crystallization of magmatic iron meteorites the role of mixing in the molten core the iiiab group is the largest of the magmatic iron meteorite groups and consequently is commonly used to test models of asteroid core crystallization simple fractional crystallization calculations appear to reproduce the general shape of the elemental trends observed in the iiiab group when these trends are plotted vs ni as is traditionally done however when the elemental trends are examined vs another element such as ge vs ir simple fractional crystallization fails to match a significant portion of the trend specifically meteorites formed during the final stages of crystallization our simple mixing model which attempts to account for the possibility of inhomogeneities in the molten metallic core is able to reproduce the entire iiiab trend observed this model is a variant of simple fractional crystallization and involves mixing between a zone of liquid involved in the crystallization process and a second zone too far from the crystallizing solid to be actively involved in crystallization this model does not suggest one unique solution for the method by which an asteroidal core crystallizes rather it demonstrates that including the effects of mixing in the molten core can account for the observed iiiab elemental trends particularly the late stage crystallizing members which other models have difficulty explaining

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

explosive volcanism and the compositions of cores of differentiated asteroids eleven iron meteorite groups show correlations between ni and siderophile trace elements that are predictable by distribution coefficients between liquid and solid metal in fractionally crystallizing metal magmas these meteorites are interpreted to be fragments of the fractionally crystallized cores of eleven differentiated asteroids many of these groups crystallized from s depleted magmas which we propose resulted from removal of the first partial melt the fe nifes cotectic melt by explosive pyroclastic volcanism of the type envisaged by wilson and keil 8 we show that these dense negatively buoyant melts can be driven to asteroidal surfaces due to the presence of excess pressure in the melt and the presence of buoyant bubbles of gas which decrease the density of the melt we also show that in typical asteroidal materials veins will form which grow into dikes and serve as pathways for migration of melt and gas to asteroidal surfaces since cotectic fe nifes melt consists of about 85 wt fes and 15 wt fe ni removal of small volumes of eutectic melts results in major loss of s but only minor loss of fe ni thus leaving sufficient fe ni to form sizeable asteroidal cores

volatile degassing of basaltic achondrite parent bodies evidence from alkali elements and phosphorus abstract the volatile alkali element na k rb cs and p abundances in eucrites diogenites and basaltic clasts from polymict eucrites howardites and mesosiderites were compared with an average of highly incompatible refractory ahir elements ba la ce nd sm hf th u normalized to cosmic abundances ratios of e g rb ahir remain constant during normal magmatic differentiation because the very low and approximately equal mineral melt partition coefficients for these elements are inefficient in causing fractionation in the melt the na ahir ratio in eucrites is essentially constant at 0 050 cosmic basaltic eucrites and basaltic clasts show a positive correlation between k rb and cs and alkali element ahir ratios the averages for basaltic eucrites are k ahir 0 043 rb ahir 0 0088 cs ahir 0 0040 cosmic the positive correlations between alkali elements and alkali ahir are due to progressive volatile loss of the alkali elements from the basalts and not to source region heterogeneities this process probably had a minor effect on the parent body inventory of volatile elements in diogenites the alkali ahir ratios are systematically greater than in basaltic eucrites and are more variable the lower values are 1 4 2 the means for basaltic eucrites while the higher values are similar to those of cv and co chondrites for k rb and cs the highest alkali ahir ratios were obtained on the most incompatible poor of the diogenites and indicate minimum ratios for the parent body of na ahir 0 24 k ahir 0 49 rb ahir 0 55 and cs ahir 0 14 cosmic there are well defined negative correlations between k rb cs and alkali ahir ratio in the diogenites the data are consistent with progressive alkali loss through volatile outgassing during crystallization of one or more parent magmas this process resulted in 90 loss of the volatile element inventory from the parent body phosphorus also shows evidence for volatile loss from the basaltic eucrites and elevated p ahir ratios in diogenites the low p ahir ratio in eucrites is largely controlled by volatile loss rather than through equilibration with a metallic liquid as previously suggested the parent body of the basaltic achondrites appears to have initially contained alkali element and presumably other volatile element abundances comparable to those in co and cv chondrites and perhaps higher

petrology and geochemistry of a silicate clast from the mount padbury mesosiderite implications for metal silicate mixing events of mesosiderite petrological and bulk geochemical studies were performed on a large silicate clast from the mount padbury mesosiderite the silicate clast is composed mainly of pyroxene and plagioclase with minor amounts of ilmenite spinel and other accessory minerals and it shows subophitic texture pyroxenes in the clast are similar to those in type 5 eucrites and could have experienced prolonged thermal metamorphism after rapid crystallization from a near surface melt ilmenite and spinel vary chemically indicating growth under disequilibrium conditions the clast seems to have experienced an episode of rapid reheating and cooling possibly as a result of metal silicate mixing abundances of siderophile elements are obviously higher than in eucrites although the clast is also extremely depleted in highly siderophile elements the fractionated pattern can be explained by injection of fe fes melts generated by partial melting of metallic portions during metal silicate mixing the silicate clast had a complex petrogenesis that could have included 1 rapid crystallization from magma in a lava flow or a shallow intrusion 2 prolonged thermal metamorphism to equilibrate the mineral compositions of pyroxene and plagioclase after primary crystallization 3 metal silicate mixing probably caused by the impact of solid metal bodies on the surface of the mesosiderite parent body and 4 partial melting of metal and sulfide portions and silicate in some cases caused by the collisional heating which produced fe fes melts with highly fractionated siderophile elements that were injected into silicate portions along cracks and fractures

compositional trends among iid irons their possible formation from the p rich lower magma in a two layer core group iid is the fifth largest group of iron meteorites and the fourth largest magmatic group i e that formed by fractional crystallization we report neutron activation data for 19 of 21 known iid irons these confirm earlier studies showing that the group has a relatively limited range in ir concentrations a factor of 5 this limited range is not mainly due to incomplete sampling instead it seems to indicate low solid liquid distribution coefficients reflecting very low s contents of the parental magma the same explanation responsible for the limited range in group iva despite this similarity these two groups have very different volatile patterns group iva has very low abundances of the volatile elements ga sb and ge whereas in group iid ga and sb abundances are the highest known in a magmatic group of iron meteorites and ge abundances are the second highest after group iiab group iid appears to be the only large magmatic group having high volatile abundances but low s in the volatile depleted groups iva and ivb it is plausible that s was lost as a volatile from a chondritic precursor material because group iid seems to have experienced minimal loss of volatiles we suggest that s was lost as an early melt having a composition near that of the fe fes eutectic 315 mg g s when temperatures had risen 400 500 k higher p rich melts formed became gravitationally unstable and drained through the first melt to form an inner core that was parental to the iid irons as discussed by kracher a wasson j t 1982 the role of s in the evolution of the parental cores of the iron meteorites geochim cosmochim acta46 2419 2426 it is plausible that a metal rich inner core and a s rich outer core could coexist metastably because stratification near the interface permitted only diffusional mixing the initial liquidus temperature of the inner p rich core is estimated to have been 1740 k after 60 crystallization the increase in p and the decrease in temperature may have permitted immiscibility with the s rich outer core we have not recognized samples of the outer core

ureilites trace element clues to their origin carbonaceous vein separates from kenna and havero as well as bulk kenna were analyzed by rnaa for ag au bi br cd cs ge in ir ni pd os rb re sb se te tl u and zn the data are reviewed together with four earlier chicago analyses of bulk ureilites linear regressions confirm the presence of two metal components with the following cl normalized ratios ir ni 14 6 1 ge ni 5 4 2 4 au ni 2 3 0 9 the high ir component is enriched in vein separates and hence belongs to veins the lowir component belongs to the ultramafic rock vein material is enriched in all elements analyzed by us except zn and accounts for most of the c noble gases and presumably siderophiles in the meteorite most of the properties of ureilites apparently can be explained by the cumulate model of berkley et al 1980 with certain modifications comparison of ureilites with three other ultramafic rocks from different planets earth s mantle lunar dunite and chassigny suggests that the ureilite parent body had a primitive chondritic composition similar to c3v chondrites but richer in metal and carbon it melted causing depletion of incompatibles to a mean abundance of 0 02 cl and incomplete segregation of metal fes and c fractional crystallization or melting of metal in the presence of s and c apparently can explain the fractionations of ir re ni au and perhaps ge obviating the need for extraneous sources of vein metal or unusual parent body compositions noble gases from the parent material may have been retrapped in carbon during magmatism provided the system was closed

copper and nickel partitioning in iron meteorites trace element analyses using proton induced x ray emission pixe and synchrotron x ray fluorescence sxrf have been made on metal and troilite from 9 iron meteorites representing 5 geochemical groups nickel and copper distribution coefficients d troilitemetal vary by factors of 600 and 20 respectively correlate positively with kamacite band width and correlate negatively with bulk nickel content meteorites with bulk ni 1 while those with ni 10 have cu depleted troilite dcu 1 since magmatic evolution and partial melting will produce cu enriched troilite only the observation of cu depleted troilite in ni rich meteorites is evidence for subsolidus re equilibration a process which reduces dcu by transfer of cu to exsolving metal elemental redistribution may have played an important role in establishing the present chemical trends in iron meteorites

partitioning of moderately siderophile elements among olivine silicate melt and sulfide melt constraints on core formation in the earth and mars abstract this study investigates the effects of variations in the fugacities of oxygen and sulfur on the partitioning of first series transition metals v cr mn fe co ni and cu and w among coexisting sulfide melt silicate melt and olivine experiments were performed at 1 atm pressure 1350 c with the fugacities of oxygen and sulfur controlled by mixing co2 co and so2 gases starting compositions consisted of a cao mgo al2o3 sio2 feo na2o analog for a barred olivine chondrule from an ordinary chondrite and a synthetic komatiite the f o 2 f s 2 conditions ranged from log f o 2 7 9 to 10 6 wi log f s 2 values ranging from 1 0 to 2 5 our experimental results demonstrate that the f o 2 f s 2 dependencies of sulfide melt silicate melt partition coefficients for the first series transition metals are proportional to their valence states the f o 2 f s 2 dependencies for the partitioning of fe co ni and cu are weaker than predicted on the basis of their valence states variations in f o 2 f s 2 conditions have no significant effect on olivine melt partitioning other than those resulting from f o 2 induced changes in the valence state of a given element the strong f o 2 f s 2 dependence for the olivine silicate melt partitioning of v is attributable to a change of valence state from 4 to 3 with decreasing f o 2 our experimentally determined partition coefficients are used to develop models for the segregation of sulfide and metal from the silicate portion of the early earth and the shergottite parent body mars we find that the influence of s is not sufficient to explain the overabundance of siderophile and chalcophile elements that remained in the mantle of the earth following core formation important constraints on core formation in mars are provided by our experimental determination of the partitioning of cu between silicate and sulfide melts when combined with existing estimates for siderophile element abundances in the martian mantle and a mass balance constraint from fe the experiments allow a determination of the mass of the martian core 17 to 22 wt of the planet and its s content 0 4 wt these modeling results indicate that mars is depleted in s and that its core is solid

origin and evolution of the ureilite parent magmas multi stage igneous activity on a large parent body analyses of olivine and pigeonite cores interpreted as cumulate crystals in eight low shock ureilites show well defined correlations between fe x where x is one of the minor elements mn cr ca al ti p or ni and fe mg ratios for the lithophile minor elements these trends are linear with positive slope and pass through or near the origin indicating various degrees of feo reduction of the parent magmas the trends shown by p and ni are consistent with this interpretation and additionally require equilibrium crystallization of 20 27 mole metal graphite was most likely the reducing agent which implies that the ureilites equilibrated at different pressures because the graphite fo2 buffer is pressure dependent there is no evidence for fractionation of olivine or pryroxene in any of these trends the ca ti and al trends show significant scatter indicating that there must have been multiple parent magmas with very similar mg mn and mgcr ratios but different mgal mgca and mgti ratios cao al2o3 ratios vary by a factor of 3 among the eight magmas and are superchondritic 14 32 molar similar to lunar mare basalts we propose the following model for generation and crystallization of ureilite parent magmas upm chondritic material undergoes 10 25 partial melting leaving a plagioclase depleted residue the magma or magmas form a differentiated crust on the ureilite parent body upb with mafic cumulates and plagioclase cumulates spatially separated later remelting 10 of the mafic cumulates produced the upm siderophile elements indicate that the source region of the upm had not experienced segregation of metal such as would occur during core formation in the source region the pressure must have been at least 0 5 1 kb because only at pressures this high does the graphite buffer allow iron oxide to be the stable phase relative to metallic iron the graphite bearing upm were emplaced at various levels in the crust where they were reduced to various extents dependent on depth ureilites are early olivine pigeonite cumulates from these magmas compositions of interstitial low ca pyroxenes indicate that the trapped interstitial liquids from which they crystallized had lower mnmg crmg camg almg and namg ratios than the upm and indicate an abrupt change in liquid composition coincident with impact excavation of ureilites this interstitial material may be derived by mixing a shock melt of nearby olivine pigeonite cumulates with residual primary liquid our model predicts that the upb had a differentiated crust did not have a core and was at least 235 km in radius the lack of a ureilite like body among spectrally analyzed asteroids suggests that either the spectral signature of the upb is not that of ureilites themselves perhaps of carbon free feldspathic material instead that the upb is not intact or that ureilites did not originate in the asteroid belt

a comparison of metallographic cooling rate methods used in meteorites abstract the primary objective of this study was to test the postulate that cooling rates acquired from metal grains in chondrites are consistent with those from iron meteorites both types of metal occur in some group iab meteorites which are mixtures of massive metal with well developed widmanstatten structures and chondritic inclusions with dispersed metal grains the grains have textures and compositions similar to chondritic metal including negligible p the meteorites studied show little or no sign of shock reheating and textural evidence indicates that silicates and metal were mixed before widmanstatten patterns formed during cooling cooling rates were obtained by comparing measured to modeled taenite grain or lamellae dimensions and central ni contents modeling entails solving diffusion equations using experimental diffusion coefficients phase relations and bulk or local ni and p contents taking into account geometry undercooling and impingement there is one set of parameters for grains and another quite different set for widmanstatten lamellae including a factor of 30 difference in diffusion coefficients yet cooling rates obtained from widmanstatten structures and metal grains in chondritic inclusions of the same meteorite are consistent uncertainties in the best data are 10 ma equivalent to a factor of 1 0 25 this agreement implies that the data and models are correct or contain fortuitously offsetting errors which is quite unlikely cooling rates range from 40 ma to 70 ma in iab meteorites that contain both grains and widmanstatten structures rates based on grains in ni poor and ni rich meteorites lacking widmanstatten patterns expand the range from 30 ma to perhaps 200 ma cooling rates correlate with ni content ni poor meteorites have slower rates than ni rich ones evidently iab meteorites were radially distributed over 30km in a body with a radius 50km a comparison of the available ar ages with cooling times inferred from the cooling rates suggests that the parent body cooled more slowly after the metallographic cooling rates were established

a petrological and chemical reexamination of main group pallasite formation abstract the large number of pallasite parent bodies suggests that their formation was a common event in asteroid evolution new microprobe data has been generated for pallasite olivines and chromites that indicate subsolidus redox processes while literature data has been collated and correlated to form new results and interpretations the new data include information on metal compositions the metal cooling rates the formation of round and fragmental olivine the close packing of olivine the crystallization of the phosphates phosphoran olivine and orthopyroxene and the bulk pallasite p and s contents a model for main group pallasites has been developed using fractional melting of a chondritic precursor to make a multi layered parent body one portion of this contains a gradational metal olivine pallasite texture concentric downward crystallization of the molten metal is indicated based on cooling rates and metal compositions fractional crystallization of the residual silicate and metallic melts produces the minor phases on cooling comparisons to and a brief review of past models particularly those utilizing mixing have been made

partitioning of siderophile elements in the fenis system 1 bar to 80 kbar abstract partition coefficients for au ni p and ge between solid fe ni metal and sulfur bearing metallic liquids have been measured at 7 27 and 80 kbar to our knowledge these are the only such data for au p and ge at high pressure comparison of our partitioning results to those obtained at 1 bar indicate that only the 80 kbar ge data differ significantly from the 1 bar experiments thus many low pressure partitioning experiments in the fe ni s p system may have applicability to the greater portion of the earth s upper mantle or alternatively the entire mantle of mars

molybdenum in eucrites evidence for a metal core in the eucrite parent body molybdenum concentrations from 15 to 70 ppb in eucrites have been determined with a new method of neutron activation these values are very low compared to chondrites and are in agreement with the large depletions of other siderophile elements in eucrites the eucrites have a mean mo la ratio of 0 0066 which is a factor of 570 lower than the ci chondritic ratio 3 75 the depletions of siderophiles in eucrites were presumably caused by segregation of metal molybdenum however is fractionated as an incompatible element similar to la among the individual eucrites the variable concentrations but roughly constant ratio of mo to la indicate that metal was absent during the fractionation that produced the eucrites and imply that core formation occurred before the fractionation the need for a relatively large degree of partial melting to completely segregate metal argues for a fractional crystallization model for the production of the eucrite magmas the amount of metal in the eucrite parent body epb required to obtain the observed depletion of the mo la ratio depends on the degree of partial melting during metal silicate equilibrium the depletion of co has a dependence on the degree of partial melting different from mo and w and using co together with mo and w the metal content and degree of partial melting can be estimated a metal content of 30 to 50 and metal segregation at approximately 50 partial melting of the silicates is consistent with the data if the epb had a ratio of co to refractory lithophile elements less than ci chondrites a metal content as low as 20 with metal segregation at 20 partial melting is possible a large metal core in the epb vesta may therefore be detectable from a spacecraft

metalliferous asteroids as potential sources of precious metals recent discoveries of near earth asteroids neas and chemical analyses of fragments of asteroids meteorites suggest that there may be a gold mine literally in near earth space judged from meteorite analyses two types of asteroids offer particularly bright prospects for recovery of large quantities of precious metals defined as au pt ir os pd rh and ru the ordinary ll chondrites which contain 1 2 5 3 fe ni metal containing 50 220 ppm of precious metals and metallic asteroids which consist almost wholly of fe ni phases and contain variable amounts of precious metals up to several hundred ppm the pulverized regolith of ll chondrite asteroids could be electromagnetically raked to separate the metallic grains suitable metallic asteroids could be processed in their entirety statistically there should be approximately six metallic neas larger than 1 km in diameter that contain over 100 ppm of precious metals successful recovery of 400 000 tons or more of precious metals contained in the smallest and least rich of these metallic neas could yield products worth 5 1 trillion us at recent market prices if marketed over 20 years this would represent a 10 fold increase over the recent global production rate of all precious metals combined the market response to the hypothetical introduction of such large quantities of precious metals is difficult to predict because these metals historically have not obeyed normal economic laws of the market an empirical model suggests that the effects on market prices of a hypothetical increase in production of a given metal can be predicted given existing knowledge of the metal s present market value and production rate this model suggests that the total value of 400 000 metric tons of precious metals if marketed over 20 years would decline to about 320 billion 16 billion per year except for au for which production and prices would be marginally affected the market prices of precious metals may decline by 1 to 2 orders of magnitude if one of the six asteroids were to be mined to depletion over 20 years less conservatively there is a 50 chance that the richest metallic nea contains at least 1 9 million metric tons of precious metals this quantity if marketed over 40 years may be worth approximately 900 billion at collapsed market prices 22 5 billion per year the actual economic and technological impact of asteroidal metals may be considerably greater due to the increased availability and reduced prices of these resources despite this great potential first order technological scientific and economic uncertainties remain before the feasibility of exploitation of asteroids for precious metals can be ascertained

re os systematics in pallasite and mesosiderite metal re os data on pallasite metal yield a sufficient range in re os to permit the determination of a whole rock isochron if pallasites from different groups are considered together the results are indistinguishable from the data obtained on groups iab iiab iid iiiab iva and ivb iron meteorites pallasite data were obtained on samples of the main group which have been considered related to group iiiab irons and on samples of the distinctive pallasites eagle station finmarken and marjalahti it appears that iron meteorites and pallasites crystallized within a time interval of 20 ma this result is consistent with the evidence from the short lived chronometers 107 pd 107 ag and 53 mn 53 cr it is also consistent with the evidence from 182 hf 182 w that the time interval over which molten feni metal from different iron meteorites equilibrated with and became segregated from silicates was 10 ma the well defined 187 re 187 os whole rock isochron for pallasites and iron meteorites requires that they were formed in parent bodies that had been partially molten and segregated feni metal and fes and then all crystallized within a 20 ma time interval this conclusion appears to apply to members of distinct groups of iron meteorites and pallasites which are viewed as not being cogenetic the short time scale requires that the irons and pallasites were formed in small parent planets of 10 km radius or near the surface of larger bodies in contrast to the pallasites we find that feni samples from mesosiderites have a narrow range in re os so that it is not possible to determine a whole rock isochron for these samples the mesosiderite data also lie somewhat displaced from the iron meteorite and pallasite isochron indicating a more complex multi stage evolution based on sm nd data on mesosiderite silicate clasts the re os results on mesosiderite metal are compatible with the model of melted or partially molten feni cores or pods still preserved in protoplanetary bodies when these bodies were disrupted up to 150 ma after early formation and the feni was splashed onto the surfaces of other small differentiated planetesimals

examining the compositions of impactors striking the moon using apollo impact melt coats and anorthositic regolith breccia meteorites abstract impactors striking the moon since the formation of its crust have left an indelible imprint on the lunar surface in the guise of craters and associated impact rocks the lunar crust has low intrinsic abundances of the highly siderophile elements hse re os ir ru pt rh pd au at greater than 3000 times lower than in chondrite meteorites consequently during impact bolides with chondritic or differentiated iron rich compositions should impart elevated hse signatures to the lunar crust here we examine glassy lunar impact melt coats imc from the outside rims of apollo 16 cataclastic anorthosites 60015 65325 and breccias 65035 as well as both fragments and powders of antarctic anorthositic regolith breccia arb meteorites miller range 090034 36 70 75 and macalpine hills 88105 for their petrography mineral chemistry and bulk rock compositions the hse concentrations for imc range from 0 001 to 0 1 ci chondrite with measured 187os 188os between 0 1189 and 0 1366 anorthositic regolith breccia meteorites which have components with 2 6 4 1 ga ages have similar hse concentrations to imc but typically have lower 187os 188os 0 1164 0 1284 these latter os ratios are generally less radiogenic than those measured in 3 8 3 9 ga apollo impact melt breccias the apollo 16 imc are not well dated but their kreepy trace element signatures and associated ages of 3 7 to 3 8 ga for apollo 16 glasses might imply at least in part an origin from the imbrium or serenitatis basin forming impacts within the imc metal schreibersite troilite assemblages record significant inter element hse fractionation which is also reflected in bulk hse patterns for both imc and arb meteorites variations in relative and absolute hse compositions directly reflect the control of metal and sulfide segregation within and between impact melt and breccia lithologies collectively imc and arb meteorites exhibit approximately 50 of the variation in ru ir and 187os 188os observed in lunar impact melt breccias these results imply that significant variations in inter element compositions can occur during impact brecciation and melting and so some impact melt rock hse compositions may not record the compositions of impactors that struck the moon with fidelity nonetheless the generally low 187re 188os of lunar impact melt rocks means that osmium isotope ratios provide evidence for impact composition and a change from ordinary to carbonaceous like impactors either with time or location striking the moon

quantifying the effect of solid phase composition and structure on solid liquid partitioning of siderophile and chalcophile elements in the iron sulfur system we report experimentally determined partition coefficients between solid and liquid phases for bulk compositions on either side of the fe fes eutectic for a suite of siderophile chalcophile and lithophile elements experiments were performed at conditions of 1 5 and 2 gpa in pressure p 1323 k in temperature t and virtually identical eutectic sulfide liquid compositions in equilibrium with either solid face centered cubic fe or solid fes this enabled isolation of the effect of solid phase composition and structure from pressure temperature melt composition effects solid phase liquid metal partition coefficients d values for ge re ni co cr mn v sn pb re and w differ significantly if partitioning occurs between identical metallic liquids but different solid phases whereas zn cu and mo are virtually unaffected for all elements except ge and sn measured solid fe liquid sulfide partition coefficients at 1 5 and 2 gpa are inconsistent with model predictions based on atmospheric pressure experiments indicating that such models may not be appropriate for modeling core crystallization processes at non ambient pressure the framework of a lattice strain based model of solid liquid metal partitioning stewart et al 2009 enables us to quantify the effect of the solid phase changing the solid phase from fe to fes leads to systematic increases in r0 from 1 54 to 1 65 a and apparent young s modulus e from 112 to 178 gpa these systematic changes can be used to predict element partitioning in eutectic solid fes bearing systems from measurements in eutectic solid fe bearing systems although changing the solid phase from face centered cubic fe to fes is an end member example our data suggest that changes with pressure in the structure e g to hexagonally close packed at high pressure and composition e g to higher s content at high pressure of solid iron could affect the partitioning of elements between fe and liquid metal during the solidification of planetary cores

magmatic activity on the iva parent body evidence from silicate bearing iron meteorites abstract four of the magmatic iva iron meteorites contain tridymite or clinobronzite orthobronzitetridymite which are quite unlike silicate assemblages in other iron meteorites the textures the bulk chemistry and the zoning preserved in the pyroxenes strongly suggest that they are igneous cumulates the pyroxenes have extremely low fe mn ratios less than 20 and low contents of rees and other incompatible elements these cumulates crystallized from magmas of unusual composition with some similarity to terrestrial boninites at the protobronzite tfdymite cotectic in the olivine plagioclase silica system a liquidus temperature in the range 1400 1350 c was inferred for the steinbach meteorite from the estimated distribution coefficient for cr in pyroxene dcrsolid liquid 0 66 the low levels of incompatible elements show that less than 1 of the residual liquid was trapped in the cumulates during cooling at subsolidus temperatures most of the protobronzite transformed to orthobronzite and the rest inverted to a fine inter growth of clino and orthobronzite in addition the igneous zoning of ca fe mg and mn was modified by diffusion whereas ti al and cr were not or only slightly affected the silica saturated magmas could not have evolved in an olivine rich mantle we assume that the magmas became incorporated in the metal core possibly due to solidification shrinkage of the metal we propose that the iva parent magmas were formed by high degrees of partial melting 40 of a chondritic precursor along the olivine pyroxene peritectic reaction curve in the olivine plagioclase silica system at low pressures the precursor may have been depleted in incompatible elements by a preceding melting episode the partial melts were then separated from the olivine residue and subsequently reduced to account for the low fe mn ratios and the unusually high si content

re os and pd ag systematics in group iiiab irons and in pallasites using improved analytical techniques which reduce the re blanks by factors of 8 to 10 we report new re os data on low re and low pge pallasites pal anom and iiiab irons the new pallasite samples nearly double the observed range in re os for pallasites and allow the determination of an isochron of slope 0 0775 0 0008 t 4 50 0 04 ga using the adjusted 187 re 1 66 10 11 a 1 and initial 187 os 188 os 0 0 09599 0 00046 if the data on different groups of pallasites including the anomalous pallasites are considered to define a whole rock isochron their formation would appear to be distinctly younger than for the iron meteorites by 60 ma five iiiab irons acuna bella roca chupaderos grant and bear creek with re contents ranging from 0 9 to 2 8 ppb show limited re os fractionation and plot within errors on the iiab iron meteorite isochron of slope 0 07848 0 00018 t 4 56 0 01 ga and initial 187 os 188 os 0 0 09563 0 00011 many of the meteorites were analyzed also for pd ag and show 107 ag enrichments correlated with pd ag requiring early formation and fractionation of the feni metal in a narrow time interval after injection of live 107 pd t 1 2 6 5 ma into the solar nebula based on pd ag the typical range in relative ages of these meteorites is 10 ma the pd ag results suggest early formation and preservation of the 107 pd 107 ag systematics both for iiiab irons and for pallasites while the younger re os apparent age for pallasites suggests that the re os system in pallasites was subject to re equilibration the low re and low pge pallasites show significant re os fractionation higher re os as the re and pge contents decrease by contrast the iiiab irons show a restricted range in re os even for samples with extremely low re and pge contents there is a good correlation of re and ir contents the correlation of re and os contents for iiiab irons shows a similar complex pattern as observed for iiab irons morgan et al 1995 and neither can be ascribed to a continuous fractional crystallization process with uniform solid metal liquid metal distribution coefficients

nitrogen isotopic compositions of iron meteorites abstract iron meteorites analyzed in this study have nitrogen concentrations 70 g g and 15n from 90 to 150 although the iron meteorites have a large range of 15n most have values more negative than 50 the nitrogen isotopic compositions were established by cosmochemical processes and were little modified by fractional crystallization or other chemical processes within the parent bodies the data do not suggest the existence of a well mixed solar nebular reservoir for nitrogen as was already inferred from data from stony meteorites the range of 1100 observed for 15n in bulk meteorites is probably too large to be accounted for by physical and chemical mass dependent fractionation processes in the solar nebula and thus reflects nebular inhomogeneities of nucleosynthetic origin nitrogen isotopes may serve as fingerprints for different reservoirs preserved in various parent bodies in a manner similar to that observed for oxygen isotopes in stony meteorites and oxygen bearing phases of stony irons

experimental investigations of trace element fractionation in iron meteorites ii the influence of sulfur abstract we have investigated the partitioning of ir ge ga w cr au p and ni between solid metal and metallic liquid as a function of temperature and s concentration of the metallic liquid partition coefficients for siderophile elements such as ir w ga and ge increase by factors of 10 100 as the sconcentration of the metallic liquid increases from 0 30 wt partition coefficients for other siderophile elements such as ni au and p increase by only factors of 2 3 in contrast partition coefficients for the more chalcophile element cr decrease these experimentally determined partition coefficients have been used in conjunction with a fractional crystallization model to reproduce the geochemical behavior of ni p au and ir during the magmatic evolution of groups iiab iiiab iva and ivb iron meteorites the mean s concentration for each group increases in the order ivb iva iiiab iiab in accord with cosmochemical prediction however we are unable to reproduce the geochemical behavior of ge ga w and cr in an internally consistent way we conclude that the magmatic histories of these iron meteorite groups are more complex than has been generally assumed

missing evidence for stepwise postglacial sea level rise and an approach to more precise determination of former sea levels on east china sea shelf abstract the vertical range of past sea level indicated by tidal flat deposits is equal to the sum of the sediment thickness and tidal range defining position of the dated sample in tidal flat deposits can increase the accuracy of former sea level determinations the salt water peaty layer often contains mixtures of autochthonous and allochthonous organic carbon and the tidal flat deposits may contain biological remains transported from shallow marine sources these allochthonous remains have older ages than those of the layers containing the dated materials most published 14 c data on the east china sea shelf are inadequate to define a precise postglacial sea level rise curve good results in determining both precise former sea levels and reliable 14 c ages can be provided by selecting large plant roots and wood fragments from peaty layers and selecting less worn foraminifera tests and mollusk shells of dominant species in modern equivalent environments such as foraminifera ammonia beccarii vars or mollusk corbicula fluminea from the peaty layer its close overlying deposits and the base of tidal flat deposits without a basal peaty layer the stepwise model for postglacial sea level rise on the east china sea shelf liu 2001 liu et al 2004 was mostly determined by freshwater peat shallow marine and subtidal samples which cannot provide precise water levels the reliability of many 14 c ages used is also questionable due to unknown proportions of allochthonous material thus the stepwise model is unsupported by available data a rough sea level curve in last 13 000 yr on the east china sea shelf is herein proposed it seems simple but is effective based on valid data

the combined influence of pacific decadal oscillation and atlantic multidecadal oscillation on central mexico since the early 1600s abstract periodic droughts have been one of the most serious environmental issues in central mexico since the earliest times the impacts of future droughts are likely to become even more severe as the current global warming trend increases potential evaporation and moisture deficits a full understanding of the mechanism underlying climate variability is imperative to narrow the uncertainty about future droughts and predict water availability the climatic complexity generated by the combined influence of both atlantic and pacific forcings however causes considerable difficulty in interpreting central mexican climate records also the lack of high resolution information regarding the climate in the recent past makes it difficult to clearly understand current drought mechanisms our new high resolution 18 o record from hoya rincon de parangueo in central mexico provides useful information on climate variations since the early 1600s according to our results the central mexican climate has been predominantly controlled by the combined influence of the 20 year pacific decadal oscillation pdo and the 70 year atlantic multidecadal oscillation amo however the amo probably lost much of its influence in central mexico in the early 20th century and the pdo has mostly driven climate change since marked dryness was mostly associated with co occurrence of highly positive pdo and negative amo between 1600 and 1900

pt re os systematics of group iiab and iiiab iron meteorites the pt re os isotopic and elemental systematics of 13 group iiab and 23 group iiiab iron meteorites are examined as has been noted previously for iron meteorite groups and experimental systems solid metal liquid metal bulk distribution coefficients d values for both iiab and iiiab systems show dos dre dpt 1 during the initial stages of core crystallization assuming closed system crystallization the latter stages of crystallization for each core are generally characterized by dpt dre dos the processes governing the concentrations of these elements are much more complex in the iiiab core relative to the iiab core several crystallization models utilizing different starting parameters and bulk distribution coefficients are considered for the re os pair each model has flaws but in general the results suggest that the concentrations of these elements were dominated by equilibrium crystallization and subsequent interactions between solid metal and both equilibrium and evolved melts late additions of primitive metal to either core were likely minor or nonexistent the 187re 187os systematics of the iiab and iiiab groups are consistent with generally closed system behavior for both elements since the first several tens of ma of the formation of the solar system consistent with short lived chronometers the re os isochron ages for the complete suites of iiab and iiiab irons are 4530 50 ma and 4517 32 ma respectively and are similar to previously reported re os ages for the lower ni endmembers of these two groups both isochrons are consistent with but do not require crystallization of the entire groups within 10 30 ma of the initiation of crystallization the first high precision 190pt 186os isochrons for iiab and iiiab irons are presented the pt os isochron ages for the iiab and iiiab irons calculated using the current best estimate of the for 190pt are 4323 80 ma and 4325 26 ma respectively the re os and pt os ages do not overlap within the uncertainties the younger apparent ages recorded by the pt os system likely reflect error in the 190pt decay constant the slope from the pt os isochron is combined with the age from the re os isochron for the iiiab irons to calculate a revised of 1 415 10 12 a 1 for 190pt although additional study of this decay constant is still needed

mantle material in the main belt battered to bits the complete or near complete differentiation of a chondritic parent body is believed to result in an object with an fe ni core a thick olivine dominated mantle and a thin plagioclase pyroxene crust compositional groupings of iron meteorites give direct evidence that at least 60 chondritic parent bodies have been differentiated and subsequently destroyed a long standing problem has been that our meteorite collections and apparently our asteroid observations as well show a great absence of olivine dominated metal free mantle material while the basaltic achondrites hed meteorites represent metal free pyroxene dominated crustal samples the isotopic and geochemical evidence implies that this class is derived from only one parent body perhaps vesta thus the meteoritic and perhaps astronomical evidence also suggests a great absence of crustal material resulting from the collisional disruption of numerous parent bodies

the geochemical coherence of pu and nd and the pu 244 u 238 ratio of the early solar system abstract two very different sets of 244 pu 238 u ratios have been reported for early solar system materials one group of samples yields high 0 015 0 016 ratios podosek 1970a 1972 drozd et al 1977 and calculations based on another group of analyses yield low ratios 0 004 marti et al 1977 recently measured partition coefficients for pu and sm are used to evaluate the data of marti et al and sol 244 pu 238 u ratios from other sources are also considered a low sol 244 pu 238 u ratio 0 005 is favored and some implications of this low ratio to galactic nucleosynthesis and meteorite age dating are briefly discussed

an investigation of the behavior of cu and cr during iron meteorite crystallization the measured cu and cr contents in magmatic iron meteorites appear to contradict the behavior predicted by experimental fractional crystallization studies currently available to investigate the origin of cu and cr concentrations observed in these meteorites a thorough set of solid metal liquid metal experiments were conducted in the fe ni s system in addition to cu and cr partitioning values were also determined for as au bi co mo ni pb rh ru sb sn v and zn from the experiments experimental results for cu and cr showed similar chalcophile partitioning behavior whereas these elements have differently sloped trends within magmatic iron meteorite groups thus fractional crystallization alone cannot control both the cu and cr concentrations in these iron meteorite groups a simple fractional crystallization model based on our experimental cu partitioning results was able to match the cu versus au trend observed in the s poor ivb iron meteorite group but not the decreasing cu versus au trends in the iiab and iiiab groups or the unique s shaped cu versus au trend in the iva group however the crystallization model calculations were found to be very sensitive to the specific choice for the mathematical expression of d cu suggesting that any future refinement of the parameterization of d cu should include a reassessment of the cu fractional crystallization trends the cr versus au trends in magmatic iron meteorite groups are steeper than those of cu and not explained by fractional crystallization other influences such as the removal of chromite from the crystallizing system or sampling biases during iron meteorite compositional analyses are likely responsible for the cr trends in magmatic iron meteorite groups

siderophile trace elements in metals and sulfides in enstatite achondrites record planetary differentiation in an enstatite chondritic parent body abstract siderophile element concentrations were measured by la icp ms in metals and sulfides from five aubrite meteorites siderophile element patterns in aubrites are either similar to those in metal from enstatite chondrites or can be derived by crystallization from metallic liquids derived by partial melting of e chondrites some metal grains in mt egerton cumberland falls and aubres show moderate to severe depletion in compatible highly siderophile elements re os ir ru which are consistent with solid metal liquid metal differentiation of enstatite chondrite like metal metals from chondrite inclusions in cumberland falls show more extremely fractionated patterns than those from the aubritic matrix potentially hinting at fractionation and partial melting processes affecting not only the aubrite parent body but the chondrite body from which the inclusions were derived as well models using experimental partition coefficients show that aubrite metal chemically corresponds to solid metal segregated during differentiation of primary metallic liquids of eh el composition that contained both substantial s and c contents this result is consistent with a genetic link between enstatite chondrites and aubrites but as to whether aubrites were derived from the same body ies as enstatite chondrites or have their origin in multiple and potentially separated bodies cannot be answered unequivocally with chemical or isotopic data alone

the evolution of partially differentiated planetesimals evidence from iron meteorite groups iab and iiicd group iab irons probably come from a parent body that partially differentiated into a sulfur rich molten core and a primitive silicate outer layer in which only minor or localized melting occurred although the complexity of meteorites from this parent body or bodies irons with and without massive silicate inclusions stony meteorites like winona sulfide rich meteorites like mundrabilla etc implies a complicated history the partial differentiation model is consistent with most properties of these meteorites the behavior of five model parent bodies of different composition during heating to 1400 k the temperature recorded in many i ab silicates shows that meteorites like iab irons can form by this process sulfide rich parent bodies differentiate into a liquid s rich core and a primitive metal bearing mantle around 1230 k but in parent bodies lower in sulfide abundance this process may be delayed until minor melting of silicates occurs 1370 30 k in most cases any silicate melt that may have formed resolidified in situ the sulfide melt fractionally crystallized along the fe ni fes cotectic some melt was injected into silicates to form copiapo type breccias considerations of graphite metal silicate equilibrium and the retention of noble gases suggest that brecciated iabs formed at depth inside a moderately large a few tens to a hundred kilometers parent body the correlation of i xe ages with ni content is interpreted as the result of an excess 129xe component associated with the s rich melt which increases with melt evolution

paucity of sulfide in a large slab of esquel new perspectives on pallasite formation the 0 7 ton esquel meteorite found in patagonia before 1951 is a typical main group pallasite in most respects we examined petrographically a slab 1 m long having an area of 3000 cm2 that shows the typical pallasitic texture with fragmental olivine phase abundances in vol are olivine 66 metal 32 schreibersite 0 76 troilite 0 46 and chromite 0 31 esquel can be divided into four lithologies 1 pallasitic matrix consisting of olivine fragments embedded in metal 81 2 large 5 cm olivine nodules having low metal contents 18 3 massive metal 0 3 and 4 zones dominated by fes and fine olivine 0 7 main group pallasites appear to have formed by the intrusion of a highly evolved low ir high ni au and s metallic magma into fragmented olivine this model implies that fes should be abundant in main group pallasites and we had speculated that examination of an exceptionally large slab might reveal a high troilite content we found instead an exceptionally low fes content new compositional data confirm that esquel has a lower au content than other main group pallasites having similar ir contents literature data based however on relatively small sections suggest that high au pallasites have higher s contents than esquel but have lower s contents than expected from a trapped melt model we conclude that a relatively complex model is required to explain the origin of main group pallasites after intrusion the degree of crystallization of the metallic magma varied from location to location but in almost all cases an fes rich liquid either escaped or formed fes rich pallasitic rocks that are underrepresented in the meteorite inventory

group iva irons new constraints on the crystallization and cooling history of an asteroidal core with a complex history we report analyses of 14 group iva iron meteorites and the ungrouped but possibly related elephant moraine eet 83230 for siderophile elements by laser ablation icp ms and isotope dilution eet was also analyzed for oxygen isotopic composition and metallographic structure and fuzzy creek currently the iva with the highest ni concentration was analyzed for metallographic structure highly siderophile elements hse re os and ir concentrations vary by nearly three orders of magnitude over the entire range of iva irons while ru pt and pd vary by less than factors of five chondrite normalized abundances of hse form nested patterns consistent with progressive crystal liquid fractionation attempts to collectively model the hse abundances resulting from fractional crystallization achieved best results for 3 wt s compared to 0 5 or 9 wt s consistent with prior studies concentrations of hse and other refractory siderophile elements estimated for the bulk iva core and its parent body are in generally chondritic proportions projected abundances of pd and au relative to more refractory hse are slightly elevated and modestly differ from l ll chondrites which some have linked with group iva based on oxygen isotope similarities abundance trends for the moderately volatile and siderophile element ga cannot be adequately modeled for any s concentration the cause of which remains enigmatic further concentrations of some moderately volatile and siderophile elements indicate marked progressive depletions in the iva system however if the iva core began crystallization with 3 wt s depletions of more volatile elements cannot be explained as a result of prior volatilization condensation processes the initial iva core had an approximately chondritic ni co ratio but a fractionated fe ni ratio of 10 indicates an fedepleted core this composition is most easily accounted for by assuming that the surrounding silicate shell was enriched in iron consistent with an oxidized parent body the depletions in ga may reflect decreased siderophilic behavior in a relatively oxidized body and more favorable partitioning into the silicate portion of the parent body phosphate inclusions in eet show d 17 o values within the range measured for silicates in iva iron meteorites eet has a typical ataxitic microstructure with precipitates of kamacite within a matrix of plessite chemical and isotopic evidence for a genetic relation between eet and group iva is strong but the high ni content and the newly determined rapid cooling rate of this meteorite show that it should continue to be classified as ungrouped previously reported metallographic cooling rates for iva iron meteorites have been interpreted to indicate an inwardly crystallizing 150 km radius metallic body with little

shock melted material in the krymka ll3 1 chondrite behavior of the opaque minerals six large millimeter to centimeter size regions of one specimen of the krymka ll3 1 ordinary chondrite show evidence of having been completely or nearly completely shock melted in situ a phenomenon rarely observed in primitive chondrites the shock pressure nominally in the range of 75 90 gpa could only have been 30 35 gpa in a porous material like fine grained matrix the melted regions have an igneous texture and their silicates are zoned and unequilibrated large metal troilite intergrowths formed in these regions the metal has a nickel content corresponding to martensite and the troilite contains up to 4 2 wt nickel melting must have been very short and cooling very fast 100 c h at high temperature the metal contains up to 0 7 wt phosphorus abundant chromite crystals and sodium iron phosphate glass globules are found in troilite the differences in composition between the opaque phases found in the melted regions and those generally observed in unmetamorphosed chondrules are assigned to melting under closed system conditions surprisingly high co concentrations up to 13 wt were found in some metal grains in or at the periphery of melted regions they likely resulted from sulfurization of metal by sulfur vapor produced during the shock after solidification at least one other shock led to mechanical effects in the melted regions