the mineralogy of ordinary chondrites and implications for asteroid spectrophotometry abstract the normative mineralogies of 94 ordinary chondrites have been calculated from high quality bulk chemical analyses relative proportions of olivine orthopyroxene and metal are more variable within the h l and ll classes than has been recognized previously mineralogy within each chondrite class varies systematically with oxidation state these data have important implications for the design of reflectance spectroscopy studies that match asteroid and meteorite spectral features and for quantitative spectral interpretations of asteroids they also provide a tool with which to predict mineral abundances in unsampled asteroids that formed under different redox conditions

intraplate extension prior to continental extrusion along the ailao shan red river shear zone left lateral movement of the ailao shan red river shear zone lends support to the hypothesis of continental extrusion resulting from the collision of india with asia our new observations from northwestern yunnan china and northwestern vietnam on different sides of the shear zone demonstrate that the sinistral offset was 600 km according to correlations of permian triassic flood basalt successions and late paleogene highly potassic mafic magmas we conclude that the shear was propagating on the south china continental margin and does not correspond to a suture between south china and indochina furthermore the highly potassic magmas were emplaced from ca 40 to 30 ma before the shear movement which was caused by the late oligocene to early miocene ca 27 22 ma extrusion activity this suggests that a late eocene to early oligocene intraplate extension possibly induced by delamination of thickened continental lithosphere took place in northwestern yunnan or eastern tibet as a response to the india asia collision this extension and sea floor spreading of the south china sea that began ca 30 ma could have accounted for the initiation of the ailao shan red river shear zone

throughgoing crustal faults along the northern margin of the south china sea and their role in crustal extension a study of the rifled northern margin of the south china sea based on new single ship and two ship multichannel seismic data has revealed the presence of highly reflective steeply dipping normal faults which cut through the entire crust these faults with dips of 25 40 are associated with localized crustal thinning of 5 15 km and represent horizontal extension of the order of tens of kilometers a range of 5 20 km of horizontal extension has been measured along individual faults although the relative amount of horizontal extension taken up by these faults is modest compared to the total extension across the margin only about 15 or less the amount of localized crustal thinning which takes place along these fault surfaces is quite significant at one fault location the crustal thickness is reduced by 40 although the faults described have been recognized at three separate locations along the margin their trends appear to differ at each in two examples the faults are oblique to the inferred general north south extension direction of the margin this characteristic taken with the fact that some of the crustal faults also appear to serve as abrupt boundaries between broad regions of differing crustal thickness and reflectivity of the lower crust suggests that at least some of these faults were present prior to rifting such faults are postulated to have been former thrust faults perhaps associated with a triassic suture zone and or cretaceous subduction margin which have been reactivated as normal faults during subsequent rifting alternatively some of the throughgoing faults may represent crustal detachment surfaces that first formed as the result of a simple shear response to crustal extension

oxygen isotope relationships in iron meteorites iron meteorites with oxygen bearing phases can be classified in terms of their oxygen isotopic abundances these iron meteorite classes are isotopically similar to various stony meteorite classes which may indicate a common origin the group iab and iiicd irons may be related to the winonaites group iie irons may be related to h chondrites group iva irons may be related to l or ll chondrites

metal sulfide silicate intergrowth textures in el3 meteorites origin by impact melting on the el parent body abstract we document the petrographic setting and textures of fe ni metal the mineralogy of metallic assemblages and the modal mineral abundances in the el3 meteorites asuka a 881314 a 882067 allan hills 85119 elephant moraine eet 90299 eet 90992 lapaz icefield 03930 macalpine hills mac 02635 mac 02837 mac 02839 mac 88136 northwest africa nwa 3132 pecora escarpment 91020 queen alexandra range que 93351 que 94321 que 94594 and higher petrologic type els dar al gani 1031 el4 sayh al uhaymir 188 el4 mac 02747 el4 que 94368 el4 and nwa 1222 el5 large metal assemblages often containing schreibersite and graphite only occur outside chondrules and are usually intergrown with silicate minerals euhedral to subhedral enstatite silica and feldspar sulfides troilite daubreelite and keilite are also sometimes intergrown with silicates numerous authors have shown that metal in enstatite chondrites that are interpreted to have been impact melted contains euhedral crystals of enstatite we argue that the metal sulfide silicate intergrowths in the els we studied were also formed during impact melting and that metal in el3s thus does not retain primitive i e nebular textures likewise the el4s are also impact melt breccias modal abundances of metal in the el3s and el4s range from approximately 7 to 30 wt these abundances overlap or exceed those of el6s and this is consistent either with pre existing heterogeneity in the parent body or with redistribution of metal during impact processes

ree geochemistry of oldhamite dominated clasts from the norton county aubrite igneous origin of oldhamite abstract oldhamite dominated lithic clasts represent a new igneous lithology of the aubrite parent body they contain single crystals of oldhamite up to 2 cm in size with inclusions of ferromagnesian alabandite troilite daubreelite caswellsilverite and fe ni metal they are usually in intimate contact with a silicate portion consisting of enstatite forsterite and or plagioclase textural evidence for igneous origin includes apparent primary igneous grain boundaries between oldhamite and forsterite coarse grain size and the presence of round droplet like mn fe mg cr na sulfide inclusions within oldhamite which appear to represent an immiscible sulfide liquid we propose that the oldhamite dominated lithology formed during the melting and fractionation of enstatite chondrite like precursor material and represents a locally cas rich facies during melting two mutually immiscible sulfide liquids a ca sulfide and a mg fe mn cr na sulfide formed in the silicate magma upon cooling the immiscible sulfides crystallized forming large oldhamite crystals containing inclusions of mn fe mg cr na bearing sulfides forsterite enstatite and plagioclase crystallized from the surrounding silicate melt at subsolidus temperatures tiny ferromagnesian alabandite crystals exsolved from oldhamite ree abundances in oldhamite are high about 200 ci but ree patterns are nearly identical within single crystals and from clast to clast indicating equilibrium conditions high ree abundances have been cited as evidence that oldhamite grains in aubrites are nebular relics however we find it difficult to imagine that the rather homogeneous ree patterns of oldhamite in the oldhamite dominated lithology of norton county are not the result of equilibration of the rees with a silicate melt during formation of the igneous aubrites through parent body melting differentiation fractionation and cooling where peak temperatures of around 1450 1500 c must have been reached we conclude that oldhamite in the oldhamite dominated lithology of norton county is of igneous origin and that its ree abundances were established by equilibration with the aubrite silicate melt

major and trace element and sr nd isotope constraints on the origin of paleogene volcanism in south china prior to the south china sea opening abstract paleogene volcanic rocks crop out in three sedimentary basins namely sanshui heyuan and lienping in the attenuated continental margin of south china lavas from the sanshui basin which erupted during 64 43 ma are bimodal consisting of intraplate tholeiitic basalt and trachyte rhyolite associations similar to cretaceous a type granites from the nearby region the felsic member shows peralkaline nature na2o k2o 10 12 na k al 0 98 1 08 general enrichment in the incompatible trace elements and significant depletion in ba sr eu p and ti although both types of the sanshui lavas have rather uniform nd isotope compositions nd t 6 to 4 that are comparable to late cenozoic basalts around the south china sea the felsic rocks possess apparently higher initial sr isotope ratios isr up to 0 713 and form a horizontal array to the right in the nd vs sr isotope plot closed system differentiation of mantle derived magmas in a double diffusive magma chamber is considered for the bimodal volcanism in which the trachytes and rhyolites represent a type melts after extensive crystal fractionation in the upper portion of the chamber such a type melts were later contaminated by small amounts 1 3 of upper crustal materials during ascent on the other hand composition of lavas in the other two basins varies from tholeiitic basalt to andesite their sr and nd isotope ratios isr 0 705 to 0 711 nd t 1 to 5 and generally correlative nb ta depletions suggest a distinct magma chamber process involving fractional crystallization concomitant with assimilation of the country rock we conclude that these paleogene volcanic activities resulted from the lithospheric extension in south china that migrated southwards and eventually led to opening of the south china sea during 30 16 ma

portales valley a meteoritic sample of the brecciated and metal veined floor of an impact crater on an h chondrite asteroid a shower of meteorite fragments fell at 0730 h local time on 1998 june 13 near the town of portales new mexico thus far 51 pieces of the portales valley h6 meteorite have been recovered this meteorite has an unusually large number of metallic veins some of these veins are also unusually thick having widths on the order of centimeters these wide veins have fine widmanstatten structure which is the first time it has been seen in an ordinary chondrite this structure indicates the metallic veins and the host chondrite cooled slowly these veins appear to have been produced by shock metamorphic processes which we infer produced a 20 km diameter impact crater on an h chondrite planetesimal

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

shock effects in eh6 enstatite chondrites and implications for collisional heating of the eh and el parent asteroids abstract of the six chondrites that were listed as eh6 or eh6 an during the course of this study we confirm the eh classification of y 8404 y 980211 and y 980223 and the eh an classification of y 793225 two chondrites a 882039 and y 980524 are reclassified as el the former contains ferroan alabandite and both contain kamacite with 1 wt si all of the meteorites contain euhedral enstatite grains surrounded by metal sulfide although this texture is rare in y 793225 consistent with enstatite crystallizing from a mixed melt all contain enstatite with 0 5 mg

a petrologic study of the iab iron meteorites constraints on the formation of the iab winonaite parent body abstract we have studied iab iron meteorites and their silicate bearing inclusions to elucidate the origin of their parent body we have divided iab irons into five categories which best describe the inclusions and other properties of the irons these are 1 sulfide rich irons such as mundrabilla that contain a significant volume fraction of troilite with or without associated silicates 2 irons with non chondritic silicate inclusions such as basaltic inclusions in caddo county 3 irons with rounded troilite and or graphite inclusions sometimes containing embedded angular silicate inclusions 4 irons with angular silicate inclusions similar to those of the winonaites where silicates may comprise up to 40 vol of the rock and 5 irons with phosphate bearing inclusions such as the brianite bearing iab san cristobal similarities in mineralogy mineral and oxygen isotopic compositions suggest that iab irons and winonaites are almost certainly from a common parent body iiicd irons may also have originated on this parent body although evidence supporting this suggestion is less convincing previous models for the origin of iab irons propose that they are either products of selective impact melting in the megaregoliths of a chondritic asteroid or products of partial melting and fractional crystallization during formation of a sulfur rich core the former model suffers from the fact that impact is incapable of causing preferential melting of metal and sulfide on a large scale and pooling of these selective melts into sizable bodies the latter model does not readily explain the occurrence of often fragmentary angular silicate clasts in a metallic fe ni host we propose a new model for the origin of iab irons that incorporates aspects of both previous models we suggest that a chondritic possibly heterogeneous parent body was heated by a non collisional heat source which caused partial but not complete melting of the body after peak temperatures were reached and cooling and crystallization had begun a major impact catastrophically fragmented this body and the debris gravitationally reassembled scrambling the parent body lithologies we suggest that the impact caused extensive mixing of molten metallic fe ni and solid silicate material resulting in the formation of iab irons with silicate inclusions whereas winonaites represent metamorphosed rocks and partial melt residues of the precursor silicate lithologies high temperatures of some of the reassembled debris and deep burial resulted in continued fractionation and formation of the 2

compositional continuity of enstatite chondrites and implications for heterogeneous accretion of the enstatite chondrite parent body abstract chemical compositions of eleven enstatite chondrites 1 eh5 4 eh3 3 el3 i el5 and 2 el6 and their metal and nonmagnetic fractions determined by instrumental neutron activation analysis are reported the abundances of nonvolatile lithophile elements such as al sc and mg increase from eh to el while those of siderophile and moderately volatile elements decrease in the sequence of eh5 to eh3 el3 and el5 6 the continuity in compositions of enstatite chondrites and in particular the inverse variations of moderately volatile element abundances with petrographic type between eh and el groups demonstrate that enstatite chondrites have been derived from a common parent body the enstatite chondrite parent body formed by heterogeneous accretion of materials available in the accretional region metal was effectively accreted into the core where eh5 chondrites were derived and the abundances of metal decreased as accretion proceeded in complement silicate abundances gradually increased in the layers that accreted later the lack of correlation between volatile element abundances and metamorphic degree demonstrates that losses of moderately volatile elements from enstatite chondrites cannot have resulted from parent body processes furthermore it is expected that during accretion various components would mix and thus erase any early fractionation of moderately volatile elements of a single parent body variations of ambient gas temperatures during accretion are also impossible to produce the volatile element pattern of enstatite chondrites it is suggested that moderately volatile elements in the enstatite chondrites were lost during local heating the chondrule formation process formation of chondrules appears to have proceeded during the accretion of the enstatite chondrite parent body and lasted for a certain period the mineralogical and textural features of enstatite chondrites can be explained in terms of two stages of metamorphism the metamorphic trend from eh3 to eh5 resulted from internal heating whereas the trend from el3 to el6 was due to an external heat source it seems very likely that the eh chondrites were metamorphosed during accretion thus cooled rather rapidly activities of the early sun could serve as an energy source for the external heating

hf w chronometry of the iab iron meteorite parent body abstract the iab iron meteorites are unusual in having abundant silicate inclusions but the relationship of metal and silicates is not well understood in order to study the sequence and the timing of events that led to the observed structure of iab iron meteorites the hf w isotope system was applied to metal and silicate separates from seven iab iron meteorites a major metal segregation event on the iab parent body is dated by the average 182 w 184 w ratio of the iab metal phases 3 1 0 2 e 182 w units corresponding to a time of 1 8 2 3 2 0 ma after the formation of ca al rich inclusions cais the 182 hf 182 w data for three bulk inclusions from landes copiapo and el taco define a linear trend interpreted as an isochron corresponding to an age of 2 5 2 3 2 0 ma after cai formation this age most likely dates silicate differentiation on the iab parent body and is within analytical uncertainty indistinguishable from the major metal segregation event several silicate inclusions are displaced from the iab silicate isochron towards less radiogenic w compositions most likely reflecting thermal events on the parent body metal segregation and silicate differentiation occurred early enough so that these processes could have at least partially been triggered by an internal heat source e g 26 al basaltic inclusions of the caddo county specimen define an age of 4 6 0 7 0 6 ma most likely dating a partial melting event on the iab parent body three silicate separates from another el taco inclusion yield an age that postdates cai formation by 10 8 2 4 2 0 ma and the iab silicate isochron by 8 3 ma this prolonged interval of local thermal events on the iab parent body are best explained by collisional heat sources i e impacts the hf w ages for thermal events on the iab parent body are within the range of previously reported 129 i 129 xe ages but are significantly older than published 87 rb 87 sr 147 sm 143 nd and most 40 k 40 ar ages most likely these chronometers were reset during later thermal events that had only a minor effect on the 182 hf 182 w and 129 i 129 xe systems

origin and history of impact melt rocks of enstatite chondrite parentage we have conducted petrologic chemical and isotopic studies of two impact produced rocks of enstatite chondrite parentage ilafegh 009 is a total impact melt rock with no residual lithic clasts formation on the el chondrite parent body is suggested by its mineralogy and mineral compositions cooling of the impact melt was rapid at melt temperatures and decreased at subsolidus temperatures in contrast to previous studies we show that happy canyon is not a new enstatite achondrite but an impactmelt breccia of enstatite chondrite and not aubrite parentage this rock formed by impact melting and incorporation into the melt of clastic material which resulted in relatively rapid cooling at all temperatures mineralogical and bulk compositional data probably biased by the heterogeneous nature of this rock do not allow unequivocal determination of its parent body i e el vs eh although some data such as bulk total fe content seem to favor el parentage both rocks were subjected to post solidification shock which was more severe for ilafegh 009 than for happy canyon it appears that both impact melt rocks could have formed by impact melting 4 57 ga ago as is indicated by the nearly identical ixe closure ages of 1 6 and 1 4 ma before bjurbole for ilafegh 009 and happy canyon respectively an apparently younger 39ar40ar age of 4 53 ga for happy canyon may be due to small biases in the intercalibration of the ixe and 39ar40ar chronometers whereas the much younger 4 34 4 44 ga age for ilafegh 009 reflects thermal resetting during shock metamorphism shallowater which was impact derived from a different enstatite achondrite parent body has an ixe closure age 0 4 ma younger than that for ilafegh 009 and an 39ar40ar age of 4 53 ga the ancient ages of these three rocks attest to the intense early bombardment in this region of the solar system

intraplate tectonics in asia a precise age for large scale miocene movement along the ailao shan red river shear zone china tertiary left lateral movement along the 1000 km long ailao shan red river shear zone appears to have played an important role in absorbing post collisional northward penetration of india into asia crustal strike slip shear along this zone caused the formation of a gneiss belt metamorphosed to amphibolite grade including anatectic melting both metamorphism and melting were induced by ductile deformation yielding the possibility to date the major tectonometamorphic event that shaped the ailao shan red river belt 17 u pb isotope analyses were performed on small size fractions of zircon monazite and xenotime extracted from two different leucogranitic layers the two samples are located about 50 km apart in the central segment of ailao shan in structurally well controlled settings where the melts crystallized within the strongly foliated gneisses during late stages of deformation all mineral u pb analyses had to be corrected for excess or deficit amounts of radiogenic 206 pb originating from initial 230 th disequilibrium in the 238 u decay series in monazite such disequilibrium 206 pb reaches 20 of total radiogenic 206 pb the corrected u pb ages of monazite and xenotime lie between 22 1 and 23 9 ma whereas zircon yields significantly discordant u pb ages between 30 5 and 33 9 ma pointing to precambrian material in the magma source region inherited components could also be detected in monazite the set of u pb data shows that monazite and xenotime formed simultaneously in both localities substantiating an early miocene age of 23 0 0 2ma for late kinematic crystallization of anatectic melts in the metamorphic belt of the ailao shan red river shear zone

a petrologic and isotopic study of lodranites evidence for early formation as partial melt residues from heterogeneous precursors abstract we have conducted petrologic chemical and isotopic studies of lodranites in an attempt to constrain their genesis lodran gibson y 791491 y 791493 y 74357 y 8002 y 75274 mac 88177 lew 88280 eet 84302 fro 90011 and que 93148 are classified as lodranites lodranites and acapulcoites are indistinguishable on the basis of oxygen isotopic compositions but are distinct in average grain sizes of their mafic silicates with lodranites being significantly coarser grained lodranites exhibit a diverse range of petrologic and mineralogic features they range widely in mafic silicate compositions fa3 13 plagioclase 0 11 4 vol fe ni metal 0 5 20 vol and troilite 0 2 5 3 vol contents and shock levels s1 s4 they appear to have experienced high peak temperatures and rapid cooling in the temperature range recorded by metallographic cooling rates i e 700 350 c the only dated lodranite gibson cooled to ar closure temperatures at 4 49 0 01 ga lodranites formed from chemically and isotopically heterogeneous precursors in which the mineral and oxygen isotopic compositions were correlated heating of their parent body to temperatures between 1050 1200 c resulted in formation of fe ni fes and basaltic partial melts depletions of troilite and or plagioclase in most lodranites testify to the removal of some of these partial melts although melt migration was complex lodranites appear to have experienced a complex cooling history of slow cooling at high temperatures followed by rapid cooling at intermediate temperatures possibly related to breakup of the parent body lodranites were liberated from their parent body during 1 3 impact events with most having cosmic ray exposure ages of 5 5 7 ma the acapulcoites are samples from the same parent body but were heated to lower temperatures and thus experienced lower degrees of partial melting

petrogenesis of anomalous queen alexandra range enstatite meteorites and their relation to enstatite chondrites primitive enstatite achondrites and aubrites queen alexandra range que meteorite 94204 is an anomalous enstatite meteorite whose petrogenesis has been ascribed to either partial melting or impact melting we studied the meteorite pairs que 94204 97289 97348 99059 99122 99157 99158 99387 and yamato y 793225 these were previously suggested to represent a new grouplet we present new data for mineral abundances mineral chemistries and siderophile trace element compositions of fe ni metal in these meteorites we find that the texture and composition of y 793225 are related to el6 and that this meteorite is unrelated to the ques the mineralogy and siderophile element compositions of the ques are consistent with petrogenesis from an enstatite chondrite precursor we caution that potential re equilibration during melting and recrystallization of enstatite chondrite melt rocks make it unreliable to use mineral chemistries to assign a specific parent body affinity i e eh or el the ques have similar mineral chemistries among themselves while slight variations in texture and modal abundances exist between them they are dominated by inclusion bearing millimeter sized enstatite average en99 1 99 5 with interstitial spaces filled predominantly by oligoclase feldspar sometimes zoned kamacite si approximately 2 4 wt troilite 2 4 wt ti and cristobalite siderophile elements that partition compatibly between solid metal and liquid metal are not enriched like in partial melt residues itqiy and northwest africa nwa 2526 we find that the modal compositions of the ques are broadly unfractionated with respect to enstatite chondrites we conclude that a petrogenesis by impact melting not partial melting is most consistent with our observations

the blithfield meteorite and the origin of sulfide rich metal poor clasts and inclusions in brecciated enstatite chondrites abstract blithfield el6 is one of five known enstatite chondrite breccias it consists of troilite rich clasts 35 5vol embedded in an extremely metallic fe ni rich matrix 65 5 vol that contains metal nodules up to 17 mm in size clasts and matrix agglomerated independently in the solar nebula under conditions of high and low ps 2 po 2 ratios respectively the matrix accreted to an el chondrite planetesimal and was metamorphosed to 1000 c above the fe ni s eutectic chondrule textures were obliterated the s rich eutectic melt was lost from the matrix the matrix material was buried to a depth of at least 3 km accreting troilite rich material was incorporated into the matrix as clasts the breccia cooled through 500 c at 1000 10 000 c myr after cooling below 500 c blithfield was quenched possibly by impact excavation from depth and deposition onto the surface clasts or inclusions that are enriched in sulfide and depleted in metallic fe ni are common in brecciated enstatite chondrites variations in the ps 2 po 2 ratio in the nebular regions where these materials formed may explain many of their petrologic properties the silica rich clasts in adhi kot eh4 formed at very high ps 2 po 2 ratios 10 27 niningerite free silica and troilite were produced from the sulfurization of enstatite and metallic fe the troilite rich clasts in blithfield and atlanta el6 as well as the troilite rich regions of the hvittis el6 matrix formed at somewhat lower ps 2 po 2 ratios where sulfurization of metalic fe produced troilite the ni content of the residual metal increased forming some metal of martensitic composition the dark inclusions in abee eh 4 which contain up to 9 wt oldhamite formed at high ps 2 po 2 ratios in the presence of an additional ca rich component

introduction to special section magmatism and extension the relationship between magmatism and the formation of continental rift zones is the subject of much controversy in particular the cause and effect relationships between magmatism and extension and the mode of generation of magma during the process of extension are still hotly debated this controversy served as the theme of a symposium on cenozoic magmatism in the colorado river extensional corridor and adjacent areas and a field trip held as part of the geological society of america cordilleran rocky mountain section meeting in reno nevada metcalf et al 1993 it was clear from data presented at the symposium that a considerable amount of new information has become available regarding magmatism and extension since the last special section on this topic published by the journal of geophysical research june 1989

partial melting of the indarch eh4 meteorite a textural chemical and phase relations view of melting and melt migration to test whether aubrites can be formed by melting of enstatite chondrites and to understand igneous processes at very low o fugacities we have conducted partial melting experiments on the indarch eh4 chondrite at 1000 1500 c silicate melting begins at 1000 c and indarch is completely melted by 1500 c the metal sulfide component melts completely at 1000 c substantial melt migration occurs at 1300 1400 c and metal migrates out of the silicate charge at 1450 c and 50 silicate partial melting as a group our experiments contain three immiscible metallic melts si p and c rich two immiscible sulfide melts fe and femgmnca rich and silicate melt our partial melting experiments on the indarch eh4 enstatite chondrite suggest that igneous processes at low fo2 exhibit several unique features the complete melting of sulfides at 1000 c suggests that aubritic sulfides are not relics aubritic oldhamite may have crystallized from ca and s complexed in the silicate melt significant metal sulfide melt migration might occur at relatively low degrees of silicate partial melting substantial elemental exchange occurred between different melts e g s between sulfide and silicate si between silicate and metal a feature not observed during experiments at higher fo2 this exchange may help explain the formation of aubrites from known enstatite chondrites

an experimental investigation of volatile exsolution in evolving magma chambers abstract previous laboratory experiments investigating the fluid dynamics of replenished magma chambers have been extended to model effects resulting from the release of gas turbulent transfer of heat between a layer of dense hot and volatile rich mafic magma overlying cooler more evolved magma can lead to crystallization and exsolution of volatiles in the lower layer small gas bubbles can cause the bulk density to decrease to that of the upper layer and thus produce sudden overturning and initiate mixing followed by further exsolution of gas and explosive eruption these processes have been modelled in the laboratory using a chemical reaction between sodium or potassium carbonate and nitric acid to release small bubbles of co 2 we have investigated both the initial overturning produced by gas release in the lower layer and the subsequent evolution of gas due to intimate mixing of the two layers the latter experiments in which the reactants remained isolated in the two layers until overturning occurred demonstrated unambiguously that the fluxes of chemical components across the interfaces between convecting layers are very slow compared to the flux of heat this shows that the evolution of layers of magma of different origins and composition can take place nearly independently of each other the magmas can coexist in the same stratified chamber until their bulk densities become equal and they mix together the processes illustrated in these experiments could occur in h 2 o bearing magmas such as in the calcalkaline association and in co 2 bearing mafic magmas such as in silica undersaturated suites

shock metamorphism of enstatite chondrites abstract we have extended the shock metamorphism classification scheme of stoffler et al 1991 to account for shock effects in orthopyroxene and applied it to sixty enstatite chondrites orthopyroxene exhibits the following sequence of progressive shock effects shock stage s1 unshocked sharp optical extinction s2 very weakly shocked undulose extinction s3 weakly shocked development of clinoenstatite lamellae parallel to 100 s4 moderately shocked weak mosaicism s5 strongly shocked strong mosaicism as in the stoffler et al 1991 scheme stage s5 is characterized principally by the solid state transformation of crystalline plagioclase into maskelynite most el3 chondrites exhibit foliations caused by impact deformation of chondrules and metal particles el5 and el6 chondrites are all shock stage s2 because opaque veins are rare in s2 ordinary chondrites it is plausible that the centimeter size kamacite veins in atlanta and blithfield and the 1 6 cm long oldhamite rich vein in jajh deh kot lalu all el6 formed when their hosts were shocked to s3 s5 levels because removal of shock stage s3 s5 features including repair of shock damaged orthopyroxene requires levels of metamorphism comparable to those experienced by petrologic type 5 to 6 chondrites we infer that proto el6 material was shocked to s3 s5 levels prior to peak metamorphism and shocked again to stage s2 after metamorphism overall enstatite chondrites appear to have suffered greater shock damage than ordinary and carbonaceous chondrites

itqiy a metal rich enstatite meteorite with achondritic texture itqiy is a unique coarse grained metal rich enstatite meteorite that was found in the western sahara and consists of two rocks together weighing 4 72 kg which are both completely coated with fusion crust we report results from our electron microprobe and instrumental neutron activation analysis techniques itqiy consists of subhedral equigranular millimeter sized enstatite 25 vol of millimeter sized kamacite and a few tiny intergrowths of sulfides and kamacite relic chondrules are absent pyroxene fs0 2 is chemically similar to enstatite in el chondrites but the metal is closer in composition to that in eh chondrites sulfides resemble those in e chondrites but their compositions are distinct from those in both el and eh chondrites itqiy clearly formed under very reducing conditions but it does not appear to have formed from eh or el chondrites two thermal events can be distinguished silicate compositions including rare earth element abundances indicate loss of partial melt and slow cooling heterogeneous sulfides indicate a subsequent reheating and quenching event which may have been due to shock as many enstatite grains show shock stage s3 features

que 94204 a primitive enstatite achondrite produced by the partial melting of an e chondrite like protolith abstract queen alexandra range que 94204 an enstatite achondrite is a coarse grained highly recrystallized chondrule free and unbrecciated rock dominated about 70 vol by anhedral equigranular crystals of orthoenstatite of nearly endmember composition fs0 1 0 4 wo0 3 0 4 with interstitial plagioclase kamacite and troilite abundance of approximately 120 triple junctions and the close association of metal sulfide and plagioclase rich melts indicate that que 94204 has undergone limited partial melting with inefficient melt extraction mineral chemistry indicates a high degree of thermal metamorphism kamacite in que 94204 contains between 2 09 and 2 55 wt si similar to highly metamorphosed el chondrites plagioclase has between 4 31 and 6 66 wt cao higher than other e chondrites but closer in composition to plagioclase from metamorphosed el chondrites que 94204 troilite contains up to 2 55 wt ti consistent with extensive thermal metamorphism of an e chondrite like precursor results presented in this study indicate that que 94204 is the result of low degree about 5 20 vol probably toward the lower end of this range partial melting of an e chondrite protolith textural and chemical evidence suggests that during the metamorphism of que 94204 melts formed first at the fe ni fes cotectic near approximately 900 c followed by plagioclase pyroxene silicate partial melts near approximately 1100 c neither the fe ni fes nor the plagioclase pyroxene melts were efficiently segregated or extracted que 94204 belongs to a grouplet of similar primitive enstatite achondrites that are analogous to the acapulcoites lodranites but that have resulted from the partial melting of an e chondrite like protolith

portales valley petrology of a metallic melt meteorite breccia portales valley pv is an unusual metal veined meteorite that has been classified as an h6 chondrite it has been regarded either as an annealed impact melt breccia as a primitive achondrite or as a meteorite with affinities to silicated iron meteorites we studied the petrology of pv using a variety of geochemical mineralogical techniques our results suggest that pv is the first well documented metallic melt meteorite breccia mineral chemical and other data suggest that the protolith to pv was an h chondrite the composition of feni metal in pv is somewhat fractionated compared to h chondrites and varies between coarse vein and silicate rich portions it is best modeled as having formed by partial melting at temperatures of 940 1150 c with incomplete separation of solid from liquid metal solid metal concentrated in the coarse vein areas and s bearing liquid metal concentrated in the silicate rich areas possibly as a result of a surface energy effect both carbon and phosphorus must have been scavenged from large volumes and concentrated in metallic liquid graphite nodules formed by crystallization from this liquid whereas phosphate formed by reaction between p bearing metal and clinopyroxene components depleting clinopyroxene throughout much of the meteorite and growing coarse phosphate at metal silicate interfaces some phosphate probably crystallized from p bearing liquids but most probably formed by solid state reaction at 975 725 c phosphate forming and feo reduction reactions were widespread in pv and entailed a change in the mineralogy of the stony portion on a large scale portales valley experienced protracted annealing from supersolidus to subsolidus temperatures probably by cooling at depth within its parent body but the main differences between pv and h chondrites arose because maximum temperatures were higher in pv a combination of a relatively weak shock event and elevated pre shock temperatures probably produced the vein and breccia texture with endogenic heating being the main heat source for melting and with stress waves from an impact event being an essential trigger for mobilizing metal portales valley is best classified as an h7 metallic melt breccia of shock stage s1 the meteorite is transitional between more primitive chondritic and evolved achondrite iron meteorite types and offers clues as to how differentiation could have occurred in some asteroidal bodies

siderophile element systematics of iab complex iron meteorites new insights into the formation of an enigmatic group siderophile trace element abundances and the 187re 187os isotopic systematics of the metal phases of 58 iab complex iron meteorites were determined in order to investigate formation processes and how meteorites within chemical subgroups may be related close adherence of 187re 187os isotopic data of most iab iron meteorites to a primordial isochron indicates that the siderophile elements of most members of the complex remained closed to elemental disturbance soon after formation minor presumably late stage open system behavior however is observed in some members of the slm slh shl and shh subgroups the new siderophile element abundance data are consistent with the findings of prior studies suggesting that the iab subgroups cannot be related to one another by any known crystallization process equilibrium crystallization coupled with crystal segregation solid liquid mixing and subsequent fractional crystallization can account for the siderophile element variations among meteorites within the iab main group mg the data for the slm subgroup are consistent with equilibrium crystallization combined with crystal segregation and mixing by contrast the limited fractionation of siderophile elements within the sll subgroup is consistent with metal extraction from a chondritic source with little subsequent processing the limited data for the other subgroups were insufficient to draw robust conclusions about crystallization processes involved in their formation collectively multiple formational processes are represented in the iab complex and modeling results suggest that fractional crystallization within the mg may have been a more significant process than has been previously recognized

tungsten and hafnium distribution in calcium aluminum inclusions cais from allende and efremovka recent 182hf 182w age determinations on allende ca al rich refractory inclusions cais and on iron meteorites indicate that cais have initial e182w 3 47 0 20 2 identical to that of magmatic iron meteorites after correction of cosmogenic 182w burn out 3 47 0 35 2 either the allende cais were isotopically disturbed or the differentiation of magmatic irons groups iiab iid iiiab and ivb all occurred 103 which is lowered by the ubiquitous presence of metal inclusions to 180hf 184w 10 in bulk fassaite metal alloy ni 50 is the sole host of w 500 ppm in ef2 while opaque assemblages oas and secondary veins are the hosts of w in golfball a large metal alloy grain from ef2 em2 has 180hf 184w 100 ppm w with no detectable pt or s this vein provides evidence for transport of oxidized w in the cai because of the ubiquitous distribution of oas interpretations of the 182hf 182w isochron reported for allende cais include i all w in the oas was derived by alteration of cai metal or ii at least some of the w in oas may have been equilibrated with radiogenic w during metamorphism of allende since ii cannot be ruled out new 182hf 182w determinations on cais from reduced cv3 chondrites are needed to firmly establish the initial w isotopic composition of the solar system

the lueders texas iab iron meteorite with silicate inclusions the lueders iron meteorite with silicate inclusions was recovered as a single specimen of 35 4 kg in shackelford county texas in 1973 and recognized as a meteorite in 1993 siderophile element concentrations indicate chemical classification as a low ni iab iron meteorite closely related to landes like landes it has a cu content 4 above the main iab iiicd trend and therefore we also designate lueders as an anomalous member of iab the metallic host is composed of equigranular kamacite but with a suggestion of octahedral structure and with a bandwidth of 1 4 mm suggesting structural classification as a coarse octahedrite og the meteorite contains 23 wt of roughly millimeter to centimeter sized angular silicate inclusions classification as a iab is confirmed by o isotopic analysis of silicate inclusions these inclusions contain an assemblage rich in silicates troilite and graphite lack certain minor phases e g daubreelite and have angular shapes a variety of processes e g fragmentation partial melting reduction appear to have played a significant role in the formation of lueders and all iab iron meteorites petrologic and chemical differences confirm that lueders is not paired with the widely distributed odessa meteorite

northwest africa 2526 a partial melt residue of enstatite chondrite parentage nwa 2526 is a coarse grained achondritic rock dominated by equigranular grains of polysynthetically twinned enstatite 85 vol with frequent 120 triple junctions and 10 15 vol of kamacite terrestrial weathering products all other phases including troilite daubreelite schreibersite and silica normative melt areas make up 1 vol of the rock oxygen isotopic analyses are well within the range of those for enstatite chondrites and aubrites we show that the enstatite achondrite russell et al 2005 northwest africa nwa 2526 is actually a partial melt residue of an enstatite chondrite like lithology that experienced 20 vol partial melting we suggest that the heat source was internal to the parent body the fes fe ni and plagioclase enstatite partial melts were removed from the parent lithology leaving nwa 2526 as a residue highly depleted in troilite and lacking plagioclase sub solidus slow cooling and annealing is responsible for the coarsegrained recrystallized texture of the rock we also suggest that the parent lithology of nwa 2526 prior to partial melting experienced a shock event which formed the curvilinear trails of blebs of minor troilite and rare metal that are enclosed in enstatite crystals thus these represent relicts after partial melting and annealing nwa 2526 experienced a second relatively mild impact event 20 gpa that caused formation of the polysynthetic twinning in the enstatite we suggest that the meteorite zaklodzie which has been referred to as a primitive enstatite achondrite przylibski et al 2005 did not form from a magma of internal origin but that it is an impact melt breccia of enstatite chondrite like parentage as previously discussed by burbine et al 2000 and keil 2007 finally the metal rich enstatite meteorite with achondritic texture itqiy patzer et al 2001 formed by processes very similar to those responsible for formation of nwa 2526 and is also the residue of 20 vol partial melting of an enstatite chondrite like parent lithology with the fes fe ni and plagioclase enstatite partial melts having been removed from the residue it also experienced an impact event after partial melting that was responsible for the formation of the mixed mg mn fesulfides and the shock stage s3 features of the enstatite these similarities indicate that nwa 2526 and itqiy may have formed on the same parent body this body was different from the eh el shallowater and aubrite parent bodies and nwa 2526 and itqiy may represent samples from yet another fifth enstatite meteorite parent body

explicating the behavior of mn bearing phases during shock melting and crystallization of the abee eh chondrite impact melt breccia literature data show that among eh chondrites the abee impact melt breccia exhibits unusual mineralogical characteristics these include very low mno in enstatite 0 04 wt higher mn in troilite 0 24 wt and oldhamite 0 36 wt than in eh4 indarch and eh3 kota kota which are not impact melt breccias low mn in keilite 3 6 4 3 wt high modal abundances of keilite 11 2 wt and silica 7 wt but ranging up to 16 wt in some regions low modal abundances of total silicates 58 8 wt and troilite 5 8 wt and the presence of acicular grains of the amphibole fluor richterite these features result from abees complex history of shock melting and crystallization impact heating was responsible for the loss of mno from enstatite and the concomitant sulfidation of mn troilite and oldhamite grains that crystallized from the impact melt acquired relatively high mn contents abundant keilite and silica also crystallized from the melt these phases along with metallic fe were produced at the expense of enstatite niningerite and troilite melting of the latter two phases produced a s rich liquid with higher fe mg and fe mn ratios than in the original niningerite allowing the crystallization of keilite prior to impact melting f was distributed throughout abee perhaps in part adsorbed onto grain surfaces after impact melting most of the f that was not volatilized was incorporated into crystallizing grains of fluor richterite other eh chondrite impact melt breccias and impact melt rocks exhibit some of these mineralogical features and must have experienced broadly similar thermal histories

chemical classification of iron meteorites xi multi element studies of 38 new irons and the high abundance of ungrouped irons from antarctica abstract we report concentrations of 14 elements in the metal of 38 iron meteorites and a pallasite the meteorites are classified based on these data and on structural observations three samples are paired with previously classified irons thus these additional 35 irons raise the number of well classified independent iron meteorites to 598 one yamato iron contains 342 mg g ni the second highest ni content in an iab iron after oktibbeha county two small irons from western australia appear to be metal nodules from mesosiderites several of the new irons are from antarctica of 24 independent irons from antarctica 8 are ungrouped the fraction 0 333 is much higher than the fraction 0 161 among all 598 classified irons statistical tests show that it is highly improbable 2 9 probability that the antarctic population is a random sample of the larger population the difference is probably related to the fact that the median mass of antarctic irons is about two orders of magnitude smaller than that of non antarctic irons it is doubtful that the difference results from fragmentation patterns yielding different size distributions favoring smaller masses among ungrouped irons more likely is the possibility that smaller meteoroids tend to sample a larger number of asteroidal source regions perhaps because small meteoroids tend to have higher ejection velocities or because small meteoroids have random walked a greater increment of orbital semimajor axis away from that of the parent body

new findings for the equilibrated enstatite chondrite grein 002 we report new petrographic and chemical data for the equilibrated el chondrite grein 002 including the occurrence of osbornite metallic copper abundant taenite and abundant diopside as inferred from low si concentrations in kamacite the presence of ferroan alabandite textural deformation chemical equilibration of mafic silicates and a subsolar noble gas component we concur with grein 002s previous classification as an el4 5 chondrite furthermore the existence of pockets consisting of relatively coarse euhedral enstatite crystals protruding large patches of fe ni alloys suggests to us that this el4 5 chondrite has been locally melted we suspect impact induced shock to have triggered the formation of the melt pockets mineralogical evidence indicates that the localized melting of metal and adjacent enstatite must have happened relatively late in the meteorites history the deformation of chondrules equilibration of mafic silicates and generation of normal zoning in fe zn sulfides took place during thermal alteration before the melting event following parent body metamorphism daubreelite was exsolved from troilite in response to a period of slow cooling at subsolidus temperatures exsolution of schreibersite from the coarse metal patches probably occurred during a similar period of slow cooling subsequent to the event that induced the formation of the melt pockets overall shock features other than localized melting correspond to stage s2 and were likely established by the final impact that excavated the grein 002 meteoroid

evaluation of image classification routines for determining modal mineralogy of rocks from x ray maps the mineral proportions of rocks or rock fragments in thin section can be retrieved from electron microprobe element x ray maps using multispectral image classification software however different image classification algorithms can yield different inferred mineral proportions and thus deviate from the true modal mineralogy several image classification algorithms implemented in the programs erdas imagine and multispec were evaluated on a very simple rock a fragment of noritic anorthositic granulite in a thin section of lunar meteorite alha81005 this fragment contains only plagioclase orthopyroxene olivine clinopyroxene and chromite each has a constant composition across the fragment the true reference mineral proportions of the fragment were measured manually from the x ray maps results of classification algorithms varied widely in accuracy from 1 to more than 15 misclassified pixels compared to the reference the best match for this fragment comes from an unsupervised classification using the isodata algorithm most minerals were placed into multiple classes which had to be manually recombined and the second closest match was a supervised classification using a euclidean distance classifier the results of supervised classifications depend strongly on the user s selection of training areas and small errors in defined training areas e g a training area for one phase contains an inclusion of another can produce large errors the most common source of misclassification is mixed pixels those that contain signatures from multiple phases in this example pixels including both plagioclase and olivine tend to be classified as clinopyroxene thus mineral proportions calculated from classification of x ray maps must be considered critically even for simple rocks in more complex samples like those with more phases or with chemically zoned phases mineral classification of x ray maps are likely to require care and may be difficult to validate

crystallization of a s saturated fe ni melt and the origin of iron meteorite groups iab and iiicd new data on trace element partitioning in the fe ni s p system suggest that iron meteorites of groups iab and iiicd come from one or more s rich parent bodies heating of the iab parent body melted all troilite but caused only very minor partial melting of silicates cocrystallization of metal and troilite produced the high ni tail of group iab the trace element patterns are consistent with a parent body with ci sulfur and siderophile abundances compositionally the parent body was intermediate between ordinary and e4 chondrites its thermal history was in between ordinary chondrite and fully differentiated achondrite group iiiab iron parent bodies iab and iiicd may be complementary fractional crystallization partial melting suites from the same parent body since the model also explains some unusual irons it seems for the first time possible to understand the genesis of almost all irons and link them to the major groups of primitive and differentiated meteorites

the silicate inclusions of the ocotillo iab iron meteorite the ocotillo iab iron meteorite contains small silicate inclusions consisting of olivine low ca pyroxene chromian diopside plagioclase magnesiochromite apatite troilite and metal the ferromagnesian silicates have a small range of fe fe mg ratios that are not due to zoning these phases appear to be not well equilibrated the feo content of magnesiochromite is lower than values normally seen in silicate assemblages in iab iron meteorites the minerals in ocotillo are generally like silicate assemblages in other iab meteorites covering similar composition ranges and exhibiting a metamorphic granoblastic texture an estimate was made of the bulk composition of ocotillo silicate inclusions the bulk composition is close to that of ordinary chondrites with the exception of a deficiency in cao that might be due to a sampling problem associated with the method used to estimate the bulk composition

petrographic and mineralogical study of new eh melt rocks and a new enstatite chondrite grouplet enstatite chondrites are classified into eh and el groups and some of them were melted once in this paper we report petrography and mineral chemistry of five new antarctic enstatite chondrites yamato 793225 is characterized by intermediate properties between the eh and el groups and probably represents a new grouplet of enstatite chondrites the three paired meteorites y 8404 y 8414 and y 86004 may have cooled rapidly near the surface of the parent body as impact eh melt rocks yamato 82189 is also classified as eh melt rock but it experienced slow cooling as did y 793225 yamato 82189 contains the first occurrence of phlogopite in enstatite meteorites