origin of fayalitic olivine rims and lath shaped matrix olivine in the cv3 chondrite allende and its dark inclusions we have studied an allende dark inclusion by optical microscopy scanning electron microscopy electron microprobe analysis and transmission electron microscopy the inclusion consists of chondrules isolated olivines and matrix which as in the allende host is mainly composed of 5 20 m long lath shaped fayalitic grains with a narrow compositional range fa42 2 and nepheline olivine phenocrysts in chondrules and isolated olivine grains show various degrees of replacement by 5 10 m wide fayalitic rims fa39 2 and 100 1000 m wide translucent zones which consist of 5 20 m long lath shaped fayalitic grains fa41 1 intergrown with nepheline these fayalitic olivines like those in the matrix of the dark inclusion contain 10 20 nm sized inclusions of chromite hercynite and fe ni sulfides the fayalitic rims around remnant olivines are texturally and compositionally identical to those in allende host suggesting that they have similar origins chondrules are surrounded by opaque rims consisting of tiny lath shaped fayalitic olivines 1 3 m long intergrown with nepheline as in the allende host fayalitic olivine veins may crosscut altered chondrules fine grained chondrule rims and extend into the matrix indicating that alteration occurred after accretion we infer that fayalitic olivine rims and lath shaped fayalites in allende and its dark inclusions formed from phyllosilicate intermediate phases this explanation accounts for 1 the similarity of the replacement textures observed in the dark inclusion and allende host to aqueous alteration textures in cm chondrites 2 the anomalously high abundances of al and cr and the presence of tiny inclusions of spinels and sulfides in fayalitic olivines in allende and allende dark inclusions 3 abundant voids and defects in lath shaped fayalites in the allende dark inclusion which may be analogous to those in partly dehydrated phyllosilicates in metamorphosed cm ci chondrites we conclude that the matrix and chondrule rims in allende were largely converted to phyllosilicates and then completely dehydrated the allende dark inclusions experienced diverse degrees of aqueous hydrothermal alteration prior to complete dehydration the absence of low ca pyroxene in the dark inclusion and its significant replacement by fayalitic olivine in allende is consistent with the lower resistance of low ca pyroxene to aqueous alteration relative to forsteritic olivine hydro thermal processing of allende probably also accounts for the low abundance of planetary noble gases and interstellar grains and the formation of nepheline sodalite salite hedenbergite pyroxenes wollastonite kirschsteinite and andradite in chondrules and ca al rich inclusions

nebular and asteroidal modification of the iron isotope composition of chondritic components abstract the fe isotope signatures 56feirmm and 57feirmm of twelve chondrules three ca al rich inclusions cais and multiple matrix aliquots from two type 3 chondrites allende cv3 and chainpur ll3 were measured by mc icp ms the mass fractionation line for these samples is 57feirmm 1 45 0 05 56feirmm 0 01 0 02 with r2 0 9995 two processes appear to affect the fe isotope composition of these chondritic components initially short lived high temperature events within the solar nebula drove isotopic signatures towards isotopically heavier compositions subsequently metasomatism shifted isotopic signatures of chondrules and cais towards bulk like compositions the fe isotope composition of allende matrix 56feirmm 0 07 0 02 n 6 is similar to that of bulk allende 56feirmm 0 05 0 05 the range of chainpur matrix compositions is 0 03 56feirmm 0 02 this relatively homogeneous matrix composition appears to be governed by metasomatic exchange of iron although the possibility that matrix is heterogeneous on a scale smaller than the sample size cannot be ruled out cais from allende in which iron has been metasomatically introduced from the matrix are mainly unfractionated with respect to the bulk composition of allende the least altered cai which contains minor bulk feo 0 4 wt is isotopically lighter 56feirmm 0 16 with respect to bulk allende this signature may reflect condensation of isotopically light iron in the nebula or preferential incorporation of the lighter iron isotopes contained within the matrix chondrule fe isotope compositions 1 33 56feirmm 0 65 are driven by both high temperature nebular events and subsequent metasomatic exchange the relationship between compositional and isotopic signatures clearly points to an evaporation origin for type i chondrules which are isotopically heavier than type ii chondrules subsequent metasomatic exchange with matrix iron has driven some chondrules most notably the smaller chondrules towards a bulk like fe isotopic signature

comparative stable isotope geochemistry of ni cu zn and fe in chondrites and iron meteorites high precision ni isotopic variations are reported for the metal phase of equilibrated and unequilibrated ordinary chondrites carbonaceous chondrites iron meteorites mesosiderites and pallasites we also report new zn and cu isotopic data for some of these samples and combine them with literature fe cu and zn isotope data to constrain the fractionation history of metals during nebular vapor solid and planetary metal sulfide silicate phase changes the observed variations of the 62ni 58ni 61ni 58ni and 60ni 58ni ratios vary linearly with mass difference and define isotope fractionation lines in common with terrestrial samples this implies that ni was derived from a single homogeneous reservoir while no 60ni anomaly is detected within the analytical uncertainties ni isotopic fractionation up to 0 45 per mass difference unit is observed the isotope compositions of ni and zn in chondrites are positively correlated we suggest that in ordinary chondrites exchange between solid phases in particular metal and silicates and vapor followed by mineral sorting during accretion are the main processes controlling these isotopic variations the positive correlation between ni and zn isotope compositions contrasts with a negative correlation between ni and zn and cu isotope compositions which when taken together do not favor a simple kinetic interpretation the observed transition element similarities between different groups of chondrites and iron meteorites are consistent with the genetic relationships inferred from oxygen isotopes iiia pallasites and iva l chondrites copper is an exception which we suggest may be related to separate processing of sulfides either in the vapor or during core formation

fayalitic olivine in cv3 chondrite matrix and dark inclusions a nebular origin fayalitic olivine fa32 is the major component of the matrices and dark inclusions of cv3 and other unequilibrated chondrites it occurs most commonly as rims veins and halos in and around chondrule silicates in the allende type cv3oxa chondrites and to a much lesser extent in the reduced cv3r and bali type cv3oxb chondrites the olivines have distinctive platy tabular and lath or irregular shaped crystals with the ratio of the two types varying widely in cv3oxb chondrites matrix fayalitic olivines range up to fag99 9 whereas in the other cv3 chondrites the range is much smaller the platy and tabular anisotropic forms of the fayalitic olivines strongly suggest growth from a vapor and the nature of the occurrences suggests that cv3 matrices are unequilibrated mixtures of nebular materials we argue that the parent body hydration dehydration model has numerous inconsistencies that make this hypothesis highly unlikely these include 1 there is no direct evidence linking fayalitic olivine to precursor phyllosilicates 2 dehydration of phyllosilicates cannot explain the wide range of morphologies of the fayalitic olivines 3 fayalitic olivine clearly predates the formation of the hydrous phases in cv3 chondrites and is one of the phases that breaks down to form phyllosilicates keller et al 1994 4 the unequilibrated nature of the matrix including fine scale zoning in 10 m sized fayalitic olivine crystals would not survive the parent body metamorphism required in the dehydration model 5 a dark inclusion in the ningqiang chondrite contains fayalitic olivine rimmed by glassy and microcrystalline material zolensky et al 1997 which probably formed by radiation damage this indicates that the fayalitic olivine was exposed to solar radiation in a nebular setting 6 some allende chondrules contain unaltered primary anhydrous glassy mesostasis in contact with the host matrix e g ikeda and kimura 1995 chondrule mesostases would not have survived parent body hydration without becoming hydrated and would probably not survive the metamorphic heating required in the dehydration scenario 7 single platy and barrel shaped crystals of fayalitic olivine are present in accretionary rims in calcium aluminum rich inclusions cais macpherson and davis 1997 which developed in the nebula 8 matrix lumps completely encased in chondrules in ordinary chondrites contain mainly fayalitic olivine scott et al 1984 which indicates a nebular origin 9 oxygen isotopic compositions of allende matrix and dark inclusions strongly indicate little or no hydration for allende and its components clayton 1997 we favor a nebular vaporization recondensation model in which vaporization of chondritic dust produced a fayalite rich vapor followed by formation of the fayalitic olivine by direct recondensation from the vapor epitactic growth on surfaces of existing forsterite and enstatite in chondrules and replacement of existing forsterite and enstatite by gas solid exchange

iron isotope differences between earth moon mars and vesta as possible records of contrasted accretion mechanisms the iron isotope compositions of shergotty nakhla chassigny snc meteorites thought to come from mars eucrites and diogenites assumed to sample asteroid 4 vesta and rocks from the moon and earth have been measured using high precision plasma source mass spectrometry the means of eight samples from mars and nine samples from vesta are within error identical despite a range of rock types they are lighter by 0 1 in 57fe 54fe compared to the average of 13 terrestrial mantle derived rocks the latter value is identical within uncertainty with a previously published mean of 46 igneous rocks from the earth the average for 14 lunar basalts and highland plutonic rocks covering a broad spectrum of major element composition is heavier by 0 1 in 57fe 54fe relative to our estimate for the earth s mantle and therefore 0 2 heavier than the eucrites diogenites and snc meteorites however the data scatter somewhat and the apollo 15 green glass and apollo 17 orange glass are identical to samples from mars and vesta there is no clear relationship between petrological characteristics and fe isotope composition despite a wide spectrum of samples instead contrasted planetary isotopic signatures are clearly resolved statistically after evaluating alternative scenario it appears that the most plausible explanation for the heavier fe in the earth and moon is that both objects grew via processes that involved partial vaporisation leading to kinetic iron isotope fractionation followed by minor loss this is consistent with the theory in which the moon is thought to have originated from a giant impact between the proto earth and another planet combined with numerical simulations fe isotope data can offer the potential to provide constraints on the processes that occurred in planetary accretion

thrust fault mechanics and origin of a frontal ramp several features of thrust sheets in nonmetamorphic and relatively low grade metamorphic terrains indicate that the modes of faulting included both gradual and stick slip types of motion a first order quantitative analysis is made of the mechanics of this type of faulting in the analysis part of a wedge shaped thrust belt slips forward over a fault area of length l and unit width w producing a strain concentration in subhorizontal strata at the front of the growing thrust where a frontal ramp develops by shear fracturing equations for net slip x and the conditions for ramping are evaluated with limit equilibrium principles and coupled oscillator theory the quantitative analysis takes into account anisotropy resulting from contrasts in competency and facies changes a special case solution for maximum net slip associated with stick slip is lw ki 1 cos ot o 12 1 cos 1t where is the stress drop ki are spring constants of the strata 0 and 1 are natural frequencies of the system and t evaluated from another equation is the time for net slip the spring constants and natural frequencies can be evaluated from seismic and stratigraphic data a preliminary solution for piggy back thrusting indicates that ramp spacing can be systematic and that it depends upon the thickness of the deforming package the elastic parameters of the deforming package the slope of the topographic surface and the stresses along the trailing edge of the package

iron isotopes in hot springs along the juan de fuca ridge we present fe isotope data for high temperature hydrothermal fluids sulfides and basalts from the juan de fuca ridge usgs basalt standard bcr 2 and the canyon diablo iron meteorite the fe isotopic compositions of all samples fall within a range of 1 however there is a small but statistically significant isotopic variation between fe dissolved in hydrothermal fluids and fe in canyon diablo specifically the hydrothermal fluids are lighter compared to canyon diablo yielding 56fe 56fe 54fe sample 56fe 54fe canyon diablo 1 1000 values from 0 30 to 0 77 in contrast the bcr 2 basalt and canyon diablo are isotopically indistinguishable measurements of 56fe in juan de fuca ridge basalts and sulfides suggest that fe isotopes are fractionated in hydrothermal systems however the data do not exclude the possibility of small 56fe differences between the rocks from which fluid fe is derived and other igneous fe in which case fractionation in igneous systems is indicated regardless these results are evidence of non biological mass fractionation of fe isotopes in nature further venting of hydrothermal fluids into the overlying water column could provide a source of isotopically light fe to the deep oceans contributing to fe isotopic variations observed in deep sea sediments

homogeneous distribution of fe isotopes in the early solar nebula to examine the iron fe isotopic heterogeneities of ci and ordinary chondrites we have analyzed several large chips approximately 1 g from three ci chondrites and three ordinary chondrites ll5 l5 and h5 the fe isotope compositions of five different samples of orgueil one from ivuna and one from alais ci chondrites are highly homogeneous this new dataset provides a 56fe average of 0 02 0 04 2se n 7 which represents the best available value for the fe isotopic composition of ci chondrites and probably the best estimate of the bulk solar system we conclude that the homogeneity of ci chondrites reflects the initial fe isotopic homogeneity of the well mixed solar nebula in contrast larger up to 0 26 in 56fe isotopic variations have been found between separate approximately 1 g pieces of the same ordinary chondrite sample the fe isotope heterogeneities in ordinary chondrites appear to be controlled by the abundances of chondritic components specifically chondrules whose fe isotope compositions have been fractionated by evaporation and recondensation during multiple heating events

matrices of carbonaceous chondrite meteorites the morphology classification and chemistry of the matrices of carbonaceous chondrite cc meteorites is reviewed based on recent research results the various kinds of ccs are examined in terms of their matrix mineralogy alteration processes in ccs are discussed

fe and o isotope composition of meteorite fusion crusts possible natural analogues to chondrule formation meteorite fusion crust formation is a brief event in a high temperature 2000 12 000 k and high pressure 2 5 mpa regime we studied fusion crusts and bulk samples of 10 ordinary chondrite falls and 10 ordinary chondrite finds the fusion crusts show a typical layering and most contain vesicles all fusion crusts are enriched in heavy fe isotopes with d 56 fe values up to 0 35 relative to the solar system mean on average the d 56 fe of fusion crusts from finds is 0 23 which is 0 08 higher than the average from falls 0 15 higher d 56 fe in fusion crusts of finds correlate with bulk chondrite enrichments in mobile elements such as ba and sr the d 56 fe signature of meteorite fusion crusts was produced by two processes 1 evaporation during atmospheric entry and 2 terrestrial weathering fusion crusts have either the same or higher d 18 o 0 9 1 5 than their host chondrites and the same is true for d 17 o the differences in bulk chondrite and fusion crust oxygen isotope composition are explained by exchange of oxygen between the molten surface of the meteorites with the atmosphere and weathering meteorite fusion crust formation is qualitatively similar to conditions of chondrule formation therefore fusion crusts may at least to some extent serve as a natural analogue to chondrule formation processes meteorite fusion crust and chondrules exhibit a similar extent of fe isotope fractionation supporting the idea that the fe isotope signature of chondrules was established in a high pressure environment that prevented large isotope fractionations the exchange of o between a chondrule melt and an 16 o poor nebula as the cause for the observed nonmass dependent o isotope compositions in chondrules is supported by the same process although to a much lower extent in meteorite fusion crusts

sr stable isotope composition of earth the moon mars vesta and meteorites abstract high precision stable sr isotopic variations 88 sr 86 sr are reported in a variety of terrestrial samples martian and lunar meteorites hed undifferentiated primitive meteorites chondrules and refractory inclusions almost all the whole rock samples are isotopically indistinguishable at a 50 parts per million ppm level the exceptions are cv and co chondrites which are isotopically light and for which we believe that their isotopic composition is controlled by the proportion of refractory material five separated chondrules and one refractory inclusion from allende are isotopically light with 88 86 sr fractionations up to 1 73 whereas the matrix is enriched in the heavy isotopes 88 86 sr 0 66 the depletion in heavy isotopes observed in chondrules and refractory inclusions could be attributed to the condensation of a material already depleted in sr however in that case more than 60 of the original material would be unaccounted we propose instead that isotopic fractionation by electromagnetic sorting of ionized heavy sr from neutral sr in the early solar system for the origin of the fractionation observed in refractory inclusions and redistribution of sr by aqueous alteration for the origin of the fractionation observed in chondrules and matrix we conclude that cv and co chondrites are not the primary building blocks for earth and mars

introduction to the special section on fault behavior and the earthquake generation process this special section is a direct outgrowth of an agu chapman conference on fault behavior and the earthquake generation process that was convened in snowbird utah from october 11 to 15 1982 the body of information concerning the behavior of faults in space and time especially with regard to earthquake recurrence fault zone geometry and the mechanical and physical properties of fault zones that control rupture processes has been increasing at an enormous rate and perhaps more importantly our understanding of these subjects has direct societal application in the mitigation of earthquake hazards especially the identification of potential seismogenic sources and the characterization of these sources in terms of quantitative estimates of the location and probability of earthquake occurrence the motivation behind the conference was the desire to pause and reflect on the recent progress in the field and to evaluate and perhaps identify potentially important new areas of research

mineralogical and isotopic constraints on chondrule formation from shock wave thermal histories when a shock wave passes through a nebular gas increasing water enrichment leads to higher temperatures and postshock p tot but lower cooling rates a kinetic evaporation model is developed for tracking the chemical and isotopic changes that would occur in a clump of chondrule precursor dust surrounded by nebular gas in a closed system traversed by a nebular shock wave taking into account effects of non equilibrium melting and fractional crystallization on the liquid composition and the temperature difference between the gas and the droplet a range of shock wave temperature pressure histories computed for systems enriched relative to solar composition by factors of 550 in water to achieve the redox state of chondrules and 600 in dust to retard evaporation are employed and redox changes are assumed to occur on the time scale of heating and cooling in each two different system compositions are assumed with the mean fe si ratios of types i and ii chondrules two different textural outcomes are modeled po in which nuclei are preserved and olivine crystallization begins immediately upon reaching saturation and bo in which no nuclei are preserved and olivine crystallization begins only after 300 400 k of supersaturation in all cases all iron evaporates regardless of its oxidation state as well as alkalis and smaller fractions of mg and si in most cases recondensation occurs on the time scale of cooling resulting in droplets whose bulk compositions have small isotopic anomalies in mg si and fe comparable to those seen in bulk chondrules because fractional crystallization of olivine occurs before recondensation is complete however large isotopic variations especially for iron would have been recorded both within olivine crystals and between olivine and glass within these objects even after diffusive relaxation during crystal growth and cooling variations in d 25 mg of several tenths of a to several i nd 29 si of 0 1 to several and in d 56 fe of several would be measurable within large grains that grew throughout the olivine crystallization interval in many cases and olivine glass differences of pseveral tenths of a in d 29 si and of several in d 56 fe would be preserved such internal isotopic heterogeneities have not yet been observed in chondrules suggesting that the latter did not form in these shock wave thermal histories suppression of production of internal isotopic variations requires heating times that are shorter by a factor of 100 combined with dust enrichments p 6 10 4 and or p tot p10 2 bar together with relatively high fo2 these

trace element distributions within ordinary chondrite chondrules implications for chondrule formation conditions and precursors the concentrations of thirty six lithophile elements in the major silicate phases of chondrules have been determined by ion microprobe two types of porphyritic chondrule olivines normal and incompatible rich have been identified the incompatible rich olivines which have forsterite contents that are greater than fo98 appear to have formed under highly reducing conditions the olivine and low ca pyroxene grains in porphyritic chondrules have relatively unfractionated l chondrite normalized ree abundances compared to expectations from equilibrium distribution coefficients this is consistent with porphyritic chondrules having formed at cooling rates of the order of 1000 c h or faster the alkali metal na k rb and cs to al ratios in chondrule glasses vary by two orders of magnitude the lower values possibly being the result of volatile loss from chondrules no correlation was found between alkali metal depletions in chondrule glasses and enrichments in the surrounding rims but any such correlation may have been destroyed by the diffusive redistribution of these elements in rims that is likely to occur even under mild metamorphic conditions the refractory lithophiles with the exception of mg are all highly incompatible in silicate melts and are as a result concentrated in the chondrule glasses the refractory lithophiles in the glass are in most cases unfractionated from one another suggesting that neither vapor phase nor crystal liquid fractionation played a major role in the formation of chondrules or their precursors based largely on the observation that the refractory lithophiles are concentrated in the chondrule glasses it is suggested that previously reported correlations between these elements in bulk chondrule analyses are due to chondrules themselves as opposed to refractory condensates having been their own immediate precursors a simple monte carlo simulation using the compositions of the major chondrule silicate phases reported here supports this hypothesis it is argued that the upper limit of about one observed in the atomic na al ratios of chondule glasses is not due to an albitic precursor but simply reflects that for na to be incorporated into the structure of the melt it must take part in the coupled substitution naalo2 sio2

indicators of faulting events and periods of quiescence in desert alluvial fans the main objective of the present paper is to present an integrated approach to the study of faulting stages in coarse alluvial deposits the study site is the alluvial fan of nahal shehoret presently under an extremely arid regime the exposure of fault scarps on the fan surfaces enables the identification of several geomorphic indicators employed in analysing and separating faulting events and periods of quiescence a the nature of fault scarp topography b soil catenary development on fault scarps c buried paleosols in alluvial deposits d colluvial bodies e rotated clasts along fault planes these indicators enable us to determine at all sites studied the total amount of displacement to separate faulting events and the amount of their displacement per event and to evaluate scarp stability and the duration of periods of tectonic quiescence

iron isotopes in chondrules implications for the role of evaporation during chondrule formation we have measured the 57fe of olivines in nine chainpur chondrules all are within error of normal typically 2 1 2 0 most of the chondules could not have lost more than 20 of their feo by rayleigh evaporation and none can have lost more than 61 yet the range of fo contents in these chondrules is fo78 99 9 the isotopic compositions of the chondrules clearly demonstrate that for instance type i chondrules cannot form from type ii chondrules by evaporation of feo under rayleigh conditions the isotopic compositions also place constraints on the minimum cooling rates these chondrules could have experienced these cooling rates must also be equal to or slower than those required to produce the chondrule textures assuming flash heating and evaporation rates like those measured in vacuum the minimum cooling rates necessary to prevent detectable fe isotopic fractionation via rayleigh evaporation approach those needed to produce barred and porphyritic textures the presence of hydrogen in the nebula non linear cooling and other effects will all tend to increase the cooling rates required to prevent 57fe 1 2 0 perhaps by as much as 1 2 orders of magnitude the two most likely ways that the cooling rates required to prevent 57fe 1 2 0 can be kept below those needed to produce barred and porphyritic textures are 1 the ph2 in the nebula was low enough to keep evaporation rates close to those in vacuum or 2 back reaction of chondrules with fe in the gas suppressed isotopic fractionation

an isotopic and petrologic study of calcium aluminum rich inclusions from co3 meteorites we have studied the mineralogy and petrology of 229 calcium aluminum rich inclusions cais from ten co3 meteorites of petrologic types 3 0 3 7 subsets of these inclusions were measured by ion probe for magnesium calcium and titanium isotopes and ree abundances most cais from co3 meteorites fall into three major types 1 melilite rich inclusions which also contain spinel hercynite perovskite and occasionally hibonite 2 spinel pyroxene inclusions and 3 hibonite hercynite inclusions in addition several isolated hibonite grains two grossite caal 4o 7 bearing cais two hibonite fassaite microspherules and one anorthite spinel pyroxene inclusion were found cais from co3 meteorites exhibit all of the ree patterns commonly seen in inclusions from cv3 and cm2 chondrites most exhibit evidence of 26 ai and many have inferred 26 ai 27 ai o 5 10 5 the relative abundances of different types of cais in co3 chondrites differ from those in cv3 and cm2 chondrites cais in co3 chondrites have experienced considerable secondary alteration both before and after accretion signatures of nebular alteration include wark lovering rims and the high fe contents in spinels from all hibonite hercynite inclusions occasionally melilite and anorthite show evidence of nebular alteration to feldspathoids and pyroxene the magnesium aluminum systematics of some melilite rich inclusions were apparently disturbed prior to final accretion of the parent body parent body alteration is indicated by correlations between cai characteristics and the petrologic type of the host meteorite spinel in melilite rich and coarse grained spinel pyroxene inclusions becomes more fe rich with the development of relatively homogeneous hercynitic spinel 50 60 mol in cais from metamorphic grades 3 4 perovskite has been converted to ilmenite in types 3 4 melilite rich inclusions are abundant in co3 0 3 3 meteorites rare in 3 4 meteorites and absent meteorites of types 3 5 3 7 melilite rich cais are probably replaced by inclusions rich in feldspathoids pyroxene and fe rich spinel isotopic disturbance of the magnesium aluminum systematics may be more severe in higher petrologic types hibonite seems to be unaffected by this level of metamorphism three isotopically unusual inclusions were found one single crystal hibonite isna sp16 has a ree pattern strongly depleted in ce and y 26 ai 27 ai o 2 4 0 3 10 5 and mass fractionated calcium f ca 12 2 amu but no resolvable nuclear anomalies in neutron rich calcium isotopes the ree pattern which is thought to reflect nebular conditions and mass fractionated calcium indicative of evaporation are similar to those of the fun inclusion hal and related hibonites indicating similar formation conditions the absence in isna sp16 of the nuclear anomalies observed in hal and the difference in 26 ai 27 ai o between hal and isna sp16 indicate that the processes that produced hal type hibonites operated on diverse materials two hibonite bearing microspherules colony sp1 and alh82101 sp15 exhibit nearly flat ree patterns with negative europium anomalies and slightly negative 26 mg alh82101 sp15 has resolved excesses of 48 ca and 50 ti these characteristics are similar to those of previously described microspherules from murchison and lance implying that the microspherules formed via a single process from related but not identical source materials

trace element geochemistry of ordinary chondrite chondrules the type i type ii chondrule dichotomy we report trace element concentrations of silicate phases in chondrules from ll3 ordinary chondrites bishunpur and semarkona results are similar to previously reported data for carbonaceous chondrites with rare earth element ree concentrations increasing in the sequence olivine 10 k h than type i chondrules appreciable na concentrations 3 221 ppm are measured in olivine from both chondrule types type ii chondrules seem to have behaved as closed systems which may require chondrule formation in the vicinity of protoplanets or planetesimals at any rate higher solid concentrations in type ii chondrule forming regions may explain the higher oxygen fugacities they record compared to type i chondrules type i and type ii chondrules formed in different environments and the correlation between high solid concentrations and or oxygen fugacities with rapid cooling rates is a key constraint that chondrule formation models must account for

a nebula setting as the origin for bulk chondrule fe isotope variations in cv chondrites abstract we combined micro computer tomography with fe and si isotope measurements of mokoia allende and grosnaja chondrules ten mokoia chondrules contain 0 9 to 11 8 vol opaque phases metal sulfide and 6 allende chondrules contain 0 0 to 6 6 vol opaque phases hence the fe isotope composition of many chondrules is dominated by the fe isotope composition of their opaque phases we studied fe isotopes of 35 bulk chondrules the range is different for each of the three meteorites studied and largest for allende with 56 fe ranging from 0 82 to 0 37 six out of seven chondrules analysed for their si isotope composition in mokoia and grosnaja have similar 29 si of around 0 12 one anomalous chondrule in mokoia has a 29 si of 0 58 we exclude isotopically heterogeneous chondrule precursors and different isotopic chondrule reservoirs as the source of the observed fe isotope variation among bulk chondrules we conclude that the observed bulk chondrule fe isotope variation is the result of evaporation and re condensation processes in a nebula setting with high dust densities required to explain the comparatively low isotope fractionations subsequent parent body alteration slightly overprinted this pre accretionary fe isotope variation

high precision iron isotope measurements of terrestrial and lunar materials we present the analytical methods that have been developed for the first high precision fe isotope analyses that clearly identify naturally occurring mass dependent isotope fractionation a double spike approach is used which allows rigorous correction of instrumental mass fractionation based on 21 analyses of an ultra pure fe standard the external precision 1 sd for measuring the isotopic composition of fe is 0 14 mass for demonstrated reproducibility on samples this precision exceeds by at least an order of magnitude that of previous attempts to empirically control instrumentally produced mass fractionation dixon et al 1993 using the double spike method 15 terrestrial igneous rocks that range in composition from peridotite to rhyolite 5 high ti lunar basalts 5 fe mn nodules and a banded iron formation have been analyzed for their iron isotopic composition the terrestrial and lunar igneous rocks have the same isotopic compositions as the ultra pure fe standard providing a reference fe isotope composition for the earth and moon in contrast fe mn nodules and a sample of a banded iron formation have iron isotope compositions that vary over a relatively wide range from 56fe 0 9 to 1 2 this range is 15 times the analytical errors of our technique these natural isotopic fractionations are interpreted to reflect biological vital effects and illustrate the great potential fe isotope studies have for studying modern and ancient biological processes

an experimental study of chondrule formation from chondritic precursors via evaporation and condensation in knudsen cell shock heating model of dust aggregates abstract chondrules igneous objects of 1 mm in diameter formed in the earliest solar system via a transient heating event are divided into two types main type i feo poor and minor type ii feo rich using various chondritic materials for different redox conditions and grain sizes chondrule reproduction experiments were carried out at iw 2 to iw 3 8 with cooling rates mainly 100 c h with peak temperatures mainly at 1450 c and mainly at 100 pa in a knudsen cell providing near chemical equilibrium between the charge and the surrounding gas at the peak temperatures vapor pressures in the capsule were controlled using solid buffers after and during the significant evaporation of the iron component from the metallic iron poor starting materials in near equilibrium crystallization occurred this resulted in the formation of a product similar to the type i chondrules dusty olivine grains occurred in charges that had precursor type ii chondrules containing coarse ferroan olivine but such grains are not common in type i chondrules therefore fine grained ferroan matrices rather than type ii chondrules are main precursor for type i chondrules the type i chondrules would have evolved via evaporation and condensation in the similar conditions to the present experimental system residual gas which escaped in experiments could have condensed to form matrices leading to complementary compositions clusters of matrices and primordial chondrules could have been recycled to form main generation chondrules originated from the shock heating

new dust opacity mapping from viking infrared thermal mapper data global dust opacity mapping for mars has been carried forward using the approach described by martin 1986 for viking ir thermal mapper data new maps are presented for the period from the beginning of viking observations until ls 210 in 1979 1 36 mars years this range includes the second and more extensive planet encircling dust storm observed by viking known as storm 1977b improvements in approach result in greater time resolution and smaller noise than the earlier work a strong local storm event filled the hellas basin at ls 170 prior to the 1977a storm dust is retained in equatorial regions following the 1977b storm far longer than in mid latitudes minor dust events appear to raise the opacity in northern high latitudes during northern spring additional mapping with high time resolution has been done for the periods of time near the major storm origins in order to search for clues to the mechanism of storm initiation the first evidence of the start of the 1977b storm is pushed back to ls 274 2 preceding signs of the storm in images by about 15 hours

formation conditions of pyroxene olivine and magnesian olivine chondrules abstract dynamic crystallization experiments performed on a type i and an intermediate type ii iii composition demonstrate chondrule texture is controlled by the abundance of heterogeneous nuclei this factor is controlled by the maximum initial temperature relative to the olivine disappearance temperature for the given heating time glassy and excentroradial textures form from superheated melts with no nuclei and barred textures form with initial temperatures near the olivine disappearance temperature porphyritic and granular textures form with initial temperatures below the liquidus with abundant nuclei comparison of zoning characteristics of experimental olivines with those in natural porphyritic olivine chondrules indicates cooling rates of 100 to 1000 c h whereas olivine morphology indicates barred olivines cooled at rates of 250 to 1000 c h texture composition relationships suggest chondrules experienced initial temperatures below 1750 c these observations are consistent with the idea that chondrules formed by transient heating in dust rich regions i e in the midplane of the solar nebula

the lack of potassium isotopic fractionation in bishunpur chondrules in a search for evidence of evaporation during chondrule formation the mesostases of 11 bishunpur chondrules and melt inclusions in olivine phenocrysts in 7 of them have been analyzed for their alkali element abundances and k isotopic compositions except for six points all areas of the chondrules that were analyzed had 41k compositions that were normal within error typically 3 2s the six anomalous points are probably all artifacts experiments have shown that free evaporation of k leads to large 41k enrichments in the evaporation residues consistent with rayleigh fractionation under rayleigh conditions a 3 enrichment in 41k is produced by 12 loss of k the range of l chondrite normalized k al ratios a measure of the k elemental fractionation in the areas analyzed vary by almost three orders of magnitude if all chondrules started out with l chondrite like k abundances and the k loss occurred via rayleigh fractionation the most k depleted chondrules would have had compositions of up to 41k 200 clearly k fractionation did not occur by evaporation under rayleigh conditions yet experiments and modeling indicate that k should have been lost during chondrule formation under currently accepted formation conditions peak temperature cooling rate etc invoking precursors with variable alkali abundances to produce the range of k al fractionation in chondrules does not explain the k isotopic data because any k that was present should still have experienced sufficient loss during melting for there to have been a measurable isotopic fractionation if k loss and isotopic fractionation was inevitable during chondrule formation the absence of k isotopic fractionation in bishunpur chondrules requires that they exchanged k with an isotopically normal reservoir during or after formation there is evidence for alkali exchange between chondrules and rim matrix in all unequilibrated ordinary chondrites however melt inclusions can have alkali abundances that are much lower than the mesostases of the host chondrules which suggests that they at least remained closed since formation if it is correct that some or all melt inclusions remained closed since formation the absence of k isotopic fractionation in them requires that the k isotopic exchange took place during chondrule formation which would probably require gas chondrule exchange potassium evaporated from fine grained dust and chondrules during chondrule formation may have produced sufficient k vapor pressure for gas chondrule isotopic exchange to be complete on the timescales of chondrule formation alternatively our understanding of chondrule formation conditions based on synthesis experiments needs some reevaluation

retention of sodium during chondrule melting abstract type i chondrules in unequilibrated ordinary and carbonaceous chondrites tend to be enriched in refractory elements and depleted in volatiles relative to bulk ci type ii chondrules show chondritic concentrations of major and minor lithophile elements with na in particular at or slightly above the appropriate bulk rock values element ratio diagrams for chondrule bulk compositions show that type ii chondrules plot on a mixing line between forsterite and a na phase with na al 1 1 whereas type i compositions can be explained by mixing forsterite with melilite or cai or other refractory component with little na calcium in type ii chondrules must have been added as an al free phase such as diopside if their bulk compositions are manipulated subtracting ca as diopside and fe mg as olivine the residue is 90 albite and 10 silica which would be in pyroxene albite was incorporated into the precursors of type ii chondrules which clearly have not been depleted in na although their initial temperatures overlap with those of type i there is no negative correlation between type i liquidus temperatures 1450 1900 c and na al ratios and hence no indication of na loss from the melt if type i precursors contained albite the most aluminous chondrules would have suffered the most extreme na loss but these have the lowest liquidus temperatures their precursors were na depleted whereas those of type ii were na enriched as a function of the abundance of albite the simplest way to obtain the bulk compositions of chondrules is to assemble condensates into precursors at different temperatures and concentration of solids in the nebula or exceptionally rapid heating is required to preserve na in chondrules after melting substantial exchange of na occurred between chondrules and chondrite matrix during parent body metamorphism to petrologic type 3 6

secondary calcium iron rich minerals in the bali like and allende like oxidized cv3 chondrites and allende dark inclusions we have characterized ca fe rich silicates salite hedenbergite pyroxenes fs10 50wo45 50 andradite ca3fe2si3o12 kirschsteinite cafesio4 and wollastonite ca3si3o9 in the type i chondrules and matrices in the bali like and allende like oxidized cv3 chondrites and allende dark inclusions in type i chondrules in the bali like cv3 chondrites metal is oxidized to magnetite magnetite sulfide nodules are replaced by ca fe rich pyroxenes with minor andradite and pure fayalite we infer that ca fe rich pyroxenes andradite fayalite magnetite and phyllosilicates which occur in mesostases formed at relatively low temperatures 300 c in the presence of aqueous solutions thermodynamic analysis of phase relations in the si fe ca o h system and large o isotopic fractionation of the coexisting magnetite and fayalite 20 krot et al 1998 are consistent with this interpretation in type i chondrules in the allende like cv3 chondrites and dark inclusions magnetite sulfide nodules are replaced by ca fe rich pyroxenes and ferrous olivine low ca pyroxene and forsterite phenocrysts are rimmed and veined by ferrous olivine it appear that the ca fe rich pyroxenes predate formation of ferrous olivine the latter postdates formation of talc and biopyriboles brearley 1997 the allende dark inclusions are crosscut by ca fe pyroxene andradite veins and surrounded by ca rich rims that consist of ca fe rich pyroxenes andradite wollastonite and kirschsteinite calcium rich veins and rims formed after aggregation and lithification of the dark inclusions the rimmed dark inclusions show zoned depletion in ca which is due to a lower abundance of ca fe rich pyroxenes close to the rim calcium was probably leached from the inclusions and redeposited along their edges we infer that the allende like chondrites and dark inclusions experienced similar aqueous alteration to the bali like chondrites and were metamorphosed subsequently which resulted in loss of aqueous solutions and dehydration of phyllosilicates we conclude that ca fe rich silicates in the oxidized cv3 chondrites and allende dark inclusions are secondary and resulted from aqueous fluid rock interactions during progressive metamorphism of a heterogeneous mixture of hydrous ices and anhydrous materials the latter were possibly mineralogically similar to the reduced cv3 chondrites

gas melt interactions and their bearing on chondrule formation interactions between nebular gas and molten silicates or oxides could have played a major role in the formation and differentiation of the first solids formed in the solar system in order to simulate such interactions we set up a new experimental device in which isothermal condensation experiments have been conducted partially molten chondrule like samples have been exposed to high sio g partial pressures for intervals between 80 and 300 s and at temperatures ranging from 1600 to 1750 k results show that silica entering from the gas phase could be responsible for several textural and mineralogical features observed in natural chondrules for instance these experiments reproduce not only the mineralogical zonation of porphyritic olivine pyroxene chondrules with the peripheral location of pyroxenes but also olivine resorption textures and the common poikilitical enclosure of olivines in pyroxenes in the light of these similarities we advocate that gas melt interactions through condensation are viable mechanisms for chondrule formation and hence may place severe constraints on the history of these primitive objects in the nebula high sio g partial pressures could have been established by the volatilization of regions with high dust gas ratio a possible scenario for this stochastic thermal activity is the intense activity of the protosun in its young stellar object phase

earthquakes of the holocene geologic studies of earthquakes involve mapping of coseismic surface faulting and secondary deformation from historical events trenching and geomorphic analysis to define the timing and size of past earthquakes and investigations of fault zone structure and geometry in both unconsolidated sediments and bedrock this research is now being referred to as paleoseismology seismic geology and earthquake geology since the mid 1970s it has led to some of the most exciting and important contributions to the understanding of earthquake behavior in space and time hanks 1985 allen 1986 the present report is the first of what is hoped will become a regularly contributed summary of the geologic aspects of the study of earthquakes