chemical studies of h chondrites i mobile trace elements and gas retention ages abstract trends for 16 trace elements ag as au bi cd co cs ga in k rb sb se te tl and zn chosen to span a broad geochemical and thermal response range in 44 h4 6 chondrites differ widely from those in l4 6 chondrites in particular h chondrites classified as heavily shocked petrologically do not necessarily exhibit 40 ar loss and vice versa the clear cut causal relationship between siderophile and mobile element loss with increasing late shock seen in l chondrites is not generally evident in the h group h chondrite parent material experienced an early high temperature genetic episode that mobilized a substantial proportion of these trace elements so that later thermal episodes resulted in more subtle collateral fraction ations mildly shocked l chondrites escaped this early high temperature event indicating that the two most numerous meteorite groups differ fundamentally in genetic history

187re 187os systematics highly siderophile element s se te abundances in the components of unequilibrated l chondrites abstract the 187re 187os systematics abundances of highly siderophile elements hse re platinum group elements and au te se and s as well as major and minor elements were determined in separated components of two unequilibrated l chondrites que 97008 l3 05 and ceniceros l3 7 the 187re 187os systematics are disturbed in the components of both meteorites most likely due to open system behavior of re during terrestrial weathering of que 97008 and alteration on the l chondrite parent body as indicated by an internal errorchron generated for components of ceniceros the hse abundance patterns suggest that the bulk rock abundances were mainly controlled by two different end members non magnetic fractions display lower re os and hse ir than ci chondrites chondrules metal troilite spherules and fine magnetic fractions are depleted in refractory hse and show higher rh ir pd ir and au ir than in ci chondrites the different hse compositions indicate the presence of unequilibrated alloys and loss of refractory hse rich carrier phases from the precursors of some l chondrite components gold is decoupled from other hse in magnetic fractions and shows chalcophile affinities with a grain size dependent variation similar to s and se presumably inherited from preaccretionary processes tellurium is depleted in all components compared to other analysed siderophile elements and its abundance was most likely controlled by fractional condensation and different geochemical affinities the volatility dependent depletion of te requires different physical and chemical conditions than typical for the canonical condensation sequence as represented by carbonaceous chondrites tellurium also shows variable geochemical behavior siderophile in ceniceros predominantly chalcophile in que 97008 these differences may have been inherited from element partitioning during chondrule formation selenium and s on the other hand are almost unfractionated from each other and only show complementary s se in a few components presumably due to the effects of volatility or metal silicate partitioning during chondrule formation terrestrial weathering had negligible effects on the s se and te systematics

chemical characteristics and origin of h chondrite regolith breccias abstract we report petrologic data and contents of ag bi cd co cs ga in rb se te tl and zn trace elements spanning the volatility mobility range in light and dark portions of h chondrite regolith breccias and l chondrite fragmental breccias the chemical petrologic characteristics of h chondrite regolith breccias differ from those of non brecciated chondrites or fragmentai breccias petrologic characteristics and at least some trace element contents of h chondrite regolith breccias reflect primary processes contents of the most volatile mobile elements may reflect either primary or secondary processing possibly within layered h chondrite parent object s chemical petrologic differences existed in different regions of the parents regolith formation and gardening and meteoroid compaction were not so severe as to alter compositions markedly

chemical studies of l chondrites vi variations with petrographic type and shock loading among equilibrated falls to study compositional trends associated with open and closed system metamorphism and or shock induced heating of the l4 6 chondrite parent s we used icpms and rnaa to quantify 51 trace elements in 48 chemically representative fall samples with these data we used graphic and two multivariate statistical methods for examining evidence for compositional differences with respect to petrographic type and degree of shock loading comparisons of mildly shocked s1 s3 l5 and l6 suites 9 and 8 chondrites respectively yield no convincing statistical evidence for a difference in trace element content our multivariate comparisons show a difference on a model dependent basis but yield indeterminate results on a model independent basis compositionally suites of strongly shocked s4 s6 and mildly shocked l4 6 chondrites 26 and 19 samples respectively can be distinguished at statistically significant levels on both model dependent and independent bases in the strongly shocked suite contents of refractory lithophiles are higher and siderophiles and volatiles are lower than those of the mildly shocked suite at moderately p 0 05 to highly significant p 0 01 levels our studies suggest that chemical differences from vaporization and loss of volatiles along with metal silicate partitioning are present from extended cooling of shock heated bodies produced by intermittent impacts especially the massive impact s that disrupted the l chondrite parent s 500 ma ago

imaging a crustal low velocity layer using reflected seismic waves from the 2014 earthquake swarm at long valley caldera california the magmatic system roof

an igneous textured clast in the peace river meteorite insights into accretion and metamorphism of asteroids in the early solar system the mineralogy petrology and geochemistry of an igneous textured clast in the peace river l6 chondrite meteorite was examined to determine the roles of nebular processes accretion and parent body metamorphism in its origin the centimetre scale clast is grey and fine grained and is in sharp contact with the host chondrite two sub millimetre veins cut across both the clast and host indicating that the clast formed prior to the impact shock event s that produced the numerous veins present in the peace river meteorite the clast and host are indistinguishable in terms of mineral compositions in contrast there are differences in modal mineralogy texture as well as trace element and oxygen isotope composition between the clast and host these differences strongly suggest that the clast was formed by impact melting of ll group chondritic material involving loss of fe fes and phosphate components followed by relatively rapid cooling and incorporation into the peace river host meteorite subsequent metamorphism on the peace river parent body caused recrystallization of the clast and homogenization of mineral compositions and thermally labile element abundances between the clast and host shock metamorphism including formation of shock melt veins occurred post metamorphism during fragmentation of the l chondrite parent body the results suggest that the formation of the peace river parent asteroid included the incorporation of material from other asteroids and that the pre metamorphic protolith was a breccia accordingly we propose that the peace river meteorite be reclassified as a polymict breccia

post shock annealing of miller range 99301 ll6 implications for impact heating of ordinary chondrites mil 99301 is an ll chondrite that has experienced successive episodes of thermal metamorphism shock metamorphism and annealing the first recognizable petrogenetic episode resulted in thermal metamorphism of the rock to petrologic type 6 as indicated by homogeneous olivine compositions significant textural recrystallization and the presence of coarse grains of plagioclase metallic fe ni and troilite the source of heat for this thermal episode is not identified the rock also experienced shock metamorphism to shock stage s4 as indicated by extensive silicate darkening caused by the dispersion within silicate grains of thin chromite melt veins and trails of metallic fe ni and troilite blebs polycrystalline troilite myrmekitic plessite a relatively high occurrence abundance oa of metallic cu 3 6 the presence of numerous chromite plagioclase assemblages and coarse grains of low ca clinopyroxene with polysynthetic twinning the shock event responsible for these effects must have occurred after the epoch of thermal metamorphism to type 6 levels otherwise the polycrystallinity of the troilite would have disappeared and the low ca clinopyroxene would have transformed into orthopyroxene despite abundant evidence of strong shock olivine and plagioclase in mil 99301 exhibit sharp optical extinction consistent with shock stage s1 and characteristic of an unshocked rock this implies that an episode of post shock annealing healed the damaged olivine and plagioclase crystal lattices and thereby changed undulose extinction into sharp extinction the rock was probably annealed to metamorphic levels approximating petrologic type 4 more significant heating would have transformed the low ca clinopyroxene into orthopyroxene it is not plausible that an episode of annealing occurring after the epoch of thermal metamorphism could have been caused by the decay of 26al because this isotope would have decayed away by that time impact heating is a more plausible source of post metamorphic annealing of rocks in the vicinity of impact craters on low density high porosity asteroids with rubble pile structures

wave equation multiple imaging wemi swimming between the flags abstract deghosting dual sensor streamer seismic data requires separation of the total recorded wavefield into upgoing and downgoing components during seismic preprocessing imaging of multiples exploits the wavefield separation process by using the downgoing wavefield at each receiver location as a virtual source multiple imaging has been shown to effectively remove shallow acquisition related artifacts commonly seen on wide tow marine seismic surveys and improve imaging quality of the shallow overburden acquisition design factors imaging condition and cross talk noise are the main factors that determine the effectiveness of separated wavefield imaging swim of dual sensor streamer data swim is a common nontrademarked acronym used in the literature to describe multiple imaging of separated wavefields sampled from dual sensor streamer data in a field trial acquisition design did not allow for proper sampling of the total wavefield with the result that multiple imaging did not prove to be as rob

low δ 18 o tephra from a compositionally zoned magma body fisher caldera unimak island aleutians abstract we present the results of an oxygen isotope study of phenocrysts in pumice clasts and ash layers produced by the 9100 yr bp composite dacite basaltic andesite climactic eruption that formed fisher caldera in the eastern aleutians products of the eruption represent a low 18o magma with 18o plagioclase 4 79 0 24 and clinopyroxene 3 81 0 23 corresponding to equilibrium at magmatic temperatures dacitic and overlying basaltic andesitic tephra of the climactic eruption subsequent intracaldera basaltic to andesitic lavas and a cumulate inclusion are similarly low in 18o other analyzed lavas and pyroclastics of unimak island and the lower alaska peninsula as well as precaldera fisher basalt have normal 18o magmatic values 5 5 we propose a model in which prior to 9100 yr bp normal mantle derived basaltic magma coalesced in a large shallow precaldera magma chamber during late wisconsin glaciation lowering of magmatic 18o resulted then from long term assimilation of 5 10 of syn glacial hydrothermally altered country rocks differentiation of basaltic magma was concurrent with this assimilation and produced low 18o fisher dacites cumulates and post caldera crystal richer lavas we propose the use of 18o values of phenocrysts especially alteration resistant pyroxene in tephra as a tool for tephrochronological and tephrostratigraphic correlation distinctly low 18o values are useful in identification of the fisher ash in the eastern aleutians and in the lower alaska peninsula

prestack waveform inversion of three dimensional seismic data an example from the rock springs uplift wyoming usa abstractapplying seismic inversion to estimate subsurface elastic earth properties for reservoir characterization is a challenge in exploration seismology in recent years waveform based seismic inversions have gained popularity but due to high computational costs their applications are limited and amplitude variation with offset angle inversion is still the current state of the art we have developed a genetic algorithm based prestack seismic waveform inversion methodology by parallelizing at multiple levels and assuming a locally 1d structure such that forward computation of wave equation synthetics is computationally efficient this method is capable of inverting 3d prestack seismic data on parallel computers applying this inversion to a real prestack seismic data volume from the rock springs uplift rsu located in wyoming usa we determined that our method is capable of inverting the data in a reasonable runtime and producing much higher quality results than amplitude variation with offset ang

compositions of three low feo ordinary chondrites indications of a common origin with the h chondrites abstract burnwell eet 96031 and lap 04575 are ordinary chondrites oc that possess lower than typical olivine fa content than has been established for the h chondrites

formation of low δ18o rhyolites after caldera collapse at yellowstone wyoming usa we present a new model for the genesis of low 18o rhyolites of the yellowstone caldera based on analyses of zircons and individual quartz phenocrysts low 18o rhyolites were erupted soon after the massive caldera forming lava creek tuff eruption 602 ka 1000 km3 and contain xenocrysts of quartz and zircon inherited from precaldera rhyolites these zircons are isotopically zoned and out of equilibrium with their host low 18o melts and quartz diffusion modeling predicts that magmatic disequilibria of oxygen isotopes persists for as much as tens of thousands of years following nearly total remelting of the hydrothermally altered igneous roots of the depressed cauldron in which the alteration resistant quartz and zircon initially retained their 18o values these results link melting to caldera collapse rule out rapid or catastrophic magma meteoric water interaction and indicate wholesale melting rather than assimilation or partial melting

overview and classification of wavefield seismic imaging methods the literature and seismic processing practice overflow with a diversity of wave equation migration methods so what makes wave equation migration a wave equation migration what is the commonality of these methods and how do they differ this article provides a framework for understanding different wave equation migration methods

yamato 86029 aqueously altered and thermally metamorphosed ci like chondrite with unusual textures we describe the petrologic and trace element characteristics of the yamato 86029 y 86029 meteorite y 86029 is a breccia consisting of a variety of clasts and abundant secondary minerals including coarse and fine grained phyllosilicates fe ni sulfides carbonates and magnetite there are no chondrules but a few anhydrous olivine rich grains are present within a very fine grained phyllosilicate rich matrix analyses of 14 thermally mobile trace elements suggest that y 86029 experienced moderate open system thermal metamorphism comparison with data for other heated carbonaceous chondrites suggests metamorphic temperatures of 500 600 oc for y 86029 this is apparent petrographically in partial dehydration of phyllosilicates to incompletely re crystallized olivine this transformation appears to proceed through intermediate highly disordered poorly crystalline phases consisting of newly formed olivine and residual desiccated phyllosilicate and their mixtures periclase is also present as a possible heating product of mg rich carbonate precursors y 86029 shows unusual textures rarely encountered in carbonaceous chondrites the periclase occurs as unusually large fe rich clasts 300 500 m fine grained carbonates with uniform texture are also present as small 10 15 m in diameter rounded to sub rounded shells of ankerite siderite enclosing magnetite these carbonates appear to have formed by low temperature aqueous alteration at specific thermal decomposition temperatures consistent with thermodynamic models of carbonate formation the fine and uniform texture suggests crystallization from a fluid circulating in interconnected spaces throughout entire growth one isolated aggregate in y 86029 also consists of a mosaic of polycrystalline olivine aggregates and sulfide blebs typical of shock induced melt re crystallization except for these unusual textures the isotopic petrologic and chemical characteristics of y 86029 are quite similar to those of y 82162 the only other heated ci like chondrite known they were probably derived from similar asteroids rather than one asteroid and hence may not necessarily be paired

meteorites found on misfits flat dry lake nevada meteorites have been found on the small misfits flat dry lakebed near stagecoach nevada 119 382w 39 348n since the first find on sept 22 2013 a total of 58 stones of weathering stage w2 3 with a combined mass of 339 g have been collected in 19 visits to the area this small 3 3 3 6 km lakebed is now a newly designated dense collection area dca most meteorites were found in a small 350 180 m area along the north shore and most are fragments of several broken individual stones three of these fragments were classified as an ll4 5 of shock stage s2 now named misfits flat 001 one of which stone mf33 fell 8 1 1 3 ka ago based on the 14c terrestrial age assuming it came from a 20 80 cm diameter meteoroid in addition a small darkly crusted meteorite mf34 now named misfits flat 002 was found 820 m wsw from the main mass this meteorite is classified as an ll5 ordinary chondrite with shock stage s4 5 the meteorite is saturated in 14c at 63 dpm kg 1 suggesting it originated from the center of a 0 5 m diameter meteoroid or deep inside a 1 0 m meteoroid less than 300 yr ago accounts exist of a fireball seen at 13 15 ut on march 2 1895 that are consistent with the find location of misfits flat 002

chemical studies of l chondrites iii mobile trace elements and ar 40 ar 39 ages abstract we report data for trace elements ag au bi cd co cs ga in rb se te tl and zn determined by radiochemical neutron activation analysis in l4 6 chondrites with undisturbed 40 ar release patterns or with patterns showing some disturbance in the 4 4 4 6 gyr plateau indicating shock induced loss mean concentrations are lower many significantly so in 16 chondrites with disturbed patterns than in 4 with undisturbed ones consistent with shock induced mobilization similar trends were noted earlier in l4 6 chondrites having mineralogically observable shock indicators mean concentrations are lower in strongly shocked i e 22 gpa than in mildly shocked 40 ar release patterns are mildly shocked but chondrites with disturbed patterns are more strongly heated on average than those of shock facies d f i e 22 to 57 gpa pooling these populations significantly lower mean concentrations of nearly all trace elements in 26 strongly shocked l4 6 chondrites than in 14 mildly shocked ones indicate loss in shock formed fes fe eutectic and or by vaporization during cooling of shock heated collisional debris two element correlations and the pattern of them i e correlation profiles are also consistent with this picture trace elements can act as thermometers for collisional episodes in l4 6 chondrites but not for earlier thermal fractionations unless compensation can be made for late shock heating

authenticating the recovery location of meteorites the case of castenaso this forensic work aims to authenticate the recovery location of castenaso a 120 g ordinary chondritic l5 meteorite reportedly found in 2003 along the sandy bank of the idice stream near the village of castenaso bologna emilia romagna italy using the hypothesis that castenaso was instead a hot desert meteorite we conducted a comparative mineralogical and geochemical study of major weathering effects on european and saharan ordinary chondrites as potential markers of the environment where castenaso resided during its terrestrial lifetime inductively coupled plasmamass spectrometry icp ms data reveals that castenaso is significantly enriched in sr ba tl and u and suggests geochemical alteration in a hot desert environment the alteration is minor castenaso is not coated by desert varnish and does not show significant light rare earth element lree enrichment or loss of ni and co the apparent contrast in size morphology and composition between the soil particles filling the external fractures of castenaso and those from the bank of the idice stream observed under the scanning electron microscope sem suggests that castenaso did not reside at the reported find location abraded quartz grains up to 1 mm in size in castenaso are undoubtedly from a hot desert eolian environment they are well rounded and show external surfaces characterized by the presence of dish shaped concavities and upturned silica plates that have been subject to solution precipitation and subsequent smoothing we therefore conclude that castenaso is one of the many hot desert ordinary chondrite finds probably from the sahara that is currently available on the market this forensic work provides the scientific grounds for changing the name of this meteorite

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

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

mount dutton volcano alaska aleutian arc analog to unzen volcano japan abstract holocene eruptions from mount dutton a small late quaternary volcano near the tip of the alaska peninsula bear strong physical and petrologic similarities to the 1990 1995 unzen fugendake eruption in japan the volcano had a protracted phase of effusive calcalkaline andesitic 54 59 wt sio 2 cone building in the late pleistocene followed by an abrupt switch to more silicic 65 wt sio 2 lavas emplaced as a central summit cluster of steep sided domes beginning in the early holocene the flanks of the volcano are mantled by pyroclastic flows debris flows and talus formed as a result of gravitational dome collapse disequilibrium mineral assemblages including coexisting quartz and olivine in eruptive episodes ranging from the initial cone building basaltic andesite lavas to the latest holocene dacite domes suggest extensive magma mixing in addition up to meter sized pillow like cognate mafic enclaves of hornblende plagioclase glass are common in the latest of the summit dacite domes mineralogical evidence and bulk chemical data indicate the enclaves represent a high alumina basalt parent with variable and subordinate reservoir contaminant and the host lava is reservoir magma with variable and subordinate basaltic contaminant mount dutton s history and petrology can be interpreted as reflecting the monotonous repetitive intrusion of mantle derived mafic magma into a silicic crystal rich crustal reservoir during the holocene these injections resulted in the extrusion of partially crystallized viscous sticky central domes which typically failed by collapse resulting in small volume merapi type flowage deposits we speculate that slow introduction of mafic magma into the silicic chamber leads both to enclave formation and to the effusive eruption style mount dutton volcano experienced severe shallow earthquake swarms in 1984 1988 and to a lesser extent in 1991 although none of these swarms resulted in an eruption their epicenter distribution and volcanic tectonic character indicate they recorded the movement of magma at shallow depth beneath the volcano the lessons of unzen and montserrat suggest that despite limited tephra production these small volcanic centers with central dome complexes present a serious hazard to life and property

h l chondrite lapaz icefield 031047 a feather of icarus abstract antarctic meteorite lap 031047 is an ordinary chondrite composed of loosely consolidated chondritic fragments its petrography oxygen isotopic composition and geochemical inventory are ambiguous and indicate an intermediate character between h and l chondrites petrographic indicators suggest lap 031047 suffered a shock metamorphic overprint below 10 gpa which did not destroy its unusually high porosity of 27 vol metallographic textures in lap 031047 indicate heating above 700 c and subsequent cooling which caused massive transformation of taenite to kamacite the depletion of thermally labile trace elements the crystallization of chondritic glass to microcrystalline plagioclase of unusual composition and the occurrence of coarsely crystallized chondrule fragments is further evidence for post metamorphic heating to 700 750 c however this heating event had a transient character because olivine and low ca pyroxene did not equilibrate nearly complete degassing up to very high temperatures is indicated by the thorough resetting of lap 031047 s ar ar reservoir 100 55 ma ago a noble gas cosmic ray exposure age indicates it was reduced to a meter size fragment at

trace element and textural evidence favoring lunar not terrestrial origin of the mini granite in apollo sample 14321 abstract the 14321 lunar granite 14321g has recently been reinterpreted bellucci et al 2019 as a piece of the hadean earth impact transported to the moon in principle samples of such derivation may afford important insights into the nature of earth s hadean crust we have tested the terrestrial provenance hypothesis by comparing trace element data from 14321g versus other lunar evolved rocks and a large data base for terrestrial granites volatile trace metals zn ga and ge are depleted in 14321g below the terrestrial granite ranges and relative to the terrestrial granite averages by factors of 27 2 0 and 18 respectively evidence from shocked chondrites martian meteorites and impact shock studies in general indicates that such major depletions are unlikely to develop without near complete shock melting which clearly did not occur in the mostly still crystalline 14321g moreover other aspects of compositional disparity between 14321g and terrestrial granites involve exclusively refractory trace elements compared to terrestrial granites of similarly high ba content 14321 is enriched in ta by a factor of 10 and the few terrestrial granites that are as ta rich as 14321 have 10 times lower ba the refractory element ratio lu sm is also close to 10 times higher in 14321g than in terrestrial granites of similarly high ba content other highly evolved lunar rocks felsites strongly resemble 14321g in all these respects we conclude that 14321g is probably of wholly lunar derivation this finding stands in contradiction to a recent inference from ti in quartz modeling bellucci et al 2019 that 14321g crystallized at a pressure of 0 69 gpa the geodynamically limited moon was presumably never capable of forming or burying such a highly granitic material 100 km below the base of its crust nor of excavating material from such a depth to the surface

chemical studies of l chondrites ii shock induced trace element mobilization abstract previous studies of chondrites heated in the laboratory for extended periods under conditions approximating those in shock heated collisional debris indicate that au co se ga rb cs te bi in ag zn tl and cd progress in mobility we report data for these 13 trace elements in 14 l4 6 chondrites of established shock history and discuss these and 13 additional chondrites studied earlier trace element contents vary with petrologic type s fe sub group and shock history the last dominating strongly absolute abundances and interelement relationships for the 6 or 7 most mobile elements vary with degree of shock loading i e residual temperatures established from mineralogic petrologic study a tertiary process shock heating previously known to have affected radiogenic 40 ar and or 4 he in meteorites but not other elements apparently was at least as effective as other open system processes secondary parent body and primary nebular and or accretionary episodes in establishing mobile trace element contents of l chondrites and probably others if conditions during early genetic episodes are to be deduced from compositional information shocked meteorites should be avoided or effects of later processes should be compensated for

oxygen isotope constraints on the petrogenesis of aleutian arc magmas the first measurement of 18 o 16 o ratios of plagioclase clinopyroxene orthopyroxene and titanomagnetite phenocrysts from modern aleutian island arc lavas provides new insight and independent constraints on magma sources and intracrustal processes basalts are heterogeneous on the scale of the entire arc 18 oplag 5 5 6 7 and individual volcanic centers ranges of 0 3 to 0 8 combined with sr isotope and trace element data 18 oplag values suggest a variable magma source characterized by differences in the mantle wedge or the subducted sediment component along the volcanic front seven tholeiitic basalt to rhyodacite 50 71 sio 2 lavas from the seguam volcanic center have nearly identical 18 oplag values of 6 0 0 2 reflecting extensive closed system plagioclase dominated crystal fractionation oxygen isotope thermometry and pyroxene and oxide equilibria indicate that differentiation occurred between 1150 100 c basalt and 950 100 c rhyodacite in contrast 18 oplag values of 12 calc alkalic basaltic andesites and andesites 53 62 sio 2 from the smaller kanaga volcanic center span a broader range of 5 9 6 6 and consist of mostly higher values isotopic disequilibrium in the kanaga system is manifest in two ways two types of basaltic inclusions with contrasting 18 o values 6 0 and 6 5 occur in one andesite 18 oplag 6 6 and in two other andesites plagioclase titanomagnetite and clinopyroxene titanomagnetite oxygen isotope temperatures are inconsistent one andesite however yields concordant oxygen isotope temperatures of 850 100 c and a two pyroxene temperature of 920 50 c assimilation of isotopically heavy crustal wall rock and mixing off basaltic and andesitic magmas explains the variable and high 18 oplag values and low eruptive temperature of the kanaga andesites

chemical studies of h chondrites 8 on contemporary meteoroid streams using date and time of fall and petrographic classification as criteria many equilibrated h chondrites that fell during september and october from 1812 to the present form four significant clusters denoted as cluster 2 through cluster 5 on day year plots using radiochemical neutron activation analysis we determined 15 trace elements u au co sb ga rb ag se cs te zn cd bi tl and in ordered by increasing putative volatility during nebular condensation in 27 members of these four clusters we used model dependent and model independent multivariate statistical techniques to compare contents of the 10 most volatile elements separately in the four clusters with those of a 33 member suite of random h chondrite falls from 1773 to 1970 the clusters 2 and 5 suites that fell in september 1880 1991 and october 1919 1984 respectively each of which is represented by 10 h chondrite falls are not compositionally distinguishable from the suite of random falls however the 17 member combined suite of clusters 3 and 4 chondrites that fell during september october 1812 1992 proves compositionally distinguishable from random falls at moderate to strong significance levels of 0 01 0 001 this 17 member suite is less readily distinguished from random falls than are the previously reported suite of cluster 1 falls may 1855 1895 or antarctic h chondrites with nominal terrestrial ages 50 kyr each of which is highly significant at 0 001 levels all suites are genomict and exhibit a range of cosmic ray exposure ages with a plurality having 6 8 ma ages inconclusive results are obtained in the cases of clusters 2 and 5 however three h chondrite suites clusters 1 3 and 4 distinguishable from the random background by one property time of fall are also distinguishable by another contents of volatile trace elements or thermal history temporal change of h chondrite sources sampled by earth are indicated by these data

silver isotope variations in chondrites volatile depletion and the initial 107pd abundance of the solar system abstract the extinct radionuclide 107pd decays to 107ag half life of 6 5 ma and is an early solar system chronometer with outstanding potential to study volatile depletion in the early solar system here a comprehensive ag isotope study of carbonaceous and ordinary chondrites is presented carbonaceous chondrites show limited variations e107ag 2 1 to 0 8 in ag isotopic composition that correlate with the pd ag ratios assuming a strictly radiogenic origin of these variations a new initial 107pd 108pd of 5 9 2 2 10 5 for the solar system can be deduced comparing the pd ag and mn cr data for carbonaceous chondrites suggests that mn cr and pd ag fractionation took place close to the time of calcium aluminium rich inclusion cai and chondrule formation 4568 ma ago using the new value for the initial 107pd abundance the revised ages for the iron rich meteorites gibeon iva 8 5 3 2 4 6 ma grant iiiab 13 0 3 5 4 9 ma and canyon diablo ia 19 5 24 1 10 4 ma are consistent with cooling rates and the closure temperature of the pd ag system in contrast to carbonaceous chondrites ordinary chondrites show large stable isotope fractionation of order of 1 permil for 107ag 109ag this indicates that different mechanisms of volatile depletion were active in carbonaceous and ordinary chondrites nebular processes and accretion as experienced by carbonaceous chondrites did not led to significant ag isotope fractionation while the significant ag isotope variations in ordinary chondrites are most likely inflicted by open system parent body metamorphism

sediment dispersed extraterrestrial chromite traces a major asteroid disruption event abundant extraterrestrial chromite grains from decomposed meteorites occur in middle ordovician 480 million years ago marine limestone over an area of 250 000 square kilometers in southern sweden the chromite anomaly gives support for an increase of two orders of magnitude in the influx of meteorites to earth during the mid ordovician as previously indicated by fossil meteorites extraterrestrial chromite grains in mid ordovician limestone can be used to constrain in detail the temporal variations in flux of extraterrestrial matter after one of the largest asteroid disruption events in the asteroid belt in late solar system history

silver contents and cu ag ratios in martian meteorites and the implications for planetary differentiation abstract silver and cu show very similar partitioning behavior in sulfide melt silicate melt and metal silicate systems at low and high pressure temperature p t experimental conditions implying that mantle melting fractional crystallization and core mantle differentiation have at most modest within a factor of 3 effects on cu ag ratios for this reason it is likely that cu ag ratios in mantle derived magmatic products of planetary bodies reflect that of the mantle and in some circumstances also the bulk planet composition to test this hypothesis new ag mass fractions and cu ag ratios in different groups of martian meteorites are presented and compared with data from chondrites and samples from the earth s mantle silver contents in lherzolitic olivine phyric and basaltic shergottites and nakhlites range between 1 9 and 12 3 ng g the data display a negative trend with mgo content and correlate positively with cu contents in spite of displaying variable initial 143 nd values and representing a diverse spectrum of magmatic evolution and physiochemical conditions shergottites and nakhlites display limited variations of cu ag ratios 1080 320 1 s n 14 the relatively constant cu ag suggests limited fractionation of ag from cu during the formation and evolution of the parent magmas irrespectively of whether sulfide saturation was attained or not the mean cu ag ratio of martian meteorites thus reflects that of the martian mantle and constrains its ag content to 1 9 0 7 ng g 1 s carbonaceous and enstatite chondrites display a limited range of cu ag ratios of mostly 500 2400 ordinary chondrites show a larger scatter of cu ag up to 4500 which may have been caused by ag redistribution during parent body metamorphism the majority of chondrites have cu ag ratios indistinguishable from the martian mantle value indicating that martian core formation strongly depleted cu and ag contents but probably did not significantly change the cu ag ratio of the mantle compared to bulk mars bulk mars is richer in moderately volatile elements than earth however the martian mantle displays a much stronger depletion of the moderately volatile elements cu and ag e g by a factor of 15 for cu this observation is consistent with experimental studies suggesting that core formation at low p t conditions on mars led to more siderophile behavior of cu and ag than at high p t conditions as proposed for earth in contrast cu ag ratios of the mantles of mars and earth cu ag earth 3500 1000 display only a difference by a factor of 3 which implies restricted fractionation of cu and ag even at high p t conditions the concentration data support the notion that siderophile element partitioning during planetary core formation scales with the size of the planetary body which is particularly important for the differentiation of large terrestrial planets such as earth collectively the ag and cu data on magmatic products from the mantles of mars and earth and the data on chondrites confirm experimental predictions and support the limited fractionation of cu and ag during planetary core formation and high temperature magmatic evolution and probably also in early solar nebular processes

an impact origin for the foliation of chondrites abstract the origin of the foliation of ordinary chondrites is addressed through the study of the anisotropy of magnetic susceptibility ams of these meteorites based on a database of 295 different meteorites including 35 achondrites hed and snc ams is a reliable proxy for preferred orientation in ordinary chondrites moreover the intensity of magnetic anisotropy is constant over a given chondrite the intensity of l chondrite petrofabric evaluated by means of ams measurements and shock stage determined independently by microscopic observation of shock features in silicates appear to be positively correlated this suggests that the oblate petrofabric in l chondrites is shock induced the inverse correlation between porosity and ams intensity suggests that hypervelocity impacts compacted and lithified an originally loose material causing the deformation of metallic fe ni grains ll and h ordinary chondrites cannot be studied as easily as l chondrites tetrataenite in ll chondrites affects the ams in an unpredictable way and shape anisotropy due to the strong susceptibility of h chondrites imposes strong constraints on sample shape our limited dataset for carbonaceous chondrites is compatible with a shock induced foliation model the very low degrees of ams in rumuruti chondrites testify that surprisingly the easily deformable fe sulfides in these chondrites show almost no deformation suggesting that sulfidation is a secondary phenomenon that occurred after the major impacts on the rumuruti parent body the weak magnetic anisotropy of martian meteorites implies that no significant macroscopic deformation took place during the severe impacts suffered by these rocks which is explained by the weak initial porosity and high compressive strength of these igneous products

comparison of the trace element composition of tagish lake with other primitive carbonaceous chondrites a meteorite fall on 18 january 2000 was detected by u s defense department satellites which established its pre impact orbit fresh samples were collected from frozen tagish lake in british columbia a week later and some properties of these samples reveal it to be a unique meteorite we characterized tagish lake and 8 other samples using icpms and rnaa data for 47 elements reveal that each of 9 carbonaceous chondrites of different type exhibit the orgueil normalized plateaus expected for members of such types trends evident in tagish lake differ from all other carbonaceous chondrites including ci and cm samples of tagish lake collected later show similar patterns affected by weathering

fall recovery and characterization of the novato l6 chondrite breccia the novato l6 chondrite fragmental breccia fell in california on 17 october 2012 and was recovered after the cameras for allsky meteor surveillance cams project determined the meteor s trajectory between 95 and 46 km altitude the final fragmentation from 42 to 22 km altitude was exceptionally well documented by digital photographs the first sample was recovered before rain hit the area first results from a consortium study of the meteorite s characterization cosmogenic and radiogenic nuclides origin and conditions of the fall are presented some meteorites did not retain fusion crust and show evidence of spallation before entry the meteoroid was 35 5 cm in diameter mass 80 35 kg with a cosmic ray exposure age of 9 1 ma if it had a one stage exposure history a two stage exposure history is more likely with lower shielding in the last few ma thermoluminescence data suggest a collision event within the last similar to 0 1 ma novato probably belonged to the class of shocked l chondrites that have a common shock age of 470 ma based on the u th he age of 420 220 ma the measured orbits of novato jesenice and innisfree are consistent with a proposed origin of these shocked l chondrites in the gefion asteroid family perhaps directly via the 5 2 mean motion resonance with jupiter novato experienced a stronger compaction than did other l6 chondrites of shock stage s4 despite this a freshly broken surface shows a wide range of organic compounds less

ordinary chondrites multivariate statistical analysis of trace element contents abstract we used standard multivariate discriminant functions borrowed from linear discriminant analysis and logistic regression to compare contents of mobile trace elements co au sb ga se rb cs te bi ag in tl zn cd in various populations of l4 6 or h4 6 chondrites to aid in our analysis we developed a non standard randomization simulation method that permits probability assignments on a distribution free basis because the sample database for antarctic l4 6 chondrites is very limited we could not establish that mildly and strongly shocked

chemical studies of h chondrites 4 new data and comparison of antarctic suites we report data for the trace elements au co sb ga rb ag se cs te zn cd bi tl and in ordered by putative volatility during nebular condensation and accretion determined by neutron activation analysis in 13 h5 chondriles from victoria land and 20 h4 6 chondrites from queen maud land antarctica these and earlier results provide antarctic sample suites of 34 chondrites from victoria land and 25 from queen maud land treatment of data for the most volatile 10 elements rb in in these suites by multivariate statistical techniques more robust as well as more conservative than conventional linear discriminant analysis and logistic regression demonstrates that compositions differ at marginally significant levels this difference cannot be explained by trivial terrestrial causes and becomes more significant despite the smaller size of the database when comparisons are limited to data from a single analyst and when all upper limits arc eliminated from consideration the victoria land and queen maud land suites have different mean terrestrial ages 300 kyr and 100 kyr respectively and age distributions suggesting that a time dependent variation of chondritic sources with different thermal histories is responsible as a result these two antarctic suites are on average chemically distinguishable from each other since h chondrites serve as a paradigm for other meteorite classes these results indicate that the near earth populations of planetary materials varied with time on the 105 year timescale

archaean fluid assisted crustal cannibalism recorded by low δ 18 o and negative e hf t isotopic signatures of west greenland granite zircon the role of fluids during archaean intra crustal magmatism has been investigated via integrated shrimp u pb 18o and la mc icpms 176hf isotopic zircon analysis six rock samples studied are all from the nuuk region southern west greenland including two 3 69 ga granitic and trondhjemitic gneisses a 3 64 ga granitic augen gneiss a 2 82 ga granodioritic ikkattoq gneiss a migmatite with late neoarchaean neosome and a homogeneous granite of the 2 56 ga qorqut granite complex qgc all zircon grains were thoroughly imaged to facilitate analysis of magmatic growth domains within the zircon analysed there is no evidence for metamictization initial ehf zircon values n 63 are largely sub chondritic indicating the granitic host magmas were generated by the remelting of older un radiogenic crustal components zircon from some granite samples displays more than one 207pb 206pb age and correlated with 176hf 177hf compositions can trace multiple phases of remelting or recrystallization during the archaean model ages calculated using lu hf arrays for each sample indicate that the crustal parental rocks to the granites granodiorites and trondhjemites segregated from a chondrite like reservoir at an earlier time during the archaean corresponding to known formation periods of more primitive tonalite trondhjemite granodiorite ttg gneisses zircon from the 3 69 ga granite the migmatite and qgc granite contains eoarchaean cores with chondritic 176hf 177hf and mantle like 18o compositions the age and geochemical signatures from these inherited components are identical to those of surrounding tonalitic gneisses further suggesting genesis of these granites by remelting of broadly tonalitic protoliths zircon oxygen isotopic compositions n 62 over nine age populations six igneous and three inherited have weighted mean or mean 18o values ranging from 5 8 0 6 to 3 7 0 5 the 3 64 ga granitic augen gneiss sample displays the highest 18o with a mildly supra mantle composition of 5 8 0 6 inherited eoarchaean ttg derived zircon shows mantle like values igneous zircon from all other samples spanning more than a billion years of archaean time record low 18o sub mantle compositions these are the first low 18o signatures reported from archaean zircon and represent low 18o magmas formed by the remelting and metamorphism of older crustal rocks following high temperature hydrothermal alteration by meteoric water meteoric fluid ingress coupled with crustal extension associated high heat flow and intra crustal melting are a viable mechanism for the production of the low 18o granites granodiorites and trondhjemites reported here both high and low 18o magmas may have been generated in extensional environments and are distinct in composition from phanerozoic i type granitic plutonic systems which are typified by increasing 18o during intra crustal reworking this suggests that archaean magmatic processes studied here were subtly different from those operating on the modern earth and involved extensional tectonic regimes and the predominance of remelting of hydrothermally altered crystalline basement

experimental shock metamorphism of the l4 ordinary chondrite saratov induced by spherical shock waves up to 400 gpa abstract we carried out shock experiments on macroscopic spherical samples of the l4 ordinary chondrite saratov natural shock stages s2 s3 using explosively generated spherical shock waves with maximum peak pressures of 400 gpa and shock induced temperatures 800 c up to several thousands c the evolution of shock metamorphism within a radius of the spherical samples was investigated using optical and scanning electron microscopy microprobe and magnetic analyses as well as mossbauer spectroscopy and x ray diffraction techniques petrographic analyses revealed a shock induced formation of three different concentric petrographic zones within the shocked samples zone of total melting i zone of partial melting ii and zone of solid state shock features iii we found a progressive pressure induced oxidation of fe ni metal whose degree increased with increasing shock peak pressure the amount of feo within zone i increased the factor of 1 4 with respect to its amount in the unshocked saratov sample this suggests that within zone i about 70 wt of the initial metallic iron was oxidized whereas magnetic analyses showed that about 10 wt of it remained intact this strongly supports the hypothesis that in addition to oxidation a migration of metallic iron from the central heavily shocked zone i toward less shocked peripheral zone took place as well likely through shock veins where metallic droplets were observed magnetic analyses also showed a shock induced transformation of tetrataenite to taenite within all shocked subsamples resulting in magnetic softening of these subsamples decrease in remanent coercivity these results have important implications for extraterrestrial paleomagnetism suggesting that due to natural impact processes the buried crustal rocks of heavily cratered solid solar system bodies can have stronger remanent magnetism than the corresponding surface rocks

late quaternary caldera forming eruptions in the eastern aleutian arc alaska late quaternary calderas have been identified at 12 of 40 volcanic centers in the eastern aleutian arc and sufficient radiocarbon dates and geologic information have now been obtained to either date or constrain the timing of the climactic caldera forming eruptions at least eight major caldera forming events each characterized by estimated eruption volumes of more than 10 km 3 occurred at seven different volcanic centers in the holocene and as many as six of these had estimated eruption volumes of more than 50 km 3 eruptions of similar magnitude formed two other calderas in wisconsin time the dating of these hitherto little known events adds significantly to the previously existing chronology of large prehistoric eruptions this refined chronology is important in understanding eruption induced climate changes in assessing volcanic hazards and in developing a tephrochronology for northwestern north america

variable tl pb and cd concentrations and isotope compositions of enstatite and ordinary chondrites evidence for volatile element mobilization and decay of extinct 205pb new tl pb and cd concentration and tl pb isotope data are presented for enstatite as well as l and ll type ordinary chondrites with additional cd stable isotope results for the former all three chondrite suites have tl and cd contents that vary by more than 1 2 orders of magnitude but pb concentrations are more uniform as a result of terrestrial pb contamination model calculations based on pb isotope compositions indicate that for more than half of the samples more than 50 of the measured pb contents are due to addition of modern terrestrial pb in part this is responsible for the relatively young and imprecise pb pb ages determined for eh l and ll chondrites which are hence only of limited chronological utility in contrast four particularly pristine el chondrites define a precise pb pb cooling age of 4559 6 ma the enstatite chondrites ecs have highly variable e114 110cd of between about 3 and 70 due to stable isotope fractionation from thermal and shock metamorphism furthermore nearly all enstatite meteorites display e205tl values from 3 3 to 0 8 while a single anomalous sample is highly fractionated in both tl and cd isotopes the majority of the ecs thereby define a correlation of e205tl with e114 110cd which suggests that at least some of the tl isotope variability reflects stable isotope fractionation rather than radiogenic ingrowth of 205tl from 205pb decay considering l chondrites most e205tl values range between 4 and 1 while two outliers with e205tl 10 are indicative of stable isotope fractionation considering only those l chondrites which are least likely to feature pb contamination or stable tl isotope effects the results are in accord with the former presence of live 205pb on the parent body with an initial 205pb 204pb 1 5 1 4 10 4 which suggests late equilibration of the pb tl system 26 113 ma after carbonaceous chondrites ccs the ll chondrites display highly variable e205tl values from 12 5 to 14 9 also indicative of stable isotope effects however the data for three pristine ll3 ll4 chondrites display an excellent correlation between e205tl and 204pb 203tl this defines an initial 205pb 204pb of 1 4 0 3 10 4 equivalent to a 205pb 205tl cooling age of 55 12 24 ma 31 67 ma after ccs

the isotope composition of selenium in chondrites constrains the depletion mechanism of volatile elements in solar system materials abstract solar nebula processes led to a depletion of volatile elements in different chondrite groups when compared to the bulk chemical composition of the solar system deduced from the sun s photosphere for moderately volatile elements this depletion primarily correlates with the element condensation temperature and is possibly caused by incomplete condensation from a hot solar nebula evaporative loss from the precursor dust and or inherited from the interstellar medium element concentrations and interelement ratios of volatile elements do not provide a clear picture about responsible mechanisms here the abundance and stable isotope composition of the moderately to highly volatile element se are investigated in carbonaceous ordinary and enstatite chondrites to constrain the mechanism responsible for the depletion of volatile elements in planetary bodies of the inner solar system and to define a 82 78 se value for the bulk solar system the 82 78 se of the studied chondrite falls are identical within their measurement uncertainties with a mean of 0 20 0 26 2 s d n 14 relative to nist srm 3149 despite se abundance depletions of up to a factor of 2 5 with respect to the ci group the absence of resolvable se isotope fractionation rules out a kinetic rayleigh type incomplete condensation of se from the hot solar nebula or partial kinetic evaporative loss on the precursor material and or the parent bodies the se depletion if acquired during partial condensation or evaporative loss therefore must have occurred under near equilibrium conditions to prevent measurable isotope fractionation alternatively the depletion and cooling of the nebula could have occurred simultaneously due to the continuous removal of gas and fine particles by the solar wind accompanied by the quantitative condensation of elements from the pre depleted gas in this scenario the condensation of elements does not require equilibrium conditions to avoid isotope fractionation the results further suggest that the processes causing the high variability of se concentrations and depletions in ordinary and enstatite chondrites did not involve any measurable isotope fractionation different degrees of element depletions and isotope fractionations of the moderately volatile elements zn s and se in ordinary and enstatite chondrites indicate that their volatility is controlled by the thermal stabilities of their host phases and not by the condensation temperature under canonical nebular conditions

chemical alteration and ree mobilization in meteorites from hot and cold deserts the effects of terrestrial weathering on ree mobilization are evaluated for a variety of uncommon meteorites found in antarctica and in hot deserts the meteorites analyzed include 7 non cumulate eucrites 10 shergottites 3 nakhlites 2 lunar meteorites 4 angrites 10 acapulcoites 1 winonaite and 1 brachinite in situ concentration measurements of lanthanides and selected other minor and trace elements were made on individual grains by secondary ion mass spectrometry sims in antarctic meteorites oxidation converts ce 3 to ce 4 which is less soluble than the trivalent ree resulting in ce anomalies the mineral most affected is low ca pyroxene however not all grains of a given mineral are and distinct analyses of a single grain can even yield ree patterns with and without ce anomalies the effect is most pronounced for antarctic eucrites in which ce anomalies are observed not only in individual minerals but also in whole rock samples although ce anomalies are observed in meteorites from hot deserts as well the most characteristic signs of chemical alteration in this environment are a lree enrichment with a typical crustal signature as well as sr ba and u contaminations these can modify the whole rock ree patterns and disturb the isotope systematics used to date these objects the lree contamination is highly heterogeneous affecting some grains and not others of a given mineral mainly olivine and low ca pyroxene the two minerals with the lowest ree concentrations the major conduit for ree movement is through shock induced cracks and defects and the highest levels of contamination are found in altered material filling such veins and cracks meteorites that experienced low shock levels and those that are highly recrystallized are the least altered copyright 2003 elsevier ltd

oxygen isotope geochemistry of oceanic arc lavas variations of oxygen isotope ratios in arc related lavas can constrain the contributions of subducted crustal igneous rocks sediments and fluids to the sub arc mantle we have measured oxygen isotope ratios in 72 arc and back arc lavas from five ocean ocean subduction zone systems using laser fluorination analyses of olivine and other phenocrysts and glass eighty percent of our samples have delta 18o values for any given phase olivine plagioclase glass or biotite within 0 2 per thousand of the average value for that phase in upper mantle peridotites and mid ocean ridge basalt morb the range for each phase is 1 0 per thousand this result contrasts with previous studies of whole rock samples which are significantly more variable even after exclusion of samples believed to be altered or fractionated by magmatic differentiation and demonstrates that most arc related lavas contain 1 2 of 18o enriched crustal oxygen from any source i e assimilation or subducted contributions elevations in delta 18o that do occur in these basic arc derived magmas relative to values most common for mantle derived lavas are associated both with enriched radiogenic isotope signatures and even more strongly with chemical indices consistent with high integrated extents of melting of their peridotite sources we interpret these relationships as evidence that melting in the sources of the high delta 18o lavas we have studied was fluxed by addition of high delta 18o aqueous fluid or perhaps a hydrous melt from the subducted slab such that sources that contain relatively large components of slab derived fluid or melt are both relatively 18o enriched and also experienced relatively large amounts of melting we have developed a quantitative model linking the amount of melting to the extents of 18o radiogenic isotope and trace element enrichment in a mantle source being fluxed by addition of aqueous fluid comparison of this model with observed variations in the geochemistry of lavas from the vanuatu fiji new caledonia region the suite of related samples showing the greatest range in delta 18o observed in this study constrains the amounts and chemical and isotopic compositions of slab derived phases in the sources of these arc related lavas assuming a delta 18o value of 20 per thousand for the slab derived fluid 0 5 1 0 wt is added to the sources of most mantle derived arc magmas the maximum amount of slab derived flux in the sources of arc magmas according to our results is 2 5 wt

holocene tephrochronology of the cold bay area southwest alaska peninsula abstract the major element glass geochemistry of 92 tephra samples from the southwest alaska peninsula provides the basis for establishing a holocene tephrochronology for the region electron microprobe analysis has been combined with field descriptions of samples stratigraphic relationships between tephra samples and sample localities and glass shard micro morphology to correlate these sampled distal tephra units throughout the area of cold bay and adjacent morzhovoi bay radiocarbon dating provides age constraints on correlated horizons previous research had clearly delineated only one horizon in the region the so called funk fisher ash dating to between 8425 350 and 9130 140 14 c yr bp in addition to constraining the bimodal andesitic and dacitic glass chemistry of that horizon this study has recognized six additional tephra layers in the area two horizons pre date the funk fisher ash and four are younger than it a tephra containing dacitic and andesitic components was identified in the vicinity of morzhovoi bay with a minimum age of 9300 80 14 c yr bp and a maximum age of 10 200 75 14 c yr bp a rhyolitic horizon composed of cm sized rounded pumice clasts was identified in the vicinity of cold bay it has been correlated to the ca 9500 bp eruption of roundtop volcano on unimak island the four younger tephra beds date to between 6070 340 and 3600 140 14 c yr bp the oldest of the four is rhyodacitic followed by a mixed rhyodacitic andesitic horizon another rhyodacitic horizon and finally an andesitic layer comparison of all the correlated horizons to proximal samples collected on unimak island provides conclusive geochemical evidence that the ca 9100 bp caldera forming eruption of fisher volcano is the source of the funk fisher ash correlation between the rhyodacitic tephra horizons and proximal samples from fisher volcano suggests that fisher caldera is the source of one of the rhyodacitic tephra horizons that post dates the funk fisher ash additional tephra samples from the southwest alaska peninsula and unimak island that were collected prior to this study correlate to the tephra horizons identified in the cold bay area and identify one additional horizon

chemical compositions and classifica tion of five thermally altered carbonaceous chondrites to establish the chemical group provenance of the five thermally altered carbonaceous chondrites asuka a 881551 asuka 882113 elephant moraine eet 96026 mulga west and northwest africa nwa 3133 we quantified 44 trace elements in each of them we also analyzed larkman nunatak lar 04318 ck4 miller range mil 090001 cr2 roberts massif rbt 03522 ck5 as reference samples as their chemical group affinity is already recognized we conclude that asuka 881551 asuka 882113 and mulga west are thermally metamorphosed ck chondrites compositionally elephant moraine 96026 most resembles the cv chondrites nwa 3133 is the most significantly thermally altered carbonaceous chondrite in our suite of samples it is completely recrystallized no chondrules or matrix remain but its bulk composition is consistent with a cv ck clan provenance the thermally labile element e g se te zn and bi depletion in nwa 3133 indicates a chemically open system during the heating episode it remains unclear if the heat necessary for its thermal alteration of nwa 3133 was due to the decay of 26al or was impact related finally we infer that mil 090001 mulga west and nwa 3133 show occasional compositional signatures indicative of terrestrial alteration the alteration is especially evident within the elements sr ba la ce th u and possibly sb despite the alteration we can still confidently place each of the altered chondrites within an established chemical group or clan

chemical studies of h chondrites 9 volatile trace element composition and petrographic classification of equilibrated h chondrites to examine the thermal history of the parent body bodies of equilibrated h chondrites we treated data for 11 volatile trace elements co rb ag se cs te zn cd bi tl and in in order of putative volatility in 90 falls 15 h4 46 h5 and 29 h6 using univariate statistical tests contents of few of these elements differ significantly between any two of these suites one element cs differs systematically between all three pairs of suites co and tl differ between two pairs of suites for co and cs contents varied as h4 h5 h6 while for tl contents varied as h4 600 k with the suites deriving from a stratified parent body bodies

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

eruptive history of fisher caldera alaska usa abstract new field compositional and geochronologic data from fisher caldera the largest of 12 holocene calderas in alaska provide insights into the eruptive history and formation of this volcanic system prior to the caldera forming eruption cfe 9400 years ago the volcanic system consisted of a cluster of several small 3 km 3 stratocones which were independently active between 66 144 and 9 4 0 2 ka fisher caldera formed through a single eruption which produced a thick dacitic fall deposit and two pyroclastic flow deposits a small dacitic flow and a compositionally mixed basaltic dacitic flow thickness and grain size data indicate that the fall deposit was dispersed primarily to the northeast whereas the two flows were oppositely directed to the south and north after the cataclysmic eruption a lake filled much of the caldera during what may have been a significant quiescent period volcanic activity from intracaldera vents gradually resumed producing thick successions of scoria fall interbedded with lake sediments several holocene stratocones have developed one of which has had a major collapse event the caldera lake catastrophically drained when a phreatomagmatic eruption generated a large wave that overtopped and incised the southwestern caldera wall multiple accretionary lapilli bearing deposits inside and outside the caldera suggest significant holocene phreatomagmatic activity the most recent eruptive activity from the fisher volcanic system was a small explosive eruption in 1826 and current activity is hydrothermal late pleistocene to holocene magma eruption rates range from 0 03 to 0 09 km 3 ky 1 km 1 respectively the fisher volcanic system is chemically diverse 48 72 wt sio 2 with at least seven dacitic eruptions over the last 82 14 ka that may have become more frequent over time least squares calculations suggest that prior to the cfe fisher volcano products were not derived from a single large magma reservoir and were likely erupted from multiple compositionally independent magma reservoirs after the cfe the majority of products appear to have derived from a single reservoir in which magma mixing has occurred

oxygen isotope geochemistry of the lassen volcanic center california resolving crustal and mantle contributions to continental arc magmatism this study reports oxygen isotope ratios determined by laser fluorination of mineral separates mainly plagioclase from basaltic andesitic to rhyolitic composition volcanic rocks erupted from the lassen volcanic center lvc northern california plagioclase separates from nearly all rocks have o values 6 1 8 4o higher than expected for production of the magmas by partial melting of little evolved basaltic lavas erupted in the arc front and back arc regions of the southernmost cascades during the late cenozoic most lvc magmas must therefore contain high o crustal material in this regard the o values of the volcanic rocks show strong spatial patterns particularly for young rhyodacitic rocks that best represent unmodified partial melts of the continental crust rhyodacitic magmas erupted from vents located within 3 5 km of the inferred center of the lvc have consistently lower o values average 6 3o 0 1o at given sio2 contents relative to rocks erupted from distal vents 47 0 km average 7 1o 0 1o further magmas erupted from vents situated at transitional distances have intermediate values and span a larger range average 6 8o 0 2o basaltic andesitic to andesitic composition rocks show similar spatial variations although as a group the o values of these rocks are more variable and extend to higher values than the rhyodacitic rocks these features are interpreted to reflect assimilation of heterogeneous lower continental crust by mafic magmas followed by mixing or mingling with silicic magmas formed by partial melting of initially high o continental crust 9 0o increasingly hybridized by lower o 6 0o mantle derived basaltic magmas toward the center of the system mixing calculations using estimated endmember source o values imply that lvc magmas contain on a molar oxygen basis approximately 42 to 4 isotopically heavy continental crust with proportions declining in a broadly regular fashion toward the center of the lvc conversely the o values of the rhyodacitic rocks suggest that the continental crust in the melt generation zones beneath the lvc has been substantially modified by intrusion of mantlederived basaltic magmas with the degree of hybridization ranging on a molar oxygen basis from approximately 60 at distances up to 12 km from the center of the system to 97 directly beneath the focus region these results demonstrate on a relatively small scale the strong influence that intrusion of mantle derived mafic magmas can have on modifying the composition of pre existing continental crust in regions of melt production given this result similar but larger scale regional trends in magma compositions may reflect an analogous but more extensive process wherein the continental crust becomes progressively hybridized beneath frontal arc localities as a result of protracted intrusion of subduction related basaltic magmas

volcanic and tectonic deformation on unimak island in the aleutian arc alaska 3 3 10 6 m 3 yr 1 the gps data indicate that inflation may have slowed down slightly compared with interferometric synthetic aperture radar insar observations between 1993 and 1998 the accumulated subsurface volume increase during the gps and insar observation period 1993 2001 70 10 6 m 3 already accounts for at least 15 more than the erupted volume from the last eruption in 1991 1992 fisher caldera shows subsidence and contraction across the caldera center the data are fit best with a rectangular dislocation source at a shallow depth it is 14 km long and 0 5 km wide dips 80 to the nw and strikes n35 e with rather large uncertainties for most of these parameters its volume decrease is 2 0 10 6 m 3 yr 1 the main mechanisms to explain the subsidence and contraction are degassing and contractional cooling of a shallow magma body and depressurization of fisher s hydrothermal system possibly triggered by an earthquake in 1999 at the 95 confidence level no significant strain accumulation due to subduction is observed across the entire island the low coupling across the rupture zone of the 1946 earthquake is a strong argument for the idea that the earthquake and tsunami did not result from a purely double couple earthquake source index terms 1206 geodesy and gravity crustal movements interplate 8155 8135 tectonophysics evolution of the earth hydrothermal systems 8424 8145 tectonophysics evolution of the earth physics of magma and magma bodies 8150 tectonophysics evolution of the earth plate boundary general 3040 keywords volcanic deformation caldera deformation aleutian arc alaska unimak island citation mann d and j freymueller volcanic and tectonic deformation on unimak island in the aleutian arc alaska j geophys res 108 b2 2108 doi 10 1029 2002jb001925 2003

three dimensional petrography of metal phases in equilibrated l chondrites effects of shock loading and dynamic compaction abstract to examine the role of impacts in the evolution of asteroids as seen through their chondritic offspring we have performed a quantitative three dimensional 3d study of metal grains in a suite of increasingly shocked l chondrites with synchrotron x ray microtomography xmt our data allow rigorous quantification of size number distributions and collective morphology of fe ni metal phases in chondritic meteorites at the resolution of our xmt measurements 8 4 17 9 m voxel the number of metal particles increase with higher degrees of petrographically identified shock loading indicating a coalescing of fe ni metal at or below this scale our results demonstrate that collective degrees of metal grain preferred orientation increase with greater degrees of impact related compaction and shock loading ductile metal grains in l chondrites begin to show foliation at peak shock pressures

compositions geochemistry and shock histories of recrystallized ll chondrites abstract to examine compositional changes associated with high degrees of apparent thermal metamorphism among the ll chondrites we have examined seven ll chondrites originally classified as being petrographic type 7 for comparison we also analyzed the l6 7 chondrite y 790124 we found that a 880933 is actually an ll4 6 genomict breccia and y 790124 is best described as an l6 s3 chondrite the remaining six chondrites eet 92013 uden y 74160 y 790144 y 791067 y 82067 are clearly of ll provenance and each experienced temperatures high enough for them to have been recrystallized in four of these samples eet 92013 uden y 74160 y 790144 we find elemental patterns suggesting fe ni fes mobilization others y 791067 y 82067 have compositions identical to average equilibrated ll chondrites from our compositional data we infer that eet 92013 uden y 74160 y 790144 experienced very low degrees of partial melting prior to recrystallization but y 791067 and y 82067 experienced isochemical solid state recrystallization the heat source responsible for the high degrees of thermal alteration of these meteorites is limited to either the decay of now extinct radionuclides 26 al or impact related heating to evaluate the nature of the heat source we use 40 ar 39 ar literature data and petrographic examinations to infer the cooling history and shock history of these chondrites we find that heating due to impact is the most likely heat source for the heating of the recrystallized chondrites the potential impacts occurred well after the initial stages of ll chondrite thermal metamorphism but still early in the ll parent body s history probably 4 2 4 3 ga ago these rocks experienced mild shock histories following their recrystallization

stony meteorite characterization by non destructive measurement of magnetic properties four parameters of low field magnetic susceptibility bulk value frequency dependence degree of anisotropy and ellipsoid shape have been determined for 321 stony meteorites from the national collection of canada these parameters provide a basis for rapid non destructive and accurate meteorite classification as each meteorite class tends to have a distinct range of values chondrites show a clear trend of increasing bulk susceptibility from ll to l to h to e within the 3 6 to 5 6 log in 10 9 m 3 kg range reflecting increasing fe ni metal and fe ni sulfide content achondrite values range in log from 2 4 to 4 7 and primitive achondrites from 4 2 to 5 7 frequency dependence is observed using 19 000 hz and 825 hz with variations in strength among meteorite classes and individual specimen dependence ranging from 1 25 6 degrees of anisotropy range from 1 to 53 with both oblate and prolate ellipsoids present the aubrite class is marked by high degrees of anisotropy low bulk magnetic susceptibility and prolate fabric camel donga is set apart from other eucrites marked by higher bulk susceptibility degree of anisotropy and magnitude of oblate ellipsoid shape the shergotty nakhla and chassigny snc meteorites show subclass distinction using frequency dependence and chassigny is set apart with a relatively strong oblate fabric the presence of both strong oblate and prolate fabrics among and within meteorite classes of chondritic and achondritic material points to a complex multi mechanism origin for anisotropy more so than previously thought and likely dominated by impact processes in the later stages of stony parent bodyformation

extent and timing of the last glacial maximum in southwestern alaska a glacier complex composed of confluent alpine glaciers island ice caps and piedmont lobes covered much of the alaska peninsula and kodiak island during the last glacial maximum lgm because this glacier complex formed the southeastern border of beringia its dynamics may have been important in the timing and feasibility of the northwest coast route for human migration into lower latitude north america radiocarbon dates from stratigraphic sections on kodiak island and in the bristol bay lowlands bracket the lgm in southwestern alaska between 23 000 and 14 700 yr b p reconstruction of ice thickness based on glacier trimlines moraines and calculations of basal shear stress depict the alaska peninsula glacier complex flowing to the outer edge of the continental shelf in the gulf of alaska equilibrium line altitudes elas were 300 to 700 m lower than today and approached sea level on the southwestern alaska peninsula in northeastern areas where elas were higher bedrock topography largely controlled ice flow except where ice saddles bridged straits and inlets

magmatic differentiation at an island arc caldera okmok volcano aleutian islands alaska okmok volcano is situated on oceanic crust in the central aleutian arc and experienced large 15 km caldera forming eruptions at 12 000 years bp and 2050 years bp each caldera forming eruption began with a small plinian rhyodacite event followed by the emplacement of a dominantly andesitic ash flow unit whereas effusive interandpost caldera lavashavebeenmorebasaltic phenocryst assemblages are composed ofolivine pyroxene plagioclase fe ti oxides and indicate crystallization at1000 11008cat 0 1 0 2gpa in thepresence of 0 4 h2o the erupted products follow a tholeiitic evolutionary trend and calculated liquid compositions range from 52 to 68 wt sio2 with 0 8 3 3 wt k2o major and trace element models suggest that the more evolved magmas were produced by 50 60 in situ fractional crystallization around the margins of the shallow magma chamber oxygen and strontium isotope data o 4 4 4 9o sr sr 0 7032 0 7034 indicate interaction with a hydrothermally altered crustal component which led to elevated thorium isotope ratios in some caldera forming magmas this compromises the use of uranium thorium disequilibria th u 1 4 0 849 0 964 to constrain the time scales of magma differentiation but instead suggests that the age of the hydrothermal system is 100 ka modelling of the diffusion ofstrontiuminplagioclase indicates thatmanyevolved crystal rims formed less than 200 years prior to eruption this addition of rim material probably reflects the remobilization of crystals from the chamber margins following replenishment basaltic recharge led to the expansion of the magma chamber which was responsible for the most recent caldera forming event

assimilation of felsic crust by basaltic magma thermal limits and extents of crustal contamination of mantle derived magmas basaltic magma is the most widely used probe of mantle chemistry and structure chemical characterization of the sources for mantle derived magma is possible only after quantitative evaluation of possible contamination by crustal wall rocks the most commonly invoked contamination mechanisms are the combined processes of crustal assimilation and magmatic fractional crystallization afc because of the energetic link between these processes predicting assimilation and crystallization progress requires modeling both heat and mass transfer in the magma country rock system using the melts software package we modeled isenthalpic heat balanced afc between two common types of basaltic magma and several types of felsic crust at 1 kbar our simulations show that during the early stages of isenthalpic afc the ratio of rates of assimilation to crystallization r may be substantially greater than one 2 0 2 7 allowing assimilation of a mass of country rock up to 5 18 of the initial magma mass with only 3 7 crystallization the second stage of afc beginning with plagioclase and or pyroxene saturation is characterized by lower r 0 5 1 0 the initial high r stage results from suppression of crystallization associated with the change in magma composition as assimilation progresses under certain conditions even small degrees of crystallization of olivine alone coupled with cooling of the magma can accommodate relatively large amounts of crustal assimilation and cause large shifts in isotopic and trace element geochemical indices with little differentiation