minor and trace element partitioning between pyroxene and melt in rapidly cooled chondrules we present minor and trace element ree sr y and zr data for pyroxenes and mesostases in four porphyritic chondrules from the semarkona ordinary chondrite apparent partition coefficients for clinoenstatite orthoenstatite pigeonite and augite are compared with experimental and petrologic data from the literature and the effects on apparent partition coefficients of the rapid cooling rates at which chondrules crystallized are evaluated for most elements the effects of cooling at rates of hundreds of degrees per hour cannot be distinguished from variations in equilibrium data resulting from differences in temperature or composition however for lree apparent partition coefficients are significantly higher than comparable equilibrium data and the ratio of hree lree partition coefficients is lower particularly for ca poor pyroxene we attribute this flattening of ree patterns to the effect of rapid cooling apparent partition coefficients of all ree and y in augite are higher than equilibrium data particularly in one chondrule with a high al 2o3 content we suggest that this may be attributed to an increase in the uptake of trivalent trace element cations in the pyroxene crystal structure as a result of charge balanced substitutions with al 31 cations

geochronology geochemistry and tectonic evolution of the western and central cordilleras of colombia abstract autochthonous rocks of the pre cretaceous continental margin of nw south america the tahami terrane are juxtaposed against a series of para autochthonous rock units that assembled during the early cretaceous allochthonous oceanic crust of the caribbean large igneous province collided with and accreted onto the margin during the late cretaceous we present the first regional scale dataset of zircon u pb la icp ms ages for intrusive and metamorphic rocks of the autochthonous tahami terrane early cretaceous igneous para autochthonous rocks and accreted oceanic crust the u pb zircon data are complemented by multiphase 40ar 39ar crystallization and cooling ages the geochronological data are combined with whole rock major oxide trace element and ree data acquired from the same units to constrain the tectonic origin of the rock units and terranes exposed in the western cordillera cauca patia valley and the central cordillera of colombia the tahami terrane includes lower paleozoic orthogneisses 440 ma that may have erupted during the active margin stage of the rheic ocean basement gneisses were intruded by permian continental arc granites during the final assembly of pangea triassic sedimentary rocks were subsequently deposited in rift basins and partially melted during high t metamorphism associated with rifting of western pangea during 240 220 ma continental arc magmatism during 180 145 ma is preserved along the whole length of the central cordillera and was followed by an early cretaceous out board step of the arc axis and the inception of the quebradagrande arc that fringed the continental margin back stepping of the arc axis may have been caused by the collision of buoyant seamounts which were coeval with plateau rocks exposed in the nicoya peninsular of costa rica rapid westward drift of south america closed the quebradagrande basin in the late aptian and caused medium high p t metamorphic rocks of the arquia complex to exhume and obduct onto the continental margin subduction beneath hot spot derived rocks of the caribbean plateau 100 92 ma formed an intra oceanic arc 92 75 ma which collectively comprise the late cretaceous caribbean large igneous province the remnant ocean basin located between south america and the caribbean large igneous province was partly consumed via continental subduction forming the large antioquia batholith the caribbean large igneous province collided and accreted to south america during 75 70 ma along the cauca almaguer fault resulting in the cessation of both arcs and the paleocene onset of subduction beneath the accreted oceanic crust

tracking ancient magmatism and cenozoic topographic growth within the northern andes forearc constraints from detrital u pb zircon ages a comparison of detrital zircon ages from the cenozoic forearc basins in sw ecuador nw peru and those derived from the amotapes massif offers a singular opportunity to shed some light on the evolution of the nw andean margin of south america an extensive set of pre andean u pb zircon ages older than late cretaceous was obtained from 18 detrital samples coming from the talara zorritos progreso and gulf of guayaquil tumbes forearc basins as well as from two metasedimentary rocks of the amotapes massif u pb age analysis on detrital zircon grains allows a general source to sink scenario to be determined for this section of the andes these analyses give consistent insights into the early topographic growth during the modern andean orogeny the occurrence of proto andean magmatic periods in the western margin of gondwana and the evolution of the amotapes massif within the geodynamical setting of northwestern south america zircon ages at ca 320 ma and neoproterozoic clusters for the amotapes metasediments show significant similarities along the strike of the metamorphic belt thus defining a common metasedimentary origin and can be compared with previously dated rocks from the eastern cordillera of peru these strong similarities demonstrate the presence of a wide polyphased metamorphic belt that includes the western parts of the eastern cordillera the northern section of the occidental cordillera of peru and the amotapes massif detrital sediments of the forearc are marked by a major gap in early cretaceous jurassic ages and the minor presence of ordovician famatinian orogeny and carboniferous permian 340 285 ma ages the age spectrum is not compatible with sediment input from the eastern andes and suggests that the external areas of the andean chain e g the western cordillera were already uplifted prior to or immediately after forearc initiation during the paleocene thus representing a pervasive drainage divide throughout all of the cenozoic the amotapes massif or other terrains sharing the same age affinities e g the olmos terrane occidental cordillera of peru are likely to represent the best potential sources for pre andean zircon grains in the forearc age clusters in both source and sink at ca 600 ma 570 560 ma and 520 ma most likely define magmatic periods and may represent first order constraints for the difficult correlation between proto andean magmatic and orogenic periods to the north and south of the huancabamba deflection it is unlikely that the neoproterozoic zircon grains came from the pan african braziliano belts thus suggesting the onset of significant magmatic activity in this section of the andes at around 570 560 ma and probably ca 600 ma the 520 ma cluster is compatible with the most prominent magmatic period in the southern andes related to the end of the pampean orogeny thus suggesting the existence of a regional proto andean magmatic arc during early cambrian times

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

palaeozoic and proterozoic zircons from the mid atlantic ridge according to the theory of plate tectonics rocks found in the vicinity of mid ocean ridges where oceanic plates are created should be relatively young at most several myr old here we report the discovery of zircons with ages of about 330 and 1 600 myr that were drilled from exposed gabbros beneath the mid atlantic ridge near the kane fracture zone1 2 3 4 age determinations were made using the 207pb 206pb evaporation method5 and confirmed with conventional u pb dating and ion microprobe shrimp analysis we suggest two plausible explanations for the origin of these unusually old zircons during the opening of the atlantic sheared crustal material or delaminated continental lithosphere sank into small roll like circulation cells6 7 that developed in the shallow mantle at each side of the ridge axis and the material was then transported through these cells to the ridge axis alternatively material from the continental crust has been trapped within the kane fracture zone since the opening of the atlantic ocean basin through a series of transform migrations and ridge jumps8 9 with portions of this material subsequently migrating down the ridge axis

the role of basalt replenishment in the generation of basaltic andesites of the ongoing activity at arenal volcano costa rica evidence from clinopyroxene and spinel the bulk composition of magma erupted from volcan arenal has remained nearly constant sio2 53 6 54 9 wt mgo 5 0 4 5 wt during almost 30 years of continuous activity 1969 1996 none the less clinopyroxene cpx phenocrysts and their spinel inclusions record a much more complex open system evolution in which steady state production of the erupted basaltic andesitic magma is linked to episodic injections of basalt into arenal s magma conduit reservoir system high resolution major element zoning profiles electron microprobe on a large number of phenocrysts 14 000 analyses tied to back scattered electron bse images have been used to assess the compositional characteristics of the magmatic end members as well as the timing and dynamics of magma replenishment events no two cpx phenocrysts have exactly the same zoning profile the vast majority of our analyses record the crystallization of cpx cr2o3 0 12 wt mg 65 79 al ti 2 7 from a liquid comparable to or more evolved than erupted magma compositions however half of all cpx grains are cored by high cr cpx cr2o3 0 2 0 72 wt or contain similar basaltic compositions as abrupt growth bands in phenocrysts with and without high cr cores phenocrysts with high cr cpx occur throughout the ongoing activity in a few cases high cr cpx occurs very near the outer margin of the grain without an apparent growth hiatus particularly in 1968 69 and 1992 93 the main conclusions are 1 all basaltic andesitic lavas erupted at arenal during the ongoing activity that began in july 1968 are the products of magma mixing 2 clinopyroxenes record multiple replenishment events of basaltic magma in contrast to the near constancy of erupted bulk compositions 3 some phenocrysts preserve records of multiple interactions with basaltic magmas requiring magmatic processes to operate on timescales shorter than residence times of some phenocrysts 4 multiple occurrences of clinopyroxene with high cr rims suggest that basalt replenishment events have occurred with sub decadal frequency and may predate eruption by months or less from this we infer that arenal volcano is underlain by a continuously active small volume magmatic reservoir maintained in quasi steady state by basalt recharge over several decades the monotony of erupting arenal magmas implies that fractionation recharge ascent and eruption are well balanced in order for magmas to be essentially uniform while containing phenocrysts with vastly different growth histories at the time of eruption

petrogenesis of angrites abstract the recent discovery of two new angrites sahara 99555 and d orbigny has revived interest in this small group of achondrites we measured trace element abundances in the individual minerals of these two angrites and compared them with the three antarctic angrites lew 86010 lew 87051 and asuka 881371 trace element variations in four of these meteorites lew 87051 asuka 881371 sahara 99555 and d orbigny indicate rapid crystallization under near closed system conditions consistent with their mineralogical and textural features all four appear to be closely related and crystallized from very similar magmas discrepancies between their bulk ree compositions and melts calculated to be in equilibrium with the major phases may be due in part to kinetic effects of rapid crystallization prior crystallization of olivine and or plagioclase may also account for the elevated parent melt composition of clinopyroxene in some of the angrites such as asuka 881371 lew 86010 also crystallized from a melt and represents a liquid composition but trace element trends in clinopyroxene and olivine differ from those of the other angrites this meteorite seems to have crystallized from a different source magma

lead isotopic signatures of porphyry copper deposits in oceanic and continental settings colombian andes abstract three discrete sub belts of porphyry copper type mineralization are recognized in the colombian andes a western eocene sub belt an eastern jurassic to early cretaceous sub belt and between them a central miocene sub belt the western sub belt is part of an oceanic domain constituted by oceanic crust and overlying immature island arc rocks the eastern sub belt is within a continental domain underlain by the leading edge of the guayana shield and the central sub belt spans the faulted boundary between them the thicker continental crust includes important granulitic rocks which crop out locally as in the vicinity of the mocoa porphyry copper deposit pb isotopic ratios were determined for pyrite samples collected from 6 porphyry copper centers 3 in the western sub belt 2 in the eastern sub belt and one in the central sub belt ratios fall into 3 discrete populations the most radiogenic values represent the western sub belt the least radiogenic represent the eastern sub belt and an intermediate value corresponds to the central sub belt ratios therefore become progressively less radiogenic from the western oceanic domain to the eastern cratonic domain comparison of the pb isotopic ratios with those given in the literature for possible source materials for colombian porphyry copper leads enables the subcontinental mantle wedge subducted oceanic crust and subducted metalliferous sediments to be discounted as principal sources the relatively radiogenic signatures of 5 of the porphyry copper centers appear to be broadly compatible with either a subducted pelagic sediment source or an upper continental crust source whereas the sixth center mocoa is characterized by a distinctly less radiogenic 206 pb 204 pb ratio an admixture of a relatively small percentage of non radiogenic pb from granulitic material in the upper crust with the more radiogenic pb typical of the western sub belt centers could account for the mocoa data because much of the upper crustal section in the region of mocoa is thought to be dominated by granulitic material the radiogenic pb component could be best derived from subducted pelagic sediments if this is the case then a subducted pelagic sediment source could dominate the pb isotopic signatures of all colombian porphyry copper mineralization with some of the pb isotopic differences being produced by selective contamination with upper crustal leads of variable character if this conclusion is correct then the fundamental metal budgets of colombian porphyry copper systems are probably largely independent of upper crustal influences

crustal evolution in the southern appalachian orogen evidence from hf isotopes in detrital zircons abstract paired u pb and lu hf isotopic compositions of individual detrital zircons reveal that mesoproterozoic lithosphere in the southernmost appalachian orogen evolved almost continuously from 1200 to 900 ma with a noticeable concentration only at 1050 ma the evolution of the lu hf system in this crust is characterized by a systematic linear evolution pattern of 176hf 177hf versus age over this interval which suggests a limited range of mixing ratios between juvenile and older crustal components to create a new mesoproterozoic lithospheric reservoir extrapolation of the lu hf systematics of this reservoir from the mesoproterozoic to the paleozoic using typical crustal values shows general coincidence with the lower part of the hf isotopic array defined by paleozoic detrital zircons because the paleozoic array extends to both much higher and much lower ehf values than are likely to develop from extrapolations of the mesoproterozoic array the paleozoic array is most compatible with an orogen fo

geology of el domo deposit in central ecuador a vms formed on top of an accreted margin el domo is a cu au zn ag volcanogenic massive sulfide deposit vms within the paleocene eocene submarine arc rocks of the macuchi formation in central ecuador the mineralization includes massive and semi massive sulfides located at the interface between a submarine rhyodacite dome complex and overlying mafic volcanic and volcaniclastic rocks including chaotic and unsorted debris flow deposits whole rock geochemical analyses show that the felsic rocks belong to a calc alkaline sequence whereas the mafic assemblages have a tholeiitic affinity reflecting a magmatic association typically found in many vms districts this geochemical change temporally coincides with the formation of the massive sulfides new zircon hf isotopic data collected from a hanging wall rhyodacite suggest that the presence of an over thickened crust of oceanic plateau affinity may have influenced the composition of the magmatic rocks the timing of massive sulfide formation is constrained by an 40ar 39ar age of 41 49 0 37 ma 2 from the volcaniclastic sequence which hosts the massive sulfides and a u pb zircon age date of 42 13 0 54 ma 2 from the footwall rhyodacite both ages are interpreted to provide minimum and maximum ages for mineralization the massive sulfides are zoned and contain variable proportions of pyrite sphalerite and chalcopyrite minor accumulations of galena bornite tennantite stromeyerite and proustite are also present the latter two minerals are the major carriers of silver whereas gold typically occurs as minute free grains or in solid solution with silver ore minerals are intergrown with variable proportions of chlorite barite quartz anhydrite and smectite the footwall rhyodacite hosts an irregular stringer zone which is rich in pyrite and gypsum and is characterized by intense quartz sericite pyrite alteration the hanging wall volcaniclastic lithologies exhibit pervasive but vertically limited 8 10 m silicification chloritization and argillization both felsic and mafic glass rich rocks located near the contact between rhyodacites and mafic volcaniclastic rocks are replaced by massive to disseminated mineralization likely controlled by their originally reactive and porous nature replacement appears to have taken place at very shallow depths 10 20 m below the seafloor as shown by 1 the presence of massive sulfide clasts within the uppermost volcaniclastic horizons and 2 the absence of mineralization and alteration in the overlying debris flows no evidence of exhalation on the seafloor has been found and replacement below the sea floor was probably an efficient mechanism for preserving the vms within such a highly dynamic volcanic system regionally mineralization occurred within an extensional submarine arc setting whereby massive sulfides were preferentially controlled by the reactivation of nne ssw crustal scale strike slip faults created during an earlier plate collision local stress regimes resulted in the formation of pull apart basins within a regional transpressive setting and provided conduits for magmatic and hydrothermal discharge the onset of volcanism and local extension coincided with an increase in the oblique convergence of the farallon and the south american plate

mode and timing of terrane accretion in the forearc of the andes in ecuador the volcanic basement of the ecuadorian western cordillera pallatanga formation and san juan unit is made up of mafi c and ultramafi c rocks that once formed an oceanic plateau radiometric ages from these rocks overlap with a hornblende 40 ar 39 ar plateau age of 88 1 6 ma age obtained for oceanic plateau basement rocks of the pinon formation in coastal ecuador and with ca 92 88 ma ages reported for oceanic plateau sequences in the caribbean and western colombia these results suggest that the oceanic plateau rocks of the western cordillera and fl at forearc in ecuador are derived from the late cretaceous caribbean colombia oceanic plateau intraoceanic island arc sequences rio cala group overlie the plateau in the western cordillera and yield crystallization ages that range between ca 85 and 72 ma the geochemistry and radiometric ages of island arc lavas from the rio cala group combined with the age range and geochemistry of their turbiditic volcaniclastic products indicate that the arc was initiated by westward subduction beneath the caribbean plateau they are coeval with island arc rocks of coastal ecuador las orquideas san lorenzo and cayo formations and colombia ricaurte arc these island arc

origin and cretaceous tectonic history of the coastal ecuadorian forearc between 1 n and 3 s paleomagnetic radiometric and fossil evidence the presence of at least one block pallatanga block derived from the caribbean colombian oceanic plateau has been proposed in the western cordillera of ecuador new 40ar 39ar dating of the basement of the coastal blocks 90 87 ma and fossil ages in the overlying calentura fm 89 84 ma argue for late turonian coniacian extrusion of the basement of the pinon block paleomagnetic inclinations indicate that the mafic basement of the pinon and san lorenzo blocks extruded at equatorial low southern latitudes no subsequent significant latitudinal drift can be observed during the cretaceous rapid changes in paleomagnetic declination between 73 and 70 ma in both the pinon and the san lorenzo blocks indicate that the large igneous province which originally hosted the coastal blocks experienced a major tectonic event during late campanian times similar late turonian coniacian basement ages and late campanian tectonic events are reported in the pallatanga block of the western cordillera we propose that the pinon san lorenzo and pallatanga blocks all derive from a common large igneous province the caribbean colombian oceanic plateau and were incorporated into the south american margin by collision during the late campanian 73 70 ma

thermoluminescence and compositional zoning in the mesostasis of a semarkona group a1 chondrule and new insights into the chondrule forming process a large group a1 porphyritic olivine chondrule in the semarkona ll3 0 chondrite with induced thermoluminescence tl and compositional zoning in its mesostasis has been discovered the chondrule has ca rich and fe poor olivine cao 0 36 0 40 wt fa0 3 0 5 and its mesostasis is highly anorthite normative 52 5 wt the chondrule shows an intense induced tl peak at 300 c with a half width of 180 c the induced tl in the 40 440 c range increases monotonically by a factor of 6 from center to rim while sio2 na2o and mno increase by factors of 1 1 3 6 and 6 respectively the spectrum of the induced tl over the 200 350 c range i e a dominant peak at 570 nm with a half width of about 100 nm and the mn tl correlation suggest mn activated plagioclase is an important constituent of the refractory mesostases in group a1 chondrules the zoning may reflect fractional crystallization soret diffusion transport of volatiles into the chondrule by aqueous alteration a zoned precursor reduction of precursor dust aggregate or recondensation of volatiles lost during chondrule formation the first four possibilities seem unlikely explanations for the zoning of the mesostasis but both reduction of precursor dust aggregate and recondensation of volatiles seem to have played significant roles to the formation of the zoning reducing conditions at high temperatures are suggested to have prevailed during the formation of group a1 chondrules

radiogenic isotopic mapping of late cenozoic eolian and hemipelagic sediment distribution in the east central pacific pelagic clay of the east central pacific province is shown to be a mixture of three primary detrital components reflecting continental source areas in asia north america and central and south america relative contributions from each source area are a function of geography and this distribution appears to have remained constant over the past five million years despite changing flux rates a q mode factor analysis of downcore records for pb sr and nd isotopes identified three factors that account for 98 of the total variance these factors represent the radiogenic isotopic signatures of 1 late cenozoic asian dust which dominates in the central north pacific 2 north american continental hemipelagic eolian sources restricted mainly to the easternmost north pacific at 30 n latitude and 3 central and south american sources restricted to areas east of 100 w longitude south of the intertropical convergence zone 6 n the asian dust signature diminishes abruptly we conclude that late cenozoic asian dust sources can be isotopically differentiated downcore from both north american and south and central american sources in the eastcentral pacific this approach has a utility for identifying changes in long term cenozoic atmospheric circulation patterns

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

electrodynamic parameters in the nighttime sector during auroral substorms the characteristics of the large scale electrodynamic parameters field aligned currents facs electric fields and electron precipitation which are associated with auroral substorm events in the nighttime sector have been obtained through a unique analysis which places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images a generic bulge type auroral emission region has been deduced from auroral images taken by the dynamics explorer 1 de 1 satellite during a number of isolated substorms and the form has been divided into six sectors based on the peculiar emission characteristics in each sector west of bulge surge horn surge middle surge eastern bulge and east of bulge by comparing the location of passes of the dynamics explorer 2 de 2 satellite to the simultaneously obtained auroral images each pass is placed onto the generic aurora the organization of de 2 data in this way has systematically clarified peculiar characteristics in the electrodynamic parameters an upward net current mainly appears in the surge with little net current in the surge horn and the west of bulge the downward net current is distributed over wide longitudinal regions from the eastern bulge to the east of bulge near the poleward boundary of the expanding auroral bulge a pair of oppositely directed fac sheets is observed with the downward fac on the poleward side this downward fac and most of the upward fac in the surge and the middle surge are associated with narrow intense antisunward convection corresponding to an equatorward directed spikelike electric field this pair of currents decreases in amplitude and latitudinal width toward dusk in the surge and the west of bulge and the region 1 and 2 facs become embedded in the sunward convection region the upward fac region associated with the spikelike field on the poleward edge of the bulge coincides well with intense electron precipitation and aurora appearing in this western and poleward portion of the bulge the convection reversal is sharp in the west of bulge and surge horn sectors and near the high latitude boundary of the upward region 1 fac in the surge the convection reversal is near the low latitude boundary of the upward region 1 with a near stagnation region often extending over a large interval of latitude in the eastern bulge and east of bulge sectors the region 1 and 2 facs are located in the sunward convection region while a spikelike electric field occasionally appears poleward of the aurora but usually not associated with a pair of fac sheets in the eastern bulge magnetic field data show complicated fac distributions which correspond to current segments and filamentary currents

ancient lithospheric source for quaternary lavas in hispaniola fragments of ancient continental lithosphere entrained in the shallow oceanic mantle have been found in a number of locations in the southern hemisphere isotopic analyses of material from quaternary volcanic centres in hispaniola indicate that the lavas are derived from an ancient lithospheric fragment with affinities to the supercontinent gondwana

geomagnetic quiet day selection based on published literature and the response to a questionnaire sent to geomagnetic field ionospheric and magnetospheric researchers several methods of choosing periods of quiet conditions based on geomagnetic records as well as other observed parameters have been identified caveats with respect to using geomagnetic indices to select quiet periods include the following 1 geomagnetic disturbances are strongly local even if the data from all available observatories indicate quiet behavior there is the distinct possibility that some other location not sampled may be disturbed 2 geomagnetic indices are convenient but imperfect indicators of geomagnetic activity indices based on a quiet day reference level have uncertainties comparable to the threshold value for quiet conditions indices representing average conditions during a 24 hr ut day may not be appropriate 3 geomagnetic activity does not fully reflect the range of possible factors that influence the ionosphere or magnetosphere

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

evidence for middle eocene and younger land emergence in central panama implications for isthmus closure the rise of the isthmus of panama linked to a number of climatic paleoceanographic and biological events has been studied mostly from indirect often distal geochemical and biotic evidence we have upgraded existing geologic mapping in central panama with more than 2000 field stations over 40 petrographic analyses and more than 30 new geochronological and thermochronological analyses this data set suggests that the isthmus was an uninterrupted chain above sea level from late eocene until at least late miocene times the basement complex of central panama is a folded faulted 3 km thick arc sequence intruded by granitoid bodies and onlapped by mildly deformed upper eocene and oligocene strata six u pb zircon ages in the granitoids along with published geochronological data reveal intense late paleocene to middle eocene magmatism 58 39 ma a temporary cessation of magmatic activity between 38 and 27 ma and renewed magmatism between 25 and 15 ma in a position 75 km south of the former magmatic axis thermochronological analyses in zircon eight u th he ages and in apatite crystals four u th he ages and nine fission track ages obtained from a subset of 58 54 ma granitoid bodies record a concordant lutetian age 47 42 ma cooling from 200 c to 70 c in 5 m y and cooling below 40 c between 12 and 9 ma cooling is linked to exhumation by an angular unconformity that separates the deformed basement complex below from mildly deformed upper eocene to oligocene terrestrial to shallow marine strata above exhumation and erosion of the basement complex are independently confirmed by lower miocene strata that have a detrital zircon signature that closely follows the central panama basement complex age distribution these results greatly restrict the width and depth of the strait separating southern central america from south america and challenge the widely accepted notion that the central american seaway closed in late pliocene time when the ice age began

70 m y history 139 69 ma for the caribbean large igneous province it is commonly accepted that large igneous provinces are formed through catastrophic volcanic events occurring over vast areas within a few million years at the initiation of hotspots mantle plumes new 40ar 39ar ages 111 139 ma and geochemical results from the nicoya peninsula costa rica extend the age range of volcanism in the caribbean large igneous province to 70 m y 69 139 ma our results are not consistent with the formation of this vast igneous province through a single plume head at the initiation of a mantle plume such as the galapagos instead we propose that multiple oceanic intraplate igneous structures such as plateaus and hotspot tracks were accumulated through the subduction process the igneous structures could be remnants of the earlier history of the galapagos hotspot making it one of the oldest active hotspots on earth alternatively they could have been derived from several spatially distinct mantle melting events that sampled similar source material e g oceanic lithosphere of similar age

average electron precipitation patterns and visual aurora characteristics during geomagnetic quiescence the pattern of polar region electron precipitation during geomagnetically quiet periods i e mostly under the positive imf bz condition is examined on the basis of measurements from the defense meteorological satellite program dmsp f2 f3 and f4 satellites the auroral electron precipitation during quiet times consists of two distinct types that differ from those found in active times the high latitude part of the precipitation region has average electron energies lower than 500 ev and has bursty spatial structures but the average energies of the low latitude part are generally greater than 500 ev with smoother spatial structures the boundary between the hard and soft regions is called the transition boundary it is found that during quiet times the average location of the poleward boundary of electron precipitation is at particularly high geomagnetic latitudes about 82 to 84 in the morning noon and evening sectors and at about 81 to 82 in the midnight sector the equatorward edge is at about 70 in the morning noon and evening sectors and about 69 in the midnight sector the latitudinal width of the region of auroral electron precipitation is unusually large indicating a widening of the auroral oval in periods of geomagnetic quiescence dmsp auroral imaging data further reveal that there are no bright discrete optical auroras in the region of low average energy electron precipitation faint stable auroral arcs are sometimes seen in the equatorial region of the high average energy electron precipitation the location of the poleward boundary is affected by the magnitude of the northward interplanetary magnetic field imf component however the location of the equatorward and transition boundaries of precipitation are not related to the positive imf bz magnitude these results indicate that the unusually high latitude of the poleward boundary of precipitation is the consequence of a very small number of geomagnetic flux lines interconnected with the imf during a northward imf condition the results also imply that the size of the polar cap is controlled inversely by the magnitude of the northward bz component the insensitivity of the transition and equatorward precipitation boundaries to the northward imf bz can be used to imply that the boundary dynamics are dominated by internal magnetospheric processes and not by direct solar magnetospheric interactions

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

distinguishing between in situ and accretionary growth of continents along active margins abstract active continental margins represent both sites of crustal loss by tectonic erosion and gain by juvenile magmatism and oceanic arc plateau accretion estimating the gain component depends on distinguishing between growth of margins by either accretion of oceanic crust or juvenile magmatic addition during extension distinguishing between these allochthonous and autochthonous models is frequently ambiguous due to difficulties in interpreting the tectonic origin of mafic rocks we present a detailed geochronological and isotopic study constraining the evolution of the continental margin of northwest south america during 190 113 ma semi continuous slab rollback resulted in crustal thinning juvenile magmatism and ultimately the removal of continental slivers negating previous hypotheses that the margin evolved by the accretion of an island arc however basic crustal growth estimates along with comparisons between rocks exposed in northern peru and the northern andes imply that uneven losses of continental crust occurred along northwest south america for mesozoic exposures and that the accretion of the caribbean large igneous province clip onto nw south america probably played a crucial role in maintaining and preserving mesozoic continental crust north of the huancabamba deflection

trace element variations in olivine from the eastern deeps intrusion at voisey s bay labrador as a monitor of assimilation and sulfide saturation processes ni cu co sulfide mineralization in the mesoproterozoic voisey s bay intrusion is spatially and genetically associated with magmatic breccia zones within an extensive dike system and in particular with the entry point of this system into a larger intrusion eastern deeps intrusion the massive sulfide and breccia bearing inner basal margin of this upper intrusion is enveloped by weakly mineralized variable textured troctolite that progressively decreases in sulfide content contains fewer paragneiss fragments and eventually grades into a largely sulfide barren homogeneous plagioclase and olivine meso cumulate termed normal troctolite toward the top of the intrusion olivine from the troctolite lithologies the mineralized basal breccia sequence and the uppermost olivine gabbro in the eastern deeps intrusion was analyzed with secondary ion mass spectrometry sims we present multitrace element data including petrogenetically important elements v cr mn co ni zn sr and y which indicate that four petrological processes important to ore formation are recorded by the compositional diversity of olivine the heterogeneous ni co fo contents of olivine in the eastern deeps intrusion co ni ratios in olivine gabbro 0 18 1 1 normal troctolite 0 07 0 26 variable textured troctolite 0 11 0 29 basal breccia sequence 0 20 0 34 are due to variable modification of original compositions by reequilibration with trapped silicate and sulfide liquids the abrupt changes of fe fo79 fo66 and ni 2 500 to 80 ppm contents and some trace elements up to 1 5x more sr up to 3x more y than average normal troctolite olivine in olivine from intervals in the barren upper normal troctolite indicate that olivine crystallized from separate pulses of magma with variable degrees of differentiation the severe ni depletion of one troctolite melt batch 10 ppm ni vs 290 ppm ni for average normal troctolite magma is likely a result of sulfide saturation and segregation either in a deeper staging chamber or in the conduit prior or contemporary to olivine precipitation and emplacement the gradational increase of fe from fo80 to fo62 mn from 4 000 to 10 000 ppm and zn from 400 to 620 ppm in olivine from the weakly mineralized and contaminated lower part of the variable textured troctolite downward into the massive sulfide bearing and strongly contaminated basal breccia sequence is consistent with olivine fractionation from an increasingly country rock contaminated sulfide saturated silicate magma olivine here crystallized prior to or contemporaneously with the segregation of an immiscible sulfide liquid olivine grains that are enclosed by sulfide display homogeneous forsterite contents fo62 with pronounced intragrain trace element zonation mn ni co zn diffusion profiles mn ni indicate a relatively short immersion time 1 000 years of these olivine grains in sulfide liquid which likely resulted in the partial retention of ni in the olivine olivine in the vicinity of massive sulfides in the eastern deeps intrusion from variable textured troctolite to basal breccia sequence has a distinct minor and trace element signature v cr mn co ni zn compared to olivine from the barren troctolite and olivine gabbro a multiple regression analysis using these data yields an index that describes the relative vertical proximity of olivine to massive sulfides in conjunction with the stratigraphic context of the host lithology this information could potentially be used as a vector toward massive sulfide mineralization in the eastern deeps intrusion and also support ni exploration in other olivine bearing mafic intrusions with similar style of sulfide mineralization

space weathering by simulated micrometeorite bombardment on natural olivine and pyroxene a coordinated ir and tem study abstract we studied space weathering effects caused by micrometeorite bombardment simulated by pulsed intense infrared laser generating 15 mj per pulse in high vacuum for our investigation we selected a natural olivine san carlos olivine fo91 and a natural pyroxene bamble orthopyroxene en87 as important rock forming minerals of the earth upper mantle as well as key planetary minerals irradiated areas of powdered pressed samples were examined by optical reflection spectroscopy in a broad optical and infrared wavelength range visible near and mid infrared and transmission electron microscopy to identify changes due to micrometeorite impacts the present study aims to investigate especially the effects of micrometeorite bombardment on reflectance spectra in the mid ir in preparation for future space missions as well as for the mertis experiment onboard the bepicolombo mission for both irradiated samples we found a reduction in albedo and in the reflectance of characteristic reststrahlen bands and an increase of the transparency feature vis and nir spectra of both minerals show the typical darkening and reddening as described for other space weathered samples tem studies revealed that space weathered layers in olivine and pyroxene differ in their respective thickness 450 nm in olivine 100 250 nm in pyroxene as well as in developed nanostratigraphy of laser ablated material like nanophase iron npfe in conclusion our spectral and structural findings were compared to samples in which space weathering was caused by different processes a comparison with these data demonstrates that there is no difference in optical reflectance spectroscopy but a significant difference in the microstructure of minerals due to the weathering source in space as there are solar wind and solar flares cause other structural and chemical changes as the bombardment with micrometeorites

differentiation and evolution of the iva meteorite parent body clues from pyroxene geochemistry in the steinbach stony iron meteorite we analyzed the steinbach iva stony iron meteorite using scanning electron microscopy sem electron microprobe analysis empa laser ablation inductively coupled plasma mass spectroscopy la icp ms and modeling techniques different and sometimes adjacent low ca pyroxene grains have distinct compositions and evidently crystallized at different stages in a chemically evolving system prior to the solidification of metal and troilite early crystallizing pyroxene shows evidence for disequilibrium and formation under conditions of rapid cooling producing clinobronzite and type 1 pyroxene rich in troilite and other inclusions subsequently type 2 pyroxene crystallized over an extensive fractionation interval steinbach probably formed as a cumulate produced by extensive crystal fractionation 6070 fractional crystallization from a high temperature 1450 1490 c silicate metallic magma the inferred composition of the precursor magma is best modeled as having formed by 30 50 silicate partial melting of a chondritic protolith if this protolith was similar to an ll chondrite as implied by o isotopic data then olivine must have separated from the partial melt and a substantial amount 53 56 of feo must have been reduced in the silicate magma a model of simultaneous endogenic heating and collisional disruption appears best able to explain the data for steinbach and other iva meteorites impact disruption occurred while the parent body was substantially molten causing liquids to separate from solids and oxygen bearing gas to vent to space leading to a molten metal rich body that was smaller than the original parent body and that solidified from the outside in this model can simultaneously explain the characteristics of both stony iron and iron iva meteorites including the apparent correlation between metal composition and metallographic cooling rate observed for metal

campanian miocene tectonostratigraphy depocenter evolution and basin development of colombia and western venezuela abstract the position of the central axis of deposition over colombian and venezuelan continental crust has varied markedly through time the axis migrated from west to east from late cretaceous to oligocene but at times secondary drainage divides were established by local uplift events in oligocene times with initial inversion of the eastern cordillera the central axis of deposition was divided into two main axes the proto magdalena and the proto orinoco systems the west to east migration of the central axis of deposition had a tectonic origin and it occurred in combination with tectonically driven changes in accommodation space depocenter evolution can be divided as follows 1 the axis of the campanian and early maastrichtian depocenter was located few km east of the present position of the central cordillera of colombia it migrated east with gradual uplift of the central cordillera 2 the central axis of late maastrichtian deposition is positioned approximately over the present day western foothills of the eastern cordillera possibly crosses the eastern cordillera over the santander massif and continues into the maracaibo lake in western venezuela accommodation space decreased from campanian to maastrichtian times in cretaceous tertiary boundary times parts of the eastern margin of the eastern cordillera were uplifted by an initial phase of inversion of deeply rooted jurassic and early cretaceous normal faults 3 in paleocene times the central axis of deposition was located along the spine of the eastern cordillera and extended into the maracaibo basin and accommodation space continued to decrease 4 in latest paleocene times the central axis of deposition shifted to eastern regions of the eastern cordillera and accommodation space decreased 5 the early eocene central axis of deposition was located along the present day eastern foothills of the eastern cordillera accommodation space continued to decrease and the regional middle eocene unconformity began to develop in middle eocene times a regional unconformity developed marking the climax of the pre andean orogeny deposition during these times was dominant in the maracaibo basin area where large amounts of sediment derived from vast exposed areas accumulated 6 the late eocene central axis of deposition was confined to the present position of the llanos foothills late eocene deposition reflects a regional increase in accommodation space in oligocene times the initial uplift of the eastern cordillera divided the main depocenter into two central axes accommodation space diminished in uplifted regions but continued to increase in the depocenters allowing sporadic marine ingressions into the present position of the llanos foothills as uplift of the eastern cordillera continued the eastern depocenter axis proto orinoco migrated east and the western depocenter axis proto magdalena migrated west this process continued through the rest of the cenozoic

observations of auroral substorms occurring together with preexisting quiet time auroral patterns although horse collar configurations are commonly associated with relatively quiescent magnetospheric conditions observations made with the viking uv imager are presented which clearly demonstrate that classical auroral substorms can coexist with such patterns thus the magnetospheric conditions that lead to the production of these quiet time auroral patterns are not necessarily stable to the growth of the instability that leads to substorm onset as well the horse collar patterns provide useful natural indicators of the underlying magnetospheric topology and can be used to determine in a relative sense the source location of the substorm activity we conclude that the horse collar patterns have a distant tail source location and that the substorm onsets have a source location on closed field lines with much lower l values the observations are therefore inconsistent with the onset mechanisms proposed in either the thermal catastrophe model or the boundary layer dynamics model in addition we have shown that substorm related plasma sheet dropouts are not always produced by a neutral line induced thinning of the plasma sheet but could also be caused by the large scale motion of the open field line region plasma data from dmsp f6 dmsp f7 isee 1 and viking are consistent with the interpretation that the regions between the horse collar arcs are threaded by open lobe field lines and that open field lines can also partially intrude into the space between the transpolar arcs and the lower latitude main oval the significance of these results for substorm activity which occurs together with quiet time auroral patterns is discussed in terms of the flux rope plasmoid version of the near earth neutral line model depending upon the location and extent of the open field line regions relative to the developing plasmoid it is suggested that several possibilities exist for the subsequent evolution of the substorm activity in the tail and that these different scenarios can be related to specific types of auroral morphologies in the ionosphere

on the conjugacy of the aurora high and low latitudes on december 5 1983 the polar orbiting satellites noaa 7 and dmsp f6 while in opposite hemispheres each crossed a narrow structure of auroral electron precipitation at nearly the same universal time ut and at the same high magnetic latitude local time location the operational line scanner ols image taken by dmsp f6 of the northern polar region showed that this enhancement was associated with a sun aligned arc saa which extended poleward from the midnight sector auroral zone and passed across the polar cap very near to the magnetic pole the dmsp f6 plasma measurements showed particle precipitation at background levels in the regions adjacent to the saa suggesting that those magnetic field lines connected to the interplanetary medium via the northern tail lobe noaa 7 observations show that the analogous precipitation structure in the southern hemisphere was embedded in a region of low intensity ion and electron precipitation which had the characteristics of the plasma sheet population comparisons between the boundaries of the lower latitude auroral zone precipitation and the electron distributions at those boundaries were also made the results of these comparisons together with those made for the high latitude saa argue for a high degree of inter hemisphere conjugacy in the electron precipitation over a wide range of invariant magnetic latitudes

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

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

the shift of the auroral electron precipitation boundaries in the dawn dusk sector in association with geomagnetic activity and interplanetary magnetic field on the basis of the auroral precipitating electron data along the dawn dusk meridian from a defense meteorological satellite program satellite dmsp f2 we show that the electron precipitation region extends poleward often to the geomagnetic latitudes 85 from the average oval location during quiet periods namely during periods of a large positive bz component this result may be interpreted as an indication that only a small amount of geomagnetic flux interconnects with the northward imf resulting in a contracted open region the polar cap the control of the location of the poleward boundary by the north south component of the imf is also statistically examined the rate of latitudinal movement is about 0 4 to 0 8 per 1 nt for positive imf bz values and about 0 7 to 1 1 per 1 nt for negative imf bz values the shift of the equatorward boundary is not obvious however for positive imf bz values as geomagnetic activity increases both the poleward and equatorward boundaries shift toward lower latitudes the equatorward shift of the poleward boundary is greater than that of the equatorward boundary resulting in a thinner electron precipitation region during disturbed periods than during quiet periods

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

analysis of auroral morphology substorm precursor and onset on january 10 1997 the solar wind interaction with the geomagnetic field is studied using global auroral images obtained by the ultraviolet imager uvi on polar we study the dynam cs of the poleward and equatorward boundaries of the auroral oval in response to the solar wind imf on january 10 1997 using a neural network algorithm to perform an automated morphological analysis poleward and equatorward boundaries identified by the algorithm demonstrate a clear growth motion with the southward turning of the imf and growth and poleward expansion at substorm onset the area poleward of the oval polar cap is found to increase in size coincident with the southward turning of the imf bz component at 0220 ut and peaks at substorm onset at 0334 ut the area of the oval however decreases continuously throughout the period of the polar cap area increase with a slight recovery observed during the substorm onset these observations are consistent with the concept that magnetospheric dynamics are directly driven by the solar wind geomagnetic field interactions

relationship of the ionospheric convection reversal to the hard auroral precipitation boundary plasma drift and particle measurements from the dmsp spacecraft for the three geospace environmental modeling gem campaign periods january 27 29 march 28 29 and july 20 21 1992 have been used to study the relationship between the plasma convection reversal and the poleward boundary of the diffuse auroral zone characterized by hard electron precipitation with energies greater than 0 45 kev this boundary named the hard auroral precipitation boundary or the hap boundary in this paper is often regarded as the ionospheric footprint of the boundary between the central plasma sheet cps and the boundary plasma sheet bps by examining simultaneous ion drift and particle measurements from about 500 satellite passes we find that the large scale plasma flow in the morning and evening sectors changes its direction within the auroral oval at the hap boundary however exceptions are found in the early morning sector between 0300 and 0600 mlt where the convection reversal is sometimes in 30 of the dmsp crossings displaced poleward relative to the hap boundary it is shown that the shape of the region bordered by the hap boundary can be roughly represented by a circle whose size is influenced by the imf bz component there is roughly a linear correlation between the diameter of this circle and the cross polar cap potential drop with the best correlation coefficient of 0 65 for winter season our study suggests that the hap boundary corresponds to the magnetospheric boundary between the quasi dipolar region and the region with more stretched field lines and the source of the region 1 field aligned current is located near the hap boundary a by dependent shift of the hap boundary with respect to the noon midnight meridian is also found in the northern hemisphere the shift is dawnward for positive by and duskward for negative by in the southern hemisphere the shift is opposite to that in the northern hemisphere

controls on magmatism in an island arc environment study of lavas and sub arc xenoliths from the tabar lihir tanga feni island chain papua new guinea the tabar lihir tanga feni tltf islands of papua new guinea mainly comprise high k calc alkaline and silica undersaturated alkaline rocks that have geochemical features typical for subduction related magmatism numerous sedimentary mafic and ultramafic xenoliths recovered from tubaf seamount located on the flank of lihir island provide a unique opportunity to study the elemental and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism in the region the sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences and oceanic crust with pacific affinity a majority of the ultramafic xenoliths contain features indicating wide spread metasomatism in the mantle wedge under the tltf arc leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete labile phases that can account for up to 50 or more of elements such as cu zn rb u pb and light ree rare earth elements most likely introduced in the xenoliths via hydrous fluids released from a subducted slab the leaching experiments demonstrated that the light ree enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases exhibit ree patterns that range from flat to light ree depleted sr nd isotopic data for the ultramafic residues show a coupled behavior of increasing 87sr 86sr with decreasing 143nd 144nd ratios the labile phases in the ultramafic xenoliths represented by the leachates show decoupling between sr and nd with distinctly more radiogenic 87sr 86sr than the residues both leachates and residues exhibit very wide range in their pb isotopic compositions indicating the involvement of three components in the mantle wedge under the tltf islands two of the components can be identified as pacific oceanic mantle and pacific sediments some of the ultramafic samples and clinopyroxene separates however exhibit relatively low 206pb 204pb at elevated 207pb 204pb suggesting that the third component is either indian ocean type mantle or australian subcontinental lithospheric mantle geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized it did not significantly contribute to the isotopic and incompatible trace element compositions of tltf lavas compared to the mantle samples the tltf lavas have very restricted pb isotopic compositions that lie within the pacific morb range indicating that magma compositions were dominated by melts released from a stalled subducted slab with pacific morb affinity interaction of slab melts with depleted peridotitic component in the mantle wedge followed by crystal fractionation most likely generated the geochemical characteristics of the lavas in the area

neogene stratigraphy paleoceanography and paleobiogeography in northwest south america and the evolution of the panama seaway abstract the atrato basin in the pacific coastal region of nw south america colombia exhibits stratigraphic and biostratigraphic characteristics similar to other neogene pacific coastal basins of southern central america and northern south america an evaluation of the neogene stratigraphy and foraminiferal biostratigraphy with emphasis on paleoceanographic and paleobiogeographic phenomena indicate the following 1 previous to the middle miocene well aerated deep and open oceanic conditions and free and active water circulation prevailed along the steep continental margins of nw south america 2 during the early middle miocene right after the neogene hiatus nh 2 15 2 ma tectonic disturbances triggered the initial uplift of the panama sill bottom water circulation foraminiferal and sedimentation changes are observed in both the pacific and caribbean coastal areas of nw south america 3 during the middle miocene coinciding with the neogene hiatus nh 3 12 9 11 8 ma an abrupt paleobathymetric change from lower to middle bathyal depths indicates a major uplift of the panama sill to about 1000 m reflecting the middle miocene tectonic disturbances in nw south america 4 during the late middle miocene immediately following this uplift a distinctive pacific benthic foraminiferal fauna abruptly appeared and extended from ecuador to california because these assemblages do not occur in the adjacent caribbean region a circulation barrier between the atlantic and pacific oceans as a result of intensification of the cool marginal california current is proposed anoxic middle to upper bathyal environments prevailed under well aerated cool surface waters in the pacific coastal areas of nw south america 5 during the latest miocene immediately following the neogene hiatus nh 6 7 0 6 3 ma surface water circulation between the caribbean and the pacific was re established and the previous influence of the california current disappeared a rapid filling and shallowing of the basin to mostly neritic depths is indicated along the pacific and caribbean coastal basins of colombia as well as a progressive increase in similarity between the coastal pacific and caribbean benthic biotas this shallowing in the early late miocene also appears to have been associated with the earliest terrestrial interchange of ground sloths to north america and raccoons and their allies to south america 6 immediately following early pliocene time the panamanian isthmus became completely emergent providing a terrestrial environment favourable for intermingling of terrestrial faunas and floras between north and south america i correlate this pliocene uplift with the neogene hiatus nh 8 3 7 3 1 ma four regional unconformities associated with tectonic disturbances in the nw corner of south america have also been recognized and correlated with neogene hiatuses recorded from the oceans early middle miocene middle miocene late miocene and early pliocene

geological history of the cretaceous ophiolitic complexes of northwestern south america colombian andes abstract the western cordillera of colombia was formed by intense alpine type nappe forming folding and thrusting the cretaceous 80 120 ma b p tholeiitic material of the western cordilleran nappes has been obducted onto the paleozoic and precambrian polymetamorphic micaschists and gneiss of the central cordillera near yarumal the antioquia batholith 60 80 ma b p intrudes both obducted cretaceous oceanic material and the polymetamorphic basement rock of the central cordillera therefore nappe emplacement and obduction onto the central cordillera occurred during late senonian to early paleocene the nappes travelled from northwest to southeast so that the highest unit the rio calima nappe therefore has the most northwestern source whereas the lowest units originated from a more southeastward direction sedimentological analysis of the volcanoclastic and sandy turbidite material from each unit suggests a marginal marine environment during cretaceous times the opening of this marginal sea from now on called the colombia marginal basin probably originated by detachment of a block from the south american continent related to the farallon south america plate convergence in the popayan area southern colombia the central cordilleran basement exhibits glaucophane schist facies metamorphism this high pressure low temperature metamorphism is of early cretaceous 125 ma b p age and is related to an undated metaophiolitic complex the ophiolitic material originating from the western cordilleran is thrust over both the blueschist belt and the metaophiolitic complex these data suggest that the occidente colombiano suffered at least two phases of ophiolitic obduction during mesozoic time

viking observations of a reverse convection cell developing in response to a northward turning of the interplanetary magnetic field observations of a developing reverse convection cell on september 23 1986 are examined using auroral images acquired with the uv imager on board the viking spacecraft the cell possesses a counter clockwise rotational sense and evolves on the duskside of the northern auroral distribution between the duskside oval and a complex duskside transpolar arc system as the structure evolves it grows and appears to displace the transpolar arc toward the dawn we interpret these observations in terms of a model proposed by burch et al 1992 in which open field lines produced by merging on the high latitude magnetopause produce reverse convection cells between the oval and the transpolar arcs associated with horse collar or teardrop auroral patterns

accreted oceanic terranes in ecuador southern edge of the caribbean plate the western part of ecuador is made of several oceanic terranes which comprise two oceanic plateaus of early 120 ma and late cretaceous age 90 ma respectively the older oceanic plateau was accreted to the andean margin in the late campanian 75 ma fragments of the turonian coniacian plateau were accreted to the ecuadorian margin in the late maastrichtian 68 ma guaranda terrane and late paleocene 58 ma pinon naranjal terrane the guaranda terrane received either fine grained oceanic sediments of coniacian maastrichtian age or island arc back arc volcanic suites of middle campanian middle maastrichtian age the pinon naranjal terrane recorded a comparable history completed in the maastrichtian paleocene either by pelagic cherts or by island arc products macuchi arc the late cretaceous plateau of ecuador is interpreted as part of the caribbean oceanic plateau cop because their evolutions are comparable if so the cop was not formed by the galapagos hotspot but on the farallon oceanic plate south of ecuador and close to the south american margin the cop belonged to the farallon plate until a subduction zone separated both plates in the middle campanian giving way to a well developed mid campanian mid maastrichtian island arc accretion in the late maastrichtian triggered a change in the subduction system and the development of a new arc system of late maastrichtian late paleocene age which crosscut the south america cop plate boundary the last accretion occurred in the late paleocene

temporal and spatial variations of the polar cap dimension inferred from the precipitation boundaries we examine temporal and spatial variations of the distance between the electron precipitation boundaries across the polar cap the defense meteorological satellite program satellite data within a few hours of the dawn dusk and noon midnight meridians are used it is found that changes in the distance between the precipitation boundaries across the polar region in both the dawn dusk and noon midnight sectors tend to follow grossly changes of the ae index on the basis of the simultaneous isee 3 data it is found that the increase of the distance begins at about the time when the southward turning of the interplanetary magnetic field reaches the magnetosphere but prior to the corresponding increase of the ae index the period of the maximum distance approximately coincides with the period of the maximum ae value the decrease of the distance begins at about the same time as the substorm activity begins to subside but it continues well after the ae index has reduced to very small values these findings are discussed in terms of changes of the total open magnetic flux by assuming that the precipitation boundaries coincide approximately with the boundaries of the open field line region

cretaceous and tertiary terrane accretion in the cordillera occidental of the andes of ecuador new field geochronological geochemical and biostratigraphical data indicate that the central and northern parts of the cordillera occidental of the andes of ecuador comprise two terranes the older pallatanga terrane consists of an early to late cretaceous oceanic plateau suite late cretaceous marine turbidites derived from an unknown basaltic to andesitic volcanic source and a tectonic melange of probable late cretaceous age the younger macuchi terrane consists of a volcanosedimentary island arc sequence derived from a basaltic to andesitic source a previously unidentified regionally important dextral shear zone named the chimbo toachi shear zone separates the two terranes regional evidence suggests that the pallatanga terrane was accreted to the continental margin the already accreted cordillera real in campanian times producing a tectonic melange in the suture zone the macuchi terrane was accreted to the pallatanga terrane along the chimbo toachi shear zone during the late eocene probably in a dextral shear regime the correlation of cretaceous rocks and accretionary events in the cordillera occidental of ecuador and colombia remains problematical but the late eocene event is recognised along the northern andean margin

cenozoic continental arc magmatism and associated mineralization in ecuador most of the economic ore deposits of ecuador are porphyry cu and epithermal style gold deposits associated with tertiary continental arc magmatism this study presents major and trace element geochemistry as well as radiogenic isotope pb sr signatures of continental arc magmatic rocks of ecuador of eocene to late miocene 50 9 ma elm and late miocene to recent 8 0 ma lmr ages the most primitive elm and lmr rocks analyzed consistently display similar trace element and isotopic signatures suggesting a common origin most likely an enriched morb type mantle in contrast major and trace element geochemistry as well as radiogenic isotope systematics of the whole sets of elm and lmr samples indicate strikingly different evolutions between elm and lmr rocks the elm rocks have consistently low sr y increasing rb sr and decreasing eu gd with sio2 suggesting an evolution through plagioclase dominated fractional crystallization at shallow crustal levels 20 km and most likely at sub crustal levels 40 50 km the change in geochemical signatures of tertiary magmatic rocks of ecuador from the elm to the lmr type coincides chronologically with the transition from a transpressional to a compressional regime that occurred at 9 ma and has been attributed by other investigations to the onset of subduction of the aseismic carnegie ridge

experimentally determined mineral melt partition coefficients for sc y and ree for olivine orthopyroxene pigeonite magnetite and ilmenite our current lack of understanding of the partitioning behavior of sc y and the ree rare earth elements can be attributed directly to the lack of a sufficiently large or chemically diverse experimental data set to address this problem we conducted a series of experiments using several different natural composition lavas doped with the elements of interest as starting compositions microprobe analyses of orthopyroxene pigeonite olivine magnetite ilmenite and co existing glasses in the experimental charges were used to calculate expressions that describe ree partitioning as a function of a variety of system parameters using expressions that represent mineral melt reactions versus element ratio distribution coefficients it is possible to calculate terms that express low ca pyroxene melt partitioning behavior and are independent of both pyroxene and melt composition compositional variations suggest that sc substitution in olivine involves either a paired substitution with al or more commonly with vacancies the partitioning of sc is dependent both on melt composition and temperature our experimentally determined olivine melt ree ds partition coefficients are similar to but slightly higher than those reported by mckay 1986 and support their conclusions that olivines are strongly lree depleted y and ree mineral melt partition coefficients for magnetite range from 0 003 for la to 0 02 for lu ilmenite partition coefficients range from 0 007 for la to 0 029 for lu these experimental values are two orders of magnitude lower than many of the published values determined by phenocryst matrix separation techniques

electrodynamic patterns in the polar region during periods of extreme magnetic quiescence the electrodynamic patterns in the polar region during periods of extreme magnetic and auroral quiescence are analyzed for the existence of region 1 and region 2 field aligned current systems convection patterns and electron precipitation boundaries within the capabilities of identification the large scale region 1 and region 2 currents are usually absent during quiet periods except in the cusp region though a seasonal effect is observed with lowest probabilities of identifying these regions occurring in the winter hemisphere field aligned currents continue to be present but as small scale structures distributed through the auroral oval and over the entire polar cap convection patterns are always found in the summer hemisphere though with large amplitude irregularities superimposed but their probability for existence decreases significantly around the winter solstice structured electron precipitation extends over the entire polar region independent of season the likelihood of finding separate regions of diffuse and structured electron precipitation is fairly small except in the cusp no consistent relationship exists between the high latitude boundary of 35 kev electron precipitation and boundaries for lower energy electrons the extreme quiescent periods for the northern hemisphere seem to exist when the interplanetary magnetic field has a northward component and a high probability of a sunward component producing minimal merging sites for the northern polar cap field lines

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

fracturing of the panamanian isthmus during initial collision with south america tectonic collision between south america and panama began at 23 25 ma the collision is significant because it ultimately led to development of the panamanian isthmus which in turn had wide ranging oceanic climatic biologic and tectonic implications within the panama canal zone volcanic activity transitioned from hydrous mantle wedge derived arc magmatism to localized extensional arc magmatism at 24 ma and overall marks a permanent change in arc evolution we interpret the arc geochemical change to result from fracturing of the panama block during initial collision with south america fracturing of the panama block led to localized crustal extension normal faulting sedimentary basin formation and extensional magmatism in the canal basin and bocas del toro synchronous with this change both panama and inboard south america experienced a broad episode of exhumation indicated by u th he and fission track thermochronology coupled with changing geographic patterns of sedimentary deposition in the colombian eastern cordillera and llanos basin such observations allow for construction of a new tectonic model of the south america panama collision northern andes uplift and panama orocline formation finally synchroneity of panama arc chemical changes and linked uplift indicates that onset of collision and isthmus formation began earlier than commonly assumed

the auroral oval position structure and intensity of precipitation from 1984 onward an automated on line data base an on line data base consisting of the auroral oval boundaries structure and particle fluxes from the dmsp f7 and f9 satellites from december 1983 through the present with about an 7 month lag is announced the data are divided into distinct regions e g diffuse aurora discrete aurora cusp proper polar rain etc the magnetospheric sources of the various particle precipitation regions are identified using a sophisticated pattern recognition technique a neural network for each region the boundaries are specified in both geographic and geomagnetic coordinates and the average and peak values of the particle fluxes are given along with the average energies the dmsp satellites are continuously monitored so that data gaps are comparatively rare it is anticipated that the location of the auroral oval and precipitation intensities will be of particular interest to ground based observers although anyone interested in the state of the auroral oval may find it of value span data requests are processed on a completely automated basis with up to 48 hours of auroral oval parameters provided in response to a single request

the nature and provenance of accreted oceanic terranes in western ecuador geochemical and tectonic constraints western ecuador consists of a complex tectonic melange of oceanic terranes accreted to the continental margin from late cretaceous to eocene time new geochemical data from these accreted terranes arising from a 5 year british geological survey mapping programme indicate that they comprise rocks from a variety of oceanic tectonic settings from thickened and relatively unsubductable oceanic plateau basalts through island arc tholeiites with occasional more calc alkaline lavas to back arc basin basalt sequences this study has enabled us to construct a new geodynamic model for the cretaceous tertiary evolution of the northern andes and has placed important new constraints on the extent of oceanic plateau sequences in colombia and around the caribbean the age and nature of sediments combined with evidence for the age of peak metamorphism suggests that a prolonged 15 20 ma accretionary event occurred in late cretaceous time and involved the collision of an oceanic plateau represented by the pallatanga unit with the continental margin this accreted unit can be correlated with similar oceanic plateau sequences from the western cordillera of colombia and those within and around the caribbean region the naranjal and macuchi island arcs and the associated la portada back arc basin developed along the accreted margin from late campanian to eocene time and these arcs accreted to the continental margin along with oceanic plateau material represented by the pinon unit and pedernales esmeraldas sequences during eocene time the development of island arcs which separate the two accretionary events implies that the most westerly coastal oceanic plateau sequences both in ecuador pinon and pedernales esmeraldas and in colombia gorgona and serrania de baudo cannot belong to the caribbean colombian oceanic plateau ccop it therefore appears that at least two different oceanic plateaux are preserved within the accreted oceanic terranes of the northern andes it is possible that the ccop formed over the galapagos hotspot as previously proposed but the more westerly coastal plateau was derived from a more southerly hotspot source region such as sala y gomez in the se pacific

ionospheric and birkeland current distributions for northward interplanetary magnetic field inferred polar convection a new stable system of birkeland and ionospheric currents has been added to the existing region 1 region 2 auroral electrojet system when the interplanetary magnetic field imf is directed northward these currents exist poleward of the region 1 system across the polar region and have been called the northward imf bz nbz current system a companion paper iijima et al this issue describes in detail the birkeland currents magnetic signature a w shaped pattern in the sunward antisunward direction and the nbz system dependence on the various components of the imf the companion study used mag sat magnetometer data from 146 orbits over the southern polar regions from november 1979 to january 1980 data from two days november 11 1979 and january 8 1980 have been analyzed in great detail with regard to the nbz w signature of birkeland currents in addition a fourier technique has been used in an analysis of the disturbance component parallel to the main field to derive the ionospheric current system during those two days the nbz ionospheric current system has shown a similar w shaped pattern with antisunward current implying sunward convection over the magnetic pole and with return currents on either side multicell patterns emerge from the nbz ionospheric patterns and an asymmetry appears to be caused by the imf by component as well suggestions are given for multicell patterns over the polar regions for northward imf during an imf by sign change

multi element variations in olivine as geochemical signatures of ni cu sulfide mineralization in mafic magma systems examples from voisey s bay and pants lake intrusions labrador canada olivine from the olivine gabbro to troctolite intrusions at voisey s bay and at pants lake labrador was analyzed for multiple elements ca sc mg si ti v cr mn fe co ni cu zn sr y and zr with secondary ion mass spectrometry sims both intrusions have similar lithologies and petrographic characteristics and are approximately coeval 1 34 and 1 32 ga respectively members of the mesoproterozoic nain plutonic suite the voisey s bay intrusion hosts a producing economic ni cu sulfide deposit whereas the pants lake intrusion displays evidence of ni cu sulfide mineralization but to date a viable ore deposit has not been discovered olivine from both barren and mineralized lithologies was analyzed to assess the potential of the olivine composition for providing a record of silicate melt evolution and sulfide saturation related to formation of a massive sulfide deposit two detailed transects were sampled using five diamond drill holes that laterally approach the basal massive sulfide in the eastern deeps portion of the voisey s bay intrusion from the barren central part olivine displays distinct trace element distributions that vary coherently with host lithology and proximity to sulfide mineralization in particular olivine shows an increase in fe fo80 to fo60 mn 2500 to 5000 ppm and zn 280 to 700 ppm generally coupled with a decrease in ni 1600 to 900 ppm co 270 to 190 ppm and cr 110 to 45 ppm from barren troctolite normal troctolite nt and weakly mineralized troctolite variable textured troctolite vtt towards the heavily mineralized brecciated basal succession basal breccia sequence bbs the enrichment in fe mn zn is most pronounced in samples that laterally approach but do not directly intersect the massive sulfide deposit at the base of the intrusion particularly in samples from the lower variable textured troctolite and the basal breccia olivine from gabbro lithologies within the basal gabbro bg and upper gabbro ug of the pants lake intrusion is fairly homogeneous fo60 4300 ppm mn 460 ppm zn 340 ppm ni 50 ppm cr with a more evolved composition 2100 ppm ca 20 ppm sc 300 ppm ti 4 ppm y than the average olivine of the voisey s bay intrusion 570 ppm ca 6 ppm sc 65 ppm ti 0 3 ppm y pants lake olivine also commonly lacks the mutually mn zn rich signature of olivine from the basal breccia at voisey s bay that is characteristic of contamination of the parent magma by country rock gneiss and there reflects a close proximity to massive sulfides in conjunction with petrographic observation and the stratigraphic context a multiple element v cr mn fe co ni zn regression can be applied to calculate a lateral proximity of olivine to massive sulfide mineralization in the eastern deeps part of the voisey s bay intrusion over a distance exceeding 150 m the compositional variations in olivine from the economic voisey s bay intrusion bimodal and primitive and the mainly barren pants lake intrusion homogeneous and evolved provide potential as a regional scale mineralogical fertility indicator for mafic intrusions with comparable types of sulfide mineralization

the geological history of northwestern south america from pangaea to the early collision of the caribbean large igneous province 290 75ma abstract northwestern south america preserves a record of the assembly of western pangaea its disassembly and initiation of the far western tethys wilson cycle subsequent pacific margin magmatism and ocean plateau continent interaction since the late cretaceous numerous models have been presented for various time slices although they are based on either spatially restricted datasets or dates that are inaccurate estimates of the time of crystallisation here we review a very large quantity of geochronological geochemical thermochronological sedimentological and palaeomagnetic data that collectively provide tight constraints for geological models these data have been collected over a trench pacific parallel distance of 1500 km colombia and ecuador and reveal important temporal trends in rifting and subduction the temporal framework for our model constraints are obtained from robust concordant zircon u pb ages of magmatic rocks during 290 75 ma the late cretaceous thermal history of the margin

contemporaneous paleogene arc magmatism within continental and accreted oceanic arc complexes in the northwestern andes and panama abstract the basement rocks of the northwestern andes are composed of accreted fault bound slivers of the cretaceous caribbean large igneous province clip previous geologic mapping geochronological and geochemical data show that paleogene arc like units make up part of the accreted clip in sw colombia timbiqui complex nw colombia santa cecilila la equis complex and at the boundary between colombia and panama acandi region similar paleogene igneous units have been reported in ecuador silante and macuchi arc and panama chagres bayano arc these arc like rocks are commonly interpreted to have formed as part of an intra oceanic arc before accretion to the continental margin alternative explanations suggest that the paleogene igneous rocks in ecuador were formed within a continental magmatic arc we compare paleogene igneous rocks in the san blas complex chagres bayano arc and acandi region santa cecilia la equis complex and the sw complex timbiqui complex silante and macuchi arcs using geochemical major and trace elements geochronological zircon u pb and isotopic zircon hf and whole rock pb sr nd isotopes data zircon u pb analyses yield ages of 46 42 ma for the acandi region 59 51 ma for the santa cecilia la equis complex and 44 42 ma for the timbiqui complex major and trace element data show low k to calc alkaline compositions with negative nb anomaly and lree hree enrichment typically attributed to arc like signatures for the three complexes pb sr and nd whole rock and zircon lu hf isotopic data for the santa cecilia la equis and the san blas complexes are consistent with a depleted mantle source mixed with pacific sediments however the presence of inherited proterozoic zircons elevated 207pb 204pb ratios in some evolved igneous rocks and crystallization under low pressure conditions suggest assimilation of continental derived sediments these data indicate that the santa cecilia la equis and the san blas complexes likely originated in an intra oceanic island arc setting in a close proximity to the south american margin before accretion to the andean margin in the case of the sw complex the absence of eu anomaly slightly depleted hree as well as increased sr y values with fractionation indicate that these magmas formed in a moderately mature arc within a mid shallow crust level moreover radiogenic pb and sr isotopic values for the less evolved igneous rocks of the sw complex evolving to less radiogenic values suggest involvement of continental sediments in the parental magma source and assimilation of the mafic basement crust this indicates that the paleogene magmatic rocks of the sw complex originated in a continental arc like setting within previously accreted slivers of the clip

dynamic variation of the auroral oval during intense magnetic storms the latitudinal variations of the noon sector polar cusp region and the nightside auroral oval were examined to investigate the auroral oval dynamics during three intense geomagnetic storms the variations were compared with the variation in ring current intensity i e dst and with changes of the interplanetary magnetic field bz component to determine the dominant parameter of the large scale polar region configuration changes during magnetic storms deviations of the polar cusp and the midnight auroral oval from their normal quiet time position are also compared to determine the occurrence of the large scale reconfiguration of the polar region geometry it is found that a the large 10 geomagnetic latitude equatorial shift of the auroral oval occurs in coordination with the equatorward motion of the polar cusp region b the polar cusp is displaced by a few degrees more than the nightside auroral oval near the peak of a magnetic storm c the midnight auroral oval recovers more slowly than the polar cusp region during the storm recovery phase and d the midnight oval and the noon sector polar cusp region move coherently with the southward variation of the interplanentary magnetic field bz but not necessarily with the dst intensity variations

queen alexandra range 93148 a new type of pyroxene pallasite trace elements including the rare earth elements were measured in olivine and orthopyroxene from queen alexandra range que 93148 and in olivine from two main group pallasites springwater and mount vernon although que 93148 was originally classified as a lodranite a variety of data including oxygen isotopic compositions goodrich and righter 2000 preclude a genetic relationship with the acapulcoites lodranites incompatible trace element e g ti zr distributions in orthopyroxene do indicate large amounts of melting and are consistent with the ultramafic assemblage observed in this meteorite trace element abundances in olivine are consistent with suggestions that que 93148 may be related to the main group pallasites goodrich and righter 2000 although there are some inconsistencies its trace element distributions are most like those of the pyroxene pallasites suggesting that it may have formed in a similar manner que 93148 may represent a new type of pyroxene pallasite with links to the main group pallasites

cretaceous basaltic terranes in western colombia elemental chronological and sr nd isotopic constraints on petrogenesis accreted terranes comprising mid to late cretaceous picrites basalts and dolerites occur in three north south trending belts in western colombia in the central cordillera western cordillera and along the pacific coast the geochemistry of these rocks is consistent with an oceanic plateau plume related origin and they most probably formed in the pacific as part of the caribbean oceanic plateau these igneous rocks display small but significant inter cordillera variations being younger and more depleted in incompatible trace element ratios and with more positive end values to the west the igneous rocks of the pacific coast serrania de baudo are dated at 73 78 ma 40ar 39ar and those of the western cordillera at 90 ma whereas the volcanics of the central cordillera are believed to be older than 100 ma most of the igneous rocks are basaltic and it is suggested that they have fractionated from picritic primary magmas generated by partial melting within a hot mantle plume variable and positive end values reveal that the plume must have been heterogeneous originating from a mantle source with a long term history of depletion partial melt modelling suggests that the composition of the basalts requires at least some input from a mantle source region containing garnet and that the extent of partial melting required to reproduce the composition of the erupted basalts is of the order of 20 mixing of melts from different depths either in the mantle melting column or during fractionation in lithospheric magma chambers can explain the relative homogeneity of basaltic lavas erupted to form this and other oceanic plateaux the caribbean colombian oceanic plateau may have formed at an oceanic spreading centre and valuable comparisons can be made between iceland and the caribbean colombian plateau

latitudinal electron precipitation patterns during large and small imf magnitudes for northward imf conditions we demonstrate that there are distinct differences in the electron precipitation patterns or the polar cap size geomagnetic activity and field aligned currents in the highest latitude region for small and large imf bz values when the imf bz component is positive first during periods of weakly northward imf there is a distinct area in the highest latitude region in which the electron precipitation is absent except for the polar rain by contrast during strongly northward imf the entire polar region is often filled with burst type soft electron precipitations second geomagnetic disturbances and field aligned current intensitites in the highest latitude region 80 in geomagnetic latitude are less during a weak imf bz condition than those during a strongly northward imf bz condition geomagnetic activity in the auroral zone for both conditions is absent or very weak

auroral morphology of the midday oval auroral displays in the noon sector were examined by using hundreds of defense meteorological satellite program auroral images taken over the southern polar region in austral winters in order to determine the morphology the auroral displays in the midday part of the auroral oval can be grouped into five characteristic types depending on the geomagnetic activity and the bz component of the interplanetary magnetic field an important characteristic is the clear disconnection in appearance between the noonside and the nightside discrete auroras also there is the lack of correlation between the concurrent nightside substorm activity and midday discrete auroral activity thus the occurrence of bright discrete auroras in the midday oval may be caused by the local injection of the magnetosheath plasma into the dayside boundary layer the observed discontinuity of discrete auroras between the dayside and the nightside ovals is consistent with the existence of two separated major injection regions along the auroral oval the dayside cusp and the nightside plasma sheet

unexpected features of the ion precipitation in the so called cleft low latitude boundary layer region association with sunward convection and occurrence on open field lines ion precipitations along the boundary of the polar cap are studied using the akebono exos d observations of plasma electric field and magnetic field during the dawn dusk crossings across the dayside polar cap at the altitudes of 7500 km to 10 000 km only the cases where the convection streamlines consisted of two cells rather than four have been chosen for analysis and hence the kp index was moderate or high and the imf polarity was southward in most of the intervals studied it is found that the ion precipitations in morning and evening sectors along the polar cap boundary are associated with the sunward convection at the above heights and judging from the presence of the polar rain the high latitude part of these precipitations occur in the region of open field lines association with the sunward convection and occurrence on open field lines contradict the view that these ions originate from the low latitude boundary layer llbl on closed field lines we propose to designate these ions as circumpolar ion precipitation cpip in order to avoid questionable presumptions on their source region the sunward convection in the morning and evening sectors of the cpip is observed systematically has speeds of about 1 km s and often extends over a few degrees or more in invariant latitude the cpip ions may have leaked from the boundary region of the plasma sheet may have been generated in the distant neutral sheet or may have entered from the magnetosheath only within a few hours of the noon meridian the cpip ions are associated with the antisunward flow at the height range of the akebono observations

extremely high latitude auroras over 1600 defense meteorological satellite program north polar images from the winters 1972 1973 1974 1975 were studied for the occurrence of auroral arcs at corrected geomagnetic latitudes greater than 80 auroras at these latitudes are designated extremely high latitude auroras and are categorized by the local time sectors in which they occur and their relationship to discrete oval arcs a data base of 300 cases of cap auroras was formed for correlation studies with geomagnetic activity indices and solar wind parameters by far the greatest number of extremely high latitude auroras occur in the morning sector and appear to be expansions of the system of morning oval arcs poleward the remaining categories include evening arcs expanding into the cap single sun aligned arcs and midnight sector oval auroras expanding into the cap except for the last category the auroras occur for similar magnetospheric conditions moderate values of kp and dst but comparatively low values of ae they also occur preferentially for high solar wind speed and for positive bz while far less clear their location in the cap shows a dependence on by morning evening arcs occur with greater frequency for negative positive by

dubious case for slab melting in the northern volcanic zone of the andes it was first suggested by r w kay that adakites may represent melts of subducted slab since then the term adakite has become synonymous with slab melts based on their unusual geochemical signature this contribution using the northern volcanic zone of the andes as an example aims 1 to expose the weakness in simply associating a geochemical signature with a genetic mechanism and 2 to underline the importance of using several integrated lines of evidence in assessing the viability of slab melting we conclude that although slab melting probably occurs in some arcs regional geochemical trends in the northern volcanic zone and their relationship to the subduction zone architecture are not indicative of slab melting and can be accounted for by normal arc magmatic processes acting on wedge derived basaltic magmas

arc continent collision and orocline formation closing of the central american seaway closure of the central american seaway was a local tectonic event with potentially global biotic and environmental repercussions we report geochronological six u pb la icp ms zircon ages and geochemical 19 xrf and icp ms analyses data from the isthmus of panama that allow definition of a distinctive succession of plateau sequences to subduction related protoarc to arc volcaniclastic rocks intruded by late cretaceous to middle eocene intermediate plutonic rocks 67 6 1 4 ma to 41 1 0 7 ma paleomagnetic analyses 24 sites 192 cores in this same belt reveal large counterclockwise vertical axis rotations 70 9 6 7 and moderate clockwise rotations between 40 4 1 and 56 2 11 1 on either side of an east west trending fault at the apex of the isthmus rio gatun fault consistent with isthmus curvature an oligocene miocene arc crosscuts the older deformed and segmented arc sequences and shows no significant vertical axis rotation or deformation there are three main stages of deformation 1 left lateral strike slip offset of the arc 100 km and counterclockwise vertical axis rotation of western arc segments between 38 and 28 ma 2 clockwise rotation of central arc segments between 28 and 25 ma and 3 orocline tightening after 25 ma when this reconstruction is placed in a global plate tectonic framework and published exhumation data is added the central american seaway disappears at 15 ma suggesting that by the time of northern hemisphere glaciation deep water circulation had long been severed in central america

the paleogene arcs of the northern andes of colombia and panama insights on plate kinematic implications from new and existing geochemical geochronological and isotopic data abstract the northern andes of colombia comprise two non cogenetic paleogene arcs formed in contrasting geodynamic settings including continental and oceanic domains new whole rock geochemistry and isotopic constraints together with a review of 332 geochemical 76 isotopic and 204 geochronological data from paleocene to eocene volcanic and plutonic rocks exposed in central and western colombia and panama are used to evaluate cause effect relations between regional plate kinematics and the spatio temporal distribution of the circum caribbean magmatic arcs short lived 60 45 ma arc like magmatism in the central cordillera of colombia was emplaced in a thickened continental crust due to the oblique subduction of the caribbean oceanic plate underneath south america as suggested by the high sr y ratios conversely the panama arc and its poorly explored extension in the northwestern segment of the western cordillera of colombia document a major phase of tholeiitic to calc alkaline arc magmatism between 71 ma and 34 ma mostly derived from a hydrated mantle wedge and emplaced in an oceanic plateau crust this arc record a major phase of magmatic activity between 40 ma and 50 ma that can be associated to changes in the convergence direction and subduction rates of the farallon plate farther to the south in the pacific region of colombia eocene arc related rocks from the timbiqui complex show a geochemical signature that suggests a magmatic origin from melting of both a subduction modified mantle and lower tectonically thickened crust a possible interpretation for the pacific and western colombia panama arc systems and its analogous in ecuador recall an aleutian type convergent margin in which the continental arc laterally switches to a purely oceanic system along the trailing edge of the caribbean plate as consequence of eastward subduction of the farallon plate

clinopyroxene ree distribution coefficients for shergottites the ree content of the shergotty melt abstract ree distribution coefficients were measured between synthetic pyroxenes and melts similar in composition to the inferred shergotty intercumulus liquid by in situ analysis with the electron microprobe using samples doped to per cent concentration levels these coefficients exhibit a strong positive correlation with pyroxene wollastonite content recommended values for augites similar in composition to the shergotty cumulus augite cores wo32en48 are d la 0 023 d ce 0 039 d nd 0 10 d sm 0 17 d eu 0 16 d gd 0 20 d yb 0 29 and d lu 0 30 recommended values for pigeonites similar in composition to the shergotty cumulus pigeonite cores wo12en58 are d la 0 002 d ce 0 004 d nd 0 019 d sm 0 031 d yb 0 13 and d lu 0 13 these values resemble distribution coefficients measured for terrestrial basalts of higher mg fe than the shergotty melt rather than those of more evolved siliceous rocks of similar mg fe to the shergotty melt thus it appears that mg fe is considerably less important than other factors in determining distribution coefficient values for pyroxenes ree abundances were computed for the shergotty intercumulus melt using distribution coefficients measured for the natural phase compositions these computed abundances display lree depletion such lree depleted abundance patterns cannot be generated by simple partial melting of a mafic mineral assemblage having the lree enriched patterns required for a two stage sm nd evolution history this result implies that either 1 the melting process was complex e g continuous melting 2 the source region had a more complex history than that of the simple 2 stage model or 3 shergotty does not satisfy the closed system assumption of the calculation and has suffered post magmatic lree enrichment

chondrule thermal history from unequilibrated h chondrites a transmission and analytical electron microscopy study sixteen texturally different porphyritic barred radial cryptocrystalline feo rich chondrules from the unequilibrated ordinary chondrites brownfield frontier mountain fro 90003 and fro 90032 were characterized by optical and scanning electron microscopy and then thoroughly studied by transmission and analytical electron microscopy nanotextural and nanochemical data indicate similar thermal evolution for chondrules of the same textural groups minor yet meaningful differences occur among the different groups olivine is the earliest phase formed and crystallizes between 1500 and 1400 c protoenstatite crystallizes at temperatures higher than 1350 1200 c it later inverts to clinoenstatite in the 1250 1200 c range enstatite is surrounded by pigeonitic or less frequently augitic rims the minimal crystallization temperature for the rims is 1000 c high pigeonite later inverts to low pigeonite between 935 and 845 c the outer pigeonitic or augitic rims are constantly exsolved producing sigmoidal augite or enstatite precipitates sigmoidal precipitates record exsolution temperatures between 1000 and 640 c cooling rate determined using the speedometer based upon ortho clinoenstatite intergrowth was in the order of 50 3000 c h at the clinoenstatite orthoenstatite transition temperature close to 1250 1200 c but decreased to 5 10 c h or slower at the exsolution temperature between 1000 and 650 c thus revealing nonlinear cooling paths nanoscale observations indicate that the individual chondrules formed and cooled separately from 1500 down to at least 650 c accretion into chondritic parent body occurred at temperatures lower than 650 c

setting characteristics and age of the andean porphyry copper belt in colombia three distinct porphyry copper subbelts are defined in the colombian andes 1 a western subbelt emplaced during the early to middle eocene in an oceanic domain underlain by ophiolitic basement along the western side of the western cordillera 2 a composite eastern subbelt generated during the middle jurassic to earliest cretaceous in a continental domain underlain by elements of the guyana shield along the eastern side of the central cordillera and the western side of the eastern cordillera and between the two 3 a central subbelt formed in the early to late miocene at the contact of the oceanic and continental domains along the cauca patia depression and its flanks k ar geochronometry undertaken as part of this study provided preferred minimum ages of 54 7 or 1 3 m y for murindo 48 1 or 1 0 m y for acandi and 42 7 or 0 9 m y for pantanos pegadorcito in the western subbelt 166 or 4 m y for mocoa 166 or 4 m y for dolores 144 or 3 m y for california and 131 or 2 m y for infierno chili in the eastern subbelt and 17 4 or 0 4 m y for piedrasentada in the central subbelt the porphyry copper prospects are associated with dacite to tonalire porphyry stocks which in the western and eastern subbelts intruded batholiths emplaced perhaps 15 to 30 m y earlier but which in the central subbelt are isolated within metamorphic or sedimentary host rocks the porphyry copper systems exhibit a variety of alteration patterns with potassium silicate and sericitic alteration important in the highest grade prospects and propylitization predominant in low grade occurrences none of the prospects displays significant supergene enrichment although porphyry coppers in the continental domain tend to be more molybdenum rich than those in the oceanic domain gold rich prospects occur in both settings as do major lode gold deposits the jurassic and miocene subbelts are interpreted to have been emplaced during eastward subduction at andean type continental margins whereas the western eocene subbelt could have been generated in either a modified andean type setting or in an intraoceanic island arc prior to its accretion to the continental edge

the early interaction between the caribbean plateau and the nw south american plate the determination of accurate and precise ages for the timing of collision between oceanic plateaus and continental crust requires an understanding of how the indenting and buttressing plates respond to the collision we present geochronological thermochronological geochemical and isotopic analyses of magmatic rocks from the ecuadorian andes which relate to the collision of the late cretaceous caribbean plateau and great arc sequence with nw south america the cessation of subduction magmatism during 65 64 ma beneath the eastern edge of caribbean plateau was synchronous with accelerated surface uplift and exhumation within the buttressing continental margin during 75 65 ma we interpret this as the collision of the leading edge of the caribbean plateau and arc sequence with the south american plate at 75 65 ma a u pb zircon shrimp age of 87 10 1 66 2 sigma ma yielded by an accreted fragment of the plateau precludes previous estimates of collision at 85 80 ma if the plateau erupted above the galapagos hotspot less