appendix meteorites a brief tutorial there are four broad categories of meteorites chondrites achondrites irons and stony irons these are subdivided into meteorite groups the basic unit of meteorite classification although no formal guideline is in place for the minimum number of meteorites needed to define a group common practice is that there should be five or more members in a group a defined meteorite group is thought to be derived from a single asteroid however some groups are genetically related and are derived from a common parent asteroid chondrites are primitive stony meteorites rocks whose compositions are little changed since their formation in the solar nebula there are fourteen defined groups of chondrites and they make up the vast majority of meteorites falling to earth in the current epoch achondrites are stony meteorites of two broad types some are primitive materials like chondrites but most are the products of igneous differentiation there are ten defined groups of achondrites of which seven are differentiated types irons are also the products of asteroidal differentiation having crystallized from metallic melts separated from chondritic precursors there are thirteen defined groups of iron meteorites stony irons are also differentiated materials and both the rocky and metallic phases were formed by igneous processes on asteroids there are two defined groups of stony iron meteorites in addition to those that fit into groups there are many meteorites that are unique or for which there are less than five examples these ungrouped meteorites make up a substantial fraction of meteorites recovered to date

upb geochronology of the central ribeira belt brazil and implications for the evolution of the brazilian orogeny abstract the ribeira belt in southeastern brazil was generated during the brazilian orogeny the south american equivalent of the pan african orogeny 700 450 ma the central sector of the belt is characterised by a series of northwest vergent crustal scale thrust stacks emplaced under amphibolite grade conditions major shear zones separate four lithotectonic domains andrel ndia to the northwest juiz de fora paraiba and coastal domains analyses of zircon monazite and titanite from a variety of rock types from the four domains lead to the following conclusions all domains record an important tectono thermal event at 590 565 ma represented by partial melting granitoid intrusion and remobilisation of older basement gneisses and corresponding to thrusting and development of dextral shear zones also recorded are older metamorphic ages 611 604 ma of unknown significance only the paraiba and coastal domains in the south record a younger metamorphic event m2 at 535 520 ma most likely associated with another episode of thrusting and shear zone development the absence of m2 metamorphism in the andrel ndia and juiz de fora domains can be explained by post 535 520 ma emplacement of the paraiba domain late to post tectonic activity is represented by 503 492 ma metamorphism precursors of the basement gneisses are 2185 2134 ma old rocks generated during the transamazonian orogeny some of which contain a significant archean component with minimum ages of 2981 2846 ma the ages presented above define specific metamorphic events for the brazilian orogeny for the first time in southeastern brazil in addition together with previously published data they indicate that the basement rocks found in the allochthonous domains of the central ribeira belt represent an extension of the units present in the southern sao francisco craton and indicate that the foreland was more extensive than the presently defined craton

vp vs ratios in the yellowstone national park region wyoming abstract in this paper we study the variation of v p v s and poisson s ratio in the yellowstone national park region using earthquakes which were well recorded by a local seismic network we find that the average v p v s value within the geothermally active yellowstone caldera is about 7 lower than in the area outside the caldera within the caldera itself there may be a further 2 7 reduction of v p v s in the hydrothermally active norris geyser basin the upper and lower geyser basins and the yellowstone lake and mud volcano regions after considering various possible causes for v p v s changes such as geologic and structural differences thermal effects partial melting and hydrothermal activity we conclude that the most plausible explanation for the observed v p v s reduction is the presence of hot water at temperatures and pore pressures near the water steam transition in the caldera geothermal reservoirs

diachronic collision slab break off and long term high thermal flux in the brasiliano pan african orogeny implications for the geodynamic evolution of the mantiqueira province abstract the mantiqueira province in se brazil and uruguay is a complex brasiliano pan african orogenic province that has recently been the topic of conflicting geodynamic models focusing the possibility of multiple collision events during the ediacaran ordovician assembly of the western gondwana supercontinent a thorough compilation of recently obtained high resolution u pb geochronology and geochemistry in the mantiqueira province provided the following results a considering the entire orogenic system the pre syn and post orogenic periods range from 670 595 ma 620 550 ma and 560 490 ma respectively and are coeval with the intrusion of 3 distinct magmatic sequences within each geographic segment of this large orogenic province evolving from i calc alkaline arc related magmatism to ii syn orogenic anatectic magmatism coeval with the metamorphic climax to iii calc alkaline alkaline including shoshonitic and a type affinity magmatism that reveal significant lower crust and mantle contribution b progressively younger ages are present from south to north irrespective of the orogenic period considered implying that the different geographic sectors were from south to north diachronically amalgamated into the mantiqueira province results provide important evidence for the existence of a single diachronic collision event at 630 590 ma followed by the thermal climax at 615 560 ma slab break off asthenospheric upwelling and mantle underplating that sustained long term high heat flux conditions until 540 490 ma when post tectonic calc alkaline alkaline granitoids probably admixed with small amounts of slab melts formed at depth intruded the middle low crust magmatic and metamorphic sequences during the final stages of thermal and orogenic collapse of the mantiqueira province belts

oxygen isotope studies of achondrites abstract oxygen isotope abundances provide a powerful tool for recognizing genetic relationships among meteorites among the differentiated achondrites three isotopic groups are recognized l snc mars 2 earth and moon and 3 hed howardites eucrites diogenites the hed group also contains the mesosiderites main group pallasites and silicates from iiiab irons the angrites may be marginally resolvable from the hed group within each of these groups internal geologic processes give rise to isotopic variations along a slope 1 2 fractionation line as is well known for terrestrial materials variations of 17o from one planet to another are inherited from the inhomogeneities in the solar nebula as illustrated by the isotopic compositions of chondrites and their constituents among the undifferentiated achondrites five isotopic groups are found 1 aubrites 2 winonaites and iab iiicd irons 3 brachinites 4 acapulcoites and lodranites and 5 ureilites the isotopic compositions of aubrites coincide with the earth and moon and also with the enstatite chondrites these bodies apparently were derived from a common reservoir the isotopic composition of which was established at the chondrule scale by nebular processes isotopic similarities between chondrites and achondrites are seen only for the following instances 1 enstatite chondrites and aubrites 2 h chondrites and he irons and 3 l or ll chondrites and iva irons the isotopic data also support the following genetic associations 1 winonaites and iab iiicd irons 2 acapulcoites and lodranites and 3 ureilites and dark inclusions of c3 chondrites an attempt to reconcile the whole planet isotopic compositions of earth mars and the eucrite parent body with mixing models of their chemical compositions failed it is not possible to satisfy both the chemical and isotopic compositions of the terrestrial planets using known primitive solar system components

juvenile contribution of the neoproterozoic rio negro magmatic arc ribeira belt brazil implications for western gondwana amalgamation abstract the ca 790 600 ma rio negro complex rnc of the ribeira belt brazil consists of a plutonic portion of a magmatic arc built by the e vergent subduction of the ese border of the sao francisco paleoplate during the amalgamation of western gondwana the plutonic series comprises low to medium k granitoids ca 790 620 ma and high k granitoids and shoshonite rocks ca 610 605 the age span of 185 m y is suggestive of a long history of arc related magmatism continuously or not in time the nd isotopic signatures of the rnc consist of end t ratios from 3 to 5 for the medium k series shoshonite series and from 14 to 3 for the younger high k group this time dependent trend of nd isotopes is indicative of progressive maturity of the arc over time the same evolution is indicated by sr data as the medium k rocks have 87sr 86sr initial ratios the role of transform fault zones such as the luanda shear zone is emphasized in order to explain the consumption of a wide oceanic plate in the inner portion of western gondwana

isotopic fractionation of germanium in iron meteorites significance for nebular condensation core formation and impact processes abstract the siderophile and volatile nature of germanium allow ge isotopes to be used to investigate the early history of planetesimals by tracing processes of core formation and impact through the study of magmatic and non magmatic iron meteorites germanium isotopic compositions were measured using a hexapole collision cell mc icpms with an external reproducibility of 0 06 amu iron meteorites display heavy ge isotopic compositions 74ge 74ge 70ge 0 27 to 1 92 with respect to a jmc ge standard which contrast with the light isotopic compositions of the studied terrestrial materials aldrich ge standard 74ge 1 68 1 72 0 22 sphalerite 74ge 0 69 all the data fall on the theoretical mass fractionation line indicating no isotopic anomalies the samples from magmatic irons iia iib iiiab iic have relatively homogeneous ge isotopic compositions of 74ge 1 77 0 22 2 for a large range of ge contents 36 189 ppm in contrast the samples of the non magmatic groups iab iie have lower and more variable ge isotopic compositions with 74ge 1 15 0 20 for iab group ge 254 493 ppm 74ge 0 27 to 0 43 for iie old group ge 63 69 ppm and 74ge 1 40 0 22 for the watson sample ge 50 ppm from the iie young group except for iie samples no within group correlation of 74ge with ir or ge contents is observed demonstrating that fractional crystallization or crystal segregation do not fractionate ge isotopes the lack of inter group correlation between the ge isotopic composition of magmatic irons and ni content an indicator of redox processes suggests that if ge isotopes are fractionated by redox induced diffusion during metal silicate segregation isotopic equilibration towards the initial composition must also occur during isotopic exchange at high temperature thus the average 74ge value of 1 77 0 22 should be representative of their parent body precursors the lack of correlation between the ge isotopic composition and ge ni ratios and the high ge ni ratio of iia samples ge ni in ci chondrites may reflect the high proportion of metal condensates present in the parent body before metal silicate differentiation these metal phases would carry the ge isotopic signature of a ge enriched gas phase of the solar nebula for the iab non magmatic group the light and homogeneous ge isotopic composition may result from condensation from a 70ge enriched vapour subsequent to partial melting within a porous parent body the origin of the heating event 26al decay or impact on the surface of the chondritic body cannot be distinguished in contrast it can be demonstrated that impact events are involved in the formation of iie irons the negative 74ge ge correlation with increasing 74ge values from the old group to the watson sample of the young group would result from non rayleigh evaporation of 70ge during impact with gas melt back reaction the high 74ge of the watson sample young group is consistent with successive impacts on the surface of the iie parent body the very light ge isotopic composition of the old less impacted iie irons compared to other studied iron meteorites suggests a light ge isotopic composition for the iie parent body

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

tectonic seismicity at stromboli volcano italy from historical data and seismic records abstract we describe the main features of crustal tectonic seismicity in the area of stromboli aeolian islands a volcano known worldwide for its typical strombolian activity we consider data from the italian catalogues of earthquakes along with records of the local seismic network covering the time span from 1885 to 1998 in the historical catalogues we find a low occurrence rate of earthquakes with a small number of seismic events having intensity viii in the mercalli cancani sieberg scale recent instrumental data confirm this finding achieving a maximum magnitude m d 3 7 we explain this low tectonic seismicity as the effect of conditions which allow only a limited storage of stress from a seismic viewpoint the aeolian archipelago shows two different seismogenic domains spatially separated by the aeolian tindari giardini fault system one domain which includes the western and southern parts of the aeolian islands is characterized by crustal seismic activity the second domain which includes the eastern part of the archipelago where stromboli lies has a large number of intermediate and deep earthquakes but little crustal seismicity we surmise that each domain has vertical heterogeneity separating ductile deformation from brittle failure of rock induced by temperature conditions and or hydration dehydration reactions which affect the mechanical strength of the medium

seismic features of the june 1999 tectonic swarm in the stromboli volcano region italy abstract crustal tectonic seismicity in the southern tyrrhenian sea is characterized by the high occurrence rates of earthquakes to the west of the alignment of salina lipari and vulcano islands in the aeolian archipelago only a few earthquakes affect the crustal region east of these islands whereas intermediate and deep seismicity plays a relevant role based on this evidence two aspects of the seismic swarm recorded at the aeolian island seismic network between june 6 and 17 1999 looked anomalous the first aspect concerned the number of earthquakes 78 that affected the stromboli submarine edifice in a short time interval secondly despite the low maximum magnitude md 3 2 reached the cumulative strain release was conspicuous in comparison with previous swarms in this region we localized the swarm about 6 km northeast of stromboli at a depth between 8 and 12 km the source region was identified using standard methods of hypocentral location as well as azimuth analysis it is worth noting that the volcanic activity at stromboli did not change significantly during the swarm nor throughout the following months therefore the seismic swarm had no link with volcanic activity observed at the surface most of the earthquakes shared similar waveform and frequency content and can be divided into families we identified some earthquakes with magnitude up to md 3 having relatively low frequency content at different seismic stations this anomalous feature leads us to hypothesize the presence of fluid circulation and or propagation of seismic waves in a ductile medium our hypothesis is in agreement with studies on marine geology which highlight various forms of submarine volcanism in the southern basin of the tyrrhenian sea

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

long period events the most characteristic seismicity accompanying the emplacement and extrusion of a lava dome in galeras volcano colombia in 1991 abstract since its reactivation in 1988 the principal eruptions of galeras volcano occurred on may 4 9 1989 july 16 1992 and january 14 march 23 april 3 april 14 and june 7 1993 the initial eruption was a phreatic event which clearly marked a new period of activity a lava dome was extruded within the main crater in october 1991 and subsequently destroyed in an explosive eruption on july 16 1992 the eruptions that followed were all vulcanian type explosions the seismicity accompanying the emplacement extrusion and destruction of the lava dome was dominated by a mix of long period lp events and tremor displaying a variety of waveforms repetitive lp events with dominant periods in the range 0 2 1 s were observed in october and november 1991 and visually correlated with short energetic pulses of gas venting through a crack bisecting the dome surface each lp event was characterized by a weak precursory signal with dominant periods in the range 0 05 0 1 s lasting roughly 7 s using the fluid driven crack model of chouet 1988 1992 we infer that two distinct cracks may have acted as sources for the lp and precursor signals spectral analyses of the data yield the following parameters for the lp source crack length 240 360 m crack width 130 150 m crack aperture 0 5 3 4 mm crack stiffness 100 500 sound speed of fluid 880 m s and excess pressure 0 01 0 19 mpa similar analyses yield the parameters of the precursor source crack length 20 30 m crack width 15 25 m crack aperture 2 3 8 7 mm crack stiffness 5 15 sound speed of fluid 140 m s and excess pressure 0 06 0 15 mpa combined with geologic and thermodynamic constraints obtained from field observations these seismic parameters suggest a gas release mechanism in which the episodic collapse of a foam layer trapped at the top of the magma column subjacent to the dome releases a slug of pressurized gas which escapes to the surface while dilating a preexisting system of cracks in the dome structure accordingly the fracture observed on the crystallized dome body is the surface extension of the lp source crack where lp activity is induced by the rapid emission and expansion of gas flowing through this conduit the width and aperture of the crack estimated in the model are in good agreement with the length and aperture of the fracture estimated from visual observations the source parameters of the precursor signal are suggestive of a nozzle like conduit connecting the lp source crack to the underlying magma reservoir excitation of this conduit segment is attributed to the rapid emission and acceleration of the frothy fluid resulting from the collapse of the foam layer at the top of the reservoir the calculated periodicity of foam collapse events is in agreement with the observed average rate of thirteen lp events per hour

rare earth element geochemistry and petrogenesis of miles iie silicate inclusions an ion probe study of rare earth element ree geochemistry of silicate inclusions in the miles he iron meteorite was carried out individual mineral phases among inclusions have distinct ree patterns and abundances most silicate grains have homogeneous ree abundances but show considerable intergrain variations between inclusions a few pyroxene grains display normal igneous ree zoning phosphates whitlockite and apatite are highly enriched in rees 50 to 2000 x ci with a relatively light rare earth element lree enriched ree pattern they usually occurred near the interfaces between inclusions and fe host in miles albitic glasses exhibit two distinctive ree patterns a highly fractionated lree enriched ci normalized la sm similar to 15 pattern with a large positive eu anomaly and a relatively heavy rare earth element hree enriched pattern ci normalized lu gd similar to4 with a positive eu anomaly and a negative yb anomaly the glass is generally depleted in rees relative to ci chondrites

concordant 3 676 myr u pb formation age for the kodaikanal iron meteorite the kodaikanal iron meteorite contains evidence of intensive chemical activity which according to rb sr studies1 occurred 800 myr after the formation of the solar system here we report the results of u pb isotope analyses on a silicate inclusion from kodaikanal leaching experiments on the two major phases clinopyroxene cpx and alkali rich glass indicate a contamination by terrestrial pb attributable to previous curatorial cutting of this iron meteorite the leached minerals define concordant u pb ages that validate the precise pb pb age of 3 676 3 myr these data are consistant with rb sr age determinations1 the absence of initial pb indicates a fast cooling of the silicate material this has already been established for the metal phase and a collisional origin for kodaikanal is favoured

impact induced perturbations of atmospheric sulfur abstract asteroids and comets that are vaporized during hypervelocity impact events can inject large masses of s into the stratosphere where it can potentially affect the radiation budget of the earth alter the chemistry of the ozone layer and eventually be converted to sulfuric acid rain relatively small carbonaceous asteroids 0 3 km in diameter contain 5 times more s than the entire modern stratosphere and these objects hit the earth at an average rate of 1 per 10 000 years larger impact events capable of injecting 10 15 g of s into the stratosphere occur at an average rate of 1 per 1 million years calculations indicate there is sufficient o and h in the vapor plumes of most impact events to convert the s to sulfuric acid aerosols if this conversion occurs then the larger impact events could depress mean surface temperatures by more than 2 c for 3 years or longer

isotopic variations of germanium in iron and stony iron meteorites abstract germanium isotopic ratios for six iron meteorites of various groups metallic phase of pallasite stony iron meteorites three chemical reagents and terrestrial germanite sample from tsumeb mine namibia have been determined using a multiple collector inductively coupled plasma mass spectrometer mc icpms in this study ga external correction technique which enables us to detect the isotopic fractionation of the elements during the sample formation was applied for the isotopic analysis of ge resultant germanium isotopic data for terrestrial and meteoritic samples were found to have significant variations variations in germanium isotopic ratios for three chemical reagents and germanite samples were about 0 1 0 3 for 70 ge 73 ge and 0 5 for 72 ge 73 ge ratios whereas germanium isotopic variations for meteoritic samples were about 4 for 70 ge 73 ge and 3 for 72 ge 73 ge ratios several points emerge from the germanium isotopic data the first is that the germanium isotopic data for meteorites and terrestrial samples were found to define a straight line having slope of 1 3 on 70 ge 73 ge vs 72 ge 73 ge diagram ge three isotopes plot this indicates clearly that the meteoritic ge had been isotopically fractionated during the sample formation a second feature of the germanium isotopic data is that all the meteoritic ge has systematically heavy isotopic compositions low in 70 ge 73 ge and 70 ge 73 ge ratios compared with those for terrestrial ge if the germanium isotopic ratios for germanite and chemical reagents reflect the germanium isotopic compositions for crustal materials of the earth this systematic deviations between meteorites and the terrestrial samples could be explained by the separation of crust from mantle alternatively heavy isotopic compositions for meteorites might be attributed to the larger evaporation loss of ge from the meteoritic materials detailed mechanisms for the systematical difference in germanium isotopic ratios between meteorites and terrestrial materials will be given in this paper the germanium isotopic data presented here demonstrate clearly that the isotopic fractionation data for medium or heavy mass range elements can give many clues to the origin and thermal evolution sequence of the metallic meteorites

a synthesis of the recent activity of galeras volcano colombia seven years of continuous surveillance 1989 1995 abstract the current period of re activation since 1988 at galeras volcano colombia has been characterized mainly by the following events 1 a semi continuous series of vulcanian eruptions during 5 9 may 1989 2 emplacement of an andesitic lava dome at the bottom of the main crater in october november 1991 3 six vulcanian eruptions during 1992 1993 the first of which destroyed most of the dome on 16 july 1992 and 4 three volcano tectonic seismic crises in april 1993 november december 1993 and march 1995 during much of this seven year period several small ash and gas emissions also have taken place the 4 9 may 1989 eruptions originated from the secondary crater el pinta and deposited ash lapilli and blocks in the crater area the 1992 1993 eruptions originated from the main crater and were associated with obstruction of the conduit by magma from dome emplacement in late 1991 causing overpressurization of the system for the 1992 1993 eruptions pre eruptive seismicity deformation and so2 flux all exhibited very low levels the eruptions were characterized by their sudden initiation low intensity vei 1 small eruption columns and small volumes of erupted material the source of the volcano tectonic seismic crises is located approximately 3 km north and northeast of the crater some of these events were felt in pasto and other towns located around the volcano on one occasion causing loss of life injuries and damage to buildings

timing of terrane accretion in the neoproterozoic eopaleozoic ribeira orogen se brazil abstract new id tims u pb geochronological data of the oriental terrane combined with detailed geological data of other terranes support the presented tectonic evolutionary model for the central segment of the neoproterozoic ribeira belt southeastern brazil that can be subdivided in four major tectonic units a the occidental terrane regarded as the reworked margin of the sao francisco craton to the northwest b the paraiba do sul klippe c the oriental terrane comprising arc complexes and d the cabo frio terrane to the southeast the oriental terrane and the cabo frio terrane were progressively accreted to the reworked continental margin of the sao francisco paleoplate the occidental terrane contains pre 1 8 ga basement slivers interlayered with neoproterozoic passive margin successions andrel ndia group the oriental terrane focus of the work was firstly accreted to the sao francisco margin at ca 580 ma it displays three structural domains thrusted to nw toward the reworked sao francisco margin from west to east a the cambuci domain that is interpreted as the fore arc association b the costeiro domain that contains a distal passive margin succession intruded by the ca 790 and 635 620 ma rio negro arc complexes and c the italva domain that comprises marbles psamitic gneisses and amphibolites of ca 840 and is regarded both as the proximal passive margin of the oriental terrane or a back arc basin syn to late collisional granitoids and three metamorphic episodes ca 605 580 and 550 ma indicate a complex history for the collision i they were also described for the kaoko damara and west congo only the youngest one pan african belts located at occidental africa and are supposed to record the main episode of convergence in western gondwana a second collisional stage at ca 530 ma accreted the cabo frio terrane to the oriental terrane and is possibly related with the closure of a back arc basin located between the oriental terrane and kalahari craton that remained open until the ordovician the effects of the collision ii were also detected at the oriental terrane finally late to post tectonic granites marked the tectonic collapse of the orogenic building between 513 480 ma

rhyacian orosirian isotopic records from the basement of the aracuai ribeira orogenic system se brazil links in the congo sao francisco palaeocontinent abstract assembled during the rhyacian orosirian boundary the congo sao francisco palaeocontinent remained linked by a continental bridge from ca 2 ga to the atlantic ocean opening in the cretaceous to the south of that bridge precursor basins and orogenic outcomes encompassed by the neoproterozoic cambrian aracuai ribeira orogenic system aros and its african counterparts developed the aros discloses reworked basement units like the juiz de fora pocrane and quirino complexes which are key units to retrieve the palaeoproterozoic history of the congo sao francisco palaeocontinent this paper presents new u pb ages and the first lu hf in zircon signatures from those complexes all magmatic zircon grains show rhyacian and orosirian ages ranging from c 2250 ma to c 1850 ma the best concordia ages range from 2184 9 ma to 2080 19 ma hf in zircon and whole rock sm nd data disclose mantle related signatures for both the pocrane and juiz de fora complexes suggesting igneous protoliths with no or very few involvement of continental crust similarly to juvenile magmatic arcs conversely hf in zircon data for the quirino complex show an evolved signature compatible with a magmatic arc developed on a continental older crust with archaean components together with a thorough compilation on rhyacian orosirian orogenic systems roos our data allow us to envisage links within the congo sao francisco palaeocontinent comprising orogens characterised by mostly evolved isotopic signatures and zircon inheritance they are grouped into the i w roos including the mantiqueira complex the west segment of the eastern bahia belt intrusions in the south gaviao and porteirinha blocks and parts of the mineiro belt and ii e roos comprising the quirino complex basement rocks of the cabo frio tectonic domain the coastal eastern bahia belt and in africa eburnean and kimezian complexes as well as the western angola shield assembled between the w roos and e roos an orogenic system with prominent juvenile to moderately juvenile isotopic signature ju roos includes the juiz de fora and pocrane complexes and the buerarema complex at the central segment of the eastern bahia belt

volcano tectonic seismicity at unzen volcano japan 1985 1999 abstract the eruption of unzen volcano in southwestern japan began on november 17 1990 and an extrusion of a dacite lava dome lasted from may 20 1991 until early february 1995 the seismic network operated by the shimabara earthquake and volcano observatory of kyushu university sevo located about 4000 volcano tectonic earthquakes during the period from 1985 to 1999 the seismicity was greatest in the period between one year before the eruption and the start of the lava dome extrusion the hypocenters were mostly distributed within a 12 20 km area on the western side of the volcano an inclined seismic aseismic boundary shallows eastward to the summit probably reflecting the thermal structure beneath the volcano within 4 km west from the summit however the seismicity is very low it was suggested that the high temperature ductile and low q bodies exist there we obtained focal mechanism solutions for about 90 earthquakes among them orientations of p axes for the earthquakes occurring on the upper side of the inclined seismic aseismic boundary suggest a pressure source located below this boundary although most of the focal mechanisms have t axes nearly horizontal in the n s direction consistent with the n s extensional tectonic stress several earthquakes whose focal mechanisms are inconsistent with the tectonic stress were observed within a horizontal distance of 5 km from the summit this is because the pressurization of volcanic gas or magma beneath the summit produced a strong local stress in the summit surroundings

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

correlation of neoproterozoic terranes between the ribeira belt se brazil and its african counterpart comparative tectonic evolution and open questions

propagation of strike slip faults across holocene volcano sedimentary deposits pasto colombia this study contributes to the understanding of shear failure development on the basis of macroscopic field data collected in latest pleistocene holocene pyroclastic and fluvio lacustrine deposits in the pasto valley sw colombia here there is a pervasive system of microfaults and joints right lateral strike slip microfaults strike n0658 whereas left lateral strike slip microfaults strike n1208 three main joint sets strike n n0658 and n0208 in decreasing order of frequency stress computation gives a horizontal s1 trending wn0608 and a horizontal s3 trending wn1508 consistent with earthquake focal mechanisms and stress inversion of main faults synthetic shears dominate resulting from nucleation of older cracks in the basement cropping out northeast of pasto the ne to ene striking buesaco aranda and pasto faults show evidence of latest pleistocene holocene right lateral strike slip motions the structures in the pasto valley can be interpreted as a mode iii damage zone representing the up dip propagation of the main faults across the young volcano sedimentary deposits q 2005 elsevier ltd all rights reserved

chronology and petrology of silicates from iie iron meteorites evidence of a complex parent body evolution iie iron meteorites contain silicate inclusions the characteristics of which suggest a parent body similar to that of h chondrites however these silicates show a wide range of alteration ranging from netschaevo and techado the inclusions of which are little altered to highly differentiated silicates like those in kodaikanal weekeroo station and colomera which have lost metal and sulfur and are enriched in feldspar we find these inclusions to show varying degrees of shock alteration we made 39ar 40ar age determinations of watson techado miles colomera and sombrerete watson has an ar ar age of 3 677 0 007 gyr similar to previously reported ages for kodaikanal and netschaevo we suggest that the various determined radiometric ages of these three meteorites were probably reset by a common impact event the space exposure ages for these three meteorites are also similar to each other and are considerably younger than exposure ages of other iies 39ar 40ar ages inferred for the other four meteorites analyzed are considerably older than watson and are techado 4 49 0 01 gyr miles 4 405 0 012 gyr colomera 4 470 0 010 gyr and sombrerete 4 541 0 0012 gyr these ages are in fair agreement with previously reported rb sr isochron ages for colomera and weekeroo station although several mechanisms to form iie meteorites have been suggested it is not obvious that a single mechanism could produce a suite of meteorites with very different degrees of silicate differentiation and with isotopic ages that differ by 0 8 gyr we suggest that those iies with older isotopic ages are a product of partial melting and differentiation within the parent body followed by mixing of silicate and metal while both were relatively hot netschaevo and watson may have formed by this same process or by impact mixing 4 5 gyr ago but their isotopic ages may have been subsequently reset by shock heating kodaikanal apparently is required to have formed more recently in which case impact melting and differentiation seems the only viable process we see no compelling reasons to believe that iie silicate and metal derived from different parent bodies or that the parent body of iies was the same as that of h chondrites

microearthquakes on the endeavour segment of the juan de fuca ridge 1 we report the results of a 55 day microearthquake experiment on the endeavour segment of juan de fuca ridge the network covered a 5 km section of the ridge axis centered on the main vent field and extended 15 km off axis on the west flank the ridge axis and flanks were seismically active and 1750 earthquakes were located with a minimum of five travel time picks including at least one s wave over half the earthquakes occurred in swarms and a waveform cross correlation technique was used to obtain relative locations on the western flank the hypocenters for four swarms at midcrustal depths are compatible with steeply dipping fault planes that strike at 035 050 n and oblique to the abyssal hills focal mechanisms determined from p wave first motions and p s amplitude ratios are predominantly strike slip with north south compression and appear to be affected by the reorganization of the explorer plate earthquakes beneath the ridge axis are concentrated in a band of intense seismicity at 1 5 3 5 km depth to the north of high rise vent field the seismicity defines a plane striking parallel to the ridge axis and dipping east at 70 and the earthquakes appear to extend beneath an axial reflector previously imaged at 2 3 km depth farther south the hypocenters are not compatible with a single fault plane focal mechanisms are characterized by subhorizontal tension axes oriented in all directions except parallel to the ridge and suggest a stress field that is about equally influenced by ridge spreading and hydrothermal cooling

primordial noble gases in a graphite metal inclusion from the canyon diablo iab iron meteorite and their implications we have carried out noble gas measurements on graphite from a large graphite metal inclusion in canyon diablo the ne data of the low temperature fractions lie on the mixing line between air and the spallogenic component but those of high temperatures seem to lie on the mixing line between ne hl and the spallogenic component the ar isotope data indicate the presence of q in addition to air spallogenic component and ar hl as the elemental concentration of ne in q is low we could not detect the ne q from the ne data on the other hand we could not observe xe hl in our xe data as the xe concentration and the xe ne ratio in q is much higher than that in the hl component it is likely that only the contribution of q is observed in the xe data xenon isotopic data can be explained as a mixture of q air and el taco xe the canyon diablo graphite contains both hl and q very much like carbonaceous chondrites retaining the signatures of various primordial noble gas components this indicates that the graphite was formed in a primitive nebular environment and was not heated to high igneous temperatures furthermore a large excess of 129xe was observed which indicates that the graphite was formed at a very early stage of the solar system when 129i was still present the hl q ratios in the graphite in canyon diablo are lower than those in carbonaceous chondrites indicating that some thermal metamorphism occurred on the former we estimated the temperature of the thermal metamorphism to about 500600 c from the difference of thermal retentivities of hl and q it is also noted that el taco xe is commonly observed in many iab iron meteorites but its presence in carbonaceous chondrites has not yet been established

emplacement and petrological evolution of the andesitic dome of galeras volcano 1990 1992 abstract since the reactivation of galeras volcano in 1988 the emplacement of a lava dome in the main crater of the volcano has been its most significant magmatic activity changes leading to the emplacement of the dome were present as early as june july 1990 when the trend of degassing for the major species mainly so 2 and co 2 in the summit fumaroles changed drastically and small vulcanian explosions become more frequent moreover after august 1990 an inflationary trend as measured by an electronic tiltmeter was observed to increase over background levels and the temperature of a newly formed fumarole besolima increased from 130 to 738 c within a month the dome was seen for the first time in early october 1991 at the base of the main crater the dome grew for about a month to a total volume of 4 10 5 m 3 during this time the growth rate was calculated to be 12 10 3 m 3 day or 0 14 m 3 s an explosive eruption on 16 july 1992 destroyed most of the dome the chemical composition of the dome ranges from 59 4 to 61 1 wt sio 2 anhydrous the presence of 1 large plagioclase phenocrysts with strongly inversely zoned rims and 2 small euhedral plagioclase crystals with normal zoning and ca rich cores indicates that a magma mixing event occurred early in the history of dome emplacement large amounts of crystal fractionation then took place about 70 fractionation can explain the mineral suite and the major and trace element compositions of the glasses the uppermost part of the dome degassing freely at the surface for ten months was able to isolate the magmatic system producing a highly evolved melt in the dome interior the july 1992 eruption tapped not only the uppermost part of the dome but also a deeper h 2 o saturated magma analyses from these samples show that most of the h 2 o was degassed while cl and b contents were higher than in the uppermost part of the dome however the f content is lower in this deeper part of the conduit while it increases in the matrix glass of the degassed upper part of the dome

the portales valley meteorite breccia evidence for impact induced melting and metamorphism of an ordinary chondrite abstract the portales valley h chondrite fall is an annealed impact melt breccia with coarse metal interstitial to angular and subrounded silicate clasts the large metal rich regions exhibit a widmanstatten structure and contain very little troilite we were able to examine a 16 5 kg metal rich specimen of portales valley silicates contain numerous flecks of metallic cu and curvilinear trails of tiny metallic fe ni blebs characteristic of shocked and annealed chondrites one silicate clast appears to have experienced little

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

comment on tectono metamorphic evolution of the central ribeira belt brazil a case of late neoproterozoic intracontinental orogeny and flow of partially molten deep crust during the assembly of west by meira et al

1989 1995 earthquake sequences in the galeras volcano region sw colombia and possible volcano earthquake interactions abstract the galeras volcanic complex located in the narino department sw colombia includes the most historically active volcano in the country galeras a 4276 m high stratovolcano located 9 km west of the city of san juan de pasto 400 000 inhabitants the area is also affected by the continental faulting represented by the buesaco aranda and pasto faults belonging to one of the most seismically active structures in colombia the romeral fault system several moderate to strong shallow earthquakes affected the city of san juan de pasto and its neighbouring region since the xvii century the coexistence of an active volcanic complex and an active fault system complicates the study and interpretation of the different processes taking place in the region as well as the identification of any connection or interaction among them the reactivation of the volcano in 1989 was characterized by three main magmatic events 1 a series of vulcanian eruptions during 4 9 may 1989 with semi continuous ash emissions from a secondary crater 2 the emplacement of an andesitic lava dome at the bottom of the main crater from september 1990 until december 1991 and 3 six vulcanian eruptions from the main crater during 1992 1993 with destruction of most of the dome during the first one on 16 july 1992 during the same period four earthquake sequences were located in a limited area n and ne of galeras volcano on august september 1989 aug1989 april june 1993 apr1993 november december 1993 nov1993 and march august 1995 mar1995 the last one included a ml4 7 main shock on 4 march 1995 producing moderate to high damage in the epicentral region msk maximum intensity viii and in the city of san juan de pasto vi vii the last damaging earthquake in the region was a msk intensity viii ix in 1947 a detailed analysis of the spatio temporal characteristics of the four earthquake sequences allowed identifying their different origin and suggesting some interrelationship between the reactivated eruptive process and the contemporaneous seismic activity the aug1989 sequence presents a typical volcanic swarm like pattern most probably related with the process of magma intrusion from depth at the beginning of the volcano s reactivation the apr1993 the nov1993 and the mar1995 sequences show a clear tectonic origin with events occurring on rupture planes almost vertical that can be associated to the active faults in the area the seismogenic process of these three sequences could have been activated or accelerated by the main eruptions during 1992 1993 these results suggest that constrains provided by improved relocations and the detailed analysis of the space time characteristics of earthquake sequences in galeras volcanic environment allow to establish the different generation mechanisms involved and to suggest feasible explanations on the possible interrelationships of the magmatic volcanic processes and the seismicity observed

contrasting triggering mechanisms of the 2001 and 2002 2003 eruptions of mount etna italy abstract mount etna produced two significant eruptions in 2001 and 2002 2003 which we have analysed using geological seismic and deformation data these eruptions showed some similarities such as the activating of two magmatic plumbing systems central lateral and eccentric but they differed in their triggering mechanisms while the 2001 eruption was largely the result of the emplacement of a n s eccentric dike independent from the central conduits consistent with e w regional extension the 2002 2003 eruption occurred in response to a major flank slip on the eastern and southeastern sides of the volcano this is demonstrated by the spatial and temporal distribution of seismicity and deformation preceding and accompanying the two eruptions during the months prior to the 2001 eruption most epicenters were concentrated on the southern flank at depths of 5 15 km below sea level during the 4 days before the eruption earthquake hypocenters migrated to shallower levels from 5 km bsl upward indicating the emplacement of the eccentric dike this is confirmed by the patterns of ground fracturing observed in the field and deformation documented by electronic distance measurements edm in contrast the months before the 2002 2003 eruption were characterised by shallower seismicity mainly concentrated along the active faults bordering the slipping flank sector flank slip accelerated in september 2002 and a second more vigorous acceleration of flank slip occurred on 26 27 october 2002 accompanying the opening of eruptive vents the very short 2 h seismic crisis preceding the onset of eruptive activity stands in neat contrast with the 4 days of intense seismicity before the 2001 eruption subsequently flank slip deformation extended all over the eastern and southeastern flanks of the volcano causing serious damage in this sector the events of 2001 2003 can be seen as a continuous chain of intimately interacting processes including regional tectonics magma accumulation and eruption and flank instability in this scenario the 2001 eruption led to increased flank instability that subsequently accelerated and culminated with the massive flank slip which in turn facilitated the 2002 2003 eruption this sequence of events points to a long term feedback mechanism between magmatism and flank instability at etna

tectonic setting and u pb zircon dating of the plutonic socorro complex in the transpressive rio paraiba do sul shear belt se brazil the precambrian rio paraiba do sul shear belt comprises a 200 km wide anastomosing network of ne sw trending ductile shear zones extending over 1000 km of the southeastern coast of brazil granulitic gneissic migmatitic and granitoid terrains as well as low to medium grade metavolcano sedimentary sequences are included within it these rocks were affected by strong contractional tangential tectonics due to west northwestward oblique convergence of continental blocks subsequent transpressional tectonics accomodated large dextral orogen parallel movements and shortening the plutonic socorro complex is one of many deformed granites with a foliation subparallel to that of the shear belt and exposes crosscutting relationships between its tectonic magmatic and metamorphic structures these relationships point to a continuous magmatic evolution related to regional thrusts and strike slip ductile shear zones the tectonic and magmatic structural features of the serra do lopo granite provide a model of emplacement by sheeting along shear zones during coeval strike slip and cross shortening of country rocks geochronological data indicate that the main igneous activity of socorro complex spanned at least 55 million years from the late stage of the northwestward ductile thrusting 650 ma through right lateral strike slip 595 ma deformation the country rocks yield discordant age data which reflect a strong imprint of the transamazonian tectono metamorphic event 1 9 to 2 0 ma we propose a model for the origin of calc alkaline granites of the ribeira belt by partial melting of the lower crust with small contributions of the lithospheric mantle during transpressional thickening of plate margins which were bounded by deep shear zones the transpressional regime also seems to have focused granite migration from deeper into higher crustal levels along these shear zones

morphometry of late pleistocene holocene faulting and volcanotectonic relationship in the southern andes of colombia diffuse seismicity and the active galeras volcano characterize the southern andes of colombia whose inhabitants are prone to a high natural risk we used field geological structural and quantitative morphological data to define the geometry and kinematics of late quaternary faults and the volcanotectonic relationships in this area the main ne sw buesaco fault has a late pleistocene holocene 29 km rupture length offset of dated landforms yields 188 13 m of horizontal right lateral long term cumulative component of slip the vertical component of uplift of the nw block being 67 12 m the long term net slip rate is 1 48 0 12 mm yr the other main structure here named aranda fault also strikes ne sw and is 13 km long offset of landforms yields 160 10 m of horizontal right lateral long term component of slip and 3 9 1 9 m of vertical component of uplift of the nw block the long term net slip rate is 1 19 0 08 mm yr on the galeras edifice e w to nw se faults have normal motions and slip rates from 0 02 to 0 23 mm yr both the galeras and a late pleistocene pyroclastic cone lie along the ideal extension of the buesaco fault magma uprising in the crust can occur along the ne striking fractures when magma pressure mp is greater than the normal to fault component of tectonic 1 magma uprising in the uppermost crust i e in the cone can occur by rock cracking along e w planes with mp 3 up to nw se planes with mp greater than shear stress component of the horizontal greatest principal stress hmax

petrology of silicate inclusions in the sombrerete ungrouped iron meteorite implications for the origins of iie type silicate bearing irons the petrography and mineral and bulk chemistries of silicate inclusions in sombrerete an ungrouped iron that is one of the most phosphate rich meteorites known was studied using optical scanning electron microscopy sem electron microprobe analysis empa and secondary ion mass spectrometry sims techniques inclusions contain variable proportions of alkalic siliceous glass 69 vol of inclusions on average aluminous orthopyroxene 9 wo1 4fs2535 up to 3 wt al plagioclase 8 mainly an7092 cl apatite 7 chromite 4 yagiite 1 phosphaterich segregations 1 ilmenite and merrillite ytterbium and sm anomalies are sometimes present in various phases positive anomalies for phosphates negative for glass and orthopyroxene which possibly reflect phosphate melt gas partitioning under transient reducing conditions at high temperatures phosphate rich segregations and different alkalic glasses k rich and na rich formed by two types of liquid immiscibility yagiite a k mg silicate previously found in the colomera iie iron appears to have formed as a late stage crystallization product possibly aided by na k liquid unmixing trace element phase compositions reflect fractional crystallization of a single liquid composition that originated by low degree 48 equilibrium partial melting of a chondritic precursor compositional differences between inclusions appear to have originated as a result of a filter press differentiation process in which liquidus crystals of cl apatite and orthopyroxene were less able than silicate melt to flow through the metallic host between inclusions this process enabled a phosphoran basaltic andesite precursor liquid to differentiate within the metallic host yielding a dacite composition for some inclusions solidification was relatively rapid but not so fast as to prevent flow and immiscibility phenomena sombrerete originated near a cooling surface in the parent body during rapid probably impact induced mixing of metallic and silicate liquids we suggest that sombrerete formed when a planetesimal undergoing endogenic differentiation was collisionally disrupted possibly in a breakup and reassembly event simultaneous endogenic heating and impact processes may have widely affected silicate bearing irons and other solar system matter

u pb age of detrital zircon from the embu sequence ribeira belt se brazil abstract the embu complex crops out in the southeastern part of sao paulo state in brazil and consists mainly of neoproterozoic metasedimentary rocks belonging to the interference zone between the brasilia and ribeira fold belts we present u pb ages la ipcms supported by cathodoluminescence images for detrital zircon from these rocks in order to constrain the formation and evolution of the source area of this complex in this paper we choose to individualize within the embu complex a paleoproterozoic basement a metasedimentary sequence the embu sequence and neoproterozoic intrusive bodies now largely orthogneisses six distinct rock types were recognized in the embu sequence biotite gneiss biotite schist quartzite amphibolite calc silicate rock and talc schist the sequence is strongly deformed by several deformation phases and metamorphosed into amphibolite facies an important shear zone the buquira shear zone separates the embu complex from the socorro guaxupe nappe to the north considered to be part of the upper plate of the southern brasilia belt collision however our mapping showed that the shear zone cuts obliquely into the embu complex diminishing considerably its regional importance three samples were separated for dating a schist north of the shear zone and two quartzites south of the shear zone the results show that the ages of detrital zircon of these samples are mainly paleoproterozoic with a minor archean component the youngest igneous zircon grain from the schist produced an age of about 1000 ma indicating the maximum sedimentation age for the sequence no significant difference appears between the samples north and south of the shear zone reforcing the interpretation that this zone is only of local importance important metamorphic overgrowth of zircon in the age range 670 640 is present in all three samples it is interpreted as related to the installment of a continental magmatic arc in the socorro guaxupe nappe and in the embu complex a second metamorphic pulse between 621 and 599 ma is only apparent in one sample according to the literature this was the period of continental collision between the paranapanema and sao francisco paleocontinents leading to the formation of the southern brasilia belt possibly the fact that the analyzed samples are localized some 30 km away from the suture in the upper plate explains that the increase in temperature during the collision in this area was insufficient to grow significant new zircon a third metamorphic pulse with ages of 576 574 ma possibly related to collision in the ribeira belt is only detected in three analyses of metamorphic rims in the schist sample earlier reported ages of about 790 ma both for igneous crystallization and metamorphism in a more westerly part of the embu complex are only recognized in one analysis of metamorphic overgrowth 794 24 ma from the schist sample and two probably hybrid analyses of 830 19 ma and 767 14 ma from the northern quartzite sample it is concluded that the results show that the detritic zircon is probably derived from the paranapanema paleocontinent

active intracontinental transpressional mountain building in the mongolian altai defining a new class of orogen abstract mountain ranges that are actively forming around the western and northern perimeter of the indo eurasia collisional deformational field such as the mongolian altai comprise a unique class of intracontinental intraplate transpressional orogen with structural and basinal elements that are distinct from contractional and extensional orogens late cenozoic uplift and mountain building in the mongolian altai is dominated by regional scale dextral strike slip faults that link with thrust and oblique slip faults within a 300 km wide deforming belt sandwiched between the more rigid junggar basin block and hangay precambrian craton dominant orogenic elements in the mongolian altai include double restraining bends terminal restraining bends partial restraining bends single thrust ridges thrust ridges linked by strike slip faults and triangular block uplifts in areas of conjugate strike slip faults the overall pattern is similar to a regional strike slip duplex array however the significant amount of contractional and oblique slip displacement within the range and large number of historical oblique slip seismic events renders the term transpressional duplex more accurate intramontane and range flanking basins can be classified as ramp basins half ramp basins open sided thrust basins pull apart basins and strike slip basins neither a classic fold and thrust orogenic wedge geometry nor a bounding foredeep exists the manner in which upper crustal transpressional deformation is balanced in the lower crust is unknown however crustal thickening by lower crustal inflation and northward outflow of lower crustal material are consistent with existing geological and geodetic data and could account for late cenozoic regional epeirogenic uplift in the russian altai and sayan regions

oxygen isotope and petrological study of silicate inclusions in iie iron meteorites and their relationship with h chondrites abstract the origin of silicate bearing irons especially those in groups iab iiicd and iie is poorly understood as silicate should have separated rapidly from molten metal here we report the results of high precision oxygen isotope analysis of silicate inclusions in eleven group iie meteorites and a petrological study of silicate inclusions in ten iie irons including those in garhi yasin and tarahumara which have not been described in detail before oxygen isotopes have also been analysed in 20 h chondrites to investigate their possible relationship with the iie irons based on petrographic observations and mineral analysis the silicate bearing iie meteorites have been divided into four types according to the nature of their silicate inclusions 1 primitive chondritic 2 evolved chondritic 3 differentiated with 10 vol orthopyroxene and 4 differentiated with 17 o variation a wide range of values is seen in the dataset as a whole group iie irons with differentiated silicates with the exception of colomera have a range of mean 17 o values that is essentially identical to those of the h4 6 chondrites 0 60 0 77 and 0 61 0 76 respectively colomera inclusions which are differentiated with 17 o value and plot 2 away from the next nearest iie iron however in view of the textural similarities to other iie inclusions a separate source for colomera is deemed unlikely three iie irons with primitive chondritic inclusions garhi yasin netschaevo and techado have relatively low mean 17 o values of 0 56 0 57 as well as relatively reduced silicates with fa 15 17 olivine which have been called hh chondrites given the significant overlap in their oxygen isotope compositions a genetic relationship between iie irons and h chondrites is supported by our new data however derivation of both groups from one parent body seems unlikely instead both groups probably sampled similar precursor materials and accreted at a similar nebular location our data suggest that the iie meteorites formed on an internally heated h hh chondrite like body that experienced the initial stages of differentiation in response to radiogenic heating however prior to full differentiation the iie parent body experienced a major hit and run style collision that resulted in silicate metal mixing the initial stages of this event involved a phase of rapid cooling that prevented unmixing of metal and silicates reassembly of the iie parent body produced a large regolith blanket that facilitated subsequent slow cooling the iie parent body has probably experienced numerous subsequent less catastrophic collisions the development of alkali glass textures in some differentiated inclusions is probably the result of one of these later events

formation of non magmatic iron meteorite group iie abstract instrumental neutron activation inaa data for metal in 22 nonmagmatic iie meteorites show narrow ranges in ir and other refractory siderophiles the ir range is a factor of 2 6 a factor of 2 smaller than in nonmagmatic iab mg and orders of magnitude smaller than in the large magmatic groups siderophile data show no evidence of fractional crystallization iie irons can be split into two sets a larger main set and a set of 6 cu or fes rich irons elemental concentrations in metal from veins in h5 chondrite portales valley fall within the iie range with the exceptions of co high and ga low h group chondrite and au normalized iie abundances for siderophiles show that iie irons are 30 higher than h in refractory siderophiles re ir and w and are about 30 lower than h chondrites in the volatiles ga and sb inconsistent with proposals that iie irons formed from h chondrites the iie fractionations contrast with those in l chondrites which are about 15 lower than h in the three refractory elements and 40 higher than h in volatiles indicating that iie irons did not form from h chondrites but from a more reduced and siderophile rich kind of ordinary chondrite hh chondrites most o isotope data support a close relationship between iie irons and h or hh chondrites lower 17o in primitive chondritic silicates support an hh classification literature isotopic data for ru and mo also show that iie metal formed from an ordinary chondrite parent it appears that the silicates and metal were formed by melting of a single asteroid there is no evidence for radiogenic 26al heating this the rapid cooling recorded in the sizes of parental gamma crystal in the metal and the absence of fractional crystallization strongly support the hypothesis that iie melting was the result of impacts to summarize the weight of the evidence favors the conclusion that iie meteorites were formed by one or more impacts on an hh asteroid the target probably had a composition like the chondritic materials in netschaevo but was unequilibrated and had much higher porosity

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

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

formation of large metal nodules in ordinary chondrites literature data show that many large 2 30 mm metallic fe ni nodules in ordinary chondrites are depleted in refractory siderophiles e g re os ir ru pt and rh and cu relative to fine grained metal but are not depleted in normally refractory w and mo common siderophiles e g ni and co and volatile siderophiles e g au as and sb it was recently proposed that metamorphic diffusion could form the metal nodules and that partitioning of siderophile elements between kamacite and taenite could account for the nodules unusual compositions however the nodules have the same compositional trends as metal nodules and veins in shocked meteorites that clearly formed by impact related processes if large metal nodules formed during metamorphism type 6 chondrites should contain abundant large nodules instead the mean metal grain size in h6 chondrites is 0 12 0 15 mm far smaller than any of the nodules it seems likely that the metal nodules formed by impact vaporization oxidation of w and mo to form volatile oxides concomitant partial oxidation of re and os rapid fractional condensation of refractory siderophiles transport of the residual vapor condensation of cu s and se into vugs and fractures to form sulfide and condensation of the remaining siderophiles to form metal nodules and veins during vapor cooling and condensation or during subsequent parent body annealing w mo re and os oxides were reduced

precise determination of the isotopic composition of potassium application to terrestrial rocks and lunar soils we detail a method for the precise and accurate determination of isotopic variations in the 41k39k ratio with a precision of 0 5 2 m purified potassium is chemically extracted from rocks soils minerals or solutions by ion exchange chromatography complete chemical yields 99 8 are achieved in order to avoid laboratory induced isotopic fractionations the purified potassium is converted to a glass by melting with barium borate flux and the resultant bead is mounted for ion probe analysis the sims method utilized by the ion probe produces extremely stable k ion beams with no measurable temporal variability in the isotope ratio the instrumental fractionation is steady at about 4 and is corrected for by measurement of a standard the measurement of gravimetrically prepared isotopic standards indicates that the method is accurate at the stated level of precision and free of egregious errors analysis of terrestrial samples including peridotite basalts granites carbonatite biotite schists and seawater indicate the complete absence of isotopic variations in 41 1k among terrestrial materials at the 0 5 o level application to lunar soils and a regolith breccia confirms previously observed large isotopic fractionation effects garner et al 1975a church et al 1976 some lunar soils e g 14163 are shown to have large sample heterogeneity 7 o while others e g soils and a regolith breccia at several apollo 15 sites station 79 are homogeneous at the level of analytical precision the presence of potassium isotopic effects in bulk soils up to 12 7 in this study with magnitudes comparable to the rayleigh fractionation factor 25 indicates that volatility during micrometeorite impact melting played a large role in lunar regolith formation as much as 15 of the regolith potassium has been lost from the moon through the tenuous lunar atmosphere

a model of vulcanian eruptions at galeras volcano colombia abstract a series of six vulcanian eruptions at galeras volcano colombia occurred during 1992 1993 these eruptions followed the emplacement of a lava dome in late 1991 which was accompanied by cooling crystallizing and partial solidification of the column of magma plugging the conduit five of the eruptions were preceded by episodes of monochromatic long period seismicity suggesting cyclic pressurization of the magma the quiescent intervals between eruptions generally decreased with each successive eruption we propose a model of gas release from magma into confined stationary pore spaces by 5 10 crystallization of anhydrous minerals causing repeated overpressurization of the magmatic system using initial lithostatic pressures of 20 100 mpa 800 3900 m depth gas buildup by crystallization indicates overpressures of 3 17 mpa which is the approximate fracture criterion for a magma chamber our galeras data suggest two models of pressurization 1 gas exsolution may have occurred at similar depths but with progressively smaller overpressures being developed before each successive eruption according to this model the comparatively long quiescent periods between the first eruptions resulted in relatively large amounts of gas exsolution from the magma and accumulation in pore spaces whereas the shorter quiescent periods for the later eruptions exsolved less gas the decrease in overpressure with time may have resulted from progressive fracturing of partly solidified magma during the eruptions which progressively decreased the tensile strength and increased the permeability of the magma 2 since overpressures are smaller at shallower depths for a given amount of crystallization the first eruptions of 1992 1993 were caused by gas exsolution at shallow depths while the last eruptions having shorter quiescent intervals resulted from a greater amount of gas exsolution at greater depths in this case the rate of gas exsolution varied as a function of depth the two scenarios for pressurization may be combined into a more general model involving edifice weakening and the development of a shallow hydrothermal system from continued degassing of the magma plug as weakening of the edifice formed an extensive system of fractures the shallow hydrothermal system deepened due to meteoric water input and magmatic degassing with time a greater number of fractures were filled and partly sealed by hydrothermal minerals at progressively deeper levels as a result the source of the overpressure also deepened with time

neoproterozoic oceanic remnants in eastern brazil further evidence and refutation of an exclusively ensialic evolution for the aracuai west congo orogen the aracuai eastern brazil and west congo southwestern africa belts are counterparts of the same neoproterozoic orogen located between the sao francisco and congo cratons the macaubas group represents a major passive margin sequence and is a key unit for interpreting the evolution of that orogen the salinas formation is the distal rock assemblage of the macaubas group and consists of a deep sea sand mud sequence and a volcanic sedimentary unit called the ribeirao da folha facies the latter includes metamorphosed volcanic exhalative sediments associated with ocean floor basalts amphibolites the magmatic protoliths of these amphibolites crystallized at about 816 72 ma sm nd whole rock isochron nd t 3 8 0 2 regional metamorphism reached the amphibolite facies at about 630 ma rb sr whole rock isochron when slabs of ultramafic rocks were tectonically emplaced over the ribeirao da folha facies we consider this volcanic sedimentary facies and the coeval slabs of ultramafic rocks to be remnants of a branch of the adamastor brazilide ocean the extensive occurrence of syntectonic to late tectonic calc alkalic granitoids along the internal domain of the aracuai belt implies that a reasonably large amount of ocean crust was consumed via an east dipping subduction zone during formation of the aracuai west congo orogen

main group pallasites thermal history relationship to iiiab irons and origin we have determined metallographic cooling rates below 975 k for eight main group mg pallasites from ni profiles across taenite lamellae of known crystallographic orientation in metallic regions with widmanstatten patterns comparison with profiles generated by modeling kamacite growth gave cooling rates ranging from 2 5 to 18 k myr relative cooling rates were also inferred from the sizes of cloudy zone particles in 28 mg pallasites 86 170 nm and tetrataenite bandwidths in 20 mg pallasites 1050 2170 nm as these parameters are positively correlated with each other and negatively correlated with the metallographic cooling rates these three different techniques show that mg pallasites cooled below 975 k at significantly diverse rates since samples from the core mantle boundary should have indistinguishable cooling rates mg pallasites could not have cooled at this location group iiiab irons which were previously thought to be core samples from the mg pallasite body have faster cooling rates 50 350 k myr and smaller cloudy zone particle sizes and tetrataenite bandwidths this shows that iiiab irons cooled faster than mg pallasites and could not plausibly be from the same body the absence of related iron meteorites and achondrites and our thermal constraints suggest that mg pallasites cooled at diverse depths in a pallasitic body consisting of well mixed olivine and metallic fe ni such a body may have formed during an impact on a differentiated asteroid or protoplanet that mixed olivine mantle fragments with residual ir poor molten metal from the out ermost part of a core that chemically resembled the iiiab core and was 80 fractionally crystallized separation of the solid core and most of the associated mantle may have resulted from a grazing hit and run impact with a larger protoplanet or asteroid thermal calculations suggest that the radius of the pallasitic body was 400 km but the likely presence of a regolith would reduce this estimate considerably 2010 elsevier ltd all rights reserved

unusual low frequency volcanic seismic events with slowly decaying coda waves observed at galeras and other volcanoes abstract events with slowly decaying coda waves are observed in active andesitic volcanoes associated mainly with vulcanian type eruptions or large gas emissions however these signals are also recorded in some quiescent volcanoes these unusual signals are considered to be related to magmatic activity and generally occur beneath the active crater the signals have been observed to be a short term precursor galeras volcano colombia 1992 1993 asama yama volcano japan 1983 after eruption tokachi dake volcano japan 1989 during seismic swarms meakan dake volcano japan 1982 and during quiescence puracevolcano colombia 1994 1995 tarumai volcano japan 1970 1971 1975 spectral analysis reveals common characteristics for this type of signal the spectrum is characterized by one or several sharp frequency peaks fundamental frequencies are not affected by epicentral distance azimuth or travel time indicating a source effect the damping coefficient for coda waves ranges from 0 002 to 0 02 and is related to large values of the quality factor q c ranging from 250 to 25 respectively these parameters may be the result of large amounts of gas bubbles in the magma body

tornillo type seismic signals at galeras volcano colombia 1992 1993 abstract unusual low frequency seismic events called tornillos screws at the observatorio vulcanologico y sismologico de pasto ovsp have been observed at galeras volcano during 1992 1993 of six eruptions that occurred between july 1992 and june 1993 five were preceded by episodes of tornillo signals these signals are characterized by 1 a waveform having a homogeneous distribution of frequencies 2 a long coda lasting up to several minutes 3 a small amplitude compared to the duration and 4 a slow decay of the coda the tornillo signals have been grouped into thirteen principal forms the 1992 1993 tornillos showed four main periods of occurrence each of which was followed by an eruption episode i occurred from 11 to 16 july 1992 and consisted of nine events episode ii lasted from 23 december 1992 to 14 january 1993 with twenty events episode iii was observed from 13 february to 23 march 1993 with 74 events episode iv lasted from 10 april to 7 june 1993 with 109 events the seismic activity at galeras in april may 1993 was characterized mainly by the occurrence of tornillo signals the behavior of these signals was similar to that before previous episodes ending with eruptions these observations permitted us to forecast the 7 june 1993 eruption several days to weeks beforehand the occurrence of tornillos is the most important criterion for determining the probability of an eruption at galeras in the current period of reactivation the maximum daily number of tornillos and the longest duration of individual events occur near the end of each episode within an individual episode the durations of single events increase progressively near to the point of eruption a positive correlation is observed between the total number of pre eruptive tornillo signals during an episode and the volume of material ejected by the eruption these observations may suggest that the presence of tornillos is an indication of variations in the physical conditions between the fluid and the surrounding solid material within the volcano and they constitute an important tool for forecasting future eruptions at galeras

kinematic evolution of the aracuai west congo orogen in brazil and africa nutcracker tectonics during the neoproterozoic assembly of gondwana abstract the neoproterozoic aracuai west congo a wc orogen is one of many brasiliano pan african orogens that developed during the assembly of west gondwana this orogen was split apart in mesozoic time due to opening of the south atlantic the aracuai orogen now underlies eastern brazil whereas the west congo belt fringes central africa s atlantic coast significantly at the time it formed the a wc orogen was bounded on the west north and east by the sao francisco congo craton a crustal block that had the shape of a lopsided upside down u thus the orogen was partially confined during tectonism in that it occupied an enclave surrounded on three sides by cratonic crust formation of the a wc orogen resulted in kinematically complex deformation substantial crustal shortening and production of a large volume of magma how such features could develop in this particular setting has long been a mystery our field studies in the aracuai orogen together with published data on the west congo belt characterize the kinematic picture of the a wc orogen and lead to a tectonic model that addresses its evolution in our model the a wc orogen formed in response to closure of the macaubas basin this basin was underlain by oceanic crust in the south but tapered northward into a continental rift which terminated against the cratonic bridge linking the eastern and western arms of the sao francisco congo craton closure occurred when the western arm now the sao francisco craton rotated counterclockwise towards the eastern arm now the congo craton this closure may have been driven by collision of the paranapanema amazonian and kalahari cratons against the external margins of the sao francisco congo craton rather than by slab pull associated with subduction of the macaubas basin s floor thus the process of forming the a wc orogen resembled the process of crushing of a nut between two arms of a nutcracker such nutcracker tectonics led to a series of kinematically distinct deformation stages initially internal portions of the orogen flowed northwards then substantial crustal thickening occurred in the orogen s interior and the deformation front migrated outwards producing thrust belts that overlapped the internal margins of the craton with continued closure space in the enclave became restricted and the orogen s interior underwent lateral escape to the south late stage extensional collapse triggered both production of late to post collisional granites and exhumation of high grade rocks from mid crustal levels

comparative petrology of silicates in the udei station iab and miles iie iron meteorites implications for the origin of silicate bearing irons abstract the textures and mineral chemistries of silicate inclusions in the udei station iab and miles fractionated iie iron meteorites were studied using optical and electron microscopy sem empa and la icp ms techniques to better understand the origin of silicate bearing irons inclusions in udei station include near chondritic basaltic gabbroic feldspathic orthopyroxenitic and harzburgitic lithologies in miles most inclusions can be described as feldspathic pyroxenite or pyroxene enriched basalt gabbro the trace element compositions of both orthopyroxene and plagioclase grains are similar in different lithologies from udei station whereas in different inclusions from miles the compositions of orthopyroxene grains are similar while those of clinopyroxene plagioclase and especially cl apatite are variable orthopyroxene in miles tends to be enriched in ree compared to that in udei station but the reverse is true for plagioclase and clinopyroxene the data can be explained by models involving partial melting of chondritic protoliths silicate melt migration and redox reactions between silicate and metal components to form phosphate the extent of heating melt migration and phosphate formation were all greater in miles silicates in miles were formed from liquids produced by 30 partial melting of a chondritic precursor brought to a peak temperature of 1250 c this silicate melt crystallized in two stages during stage 1 crystallizing minerals orthopyroxene clinopyroxene chromite and olivine were largely in equilibrium with an intercumulus melt that was evolving by igneous fractionation during slow cooling with a residence time of 20 ka at 1150 c during stage 2 following probable re melting of feldspathic materials and after the silicate mush was mixed with molten metal plagioclase and phosphate fractionally crystallized together during more rapid cooling down to the solidus in udei station despite a lower peak temperature we suggest that a model involving major collisional disruption and mixing of partly molten endogenically heated planetesimals can best explain the data for iab and fractionated iie silicate bearing irons the extent of endogenic heating was different less for the iabs and the amount of parent body disruption was different scrambling with collisional unroofing for the iab iiicd winonaite body more complete destruction for the fractionated iie body but both bodies were partly molten and incompletely differentiated at the time of impact we suggest that the post impact secondary body for iab iiicd winonaite meteorites was mineralogically zoned with ni poor metal in the center and that the secondary body for fractionated iie meteorites was a relatively small melt rich body that had separated from olivine during collisional break up

u pb geochronology of the granite magmatism in the embu terrane implications for the evolution of the central ribeira belt se brazil abstract the embu domain represents the central part of the ribeira fold belt in sao paulo brazil it hosts several granitic occurrences of varied composition ranging from small granitic bodies that outcrop in a domain bounded by the intersection of two major sutures taxaquara and guararema faults and batholitic masses outcropping to the east understanding the evolution of such granites is of vital importance to better constrain evolutionary models for the ribeira belt in the present study a set of samples from eleven main granite occurrences from the east of sao paulo state was selected for geochronological investigation using laser ablation multicollector inductively coupled plasma spectrometry la mc icp ms and thermal ionization mass spectrometry tims u pb dating of zircon and monazite crystals respectively the results indicate a remarkable cluster of ages around 590 ma with older events of granite magmatism between 660 and 600 ma registered for four plutons indicating a long history of crustal reworking and magma generation the ages of reworked sources were evaluated from inherited zircon cores although highly discordant these point to the predominance of paleoproterozoic 2 4 2 1 ga sources with minor contributions from mesoproterozoic 1 1 0 9 ga and archean sources 3 1 ga the new data bring important insights into the role played by the embu domain on the paleogeography and evolution of the ribeira belt

high precision relocation of microearthquakes at mt etna 1991 1993 eruption onset a tool for better understanding the volcano seismicity abstract the beginning of the 1991 1993 large eruption at mt etna was marked by a seismic swarm and a sudden increase of volcanic tremor amplitude several earthquakes were recognized as clusters of shallow events with nearly identical waveforms multiplets we have analyzed these signals with a cross spectral technique that allows us to locate the relative position of the events within a multiplet obtaining a precision higher than with traditional routines the relocated shocks occurred some kilometers away from the propagating eruptive fracture east of both the summit craters and the modeled eruptive dykes they clustered in a relatively small volume elongated in the nnw sse direction the depth of which ranged from about 1100 to 2000 m below sea level these earthquakes are interpreted as the passive brittle response of the volcanic basement to the intrusion of the eruptive dyke rather than being directly linked to the magma transport

chemical compositions of siderophile element rich opaque assemblages in an allende inclusion abstract ten opaque assemblages or fremdlinge weighing 0 17 19 g from an allende type b ca al rich coarse grained inclusion were analyzed by inaa for ca na mn sc ta ti v cr fe ni co w re os ir mo ru pt rh au as sb and se refractory siderophiles are usually enriched by factors of 103 104 relative to cl chondrites but not uniformly from sample to sample c1 chondrite normalized re os ir pt os ru and pt rh ratios vary from 0 81 1 03 0 70 2 25 0 67 3 32 and 0 45 1 43 respectively attempts to model the abundances of refractory siderophiles by condensation from the solar nebula into a single phase assuming ideal solution fail to match the observed combination of subchondritic re os and ir pt ratios even allowing for fractionation of the highest temperature condensates a different model in which all metals whose high temperature crystal structures are hcp condense into one alloy bcc into another and fcc into a third is capable of matching virtually all refractory siderophile fractionations in these fremdlinge in this model all isostructural metals are assumed to form ideal solutions with one another and metals of different structure are assumed to be insoluble in one another after fractionation of high temperature hcp and fcc alloys individual proto fremdlinge formed by mixing of high and low temperature varieties of each of the three alloys either in the nebula before and during cai accretion or during later partial melting of the silicate oxide and metal components of cais

a nonmagmatic origin of group iie iron meteorites abstract new neutron activation data on 10 elements in 12 iie and iie related irons lead to a reclassification of several irons seymchan and lonaconing are removed from iie and leshan added four iie members are designated iie an to call attention to some anomalous properties the eight normal iie members define element ni trends generally similar to those in the nonmagmatic group iab the small negative slopes on w ni and ir ni diagrams are strongly indicative of a nonmagmatic origin of the iie irons we propose that iie irons like iab irons originated as individual pools of impact produced melt in the near surface region of a chondritic parent body the positive as ni and au ni trends are the only evidence suggesting fractional crystallization but their slopes are lower than those in magmatic group iiiab and only slightly higher than those of cu and sb in iab we suggest that the s and c contents of the iie precursor materials were much lower than those of the iab precursors thus higher temperatures were required to generate enough metallic melt to segregate into pools these higher temperatures are also reflected in the nonchondritic compositions of the silicate inclusions

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

two stage formation of pallasites and the evolution of their parent bodies revealed by deformation experiments abstract pallasites stony iron meteorites predominantly composed of olivine crystals and fe ni metal are samples of the interior of early solar system bodies and can thus provide valuable insights into the formation of terrestrial planets however pallasite origin is controversial either sampling the core mantle boundary or the shallower mantle of planetesimals that suffered an impact we present high strain rate deformation experiments with the model system olivine fes melt gold melt to investigate pallasite formation and the evolution of their parent bodies and compare the resulting microstructures to two samples of seymchan pallasite our experiments reproduced the major textural features of pallasites including the different olivine shapes olivine aggregates and the distribution of the metal and sulfide phases these results indicate that pallasites preserve evidence for a two stage formation process including inefficient core mantle differentiation and an impact causing disruption metal melt injection and fast cooling within months to years olivine aggregates important constituents of angular pallasites are reinterpreted as samples of a partially differentiated mantle containing primordial metallic melt not stemming from the impactor the long term retention of more than 10 vol of metal melt in a silicate mantle sampled by olivine aggregates indicates high effective percolation thresholds and inefficient metal silicate differentiation in planetesimals not experiencing a magma ocean stage

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

the cu isotopic composition of iron meteorites abstract high precision cu isotopic compositions have been measured for the metal phase of 29 iron meteorites from various groups and for four terrestrial standards the data are reported as the 65cu permil deviation of the 65cu 63cu ratio relative to the nist srm 976 standard terrestrial mantle rocks have a very narrow range of variations and scatter around zero in contrast iron meteorites show 65cu approximately 2 3 variations different groups of iron meteorites have distinct 65cu values nonmagmatic iab iiicd iron meteorites have similar 65cu 0 03 0 08 and 0 12 0 10 respectively close to terrestrial values approximately 0 the other group of nonmagmatic irons iie is isotopically distinct 0 69 0 15 ivb is the iron meteorite group with the strongest elemental depletion in cu and samples in this group are enriched in the lighter isotope 65cu down to 2 26 evaporation should have produced an enrichment in 65cu over 63cu 65cu 0 and can therefore be ruled out as a mechanism for volatile loss in ivb meteorites in silicate bearing iron meteorites 17o correlates with 65cu this correlation between nonmass dependent and mass dependent parameters suggests that the cu isotopic composition of iron meteorites has not been modified by planetary differentiation to a large extent therefore cu isotopic ratios can be used to confirm genetic links cu isotopes thus confirm genetic relationships between groups of iron meteorites e g iab and iiicd iiie and iiiab and between iron meteorites and chondrites e g iie and h chondrites several genetic connections between iron meteorites groups are confirmed by cu isotopes e g iab and iiicd iiie and iiiab and between iron meteorites and chondrites e g iie and h chondrites

aftershock sequence of the 9 may 1989 canary islands earthquake abstract the 9 may 1989 canary islands earthquake ml 5 2 was located at 27 56 8 n 16 12 0 w between the islands of tenerife and gran canaria and is one of the strongest shocks that have occurred in the region station tfmb on tenerife recorded 236 aftershocks during the year following the mainshock from this aftershock sequence both p and b values and the cumulative energy release were calculated regional v p v s ratios on tenerife and gran canaria were obtained using modified wadati diagrams the results indicate the tectonic character of the sequence and suggest the presence of a fractured geothermal reservoir under tenerife as a plausible explanation for the v p v s value obtained the observed differences in the regional v p v s ratios together with the geographical limitations on azimuthal station coverage could to a large extent explain the poor hypocentre determinations in the region that have been hitherto obtained by standard methods

seismic evolution of the 1989 1990 eruption sequence of redoubt volcano alaska abstract redoubt volcano in south central alaska erupted between december 1989 and june 1990 in a sequence of events characterized by large tephra eruptions pyroclastic flows lahars and debris flows and episodes of dome growth the eruption was monitored by a network of five to nine seismic stations located 1 to 22 km from the summit crater notable features of the eruption seismicity include 1 small long period events beginning in september 1989 which increased slowly in number during november and early december 2 an intense swarm of long period events which preceded the initial eruptions on december 14 by 23 hours 3 shallow swarms 0 to 3 km of volcano tectonic events following each eruption on december 15 4 a persistent cluster of deep 6 to 10 km volcano tectonic earthquakes initiated by the eruptions on december 15 which continued throughout and beyond the eruption 5 an intense swarm of long period events which preceded the eruptions on january 2 and 6 nine additional intervals of increased long period seismicity each of which preceded a tephra eruption hypocenters of volcano tectonic earthquakes suggest the presence of a magma source region at 6 10 km depth earthquakes at these depths were initiated by the tephra eruptions on december 15 and likely represent the readjustment of stresses in the country rock associated with the removal of magma from these depths the locations and time history of these earthquakes coupled with the eruptive behavior of the volcano suggest this region was the source of most of the erupted material during the 1989 1990 eruption this source region appears to be connected to the surface by a narrow pipe like conduit as inferred from the hypocenters of volcano tectonic earthquakes concentrations of shallow volcano tectonic earthquakes followed each of the tephra eruptions on december 15 these shocks may represent stress readjustment in the wall rock related to the removal of magma and volatiles at these depths this shallow zone was the source area of the majority of long period seismicity through the remainder of the eruption the long period seismicity likely reflects the pressurization of the shallow portions of the magmatic system

the bagnone iron meteorite tuscany italy history mineralogy and chemical classification bagnone the largest italian iron meteorite 48 kg was found as a single mass at the beginning of the 20th century while ploughing a field close to the bagnone castle massa carrara northern tuscany the morphology of the external surface suggests that bagnone represents a complete individual it is classified as a medium octahedrite average bandwidth 0 96 mm of the iiiab chemical group based on inductively coupled plasma mass spectrometry analyses

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

petrology and geochemistry of chondrules and metal in nwa 5492 and gro 95551 a new type of metal rich chondrite abstract northwest africa nwa 5492 and grosvenor mountains gro 95551 are metal rich chondrites having silicate olivine and pyroxene compositions that are more reduced than those in other metal rich chondrites such as the ch and cb chondrites additionally sulfides in nwa 5492 and gro 95551 are more abundant and not related to the metal as in the cb chondrites average metal compositions in nwa 5492 and gro 95551 are close to h chondrite metal oxygen isotope ratios of nwa 5492 and gro 95551 components chondrules and fragments show a range of compositions with most having 17o values 0 since there is no matrix component their average chondrule fragment oxygen isotopic compositions are considered to be representative of whole rock and 17o values are sandwiched between the values for enstatite e and ordinary o chondrites these data argue for a close relationship between nwa 5492 and gro 95551 and suggest that they are the first examples of a new type of metal rich chondrite oxygen isotope ratios of chondrules in nwa 5492 and gro 95551 show considerable overlap with chondrules in o e and r chondrites with average compositions indistinguishable from ll3 chondrules suggesting considerable mixing between these solar system materials during chondrule formation and or that their precursors experienced similar formation environments and or processes another characteristic shared between nwa 5492 and gro 95551 and o e and r chondrites is that they are all relatively dry low abundances of hydrated minerals compared to many c chondrites and have fewer smaller cais than many c chondrites no cais were found in nwa 5492 or gro 95551 but they contain rare al rich chondrules we suggest that o e r and the nwa 5492 and gro 95551 chondrites are closely related solar system materials

a ab complex iron meteorite containing low ca clinopyroxene abstract northwest africa 468 nwa 468 is a new ungrouped silicate rich member of the iab complex of nonmagmatic iron meteorites the silicates contain relatively coarse 300 m grains of low ca clinopyroxene with polysynthetic twinning and inclined extinction low ca clinopyroxene is indicative of quenching from high temperatures either from protoenstatite in a few seconds or high temperature clinoenstatite in a few hours it seems likely that nwa 468 formed by impact melting followed by rapid cooling to 660 c after the loss of a metal sulfide melt from the silicates sulfide was reintroduced either from impact mobilized fes or as an s 2 vapor that combined with metallic fe to produce fes the o isotopic composition 17 o 1 39 indicates that the precursor material of nwa 468 was a metal rich e g cr carbonaceous chondrite lodranites are similar in bulk chemical and o isotopic composition to the silicates in nwa 468 the mac 88177 lodranite which also contains low ca clinopyroxene is close in bulk chemical composition both nwa 468 and mac 88177 have relatively low abundances of rare earth and plagiophile elements siderophiles in the metal rich areas of nwa 468 are similar to those in the mac 88177 whole rock both samples contain low ir and relatively high fe cu and se most unweathered lodranites contain 20 to 38 wt metallic fe ni these rocks may have formed in an analogous manner to nwa 468 i e by impact melting of metal rich carbonaceous chondrite precursors but with less separation of metal rich melts from silicates

trace element studies of silicate rich inclusions in the guin ungr and kodaikanal iie iron meteorites a devitrified glass inclusion from the guin ungr iron consists of cryptocrystalline feldspars pyroxenes and silica and is rich in sio2 al2o3 and na2o it contains a rutile grain and is in contact with a large cl apatite the latter is very rich in rare earth elements rees 80 x ci which display a flat abundance pattern except for eu and yb which are underabundant the devitrified glass is very poor in rees light rare earth elements lrees and deficits in eu and yban ultrarefractory pattern the genetic models suggested so far cannot explain what is observed and consequently we offer a new model for silicate inclusion formation in iie and related irons nebular processes and a relationship with e meteorites guin or ca al rich inclusions cais kodaikanal are indicated a sequence of condensation cas tin or refractory pyroxene rich liquids and vapor solid elemental exchange can be identified that took place beginning under reducing and ending at oxidizing conditions phosphate rutile formation alkali and fe 2 metasomatism metasomatic loss of rees from glasses

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

14c ages and activity for the past 50 ka at volcan galeras colombia abstract volcan galeras is the southernmost colombian volcano with well recorded historic activity the volcano is part of a large and complex volcanic center upon which 400 000 people live historic activity has centered on a small volume cone inside the youngest of several large amphitheaters that breach the west flank of the volcano away from the city of pasto population 300 000 lava flows sio2 between 54 6 and 64 7 wt have dominated activity for more than 1 ma but explosive events have also occurred joint studies by volcanologists from colombia ecuador peru bolivia argentina and the united states produced 24 new14c ages and more than 100 stratigraphic sections to interpret the past 50 ka of activity at galeras including sector collapse events the youngest collapse event truncated 12 8 ka lava flows and may have occurred as recently as 8 to 10 ka tephra fall material rapidly thins and becomes finer away from the vent area the only widespread marker in the

relocation of seismicity preceding the 1984 eruption of mauna loa volcano hawaii delineation of a possible failed rift abstract waveform cross correlation based repicking of p arrival times and subsequent relocation of 187 earthquakes that occurred near the summit of mauna loa hawaii prior to the march 1984 eruption has illuminated a previously obscured structure beneath the northwestern flank simultaneous inversion for hypocenters and velocity model parameters using the refined arrival times resulted in well constrained relative earthquake locations with very low arrival time misfits average rms 0 041 s pre eruption seismicity from this time period occurred in two groups a shallow group located near mauna loa s summit at depths of 1 3 km and a deeper group 5 10 km located 4 6 km northwest of the summit after relocation we found that most of the northwest flank earthquakes occurred along a 3 km long planar feature striking about 60 west of north in a thin band about 1 km thick there is a temporal migration of epicenters from the northwestern edge of the zone migrating towards the summit focal mechanisms in this zone reveal a change in faulting from strike slip in the southeast to a mix of strike slip and normal faulting in the northwest this feature we interpret to be related to a rift zone that was stunted by the buttressing of the adjacent volcanoes hualalai and mauna kea previous gravity and magnetic studies provide supporting evidence for the existence of a rift zone there was only a modest increase in clustering of the summit events after relocation hypocenters that were previously located within a diffuse zone 4 5 km were reduced to a 2 5 km wide zone they extend from a depth of 6 km to about sea level trending toward the summit the focal mechanisms from the summit events are mix of faulting types without a consistent pattern

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

first finding of impact melt in the iie netschaevo meteorite about half of the iie nonmagmatic iron meteorites contain silicate inclusions with a primitive to differentiated nature the presence of preserved chondrules has been reported for two iie meteorites so far netschaevo and mont dieu which represent the most primitive silicate material within this group in this study silicate inclusions from two samples of netschaevo were examined both silicate inclusions are characterized by a porphyritic texture dominated by clusters of coarse grained olivine and pyroxene set in a fine grained groundmass that consists of new crystals of olivine and a glassy appearing matrix this texture does not correspond to the description of the previously examined pieces of netschaevo which consist of primitive chondrule bearing angular clasts detailed petrographic observations and geochemical analyses suggest that the investigated samples of netschaevo consist of quenched impact melt this implies that netschaevo is a breccia containing metamorphosed and impact melt rock imr clasts and that collisions played a major role in the formation of the iie group