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

a 2 14 myr astronomically tuned record of relative geomagnetic paleointensity from the western philippine sea 1 we present a 2 14 myr astronomically tuned relative geomagnetic paleointensity record from the western philippine sea pseudosingle domain titanomagnetite is the only magnetic mineral identified and variations in titanomagnetite concentration fall well within the accepted limits for relative paleointensity variations no significant temporally persistent periodicities are observed in wavelet analyses of the paleointensity time series or in the rock magnetic parameters used for relative paleointensity normalization this suggests that our paleointensity record is largely free of rock magnetic or lithological artefacts and that it represents a reliable record of geomagnetic behavior with no evidence for modulation of the field at earth orbital periods the paleointensity record is highly coherent with the sint 800 global paleointensity stack for the last 800 kyr and with a coeval record from the west caroline basin our record confirms that it is normal for the geomagnetic field to undergo dynamic changes within polarity intervals with relatively frequent collapses of the field to low paleointensities and concomitant deviations away from the stable field direction we do not observe an asymmetrical sawtooth form in our paleointensity record which might suggest that previously observed asymmetrical sawtooth paleointensities result from rock magnetic artefacts also we do not observe a persistent 100 kyr inclination periodicity in contrast to the suggestion that geomagnetic field directions are modulated by orbital eccentricity good agreement between our paleointensity record and the coeval west caroline basin record provides the beginning of a detailed view of geomagnetic field behavior between 0 8 and 2 14 ma

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

why are geomagnetic excursions not always recorded in sediments constraints from post depositional remanent magnetization lock in modelling abstract many geomagnetic excursions have been documented during the brunhes chron however high resolution paleomagnetic studies of sediments often provide evidence of few if any excursions we have investigated the reasons for this observation by modelling the post depositional remanent magnetization pdrm lock in process a cubic lock in function produces more rapid lock in compared to linear and exponential functions proposed in the literature we therefore used a cubic function to model best case scenarios for the quality of the paleomagnetic record when a high frequency geomagnetic input signal is convolved with the sediment lock in function for a wide range of sedimentation rates even for a lock in depth of 10 cm where 95 of the pdrm is locked in 5 cm below the surface mixed layer an input signal containing abundant excursions with 1 kyr duration is smoothed so drastically at low sedimentation rates 1 3 cm kyr that no excursions are recorded excursions are further attenuated by increasing the lock in depth as well as by sampling at discrete stratigraphic intervals the pdrm process acts as a low pass filter for high frequency secular variation signals even at moderate sedimentation rates 10 cm kyr we have also modelled the effects of modulation of sedimentation rate by a climatic forcing function e g insolation even though the average sedimentation rate might be substantial pdrm recording becomes much less reliable in intervals where sedimentation rates are lowest in these cases some excursions are reliably recorded in detail while others may not be recorded at all our modelling suggests that in order to consistently detect the presence of geomagnetic excursions it is ideal to work with sediments that maintain minimum sedimentation rates above 10 cm kyr if the normal spectrum of geomagnetic field behaviour contains abundant excursions failure to document excursions in many apparently high resolution analyses probably results from pdrm lock in within relatively slowly deposited sediments or to unrecognized intervals of slow sedimentation in environments with higher average sedimentation rates

10be evidence for delayed acquisition of remanent magnetization in marine sediments implication for a new age for the matuyama brunhes boundary abstract fluxes of the meteoric cosmogenic radionuclide 10 be vary with changes in the incoming cosmic rays modulated by geomagnetic field intensity variations the variability in the 10 be flux can be used to synchronize ice cores as well as marine sediments by comparison with the relative paleointensity variations of the geomagnetic field however lock in of the paleomagnetic signal at some depth below the sediment water interface in marine sediments through acquisition of a post depositional remanent magnetization pdrm adds uncertainty to synchronization despite the long history of such studies the magnitude of the pdrm lock in depth remains controversial in this article we present clear evidence for a downward offset of the paleointensity minimum relative to the 10 be flux anomaly at the matuyama brunhes m b geomagnetic polarity boundary which we interpret to result from a 15 cm pdrm lock in depth this lock in depth indicates that up to several tens of thousands years of age offset probably occurs when a paleomagnetic record is used for dating marine sediments and the age of the m b boundary should be revised to ca 10 kyr younger which is consistent with a younger ice core derived age of 770 6 ka 2 this cosmogenic age tuning strategy will contribute to refining paleomagnetic based age models for marine sediments and identifying of lead lag relationships for global abrupt environmental changes

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

paleomagnetic reconstruction of the global geomagnetic field evolution during the matuyama brunhes transition iterative bayesian inversion and independent verification abstract the earth s magnetic field changed its polarity from the last reversed into today s normal state approximately 780 000 years ago while before and after this so called matuyama brunhes reversal the earth magnetic field was essentially an axial dipole the details of its transitional structure are still largely unknown here a bayesian inversion method is developed to reconstruct the spherical harmonic expansion of this transitional field from paleomagnetic data this is achieved by minimizing the total variational power at the core mantle boundary during the transition under paleomagnetic constraints the validity of the inversion technique is proved in two ways first by inverting synthetic data sets from a modeled reversal here it is possible to reliably reconstruct the gauss coefficients even from noisy records second by iteratively combining four geographically distributed high quality paleomagnetic records of the matuyama brunhes reversal into a single geometric reversal scenario without assuming an a priori common age model the obtained spatio temporal reversal scenario successfully predicts most independent matuyama brunhes transitional records therefore the obtained global reconstruction based on paleomagnetic data invites to compare the inferred transitional field structure with results from numerical geodynamo models regarding the morphology of the transitional field it is found that radial magnetic flux patches form at the equator and move polewards during the transition our model indicates an increase of non dipolar energy prior to the last reversal and a non dipolar dominance during the transition thus the character and information of surface geomagnetic field records is strongly site dependent the reconstruction also offers new answers to the question of existence of preferred longitudinal bands during the transition and to the problem of reversal duration different types of directional variations of the surface geomagnetic field continuous or abrupt are found during the transition two preferred longitudinal bands along the americas and east asia are not predicted for uniformly distributed sampling locations on the globe similar to geodynamo models with cmb heatflux derived from present day lower mantle heterogeneities a preference of transitional vgps for the pacific hemisphere is found the paleomagnetic duration of reversals shows not only a latitudinal but also a longitudinal variation even the paleomagnetically determined age of the reversal varies significantly between different sites on the globe the described bayesian inversion technique can easily be applied to other high quality full vector reversal records also its extension to inversion of secular variation and excursion data is straightforward

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

a model for planetesimal meltdown by 26al and its implications for meteorite parent bodies the melting of planetesimals heated by 26 al has been modelled using a new finite difference method that incorporates convection as an example we consider a planetesimal with a radius of 64 km which accretes instantaneously at t 0 75 myr after the formation of calcium aluminum rich inclusions from cold 250 k nebular dust with 50 porosity at t 0 9 myr t 700 k the planetesimal shrinks to a radius of 50 km due to sintering at t 1 2 myr t 1425 k the fully insulated interior deeper than a few kilometers starts to melt and at t 1 5 myr t 1725 k with 50 melting convection starts by t 2 myr the planetesimal is a globe of molten convecting slurry inside a thin residual crust from about t 2 5 myr the crust thickens rapidly as the power of 26 al fades planetesimals probably melt in this manner when they accrete before t 1 3 myr and are large enough to insulate themselves r 20 km for accretion at t 0 rising to 80 km at t 1 3 myr melting behavior will also be affected by the level of 60 fe in nebular dust by the extent of devolatilization reactions and basalt segregation during heating and by gradual accretion the model suggests that a the parent bodies of differentiated meteorites had accreted before about t 1 5 to 2 myr and before most chondritic parent bodies had formed and b that molten planetesimals may be a source for chondrule melt droplets

orbital modulation of the earth s magnetic field intensity more than 20 years ago on the basis of data from a pacific sediment core it was suggested that geomagnetic field intensity may vary with the earth s orbital obliquity centred on a period of 41 kyr as a result of the effect of obliquity on precessional forces in the earth s core1 it had also been proposed that precession plays an important role in the energy budget of the earth s geodynamo2 but subsequent analyses indicated that the energy available from precession is at least an order of magnitude less than that required to drive the geodynamo3 here however we report a spectral analysis of sedimentary records of relative geomagnetic palaeointensity from two north atlantic sites which shows significant power both at orbital eccentricity 100 kyr and obliquity 41 kyr the eccentricity power is also present in bulk magnetic properties such as susceptibility and is therefore attributable to lithological variations controlled by eccentricity driven climate change the obliquity power however is not apparent in bulk magnetic properties and seems to be a property of the geomagnetic field itself thus providing evidence for the orbital forcing of geomagnetic field intensity

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

geomagnetic dipole strength and reversal rate over the past two million years independent records of relative magnetic palaeointensity from sediment cores in different areas of the world can be stacked together to extract the evolution of the geomagnetic dipole moment1 2 and thus provide information regarding the processes governing the geodynamo so far this procedure has been limited to the past 800 000 years 800 kyr ref 3 which does not include any geomagnetic reversals here we present a composite curve that shows the evolution of the dipole moment during the past two million years this reconstruction is in good agreement with the absolute dipole moments derived from volcanic lavas which were used for calibration we show that at least during this period the time averaged field was higher during periods without reversals but the amplitude of the short term oscillations remained the same as a consequence few intervals of very low intensity and thus fewer instabilities are expected during periods with a strong average dipole moment whereas more excursions and reversals are expected during periods of weak field intensity we also observe that the axial dipole begins to decay 60 80 kyr before reversals but rebuilds itself in the opposite direction in only a few thousand years

episodic carbonate precipitation in the cm chondrite alh 84049 an ion microprobe analysis of o and c isotopes we have determined the o and c isotope compositions of dolomite grains and the c isotope compositions of calcite grains in the highly altered cm1 chondrite alh 84049 using secondary ion mass spectrometry sims chemically zoned dolomite constitutes 0 8 volume percent vol of the sample and calcite 0 9 vol thirteen separate dolomite grains have 13c values that range from 37 to 60 2 18 o values from 25 to 32 3 and 17 o values from 10 to 16 3 vsmow intragrain 13 c values in dolomite vary up to 10 the 13 c values of three calcite grains are distinct from those of dolomite and range from 10 to 13 2 pdb calcite and dolomite appear to record different precipitation episodes carbon isotope values of both dolomite and calcite in this single sample encompass much of the reported range for cm chondrites our results imply that bulk carbonate c and o isotope analyses may oversimplify the history of carbonate precipitation multiple generations of carbonates with variable isotope compositions exist in alh 84049 and perhaps in many cm chondrites this work shows that one should exercise caution when using a clumped isotope approach to determine the original temperature and the isotopic compositions of water for cm chondrite carbonates less altered cm meteorites with more homogeneous c isotope compositions however may be suitable for bulk carbonate analyses but detailed carbonate petrologic and isotopic characterization of individual samples is advised

oxygen isotopes in asteroidal materials measurement of the variation of the 17o 16o and 18o 16o ratios of extraterrestrial materials has proven to be a key tool in understanding the formation and evolution of the solar system this chapter attempts to collate the huge data set of oxygen isotopic measurements of asteroidal material as sampled by meteorites in order to understand some of the processes involved in the formation of these primitive bodies and how such events have affected the oxygen isotopic ratios ultimately offering a window back to the very origin of solar system and its primordialoxygen isotopic heterogeneity oxygen is generally a major element in the gas liquid and solid phases that have interacted throughout the evolution of the solar system its chemical properties ensuring that there is abundant opportunity for reactions and exchange the details of which can be recorded in the isotopic ratios of the products such processes can be crudely subdivided into the following six categories early solar nebular isotopic heterogeneity high temperature modifi cation of nebular components metamorphism and melting in asteroids aqueous alteration in asteroids mixing of components brecciation terrestrial weathering it is clear from the isotopic variation in the earliest formed nebular components such as refractory inclusions and chondrules that there was considerable oxygen isotopic heterogeneity in the early solar nebula spanning over 50 and perhaps much greater in both 17o and 18o the origin of this variation is discussed elsewhere in this volume however isotopic exchange between the 16o rich solids and 17o and 18o rich gas phase s resulted in a wide range of isotopic variation in these early formed components offering insight into the conditions of these processes and the evolution of the reservoirs with time and or space once accreted onto the early planetisemals much of the primitive nebular components experienced considerable modification the metamorphism and melting experienced by some of these bodies most probably the result of heating from the decay of short lived radionuclides e g 26al resulted in progressive homogenization within individual bodies of any original isotopic heterogeneity complete homogenization appears to have been achieved in the hed parent body indicating very high levels of melting and the development of a large magma ocean the carbonaceous and ordinary chondrite parent bodies experienced more modest heating sufficient to mobilize water ice that accreted along with the more refractory materials the subsequent reactions between the liquid and solid phases at low temperature imparted large isotopic fractionations which record some of the conditions of this alteration such as water rock ratio temperature and isotopic signature and thus potentially the origin of the water ice the evolution of the asteroidal bodies has been far from simple and an important use of oxygen isotope measurements has been in the identifi cation of the components present in some meteorites the result of breakup reassembly and mixing of different bodies during impact processing the final oxygen isotopic signatures imparted into meteorite samples are those associated with terrestrial weathering the isotopic compositions of the weathering products depend on location primarily latitude while the magnitude of the weathering is dependent on many factors with some meteorites e g highly reduced enstatite chondrites and the matrix rich co3s particularly susceptible to such effects

10 be evidence for the matuyama brunhes geomagnetic reversal in the epica dome c ice core every few hundred thousand years earth s north and south magnetic poles swap places for the first time a record of one of these geomagnetic reversals has been found in an ice core because the strength of the field is very low during a reversal the intensity of cosmic rays in the earth s atmosphere is increased this leads to an increase in the quantity of 10be which like 14c is formed by cosmic rays in the atmosphere deposited in the ice it is this 10be enhancement in the ice that has been detected an ice core drilled at dome c antarctica is the oldest ice core so far retrieved1 on the basis of ice flow modelling and a comparison between the deuterium signal in the ice with climate records from marine sediment cores the ice at a depth of 3 190 m in the dome c core is believed to have been deposited around 800 000 years ago2 offering a rare opportunity to study climatic and environmental conditions over this time period however an independent determination of this age is important because the deuterium profile below a depth of 3 190 m depth does not show the expected correlation with the marine record2 here we present evidence for enhanced 10be deposition in the ice at 3 160 3 170 m which we interpret as a result of the low dipole field strength during the matuyama brunhes geomagnetic reversal which occurred about 780 000 years ago if correct this provides a crucial tie point between ice cores marine cores and a radiometric timescale

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

mean mixed depth of sediments the wherefore and the why the activities and consequently the bioturbational effects of deposit feeding organisms are largely restricted to a narrow surficial zone of marine sediments with a worldwide environmentally invariant mean of 9 8 cm with a standard deviation of 4 5 cm currently available theories of infaunal behavior cannot predict quantitatively this observation a new simple model that accounts for the feedback between resource food abundance its reactivity and the intensity of bioturbation leads to a quantitative estimate of 9 7 cm this model constitutes a fundamental advance in our understanding bioturbation

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

a 200 kyr record of cosmogenic radionuclide production rate and geomagnetic field intensity from 10be in globally stacked deep sea sediments the reconstruction of geomagnetic field intensity variations during the last 200 kyr from paleomagnetic data is at present the subject of numerous studies and major debate there is currently no generally accepted record here we present a global stacked record of 230thex normalized 10be deposition in marine sediments representing relative variations in 10be production rate which are translated into field intensity variations the record shows major periods during which the field intensity was between 10 and 40 of the present day value namely 30 42 60 75 85 110 and 180 192 kyr b p our results are compared to independently derived paleomagnetic studies and th u calibrations of 14c dates on corals during most of the observed period the geomagnetic field intensity was weaker than today resulting in an overall 30 reduced value for the last 200 kyr

the fate of magmas in planetesimals and the retention of primitive chondritic crusts abstract high abundances of short lived radiogenic isotopes in the early solar system led to interior melting and differentiation on many of the first planetesimals petrologic isotopic and paleomagnetic evidence suggests that some differentiated planetesimals retained primitive chondritic material the preservation of a cold chondritic lid depends on whether deep melts are able to ascend and breach the chondritic crust we evaluate the likelihood of melt ascent on a range of chondritic parent bodies we find that due to the efficient ascent of free volatiles in the gas and supercritical fluid phases at temperatures still below the solidus for silicates and metals mobile silicate melts on planetesimals were likely volatile depleted by calculating the densities of such melts we show that silicate melts likely breached crusts of enstatite chondrite compositions but did not ascend in the cv and cm parent bodies ordinary chondrite melts represent an intermediate case these predictions are consistent with paleomagnetic results from cv and cm chondrites as well as spectral observations of large e type asteroids

depositional remanent magnetization toward an improved theoretical and experimental foundation the first theoretical predictions for the behavior of magnetic particles in water were that sedimentary magnetizations would be fully aligned with the ambient field yet redeposition experiments showed a strong and quasi linear dependence on the external field this empirically observed linearity has served as the fundamental assumption of sedimentary paleointensity studies for decades we present redeposition experiments which suggest instead that the relationship between depositional remanence drm and applied field may frequently be curved for magnetic fields in the range of the earth s numerical simulations using a flocculation model can explain the redeposition data and suggest that drm will be significantly non linear when the flocs are small several microns there is a strong dependence of floc size on salinity particularly in low salinity environments floc size has a profound influence on the efficiency of drm hence low salinity environment may give results with poor reproducibility the size of the floc in which magnetic particles are embedded is not accounted for in current methods of normalization yet is the most important parameter on the bright side however it now seems possible to quantitatively explain paleointensity in sedimentary systems opening the door to absolute paleointensity estimates from sediments whose key parameters of floc size distribution and settling times can be constrained

highly resolved beryllium 10 record from odp site 1089 a global signal abstract in this study a highly resolved beryllium 10 record from a rapidly accumulating sediment core located in the southern cape basin odp leg 177 site 1089 is presented to extract the global 10 be production signal from the sedimentary record a dedicated correction procedure is applied first sediment redistribution is quantified by applying thorium 230 normalization then lateral transport of 10 be and 230 th is assessed using simple box model calculations the model results indicate that only minor transport corrections have to be applied thus site 1089 combines the advantage of good time resolution with nearly negligible oceanic transport of 10 be leading to a favored site for the reconstruction of global 10 be production changes from marine sediments our results suggest that the transport corrected 10 be record at site 1089 reflects long term variations of the global 10 be production rate over the past 300 kyr the absolute values of the youngest holocene samples are in good agreement with the estimated recent long term averaged global 10 be production rate deduced from observations the variability of the 10 be flux over the past 300 kyr corresponds well to the expected range deduced from model calculations the comparison with other reconstructions of geomagnetic paleointensity and with records of cosmogenic nuclides further emphasizes the global character of this 10 be record therefore our results show that it is possible to quantitatively extract the global 10 be production rate from a single marine record if transport processes are corrected for since the error of a single reconstruction is large a combination of several highly resolved and transport corrected 10 be records may lead to a significantly improved reconstruction of geomagnetic variability and of global cosmic ray flux and over the past 300 kyr

oxygen isotopes of chondritic components we review the oxygen isotopic compositions of chondrite components refractory inclusions chondrules and matrix and their inter and intra crystalline oxygen isotopic distributions primary oxygen isotopic compositions acquired before planetesimal accretion are easily disturbed by parent body processes such as aqueous alternation and thermal metamorphism primary or original oxygen isotopic compositions of refractory inclusions ca al rich inclusions and amoeboid olivine aggregates distribute along a slope 1 line on the three oxygen isotope diagram over the range of 60 17 o 18 o 16 o rich and 17 o 18 o rich during refractory inclusion formation on the other hand primary minerals of most chondrules have small isotopic variations and are enriched in 17 o and 18 o relative to refractory inclusions suggesting that chondrule formation occurred in 17 o 18 o rich nebular gas however rare 16 o rich chondrules have been found suggesting some overlap in the timing of formation of chondrules and refractory inclusions in the solar nebula oxygen isotopic compositions of matrix grains distribute along the slope 1 line over the same range as refractory inclusions and chondrules the similarity in oxygen isotopic distributions suggests that matrix was originally a mechanical mixture of nebular dusts co generated with chondrules and refractory inclusions presolar grains with oxygen isotopic compositions that are clearly distinct from those of solar nebular materials are a trace component of chondrite matrices based on these oxygen isotopic characteristics more than 99 5 of the solid materials in the nebula formed locally in the solar nebula and the remainder formed in interstellar space the astrophysical setting of chondrite component formation in the early solar system is also discussed refractory inclusions and 16 o rich matrix dusts formed around the inner edge of the solar nebula on the other hand most chondrules and 17 o 18 o rich matrix dusts seem to have formed elsewhere in the solar nebula efficient large scale radial mixing of the solar nebular materials may have been an essential process in the formation of chondritic planetesimals

matuyama brunhes reversal and kamikatsura event on maui paleomagnetic directions 40 ar 39 ar ages and implications abstract eighty nine basaltic lava flows from the northwest wall of haleakala caldera preserve a concatenated paleomagnetic record of portions of the matuyama brunhes m b reversal and the preceding kamikatsura event as well as secular variation of the full polarity reversed and normal geomagnetic field they provide the most detailed volcanic record to date of the m b transition the 24 flows in the transition zone show for the first time transitional virtual geomagnetic poles vgps that move from reverse to normal along the americas concluding with an oscillation in the pacific ocean to a cluster of vgps east of new zealand and back finally to stable polarity in the north polar region all but one of the 16 kamikatsura vgps cluster in central south america the full polarity flows with 40ar 39ar ages spanning a total of 680 kyr pass a reversal test and give an average vgp insignificantly different from the rotation axis with standard deviation consistent with that for other 0 5 ma lava flows of similar latitude precise 40ar 39ar dating consisting of 31 incremental heating experiments on 12 transitional flows yields weighted mean ages of 775 6 1 9 and 900 3 4 7 ka for the m b and kamikatsura transitional flows respectively this matuyama brunhes age is 16 kyr younger than ages for m b flows from the canary islands tahiti and chile that were dated using exactly the same techniques and standards suggesting that this polarity transition may have taken considerably longer to complete and been more complex than is generally believed for reversals

astronomical calibration of the matuyama brunhes boundary consequences for magnetic remanence acquisition in marine carbonates and the asian loess sequences we have compiled 19 records from marine carbonate cores in which the matuyama brunhes boundary mbb has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes by correlation of the 18o data to a timescale based on astronomical forcing we estimate astronomical ages for each of the mbb horizons in all but one record the mbb occurs within stage 19 most magnetostratigraphic sections in asian loess place the mbb within a loess interval since loess deposition is presumed to be associated with glacial intervals loess horizons should correspond to even numbered oxygen isotope stages a glacial age for the mbb is at odds with the results presented here which firmly place the mbb within interglacial stage 19 inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed the mean of the stage 19 age estimates for the mbb is 777 9 1 8 n 18 inclusion of the single stage 20 age results in a mean of 778 8 2 5 n 19 the astronomical age estimate of the mbb compares favorably with an unweighted mean of 778 2 3 5 n 10 from a compilation of 40ar 39ar results of transitional lava flows combining the two independent data sets yields a grand mean of 778 0 1 7 n 28 the new compilation shows virtually no trend in placement of the mbb within isotope stage 19 as a function of sediment accumulation rate we interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment water interface separating the cores into two geographic regions an indo pacific caribbean ipc group and an atlantic group results in a significant difference in the position of the mid point of the reversal with respect to the astronomical time scale the data presented here suggest a difference of several thousand years between the two regions this observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest systematic differences in remanence acquisition or by genuine differences in the timing of the directional changes between the two regions

metamorphism at a late mesozoic accretionary margin a study from the coast belt of the north american cordillera the late mesozoic and cenozoic metamorphic evolution of the western north american continental margin is recorded in a belt of homogeneous metapelitic rocks the kluane metamorphic assemblage kma in the northern coast belt of yukon territory a record of late cretaceous medium pressure and temperature c 7 kbar 500 c metamorphism m1 is preserved in ca rich garnet and na rich plagioclase cores in rocks that were little affected by later events m1 was synchronous with mylonitization and is attributed to accretion of the kma to the ancient continental margin isothermal decompression during rapid uplift was followed by early eocene emplacement of the ruby range batholith rrb part of a magmatic arc produced by subduction of the kula plate the intrusion of the rrb led to a contact metamorphic overprint m2 producing a 5 6 km wide aureole in which the grade ranges from subgarnet zone to garnet cordierite k feldspar zone pressure and temperature estimates for m2 calculated from mineral equilibria are 3 5 4 5 kbar and 530 720 c generally consistent with the stability limits of the observed mineral assemblages comparison of mineral assemblages and p t conditions in the kma with those in the mclaren glacier metamorphic belt in alaska does not support the correlation of the two metamorphic sequences this weakens the hypothesis proposing 400 km of dextral slip along the denali fault zone

orbital and millennial antarctic climate variability over the past 800 000 years a high resolution deuterium profile is now available along the entire european project for ice coring in antarctica dome c ice core extending this climate record back to marine isotope stage 20 2 800 000 years ago experiments performed with an atmospheric general circulation model including water isotopes support its temperature interpretation we assessed the general correspondence between dansgaard oeschger events and their smoothed antarctic counterparts for this dome c record which reveals the presence of such features with similar amplitudes during previous glacial periods we suggest that the interplay between obliquity and precession accounts for the variable intensity of interglacial periods in ice core records

a precursor to the matuyama brunhes transition field instability as recorded in pelagic sediments abstract the period some 20 25 kyr just prior to the most recent generally recognized geomagnetic field polarity transition the matuyama to brunhes reversal appears to be marked by significant geomagnetic variability manifested as pronounced oscillations in intensity we compare several previously published paleomagnetic records with new high resolution paleomagnetic data obtained from five pelagic marine sites north atlantic dsdp hole 609b equatorial atlantic odp hole 665a and western equatorial pacific odp holes 803b 804c and 805b using standard rock magnetic normalization for all of the samples as well as a thellier thellier method on the sediments of hole 804c we consistently find a decrease in paleointensity dip of 1 approximately 15 kyr prior to the matuyama to brunhes transition in the five new records as well as in the previously published records despite sedimentation accumulation rates sar that range from 11 cm kyr to 1 cm kyr these sequences yield paleointensity curves that are broadly similar in form even at the lowest sars the intensity of the pre reversal low dip1 appears to be of the same magnitude as that of the transition itself dip2 of 1 in some of the records a directional excursion to nearly full normal polarity accompanies dip1 and remains after alternating field af and or thermal demagnetization whereas in other records the directional changes vanish with demagnetization and appear to be caused by overprinting a viscous remanent magnetization vrm contribution to nrm was identified in two of the records and until removed by af or thermal demagnetization was found to blur the double dip nature of the paleointensity profiles into an apparent single dip and also resulted in an apparent but erroneous sawtooth like post transitional sudden increase in paleointensity after appropriate normalization the magnitude of the post transitional recovery was much reduced the magnetic directions of three of the new records after cleaning and adjusting the stable declinations to either 0 normal or 180 reverse map to vgp positions lying in the pacific the directional variations however are far less consistent than the intensity variations the confirmed global existence of this dip so closely preceding a major reversal invites questions about its relation to the reversal itself the apparent normal character of this interval can also present problems for magnetostratigraphical interpretations based on coarse or incomplete sampling by mimicking the brunhes matuyama reversal some 15 kyr earlier in the section than its true location

is burial velocity a master parameter for bioturbation abstract the intensity of biological mixing in a marine sediment is widely believed to be related to the flux of labile organic matter at the same time this flux has been shown to be strongly correlated to the burial velocity one would then expect a significant correlation between biological mixing coefficient d b and burial velocity a nonlinear regression of existing data with a power law eqns 2 and 3 of the text produces a statistically significant correlation the statistical significance of the regression in no way implies causation and in fact it accounts for only about 22 of the total variance in the dataset thus reducing its physical significance a number of additional processes contribute to the large observed variance including timing of mixing events temporal and spatial heterogeneity differences in temporal scales sampling artifacts and specific particle associations with due care the d b relationship may prove useful in defining the likely range of d b values in a particular environment a similar analysis of the tracer identified surface mixed depth shows that it is essentially independent of burial velocity with a worldwide mean of 9 8 4 5 cm depth of mixing is probably more closely related to the energetic costs of deep burrowing

stable calcium isotopic composition of meteorites and rocky planets abstract new measurements of mass dependent calcium isotope effects in meteorites lunar and terrestrial samples show that earth moon mars and differentiated asteroids e g 4 vesta and the angrite and aubrite parent bodies are indistinguishable from primitive ordinary chondritic meteorites at our current analytical resolution 0 07 sd for the 44 ca 40 ca ratio in contrast enstatite chondritic meteorites are slightly enriched in heavier calcium isotopes ca 0 5 and primitive carbonaceous chondritic meteorites are depleted in heavier calcium isotopes ca 0 5 the calcium isotope effects cannot be easily ascribed to evaporation or intraplanetary differentiation processes the isotopic variations probably survive from the earliest stages of nebular condensation and indicate that condensation occurred under non equilibrium undercooled nebular gas conditions some of this early high temperature calcium isotope heterogeneity is recorded by refractory inclusions niederer and papanastassiou 1984 and survived in planetesimals but virtually none of it survived through terrestrial planet accretion the new calcium isotope data suggest that ordinary chondrites are representative of the bulk of the refractory materials that formed the terrestrial planets enstatite and carbonaceous chondrites are not the enrichment of light calcium isotopes in bulk carbonaceous chondrites implies that their compositions are not fully representative of the solar nebula condensable fraction

conditions for pore water convection within carbonaceous chondrite parent bodies implications for planetesimal size and heat production abstract the critical rayleigh number for convective flow of pore water through a permeable spherical body is derived the result affords constraints on carbonaceous chondrite planetesimal sizes and rates of heat production if the chemical compositions of carbonaceous chondrites are taken as evidence against flow of pore water then the present calculations suggest that the parent bodies were less than 80 km in diameter the maximum diameter for absence of flow depends in part on the high permeabilities required by estimates of high water rock ratios in altered carbonaceous chondrites pore water would have flowed through parent bodies larger than 120 km in diameter with typical rock like permeabilities convective flow and isochemical alteration are not compatible and our calculations can be used to identify the combinations of parent body size and heat generation consistent with one or the other

14c activity and global carbon cycle changes over the past 50 000 years a series of 14c measurements in ocean drilling program cores from the tropical cariaco basin which have been correlated to the annual layer counted chronology for the greenland ice sheet project 2 gisp2 ice core provides a high resolution calibration of the radiocarbon time scale back to 50 000 years before the present independent radiometric dating of events correlated to gisp2 suggests that the calibration is accurate reconstructed 14c activities varied substantially during the last glacial period including sharp peaks synchronous with the laschamp and mono lake geomagnetic field intensity minimal and cosmogenic nuclide peaks in ice cores and marine sediments simulations with a geochemical box model suggest that much of the variability can be explained by geomagnetically modulated changes in 14c production rate together with plausible changes in deep ocean ventilation and the global carbon cycle during glaciation

in phase anomalies in beryllium 10 production and palaeomagnetic field behaviour during the iceland basin geomagnetic excursion increases in the production rate of cosmogenic radionuclides associated with geomagnetic excursions have been used as global tie points for correlation between records of past climate from marine and terrestrial archives we have investigated the relative timing of variations in 10be production rate and the corresponding palaeomagnetic signal during one of the largest pleistocene excursions the iceland basin ib event ca 190 kyr as recorded in two marine sediment cores odp sites 1063 and 983 with high sedimentation rates variations in 10be production rate during the excursion were estimated by use of 230thxs normalized 10be deposition rates and authigenic 10be 9be resulting 10be production rates are compared with high resolution records of geomagnetic field behaviour acquired from the same discrete samples we find no evidence for a significant lock in depth of the palaeomagnetic signal in these high sedimentation rate cores apparent lock in depths in other cores may sometimes be the result of lower sample resolution our results also indicate that the period of increased 10be production during the ib excursion lasted longer and most likely started earlier than the corresponding palaeomagnetic anomaly in accordance with previous observations that polarity transitions occur after periods of reduced geomagnetic field intensity prior to the transition the lack of evidence in this study for a significant palaeomagnetic lock in depth suggests that there is no systematic offset between the 10be signal and palaeomagnetic anomalies associated with excursions and reversals with significance for the global correlation of climate records from different archives

progressive alteration in cv3 chondrites more evidence for asteroidal alteration abstract the oxidized cv3 chondrites can be divided into two major subgroups or lithologies bali like cv3oxb and allende like cv3oxa in which chondrules calcium aluminum rich inclusions cais and matrices show characteristic alteration features weisberg et al 1997 krot et al 1997d kimura and ikeda 1997 the cv3oxb lithology is present in bali kaba parts of the mokoia breccia and possibly in grosnaja and allan hills alh 85006 it is characterized by the presence of the secondary low ca phyllosilicates saponite and sodium phlogopite magnetite ni rich sulfides fayalite fa 90 ca fe rich pyroxenes fs10 50wo45 50 and andradite phyllosilicates replace primary ca rich minerals in chondrules and cais which suggests mobilization of ca during aqueous alteration magnetite nodules are replaced to various degrees by fayalite ca fe rich pyroxenes and minor andradite fayalite veins crosscut fine grained rims around chondrules and extend into the matrix thermodynamic analysis of the observed reactions indicates that they could have occurred at relatively low temperatures 300 c in the presence of aqueous solutions oxygen isotopic compositions of the coexisting magnetite and fayalite plot close to the terrestrial fractionation line with large 18ofayalite magnetite fractionation 20 we infer that phyllosilicates magnetite fayalite ca fe rich pyroxenes and andradite formed at relatively low temperatures 300 c by fluid rock interaction in an asteroidal environment secondary fayalite and phyllosilicates are virtually absent in chondrules and cais in the cv3oxa lithology which is present in allende and its dark inclusions axtell alha81258 alh 84028 lewis cliff lew 86006 and parts of the mokoia and vigarano breccias instead secondary nepheline sodalite and fayalitic olivine are common fayalitic olivine in chondrules replaces low ca pyroxenes and rims and veins forsterite grains it also forms coarse lath shaped grains in matrix secondary ca fe rich pyroxenes are abundant we infer that the cv3oxa lithology experienced alteration at higher temperatures than the cv3oxb lithology the presence of the reduced and cv3oxa lithologies in the vigarano breccia and cv3oxa and cv3oxb lithologies in the mokoia breccia indicates that all cv3 chondrites came from one heterogeneously altered asteroid the metamorphosed clasts in mokoia krot and hutcheon 1997 may be rare samples of the hotter interior of the cv asteroid we conclude that the alteration features observed in the oxidized cv3 chondrites resulted from the fluid rock interaction in an asteroid during progressive metamorphism of a heterogeneous mixture of ices and anhydrous materials mineralogically similar to the reduced cv3 chondrites

chondritic lithic clasts in the cb ch like meteorite isheyevo fragments of previously unsampled parent bodies the cb ch like chondrite isheyevo is characterized by the absence of fine grained interchondrule matrix material the only present fine grained material is found as chondritic lithic clasts in contrast to the pristine high temperature components of isheyevo these clasts experienced extensive aqueous alteration in an asteroidal setting hence the clasts are foreign objects that either accreted together with the high temperature components or were added later to the final isheyevo parent body during regolith gardening in order to constrain the origin and secondary alteration of the clasts in isheyevo we studied their mineralogy petrography structural order of the polyaromatic carbonaceous matter and oxygen isotopic compositions of carbonates three main groups of clasts were defined based on mineralogy and petrology group i clasts consist of phyllosilicates carbonates magnetite and lath shaped fe ni sulfides group ii clasts contain different abundances of anhydrous silicates embedded in a hydrated matrix sulfides magnetite and carbonates are rare with only a few exceptions groups i and ii clasts did not experienced significant thermal metamorphism group iii clasts are characterized by the absence of magnetite and the presence of fe ni metal in addition to aqueous alteration they experienced thermal metamorphism as reflected by the structure of their polyaromatic carbonaceous matter while there are some similarities between the isheyevo clasts ci chondrites and the matrices of cm and cr chondrites on the whole the characteristics of the clasts do not match those of any of these aqueously altered meteorite classes nor do they match those of similar material in various types of chondritic clasts present in other meteorite groups we conclude that the isheyevo clasts represent fragments of previously unsampled parent bodies

an ice core perspective on the age of the matuyama brunhes boundary abstract two intervals of enhanced 10be flux thought to be associated with periods of low dipole intensity and identified as the matuyama brunhes transition and a precursor event have been observed in the bottom section of the epica dome c ice core the peaks span 764 776 ka and 788 798 ka on the new edc3 chronology with a stated absolute age uncertainty of 6 ka 2 this chronology uses orbital tuning of atmospheric oxygen 18 18oatm to correct for anomalies in ice flow in the bottom 500 m of the core an additional 28 18oatm data points have been measured to improve resolution and verify the accuracy of the tuning and the stated timescale uncertainty both the dating of the increased 10be and that relative to climatic records are compared to paleointensity records found in orbitally tuned marine sediments the mid point of the 10be peak associated with the m b is approximately 10 ka younger than the age determined radioisotopically from lavas with transitional orientations taking into account recent revisions to the 40ar 39ar dating standard and improved precision climatic constraints on the edc3 agescale make an error of this magnitude in the ice chronology implausible this age difference however is consistent with recent modeling suggesting that directional changes are spatially asynchronous and may precede the dipole intensity minimum in some locations although formally less precise than the published age from astrochronologically dated marine sediments ice core ages are potentially more accurate because they are not subject to lock in depth uncertainties

record of low temperature alteration in asteroids most chondritic materials experienced diverse styles of secondary alteration resulting in formation of hydrous and anhydrous oxygen bearing minerals silicates oxides and carbonates phyllosilicates magnetite calcite dolomite breunnerite ferrous olivine hedenbergite wollastonite grossular monticellite forsterite andradite nepheline sodalite brearley 2003 2005 and references therein petrographic mineralogic oxygen and short lived isotope systematics 26al 26mg 53mn 53cr 129i 129xe suggest that alteration occurred in the presence of aqueous solutions under variable physico chemical conditions temperature water rock ratio ph f o2 and fluid compositions in an asteroidal setting it started within 1 2 m y after formation of the cv cais was multistage and lasted up to 15 m y krot et al 2006 and references therein here we review bulk o isotopic compositions of chondritic materials o isotopic compositions of secondary minerals produced during asteroidal alteration and possible effects of fluid assisted thermal metamorphism on o isotopic exchange in primary high temperature minerals melilite and anorthite in the cv and co cais the implications of these data for understanding temperatures of alteration water rock ratios and oxygen isotopic composition of water that probably accreted into chondrite parent bodies in the form of ice are discussed the inferred or measured o isotopic composition of the meteoritic water is close to the terrestrial fractionation line 17o 2 it is very different from the inferred o isotopic compositions of the sun 17o 25 clayton 2002 yurimoto and kuramoto 2004 hashizume and chaussidon 2005 lyons and young 2005 suggesting significant evolution of o isotopic composition of the gaseous reservoir in the inner protoplanetary disk over its short 3 5 m y life time

the oxygen isotope evolution of parent body aqueous solutions as recorded by multiple carbonate generations in the lonewolf nunataks 94101 cm2 carbonaceous chondrite abstract the cm2 carbonaceous chondrite lon 94101 contains aragonite and two generations of calcite that provide snapshots of the chemical and isotopic evolution of aqueous solutions during parent body alteration aragonite was the first carbonate to crystallize it is rare heterogeneously distributed within the meteorite matrix and its mean oxygen isotope values are 18o 39 9 0 6 17o 0 3 1 0 1 calcite precipitated soon afterwards and following a fall in solution mg ca ratios to produce small equant grains with a mean oxygen isotope value of 18o 37 5 0 7 17o 1 4 1 1 1 these grains were partially or completely replaced by serpentine and tochilinite prior to precipitation of the second generation of calcite which occluded an open fracture to form a millimetre sized vein and replaced anhydrous silicates within chondrules and the matrix the vein calcite has a mean composition of 18o 18 4 0 3 17o 0 5 0 5 1 petrographic and isotopic results therefore reveal two discrete episodes of mineralisation that produced calcite generations with contrasting 18o and mean 17o values the aragonite and equant calcite crystallized over a relatively brief period early in the aqueous alteration history of the parent body and from static fluids that were evolving chemically in response to mineral dissolution and precipitation the second calcite generation crystallized from solutions of a lower 17o and a lower 18o and or higher temperature as two generations of calcite whose petrographic characteristics and oxygen isotopic compositions are similar to those in lon 94101 occur in at least one other cm2 multiphase carbonate mineralisation could be the typical outcome of the sequence of chemical reactions during parent body aqueous alteration it is equally possible however that the second generation of calcite formed in response to an event such as impact fracturing and concomitant fluid mobilisation that affected a large region of the common parent body of several cm2 meteorites these findings show that integrated petrographic chemical and isotopic studies can provide new insights into the mechanisms of parent body alteration including the spatial and temporal dynamics of the aqueous system

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

time dependent oxygen isotopic effects of co self shielding across the solar protoplanetary disk abstract optically thin surfaces of the solar circumstellar disk were likely sites for generating 16 o isotope variability in the early solar system astrochemical reaction network calculations predict that a robust feature of these photoactive horizons of the disk was conversion of co gas to 16 o poor high 17 o h 2 o ice on a timescale of 10 5 yr within several au of the central star ultraviolet fluxes were too great for the oxygen isotopic effects of co photodissociation to be sequestered in h 2 o meaning that the co self shielding oxygen isotopic effect was an outer disk phenomenon calculations depicting transport in the circumstellar disk suggest that co photodissociation at disk surfaces triggered a wave of high 17 o h 2 o that passed from surface regions through the outer disk and into the rocky planet forming region on a timescale of 10 5 to 10 6 yr

fluid flow and chemical alteration in carbonaceous chondrite parent bodies abstract carbonaceous chondrites are a class of primitive meteorites that likely represent early solar system materials thus providing critical insights about planetary evolution however evidence shows that they have undergone considerable processing elucidating the complex formation history of aqueously altered chondrite groups has been a principal objective of cosmochemistry fulfilling this objective requires understanding the nature of the water rock interactions involved many current alteration models are generally founded upon an assumption of uniform alteration in a closed system in these models different chondrite groups must originate on separate parent bodies contrary to this common view we show that chondrite data are consistent with non uniform alteration in a convecting system so that multiple chondrite groups can form within a single body

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

a geomagnetic paleointensity stack between 0 8 and 3 0 ma from equatorial pacific sediment cores 1 we have conducted a paleomagnetic study of six sediment cores taken from the equatorial pacific three from the west caroline basin and three from the manihiki plateau in order to make a relative paleointensity stack during the matuyama and late gauss chrons the age of the bottom of the cores ranges from 1 2 to 3 0 ma the sediments show little downcore changes in the proxies of magnetic grain size and mineralogy and are hence suitable for estimating relative paleointensity the age of the cores is controlled by correlating variations in magnetic concentration magnetic susceptibility and or anhysteretic remanent magnetization arm to target oxygen isotope 18o curves all cores from the manihiki plateau show an upward decrease of the natural remanent magnetization nrm intensity normalized by arm and isothermal remanent magnetization irm with a decrease in sedimentation rate such a trend was removed before being converted to relative paleointensity this observation implies that a sedimentation rate change can affect relative paleointensity estimation relative paleointensity records from the six cores coincide well with each other within uncertainty of age we constructed a stacked curve between 0 8 and 3 0 ma the equatorial pacific paleointensity stack epapis 3ma after adjusting age the number of cores stacked is three to four throughout the record quasiperiodic paleointensity lows occur in the epapis 3ma and corresponding paleointensity lows can be found in previously published records with some shifts in age at least parts of the paleointensity minima seem to be accompanied by geomagnetic excursions the asymmetric sawtooth pattern of paleointensity variations is not observed at the matuyama gauss boundary and thereafter in our record a spectral analysis shows that 100 kyr orbital eccentricity frequency may exist in paleointensity variations during the matuyama and late gauss chrons

why aqueous alteration in asteroids was isochemical high porosity high permeability carbonaceous chondrite meteorites are the most compositionally primitive rocks in the solar system but the most chemically pristine ci1 and cm2 chondrites have experienced pervasive aqueous alteration apparently within asteroid parent bodies unfractionated soluble elements suggest very limited flow of liquid water indicting a closed system at scales large than 100 s m consistent with data from oxygen isotopes and meteorite petrography however numerical studies persistently predict large scale 10 s km water transport in model asteroids either in convecting cells or via exhalation flow an open system at scales up to 10 s km these models have tended to use permeabilites in the range 10 13 to 10 11 m2 we show that the permeability of plausible chondritic starting materials lies in the range 10 19 to 10 17 m2 0 1 10 d around six orders of magnitude lower than previously assumed this low permeability is largely a result of the extreme fine grain size of primitive chondritic materials applying these permeability estimates in numerical models we predict very limited liquid water flow distances of 100 s m at most even in a high porosity water saturated asteroid with a high thermal gradient over millions of years isochemical alteration with flow over minimal lengthscales is not a special circumstance it is inevitable once we consider the fundamental material properties of these rocks to achieve large scale flow it would require average matrix grain sizes in primitive materials of 10 s 100 s m orders of magnitude larger than observed finally in addition to reconciling numerical modelling with meteorite data our work explains several other features of these enigmatic rocks most particularly why the most chemically primitive meteorites are also the most altered

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

petrographic and oxygen isotopic study of refractory forsterites from r chondrite dar al gani 013 r3 5 6 unequilibrated ordinary and carbonaceous chondrites we have conducted petrographic chemical and in situ oxygen isotopic studies of refractory forsterites from unequilibrated ordinary and carbonaceous chondrites as well as an unequilibrated r chondrite refractory forsterites occur in all types of unequilibrated chondrites and all have very similar chemical composition with low feo and high refractory lithophile element rle contents refractory forsterites are typically enriched in 16o relative to normal olivine independent of the bulk o isotope ratios of the parent meteorites analyses of refractory forsterites spread along a 17o mixing line with 17o ranging from 2 to 10 due to similarities in oxygen isotopes and chemical compositions we conclude that refractory forsterites of various types of chondrites come from a single common reservoir implications of this hypothesis for the chemical and o isotope evolution of silicates in the early solar nebular are discussed

oxygen isotopes in separated components of ci and cm meteorites oxygen isotopic compositions have been determined for whole rock and separated components from the ci chondrites alais ivuna and orgueil and from the atypical cm chondrites bells and essebi the composition of ci matrix is in agreement with previous measurements and is systematically different from that of cm matrix probably due to warmer and wetter conditions of aqueous alteration for ci matrix although bells matrix is like that of other cm chondrites the matrix of essebi falls in the ci field essebi also contains abundant magnetite like cis and unlike other cms the isotopic compositions of magnetite in alais ivuna orgueil and essebi define a trend with 17o of 1 3 to 1 8 distinctly out of isotopic equilibrium with the phyllosilicates in each meteorite the magnetite serves as an isotopic tracer for the fluid which brought about aqueous alteration and oxidation and thus provides additional evidence for a gaseous reservoir in the solar nebula which was more 16o depleted than the earth

oxygen isotopic variations in the outer margins and wark lovering rims of refractory inclusions abstract oxygen isotopic variations across the outer margins and wark lovering wl rims of a diverse suite of six coarse grained types a and b refractory inclusions from both oxidized and reduced cv3 chondrites suggest that cais originated from a 16 o rich protosolar gas reservoir and were later exposed to both relatively 17 18 o rich and 16 o rich reservoirs the o isotope profiles of cais can be explained by changes in the composition of gas near the protosun or the migration of cais through a heterogeneous nebula variability within the inclusion interiors appears to have been set prior to wl rim growth modeling the isotopic zoning profiles as diffusion gradients between inclusion interiors and edges establishes a range of permissible time temperature combinations for their exposure in the nebula at mean temperatures of 1400 k models that match the isotope gradients in the inclusions yield timescales ranging from 5 10 3 to 3 10 5 years assuming cais originated with a relatively 16 o rich protosolar isotopic composition differences among the melilite interiors and the isotopic gradients in their margins imply the existence of a number of isotopically distinct reservoirs evidence at the edges of some cais for subsequent isotopic exchange may relate to the beginning of rim formation in the wl rim layers surrounding the interiors spinel is relatively 16 o rich but subtly distinct among different cais melilite is often relatively 16 o poor but rare relatively 16 o rich grains also exist pyroxene generally exhibits intermediate o isotope compositions and isotopic zoning olivine in both wl and accretionary rims when present is isotopically heterogeneous the extreme isotopic heterogeneity among and within individual wl rim layers and in particular the observed trends of outward 16 o enrichments suggest that rims surrounding cais contained in cv3 chondrites like the inclusions themselves formed from a number of isotopically distinct gas reservoirs collectively these results support numerical protoplanetary disk models in which cais were transported between several distinct nebular reservoirs multiple times prior to accretion onto a parent body

geomagnetic paleointensity and environmental record from labrador sea core md95 2024 global marine sediment and ice core chronostratigraphy for the last 110 kyr piston core md95 2024 from the labrador rise provides a continuous record of rapidly deposited detrital layers denoting laurentide ice sheet lis instability the core also provides a high resolution record of geomagnetic paleointensity that is consistent with but at higher temporal resolution than previous labrador sea records correlation to the greenland summit ice cores grip gisp2 is achieved by assuming that labrador sea detrital layers correspond to cold stadials in the ice cores this allows a gisp2 official chronology to be placed on md95 2024 which is consistent with the inverse correlation between paleointensity and the flux of cosmogenic isotopes 10 be and 36 cl in greenland ice cores synchronous millennial scale variability observed from the md95 2024 paleointensity

paleointensity record from pleistocene sediments 1 4 0 ma off the california margin high resolution magnetic measurements of 45 m of sediment from the delgada fan area at ocean drilling program site 1021 39 5 n 127 46 w were performed in order to extract a reliable signal of relative paleointensity a detailed chronology was established by correlation of the magnetic susceptibility variations to the insolation curve for the past 1 4 myr covered by the record the remanence carrier is pseudo single domain sized magnetite that varies in concentration by a factor 3 stepwise alternating field demagnetization isolated a very stable natural remanent magnetization nrm which showed the succession of the cobb and jaramillo subchrons and directional changes associated with the punaruu event 1 105 ma two techniques have been used to extract the signal of relative paleointensity a significant climatic component remained present in the curve obtained from the slopes of the nrm versus the anhysteretic remanent magnetization arm in contrast the nrm and the susceptibility k lf both being driven by orbital forcing normalization by susceptibility is not contaminated by climatically driven changes and is thus taken as our best estimate of relative paleointensity comparison with other high resolution records and compilations reinforces the actual geomagnetic origin of the variations displayed by the nrm k signal overall this record confirms the succession of distinct highs and lows which are concomitant with full field reversals and short term geomagnetic events present during the matuyama chron the presence of large oscillations of the field with no periodicity confirms that the geomagnetic field at least his dipolar part is highly unstable and may have remained in this mode for a long period of time

thermal evolution and physics of melt extraction on the ureilite parent body we develop a physical model of the thermal history of the ureilite parent body upb that numerically tracks the history of its heating hydration dehydration partial melting and smelting as a function of its formation time and the initial values of its composition formation temperature and water ice content petrologic and chemical data from the main group non polymict ureilite meteorites which sample the interior of the upb between depths corresponding to pressures in the range 3 10 mpa are used to constrain the model we find that to achieve the 30 melting inferred for ureilites from all sampled depths the upb must have had a radius between 80 and 130 km and must have accreted about 0 55 ma after cai formation melting began in the body at 1 ma after cai and the time at which 30 melting was reached varied with depth in the asteroid but was always between 4 5 and 5 8 ma after cai the total rate at which melt was produced in the upb varied from more than 100 m 3 s 1 in the very early stages of melting at 1 ma after cai to 5m 3 s 1 between 2 and 3 ma after cai decreasing to extremely small values as the end of melting was approached beyond 5 ma although the initial period of high melt

carbonaceous chondrites as analogs for the composition and alteration of ceres the mineralogy and geochemistry of ceres as constrained by dawn s instruments are broadly consistent with a carbonaceous chondrite cm ci bulk composition differences explainable by ceres s more advanced alteration include the formation of mg rich serpentine and ammoniated clay a greater proportion of carbonate and lesser organic matter amounts of magnetite sulfide and carbon that could act as spectral darkening agents and partial fractionation of water ice and silicates in the interior and regolith ceres is not spectrally unique but is similar to a few other c class asteroids which may also have suffered extensive alteration all these bodies are among the largest carbonaceous chondrite asteroids and they orbit in the same part of the main belt thus the degree of alteration is apparently related to the size of the body although the ammonia now incorporated into clay likely condensed in the outer nebula we cannot presently determine whether ceres itself formed in the outer solar system and migrated inward or was assembled within the main belt along with other carbonaceous chondrite bodies

an atlantic lead over pacific deep water change across termination i implications for the application of the marine isotope stage stratigraphy globally synchronous changes in glacioeustatic sea level provide a particularly useful stratigraphic reference in the study of glacial interglacial climate change and form the basis for the marine isotope stage mis system which posits a dominant glacioeustatic signal in benthic foraminferal 18o however benthic 18o strictly does not represent a pure sea level record and it is shown here that due to local changes in deep water hydrography the mis 2 1 boundary i e benthic 18o change occurred 4000 years later in the deep eastern equatorial pacific than in the deep northeast atlantic this discrepancy is too large to be attributed entirely to the pacific lag that may be expected due to the mixing time of the ocean basins instead multiple proxies including benthic foraminiferal mg ca suggest that the large offset in benthic 18o most probably resulted from a late rise in deep water temperature at the pacific site and a series of coupled deep water temperature and 18o changes at the northeast atlantic site these findings have important implications for the application of the mis stratigraphy and the interpretation of benthic 18o records in general as they show that benthic 18o fluctuations and mis boundaries from different hydrological settings may be significantly diachronous these results call into question the leads and lags that we might infer from a naive interpretation of benthic 18o as a sea level proxy with a direct bearing on the mechanisms that we may propose to explain glacial interglacial climate transitions

beryllium 10 in deep sea sediments a tracer for the earth s magnetic field intensity during the last 200 000 years abstract we present a new approach for the reconstruction of the earth s magnetic field intensity during the last 200 000 years based on 10 be in deep sea sediments a simple box model that is able to describe boundary scavenging of 10 be in the ocean is used to study the sensitivity of 10 be to changes of water circulation and or varying particle rain rates the model is applied to separate transport and production signal in single 10 be records vertical sediment accumulation rates are used as input values for the model calculations the calculated scavenging factors are applied to correct the 10 be records for boundary scavenging the model corrected 10 be based paleointensity record correlates well with other records from very different archives

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

post depositional remanent magnetization lock in and the location of the matuyama brunhes geomagnetic reversal boundary in marine and chinese loess sequences article i nfo pdrm lock in loess paleosol bioturbation disturbs detrital magnetic particles after deposition so accurate recording of the ancient geomagnetic field in bioturbated sediments is widely attributed to acquisition of a post depositional remanent magnetization pdrm whereby the geomagnetic field exerts a torque on a magnetic particle and aligns it with the field after the final mixing event experienced by the particle the relationship between the matuyama brunhes boundary mbb and oxygen isotope age tie points in marine sediments has been widely used to determine the depth at which the paleomagnetic signal is locked in however such analyses can be badly affected by age discrepancies among different paleoclimatic proxies and by varying isotopic compositions of seawater in different locations and from the presence of different water masses at different depths at the same location it is therefore necessary to separately compare paleomagnetic data with respect to either benthic or planktonic foraminiferal oxygen isotope records for sites from the same water mass to avoid inadvertently introducing age differences to the analysis when the global data set is subjected to such a rigorous analysis few reliable data remain for the mbb using two complementary approaches we estimate that the mbb is on average shifted 20 cm below its true position in marine sediments this offset is the sum of the thickness of the bioturbated surface mixed layer which is possibly dominant and the pdrm lock in depth there is also controversy concerning observed differences in the position of the mbb relative to paleoclimatic proxies in marine sediments and chinese loess deposits for the chinese loess quartz grain size is insensitive to pedogenic alteration and is a useful parameter for determining the true position of the mbb with respect to paleoclimatic boundaries we conclude that the mbb occurs late in marine oxygen isotope stage 19 and in the upper part of chinese paleosol unit s8 rather than at the mid or lower part of loess unit l8 our results require adjustment of the generally accepted positions for the mbb and resolve a longstanding chronological conundrum for marine and chinese loess sequences

depth of post depositional remanence acquisition in deep sea sediments a case study of the brunhes matuyama reversal and oxygen isotopic stage 19 1 although post depositional magnetization pdrm in deep sea sediments appears to be acquired during the earliest stages of sediment compaction the natural variability of the pdrm lock in depth in deep sea sediments is poorly understood and as yet unquantified here we consider variations in the relative stratigraphic positions of oxygen isotopic interglacial stage 19 1 and the brunhes matuyama b m chronozone reversal for eight deep sea sediment cores results from a similar study of the displacement between a widespread microtektite layer and the b m boundary are also included 1 the pdrm lock in depth and the temporal relationships between the b m and stage 19 1 datums can be determined from the offsets between the paleomagnetic and the isotopic and microtektite stratigraphies for cores with sedimentation rates greater than 1 cm kyr a the depth offset between the paleomagnetic and isotopic datums is a linear function of sedimentation rate a simple model r 2 0 77 demonstrates that 1 pdrm acquisition occurs 16 cm below the sediment surface and 2 the b m reversal occurs 6 kyr 4 2 kyr after the stage 19 1 datum and the stage 19 1 datum occurs 9 kyr 4 3 kyr after the deposition of the australasian microtektite strewnfield this example argues against simple geophysical models linking geomagnetic field reversals to climate change or impact events the b m boundary is anomalously deep 30 50 cm in very low accumulation rate sediments 1 cm kyr 1 and this may reflect the unusual physical properties of these sediments a review of the geotechnical literature suggests that very low accumulation rate sediments have sufficient time to develop enhanced interparticle rigidity structural strength which inhibits early compaction and hence pdrm acquisition

cosmogenic nuclides during isotope stages 2 and 3 abstract changes in the concentration of cosmogenic nuclides measured in natural archives are related mainly to changes in the cosmic ray induced production rate in the atmosphere but also to the subsequent transport and deposition processes which depend on the geochemical properties of the nuclides a comparison of 10 be and 36 cl data from ice cores from greenland ice cores and 14 c data from tree rings and other archives provides the following information for the isotope stages 2 and 3 1 changes in the cosmogenic nuclide production rate on time scales longer than approximately 2 kyr agree well with paleomagnetic data derived from deep sea sediments in particular the geomagnetic excursions leading to weak fields around 40 kyr bp laschamp and 32 kyr bp mono lake can be clearly seen 2 production variations on shorter time scales contain information on solar variability as an example the 205 yr devries cycle already known from the analysis of 14 c tree ring data during the holocene is also present in the 10 be data of the time period 25 55 kyr bp 3 a comparison of 14 c calculated using 10 be data with the measured 14 c record points to a reduction of the global deep water formation during the younger dryas cold event 4 using information available on the production rates of cosmogenic nuclides in the past derived from paleomagnetic data the paleoprecipitation rate can be calculated this is crucial for establishing precise time scales

comprehensive study of carbon and oxygen isotopic compositions trace element abundances and cathodoluminescence intensities of calcite in the murchison cm chondrite we have performed in situ analyses of c and o isotopic compositions trace element concentrations and cathodoluminescence cl intensities on calcite in murchison a weakly altered cm chondrite we found that the trace element mg mn and fe concentrations are heterogeneous within single calcite grains grain to grain heterogeneity is even more pronounced the analyzed calcite grains can be separated into two distinct types with respect to their c isotopic ratios trace element concentrations and cl characteristics calcite grains with higher 13cpdb values 75 have low trace element concentrations and uniformly dark cl while grains with lower 13c values 35 have higher trace element concentrations and cl zoning in contrast to the c isotopic ratios o isotopic ratios are similar for both types of calcites 18osmow 34 the o isotopic ratios trace element concentrations and cl characteristics provide no evidence for c isotope evolution in fluids from a single c reservoir by rayleigh type isotope fractionation i e removal of c bearing gaseous species also it seems difficult to explain the o and c isotopic compositions of the two types of calcites by their formation at different temperatures from a single fluid instead the 13c variation suggests the presence of at least two c reservoirs with different isotopic ratios in the aqueous fluids from which the calcites precipitated the c reservoirs with lower 13c values are likely to be organic matter the same holds for the c reservoirs with higher 13c values which might have significant contributions from the 13c enriched grains identified in meteoritic insoluble organic matter thermodynamic calculations show that calcite with lower fe concentrations formed under more reduced conditions than calcite with higher fe concentrations if this is the case the 13c rich organic grains may have been destroyed and dissolved in the fluids under more reduced conditions than other organic components the fact that the two types of calcites were found in different domains in the same thin section suggests that microenvironments with diverse physicochemical conditions such as redox states were present at scales of 100 s m

millennial scale instability in the geomagnetic field prior to the matuyama brunhes reversal changes in the earth s magnetic field have global significance that reach from the outer core extending out to the uppermost atmosphere paleomagnetic records derived from sedimentary and volcanic sequences provide important insights into the geodynamo processes that govern the largest geomagnetic changes polarity reversals but dating uncertainties have hindered progress in this understanding here we report a paleomagnetic record from multiple lava flows on tahiti that bracket the matuyama brunhes m b polarity reversal 771 thousand years ago our high precision 40 ar 39 ar ages constrain several rapid and short lived changes in field orientation up to 33 000 years prior to the m b reversal these changes are similar to ones identified in other less well dated lava flows in maui chile and la palma that occurred during an extended period of reduced field strength recorded in sediments we use a simple stochastic model to show that these rapid polarity changes are highly attenuated in sediment records with low sedimentation rates this prolonged 33 000 year period of reduced field strength and increased geomagnetic instability supports models that show frequent centennial to millennial scale polarity changes in the presence of a strongly weakened dipole field

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

magma oceans in the inner solar system theory and observations point to the occurrence of magma ponds or oceans in the early evolution of terrestrial planets and in many early accreting planetesimals the apparent ubiquity of melting during giant accretionary impacts suggests that silicate and metallic material may be processed through multiple magma oceans before reaching solidity in a planet the processes of magma ocean formation and solidification therefore strongly influence the earliest compositional differentiation and volatile content of the terrestrial planets and they form the starting point for cooling to clement habitable conditions and for the onset of thermally driven mantle convection and plate tectonics this review focuses on evidence for magma oceans on planetesimals and planets and on research concerning the processes of compositional differentiation in the silicate magma ocean distribution and degassing of volatiles and cooling

geomagnetic palaeointensities and astrochronological ages for the matuyama brunhes boundary and the boundaries of the jaramillo subchron palaeomagnetic and oxygen isotope records from odp site 983 we have measured relative geomagnetic palaeointensity proxies palaeomagnetic directions and 18 o for the 700 1100 ka interval from odp site 983 gardar drift north atlantic where mean sedimentation rates are ca 13 cm kyr 1 the age model was generated by matching the benthic 18 o data to the ice volume model and confirmed by tuning the precessional components of both signals for the matuyama brunhes boundary mbb and the boundaries of the jaramillo subchronozone the duration of the polarity reversal process defined by virtual geomagnetic polar latitudes of less than 45 is ca 5 kyr whereas the generally accepted astrochronological estimates for the boundaries of the jaramillo subchronozone lie within the polarity transitions as recorded at site 983 the astrochronological age for the matuyama brunhes polarity transition 780 ka is ca 5 kyr older than the onset of this transition at site 983 775 ka the polarity reversals lie within palaeointensity lows with abrupt recovery of palaeointensity post reversal there is no progressive sawtooth decrease in palaeointensity within the jaramillo subchronozone or between the top of the jaramillo and the mbb but rather within polarity chrons several short intervals of low palaeointensity which sometimes coincide with high amplitude secular variation orbital 100 and 41 kyr periods are present in the palaeointensity record as they are not obviously attributable to climate lithology in these records they may be a feature of the geomagnetic field itself

a mediterranean perspective on 10be sedimentation and climate around the matuyama brunhes boundary les liaisons dangereuses abstract the 10be 9be ratio is commonly employed as a tool for establishing the stratigraphic position of paleomagnetic excursions and reversals whenever the traditional paleomagnetic approach fails to provide conclusive results in particular it is held that 10be production rates in the atmosphere depend on the strength of the earth s magnetic field and the fallout and deposition of cosmogenic beryllium at the surface happen on a very short time scale however investigations performed on terrestrial and marine successions demonstrate that the 10be record and the paleomagnetic signal are often asynchronous mechanisms that control the conveyance and deposition of cosmogenic 10be to the seafloor are still ambiguous and poorly documented here we discuss the dynamics of 10be in a central mediterranean marginal marine depositional scenario characterized by a pervasive terrigenous influx our data show that a very close correlation exists between 10be concentrations and the local proxy of rainfall rates and regimes pollen indicating that a considerable 10be transport from the mainland may occur in response to the remobilization of terrestrial reservoirs during periods of increased runoff superimposed is a dynamic oceanographic setting that further controls the preservation potential of 10be at the bottom in terms of changing water chemistry and or composition of the sedimentary flux to the seafloor results of our investigation suggest that in particular environmental and depositional settings the interplay between climate terrigenous yield and oceanography may jeopardize the sedimentary depiction of the meteoric 10be contribution thus challenging the use of 10be for tracking the stratigraphic position of geomagnetic reversals

bright carbonate veins on asteroid 101955 bennu implications for aqueous alteration history introduction carbonaceous asteroids formed early in solar system history and experienced varying degrees of aqueous water rock and thermal alteration most models of the evolution of these asteroids suggest that aqueous alteration was driven by hydrothermal convection however it is debated whether this alteration occurred in a chemically closed or open system the bulk chemical compositions of the carbonaceous chondrite meteorites imply that the system was closed models predict that large scale fluid flow in an open system took place on at least some asteroids in this scenario fluids would have flowed through fractures from the interior and minerals would have precipitated into these fractures forming veins rationale global spectral observations by the osiris rex origins spectral interpretation resource identification and security regolith explorer spacecraft have shown that carbon bearing materials including organics and or carbonates are widespread on the surface of near earth asteroid 101955 bennu to understand the composition of these carbon bearing materials and their implications for bennu s aqueous alteration history we examined visible near infrared spectra obtained at resolutions of 4 m in the region around the mission s primary sampling site nightingale we compared these spectra with laboratory spectra of organics and carbonates broadband panchromatic images of the nightingale region at pixel scales of 1 4 cm allowed us to search for surface features that could contextualize the spectral data results on the basis of the wavelength position and shape of spectral absorption features the closest matches to a subset 15 of spectra collected in the nightingale region are the carbonate minerals calcite dolomite and magnesite with calcite being the most prevalent these minerals are found in the highly aqueously altered cm and ci group carbonaceous chondrites proposed meteorite analogs of bennu bright centimeters thick roughly meter long veins are apparent in images of some boulders near the nightingale site the veins have albedos of 10 to at least 19 much brighter than bennu s average of 4 4 these albedos are consistent with the veins being filled by carbonates with minor abundances of opaque materials such as magnetite and organics the albedos are too high for the veins to be dominantly composed of organics or any other material that has been identified on bennu except carbonates there are only a few bright vein filling materials found in meteorites and carbonate is the only one of these that is plausibly present on bennu linear mixture modeling suggests that only a small fraction of a spectrometer field of view conclusion the observed bright veins on bennu are most likely dominantly composed of carbonates the detection of these meter sized putative carbonate veins indicates that extensive water flow occurred in a chemically open hydrothermal system on the larger parent asteroid of bennu before the catastrophic disruption of the parent asteroid led to bennu s reaccumulation as a rubble pile kinetic modeling assuming a calcite vein composition shows that a hydrothermal system kilometers in size with alteration likely taking place over thousands to millions of years would have been needed to create veins of the dimensions preserved in the constituent boulders of bennu some cm and ci group meteorites have very small micrometers to millimeters carbonate veins or presumed vein fragments veins on the scale that we observe on bennu are not present in meteorites it is possible that such veins are present on other carbonaceous asteroids that have not been observed by spacecraft we predict that the sample of bennu s surface material that is planned to be returned to earth by the osiris rex spacecraft will contain carbonates and that they will be distributed with a structure and scale distinct from those that occur in meteorites

oxidation during metamorphism of the ordinary chondrites subtle but progressive changes in the compositions and relative abundances of olivine pyroxene and metal with increasing metamorphic grade in equilibrated types 4 6 ordinary chondrites indicate that these meteorites experienced oxidation of metallic fe during heating oxygen fugacities calculated from these mineral assemblages increase with petrologic type the hypothesis of oxidation during chondrite metamorphism differs from a commonly accepted idea that chondritic fe was reduced by graphite the mineralogies of unequilibrated type 3 chondrites do not conform to the progressions in equilibrated chondrites allowing the possibility that fe may first have been reduced through reaction with graphite at the onset of metamorphism although there are conflicting observations supporting both oxidation and reduction in these meteorites as temperatures increased to levels appropriate for types 4 6 chondrites the activity of graphite may have been lowered by its dissolution in taenite and in any case oxygen fugacities appear to have been well within the stability field of graphite precluding its reaction at these temperatures oxidation state was largely controlled by equilibrium between ferromagnesian silicates and metal oxygen fugacities calculated from chondrite mineral equilibria are 2 3 log units below previously reported intrinsic o2 measurements we propose that progressive oxidation during metamorphism was promoted by interaction with small amounts of an oxidizing vapor derived by heating ices originally accreted into the parent asteroids assuming that this vapor was pure h2o the watenrock weight ratio required to account for the observed oxidation of fe in h and l chondrites is very modest less than several per mil progressive oxidation in the metamorphic sequence resulted from different degrees of reaction with such a fluid possibly caused by temperature variations or evolution of the composition of the fluid as it permeated through the body the presence of even minor amounts of a vapor phase during metamorphism affects interpretations of the volatile trace element and oxygen isotopic fractionations observed in ordinary chondrites and inferred to have formed by nebular or parent body processes