the geochemical fingerprints of different tectonomagmatic environments using hygromagmatophile element abundances of tholeiitic basalts and basaltic andesites abstract using hygromagmatophile element abundances normalized to primordial mantle composition it is possible to discriminate between the tectonic settings of tholeiitic basalts and basaltic andesites 47 0 53 5 sio2 based on field and chemical evidence data from provinces or grouped localities have been compiled in order to establish chemical reference data sets for ocean island continental ocean ridge and floor subduction zone and back arc environments the disturbing effects of crustal contamination and hydrothermal alteration on the hygromagmatophile h element patterns are of minor significance in the recognition of plate tectonic settings on the basis of the data sets magmas from a number of ambiguous tectonic settings have been recognized using their h element patterns attention is directed to problems concerning rifting and drifting related basalts in an apparent continental setting and to the origin of ophiolite complexes using available data the pillow lavas of the troodos ophiolite complex cyprus are recognized as basalts erupted in a subduction zone environment

the isotopic composition of uranium and lead in allende inclusions and meteoritic phosphates we measured the isotopic compositions of u and pb in eight ca al rich inclusions from the allende chondrite in whitlockite from the st severin chondrite and in a whitlockite and a whole rock sample from the angra dos reis achondrite ador the total range in 235 u 238 u in the acid soluble phases of the allende inclusions and meteoritic whitlockites is from 1 137 6 to 1 138 3 and to within the error limits 3 20 indistinguishable from normal terrestrial u the 235 u 238 u ratios for whitlockites from st severin and ador are the same as that in the bulk meteorites and are normal the pb isotopic compositions of the acid soluble phases of the allende coarse grained inclusions except c 1 give a mean 207 pb 206 pb model age of4 559 0 015ae which is consistent with the observation of normal u isotopic composition the pb isotopic compositions of coarse grained fun inclusion c 1 and of two fine grained inclusions from allende indicate that the u pb systems of these samples have evolved differently from the rest of the meteorite the 207 pb 206 pb model ages for the whitlockites and for bulk ador are the same as those for st severin whitlockite 4 551 0 004ae the pb results from st severin indirectly constrain the value of the u isotopic abundance to be within 7 4 of normal both the directly measured235u 238u and the pb data on st severin are in complete disagreement with claims of large 235 u excess reported by other workers on the same meteorite the th u ratios of some acid soluble phases of allende inclusions and of meteoritic whitlockite show varying degrees of enrichment of th relative to u by factors of 2 to 25 relative to the bulk meteorites previous studies have shown that 244 pu 238 u ratios in whitlockite are also fractionated relative to the bulk meteorites the observation of no isotopic effects in u reported here does not support the claims of either fractionation of 247 cm 238 u or of a large amount of live 247 cm at the early stage of formation of the solar system using the present data we obtain a limit of 247 cm 235 u 4 10 3 at the time of meteorite formation or differentiation this implies that the last major r process contribution to the material in the protosolar nebula was 10 8 years prior to 26 al formation and injection

implications of the carbonaceous chondrite mn cr isochron for the formation of early refractory planetesimals and chondrules abstract an excellent 53 mn 53 cr isochron for bulk ci cm co cv cb and ungrouped c3 chondrites seems to suggest that each carbonaceous chondrite group acquired its mn cr ratio 4568 1 myr ago this age is indistinguishable from the age of ca al rich inclusions cais which is considered to be the start of the solar system t 0 however carbonaceous chondrites were not assembled until at least 1 5 5 myr after t 0 to judge by the 207 pb 206 pb and 26 al 26 mg ages of the chondrules within them and by the fact that they were not melted by heat from the decay of 26 al presumably therefore these meteorites inherited their bulk mn cr isochron from precursor materials which experienced mn cr fractionation at t 0 as a possible physical mechanism for how the isochron was established initially and later inherited by the carbonaceous chondrites we propose the rapid formation at t 0 of planetesimals that were variably depleted in moderately volatile elements and hence had variably low mn cr the planetesimals and the undepleted high mn cr primitive dust from which they were made shared the same initial e 53 cr and therefore evolved on an isochron we suggest that later impact disruption of the planetesimals produced dusty debris which became mixed in various proportions with unprocessed high mn cr dust before accreting to the carbonaceous chondrite parent bodies with mixing in a closed system the isochron was unchanged we infer that some debris rich material was converted to chondrules prior to accretion the chondrules could have been formed by flash melting of the mixed dust or could instead have been made directly by the impact splashing of molten planetesimals or by condensation from impact generated vapor plumes

mn cr isotope systematics of the d orbigny angrite we have conducted a detailed study of the mn cr systematics of the angrite d orbigny here we report cr isotopic abundances and mn cr ratios in olivine pyroxene glass chromite and bulk rock samples from d orbigny 53cr excesses in these samples correlate well with their respective mn cr ratios and define an isochron with a slope that corresponds to an initial 53mn 55mn ratio 3 24 0 04 x 10 6 and initial 53cr 52cr ratio of 53 0 30 0 03 at the time of isotopic closure the 53mn 55mn ratio of the d orbigny bulk rock is more than two fold the 53mn 55mn ratio of the angrites lewis cliff 86010 lew and angra dos reis ador and implies an older mn cr age of 4562 9 0 6 ma for d orbigny relative to a pb pb age of 4557 8 0 5 ma for lew and ador one of the most unusual aspects of d orbigny is the presence of glass a phase that has not been identified in any of the other angrites the mn cr data for glass and a pyroxene fraction found in druses indicate that they formed contemporaneously with the main phases of the meteorite since the mn cr age of d orbigny is 5 ma years older than the angrites lew and ador d orbigny likely represents an earlier stage in the evolution of the angrite parent body

revisiting the tectonic evolution of the triassic palaeo tethys convergence zone in northern thailand inferred from detrital zircon u pb ages

the western ailaoshan volcanic belts and their se asia connection a new tectonic model for the eastern indochina block the western ailaoshan wal volcanic belts mark the western boundary of the south china indochina suture zone along the ailaoshan fold belt geochronological and geochemical evidence suggests that the wal volcanic belts vary in ages and origins and includes 1 l carboniferous earliest m permian ca 300 270 ma incipient backarc basin magmatism 2 m permian ca 265 ma island arc mature backarc basin magmatism 3 early l permian ca 260 255 ma wal indochina arc continent collision and the associated syn collisional granitic magmatism as well as subsequent 4 e triassic ca 250 245 ma post collisional magmatism regional geological comparison indicates that the post l carboniferous wal tectonic evolution was closely related to the eastern indochina block we present a new g plate geodynamic evolution model for the ailaoshan fold belt and its surrounding south china indochina region based on new and published u pb zircon ages together with whole rock and pb isotope geochemical data

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

crystallization recrystallization and impact metamorphic ages of eucrites y792510 and y791186 abstract monomict eucrites y792510 and y791186 of pyroxene metamorphic type 6 contain areas of recrystallized mesostasis and granoblastic pyroxene textural comparisons suggest that the mesostasis areas in these eucrites originally resembled analogous areas in the pristine basaltic eucrite clast y75011 84 39ar 40ar ages were determined for microbreccias y791186 87 and y792510 62 and crystalline clast y792510 65 the 39ar 40ar ages for y791186 87 and y792510 62 are identical within error limits at 3 46 0 18 ga and 3 40 0 16 ga 2 respectively both these samples were involved in an impact event in which ar was quantitatively degassed 3 43 0 10 ga ago y792510 65 has an apparently older 39ar 40ar age of 3 60 0 04 ga 2 suggesting that it was degassed in a separate event or to a lesser degree rb sr and sm nd ages were determined for both y792510 clasts the rb sr age of 3 26 0 24 ga 2 for y792510 65 was totally reset by the impact event s whereas the rb sr age of 4 26 0 62 ga 2 for y792510 62 either was partially reset by the same event s or by earlier recrystallization of mesostasis the 147sm 143nd age calculated for y792510 65 including all data is 4 67 0 22 ga 2 but is likely to have been affected by weathering in the antarctic it becomes 4 57 0 09 ga if only data for the major minerals plagioclase and pyroxene are included in the isochron the 147sm 143nd age of y792510 62 is 4 34 0 06 ga 2 for the same mineral phases but may be biased by the presence of weathered ca phosphates in the pyroxene separate the 146sm 142nd age of y792510 65 is 4 42 0 06 0 04 ga corresponding to initial 146sm 144sm 0 0030 0 0010 relative to 146sm 144sm lew 0 0076 0 0009 for the lew86010 angrite at 4 558 ga ago the y792510 65 ages from the two sm nd chronometers are concordant at 4 48 ga which may be the crystallization age of this eucritic basalt late crystallization alternatively these sm nd ages may have been affected by the event in which ca phosphate and augite in mesostasis were recrystallized and augite and chromite in granoblastic areas were derived from cloudy pyroxenes late recrystallization the subsolidus recrystallization may be a consequence of a thermal spike in the midst of an otherwise monotonically cooling environment resulting from impact cratering of the hot crust of the newly formed eucrite parent body pyroxene homogenization probably occurred prior to formation of granoblastic pyroxene and close to the time of initial crystallization mn cr data provide only a hint of live 53mn t1 2 3 7ma and imply either chromium isotopic equilibration after decay of 53mn or late crystallization impact brecciation and lithification probably accompanied thermal annealing 3 4 3 6 ga ago as given by the 39ar 40ar ages the rb sr age of y792510 65 is consistent with this interpretation the 39ar 40ar and rb sr systematics of minute k rich inclusions in plagioclase and pyroxene formed by previous impact and thermal annealing events probably were reset then these young impact metamorphic ages demonstrate that impact resetting of radiometric ages on the hed parent body asteroid 4 vesta occurred over an interval of at least 0 7 ga

crystal bearing lunar spherules impact melting of the moon s crust and implications for the origin of meteoritic chondrules crystal bearing lunar spherules clss in lunar breccia 14313 14315 14318 soil 68001 24105 and impact melt rock 62295 samples can be classified into two types feldspathic and olivine rich feldspathic clss contain equant tabular or acicular plagioclase grains set in glass or a pyroxene olivine mesostasis the less common olivine rich clss contain euhedral or skeletal olivine set in glass or possess a barred olivine texture bulk chemical and mineral chemical data strongly suggest that feldspathic clss formed by impact melting of mixtures of ferroan anorthosite and mg suite rocks that compose the feldspathic crust of the moon it is probable that olivine rich clss also formed by impact melting but some appear to have been derived from distinctively magnesian lunar materials atypical of the moon s crust some clss contain reversely zoned relic plagioclase grains that were not entirely melted during cls formation thin 5 m thick rims of troilite or phosphate and chemical gradients in glassy mesostases attributed to metasomatism in a volatile rich na k p rich environment crystal bearing lunar spherules were rimmed and metasomatized prior to brecciation compound cls objects are also present these formed by low velocity collisions in an environment probably an ejecta plume that contained numerous melt droplets factors other than composition were responsible for producing the crystallinity of the clss we agree with previous workers that relatively slow cooling rates and long ballistic travel times were critical features that enabled these impact melt droplets to partially or completely crystallize in free flight moreover incomplete melting of precursor materials formed nucleation sites that aided subsequent crystallization clearly clss do not resemble meteoritic chondrules in all ways the two types of objects had different precursors and did not experience identical rimming processes and vapor fractionation appears to have played a less important role in establishing the compositions of clss than of chondrules however the many detailed similarities between clss and chondrules indicate that it is more difficult to rule out an origin for some chondrules by impact melting than some have previously argued differences between clss chondrules and their host rocks possibly can be reconciled with an impact melt origin for some chondrules when different precursors the higher gravity of the moon compared to chondrite parent bodies and the likely presence of nebular gas during chondrule formation are taken into account

chondrules from chainpur ll 3 reduced parent rocks and vapor fractionation abstract nineteen chondrules from the chainpur ll 3 chondrite were analyzed for their bulk lithophile element contents and mineral chemistries chondrule compositions are highly fractionated the following compositional groups can be distinguished 1 refractory depleted in na and k 2 k depleted k ci na ci 3 approximately chondritic na k ci na ci and 4 k enriched k ci na ci the most refractory chondrules are depleted in k 0 11 ci abundances normalized to sc na 0 26 mn 0 52 and cr 0 71 therefore we conclude that the refractory group was mainly produced by vapor fractionation during the chondrule forming event the majority of chondrules in enriched in na 1 37 1 40 ci but fractionated in k which ranges from 0 4 ci k depleted to 2 84 ci k enriched for the fractionation of k from na we suggest a separation mechanism via alkali sulfides uncorrelated fractionations of ca mn cr eu yb sm give supporting evidence for such a mechanism these findings imply that the pre chainpur matter went through highly reducing conditions high h2 o2 and s2 o2 ratios before it was sampled by the chondrule forming process most chondrules of our suite have similar fa contents in olivine 11 21 and are enriched in volatile elements from this we conclude that these chondrules were formed under oxidizing conditions in a relatively dense atmosphere with high partial pressures of volatile elements alkalis halogens a proto planetary atmosphere could have provided these conditions the fractionation process proposed should also be applicable to fractionations on a large scale like the na k and mg si fractionations among chondrites

a tungsten isotope approach to search for meteoritic components in terrestrial impact rocks abstract the identification of meteorite impact structures on earth is based on two main criteria the presence of shock metamorphic effects in the crater rock ejecta and or the confirmation of an extraterrestrial meteoritic component in breccias or melt rocks for the latter both high elemental abundances of siderophile elements especially the platinum group elements with corresponding meteoritic inter element ratios and the osmium os and chromium cr isotopic signatures characteristic of meteorites have been used successfully inspired by earlier suggestions of a meteoritic component in archean rocks based on tungsten w isotope anomalies here we explore the possible use of 182 w which has been produced by the decay of now extinct 182 hf t 1 2 8 9 ma as a tracer of meteoritic component in terrestrial material each group of meteorites has w isotopic compositions that are distinct from each other and from the terrestrial crust 182 w has already been used to try to identify the impactor at the k t boundary by analyzing the sediments and ni rich spinel in the present study we broaden the field of investigation by choosing a different approach namely analyzing a variety of known impact rocks we measured the w isotope composition in four samples from different impact structures gardnos norway morokweng south africa vredefort south africa ries germany as well as in two samples from a distal ejecta layer k t boundary samples from gams austria and berwind canyon usa all these samples are unambiguously impact produced and in several of those materials a meteoritic component has unequivocally been identified by other geochemical proxies in all these samples the isotopic composition of w is identical with analytical error to that of the earth s continental crust and no 182 w anomalies are present even in the samples containing a significant percent level meteoritic component therefore we conclude that in contrast to the cr or os isotopes w isotopes are not suitable for the identification of meteoritic components in terrestrial rocks

a short duration of chondrule formation in the solar nebula evidence from 26al in semarkona ferromagnesian chondrules abstract the 26 al 26 mg systems of five ferromagnesian chondrules from the least metamorphosed ordinary chondrite semarkona ll3 0 were studied using a secondary ion mass spectrometer their glass or plagioclase portions contain excesses of 26 mg and in two chondrules the 26 mg excesses are well correlated with 27 al 24 mg which demonstrates the in situ decay of 26 al the initial 26 al 27 al ratios in these chondrules obtained from the slope of isochrons show a narrow range of between 6 10 6 and 9 10 6 indicating their short formation duration of less than 1 my if the solar nebula was initially homogeneous in al isotopes the chondrule formation ages are 2 my younger than those of cais our results based on the study of the least metamorphosed uoc are consistent with the previous studies on al rich chondrules that the chondrule formation started at least 2 my after cais alternatively the older records before 2 my were erased by chondrule recycling process it further suggests that the young apparent ages 3 to 5 my after cais for chondrules in type 3 4 uocs are due to the disturbance of the 26 al 26 mg system possibly during parent body metamorphism the result is not consistent with the extended nebular time scale of 5 my and the chondrule formation by planetary processes the ni isotopic analysis of the feo rich olivines in a type ii chondrule in semarkona did not show any detectable excess 60 ni in spite of their high fe ni ratios the upper limit of the initial 60 fe 56 fe ratio of the solar system was estimated to be 3 4 10 7 which is consistent with the previous estimate 0 2 1 9 10 7 from eucrites this result confirms that the 60 ni excess previously observed from cais was not due to the decay of the short lived nuclide 60 fe but a ni isotopic anomaly of nucleosynthetic origin

refractory forsterite in primitive meteorites condensates from the solar nebula all groups of chondritic meteorites contain discrete grains of forsteritic olivine with feo contents below 1 wt and high concentrations of refractory elements such as ca al and ti ten such grains 52 to 754 g with minor amounts of adhering matrix were separated from the allende meteorite after bulk chemical analysis by instrumental neutron activation analysis inaa some samples were analyzed with an electron microprobe and some with an ion microprobe matrix that accreted to the forsterite grains has a well defined unique composition different from average allende matrix in having higher cr and lower ni and co contents which implies limited mixing of allende matrix all samples have approximately chondritic relative abundances of refractory elements ca al sc and rare earth elements ree although some of these elements such as al do not quantitatively reside in forsterite whereas others e g ca are intrinsic to forsterite the chondritic refractory element ratios in bulk samples the generally high abundance level of refractory elements and the presence of ca al ti rich glass inclusions suggest a genetic relationship of refractory condensates with forsteritic olivine the ca al ti rich glasses may have acted as nuclei for forsterite condensation arguments are presented that exclude an origin of refractory forsterite by crystallization from melts with compositions characteristic of allende chondrules a all forsterite grains have cao contents between 0 5 and 0 7 wt with no apparent zoning requiring voluminous parental melts with 18 to 20 wt cao far above the average cao content of allende chondrules similar arguments apply to al contents b the low feo content of refractory forsterite of 0 2 0 4 wt imposes an upper limit of 1 wt of feo on the parental melt too low for ordinary and carbonaceous chondrule melts c the mn contents of refractory forsterites are between 30 to 40 ppm this is at least one order of magnitude below the mn content of chondrule olivines in all classes of meteorites the observed mn contents of refractory forsterite are much too low for equilibrium between olivine and melts of chondrule composition d as shown earlier refractory forsterites have o isotopic compositions different from chondrules weinbruch et al 1993a refractory olivines in carbonaceous chondrites are found in matrix and in chondrules the compositional similarity of both types was taken to indicate that all refractory forsterites formed inside chondrules e g jones 1992 as refractory forsterite cannot have formed by crystallization from chondrule melts we conclude that refractory forsterite from chondrules are relic grains that survived chondrule melting and probably formed in the same way as refractory forsterite enclosed in matrix we favor an origin of refractory forsterite by condensation from an oxidized nebular gas

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

late triassic post collisional granites related to paleotethyan evolution in se thailand geochronological and geochemical constraints abstract a set of new u pb geochronological elemental and sr nd hf isotopic data is presented to constrain the formation age and petrogenesis of granites in se thailand which are significant for understanding eastern paleotethyan tectonic evolution and regional correlation in se asia six granite samples from the chonburi rayong bang lamung and chanthaburi plutons yielded similar crystallization ages of 222 218 ma these granites can be geochemically classified into s type group 1 and i type group 2 granites both groups have similar chondrite normalized ree and primitive mantle normalized multi element patterns but distinctly different sr nd hf isotopic compositions group 1 is characterized by more enriched initial 87sr 86sr i ratios 0 7101 0 7272 and lower end t values 12 0 to 13 2 compared with those of group 2 with 87sr 86sr i of 0 7027 0 7053 and end t of 0 6 to 1 2 consistently zircons from group 1 yield negative ehf t values of 7 3 to 27 4 and mesoproterozoic or older hf model ages tdm 1 53 2 63 ga distinct from those of group 2 with positive ehf t values 5 0 11 3 and younger tdm 0 42 0 67 ga group 1 granites mainly originated from ancient greywackes but group 2 from juvenile mafic crust with the input of meta sedimentary component these data along with available observations synthetically indicate that these granites formed in the post collisional gravitational collapse setting of the thickened crust in response to the assemblage of the indochina with sibumasu blocks they linked northerly and easterly to the late triassic granitoids in nw thailand and east malay peninsula related to the paleotethyan tectonic regime respectively

evidence for the presence of 53mn in the early solar system chromium isotopic anomalies appear to be widespread in the allende inclusions clear anomalies are shown in the 53cr 52cr isotopic ratio deviations from the terrestrial value are correlated with the mn cr ratios indicating that 53mn half life t 3 7 106y existed at the time of formation of the meteorite inclusions the initial 53cr 52cr in the inclusions is lower than the terrestrial value recent results have shown that chromium isotopic anomalies are frequent on the 54cr isotope birck and allegre 1984 at the same time our results suggested the possibility of some anomalies on the 53cr isotope which can be produced by the decay of 53mn the present article presents improved precision measurements on the same inclusions of the allende meteorite and an extension of the measurements to mineral separates these results are the first evidence for live 53mn in the early solar system noted 53mn in the following

chromium isotopic systematics of the sutter s mill carbonaceous chondrite implications for isotopic heterogeneities of the early solar system recent studies have shown that major meteorite groups possess their own characteristic 54cr values demonstrating the utility of cr isotopes for identifying genetic relationships between the planetary materials in conjunction with other classical tools such as oxygen isotopes in this study we performed cr isotope analyses for whole rocks and chemically separated phases of the new cm2 chondrite sutter s mill sm 43 and 51 the two whole rocks of sutter s mill show essentially identical e54cr excesses sm 43 0 95 0 09e sm 51 0 88 0 07e relative to the earth these values are the same within error with that of the cm2 type murchison 0 89 0 08e suggesting that parent bodies of sutter s mill and murchison were formed from the same precursor materials in the solar nebula large e54cr excess of up to 29 40e is observed in the silicate phase of sutter s mill while that of murchison shows 15 74e importantly the leachate fractions of both sutter s mill and murchison form a steep linear anticorrelation between e54cr and e53cr cross cutting the positive correlation previously observed in carbonaceous chondrites the fact that l4 acid leachate fraction contains higher 54cr excesses than that of l5 step designed to dissolve refractory minerals suggests that spinel is not a major 54cr carrier we also note that l5 contains 53cr anomalies lower than the solar initial value suggesting it carries a component of nucleosynthetic anomaly unrelated to the 53mn decay we have identified five endmember components of nucleosynthetic origin among the early solar system materials

extinct i129 in c3 chondrites abstract eight c3 chondrites were examined by the i129 xe129 dating method to see whether their i xe ages better initial i 129 i 127 ratios r0 correlate with any other properties the r0 s range from 1 60 10 4 to 1 09 10 4 corresponding to i xe ages from 2 0 myr before to 6 7 myr after murchison magnetite three c3o s lance felix ornans have essentially indistinguishable r0 s of 1 41 0 13 to 1 17 0 10 10 4 the fourth c3o warrenton is undatable owing to homogenization of radiogenic and trapped xe four c3v s show a distinct spread vigarano and grosnaja are highest r0 1 60 0 07 and 1 57 0 14 10 4 mokoia is intermediate and kaba is lowest r0 1 38 0 06 and 1 09 0 10 10 4 literature values for allende place it near kaba these r0 s correlate inversely with 4 other properties i br and cd content and olivine composition both percent mean deviation pmd and proportion of iron poor olivine grains 2 fayalite it is difficult to accept the 9 myr spread in r0 as a true age reflecting either nebular or parent body processes this time span is more than an order of magnitude longer than the lifetime of the solar nebula inferred from astronomical evidence nor does the degree of thermal metamorphism which is slight for c3 s anyway correlate with r0 a more plausible interpretation is that the variations in r0 reflect mainly isotopic heterogeneity of iodine the simplest model that accounts for the correlations with r0 involves mixing of two iodine components in the solar nebula associated with gas and grains respectively the second of lower i 129 i 127 ratio predominated at later times and thus became enriched in late formed meteorites along with other volatiles such as cd and br the low fe content and large pmd of olivine may reflect either less metamorphism owing to shallow location in the parent body or greater reduction of fe2 during chondrule formation

structure of the mae on depression chiang mai province based on gravity modelling and geological field observation implications for tectonic evolution of the chiang mai chiang rai suture zone northern thailand abstract we propose a structural model for the tectonic evolution of the mae on depression area along the chiang mai chiang rai suture zone cmcrsz the study location is the central mae on district where a topographic depression is present that is continuous with the eastern margin of chiang mai rift basin yet volcanic exposures mapped in topographic lows indicate the depression has not developed as part of the syn rift or post rift basin with sediment fill terrestrial gravity was acquired to visualize the subsurface extent of the volcanic exposures which are part of a melange complex associated with the sibumasu sukhothai terrane collision during the permo triassic indosinian orogeny gravity anomaly highs correspond to the asymmetrical trapezoid geometry of a relatively dense volcanic block with a thickness of around 750 900 m overlain by lower density units comprising carbonate and pelagic formations interpreted as typical ocean plate stratigraphy associated with a volcanic seamount gravity gradient analysis fits with the geological interpretation of a klippe of mae tha formation derived from the western margin of the sukhothai terrane in thrust contact with the cmcrsz melange the structural interpretation of the gravity modelling provides important constraints on the tectonic contacts along the cmcrsz that fits the eastward subduction model of the palaeo tethys beneath the sukhothai terrane the mae on depression is one of the best localities in northern thailand for demonstrating the thickness and extent of blocks of palaeo tethys volcanics associated with carbonates which are interpreted as representing former seamounts

tectonic setting of the permo triassic chiang khong volcanic rocks northern thailand based on petrochemical characteristics abstract the inferred permo triassic chiang khong volcanic belt is composed of felsic to mafic volcanic rocks and their pyroclastic equivalents almost all the least altered mafic volcanic rocks are lava flows a few might have occurred as dykes these mafic volcanic rocks are non foliated to weakly foliated and mostly have porphyritic textures the phenocrysts microphenocrysts in porphyritic samples are commonly plagioclase and may include clinopyroxene olivine fe ti oxide apatite and amphibole the matrix of lava flows ranges texturally from felty to trachytic but a few samples show felty to ophitic subophitic and glassy textures whereas that of possible dyke samples is holocrystalline the primary matrix constituents are largely plagioclase and variable proportions of clinopyroxene fe ti oxide amphibole olivine apatite quartz alkali feldspar and or glass all the studied samples have been subjected to greenschist facies regional metamorphism chemically the samples show narrow ranges of least mobile incompatible element ratios and range compositionally from dacite to basalt of tholeiitic series these samples are chemically analogous to those of the tertiary andesite from sardinian rift sardinia italy particularly in terms of least mobile incompatible element ratios accordingly the studied mafic volcanic rocks are interpreted to have formed in a continental volcanic arc however the problem related to the geometry of plate convergence giving rise to the continental volcanic arc still exists

matrix material in type 3 chondrites occurrence heterogeneity and relationship with chondrules abstract matrix material in type 3 chondrites forms rims on chondrules metal sulfide aggregates ca al rich inclusions and chondritic clasts it also forms lumps up to a millimeter in size which may contain coarser silicates chondrules of all types were found with internal matrix lumps that appear to have entered the chondrules before the latter had crystallized mean concentrations of mg na al and ca in matrix occurrences show up to fivefold variations in a single chondrite variations between mean matrix compositions of individual type 3 ordinary chondrites are almost as large and partly reflect systematic differences between h l and ll matrices such variations are probably a result of nebular separation of feldspathic material and ferromagnesian silicates compositions of chondrules and their matrix rims are normally unrelated although rim compositions are correlated with those of matrix lumps inside chondrules a single chondrule was found with a composition nearly identical to that of its internal matrix lump suggesting that some chondrules may have formed from matrix material matrix lumps are as heterogeneous as chondrules but mean chondrule and matrix compositions differ even allowing for possible loss of metallic fe ni during chondrule formation since bulk compositions of matrix lumps and rims have probably not changed significantly since their formation except for fe mg exchange our matrix samples cannot represent typical chondrule precursor materials

rare earth element mobility around the volcanogenic polymentallic massive sulfide deposit at que river tasmania australia abstract the que river volcanogenic base and precious metal deposit is located in the cambrian mt read volcanics of western tasmania the host andesites and dacites adjacent to mineralization have been hydrothermally altered during mineralization and the entire sequence has undergone devonian regional prehnite pumpellyite facies metamorphism rocks petrographically recognizable as andesites exhibit chondrite normalized ree patterns similar to modern orogenic high k andesitic rocks with ce yb n ratios ranging from 6 4 to 8 6 dacites are more fractionated with ce yb n ranging from 10 6 to 16 0 like the andesites the dacites are characterized by negative eu anomalies eu eu 0 73 0 85 except for a more extreme eu anomaly in one sample eu eu 0 57 polymict volcaniclastic rocks have patterns very similar to those of the andesites the chondrite normalized ree patterns for the hydrothermally altered volcanics fall into three groups that can also be distinguished petrographically a single chloriterich sample has a substantial eu anomaly and shows less relative depletion in the middle ree than do either the andesites or the dacites highly silicified samples are all characterized by large negative eu anomalies eu eu 0 37 0 46 and many are relatively unfractionated with ce yb n ratios ranging from 3 5 to 7 7 a third group of samples characterized by the development of phengitic mica and carbonate show ree patterns similar to the andesites although their eu anomalies are generally smaller a sample of massive zn 1bpb ore has very low ree abundances showing light ree enrichment and a small negative eu anomaly eu eu 0 82 massive pyrite has a similar pattern except for higher overall abundances and a small positive eu anomaly eu eu 1 10 except for the eu anomalies the andesites and dacites show no evidence for ree mobility eu anomalies in the andesites are probably due to alteration of phenocrystic plagioclase and subsequent removal of divalent eu substantial ree mobility has occurred in the hydrothermally altered volcanic rocks the degree of mobility correlating with alteration style and intensity eu has been leached from both former plagioclase phenocrysts and the groundmass some of the eu leached from zones of the most intense alteration may have been redeposited in lower temperature more distal alteration zones

age and isotopic relationships among the angrites lewis cliff 86010 and angra dos reis results of a wide ranging isotopic investigation of the unique antarctican angrite lew 86010 lew are presented together with a reassessment of the type angrite angra dos reis ador the principal objectives of this study are to obtain precise radiometric ages initial sr isotopic compositions and to search for the erstwhile presence of the short lived nuclei 146sm and 26al via their daughter products the isotopic compositions of sm u ca and ti were also measured this allows a detailed appraisal to be made of the relations between and the geneology of these two angrites lew proves to be severely contaminated with modern terrestrial pb which is shown to result from terrestrial weathering nevertheless concordant pb pb model ages of pyroxene separates were obtained 2 4 55784 52 ga for lew and 4 55780 42 ga for ador uranium isotopic compositions are normal within error the inferred initial pb isotopic composition is within error of primordial pb as defined by canon diablo troilite because of the extreme upb ratios of both angrites this places an upper limit of 2 ma on the time between volatile element loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates initial 87sr86sr ratios were found to be indistinguishable in lew ador and the cumulate eucrite moore county the derived initial 87sr86sr for all three meteorites is 0 698970 15 2 a 147sm 143nd isochron for lew was obtained yielding an age of 4 553 34 ga with an initial 143nd144nd of 0 506682 49 2 146sm142nd isotope systematics were also measured anomalies on 142nd arising from extinct 146sm were clearly resolved resulting in an initial 146sm144sm of 0 0071 17 with nd142 2 57 0 62 at the time of isotopic closure both of these sm nd methods imply derivation of lew from a reservoir with chondritic smnd ratio they are also quite consistent with the previously reported systematics for ador overall the age and isotopic similarities between lew and ador are striking it suggests almost simultaneous production on the same asteroid even though recent experimental studies imply that the two are not comagmatic calcium and titanium do not exhibit enrichments in the n rich isotopes in contrast to cais making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula no evidence was found for live 26al in lew when combined with the pb pb age and initial 87sr86sr data this firmly excludes 26al being an important heat source in the early solar system heat supplied for for example 1 the differentiation of the angrite and eucrite parent bodies and 2 metamorphism of the chondrite parent bodies must come from some other source the debate over live vs extinct 26al in the earliest solar system remains unresolved by this new data from lew however both the angrite and eucrite parent bodies have extremely low rbsr and high upb ratios compared to the solar nebula therefore the common initial 87sr86sr of 0 698970 can be firmly associated with the u pb angrite age above and the formation of both bodies this enables an absolute chronology of the early solar system to be established based upon 87sr86sr differences the oldest cais are a minimum of 11 4 ma older than the angrites i e 4 569 5 ga the age and isotopic constraints are discussed with respect to current collapse condensation and accretion timescales calculated for the solar nebula

the hf w isotopic system and the origin of the earth and moon abstract the earth has a radiogenic w isotopic composition compared to chondrites demonstrating that it formed while 182hf half life 9 myr was extant in earth and decaying to 182w this implies that earth underwent early and rapid accretion and core formation with most of the accumulation occurring in 10 myr and concluding approximately 30 myr after the origin of the solar system the hf w data for lunar samples can be reconciled with a major moon forming impact that terminated the terrestrial accretion process 30 myr after the origin of the solar system the suggestion that the proto earth to impactor mass ratio was 7 3 and occurred during accretion is inconsistent with the w isotope data the w isotope data is satisfactorily modeled with a mars sized impactor on proto earth proto earth to impactor ratio of 9 1 to form the moon at 30 myr

east flank of the sibumasu block in nw thailand and myanmar and its possible northward continuation into yunnan a review and suggested tectono stratigraphic interpretation abstract the east flank of the sibumasu block was a passive continental margin and in nw thailand is marked by the absence of the autochthonous middle permian triassic platform carbonates which are widespread across the rest of sibumasu further west instead the carbonates are represented by hemipelagic cherts mudstones and sandstones including turbidites during the northward drift of sibumasu following its early permian rifting from gondwana an accretionary complex was present where palaeotethyan pelagic rocks as old as devonian were subducted beneath the sukhothai volcanic arc at the time of sibumasu s collision with the sukhothai arc beginning in the middle triassic the accretionary complex was thrust westwards across the east flank of sibumasu it is suggested that in the late triassic the thrust pile which had been the accretionary complex underwent erosion and was the source of terrigenous clastic rocks deposited further west in a foredeep basin the boundary of sibumasu s east flank with the permo triassic carbonate platform further west is the arcuate mae ping nam teng fault system notwithstanding later cenozoic strike slip displacement those faults as well as the mae yuam fault are interpreted to have had an earlier history of westward directed indosinian thrusting northwards in myanmar and yunnan the sibumasu permo triassic carbonate shelf continues as the shan plateau and baoshan block the east flank is represented by the changning menglian belt and the palaeotethys cryptic suture in thailand possibly joins with the lancangjiang suture

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

tectonics and metallogeny of mainland southeast asia a review and contribution abstract the southeast asia region is endowed with a diversity of mineral resources notably porphyry related skarn epithermal and sediment hosted orogenic gold deposits formation of these deposits was associated with a long and complex tectonic history of gondwana supercontinent break up arc magmatism backarc basin development together with arc continent and continent continent collisions that created the present day mainland se asia this paper summarizes historical and current se asian geological research and ore deposit studies here we present a new tectonic and metallogenic model for phanerozoic mainland se asia from this model we conclude that incipient arc backarc basin magmatism is the key to the formation of many important ore deposits in the truong son and loei fold belts the two major metallogenic belts in mainland se asia in addition triassic to cenozoic arc continent and continent continent collisions have led to the formation of many sediment hosted orogenic gold deposits in the sukhothai and the sibumasu terranes oblique cretaceous to recent subduction along the andaman sunda trench was responsible for gold and copper gold molybdenum porphyry and epithermal mineralization along the kawlin wutho mt popa arc in myanmar in the north and the sumatran volcanic arc in the south we anticipate that the present emphasis on exploring for shallower level epithermal deposits will increasingly lead to exploration for deeper level porphyry and porphyry related skarn systems in the coming decades

cr diffusion in orthopyroxene experimental determination 53mn 53cr thermochronology and planetary applications we have determined cr diffusion coefficients d in orthopyroxene parallel to the a b and c axial directions as a function temperature at f o2 corresponding to those of the wustite iron wi buffer diffusion is found to be significantly anisotropic with d c d b d a conforming to an earlier theoretical prediction increase of f o2 from wi buffer conditions to 4 5 log unit above the buffer at 950 and 1050 c leads to decrease of d cr by a factor of two to three possibly suggesting significant contribution from an interstitial diffusion mechanism we have used the diffusion data to calculate the closure tem peratures tc of the mn cr decay system in orthopyroxene as a function of initial temperature t0 grain size a and cooling rate for spherical and plane sheet geometries we also present graphical relations that permit retrieval of cooling rates from knowledge of the resetting of mn cr ages in orthopyroxene during cooling t0 and a application of these relations to the mn cr age data of the cumulate eucrite serra de mageyields a tc of 830 980 c and cooling rates of 2 27 c myr at tc and 1 13 c myr at 500 c it is shown that the cooling of serra de mageto the closure temperature of the mn cr system took place at its original site in the parent body and thus implies a thickness for the eucrite crust in the commonly accepted hed parent body vesta of greater than 30 km this thickness of the eucrite crust is compatible only with a model of relatively olivine poor bulk mineralogy in which olivine constitutes 19 7 of the total asteroidal mass

origin and chronology of chondritic components a review abstract mineralogical observations chemical and oxygen isotope compositions absolute 207pb 206pb ages and short lived isotope systematics 7be 7li 10be 10b 26al 26mg 36cl 36s 41ca 41k 53mn 53cr 60fe 60ni 182hf 182w of refractory inclusions ca al rich inclusions cais and amoeboid olivine aggregates aoas chondrules and matrices from primitive unmetamorphosed chondrites are reviewed in an attempt to test i the x wind model vs the shock wave model of the origin of chondritic components and ii irradiation vs stellar origin of short lived radionuclides the data reviewed are consistent with an external stellar origin for most short lived radionuclides 7be 10be and 36cl are important exceptions and a shock wave model for chondrule formation and provide a sound basis for early solar system chronology they are inconsistent with the x wind model for the origin of chondritic components and a local irradiation origin of 26al 41ca and 53mn 10be is heterogeneously distributed among cais indicating its formation by local irradiation and precluding its use for the early solar system chronology 41ca 41k and 60fe 60ni systematics are important for understanding the astrophysical setting of solar system formation and origin of short lived radionuclides but so far have limited implications for the chronology of chondritic components the chronological significance of oxygen isotope compositions of chondritic components is limited the following general picture of formation of chondritic components is inferred cais and aoas were the first solids formed in the solar nebula 4567 4568 myr ago possibly within a period of 10 and have 26al 27al 0 similar to those in chondrules 5 nebular gas the majority of chondrules within a chondrite group may have formed over a much shorter period of time

understanding sibumasu in the context of ribbon continents abstract well described ribbon continent collision belts and modern examples with 2d seismic coverage particularly zealandia provide analogues for the permo triassic development of sibumasu two key parameters for the ribbon continents are their width and how rifting developed which in turn impact the structural metamorphic igneous and stratigraphic development of ribbon continents in ways that make them significantly different from the collisional development of major continents key characteristics of the tectonics of sibumasu are 1 a tendency towards basin inversion during the late triassic and possibly early jurassic and mixed thick skinned and thin skinned style deformation 2 extensional collapse around 215 ma is an important feature of sibumasu particularly in northern thailand and probably other areas too 3 slab delamination and extension related decompression melting were probably important mechanisms for generating granites in addition to crustal thickening 4 metamorphism is relatively high temperature lower pressure

nature of accretion related to paleo tethys subduction recorded in northern thailand constraints from melange kinematics and illite crystallinity abstract we reconstructed the accretion process related to paleo tethys subduction recorded in northern thailand based on melange and thrust structures and metamorphic temperatures derived from illite crystallinity data melange formation was characterized by hydrofracturing and cataclastic deformation with mud injection under semi lithified conditions followed by shear deformation and pressure solution illite crystallinity data suggest metamorphic temperatures below 250 c during melange formation the combined structural and metamorphic data indicate that during melange formation the accretionary complex related to paleo tethys subduction developed at shallow levels within an accretionary prism asymmetric shear fabrics in melange indicate top to south shear after correction for rotation associated with collision between the indian and eurasian continents the trend of the paleo tethys subduction zone is estimated to have been n80 e we conclude that the paleo tethys was subducted northward beneath the indochina block from the permian to triassic

mn cr systematics in primitive meteorites insights from mineral separation and partial dissolution abstract cr isotopic compositions have been measured on carbonaceous chondrites cc tafassasset paris niger i nwa 5958 nwa 8157 and jbilet winselwan in bulk samples the 54 cr 52 cr ratios expressed as e 54 cr range from 0 93 to 1 58 e units these values are in agreement with values characteristic for distinct petrologic types despite this 54 cr heterogeneity the variability in the 53 cr 52 cr ratios expressed as e 53 cr of 0 2 e units and the mn cr ratios is consistent with the previous finding of an isochron in the mn cr evolution diagram the mn cr ratio in cc corresponds to variable abundances of high t condensate formed and separated at the beginning of the solar system thus the canonical 53 mn 55 mn ratio can be defined based on a consistent chronology for u pb and mn cr between the earliest objects formed in the solar nebula and the d orbigny angrite we define a canonical 53 mn 55 mn ratio and e 53 cr i of 6 8 10 6 and 0 177 respectively the internal mn cr systematics in tafassasset and paris were studied by two approaches leaching technique and mineral separation despite variable e 54 cr values up to 30 e linear co variations were found between e 53 cr and mn cr ratio the mineral separates as well as the leachates of tafassasset fall on a common isochron indicating that 1 cooling of the tafassasset s parent body occurred at 4563 5 0 25 ma and that 2 54 cr is decoupled from the other isotopes even though temperatures 900 c have been reached during metamorphism in the case of paris the leachates form an alignment with a 53 mn 55 mn ratio higher than the canonical value this alignment is not an isochron but rather a mixing line based on leachates from various cm and ci we propose the occurrence of three distinct cr reservoirs in meteoritic material pure54 high53 and low53 characterized by a e 53 cr and e 54 cr of 0 and 25 000 2 17 and 8 and 0 5 and 151 respectively pure54 has already been described and is carried by highly refractory nano spinel high53 is mn rich and most probably carried by sulfides in the matrix whereas low53 is characterized by low mn cr ratios and it is sensitive to metamorphism this component could correspond to mineral phases such as refractory oxides and carbide variable mixing proportions of high53 and low53 would explain the larger than expected uncertainty mswd of 5 5 on the cc bulk regression line a monte carlo simulation allows us to evaluate the impact of the dispersion of the initial cr isotopic ratios as a function of variable high53 the co variation of the mn cr ratio and the e 53 cr defined by the mineral separates from paris corresponds to an age of 4566 44 0 66 0 75 ma while their e 54 cr still differ by at least 0 42 e this age is likely to date the segregation of forsteritic olivines most probably from type i chondrules from fayalitic olivines from type ii chondrules and given the sampling procedure by handpicking of hundreds of grains corresponds to the average age of chondrule formation

petrochemistry u pb zircon age and palaeotectonic setting of the lampang volcanic belt northern thailand two ne trending belts of mainly subaerial dacitic to rhyolitic flows and tuffs occur in the area between the towns of lampang and denchai in northern thailand in the western belt doi ton the rocks have been pervasively altered to quartz keratophyre rocks in the eastern belt doi luang are generally less altered mobile chemical components such as na 2 o and k 2 o show wide variation particularly in samples from the doi ton belt however low zr tio 2 ratios and low nb and ta contents support an origin at a convergent plate margin a positive epsilon nd value of 4 9 for rhyolite from the doi luang belt supports derivation from a primitive crustal source a rhyolite sample from the doi luang belt yielded a u pb zircon age of 240 1 ma early mid triassic the doi ton and doi luang belts are part of the lampang volcanic belt which can be traced to the north into the lincang jinghong volcanic belt in southern china comparison with published petrological data from the lincang jinghong belt shows strong similarity including the widespread development of keratophyric mineralogy and chemistry the lampang lincang jinghong belt formed at an early mid triassic convergent plate margin and is similar in age and tectonic setting to the more mafic phetchabun volcanic belt on the east side of the nan river suture zone these data constrain the timing of final amalgamation between the indochina and shan thai terranes to mid triassic or younger

diffusion kinetics of cr in olivine and 53mn 53cr thermochronology of early solar system objects we have determined the diffusion coefficient of cr in olivine as function of temperature oxygen fugacity fo2 and crystallographic orientation and used these data to develop a quantitative understanding of the resetting of the short lived 53 mn 53 cr decay system in olivine during cooling within meteorite parent body the diffusion of cr in olivine was found to be anisotropic and effectively indepen dent of fo2 between wustite iron buffer and two orders of magnitude above this buffer the diffusion data were used to calculate the spatially averaged mean closure temperature of the 53 mn 53 cr decay system in olivine as function of the initial temperature cooling rate and grain size and also the closure age profile of this system in olivine single crystal as function of radial distance and a dimensionless parameter that incorporates the effects of various parameters that affect the closure age we also present a thermochronolgic formulation that permits retrieval of cooling rates from the extent of resetting of the bulk 53 mn 53 cr closure age of olivine during cooling this method was applied to determine the cooling rate of the pallasite omolon which showed 53 mn 53 cr bulk age of olivine that is 10 myr younger than the age of the solar system the calculated cooling rate which is 20 40 c myr at 985 1000 c is in good agreement with the metallographic cooling rate at 500 c when the two results are considered in terms of a cooling model in which the reciprocal temperature increases linearly with time the inferred cooling rate of omolon which seems to be a sample from the core mantle boun dary yields a burial depth of 30 km in a parent body of at least 100 km radius

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

manganese chromium formation intervals for chondrules from the bishunpur and chainpur meteorites whole chondrule mn cr isochrons are presented for chondrules separated from the chainpur ll3 4 and bishunpur ll3 1 meteorites the chondrules were initially surveyed by instrumental neutron activation analysis ll chondrite normalized mn cr mn fe and sc fe served to identify chondrules with unusually high or low mn cr ratios and to correlate the abundances of other elements to sc the most refractory element measured a subset of chondrules from each chondrite was chosen for analysis by a scanning electron microscope equipped with an energy dispersive x ray spectrometer prior to high precision cr isotopic analyses 53cr 52cr correlates with 55mn 52cr to give initial 53mn 55mn i 9 4 1 7 10 6 for chainpur chondrules and 53mn 55mn i 9 5 3 1 10 6 for bishunpur chondrules the corresponding chondrule formation intervals are respectively tlew 10 1 ma for chainpur and 10 2 ma for bishunpur relative to the time of igneous crystallization of the lewis cliff lew 86010 angrite because mn sc correlates positively with mn cr for both the chainpur and bishunpur chondrules indicating dependence of the mn cr ratio on the relative volatility of the elements we identify the event dated by the isochrons as volatility driven elemental fractionation for chondrule precursors in the solar nebula thus our data suggest that the precursors to ll chondrules condensed from the nebula 5 8 2 7 ma after the time when initial 53mn 55mn i 2 8 0 3 10 5 for calcium aluminum rich inclusions cais our preferred value determined from data for a mineral separates of type b allende cai br1 b spinels from efremovka cai e38 and c bulk chondrites mn cr formation intervals for meteorites are presented relative to average i mn 53mn 55mn ch 9 46 10 6 for chondrules mn cr ratios for radiogenic growth of 53cr in the solar nebula and later reservoirs are calculated relative to average i mn 53cr i 9 46 0 08 10 6 0 23 0 08 for chondrules inferred values of mn cr lie within expected ranges thus it appears that evolution of the cr isotopic composition can be traced from condensation of cais via condensation of the ferromagnesian precursors of chondrules to basalt generation on differentiated asteroids measured values of 53cr for individual chondrules exhibit the entire range of values that has been observed as initial 53cr values for samples from various planetary objects and which has been attributed to radial heterogeneity in initial 53mn 55mn in the early solar system estimated 55mn 52cr 0 42 0 05 for the bulk earth combined with 53cr 0 for the earth plots very close to the chondrule isochrons so that the earth appears to have the mn cr systematics of a refractory chondrule thus the earth apparently formed from material that had been depleted in mn relative to cr contemporaneously with condensation of chondrule precursors if as seems likely the earth s core formed after complete decay of 53mn there must have been little differential partitioning of mn and cr at that time

the bentong raub suture zone abstract it is proposed that the bentong raub suture zone represents a segment of the main devonian to middle triassic palaeo tethys ocean and forms the boundary between the gondwana derived sibumasu and indochina terranes palaeo tethyan oceanic ribbon bedded cherts preserved in the suture zone range in age from middle devonian to middle permian and melange includes chert and limestone clasts that range in age from lower carboniferous to lower permian this indicates that the palaeo tethys opened in the devonian when indochina and other chinese blocks separated from gondwana and closed in the late triassic peninsular malaysia segment the suture zone is the result of northwards subduction of the palaeo tethys ocean beneath indochina in the late palaeozoic and the triassic collision of the sibumasu terrane with and the underthrusting of indochina tectonostratigraphic palaeobiogeographic and palaeomagnetic data indicate that the sibumasu terrane separated from gondwana in the late sakmarian and then drifted rapidly northwards during the permian triassic during the permian subduction phase the east malaya volcano plutonic arc with i type granitoids and intermediate to acidic volcanism was developed on the margin of indochina the main structural discontinuity in peninsular malaysia occurs between palaeozoic and triassic rocks and orogenic deformation appears to have been initiated in the upper permian to lower triassic when sibumasu began to collide with indochina during the early to middle triassic a type subduction and crustal thickening generated the main range syn to post orogenic granites which were emplaced in the late triassic early jurassic a foredeep basin developed on the depressed margin of sibumasu in front of the uplifted accretionary complex in which the semanggol formation rocks accumulated the suture zone is covered by a latest triassic jurassic and cretaceous mainly continental red bed overlap sequence

iodine xenon chemical and petrographie studies of semarkona chondrules evidence for the timing of aqueous alteration we have performed inaa petrographie and noble gas analyses on seventeen chondrules from the semarkona meteorite ll3 0 primarily to study the relationship of the i xe system to other measured properties we observe a range of 10 ma in apparent i xe ages the three latest apparent ages fall in a cluster suggesting the possibility of a common event the initial 129i 127i ratio r0 is apparently related to chondrule type and or mineralogy with nonporphyritic and pyroxene rich chondrules showing evidence for lower r0 s later apparent i xe ages than porphyritic and olivine rich chondrules in addition chondrules with sulfides on or near the surface have lower r0s than other chondrules the 129xe 132xe ratio in the trapped xe component anticorrelates with r0 consistent with evolution of a chronometer in a closed system or in multiple similar systems on the basis of these correlations we conclude that the variations in r0 represent variations in ages and that later event s possibly aqueous alteration preferentially affected chondrules with nonporphyritic textures and or sulfide rich exteriors about 10 ma after the formation of the chondrules

geochemistry and tectonic setting of the central loei volcanic rocks pak chom area loei northeastern thailand abstract the central loei volcanic rocks as evidenced by those in the pak chom area were formed in the late devonian early carboniferous and can be separated into three magmatic groups transitional tholeiitic basalt tholeiitic microgabbro and calc alkalic basalt andesite on the basis of immobile element contents and ratios of least altered samples all the tholeiitic microgabbro possibly occurred as dikes chemically the transitional tholeittic basalt and tholeiitic microgabbro have higher abundances of tio 2 ni and cr relative to the calc alkalic basalt andesite at similar values for feo mgo they also contain higher ti zr but lower zr nb the transitional tholeiitic basalt has higher concentrations of p 2 o 5 and nb relative to the tholeiitic microgabbro at similar levels of feo mgo and also has higher ratios of nb y and ti v but lower values for ti zr and zr nb in terms of chondrite normalized ree and n morb normalized patterns the transitional tholeiitic basalt tholeiitic basalt and calc alkalic basalt andesite are analogous to those from north atlantic southwest indian ridge and new britain arc on this basis the central loei volcanic rocks are comprised of morbs and oceanic island arc lavas these arc lavas may have erupted on an oceanic basement in the same ocean basin as those in the chiang rai chiang mai volcanic belt

late triassic post collisional granites related to paleotethyan evolution in northwestern lao pdr geochronological and geochemical evidence abstract the late paleozoic to early mesozoic granites exposed in northwestern lao pdr provide important constraints on the tectonic evolution of the eastern paleotethyan ocean and regional correlation with the giant granitic belt in southeast asia new geochronological data show that the granites have late triassic zircon u pb ages of 231 220 ma they are dominated by monzogranite and biotite granite with an i type geochemical affinity these granites are enriched in lrees and liles and depleted in hfses the geochemical variations for these granites indicate the fractional crystallization of plagioclase k feldspar biotite apatite and fe ti oxides their initial 87sr 86sr ratios range from 0 7021 to 0 7105 and end t values from 1 6 to 7 3 zircon in situ ehf t values are in range of 6 0 5 3 peaks at 2 8 and 2 5 respectively hf model ages of 0 67 1 64 ga peaks at 0 83 ga and 1 45 ga respectively and 18o values of 6 6 8 0 similar to the late triassic granitoids from the eastern province in southeast asia these granites originated from a mixed source of ancient metamorphic rocks with juvenile mafic crust the late triassic granites in northwestern lao pdr formed in a post collisional setting in response to the thickened crustal collapse during the assembly of the sibumasu with indochina blocks these granites can southerly link with the eastern granite province of the eastern paleotethyan domain in southeast asia

closure of the east paleotethyan ocean and amalgamation of the eastern cimmerian and southeast asia continental fragments abstract the phanerozoic record of southeast asia preserves the history of opening and subsequent consumption of the paleotethyan ocean and the assembly of continental fragments into asia as part of the broader scale reconstruction of pangea however uncertainty remains as to which of the many suture zones in southeast asia represents the relict of the main ocean when final ocean closure occurred and the assembly history of the eastern cimmerian and southeast asia continental fragments our overview of the geological features of the suture zones the bounding continental fragments and their magmatic metamorphic and sedimentary records resolves many of these key issues the sedimentary biogeographical structural lithological geochemical and geochronological data from the changning menglian inthanon and bentong raub suture zones argue for their linkage with the longmu co shuanghu suture zone in central tibet and together constitute the main east paleotethyan ocean relict the eastward subduction of the ocean resulted in the development of a series of magmatic arc back arc basin and continental fragments in southeast asia including from west to east the lincang sukhothai east malaya arc the jinghong nan sa kaeo back arc basin the simao west indochina fragment the luang prabang loei back arc basin the south indochina fragment the wusu and truong son back arc basins the north indochina fragment the jinshajiang ailaoshan song ma branch back arc basin and the south china block assembly of these fragments resulted in indosinian high temperature and high pressure metamorphism and related tectonothermal event available data indicate a switch from subduction of the main east paleotethyan ocean to the collision of the sibumasu with simao indochina blocks at 237 ma with subsequent syn and post collisional events at 237 230 ma and 230 200 ma respectively along the changning menglian inthanon and bentong raub suture zones the timing of initial syn and post collision events along the jinshajiang ailaoshan song ma suture zone with its record of back arc basin closure is at 247 ma 247 237 ma and 237 200 ma generally 10 ma older than that along the changning menglian inthanon and bentong raub suture zones our synthesis of all available data enables establishment of a comprehensive geodynamic model for the east paleotethyan evolution this model links the spatial temporal pattern across southeast asia into a series of tectonic events including ocean back arc basin opening subduction closure subsequent assemblage and orogenic collapse along with associated igneous metamorphic and sedimentary activities

compositions and textures of relic forsterite in carbonaceous and unequilibrated ordinary chondrites several percent of the olivine in the c2 c3 and unequilibrated ordinary chondrites uoc can be distinguished by blue cathodoluminescence cl and an unusual composition for forsterite this olivine has the following textural features 1 1 forms cores in single olivine grains 2 2 shows subhedral to euhedral boundaries against rim olivine 3 3 rarely contains inclusions 4 4 has embayments containing olivine like that of the rim 5 5 occurs within chondrules especially in uoc meteorites the blue olivine is always fe poor 0 25 feo 1 0 and shows the following average and maximum values al2o3 0 25 0 5 tio2 0 05 0 09 cao 0 5 0 8 cr2o3 0 15 0 5 and mno 0 02 0 15 vanadium is present within a single olivine and within all blue olivines al ca and ti are strongly positively correlated as are mn fe and cr in olivine surrounding the blue the blue cores are not zoned but each element shows a marked change at the boundary of the blue with al showing the most rapid change these are interpreted as diffusion profiles between rim and core olivine textures suggest initial free growth probably from a gas and later addition of olivine by liquid crystallization to form single crystals or chondrules the unusual olivine composition indicates high temperature growth from a refractory rich reservoir with al entering olivine in tetrahedral coordination vapor growth is suggested as the process allowing the high minor element levels the occurrence of blue olivine in all primitive meteorites indicates that it is relic material which was widespread prior to chondrule and hence meteorite formation similarities in composition exist between this relic olivine and olivine of cosmic dust and deep sea particles pointing to this olivine being a common component in all primitive extraterrestrial material

a cr isotopic study of the bon accord nio body in the barberton greenstone belt south africa abstract an unusual oxide body enriched in ni and the platinum group elements was found in the north western part of the barberton greenstone belt south africa in the 1920s with mineralogy and geochemistry which are extremely anomalous and do not conform to that of any other ni rich crustal rocks it has been interpreted as either an archaean paleo meteorite or of terrestrial deep mantle origin cr isotopic ratios were used to evaluate the merits of these two possibilities isotopic analyses returned values for e 53 of 0 00 epsilon units within a 2 s error relative to a terrestrial standard we interpret this information together with trace element data from the surrounding peridotites as pointing to a terrestrial origin for the body and conclude that the current dataset is not inconsistent with a deep mantle origin as a remnant of core formation with subsequent transport to and emplacement in the lithosphere by a mantle plume

an upper limit for ice in shackleton crater as revealed by lro mini rf orbital radar 1 although diverse measurements have indicated h oh or h2o species in the lunar polar regions pinpointing its location form and abundance in specific reservoirs has proven elusive here we report on the first orbital radar measurements of shackleton crater near the lunar south pole mini rf observations indicate a patchy heterogeneous enhancement in cpr circular polarization ratio on the crater walls whose strength decreases with depth toward the crater floor a result that is most consistent with a roughness effect due to less mature regolith present on the crater wall slopes however the results do not rule out a modest ice contribution and an upper limit of 5 10 wt h2o ice up to 30 vol present in the uppermost meter of regolith is also consistent with the observations

manganese chromium isotope systematics of carbonaceous chondrites abstract in this article we present the results of cr isotope investigations of different types of carbonaceous chondrites and of the pallasite eagle station the 53 cr 52 cr ratios in the bulk samples of carbonaceous chondrites are correlated with 55 mn 52 cr ratios the slope of the correlation line yields a 53 mn 55 mn ratio of 8 5 1 5 10 6 at the time of mn cr fractionation mapping this ratio onto an absolute time scale yields a time for this event of 4568 1 0 8 1 1 ma ago this time is very similar to the formation age of efremovka cais of 4567 2 0 6 ma y amelin a n krot i d hutcheon a a ulyanov lead isotopic ages of chondrules and calcium aluminum rich inclusions science 297 2002 1678 1683 to a time of the chondrule formation of 4568 1 ma ago l e nyquist d lindstrom d mittlefehldt c y shih h wiesmann s wentworth r martinez manganese chromium formation intervals for chondrules from the bishunpur and chainpur meteorites meteorit planet sci 36 2001 911 938 which most likely constrains early global high temperature mn cr fractionation in a nebular setting the bulk samples of carbonaceous chondrites exhibit clear 54 cr excesses 54 cr that are correlated with the 53 cr excesses 53 cr and also with mn cr ratios one possible explanation of this correlation is that 54 cr is also radiogenic like 53 cr and was formed by the decay of the short lived parent radionuclide 54 mn the very short half life of 54 mn of 312 days would require that both short lived radionuclides 53 mn and 54 mn were generated locally in spallation reactions during the early period of an active sun the alternative and possibly more plausible explanation is the heterogeneous addition of presolar material the presolar component enriched in 54 cr is mostly contained in the matrix of carbonaceous chondrites the relative amount of matrix decreases in the sequence ci cm co cv a large proportion of mn is associated with the matrix while cr preferentially resides in the chondrules thus the mn cr ratio also follows the sequence ci cm co cv and is correlated with 54 cr the acid resistant residues of carbonaceous chondrites are characterized by relatively large excesses of 54 cr and moderate deficits of 53 cr the magnitude of these excesses and deficits decreases in the sequence ci cm cv and may imply that cr in the cm and cv residues is increasingly more equilibrated with cr from the rest of the meteorites the 53 cr 52 cr and 54 cr 52 cr ratios in the residues are anti correlated indicating that there are at least two cr components of possibly presolar origin all residues have large excesses of the most neutron rich ti isotope 50 ti the pattern of 50 ti does not exactly follow that observed for 54 cr but it is generally similar the cr isotope systematic of the pallasite eagle station indicates that the precursor of this meteorite was a cv type material the 53 mn 53 cr system indicates that the cr isotopes equilibrated in this meteorite 4557 5 0 6 ma ago

iodine xenon analysis of ordinary chondrite halide implications for early solar system water we report the results of iodine xenon analyses of irradiated halide grains extracted from the h chondrite monahans 1998 and compare them with those from zag whitby et al 2000 to address the timing of aqueous processing on the h chondrite parent body xe isotopic analyses were carried out using the relax mass spectrometer with laser stepped heating the initial 129i 127i ratio in the monahans halide was determined to be 9 37 0 06 10 5 with an iodine concentration of 400 ppb significant scatter especially in the zag data indicates that a simple interpretation as a formation age is unreliable instead we propose a model whereby halide minerals in both meteorites formed 5 ma after the enstatite achondrite shallowater at an absolute age of 4559 ma this age is in agreement with the timing of aqueous alteration on the carbonaceous chondrite parent bodies and ordinary chondrite metamorphism and is consistent with the decay of 26al as a heat source for heating and mobilisation of brines on the h chondrite parent body post accretion surface impact events may have also contributed to the heat source

the i xe chronometer and the early solar system we review the development of the i xe technique and how its data are interpreted and specify the best current practices individual mineral phases or components can yield interpretable trends in initial 129i 127i ratio whereas whole rock i xe ages are often hard to interpret because of the diversity of host phases many of which are secondary varying standardizations in early work require caution only samples calibrated against shallowater enstatite or bjurble can contribute reliably to the emerging i xe chronology of the early solar system although sparse data for which i xe and mn cr can be compared suggest that the two systems are concordant among ordinary chondrite samples we derive a new age for the closure of the shallowater enstatite standard of 4563 3 0 4 myr from the relationship between the i xe and pb pb systems this yields absolute i xe ages and allows data from this and other systems to be tested by attempting to construct a common chronology of events in the early solar system absolute i xe dates for aqueous and igneous processes are consistent with other systems consideration of the i xe host phases in cais and dark inclusions demonstrates that here the chronometer records aqueous alteration of pre existing material the ranges of chondrule ages deduced from the al mg and i xe systems in semarkona ll3 0 and chainpur ll3 4 are consistent chainpur i xe data exhibit a greater range of ages than semarkona possibly reflecting a greater degree of parent body processing however individual chondrules show little or no evidence of such processing determining the host phase s responsible for high temperature correlations may resolve the issue

basaltic activity preserved in an upper permian radiolarian chert from the paleo tethys in the inthanon zone northern thailand basaltic sandstone composed mainly of clay to sand sized fragments of glass and basaltic rock has been discovered in a pelagic chert sequence in northern thailand the sandstone conformably underlies and is partly intercalated with the upper permian wuchiapingian early changhsingian radiolarian bearing chert geological age dating based on radiolarians petrographic examination of the bedded chert with the basaltic sandstone and geochemical studies were conducted to investigate the origin and sedimentary mode of the lithologic sequence the bedded chert overlying the basaltic sandstone yielded late permian radiolarians and was correlatable with the wuchiapingian follicucullus charveti albaillella yamakitai assemblage zone as well as the early changhsingian neoalbaillella ornithoformis assemblage zone lithofacies and microfacies of the bedded chert indicated deposition in a pelagic realm of the paleo tethys although the tectonic setting was difficult to estimate based on geochemical features because of element mobility and alteration the basaltic fragments likely originated as oceanic basalt such as oceanic island basalt microfacies and sedimentary structures of the chert and sandstone indicated that the studied sequence is composed of intermittent high sedimentation rates mainly gravity currents of basaltic fragments in a background setting of very slowly accumulating radiolarian chert the sedimentary mode of the sequence the rare terrigenous influx and the presence of fusulinoidean or other foraminiferal tests indicate that the basaltic rocks fragment were originally sourced from an oceanic topographic high such as a seamount or an oceanic plateau and that the volcanic activity occurred in the chert accumulated oceanic basin of the paleo tethys in the wuchiapingian

the distribution of short lived radioisotopes in the early solar system and the chronology of asteroid accretion differentiation and secondary mineralization abstract we evaluate initial 26 al 27 al i 53 mn 55 mn i and 182 hf 180 hf i ratios together with 207 pb 206 pb ages for igneous differentiated meteorites and chondrules from ordinary chondrites for consistency with radioactive decay of the parent nuclides within a common closed isotopic system i e the early solar nebula the relative initial isotopic abundances of 26 al 53 mn and 182 hf in differentiated meteorites and chondrules are consistent with decay from common solar system initial values here denoted by i al ss i mn ss and i hf ss respectively i mn ss and i hf ss 9 1 1 7 10 6 and 1 07 0 08 10 4 respectively correspond to canonical i al ss 5 1 10 5 i hf ss so determined is consistent with i hf ss 9 72 0 44 10 5 directly determined from an internal hf w isochron for cai minerals i mn ss is within error of the lowest value directly measured for cais we suggest that erratically higher values measured for cais in carbonaceous chondrites may reflect proton irradiation of unaccreted cais by the early sun after other asteroids destined for melting by 26 al decay had already accreted the 53 mn incorporated within such asteroids would have been shielded from further local spallogenic contributions from within the solar system the relative initial isotopic abundances of the short lived nuclides are less consistent with the 207 pb 206 pb ages of the corresponding materials than with one another the best consistency of short and long lived chronometers is obtained for 182 hf 180 hf i and the 207 pb 206 pb ages of angrites 182 hf 180 hf i decreases with decreasing 207 pb 206 pb ages at the rate expected from the 8 90 0 09 ma half life of 182 hf the model solar system age thus determined is t ss hf w 4568 3 0 7 ma 26 al 27 al i and 53 mn 55 mn i are less consistent with 207 pb 206 pb ages of the corresponding meteorites but yield t ss mn cr 4568 2 0 5 ma relative to i al ss 5 1 10 5 and a 207 pb 206 pb age of 4558 55 0 15 ma for the lew86010 angrite the mn cr method with i mn ss 9 1 1 7 10 6 is useful for dating accretion if identified with chondrule formation primary igneous events and secondary mineralization on asteroid parent bodies all of these events appear to have occurred approximately contemporaneously on different asteroid parent bodies for i mn ss 9 1 1 7 10 6 parent body differentiation is found to extend at least to 5 ma post t ss i e until differentiation of the angrite parent body 4563 5 ma ago or 4564 5 ma ago using the directly measured 207 pb 206 pb ages of the d orbigny clan angrites the 1 ma difference is characteristic of a remaining inconsistency for the d orbigny clan between the al mg and mn cr chronometers on one hand and the 207 pb 206 pb chronometer on the other differentiation of the iiiab iron meteorite and ureilite parent bodies probably occurred slightly later than for the angrite parent body and at nearly the same time as one another as shown by the mn cr ages of iiiab irons and ureilites respectively the latest recorded episodes of secondary mineralization are for carbonates on the ci carbonaceous chondrite parent body and fayalites on the cv carbonaceous chondrite parent body both extending to 10 ma post t ss

evidence for primitive nebular components in chondrules from the chainpur chondrite abstract the least equilibrated ordinary chrondrites contain chondrules which have experienced little change since the time of their formation in the early solar system these chondrules are excellent indicators of the physical and chemical nature of the solar nebula we separated 36 chondrules from the chainpur ll3 4 chondrite and analyzed each for 20 elements and petrographic properties sampling biases were minimized as far as possible chondrules seem to have formed through the melting of random mixtures of grains comprising a limited number of nebular components the identity of these components can be deduced from chondrule compositions the dominant components appear to be 1 a mixture of metal and sulfide with composition similar to whole rock metal and sulfide 2 refractory ir rich metal 3 refractory olivine rich silicates 4 low temperature pyroxene rich silicates and possibly 5 a component containing the more volatile lithophiles most of the textural types of chondrules formed from the same set of precursor components in some cases chondrules having different textures are almost identical in composition a few unusual chondrule types seem to mainly consist of uncommon nebular components possibly indicating different modes of formation etching experiments confirm that chondrule rims are enriched in metal troilite and moderately volatile elements relative to the bulk chondrules however a large fraction of the volatiles remains in the unetched interior

the lack of potassium isotopic fractionation in bishunpur chondrules in a search for evidence of evaporation during chondrule formation the mesostases of 11 bishunpur chondrules and melt inclusions in olivine phenocrysts in 7 of them have been analyzed for their alkali element abundances and k isotopic compositions except for six points all areas of the chondrules that were analyzed had 41k compositions that were normal within error typically 3 2s the six anomalous points are probably all artifacts experiments have shown that free evaporation of k leads to large 41k enrichments in the evaporation residues consistent with rayleigh fractionation under rayleigh conditions a 3 enrichment in 41k is produced by 12 loss of k the range of l chondrite normalized k al ratios a measure of the k elemental fractionation in the areas analyzed vary by almost three orders of magnitude if all chondrules started out with l chondrite like k abundances and the k loss occurred via rayleigh fractionation the most k depleted chondrules would have had compositions of up to 41k 200 clearly k fractionation did not occur by evaporation under rayleigh conditions yet experiments and modeling indicate that k should have been lost during chondrule formation under currently accepted formation conditions peak temperature cooling rate etc invoking precursors with variable alkali abundances to produce the range of k al fractionation in chondrules does not explain the k isotopic data because any k that was present should still have experienced sufficient loss during melting for there to have been a measurable isotopic fractionation if k loss and isotopic fractionation was inevitable during chondrule formation the absence of k isotopic fractionation in bishunpur chondrules requires that they exchanged k with an isotopically normal reservoir during or after formation there is evidence for alkali exchange between chondrules and rim matrix in all unequilibrated ordinary chondrites however melt inclusions can have alkali abundances that are much lower than the mesostases of the host chondrules which suggests that they at least remained closed since formation if it is correct that some or all melt inclusions remained closed since formation the absence of k isotopic fractionation in them requires that the k isotopic exchange took place during chondrule formation which would probably require gas chondrule exchange potassium evaporated from fine grained dust and chondrules during chondrule formation may have produced sufficient k vapor pressure for gas chondrule isotopic exchange to be complete on the timescales of chondrule formation alternatively our understanding of chondrule formation conditions based on synthesis experiments needs some reevaluation

preliminary geochemical study of volcanic rocks in the pang mayao area phrao chiang mai northern thailand tectonic setting of formation abstract the least altered permian mafic volcanic rocks from the pang mayao area phrao district chiang mai province part of chiang rai chiang mai volcanic belt have been analyzed and are found to be mid ocean ridge and ocean island basalts the mid ocean ridge basalts occur as lava flows or dike rocks they are equigranular fine to medium grained and consist largely of plagioclase clinopyroxene and olivine these basalt samples are tholeiitic and have compositions very similar to t morb from the region where the du toit fracture zone intersects the southwest indian ridge the ocean island basalt occurs as pillow breccia and lava flows or dike rocks they are slightly to moderately porphyritic with phenocrysts microphenocrysts of clinopyroxene olivine plagioclase and or fe ti oxide the groundmass is very fine grained and made up largely of felty plagioclase laths with subordinate clinopyroxene these basalt samples are alkalic and chemically analogous to those from haleakala volcano maui hawaiian chain these mafic volcanic rocks may have been formed in a major ocean basin rather than in a mature back arc basin

petrology and mineralogy of type ii feo rich chondrules in semarkona ll3 0 origin by closed system fractional crystallization with evidence for supercooling abstract the petrology of type ii porphyritic olivine chondrules in semarkona ll3 0 has been studied in detail olivines in these chondrules are euhedral fe richand are strongly zoned from cores to rims of grains in feo fa10 30 cr2o3 0 2 0 6 wt mno 0 2 0 7 wt and cao 0 1 0 4 wt interstitial mesostasis is rich in si aland ca and is glassy with abundant microcrystallites minor minerals include troilite fe ni metaland chromite some olivine grains contain euhedral fayalite rich cores that are probably produced during initial supercooling of the chondrule melt rare relict grains of forsteritic olivine have compositions very similar to olivines in type ia chondrules in semarkona and may result from disaggregation of such chondrules apart from these relics all properties of type ii chondrules can be described by closed system fractional crystallization of droplets which were essentially entirely molten the chondrules did not originate as aggregates of large olivine grains and are unlikely to be fragments of precursor igneous rocks cooling rates of the order of 1000 c h are indicated from peak temperatures of around 1600 c the chondrules show considerable evidence for supercooling and large departures from equilibrium during crystallization there is no evidence for chondrule nebula exchange either during or after crystallization or any secondary processes such as aqueous alteration or thermal metamorphism bulk composition data suggest that precursor material included a refractory ca al ti rich component similar to one of the precursor components of type ia chondrulesand a non refractory fe and si rich component type ia chondrules may have formed from type ii chondrules by loss of fe and volatiles alternatively the two chondrule types may have formed in regions of considerable diversity in the solar nebula from precursor materials with different fe mg ratios

testing an integrated chronology i xe analysis of enstatite meteorites and a eucrite we have determined initial 129i 127i ratios for mineral concentrates of four enstatite meteorites and a eucrite in the case of the enstatite meteorites the inferred ages are associated with the pyroxene rich separates giving pyroxene closure ages relative to the shallowater standard of indarch eh4 0 04 0 67 ma khairpur el6 4 22 0 67 ma khor temiki aubrite 0 06 ma and itqiy enstatite achondrite 2 6 2 6 ma negative ages indicate closure after shallowater no separate from the cumulate eucrite asuka a 881394 yielded a consistent ratio though excess 129xe was observed in a feldspar separate suggesting disturbance by thermal metamorphism within 25 ma of closure in shallowater iodine 129 ages are mapped to the absolute pb pb time scale using the calibration proposed by gilmour et al 2006 who place the closure age of shallowater at 4563 3 0 4 ma comparison of the combined 129i pb data with associated 53mn ages for objects that have been dated by both systems indicates that all three chronometers evolved concordantly in the early solar system the enstatite chondrites are offset from the linear array described by asteroid belt objects when 53mn ages are plotted against combined 129i pb data supporting the suggestion that 53mn was radially heterogeneous in the early solar system

evolution of alkalic lavas at haleakala volcano east maui hawaii the postshield and posterosional stages of haleakala volcano contain intercalated alkalic basalt and evolved alkalic lavas isotopic and incompatible element abundance ratios in the haleakala postshield basalts changed systematically with time providing evidence for significant temporal changes in the mantle components contributing to the magmatic sources specifically a depleted i e low87sr 86sr and high143nd 144nd mantle component is more abundant in younger lavas however as magma production rates decreased during the postshield and posterosional stages basaltic melts in magma reservoirs cooled and fractionated leading to evolved residual melts such as hawaiite because primary basalt compositions changed with time the evolved haleakala lavas formed from a range of parental compositions however basalts and evolved lavas of similar age and isotopic ratios sr and nd have major and trace element contents that are consistent with a crystal fractionation model although alkalic basalt and hawaiite are the dominant lavas of the postshield stages of both haleakala and mauna kea volcanoes there are important differences between their lavas for example compositional differences between the hawaiite suites at haleakala and mauna kea indicate that on average the evolved lavas at haleakala formed at lower pressures also at haleakala basalts are intercalated with hawaiites whereas at mauna kea basalts and hawaiites are separated by a sharp boundary these differences probably reflect a higher magma supply rate to the haleakala volcano

on early solar system chronology implications of an heterogeneous spatial distribution of 26al and 53mn abstract early solar system chronology is usually built with the assumption that the distribution of short lived radionuclides was homogeneous through the solar accretion disk at present there is no unambiguous evidence for a homogeneous distribution of short lived radionuclides in the solar accretion disk while some data point to a heterogeneous distribution of short lived radionuclides in this paper we explore a possible chronology based on a heterogeneous distribution of 26 al and 53 mn in the accretion disk our basic assumption is that the different abundances of extinct short lived radionuclides in calcium aluminium rich inclusions cais and chondrules are due to spatial rather than temporal differences we develop a simple model where cais and chondrules form contemporaneously in different spatial locations and are characterised by distinct initial 26 al and 53 mn abundances in this model all evolved bodies are supposed to be originally chondritic i e to be made of a mixture of cais chondrules and matrix this mixture determines the initial content in 26 al and 53 mn of a chondritic parent body as a function of its cai and chondrule abundance fraction this approach enables us to calculate coherent 26 al and 53 mn ages from the agglomeration of the parent body precursors cais and chondrules until the isotopic closure of 26 al and 53 mn thereafter called 26 al 53 mn age we calculate such 26 al 53 mn ages for a diversity of evolved objects with the constraint that they should be found for realistic chondritic parent body precursors i e objects having similar or identical petrograpy to the existing chondrite groups the so defined age of the d orbigny angrite is 4 3 1 1 myr for the asuka 881394 eucrite 2 8 1 0 myr for the h4 chondrite sainte marguerite 3 myr and for h4 forest vale 5 myr the calculated 26 al 53 mn ages give timescales for the evolution of the respective parent bodies meteorites that can be investigated in the light of further petrographic studies we anchor the calculated relative chronology to an absolute chronology using absolute pb pb ages and relative hf w ages of the objects under scrutiny the precursors of sainte marguerite and forest vale agglomerated at the same time 4565 8 1 2 ma ago the precursors of eucrites asuka 881394 agglomerated 4564 8 1 2 ma ago the precursors of angrites agglomerated late 4561 5 1 8 ma ago our model provides a fully compatible al mg mn cr pb pb chronology and is shown to be robust to reasonable changes in the input parameters the calculated initial 26 al 27 al ratios are high enough to have 26 al as a possible heat source for differentiation

permian tectonic framework and palaeogeography of se asia abstract the principal continental lithospheric terranes now comprising se asia and that existed in the permian are the south china indochina super terrane and the sibumasu simao west burma and sw borneo semitau terranes the simao terrane is here regarded a separate terrane derived from south china by back arc spreading in the lower carboniferous rather than as an extension of indochina the main palaeo tethys ocean in thailand is identified as being represented by the chiang mai and sra kaeo sutures and the eastern boundary of the sibumasu terrane is thus interpreted to lie farther west in thailand than previously inferred the nan uttaradit suture is interpreted as representing a marginal back arc basin and contiguous with the ailaoshan and jinshajiang sutures in sw china the south china indochina super terrane and the simao terrane were located within the palaeo tethys in equatorial latitudes during the permian where they together with north china exhibit cathaysian floras and faunas the sibumasu terrane was located on the nw australian margin of gondwana until the sakmarian major shifts in faunal affinities from gondwana province faunas to an independent sibumasu province and then to assimilation into the cathaysian province records the separation and rapid northward drift of sibumasu after the late sakmarian the principal permian palaeogeographic models for the region are assessed for internal consistencies and conflicts