back arc strain in subduction zones statistical observations versus numerical modeling 1 recent statistical analysis by lallemand et al 2008 of subduction zone parameters revealed that the back arc deformation mode depends on the combination between the subducting vsub and upper vup plate velocities no significant strain is recorded in the arc area if plate kinematics verifies vup 0 5 vsub 2 3 cm a in the hs3 reference frame arc spreading shortening occurs if vup is greater lower than the preceding relationship we test this statistical law with numerical models of subduction by applying constant plate velocities far away from the subduction zone the subducting lithosphere is free to deform at all depths we quantify the force applied on the two converging plates to sustain constant surface velocities the simulated rheology combined viscous non newtonian and brittle behaviors and depends on water content the influence of subduction rate vs is first studied for a fixed upper plate after 950 km of convergence steady state slab pull the transition from extensional to compressive stresses in the upper plate occurs for vs 1 4 cm a the effect of upper plate velocity is then tested at constant subduction rate upper plate retreat advance with respect to the trench increases extension compression in the arc lithosphere and increases decreases the subducting plate dip our modeling confirms the statistical kinematic relationship between vsub and vup that describes the transition from extensional to compressive stresses in the arc lithosphere even if the modeled law is shifted toward higher rates of upper plate retreat using our set of physical parameters e g 100 km thick subducting oceanic plate and short term simulations our results make valid the choice of the hs3 reference frame for assessing plate velocity influence on arc tectonic regime the subduction model suggests that friction along the interplate contact and the mantle stokes reaction could be the two main forces competing against slab pull for upper mantle subductions besides our simulations show that the arc deformation mode is strongly time dependent

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

rise of the ellsworth mountains and parts of the east antarctic coast observed with gps 1 using gps observations from 1996 to 2011 we constrain postglacial rebound in antarctica sites in the ellsworth mountains west antarctica are rising at 5 4 mm yr 95 confidence limits as in the postglacial rebound model of peltier but 10 mm yr slower than in the model of ivins and james therefore significant ice loss from the ellsworth mountains ended by 4 ka and current ice loss there is less than inferred from grace gravity observations in studies assuming the model of ivins and james three sites along the coast of east antarctica are rising at 3 to 4 2 mm yr in viscous response to holocene unloading of ice along the queen maud land coast and elsewhere kerguelen island and seven sites along the coast of east antarctic are part of a rigid antarctica plate o higgins northern antarctic peninsula is moving southeast at 2 3 0 6 mm yr relative to the antarctic plate

oxygen isotopes in r chondrite magnetite and olivine links between r chondrites and ordinary chondrites ion microprobe studies yield 17o 17o 0 52 18o values in magnetite from the rumurti chondrite rc pca91241 which is paired with pca91002 of 3 1 to 3 9 slightly higher than that in o from whole rock r samples despite 17o values in whole rock rcs that are much by ca 1 6 higher than in whole rock ll chondrites the 17o in r magnetite is much lower by ca 2 than the values 4 to 7 from ll3 semarkona and ngawi choi et al 1998 the 18o values in pca magnetite 15 to 10 are the lowest known in meteorites well below the range in semarkona 4 to 9 on a 17o 18o diagram both magnetite data sets form linear arrays with slopes of ca 0 7 indicating mixing of o from different isotopic reservoirs the slopes and intercepts of the two arrays are similar enough to permit them to be segments of a single array this suggests that in rcs and ll chondrites magnetite formed from the same raw materials by the same processes probably by aqueous alteration of metal in an asteroidal setting we observed 17o values in olivines and pyroxene from rcs ranging from 1 2 to 2 9 and 18o from 1 4 to 9 1 these compositions scatter in the same general range observed in chondrules from ordinary chondrites the similarity in the o isotopic composition of minerals that preserve a record of formation in the solar nebula supports a model in which rcs formed from nebular components similar to those in h chondrites but with a matrix chondrule ratio several times higher in the rcs and with more extensive aqueous alteration in the rcs than in known h chondrites we postulate that the matrix in r chondrites has 17o higher than whole rock values we suggest that the original 17o value of h2o in the rc body was similar to that incorporated into the ordinary chondrites previously estimated by choi et al 1998 to be ca 7 in the ll parent body

the oxygen isotope record in murchison and other carbonaceous chondrites the ranges of 18o and 17o in components of the murchison c2 chondrite exceed those in all other meteorites analyzed previous authors have proposed that c2 chondrites are the products of aqueous alteration of anhydrous silicates a model is presented to determine whether the isotopic variations can be understood in terms of such alteration processes the minimum number two of initial isotopic reservoirs is assumed two major stages of reservoir interaction are required one at high temperature to produce the16o mixing line observed for the anhydrous minerals and another at low temperature to produce the matrix minerals the isotopic compositions severely constrain the conditions of the low temperature process requiring temperatures 44 extension of the model to the data on c1 chondrites requires aqueous alteration in a warmer wetter environment

phosphate minerals in ll chondrites a record of the action of fluids during metamorphism on ordinary chondrite parent bodies ordinary chondrites contain two phosphate minerals merrillite and chlorapatite both of which are secondary minerals that developed in response to metamorphism on the chondrite parent bodies we have studied the phosphate mineralogy of four ll chondrites of petrologic types 3 9 6 in order to determine the petrogenesis of the two minerals and interpret the conditions under which they formed characterization of merrillite and apatite includes textural observations mineral compositions determined by electron probe microanalysis and ion microprobe analyses of trace element and volatile anion elemental abundances initial formation of phosphate minerals during mild metamorphism to petrologic type 4 conditions resulted in oxidation of p that was originally incorporated in metal and growth of merrillite as inclusions within metal grains subsequent development of both phosphate minerals occurred in response to diffusional equilibration possible precipitation from fluids as well as replacement reactions resulting from interactions with fluids porosity and vein filling textures in both merrillite and chlorapatite as well as textures indicating replacement of merrillite by chlorapatite support a model in which fluid played a significant role and suggest an interface coupled dissolution reprecipitation mechanism during metasomatism some associations of phosphate minerals with chromite plagioclase assemblages suggest that phosphate minerals could also be related to impact processes either as precipitation from an impact melt or as a result of interactions with a fluid or vapor derived from an impact melt fluid compositions may have been water bearing initially at low temperatures of metamorphism but later evolved to become halogen rich and very dry late stage halogen rich fluids that dominated during cooling of ll chondrite material may have been derived from vaporization of partial melts in the interior of the parent body overall the ll chondrite parent body underwent a complex chemical evolution in which metasomatism played a significant role

oxygen isotopes in asteroidal materials measurement of the variation of the 17o 16o and 18o 16o ratios of extraterrestrial materials has proven to be a key tool in understanding the formation and evolution of the solar system this chapter attempts to collate the huge data set of oxygen isotopic measurements of asteroidal material as sampled by meteorites in order to understand some of the processes involved in the formation of these primitive bodies and how such events have affected the oxygen isotopic ratios ultimately offering a window back to the very origin of solar system and its primordialoxygen isotopic heterogeneity oxygen is generally a major element in the gas liquid and solid phases that have interacted throughout the evolution of the solar system its chemical properties ensuring that there is abundant opportunity for reactions and exchange the details of which can be recorded in the isotopic ratios of the products such processes can be crudely subdivided into the following six categories early solar nebular isotopic heterogeneity high temperature modifi cation of nebular components metamorphism and melting in asteroids aqueous alteration in asteroids mixing of components brecciation terrestrial weathering it is clear from the isotopic variation in the earliest formed nebular components such as refractory inclusions and chondrules that there was considerable oxygen isotopic heterogeneity in the early solar nebula spanning over 50 and perhaps much greater in both 17o and 18o the origin of this variation is discussed elsewhere in this volume however isotopic exchange between the 16o rich solids and 17o and 18o rich gas phase s resulted in a wide range of isotopic variation in these early formed components offering insight into the conditions of these processes and the evolution of the reservoirs with time and or space once accreted onto the early planetisemals much of the primitive nebular components experienced considerable modification the metamorphism and melting experienced by some of these bodies most probably the result of heating from the decay of short lived radionuclides e g 26al resulted in progressive homogenization within individual bodies of any original isotopic heterogeneity complete homogenization appears to have been achieved in the hed parent body indicating very high levels of melting and the development of a large magma ocean the carbonaceous and ordinary chondrite parent bodies experienced more modest heating sufficient to mobilize water ice that accreted along with the more refractory materials the subsequent reactions between the liquid and solid phases at low temperature imparted large isotopic fractionations which record some of the conditions of this alteration such as water rock ratio temperature and isotopic signature and thus potentially the origin of the water ice the evolution of the asteroidal bodies has been far from simple and an important use of oxygen isotope measurements has been in the identifi cation of the components present in some meteorites the result of breakup reassembly and mixing of different bodies during impact processing the final oxygen isotopic signatures imparted into meteorite samples are those associated with terrestrial weathering the isotopic compositions of the weathering products depend on location primarily latitude while the magnitude of the weathering is dependent on many factors with some meteorites e g highly reduced enstatite chondrites and the matrix rich co3s particularly susceptible to such effects

an fe isotope study of ordinary chondrites the fe isotope composition of ordinary chondrites and their constituent chondrules metal and sulphide grains have been systematically investigated bulk chondrites fall within a restricted isotopic range of 1 0 84 to 0 21 relative to the irmm 14 fe standard fe isotope compositions do not vary systematically with the very large differences in total fe concentration or oxidation state of the h l and ll chondrite classes similarly the fe isotope compositions of chondrules do not appear to be determined by the h l or ll classification of their host chondrite this may support an origin of the three ordinary chondrite groups from variable accretion of identical fe bearing precursors a close relationship between isotopic composition and redistribution of fe during metamorphism on ordinary chondrite parent bodies was identified the largest variations in chondrule compositions were found in chondrites of the lowest petrologic types the clear link between element redistribution and isotopic composition has implications for many other non traditional isotope systems e g mg si ca cr isotopic compositions of chondrules may also be determined by their melting history porphyritic chondrules exhibit a wide range in isotope compositions whereas barred olivine and radial pyroxene chondrules are generally isotopically heavier than the ordinary chondrite mean very large chondrules preserve the greatest heterogeneity of fe isotopes the mean fe isotope composition of bulk ordinary chondrites was found to be 0 06 0 12 2 sd this is isotopically lighter than the terrestrial mean composition and all other published non chondritic meteorite suites e g lunar and martian samples eucrites pallasites and irons ordinary chondrites though the most common meteorites found on earth today were not the sole building blocks of the terrestrial planets

the crystalline lunar spherules their formation and implications for the origin of meteoritic chondrules crystalline lunar spherules cls from three thin sections of apollo 14 regolith breccias 14318 6 14318 48 and 14315 20 have been examined the objects have been classified and their abundances size distributions bulk compositions and where possible plagioclase compositions determined by number 64 consist predominantly of very fine grained equant plagioclase grains but can also contain larger 50 m feldspar crystals type x while 22 contain plagioclase lathes in a fine grained mafic mesostasis type y plagioclase in both spherule types displays bright yellow cathodoluminescence that is conspicuous among the blue cl of the normal feldspar of the breccias type z spherules 5 contain feldspar with blue cl and minor amounts of olivine and pyroxene type q spherules 4 contain feldspar with yellow cl but in a luminescent mesostasis of quartz or feldspar a few spherules are mixtures of type y and type x textures most type x spherules and a few type y spherules have fine grained opaque rims compound objects were also found and consist of two or more cls that appear to have collided while still plastic or molten the cls are thought to be impact spherules that crystallized in free flight their coarse textures suggesting fairly slow cooling rates 1 c s the abundance of the cls resembles that of chondrules in the cm chondrite murchison and their cumulative size frequency distributions are very similar to those of the chondrules in several meteorite classes the bulk compositions of the cls do not resemble regoliths at any of the apollo sites including apollo 14 or any of the common impact glasses but they do resemble the bulk compositions of several lunar meteorites and the impact glasses they contain the apollo 14 site is located on a region containing imbrium ejecta and we suggest that the cls derive from the imbrium impact ballistic calculations indicate that only impact events of this size on the moon are capable of producing melt spherules with the required free flight times and slow cooling rates smaller impacts produce glassy spherules and agglutinates as has been pointed out many times the cls have many properties in common with meteoritic chondrules while much remains unclear difficulties with a nebular origin and new developments in chondrule chronology studies of asteroid surfaces and impact ejecta behavior and the present observations indicate that meteoritic chondrules could have formed by impact

chondrule fragments from comet wild2 evidence for high temperature processing in the outer solar system abstract terminal grains from c2063 1 154 1 0 track 154 and c2061 1 113 5 track 113 from the stardust collection of comet wild2 s coma have been studied by tem and nanosims terminal grain 2 of c2063 1 154 1 0 consists of an al rich diopside en 97 99 al 2 o 3 9 11 wt pigeonite en 85 wo 15 with tio 2 and al 2 o 3 contents of 0 5 and 5 2 wt and minor forsterite and enstatite the mineral assemblage and al rich ti poor composition of the grain are consistent with being a fragment of an al rich chondrule similar to those present in carbonaceous chondrites the oxygen isotopic composition of the c2063 1 154 1 0 grain was determined by nanosims analyses and found to be 17 o 10 6 5 7 18 o 7 5 2 5 and 17 o 1 4 4 3 18 o 6 5 1 6 1 errors for the two sections these figures are distinct from cais and consistent with an origin as al rich chondrule fragments terminal grain 5 of c2061 1 113 5 consists of low ca pyroxene en 86 87 fs 10 11 wo 3 4 and 2 wt al 2 o 3 and in one section 5 10 of a na rich silicate phase this assemblage may be a fragment of a low ca pyroxene bearing chondrule and mesostasis the original chondrule diameter for the c2063 1 154 1 0 and c2061 1 113 5 samples by analogy with carbonaceous chondrite chondrules might have been in the range 0 2 1 0 mm if they were of that size then the presence of large grains of high temperature material e g 1500 k for such refractory assemblages could be explained through commonly invoked models of radial drift from inner to outer solar system but only if the chondrules were first fragmented to dust within the inner solar system an alternative scenario is that some chondrule formation was associated with high temperature processing and planetesimals in the outer solar system

mineralogical controls on the oxygen isotopic compositions of uocs abstract we report the results of oxygen isotopic analyses made on mineral separates prepared from chondrules and whole meteorites uocs feldspar glass mainly chondrule mesostasis is more 16o poor than the olivine low ca pyroxene from which it was separated this relationship is true both for chondrules and whole meteorites from the three uoc groups feldspar and glass have exchanged to a higher degree with 16o poor gases than ferromagnesian minerals because of the susceptibility of framework silicates and glass to gas solid diffusion olivine and low ca pyroxene have preserved primitive isotopic compositions in this way mineralogy is the dominant control on the oxygen isotopic compositions of uocs the similarity of chondrule and whole meteorite isotope compositions suggests that for many uocs matrix and chondrules have undergone exchange with the same isotope reservoirs this exchange postdates chondrule formation and is believed to have occurred after consolidation with the surrounding chondrite matrix in a parent body regolith after this gas solid exchange some isotopic equilibration took place between feldspar and the other relatively 16o rich solids as a result of metamorphism a progressive decrease in the values of 18ofeldspar glass 18oolivine pyroxene for bulk meteorites occurs with increasing petrologic subtype from 8 0 00 for a type 3 2 to 2 5 for a type 3 9 a chemical origin for the initial 16o poor gas reservoirs in the h l and ll parent bodies by processes of mass independent fractionation is considered more likely than nucleosynthesis feldspar glass separated from adrar 003 l ll3 2 may be in or close to having isotopic equilibrium with the 16o poor gases for l or ll chondrites on this basis and assuming that the gas was dominated by h2o and its isotopic composition lay along a line of slope 1 corresponding to what has been called the equilibrated chondrite line a tentative upper limit for gas solid isotopic exchange in uocs can be calculated which gives 370 c

polarity reversal in the solomon islands arc mckenzie1 proposed that the direction of subduction beneath an island arc will reverse following arc continent collision dewey and bird2 suggested that such an arc polarity reversal occurred in northern new guinea johnson and molnar3 however proposed two alternative models of plate convergence that could explain the observed seismicity we present here a spatial seismicity study of the solomon islands region which has revealed the existence of two juxtaposed wadati benioff w b zones of opposite polarity shallow and intermediate foci define a north east dipping w b zone associated with active subduction along the new britain and san cristobal trenches and a south west dipping w b zone associated with the inactive north solomon trench nst these data provide the first direct seismic evidence of a reversal in subduction polarity at an island arc

topographic asymmetry of the south atlantic from global models of mantle flow and lithospheric stretching the relief of the south atlantic is characterized by elevated passive continental margins along southern africa and eastern brazil and by the bathymetric asymmetry of the southern oceanic basin where the western flank is much deeper than the eastern flank we investigate the origin of these topographic features in the present and over time since the jurassic with a model of global mantle flow and lithospheric deformation the model progressively assimilates plate kinematics plate boundaries and lithospheric age derived from global tectonic reconstructions with deforming plates and predicts the evolution of mantle temperature continental crustal thickness long wavelength dynamic topography and isostatic topography mantle viscosity and the kinematics of the opening of the south atlantic are adjustable parameters in thirteen model cases model predictions are compared to observables both for the present day and in the past present day predictions are compared to topography mantle tomography and an estimate of residual topography predictions for the past are compared to tectonic subsidence from backstripped borehole data along the south american passive margin and to dynamic uplift as constrained by thermochronology in southern africa comparison between model predictions and observations suggests that the first order features of the topography of the south atlantic are due to long wavelength dynamic topography rather than to asthenospheric processes the uplift of southern africa is best reproduced with a lower mantle that is at least 40 times more viscous than the upper mantle

patterns of late cenozoic exhumation deduced from apatite and zircon u he ages from fiordland new zealand new apatite and zircon u th he ages from the fiordland region of new zealand s south island expand on earlier results and provide new constraints on patterns of late cenozoic exhumation and cooling across this region zircon u th he cooling ages in combination with increased density of apatite ages show that in addition to a gradual northward decrease in cooling ages that was seen during an earlier phase of this study there is also a trend toward younger cooling ages to the east distinct breaks in cooling age patterns on southwestern fiordland appear to be correlated to the location of previously mapped faults the northward decrease in ages may reflect asynchronous cooling related to migration in the locus of exhumation driven by subduction initiation or it may reflect synchronous regional exhumation that exposed different structural levels across fiordland or some combination of these effects in either case differential exhumation accommodated by major and minor faults that dissect fiordland basement rocks apparently played an important role in producing the resulting age patterns

relict olivine chondrule recycling and the evolution of nebular oxygen reservoirs chondrules often contain relict olivine grains that did not crystallize in situ providing opportunities to decipher how chondrule components evolved we studied olivine in the sahara 97210 ll3 2 wells ll3 3 and chainpur ll3 4 chondrites using sem empa and sims techniques oxygen isotopes were analyzed in 16 objects from all three meteorites and trace elements were analyzed in sahara 97210 and chainpur two groups of olivine are identified based on oxygen isotope compositions one group is enriched in 16 o 17 o 8t o 4 and falls close to the ccam mixing line it includes forsterite and mg rich olivine present as relict grains in type ii ferrous chondrules and the forsteritic cores of some isolated grains these low 17 o grains are poor in mno 0 2 wt and are usually enriched in cao 0 3 0 65 wt the other group is less enriched in 16 o 17 o 3t o 4 it includes normal non relict ferrous olivine in type ii chondrules normal non relict mg rich olivine in type i magnesian chondrules dusty olivine relict grains in type i chondrules and mg rich olivine relicts in a type ii and a type i chondrule these high 17 o grains have variable cao 0 0 95 wt and mno 0 0 45 wt contents with the more calcic and mn poor compositions associated with forsteritic olivine trace element data show that forsteritic olivine grains in both oxygen groups are similarly enriched in refractory elements al sc y ca ti v and depleted in volatile elements cr mn p rb sometimes k and na compared to normal ferrous olivine suggesting that variations in chemical composition reflect the extent of thermal processing greater for magnesian olivine the data are consistent with a model in which nebular reservoirs became less enriched in 16 o with time an earlier episode of chondrule formation produced type i chondrules and isolated forsterites in carbonaceous chondrites and forsteritic grains that were incorporated into ordinary chondrites as relict grains in chondrules and as isolated grains later episodes of chondrule formation produced both type i and type ii chondrules in ordinary chondrites under a variety of thermal and redox conditions 2007 elsevier b v all rights reserved

phosphate and feldspar mineralogy of equilibrated l chondrites the record of metasomatism during metamorphism in ordinary chondrite parent bodies in ordinary chondrites ocs phosphates and feldspar are secondary minerals known to be the products of parent body metamorphism both minerals provide evidence that metasomatic fluids played a role during metamorphism we studied the petrology and chemistry of phosphates and feldspar in petrologic type 4 6 l chondrites to examine the role of metasomatic fluids and to compare metamorphic conditions across all three oc groups apatite in l chondrites is cl rich similar to h chondrites whereas apatite in ll chondrites has lower cl f ratios merrillite has similar compositions among the three chondrite groups feldspar in l chondrites shows a similar equilibration trend to ll chondrites from a wide range of plagioclase compositions in petrologic type 4 to a homogeneous albitic composition in type 6 this contrasts with h chondrites which have homogeneous albitic plagioclase in petrologic types 4 6 alkali and halogen rich and likely hydrous metasomatic fluids acted during prograde metamorphism on oc parent bodies resulting in albitization reactions and development of phosphate minerals fluid compositions transitioned to a more anhydrous cl rich composition after the asteroid began to cool differences in secondary minerals between h and l ll chondrites can be explained by differences in fluid abundance duration or timing of fluid release phosphate minerals in the regolith breccia kendleton show lithology dependent apatite compositions bulk cl f ratios for ocs inferred from apatite compositions are higher than measured bulk chondrite values suggesting that bulk f abundances are overestimated and that bulk cl f ratios in ocs are similar to ci

elemental redistribution in tieschitz and the origin of white matrix two types of mesostasis coexist within some porphyritic chondrules in tieschitz one type is smooth the other confined to chondrule margins is blocky on a 5 10 m scale mesostases in one porphyritic olivine pyroxene pop chondrule and one porphyritic olivine po chondrule were analysed by scanning electron microscopy sem and energy dispersive x ray spectrometry eds as was white matrix nearby mesostases in the po chondrule and in four others were analysed by ion probe pyroxene phenocrysts or dendrites extend across contacts between smooth and blocky mesostasis with no compositional change relative to smooth mesostasis blocky mesostasis is enriched in al alkalis ba f and cl but depleted in si fe and ca white matrix fills channels between the chondrules it is physically and chemically similar to blocky mesostasis but three ion probe analyses indicate that unlike the mesostases it is poor in sc and has variable and fractionated rare earth elements rees smooth mesostasis is interpreted as solidified primary chondrule liquid whereas blocky mesostasis is its alteration product or less likely a precipitate replacing smooth mesostasis leached out by aqueous fluid white matrix may have formed by secondary alteration or replacement of mesostases that had been expelled from chondrules during accretion or as a precipitate filling interchondrule voids iron may have been lost from the bulk meteorite but most other elements merely underwent internal redistribution disturbed isotopic systems indicate that aqueous fluid may have been active on the tieschitz parent body only 2 ga ago if correct this would be the first evidence that an ordinary chondrite parent body underwent internal reprocessing significantly later than 4 5 ga ago

subduction duration and slab dip the dip angles of slabs are among the clearest characteristics of subduction zones but the factors that control them remain obscure here slab dip angles and subduction parameters including subduction duration the nature of the overriding plate slab age and convergence rate are determined for 153 transects along subduction zones for the present day we present a comprehensive tabulation of subduction duration based on isotopic ages of arc initiation and stratigraphic structural plate tectonic and seismic indicators of subduction initiation we present two ages for subduction zones a long term age and a reinitiation age using cross correlation and multivariate regression we find that 1 subduction duration is the primary parameter controlling slab dips with slabs tending to have shallower dips at subduction zones that have been in existence longer 2 the long term age of subduction duration better explains variation of shallow dip than reinitiation age 3 overriding plate nature could influence shallow dip angle where slabs below continents tend to have shallower dips 4 slab age contributes to slab dip with younger slabs having steeper shallow dips and 5 the relations between slab dip and subduction parameters are depth dependent where the ability of subduction duration and overriding plate nature to explain observed variation decreases with depth the analysis emphasizes the importance of subduction history and the long term regional state of a subduction zone in determining slab dip and are consistent with mechanical models of subduction

fore arc basalts and subduction initiation in the izu bonin mariana system recent diving with the jamstec shinkai 6500 manned submersible in the mariana fore arc southeast of guam has discovered that morb like tholeiitic basalts crop out over large areas these fore arc basalts fab underlie boninites and overlie diabasic and gabbroic rocks potential origins include eruption at a spreading center before subduction began or eruption during near trench spreading after subduction began fab trace element patterns are similar to those of morb and most izu bonin mariana ibm back arc lavas however ti v and yb v ratios are lower in fab reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid ocean ridges and back arc basins some fab also have higher concentrations of fluid soluble elements than do spreading center lavas thus the most likely origin of fab is that they were the first lavas to erupt when the pacific plate began sinking beneath the philippine plate at about 51 ma the magmas were generated by mantle decompression during near trench spreading with little or no mass transfer from the subducting plate boninites were generated later when the residual highly depleted mantle melted at shallow levels after fluxing by a water rich fluid derived from the sinking pacific plate this magmatic stratigraphy of fab overlain by transitional lavas and boninites is similar to that found in many ophiolites suggesting that ophiolitic assemblages might commonly originate from near trench volcanism caused by subduction initiation indeed the widely dispersed jurassic and cretaceous tethyan ophiolites could represent two such significant subduction initiation events

primordial oxygen isotope reservoirs of the solar nebula recorded in chondrules in acfer 094 carbonaceous chondrite abstract highly precise and accurate ion microprobe analyses of oxygen three isotope ratios in chondrules from the acfer 094 one of the most primitive carbonaceous chondrites show that chondrules preserve evidence for oxygen isotope heterogeneity in chondrule forming regions of the solar nebula identical 17 o values in most minerals and glass within each chondrule indicate that the oxygen isotope ratio in chondrule melt did not change during or after crystallization nearly half of the chondrules studied contain small amounts of olivine grains that have an oxygen isotope anomaly relative to other minerals and glass in the same chondrule most chondrules in acfer 094 can be classified into two oxygen isotope groups 17 o 2 and 17 o 5 indicating that the final melting of chondrules occurred within two distinct oxygen isotope reservoirs probably representing the local protoplanetary disk immediately before planetesimal formation one of these reservoirs 17 o 2 is observed from chondrules in other carbonaceous chondrites and from crystalline silicates in comet wild 2 suggesting that crystalline silicates formed in an oxygen isotope reservoir of 17 o 2 were widely distributed in the outer asteroid belt and throughout the outer solar nebula oxygen three isotope ratios of minerals in chondrules from acfer 094 are distributed along a newly defined primitive chondrule minerals pcm line which has slope 1 17 o 0 987 0 013 18 o 2 70 0 11 and intersects the terrestrial fractionation line at 18 o 5 8 0 4 these data are distinct from and plot between the ccam and young and russell lines the pcm line is interpreted to represent the mixing trend of extreme oxygen isotope reservoirs in the early solar system that were the primary oxygen isotope reservoir of solids that accreted to form planets including the earth

the meteoritical bulletin no 94 september 2008 the meteoritical bulletin 94 reports over 900 119 non antarctic and 803 antarctic newly approved meteorite names and their recovery histories macroscopic descriptions petrography mineral compositions and geochemistry meteorites reported include lunar martian howardites eucrites diogenites mesosiderites aubrites irons angrites ureilites acapulcoites lodranites and h l ll r co and cv chondrites eight of the meteorites in this bulletin are falls including 4 from india jodiya kaprada kavapura mahadevpur 1 from mali chergach turkey didim libya werdama and the u s berthoud

diffusion in silicate minerals and glasses a data digest and guide to the literature the rates of diffusion of cations oxygen and hydrogen in silicates are of fundamental importance to a wide range of petrological and geochronological problems in order that the earth scientist may readily apply the available experimentally determined diffusion rates to such problems a compilation of data for diffusion in silicate minerals and glasses is presented along with a working guide for its use published data and experimental conditions are listed in tables and these are preceded by an outline of diffusion mechanisms terminology equations and units diffusion regimes factors controlling diffusion rates measurement techniques and diffusional behaviour

geochemical and oxygen isotope perspective of a new r chondrite dhofar 1671 affinity with ordinary chondrites dhofar 1671 is a relatively new meteorite that previous studies suggest belongs to the rumuruti chondrite class major and ree compositions are generally in agreement with average values of the r chondrites rcs moderately volatile elements such as se and zn abundances are lower than the r chondrite values that are similar to those in ordinary chondrites ocs porphyritic olivine pyroxene pop radial pyroxene rp and barred olivine bo chondrules are embedded in a proportionately equal volume of matrix one of the characteristic features of rcs microprobe analyses demonstrate compositional zoning in chondrule and matrix olivines showing fa poor interior and fa rich outer zones precise oxygen isotope data for chondrules and matrix obtained by laser assisted fluorination show a genetic isotopic relationship between ocs and rcs on the basis of our data we propose a strong affinity between these groups and suggest that oc chondrule precursors could have interacted with a 17o rich matrix to form rc chondrules i e 17o shifts from 1 to 3 these interactions could have occurred at the same time as exotic clasts in brecciated samples formed such as nwa 10214 ll3 6 parnallee ll3 pca91241 r3 8 6 and dhofar 1671 r3 6 we also infer that the source of the oxidation and 17o enrichment is the matrix which may have been enriched in 17o rich water the abundance of matrix in rcs relative to ocs ensured that these rocks would be apparently more oxidized and appreciably 17o enriched in situ analysis of dhofar 1671 is recommended to further strengthen the link between ocs and rcs

the role of the overriding plate thermal state on slab dip variability and on the occurrence of flat subduction 1 slab dip varies significantly both between different and along single subduction zones provided that old subducting plates are colder and denser than young plates variations in the slab dip should correlate with slab age however recent statistical analyses do not show this expected correlation we present the results of non newtonian numerical dynamic models where subduction is driven by means of a kinematic boundary condition we systematically vary the age of both the overriding and subducting plates in order to test these effects on the slab dip at different depth ranges we find that colder overriding plates result in shallower slab dips and episodes of flat slab subduction as a result of the increased suction force in the mantle wedge the influence of the thermal state of the overriding plate on slab dip is shown here to be more important than that of the age of subducting lithosphere modeling results are qualitatively compared to the large dip variability of the cocos slab including a flat slab segment we suggest that this variability is likely related to the change of the thermal state of the overriding plates with flat subduction occurring under cold lithosphere in southwestern mexico and steep subduction under the warmer lithosphere of the northwestern caribbean plate

high precision sims oxygen three isotope study of chondrules in ll3 chondrites role of ambient gas during chondrule formation we report high precision sims oxygen three isotope analyses of 36 chondrules from some of the least equilibrated ll3 chondrites and find systematic variations in oxygen isotope ratios with chondrule types feo poor type i chondrules generally plot along a mass dependent fractionation line d 17 o 0 7 with d 18 o values lower in olivine rich ia than pyroxenerich ib chondrules data from feo rich type ii chondrules show a limited range of d 18 o and d 17 o values at d 18 o 4 5 d 17 o 2 9 and d 17 o 0 5 which is slightly 16 o enriched relative to bulk ll chondrites d 17 o 1 3 data from four chondrules show 16 o rich oxygen isotope ratios that plot near the ccam carbonaceous chondrite anhydrous mineral line glass analyses in selected chondrules are systematically higher than co existing minerals in both d 18 o and d 17 o values whereas high ca pyroxene data in the same chondrule are similar to those in olivine and pyroxene phenocrysts our results suggest that the ll chondrite chondrule forming region contained two kinds of solid precursors 1 16 o poor precursors with d 17 o 1 6 and 2 16 o rich solid precursors derived from the same oxygen isotope reservoir as carbonaceous chondrites oxygen isotopes exhibited open system behavior during chondrule formation and the interaction between the solid and ambient gas might occur as described in the following model significant evaporation and recondensation of solid precursors caused a large mass dependent fractionation due to either kinetic or equilibrium isotope exchange between gas and solid to form type ia chondrules with higher bulk mg si ratios type ii chondrules formed under elevated dust gas ratios and with water ice in the precursors in which the ambient h2o gas homogenized chondrule melts by isotope exchange low temperature oxygen isotope exchange may have occurred between chondrule glasses and aqueous fluids with high d 17 o 5 in ll the parent body according to our model oxygen isotope ratios of chondrules were strongly influenced by the local solid precursors in the proto planetary disk and the ambient gas during chondrule melting events 2010 elsevier ltd all rights reserved

contrasting oxygen diffusion in nepheline diopside and other silicates and their relevance to isotopic systematics in meteorites oxygen self diffusion coefficients have been determined in natural nepheline na3 na k al4si4o16 and synthetic diopside camg sio3 2 at one atmosphere by monitoring the rate of 18o exchange between co2 gas and minerals of pre determined grain size 10 12 5 and 15 m radius oxygen self diffusion in nepheline between 1000 and 1300 c fits a linear arrhenius relation of the form d 5 9 10 9 exp 25 0 2 5 103calrt and for diopside between 1150 and 1350 c d 6 3 exp 96 7 5 8 103calrt the activation energy for oxygen diffusion in nepheline is similar to that of melilite 31 9 0 1 kcal mol but the oxygen diffusivity is lower by two orders of magnitude the diffusion of oxygen in diopside is similar to that of nepheline at 1375 c however the activation energy is almost four times higher this contrast suggests a difference in the crystal chemical and structural controls of the minerals both nepheline and melilite are characterized by strongly distorted alkali sites within their frameworks which could account for their relatively low activation energies and high diffusivities more importantly nepheline and melilite have sheet like structures and high anionic porosities which may provide ready migration paths for oxygen the concentration of open space between the silicate sheets in melilite may explain the greater diffusivity of oxygen in melilite than in nepheline diopside forsterite anorthite spinel are characterized by high activation energies 80 to 100 kcal and are refractory with very stable bonds and low anion porosities furthermore these minerals are much denser and closer packed than nepheline or melilite making it difficult for oxygen to migrate within the crystalline structure and have access to the cation oxygen bonds oxygen diffusion studies constrain interpretations of post solidus oxygen exchange among minerals and may be used to explain the stable isotope systematics observed in ca al rich inclusions of carbonaceous chondrites the slow exchange and diffusion of oxygen in diopside suggests that the isotopic composition of clinopyroxenes in ca al rich inclusions of carbonaceous chondrites has not changed appreciably since their incorporation into the meteorite however minerals such as nepheline and melilite which readily exchange oxygen record the isotopic composition of the last nebular gas to which they were exposed

oxygen isotope studies of ordinary chondrites several stages in the evolution of ordinary chondritic meteorites are recorded in the oxygen isotopic composition of the meteorites and their separable components chondrules fragments clasts and matrix the whole rock isotopic compositions reflect the iron group of the meteorite h l or ll isotopic uniformity of h3 to h6 and l3 to l6 are consistent with closed system metamorphism within each parent body ll3 chondrites differ slightly from ll4 to ll6 implying a small degree of opensystem aqueous alteration and carbon reduction on the scale of individual chondrules the meteorites are isotopically heterogeneous allowing recognition of the solar nebular processes of chondrule formation chondrules for all classes of ordinary chondrites are derived from a common population which was separate from the population of chondrules in carbonaceous or enstatite chondrites chondrules define an isotopic mixing line dominated by exchange between 16o rich and 16o poor reservoirs the oxygen isotopic compositions of chondrites serve as fingerprints for identification of genetic association with other meteorite types achondrites and iron and for recognition of source materials in meteoritic breccias

nwa 1152 and sahara 00182 new primitive carbonaceous chondrites with affinities to the cr and cv groups we have investigated the mineralogy petrography bulk chemistry and light element isotope composition of the ungrouped chondrites north west africa nwa 1152 and sahara 00182 nwa 1152 contains predominantly type 1 porphyritic olivine po and porphyritic olivinepyroxene pop chondrules chondrule silicates are magnesium rich fo98 8 1 2 n 36 fs2 3 2 1 wo1 2 0 3 n 23 matrix comprises 40 vol of the sample and is composed of a micron sized silicate groundmass with larger silicate sulfide magnetite and fe ni metal ni 50 wt grains phyllosilicates were not observed in the matrix refractory inclusions are rare 0 3 vol and are spinel pyroxene aggregates or amoeboid olivine aggregates melilite is absent from the refractory inclusions sahara 00182 contains predominantly type 1 po chondrules pop chondrules are less common most chondrules contain blebs of and are often rimmed with fe ni metal and sulfide chondrule phenocrysts are magnesium rich fo92 2 0 6 n 129 fs4 4 1 8 wo1 3 1 1 n 16 matrix comprises 30 vol of the meteorite and is predominantly sub micron silicates with rare larger silicate gains matrix fe ni metal mean ni 5 8 wt and sulfide grains are up to mm scale no phyllosilicates were observed in the matrix refractory inclusions are rare 1 1 vol and melilite is absent the oxygen isotope composition of nwa 1152 falls within the range of the cv chondrites with 17o 3 43 18o 0 70 and is similar to sahara 00182 17o 3 89 18o 0 19 grossman and zipfel 2001 based on mineralogical and petrographic characteristics we suggest nwa 1152 and sahara 00182 show many similarities with the cr chondrites however oxygen isotopes suggest affinity with the cvs thus neither sample can be assigned to any of the currently known carbonaceous chondrite groups based on traditionally recognized characteristics both samples demonstrate the complexity of inter and intra group relationships of the carbonaceous chondrites whatever their classification nwa 1152 and sahara 00182 represent a source of relatively pristine solar system material

overriding plate thickness control on subducting plate curvature subducting plate sp curvature exerts a key control on the amount of bending dissipation associated with subduction and the magnitude of the subduction resisting bending force however the factors controlling the development of sp curvature are not well understood we use numerical models to quantify the role of sp rheology on the minimum radius of curvature rmin we find that rmin depends strongly on the sp thickness when the rheology is viscous this dependence is substantially reduced when the sp behaves plastically in line with the lack of correlation between rmin and sp thickness on earth in contrast plasticity leads to a strong positive correlation between rmin and the overriding plate op thickness using an analysis of rmin versus op thickness we show that such a positive correlation exists on earth this suggests that op structure in conjunction with sp plasticity is crucial in generating slab curvature systematics on earth

thermal modeling of the geometry of subduction with implications for the tectonics of the overriding plate the path which subducting lithosphere follows can be broken into three zones zone i a subhorizontal region of nearly constant dip near the trench zone ii a region of rapidly increasing dip and zone iii a region generally deeper than 125 km again of nearly constant dip we have modeled the thermal evolution of slabs as they pass through these three zones using the method of lines a numerical technique well suited to the solution of time dependent partial differential equations by use of appropriate initial conditions and time dependent boundary conditions we can simulate subduction with variations in age of subducting lithosphere slab velocities and paths followed by the slab thermal modeling results indicate that the geometry of the slab in region ii can be empirically related directly to the thermal structure of the slab in that region and thus indirectly to age and velocity it follows that low angle subduction can extend several hundred kilometers inland only in cases of high convergent velocities 15 cm yr 1 and old 120 m y lithosphere we further suggest that rapid variations in the convergence velocities can provide an enabling mechanism for back arc spreading while slow changes in convergence velocities and or age variations in the subducting lithosphere lead to migration of volcanic arcs

repeated crustal thickening and recycling during the andean orogeny in north chile 21 26 s posttectonic granitoids 2 lower crustal p wave velocities of 7 3 7 7 km s 1 compatible with underplated mafic crust and 3 results from recent experimental petrologic work showing garnet stability in mafic mineralogies 12 kbar 40 km crustal thickness analogous to older andean magmatic episodes in north chile newly underplated basaltic crust may account for the remaining deficit in neogene crustal thickening similar evolutionary patterns in geochemistry and initial sr and nd isotopic characteristics of andean 200 ma to present magmatic rocks suggest that the andean orogeny in this region evolved by a combination of processes of repeated arc migration tectonic and magmatic crustal thickening and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust index terms 1020 geochemistry composition of the crust 1030 geochemistry geochemical cycles 0330 8159 evolution of the earth rheology crust and lithosphere 9360 information related to geographic region south america keywords andean orogeny continental crust recycling adakite ttg crustal thickening

correlated mineralogy chemical compositions oxygen isotopic compositions and size of chondrules we report results of a study into the size chemical composition mineralogy and oxygen isotopic compositions of 40 chondrules in the size range 1 9 mm diameter separated from chainpur ll3 4 and parnallee ll3 6 these 4 parameters were found to be interdependent the main control on the bulk compositions of chondrules is the relative volatilities of the elements the major elements mg al ti ca show strong correlations among one other and most of the remaining more volatile elements form another grouping increasing proportions of refractory to volatile elements correlate with chondrule size smaller chondrules tend to have more glassy sio2 rich mesostases than larger chondrules which have a higher proportion of mesostasis with feldspathic compositions chondrules with high sio2 glassy mesostases have exchanged with 16o poor gases to a greater degree than low sio2 mesostases containing crystalline feldspar the state of mesostases of the chondrules is the most important control on their oxygen isotopic compositions although there also seems to be a size effect in chainpur smaller chondrules in chainpur have more 16o poor compositions suggesting that the relatively large surface areas of smaller chondrules facilitated gas solid exchange this correlation is obscured in parnallee because it has undergone more extensive devitrification than chainpur leading to an increase in the proportion of crystalline mesostasis the model we propose to explain the interdependence of bulk compositions size and oxygen isotopes is in two stages chondrules which were still partially molten experienced an influx of relatively volatile elements from ambient gases smaller chondrules experienced a relatively greater influx than larger chondrules because of their greater surface area volume ratio the second stage following crystallisation of the chondrules and limited devitrification of the mesostases is the influx of an 16o poor gas which led to the isotopic exchange we believe the most likely setting for the formation of these chondrules and the associated volatile rich and 16o poor gas influxes is a planetary regolith setting

water in snc meteorites evidence for a martian hydrosphere the shergotty nakhla chassigny snc meteorites purportedly of martian origin contain 0 04 to 0 4 percent water by weight oxygen isotopic analysis can be used to determine whether this water is extraterrestrial or terrestrial such analysis reveals that a portion of the water is extraterrestrial and furthermore was not in oxygen isotopic equilibrium with the host rock lack of equilibrium between water and host rock implies that the lithosphere and hydrosphere of the snc parent body formed two distinct oxygen isotopic reservoirs if mars was the parent body the maintenance of two distinct reservoirs may result from the absence of plate tectonics on the planet

spinel group minerals in ll3 00 6 chondrites indicators of nebular and parent body processes we carried out a systematic study of spinel group minerals in ll3 00 3 9 and ll4 6 chondrites with increasing petrologic type the size and abundance of spinel increase the compositions of spinel group minerals in type 3 chondrites depend on the occurrence mg al rich spinel occurs mainly in chondrules some chromite occurs in chondrules and matrix and nearly pure chromite is exclusively encountered in the matrix the occurrence of nearly pure chromite and the wide compositional variations distinguish spinel group minerals in types 3 00 3 3 from those in the other types spinel group minerals in types 3 5 3 9 show a narrower range of compositions and those in types 4 6 are homogeneous the changes in composition and abundance of spinel in type 3 chondrites are most likely due to thermal metamorphism therefore the chemistry of spinel group minerals could be used as a sensitive indicator of metamorphic conditions not only for type 3 6 but also 3 00 3 9 they can be applied to identify the most primitive least metamorphosed chondrites the bulk compositions of spinel bearing chondrules and the textural setting of the spinel indicate that most spinel group minerals crystallized directly from chondrule melts however some spinel grains especially those enclosed in olivine phenocrysts can not be explained by in situ crystallization in the chondrule we interpret these spinel grains to be relic phases that survived chondrule melting this is supported by the oxygen isotopic composition of a spinel grain which has significantly lighter oxygen than the coexisting olivine the oxygen isotopic composition of this spinel is similar to those of al rich chondrules our discovery of relic spinel in chondrules is an indication of the complexities in the early solar nebular processes that ranged from formation of refractory inclusion through al rich chondrule to ferromagnesian chondrules and attests to the recycling of earlier formed materials into the precursors of later formed materials the characteristic features of spinel group minerals are not only sensitive to thermal metamorphism but also shed light on chondrule formation processes

the semarkona meteorite first recorded occurrence of smectite in an ordinary chondrite and its implications abstract semarkona is an unshocked unequilibrated ordinary chondrite much of the interchondrule matrix and the mesostases of some chondrules and clasts have been hydrothermally altered analyses of altered materials and opaque matrix are presented ca and fe have been redistributed from primary anhydrous phases into calcite and na fe smectite respectively in semarkona the process possibly requires open system behavior inconsistent with the conventional view of metamorphism of the ordinary chondrites the low petrologic type previously assigned to semarkona is the result of retrograde metamorphism under aqueous conditions of a higher temperature mineral assemblage semarkona and possibly bishunpur should be assigned to petrologic type 2

chemical and physical studies of type 3 chondrites vii annealing studies of the dhajala h3 8 chondrite and the thermal history of chondrules and chondrites abstract samples of the dhajala h3 8 chondrite have been annealed for 10 hours at 600 700 800 900 and 1000 c and at 1000 c for 1 2 20 and 100 h and their thermoluminescence tl properties measured the tl sensitivity decreased by a factor of 2 after annealing at but at higher temperatures fell by an order of magnitude an abrupt increase in the temperature of the tl peak from 172 9 cto231 8 c and a steady increase in the width of the peak from 169 7 cto212 5 c were caused by the annealing treatment the tl phosphor in dhajala is thought to be feldspar predominantly in the high temperature disordered form but the present data indicate that a contribution from the low temperature form is also present and that this low temperature component is converted to the high form by the annealing treatment the low temperature feldspar is located in a few of the chondrules 20 of those separated from the meteorite which are also noteworthy for having high tl sensitivities these chondrules must have suffered greater crystallization of their mesostasis than the other chondrules and equilibrated to lower temperatures it is argued that for compositional reasons their mesostasis constituted less of a barrier to diffusion and therefore equilibration presumably the post metamorphic cooling rate of the meteorite through the stability field of the low form was slow enough to permit some crystallization and the width and temperature of the tl peaks for petrologic types 3 5 3 9 are somehow related to cooling rate based on tl there is no indication of a correlation between petrologic type and cooling rate for types 3 5 3 9 this is not consistent with a simple single internally heated meteorite parent body

carbide magnetite assemblages in type 3 ordinary chondrites abundant carbide magnetite assemblages occur in matrix chondrules and chondrule rims in several h3 l3 and ll3 chondrites carbides cohenite fe ni 3c and haxonite fe ni 23c6 show compositional variations between different meteorites and appreciable ranges within meteorites carbides in h chondrites have lower co contents 0 0 6 wt than those in l and ll chondrites 0 3 1 2 wt metal associated with carbides and magnetite consists of high ni 50 70 wt taenite and in l and ll chondrites co rich up to 35 wt kamacite minor element contents of troilite and magnetite are very low textural observations indicate that carbide magnetite assemblages formed by replacement of metal sulfide nodules the high co contents of residual kamacite in association with carbides indicates that co is not incorporated into carbides i e fe co is much higher in the carbides than in kamacite because ni in carbides and magnetite is low the ni contents of residual taenite tend to be high ni rich sulfides were found only in ll3 chondrites possibly indicating their more extensive oxidation and or aqueous alteration we suggest that carbide magnetite assemblages in type 3 ordinary chondrites formed as the result of hydrothermal alteration of metallic fe in metal troilite nodules by a coh bearing fluid on their parent bodies this alteration resulted in carbidization of feni metal probably by co gas e g 15 fe s 4 co g fe3c s fe3o4 s or 3 fe s 2 co g fe3c s co2 g and oxidation probably by h2o gas e g 3fe s 4h2o g fe3o4 s 4h2 g the coh bearing fluids which were possibly released during metamorphism and transported through zones of high permeability may have been derived from ices adsorbed gases or hydrated minerals the co may be the result of the reaction of carbon compounds e g hydrocarbons with water vapor or magnetite

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

great earthquakes and slab pull interaction between seismic coupling and plate slab coupling great earthquakes the few largest earthquakes that account for most of the earth s seismic energy release have occurred at only a few subduction zones around the world strong locking or seismic coupling of the interface between plates at certain subduction zones is often invoked to explain these great earthquakes although past studies have correlated strong seismic coupling with a compressional stress environment that is characterized by back arc compression and caused by trenchward motion of the overriding plate the consequences of this compressional environment for the tectonic forces that drive global plate motions are not yet clear to examine these consequences we compared subduction zone earthquake magnitudes to tectonically constrained estimates of the degree to which each slab transmits its excess weight as a direct pull force on a subducting plate at seismically uncoupled subduction zones that generate only moderate sized earthquakes we find that slabs must transmit nearly their entire upper mantle weight as a pull force on the subducting plate at seismically coupled subduction zones that produce great earthquakes however we find that slabs must be nearly completely detached from their subducting plates this suggests that slabs subducting in a compressional environment experience stress induced weakening that prevents the effective transmission of the slab pull force convergent mantle flow above a descending slab that becomes decoupled from its surface plate may induce additional surface compression that further locks the subduction zone and leads to additional slab decoupling and detachment the resulting redistribution of plate driving forces may be responsible for rapid changes in plate motion c 2003 elsevier b v all rights reserved

oxygen isotope systematics of chondrules in the allende cv3 chondrite high precision ion microprobe studies the oxygen three isotope systematics of 36 chondrules from the allende cv3 chondrite are reported using high precision secondary ion mass spectrometer cameca ims 1280 twenty six chondrules have shown internally homogenous 17 o values among olivine pyroxene and spinel within a single chondrule the average 17 o values of 19 feo poor chondrules 13 porphyritic chondrules 2 barred olivine chondrules and 4 chondrule fragments show a peak at 5 3 0 6 2sd another 5 porphyritic chondrules including both feo poor and feo rich ones show average 17 o values between 3 and 2 and 2 other feo poor barred olivine chondrules show average 17 o values of 3 6 and 0 these results are similar to those for acfer 094 chondrules showing bimodal 17 o values at 5 and 2 nine porphyritic chondrules contain olivine grains with heterogeneous 17 o values as low as 18 indicating that they are relict olivine grains and some of them were derived from precursors related to refractory inclusions however most relict olivine grains show oxygen isotope ratios that overlap with those in homogeneous chondrules the 17 o values of four barred olivine chondrules range from 5 to 0 indicating that not all bo chondrules plot near the terrestrial fractionation line as suggested by previous bulk chondrule analyses based on these data we suggest the presence of multiple oxygen isotope reservoirs in local dust rich protoplanetary disk from which the cv3 parent asteroid formed a compilation of 225 olivine and low ca pyroxene isotopic data from 36 chondrules analyzed in the present study lie between ccam carbonaceous chondrite anhydrous mineral and young and russell lines these data define a correlation line of 17 o 0 982 0 019 18 o 2 91 0 10 which is similar to those defined by chondrules in cv3 chondrites and acfer 094 in previous studies plagioclase analyses in two chondrules plot slightly below the ccam line with 17 o values of 2 6 which might be the result of oxygen isotope exchange between chondrule mesostasis and aqueous fluid in the cv parent body

subduction initiation at relic arcs although plate tectonics is well established how a new subduction zone initiates remains controversial based on plate reconstruction and recent ocean drilling within the izu bonin mariana we advance a new geodynamic model of subduction initiation si we argue that the close juxtaposition of the nascent plate boundary with relic oceanic arcs is a key factor localizing initiation of this new subduction zone the combination of thermal and compositional density contrasts between the overriding relic arc and the adjacent old pacific oceanic plate promoted spontaneous si we suggest that thermal rejuvenation of the overriding plate just before 50 ma caused a reduction in overriding plate strength and an increase in the age contrast hence buoyancy between the two plates leading to si the computational models map out a framework in which rejuvenated relic arcs are a favorable tectonic environment for promoting subduction initiation while transform faults and passive margins are not

origin of mass independent oxygen isotope variation among ureilites clues from chondrites and primitive achondrites ureilite meteorites are abundant carbon rich primitive achondrites made of coarse grained equilibrated olivine and pyroxene usually pigeonite they probably sample the baked heterogeneous melt depleted mantle of a large once chondritic parent body that was broken up catastrophically while still young and hot heterogeneity in the parent body is inferred from a considerable slope 1 variation from one meteorite to another in oxygen isotopes 2 5 17o 0 2 which correlates with both molar feo mgo range 0 03 0 35 and molar feo mno range 3 57 i e 17o correlates with the redox state no consensus has yet emerged on the cause of these correlated trends one view favors their inheritance via silicates from hot nebular preaccretion processes another invokes smelting reduction of feo by c in the hot parent body here guided mainly by similar trends among equilibrated ordinary and r chondrites studies of their unequilibrated counterparts and work on other primitive achondrites we propose a new model for ureilites in which the parent body accreted nebular ice with high 17o mg rich silicates with low 17o and varying amounts of metallic iron water from the thawing ice then oxidized the metal yielding secondary feo bearing minerals with high 17o that with metamorphism became incorporated into the ureilite silicates feo mgo feo mno and 17o correlate because they rose in unison by amounts that varied spatially depending on the local amount of metal that was oxidized we suggest that the parent body was so large radius 100 km that smelting was inhibited and that carbon played a passive role in ureilite evolution although ureilites are regarded as complicated meteorites we believe our analysis explains their mass independent oxygen isotope trend and related feo variation through well understood processes and enlightens our understanding of the evolution of early planetesimals from cold wet bodies to hot dry ones

oxygen isotope heterogeneity in chondrules from the mokoia cv3 carbonaceous chondrite we report a study of the oxygen isotope ratios of chondrules and their constituent mineral grains from the mokoia oxidized cv3 chondrite bulk oxygen isotope ratios of 23 individual chondrules were determined by laser ablation fluorination and oxygen isotope ratios of individual grains mostly olivine were obtained in situ on polished mounts using secondary ion mass spectrometry sims our results can be compared with data obtained previously for the oxidized cv3 chondrite allende bulk oxygen isotope ratios of mokoia chondrules form an array on an oxygen three isotope plot that is subparallel to and slightly displaced from the ccam carbonaceous chondrite anhydrous minerals line the best fit line for all cv3 chondrite chondrules has a slope of 0 99 and is displaced significantly by 17o 2 5 from the young and russell slope one line for unaltered calcium aluminum rich inclusion cai minerals oxygen isotope ratios of many bulk cais also lie on the cv chondrule line which is the most relevant oxygen isotope array for most cv chondrite components bulk oxygen isotope ratios of most chondrules in mokoia have 18o values around 0 and olivine grains in these chondrules have similar oxygen isotope ratios to their bulk values in general it appears that chondrule mesostases have higher 18o values than olivines in the same chondrules our bulk chondrule data spread to lower 18o values than any ferromagnesian chondrules that have been measured previously two chondrules with the lowest bulk 18o values 7 5 and 11 7 contain olivine grains that display an extremely wide range of oxygen isotope ratios down to 17o 18o around 50 in one chondrule in these chondrules there are no apparent relict grains and essentially no relationships between olivine compositions which are homogeneous and oxygen isotopic compositions of individual grains heterogeneity of oxygen isotope ratios within these chondrules may be the result of incorporation of relict grains from objects such as amoeboid olivine aggregates followed by solid state chemical diffusion without concomitant oxygen equilibration alternatively oxygen isotope exchange between an 16o rich precursor and an 16o poor gas may have taken place during chondrule formation and these chondrules may represent partially equilibrated systems in which isotopic heterogeneities became frozen into the crystallizing olivine grains if this is the case we can infer that the earliest nebular solids from which chondrules formed had 17o and 18o values around 50 similar to those observed in refractory inclusions

variations of slab dip and overriding plate tectonics during subduction insights from analogue modelling we present small scale laboratory models of oceanic subduction in which plates motion is imposed by lateral boundary conditions the oceanic plate moves trenchward at constant speed and subducts below a fixed overriding plate in this configuration the long term process of subduction is not steady state slab interaction with the upper mantle lower mantle boundary results in periods of slab flattening during which the dip of the slab diminishes followed by periods of slab steepening the overriding plate tectonic regime is influenced by the dynamics of subduction slab anchoring favouring trench perpendicular shortening when the slab is anchored slab flattening further favours shortening while slab steepening favours extension or smaller shortening rates non steady state long term subduction may explain part of the variability of slab geometries evidenced by statistical analyses of present day subduction zones experiments suggest that despite boundary conditions applied on the converging plates do not change tectonics pulses within the overriding plate may be caused by this non steady state behaviour

16 o enrichments in aluminum rich chondrules from ordinary chondrites the oxygen isotopic compositions of seven al rich chondrules from four unequilibrated ordinary chondrites were measured in situ using an ion microprobe on an oxygen three isotope plot the data are continuous with the ordinary chondrite ferromagnesian chondrule field but extend it to more 16o enriched values along a mixing line of slope 0 83 0 09 with the lightest value recorded at 18o 15 7 1 8 and 17o 13 5 2 6 if al rich chondrules were mixtures of ferromagnesian chondrules and cai material their bulk chemical compositions would require them to exhibit larger 16o enrichments than we observe therefore al rich chondrules are not simple mixtures of these two components three chondrules exhibit significant internal isotopic heterogeneity indicative of partial exchange with a gaseous reservoir porphyritic al rich chondrules are consistently 16o rich relative to nonporphyritic ones suggesting that degree of melting is a key factor and pointing to a nebular setting for the isotopic exchange process because al rich chondrules are closely related to ferromagnesian chondrules their radiogenic mg isotopic abundances can plausibly be applied to help constrain the timing or location of chondrule formation

the oxygen isotope composition of chondrules and isolated forsterite and olivine grains from the tagish lake carbonaceous chondrite abstract the oxygen isotope compositions obtained by laser fluorination of hand picked separates of isolated forsterite isolated olivine and chondrules from the tagish lake carbonaceous chondrite describe a line 17o 0 95 18o 3 24 r2 0 99 similar to the trend known for chondrules from other carbonaceous chondrites the isolated forsterite grains fo99 6 99 8 18o 7 2 to 5 5 17o 9 6 to 8 2 are more 16o rich than the isolated olivine grains fo39 6 86 8 18o 3 1 to 5 1 17o 0 3 to 2 2 and have chemical and isotopic characteristics typical of refractory forsterite chondrules contain olivine fo97 2 99 8 with oxygen isotope compositions 18o 5 2 to 5 9 17o 8 1 to 1 2 that overlap those of isolated forsterite and isolated olivine an inverse relationship exists between the 17o values and fo contents of tagish lake isolated forsterite and chondrules the chondrules likely underwent greater exchange with 16o poor nebular gases than the forsterite the oxygen isotope compositions of the isolated olivine grains describe a trend with a steeper slope 1 1 0 1 r2 0 94 than the carbonaceous chondrite anhydrous mineral line ccamslope 0 95 the isolated olivine may have crystallized from an evolving melt that exchanged with 16o poor gases of somewhat different composition than those which affected the chondrules and isolated forsterite the primordial components of the tagish lake meteorite formed under conditions similar to other carbonaceous chondrite meteorite groups especially cms its alteration history has its closest affinities to ci carbonaceous chondrites

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

kinematic reconstruction of the caribbean region since the early jurassic abstract the caribbean oceanic crust was formed west of the north and south american continents probably from late jurassic through early cretaceous time its subsequent evolution has resulted from a complex tectonic history governed by the interplay of the north american south american and paleo pacific plates during its entire tectonic evolution the caribbean plate was largely surrounded by subduction and transform boundaries and the oceanic crust has been overlain by the caribbean large igneous province clip since 90 ma the consequent absence of passive margins and measurable marine magnetic anomalies hampers a quantitative integration into the global circuit of plate motions here we present an updated quantitatively described kinematic reconstruction of the caribbean region back to 200 ma integrated into the global plate circuit and implemented with gplates free software our reconstruction includes description of the tectonic units in terms of euler poles and finite rotation angles our analysis of caribbean tectonic evolution incorporates an extensive literature review to constrain the caribbean plate motion between the american continents we use a novel approach that takes structural geological observations rather than marine magnetic anomalies as prime input and uses regionally extensive metamorphic and magmatic phenomena such as the great arc of the caribbean the clip and the caribbean high pressure belt as correlation markers the resulting model restores the caribbean plate back along the cayman trough and major strike slip faults in guatemala offshore nicaragua offshore belize and along the northern andes towards its position of origin west of the north and south american continents in early cretaceous time we provide the paleomagnetic reference frame for the caribbean region by rotating the global apparent polar wander path into coordinates of the caribbean plate interior cuba and the chortis block we conclude that formation of the caribbean plate west of the north and south americas as a result of panthalassa pacific spreading leads to a much simpler plate kinematic scenario than proto caribbean atlantic spreading placing our reconstruction in the most recent mantle reference frames shows that the clip originated 2000 3000 km east of the modern galapagos hotspot and may not have been derived from the corresponding mantle plume finally our reconstruction suggests that most if not all modern subduction zones surrounding the caribbean plate initiated at transform faults two of these along the southern mexican and nw south american margins evolved diachronously as a result of migrating trench trench transform triple junctions

mass independent fractionation of oxygen isotopes in the mesostasis of a chondrule from the semarkona ll3 0 ordinary chondrite abstract a large chondrule from semarkona the most primitive ordinary chondrite known has been discovered to contain a record of mass transport during its formation in most respects it is a normal type i group a1 low feo chondrule that was produced by reduction and mass loss during the unidentified flash heating event that produced the chondrules the most abundant structural component in primitive meteorites we have previously measured elemental abundances and abundance profiles in this chondrule we here report oxygen isotope ratio abundances and ratio abundance profiles we have found that the mesostasis is zoned in oxygen isotope ratio with the center of the chondrule containing isotopically heavier oxygen than the outer regions the outer regions being volatile rich from the diffusion of volatiles into the chondrule during cooling the 17o values range from 2 0 to 9 9 while 18o range from 1 9 to 9 6 more importantly a plot of 17o against 18o has a slope of 1 1 0 2 1 and 0 88 0 10 1 when measured by two independent methods co variation of 17o with 18o that does not follow mass dependent fractionation has often been seen in primitive solar system materials and is usually ascribed to the mixing of different oxygen reservoirs we argue that petrographic and compositional data indicate that this chondrule was completely melted at the time of its formation so that relic grains could not have survived furthermore there is petrographic and compositional evidence that there was no aqueous alteration of this chondrule subsequent to its formation although it is possible to formulate a series of exchanges between the chondrule and external 16o rich and 16o poor reservoirs that may explain the detailed oxygen isotope systematics of this chondrule such a sequence of events looks very contrived we therefore hypothesize that reduction devolatilization and crystallization of the chondrule melt may have produced 16o rich olivines and 16o poor mesostasis plotting on a slope one line as part of the chondrule forming process in an analogous fashion to known chemical mass independent isotopic fractionation mechanisms during cooling volatiles and oxygen near the terrestrial line in oxygen isotope composition produced the outer zone of volatile rich and 16o rich mesostasis the chondrule therefore not only retains a record of considerable mass transport accompanying formation but also may indicate that the isotopes of oxygen underwent mass independent fractionation during the process

halite and stable chlorine isotopes in the zag h3 6 breccia zag is an h3 6 chondrite regolith breccia within which we have studied 14 halite grains 3mm the purity of the associated nacl h2o brine is implied by freezing characteristics of fluid inclusions in the halite and epma analyses together with a lack of other evaporite like phases in the zag h3 6 component this is inconsistent with multistage evolution of the fluid involving scavenging of cations in the zag region of the parent body we suggest that the halite grains are clastic and did not crystallise in situ halite and water soluble extracts from zag have light cl isotopic compositions 37 cl 1 4 to 2 8 i previously reported bulk carbonaceous chondrite values are approximately 37 cl 3 to 4 i this difference is too great to be the result of fractionation during evaporation and instead we suggest that cl isotopes in chondrites are fractionated between a light reservoir associated with fluids and a heavier reservoir associated with higher temperature phases such as phosphates and silicates extraterrestrial carbon released at 600 degrees celsius from the h3 4 matrix has 13 c 20 i consistent with poorly graphitised material being introduced into the matrix rather than indigenous carbonate derived from a brine we have also examined 28 other h chondrite falls in order to ascertain how widespread halite or evaporite like mineral assemblages are in ordinary chondrites we did not find any more to add to zag h3 6 and monahans h5 which suggests that such highly soluble phases were not usually preserved on the parent bodies

revised age of aleutian island arc formation implies high rate of magma production radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the aleutian island arc provides insight into the timing of arc formation in the middle eocene twenty eight 40 ar 39 ar ages constrain the duration of arc magmatism to the last 46 m y basaltic lavas from the finger bay volcanics the oldest exposed rocks in the arc gave an isochron age of 37 4 0 6 ma which is 12 17 m y younger than a widely cited age of 55 50 ma three main pulses of arc wide magmatism occurred at 38 29 16 11 and 6 0 ma which coincide with periods of intense magmatism in other western pacific island arcs using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m y we calculate a time averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude