26al 26mg systematics in d orbigny and sahara 99555 angrites implications for high resolution chronology using extinct chronometers abstract we report on an investigation of the 26al 26mg isotope systematics in the d orbigny and sahara 99555 angrites high precision mg isotope compositions and al mg ratios were measured in mineral separates and whole rock samples from d orbigny and sahara 99555 using multiple collector inductively coupled plasma mass spectrometry mc icpms plagioclase separates from both angrites have resolvable excesses in 26mg 26mg that correlate with their respective al mg ratios 26al 26mg systematics in the mineral separates and whole rocks define precise isochrons that correspond to 26al 27al ratios of 5 06 0 92 10 7 and 5 13 1 90 10 7 and initial 26mg values of 0 006 0 040 and 0 016 0 047 for d orbigny and sahara 99555 respectively the slopes and initial 26mg values are identical for these two meteorites within errors and the data for both angrites considered together define an isochron corresponding to a 26al 27al ratio of 5 10 0 55 10 7 and initial 26mg value of 0 012 0 019 relative to the efremovka e60 cai the 26al 27al values reported here for these angrites imply 26al 26mg ages of 4562 42 0 29 ma and 4562 43 0 53 ma for d orbigny and sahara 99555 respectively these 26al 26mg ages are concordant with model ages determined using other extinct radionuclide chronometers e g 53mn 53cr and 182hf 182w but are 2 myr younger than the absolute 207pb 206pb ages that have been reported recently for these angrites the reason for this discrepancy is not presently known but may imply disturbance of one or more of the isotope systems under consideration or a possible bias in the 207pb 206pb ages of the angrites resulting from natural or analytical causes

thermal history of the h chondrite parent body implications for metamorphic grade and accretionary time scales abstract multiple temperature age constraints for eight h chondrite samples have been used to provide insight into the thermal history of their parent body through combination with numerical models of thermal evolution assuming internal heating by 26al and conductive cooling the effect of spreading accretion out over time is the principal focus of this work a wide range of body size and date of accretion is systematically tested for different values of accretion rate in order to quantify and illustrate the parameter space that is consistent with the available thermo chronological data we conclude that the h chondrite samples considered have a thermal history consistent with a parent body that at some stage had a concentric onion shell internal structure that body had a radius no larger than 130 km and accretion most probably took place over a time interval on the order of 0 0 0 2 myr approximately 2 myr after cai condensation in any case the time interval of accretion is unlikely to have been more than 0 5 myr supporting evidence in favour of rapid accretion possibly through reassembly of the fragments of an earlier generation of bodies furthermore the h chondrites studied here are inferred to have come from a wide depth range within the body where they experienced metamorphism indicating that preservation of the onion shell structure is unlikely the presence of an insulating regolith does not modify this conclusion as appropriate thermal histories for the three h6 samples considered cannot be reproduced at depths near the surface asteroid 6 hebe may be the parent body of the h chondrites but the high bulk density of the latter is difficult to reconcile with a rubble pile structure of pure h chondrite material finally optimized thermal histories are used to constrain the temperatures characterizing boundaries between petrological types 800 1000 and 1140 k for the h3 4 h4 5 and h5 6 boundaries respectively in detail the type 6 samples studied here are inferred to have experienced a similar peak temperature on the order of 1180 k but spent different times at that temperature while samples of lower metamorphic grade experienced different peak temperature but spent more or less the same time within 10 k of peak temperature on the order of 1 ma these results constitute a quantitative framework within which variations in petrographic and textural properties may be interpreted

mn cr isotope systematics of the d orbigny angrite we have conducted a detailed study of the mn cr systematics of the angrite d orbigny here we report cr isotopic abundances and mn cr ratios in olivine pyroxene glass chromite and bulk rock samples from d orbigny 53cr excesses in these samples correlate well with their respective mn cr ratios and define an isochron with a slope that corresponds to an initial 53mn 55mn ratio 3 24 0 04 x 10 6 and initial 53cr 52cr ratio of 53 0 30 0 03 at the time of isotopic closure the 53mn 55mn ratio of the d orbigny bulk rock is more than two fold the 53mn 55mn ratio of the angrites lewis cliff 86010 lew and angra dos reis ador and implies an older mn cr age of 4562 9 0 6 ma for d orbigny relative to a pb pb age of 4557 8 0 5 ma for lew and ador one of the most unusual aspects of d orbigny is the presence of glass a phase that has not been identified in any of the other angrites the mn cr data for glass and a pyroxene fraction found in druses indicate that they formed contemporaneously with the main phases of the meteorite since the mn cr age of d orbigny is 5 ma years older than the angrites lew and ador d orbigny likely represents an earlier stage in the evolution of the angrite parent body

radiometric ages of basaltic achondrites and their relation to the early history of the solar system pbpb mineral isochrons were determined for three cumulate eucrites moama moore county and serra de mage and three noncumulate eucrites nuevo laredo bouvante and stannern two noncumulate eucrites bereba and cachari show disturbed pb isotope patterns the mineral isochron ages of cumulate eucrites range from 4 40 ga to 4 48 ga the latter is the age of moore county 4 484 0 019 ga which was also dated by the smnd method at 4 456 0 025 ga nuevo laredo and bouvante give the same pb pb age 4 514 0 015 ga and 4 510 0 004 ga respectively the pb isochron of stannern corresponds to 4 128 0 016 ga which is a reequilibration age both cachari and bereba show evidence of isotope resetting at about 4 ga a pb pb multicotrelation approach is introduced and applied to the data the younger ages of the cumulate eucrites preclude their formation by fractional crystallization from the melts that produced the noncumulate eucrites the 1 of the source of the cumulate eucrites is 14 whereas that of the source of the noncumulate eucrites is 150 this large difference in 1 values suggests that eucrites were derived from separate planetesimals or from a single heterogeneous layered parent body

bending anisotropy a mechanical constraint on the orientation of fold axes in an anisotropic medium abstract bending anisotropy is the property of a layer of material whereby it bends more easily in some directions than it does in others in macroscopically homogeneous layers bending anisotropy results from the material itself being rheologically anisotropic in the plane of the layering in this paper we investigate bending anisotropy in materials with orthorhombic symmetry and linear elastic or viscous behaviour although such models are rheologically simple we believe they may provide a first approximation to the behaviour of some tectonites with linear and planar fabrics thus we predict that in rocks with strong linear fabrics folds will form with axes nearly parallel to the linear fabrics for a variety of different stress fields this has important geological consequences

a general harmonic coordinate transformation to simulate the states of strain in inhomogeneously deformed rocks in order to investigate the possible states of strain that may exist in an inhomogeneously deformed body a general deformation is proposed in the form of a harmonic coordinate transformation this deformation differs from those previously considered by jaeger ramsay graham and hobbs in that it is capable of representing a wide variety of deformations previous efforts had inbuilt assumptions regarding the mechanism of deformation the transformation contains adjustable terms all of which have distinct geometrical significance some represent a homogeneous deformation some represent inhomogeneous shortening some represent inhomogeneous shear and others correspond to a pinch and swell type of deformation by combining these terms with different degrees of emphasis many kinds of deformation may be simulated in this paper two constraints are developed in conjunction with the general harmonic transformation these are the conditions for constant volume deformation both locally and generally and the condition for zero shear strain of lines initially normal to the distorted layer again both locally and generally

accretion timescales and style of asteroidal differentiation in an 26al poor protoplanetary disk the decay of radioactive 26al to 26mg half life of 730 000 years is postulated to have been the main energy source promoting asteroidal melting and differentiation in the nascent solar system high resolution chronological information provided by the 26al 26mg decay system is therefore intrinsically linked to the thermal evolution of early formed planetesimals in this paper we explore the timing and style of asteroidal differentiation by combining high precision mg isotope measurements of meteorites with thermal evolution models for planetesimals in detail we report mg isotope data for a suite of olivine rich al mg 0 achondritic meteorites as well as a few chondrites main group pyroxene and the zinder pallasites as well as the lodranite all record deficits in the mass independent component of 26mg 26mg relative to chondrites and earth this isotope signal is expected for the retarded ingrowth of radiogenic 26mg in olivine rich residues produced through partial silicate melting during 26al decay and consistent with their marginally heavy mg isotope composition relative to ordinary chondrites which may reflect the early extraction of isotopically light partial melts from the source rock we propose that their parent planetesimals started forming within 250 000 years of solar system formation from a hot 500 k inner protoplanetary disk region characterized by a reduced initial 26al 27al 0 abundance 1 2 10 5 relative to the 26al 27al 0 value in cais of 5 25 10 5 this effectively reduced the total heat production and allowed for the preservation of solid residues produced through progressive silicate melting with depth within the planetesimals these non carbonaceous planetesimals acquired their mass throughout an extended period 3 myr of continuous accretion thereby generating onion shell structures of incompletely differentiated zones consisting of olivine rich residues overlaid by metachondrites and undifferentiated chondritic crusts in contrast individual olivine crystals from eagle station pallasites record variable 26mg excesses suggesting that these crystals captured the 26mg evolution of a magmatic reservoir controlled by fractional crystallization processes during the lifespan of 26al similar to previous suggestions based on isotopic evidence we suggest that eagle station pallasites formed from precursor material similar in composition to carbonaceous chondrites from a cool outer protoplanetary disk region characterized by 26al 27al 0 2 7 10 5 protracted planetesimal accretion timescales at large orbital distances with onset of accretion 0 3 1 myr post cais may have resulted in significant radiative heat loss and thus efficient early interior cooling of slowly accreting carbonaceous planetesimals

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

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

nucleosynthetic w isotope anomalies and the hf w chronometry of ca al rich inclusions abstract ca al rich inclusions cai are the oldest dated objects formed in the solar system and are pivotal reference points in early solar system chronology knowledge of their initial 182hf 180hf and 182w 184w is essential not only for obtaining precise hf w ages relative to the start of the solar system but also to assess the distribution of short lived radionuclides in the early solar nebula however the interpretation of hf w data for cai is complicated by nucleosynthetic w isotope variations to explore their extent and nature and to better quantify the initial hf and w isotope compositions of the solar system we obtained hf w data for several fine and coarse grained cai from three cv3 chondrites the fine grained cai exhibit large and variable anomalies in e183w e i w equals 0 01 deviation from terrestrial values extending to much larger anomalies than previously observed in cai and reflecting variable abundances of s and r process w isotopes conversely the coarse grained mostly type b inclusions show only small if any nucleosynthetic w isotope anomalies the investigated cai define a precise correlation between initial e182w and e183w providing a direct empirical means to correct the e182w of any cai for nucleosynthetic isotope anomalies using their measured e183w after correction for nucleosynthetic w isotope variations the cai data define an initial 182hf 180hf of 1 018 0 043 10 4 and an initial e182w of 3 49 0 07 the hf w formation intervals of the angrites d orbigny and sahara 99555 relative to this cai initial is 4 8 0 6 ma in good agreement with al mg ages of these two angrites this renders a grossly heterogeneous distribution of 26al in the inner solar system unlikely at least in the region were cai and angrites formed

investigation of magnesium isotope fractionation during basalt differentiation implications for a chondritic composition of the terrestrial mantle to investigate whether magnesium isotopes are fractionated during basalt differentiation we have performed high precision mg isotopic analyses by multi collector inductively coupled plasma mass spectrometry mc icp ms on a set of well characterized samples from kilauea iki lava lake hawaii usa samples from the kilauea iki lava lake produced by closed system crystal melt fractionation range from olivine rich cumulates to highly differentiated basalts with mgo content ranging from 2 37 to 26 87 wt our results demonstrate that although these basalts have diverse chemical compositions mineralogies crystallization temperatures and degrees of differentiation their mg isotopic compositions display no measurable variation within the limits of our external precision average 26 mg 0 36 0 10 and 25 mg 0 20 0 07 uncertainties are 2sd this indicates that mg isotopic fractionation during crystal melt fractionation at temperatures of 1055 c is undetectable at the level of precision of the current investigation calculations based on our data suggest that at near magmatic temperatures the maximum fractionation in the 26 mg 24 mg ratio between olivine and melt is 0 07 two additional oceanic basalts two continental basalts bcr 1 and bcr 2 and two primitive carbonaceous chondrites allende and murchison analyzed in this study have mg isotopic compositions similar to the kilauea iki lava lake samples in contrast to a recent report u wiechert a n halliday non chondritic magnesium and the origins of the inner terrestrial planets earth and planetary science letters 256 2007 360 371 the results presented here suggest that the bulk silicate earth has a chondritic mg isotopic composition 2007 published by elsevier b v

introduction the geologic mapping of vesta the purpose of this paper is to introduce the geologic mapping of vesta special issue section of icarus which includes several papers containing geologic maps of the surface of vesta made to support data analysis conducted by the dawn science team during the vesta encounter july 2011 september 2012 in this paper we briefly discuss pre dawn knowledge of vesta provide the goals of our geologic mapping campaign discuss the methodologies and materials used for geologic mapping review the global geologic context of vesta discuss the challenges of mapping the geology of vesta as a small airless body and describe the content of the papers in this special issue section we conclude with a discussion of lessons learned from our quadrangle based mapping effort and provide recommendations for conducting mapping campaigns as part of planetary spacecraft nominal missions

carbonaceous achondrites northwest africa 6704 6693 milestones for early solar system chronology and genealogy abstract northwest africa nwa 6704 6693 are medium to coarse grained achondrites with unique petrologic and geochemical traits that are distinct from the currently established meteorite groups here we report on the extinct 26al 26mg and 53mn 53cr systems to establish fine scale chronology of its formation and cr and ti isotopic anomalies to constrain the composition of the source reservoir of nwa 6704 6693 excesses in the neutron rich 54cr and 50ti isotopes due to nucleosynthetic anomalies separate nwa 6704 6693 from the vast majority of established achondrites and instead resemble the excesses seen among the carbonaceous chondrites specifically the cr type chondrites the excesses in these isotopes indicate a common feeding zone during accretion in the protoplanetary disk between the source of nwa 6704 6693 and that of the carbonaceous chondrites the 26al 26mg data for pyroxene and plagioclase from nwa 6704 yield a 26al 27al 0 3 15 0 38 10 7 mswd 0 49 and an initial 26mg 0 004 0 005 at the time of isotopic closure this initial 26al 27al 0 translates to an absolute age of 4563 14 0 38 ma relative to the d orbigny angrite however given the potential heterogeneity of 26al the d orbigny angrite might not be a good age anchor for the purpose of calculating 26al 26mg ages the 26al 26mg age relative to another carbonaceous achondrite nwa 2976 is 4562 66 0 60 ma the 53mn 53cr systematics of nwa 6704 6693 indicate a 53mn 55mn 0 of 2 59 0 34 10 6 mswd 1 2 with an evolved initial e53cr of 0 14 0 03 the 53mn 55mn 0 yields an 53mn 53cr age of 4562 17 0 76 ma relative to the d orbigny angrite concordant ages determined using the short lived 26al 26mg and 53mn 53cr systems and extant 207pb 206pb system 4562 60 0 30 ma for nwa 6704 6693 amelin et al 2019 indicate rapid cooling and nearly contemporaneous closing of multiple isotope systems the ancient crystallization ages and positive 54cr and 50ti anomalies of nwa 6704 6693 indicate widespread melting and differentiation processes occurring in both non carbonaceous nc and carbonaceous chondrite cc regions of the protoplanetary disk additionally we report the cr and ti isotopic composition for a petrologic range of cr type materials cr2 cr6 and achondrites the additional cr and ti isotopic data for these cr type materials indicates a range in isotopic composition not previously observed based on cr2 chondrites alone

stream functions and complex potentials implications for development of rock fabric and the continuum assumption abstract stream functions may be applied to study the development of ductile deformation patterns in rocks it is demonstrated how to derive the stream function and the associated complex potential from the velocity field of any particular flow finite deformation patterns are subsequently modelled analytically from this mathematical description of flow basic examples discussed are rigid body translation pure shear and composite flows due to simultaneous pure and simple shear a general stream function is obtained to describe the flow regime within a ductile deformation zone deforming by homogeneous plane strain by superposed pure and simple shear components the deformation patterns are visualized using analytical methods only the results are discussed in terms of fabric formation progressive deformation in a simple continuum modelled here mathematically using principles of fluid mechanics is compared with crystalline creep in rocks the validity of the continuum assumption is briefly outlined in view of recent experiments on particular synthetic monocrystalline aggregates octochloropropane ocp deformable at room conditions this provides a method for testing how crystalline deformation compares with an isotropic continuum

impact histories of angrites eucrites and their parent bodies abstract eucrites which are probably from 4 vesta and angrites are the two largest groups of basaltic meteorites from the asteroid belt the parent body of the angrites is not known but it may have been comparable in size to vesta as it retained basalts and had a core dynamo both bodies were melted early by 26al and formed basalts a few myr after they accreted despite these similarities the impact histories of the angrites and eucrites are very different angrites are very largely unshocked and none are breccias whereas most eucrites are breccias and many are shocked we attribute the lack of shocked and unbrecciated angrites to an impact possibly at 4558 myr ago the radiometric age of the younger angrites that extracted the angrites from their original parent body into smaller bodies these bodies which may have had a diameter of approximately 10 km suffered much less impact damage than vesta during the late heavy bombardment because small bodies retain shocked rocks less efficiently than large ones and because large bodies suffer near catastrophic impacts that deposit vastly more impact energy per kg of target our proposed history for the angrites is comparable to that proposed by bogard and garrison 2003 for the unbrecciated eucrites with ar ar ages of 4 48 gyr and that for unbrecciated eucrites with anomalous oxygen isotopic compositions that did not come from vesta we infer that the original parent bodies of the angrites and the anomalous eucrites were lost from the belt when the giant planets migrated and the total mass of asteroids was severely depleted alternatively their parent bodies may have formed in the terrestrial planet region and fragments of these bodies were scattered out to the primordial main belt as a consequence of terrestrial planet formation

the origin of fold abutments in the map pattern of the westernmost grenville province central ontario abstract the westernmost part of the proterozoic grenville orogen widely interpreted as an aggregate of structural domains and subdomains has been subjected to superimposed disharmonic buckling on the scale of kilometres the domain boundaries are characterized by abutments of upright to overturned folds attributed by others to thrust induced truncation of inherited structures the present study shows that abutments on geological maps are fold termination lines which mark narrow zones of adjustment flow caused by buckle strain heterogeneity on the kilometre scale such adjustment zones are ductile equivalents of fold decollement surfaces in the brittle upper crust and are normally characterized by high strain gradients prominent adjustment zones may be located within strain softened rocks at domain boundaries coincident with major ductile thrusts

numerical simulations of the differentiation of accreting planetesimals with 26al and 60fe as the heat sources numerical simulations have been performed for the differentiation of planetesimals undergoing linear accretion growth with 26al and 60fe as the heat sources planetesimal accretion was started at chosen times up to 3 ma after ca al rich inclusions cais were formed and was continued for periods of 0 001 1 ma the planetesimals were initially porous unconsolidated bodies at 250 k but became sintered at around 700 k ending up as compact bodies whose final radii were 20 50 100 or 270 km with further heating the planetesimals underwent melting and igneous differentiation two approaches to core segregation were tried in the first labelled a the core grew gradually before silicate began to melt and in the second labelled b the core segregated once the silicate had become 40 molten in a when the silicate had become 20 molten the basaltic melt fraction began migrating upward to the surface carrying 26al with it the 60fe partitioned between core and mantle the results show that the rate and timing of core and crust formation depend mainly on the time after cais when planetesimal accretion started they imply significant melting where accretion was complete before 2 ma and a little melting in the deep interiors of planetesimals that accreted as late as 3 ma the latest melting would have occurred at 10 ma the effect on core and crust formation of the planetesimals final size the duration of accretion and the choice of 60fe 56fe initial were also found to be important particularly where accretion was late the results are consistent with the isotopic ages of differentiated meteorites and they suggest that the accretion of chondritic parent bodies began more than 2 or 3 ma after cais

26 al 26 mg and 207 pb 206 pb systematics of allende cais canonical solar initial 26 al 27 al ratio reinstated abstract the precise knowledge of the initial 26 al 27 al ratio 26 al 27 al 0 is crucial if we are to use the very first solid objects formed in our solar system calcium aluminum rich inclusions cais as the time zero age anchor and guide future work with other short lived radio chronometers in the early solar system as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation new high precision multi collector inductively coupled plasma mass spectrometry mc icp ms measurements of 27 al 24 mg ratios and mg isotopic compositions of nine whole rock cais six mineralogically characterized fragments and three micro drilled inclusions from the cv carbonaceous chondrite allende yield a well defined 26 al 26 mg fossil isochron with an 26 al 27 al 0 of 5 23 0 13 10 5 internal mineral isochrons obtained for three of these cais a44a ajef and a43 are consistent with the whole rock cai isochron the mineral isochron of ajef with 26 al 27 al 0 4 96 0 25 10 5 anchored to our precisely determined absolute 207 pb 206 pb age of 4567 60 0 36 ma for the same mineral separates reinstate the canonical 26 al 27 al 0 of 5 10 5 for the early solar system the uncertainty in 26 al 27 al 0 corresponds to a maximum time span of 20 ka thousand years suggesting that the allende cai formation events were culminated within this time span although all allende cais studied experienced multistage formation history including melting and evaporation in the solar nebula and post crystallization alteration likely on the asteroidal parent body the 26 al 26 mg and u pb isotopic systematics of the mineral separates and bulk cais behaved largely as closed system since their formation our data do not support the supra canonical 26 al 27 al ratio of individual minerals or their mixtures in cv cais suggesting that the supra canonical 26 al 27 al ratio in the cv cais may have resulted from post crystallization inter mineral redistribution of mg isotopes within an individual inclusion this redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk cai and bulk mineral data obtained by mc icp ms the radiogenic 208 pb 206 pb ratio obtained as a by product from the pb pb age dating is used to estimate time integrated 232 th 238 u ratio value of cais limited variations among the minerals within a single cai contrasted by much larger variations among the bulk cais suggest th u fractionation occurred prior to crystallization of igneous cais if interpreted as primordial heterogeneity the value can be used to calculate the mean age of the interstellar dust from which the cais condensed

multi year pressure ridges in the canadian beaufort sea abstract pressure ridges and hummocks are the largest of the sea ice relief features seen in the ice cover which characterizes the canadian beaufort sea due to their massiveness and structural integrity multi year pressure ridges are generally considered to be the most formidable formations in this area and represent not only a significant impediment to shipping but also a major problem in the design of offshore structures here the findings of a field study designed to generate fundamental data on multi year pressure ridges in the near shore zone of the canadian beaufort sea are presented the study investigated the geometry of eleven floating multi year ridges or ridge fragments and the sail height and keel depth of four additional multi year ridge fragments the cross sections of multi year ridges with total thicknesses varying between 9 6 and 41 8 m were examined and the results suggest that they can be adequately represented by one ridge model with a constant sail to keel ratio and geometry it is also shown that the ice comprising multi year ridges is solid with the interblock voids existing at the time of their formation being completely filled with ice the data obtained from this study is being used in the engineering design of exploration and production systems for the beaufort sea in the shallow waters of this area exploratory drilling from artificial islands has been carried out since 1973 and since 1976 the exploration effort has extended into the deeper waters of the beaufort sea using drillships

the cr chondrite clan implications for early solar system processes in this paper we review the mineralogy and chemistry of calcium aluminum rich inclusions cais chondrules feni metal and fine grained materials of the cr chondrite clan including cr ch and the metal rich cb chondrites queen alexandra range 94411 hammadah al hamra 237 bencubbin gujba and weatherford the members of the cr chondrite clan are among the most pristine early solar system materials which largely escaped thermal processing in an asteroidal setting bencubbin weatherford and gujba may be exceptions and provide important constraints on the solar nebula models these constraints include 1 multiplicity of cai formation 2 formation of cals and chondrules in spatially separated nebular regions 3 formation of cais in gaseous reservoir s having o 16 rich isotopic compositions chondrules appear to have formed in the presence of o 16 poor nebular gas 4 isolation of cais and chondrules from nebular gas at various ambient temperatures 5 heterogeneous distribution of al 26 in the solar nebula and 6 absence of matrix material in the regions of cai and chondrule formation

26al 26mg dating of asteroidal magmatism in the young solar system abstract we present high precision mg isotope data for most classes of basaltic meteorites including eucrites mesosiderite silicate clasts angrites and the ungrouped northwest africa nwa 2976 measured by pseudo high resolution multiple collector inductively coupled plasma mass spectrometry and utilising improved techniques for chemical purification of mg with the exception of the angrites angra dos reis lewis cliff lew 86010 nwa 1296 and nwa 2999 and the diogenite bilanga which have either been shown to have young ages by other dating techniques or have low al mg ratios all bulk samples of basaltic meteorites have 26mg excesses 26 mg 0 0135 to 0 0392 the 26mg excesses cannot be explained by analytical artefacts cosmogenic effects or heterogeneity of initial 26al 27al al mg ratios or mg isotopes in asteroidal parent bodies as compared to earth or chondrites the 26mg excesses record asteroidal melting and formation of basaltic magmas with super chondritic al mg and confirm that radioactive decay of short lived 26al was the primary heat source that melted planetesimals model 26al 26mg ages for magmatism on the eucrite mesosiderite angrite and nwa 2976 parent bodies are 2 6 3 2 3 9 4 1 and 3 5 myr respectively after formation of calcium aluminium rich inclusions cais however the validity of these model ages depends on whether the elevated al mg ratios of basaltic meteorites result from magma ocean evolution on asteroids through fractional crystallisation or directly during partial melting mineral isochrons for the angrites sahara sah 99555 and d orbigny and nwa 2976 yield ages of 5 06 0 05 0 06 myr and 4 86 0 09 0 10 myr respectively after cai formation both isochrons have elevated initial 26 mg values given the brecciated and equilibrated texture of nwa 2976 it is probable that its isochron age and elevated initial 26 mg 0 0175 0 0034 reflects thermal resetting during an impact event and slow cooling on its parent body however in the case of the angrites the marginally elevated initial 26 mg 0 0068 0 0058 may reflect either 26 mg ingrowth in a magma ocean prior to eruption and crystallisation or in an older igneous protolith with super chondritic al mg prior to impact melting and crystallisation of these angrites or partial internal re equilibration of mg isotopes after crystallisation 26al 26mg model ages and an olivine pyroxene whole rock isochron for the angrites sah 99555 and d orbigny are in good agreement with age constraints from 53mn 53cr and 182hf 182w short lived chronometers suggesting that the 26al 26mg feldspar controlled isochron ages for these angrites may be compromised by the partial resetting of feldspar mg isotope systematics even when age constraints from the 26al 26mg angrite model ages or the mafic mineral whole rock isochron are considered the relative time difference between sah 99555 d orbigny crystallisation and cai formation cannot be reconciled with pb pb ages for sah 99555 d orbigny and cais which are ca 1 0 myr too old angrites or too young cais for reasons that are not clear this discrepancy might indicate that 26al was markedly lower ca 40 in the planetesimal and planet forming regions of the proto planetary disc as compared to cais or that cai pb pb ages may not accurately date cai formation which might be better dated by the 182hf 182w and 26al 26mg chronometers as 4568 3 0 7 burkhardt et al 2008 and 4568 5 0 3 ma herein respectively when mapped onto an absolute timescale using pb pb ages for angrites

chronology of the angrite parent body and implications for core formation in protoplanets abstract angrites formed by some of the earliest igneous activity in the solar system and provide insights into the early stages of planetary melting and differentiation moreover they are pivotal reference points for early solar system chronology in order to study the processes and timescales of metal segregation in early protoplanets and to assess the distribution of short lived radionuclides in the early solar system the 182 hf 182 w system was applied to a comprehensive suite of angrites 182 hf 182 w isochron ages for angrites are in excellent agreement with previously reported 207 pb 206 pb and 53 mn 53 cr results but are 1 myr older than ages obtained from 26 al 26 mg chronometry these inconsistencies probably reflect a disturbance of the al mg system in the angrite feldspars but could alternatively be explained by a heterogeneous distribution of 26 al in the early solar system based on the hf w results four texturally and temporally resolved groups of angrites can be identified that were derived from at least two distinct mantle sources these mantle sources are the result of separate events of core formation both of which took place within 2 myr of cai formation thus core formation in the angrite parent body did not occur as a single event of metal segregation from a global magma ocean but rather took place under varying conditions by several more local events the disparate hf w systematics of the two distinct angrite source regions indicate that convection in the magma ocean was inefficient in homogenizing the composition of the mantle possibly as a result of a continuous bombardment with small planetesimals during ongoing core formation such impacts could have constantly removed primordial and earlier formed crust facilitating rapid cooling of the magma ocean which solidified as early as 3 6 0 7 myr after cai formation

lead and mg isotopic age constraints on the evolution of the hed parent body the large collection of howardite eucrite diogenite hed meteorites allows us to study the initial magmatic differentiation of a planetesimal we report pb pb ages of the unequilibrated north west africa nwa 4215 and dhofar 700 diogenite meteorites and their mass independent 26mg isotope compositions 26mg to better understand the timing of differentiation and crystallization of their source reservoir s nwa 4215 defines a pb pb age of 4484 5 7 9 myr and has a 26mg excess of 2 3 1 6 ppm whereas dhofar 700 has a pb pb age of 4546 4 4 7 myr and a 26mg excess of 25 5 1 9 ppm we interpret the young age of nwa 4215 as a thermal overprint but the age of dhofar 700 is interpreted to represent a primary crystallization age combining our new data with published mg isotope and trace element data suggests that approximately half of the diogenites for which such data are available crystallized within the first 1 2 myr of our solar system consistent with a short lived early formed magma ocean undergoing convective cooling the other half of the diogenites including both nwa 4215 and dhofar 700 are best explained by their crystallization in slowly cooled isolated magma chambers lasting over at least 20 myr

heterogeneous distribution of 26al at the birth of the solar system evidence from refractory grains and inclusions we review recent results on o and mg isotope compositions of refractory grains corundum hibonite and calcium aluminum rich inclusions cais from unequilibrated ordinary and carbonaceous chondrites we show that these refractory objects originated in the presence of nebular gas enriched in 16o to varying degrees relative to the standard mean ocean water value the 17osmow value ranges from approximately 16 to 35 and recorded heterogeneous distribution of 26al in their formation region the inferred 26al 27al 0 ranges from approximately 6 5 10 5 to 2 10 6 there is no correlation between o and mg isotope compositions of the refractory objects 26al rich and 26al poor refractory objects have similar o isotope compositions we suggest that 26al was injected into the 26al poor collapsing protosolar molecular cloud core possibly by a wind from a neighboring massive star and was later homogenized in the protoplanetary disk by radial mixing possibly at the canonical value of 26al 27al ratio approximately 5 10 5 the 26al rich and 26al poor refractory grains and inclusions represent different generations of refractory objects which formed prior to and during the injection and homogenization of 26al thus the duration of formation of refractory grains and cais cannot be inferred from their 26al 26mg systematics and the canonical 26al 27al 0 does not represent the initial abundance of 26al in the solar system instead it may or may not represent the average abundance of 26al in the fully formed disk the latter depends on the formation time of cais with the canonical 26al 27al ratio relative to the timing of complete delivery of stellar 26al to the solar system and the degree of its subsequent homogenization in the disk the injection of material containing 26al resulted in no observable changes in o isotope composition of the solar system instead the variations in o isotope compositions between individual cais indicate that o isotope composition of the cai forming region varied because of coexisting of 16o rich and 16o poor nebular reservoirs gaseous and or solid at the birth of the solar system or because of rapid changes in the o isotope compositions of these reservoirs with time e g due to co self shielding in the disk

the geologic history of vesta inferred from combined 207pb 206pb and 40ar 39ar chronology of basaltic eucrites abstract eucrites represent samples of mafic crust of the parent body likely vesta and record a complex geologic history involving magmatism metamorphism impact processing and metasomatism for better understanding of the geologic history we present combined 207pb 206pb and 40ar 39ar chronology of four basaltic eucrites showing distinct petrologic features agoult an equilibrated eucrite with granulitic textures camel donga a brecciated equilibrated eucrite enriched in metallic iron dag 380 a partially equilibrated monomict eucrite and nwa 049 a metasomatized polymict eucrite comprising mostly unequilibrated clasts the 207pb 206pb dating in combination with the acid washing technique was applied to pyroxene rich and plagioclase rich fractions while the 40ar 39ar dating was performed mainly on plagioclase grains using the laser incremental heating method the pb isotopic data for acid leaching residues are more radiogenic than those for acid washes reflecting efficient removal of non radiogenic pb components by the acid washing among the residues however only those of the agoult and camel donga plagioclase rich fractions yielded isochrons rather than errorchrons with 207pb 206pb ages of 4532 39 0 87 ma and 4515 43 0 42 ma respectively the trace element systematics further suggest that these residues essentially represent contaminant oxide minerals such as ilmenite and chromite rather than plagioclase on the other hand the ar isotopic data for agoult and camel donga yielded plateau ages with weighted means of 4494 9 ma and 3749 25 ma in contrast to those for dag 380 and nwa 049 that did not return any plateau the 207pb 206pb and 40ar 39ar ages of agoult are interpreted to reflect the time of the u pb and k ar system closure during a prolonged thermal metamorphism by combining the ages reported here and in the literature with the calculated closure temperatures the cooling rate of agoult is determined to be 11 2 c ma this cooling rate corresponds to a burial depth of 10 20 km indicating that granulitic eucrites were originally crystallized near the surface and were subsequently buried and metamorphosed the 20 ma younger 207pb 206pb ages of camel donga than agoult are considered to reflect delayed burial metamorphism in the source region whereas the plateau 40ar 39ar age would record the time of an impact causing brecciation and efficient ar degassing of the buried crust furthermore the model 207pb 206pb dates calculated for acid washed pyroxenes imply that the burial metamorphism of agoult and dag 380 were broadly contemporaneous and also that nwa 049 was metasomatized by fluids within the uppermost crust later than 4 4 ga

petrogenesis and provenance of ungrouped achondrite northwest africa 7325 from petrology trace elements oxygen chromium and titanium isotopes and mid ir spectroscopy northwest africa nwa 7325 is an ungrouped achondrite that has recently been recognized as a sample of ancient differentiated crust from either mercury or a previously unknown asteroid in this work we augment data from previous investigations on petrography and mineral compositions mid ir spectroscopy and oxygen isotope compositions of nwa 7325 and add constraints from cr and ti isotope compositions on the provenance of its parent body in addition we identify and discuss notable similarities between nwa 7325 and clasts of a rare xenolithic lithology found in polymict ureilites nwa 7325 has a medium grained protogranular to poikilitic texture and consists of 10 15 vol mg rich olivine fo 98 25 30 vol diopside wo 45 mg 98 55 60 vol ca rich plagioclase an 90 and trace cr rich sulfide and fe ni metal we interpret this meteorite to be a cumulate that crystallized at 1200 c and very low oxygen fugacity similar to the most reduced ureilites from a refractory incompatible element depleted melt modeling of trace elements in plagioclase suggests that this melt formed by fractional melting or multi stage igneous evolution a subsequent event likely impact resulted in plagioclase being substantially remelted reacting with a small amount of pyroxene and recrystallizing with a distinctive texture the bulk oxygen isotope composition of nwa 7325 plots in the range of ureilites on the ccam line and also on a mass dependent fractionation line extended from acapulcoites the e54cr and e50ti values of nwa 7325 exhibit deficits relative to terrestrial composition as do ordinary chondrites and most achondrites its e54cr value is distinct from that of any analyzed ureilite but is not resolved from that of acapulcoites as represented by acapulco in terms of all these properties nwa 7325 is unlike any known achondrite however a rare population of clasts found in polymict ureilites the magnesian anorthitic lithology are strikingly similar to nwa 7325 in mineralogy and mineral compositions oxygen isotope compositions and internal textures in plagioclase these clasts are probably xenolithic in polymict ureilites and could be pieces of nwa 7325 like meteorites using constraints from chromium titanium and oxygen isotopes we discuss two possible models for the provenance of the nwa 7325 parent body 1 accretion in the inner solar system from a reservoir similar to that of acapulcoites in 17o e54cr and e50ti or 2 early 1 ma after cai formation accretion in the outer solar system beyond the snow line before 54cr and 50ti anomalies were introduced to this region of the solar system the mid ir emission spectrum of nwa 7325 obtained in this work matches its modal mineralogy and so can be compared with spectra of new meteorites or asteroids planets to help identify similar materials and or the parent body of nwa 7325

correlated helium 3 and tungsten isotopes in iron meteorites quantitative cosmogenic corrections and planetesimal formation times abstract core formation in planetesimals can in principle be dated using the short lived 182 hf 182 w chronometer however it has been predicted that burnout and production of w isotopes by nuclear reactions can substantially modify the compositions in iron meteorites when exposed for several hundred million years myr to galactic cosmic rays this may severely limit the use of the hf w system for determining chronologies of metal segregation as recorded in iron meteorites here we present the first experimental evidence of cosmogenic effects on w isotopes in two magmatic iron meteorites carbo iid and grant iiiab the 182 w 184 w ratio near the pre atmospheric centre of the meteorites is lower by 0 5 epsilon compared to values near the pre atmospheric surface the 182 w 184 w ratio displays excellent correlations with the 3 he concentration which in turn provides a proxy for the fluence of the relevant cosmic ray particles using new nuclear physics parameters a first order correction for cosmic ray effects on w isotopes is proposed based on the 3 he concentration and on an independent exposure age of the meteorite this method is then applied to other magmatic iron meteorites negrillos cape of good hope navajo and arispe when corrected for cosmogenic effects the w isotopic compositions of carbo grant negrillos cape of good hope and navajo are similar to the initial composition recorded in allende cais this shows that at least some magmatic meteorites from groups iiab iid iiiab and ivb segregated within less than 1 2 myr of the hf w system closure in allende cais the isotopic variations among the samples indicate that the iron meteorites studied in this paper segregated over a time interval of 4 myr carbo being the oldest iron meteorites of this study

magnesium isotopes constraints on the origin of mg rich olivines from the allende chondrite nebular versus planetary abstract high precision mg isotope measurements by multi collector ion microprobe show that refractory olivines from the allende chondrite either olivines isolated in the matrix 2 samples studied or olivines in type i chondrules 6 samples studied have variable 26mg enrichments and deficits calculated in permil as the 26mg deviation from the instrumental mass fractionation line relative to the earth most average 26mg noted 26mg av values between 10 and 20 analyses per chondrule are negative but the total range is from 0 029 0 010 2 sigma errors to 0 011 0 011 with an exception of one olivine at 0 043 0 023 these variations in 26mg av reflect the formation of the olivines from reservoirs enriched in various amounts of 26mg by the decay of short lived 26al t1 2 0 73 ma similarly 30 analyses of olivines from the eagle station pallasite show a 26mg av value of 0 033 0 008 as negative as some olivines from allende chondrules and the solar system initial 26mg value of 0 038 0 004 defined at the time of formation of type b ca al rich inclusions cais when 26al 27al 5 23 10 5 jacobsen et al 2008 because olivines are al poor and because their mg isotopic compositions are not reset during the chondrule forming events their 26mg av can be used to calculate model crystallization ages relative to various theoretical mg isotope growth curves the two end member scenarios considered are i a nebular growth in which the al mg ratio remains chondritic and ii a planetary growth in which a significant increase of the al mg ratio can be due to for instance olivine magmatic fractionation the low 26mg av value of olivines from the eagle station pallasite demonstrate that metal silicate differentiation occurred as early as 0 15 0 23 0 29 ma after cais in either of the growth scenarios similarly the variable 26mg av values of refractory olivines can be understood if they were formed in planetesimals which started to differentiate as early as the eagle station parent body accretion of these planetesimals must have been coeval to the formation of cais and their disruption could explain why their fragments mg rich olivines were distributed in the chondrule forming regions of the disk

oxygen isotope variation in stony iron meteorites asteroidal material delivered to earth as meteorites preserves a record of the earliest stages of planetary formation high precision oxygen isotope analyses for the two major groups of stony iron meteorites main group pallasites and mesosiderites demonstrate that each group is from a distinct asteroidal source mesosiderites are isotopically identical to the howardite eucrite diogenite clan and like them are probably derived from the asteroid 4 vesta main group pallasites represent intermixed core mantle material from a single disrupted asteroid and have no known equivalents among the basaltic meteorites the stony iron meteorites demonstrate that intense asteroidal deformation accompanied planetary accretion in the early solar system

the effect of melt composition on metal silicate partitioning of siderophile elements and constraints on core formation in the angrite parent body we present 275 new metal silicate partition coefficients for p s v cr mn co ni ge mo and w obtained at moderate p 1 5 gpa and high t 1683 1883 k we investigate the effect of silicate melt composition using four end member silicate melt compositions we identify possible silicate melt dependencies of the metal silicate partitioning of lower valence elements ni ge and v elements that are usually assumed to remain unaffected by changes in silicate melt composition results for the other elements are consistent with the dependence of their metal silicate partition coefficients on the individual major oxide components of the silicate melt composition suggested by recently reported parameterizations and theoretical considerations using multiple linear regression we parameterize compiled metal silicate partitioning results including our new data and report revised expressions that predict their metal silicate partitioning behavior as a function of p t x fo2 we apply these results to constrain the conditions that prevailed during core formation in the angrite parent body apb our results suggest the siderophile element depletions in angrite meteorites are consistent with a cv bulk composition and constrain apb core formation to have occurred at mildly reducing conditions of 1 4 0 5 log units below the iron wustite buffer iw corresponding to a apb core mass of 18 11 the core mass range is constrained to 21 8 mass if light elements s and or c are assumed to reside in the apb core incorporation of light elements in the apb core does not yield significantly different redox states for apb core mantle differentiation the inferred redox state is in excellent agreement with independent fo2 estimates recorded by pyroxene and olivine in angrites

high precision mg isotopic systematics of bulk chondrites abstract variations of the mass independent abundance of 26 mg 26 mg and stable mg 25 mg isotope composition of chondrites are important because they constrain the homogeneity of 26 al and mg isotopes in the proto planetary disc and the validity of the short lived 26 al to 26 mg chronometer applied to meteorites we present high precision mg isotope data and al mg ratios of chondrites representing nearly all major chondrite classes including a step leaching experiment on the cm2 chondrite murchison 26 mg variations in leachates of murchison representing acid soluble material are 30 times smaller than reported for neutron rich isotopes of ti and cr and do not reveal resolvable deficits in 26 mg 0 002 to 0 118 very small variations in 26 mg anomalies in bulk chondrites 0 006 to 0 019 correlate with increasing 27 al 24 mg ratios and 50 ti reflecting the variable presence of calcium aluminium rich inclusions cais in some types of carbonaceous chondrites similarly release of radiogenic 26 mg produced by 26 al decay from cai material in the step leaching of murchison best explains the high 26 mg observed in the last aggressive leaching steps of this experiment overall the observed variations in 26 mg are small and potential differences beyond that which result from the presence of cai like material cannot be detected within the analytical uncertainties of this study 0 004 the results do not allow radical heterogeneity of 26 al 30 or measurable mg nucleosynthetic heterogeneity 0 005 to have existed on a planetesimal scale in the proto planetary disc combined with published 26 mg data for cais the bulk chondrite data yield a precise initial 26 al 27 al 0 5 21 0 06 10 5 and 26 mg 0 0340 0 0016 for the solar system however it is not possible with the currently available data to determine with certainty whether cais and the material from which planetesimals accreted including chondrite parent bodies had precisely the same initial levels of 26 al although planetesimals and planets appear to have accreted from material with a mean initial 26 al 27 al 0 in the range of 2 1 to 6 7 10 5 the average stable mg isotope composition of all analysed chondrites with the exception of a chondrule from the cba chondrite gujba 25 mg dsm 3 0 032 0 035 is 25 mg dsm 3 0 152 0 079 2 sd and is indistinguishable from that of the earth s mantle

differentiation history of the mesosiderite parent body constraints from trace elements and manganese chromium isotope systematics in vaca muerta silicate clasts abstract we report here the results of a study of trace element microdistributions and 53mn 53cr systematics in several basaltic and orthopyroxenitic clasts from the vaca muerta mesosiderite ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that following igneous crystallization these clasts underwent extensive metamorphic equilibration that resulted in intra and inter grain redistribution of elements there is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents such as phosphorus derived from the mesosiderite metal at the time of metal silicate mixing furthermore our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal silicate mixing event after taking into account the effects of these postmagmatic secondary processes comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites respectively suggests that the primary igneous petrogenesis including parent magma and source compositions of vaca muerta silicates were similar to those of achondritic meteorites of the howardite eucrite diogenite hed association internal 53mn 53cr isochrons obtained for two basaltic pebble 16 and 4679 and two orthopyroxenitic 4659 and 4670 clasts show that chromium isotopes are equilibrated within each clast nevertheless just as for noncumulate eucrites and diogenites 53cr excesses in whole rock samples of the basaltic clasts 1 01 e in pebble 16 1 07 e in 4679 are significantly higher than in the orthopyroxene rich clasts 0 62 e in 4659 0 53 e in 4670 as in the case of the hed parent body this suggests that mn cr fractionation in the parent body of the vaca muerta silicate clasts occurred very early in the history of the solar system when 53mn was still extant however the slope of the 53mn 53cr isochron defined by the whole rock samples of vaca muerta clasts corresponding to a 53mn 55mn ratio of 3 3 0 6 10 6 is distinctly lower than that defined by the hed whole rock samples corresponding to a 53mn 55mn ratio of 4 7 0 5 10 6 indicating that the global mn cr fractionation event that established mantle source reservoirs on the parent body of the vaca muerta silicate clasts occurred 2 ma after a similar event on the hed parent body

widespread magma oceans on asteroidal bodies in the early solar system our solar system formed about 4 6 billion years ago and within 4 million years small planetary bodies had formed some melting to form volcanic and related rocks two families of meteorites the heds and angrites are thought to have originated from asteroids that melted at this time new oxygen isotope measurements confirm that these meteorites are from two distinct asteroids that underwent large scale melting in the early solar system these new results show that early global scale melting was a feature of all the differentiated planets earth moon and mars and asteroids so far sampled immediately following the formation of the solar system small planetary bodies accreted1 some of which melted to produce igneous rocks2 3 over a longer timescale 15 33 myr the inner planets grew by incorporation of these smaller objects4 5 through collisions processes operating on such asteroids strongly influenced the final composition of these planets4 including earth5 currently there is little agreement about the nature of asteroidal igneous activity proposals range from small scale melting to near total fusion and the formation of deep magma oceans2 here we report a study of oxygen isotopes in two basaltic meteorite suites the heds howardites eucrites and diogenites which are thought to sample the asteroid 4 vesta6 and the angrites from an unidentified asteroidal source our results demonstrate that these meteorite suites formed during early global scale melting 50 per cent events we show that magma oceans were present on all the differentiated solar system bodies so far sampled magma oceans produced compositionally layered planetesimals the modification of such bodies before incorporation into larger objects can explain some anomalous planetary features such as earth s high mg si ratio

u pb and al mg systematics of the ungrouped achondrite northwest africa 7325 abstract northwest africa nwa 7325 is a unique ungrouped gabbroic achondrite which has characteristics consistent with a possible link to the planet mercury in order to understand the origin of this meteorite and the nature of its parent body we have determined its crystallisation age using the long lived u pb and short lived al mg chronometers an internal pb pb isochron defined by six acid leached pyroxene fractions yields an age of 4563 4 2 6 ma assuming that the 238u 235u ratio for nwa 7325 is identical to the bulk earth and solar system value of 137 794 the al mg isotope analyses of seven fractions four plagioclase one pyroxene one olivine and one whole rock define a regression line corresponding to 26al 27al0 3 03 0 14 10 7 and an initial 26mg of 0 093 0 004 when anchored to the d orbigny angrite this initial 26al 27al yields an age of 4563 09 0 26 ma the pb pb age of 4563 4 2 6 ma and al mg age of 4563 09 0 26 ma are in complete agreement but the low u concentrations of nwa 7325 resulted in a relatively low precision pb pb age the observed excess in initial 26mg can be explained by 27al 24mg fractionation and subsequent mg isotopic evolution after planetary differentiation furthermore the parental magma of nwa 7325 most likely formed within 1 72 ma after calcium aluminium rich inclusion cai formation nwa 7325 formed near simultaneously with quenched angrites and a number of ungrouped achondrites at 4563 ma suggesting that a multitude of planetary bodies had formed and differentiated by 4 5 myr after cai formation this ancient age may be interpreted as an argument against nwa 7325 originating from mercury however it does not completely rule it out

a new source of basaltic meteorites inferred from northwest africa 011 eucrites are a class of basaltic meteorites that share common mineralogical isotopic and chemical properties and are thought to have been derived from the same parent body possibly asteroid 4 vesta the texture mineralogy and noble gas data of the recently recovered meteorite northwest africa nwa 011 are similar to those of basaltic eucrites however the oxygen isotopic composition of nwa011 is different from that of other eucrites indicating that nwa011 may be derived from a different parent body the presence of basaltic meteorites with variable oxygen isotopic composition suggests the occurrence of multiple basaltic meteorite parent bodies perhaps similar to 4 vesta in the early solar system

rb sr chronology of volatile depletion in differentiated protoplanets babi ador and all revisited abstract a strong depletion in moderately volatile elements is a characteristic feature of many planetary bodies in the inner solar system and either reflects the rapid accretion of planetesimals from an incompletely condensed solar nebula or is the result of energetic collisions during planetary accretion to better constrain the origin and timescales of this volatile depletion we have precisely measured sr isotope compositions in angrites eucrites and ca al rich inclusions cai angrites have an initial 87sr 86sr ador 0 698978 0 000004 which is indistinguishable from 87sr 86sr babi 0 698970 0 000028 obtained for eucrites in agreement with earlier studies we find that angrites and eucrites have higher initial 87sr 86sr ratios than cai at face value corresponding to model timescales for volatile loss of several millions of years however all the investigated cai are characterized by elevated 84sr 86sr ratios compared to angrites and eucrites which we interpret to reflect an excess of r process sr in the cai if this is correct then the nucleosynthetic sr isotope anomalies in the cai require an upward correction of their measured 87sr 86sr after this correction cai have an initial 87sr 86sr cai 0 698975 0 000008 which is indistinguishable from ador and babi this implies volatile loss from the angrite and eucrite parent bodies within less than 1 ma after formation of cai the volatile depleted nature of these differentiated protoplanets thus most likely reflects their rapid accretion from volatile poor dust in an incompletely condensed solar nebula

strain patterns of crescentic granitoid plutons in the archean greenstone terrain of ontario abstract granitoid plutons of crescentic outcrop pattern in the archean terrain of northwestern ontario are dominated by constrictive strains that are interpreted to be due to intrusion rather than regional deformation the strain patterns of antiformal plutons are characterized by subvertical lineations in contrast the strain patterns of synformat plutons are characterized by subhorizontal lineations presumably only the last flow increments were recorded by the deformed mineral aggregates employed as strain gauges and occurred when the granitoid magma was largely crystalline simple kinematic models are capable of explaining many but not all elements of the natural strain patterns

hf w evidence for rapid differentiation of iron meteorite parent bodies abstract new high precision w isotope data on iron meteorites are presented that provide important constraints on the timing of silicate metal segregation in planetesimals magmatic iron meteorites all have e 182 w within error or less radiogenic than initial e 182 w estimated by studies of chondritic meteorites at face value this implies that iron meteorites are as old and older than refractory calcium aluminium rich inclusions cai which are widely thought to be the oldest solar system objects moreover different meteorites from the same magmatic groups believed to be derived from the same planetissimal core display a range of e 182 w we suggest that the paradoxical e 182 w values more negative than initial solar system initial ssi are most readily explained as a result of secondary spallation reactions with cosmic rays during transit between parent body and the earth this is supported by the most negative e 182 w being found in meteorites with the oldest exposure ages and the magnitude of the effect is shown to be consistent with known nuclear reactions on the other hand it is also striking that none of the magmatic iron group meteorites have e 182 w analyses outside error more radiogenic than the estimated solar system initial ratio this suggests that core formation in parent bodies of magmatic iron meteorites occurred 1 5 myr after the formation age of cai y amelin a n krot i d hutcheon and a a ulyanov lead isotopic ages of chondrules and calcium aluminum inclusions science 297 1678 1683 2002 this extremely early metal silicate differentiation is coeval with the first chondrules m bizzarro j a baker and h haack mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions nature 431 275 278 2004 a n krot y amelin p cassen and a meibom young chondrules in cb chondrites from a giant impact in the early solar system nature 436 989 992 2005 formation of later chondrules and hence the parent bodies of some chondritic meteorites must therefore have occurred in the presence of planetesimals large enough to possess iron cores we conclude that early planetary accretion and differentiation was sufficiently fast for 26 al decay to be an important heat source non magmatic iron meteorites however display more radiogenic and varied w isotope signatures this is in keeping with them being generated later by impact melting during which the metal partially re equilibrated with the then more radiogenic silicate fraction

strain in an archean greenstone belt of minnesota abstract we measured strain at more than 60 locations in metasedimentary and metavolcanic rocks of the vermilion district an e w trending archean greenstone belt in minnesota strain ellipsoid orientations and shapes correlate strongly with n s location in the belt but magnitudes do not flattening strains occur near the present vermilion fault which bounds the greenstone belt to the north with constrictional strains to the south the observed strain patterns can be mathematically modeled by deformation paths which produce the flattening strains with west plunging 1 axes by dextral shear of the constrictional strains with east plunging 1 axes using reasonable geologic constraints the shear plane must dip to the north with a subhorizontal shear direction structures throughout the district also indicate dextral shear a geometrical finite element program uses the measured strains to destrain the rocks and find the configuration which most closely satisfies strain compatibility equations the linear e w strain patterns and minor rotations about horizontal axes during the deformation preclude origin of the greenstone belt by infolding and shear off the flanks of a rising granitic diapir by accounting for rotations which result in the deformed curvature of the original surface a true estimate of 50 n s shortening across the belt can be made the data and deformation models favor the origin of the vermilion district rocks at a convergent margin most likely as a n dipping subduction zone complex with shallow slab dip the origin of the constrictional strains remains enigmatic

magnesium isotopic composition of achondrites abstract magnesium isotopic compositions of 22 well characterized differentiated meteorites including 7 types of achondrites and pallasite meteorites were measured to estimate the average mg isotopic composition of their parent bodies and evaluate mg isotopic heterogeneity of the solar system the 26mg values are 0 236 and 0 190 for acapulcoite lodranite and angrite meteorites respectively and vary from 0 267 to 0 222 in the winonaite iab iron silicate group 0 369 to 0 292 in aubrites 0 269 to 0 158 in heds 0 299 to 0 209 in ureilites 0 307 to 0 237 in mesosiderites and 0 303 to 0 238 in pallasites magnesium isotopic compositions of most achondrites and pallasite meteorites analyzed here are similar and reveal no significant isotopic fractionation however mg isotopic compositions of d orbigny angrite and some heds are slightly heavier than chondrites and the other achondrites studied here the slightly heavier mg isotopic compositions of angrites and some heds most likely resulted from either impact induced evaporation or higher abundance of clinopyroxene with the mg isotopic composition slightly heavier than olivine and orthopyroxene the average mg isotopic composition of achondrites 26mg 0 246 0 082 2sd n 22 estimated here is indistinguishable from those of the earth 26mg 0 25 0 07 2sd n 139 chondrites 26mg 0 28 0 06 2sd n 38 and the moon 26mg 0 26 0 16 2sd n 47 reported from the same laboratory the chondritic mg isotopic composition of achondrites the moon and the earth further reflects homogeneity of mg isotopes in the solar system and the lack of mg isotope fractionation during the planetary accretion process and impact events

distinct 238 u 235 u ratios and ree patterns in plutonic and volcanic angrites geochronologic implications and evidence for u isotope fractionation during magmatic processes angrites are differentiated meteorites that formed between 4 and 11 myr after solar systemformation when several short lived nuclides e g 26 al 26 mg 53 mn 53 cr 182 hf 182 w were still alive as such angrites are prime anchors to tie the relative chronology inferred from these short lived radionuclides to the absolute pb pb clock the discovery of variable u isotopic composition at the sub permil level calls for a revision of pb pb ages calculated using an assumed constant 238 u 235 u ratio i e pb pb ages published before 2009 2010 in this paper we report high precision u isotope measurement for six angrite samples nwa 4590 nwa 4801 nwa 6291 angra dos reis d orbigny and sahara 99555 using multi collector inductively coupled plasma mass spectrometry and the irmm 3636 u double spike the age corrections range from 0 17 to 1 20 myr depending on the samples after correction concordance between the revised pb pb and hf w and mn cr ages of plutonic and quenched angrites is good and the initial 53 mn 55 mn 0 ratio in the early solar system ess is recalculated as being 7 1 10 6 at the formation of the solar system the error bar incorporates uncertainty in the absolute age of calcium aluminum rich inclusions cais an uncertainty remains as to whether the al mg and pb pb systems agree in large part due to uncertainties in the pb pb age of cais a systematic difference is found in the u isotopic compositions of quenched and plutonic angrites of 0 17 a difference is also found between the rare earth element ree patterns of these two angrite subgroups the 238 u values are consistent with fractionationduring magmatic evolution of the angrite parent melt stable u isotope fractionation due to a change in the coordination environment of u during incorporation into pyroxene could be responsible for such a fractionation in this context pb pb ages derived from pyroxenes fraction should be corrected using the u isotope composition measured in the same pyroxene fraction

do principal surfaces of stress and strain always exist variations of stress and strain are commonly expressed by patterns of stress or strain trajectories three mutually orthogonal families of continuous lines parallel to maximum intermediate and minimum stress or strain axes it might be assumed that there are equivalent continuous principal surfaces of stress or strain for any state of continuously varying stress or strain we demonstrate that this will not generally be the case for three dimensionally varying states of stress or strain whether or not principal surfaces of stress or strain exist is governed by the abnormality of the vector field of principal trajectories we consider the z vector fields for examples of many types of three dimensional heterogeneous deformation and show that most of these do not lead to definable principal xy strain surfaces an alternative geometric test is presented termed the continuity loop for simply demonstrating the existence or not of principal surfaces using geometrical and orientational information it is important to the understanding of geological structures to know which kinds of heterogeneous deformation give rise to principal surfaces of stress or strain we conclude with examples of structures which might be indicative of the absence of continuous principal surfaces of stress segmented faults echelon veins and dykes a discussion of the implication for strain fabrics and foliations and a warning that foliation trace trajectories on maps or sections may not necessarily indicate the existence of real foliation surfaces in three dimensions

is vesta an intact and pristine protoplanet abstract it is difficult to find a vesta model of iron core pyroxene and olivine rich mantle and hed crust that can match the joint constraints of a vesta s density and core size as reported by the dawn spacecraft team b the chemical trends of the hed meteorites including the depletion of sodium the feo abundance and the trace element enrichments and c the absence of exposed mantle material on vesta s surface among vestoid asteroids or in our collection of basaltic meteorites these conclusions are based entirely on mass balance and density arguments independent of any particular formation scenario for the hed meteorites themselves we suggest that vesta either formed from source material with non chondritic composition or underwent after its formation a radical physical alteration possibly caused by collisional processes that affected its global composition and interior structure

influence of pre or post diapiric deformation on the foliation and strain pattern of a diapiric ridge abstract foliation and total strain patterns of diapiric structures are very sensitive to pre or post diapiric deformation this is illustrated quantitatively by superimposing homogeneous pure and simple shear on mareschal and west s 1977 finite element model of an antiformal diapiric ridge in models with post diapiric homogeneous deformation type 1 the foliation patterns typical for diapirism are obliterated in models with pre diapiric homogeneous deformation type 2 diapiric deformation was too weak to overprint pre diapiric foliation patterns however total strain patterns can be used successfully for identifying a diapiric component in the deformation history of both type 1 and type 2 models provided post or pre diapiric deformation is not too high

recognition of pervasive prestrain in the total strain pattern of large folds abstract two methods are introduced for identifying states of pervasive prefold strain in the total strain field of large domes or basins the first method relies on the symmetry rule and is restricted to structures in which the planes of effective symmetry of the total strain field are non coincident with those of the fold form the second method seeks to demonstrate the existence of a strain induced anisotropy foliation or lineation prior to upright folding and utilizes narrow decollement zones within homogeneous rock units of domes or basins the methods are applied to large oval structures in two precambrian gneiss terranes of ontario

reassessing the origin and chronology of the unique achondrite asuka 881394 implications for distribution of 26al in the early solar system abstract the achondrite asuka 881394 has mineralogy broadly similar to that of eucrites but is isotopically chemically and texturally distinct from them previous u pb chronology shows that it is very old forming within the first 0 8 ma of the formation of calcium aluminum rich inclusions cais however the age difference between asuka 881394 and other very old solar system materials cais quenched angrites measured with the 26al 26mg and 53mn 53cr extinct radionuclide chronometers and 207pb 206pb chronometer is not the same this could be interpreted to reflect heterogeneity in the distribution of 26al and 53mn in the early solar system given the significant implications for the early solar system chronology if 26al and 53mn are indeed heterogeneously distributed in this study we further investigate the origin of asuka 881394 and the apparent age discordance between short lived and absolute chronometers by focusing on measurement of its e54cr composition renewed measurements of the absolute pb pb age and new high precision measurements of its 26al 26mg systematics new cr isotope data places additional constraints on the origin of asuka 881394 its e 54cr value of 0 37 0 10e is resolvable outside of uncertainty from hed meteorites 0 72 0 10e reinforcing evidence from oxygen isotopic analyses that suggest it originated from a distinct parent body unlikely to be 4 vesta new pb pb analyses combined with using a directly measured 238u 235u ratio for age calculation result in a recalculated pb pb age of 4564 95 0 53 ma 1 5 ma younger than the previously reported age with this age asuka 881394 remains one of the oldest known achondrites in our solar system new high precision 26al 26mg data produce an initial 26al 27al ratio of 1 48 0 12 10 6 within error of previous data this ratio corresponds to an al mg age of 4563 69 0 36 ma or 4564 83 0 21 ma relative to cais and the d orbigny angrite respectively thus depending on which age anchor is used the 26al 26mg age is either 1 26 ma or 0 12 ma younger than the new pb pb age the latter being unresolved within analytical uncertainty though a potential age discrepancy between 26al 26mg and pb pb could be a result of heterogeneous distribution of 26al we demonstrate with our new high precision mg isotope data in conjunction with petrographic evidence that the mg isotope system has been disturbed in asuka 881394 we suggest that the 26al 26mg system closed to diffusion after the u pb system either due to slow cooling on the parent body or low grade metamorphic re equilibration of mg thus we can satisfactorily explain the observed age discrepancy between 26al 26mg and u pb systems in asuka 881394 without invoking heterogeneous distribution of 26al 27al ratio in the early solar system comparison of the asuka 881394 data with that of other anomalous achondrites from distinct parent bodies suggests that these could also have evolved from a source region with a canonical 26al 27al ratio because these achondrites have significant differences in their e54cr 17o systematics which could be indicative of location within the early protoplanetary disk it is consistent with homogeneous distribution of 26al in the early solar system angrites remain an outlier either because they evolved from a source with a lower 26al 27al ratio or because their 26al 26mg or pb pb data are problematic in either case this suggests that basaltic angrites are questionable as the age anchor for the entire solar system as a whole and other very old well preserved achondrites should be considered for that role

the u pb systematics of angrite sahara 99555 abstract angrite sahara 99555 hereafter sah precisely dated by baker et al baker j bizzarro m wittig n connelly j and haack h 2005 early planetesimal melting from an age of 4 5662 gyr for differentiated meteorites nature 436 1127 1131 has been proposed as a new reference point for the early solar system timescale and for calculation of the revised minimum age of our solar system the pb pb age of sah of 4566 18 0 14 ma reported by baker et al differs from the pb pb age of d orbigny another basaltic angrite of 4564 42 0 12 ma amelin y 2008 u pb ages of angrites geochim cosmochim acta 72 221 232 despite the fact that the relative 53mn 53cr and 182hf 182w ages of these meteorites are identical here i report u pb data for 21 whole rock and pyroxene fractions from sah analyzed using the same approach as d orbigny amelin 2008 these fractions contain between 1 3 and 8 9 pg of total common pb slightly more than analytical blank measured 206pb 204pb ratios are between 625 and 2817 for d orbigny blank corrected 206pb 204pb ratios are between 1173 and 6675 eight acid washed whole rock fractions yielded an isochron age of 4564 86 0 38 ma mswd 1 5 data for pyroxene fractions plot mostly above the whole rock isochron and do not form a linear array in 207pb 206pb vs 204pb 206pb isochron coordinates the 207pb 206pb model dates of the pyroxene fractions vary from 4563 8 0 3 to 4567 1 0 5 ma the difference between whole rock and pyroxene u pb systematics may be a result of re distribution of radiogenic pb at a mineral grain scale several million years after crystallization complexities of sm nd lu hf and possibly 26al 26mg mineral systematics of sah described previously may be related to the same process that caused the re distribution of radiogenic pb disturbance of isotopic chronometers renders sah an imperfect anchor for the early solar system timescale the problems with age determination revealed by the studies of sah call for greater attention in pb isotopic dating of angrites and other achondrites

53 mn 53 cr radiometric dating of secondary carbonates in cr chondrites timescales for parent body aqueous alteration abstract we present 53mn 53cr ages of secondary carbonates in renazzo like cr chondrites determined by secondary ion mass spectrometry the timing of aqueous alteration in cr chondrites has been unconstrained in the literature we measured 53mn 53cr isotope systematics in carbonates from three different cr chondrite lithologies calcite in the interchondrule matrix of renazzo calcite in the matrix of gro 95577 and dolomite in a dark inclusion of renazzo all show excesses in 53cr interpreted as the daughter product from the decay of 53mn the renazzo calcite yields an initial ratio of 53mn 55mn 0 3 6 2 7 10 6 and the renazzo dark inclusion dolomite ranges from 53mn 55mn 0 3 1 1 4 10 6 corrected to the rsf of a calcite standard to 3 7 1 7 10 6 corrected to an inferred dolomite rsf when anchored to the d orbigny angrite the renazzo carbonates yield ages between 4563 6 and 4562 6 ma or 4 3 5 3 myr after the formation of cv cais calcite measured in the heavily altered specimen gro 95577 yields a shallower slope of 53mn 55mn 0 7 9 2 8 10 7 corresponding to a much younger age of 4555 4 ma or 12 6 myr after cai formation the two renazzo ages are contemporaneous with mn cr ages of carbonates in tagish lake ci and cm chondrites but the gro 95577 age is uniquely young these findings suggest that early aqueous alteration on chondritic parent bodies was a common occurrence likely driven by internal heating from 26al decay after accretion the young carbonate ages of gro 95577 suggest that either the cr parent body was sufficiently large to sustain heating from 26al for 8 myr or that late stage impact events supplied heat to the region where gro 95577 originated

differentiation and magmatic activity in vesta evidenced by 26al 26mg dating in eucrites and diogenites abstract the 26 al 26 mg short lived chronometer has been widely used for dating ancient objects in studying the early solar system here we use this chronometer to investigate and refine the geological history of the asteroid 4 vesta ten meteorites widely believed to come from vesta 4 basaltic eucrites 3 cumulate eucrites and 3 diogenites and the unique achondrite asuka 881394 were selected for this study all samples were analyzed for their 26 mg and 27 al 24 mg ratios in order to construct both whole rock and model whole rock isochrons mineral separation was performed on 8 of the hed s in order to obtain internal isochrons while whole rock al mg analyses of hed s plot on a regression that could be interpreted as a vestan planetary isochron internal mineral isochrons indicate a more complex history crystallization ages obtained from internal 26 al 26 mg systematic in basaltic eucrites show that vesta s upper crust was formed during a short period of magmatic activity at 2 66 0 58 1 39 million years ma after calcium aluminum inclusions after cai we also suggest that impact metamorphism and subsequent age resetting could have taken place at the surface of vesta while 26 al was still extant cumulate eucrites crystallized progressively from 5 48 0 60 1 56 to 7 25 ma after cai model ages obtained for both basaltic and cumulate eucrites are similar and suggest that the timing of differentiation of a common eucrite source from a chondritic body can be modeled at 2 88 0 12 0 14 ma after cai i e contemporaneously from the onset of the basaltic eucritic crust based on their cumulate texture we suggest cumulate eucrites were likely formed deeper in the crust of vesta diogenites have a more complicated history and their 26 al 26 mg systematics show that they likely formed after the complete decay of 26 al and thus are younger than eucrites this refined chronology for eucrites and diogenites is consistent with a short magma ocean stage on 4 vesta from which the basaltic eucrites rapidly crystallized in order to explain the younger age and the complex history of diogenites we postulate that a second episode of magmatism was possibly triggered by a mantle overturn we bring a refined chronology of the geological history of vesta that shows that the asteroid has known a more complex differentiation than previously thought

sea ice thickness measurement using a small airborne electromagnetic sounding system the evaluation of a small electromagnetic induction sounding system for use in airborne measurement of sea ice thickness is discussed as are the results from arctic field testing also outlined are the system noise and drift problems encountered during arctic field evaluation problems which adversely affected the quality of the sounding data the sea ice sounding results indicate that for ice floes with moderate relief it should be possible to determine thickness to within 5 percent but that because of sounding footprint size and current model algorithm constraints steep sided pressure ridge keels cannot be well defined the findings also indicate that with further system improvement the day of routine sea ice thickness profiling from an airborne platform is close at hand

excess hafnium 176 in meteorites and the early earth zircon record 1 the long lived176lu to 176hf decay system is a powerful tool to understand ancient chemical fractionation events associated with planetary differentiation detrital hadean zircons 3 8 gyr from the jack hills metasedimentary belt of western australia record extremely enriched hf isotope signals suggesting early extraction of a continental crust 4 5 gyr but fail to identify a prevalent complementary depleted mantle reservoir suggesting that crust formation processes in the early earth were fundamentally distinct from today however this conclusion assumes that the hf isotope composition of bulk chondrite meteorites can be used to estimate the composition of earth prior to its differentiation into major silicate reservoirs namely the bulk silicate earth bse we report a176lu 176hf internal mineral isochron age of 4869 34 myr for the pristine sah99555 angrite meteorite this age is 300 myr older than the age of the solar system confirming the existence of an energetic process yielding excess 176hf in affected early formed solar system objects through the production of the 176lu isomer t1 2 3 9 hours thus chondrite meteorites contain excess 176hf and their present day composition cannot be used to infer the lu hf parameters of bse using a revised bse estimate based on the sah99555 isochron we show that earth s oldest zircons preserve a record of coexisting enriched and depleted hafnium reservoirs as early as 4 3 gyr in earth s history with little evidence for the existence of continental crust prior to 4 4 gyr this new view suggests continuous juvenile crustal growth and recycling throughout the hadean and archean eras perhaps analogous to modern plate tectonics

kinematic modeling of cross sectional deformation sequences by computer simulation we derive a kinematic algorithm which permits simulation of postulated cross sectional deformation sequences in sedimentary rocks affected by faulting fault related folding and simple shear the transformations are to a more deformed state and are formulated analytically in terms of the less deformed configuration forward modeling lagrangian description the medium is subdivided into domains of constant dip and homogeneous displacement vector fields which are delimited by the axial planes of the fault inflections the displacement trajectory is of constant length for all the displaced particles throughout the medium and parallel to the underlying active fault segment as a consequence the deformation path is continuous but not smooth and causes an angular style of parallel folding except at the deformation front where the layer thickness is not conserved the inhomogeneity of the displacement vector field across axial planes introduces longitudinal and angular shear strains whereas the area of the medium remains constant these strains which are superposed on the externally applied simple shear make the mapping transformation unconformal we define the stratigraphic layering of the undeformed medium by an approximating body of finite quadrilateral elements evaluate the defined displacement functions for the grid nodes and graph the deformed mesh eventually we create with the algorithm synthetic deformation sequences for simple boundary conditions and try in a specific case study to match the resulting synthetics with the available observational and experimental data

magnesium isotopic fractionation in chondrules from the murchison and murray cm2 carbonaceous chondrites we present high precision measurements of the mg isotopic compositions of a suite of types i and ii chondrules separated from the murchison and murray cm2 carbonaceous chondrites these chondrules are olivine and pyroxene rich and have low 27 al 24 mg ratios 0 012 0 316 the mg isotopic compositions of murray chondrules are on average lighter d 26 mg ranging from 0 95 to 0 15 relative to the dsm 3 standard than those of murchison d 26 mg ranging from 1 27 to 0 77 taken together the cm2 chondrules exhibit a narrower range of mg isotopic compositions than those from cv and cb chondrites studied previously the least altered cm2 chondrules are on average lighter average d 26 mg 0 39 0 30 2se than the moderately to heavily altered cm2 chondrules average d 26 mg 0 11 0 21 2se the compositions of cm2 chondrules are consistent with isotopic fractionation toward heavy mg being associated with the formation of secondary silicate phases on the cm2 parent body but were also probably affected by volatilization and recondensation processes involved in their original formation the low al cm2 chondrules analyzed here do not exhibit any mass independent variations in 26 mg from the decay of 26 al with the exception of two chondrules that show only small variations just outside of the analytical error in the case of the chondrule with the highest al mg ratio a type iab chondrule from murchison the lack of resolvable 26 mg excess suggests that it either formed 1 ma after calcium aluminum rich inclusions or that its al mg isotope systematics were reset by secondary alteration processes on the cm2 chondrite parent body after the decay of 26 al

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

sims pb pb and u pb age determination of eucrite zircons at 5 μm scale and the first 50 ma of the thermal history of vesta eucrites a subgroup of basaltic achondrites represent the remnants of the earliest magmatic stage of the solar system formation possibly on the asteroid 4 vesta the precise determination of their crystallization age is therefore important for constraining the evolutionary history of the asteroid including melting and differentiation processes of the parent body zircons would be the best mineral for such purpose however extra terrestrial zircons are rare and those found in eucrites are typically small 5 m using a cameca ims 1280 high resolution ion microprobe precise uranium lead u pb and pb pb ages of the non cumulate basaltic eucrite zircons in bereba cachari caldera camel donga and juvinas were determined successfully at scale of 5 m we obtained a weighted average 207pb 206pb age of 4541 11 ma and u pb concordia age of 4525 24 ma for the five eucrite samples studied a synthesis of available age data of howardites eucrites and diogenites indicate a protracted magmatic history on vesta with a most pronounced core mantle differentiation event marked at 4564 2 ma followed by a peak of basaltic magmatism at 4552 7 ma which gradually diminished over the next 50 ma thermal modeling of vesta suggests that partial melt fraction of greater than 10 with temperature well exceeding 1500 k could be maintained over this period throughout its interior this is consistent with the chronological records of eucrites suggesting persistent magmatism on vesta during the first 50 ma in the early solar system

interplay between folding and ductile shearing in the proterozoic crust of the muskoka parry sound region central ontario grenville gneiss of the central georgian bay region was subjected to ductile deformation that produced narrow mylonite zones as well as three sets of superimposed folds differing greatly in structural signature size and orientation some mylonite zones are concordant to gneissosity and are repeatedly folded others cut gneissosity and postdate the folding gneissosity was generated as a regionally subhorizontal feature either by crustal thinning or like the early mylonite zones by low angle reverse shearing an attempt is made to account for the initially subhorizontal gneissosity the mylonite zones and the folds in a regime of large scale reverse shearing that strikes parallel to the grenville front upright northwest southwest to north south buckle folds dominate the map pattern and are subperpendicular to the reverse grenville front boundary fault these set 2 folds cannot be attributed to reverse simple shearing but require a large component of east west compression such stress could have been

petrology and geochemistry of d orbigny geochemistry of sahara 99555 and the origin of angrites petrology and geochemistry of d orbigny geochemistry of sahara 99555 and the origin of angrites

reconciliation of the excess 176 hf conundrum in meteorites recent disturbances of the lu hf and sm nd isotope systematics abstract the long lived 176 lu 176 hf and 147 sm 143 nd radioisotope systems are commonly used chronometers but when applied to meteorites they can reveal disturbances specifically lu hf isochrons commonly yield dates up to 300 myr older than the solar system and varying initial 176 hf 177 hf values we investigated this problem by attempting to construct mineral and whole rock isochrons for eucrites and angrites meteorites from different parent bodies exhibit similar disturbance features suggesting that a common process is responsible minerals scatter away from isochron regressions for both meteorite classes with low hf phases such as plagioclase and olivine typically being most displaced above or left of reference isochrons relatively hf rich pyroxene is less disturbed but still to the point of steepening lu hf errorchrons using our lu hf and sm nd data we tested various hf and lu redistribution scenarios and found that decoupling of lu hf from 176 hf 177 hf must postdate the accumulation of significant radiogenic 176 hf therefore early irradiation or diffusion cannot explain the excess 176 hf instead disturbed meteorite isochrons are more likely caused by terrestrial weathering contamination or common laboratory procedures the partial dissolution of phosphate minerals may predominantly remove rare earth elements including lu leaving relatively immobile and radiogenic hf behind robust lu hf and improved sm nd meteorite geochronology will require the development of chemical or physical methods for removing unsupported radiogenic hf and silicate hosted terrestrial contaminants without disturbing parent daughter ratios

u pb ages of angrites abstract precise u pb ages determined with double spike 202 pb 205 pb thermal ionization m1ass spectrometry are reported for angrites angra dos reis ador lewis cliff 86010 lew and d orbigny nineteen of 23 acid washed pyroxene fractions from these meteorites and whole rock fractions from d orbigny contain between 0 5 and 1 3 pg of total common pb indistinguishable from analytical blank measured 206 pb 204 pb ratios in these fractions are between 6300 and 14 100 for ador 1160 4500 for lew and 608 8500 for d orbigny blank corrected 206 pb 204 pb ratios for all three meteorites vary from 2160 to over 100 000 these fractions yielded precise and reproducible 207 pb 206 pb dates with the average values of 4557 65 0 13 ma for ador 4558 55 0 15 ma for lew and 4564 42 0 12 ma for d orbigny pb pb isochrons including data with slightly elevated common pb and u pb upper concordia intercepts yield similar dates the implications of these new pb isotopic ages of angrites are threefold first they demonstrate that ador and lew are not coeval and the group of slowly cooled angrites is therefore genetically diverse second the new age of lew suggests an upward revision of 53 mn 53 cr absolute ages by 0 7 ma third a precise age of d orbigny allows consistent linking of the 53 mn 53 cr and 26 al 26 mg extinct nuclide chronometers to the absolute lime scale

estimating the thickness of sea ice sea ice freeboard thickness and snow depth were measured from a series of closely spaced 5 to 10 m drill hole sites from five free floating multiyear ice floes in the beaufort sea during the spring of 1986 and 1987 a regression of ice thickness on ice draft was performed on the data from each floe and for the combined data set regressions were performed both on nonscreened data and data screened to eliminate pressure ridges a predictive equation was developed permitting the total ice thickness to be estimated from measured values of ice draft e g from submarine echo sounder traces t 1 115 d the keel sail height ratio for pressure ridges was found constant at 3 3 to 1 in agreement with past observations

mg diffusion in anorthite implications for the formation of early solar system planetesimals we have measured the self diffusion coefficients of mg ca and sr in anorthitic plagioclase in order to assess the potential of mg isotopic heterogeneities in early solar system planetesimals to survive thermal metamorphism diffusion couples were constructed from polished single crystals of natural anorthite and synthetic isotopically enriched anorthite glass couples were annealed at atmospheric pressure and 1200 1400 c and isotopic concentration profiles were measured with an ion microprobe the results show that mg diffusion in anorthite is surprisingly fast with d mg being over 2 orders of magnitude greater than d sr this indicates that the diffusion coefficient of mg in anorthite cannot be approximated with that for sr mg diffusion in the c direction is also slightly faster than in the b direction while ca and sr diffusion appear to be isotropic the results provide important constraints on the thermochronological history of anorthite bearing mineral assemblages that preserve radiogenic 26 mg excesses in a planetesimal heated by the decay of 26 al the temperature at any point depends on the planetesimal size time of formation thermal conductivity and depth within the planetesimal given sufficient heating 26 mg heterogeneities produced by the in situ decay of 26 al in ca al rich inclusions cais and chondrules will be erased by diffusive equilibration using the self diffusion coefficient for mg in anorthite measured in this study we show that the common occurrence of 26 mg excesses in these inclusions requires that they must be stored in small 15 km bodies or the outermost rims of larger bodies for the first 1 2 million years of the solar system s history for early formed bodies larger than 15 km most of the mass will have been heated sufficiently for any radiogenic 26 mg to have been diffusively homogenized in the mg rich planetary environment

new constraints on early solar system chronology from al mg and u pb isotope systematics in the unique basaltic achondrite northwest africa 2976 abstract we report elemental abundances and the isotopic systematics of the short lived 26 al 26 mg half life of 0 73 ma and long lived u pb radiochronometers in the ungrouped basaltic meteorite northwest africa nwa 2976 the bulk geochemical composition of nwa 2976 is clearly distinct from that of the eucrites and angrites but shows broad similarities to that of the paired nwa 001 and 2400 ungrouped achondrites indicating that it is likely to also be paired with these two samples the major and trace element abundances in nwa 2976 further indicate that it formed by extensive melting and magmatic fractionation processes on its parent body the al mg and pb pb isotope systematics indicate that this meteorite represents the earliest stages of crust formation on a differentiated parent body in the early solar system the absolute pb pb internal isochron age of nwa 2976 obtained from acid leaching residues of three whole rock samples and two pyroxene separates is 4562 89 0 59 ma mswd 0 02 this pb pb age is calculated using the measured 238 u 235 u ratio of a nwa 2976 whole rock of 137 751 0 018 2 which was determined relative to the recently revised value of 137 840 0 008 for the srm 950a u isotope standard the al mg systematics reveal the presence of 26 mg isotopic anomalies produced by the decay of 26 al with an 26 al 27 al 0 of 3 94 0 16 10 7 and indicate a time of formation of 0 26 0 18 ma after the d orbigny angrite using the revised pb pb age of 4563 36 0 34 ma for the d orbigny anchor corrected for its u isotopic composition we deduce an al mg model age of 4563 10 0 38 ma for nwa 2976 which is consistent with its pb pb internal isochron age the concordance of the pb pb and al mg chronometers when taking into account the differences in the u isotopic compositions of the d orbigny and nwa 2976 achondrites whose parent bodies likely formed in distinct regions of early solar system as indicated by their different oxygen isotopic compositions implies that 26 al was homogeneously distributed in the early solar system it also suggests that igneous processes on planetesimals as represented by the formation of various basaltic meteorite groups that likely originated on distinct parent bodies e g eucrites and angrites as well as ungrouped achondrites were widespread throughout the protoplanetary disk within the first 5 ma of the history of the solar system