oxygen isotopic compositions of allende type c cais evidence for isotopic exchange during nebular melting and asteroidal metamorphism abstract in situ oxygen isotopic measurements of primary and secondary minerals in type c cais from the allende cv3 chondrite reveal that the pattern of relative enrichments and depletions of 16 o in the primary minerals within each individual cai are similar to the patterns observed in types a and b cais from the same meteorite spinel is consistently the most 16 o rich 17 o 25 to 15 followed by al ti dioside 17 o 20 to 5 and anorthite 17 o 15 to 0 melilite is the most 16 o depleted primary mineral 17 o 5 to 3 we conclude that the original melting event that formed type c cais occurred in a 16 o rich 17 o 20 nebular gas and they subsequently experienced oxygen isotopic exchange in a 16 o poor reservoir at least three of these abc ts26f1 and 93 experienced remelting at the time and place where chondrules were forming trapping and partially assimilating 16 o poor chondrule fragments the observation that the pyroxene is 16 o rich relative to the feldspar even though the feldspar preceded it in the igneous crystallization sequence disproves the class of cai isotopic exchange models in which partial melting of a 16 o rich solid in a 16 o poor gas is followed by slow crystallization in that gas for the typical not associated with chondrule materials type c cais as well for as the types a and b cais the exchange that produced internal isotopic heterogeneity within each cai must have occurred largely in the solid state the secondary phases grossular monticellite and forsterite commonly have similar oxygen isotopic compositions to the melilite and anorthite they replace but in one case cai 160 grossular is 16 o enriched 17 o 10 to 6 relative to melilite 17 o 5 to 3 meaning that the melilite and anorthite must have exchanged its oxygen subsequent to secondary alteration this isotopic exchange in melilite and anorthite likely occurred on the cv parent asteroid possibly during fluid assisted thermal metamorphism

oxygen isotopic evolution of amoeboid olivine aggregates in the reduced cv3 chondrites efremovka vigarano and leoville amoeboid olivine aggregates aoas from the reduced cv chondrites efremovka vigarano and leoville consist of forsteritic olivine feni metal and a refractory component composed of spinel al diopside anorthite secondary ferrous olivine and alkali rich minerals nepheline and sodalite commonly observed in the oxidized cvs are rare mineralogy and chemical compositions of aoas are similar to those predicted by equilibrium thermodynamic condensation models suggesting that aoas formed primarily by gas solid condensation over a narrow temperature range slightly below the temperatures over which most ca al rich inclusions cais formed aoas in the reduced cvs preserve a 1st generation 16o rich signal 17 18o 40 similar to that observed in many cais suggesting that these refractory objects originated from a common source in the solar nebula in fact aoas and many fine grained cais may have formed by the same processes but at slightly different temperatures and can be considered a single class of refractory objects alteration of the aoas is manifested by differing extents of 16o depletion in original aoa minerals feo enrichment in olivine and formation of interstitial very fine grained na bearing phases from the six aoas and one fine grained melilite pyroxene rich cai examined in this study five distinct patterns of alteration were identified 1 one unaltered aoa from vigarano is characterized by 16o rich forsterite without feo rich rims and interstitial na bearing phases 2 weak alteration in the melilite pyroxene rich cai is characterized by incomplete 16o depletion in some melilite and precipitation of na bearing phases near the cai rim 3 oxygen isotopic composition and mineralogy are correlated in two aoas from leoville with 16o rich olivine 16o poor anorthite and a range of intermediate compositions in al diopside this pattern is consistent with model diffusion between original grains and a 16o poor reservoir during a relatively short term 60 yr high temperature 900 1100 c event 4 original forsterite has been enriched in feo but remained 16o rich in one aoa from vigarano this result is consistent with the slower rate of diffusion of o than fe and mg in olivine at least some interstitial phases are 16o rich and na bearing phases are abundant in this aoa 5 in contrast oxygen isotopic composition and fo content are correlated in two aoas from efremovka the olivine in these aoas tends to have forsteritic 16o rich cores and feo rich 16o depleted rims the general correlation between oxygen isotopic composition and fo content is difficult to model by diffusion and may have formed instead by aqueous dissolution and precipitation along the margins of preexisting olivine grains independent evidence for aqueous alteration of the efremovka aoas is provided by oh rich signals detected during ion beam sputtering of some of the 16o poor olivine elevated 16oh count rates and order of magnitude increases in 16oh detected during single analyses reflect trapping of an aqueous phase in 16o depleted olivine an elevated 16oh signal was also detected in one analysis of relatively 16o poor melilite in the melilite pyroxene cai from vigarano suggesting that this object also was altered by aqueous fluid

16 o rich melilite in co3 0 chondrites possible formation of common 16 o poor melilite by aqueous alteration a problem of long standing is the origin of the high 17o 17o 0 52 18o values of melilite in the refractory inclusions of carbonaceous chondrites the spinel and usually also the diopside in the inclusions have primitive 17o 20 but in the widely studied cv3 chondrite allende the melilite commonly has a 17o of about 4 2 recent studies of oxygen diffusion coefficients in melilite and spinel have shown that the traditional model for altering the melilite diffusive exchange with the gas in the solar nebula cannot occur under plausible nebular scenarios because of increasing evidence for aqueous alteration of allende and other carbonaceous chondrites we tested the hypothesis that melilite alteration might occur during aqueous alteration by studying melilite in co chondrites showing variable degrees of alteration our results show that almost all melilite in co3 2 kainsaz and co3 3 ornans has 17o 15 whereas in co3 0 colony and inclusions in co3 0 yamato 81020 studied by itoh et al 2000 17o in the melilite is 20 with the exception of one unusual inclusion that has undergone two melting episodes we suggest that the o isotopic composition of the melilite is reset by dissolution of the primitive melilite and reprecipitation in the presence of h2o having a high 17o value similar to that inferred from studies of cv magnetite choi et al 1997 choi et al 2000 although alteration of melilite can produce phases such as nepheline andradite and hedenbergite it appears that depending on the composition of the microenvironment melilite present in a high energy state because of fine grain size dislocations other defects can dissolve and reprecipitate with an o isotopic composition intermediate between the initial composition and that of the h2o a possible implication of our study is that the slope 0 9 arrays observed in the minerals of refractory inclusions of many carbonaceous chondrites could be mainly the result of aqueous alteration

crystal growth and disequilibrium distribution of oxygen isotopes in an igneous ca al rich inclusion from the allende carbonaceous chondrite abstract ts34 is a type b1 ca al rich inclusion cai from the allende cv3 chondrite consisting of spinel melilite ti al rich clinopyroxene fassaite and minor anorthite in an igneous texture oxygen and magnesium isotopic compositions were measured by secondary ion mass spectrometry in spots of known chemical composition in all major minerals in ts34 using the sequence of formation from dynamic crystallization experiments and from chemical compositions of melilite and fassaite the oxygen isotopic evolution of the cai melt was established oxygen isotopic compositions of the constituent minerals plot along the carbonaceous chondrite anhydrous mineral line the spinel grains are uniformly 16o rich 17o 22 7 1 7 2sd while the melilite grains are uniformly 16o poor 17o 2 8 1 8 irrespective of their akermanite content and thus their relative time of crystallization the fassaite crystals exhibit growth zoning overprinting poorly developed sector zoning they generally grow from ti rich to ti poor compositions the fassaite crystals also show continuous variations in 17o along the inferred directions of crystal growth from 16o poor 17o 3 to 16o rich 17o 23 covering the full range of oxygen isotopic compositions observed in ts34 the early crystallized 16o poor fassaite and the melilite are in oxygen isotope equilibrium and formed simultaneously the correlation of oxygen isotopic compositions with ti content in the fassaite imply that the oxygen isotopic composition of the cai melt evolved from 16o poor to 16o rich during fassaite crystallization presumably due to oxygen isotope exchange with a surrounding 16o rich nebular gas formation of spinel the liquidus phase in melts of this composition predates crystallization of all other phases so its 16o rich composition is a relic of an earlier stage anorthite exhibits oxygen isotopic compositions between 17o 2 and 9 within the range of those of fassaite indicating co crystallization of these two minerals during the earliest to intermediate stage of fassaite growth the melilite and fassaite yield an 26al 26mg mineral isochron with an initial value of 26al 27al 0 5 003 0 075 10 5 corresponding to a relative age of 0 05 0 02 myr from the canonical al mg age of cais these data demonstrate that both 16o rich and 16o poor reservoirs existed in the solar nebula at least 0 05 myr after the birth of the solar system

a link between oxygen calcium and titanium isotopes in 26 al poor hibonite rich cais from murchison and implications for the heterogeneity of dust reservoirs in the solar nebula placs platy hibonite crystals and related hibonite rich calcium aluminum rich inclusions cais hereafter collectively referred to as plac like cais have the largest nucleosynthetic isotope anomalies of all materials believed to have formed in the solar system most plac like cais have low inferred initial 26al 27al ratios and could have formed prior to injection or widespread distribution of 26al in the solar nebula in this study we report 26al 26mg systematics combined with oxygen calcium and titanium isotopic compositions for a large number of newly separated plac like cais from the murchison cm2 chondrite 32 cais studied for oxygen 26 of these also for 26al 26mg calcium and titanium our results confirm 1 the large range of nucleosynthetic anomalies in 50ti and 48ca our data range from 70 to 170 and 60 to 80 respectively 2 the substantial range of 17o values 28 to 17 with 17o 17o 0 52 18o and 3 general 26al depletion in plac like cais the multielement approach reveals a relationship between 17o and the degree of variability in 50ti and 48ca plac like cais with the highest 17o 17 show large positive and negative 50ti and 48ca anomalies while those with the lowest 17o 28 have small to no anomalies in 50ti and 48ca these observations could suggest a physical link between anomalous 48ca and 50ti carriers and an 16o poor reservoir we suggest that the solar nebula was isotopically heterogeneous shortly after collapse of the protosolar molecular cloud and that the primordial dust reservoir in which anomalous carrier phases were heterogeneously distributed was 16o poor 17o 17 relative to the primordial gaseous co h2o reservoir 17o 35 however other models such as co self shielding in the protoplanetary disk are also considered to explain the link between oxygen and calcium and titanium isotopes in plac like cais

petrography and oxygen isotopic compositions in refractory inclusions from co chondrites fine grained ca al rich inclusions fgis in yamato 81020 co3 0 and kainsaz co3 1 co3 2 chondrites have been studied by secondary ion mass spectrometry the fgis from yamato 81020 consist of aggregates of hibonite spinel melilite anorthite diopside and olivine grains with no petrographic evidence of alteration in contrast the fgis from kainsaz commonly contain alteration products such as nepheline from replacement textures and chemical compositions of altered and unaltered fgis we conclude that the alteration products formed by decomposition of melilite and anorthite all phases in the yamato 81020 fgis are enriched in 16o with 17 18o 40 except for one fgi that experienced melting oxygen isotopic compositions of melilite anorthite some spinel and diopside in kainsaz fgis changed from 17 18o 40 toward 0 by aqueous alteration alteration products in fgis are depleted in 16o relative to primary phases with 17 18o 0 these results show that fgis in co chondrites commonly had 16o rich compositions in the solar nebula the original 16o rich fgis were modified to 16o poor compositions during aqueous alteration in the parent body

oxygen isotopes of chondritic components we review the oxygen isotopic compositions of chondrite components refractory inclusions chondrules and matrix and their inter and intra crystalline oxygen isotopic distributions primary oxygen isotopic compositions acquired before planetesimal accretion are easily disturbed by parent body processes such as aqueous alternation and thermal metamorphism primary or original oxygen isotopic compositions of refractory inclusions ca al rich inclusions and amoeboid olivine aggregates distribute along a slope 1 line on the three oxygen isotope diagram over the range of 60 17 o 18 o 16 o rich and 17 o 18 o rich during refractory inclusion formation on the other hand primary minerals of most chondrules have small isotopic variations and are enriched in 17 o and 18 o relative to refractory inclusions suggesting that chondrule formation occurred in 17 o 18 o rich nebular gas however rare 16 o rich chondrules have been found suggesting some overlap in the timing of formation of chondrules and refractory inclusions in the solar nebula oxygen isotopic compositions of matrix grains distribute along the slope 1 line over the same range as refractory inclusions and chondrules the similarity in oxygen isotopic distributions suggests that matrix was originally a mechanical mixture of nebular dusts co generated with chondrules and refractory inclusions presolar grains with oxygen isotopic compositions that are clearly distinct from those of solar nebular materials are a trace component of chondrite matrices based on these oxygen isotopic characteristics more than 99 5 of the solid materials in the nebula formed locally in the solar nebula and the remainder formed in interstellar space the astrophysical setting of chondrite component formation in the early solar system is also discussed refractory inclusions and 16 o rich matrix dusts formed around the inner edge of the solar nebula on the other hand most chondrules and 17 o 18 o rich matrix dusts seem to have formed elsewhere in the solar nebula efficient large scale radial mixing of the solar nebular materials may have been an essential process in the formation of chondritic planetesimals

26al 26mg chronology and oxygen isotope distributions of multiple melting for a type c cai from allende abstract disequilibrium oxygen isotopic distributions of ca al rich inclusions cais correspond to multiple melting events in the solar nebula 26al 26mg systematics may be applicable for age differences among such melting events we have carried out a coordinated study of detailed petrographic observations and in situ oxygen and magnesium isotope measurements for a type c cai ek1 04 2 from the allende cv3 meteorite to determine the melting events and their ages the cai consists mainly of spinel anorthite olivine and pyroxene and has a core and mantle structure petrography of the core suggests that the crystallization sequence of the core minerals is from spinel anorthite olivine and to pyroxene the mantle has the same mineral assemblage as the core and shows incomplete melting and solidification textures oxygen isotopic compositions of the minerals are distributed along the carbonaceous chondrite anhydrous mineral ccam line 18o 44 to 9 which indicates to preserve a chemical disequilibrium status in the cai spinel shows a 16o rich signature 18o 43 while anorthite is 16o poor 18o 8 olivine and pyroxene in the core have the same oxygen isotopic composition 18o 15 which indicates their equilibrium olivine and pyroxene in the mantle have variable oxygen isotopic compositions and are slightly depleted in 16o 18o 13 to 4 compared with the same minerals in the core the 26al 26mg systematics is consistent with the disequilibrium status observed according to the petrography and oxygen isotopes spinel is plotted on a line of 26al 27al 0 3 5 0 2 10 5 anorthite is plotted on a line of 1 5 10 7 and olivine and pyroxene in the core are plotted on a line of 1 7 10 6 plots of olivine and pyroxene in the mantle are scattered below the isochron of these minerals in the core this study indicates that the ek1 04 2 type c cai underwent multiple heating events after the formation of its cai precursor the precursor cai was formed 0 4 myr after the formation of the solar system defined by canonical cai formation at least 1 6 myr after the precursor cai formation the cai was partially melted and the melt exchanged oxygen isotopes with surrounding 16o poor nebular gas 16o poor olivine and pyroxene in the core crystallized from the melt subsequently al rich chondrules accreted onto the cai and the cai experienced partial melting again and recrystallized to form the mantle the oxygen and magnesium isotopes in anorthite were redistributed during thermal metamorphism in the allende parent body our study reveals that the cai had been retained in the solar nebula for at least 1 6 myr and underwent multiple melting events in the nebula and oxygen and 26al 26mg systematics has been partially disturbed depending on crystal sizes by metamorphism on the parent body

oxygen isotope heterogeneities in the earliest protosolar gas recorded in a meteoritic calcium aluminum rich inclusion abstract combined petrologic oxygen and magnesium isotopic and trace element analyses of a compound calcium aluminum rich inclusion cai from the efremovka reduced cv3 carbonaceous chondrite reveal that it consists of a mg rich 16 o rich xenolithic cai previously altered in the nebula that impacted an extensively molten 16 o depleted type a host cai shortly before the end of the host s crystallization convoluted regions in the xenolith were probably formed by rapid crystallization of the partial melt produced during impact oxygen isotopic ratios in the host cai are correlated both with melilite chemistry and location in the inclusion the region immediately inside the wark lovering rim of the cai consists of 16 o rich gehlenite with 17 o ranging down to 20 but melilite becomes progressively 16 o poor 17 o 0 and mg rich towards the interior in the absence of mg isotopic fractionation this variation is best attributed to o isotopic exchange between the nebular gas and the partially molten inclusion during its crystallization this event lasted less than 200 h which implies that the host cai was transported between two nebular reservoirs with distinct o isotopic compositions during this time examination of possible transport mechanisms suggests that the transport occurred over a distance of less than 1 astronomical unit the close to canonical 26 al 27 al ratio of 4 1 10 5 determined from both inclusions implies that at most 670 000 yr after the birth of the solar system the 16 o rich reservoir was spatially limited and an 16 o poor reservoir with typical planetary isotopic composition was available for planet formation

progressive alteration in cv3 chondrites more evidence for asteroidal alteration abstract the oxidized cv3 chondrites can be divided into two major subgroups or lithologies bali like cv3oxb and allende like cv3oxa in which chondrules calcium aluminum rich inclusions cais and matrices show characteristic alteration features weisberg et al 1997 krot et al 1997d kimura and ikeda 1997 the cv3oxb lithology is present in bali kaba parts of the mokoia breccia and possibly in grosnaja and allan hills alh 85006 it is characterized by the presence of the secondary low ca phyllosilicates saponite and sodium phlogopite magnetite ni rich sulfides fayalite fa 90 ca fe rich pyroxenes fs10 50wo45 50 and andradite phyllosilicates replace primary ca rich minerals in chondrules and cais which suggests mobilization of ca during aqueous alteration magnetite nodules are replaced to various degrees by fayalite ca fe rich pyroxenes and minor andradite fayalite veins crosscut fine grained rims around chondrules and extend into the matrix thermodynamic analysis of the observed reactions indicates that they could have occurred at relatively low temperatures 300 c in the presence of aqueous solutions oxygen isotopic compositions of the coexisting magnetite and fayalite plot close to the terrestrial fractionation line with large 18ofayalite magnetite fractionation 20 we infer that phyllosilicates magnetite fayalite ca fe rich pyroxenes and andradite formed at relatively low temperatures 300 c by fluid rock interaction in an asteroidal environment secondary fayalite and phyllosilicates are virtually absent in chondrules and cais in the cv3oxa lithology which is present in allende and its dark inclusions axtell alha81258 alh 84028 lewis cliff lew 86006 and parts of the mokoia and vigarano breccias instead secondary nepheline sodalite and fayalitic olivine are common fayalitic olivine in chondrules replaces low ca pyroxenes and rims and veins forsterite grains it also forms coarse lath shaped grains in matrix secondary ca fe rich pyroxenes are abundant we infer that the cv3oxa lithology experienced alteration at higher temperatures than the cv3oxb lithology the presence of the reduced and cv3oxa lithologies in the vigarano breccia and cv3oxa and cv3oxb lithologies in the mokoia breccia indicates that all cv3 chondrites came from one heterogeneously altered asteroid the metamorphosed clasts in mokoia krot and hutcheon 1997 may be rare samples of the hotter interior of the cv asteroid we conclude that the alteration features observed in the oxidized cv3 chondrites resulted from the fluid rock interaction in an asteroid during progressive metamorphism of a heterogeneous mixture of ices and anhydrous materials mineralogically similar to the reduced cv3 chondrites

correlations between oxygen isotopic composition and petrologic setting in a coarse grained ca al rich inclusion abstract oxygen isotopic compositions in conjunction with petrologic investigation have been determined for a coarse grained type b2 ca al rich inclusion cai from the reduced cv3 vigarano using secondary ion mass spectrometry the oxygen isotopic compositions of minerals are distributed along the carbonaceous chondrite anhydrous mineral line indicating mixing between 16o rich and 16o poor nebular components the o isotopic heterogeneities among and within minerals in the cai interior indicate that cai formation started in an 16o rich nebula and subsequently continued in an 16o poor nebula 16o rich signatures of melilite and fassaite in the wark lovering rim and of olivine of the accretionary rim indicate that the nebular environment during formation of the cai returned to an 16o rich composition after processing in an 16o poor nebula these o isotopic variations in the cai support multiple heating events in the solar nebula and indicate that the nebular environments fluctuated from 16o rich to 16o poor and back to 16o rich compositions during the formation of a single cai

variations in initial 26al 27al ratios among fluffy type a ca al rich inclusions from reduced cv chondrites abstract al mg mineral isochrons of three ca al rich inclusions cais that formed primarily by condensation one fine grained spinel rich inclusion and two fluffy type a cais from the reduced cv chondrites efremovka and vigarano were obtained by in situ al mg isotope measurements using secondary ion mass spectrometry the slope of the isochron obtained for the fine grained spinel rich inclusion gives an initial 26al 27al value 26al 27al 0 of 5 19 0 17 10 5 this is essentially identical to the solar system initial 26al 27al determined by whole rock al mg isochron studies for cais in cv chondrites in contrast the isochron slopes for the two fluffy type a cais from their al mg mineral isochrons 4 703 0 082 10 5 and 4 393 0 084 10 5 are significantly lower than the solar system initial value the range of 26al 27al 0 values of the three cais from 5 19 0 17 to 4 393 0 084 10 5 corresponds to a formation age spread of 0 17 0 04 myr this formation age spread is similar to that of igneous cais from cv chondrites the data suggest that condensation and melting of minerals occurred in the hot nebular gas contemporaneously for 0 2 myr at the very beginning of our solar system if 26al was distributed homogeneously in the cai forming region alternatively the observed variations in 26al 27al 0 among fluffy type a cais would also raise a possibility of heterogeneous distributions of 26al in the forming region

oxygen isotopic sims analysis in allende cai details of the very early thermal history of the solar system abstract the oxygen isotopic micro distributions within and among minerals in a coarse grained ca al rich inclusion cai 7r 19 1 from the allende meteorite were measured by in situ using secondary ion mass spectrometry sims all values of o isotopic ratios in 7r 19 1 minerals fall along the carbonaceous chondrite anhydrous mineral mixing ccam line on a 17 o smow vs 18 o smow plot major refractory minerals spinel fassaite and melilite in 7r 19 1 showed large negative anomalies of 17 o in the order spinel 21 16 o rich melilite 18 fassaite 15 to 1 16 o poor melilite 8 to 2 however the lower limit values of 17 o are similar at about 21 a value commonly observed in cais the similarity in the extreme values of the isotope anomaly anomalies suggests that crystallization of all cais started from an 16 o enrichment of 21 17 o relative to terrestrial values the order of the o isotopic anomalies observed for 7r 19 1 except for 16 o poor melilite is parallel to the crystallization sequence determined by experiment from cai liquid stolper 1982 indicating that the o isotopic exchange in 7r 19 1 occurred between cai melt and surrounding gas while 7r 19 1 was crystallizing from the 16 o enriched cai liquid 21 in 17 o in the 16 o poor solar nebula however the a single crystallization sequence during the cooling stage cannot explain the existence of 16 o poor melilite the presence of 16 o poor melilite suggests that multiple heating events occurred during cai formation the sharp contact between 16 o rich and 16 o poor melilite crystals and within 16 o rich melilite indicates that these multiple heatings occurred quickly based on the o isotopic and chemical compositions fassaite crystals were aggregates of relic crystals formed from cai melt whichthat have had various o isotopic compositions from the remelting processes the results of intra mineral distributions of o isotopes also support multiple heating events during cai formation

fluffy type a ca al rich inclusions in the allende meteorite abstract allende fluffy type a s fta s are a distinct sub group of ca al rich inclusions whose primary mineral assemblage consists of al rich melilite ak 0 33 spinel that is commonly very v rich perovskite and frequently hibonite some contain relatively coarse grained melilite up to 1 5 mm that is intensely kink banded and commonly reversely zoned hibonite and v rich spinel others contain much finer grained and strain free melilite 50 m and have not been found to contain hibonite or v rich spinel some fta s contain both coarser and finer grained melilite and textural relationships indicate that the latter is replacing the former fta s are characterized by extremely irregular shapes and 60 75 volume per cent of fine grained secondary alteration products many are aggregates of innumerable nodules each of which is surrounded by a wark lovering type rim sequence these nodules are frequently separated from one another by matrix like clastic rim material other fta s do not have nodular structure structural and mineralogical characteristics of their wark lovering rims suggest that fta s did not achieve their shapes by deformation of a liquid or a hot plastic solid in contrast to those in type b inclusions formation of reverse zoning in the coarser grained melilite crystals in fta s cannot be understood in terms of crystallization from a liquid but are readily explainable by condensation from a solar nebular gas during a period of falling pressure further evidence against a liquid origin is the wide range of spinel compositions within individual coarser grained fta s the fact that the reversely zoned melilite crystals cannot have been produced in any kind of sublimation or distillation process precludes formation of these inclusions as volatilization residues fta s are aggregates in some of which are preserved vapor solid condensate grains that formed at high temperature in the solar nebula

intrinsic and scattering seismic wave attenuation in the canary islands the canary islands volcanic complex is studied in terms of coda wave attenuation a multiple lapse time window method based on the hypothesis of multiple isotropic scattering with uniform distribution of scatterers is applied to the available seismic data in order to obtain the intrinsic absorption qi 1 and the scattering attenuation qs 1 in the canarian lithosphere the analysis is performed for two hypocentral distance ranges from 0 to 80 km and from 0 to 140 km results show that in both cases and for all the studied frequency bands 1 2 2 4 6 8 and 8 10 hz intrinsic absorption dominates the low albedos found in the region indicate the low degree of heterogeneity in the canarian lithosphere at the scale length of the studied frequencies on the other hand the coda attenuation qc 1 calculated on the basis of the single scattering theory gives values near qi 1 for low frequencies and near the total attenuation qt 1 for high frequencies the degree of frequency dependence of the attenuation parameters is strong in all cases a correlation of the observed attenuation parameters with the geological evidence for a hotspot type archipelago and with other geophysical studies is suggested

grain growth control of isotope exchange between rocks and fluids isotope exchange between fluid and rocks has been traditionally considered to be rate limited by two elementary processes lattice diffusion in the matrix minerals and dissolution into the fluid followed by precipitation from it in this study we show the results of high pressure experiments on 18 o water infiltration into quartzite that point to a third highly efficient process grain growth accompanied by migration of the grain boundaries gbs that are isotopically enriched by gb diffusion and surface exchange we predict on the basis of a mass transfer mechanism discrimination diagram that this gb sweeping is the primary control on isotopic equilibration under hydrothermal conditions in various fine grained rocks with low fluid fraction grain growth should be considered when interpreting and simulating isotope and chemical composition of rocks and fluids

calcium aluminum rich inclusions recycled during formation of porphyritic chondrules from ch carbonaceous chondrites abstract we report on the mineralogy petrography and o isotope compositions of 60 ca al rich inclusions cais incompletely melted during formation of porphyritic chondrules from the ch metal rich carbonaceous chondrites and isheyevo ch cb these include i relict polymineralic cais in porphyritic chondrules ii cais surrounded by chondrule like igneous rims iii igneous pyroxene rich and type c like cais and iv plagioclase rich chondrules with clusters of relict spinel grains 26al 26mg systematics were measured in 10 relict cais and 11 cai bearing plagioclase rich chondrules based on the mineralogy the ch cais incompletely melted during chondrule formation can be divided into grossite rich n 13 hibonite rich n 11 spinel melilite rich n 33 these include plagioclase rich chondrules with clusters of relict spinel grains types mineralogical observations indicate that these cais were mixed with different proportions of ferromagnesian silicates and experienced incomplete melting and gas melt interaction during chondrule formation these processes resulted in partial or complete destruction of the cai wark lovering rims replacement of melilite by na bearing plagioclase and dissolution and overgrowth of nearly end member spinel by chromium and iron bearing spinel only two relict cais and two cai bearing chondrules show resolvable excess of radiogenic 26mg the inferred initial 26al 27al ratios are 1 7 1 3 10 6 3 7 3 1 10 7 1 9 0 9 10 6 and 4 9 2 6 10 6 there is a large range of 17o among the ch cais incompletely melted during chondrule formation from 37 to 5 the unmelted minerals in individual cais however are isotopically uniform and systematically 16o enriched relative to the host chondrules and chondrule like igneous rims which have 17o ranging from 7 to 4 most of the ch cais incompletely melted during chondrule formation are mineralogically and isotopically similar to the ch cais surrounded by wark lovering rims and apparently unaffected by chondrule melting the mineralogy and o isotope compositions of the ch cai bearing chondrules are similar to those of the ch porphyritic chondrules without relict cais we conclude that ch porphyritic chondrules formed by incomplete melting of isotopically diverse solid precursors including mineralogically and isotopically unique cais commonly observed only in ch chondrites therefore the ch porphyritic chondrules must have formed in a distinct disk region where the ch cais were present at the time of chondrule formation because most ch cais avoided chondrule melting we infer that chondrule formation was highly localized these observations preclude formation of ch porphyritic chondrules by splashing of molten planetesimals by impact processing of differentiated planetesimals and by large scale nebular shocks e g shocks driven by disk gravitational instabilities or by x ray flares instead they are consistent with small scale chondrule forming mechanisms proposed in the literature such as nebular processing of dust balls by bow shocks and by current sheets

heterogeneous distribution of 26al at the birth of the solar system evidence from corundum bearing refractory inclusions in carbonaceous chondrites abstract we report on the mineralogy petrology and in situ oxygen and magnesium isotope measurements using secondary ion mass spectrometry of 10 corundum bearing calcium aluminum rich inclusions cais from the adelaide ungrouped murray and murchison cm carbonaceous chondrites we also measured in situ oxygen isotope compositions of several isolated corundum grains in the matrices of murray and murchison most of the corundum bearing objects studied are uniformly 16o rich 17o values range from 17 to 28 2 2 5 17oavr 23 5 suggesting that they formed in a 16o rich gas of approximately solar composition and largely avoided subsequent thermal processing in an 16o poor gaseous reservoir there is a large spread of the initial 26al 27al ratio 26al 27al 0 in the corundum bearing cais two adelaide cais show no resolvable excess of radiogenic 26mg 26mg the inferred 26al 27al 0 are 0 6 2 0 10 6 and 0 9 1 2 10 6 respectively slopes of the model 26al 26mg isochrons in five cais from murray and murchison are 4 4 0 2 10 5 3 3 0 3 10 5 4 1 0 3 10 5 3 9 0 4 10 5 and 4 0 2 0 10 6 respectively these values are lower than the canonical 26al 27al 0 ratio of 5 23 0 13 10 5 inferred from the whole rock magnesium isotope measurements of the cv cais but similar to the 26al 27al 0 ratio of 4 1 0 2 10 5 in the corundum bearing cai f5 from murray five other previously studied corundum bearing cais from acfer 094 ungrouped and cm carbonaceous chondrites showed no resolvable 26mg we conclude that the corundum bearing cais as well as the solar corundum grains from matrices and acid resistant residues of unequilibrated ordinary and carbonaceous chondrites recorded heterogeneous distribution of 26al in the solar system during an epoch of cai formation the 26al rich and 26al poor corundum bearing cais and solar corundum grains represent different generations of refractory objects formed during this epoch as a result its duration cannot be inferred from 26al 26mg systematics of cais oxygen isotope composition of the protoplanetary disk was probably heterogeneous during this time reflecting either initial differences in oxygen isotopic compositions of the solid and gaseous reservoirs in the early solar system or rapid isotopic evolution of these reservoirs in the protoplanetary disk with time we suggest that 26al was injected into the protosolar molecular cloud core possibly by a wind from a neighboring massive star or by or a low mass asymptotic giant branch star prior to formation of cais and refractory grains and was subsequently homogenized through the protoplanetary disk by radial mixing

oxygen isotopic alteration in ca al rich inclusions from efremovka nebular or parent body setting in situ sims oxygen isotope data were collected from a coarse grained type b1 ca al rich inclusion cai and an adjacent fine grained cai in the reduced cv3 efremovka to evaluate the timing of isotopic alteration of these two objects the coarse grained cai cgi 10 is a sub spherical object composed of elongate euhedral normally zoned melilite crystals ranging up to several hundreds of m in length coarse grained anorthite and al ti diopside fassaite all with fine grained 10 m across inclusions of spinel similar to many previously examined coarse grained cais from cv chondrites spinel and fassaite are 16 o rich and melilite is 16 o poor but in contrast to many previous results anorthite is 16 o rich isotopic composition does not vary with textural setting in the cai analyses of melilite from the core and mantle and analyses from a variety of major element compositions yield consistent 16 o poor compositions cgi 10 originated in an 16 o rich environment and subsequent alteration resulted in complete isotopic exchange in melilite the fine grained cai fgi 12 also preserves evidence of a 1st generation origin in an 16 o rich setting but underwent less severe isotopic alteration fgi 12 is composed of spinel melilite nodules linked by a mass of al diopside and minor forsterite along the cai rim all minerals are very fine grained 5 m with no apparent igneous textures or zoning spinel al diopside and forsterite are 16 o rich while melilite is variably depleted in 16 o 17 18 o from 40 to 5 the contrast in isotopic distributions in cgi 10 and fgi 12 is opposite to the pattern that would result from simultaneous alteration the object with finer grained melilite and a greater surface area volume has undergone less isotopic exchange than the coarser grained object thus the two cais were altered in different settings as the cais are adjacent to each other in the meteorite isotopic exchange in cgi 10 must have preceded incorporation of this cai in the efremovka parent body this supports a nebular setting for isotopic alteration of the commonly observed 16 o poor melilite in coarse grained cais from cv chondrites

the formation of rims on calcium aluminum rich inclusions step i flash heating abstract wark lovering rims of six calcium aluminum rich inclusions cais representing the main cai types and groups in allende efremovka and vigarano were microsurgically separated and analysed by neutron activation analysis naa all the rims have similar 4x enrichments relative to the interiors of highly refractory lithophile and siderophile elements the naa results are confirmed by ion microprobe and scanning electron microscope sem analyses of rim perovskites and rim metal grains less refractory eu yb v sr ca and ni are less enriched in the rims the refractory element patterns in the rims parallel the patterns in the outer parts of the cais in particular the rims on type b1 cais have the igneously fractionated rare earth element ree pattern of the melilite mantle below the rim and not the ree pattern of the bulk cai proving that the refractory elements in the rims were derived from the outer mantle and were not condensates onto the cais the refractory elements were enriched in an al2o3 rich residue 50 m thick after the most volatile 80 of the outermost 200 m of each cai had been volatilized including much mg si and ca some volatilization occurred below the rim and created refractory partial melts that crystallized hibonite and gehlenitic melilite the required flash heating probably exceeded 2000 c but for only a few seconds in order to melt only the outer cai and to unselectively volatilize slow diffusing o isotopes which show no mass fractionation in the rim the volatilization did however produce heavy mass fractionated mg in rims in some cais this was later obscured when normal mg diffused in from accreted olivine grains at relatively high temperature not the lower temperature meteorite metamorphism and created the 50 m set of monomineralic rim layers of pyroxene melilite and spinel

calcium aluminum rich inclusions and amoeboid olivine aggregates from the cr carbonaceous chondrites calcium aluminum rich refractory inclusions cais in cr chondrites are rare 1 vol fairly small 500 m and irregularly shaped and most of them are fragmented based on the mineralogy and petrography they can be divided into grossite hibonite rich melilite rich and pyroxene anorthite rich cais other types of refractory objects include fine grained spinel melilite pyroxene aggregates and amoeboid olivine aggregates aoas some of the pyroxene anorthite rich cais have igneous textures and most melilite rich cais share similarities to both the fluffy and compact type a cais found in cv chondrites one major difference between these cais and those in cv cm and co chondrites is that secondary mineral phases are rare in situ ion microprobe analyses of oxygen isotopic compositions of 27 cais and aoas from seven cr chondrites demonstrate that most of the cais are 16o rich 17o of hibonite melilite spinel pyroxene and anorthite 22 and isotopically homogeneous within 3 4 likewise forsterite spinel anorthite and pyroxene in aoas have nearly identical 16o rich compositions 24 17o 20 deg in contrast objects which show petrographic evidence for extensive melting are not as 16o rich 17o less than 18 o secondary alteration minerals replacing 16o rich melilite in melilite rich cais plot along the terrestrial fractionation line most cr cais and aoas are mineralogically pristine objects that largely escaped thermal metamorphism and secondary alteration processes which is reflected in their relatively homogeneous 16o rich compositions it is likely that these objects or their precursors condensed in an 16o rich gaseous reservoir in the solar nebula in contrast several igneous cais are not very enriched in 16o probably as a result of their having melted in the presence of a relatively 16o poor nebular gas if the precursors of these cais were as 16o rich as other cr cais this implies either temporal excursions in the isotopic composition of the gas in the cai forming regions and or radial transport of some cai precursors into an 16o poor gas the absence of oxygen isotope heterogeneity in the primary minerals of melilite rich cais containing alteration products suggests that mineralogical alteration in cr chondrites did not affect oxygen isotopic compositions of their cais

determination of the petrologic type of cv3 chondrites by raman spectroscopy of included organic matter abstract this paper reports the first reliable quantitative determination of the thermal metamorphism grade of a series of nine cv3 chondrites allende axtell bali mokoia grosnaja efremovka vigarano leoville and kaba the maturity of the organic matter in matrix determined by raman spectroscopy has been used as a powerful metamorphic tracer independent of the mineralogical context and extent of aqueous alteration this tracer has been used along with other metamorphic tracers such as fe zoning in type i chondrules of olivine phenocrysts presolar grain abundance and noble gas abundance bulk and p3 component the study shows that the petrologic types determined earlier by induced thermoluminescence were underestimated and suggests the following values pt allende axtell 3 6 pt bali mokoia grosnaja 3 6 pt efremovka leoville vigarano 3 1 3 4 pt kaba 3 1 the most commonly studied cv3 allende is also the most metamorphosed bali is a breccia containing clasts of different petrologic types the attribution suggested by this study is that of clasts of the highest petrologic types as pointed out by iom maturity and noble gas bulk abundance cv3 chondrites have complex asteroidal backgrounds with various degrees of aqueous alteration and or thermal metamorphism leading to complex mineralogical and petrologic patterns fe mg chemical zoning in olivine phenocrysts on the borders of type i chondrules of porphyritic olivine and pyroxene rich textural types has been found to correlate with the metamorphism grade this suggests that chemical zoning in some chondrules often interpreted as exchanges between chondrules and nebular gas may well have an asteroidal origin furthermore the compositional range of olivine matrix is controlled both by thermal metamorphism and aqueous alteration this does not support evidence of a nebular origin and does not necessarily mirror the metamorphism grade through fe mg equilibration on the other hand it may provide clues on the degree of aqueous alteration vs thermal metamorphism and on the timing of both processes in particular mokoia experienced significant aqueous alteration after the metamorphism peak whereas grosnaja which has similar metamorphism grade did not

mineralogical anatomy and implications of a ti sc rich ultrarefractory inclusion from sayh al uhaymir 290 ch3 chondrite titanium rich minerals are common in ca al rich inclusions from primitive chondrites they are important not only for testing the condensation models for a gas with a solar composition but also for constraining the redox conditions of the early solar nebula in this study we report the detailed mineralogical features and its oxygen isotope compositions of a ti sc rich ultrarefractory inclusion a0031 from a ch3 chondrite sayh al uhaymir 290 the a0031 inclusion has a compact and layered texture with the interior consisting of panguite sc rich anosovite ti rich davisite and anorthite a few hexaferrum perovskite and spinel crystals are present as inclusions in these minerals outside of ti rich davisite are a layer of al ti rich diopside and two grains of enstatite this texture strongly suggests that a0031 has a condensation origin panguite is its third occurrence in nature and similar in composition to the type panguite from the allende meteorite sc rich anosovite in a0031 has a chemical formula of ti 4 ti 3 mg sc al 3o 5 with the pseudobrookite structure this is the second report of ti 3o 5 in nature but is the first description of anosovite formed in the solar nebula as an ultrarefractory phase the discovery of sc rich anosovite in a0031 reveals the stability of ti 3o 5 in the early solar nebula and supports the prediction of previous equilibrium condensation calculations the panguite sc rich anosovite and ti rich davisite in a0031 show a large variation in ti 3 ti tot the primitive nature of a0031 implies that the variations in ti 3 ti tot among different ti rich minerals are primary features we propose that the distribution of ti 3 and ti 4 could be controlled mainly by their various competition abilities of incorporating into these ti sc al rich minerals similarity of ti 3 ti tot value between ti rich davisite from a0031 and those in other carbonaceous chondrites indicates that most refractory inclusions might have formed in highly reducing nebular settings the 16 o depleted isotope compositions of a0031 confirm the existence of diverse oxygen reservoirs for ch ca al rich inclusions in the solar nebula

oxygen isotopic composition of the solar nebula gas inferred from high precision isotope imaging of melilite crystals in an allende cai high precision isotope imaging analyses of reversely zoned melilite crystals in the gehlenitic mantle of type a cai on01 of the allende carbonaceous chondrite reveal that there are four types of oxygen isotopic distributions within melilite single crystals 1 uniform depletion of 16o 18o 10 2 uniform enrichment of 16o 18o 40 3 variations in isotopic composition from 16o poor core to 16o rich rim 18o 10 to 30 20 to 45 and 10 to 35 with decreasing akermanite content and 4 16o poor composition 18o 10 along the crystal rim hibonite spinel and perovskite grains are 16o rich 18o 45 and adjoin 16o poor melilites gas solid or gas melt isotope exchange in the nebula is inconsistent with both the distinct oxygen isotopic compositions among the minerals and the reverse zoning of melilite fluid rock interaction on the parent body resulted in 16o poor compositions of limited areas near holes cracks or secondary phases such as anorthite or grossular we conclude that reversely zoned melilites mostly preserve the primary oxygen isotopic composition of either 16o enriched or 16o depleted gas from which they were condensed the correlation between oxygen isotopic composition and akermanite content may indicate that oxygen isotopes of the solar nebula gas changed from 16o poor to 16o rich during melilite crystal growth we suggest that the radial excursions of the inner edge of the protoplanetary disk gas simultaneously resulted in both the reverse zoning and oxygen isotopic variation of melilite due to mixing of 16o poor disk gas and 16o rich coronal gas gas condensates aggregated to form the gehlenite mantle of the type a cai on01

oxygen isotopic zoning of reversely zoned melilite crystals in a fluffy type a ca al rich inclusions from the vigarano meteorite the oxygen isotopic microdistributions within melilite measured using in situ secondary ion mass spectrometry correspond to the chemical zoning profiles in single melilite crystals of a fluffy type a ca al rich inclusions cais of reduced cv3 vigarano meteorite the melilite crystals show chemical reverse zoning within an individual single crystal from the akermanite rich core to the akermanite poor rim the composition changes continuously with the crystal growth the zoning structures suggest that the melilite grew in a hot nebular gas by condensation with decreasing pressure the oxygen isotopic composition of melilite also changes continuously from 16o poor to 16o rich with the crystal growth these observations suggest that the melilite condensation proceeded with change consistent with an astrophysical setting around the inner edge of a protoplanetary disk where both 16o rich solar coronal gas and 16o poor dense protoplanetary disk gas could coexist fluffy type a cais could have been formed around the inner edge of the protoplanetary disk surrounding the early sun

origin of compact type a refractory inclusions from cv3 carbonaceous chondrites compact type a cta inclusions are one of the major types of coarse grained refractory inclusions found in carbonaceous chondrites they have not been studied in a systematic fashion leading to some uncertainties and unproven assumptions about their origin to address this situation we studied a total of eight ctas from allende efremovka and axtell by scanning electron microscopic and electron and ion microprobe techniques these inclusions are very melilite rich ranging from 60 vol to nearly monomineralic also present are mg al spinel 5 20 perovskite trace 3 and in some samples ti rich 17 wt tio2tot fassaite trace 20 and rhonite 1 melilite compositions are mostly between ak15 and ak40 chondrite normalized ree abundance patterns for melilite flat at 10 ci with positive eu anomalies and fassaite slight hree enrichment relative to lree and negative eu anomalies are like those for their counterparts in once molten type b inclusions the patterns for rhonite have positive slopes from la through lu and abundances 10 ci for la and 35 60 ci for lu features of ctas that suggest that they were once molten include rounded inclusion shapes positively correlated sc and v abundances in fassaite radially oriented melilite laths at inclusion rims and the distribution of trace elements among the phases fractional crystallization models show that with one exception the ree contents of perovskite and fassaite arose by crystallization of these phases from late residual liquids that would have resulted from prior crystallization of the observed proportions of melilite and spinel from liquids having the bulk compositions of the inclusions one allende cta ts32 however has several features irregular shape reversely zoned melilite fassaite ree contents that are not readily explained by crystallization from a melt this inclusion may have undergone little melting and may be dominated by relict grains

origin of magnetite in oxidized cv chondrites in situ measurement of oxygen isotope compositions of allende magnetite and olivine magnetite in the oxidized cv chondrite allende mainly occurs as spherical nodules in porphyritic olivine po chondrules where it is associated with ni rich metal and or sulfides to help constrain the origin of the magnetite we measured oxygen isotopic compositions of magnetite and coexisting olivine grains in po chondrules of allende by an in situ ion microprobe technique five magnetite nodules form a relatively tight cluster in oxygen isotopic composition with delta 18o values from 4 8 to 7 1 and delta 17o values from 2 9 to 6 3 seven coexisting olivine grains have oxygen isotopic compositions from 0 9 to 6 3 in delta 18o and from 4 6 to 7 9 in delta 17o the delta 17o values of the magnetite and coexisting olivine do not overlap they range from 0 4 to 2 6 and from 4 0 to 5 7 respectively thus the magnetite is not in isotopic equilibrium with the olivine in po chondrules implying that it formed after the chondrule formation the delta 17o of the magnetite is somewhat more negative than estimates for the ambient solar nebula gas we infer that the magnetite formed on the parent asteroid by oxidation of metal by h2o which had previously experienced minor o isotope exchange with fine grained silicates

formation of spinel hibonite rich inclusions found in cm2 carbonaceous chondrites we report petrography mineral chemistry bulk chemistry and bulk isotopic compositions of a suite of 40 spinel rich inclusions from the murchison cm2 carbonaceous chondrite seven types of inclusions have been identified based on mineral assemblage spinel hibonite perovskite spinel perovskite pyroxene spinel perovskite melilite spinel hibonite perovskite melilite spinel hibonite spinel pyroxene and spinel melilite anorthite hibonite bearing inclusions have ti poor spinel compared to the hibonite free ones and spinel hibonite perovskite inclusions have the highest average bulk tio 2 contents 7 8 wt the bulk cao al 2 o 3 ratios of the inclusions range from 0 005 to 0 21 well below the solar value of 0 79 hibonite spinel rich inclusions consist of phases that are not predicted by condensation calculations to coexist in the equilibrium sequence hibonite is followed by melilite which is followed by spinel therefore hibonite melilite or melilite spinel inclusions should be dominant instead one explanation for the missing melilite is that it condensed as expected but was lost due to evaporation of mg and ca during heating and melting of spherule precursors if this theory were correct melilite poor spherules would have isotopically heavy mg and ca assuming rayleigh fractionation accompanied evaporation except for one inclusion with f mg 4 3 2 6 amu and another with isotopically light ca f ca 3 4 2 0 amu however all the inclusions we analyzed have normal isotopic compositions within their 2 uncertainties thus we found no evidence for significant mass dependent fractionation conditions necessary for non rayleigh evaporation are unlikely if not unrealistic and our preferred explanation for the general lack of melilite among hibonite spinel bearing inclusions is kinetic inhibition of melilite condensation relative to spinel because of similarities between the crystal structures of hibonite and spinel it should be easier for spinel than for melilite to form from hibonite

new constraints on the relationship between 26al and oxygen calcium and titanium isotopic variation in the early solar system from a multielement isotopic study of spinel hibonite inclusions abstract we report oxygen calcium titanium and 26al 26mg isotope systematics for spinel hibonite inclusions shibs a class of calcium aluminum rich inclusions cai common in cm chondrites in contrast to previous studies our analyses of 33 shibs and four shib related objects obtained with high spatial resolution demonstrate that these cais have a uniform 17o value of approximately 23 similar to many other mineralogically pristine cais from unmetamorphosed chondrites e g cr cv and acfer 094 five shibs studied for calcium and titanium isotopes have no resolvable anomalies beyond 3 uncertainties this suggests that nucleosynthetic anomalies in the refractory elements had been significantly diluted in the environment where shibs with uniform 17o formed we established internal 26al 26mg isochrons for eight shibs and found that seven of these formed with uniformly high levels of 26al a multi cai mineral isochron yields an initial 26al 27al ratio of 4 8 10 5 but one shib has a smaller initial 26al 27al of 2 5 10 5 indicating variation in 26al 27al ratios when shibs formed the uniform calcium titanium and oxygen isotopic characteristics found in shibs with both high and low initial 26al 27al ratios allow for two interpretations 1 if subcanonical initial 26al 27al ratios in shibs are due to early formation as suggested by liu et al 2012 our data would indicate that the cai formation region had achieved a high degree of isotopic homogeneity in oxygen and refractory elements before a homogeneous distribution of 26al was achieved 2 alternatively if subcanonical ratios were the result of 26al 26mg system resetting the clustering of shibs at a 17o value of 23 would imply that a 16o rich gaseous reservoir existed in the nebula until at least 0 7 ma after the formation of the majority of cais

the origin and significance of reverse zoning in melilite from allende type b inclusions in many type b allende inclusions melilite is reversely zoned over restricted portions of each crystal textural relationships and the results of dynamic crystallization experiments suggest that the reverselyzoned intervals in these type melilites result from the co precipitation of melilite with clinopyroxene from a melt prior to the onset of anorthite precipitation when clinopyroxene begins to precipitate the al mg ratio of the melt rises causing the crystallizing melilite to become more gehlenitic an effect which is negated by crystallization of anorthite because the equilibrium crystallization sequence in these liquids is anorthite before pyroxene melilite reverse zoning can occur only when anorthite nucleation is suppressed relative to pyroxene this has been achieved in our experiments at cooling rates as low as 0 5 c hour our experiments further indicate however that reverse zoning does not form at cooling rates 50 c hour probably because the clinopyroxene becomes too al rich to drive up the al mg ratio of the liquid type inclusions with reversely zoned melilites must have cooled at rates greater than those at which anorthite begins to crystallize before clinopyroxene but 50 c hour such rates are far too slow for the type droplets to have cooled by radiation into a nebular gas but are much faster than the cooling rate of the solar nebula itself one possibility is that type b s formed in local hot regions within the nebula where their cooling rate was equal to that of their surrounding gas other possibilities are that their cooling rates reflect their movement along nebular temperature gradients or the influence of a heat source the sun or viscous drag on inclusions as they moved through the nebular gas are potential candidates for such heat sources

dyke emplacement in a layered and faulted rift zone abstract in rift zone eruptions magma is normally transported to the surface through dykes most injected dykes however do not make it to the surface but rather become arrested at certain depths in the rift zone surface deformation studies are routinely used to infer the depths to the tips as well as the geometries of arrested dykes most such studies model the rift zone as a homogeneous isotropic elastic half space all rift zones however contain normal faults and are composed of rock layers with different mechanical properties the numerical models presented here indicate that when the upper tip of a vertically propagating dyke reaches the same crustal level as the bottom tips of the boundary faults of an active graben the faults close as the tip continues its propagation up into the graben the overpressured dyke encourages reverse slip on the boundary faults the present numerical models also indicate that mechanical layering has strong effects on dyke emplacement and the surface deformation associated with arrested dykes in rift zones in particular soft low young s modulus layers and weak contacts at shallow depths contribute to offset arrest and aperture thickness variations of the dykes and suppress the dyke induced surface stresses and associated brittle deformation when the magmatic overpressure of a dyke is the only loading the dyke tends to be thicker inside soft layers than inside stiff high young s modulus layers soft layers and weak contacts discontinuities common at shallow depths in rift zones suppress dyke tip tensile stresses thereby encouraging dyke arrest and may result in little if any surface stresses and brittle deformation being induced by an arrested dyke weak contacts also encourage transfer of the dyke induced surface tensile stresses to the regions above the lateral ends of the contacts then the tensile stresses at the rift zone surface in a large area above the dyke itself become small or negligible all the tensile stress occur above the lateral ends of the contact which may be many kilometres from the dyke tip for a dyke arrested in a layered rift zone straightforward inversion of surface deformation data may therefore yield geometric results that have little relation to the actual geometry of the arrested dyke the results of the numerical models presented here are generally supported by field observations of dykes in rift zones