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

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

the age of the solar system redefined by the oldest pb pb age of a meteoritic inclusion the age of the solar system is defined by the formation of the first solid grains in the solar nebula pb pb age dating of these solids which were later trapped in a meteorite indicates that the solar system is 0 34 1 91 million years older than previously thought the age of the solar system can be defined as the time of formation of the first solid grains in the nebular disc surrounding the proto sun this age is estimated by dating calcium aluminium rich inclusions in meteorites these inclusions are considered as the earliest formed solids in the solar nebula their formation marks the beginning for several long and short lived radiogenic clocks that are used to precisely define the timescales of solar system events such as the formation and evolution of planetary bodies1 2 3 here we present the 207pb 206pb isotope systematics in a calcium aluminium rich inclusion from the northwest africa 2364 cv3 group chondritic meteorite which indicate that the inclusion formed 4 568 2 million years ago this age is between 0 3 refs 4 5 and 1 9 refs 1 6 million years older than previous estimates and is the oldest age obtained for any solar system object so far we also determined the 26al 26mg model age of this inclusion and find that it is identical to its absolute pb pb age implying that the short lived radionuclide 26al was homogeneously distributed in the nebular disc surrounding the proto sun from the consistently old ages in the studied inclusion we conclude that the proto sun and the nebular disc formed earlier than previously thought

prestack depth migration of dense wide angle seismic data abstract prestack depth migration of wide angle seismic data represents an extension of traditional imaging with near vertical incidence data because it includes a larger component of the recorded wave field to date however studies that have employed wide angle migration have suffered because only widely spaced data were available and because only very simple synthetic tests were performed although wide angle migration has the potential to increase our ability to image deep crustal structures particularly when closely spaced data are collected a thorough study of this technique has been lacking to address this we present a case study of prestack depth migration of relatively dense synthetic wide angle marine data the objectives are to identify potential benefits and limitations of this approach and answer such fundamental questions as how close the receiver spacing must be for a typical survey to image effectively with wide angle data this will facilitate the design of better seismic experiments our study employs kirchhoff prestack depth migration of variably spaced full wave field synthetic wide angle ocean bottom hydrophone obh data generated using a realistic velocity model based on the passive eastern margin of the united states we show how an increase in obh density improves the migration by increasing the lateral resolution and signal to noise ratio we also investigate the contribution of various offset ranges to the migrated image and show how the wider angle components contribute primarily to the deepest parts of the image with relatively low spatial frequency compared to the near vertical incidence components to investigate how errors in the velocity model affect imaging as a function of offset range we migrate the data using a velocity model derived from refraction and reflection traveltime inversion this example demonstrates the need to obtain an increasingly accurate model as increasingly wider angle data are migrated to effectively image structures in our 200 40 km synthetic velocity model an obh spacing of approximately 2 km is required

diffusion kinetics of lutetium in diopside and the effect of thermal metamorphism on lu hf systematics in clinopyroxene abstract we report new experimental data for lu diffusion in diopside from 1150 to 1250 c at 1 bar pressure and oxygen fugacity corresponding to the iron wustite buffer the primary experimental dataset defines the following arrhenius relation for lu diffusion parallel to the b crystallographic axis d lu b 1 37 1 2 10 13 exp 208 7 15 2 kj mol 1 rt m 2 s where t is in kelvin we find that lu diffusion in diopside is significantly faster than that of hf which could lead to spurious lu hf ages via decoupling of 176lu from 176hf if the system was subjected to protracted periods of high temperature the extent to which lu hf ages are compromised during initial cooling depends on the peak temperature grain size cooling rate and the nature of lu partitioning between clinopyroxene and its surrounding phase s however in geologically realistic scenarios the effects of diffusion on lu hf ages during initial cooling do not produce lu hf ages that differ from the age corresponding to the closure temperature of hf by more than a few million years lu hf systematics in clinopyroxene are far more susceptible to diffusion during reheating events that occur after radiogenic 176hf has already accumulated in the host phase for a given thermal pulse the lu hf age varies as a function of grain size temperature the timing and duration of reheating and the partitioning of lu between clinopyroxene and its surrounding phase s the data collected and numerical model developed in this study have been used to simulate the effect of thermal metamorphism on the internal lu hf systematics of the eucrites millbillillie and piplia kalan although our model results do not offer a comprehensive solution to the apparent pre solar lu hf ages observed in many ancient meteorites we argue that the 4 9 ga pyroxene whole rock lu hf ages of these specific samples are at least partially if not primarily due to diffusive loss of lu from the constituent minerals to a partial melt phase during high temperature metamorphism

petrogenesis of angrites abstract the recent discovery of two new angrites sahara 99555 and d orbigny has revived interest in this small group of achondrites we measured trace element abundances in the individual minerals of these two angrites and compared them with the three antarctic angrites lew 86010 lew 87051 and asuka 881371 trace element variations in four of these meteorites lew 87051 asuka 881371 sahara 99555 and d orbigny indicate rapid crystallization under near closed system conditions consistent with their mineralogical and textural features all four appear to be closely related and crystallized from very similar magmas discrepancies between their bulk ree compositions and melts calculated to be in equilibrium with the major phases may be due in part to kinetic effects of rapid crystallization prior crystallization of olivine and or plagioclase may also account for the elevated parent melt composition of clinopyroxene in some of the angrites such as asuka 881371 lew 86010 also crystallized from a melt and represents a liquid composition but trace element trends in clinopyroxene and olivine differ from those of the other angrites this meteorite seems to have crystallized from a different source magma

deep structure of the u s atlantic continental margin offshore south carolina from coincident ocean bottom and multichannel seismic data we present the results of a combined multichannel seismic reflection mcs and wide angle ocean bottom seismic profile collected in 1988 across the carolina trough on the u s atlantic continental margin inversion of vertical incidence and wide angle travel time data has produced a velocity model of the entire crust across the continent ocean transition the margin consists of three structural elements 1 rifted continental crust comprising 1 4 km of post rift sedimentary rocks overlying a 30 34 km thick subsedimentary crust 2 transitional crust a 70 to 80 km wide zone comprising up to 12 km of postrift sedimentary rocks overlying a 10 to 24 km thick subsedimentary crust and 3 oceanic crust comprising 8 km of sedimentary rocks overlying an 8 km thick crystalline crust the boundary between rifted continental and transitional crust marked by the brunswick magnetic anomaly represents an abrupt change in physical properties with strong lateral increases in seismic velocity density and magnetic susceptibility the transitional crust contains mid crustal seaward dipping reflections observed on the mcs section and has seismic velocities of 6 5 6 9 km s in the midcrust and 7 2 7 5 km s in the lower crust modeling of potential field data shows that transitional crust also produces the prominent margin parallel gravity anomaly and the brunswick and east coast magnetic anomalies these observations support the interpretation that the transitional crust was formed by magmatism during continental breakup the prodigious thickness up to 24 km of igneous material rivals that interpreted on continental margins of the north atlantic e g hatton bank and voring plateau which formed in the vicinity of the iceland hotspot these observations when combined with other transects across the margin confirm previous suggestions that the u s atlantic margin is strongly volcanic and further imply that the magmatism was not the result of a long lived mantle plume

large igneous province on the us atlantic margin and implications for magmatism during continental breakup rifted continental margins commonly include sections of igneous rock more than twice as thick as normal oceanic crust explanations for this voluminous magmatic accretion during rifting include plume models1 3 which require a deep seated thermal or chemical anomaly in upwelling mantle and non plume models4 7 which call on broad shallow thermal anomalies and or rapid upwelling of mantle through the melting zone new seismic models from two transects across the continent ocean transition on the us atlantic margin8 10 confirm the presence of a 20 25 km thick igneous section here we argue that the similarity of the crustal structure on these and two previous transects spanning 1 000 km of the margin and the association of thick igneous crust with the east coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3 2 106 km3 the distribution of volcanic and plutonic rocks details of the seismic structure and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non plume processes created the thick igneous crust

post eruptive deformation associated with the 1986 87 and 1989 lava flows of etna detected by radar interferometry we analysed 92 interferograms produced using ers1 sar images taken on etna between may 1992 and october 1993 nineteen show a local subsidence in the eastern flank of the volcano correlated with the location of the 30 october 1986 1 march 1987 and 27 september 9 october 1989 lava flows using fringe unwrapping and data gridding techniques and assuming that over the sampled time window deformation was a linear function of time we derive a map of along range rate of motion the correlation between the deformation field and the area covered by recent lavas suggests that compaction of the lava flows continues several years after the eruptions the area of maximum subsidence 47 mm yr is localised at the narrowing of the 1989 flow between 1500 and 1700 m a s l we observe that subsidence extends outside the lava flows accounting for at least 12 mm yr assuming a relaxation process of the substrate in response to loading produced by recent lavas a simple 1d maxwell visco elastic model predicts a maximum subsidence rate of 25 mm yr and a relaxation time of about 3 5 years the relaxation time agrees with those derived from post eruptive displacements observed by levelling on etna and piton de la fournaise after several eruptions we conclude that at the time of our measurements 25 to 50 of the deformation was related to relaxation of the substrate and the other part due to compaction of the lava flows

mn cr dating of fe and ca rich olivine from quenched and plutonic angrite meteorites using secondary ion mass spectrometry this research was supported by grants dp034277 and dp0666751 from the australian research council to tri and ya and an australian national university phd research scholarship

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

u pb systematics of the unique achondrite ibitira precise age determination and petrogenetic implications abstract ibitira is an unbrecciated equilibrated vesicular basaltic achondrite that is considered to have originated on a parent body distinct from all other known meteorites we present the first combined high precision u and pb isotopic data for this unique meteorite the 238 u 235 u value of 137 777 0 013 determined for the whole rock is comparable to values determined for bulk chondrites and other basaltic achondrites this value results in corrections of 1 1 ma for pb pb dates calculated using the previously assumed invariant 238 u 235 u value of 137 88 using the determined 238 u 235 u value the 7 most radiogenic pb isotopic analyses for acid leached pyroxene rich and whole rock fractions yield an isochron pb pb age of 4556 75 0 57 ma in excellent agreement with the results of mn cr chronology which give the ages of 4557 4 2 5 ma and 4555 9 3 2 ma using the u corrected pb pb age of d orbigny as a time anchor along with the previously proposed thermal history of ibitira and our closure temperature estimates for pb diffusion the pb pb age is interpreted as the timing of the last chemical equilibration and coarse pyroxene exsolution that occurred during high temperature metamorphism the metamorphism may have been caused by burial of ibitira lava under successive lava flows and if so the pb pb age should post date the crystallization by a short time interval the pb isotopic data for acid leachates suggest partial re equilibration of pb between plagioclase and phosphate perhaps during an impact event at 4 49 ga as recorded by k ar systematics the whole rock 238 u 204 pb indicates that compared to ci chondrites ibitira is less depleted in pb than in some alkali elements despite a lower condensation temperature of pb than the alkali elements the restricted pb depletion may reflect preferential concentration of metals with high fluid melt partition coefficients including pb and zn as a result of fluid exsolution and migration within the parent magma we discuss the implications of the u pb systematics for the origin and differentiation of the parent body

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

crustal structure beneath lofoten n norway from vertical incidence and wide angle seismic data summary a high quality multichannel seismic reflection line was acquired in 1987 along a 175 km long profile across the continental shelf off lofoten northern norway a seismic wide angle experiment was performed in 1988 along the same profile using seven three component ocean bottom seismographs obs with 20 25 km spacing and shotpoint intervals of 240 m the study of the data has shown that the combination of the multichannel reflection and the wide angle obs technique provides information about the crustal structure beneath the lofoten shelf that could not have been achieved using only one of the techniques the multichannel reflection data provide a detailed image of the shallow cretaceous structures which represents an important basis for inversion of the obs data the lower crust and the moho are also well mapped in some parts of the area with the multichannel reflection technique the obs data reveal that significant amounts of pre cretaceous sediments exist along almost the entire profile with a maximum thickness of about 5 km in the vestfjorden basin from the obs data the thickness of the lower crust is inferred to decrease from about 11 5 km under the rost high to about 2 km below the lofoten ridge the obs data indicate further that the moho position under the vestfjorden basin is considerably deeper than can be inferred from the reflection data about 10km below moho a strong dipping event is observed in the obs data this upper mantle reflection might be related to a possible seaward dipping master fault and or presence of layers of partially hydrated peridotite

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

magnesium isotope analysis of olivine and pyroxene by sims evaluation of matrix effects abstract the performance of multi collector secondary ion mass spectrometry mc sims for mg isotope ratio analysis was evaluated using 17 olivine and 5 pyroxene reference materials rms the mg isotope composition of these rms was accurately and precisely determined by multi collector inductively coupled plasma mass spectrometry mc icp ms and these measured isotope ratios were used to evaluate sims instrumental mass bias as a function of the forsterite fo content of olivine the magnitude of the mg isotope matrix effects were 3 in 25mg and are a complex function of olivine fo content that ranged from fo59 3 to fo100 in addition to these mg isotope matrix effects si ion yields and mg si ion ratios varied as a complex function of the fo content of the olivine rms for example si ion yields varied by 33 based on the observations we propose instrumental bias correction procedures for sims mg isotope analysis of olivine using a combination of mg si ratios and fo content of olivine using this correction method the accuracy of 25mg analyses is 0 3 except for analysis of olivine with fo86 88 where instrumental biases and mg si ratios change dramatically with fo content making it more difficult to assess the accuracy of mg isotope ratio measurements by sims over this narrow range of fo content five pyroxene rms 3 orthopyroxenes and 2 clinopyroxenes show smaller ranges of instrumental bias 1 4 in 25mg as compared to the olivine rms the instrumental bias for the 3 orthopyroxene rms do not define a linear relationship with respect to enstatite en content that ranged from en85 5 96 3 the clinopyroxene rms have similar en and wollastonite wo contents but have 25mg values that differ by 0 5 relative to their 25mg values determined by mc icp ms these results indicate that additional factors e g minor element abundances likely contribute to sims instrumental mass fractionation in order to better correct for these sims matrix effects additional pyroxene rms with various chemical compositions and known mg isotope ratios are needed

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

moho depth and crustal composition in southern africa abstract we present new results on structure thickness and composition of the crust in southern africa based on 6300 seismic receiver functions at 85 stations application of hk stacking to the entire sase dataset and use of multi frequency bands improve resolution substantially we observe a highly heterogeneous crustal structure with short wavelength variations in thickness h vp vs ratio composition and moho sharpness which defines 20 blocks that do not everywhere coincide with surface tectonic features in the zimbabwe craton the tokwe block has h 35 38 km and vp vs 1 74 1 79 whereas the thicker crust in the tati block h 47 51 km may be related to deformation of the archean crust along the cratonic margin two distinct crustal blocks with similar crustal thickness 42 46 km but significantly different vp vs ratios are recognized in the limpopo belt extreme values of 1 90 1 94 at the dyke swarms in eastern limpopo and 1 84 at the olifants river dyke swarm and easternmost bushveld intrusion complex bic indicate voluminous magmatic intrusions in the whole crust we find no evidence for magmatic intrusions in the central inferred part of bic where the crust is thick 45 50 km and vp vs is low 1 68 1 70 this thick crustal root may have deflected rising magmas to form the two bic lobes most of central kaapvaal has thin 35 40 km crust and vp vs 1 74 these characteristics are similar to the tokwe block in zimbabwe craton and may indicate delamination of pre existing lower crust which is further supported by a very sharp moho transition the exposed cross section in the vredefort impact crater is non representative of cratonic crust due to shallow moho 34 km and high vp vs 1 80 attributed to shock metamorphism high vp vs 1 76 is typical of the witwatersrand basin and anomalously low vp vs 1 66 1 67 marks the kaapvaal kheis namaqua transition highly heterogeneous crust both in thickness and vp vs ratio is typical of the namaqua natal and cape fold belts

migration of wide aperture onshore offshore seismic data central california seismic images of late stage subduction we conducted a high shot and receiver density onshore offshore seismic experiment in 1986 across the central california transform margin as part of the pacific gas and electric edge deep crustal geophysical survey the data set consists of land recordings of 406 marine air gun shots fired along a 70 km track across the santa maria basin the seismic vessel s track was collinear with 154 land seismic recorders deployed over 50 km across the sur obispo terrane and part of the salinian block the source receiver offsets range from 2 5 km to 120 km the unique recording geometry and high shot and receiver density permit imaging the deeper crustal structure of the california transform margin but pose serious challenges for conventional seismic imaging methods consequently we developed a unified approach to wide angle imaging which incorporates a tomography technique for obtaining initial migration velocity models and a prestack kirchhoff depth migration with depth focusing of the main reflectors for velocity model refinement and imaging this approach retains the velocity and dip information in the data and allows user control over the imaging process we applied this processing to subsets of the onshore offshore data to obtain an upper crustal velocity model and a 32 km long wide angle depth migrated image of the lower crust in the coastal region under the santa maria basin and the sur obispo terrane the migration image shows a part of an underplated oceanic crustal layer interpreted as the monterey plate meltzer and levander 1991 miller et al 1992 which dips coastward at depths of 18 to 28 km the plate appears to be kinked or imbricated beneath the coastline the image correlates well with existing wide angle seismic interpretations based on ray tracing walter and sharpless 1987 putzig 1988 howie et al 1993 the migrated lower crustal image has more structural information than is apparent in conventional ray based interpretations along the same crossing and adjacent profiles the crustal image demonstrates that automated imaging techniques applied to wide angle data can provide images of crustal structure comparable in fidelity to conventionally processed and migrated near vertical incidence data

wide angle reflection migration technique with an example from the polar profile northern scandinavia abstract the 440 km long polar profile runs approximately sw to ne across the baltic shield from the karelian province through the lapland granulite belt and the kola peninsula province in northern finland and northern norway the migration method developed by v n pilipenko physics of the earth n1 1983 36 42 was applied to the wide angle reflection data obtained along the profile to prepare the data for the migration p and s waves were analysed with more attention to the distribution of real reflectors in the crust and on the relation between the p and s wave velocities depth migrated images of the crustal structure were obtained for both the p and s wave fields this new analysis revealed structural features of the crust which had not before been imaged the most impressive features are several inclined boundaries in the upper crust dipping northeast beneath the lapland granulite belt another newly found element is a lower crustal boundary at a depth of 35 km in the karelian province which is well traced in the p wave migration image but is not observed in the s wave image this boundary corresponds to a strong change in the v p v s ratio in the lower crust which may be interpreted in terms of mafic crustal composition we have found that this migration technique can be applied to deep seismic sounding data resulting in new opportunities for deep crustal seismic studies nevertheless more and denser observations are necessary to continue the improvement of the resolution and the reliability of the structure images

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

147 146sm 143 142nd 176lu 176hf and 87rb 87sr systematics in the angrites implications for chronology and processes on the angrite parent body abstract angrites are a group of basaltic achondrites with distinctive mineralogic and geochemical characteristics that have the potential to provide insights into processes occurring on planetesimals in the early solar system these achondrites have been used as anchors linking the relative age information obtained from short lived extinct chronometers e g al mg hf w and mn cr with absolute chronometers e g u pb angrites provide excellent examples of early differentiation processes such as core formation and silicate differentiation on protoplanetary bodies the significant increase in the number of known angrite samples in recent years has offered the opportunity to compare several short and long lived isotopic systems in samples with different petrogenetic histories that formed on the same parent body to this end the 147sm 143nd 146sm 142nd 176lu 176hf and 87rb 87sr isotope systematics have been investigated in a suite of plutonic coarse grained nwa 4590 nwa 4801 and nwa 2999 and quenched fine grained d orbigny angrites the coupled 147 146sm 143 142nd systematics indicate possible isotopic disturbances in two angrites d orbigny and nwa 2999 resulting from post crystallization processes the internal 146sm 142nd isochrons of two coarse grained angrites nwa 4590 and nwa 4801 provide an updated best estimate of the initial solar system 146sm 144sm ratio i e at 4568 ma of 0 0084 0 0003 the 176lu 176hf isotope systematics in these angrites do not provide evidence of a previously proposed intense irradiation event in the early solar system the internal 176lu 176hf isochrons for the nwa 4590 and d orbigny angrites provide an estimate for the solar system initial 176hf 177hf ratio of 0 279775 0 000031 2 that agrees within uncertainty with the value of average chondrites reported by bouvier et al 2008 finally the calculated initial 87sr 86sr ratios based on the measured sr isotopic composition of plagioclase in these angrites yield an estimated initial 87sr 86sr ratio of 0 698980 0 000011 for the angrite parent body this is indistinguishable from a recently determined value for the solar system initial 87sr 86sr based on values measured in calcium aluminum rich inclusions cais after correcting for nucleosynthetic effects in the cais the low initial 87sr 86sr of the angrite parent body implies that it acquired its volatile element depleted characteristic within 1 8 ma of solar system formation likely because it accreted from volatile depleted planetesimals that formed in the hot inner nebula these integrated isotopic systematics suggest a complex history for the angrite parent body not previously inferred from short lived chronometers and provide new estimates for the initial isotopic composition of the early solar system

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

radar interferometry and its application to changes in the earth s surface geophysical applications of radar interferometry to measure changes in the earth s surface have exploded in the early 1990s this new geodetic technique calculates the interference pattern caused by the difference in phase between two images acquired by a spaceborne synthetic aperture radar at two distinct times the resulting interferogram is a contour map of the change in distance between the ground and the radar instrument these maps provide an unsurpassed spatial sampling density 100 pixels km 2 a competitive precision 1 cm and a useful observation cadence 1 pass month 1 they record movements in the crust perturbations in the atmosphere dielectric modifications in the soil and relief in the topography they are also sensitive to technical effects such as relative variations in the radar s trajectory or variations in its frequency standard we describe how all these phenomena contribute to an interferogram then a practical summary explains the techniques for calculating and manipulating interferograms from various radar instruments including the four satellites currently in orbit ers 1 ers 2 jers 1 and radarsat the next chapter suggests some guidelines for interpreting an interferogram as a geophysical measurement respecting the limits of the technique assessing its uncertainty recognizing artifacts and discriminating different types of signal we then review the geophysical applications published to date most of which study deformation related to earthquakes volcanoes and glaciers using ers 1 data we also show examples of monitoring natural hazards and environmental alterations related to landslides subsidence and agriculture in addition we consider subtler geophysical signals such as postseismic relaxation tidal loading of coastal areas and interseismic strain accumulation we conclude with our perspectives on the future of radar interferometry the objective of the review is for the reader to develop the physical understanding necessary to calculate an interferogram and the geophysical intuition necessary to interpret it

deflation of mount etna monitored by spaceborne radar interferometry ground based measurements of volcano deformation can be used to assess eruptive hazard but require the costly and often hazardous installation and maintenance of an instrument network here we show that spaceborne radar interferometry which has already shown its utility in mapping earthquake related deformation1 can be used to monitor long term volcano deformation two families of synthetic aperture radar images acquired from ascending and descending orbits by the satellite ers 1 and looking at mount etna from opposite sides cover the time period from 17 may 1992 to 24 october 1993 and include the second half of etna s most recent eruption despite artefacts of the interferometric technique we can observe a volcano wide deflation which is an expected consequence of the eruption but which had not previously been appreciated we quantify it using a simple model based on the change of pressure in a sphere located in an elastic half space the modelled deformation increases linearly with time until the end of the eruption our results show that it will be possible to use this technique to detect the inflation of volcanic edifices that usually precedes eruptions

slowness weighted diffraction stack for migrating wide angle seismic data in laterally varying media wide angle prestack depth migration is an important tool for studying the nature of reflecting boundaries in the earth s crust the slowness weighted diffraction stack swds method has been used to incorporate both two way traveltime constraints and slowness information in the migration for this purpose traveltimes and apparent slownesses of reflected arrivals must be calculated in the image space earlier applications of swds required a 1d or gently varying seismic velocity structure to obtain these quantities by ray tracing in the image space i show that the apparent slownesses can also be derived directly from one way traveltime maps using fermat s principle the sdws is applied to an existing onshore offshore wide angle data set and the example shows that the method can be used to image detailed reflectivity structure at great depths

53mn 53cr and 26al 26mg ages of a feldspathic lithology in polymict ureilites abstract we report 53 mn 53 cr and 26 al 26 mg isotopic data obtained by in situ sims analysis for feldspathic clasts in polymict ureilites dag 319 and dag 165 the analyzed clasts belong to the albitic lithology the most abundant population of indigenous feldspathic materials in polymict ureilites and are highly fractionated igneous assemblages of albitic plagioclase fe rich pyroxenes phosphates ilmenite silica and fe mn k p ti enriched glass glass in dag 165 clast 19 has extremely high and variable 55 mn 52 cr ratios 500 58 000 and shows correlated 53 cr excesses up to 1500 clearly indicating the presence of live 53 mn at the time of formation the slope of the well correlated isochron defined by glass and pyroxenes from this clast corresponds to 53 mn 55 mn 2 84 0 10 10 6 2 data for less 55 mn 52 cr enriched glasses from dag 319 clast b1 as well as phosphates from several other clasts are consistent with this isochron the 53 mn 55 mn ratio obtained from the isochron implies that these clasts are 0 70 0 18 ma younger than the d orbigny angrite corresponding to the absolute age of 4563 72 0 22 ma plagioclase in dag 319 clast b1 has a fairly constant 27 al 24 mg ratio of 900 and shows resolvable 26 mg excesses of 2 the slope of the isochron defined by pyroxene and plagioclase in this clast is 3 0 1 1 10 7 2 corresponding to a time difference of 5 4 0 3 0 5 ma after cai assuming the canonical initial 26 al 27 al ratio of 5 10 5 and an age 0 5 0 3 0 5 ma younger than d orbigny its absolute age relative to d orbigny is 4563 9 0 4 0 5 ma in agreement with the 53 mn 53 cr age from clast 19 these data provide the first high precision age date 5 4 ma after cai for ureilites giving a minimum estimate for the age of differentiation of their parent asteroid interpretation of this age for the thermal and physical history of that asteroid depends on a number of currently unknown or model dependent parameters including its size bulk composition and oxidation state and the petrologic relationship between the feldspathic clasts and main group ureilites

26al 26mg isotope systematics of the first solids in the early solar system high precision bulk aluminum magnesium isotope measurements of calcium aluminum rich inclusions cais from cv carbonaceous chondrites in several laboratories define a bulk 26al 26mg isochron with an inferred initial 26al 27al ratio of approximately 5 25 10 5 named the canonical ratio nonigneous cv cais yield well defined internal 26al 26mg isochrons consistent with the canonical value these observations indicate that the canonical 26al 27al ratio records initial al mg fractionation by evaporation and condensation in the cv cai forming region the internal isochrons of igneous cv cais show a range of inferred initial 26al 27al ratios 4 2 5 2 10 5 indicating that cai melting continued for at least 0 2 ma after formation of their precursors a similar range of initial 26al 27al ratios is also obtained from the internal isochrons of many cais igneous and nonigneous in other groups of carbonaceous chondrites some cais and refractory grains corundum and hibonite from unmetamorphosed or weakly metamorphosed chondrites including cvs are significantly depleted in 26al at least some of these refractory objects may have formed prior to injection of 26al into the protosolar molecular cloud and its subsequent homogenization in the protoplanetary disk bulk aluminum and magnesium isotope measurements of various types of chondrites plot along the bulk cv cai isochron suggesting homogeneous distribution of 26al and magnesium isotopes in the protoplanetary disk after an epoch of cai formation the inferred initial 26al 27al ratios of chondrules indicate that most chondrules formed 1 3 ma after cais with the canonical 26al 27al ratio

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

crustal deformation from 1992 to 1995 at the mid atlantic ridge southwest iceland mapped by satellite radar interferometry satellite radar interferometry observations of the reykjanes peninsula oblique rift in southwest iceland show that the reykjanes central volcano subsided at an average rate of up to 13 millimeters per year from 1992 to 1995 in response to use of its geothermal field interferograms spanning up to 3 12 years also include signatures of plate spreading and indicate that the plate boundary is locked at a depth of about 5 kilometers below that depth the plate movements are accommodated by continuous ductile deformation which is not fully balanced by inflow of magma from depth causing subsidence of the plate boundary of about 6 5 millimeters per year

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

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

147 sm 143 nd and 176 lu 176 hf systematics of eucrite and angrite meteorites comparative planetary geochemistry provides insight into the origin and evolutionary paths of planetary bodies in the inner solar system the eucrite and angrite achondrite groups are particularly interesting because they show evidence of early planetary differentiation we present 147sm 143nd and 176lu 176hf analyses of eight noncumulate basaltic eucrites two cumulate eucrites and three angrites which together place new constraints on the evolution and differentiation histories of the crusts of the eucrite and angrite parent bodies and their mantle mineralogies the chemical compositions of both eucrites and angrites indicate similar evolutionary paths and petrogenetic models with formation and isolation of differentiated crustal reservoirs associated with segregation of ilmenite we report a 147sm 143nd mineral isochron age for the moama cumulate eucrite of 4519 34 ma mswd 1 3 this age indicates protracted magmatism within deep crustal layers of the eucrite parent body lasting up to about 50 ma after the formation of the solar system we further demonstrate that the isotopic compositions of constituent minerals are compromised by secondary processes hindering precise determination of mineral isochron ages of basaltic eucrites and angrites we interpret the changes in geochemistry and consequently the erroneous 147sm 143nd and 176lu 176hf internal mineral isochron ages of basaltic eucrites and angrites as the result of metamorphic events such as impacts effects from pressure temperature and peak shock duration on the surfaces of the eucrite and angrite parent bodies

deep crustal seismology of continental margins this review discusses results from recent seismic experiments at continental margins conducted by u s investigators as well as those by non u s investigators necessary to place the results of u s workers into context continental margins may be characterized as either rifted transform or convergent rifted or passive margins form where continents are torn apart during continental drift transform margins accommodate transform plate motions along strike slip faults such as the san andreas fault at convergent margins oceanic crust is being thrust beneath or subducted under the continental crust all three types of margins are found in north america

the moho structure beneath the yarlung zangbo suture and its implications evidence from large dynamite shots abstract the yarlung zangbo suture yzs is the collisional front between the indian and eurasian plates the deep structure beneath the yzs has long been a focus of research its particular tectonic location and formation time provide a natural laboratory for the study of ongoing continental collisional processes lateral variations in the moho discontinuity provide evidence of basic processes governing plate tectonic evolution the depth and geometry of the moho provides first order information for restoration of complex geodynamic systems this paper presents a studying result of the moho both adjacent to and beneath the yzs obtained using five large dynamite shots along two profiles these five large shots were combined to construct two single fold profiles yzs a and yzs b and obtain high resolution images of the moho beneath the yzs both profiles show that continental crust thickness beneath the yzs is twice the global average of continental crust and the depth of the moho along profile yzs b is deeper than yzs a yzs a 81 e indicates a relatively flat moho without offset related to continent continent convergence beneath the collision zone conversely a moho structure with offset and overlapping reflections was imaged beneath profile yzs b 88 e significant lateral variations in the geometry of the moho discontinuity along the yzs may indicate that the indian lithosphere is thrusting beneath tibet at a different angle from west to east

correlated study of initial sr 87 sr 86 and al mg isotopic systematics and petrologic properties in a suite of refractory inclusions from the allende meteorite we have studied a suite of six coarse grained ca al rich inclusions cais from the allende meteorite applying in concert detailed petrographic and chemical characterization ion probe mass spectrometric analyses of the al mg isotopic system to determine the abundance and distribution of 26a1 and thermal emission mass spectrometric analyses of the rb sr system to determine initial 87sr86sr ratios the simplest inclusion is a type bl usnm 3529 z that shows evidence of minor alteration and recrystallization since solidification from a melt droplet e g some of the coarse grained anorthite in the inclusion especially near the inclusion rim apparently formed by recrystallization or melting of fine grained secondary anorthite that had replaced melilite four other type bs contain evidence of more widespread recrystallization in addition to evidence of local replacement of melilite by coarse anorthite e g melilite zoning is complex and not igneous in origin and in one inclusion usnm 3529 21 melilite and pyroxene crystals enclose relicts of an earlier generation of the same phases the sixth inclusion usnm 3898 is a type a whose properties appear to be largely metamorphic in origin all six inclusions show variations in mg isotopic compositions indicating that they incorporated 26a1 in 3529 z the relative abundance of 26a1 26al27al 4 0 0 1 10 5 is close to the canonical value 26al27al 5 10 5 for cais but both anorthite and melilite show minor departures from a strict isochron relationship between mg and al in particular the coarse anorthite near the inclusion rim has initial 26al27al ratios of 1 2 10 5 implying formation at least 1 5 million years after the first generation anorthite the other four type bs exhibit larger disturbances of the al mg system that can be understood in terms of local recrystallization and isotopic exchange primarily between anorthite and melilite only 3898 is consistent with a strict al mg isochron relationship but this cai lacks primary anorthite and the small range of al mg among the phases analyzed may preclude resolution of any small isotopic disturbances the rb sr isotopic system also typically indicates some level of disturbance some of which must have been relatively recent nevertheless phases characterized by very low rb sr permit precise identification of initial 87sr86sr our al mg and sr data are largely consistent with a simple chronological interpretation of both systems i e the 26al results suggest that all the initial 87sr86sr ratios should have the same value and with one exception our initial 87sr86sr ratios are consistent with a single value this value is somewhat higher than the primitive value all reported by gray et al 1973 however also with one exception the very low rb sr cai data reported by gray et al 1973 are consistent with the same value the exception is the same inclusion in both studies our analysis of usnm 3898 yields an initial 87sr86srhigher than that of the other cais while gray et al 1973 obtained an initial 87sr86srlower than that of the other cais for the same inclusion labeled as d7 on which all is based neither result can be readily explained as a chronological aberration since 3898 has nearly canonical 26al27al the present data are the first which establish a firm association between primitive 26al27al and primitive 87sr86sr by analysis of both isotopic systems in the same inclusions the comparison of both isotopic systems interpreted as simple nebular chronometers does not reveal any chronological inconsistencies that demand resolution in terms of a grossly heterogeneous distribution of 26al

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

dynamic deformation of etna volcano observed by satellite radar interferometry satellite radar interferometry of mt etna volcano sicily italy reveals a sequence of deformation characterized by deflation during the end of the 1993 eruption inflation from 1993 1995 with an increase in the inflation rate immediately before its resumed eruptive activity in late 1995 this was followed by very low deformation levels during the following year the source of the deformation changed from a depth of 9 km during deflation to more than 11 14 km during the subsequent inflation consistent with a model in which deflation at shallower levels is followed by inflation at greater depth as the volcano system recharges from below before its next eruption this study demonstrates that radar interferometry provides an important contribution towards understanding the dynamic deformation of volcanoes by revealing large scale changes in their pre eruption deformation rates radar interferometry could play an important role in volcano eruption monitoring

modern u pb chronometry of meteorites advancing to higher time resolution reveals new problems in this paper we evaluate the factors that influence the accuracy of lead pb isotopic ages of meteorites and may possibly be responsible for inconsistencies between pb isotopic and extinct nuclide timescales of the early solar system instrumental mass fractionation and other possible analytical sources of error presence of more than one component of non radiogenic pb migration of ancient radiogenic pb by diffusion and other mechanisms possible heterogeneity of the isotopic composition of uranium u uncertainties in the decay constants of uranium isotopes possible presence of freshly synthesized actinides with short half life e g 234u in the early solar system possible initial disequilibrium in the uranium decay chains and potential fractionation of radiogenic pb isotopes and u isotopes caused by alpha recoil and subsequent laboratory treatment we review the use of 232th 238u values to assist in making accurate interpretations of the u pb ages of meteorite components we discuss recently published u pb dates of calcium aluminum rich inclusions cais and their apparent disagreement with the extinct nuclide dates in the context of capability and common pitfalls in modern meteorite chronology finally we discuss the requirements of meteorites that are intended to be used as the reference points in building a consistent time scale of the early solar system based on the combined use of the u pb system and extinct nuclide chronometers

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

chemical alteration and ree mobilization in meteorites from hot and cold deserts the effects of terrestrial weathering on ree mobilization are evaluated for a variety of uncommon meteorites found in antarctica and in hot deserts the meteorites analyzed include 7 non cumulate eucrites 10 shergottites 3 nakhlites 2 lunar meteorites 4 angrites 10 acapulcoites 1 winonaite and 1 brachinite in situ concentration measurements of lanthanides and selected other minor and trace elements were made on individual grains by secondary ion mass spectrometry sims in antarctic meteorites oxidation converts ce 3 to ce 4 which is less soluble than the trivalent ree resulting in ce anomalies the mineral most affected is low ca pyroxene however not all grains of a given mineral are and distinct analyses of a single grain can even yield ree patterns with and without ce anomalies the effect is most pronounced for antarctic eucrites in which ce anomalies are observed not only in individual minerals but also in whole rock samples although ce anomalies are observed in meteorites from hot deserts as well the most characteristic signs of chemical alteration in this environment are a lree enrichment with a typical crustal signature as well as sr ba and u contaminations these can modify the whole rock ree patterns and disturb the isotope systematics used to date these objects the lree contamination is highly heterogeneous affecting some grains and not others of a given mineral mainly olivine and low ca pyroxene the two minerals with the lowest ree concentrations the major conduit for ree movement is through shock induced cracks and defects and the highest levels of contamination are found in altered material filling such veins and cracks meteorites that experienced low shock levels and those that are highly recrystallized are the least altered copyright 2003 elsevier ltd

mn cr ages of dolomites in ci chondrites and the tagish lake ungrouped carbonaceous chondrite abstract 53 mn 53 cr ages have been measured for dolomites in three ci chondrites and an ungrouped c2 chondrite tagish lake for an accurate age determination we utilized in contrast to previous works a matrix matched carbonate standard to appropriately correct the observed mn cr ratios the mn cr data of the dolomites most likely represent isochrons with chronological significances and the obtained ages are concentrated in 4563 8 4562 5 million years ago 4 4 5 7 million years after the birth of the solar system the ages of the ci and tagish lake dolomites are very similar to those of carbonates in cm chondrites although ci and cm chondrites are likely to have different alteration histories the similar carbonate ages in various carbonaceous chondrites suggest that the aqueous activities in hydrous asteroids initiated almost concurrently if this is the case the most plausible heat source for the aqueous alteration is the decay energy of short lived radionuclides especially 26 al we carried out a model calculation for the thermal history of the ci chondrite parent body assuming that 26 al is the dominant heat source by two different approaches to estimate the accretion time of the ci chondrite parent body we conclude that it accreted 3 0 3 9 million years after the birth of the solar system this is also likely to be the case for the cm chondrite and tagish lake parent bodies because the result of the calculation is not sensitive to the ice abundance therefore we stress that near the condensation front of water ice meteorite parent bodies with abundant ice accreted almost simultaneously at 3 4 million years after cai formation combining our mn cr ages of the dolomites with those of breunnerites reported previously the youngest one is 4554 6 million years ago we found that the aqueous alteration in the ci chondrite parent body lasted for at least 9 million years therefore the parent body should be large enough 50 km in radius so that the internal heat had been kept for 9 million years

some recent developments in the acquisition and processing of seismic data abstract industry style 3 d surveys are different in the volume of data processing techniques and expected results from academic 3 d surveys industry has developed new acquisition techniques for marine 3 d surveys multi streamer seismic ships vertical cables and dual sensor bottom cables as opposed to 2 d post stack time migration new techniques involving 2 d and 3 d pre stack depth migration are becoming important examples are given migration of wide angle data to obtain subsurface imaging is discussed attribute analyses including the use of wavelets to detect azimuthal anisotropy and lithologic contrasts are described the use of the recent developments in wavelets in seismic analysis is introduced

shock metamorphism of carbonaceous chondrites abstract we have studied shock effects in carbonaceous chondrites using optical microscopy of thin sections and find that our petrographic classification of progressive shock metamorphism in ordinary chondrites can also be applied to carbonaceous chondrites we find that sixty nine carbonaceous chondrites can be assigned to four shock stages si to s4 largely on the basis of shock effects in olivine the least shocked chondrite groups are cm2 and co3 thirty six out of thirty eight members are classified as shock stage s1 differences between the mean shock levels of certain groups of chondrites e g co3 and cv3 are probably due to stochastic differences in the sampling of their parent bodies however there is an overall tendency for the mean shock level of carbonaceous chondrites and ordinary chondrites to increase with increasing petrologic type that may reflect real differences in the shock level of near surface materials on their parent bodies caused by intrinsic variations in the chemical and physical properties of these materials petrologic type 2 and 3 chondrites are more porous and richer in volatiles than types 4 to 6 the greater porosity of types 2 3 causes higher post shock temperatures and melting at lower pressures and the higher volatile contents ensure that strongly shocked material is more readily dispersed on release from high pressure we suggest that type 2 3 material shocked above 20 30 gpa normally escapes from the parent asteroids and forms particles that are too small to survive as meteorites in the cv3 group there is a correlation between the degree of chondrule flattening and the intensity of shock metamorphism analogous to that discovered in ordinary chondrites by sneyd et al 1988 suggesting that shock rather than static overburden pressure is responsible for chondrule flattening we infer that the correlations are due to collapse of pores under shock pressures of at least 5 10 gpa and that shock is an important process affecting many physical properties of type 2 4 chondrites

tropospheric corrections of sar interferograms with strong topography application to etna the accuracy of spaceborne geodetic techniques including sar interferometry is limited by the time and spatial variation and altitude dependance of the propagation delay of electomagnetic waves in the lower troposphere particularly in mountainous areas in this paper we use a 1d model developed for tropospheric corrections of gps and doris measurements to correct sar data the differential tropospheric delay is computed at each pixel of the interferogram from ground temperature humidity and pressure using two empirical parameters calibrated from several radio soundings acquired in various latitude and climate conditions it is shown that with such a model given the 3300 meters topography of etna tropospheric variations can generate up to 4 phase rotations between the top and the bottom of the volcano in 16 out of the 20 interferograms processed with images acquired between august 1992 and october 1993 correction of the tropospheric effect reduces the number of fringes associated with the 1991 93 eruption from previous estimates the remaining deformation is consistant with a deforming source located at a depth of 14 1 km during the second half of the eruption the subsidence rate at the top of the volcano is roughly stable at 13 3 mm month these values are in good agreement with tiltmeter data collected on etna during the same period and with the estimated volume of erupted material no significant deformation can be observed during the last month of eruption inflation of the volcano seems to resume immediately after the end of the eruption at a rate of 3 mm month

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

image of the moho across the continent ocean transition u s east coast strong wide angle reflections from the moho were recorded by ocean bottom seismic instruments during the 1988 carolina trough multichannel seismic experiment in an area where the moho is difficult to detect with vertical incidence seismic data prestack depth migration of these reflections has enabled the construction of a seismic image of the moho across the continent ocean transition of a sedimented passive margin the moho rises across the margin at a slope of 10 12 from a depth of about 33 km beneath the continental shelf to 20 km beneath the outer rise this zone of crustal thinning defines a distinct 60 70 km wide continent ocean transition zone we interpret the moho in the carolina trough as a jurassic feature formed by magmatic intrusion and underplating during the rifting of pangea

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

aluminum 26 in h4 chondrites implications for its production and its usefulness as a fine scale chronometer for early solar system events in order to investigate whether or not 26al can be used as a fine scale chronometer for early solar system events we measured with an ion microprobe mg isotopes and al mg ratios in separated plagioclase olivine and pyroxene crystals from the h4 chondrites ste marguerite forest vale beaver creek and quenggouk and compared the results with the canonical 26al 27al ratio for ca al rich inclusions cais for ste marguerite sm and forest vale fv pb pb and mn cr ages have previously been determined gpel et al 1994 polnau et al 2000 polnau and lugmair 2001 plagioclase grains from these two meteorites show clear excesses of 26mg the 26al 27al ratios inferred from these excesses and from isotopically normal mg in pyroxene and olivine are 2 87 0 64 x 10 7 for sm and 1 52 0 52 x 10 7 for fv the differences between these ratios and the ratio of 5 x 10 5 in cais indicate time differences of 5 4 0 1 ma and 6 1 0 2 ma for sm and fv respectively these differences are in agreement with the absolute pb pb ages for cais and sm and fv phosphates but there are large discrepancies between the u pb and mn cr system for the relative ages for cais sm and fv for example mn cr ages of carbonates from kaidun are older than the pb pb age of cais however even if we require that cais are older than these carbonates the time difference between this adjusted cai age and the mn cr ages of sm and fv require that 26al was widely distributed in the early solar system at the time of cai formation and was not mostly present in cais a feature of the x wind model proposed by shu and collaborators gounelle et al 2001 shu et al 2001 from this we conclude that there was enough 26al to melt small planetary bodies as long as they formed within 2 ma of cais and that 26al can serve as a fine scale chronometer for early solar system events

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

multichannel seismic image of the crustal thinning at the ne iberian margin combining normal and wide angle reflection data the seismic image of the moho at the transition from the ne iberian peninsula to the western mediterranean sea is investigated using coincident steep and wide angle reflection data merged into a single stacked and migrated section the multichannel analysis of onshore offshore large aperture data provides new insight on the deep structure in areas where the steep sections lack penetration and reveals that the crust undergoes a strong but continuous thinning along the flanks of the valencia trough the combined seismic image indicates that almost half of the continental crust 14 15 km is lost in less than 60 km horizontal distance

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

the seismic moho structure of shatsky rise oceanic plateau northwest pacific ocean abstract oceanic plateaus are large igneous provinces formed by extraordinary eruptions that create thick oceanic crust whose structure is poorly known owing to the lack of deep penetration seismic data multichannel seismic mcs reflection and wide angle refraction data allow us to show moho structure beneath a large part of the shatsky rise oceanic plateau in the northwest pacific ocean moho reflectors in the two data sets can be connected to trace the interface from the adjacent abyssal plain across much of the interior the reflectors display varied character in continuity shape and amplitude similar to characteristics reported in other locations beneath normal crust the moho is observed at 13 km depth 7 km below the seafloor in mcs data and disappears at 20 km depth 17 km below the seafloor beneath the high plateau moho at the distal flanks dips downward towards the center with slopes of 0 5 1 increasing to 3 5 at the middle flanks seismic moho topography is consistent with airy isostasy confirming this widely applied assumption data from this study show that crustal thickness between the massifs in the interior of the plateau is nearly twice normal crustal thickness despite the fact that this crust records apparently normal seafloor spreading magnetic lineations the moho model allows improved estimates of plateau area 5 33 10 5 km 2 and volume 6 90 10 6 km 3 the latter assuming that the entire crust was formed by shatsky rise volcanism because the massifs formed at spreading ridges this study is unique in showing moho depth and structure over an extraordinarily large area beneath an oceanic plateau giving insight to plateau structure and formation

ancient relative and absolute ages for a basaltic meteorite implications for timescales of planetesimal accretion and differentiation abstract asuka 881394 is a unique basaltic meteorite that originated in the crust of a differentiated planetesimal in the early solar system we present high precision pb mg and cr isotopic compositions of bulk samples and mineral separates from this achondrite a 207pb 206pb internal isochron obtained from the radiogenic pyroxene and whole rock fractions of asuka 881394 yields an absolute age of 4566 5 0 2 ma which we consider to be the best estimate for the crystallization age of this basaltic achondrite the 26al 26mg systematics show some evidence of disturbance but 5 of the 6 analyzed whole rock and mineral fractions define an isochron corresponding to a 27al 26al ratio of 1 28 0 07 10 6 comparison with the 26al 26mg and pb pb systematics in the d orbigny achondrite translates to a 26al 26mg age of 4565 4 0 2 ma for asuka 881394 the 53mn 53cr systematics in whole rock silicate and chromite fractions correspond to a 53mn 55mn ratio of 3 85 0 23 10 6 compared to the most precise 53mn 53cr and pb pb systematics available for the d orbigny angrite this translates to a 53mn 53cr age of 4565 3 0 4 ma similarly a comparison with the nwa 4801 angrite yields a 53mn 53cr age of 4565 5 0 4 ma in agreement with the age obtained relative to d orbigny while the 26al 26mg and 53mn 53cr ages appear to be concordant in asuka 881394 these ages are 1 ma younger than its 207pb 206pb age this discordance might have been caused by one or more of several reasons including differences in the closure temperatures for pb versus cr and mg diffusion in their host minerals combined with slow cooling of the parent body as well as differential resetting of isotopic systems by a process other than volume diffusion e g shock metamorphism the ancient age of asuka 881394 suggests that basaltic volcanism on its parent planetesimal occurred within 3 ma of the formation of earliest solids in the solar system essentially contemporaneously with chondrule formation this requires that the asuka 881394 parent body was fully accreted within 500 000 yrs of solar system formation

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

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

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

effects of topography on the interpretation of the deformation field of prominent volcanoes application to etna we have investigated the effects of topography on the surface deformation field of volcanoes our study provides limits to the use of classical half space models considering axisymmetrical volcanoes we show that interpreting ground surface displacements with half space models can lead to erroneous estimations of the shape of the deformation source when the average slope of the flanks of a volcano exceeds 20 tilting in the summit area is reversed to that expected for a flat surface thus neglecting topography may lead to misinterpreting an inflation of the source as a deflation comparisons of mogi s model with a three dimensional model shows that ignoring topography may lead to an overestimate of the source volume change by as much as 50 for a slope of 30 this comparison also shows that the depths calculated by using mogi s solution for prominent volcanoes should be considered as depths from the summit of the edifices finally we illustrate these topographic effects by analyzing the deformation field measured by radar interferometry at mount etna during its 1991 1993 eruption a three dimensional modeling calculation shows that the flattening of the deflation field near the volcano s summit is probably a topographic effect

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

volcano wide fringes in ers synthetic aperture radar interferograms of etna 1992 1998 deformation or tropospheric effect mount etna 3300 m is the volcano that has been first and most studied by differential synthetic aperture radar previous papers gave evidence for a large scale deformation of the entire edifice consistent with unrest episodes but with a poor fit with classical elastic models also atmospheric effects on mountainous areas are known to be very significant accordingly interferograms may reflect both deformation and tropospheric effects we investigate here the possibility of evaluating and correcting tropospheric effects directly from the interferograms from 38 ers synthetic aperture radar ascending scenes taken from 1992 to 1999 we have computed 238 interferograms the amount of data allows us to analyze how the coherence is maintained over long periods of time and thus how it reveals the location of permanent scatters from a simple analysis of phase elevation relation we give evidence for the clear difference in pattern between stratified tropospheric effects and large scale deformation produced by a source in a three dimensional elastic body with topography using combination of inversion processes we analyze both deformation source and tropospheric effects for the complete set of 238 interferograms finally time evolution of the parameters and modeling probability estimation are retrieved for each sar image estimated relative change in tropospheric delay varies in time from 2 7 to 3 0 1 2 fringes they are consistent with other tropospheric estimations at etna based on ground met and global positioning system observations on the same period of time magmatic volume variations remain below error bars and thus cannot be estimated properly finally some of the interferograms may be entirely explained by the difference in tropospheric conditions between the two acquisition periods without the need to invoking a deformation of the volcano