barred olivine chondrules in ordinary chondrites barred olivine chondrules were studied in h l ll3 6 chondrites no important differences were found between barred olivine chondrules in h3 l3 and ll3 chondrites however comparison of type 3 barred olivine chondrules with all chondrule types in h l ll3 ordinary chondrites show that barred olivine chondrules are enriched in feo al2o3 na2o and k2o and are depleted in sio2 and mgo mineral abundances of barred olivine chondrules show that they are enriched in the feldspathic component compared to most chondrules pyroxene is rarely absent and is sometimes a major component pyroxene rich barred olivine chondrules in type 3 chondrites correlate negatively with feldspathic glass rich barred olivine chondrules indicating that pyroxene formed at the expense of olivine bars barred olivine chondrules in ordinary chondrites decrease systematically in cr2o3 and tio2 with increasing petrologic type feo however shows a systematic decrease with petrologic type in h chondrites no change in l chondrites and a systematic increase in ll chondrites the conclusions of this study are 1 barred olivine chondrules are mainly intermediate in major element composition between the abundant fe mg chondrules and the rare al rich chondrules 2 systematic compositional changes in barred olivine chondrules from type 3 6 in h l ll chondrites may be the result of primary differences 3 precursor components that melted to form barred olivine chondrules were enriched in olivine rich and volatile feldspar and feldspathoid rich components the alkalies may have been condensed where alumina was already available

bo xian ll3 9 oxygen isotopic and mineralogical characterisation of separated chondrules we report the results of a mineralogical and o isotopic study of 362 chondrules disaggregated from the bo xian chondrite the range of mineral compositions fa 0 8 31 2 mean 23 5 mode 27 28 are consistent with a reclassification of this meteorite from ll4 to ll3 9 chondrule diameters range from 0 20 to 3 40 mm mean 0 74 mm in the disaggregated population a lower mean diameter 0 64 mm calculated from thin section measurements partly reflects the high proportion of chondrule fragments the chondrule size distribution which is approximately log normal is consistent with size sorting mechanisms this sorting could be linked to the fragmentation of many chondrules on the parent body however in detail the variation in diameter of different chondrule types and a hiatus in the size distribution at 0 6 mm indicate that there may have been complex controls perhaps partly being determined by the chondrule formation mechanism seven percent of the sectioned chondrules 102 contain chemically fractionated mineral assemblages cristobalite bearing and al rich this significant degree of chemical heterogeneity probably resulted from both igneous and volatility controls oxygen isotopic compositions were determined on mineral separates and 16 of the sectioned chondrules three separate isotopic exchange events have been identified the dominant one is a low temperature hydrous gas solid exchange event between 16o rich solid and 16o poor gas reservoirs that lay along a slope 1 0 line on three isotope plots partial equilibration with the gas by feldspar and cristobalite which exchanged more rapidly than olivine or pyroxene led to formation of a slope 0 77 mixing line for bo xian and other ll chondrites mineralogy is the dominant control on the extent of this exchange no relationship between isotopic composition and chondrule texture or size was identified the feldspar separate and cristobalite rich chondrules have the most 16o poor compositions subsequently thermal metamorphism in the parent body led to partial isotopic equilibration between the different mineral phases a third exchange event predating the other two events is probably shown by one of the al rich chondrules this has an 16o rich composition lying below the terrestrial fractionation line another al rich chondrule has a normal ordinary chondrite isotopic composition it is not clear whether the isotopic fractionation recorded in some al rich chondrules can be achieved by the dominant gas solid exchange instead the precursor o to the mineral phases may have become 16o rich during an earlier phase of mass independent fractionation

crystal bearing lunar spherules impact melting of the moon s crust and implications for the origin of meteoritic chondrules crystal bearing lunar spherules clss in lunar breccia 14313 14315 14318 soil 68001 24105 and impact melt rock 62295 samples can be classified into two types feldspathic and olivine rich feldspathic clss contain equant tabular or acicular plagioclase grains set in glass or a pyroxene olivine mesostasis the less common olivine rich clss contain euhedral or skeletal olivine set in glass or possess a barred olivine texture bulk chemical and mineral chemical data strongly suggest that feldspathic clss formed by impact melting of mixtures of ferroan anorthosite and mg suite rocks that compose the feldspathic crust of the moon it is probable that olivine rich clss also formed by impact melting but some appear to have been derived from distinctively magnesian lunar materials atypical of the moon s crust some clss contain reversely zoned relic plagioclase grains that were not entirely melted during cls formation thin 5 m thick rims of troilite or phosphate and chemical gradients in glassy mesostases attributed to metasomatism in a volatile rich na k p rich environment crystal bearing lunar spherules were rimmed and metasomatized prior to brecciation compound cls objects are also present these formed by low velocity collisions in an environment probably an ejecta plume that contained numerous melt droplets factors other than composition were responsible for producing the crystallinity of the clss we agree with previous workers that relatively slow cooling rates and long ballistic travel times were critical features that enabled these impact melt droplets to partially or completely crystallize in free flight moreover incomplete melting of precursor materials formed nucleation sites that aided subsequent crystallization clearly clss do not resemble meteoritic chondrules in all ways the two types of objects had different precursors and did not experience identical rimming processes and vapor fractionation appears to have played a less important role in establishing the compositions of clss than of chondrules however the many detailed similarities between clss and chondrules indicate that it is more difficult to rule out an origin for some chondrules by impact melting than some have previously argued differences between clss chondrules and their host rocks possibly can be reconciled with an impact melt origin for some chondrules when different precursors the higher gravity of the moon compared to chondrite parent bodies and the likely presence of nebular gas during chondrule formation are taken into account

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

depositional facies of the miocene pliocene ogallala formation northwestern texas and eastern new mexico deposition of the basal fluvial sediments of the miocene pliocene ogallala formation in western texas and eastern new mexico was controlled by topography on the underlying erosional surface paleovalley fill facies consist of gravelly and sandy braided stream deposits interbedded with and overlain by eolian sediments deposited as sand sheets and loess uplands on the pre ogallala erosional surface are overlain primarily by similar eolian sediments calcic paleosols consisting mostly of glaebules and rhizoconcretions of caco sub 3 occur throughout the eolian facies massive to laminated and locally pisolitic brecciated and recemented pedogenic calcretes occur primarily near or at the top of the ogallala formation eolian facies preserve numerous superposed calcretes and calcic paleosols reflecting slow episodic aggradation on a savannah or grassland under arid to subhumid climatic conditions the change from fluvial to mostly eolian sedimentation probably resulted from diversion of streams that deposited fluvial sediments of the ogallala formation to form the pecos and canadian rivers source areas for eolian sediments may initially have been flood plains of ogallala braided streams and later the flood plains of the newly formed pecos and canadian rivers 38 references

chondrules from chainpur ll 3 reduced parent rocks and vapor fractionation abstract nineteen chondrules from the chainpur ll 3 chondrite were analyzed for their bulk lithophile element contents and mineral chemistries chondrule compositions are highly fractionated the following compositional groups can be distinguished 1 refractory depleted in na and k 2 k depleted k ci na ci 3 approximately chondritic na k ci na ci and 4 k enriched k ci na ci the most refractory chondrules are depleted in k 0 11 ci abundances normalized to sc na 0 26 mn 0 52 and cr 0 71 therefore we conclude that the refractory group was mainly produced by vapor fractionation during the chondrule forming event the majority of chondrules in enriched in na 1 37 1 40 ci but fractionated in k which ranges from 0 4 ci k depleted to 2 84 ci k enriched for the fractionation of k from na we suggest a separation mechanism via alkali sulfides uncorrelated fractionations of ca mn cr eu yb sm give supporting evidence for such a mechanism these findings imply that the pre chainpur matter went through highly reducing conditions high h2 o2 and s2 o2 ratios before it was sampled by the chondrule forming process most chondrules of our suite have similar fa contents in olivine 11 21 and are enriched in volatile elements from this we conclude that these chondrules were formed under oxidizing conditions in a relatively dense atmosphere with high partial pressures of volatile elements alkalis halogens a proto planetary atmosphere could have provided these conditions the fractionation process proposed should also be applicable to fractionations on a large scale like the na k and mg si fractionations among chondrites

refractory precursor components of semarkona chondrules and the fractionation of refractory elements among chondrites abstract chondrules from the semarkona ll3 0 chondrite show refractory and common lithophile fractionation trends similar to those observed among the chondrite groups it appears that chondrules are mixtures of a small number of pre existing solid components and we infer that chondrule precursor materials were related to the nebular components involved in the lithophile element fractionations recognized in ordinary chondrites compositional trends among the chondrules can be used to deduce the compositions of these components we use instrumental neutron activation analysis to measure many 20 of the lithophile elements in 30 chondrules the amounts of oxidized iron were calculated from other compositional parameters concentrations of si were estimated using mass balance considerations the data were corrected for the diluting effects of non lithophile constituents plots of lithophile elements versus a reference refractory element such as al show that there were two major chondrule silicate precursor components a refractory olivine rich feo free one and a non refractory sio 2 feo rich one the refractory component probably forms from olivine enriched condensates formed above the condensation temperature of enstatite the non refractory component must have formed from fine grained materials that were able to equilibrate down to lower nebular temperatures chondrite matrix may have had an origin similar to that of the non refractory material and constitutes a third lithophile bearing component that took part in chondrite fractionation processes the low abundance of refractories and mg in ordinary and enstatite chondrites was produced by the loss of materials having a higher refractory element mg ratio than that in the refractory component of chondrules

the compositions of incipient shock melts in l6 chondrites abstract microprobe analyses of 33 melt pocket glasses in five l6d and l6e chondrites show them to be chemically varied but typically enriched in the constituents of plagioclase relative to the host meteorites this enrichment appears to increase with the degree of melting 0 6 5 vol but other chemical variations among the glasses sodium depletion reduction of ferrous iron appear to be unrelated to shock intensity and melt abundance chemical trends for melt pocket glasses differ sharply from those reported for chondrules in ordinary chondrites thus partial shock melting of chondritic material is an inadequate explanation for the chemical properties of chondrules

extensive impact melting on the h chondrite parent asteroid during the cataclysmic bombardment of the early solar system evidence from the achondritic meteorite dar al gani 896 dag 896 is an olivine rich microporphyritic rock of komatiitic composition both the olivine composition fa 17 5 2 1 mn mg 0 0061 and the bulk oxygen isotopic composition delta 17 o 2 55 delta 18 o 3 50 indicate that dag 896 is a sample of the h chondrite parent body the bulk chemistry shows an h chondritic distribution of lithophile elements whereas chalcophile and siderophile elements are strongly depleted indicating formation through whole rock melting or nearly so of h chondrite material nearly complete loss of the metal plus sulfide component and crystallization without significant igneous fractionation superheated severely shocked chondritic relics similar to10 vol typically in the form of corroded lithic fragments 100 mum in size intimately distributed within the igneous lithology indicate that melting was triggered by a highly energetic impact which possibly induced shock pressures of similar to80 100 gpa the relatively young 3 704 0 035 ga ar 40 ar 39 crystallization age is consistent with the impact melting origin as magmatism in the asteroid belt was active only in the first hundred million years of solar system history 1ased on textural data and thermodynamic crystallization modelling we infer that dag 896 crystallized from a liquidus temperature of similar to1630degreesc under relatively slow cooling rates similar to10degreesc h 1 to 1300degreesc before quenching the two stage cooling history indicates that a reasonable formation environment might be a dike intruding cooler basement below a crater floor metal silicate fractionation may have been accomplished at least at the centimeter scale of the studied meteorite sample through differential acceleration of immiscible liquids of different density during the intense flow regimes associated with the excavation and modification stages of the cratering mechanism alternatively dag 896 may represent a surface sample of a differentiated melt body at the floor of an impact crater as gravitational settling appears to be an effective process at the surface of a chondritic parent asteroid for metal particles 1 to 10 mm in size typically observed in partially differentiated impact melt rocks stokes law indicates a settling velocity 1 in h 1 during the first few hours of crystallization on asteroidal bodies of 25 kin radius the similar to3 7 ga age of dag 896 nearly overlaps with the slightly older resetting ages of h chondrites all impact melts available from the literature indicating that the h chondrite parent asteroid underwent extensive impact melting at the enduring of the cataclysmic bombardment of the early solar system such an age overlap may also indicate early disruption of the initial h chondrite parent asteroid the close similarity between the bulk composition and degassing age of dag 896 and silicate inclusions in he iron meteorites is further evidence in support of a common origin by impact melting and metal silicate segregation on the h chondrite parent asteroid our new high precision oxygen isotopic measurements of h chondrites delta 17 o 0 77 0 04 should be extended to iies to verify this possible petrogenetic link

a survey of clasts and large chondrules in ordinary chondrites we report the results of a survey of clasts and large 5 mm chondrules macrochondrules within the 833 ordinary chondrites of the natural history museum collection thirty six macrochondrules and 24 clasts were identified and studied macrochondrules have textures and mineral assemblages like normal chondrules and so share a common origin clasts show evidence for fracturing from larger bodies and can be classified as either 1 chemically fractionated if they have major and trace element compositions differing substantially from most chondrules and clasts 2 impact melt clasts if they have microporphyritic textures and signs that indicate they are derived from shock melted chondritic material 3 microporhyritic clasts if they are similar to the last category but lack evidence for derivation through shock melting or 4 indeterminate clasts forming a diverse class that includes all those clasts that do not fit into the other categories

petrology trace element abundances and oxygen isotopic compositions of a compound cai chondrule object from allende abstract we report the petrology trace element abundances and oxygen isotopic characteristics of a compound cai chondrule object wi 025 found in the allende cv3 chondrite the wi 025 is an irregularly shaped inclusion consisting of three texturally and chemically distinct portions the interior portion the igneous rim and the intermediate zone located between these two portions the interior portion consists of anorthite spinel olivine and al bearing low ca pyroxene the major element chemistry of the interior portion corresponds to that of al rich chondrules and is of intermediate character between fine grained spinel rich cais and ferromagnesian chondrules the interior portion has abundant 16 o rich spinel 17 o 14 2 to 24 7 and displays a group ii cai like ree composition these observations indicate that the interior portion contains a cai component formed by fractional condensation the major and trace element chemistry of the interior portion indicate that the cai had subsequently assimilated chondrule materials through partial melting the maximum heating temperature of the partial melting is estimated at approximately 1400 c similar to the maximum heating temperature of type b cais the oxygen isotopic compositions of the olivine and low ca pyroxene 17 o 6 3 in the interior portion indicate that the partial melting and chondrule assimilation took place under a moderately 16 o poor nebular gas the igneous rim is texturally and chemically similar to ferromagnesian chondrules and entirely surrounds the interior portion the oxygen isotopic compositions of the olivine and low ca pyroxene in the igneous rim are indistinguishable from those of the interior olivine and al bearing low ca pyroxenes these observations indicate that a chondrule material which was melted in the same nebular gas as the interior portion was accreted to the interior portion the intermediate zone represents a reaction zone accompanying the igneous rim formation the formation history of wi 025 can be summarized by the following processes 1 original cai formation 2 partial melting and chondrule assimilation 3 igneous rim formation and 4 secondary alteration on the parent body

petrogenesis of complex veins in the chantonnay l6f chondrite abstract two cross cutting veins in the chantonnay l6f chondrite illustrate different patterns of fractionation of total chondritic shock melts the earlier vein which is dark colored and bears abundant host rock xenoliths is strongly reduced and sodium poor relative to the bulk meteorite it resembles and may be cogenetic with melt pockets in chantonnay the later vein which is lighter colored and somewhat vesicular lacks evidence of either na loss or reduction but shows modest internal differentiation its metal and total iron contents 26 5 wt are higher than normal for l group chondrites the trend of chemical fractionation recorded in the earlier chantonnay vein resembles that reported for chondrules in ordinary chondrites suggesting that chemical variations among chondrules in part reflect variations among their parental shock melts

rocks of ages propagation of hot rock cookery in western north america cook stone technology s old world roots were established by 30 000 b p and reappeared in the new world by 10 000 b p after millennia of direct fire cooking hot rock cookery which is necessary for foods that require prolonged cooking facilitated land use intensification by affording greater utilization of nutrients in available foods on a given landscape this technology gradually diversified during the early holocene in western north america by 4000 b p its initial intensification was underway final intensification began by 2000 b p and typically peaked during the last 1500 years propagation of hot rock cookery exemplifies pre columbian food crises and signals carbohydrate revolutions wherein more high cost foods feed growing populations as modeled cook stone griddles earth ovens and steaming pits with rock heating elements are more costly facilities insofar as fuel is used to heat rocks that in turn extend cooking time and temperature more expensive still is stone boiling given that fuel heats rocks that in turn heat water that cooks the food even more expensive in terms of energy expended is the manufacture of heating elements in the form of stone ceramic and metal cooking containers all of which afford further evidence of land use intensification

geology geochemistry and cooling history of the westcoast crystalline complex and related rocks meares island and vicinity vancouver island british columbia the westcoast crystalline complex is a belt of plutonic rocks along the west coast of vancouver island it is composed mainly of heterogeneous amphibolitic country rock westcoast amphibolite granitoids of trondhjemitic to gabbroic composition westcoast diorite and variable mixtures of these two components westcoast migmatite although the protolith of some deformed enclaves may be paleozoic most of these rocks were generated in a magmatic arc setting and intruded in jurassic time major and trace element chemistry of the westcoast crystalline complex shows a sub alkaline tholeiitic to calc alkaline trend the exponential cooling curves derived for westcoast diorites are not consistent with in situ crustal magma genesis but instead indicate that these rocks intruded relatively cool country rock based on age and chemistry the westcoast crystalline complex can be interpreted as the deeper crustal equivalent of the more differentiated island intrusions and bonanza volcanics taken together these roc

large silica rich igneous textured inclusions in the buzzard coulee chondrite condensates differentiates or impact melts we studied three large 2 4 mm diameter igneous textured inclusions in the buzzard coulee h4 chondrite using microanalytical techniques olm sem empa sims to better elucidate the origins of igneous inclusions in ordinary chondrites the inclusions are clasts that come in two varieties 1 white inclusions bz 1 and bz 2 represent a nearly holocrystalline assemblage of low ca and high ca pyroxene 63 66 area and cristobalite 33 36 and 2 tan inclusion bz 3 is glass rich approximately 60 with low ca and high ca pyroxene phenocrysts the bulk compositions of the inclusions determined by modal reconstruction are all sio2 rich approximately 67 wt for bz 1 and bz 2 approximately 62 for bz 3 but bz 3 is enriched in incompatible elements e g ree approximately 4 5 ci abundances whereas bz 2 and bz 1 are depleted in those elements that are most incompatible in pyroxene e g la ho approximately 0 15 0 4 ci abundances these bulk compositions do not resemble what one would expect for partial or complete shock melting of a chondritic precursor and show no evidence for overall volatility control we infer that the inclusions originated through igneous differentiation and feo reduction with bz 3 forming as an andesitic partial melt approximately 30 40 partial melting of an h chondrite precursor and bz 1 and bz 2 forming as pyroxene cristobalite cumulates from an si rich melt we suggest that both types of melts experienced a period of transit through a cold low pressure space environment in which cooling feo reduction and interaction with a vapor occurred melts may have been lofted into space by excavation or splashing during collisions or by pyroclastic volcanism our results indicate intriguing similarities between the inclusions in buzzard coulee and the silicates in some iron iie type and stony iron iva type meteorites suggesting a genetic relationship

correlation among lower to upper crustal components in an island arc the jurassic bonanza arc vancouver island canada the westcoast crystalline complex wcc island intrusions and bonanza group of vancouver island canada form three different crustal levels of the early to middle jurassic bonanza island arc differential uplift has exposed the plutonic roots and the volcanic carapace of the arc for a strike length of 500 km and for another 250 km on the queen charlotte islands at deeper crustal levels within the arc influx of mantle derived magmas was accompanied by metamorphism and melting of wrangellian basement rocks yielding the heterogeneous wcc upward mobilization and hybridization of magmas to shallower levels in the crust resulted in the batholiths of the island intrusions and the lavas and pyroclastic rocks of the bonanza group new u pb crystallization ages for plutonic rocks of the arc span an age range of 190 3 1 0 to 168 6 5 3 ma ages of the wcc and western island intrusions are indistinguishable and overlap with published fossil and isotopic ages for the bonanza group younger middle jurassic

going underground experimental carbonization of fruiting structures under hearths we show that carbonized fruits and seeds recovered from middle stone age deposits in rock shelters are likely to have been carbonized as part of post depositional processes we buried indigenous south african fruits nuts and seeds at pre determined depths and distances from the centers of experimental fires the cold ashes of the hearths and the sand surrounding them were subsequently excavated using standard archaeological techniques and dry screening the fruiting structures from the oxidizing part of the fire were burnt to ashes whereas those buried in sand under anoxic conditions survived in varying forms those buried 5 cm below the center of the fire were carbonized those buried 10 cm below the center of the fire were dehydrated and those 5 cm and 10 cm below the surface at the outer edge of the fire were unaffected size moisture or oil content of the original fruit or seed did not appear to influence whether or not carbonization took place temperatures recorded 5 cm below the experimental fires suggest that the carbonization occurred at or before a maximum temperature of 328 c and also at lower maximum temperatures 152 c that were maintained for long periods even when the quantities of a particular wood are controlled open fires may produce variable underground temperatures and the temperatures below ancient hearths would have been equally variable we suggest that cyperaceae sedge nutlets the most numerous fruiting structures in the sibudu middle stone age archaeobotanical assemblage occur in the shelter as a result of human activity subsequent burial and accidental carbonization when hearths were built directly above the buried nutlets

the mineral chemistry and origin of inclusion matrix and meteorite matrix in the allende cv3 chondrite the two textural varieties of olivine rich allende inclusions rimmed and unrimmed olivine aggregates consist primarily of a porous fine grained mafic constituent inclusion matrix that differs from the opaque meteorite matrix of cv3 chondrites by being relatively depleted in sulfides metal grains and perhaps carbonaceous material olivine is the most abundant mineral in allende inclusion matrix clinopyroxene nepheline sodalite and ti al pyroxene occur in lesser amounts olivine in unrimmed olivine aggregates type 1a inclusions is ferrous and has a narrow compositional range fo50 65 olivine in rimmed olivine aggregates type 1b inclusions is on average more magnesian with a wider compositional range fo53 96 olivine grains in the granular rims of type 1b inclusions are zoned with magnesian cores fo 80 and ferrous rinds fo 70 ferrous olivines fo 65 in both varieties of inclusions commonly contain significant amounts of al2o3 as much as 0 7 wt cao as much as 0 4 wt and tio2 as much as 0 2 wt refractory elements that probably occur in submicroscopic inclusions of ca al ti rich glass rather than in the olivine crystal structure defocussed beam analyses of allende matrix materials demonstrate that 1 inclusion matrix in type 1a inclusions is more enriched in olivine and feo than inclusion matrix in the cores of type 1b inclusions 2 opaque matrix materials are depleted in feldspathoids and enriched in sulfides and metal grains relative to inclusion matrix 3 the bulk compositions of type 1a and type 1b inclusions overlap and 4 excluding sulfides and metal the bulk compositions of allende matrix materials cluster in a complementary pattern around the bulk composition of c1 chondrites inclusion matrix and meteorite matrix in allende and other cv3 chondrites are probably relatively primitive nebular material but a careful evaluation of the equilibrium condensation model suggests that these matrix materials do not consist of crystalline phases that formed under equilibrium conditions in a relatively cool gas of solar composition allende inclusion matrix is interpreted as an aggregate of condensates that formed under relatively oxidizing non equilibrium conditions from supercooled supersaturated vapors produced during the vaporization of interstellar dust by aerodynamic drag heating in the solar nebula cv3 meteorite matrix contains in addition a proportion of interstellar material that was heated but not vaporized in the nebula granular olivine in rimmed olivine aggregates may have formed during the recrystallization and incipient melting of aggregates of inclusion matrix in the nebula the mineral chemistry of matrix olivine in allende seems to have been established by three different processes non equilibrium vapor solid condensation recrystallization and partial melting in the nebula and femg equilibration without textural homogenization in the meteorite parent body

brecciation among 2280 ordinary chondrites constraints on the evolution of their parent bodies abstract after accretion of meteorite parent bodies larger and smaller collisions have led to significant modifications of these bodies involved processes include excavation of material thermal metamorphism melting mixing of different materials re accretion and re lithification all these processes can be repeated several times in this study polished thin sections pts of 2280 chondrites 1193 h 947 l and 140 ll chondrites were investigated in order to obtain the abundance of brecciated rocks among the ordinary chondrites in addition we have determined the abundance and characteristics of shock vein bearing h l and ll chondrites and of impact melt rock clasts we also recognized xenolithic components based on o isotope studies noble gas data were considered in order to detect regolith breccias and to discuss late impact histories the investigation of 2280 samples shows that 23 276 of 1193 of the h chondrites 23 220 of 947 of the l chondrites and 79 110 of 140 of the ll chondrites are brecciated considering the heavily brecciated ll chondrites in 63 of the 140 chondrites 45 shock veins were clearly detected 57 of these 63 chondrites are brecciated rocks the investigation of the h and l chondrites has shown that about 26 310 of 1193 of the h chondrites and 40 379 of 947 of the l chondrites contain shock veins in our data set 20 of the h chondrites and 8 3 of the ll chondrites but only 3 0 of the l chondrites contain solar noble gases remarkably about 62 of all brecciated h chondrites with noble gases analyzed contain solar noble gases compared to only around 11 and 10 of the brecciated l and ll chondrites respectively the identification of xenolithic clasts e g ci cm and ureilite like lithologies in primitive type 3 chondrites indicates simultaneous accretion of clasts and chondrules these clasts must have been formed early within the first 2 ma on subsequently destroyed precursor first generation parent bodies the formation of complex breccias witnesses the collisions between asteroids of very different lithologies and heritage although the onion shell configuration of primordial parent bodies is necessary in order to form the chondrites with different degrees of metamorphic overprint petrologic types 3 6 subsequent catastrophic fragmentation and reassembly to form asteroids with a rubble pile structure are required to explain certain features discussed in this work however distinct peaks in the cosmic ray exposure age distributions indicate that not too many impacts in the last 100 ma were responsible to deliver the majority of the ordinary chondritic meteoroids to earth yet this certainly does not tell anything about the number of last generation parent bodies that exist in the asteroid belt since s type asteroids are the most abundant type of asteroid in the inner main belt and thought to be the parent bodies of ordinary chondrites

trace element fractionation by impact induced volatilization sims study of lunar hasp samples impact is a dominant process acting on the surfaces of planetary bodies and is the major process that modifies the physical and chemical nature of planetary materials on atmosphere free bodies once volcanic activity has ceased elements with relatively high volatilities are readily mobilized during impact hasp high alumina silica poor compositions are thought to be generated by impact induced volatilization processes on the moon the high al si ratio of hasp is believed to be the result of preferential loss of sio 2 relative to al 2 o 3 here we report sims analyses for hasp glass beads and devitrified glasses from the apollo 14 regolith to assess the behavior of li be b ree sr y and zr during impact events we compare the behavior of these trace elements to the behavior of major and minor elements analyzed by the electron microprobe the hasp samples we studied fall into two compositional groups with one group apparently derived from kreepy lunar lithologies and the other from anorthosite the kreepy hasp beads have an estimated mass loss of 28 compared to their kreepy protolith we estimate that 44 of sio 2 and 35 of feo were lost during impact induced volatilization the mass loss estimate for the anorthositic protolith to produce anorthositic hasp is more uncertain because of the uncertainty of the protolith composition however an estimate of approximately 25 appears reasonable with 38 loss of silica clearly impact processes in the early stages of planetary evolution are important in the modification of geochemical signatures and ordinarily geochemically coherent elements may be decoupled because of different volatilities

digital imaging technology and experimental archeology a methodological framework for the identification and interpretation of fire modified rock fmr abstract this paper presents the results of a series of experiments for the identification and analysis of fire modified rock fmr fmr is a common but frequently overlooked artifact type experiments were conducted simulating the effects of different hypothetical burning scenarios on rocks similar to those found in a south african middle stone age site a digital imaging method was then used to quantify fmr color values designed to limit intra analyst bias statistical tests and a blind test suggest that unburned rocks and experimental fmr can be separated statistically based on physical appearance two burning scenario models based on measured experimental data were applied to archaeological fmr from a south african middle stone age site named pinnacle point 5 6 pp5 6 and show that the archaeological samples are not statistically different from a simulated campfire and possibly a lithic raw material heat treatment fire

hf w sm nd and rb sr isotopic evidence of late impact fractionation and mixing of silicates on iron meteorite parent bodies abstract we report the first sm nd and rb sr isotopic analyses of silicate inclusions in four iie iron meteorites miles weekeroo station a and b and watson we also report the hf w isotopic composition of a silicate inclusion from watson and 182 w 184 w of the host feni metal in all four iies the host metal in watson has a negative w value 2 21 0 24 similar to or higher than other iron meteorites 1 35 and consistent with segregation of metal from silicate early in solar system history however the large silicate inclusion in the watson iie iron yielded a chondritic w value 0 50 0 55 thus indicating a lack of equilibration with the feni host within the practical lifetime of activity of the parent 182 hf 50 ma one of the silicate inclusions in miles is roughly chondritic in major element composition has a present day nd of 10 3 relatively non radiogenic 87 sr 86 sr 0 714177 13 and a t chur age of 4270 ma two silicate inclusions from weekeroo station and one from watson exhibit fractionated sm nd and rb sr ratios and more radiogenic 87 sr 86 sr 0 731639 12 to 0 791852 11 and non radiogenic nd values 5 9 to 13 4 the silicate inclusion in watson has a t chur age of 3040 ma in agreement with previously determined 4 he and 40 ar gas retention ages indicative of a late thermal event a later event is implied for the two silicate inclusions in weekeroo station which yield indistinguishable t chur ages of 698 and 705 ma silicate inclusions in iie iron meteorites formed over a period of 3 billion yr by impacts involving an h chondrite parent body and an feni metal parent body the lile enriched nature of some of these silicates suggests several stages of melting mixing and processing however there is little evidence to suggest that the silicates in the iie irons were ever in equilibrium with the host feni metal

where were the hearths an experimental investigation of the archaeological signature of prehistoric fire technology in the alluvial gravels of the southern plains the interpretation that a rock has been subjected to cultural utilization is a basic element of hunter gatherer research the fracture mechanics of the flaked stone tool production process are well understood and the material residues are routinely identified during fieldwork conversely non percussion processes such as hot rock technology which can result in rock fracture are less well understood an experimental study has been developed to examine the fractured gravels observed at a prehistoric site on the southern plains usa the experiment has sought to determine whether cultural or natural agencies were responsible for the production of the angularly fractured rocks the positive results of the experiment indicate that not only were humans most likely responsible for the breakages but also that the fractured rocks often exhibit distinctive morphologies that may be identified during fieldwork this simple experimental methodology is applicable to other hunter gatherer sites where the depositional environment is not conducive to the structural preservation of features such as hearths

petrology of feo poor porphyritic pyroxene chondrules in the semarkona chondrite abstract the mineralogy and petrology of feo poor porphyritic pyroxene and olivine rich chondrules in the semarkona ll3 0 chondrite are described in detail in an extension of the textural classification scheme these chondrules are designated types iab and ib in type iab chondrules the proportion of olivine phenocrysts is between 20 80 and in type ib chondrules olivine constitutes

chemical and physical studies of type 3 chondrites v the enstatite chondrites abstract we report instrumental neutron activation analysis determinations of 19 major minor and trace elements in three enstatite chondrites based on these and literature data on the bulk and mineral composition of enstatite chondrites we discuss the history of the type 3 or unequilibrated enstatite chondrites and their relationship with the other enstatite chondrites the type 3 enstatite chondrites have e chondrite lithophile element abundances and their siderophile element abundances place them with the eh chondrites well resolved from the el chondrites moderately volatile chalcophile elements are at the low end of the eh range and cr appears to be intermediate between eh and el we suggest that the type 3 enstatite chondrites are eh chondrites which have suffered small depletions of certain chalcophile elements through the loss of shock produced sulfurous liquids the oxygen isotope differences between type 3 and other enstatite chondrites is consistent with equilibration with the nebula gas 30 higher than the others or with the loss of a plagioclase rich liquid the mineral chemistry of the type 3 chondrites is consistent with either low temperature equilibration or in some instances with shock effects

a unique high mn fe microgabbro in the parnallee ll3 ordinary chondrite nebular mixture or planetary differentiate from a previously unrecognized planetary body the study of planetary materials in chondritic meteorites constrains the compositional diversity of materials in different nebular environments and provides information on the degree of differentiation of early planetary bodies we studied a unique microgabbro fragment from the parnallee ll3 unequilibrated ordinary chondrite the fragment which was originally identified by its ophitic to sub ophitic texture exhibits features characteristic of lunar and terrestrial tholeiitic basalts extreme compositional zoning in clinopyroxene wo 10 en 65 fs 25 to wo 15 en 2fs 83 a multiply saturated major element composition similar to mid ocean ridge basalt with 3 1 wt na 2o and 0 15 wt k 2o and uniformly enriched rare earth elements c 10 c1 a high bulkmno feo ratio 0 064 distinguishes the microgabbro from other basaltic rocks and suggests the precursor material formed or reached equilibrium in a reducing environment however the absence of fe metal and the extreme enrichment of feo up to 40 wt in late crystallizing pyroxferroite requires the last crystallization event to have occurred in a relatively oxidizing environment we suggest the microgabbro formed by partial melting in a planetary body after removal of metallic fe examination of possible planetary source materials such as alkali rich eucritic material a volatile depleted c1 carbonaceous chondrite or h group chondrite shows that multiple stage fractionation is required to produce a melt with the feo mgo ratio of the microgabbro from these materials the increasing number of planetary igneous fragments observed in unequilibrated ordinary chondrites uoc suggests the picture of uoc as primitive assemblages of unprocessed material is overly simplistic

three archaeomagnetic applications of archaeological interest to the study of burnt anthropogenic cave sediments abstract recent archaeomagnetic studies carried out on mid to late holocene burnt anthropogenic cave sediments have shown that under certain conditions these materials are suitable geomagnetic field recorders archaeomagnetic analyses carried out on these contexts constitute a rich source of information not only for geophysical purposes in terms of reconstructing the variation of earth s magnetic field in the past but also from the archaeological point of view for example by archaeomagnetic dating here we report three different archaeomagnetic applications to the study of burnt cave sediments i archaeomagnetic dating ii determining palaeotemperatures and iii assessing post depositional processes the first case study is a dating attempt carried out on a late holocene bronze age burnt level from el mirador cave burgos spain using the directional european secular variation curve several dating intervals were obtained for the last burning of this combustion feature considering the archaeological evidence and the independent radiometric 14 c dating available the possible ages obtained are discussed this is the first archaeomagnetic dating obtained in these contexts so far the second case study is an application of the method to determine the last heating temperatures reached by the carbonaceous facies of these fires stepwise thermal demagnetization of oriented samples can be used to quantitatively estimate heating temperatures an intermediate normal polarity component interpreted as a partial thermo remanence ptrm with maximum unblocking temperatures of 400 450 c was systematically identified revealing the last heating temperatures experienced by this facies these temperatures were confirmed with partial thermomagnetic curve experiments finally archaeomagnetic analyses on a partially bioturbated burning event were performed in order to evaluate until what spatial extent the burnt sediments were affected by post depositional mechanical alteration processes for each case study the archaeological implications are discussed highlighting the potential of archaeomagnetic methods to retrieve archaeological information

estimating the approximate firing temperature of burnt archaeological sediments through an unmixing algorithm applied to hysteresis data abstract estimating the temperature to which burnt archaeological sediments and soils have been exposed in antiquity is of interest as it may well elucidate the interpretation of specific features certain semi industrial activities such as metal working or the production of pottery are often associated with the controlled use of intense high temperature processes 500 c exposure to such high temperatures will inevitably lead to the thermal alteration of commonly found iron minerals within the soil magnetic measurements made on a series of soil samples subjected to controlled laboratory heating confirms both the sensitivity of iron minerals to thermal alteration and also suggests a correlation between the maximum exposure temperature and the hysteresis properties from this data a method for estimating the maximum exposure temperature of burnt archaeological samples recovered from similar soil types is proposed based on the application of a linear unmixing model this model compares hysteresis data from the archaeological samples to an end member data set created from the laboratory heated soil the maximum exposure temperature for the archaeological samples is estimated from the relative proportion of known temperature end members present within the final model describing the experimental hysteresis data the validity of the model is demonstrated through application to samples recovered from a series of actualistic fire experiments and a range of burnt archaeological features recovered during excavation of a multi period site at yarnton near the city of oxford uk a further application of the method to determine the fidelity of samples collected for archaeomagnetic dating is presented from the excavation of a large hearth type features at whitby north yorkshire uk

strain and shape fabric variations associated with ductile shear zones abstract the foliated and compositionally banded granitic orthogneisses in the central core of the maggia nappe a lower pennine basement nappe of the central swiss alps are shown to be the sheared equivalent of late hercynian age granitic intrusions these ductile shear zones show mineral assemblages in amphibolite facies are alpine in age and form an anastomosing network enclosing remnant lozenge shaped pods of relatively undeformed rock the foliation developed within the shear zones concomitantly with a change in shape of quartz grain aggregates from initially equidimensional through tear drop shapes to ribbon like aggregates these shape changes occurred by intracrystalline glide together with intercrystalline slip on deformation induced planar surfaces

large lake basins of the southern high plains ground water control of their origin the origin of the 40 50 topographically large lake basins on the southern high plains of texas and new mexico has been an enigma previous workers have considered deflation or evaporite dissolution at depth and subsequent collapse as the most probable mechanisms however the eolian hypotheses have been unable to provide convincing arguments as to how the wind selectively erodes the thick deflation resistant calcrete caprock that is persistent over much of the southern high plains furthermore recent detailed studies on some of the basins show no significant evaporite dissolution at depth and neither mechanism offers a satisfactory explanation as to why the basins are almost universally associated with subsurface topographic highs or why they are absent where the high plains aquifer thickness exceeds 60 m we address these latter concerns and modify the deflation hypothesis by proposing that the calcrete caprock may never have been deposited in the areas now occupied by the basins the absence of calcrete deposition is proposed to have resulted from high water tables caused by an increase in hydraulic gradient where aquifers thinned above bedrock highs a high water table close to and or intersecting the surface prevents deposition of calcrete and thus the uncemented surface would be more susceptible to deflation than the surrounding calcrete covered areas after decline of the water table the rise in water table associated with bedrock highs is documented by numerical simulation using boundary conditions and hydrologic parameters representative of the southern high plains

late quaternary landscape evolution soil stratigraphy and geoarchaeology of the caprock canyonlands northwest texas usa abstract in northwest texas usa between the southern high plains to the west and the central lowlands to the east lies a geographic boundary known as the escarpment breaks or caprock canyonlands the canyonlands contain abundant springs lithic resources shelter and plant and animal food sources that attracted hunter gatherer groups a geoarchaeological study was conducted in the canyonlands to determine the effects of late quaternary landscape evolution especially intensive erosion on the region s archaeological record geomorphic and stratigraphic field research and a total of 95 new radiocarbon age determinations 94 of which were determined on paired samples aid in reconstructing an understudied dynamic and erosive landscape and explain how the landscape has changed the pattern is similar to reported data from the central plains and western rolling plains but dissimilar to the southern high plains high rates of erosion and geological controls on the south fork of the double mountain fork of the brazos river a 4th order stream have hindered the discovery of deeply buried soils and in situ paleoindian artifacts and features but a late holocene pedocomplex is relatively intact in valley fills beneath remnants of the t 2 terrace of the south fork the eroding slopes near the edge of the caprock escarpment exposed a record of in situ archaic to protohistoric aged materials the eroding slopes should be targeted for future quantification of erosion and archaeological preservation bias for the canyonlands

petrogenesis of silicate inclusions in the weekeroo station iie iron meteorite differentiation remelting and dynamic mixing the weekeroo station iie iron meteorite contains a variety of felsic and mafic inclusions enclosed in an feni metal host petrographic emp and sims data suggest that the petrogenesis of the silicates was complex and included differentiation remelting feo reduction and dynamic mixing of phases differentiation produced a variety of olivine free inclusion assemblages ranging from pyroxene 1 plagioclase 1 tridymite with peritectic compositions to coarse orthopyroxene to plagioclase 1 tridymite and its glassy equivalent individual phases have similar trace element abundances and patterns despite large variations in inclusion textures modes and bulk compositions probably as a result of mechanical separation of pre existing phases in an impact event that dynamically mixed silicates with the metallic host trace element data imply that augite and plagioclase grains in different inclusions crystallized from the same precursor melt characterized by relatively unfractionated ree abundances of 20 30 3 ci chondrites except for a negative eu anomaly such a precursor melt could have been produced by 2 5 equilibrium partial melting of an h chondrite silicate protolith or by higher degrees of partial melting involving subsequent fractional crystallization glass appears to have formed by the remelting of pre existing plagio clase and orthopyroxene in a process that involved either disequilibrium or substantial melting of these phases during remelting silicate melt reacted with the feni metal host and feo was reduced to fe metal following remelting and metal silicate mixing inclusions apparently cooled at different rates in a near surface setting on the parent body glass or pigeonite bearing inclusions cooled more rapidly 2 5 c hr between 1000 850 c than pigeonite free largely crystalline inclusions the results of this study point to two likely models for forming iie iron meteorites both involving collision between an feni metal impactor and either a differentiated or undifferentiated silicate rich target of h chondrite affinity each model has difficulties and it is possible that both are required to explain the diverse iie group copyright 1999 elsevier science ltd

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

derivation of a heterogeneous lithic fragment in the bovedy l group chondrite from impact melted porphyritic chondrules the bovedy l group chondrite contains a light colored poikilitic lithic fragment with olivine low ca pyroxene and kamacite compositions characteristic of porphyritic chondrules from unequilibrated ordinary chondrites its texture compositional similarities to porphyritic chondrules and low na2o k2o and p2o5 content indicate that the fragment represents a solidified slightly fractionated impact melt formed from a source that was rich in porphyritic chondrules the fragment is heterogeneous with a progressive increase in the bulk mgofeo ratio and in mgo content of olivines and low ca pyroxenes across its length 39ar40ar analyses of the fragment and host indicate that the meteorite experienced extensive degassing due to reheating the approximate age of 0 5 0 94 byr dates the reheating event and not the formation of the lithic fragment or the bovedy breccia this reheating event renders the fragment s and host s metallographic cooling rate of 5 c myr through 500 c imprecise however the absence of martensite and the presence of kamacite zoned taenite and tetrataenite in the fragment and host are consistent with such slow cooling through 500 c this cooling rate must have resulted from burial of the fragment host assemblage beneath insulating material on the bovedy parent body if the thermal diffusivity of this overburden was approximately comparable to that of the lunar regolith 10 4cm2 sec then the fragment was buried at a depth 6 5 km if k 10 2 cm2 sec similar to chondritic material then the fragment was buried at a depth 65 km

late accretion and lithification of chondritic parent bodies mg isotope studies on fragments from primitive chondrites and chondritic breccias recent results of isotopic dating studies 182hf 182w 26al 26mg and the increasing number of observed igneous and metamorphosed fragments in primitive chondrites provide strong evidence that accretion of differentiated planetesimals predates that of primitive chondrite parent bodies the primitive chondrites adrar 003 and acfer 094 contain some unusual fragments that seem to have undergone recrystallization magnesium isotope analyses reveal no detectable radiogenic 26mg in any of the studied fragments the possibility that evidence for 26al was destroyed by parent body metamorphism after formation is not likely because several other constituents of these chondrites do not show any metamorphic features since final accretion of a planetesimal must have occurred after formation of its youngest components formation of these parent bodies must thus have been relatively late i e after most 26al had decayed al mg isotope data for some igneous textured clasts granitoids and andesitic fragments within the two chondrite regolith breccias adzhi bogdo and study butte reveal also no evidence for radiogenic 26mg as calculated from the upper limits the formation of these igneous clasts the incorporation into the parent body regolith and the lithification must have occurred at least 3 8 myr andesite in study butte and 4 7 myr granitoids in adzhi bogdo after calciumaluminum rich inclusions cai formation the absence of 26mg excess in the igneous inclusions does not exclude 26al from being a heat source for planetary melting in large early formed planetesimals cooling below the closure temperature of the al mg system may be too late for any evidence for live 26al in the form of 26mg excess to be preserved thus growing evidence exists that chondritic meteorites represent the products of a complex multi stage history of accretion parent body modification disruption and re accretion

x ray digital imaging petrography of lunar mare soils modal analyses of minerals and glasses it is essential that accurate modal i e volume percentages of the various mineral and glass phases in lunar soils be used for addressing and resolving the effects of space weathering upon reflectance spectra as well as for their calibration such data are also required for evaluating the resource potential of lunar minerals for use at a lunar base however these data are largely lacking particle counting information for lunar soils originally obtained to study formational processes does not provide these necessary data including the percentages of minerals locked in multi phase lithic fragments and fused soil particles such as agglutinates we have developed a technique for modal analyses sensu stricto of lunar soils using digital imaging of x ray maps obtained with an energy dispersive spectrometer mounted on an electron microprobe a suite of nine soils 90 to 150 micrometers size fraction from the apollo 11 12 15 and 17 mare sites was used for this study this is the first collection of such modal data on soils from all apollo mare sites the abundances of free mineral fragments in the mare soils are greater for immature and submature soils than for mature soils largely because of the formation of agglutinitic glass as maturity progresses in considerations of resource utilization at a lunar base the best lunar soils to use for mineral beneficiation i e most free mineral fragments have maturities near the immature submature boundary is feo approximately or 30 not the mature soils with their complications due to extensive agglutination the particle data obtained from the nine mare soils confirm the generalizations for lunar soils predicted by l a taylor and d s mckay 1992 lunar planet sci conf 23rd pp 1411 1412 abstract

the origin of chondrules and chondrites debris from low velocity impacts between molten planetesimals we investigate the hypothesis that many chondrules are frozen droplets of spray from impact plumes launched when thin shelled largely molten planetesimals collided at low speed during accretion this scenario here dubbed splashing stems from evidence that such planetesimals intensely heated by 26al were abundant in the protoplanetary disk when chondrules were being formed approximately 2 myr after calcium aluminum rich inclusions cais and that chondrites far from sampling the earliest planetesimals are made from material that accreted later when 26al could no longer induce melting we show how splashing is reconcilable with many features of chondrules including their ages chemistry peak temperatures abundances sizes cooling rates indented shapes relict grains igneous rims and metal blebs and is also reconcilable with features that challenge the conventional view that chondrules are flash melted dust clumps particularly the high concentrations of na and feo in chondrules but also including chondrule diversity large phenocrysts macrochondrules scarcity of dust clumps and heating we speculate that type i feo poor chondrules come from planetesimals that accreted early in the reduced partially condensed hot inner nebula and that type ii feo rich chondrules come from planetesimals that accreted in a later or more distal cool nebular setting where incorporation of water ice with high 17o aided oxidation during heating we propose that multiple collisions and repeated re accretion of chondrules and other debris within restricted annular zones gave each chondrite group its distinctive properties and led to so called complementarity and metal depletion in chondrites we suggest that differentiated meteorites are numerically rare compared with chondrites because their initially plentiful molten parent bodies were mostly destroyed during chondrule formation

archeomagnetic results from southwest united states and mesoamerica and comparison with some other areas abstract archeomagnetic am results from southwestern united states can be represented by an archeomagnetic apparent polar curve aapc that is distributed near the axis of rotation and contained within 75 n latitude the aapc for the past 1600 yr is located mostly on the near side with mainly easterly declinations it is characterized by loops extending outward from the axis of rotation about 10 15 every 250 300 yr at 1350 1100 800 500 and 200 ad inclination of the magnetic field varied from a minimum of 40 to a maximum of 62 with maximum values occurring at 600 1100 1350 ad and modern minimum values occurred at 800 1250 and 1500 ad declination ranged from 18 e to 14 w with maximum easterly values occurring at 500 ad and during the nineteenth century the most westerly declination occurred about 1100 ad the maximum and minimum values of declination and inclination correspond in age with loop like extension of the aapc archeomagnetic paleointensity results suggest changes from 1 5 to 0 8 times the present field with variations occurring generally in unison with the changes in direction comparison of archeomagnetic data from mesoamerica england france japan bulgaria and china with the data from the southwest united states suggests some similarities and dissimilarities of the spatial and temporal position of the aapc early age archeomagnetic results to 8500 bc from the united states confirm a similar pattern of secular variation and limit the time of the most recent reversal some available global data suggest that 200 ad may have been a time of small divergence from an axial field inclined by 7 5 from the rotational axis

macrochondrules in chondrites formation by melting of mega sized dust aggregates and or by rapid collisions at high temperatures seventy four macrochondrules with sizes 3 mm were studied considering the extraordinary size of the chondrules occasionally achieving a mass of 1000 times and more the mass of a normal sized chondrule the conditions in the formation process must have been somewhat different compared with the conditions for the formation of the common chondrules macrochondrules are typically rich in olivine and texturally similar to specific chondrule types barred radial porphyritic and cryptocrystalline of normal sized chondrules however our studies show that most of the macrochondrules are fine grained or have elongated crystals mostly bo rp and c which lead to the assumption that they were once totally molten and cooled quite rapidly porphyritic chondrules belong to the least abundant types of macrochondrules this distribution of chondrule types is highly unusual and just a reverse of the distribution of chondrule types among the typical sized chondrules in most chondrite groups except for the ch and cb chondrites new chondrule subtypes like radial olivine ro or multi radial mr chondrules are defined to better describe the textures of certain large chondrules macrochondrules may have formed due to melting of huge precursor dust aggregates or due to rapid collisions of superheated melt droplets which led to the growth of large molten spherules in regions with high dust densities and high electrostatic attraction

the iodine xenon system in clasts and chondrules from ordinary chondrites implications for early solar system chronology abstract we have studied the i xe system in chondrules and clasts from ordinary chondrites cristobalite bearing clasts from parnallee ll3 6 closed to xe loss 1 4 ma after bjurbole feline a feldspar and nepheline rich clast also from parnallee closed at 7 04 0 15 ma two out of three chondrules from parnallee that yielded well defined initial i ratios gave ages identical to bjurbole s within error a clast from barwell l6 has a well defined initial i ratio corresponding to closure 3 62 0 60 ma before bjurbole partial disturbance and complete obliteration of the i xe system by shock are revealed in clasts from julesburg l3 6 and quenggouk h4 respectively partial disturbance by shock is capable of generating anomalously high initial i ratios in some cases these could be misinterpreted yielding erroneous ages a macrochondrule from isoulane n amahar contains concentrations of i similar to ordinary chondrules but unlike most ordinary chondrules contains no radiogenic 129xe this requires resetting 50 ma or more later than most chondrules the earliest chondrule ages in the i xe mn cr and al mg systems are in reasonable agreement this and the frequent lack of evidence for metamorphism capable of resetting the i xe chronometer leads us to conclude that at least the earliest chondrule i xe ages represent formation if so chondrule formation took place at a time when sizeable parent bodies were present in the solar system

evidence for a precompaction exposure to cosmic rays in a chondrule from the h6 chondrite alh76008 abstract we studied the cosmic ray records of the h6 chondrite alh76008 in a bulk sample in a chondrule fragment and in matrix material the matrix was separated into olivine pyroxene plagioclase and metal concentrates for all samples he ne and ar isotopic measurements were performed the cosmic ray exposure age of the bulk chondrite is 1 72 0 11 ma whereas the 3he 21ne and 38ar ages of the chondrule exceed those of bulk material by 31 67 and 55 respectively the radiation environment in the early solar system is ill defined but for current production rates this translates into a precompaction exposure time of 0 90 ma furthermore the plagioclase mainly anorthite concentrate representing a few percent of the total meteorite contains solar type noble gases that reveal differential irradiation of alh76008 components we observe a trapped ratio 20ne 22ne 12 3 0 3 a value that is between those of solar wind and solar energetic particles this is the first clear identification of differential irradiation conditions between a chondrule and the bulk chondrite the short exposure age may favor the resolution of differential exposure histories

a new metal rich chondrite grouplet abstract a new grouplet of primitive metal rich chondrites here called the cb c carbonaceous b bencubbinite chondrites has been recognized it includes bencubbin weatherford hammadah a1 hamra hh 237 and queen alexandra range que 94411 paired with que 94627 their mineral compositions as well as their oxygen and nitrogen isotopic compositions indicate that they are closely related to the cr and ch chondrites all of which are members of the more inclusive cr clan cb chondrites have much greater metal silicate ratios than any other chondrite group widely increasing the range of metal silicate fractionation recorded in solar nebular processes they also have the greatest moderately volatile lithophile element depletions of any chondritic materials metal has compositional trends and zoning patterns that suggest a primitive condensation origin in contrast with metal from other chondrite groups cb chondrites as well as other cr clan chondrites have much heavier nitrogen higher 15n 14n than that in other chondrite groups the primitive characteristics of the cb chondrites suggest that they contain one of the best records of early nebular processes another chondrite grosvenor mountains 9555 1 is petrographically similar to the cb chondrites but its mineral and oxygen and nitrogen isotope compositions indicate that it formed from a different nebular reservoir

mega chondrules and large igneous textured clasts in julesberg l3 and other ordinary chondrites vapor fractionation shock melting and chondrule formation abstract a petrographic microprobe study of large metal poor igneous textured objects in julesberg l3 and other ordinary chondrites suggests that they can be classified into two petrographic types mega chondrules and large lithic clasts and into two chemical types na poor and na rich mega chondrules show textural evidence of having solidified as freely floating melt droplets whereas lithic clasts formed by the fragmentation of larger objects possibly still larger mega chondrules barred olivine or barred olivine pyroxene textures are most common for mega chondrules whereas a variety of textures occur in large lithic clasts the two petrographic types cannot be distinguished on the basis of modal bulk or phase compositions sodium poor objects are characterized by 1 plagioclase or glass with mainly bytownite composition an70 90 typical 2 subchondritic na al ratios and typically subchondritic volatile element na k and mn abundances and 3 bulk chemical trends that resemble those expected for vapor fractionation processes some na poor objects may have formed by the melting of precursors that formed as condensates or vaporization residues others may have formed by the melting of precursors that formed by fractional condensation or fractional vaporization following vapor fractionation na poor objects or their precursors appear to have reequilibrated at lower temperatures which raised the bulk na content of the objects but not to the levels seen in na rich objects enrichments of fe2 na k and p on the margins of some na poor objects suggest that they partly reacted with volatile rich surroundings both before and after brecciation sodium rich objects are characterized by 1 plagioclase or glass with oligoclase or albite composition an2 25 2 roughly chondritic na al ratios and volatile element na k mn abundances and 3 bulk chemical trends similar to those shown by melt pocket glasses in ordinary chondrites sodium rich melt objects could have formed by the shock melting of chondritic precursors feldspathic compositions for some na rich melt objects can be explained by preferential shock melting of feldspar or feldspathic glass in chondritic target materials literature data imply that the same two chemical populations of objects na poor and na rich occur among smaller normal sized chondrules in type 3 ordinary chondrites suggesting that the same processes that affected large melt objects also affected chondrules

phosphate minerals in the h group of ordinary chondrites and fluid activity recorded by apatite heterogeneity in the zag h3 6 regolith breccia phosphate minerals in ordinary chondrites provide a record of fluids that were present during metamorphic heating of the chondrite parent asteroids we have carried out a petrographic study of the phosphate minerals merrillite and apatite in metamorphosed h group ordinary chondrites of petrologic type 4 6 to understand development of phosphate minerals and associated fluid evolution during metamorphism in unbrecciated chondrites apatite is cl rich and shows textural evolution from fine grained apatite merrillite assemblages in type 4 toward larger uniform grains in type 6 the cl f ratio in apatite shows a similar degree of heterogeneity in all petrologic types and no systematic change in compositions with metamorphic grade which suggests that compositions in each meteorite are dictated by localized conditions possibly because of a limited fluid rock ratio the development of phosphate minerals in h chondrites is similar to that of l and ll chondrites despite the fact that feldspar equilibration resulting from albitization is complete in h4 chondrites but not in l4 or ll4 chondrites this suggests that albitization took place during an earlier period of the metamorphic history than that recorded by preserved apatite compositions and chemical equilibrium was not achieved throughout the h chondrite parent body or bodies during the late stages of metamorphism a relict igneous clast in the h5 chondrite oro grande has apatite rims on relict phenocrysts of possibly diopside that have equilibrated with the host chondrite apatite in the zag h3 6 regolith breccia records a complex fluid history which is likely related to the presence of halite in this meteorite the porous dark h4 matrix of zag where halite is observed has a high apatite merrillite ratio and apatite is extremely cl rich one light h6 clast contains similarly cl rich apatite in a second light h6 clast apatite compositions are very heterogeneous and more f rich apatites in both h4 matrix and h6 clasts have very low h 2o contents heterogeneous apatite compositions in zag record multiple stages of regolith processing and shock at the surface of the h chondrite parent body and apatite records either the passage of fluids of variable compositions resulting from different impact related processes or the passage of a single fluid whose composition evolved as it interacted with the chondrite regolith unraveling the history of apatite can potentially help to interpret the internal structure of chondrite parent bodies with implications for physical and mechanical properties of chondritic asteroids the behavior of halogens recorded by apatite is important for understanding the behavior of volatile elements in general if impact melt materials close to the surface of a chondritic asteroid are readily degassed the volatile inventories of terrestrial planets could be considerably more depleted than the ci carbonaceous chondrite abundances that are commonly assumed

alkali differentiation in ll chondrites abstract the ll group chondrites krahenberg krbg and bhola are heterogeneous agglomerates containing a variety of lithic fragments and chondrules as well as crystal fragments the fe fe mg content of most olivine grains is uniform fa28 although a few with distinctly lower fe contents were found fa19 both meteorites contain large cm sized fragments with high enrichments of k 12 rb 45 and cs 70 relative to ll chondrites while the ree concentrations are normal except for a negative eu anomaly na and sr are depleted 0 5 and the na k weight ratio is 0 33 compared to 11 in the host however there is no difference in the sum of na k atoms also the major elements si al mg ca and fe are nearly the same in fragments as in the host material the k rich igneous lithic fragments have a microporphyritic texture of euhedral to skeletal olivines in a partly devitrified glass with 4 k2o the main pans of both krbg and bhola contain mesostasis glasses in porphyritic chondrules and lithic fragments with varying k content 0 1 8 6 k2o and na k ratios 0 2 100 crystalline plagioclase is depleted in k with an average na k ratio of 22 i e higher than that for ordinary chondritic plagioclase 8 4 olivines in the large k rich fragments and in the host meteorites have the same iron content fa28 indicating that both formed under the same oxygen fugacity and probably on the same parent body conceivable mechanisms for the formation of the k rich rocks from normal ll chondrite parent material are 1 magmatic differentiation 2 na k exchange via a vapor phase 3 silicate liquid immiscibility 4 volatilization and condensation in impact events process 2 appears most feasible for forming a rock enriched only in k and heavier alkalies and depleted in na without noticeably changing other elements including the ree

classification metamorphic history and pre metamorphic composition of chondrules abstract sears et al 1992 and dehart et al 1992 have proposed that the existing scheme for chondrule classification which is based on modal mineralogy texture and mineral composition should be replaced by a new scheme that is based primarily on the compositions of olivine phenocrysts and mesostases but also on their cathodoluminescence we have used published and unpublished compositional data for chondrules in ordinary and carbonaceous chondrites to compare the two classification schemes we find that the new scheme does not preserve the identity of the existing chondrule groups even in the least metamorphosed chondrites because of difficulties in assigning chondrules to the proposed groups and the lack of olivine grains or mesostasis areas of sufficient size 30 of chondrules in some type 3 ordinary chondrites cannot be classified in the new scheme using compositional data for type 4 6 chondrites the existing scheme provides more information about the history of individual chondrules than the new scheme we conclude that textural and modal criteria should be retained for the classification of chondrules

shock and thermal history of northwest africa 4859 an annealed impact melt breccia of ll chondrite parentage containing unusual igneous features and pentlandite abstract northwest africa 4859 nwa 4859 is a meteorite of ll chondrite parentage that shows unusual igneous features and contains widely distributed pentlandite the most obvious unusual feature is a high proportion of large 3 cm diameter igneous textured enclaves lites interpreted as shock melts that were intruded into an ll chondrite host one such lite appears to have been produced by whole rock melting of ll chondrite initial rapid partial crystallization and subsequent slow cooling of the residual melt in the host to produce a differentiated object other unusual features include mm sized overgrowth objects fine grained plagioclase rich bands and coarse troilite 7 mm across grains all these features are interpreted as having crystallized from melts produced by a single transient shock event followed by slow cooling a subsequent shock event of moderate s3 intensity produced veining and transformed some of the pyroxene into the clinoenstatite polytype pentlandite together with associated troilite in nwa 4859 probably formed by the breakdown of a monosulfide precursor phase at low temperature 230 c following the second shock event nwa 4859 is interpreted to be an unusual impact melt breccia that contains shock melt which crystallized in different forms at depth within the parent body

variability in the chronology of late holocene aboriginal occupation on the arid margin of southeastern australia analysis of 28 radiocarbon determinations obtained from the excavation of heat retainer hearths from sturt national park in western new south wales indicates aboriginal occupation of the arid margin of australia during the last 1700 years the determinations show two phases of hearth construction separated by a period of at least 200 years and possibly 400 years when no hearths were constructed palaeoenvironmental and archaeological evidence is reviewed in an effort to account for the late holocene dates from the hearths and the existence of the hiatus in hearth construction despite the close match between the hiatus and the so called medieval warm period caution is required before extrapolating the results from an archaeological study of a small geographic region to the continent as a whole

palaeomagnetic studies of burned rocks an assemblage of rocks such as comprising a fireplace will acquire a remanent magnetization upon cooling parallel to the ambient geomagnetic field and thus will share a common direction of magnetization detailed palaeomagnetic analyses of oriented samples will reveal whether these rocks have remained in situ since the last heating event or whether these rocks have moved or represent discard material thermal demagnetization makes it possible in many cases to estimate the maximum temperature which a given rock has experienced the direction of magnetization can be used to reposition rocks to their cooling position and the cooling history of boiling stones can be reconstructed for some rocks this paper presents results obtained from burned rocks from various archaeological sites

chondrules and other components in c o and e chondrites similarities in their properties and origins we describe three types of chondrules that are common to h3 ll3 cm2 co3 and cv3 chondrites low and high feo porphyritic olivine chondrules contain olivine with fa0 3 8 and fa5 50 respectively and can easily be distinguished petrographically poikilitic pyroxene chondrules have 1 20 vol olivine fa0 2 8 enclosed by low ca pyroxene fs0 5 7 and also occur in e chondrites these three types formed in separate nebular regions which had dimensions and spacings such that a few percent of the chondrules that collided to form compound chondrules were of different types sorting of chondrule precursor material and mixing of chondrule types probably account for most variations in bulk and mineral chemistry among chondrite groups metallic fe ni grains containing 0 1 1 cr si and p are present in low feo olivine chondrules from all type 2 and least metamorphosed type 3 chondrites metal compositions reflect reduction during chondrule formation in the nebula not nebular condensation opaque matrices in type 3 ordinary and carbonaceous chondrites are somewhat similar in composition and mineralogy and probably have related origins we conclude that chondrules in all known chondrite groups share similar nebular origins

geological evidence for post 40 m y b p large scale northwestward displacement of part of southeastern alaska the leech river complex on southern vancouver island and schistose rocks on southern baranof island have very similar geologic histories both were derived from probable upper jurassic cretaceous terrigenous clastic rocks and minor associated volcanogenic rocks and chert each underwent 1 at least two penetrative deformations locally involving folding disruption and transposition of thinly layered sequences and 2 synkinematic low pressure buchan type metamorphism progressive metamorphic zones ranging from greenschist to amphibolite facies are related to granitic intrusive bodies deformation metamorphism and intrusion culminated about 40 to 42 m y ago apparently these units now separated by more than 1 100 km were once contiguous and lay at least as far south as the latitude of southern vancouver island shortly after 40 m y b p the allochthonous leech river complex was emplaced at the southern edge of vancouver island and the schists of baranof island and perhaps the entire chugach terrane began their long northwestward journey along a system of dextral transcurrent faults

implications of water supply for indigenous americans during holocene aridity phases on the southern high plains usa abstract springs in the 40 to 50 large lake basins 15 km 2 on the southern portion of the southern high plains shp were active during periods of aridity in the holocene when there may have been human habitation of the area eolian erosion of the lake floors and lunette accretion occurred as groundwater levels declined in response to decreased groundwater recharge the declining lake floor associated with eolian erosion allowed groundwater evaporative discharge to continue thus maintaining a groundwater gradient toward the lake this hydrologic condition was favorable for a relatively continuous spring discharge to the lake independent of the elevation of the lake floor to evaluate the postulated dynamic equilibrium critical to this conclusion 17 optically stimulated ages were determined from a 17 7 m deep core of a lunette adjacent to double lakes texas 33 13 15 n 101 54 08 w the core yielded sediment accumulation dates of 11 500 1100 6500 700 and 4900 500 yr b p corresponding broadly with periods of aridity known from other evidence based on analysis of this lunette it is concluded that springs in double lakes basin probably existed throughout the holocene with discharges similar to those observed historically we assumed that similar dynamic equilibrium existed in the other large lake basins in the shp and that these springs could have provided a continuous source of water for indigenous peoples during periods of prolonged aridity the dynamic equilibrium that is proposed in this study is applicable not only to other arid and semiarid geographic areas with wind erodible material but also over different geologic times

evidence for primitive nebular components in chondrules from the chainpur chondrite abstract the least equilibrated ordinary chrondrites contain chondrules which have experienced little change since the time of their formation in the early solar system these chondrules are excellent indicators of the physical and chemical nature of the solar nebula we separated 36 chondrules from the chainpur ll3 4 chondrite and analyzed each for 20 elements and petrographic properties sampling biases were minimized as far as possible chondrules seem to have formed through the melting of random mixtures of grains comprising a limited number of nebular components the identity of these components can be deduced from chondrule compositions the dominant components appear to be 1 a mixture of metal and sulfide with composition similar to whole rock metal and sulfide 2 refractory ir rich metal 3 refractory olivine rich silicates 4 low temperature pyroxene rich silicates and possibly 5 a component containing the more volatile lithophiles most of the textural types of chondrules formed from the same set of precursor components in some cases chondrules having different textures are almost identical in composition a few unusual chondrule types seem to mainly consist of uncommon nebular components possibly indicating different modes of formation etching experiments confirm that chondrule rims are enriched in metal troilite and moderately volatile elements relative to the bulk chondrules however a large fraction of the volatiles remains in the unetched interior

a planetary h group pebble in the barwell l6 unshocked chondritic meteorite abstract one stone of the barwell l6 shower contains an igneous inclusion with the oxygen isotopic signature of h group chondrites the inclusion is composed dominantly of olivine with a composition fo 75 5 identical with that of its host other phases are only partially equilibrated with the host the inclusion has a fractionated rare earth element pattern showing depletion in the heavy rare earth elements and a positive eu anomaly it is depleted in siderophile elements inclusion and host have ar ar ages 4 44 and 4 51 ga respectively and 129 xe r i ratios identical within experimental error we conclude that melting occurred very early on an h group chondritic body two objects from a single meteorite have different oxygen isotopic ratios but identical 129 xe r 127 i ratios which may indicate that the i xe systematics reflect chronological differences in closure to xe rather than isotopically heterogeneous iodine the inclusion and its host cooled to xe closure temperature contemporaneously with chondrules from unequilibrated ordinary chondrites

properties of the guin ungrouped iron meteorite the origin of guin and of group iie irons abstract guin is an ungrouped iron meteorite containing 45 g g ga and 186 g g ge it is a coarse octahedrite with an unusually high ni content 92 5 mg g its metal composition is generally similar to irons in groups iab and iiicd but its closest relative is the cruz del aire ungrouped iron guin contains 6 3 vol silicate inclusions that are petrologically and compositionally similar to those in iie irons the largest inclusion 2 4 cm consists primarily of a shock melted plagioclase rich matrix surrounding large 1 15 mm partly melted augite grains the oxygen isotopic composition of the inclusion 18 o 5 29 17 o 3 87 is near the ll chondrite field the guin inclusion is similar in bulk composition to selected melt pocket glasses in ordinary chondrites produced in situ by preferential melting of plagioclase by shock compression we suggest that the guin assemblage formed by impact melting on a chondritic parent body silicate inclusions in iie irons share many petrologic and compositional but not oxygen isotopic characteristics with the guin inclusions suggesting that iie irons also formed by impact melting of chondritic materials individual iie irons were probably derived from separate cratering events

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

petrology and mineralogy of type ii feo rich chondrules in semarkona ll3 0 origin by closed system fractional crystallization with evidence for supercooling abstract the petrology of type ii porphyritic olivine chondrules in semarkona ll3 0 has been studied in detail olivines in these chondrules are euhedral fe richand are strongly zoned from cores to rims of grains in feo fa10 30 cr2o3 0 2 0 6 wt mno 0 2 0 7 wt and cao 0 1 0 4 wt interstitial mesostasis is rich in si aland ca and is glassy with abundant microcrystallites minor minerals include troilite fe ni metaland chromite some olivine grains contain euhedral fayalite rich cores that are probably produced during initial supercooling of the chondrule melt rare relict grains of forsteritic olivine have compositions very similar to olivines in type ia chondrules in semarkona and may result from disaggregation of such chondrules apart from these relics all properties of type ii chondrules can be described by closed system fractional crystallization of droplets which were essentially entirely molten the chondrules did not originate as aggregates of large olivine grains and are unlikely to be fragments of precursor igneous rocks cooling rates of the order of 1000 c h are indicated from peak temperatures of around 1600 c the chondrules show considerable evidence for supercooling and large departures from equilibrium during crystallization there is no evidence for chondrule nebula exchange either during or after crystallization or any secondary processes such as aqueous alteration or thermal metamorphism bulk composition data suggest that precursor material included a refractory ca al ti rich component similar to one of the precursor components of type ia chondrulesand a non refractory fe and si rich component type ia chondrules may have formed from type ii chondrules by loss of fe and volatiles alternatively the two chondrule types may have formed in regions of considerable diversity in the solar nebula from precursor materials with different fe mg ratios

refractory inclusions with unusual chemical compositions from the vigarano carbonaceous chondrite abstract ten inclusions nine from vigarano and one from leoville both members of the reduced subgroup of c3v chondrites were analyzed for major and trace elements by neutron activation most have some refractory element characteristics that are common in refractory inclusions from allende a member of the oxidized subgroup of c3v chondrites for instance three leo 3537 2 vig 1623 8 vig 1623 11 are type b inclusions with group i or modified group i ree patterns three vig 1623 5 vig 1623 10 vig 1623 13 are olivine rich inclusions each of which is rather uniformly diluted in most refractory elements compared to group i inclusions and two vig 1623 14 vig 1623 16 are fine grained inclusions with group ii ree patterns six of the vigarano inclusions however have refractory element fractionations that are unusual in allende inclusions vig 1623 11 has cl chondrite normalized enrichment factors for th and refractory siderophiles that are lower than those for ree by 60 or more a positive mo anomaly mo ir 1 92 0 02 relative to cl chondrites and an unusually low zr hf ratio 0 67 0 06 relative to cl relative to cl chondrites the ir enrichment factor is 1 7 times greater than the mean enrichment factor for re os and ru in vig 1623 10 vig 1623 13 has one of the lowest ir sm ratios 0 00044 0 00007 relative to cl ever measured in an olivine chondrule vig 1623 16 has a modified group ii ree pattern which reflects removal of refractory rees from the nebular gas at unusually low temperatures vig 477 5 has a ree pattern with a small negative ce anomaly ce la 0 79 0 01 relative to cl a large negative yb anomaly yb lu 0 57 0 02 relative to cl and no eu anomaly this inclusion also has a positive mo anomaly mo ir 2 25 0 03 relative to cl and enrichment factors for zr and th that are lower than those for lu by about 65 vig 477 b has a ree pattern that may be a mixture of five parts of a component with an ultrarefractory ree pattern and ninety five parts of another with a modified group ii ree pattern vigarano apparently sampled a different population of refractory inclusions from allende presumably because refractory nebular materials were not well mixed where and when c3v chondrites accreted the vigarano and leoville coarse grained inclusions studied here like others analyzed previously tend to have lower concentrations of na and au than allende coarse grained inclusions but the two vigarano fine grained inclusions of this study have concentrations of these elements that are comparable to those of allende fine grained inclusions thus although coarse grained inclusions in the reduced subgroup of c3v chondrites were altered under different conditions from those in the oxidized subgroup some fine grained inclusions in the reduced subgroup were altered under similar conditions as some in the oxidized subgroup

a kinematic model for the formation of the siletz crescent forearc terrane by capture of coherent fragments of the farallon and resurrection plates 1 the volcanic basement of the oregon and washington coast ranges has been proposed to represent a pair of tracks of the yellowstone hotspot formed at a mid ocean ridge during the early cenozoic this interpretation has been questioned on many grounds especially that the range of ages does not match the offshore spreading rates and that the presence of continental coarse clastic sediments is difficult to reconcile with fast convergence rates between the oceanic plates and north america updates to basement geochronology and plate motion history reveal that these objections are much less serious than when they were first raised forward plate kinematic modeling reveals that predicted basement ages can be consistent with the observed range of about 55 49 ma and that the entire basement terrane can form within about 300 km of continental sources for clastic sediments this kinematic model indicates that there is no firm reason to reject the near ridge hotspot hypothesis on the basis of plate motions a novel element of the model is the resurrection plate previously proposed to exist between the farallon and kula plates by including the defunct resurrection plate in our reconstruction we are able to model the farallon hotspot track as docking against the oregon subduction margin starting about 53 ma followed by docking of the resurrection track to the north starting about 48 ma accretion of the farallon plate fragment and partial subduction of the resurrection fragment complicates the three dimensional structure of the modern cascadia forearc we interpret the so called e layer beneath vancouver island to be part of the resurrection fragment our new kinematic model of mobile terranes within the paleogene north american plate boundary allows reinterpretation of the three dimensional structure of the cascadia forearc and its relationship to ongoing seismotectonic processes

the neogene alert bay volcanic belt of northern vancouver island canada descending plate edge volcanism in the arc trench gap abstract the alert bay volcanic belt trends northeasterly across northern vancouver island coincident with the trace of the subducted juan de fuca explorer plate edge volcanism began in the west at brooks peninsula about 8 ma ago but occurred in most centers 3 5 1 ma ago there is a suggestion of eastward migration of activity and shift from basalt to dacite or rhyolite with time most of the volcanism was coincident with a time of rapid changes in the geometry of subduction as inferred from offshore magnetic patterns and with a hiatus in mainland cascade volcanic arc activity geometry and chronometry suggest this is a descending plate edge volcanic belt where disruption of steady state plate consumption patterns triggered magma genesis chemically the rocks are quite variable with divergent fractionation trends one trend resembles that of mull hebrides with a plagiophyric basalt of transitional alkaline subalkaline mildly tholeiitic and aluminous character which differentiated to clinopyroxene andesite and eventually to tholeiitic rhyolite and mildly tholeiitic calc alkaline dacite both of k poor magma type the other trend is like the cascades with aluminous aphyric calc alkaline basalt hornblende and or hypersthene andesite and k poor dacite this divergent character is also evident in ba rb nb and zr fractionation trends major and trace element discriminant diagrams generally identify the basalts as within plate types the 87 sr 86 sr isotope ratio is relatively low averaging 0 70325 and shows no trend with rock type or differentiation series oxygen in the entire suite is relatively heavy 18 o averaging 7 1 even the basalts are 18 o enriched oxygen shows no trend with degree of hydration rock type or series these isotopic and chemical data are compatible with minor crustal contamination of mafic primary magmas followed by fractional crystallization under different oxidation and hydration conditions

feo rich porphyritic pyroxene chondrules in unequilibrated ordinary chondrites abstract a suite of feo rich type ii porphyritic olivine pyroxene pop chondrules has been studied in detail data for ten chondrules from semarkona ll3 0 are emphasized and one chondrule from chainpur ll3 4 and two from parnallee ll3 6 are included as further examples of certain properties the chondrules contain phenocrysts of pyroxene and olivine in varying proportions and have fe fe mg 10 mol in the silicate minerals four pyroxene minerals are present clinoenstatite orthorhombic enstatite pigeonite and augite pyroxene phenocrysts may consist of all four minerals with progressive overgrowths in order of increasing wo content enstatite may occur independently of clinoenstatite but ca rich pyroxenes pigeonite and augite always nucleate on low ca pyroxene phenocrysts pyroxene and olivine phenocrysts may be independent of each other or may be intergrown and commonly show hopper morphologies indicative of rapid growth pyroxenes and olivines are commonly strongly zoned as a result of fractional crystallization of the chondrules in addition low ca pyroxene may contain ghost regions of slightly more feo and minor element rich material in the cores of grains these regions are partially resorbed and appear to be derived from previous crystallization of essentially the same chondrule oscillatory zoning is observed in enstatite in two chondrules and is believed to be the result of disquilibrium kinetic effects relict grains including a forsterite relict and a partially reduced feo rich relict in the same chondrule and clinoenstatite cores of pyroxene phenocrysts in another chondrule are evidence for recycling and mixing of chondrule material all of the chondrules are highly disequilibrium assemblages they appear to have cooled from essentially molten droplets with little volatile loss or recondensation occurring during the heating event peak temperatures were probably close to liquidus temperatures 1500 1700 c and cooling rates were probably of the order of hundreds of degrees per hour this description will serve as a basis for future interpretation of the effects of metamorphism on porphyritic pyroxene rich chondrules

storage or cooking pots inferring pottery use through archaeomagnetic assessment of palaeotemperatures abstract in this paper we report the results of an archaeomagnetic and rock magnetic study performed on a set of neolithic and chalcolithic potsherds from el portalon de cueva mayor site sierra de atapuerca burgos spain to investigate their thermal history and obtain information about their function during ancient times two types of palaeomagnetic behaviour have been observed one yields stable and univectorial magnetization diagrams type 1 while the other yields multivectorial diagrams with an intermedium component interpreted as a partial thermoremanence p trm type 2 acquired during a partial reheating of the pottery during their use to investigate the connection of these two types with the ancient function of the potsherds a set of modern pots was experimentally fired and then partially reheated al known temperatures the results obtained reproduced patterns similar to those observed in the archaeological samples type 1 univectorial samples that reached only a high heating temperature 600 c during their manufacture were most probably used as storage vessels by contrast type 2 multivectorial samples probably recorded a p trm acquired during their last use reheating related to cooking activities

mineral equilibrium in fractionated nebular systems we investigated the equilibrium mineral assemblages in chemically fractionated nebular systems using a computer routine that finds the set of minerals and gases which minimizes the gibbs free energy of a system with stipulated elemental abundances diagrams are presented showing the equilibrium mineralogy as a function of temperature 400 2300 k for unfractionated solar material and five fractionated systems the fractionated systems were defined by mixing in various proportions the following four volatility components that solar material can be divided into refractory dust carbonaceous matter ices and h2 gas dust enrichment is seen to increase temperatures of condensation evaporation and the fe2 content of mafic minerals and to permit existence of stable melt phases enrichment of dust and organic matter produces mineral assemblages that are similar in many ways to those of enstatite chondrites but with mafic minerals that are far more reduced fe2 poor than those in primitive enstatite chondrites enrichment of dust organics and ices leads to highly ferrous mineralogies even at the highest temperatures but does not predict the stability of hydrous phases above 450 k

relict chondrules in primitive achondrites remnants from their precursor parent bodies abstract we studied the petrography analyzed the chemical compositions constrained the closure temperatures via geothermometry and determined the oxidation states of relict chondrules in campo del cielo iab iron meteorite graves nunataks gra 98028 acapulcoite and netschaevo iie iron meteorite to constrain their formation conditions and investigate links to known meteorite groups despite having been thermally metamorphosed mineral phases within relict chondrules retain information about their precursor compositions the sizes and textures of relict chondrules and silicate and chromite compositions indicate that campo del cielo gra 98028 and netschaevo had distinct parent bodies that were similar to but different from known chondrite groups to determine the utility of relict chondrule sizes in thermally metamorphosed meteorites we determined the chondrule size distributions in the ll chondrites semarkona ll3 00 soko banja ll4 siena ll5 and saint severin ll6 and the h chondrites clovis no 1 h3 6 kesen h4 arbol solo h5 and estacado h6 as expected mean chondrule diameters increase with degree of thermal metamorphism we find that campo del cielo and gra 98028 were reduced during thermal metamorphism consistent with previous studies indicating that their precursors were initially more feo rich than their current compositions in contrast to previous studies we find no evidence for reduction of silicates in netschaevo normal zoning of olivine in netschaevo is consistent with crystallization and suggests its silicates are near their primary feo contents the presence of elongated chromite grains along olivine grain boundaries in netschaevo indicates formation during thermal metamorphism under oxidizing conditions due to the absence of reduction and the composition of chromite being distinct from that of metamorphosed h chondrites we conclude that netschaevo and by extension the iie iron meteorites are not from the h chondrite parent body

impact cratering on the h chondrite parent asteroid 1 this paper reports petrological data for lapaz icefield 02240 03922 031125 031173 031308 04462 and 04751 which are meteoritic samples of clast rich impact melt rocks from the h chondrite parent asteroid the size distribution and metallographic characteristics of fe ni metal in the melts indicate very rapid 1 to 40 c s cooling in the temperature range between 1500 and 950 c when the clast melt mixtures were thermally equilibrating cooling slowed to values between 10 3 and 10 2 c s in the temperature range between 700 and 400 c when the melt rocks were cooling to their surroundings these data suggest that the rocks cooled near the surface of the h chondrite asteroid within suevitic impact deposits integrating these data with the petrologic characteristics of other h chondrite melt rocks and their radioisotopic ages indicates that the h chondrite asteroid suffered at least one large impact event while still cooling from endogenous metamorphism at 4500 ma this impact must have degraded the asteroid s integrity but did not cause shattering impact events in the era between 4100 and 3600 ma produced melt volumes large enough to allow segregation of metal and troilite from silicate melts possibly within continuous impact melt sheets contained in craters the impact record after 3600 ma does not display such assemblages which suggests a decrease in the rate of large impact events or a catastrophic size reduction of the h chondrite parent asteroid at around this time

chemical and physical studies of chondrites x cathodoluminescence and phase composition studies of metamorphism and nebular processes in chondrules of type 3 ordinary chondrites abstract the cathodoluminescence cl properties of eight type 3 ordinary chondrites and one l5 chondrite have been determined and phenocryst and mesostasis compositions have been analyzed in the chondrules of four of them semarkona type 3 0 krymka 3 1 allan hills a77214 3 5 and dhajala 3 8 in order to investigate their origins and metamorphic history two major classes of chondrule with eight subdivisions have been identified mainly on the basis of cl properties and 95 of the chondrules can be assigned to these groups on the basis of phenocryst and mesostasis composition class a chondrules consisting of those with plagioclase normative mesostasis with bright cl are subdivided into groups a1 a2 a3 a4 and a5 class b chondrules with little or no cl and having quartz normative mesostases in the least metamorphosed chondrites which becomes feldspathic with metamorphism are subdivided into groups b1 b2 and b3 relationships between the eight chondrule groups can be deduced from their relative abundance in each of the nine chondrites groups a1 a2 b1 and a5 are present in semarkona 15 20 60 5 by number respectively group a5 chondrules in semarkona being more heterogeneous than a5 chondrules in chondrites of higher petrologic type chondrule group a1 evolves into a3 then a4 and then a5 during metamorphism while a2 evolves into a4 and then a5 chondrule group b1 evolves into b2 b3 and then a5 but higher levels of metamorphism are required to complete the series than for the a1 5 series conversion of a group to an adjacent group can be observed in allan hills a77214 3 5 where several chondrules are group b3 in their central regions and group a5 in their outer regions the present chondrule groups are essentially independent of texture since group a and group b chondrules differ in bulk composition redox state and possibly oxygen isotope systematics their relative abundance might be a factor in the creation of the nine chondrite classes

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

a methodological approach for identifying archaeological evidence of fire resulting from human activities abstract the identification of unequivocal evidence of humanly controlled fire is extremely problematic in archaeological contexts where obvious remains such as charcoal and ash have not been preserved until recently the difficulty of identification had been magnified by the lack of an established methodological approach which addressed the problem a methodological approach which can be used to identify and discriminate between archaeological evidence of fire resulting from natural processes and archaeological evidence of fire resulting from human activities is now available the approach utilizes several parameters of investigation including 1 macroscopic examinations of fire feature surfaces and profiles 2 analyses of magnetometer data 3 magnetic susceptibility studies 4 analyses of alternating field demagnetization characteristics 5 studies of isothermal remanent magnetization irm acquisition 6 analyses of coercivity of remanence characteristics and 7 palaeointensity studies the methodological approach is illustrated by comparing the results from analyses of features and samples from grass fire experiment sites isolated tree stump fire experiment sites multiple burn campfire experiment sites modern forest fire sites known archaeological hearths from modoc rock shelter illinois and known archaeological clay ovens from tell ashkelon israel for each available parameter of investigation comparisons revealed that multiple burn campfire sites exhibited basin shaped three dimensional configurations of oxidized sediments and diagnostic magnetic anomalies while multiple burn campfire samples displayed dramatic increases in magnetic susceptibility highly stable to very highly stable responses to alternating field demagnetization dramatic differences in irm acquisition and coercivity of remanence responses and diagnostic palaeointensity results when compared with the responses of adjacent unfired control samples archaeological hearth features and samples and samples from archaeological clay oven features exhibited results which were broadly consistent with the results from multiple burn campfire sites and samples the results displayed by grass fire isolated tree stump fire and forest fire sites and samples were generally found to be mutually exclusive from the results exhibited by multiple burn campfire and archaeological hearth and oven sites and samples for all parameters of investigation the results demonstrate that the methodological approach can be used to unequivocally identify archaeological evidence of humanly controlled fire resulting from multiple burn campfires fire hearths ovens and similar kinds of humanly produced fires

asteroid 6 hebe the probable parent body of the h type ordinary chondrites and the iie iron meteorites the s iv type asteroid 6 hebe is identified as the probable parent body of the h type ordinary chondrites and of the iie iron meteorites the ordinary chondrites are the most common type of meteorites falling to earth but prior to the present study no large mainbelt source bodies have been confirmed hebe is located adjacent to both the v6 and 3 1 resonances and has been previously suggested as a major potential source of the terrestrial meteorite flux hebe exhibits subtle rotational spectral variations indicating the presence of some compositional variations across its surface the silicate portion of the surface assemblage of hebe is consistent both in overall average and in its range of variation with the silicate components in the suite of h type chondrites the high albedo of hebe rules out a lunar style space weathering process to produce the weakened absorption features and reddish spectral slope in the s type spectrum of hebe linear unmixing models show that a typical ni fe metal spectrum is consistent with the component that modifies an h chondrite spectrum to produce the s type spectrum of hebe on the basis of the association between the h chondrites and the he iron meteorites our model suggests that large impacts onto the relatively metal rich h chondrite target produced melt bodies sheets or pods that differentiated to form thin laterally extensive near surface layers of ni fe metal fragments of the upper silicate portions of these melt bodies are apparently represented by some of the igneous inclusions in h chondrite breccias alternately masses of metal could have been deposited on the surface of hebe by the impact of a core or core fragment from a differentiated parent body of h chondrite composition subsequent impacts preferentially eroded and depleted the overlying silicate and regolith components exposing and maintaining large masses of metal at the optical surface of hebe in this interpretation the nonmagmatic iie iron meteorites are samples of the ni fe metal masses on the surface of hebe whereas the h chondrites are samples from between and or beneath the metal masses

secular variation of archaeomagnetic direction in the american southwest a d 750 1425 oriented archaeomagnetic samples were collected from 158 in situ features at 33 archaeological sites in the southwestern united states seventy three independently dated features were used for analysis of secular variation a moving window technique with outlier rejection was developed to compute a smoothed secular variation curve this technique incorporates weighted fisher statistics to account for imprecision in both the age of remanence acquisition and the direction of magnetization for each feature a mean direction and measure of dispersion is generated for each window of time the secular variation record covers the period a d 750 1425 during which time the virtual geomagnetic pole vgp moves in a generally counterclockwise fashion from 86 n 83 e at a d 750 to the lowest latitude position of 74 n 192 e at a d 1075 and then to 85 n 236 e at a d 1425 the median rate of vgp movement is 0 036 yr 1 similar to present day rates the southwestern record shows close correspondence to other north american archaeomagnetic limnomagnetic and speleomagnetic records the archaeomagnetic record can serve as the master curve for the derivation of archaeomagnetic dates

eileen johnson s legacy of research the discussion begins where her career on a national and international basis began at lubbock lake that starting point lead to significant broadening of her research both intellectually and geographically and was reflected in the work of her collaborators and students in paleoindian archaeology in all other archaeological time periods and traditions on the southern plains in the method and theory of zooarchaeology and quaternary vertebrate paleontology work beyond the high plains and sustained long term research programs both on and beyond the southern plains