew 86010 angrite because mn sc correlates positively with mn cr for both the chainpur and bishunpur chondrules indicating dependence of the mn cr ratio on the relative volatility of the elements we identify the event dated by the isochrons as volatility driven elemental fractionation for chondrule precursors in the solar nebula thus our data suggest that the precursors to ll chondrules condensed from the nebula 5 8 2 7 ma after the time when initial 53mn 55mn i 2 8 0 3 10 5 for calcium aluminum rich inclusions cais our preferred value determined from data for a mineral separates of type b allende cai br1 b spinels from efremovka cai e38 and c bulk chondrites mn cr formation intervals for meteorites are presented relative to average i mn 53mn 55mn ch 9 46 10 6 for chondrules mn cr ratios for radiogenic growth of 53cr in the solar nebula and later reservoirs are calculated relative to average i mn 53cr i 9 46 0 08 10 6 0 23 0 08 for chondrules inferred values of mn cr lie within expected ranges thus it appears that evolution of the cr isotopic composition can be traced from condensation of cais via condensation of the ferromagnesian precursors of chondrules to basalt generation on differentiated asteroids measured values of 53cr for individual chondrules exhibit the entire range of values that has been observed as initial 53cr values for samples from various planetary objects and which has been attributed to radial heterogeneity in initial 53mn 55mn in the early solar system estimated 55mn 52cr 0 42 0 05 for the bulk earth combined with 53cr 0 for the earth plots very close to the chondrule isochrons so that the earth appears to have the mn cr systematics of a refractory chondrule thus the earth apparently formed from material that had been depleted in mn relative to cr contemporaneously with condensation of chondrule precursors if as seems likely the earth s core formed after complete decay of 53mn there must have been little differential partitioning of mn and cr at that time

radiochronological age and correlation of proterozoic sediments in brazil abstract a review of available rb sr and k ar datings obtained on sedimentary sequences metamorphosed or not interbedded volcanics and cross cutting intrusives of the precambrian of brazil yields the following conclusions 1 1 the roraima and rio fresco formations resting upon the amazonian craton have been affected by the trans amazonian orogeny and are of lower proterozoic age 2 2 the beneficiente group in the same region seems to be of middle proterozoic lower riphean age 3 3 upon the sao francisco craton and upon the lower proterozoic rio dos remedios complex the paraguacu group might be of middle riphean age and the glacial sequences macaubas group and bebedouro formation date back to between 950 and 1000 ma 4 4 the age of the bambui and una groups in the same region remains undetermined it might be either upper riphean or upper riphean and vendian 5 5 the molassic series associated with the brazilian orogeny are dated back to between the proterozoic phanerozoic boundary and the ordovician

oxygen isotopic compositions of enstatite chondrites and aubrites oxygen isotopic compositions have been measured for ten eh chondrites eight el chondrites and nine aubrites the mean 18o and 17o values are 5 31 and 2 74 5 57 and 2 96 5 24 and 2 70 respectively within the eh group more equilibrated meteorites are fractionated toward higher heavy isotope abundances than the less equilibrated meteorites small departures of individual meteorites from a common mass fractionation occur for some of the eh samples the aubrites may be genetically related to either the eh or el group or to both almost all of the measured isotopic compositions are indistinguishable from the terrestrial oxygen isotope fractionation line suggesting a genetic relationship among the earth the moon and the enstatite the meteorites

the role of intracontinental deformation in supercontinent assembly insights from the ribeira belt southeastern brazil neoproterozoic west gondwana in this study we challenge the multiple collision model for the tectonic evolution of the neoproterozoic ribeira belt in southeastern brazil new u pb shrimp data reveal palaeoproterozoic 2153 15 ma and cryogenian 783 6 and 768 8 ma granitic rocks in the embu domain and detrital zircon data of metasedimentary units from the embu and costeiro domains suggest a coherent tectonic evolution for the whole ribeira belt rather than by multiple collisions these data are best explained by a simpler tectonic model involving continent craton volcanic arc collisions in the dom feliciano and brasilia belts that led to intracontinental crustal thickening of the adjacent thinned hinterland ribeira belt at 640 610 ma followed by widespread post collisional magmatism and rift related sedimentation at 600 540 ma we suggest that intracontinental orogeny is a relevant process during supercontinent assembly as illustrated here by the evolution of significant parts of the brasiliano orogen

constraints on melting and magma production in the crust major intrusions of granitic rocks are found in several tectonic settings and in all cases crustal melts may contribute to the volumes of granitic magma high grade metamorphism and partial melting of the crust take place predominantly under fluid absent conditions we present a model for calculating the amounts of melt that may be formed by fluid absent breakdown of micas and amphiboles in common crustal rock types pelitic quartzofeldspathic intermediate and mafic melt proportions depend mainly on the kind of source rock the pressure at which melting takes place the temperature and the hydrous mineral h2o content of the source as a consequence of the pressure dependence of water solubility in silicate melts any given source rock will produce more melt by a given fluid absent reaction at lower pressure at a given pressure higher temperature reactions can produce more melt from a given source rock based on a survey of the compositions of common rock types we show that the amounts of melt can vary from 50vol thus crustal rocks vary widely in their fertility as magma sources depending on the types and amounts of hydrous phases they contain in general muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation biotite breakdown in pelites occurs at higher temperature and because most high grade pelites below granulite grade are biotite rich can yield up to about 50 vol of granitoid melt rocks of intermediate composition and hornblende rich mafic rocks are potentially highly fertile magma sources also provided that the high temperatures necessary for biotite and hornblende breakdown are realized pyroxene rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with 30and probably 20vol melt the pressure temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria

early archean spherule beds chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type three early archean spherule beds from barberton south africa have anomalous cr isotope compositions in addition to large ir anomalies confirming the presence of meteoritic material with a composition similar to that in carbonaceous chondrites the extra terrestrial components in beds s2 s3 and s4 are estimated to be approx l 50 60 and 15 30 respectively these beds are probably the distal and possibly global ejecta from major large body impacts these impacts were probably much larger than the cretaceous tertiary event and all occurred over an interval of approx 20 m y implying an impactor flux at 3 2 ga that was more than an order of magnitude greater than the present flux

a cr isotopic study of the bon accord nio body in the barberton greenstone belt south africa abstract an unusual oxide body enriched in ni and the platinum group elements was found in the north western part of the barberton greenstone belt south africa in the 1920s with mineralogy and geochemistry which are extremely anomalous and do not conform to that of any other ni rich crustal rocks it has been interpreted as either an archaean paleo meteorite or of terrestrial deep mantle origin cr isotopic ratios were used to evaluate the merits of these two possibilities isotopic analyses returned values for e 53 of 0 00 epsilon units within a 2 s error relative to a terrestrial standard we interpret this information together with trace element data from the surrounding peridotites as pointing to a terrestrial origin for the body and conclude that the current dataset is not inconsistent with a deep mantle origin as a remnant of core formation with subsequent transport to and emplacement in the lithosphere by a mantle plume

variable tl pb and cd concentrations and isotope compositions of enstatite and ordinary chondrites evidence for volatile element mobilization and decay of extinct 205pb new tl pb and cd concentration and tl pb isotope data are presented for enstatite as well as l and ll type ordinary chondrites with additional cd stable isotope results for the former all three chondrite suites have tl and cd contents that vary by more than 1 2 orders of magnitude but pb concentrations are more uniform as a result of terrestrial pb contamination model calculations based on pb isotope compositions indicate that for more than half of the samples more than 50 of the measured pb contents are due to addition of modern terrestrial pb in part this is responsible for the relatively young and imprecise pb pb ages determined for eh l and ll chondrites which are hence only of limited chronological utility in contrast four particularly pristine el chondrites define a precise pb pb cooling age of 4559 6 ma the enstatite chondrites ecs have highly variable e114 110cd of between about 3 and 70 due to stable isotope fractionation from thermal and shock metamorphism furthermore nearly all enstatite meteorites display e205tl values from 3 3 to 0 8 while a single anomalous sample is highly fractionated in both tl and cd isotopes the majority of the ecs thereby define a correlation of e205tl with e114 110cd which suggests that at least some of the tl isotope variability reflects stable isotope fractionation rather than radiogenic ingrowth of 205tl from 205pb decay considering l chondrites most e205tl values range between 4 and 1 while two outliers with e205tl 10 are indicative of stable isotope fractionation considering only those l chondrites which are least likely to feature pb contamination or stable tl isotope effects the results are in accord with the former presence of live 205pb on the parent body with an initial 205pb 204pb 1 5 1 4 10 4 which suggests late equilibration of the pb tl system 26 113 ma after carbonaceous chondrites ccs the ll chondrites display highly variable e205tl values from 12 5 to 14 9 also indicative of stable isotope effects however the data for three pristine ll3 ll4 chondrites display an excellent correlation between e205tl and 204pb 203tl this defines an initial 205pb 204pb of 1 4 0 3 10 4 equivalent to a 205pb 205tl cooling age of 55 12 24 ma 31 67 ma after ccs

are high temperature fractionations in the solar nebula preserved in highly siderophile element systematics of the earth s mantle the relative abundances of the highly siderophile elements hse os ir ru pt rh and pd in relatively pristine lherzolites differ from solar abundance ratios and are several orders of magnitude higher than predicted for equilibrium distribution between metal silicate core mantle the samples are characterized by a mean ca al ratio of 1 18 0 09 m and a mean ca si ratio of 0 10 0 01 m overlapping with a mean ca al of 1 069 0 044 m and a mean ca si of 0 081 0 023 m found in chondrites wasson and kallemeyn 1988 interestingly the ci normalized abundance pattern shows decreasing solar system normalized abundances with increasing condensation temperatures the abundance of the moderately volatile element pd is about 2x higher than those in the most refractory siderophiles ir and os thus the hse systematics of upper mantle samples suggest that the late bombardment which added these elements to the accreting earth more closely resembles materials of highly reduced eh or el chondrites than carbonaceous chondrites in fact the hse in the earth mantle are even more fractionated than the enstatite chondritesan indication that some inner solar system materials were more highly fractionated than the latter

a type post collisional granites in the borborema province ne brazil the queimadas pluton abstract the queimadas pluton constitutes an e w elongated tabular intrusion exposed over an area of 50 km 2 it is intruded in paleoproterozoic gneiss migmatite of the alto pajeu terrane in the central tectonic domain of the borborema province and has a zircon u pb age of 570 20 ma it is cut by later shear zones with nne direction which provoked necking and disruption in the body a suite of biotite amphibole monzogranites associated with quartz diorite and later leucogranite and diabase dominates in these alkaline granites that are metaluminous to slightly peraluminous and show high feot feot mgo and k 2 o na 2 o ratios their ree patterns are moderately fractionated with ce yb n ratios of 10 16 and significant negative eu anomalies eu 0 40 0 67 geochemical and nd isotopic data indicate that the queimadas pluton magma originated by partial melting of a crustal fertile granodioritic rocks the studied granites crystallized under relatively low f o 2 and are a type post collisional granitoids

manganese chromium isotope systematics of carbonaceous chondrites abstract in this article we present the results of cr isotope investigations of different types of carbonaceous chondrites and of the pallasite eagle station the 53 cr 52 cr ratios in the bulk samples of carbonaceous chondrites are correlated with 55 mn 52 cr ratios the slope of the correlation line yields a 53 mn 55 mn ratio of 8 5 1 5 10 6 at the time of mn cr fractionation mapping this ratio onto an absolute time scale yields a time for this event of 4568 1 0 8 1 1 ma ago this time is very similar to the formation age of efremovka cais of 4567 2 0 6 ma y amelin a n krot i d hutcheon a a ulyanov lead isotopic ages of chondrules and calcium aluminum rich inclusions science 297 2002 1678 1683 to a time of the chondrule formation of 4568 1 ma ago l e nyquist d lindstrom d mittlefehldt c y shih h wiesmann s wentworth r martinez manganese chromium formation intervals for chondrules from the bishunpur and chainpur meteorites meteorit planet sci 36 2001 911 938 which most likely constrains early global high temperature mn cr fractionation in a nebular setting the bulk samples of carbonaceous chondrites exhibit clear 54 cr excesses 54 cr that are correlated with the 53 cr excesses 53 cr and also with mn cr ratios one possible explanation of this correlation is that 54 cr is also radiogenic like 53 cr and was formed by the decay of the short lived parent radionuclide 54 mn the very short half life of 54 mn of 312 days would require that both short lived radionuclides 53 mn and 54 mn were generated locally in spallation reactions during the early period of an active sun the alternative and possibly more plausible explanation is the heterogeneous addition of presolar material the presolar component enriched in 54 cr is mostly contained in the matrix of carbonaceous chondrites the relative amount of matrix decreases in the sequence ci cm co cv a large proportion of mn is associated with the matrix while cr preferentially resides in the chondrules thus the mn cr ratio also follows the sequence ci cm co cv and is correlated with 54 cr the acid resistant residues of carbonaceous chondrites are characterized by relatively large excesses of 54 cr and moderate deficits of 53 cr the magnitude of these excesses and deficits decreases in the sequence ci cm cv and may imply that cr in the cm and cv residues is increasingly more equilibrated with cr from the rest of the meteorites the 53 cr 52 cr and 54 cr 52 cr ratios in the residues are anti correlated indicating that there are at least two cr components of possibly presolar origin all residues have large excesses of the most neutron rich ti isotope 50 ti the pattern of 50 ti does not exactly follow that observed for 54 cr but it is generally similar the cr isotope systematic of the pallasite eagle station indicates that the precursor of this meteorite was a cv type material the 53 mn 53 cr system indicates that the cr isotopes equilibrated in this meteorite 4557 5 0 6 ma ago

neoproterozoic crustal accretion in central brazil recent geochronological studies have demonstrated the existence of several neoproterozoic orthogneiss and metavolcanic are suites ca 900 to 600 ma in the southern part of the tocantins province a major neoproterozoic orogenic area in central brazil exposed between the amazon and sao francisco cratons nd and sr isotopic characteristics of these metamorphic suites are primitive initial 87 sr 87 sr ratios vary from 0 7024 to 0 7042 and initial nd values are positive varying between 0 2 and 6 9 the most primitive initial sr and nd isotopic compositions are very similar to model depleted mantle compositions at the time of formation of the original magmas the mantle like characteristics of these are suites in western goias contradict previous ensialic evolution models for the tocantins province and reveal that crustal accretion processes were important in large areas of central brazil during neoproterozoic time

search for traces of the late heavy bombardment on earth results from high precision chromium isotopes abstract high precision mass spectrometric analyses of the chromium isotopic composition of metamorphosed turbiditic and pelagic sedimentary rocks and banded iron stones from the 3 7 gyr isua supracrustal belt isb in west greenland cannot be distinguished from the standard terrestrial 53 cr 52 cr ratio at our present level of resolution as a consequence our search for chemical traces of possible impact derived meteoritic components asteroidal and or cometary material or accreted cosmic dust in the earth s oldest chemical and detrital sediments was negative our results based on the 53 mn 53 cr short lived radionuclide system half life of 3 7 myr cannot confirm the recent findings by 1 r schoenberg b s kamber k d collerson s moorbath tungsten isotope evidence from approximately 3 8 gyr metamorphosed sediments for early meteorite bombardment of the earth nature 418 2002 403 405 of tungsten isotope anomalies based on the 182 hf 182 w short lived radionuclide system half life of 9 myr in these sediments which were interpreted as indicating a component derived from meteorites possible reasons for the failure to trace cosmic material in the isb metasediments are various 1 the samples studied are not representative 2 the sedimentation period did not overlap with the period of late heavy bombardment of the moon and 3 the potential chromium anomalies if present are too small to be traceable by our present levels of detection unequivocal evidence of a late heavy bombardment on the early earth therefore remains elusive and uncertain

precambrian scotland as a laurentia gondwana link origin and significance of cratonic promontories refinement of the supercontinental reconstruction for the latest precambrian that places the labrador greenland promontory of laurentia within the arica embayment along the margin of the gondwana craton juxtaposes the rockall microcontinent and northwestern british isles with the continental margin of peru the conjugate cratonic margin to the northwest caledonian foreland the hebridean shield may have been amazonia possible south american correlatives of the precambrian moinian and dalradian complexes of the scottish highlands can be identified in the reconstruction notably there are possible equivalents of the older granites within the moinian and of dalradian glacial deposits therefore the well studied precambrian rocks of the scottish highlands provide critical tests for the suggested reconstruction the latest precambrian setting of the arequipa massif with respect to the gondwana craton margin and the paleozoic intracratonic basin of peru bolivia bears a striking similarity to the early mesozoic setting of the rockall plateau and northwestern british isles with respect to the european continental margin and the north sea western approaches graben system in each case rifting appears to have been controlled by the youngest and presumably weakest lithosphere but the extremity of the labrador scotland greenland promontory was detached during final continental separation in vendian to earliest cambrian time to form the arequipa massif and in jurassic time to form the hebridean shield the promontory may have played an important tectonic role through at least 1 b y of earth history including influence on the development of the andean cordillera features of this type likely have played a major role in the development of orogenic belts of all ages

did the taconic appalachians continue into southern south america the appalachian mountains now terminating abruptly at the gulf of mexico coastal plain may have formerly continued into southern south america rocks forming the basement of the argentine andes can be interpreted as remnants of an early paleozoic orogen the famatinian belt not unlike the taconic appalachians both orogens are bordered to the west present coordinates by lower paleozoic carbonate platforms bearing the olenellid trilobite fauna that is characteristic of laurentia paleomagnetic and geologic data indicate that they could have formed as one continuous mountain chain possibly extending into antarctica during ordovician closure of an ocean basin southern iapetus between laurentia and gondwana the taconic and famatinian segments of the chain may have been truncated during late ordovician separation of laurentia and gondwana along the preexisting late neoproterozoic to cambrian rift system that initiated formation of the ouachita embayment and the southern margin of north america

explicating the behavior of mn bearing phases during shock melting and crystallization of the abee eh chondrite impact melt breccia literature data show that among eh chondrites the abee impact melt breccia exhibits unusual mineralogical characteristics these include very low mno in enstatite 0 04 wt higher mn in troilite 0 24 wt and oldhamite 0 36 wt than in eh4 indarch and eh3 kota kota which are not impact melt breccias low mn in keilite 3 6 4 3 wt high modal abundances of keilite 11 2 wt and silica 7 wt but ranging up to 16 wt in some regions low modal abundances of total silicates 58 8 wt and troilite 5 8 wt and the presence of acicular grains of the amphibole fluor richterite these features result from abees complex history of shock melting and crystallization impact heating was responsible for the loss of mno from enstatite and the concomitant sulfidation of mn troilite and oldhamite grains that crystallized from the impact melt acquired relatively high mn contents abundant keilite and silica also crystallized from the melt these phases along with metallic fe were produced at the expense of enstatite niningerite and troilite melting of the latter two phases produced a s rich liquid with higher fe mg and fe mn ratios than in the original niningerite allowing the crystallization of keilite prior to impact melting f was distributed throughout abee perhaps in part adsorbed onto grain surfaces after impact melting most of the f that was not volatilized was incorporated into crystallizing grains of fluor richterite other eh chondrite impact melt breccias and impact melt rocks exhibit some of these mineralogical features and must have experienced broadly similar thermal histories

brasiliano pan african granitic magmatism in the pajeu paraiba belt northeast brazil an isotopic and geochronological approach the neoproterozoic brasiliano pan african orogeny in the borborema province bp ne brazil is characterized by intense granitic magmatism spatially associated with continental scale shear zones and metamorphism under high temperature conditions u pb zircon and rb sr whole rock data for 14 granite intrusions from the paje u parabelt have ages that suggest more than 100 ma of intrusive magmatism which can be divided in four events the oldest granites between 620 and 600 ma are medium to slightly high k calc alkaline i type granitoids intruded into metagreywackes and gneiss granites of 0 9 1 0 ga they are related to the peak of metamorphism and to the development of a flat lying foliation the youngest intrusions 540 520 ma have geochemical signature of a type post orogenic extension related granites and are associated with sub volcanic bimodal magmatism probably contemporaneous with deposition of small sedimentary basins in the north iara jaibaras graben and sa ori and central fatima bet ania and carnatectonic domains of the borborema province they reflect post tectonic relaxation of the brasiliano orogeny between these episodes two other intrusive events were identified 1 high k calc alkaline granitoids and shoshonitic granitoids associated with mafic syenites meladiorites and hornblende biotite diorites intruded between 590 and 581 ma associated with a transcurrent deformation event and 2 alkaline post collisional granitoids having u pb zircon ages of ca 570 ma marking the final stage of the brasiliano orogeny in the south domain of the borborema province migmatization took place between 610 and 600 ma although similar ages were not found in the granitoids of the central tectonic domain field evidence suggest that migmatization followed the intrusions of the oldest 620 600 ma granitoids 2004 elsevier b v all rights reserved

spectroscopic survey of m type asteroids abstract m type asteroids as defined in the tholen taxonomy tholen d j 1984 asteroid taxonomy from cluster analysis of photometry ph d dissertation university of arizona tucson are medium albedo bodies supposed to have a metallic composition and to be the progenitors both of differentiated iron nickel meteorites and enstatite chondrites we carried out a spectroscopic survey in the visible and near infrared wavelength range 0 4 2 5 m of 30 asteroids chosen from the population of asteroids initially classified as tholen m types aiming to investigate their surface composition the data were obtained during several observing runs during the years 2004 2007 at the tng ntt and irtf telescopes we computed the spectral slopes in several wavelength ranges for each observed asteroid and we searched for diagnostic spectral features we confirm a large variety of spectral behaviors for these objects as their spectra are extended into the near infrared including the identification of weak absorption bands mainly of the 0 9 m band tentatively attributed to orthopyroxene and of the 0 43 m band that may be associated to chlorites and mg rich serpentines or pyroxene minerals such us pigeonite or augite a comparison with previously published data indicates that the surfaces of several asteroids belonging to the m class may vary significantly we attempt to constrain the asteroid surface compositions of our sample by looking for meteorite spectral analogs in the relab database and by modeling with geographical mixtures of selected meteorites minerals we confirm that iron meteorites pallasites and enstatite chondrites are the best matches to most objects in our sample as suggested for m type asteroids for 22 kalliope we demonstrate that a synthetic mixture obtained enriching a pallasite meteorite with small amounts 1 2 of silicates well reproduce the spectral behavior including the observed 0 9 m feature the presence of subtle absorption features on several asteroids confirms that not all objects defined by the tholen m class have a pure metallic composition a statistical analysis of spectral slope distribution vs orbital parameters shows that our sample originally defined as tholen m types tend to be dark in albedo and red in slope for increasing value of the semi major axis however we note that our sample is statistically limited by our number of objects 30 and slightly varying results are found for different subsets if confirmed the albedo and slope trends could be due to a difference in composition of objects belonging to the outer main belt or alternatively to a combination of surface composition grain size and space weathering effects

i xe ages of enstatite chondrites abstract in order to elucidate the early thermal history of enstatite chondrite parent bodies we determined 129i 129xe whole rock ages of enstatite chondrites 5 eh 2 el one eh impact melt relative to the shallowater reference meteorite 4562 3 0 4 ma all errors are 1 i xe ages of both el6 chondrites lon 94100 4 38 0 60 ma and neuschwanstein 3 87 0 73 ma negative sign indicates ages younger than shallowater agree well with data of other el6 chondrites lon 94100 displayed a second isochron at lower temperatures equivalent to a younger age of 5 25 1 17 ma perhaps reflecting different retention temperatures of respective carrier phases during sequential cooling the enstatite chondrites abee eh4 indarch eh4 eet 96135 eh4 5 and st marks eh5 encompass a i xe age range of 0 57 1 05 ma eet 96135 1 to 0 45 0 72 ma abee again in agreement with previously reported ages of eh chondrites only the age of st marks differs strongly from previously reported younger ages now being more in accordance with other members of the eh clan the eh3 chondrite sahara 97096 showed the youngest i xe age of 7 87 0 46 ma distinctly younger than other i xe ages of eh chondrites including other eh3s due to the apparent high retention temperature of the i xe system in enstatite estimated 800 c this young age implies a later resetting of the i xe system by a severe thermal likely impact induced event the eh impact melt lap 02225 records a similarly young thermal event though no isochron relationship could be established the data fall within an apparent i xe age range of 5 to 15 ma similar to sahara 97096 overall eh chondrite parent body experienced a thermal history determined by a complex interplay between impact disturbances and parent body metamorphism

129i 127i variations among enstatite chondrites abstract this paper presents the 129i 127i composition for 11 different enstatite chondrites there is a well defined hiatus in iodine isotopic compositions separating the enstatite chondrites the hiatus correlates with a similar hiatus defined by variations in chemical composition both major and minor elements eh 4 5 chondrites have 129i 127i ratios 20 higher 4 m y older than el 6 chondrites the enstatite chondrites are the third meteorite group joining silicates in iab irons and c3v carbonaceous chondrites in which 129i 127i correlates with chemical composition within each enstatite chondrite group the variations in initial iodine isotopic composition is less than 4 comparable to the experimental resolution the 20 difference between eh and el may indicate a maximum iodine isotopic heterogeneity between the eh and el nebular reservoirs alternatively if eh and el chondrites evolved from a homogeneous iodine isotopic reservoir then the el group must have evolved 4 0 m y longer than the eh group and either in an environment with a much lower i xe ratio than presently observed or one which was open to the loss of radiogenic 129xe the latter case may imply a nebular environment and would therefore be consistent with the open system evolution required to produce the chemical hiatus the duration of subsequent evolution in the high i xe environment must have been 1 m y and probably represents thermal metamorphism in two different parent bodies

testing an integrated chronology i xe analysis of enstatite meteorites and a eucrite we have determined initial 129i 127i ratios for mineral concentrates of four enstatite meteorites and a eucrite in the case of the enstatite meteorites the inferred ages are associated with the pyroxene rich separates giving pyroxene closure ages relative to the shallowater standard of indarch eh4 0 04 0 67 ma khairpur el6 4 22 0 67 ma khor temiki aubrite 0 06 ma and itqiy enstatite achondrite 2 6 2 6 ma negative ages indicate closure after shallowater no separate from the cumulate eucrite asuka a 881394 yielded a consistent ratio though excess 129xe was observed in a feldspar separate suggesting disturbance by thermal metamorphism within 25 ma of closure in shallowater iodine 129 ages are mapped to the absolute pb pb time scale using the calibration proposed by gilmour et al 2006 who place the closure age of shallowater at 4563 3 0 4 ma comparison of the combined 129i pb data with associated 53mn ages for objects that have been dated by both systems indicates that all three chronometers evolved concordantly in the early solar system the enstatite chondrites are offset from the linear array described by asteroid belt objects when 53mn ages are plotted against combined 129i pb data supporting the suggestion that 53mn was radially heterogeneous in the early solar system

nature of volatile depletion and genetic relationships in enstatite chondrites and aubrites inferred from zn isotopes enstatite meteorites include the undifferentiated enstatite chondrites and the differentiated enstatite achondrites aubrites they are the most reduced group of all meteorites the oxygen isotope compositions of both enstatite chondrites and aubrites plot along the terrestrial mass fractionation line which suggests some genetic links between these meteorites and the earth as well for this study we measured the zn isotopic composition of 25 samples from the following groups aubrites main group and shallowater el chondrites eh chondrites and happy canyon impact melt breccia we also analyzed the zn isotopic composition and elemental abundance in separated phases metal silicates and sulfides of the eh4 el3 and el6 chondrites the different groups of meteorites are isotopically distinct and give the following values parts per thousand aubrite main group 7 08 delta zn 66 0 37 eh3 chondrites 0 15 delta zn 66 0 31 eh4 chondrites 0 15 delta zn 66 0 27 eh5 chondrites delta zn 66 0 27 0 09 n 1 el3 chondrites 0 01 delta zn 66 0 63 the shallowater aubrite 1 48 delta zn 66 2 36 el6 chondrites 2 26 delta zn 66 7 35 and the impact melt enstatite chondrite happy canyon delta zn 66 0 37 the aubrite pena blanca spring delta zn 66 7 0 parts per thousand and the el6 north west forrest delta zn 66 7 35 are the isotopically lightest and heaviest samples respectively known so far in the solar system in comparison the range of zn isotopic composition of chondrites and terrestrial samples 1 5 delta zn 66 1 parts per thousand is much smaller luck et al 2005 herzog et al 2009 eh and el3 chondrites have the same zn isotopic composition as the earth which is another example of the isotopic similarity between earth and enstatite chondrites the zn isotopic composition and abundance strongly support that the origin of the volatile element depletion between el3 and el6 chondrites is due to volatilization probably during thermal metamorphism aubrites show strong elemental depletion in zn compared to both eh and el chondrites and they are enriched in light isotopes delta zn 66 down to 7 04 parts per thousand this is the opposite of what would be expected if zn elemental depletion was due to evaporation assuming the aubrites started with an enstatite chondrite like zn isotopic composition evaporation is therefore not responsible for volatile loss from aubrites on earth zn isotopes fractionate very little during igneous processes while differentiated meteorites show only minimal zn isotopic variability it is therefore very unlikely that igneous processes can account for the large isotopic fractionation of zn in aubrites condensation of an isotopically light vapor best explains zn depletion and isotopically light zn in these puzzling rocks mass balance suggests that this isotopically light vapor carries zn lost by the el6 parent body during thermal metamorphism and that aubritcs evolved from an el6 like parent body finally zn isotopes suggest that shallowater and aubrites originate from distinct parent bodies

granites and the geodynamic history of the neoproterozoic brasilia belt central brazil a review abstract recent field and geochronological studies have demonstrated the importance of granitic magmatism in the evolution of the neoproterozoic brasilia belt in central brazil this is an orogenic belt developed in response to the convergence between the amazon sao francisco congo and parana continental blocks the presence of neoproterozoic juvenile arc rocks and syn collisional peraluminous granites challenged previous intracontinental evolution models for the belt the granitoid intrusions reviewed in this paper record the different stages of evolution of the orogen and their field and isotopic characteristics can be used to reconstruct the tectonic history of the belt the main field and isotopic characteristics of four granite suites associated with the brasilia belt are reviewed i 1 77 1 58 ga old rift related a type granite intrusions ii ca 0 8 0 7 syn collisional granitoids iii arc metatonalites and metagranodiorites ca 0 9 to 0 63 ga and iv bimodal post orogenic suite ranging in age from ca 0 59 to 0 48 ga these rocks suggest that during most of the neoproterozoic the western margin of the sao francisco continent faced a large oceanic basin where subduction and oceanic lithosphere consumption started at ca 0 9 ga roughly coeval with the initial stages of the break up of rodinia final ocean closure happened at ca 0 63 0 60 ga with crustal thickening uplift and erosion post orogenic extension related magmatism took place between ca 0 6 and 0 5 ga and was partially contemporaneous with the deposition of the paraguay and tucavaca sedimentary successions resulting from the rifting event related to the break up of laurentia from southwestern gondwana