indicators of aqueous alteration in cm carbonaceous chondrites microtextures of a layered mineral containing fe s o and ni a petrographic and transmission electron microscopy study of the mighei murchison and murray cm carbonaceous chondrites shows that much of the cm matrix material was probably produced by aqueous alteration of olivine pyroxene sulfide and metal the amount of cm matrix appears to be proportional to the degree of alteration as suggested by mcsween 1979 and microtextures of pcp poorly characterized phase provide evidence of the progressive alteration pcp is divided into two major types one occurs in chondrules and aggregates and consists largely of an fe ni s o phase type i and the other occurs in matrix and consists of the fe ni s o phase and cronstedtite in various proportions type ii microtextures of pcp suggest that it resulted from a three stage alteration process 1 1 type i was produced by alteration of kamacite in chondrules and aggregates presumably early in the parent body regolith 2 2 as the alteration advanced olivine and pyroxene were converted to serpentine type i pcp separated from chondrules and aggregates into the matrix during regolith gardening simultaneously the fe ni s o phase reacted with si released by alteration of olivine and pyroxene producing well formed platy cronstedtite and coherent intergrowths of the fe ni s o phase and cronstedtite the fe ni s o phase also recrystallized into platy and pod like crystals fe s ni cr and p were leached out of type i pcp and were deposited as small grains of fe ni sulfides magnetite chromite and a mineral unidentified containing fe ni cr and p as a result pcp came to consist primarily of the fe ni s o phase and cronstedtite i e type ii pcp 3 3 during continued alteration the well formed crystals of the fe ni s o phase cronstedtite and their intergrowths in type ii pcp were replaced by poorly formed fibers in comparison to other cm chondrites mighei murchison and murray are relatively unaltered their matrices retain abundant amounts of the fe ni s o phase and cronstedtite commonly as distinctive pcp grains which account for a large proportion of the fe in these meteorite matrices in more altered cm chondrites much of the fe ni s o phase was probably consumed to produce cronstedtite magnetite and sulfides with further alteration cronstedtite itself reacted with the serpentine to form ferroan serpentine thus the cm matrix was increasingly enriched in mg with alteration and pcp was increasingly degraded and intimately mixed with the magnesian phyllosilicates

impact shock behavior of mg and ca sulfates and their hydrates abstract shock recovery experiments on mgso4 caso4 and their hydrates kieserite epsomite and bassanite were performed to investigate shock induced dehydration and decomposition at shock pressures up to 36 gpa the recovered solid samples indicated dehydration at pressures below 24 gpa but no clear evidence was found for possible decomposition of mgso4 and caso4 to produce mgo or cao as final products these sulfates and hydrates have been observed on the surface of mars and the present experimental results can be applied towards understanding the presence of surface water on mars and the recycling of water by impacts this finding that the sulfate hydrates undergo dehydration upon impact as well as the fact that the sulfates caso4 and mgso4 absorb moisture suggests the total amount of water on mars has remained almost unchanged since the time of formation of the planet

petrology and raman spectroscopy of high pressure phases in the gujba cb chondrite and the shock history of the cb parent body abstract high pressure phases majorite possibly majorite pyropess wadsleyite and coesite are present in the matrix and in barred olivine fragments in the gujba cb chondrite grossular pyrope was also observed in some small inclusions the cb chondrites are metal rich meteorites with characteristics that sharply distinguish them from other chondrite groups all of the cb chondrites contain impact melt regions interstitial to their chondrules fragments and metal and a major impact event or events on the cb chondrite parent body is clearly a significant stage in its history we studied three areas interstitial to chondrules and metal in the gujba cba chondrite from raman spectra the barred olivine fragments and matrix in these regions have various combinations of olivine and low ca pyroxene as well as majorite garnet mg4si4o12 a phase that forms by high pressure transformation of low ca pyroxene and wadsleyite a high pressure product of olivine compositions of the majorite suggest both majorite and majorite pyrope solid solution may be present the mineral assemblage of majorite and wadsleyite suggest minimum shock pressures and temperatures of 19 gpa and 2000 c respectively the occurrences of high pressure phases are variable from one area to another on the scale of millimeters or less suggesting heterogeneous distribution of shock and or back transformation to low pressure polymorphs throughout the meteorite the high pressure phases record a high temperature pressure impact event that is superimposed onto and thus postdates formation of the chondrules and other components in the cb chondrites the barred chondrules and metal in the cb chondrites are primary materials formed prior to the impact event either generated in an earlier planetesimal scale impact event or in the nebula

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

fe s ca al bearing carbonaceous vein in the yamato 74130 ureilite evidence for a genetic link to carbonaceous chondrites an unusually large carbonaceous vein c vein 4 7 mm 0 3 mm in size was found in the yamato 74130 ureilite a petrographie and scanning electron microscope study of the c vein shows that it contains a complex network of s rich fe s rich al rich and si rich regions the al rich areas contain ca al rich clinopyroxenes a na ca silica rich phase fe rich olivine and al rich enstatite some clinopyroxenes contain up to 12 9 wt a12o3 and 5 4 wt tio2 resembling the fassaite that is common in ca al rich refractory inclusions in cv chondrites the internal regions probably consist of intimate mixtures of carbonaceous material and extremely small particles rich in s fe s al and si the elements that occur in the internal regions s fe al ca and na are those that are strongly depleted in ureilites relative to carbonaceous chondrites the presence of those elements in the c vein suggests that the internal materials are remnants of drain channels for the depleted elements produced during formation process of ureilites from carbonaceous chondrites those elements were probably mobilized under high temperature and pressure generated by shock impacts and escaped as mixtures of liquids and vapor through fissures produced within mechanically weak c veins the ti rich fassaite may be a remnant of the cv chondrite like material the c vein provides evidence that ureilites resulted from preferential melting and recrystallization of carbonaceous chondrites by a temperature gradient zone refining process during a planetesimal scale collision or collisions

mineralogy of coexisting pyroxenes in magnesian ureilites and their formation conditions abstract the occurrence of orthopyroxene in two magnesian ureilites found in antarctica yamato y 791538 and lew85440 has been confirmed by electron probe microanalysis and by single crystal x ray diffraction the first coexisting equilibrium pigeonite orthopyroxene pair has been found in y791538 the lew85440 orthopyroxene coexists with minor augite the orthopyroxenes and olivines in y791538 y74659 and lew85440 have almost identical compositions but the ca contents of their coexisting clinopyroxenes differ considerably these phase assemblages can be explained by crystallization of mafic silicates with a similar composition oxygen fugacity and oxygen isotope anomaly at different temperature above 1220 c the high temperature crystallization of refractory mg rich silicates and the possible preservation of the oxygen isotope anomaly of precursor carbonaceous chondrites like materials may be consistent with their formation processes probably through local melting of more matrices than mg rich silicates and crystal growth induced by the planetesimal scale collision

fine structures of planar deformation features in shocked olivine a comparison between martian meteorites and experimentally shocked basalts as an indicator for shock pressure

ca rich majorite derived from high temperature melt and thermally stressed hornblende in shock veins of crustal rocks from the ries impact crater germany shock veins up to 1 1 mm thick were found within non porous lithic clasts from suevite breccia of the nordlinger ries impact structure these veins were studied by optical microscopy in transmitted and reflected light and by scanning electron microscopy in shocked amphibolites two types of ca rich majorite occur within and adjacent to the veins the first type crystallized from shock induced melts within the veins si contents of these majorites suggest dynamic pressure of 15 17 gpa implying minimum temperatures in the range of 2 150 2 230 c the second type of majorite was formed adjacent to the shock veins within pargasitic hornblende this majorite contains significant amounts of h2o 0 7 0 9 wt based on the textural setting the shrinkage cracks and the chemical compositions of both phases a solid state mechanism is deduced for the hornblende to majorite phase transition both genetic types of ca rich majorite are described for the first time from a terrestrial impact crater along with stishovite majorite constitutes the second silicate mineral displaying sixfold coordination of si at ries using micro raman spectroscopy jadeite coesite and jadeite grossular were identified within local melt glasses of alkali feldspar and plagioclase composition respectively stishovite aggregates produced by solid state reaction along with shock induced high pressure melt glasses of almandine composition were also detected in shock veins of a garnet cordierite sillimanite restite the quenched homogeneous almandine glasses point to melting temperatures of more than 2 500 c for the veins our findings demonstrate that terrestrial shock veins can give valuable information on shock induced mineral transformations and transient high pressures of host rocks during a natural impact event

shock effects and argon loss in samples of the leedey l6 chondrite experimentally shocked to 29 70 ga pressures abstract shock recovery experiments between 29 and 70 gpa were made on the leedey l6 chondrite to determine the possible loss of radiogenic ar caused directly by passage of shock waves as opposed to purely thermal loss during prolonged post shock cooling experiments were made in vacuum to prevent shock implantation of terrestrial air and following neutron irradiation the 39 ar 40 ar ratios were measured by stepwise temperature release petrographie examination of experimentally shocked leedey showed predominantly solid state effects in the form of increasing disaggregation of component minerals with increasing shock pressure but very little melting even at the highest shock levels the melting behavior of the minor feldspar phase was difficult to document but likely was complete by the higher shock levels the 39ar 40ar release patterns were complex for both shocked and reference leedey probably because of a natural shock history for the meteorite at ca 3 8 gy the average 39ar 40ar age for shocked leedey decreased with increasing shock levels from essentially no change at 29 gpa to a 20 decrease in age at 60 gpa the feldspar phases degas at lower extraction temperatures and show even greater lowering of 39ar 40ar age with increasing shock pressure the sample shocked at 69 gpa shows less mechanical disruption than samples shocked to lesser pressures and also shows a smaller loss of ar compared to samples shocked at 45 and 60 gpa we conclude that observed ar loss is caused by extensive grain fracturing and heating during shock and that greater ar loss at low extraction temperatures is enhanced by feldspar melting none of the experiments duplicated the magnitude of ar loss commonly seen in naturally shocked materials that lack evidence of pervasive melting we conclude that large scale gas loss in shocked but unmelted samples requires that the materials reside for prolonged periods of time at elevated temperatures in a relatively hot impact formation either inside or outside the crater cavity

shock effects of the leoville cv carbonaceous chondrite a transmission electron microscope study leoville is a cv carbonaceous chondrite with a strong preferred orientation of the elongated chondrules and inclusions the texture probably resulted from deformation olivine grains in most chondrules show undulatory extinction and fine planar fractures transmission electron microscope observations reveal that most olivine grains in the matrix exhibit high densities of micro cracks and dislocations with burgers vector b 001 the densities of the dislocations are comparable to those in chondrule olivines olivine in the matrix also forms aggregates in places which comprise extremely small rounded to sub rounded grains and glassy material closely resembling the recrystallized olivine commonly seen in shocked ordinary chondrites enstatite in the matrix has numerous dislocations and lamellae that are caused by 100 stacking faults these micro textures are characteristic of deformation at high strain rates and are very similar to those in shocked ordinary chondrites and experimentally shocked materials therefore this suggests that the chondrules and matrix in the leoville experienced shock probably after accretion to the meteorite parent body comparison of the textures to those in experimentally shocked materials suggests that the shock pressures experienced by leoville were in the range 5 20 gpa we believe that multiple impacts by such relatively mild shock pressures compacted the leoville meteorite and caused the deformation of chondrules and the foliation

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

weakly shocked and deformed cm microxenoliths in the pultusk h chondrite the pultusk meteorite is a brecciated h4 5 chondrite cut by darkened cataclastic zones within the breccia relict type ia ib and iia chondrules and microxenoliths of carbonaceous cm chondrite lithology occur this is the first description of foreign clasts in the pultusk meteorite the matrix of the xenoliths was identified by usage of microprobe and raman spectroscopic analyses raman spectra show distinct bands related to the presence of slightly ordered carbonaceous matter at approximately 1320 and 1580 1584 cm 1 bands related to serpentine group minerals are also visible especially a peak at 692 cm 1 and moreover other weak bands are interpreted as evidence for tochilinite we decipher the metamorphic and deformational history of the xenoliths they experienced aqueous alteration before being incorporated into the unaltered and well equilibrated parent rock of the pultusk chondrite the xenoliths are weakly shocked as indicated by defects in the crystal structure of silicates and carbonates but hydrated minerals serpentine and tochilinite are still present in the matrix the carbonaceous matter within the clasts matrix displays first order d and g raman bands that suggests it is only slightly ordered as a result of mild thermal processing distinct shear bands are present in both the xenoliths and the surrounding rock which testifies that the xenoliths were affected by a deformational event along with host rock the host rock was brittly deformed but the clasts experienced more ductile deformation revealed by semibrittle faulting of minerals kinking of the tochilinite cronstedtite matrix and injections of xenolithic material into the adjacent breccia we argue that both processes the high strain rate shear deformation and the incorporation of the xenoliths into the host pultusk breccia could have been impact related the pultusk xenoliths are thus rather spalled collisional fragments than trapped fossil micrometeorites

frictional melting processes and products in geological materials introduction and discussion this special issue is the outcome of a theme session convened by the above authors entitled frictional melting processes and products in geologic materials held at the 1990 geological society of america annual meeting in dallas texas u s a the goals of the session were threefold to bring together researchers working on various aspects of frictional melting in rock to provide a forum for the exchange of new ideas and to encourage the production of a thematic set of papers dedicated to the topic of the eight papers presented here the first two spray swanson are primarily processand mechanism oriented the next magloughlin maddock o hara deal primarily with chemical aspects of host rock pseudotachylyte relations the last three mainly provide descriptions of naturally occurring koch and masch techmer et al and artificially produced kennedy and spray friction melts natural pseudotachylytes are described from strike slip zones magloughlin swanson thrusts and their listric counterparts koch and masch techmer et al and connecting lateral ramps o hara thus it appears that there exists an association between pseudotachylyte generation and relatively long lived largedisplacement faulting and shearing the prime objective of the meeting was to promote the integration of different approaches

studies of an artificially shock loaded h group chondrite five samples of the naturally unshocked kernouve h6 meteorite were artificially shock loaded to pressures of 70 165 270 and 390 kbar and the silicates and metal examined optically by scanning and transmission electron microscopy and by thermoluminescence tl olivine deformation is closely comparable to that in naturally shocked meteorites producing dislocations with burgers vector 001 at pressures of 165 kbar these are formed in well defined slip planes at 270 kbar olivine develops optical mosaicism has high dislocation densities throughout and is also highly fractured recovery due to heating is minimal in orthopyroxene the deformation mechanism changes from the clino inversion to unit dislocation slip between 70 and 165 kbar in diopside 001 and 100 twinning was produced plagioclase is inferred to have been progressively converted to maskelynite but some is still present in 270 kbar sample the microhardness of the kamacite in the samples increases with shock pressure the transformation pressure in the kamacite is 30 40 kbar higher than observed for iron meteorites annealed kamacite displays incipient polycrystallinity and martensite and taenite sometimes contains slip lines troilite acquired cracks undulose extinction twins polycrystallinity and finally melted as the shock pressure increased at pressures over 200 kbar there was a systematic decrease in the natural tl and the tl sensitivity detailed considerations of changes in the natural tltl sensitivity ratio for various regions of the tl glow curve suggest that two processes were effective during shock thermal drainage of electron traps and a reduction in the effective trap density it is suggested that the latter process associated with the vitrification of feldspar the tl phosphor an additional sample was subjected to a shock pulse which was spiked instead of square very distinctive changes were apparent thermal effects are conspicuous and with widespread annealing 600 800 c of metal and sulfide glassy opaque veins were produced which are analogous to the black veins in shock lithified gas rich meteorites anomalous low temperature tl was induced suggesting that a new or modified phase or mineral has become the dominant tl phosphor

meteoritic and other constraints on the internal structure and impact history of small asteroids studies of the internal structure of asteroids which are crucial for understanding their impact history and for hazard mitigation appear to be in conflict for the s type asteroids eros gaspra and ida spacecraft images and geophysical data show that they are fractured coherent bodies whereas models of catastrophic asteroidal impacts family and satellite formation and studies of asteroid spin rates and other diverse properties of asteroids and planetary craters suggest that such asteroids are gravitationally bound aggregates of rubble these conflicting views may be reconciled if 10 50 km s type asteroids formed as rubble piles but were later consolidated into coherent bodies many meteorites are breccias that testify to a long history of impact fragmentation and consolidation by alteration metamorphism igneous and impact processes ordinary chondrites which are the best analogs for s asteroids are commonly breccias some may have formed in cratering events but many appear to have formed during disruption and reaccretion of their parent asteroids some breccias were lithified during metamorphism and a few were lithified by injected impact melt but most are regolith and fragmental breccias that were lithified by mild or moderate shock like their lunar analogs shock experiments show that porous chondritic powders can be consolidated during mild shock by small amounts of silicate melt that glues grains together and by friction and pressure welding of silicate and metallic fe ni grains we suggest that the same processes that converted impact debris into meteorite breccias also consolidated asteroidal rubble internal voids would be partly filled with regolith by impact induced seismic shaking consolidation of this material beneath large craters would lithify asteroidal rubble to form a more coherent body fractures on ida that were created by antipodal impacts and are concentrated in and near large craters and small positive gravity anomalies associated with the psyche and himeros craters on eros are consistent with this concept spin data suggest that smaller asteroids 0 6 6 km in size are unconsolidated rubble piles c type asteroids which are more porous than s types and their analogs the volatile rich carbonaceous chondrites were probably not lithified by shock

the porosity and permeability of chondritic meteorites and interplanetary dust particles abstract we have investigated the porosity of a large number of chondritic interplanetary dust particles idps and meteorites by three techniques standard liquid gas flow techniques a new noninvasive ultrasonic technique and image processing of backscattered images the latter technique is obviously best suited to sub kilogram sized samples we have also measured the gas and liquid permeabilities of some chondrites by two techniques standard liquid gas flow techniques and a new nondestructive pressure release technique we find that chondritic idps have a somewhat bimodal porosity distribution peaks are present at 0 and 4 porosity a tail then extends to 53 type 1 3 chondrite matrix porosities range up to 30 with a peak at 2 the bulk porosities for type 1 3 chondrites have the same approximate range as exhibited by the matrix which indicates that other components of the bulk meteorites including chondrules and aggregates have the same average porosity as the matrix these results reveal that the porosities of primitive materials at scales ranging from nanogram to kilogram are similar which implies that similar accretion dynamics operated through 12 orders of size magnitude permeabilities of the investigated chondrites vary by several orders of magnitude and there appears to be no simple dependence of permeability with degree of aqueous alteration chondrite type or porosity

fabric analysis of allende matrix using ebsd fabric analysis of the interstitial matrix material in primitive meteorites offers a novel window on asteroid formation and evolution electron backscatter diffraction ebsd has allowed fabrics in these fine grained materials to be visualized in detail for the first time our data reveal that allende a cv3 chondrite possesses a uniform planar short axis alignment fabric that is pervasive on a broad scale and is probably the result of deformational shortening related to impact or gravitational compaction interference between this matrix fabric and the larger more rigid components such as dark inclusions dis and calcium aluminium rich inclusions cais has lead to the development of locally oriented and intensified matrix fabrics in addition dis possess fabrics that are conformable with the broader matrix fabric these results suggest that dis were in situ prior to the deformational shortening event responsible for these fabrics thus providing an argument against dark inclusions being fragments from another lithified part of the asteroid kojima and tomeoka 1996 fruland et al 1978 moreover both dis and allende matrix are highly porous 25 corrigan et al 1997 mobilizing a highly porous di during impact induced brecciation without imposing a fabric and incorporating it into a highly porous matrix without significantly compacting these materials is improbable we favor a model that involves allende dis cais and matrix accreting together and experiencing the same deformation events

a unique omphacite amphibole and graphite bearing clast in queen alexandra range que 99177 a metamorphosed xenolith in a pristine cr3 chondrite abstract a metamorphosed unshocked chondritic clast containing approximately equal volumes of matrix and chondrules has been identified in the primitive cr3 chondrite que 99177 the most striking features of this clast are the presence of ferroan omphacitic pyroxenes amphiboles pargasite and pargasitic hornblende and coarse grained up to 70 m graphite laths the clast also contains relatively equilibrated feo rich olivine low ca pyroxene and sulfides pentlandite and troilite the clast s matrix contains coarse interlocking crystals the chondrules are extensively recrystallized contain no mesostasis or metal and present poorly defined boundaries based on its compositional and crystallographic features omphacite appears to have formed at high pressures 6 gpa amphibole formation probably postdated graphite formation and occurred by reaction of olivine clinopyroxene and omphacite in the presence of an oxidizing na rich fluid at high temperatures 700 c the absence of metal suggests that this fluid was also rich in s

glassy pseudotachylyte veins from the fuyun fault zone northwest china glassy pseudotachylytes occur in granitic mylonites from the fuyun fault zone northwest china powder x ray diffraction data indicate that the fine grained matrices of the fuyun pseudotachylytes vary from almost crystalline to complete glass a few to 90 wt glass powder x ray diffraction patterns also indicate that clasts in the glass type pseudotachylyte veins mostly consist of quartz grains microlites displaying quenched or rapid cooling textures vesicles and amygdules flow structures and rounded fragments are also common in the fuyun pseudotachylytes the average bulk chemical compositions of these pseudotachylytes are similar to those of the host rocks but the glass matrix has a low sio2 component which is 5 15 wt lower than that of the host rocks the pseudotachylytes contain 2 3 wt water h2o using experimental data which relate the solubility of water to pressure approximately 400 bars lithostatic pressure corresponding to approximately 1 5 km depth of pseudotachylyte formation is estimated a minimum temperature estimate of 1450 c can be estimated due to the presence of the pure sio2 glass the x ray diffraction and chemical data indicate that the fuyun pseudotachylytes formed during seismic faulting at shallow depths by selective melting under water saturated conditions rather than by total melting or by crushing of the host rocks along fault surfaces

silicate darkening in the kobe ck chondrite evidence for shock metamorphism at high temperature the kobe ck4 chondrite like most metamorphosed ck chondrites exhibits pronounced silicate darkening of matrix and chondrule mesostases our petrographic and scanning electron microscopic study reveals that the matrix of kobe consists mostly of intermixtures of two types of fine grained olivine one forms subhedral to anhedral normal crystals the other fills interstices of the subhedral to anhedral olivine crystals exhibiting a complex network of veinlets the latter type of olivine contains high densities of small spherical vesicles 0 05 3 m in diameter and grains 0 05 5 m of magnetite and pentlandite as well as round to anhedral grains 1 10 m of plagioclase low ca pyroxene diopside and chlorapatite the vesicular olivine is particularly abundant in regions of matrix that exhibit a relatively high degree of darkening and commonly fills chondrule mesostases the vesicular olivine is clearly the principal cause of the silicate darkening in kobe the internal texture of the vesicular olivine closely resembles those of local melts produced from the matrices of experimentally and naturally shocked carbonaceous chondrites the occurrence and texture of the vesicular olivine suggest that it resulted from recrystallization of partially melted matrix olivine by shock kobe exhibits light shock effects in olivine that are consistent with shock stage s2 that is too low to explain the occurrence of olivine melting we suggest that the vesicular olivine in kobe was produced by shock metamorphism at a relatively mild shock pressure 600 c thus it is probable that the shock effects in olivine manifest as fracturing and deformation were relatively minor but heating was strong enough to cause partial melting of matrix olivine

structural deformation of the leoville chondrite foliations defined by alignment of elongated chondrules have been noted previously in chondrites but none displays this effect so well as leoville cv3 the shapes of leoville chondrules were produced by deformation in situ as indicated by inclusions and clasts with similar shapes and preferred orientations to those of chondrules similarities in the aspect ratios of apparent strain ellipses measured for chondrules alone 1 9 and 2 0 by several methods and for the whole meteorite 2 0 indicate either that leoville deformed homogeneously or that it deformed as a framework of touching chondrules this amount of strain corresponds to approximately 33 uniaxial shortening assuming constant volume because the strain ellipse was measured in only one orientation this strain value is a minimum estimate for the meteorite lack of correlation between foliation and either shock or thermal effects argues that impact or metamorphism are unlikely to have produced this deformation compaction due to overburden from progressive accretion on the chondrite parent body is suggested to have been its cause estimates of maximum deviatoric stresses in the interiors of asteroid sized bodies and constraints on maximum temperatures for cv3 chondrites are consistent with diffusional flow as the deformation mechanism for olivine in these chondrules diffusional flow is also suggested by the scarcity of observed lattice dislocations deformation of leoville olivines by this mechanism at geologically reasonable strain rates appears to require higher temperatures than those believed to have been experienced by this meteorite 600 c however differences in olivine grain size the presence of water or a more complex deformation history might explain this discrepancy

correlated alteration effects in cm carbonaceous chondrites three parameters are proposed to determine the relative extent of alteration in cm chondrites the mineralogic alteration index monitors the relative progress of coupled substitutions in the progressive alteration of cronstedtite to mg serpentine and increases with increasing alteration to calculate values of this index an algorithm has been developed to estimate the average matrix phyllosilicate composition in individual cm chondrites the second parameter is the volume percent of isolated matrix silicates which decreases with progressive alteration due to mineral hydration finally the volume percent of chondrule alteration monitors the extent of chondrule phyllosilicate production and increases as alteration proceeds these parameters define the first cm alteration scale that relies on multiple indicators of progressive alteration the following relative order of increasing alteration is established by this model murchison bells pollen murray mighei nogoya cold bokkeveld the relative degree of aqueous processing cochabamba and boriskino experienced is less precisely constrained although both fall near the middle of this sequence a comparison between the mineralogic alteration index and literature values for the whole rock chemistry of cm chondrites reveals several correlations a positive nearly linear correlation between bulk h content and progressive cm alteration suggests an approximately constant production rate of new phyllosilicates relative to the mineralogical transition from cronstedtite to mg serpentine the abundance of trapped planetary 36ar decreases systematically in progressively altered cm chondrites suggesting the wholesale destruction of primary noble gas carrier phase s by aqueous reactions because low temperature fluid rock reactions are generally associated with large isotopic mass fractionation factors we also compared our model predictions with 18o values for bulk cm samples although some of these data are poorly resolved the order of increasing 18o values approximates the order of increasing alteration predicted by our model parameters multiple correlations between diverse alteration parameters strongly suggest that a different cm chondrites experienced similar kinds of processes and conditions and b cm materials experienced in situ alteration on the cm parent body or bodies

cm chondrites exhibit the complete petrologic range from type 2 to 1 we have characterized the most phyllosilicate rich members of the cm chondrite group based upon petrographic and compositional factors we conclude that these particular meteorites have experienced pervasive aqueous alteration far beyond that witnessed by typical cms the lack of anhydrous silicates cai and except as relicts chondrules merits the classification of type cmi for three meteorites eet 83334 alh 88045 and one kaidun lithology still other cms notably alh 83100 eet 90047 and yamato 82042 are clearly intermediate between types 1 and 2 brecciated cm chondrites like cold bokkeveld contain all of these varied lithologies the cm chondrites thus exhibit the complete petrologic range from 2 through 1 our results show that progressive aqueous alteration on the parent cm asteroid s was locally accompanied by significant increases in temperature to a peak of 450 c for the kaidun lithology fo2 and locally degree of chemical leaching all well beyond the conditions recorded by typical cm2s the most altered cms also are commonly deformed displaying a distinct lineation probably due to static rather than dynamic forces

heating effects of the matrix of experimentally shocked murchison cm chondrite comparison with micrometeorites micrometeorites have been significantly altered or melted by heating which has been mainly ascribed to aerodynamic drag during atmospheric entry however if a major fraction of micrometeorites are produced by impacts on porous asteroids they may have experienced shock heating before contact with the earth s atmosphere tomeoka et al 2003 a transmission electron microscope tem study of the matrix of murchison cm chondrite experimentally shocked at pressures of 10 49 gpa shows that its mineralogy and texture change dramatically mainly due to shock heating with the progressive shock pressures tochilinite is completely decomposed to an amorphous material at 10 gpa fe mg serpentine is partially decomposed and decreases in amount with increasing pressure from 10 to 30 gpa and is completely decomposed at 36 gpa at 49 gpa the matrix is extensively melted and consists mostly of aggregates of equigranular grains of fe rich olivine and less abundant low ca pyroxene embedded in si rich glass the mineralogy and texture of the shocked samples are similar to those of some types of micrometeorites in particular the samples shocked at 10 and 21 gpa are similar to the phyllosilicate serpentine rich micrometeorites and the sample shocked at 49 gpa is similar to the olivine rich micrometeorites the shock heating effects also resemble the effects of pulse heating experiments on the ci and cm chondrite matrices that were conducted to simulate atmospheric entry heating we suggest that micrometeorites derived from porous asteroids are likely to go through both shock and atmospheric entry heating processes

a chemical and isotopic study of hibonite rich refractory inclusions in primitive meteorites the ion microprobe has been used to determine the concentrations of a number of trace elements including all 14 stable rare earth elements and the isotopic compositions of ca and mg in seven hibonite rich refractory inclusions hibonite in every inclusion analyzed shows the effect of ree partitioning with other solid or liquid phases in which hibonite is enriched in light over heavy ree compared to other coexisting phases this partitioning causes cl chondritenormalized enrichments to drop by factors of 4 to 22 from lanthanum to lutetium fractionations due to gas solid partitioning of ree are sometimes superimposed on the pattern produced by partitioning between condensed phases two inclusions hal and dh h1 show evidence for formation in extremely oxidizing environments and their ca is mass fractionated in favor of the heavy isotopes by 7 and 12 amu respectively two corundum hibonite inclusions show evidence of extremely high temperature gas solid fractionation such that the more volatile light ree are depleted as a group relative to the more refractory heavy ree these inclusions both have ca that is mass fractionated in favor of the light isotopes hibonite preserves the largest nuclear isotope anomalies in ti found to date faheyet al 1985a hintonel al 1987b but there does not seem to be a simple relationship between chemical composition isotopic mass fractionation and the sign and magnitude of nuclear isotopic anomalies in hibonite rich inclusions one possibility for producing a variety of chemical compositions and degrees of mass fractionation while preserving nuclear isotope anomalies is by multiple cycles of condensation and evaporation in a turbulent protoplanetary accretion disk morfill 1983 morfill and r n clayton 1986a b these cycles could also occur in a presolar nebula with large non axisymmetric temperature and pressure distributions boss 1988 nuclear anomalies are most likely to be preserved in hibonite because hibonite is the most refractory major phase to occur commonly in meteorites

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

shock induced dehydrogenation of polycyclic aromatic hydrocarbons with or without serpentine implications for planetary accretion abstract polycyclic aromatic hydrocarbons pahs were shocked across a peak shock pressure range of 5 8 to 36 6 gpa with or without serpentine and were then evaluated with an elemental analyzer an x ray diffractometer a gas chromatograph and a gas chromatograph mass spectrometer on the basis of the results we examined the shock induced dehydrogenation of pahs and the matrix s effects on it moreover we discussed the hydrogen amounts supplied to the earth s atmosphere and interior during its formation as well as the d values of those amounts we did not find any matrix effects on the dehydrogenation of pahs in the pressure range used in this study the incipient dehydrogenation of pahs occurred at 10 gpa and dehydrogenation was almost completed at 36 5 gpa assuming that carbonaceous chondrites such as the murchison meteorite accreted on and formed the earth they should have started and completed dehydrogenation at a point where the growing earth reached a radius of 1020 km and 3060 km respectively by the end of the accretion most of the hydrogen delivered to the earth had been discharged into the atmosphere the cumulative amount of hydrogen in the earth s interior at that point and its d value were calculated to be 3 3 10 21 kg and 47 respectively these calculated data were comparable to those already established for the earth s interior however to explain the isotopically light juvenile hydrogen d 125 in the deep earth reported by hauri chem geol 183 2002 115 141 we should consider materials that have low d values e g 850 of the presolar nebular as sources of the earth s hydrogen

the paradox between low shock stage and evidence for compaction in cm carbonaceous chondrites explained by multiple low intensity impacts abstract petrographic analysis of eight cm carbonaceous chondrites eet 96029 lap 031166 lon 94101 met 01072 murchison murray sco 06043 que 93005 by electron imaging and diffraction and x ray computed tomography reveals that six of them have a petrofabric defined by shock flattened chondrules with the exception of murchison those cms that have a strong petrofabric also contain open or mineralized fractures indicating that tensional stresses accompanying the impacts were sufficient to locally exceed the yield strength of the meteorite matrix the cms studied span a wide range of petrologic subtypes and in common with rubin 2012 we find that the strength of their petrofabrics increases with their degree of aqueous alteration this correspondence suggests that impacts were responsible for enhancing alteration probably because the fracture networks they formed tapped fluid reservoirs elsewhere in the parent body two meteorites that do not fit this pattern are met 01072 and murchison both have a strong petrofabric but are relatively unaltered in the case of met 01072 impact deformation is likely to have postdated parent body aqueous activity the same may also be true for murchison but as this meteorite also lacks fractures and veins its chondrules were most likely flattened by multiple low intensity impacts multiphase deformation of murchison is also revealed by the microstructures of calcite grains and chondrule defined petrofabrics as revealed by x ray computed tomography the contradiction between the commonplace evidence for impact deformation of cms and their low shock stages most belong to s1 can be explained by most if not all having been exposed to multiple low intensity i e

dark inclusions in the mokoia cv3 chondrite evidence for aqueous alteration and subsequent thermal and shock metamorphism mokoia is a cv3 chondrite that contains abundant phyllosilicate mineralization we present a detailed petrographic and scanning electron microscopic study of 24 dark inclusions dis that we found in mokoia the overall texture and constituent minerals of the dis resemble those in the host meteorite fe bearing saponite and na rich phlogopite the same phyllosilicates as in the host meteorite occur in the dis which strongly suggests that the dis have a similar alteration history to the host meteorite however the dis show several distinct differences from the host meteorite olivine grains in the di matrices are more homogeneous in fe fe mg ratio than those in the host meteorite matrix phyllosilicates in the dis are less abundant than in the host meteorite and they have been dehydrated to various extents these characteristics suggest that the dis have experienced higher degree of thermal metamorphism than the host meteorite in addition the matrices in the dis are more compacted than those in the host meteorite most olivine grains in the dis show undulatory extinction in transmitted crossed polarized light and some show planar fractures while such olivine grains are rare in the host meteorite two of the dis contain si mg fe and o rich melt veins these characteristics indicate that most dis have been shocked to shock stage s3 s4 while the host meteorite is shock stage s1 virtually unshocked thermal metamorphism of the dis was likely caused by shock heating these results are consistent with the contention previously proposed for the dis in cv3 chondrites i e the dis have experienced aqueous alteration and subsequent dehydration on the cv parent body we suggest that thermal and shock metamorphism occurred locally to various extents after pervasive aqueous alteration in the mokoia parent body

mineralogy of antarctic ureilites and a working hypothesis for their origin and evolution abstract mineralogical and crystallographic data and electron microprobe analyses of silicate minerals for antarctic ureilites y74123 y74130 y74659 y790981 alha77257 alha78019 and alh82106 extend the chemical and textural variations among ureilites beyond those known for non antarctic ureilites low ca pyroxenes possibly orthopyroxene poikilitically enclosed in olivine and coexisting with mg rich pigeonites have been identified in y74659 and alha77257 coexisting low ca pyroxene pigeonite and augite together with the very fe rich olivine in ureilites were observed in y74130 almost fe free pigeonite and diopside coexist in alh82106 no systematic differentiation trend has been reconstructed from these data but the pyroxene pairs yield formation temperatures above 1200 c a weak anticorrelation between mno and feo delineated by these pyroxenes suggests that a reduction reaction is responsible for the chemical variations a working hypothesis for ureilite formation involves high temperature episoded and a planetesimal scale collision which might have facilitated compaction and recrystallization of mafic silicates during the episodes fe ni s eutectic melt and a partial melt rich in ca al and fe might have been lost from a carbonaceous chondrite like source material

experimental shock metamorphism of the murchison cm carbonaceous chondrite abstract a series of shock recovery experiments were carried out on the murchison cm carbonaceous chondrite by using a single stage propellant gun the murchison samples were shocked in nine experiments at peak pressures from 4 to 49 gpa the recovered samples were studied in detail by using an optical microscope a scanning electron microscope and an electron probe microanalyzer chondrules are flattened in the plane of the shock front at 4 to 30 gpa the mean aspect ratio of chondrules increases from 1 17 to 1 57 roughly in proportion to the intensity of shock pressure up to 25 gpa at 25 to 30 gpa the mean aspect ratio does not increase further and chondrules show increasingly more random orientations and degrade their preferred orientations and at 35 gpa they are extensively disrupted most coarse grains of olivine and pyroxene are irregularly fractured fracture density increases with increasing shock pressure and at 30 gpa almost all are thoroughly fractured with subgrains of local shock melting occurs as melt veins and pockets at 20 to 30 gpa fracture filling veins of fine grains of matrix are also produced at 25 to 30 gpa the melts and the fine grains seem to result mainly from frictional heating due to displacement along fractures at 35 gpa melting occurs pervasively throughout the matrix the melts are mainly produced from the matrix however they are consistently more enriched in fe s and ca which indicates that these elements are selectively incorporated into the melts the melts contain tiny spherules of fe ni metal fe sulfide and numerous vesicles at 49 gpa the matrix is totally melted and coarse grains of olivine are partially melted the melts contain much larger vesicles 50 300 m in diameter than those in the samples shocked at lower pressures which indicates that much more intense devolatilization and gas expansion took place for the purpose of comparing shock thermal effects between the experimentally shocked samples and naturally shocked targets surface materials in the murchison parent body we calculated internal energy increase for compression by multiple shock wave reflections experimental case and for compression by a single shock wave natural case the results suggest that postshock thermal effects observed at each experiment may be attained by impact on the natural targets at a considerably lower shock pressure than the peak shock pressure from the results of our experiments and calculations we conclude that if the murchison parent body were shocked on the surface at pressures higher than 25 gpa shocked material would probably undergo drastic increase in the degree of comminution and simultaneous generation of strong expansive forces on pressure release thus the results support the hypothesis of scott et al 1992 that volatile rich carbonaceous chondrites shocked above 20 to 30 gpa escaped from the parent body and formed particles that are too small to survive as meteorites

simulation experiments for shocked primitive materials in the solar system abstract shock recovery experiments were conducted on porous mixtures initial porosity 35 5 in silicate metal and silicate metal sulfide systems in an attempt to simulate impact phenomena of unconsolidated porous materials similar to ordinary chondrites in a wide range of shock pressures from 20 to 70 gpa the textures and chemical compositions of shocked samples were investigated in detail features such as grain deformation fracture density decrease and heterogeneous meltings are found at

shock induced volatile loss from a carbonaceous chondrite implications for planetary accretion solid recovery impact induced volatile loss experiments on the murchison c2m meteorite indicate that for an impact of a given velocity h 2o and total volatiles are driven from the sample in the same proportion as present initially we infer that the volatiles other than h 2o driven from the meteorite also have the same bulk composition as those of the starting material thus the early bulk composition of an impact induced atmosphere of a planet growing by accretion from material like murchison would be the same as the volatile composition of the incident planetesimals incipient devolatilization of murchison occurs at an initial shock pressure of about 11 gpa and complete devolatilization occurs at a pressure of about 30 gpa if an earth sized planet were formed from the infall of planetesimals of murchison composition incipient and complete devolatilization of accreting planetesimals would occur when the planet reached approximately 12 and 27 respectively of its final radius thus impact induced devolatilization of accreting planetesimals and of the hydrated surface would profoundly affect the distribution of volatiles within the accreting planet for example for a cold homogeneous accretion of a planet prior to metallic core formation and internal differentiation the growing planet would have a very small core with the same volatile content as the incident material a volatile depleted mantle and an extremely volatile rich surface

near field modifications of stress induced by dyke injection at shallow depth summary the deformation and stress fields accompanying dyke injection in rift zones are described in terms of a vertical crack opening in response to internal overpressure in plane strain configuration previous numerical computations of the displacement field induced by shallow dykes are found to be affected in some cases by significant distortion mainly due to the limited extension of the computational domain difficulties in handling the singularities in the proximity of the free surface and an incorrect choice of the reference frame in particular it is found that no subsidence can be ascribed to the mere opening of a tensile crack in a homogeneous elastic half space if analytical constant dislocation models are employed surface displacements compare reasonably well with crack model solutions if the upper tip is not too shallow however constant dislocation solutions present unphysical singularities along the tips which distort significantly the near field stress pattern with respect to crack solutions if the principal stress axes are computed from crack models a broad region is found on both sides of the dyke where the induced stress has a nearly vertical intermediate axis while the compressive axis is normal to the dyke plane strike slip earthquakes are expected to prevail in this region above the upper crack tip a small region is present where the tensile stress is dominant and the intermediate stress is still vertical in the proximity of the free surface typically within a few tens of metres of it induced tensile stresses are greater than the lithostatic pressure open fissures might then develop in cohesionless soil or pre faulted rock the induced pressure in the host rock is found to be negative suction in the proximity of the ground surface and positive at greater depth fluid flow within the aquifers can be significantly altered by this induced overpressure and by the anisotropic modification of the pre existing permeability according to the modified coulomb failure criterion in the short term the shallower region characterized by suction is strengthened while the deeper pressurized region is weakened these results can explain in a straightforward way the abundance of strike slip focal mechanisms in volcanic areas the switch between tensional and compressional axes inferred from focal mechanisms of earthquakes in connection with a dyke injection episode on mount etna en echelon fracture systems observed at piton de la fournaise above the feeding dyke and precursory geochemical anomalies

constraints on the role of impact heating and melting in asteroids received 1996 august 30 accepted in revised form 1997 january 30 abstract imaging of asteroids gaspra and ida and laboratory studies of asteroidal meteorites show that im pacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks however in recent years impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events the latter were proposed to explain the origin of ureilites aubrites mesosiderites the eagle station pallasites acapulcoites lodranites and the iab iiicd and iie irons we considered fundamental observations from terrestrial impact craters combined with results from laboratory shock experiments and theoretical consider ations to evaluate the efficiency of impact heating and melting of asteroids studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of aster oids will produce two types of impact melts 1 large scale whole rock melts total melts not partial melts at high shock pressure and 2 localized melts formed at the scale of the mineral constituents mineral specific or grain boundary melting at intermediate shock pressures the localized melts form minuscule amounts of melt that quench and solidify in sifu thus preventing them from pooling into larger melt bodies partial melting as defined in petrology has not been observed in natural and experimental shock metamor phism and is thermodynamically impossible in a shock wave induced transient compression of rocks the total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids and impact melt rocks are usually clast laden thermal metamorphism of country rocks by impact is extremely minor experimental and theoretical considerations suggest that 1 single dis ruptive impacts cannot raise the average global temperature of strength or gravity dominated asteroids by more than a few degrees 2 cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter 3 small crater size low gravity and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small 0 014 1 fraction of the total displaced crater volume 4 total impact melt volume formed during the typical lifetime of an asteroid is a small fraction 0 001 of the volume of impact generated debris and 5 much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and thus not retained on the asteroid the inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above and we must turn to processes other than impact such as decay of short lived radionuclides or electromagnetic induction during an early t tauri phase of the sun to explain heating and melting of the parent bodies of these meteorites