high al gabbros in the laramie anorthosite complex wyoming implications for the composition of melts parental to proterozoic anorthosite high al gabbro represents one of the latest phases of magmatism in the 1 43 ga laramie anorthosite complex lac in southeastern wyoming this lithology which is mineralogically and geochemically the most primitive in the lac forms dikes and small intrusions that cross cut monzonitic and anorthositic rocks high al gabbro is characterized by high al2o3 15 19 wt ree patterns with positive europium anomalies eu eu 1 2 3 8 and the lowest initial 87sr 86sr as low as 0 7033 and highest initial nd up to 2 in the lac their sr and nd isotopic characteristics indicate a mantle origin followed by crustal assimilation during ascent intermediate plagioclase an50 60 and mafic silicate fo54 63 compositions suggest that they are not primary mantle melts and that they differentiated prior to final emplacement high al gabbros of the lac are similar compositionally to gabbros from several other proterozoic anorthosite complexes including rocks from the harp lake complex and the hettasch intrusion in labrador and the adirondack mountains of new york these gabbros are considered to be parental to their associated anorthositic rocks a theory that is supported by recent experimental work we interpret lac high al gabbros to represent mantle derived melts produced by the differentiation of a basaltic magma in an upper mantle chamber continued evolution of this magma eventually resulted in the formation of plagioclase rich diapirs which ascended to mid crustal levels and formed the anorthositic rocks of the lac because these gabbros intrude the anorthositic rocks they do not represent directly the magma from which anorthosite crystallized and instead are younger samples of magma formed by identical processes

derivation of the 1 0 0 9 ga ferro potassic a type granitoids of southern norway by extreme differentiation from basic magmas major and trace elements sr and nd isotopic data as well as mineral compositions are presented for a selection of the 1 0 0 9 ferro potassic a type granitoids bessefjellet rustfjellet verhuskjerringi valle holum svofjell handeland tveit aseral lyngdal gabbronorites that occur close to the mandal ustaoset line mul of southern norway these hornblende biotite granitoids hbg define an extensive differentiation trend ranging from gabbronorites 50 wt sio 2 to granites 77 wt sio2 this trend is interpreted as resulting from extreme fractional crystallization of several basaltic magma batches with similar major and trace elements compositions at 930 ma the hbg suite displays a narrower range in isr 0 7027 0 7056 than in end t 1 97 down to 4 90 suggesting some assimilation of a rb depleted lower crust afc process or and source variability an age of 929 47 ma is given by a rb sr isochron on the holum granite sri 0 7046 0 0006 mswd 1 7 geothermobarometers indicate a low pressure of emplacement 1 3 2 7 kbar and an oxygen fugacity close to nno high liquidus temperatures are given by the apatite saturation thermometer 1005 1054 c and are in agreement with results from other studies the basaltic parent magmas of the hbg suite are partial melts of an hydrous mafic potassic source lying either in the lithospheric upper mantle or in the mafic lower crust derived from it this contrasts with the 930 ma anorthosite mangerite charnockite suite amc suite of the rogaland province for which a depleted lower crustal anhydrous gabbronoritic source has been indicated the present data imply the penecontemporaneous melting of two contrasting sources in southern norway the source duality could result from an increasing degree of metamorphism amphibolite to granulite from east to west an horizontal stratification of the lower crust or from the stratification of the lithosphere melting of the lower crust or upper mantle it may also indicate that the amc and hbg suites formed in two distinct crustal segments the linear alignment of the hbg suite along the mandal ustaoset shear zone suggests that a linear uprise of the asthenosphere following a lithospheric delamination under this structure could be the vector of the mantle heat 2003 elsevier science b v all rights reserved

oxygen isotope studies of achondrites abstract oxygen isotope abundances provide a powerful tool for recognizing genetic relationships among meteorites among the differentiated achondrites three isotopic groups are recognized l snc mars 2 earth and moon and 3 hed howardites eucrites diogenites the hed group also contains the mesosiderites main group pallasites and silicates from iiiab irons the angrites may be marginally resolvable from the hed group within each of these groups internal geologic processes give rise to isotopic variations along a slope 1 2 fractionation line as is well known for terrestrial materials variations of 17o from one planet to another are inherited from the inhomogeneities in the solar nebula as illustrated by the isotopic compositions of chondrites and their constituents among the undifferentiated achondrites five isotopic groups are found 1 aubrites 2 winonaites and iab iiicd irons 3 brachinites 4 acapulcoites and lodranites and 5 ureilites the isotopic compositions of aubrites coincide with the earth and moon and also with the enstatite chondrites these bodies apparently were derived from a common reservoir the isotopic composition of which was established at the chondrule scale by nebular processes isotopic similarities between chondrites and achondrites are seen only for the following instances 1 enstatite chondrites and aubrites 2 h chondrites and he irons and 3 l or ll chondrites and iva irons the isotopic data also support the following genetic associations 1 winonaites and iab iiicd irons 2 acapulcoites and lodranites and 3 ureilites and dark inclusions of c3 chondrites an attempt to reconcile the whole planet isotopic compositions of earth mars and the eucrite parent body with mixing models of their chemical compositions failed it is not possible to satisfy both the chemical and isotopic compositions of the terrestrial planets using known primitive solar system components

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

geochemical and isotopic zoning patterns of plagioclase megacrysts in gabbroic dykes from the gardar province south greenland implications for crystallisation processes in anorthositic magmas chemical and sr isotopic zoning patterns in plagioclase megacrysts from gabbroic dykes in the gardar province can be used to elucidate magma chamber and emplacement processes the megacrysts occur either as single crystals or assembled as anorthosite xenoliths the size of the megacrysts varies from 1 cm to 1 m they consist of a large core with variable zonation an58 39 and a relatively small 600 m normally zoned rim an62 27 the contact between core and rim is sharp and marked by a sharp increase in anorthite content which can reach 11 mol an this gap is interpreted as having formed during dyke emplacement due to a sudden pressure release some of the megacryst cores show a fairly constant composition whereas others exhibit an unusual wavy oscillatory zoning which has not been reported elsewhere to our knowledge the oscillatory zoning has wavelengths of up to 2 500 m and a maximum amplitude of 7 mol an it is interpreted as reflecting movements of the crystals in the magma reservoir the sr isotopic composition of one crystal shows a radiogenic inner core 87sr 86sr i 0 7044 and a less radiogenic outer core 87sr 86sr i 0 7039 0 7036 the lack of a significant change between outer core and rim 87sr 86sr i 0 7037 is consistent with formation of the more an rich rim due to pressure release variations in the core may be related to movements of the crystal and or magma mixing a trace element profile across a megacryst shows a small increase in sr and small decreases in ba and la contents of the recalculated melt composition across the core rim boundary whereas p ce nd and eu remain constant melt compositional changes upon emplacement are therefore considered to be of minor importance constant ratios of incompatible trace elements in the megacryst cores indicate a dominant influence of a lower crustal source on trace element budgets

grain boundary graphite in rocks and implications for high electrical conductivity in the lower crust the origin of zones of high electrical conductivity in the lower continental crust is a long standing mystery possible explanations include the presence of brines1 2 partial melt3 serpentine4 and graphite5 when discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6 10 here we show that a fine film of graphite is present on the grain boundaries in three rocks from the laramie anorthosite complex in two of these rocks graphite was stable during igneous crystallization but in the other it was not we maintain that in all of the rocks the grain boundary graphite precipitated from a co2 rich fluid during cooling the chemical processes that produced the grain boundary graphite in these rocks are likely to operate in many lower crustal rocks we therefore contend that because the films we observe are capable of producing the high conductivity that is seen in the lower crust grain boundary graphite should be considered as a possible cause for some conductivity anomalies

the vertical structure of toga coare convection part i radar echo distributions radar data collected by the 5 cm mit radar which was deployed aboard the r v vickers during the intensive observing period of the tropical ocean global atmosphere coupled ocean atmosphere response experiment are partitioned into convective and stratiform cartesian grid columns the vertical structure of convective echo is examined through the use of two variables echo top height and the height of the 30 dbz reflectivity contour the first of these variables has traditionally been used to describe the vertical structure characteristics of convection and the second has recently been linked to internal microphysical properties and lightning histograms of the relative frequency of convective only echo top heights and 30 dbz contour heights were constructed for the three cruises of the vickers with each cruise experiencing different phases of the intraseasonal oscillation iso cruise 1 which was dominated by the convectively inactive phase of the iso was characterized by the highest frequency of shallow convection based on echo top heights whereas cruise 2 which was dominated by a particularly well defined passage of the convective phase of the iso exhibited the tallest echo top heights cruise 3 convection was influenced by moderate westerly surface winds characteristic of postwesterly wind burst conditions and convection was of intermediate height when viewed as a function of internal vertical structure i e 30 db z contour height the frequency distributions vary less from cruise to cruise with cruises 1 and 2 having nearly identical distributions of convective 30 dbz contour heights furthermore when the contribution to convective rainfall is examined as a function of 30 dbz contour height it is seen that relatively more rain fell from vertically intense convection i e convection with tall 30 dbz contours during cruises 1 and 3 than during cruise 2 instantaneous correlations between rainfall rate and radar echo height were highly scattered about a mean value of about 0 55 whereas rainfall rate and 30 dbz contour height correlations peaked at about 0 8 and exhibited much less scatter

a new ferrosilite and fe pigeonite occurrence in the reading prong new jersey usa abstract ferrosilite and fe rich pigeonite have been discovered as discrete grains and as lamellae intergrown with hedenbergite in a fayalite bearing hedenbergite quartz syenite in the reading prong province near berkshire valley new jersey pyroxene thermometry suggests that these extremely fe rich pyroxenes probably began crystallizing above 910 c blocking temperatures indicated by the compositions of lamellae in exsolved pyroxenes were around 550 to 600 c load pressure during crystallization may have been around 6 to 6 5 kbar oxygen fugacity varied from 10 12 to 10 17 bars and was no lower than one log unit below qfm these data suggest that the part of the northwestern reading prong that contains the quartz syenite may have been approximately 22 km deep 1000 m y ago

origin of ureilites inferred from a sims oxygen isotopic and trace element study of clasts in the dar al gani 319 polymict ureilite secondary ion mass spectrometer sims oxygen isotope analyses were performed on 24 clasts representing 9 clast types in the dar al gani dag 319 polymict ureilite with precisions better than 1 olivine rich clasts with typical ureilitic textures and mineral compositions have oxygen isotopic compositions that are identical to those of the monomict ureilites and plot along the ccam carbonaceous chondrite anhydrous mineral line other igneous clasts including plagioclase bearing clasts also plot along the ccam line indicating that they were derived from the ureilite parent body upb thus we suggest that some of the plagioclase bearing clasts in the polymict ureilites represent the missing basaltic component produced by partial melting on the upb trace element concentrations mg k sc ti v cr mn fe co ni cu rb sr and ba in ureilitic plagioclase and glass from 13 clasts were obtained by using the sims high mass resolution method the trace element contents of the plagioclase generally show monotonic variations with anorthite content mol that are consistent with partial melting and fractional crystallization incompatible trace element concentrations k ti and ba are low and variable for plagioclase with an 40 indicating that the parental magmas for some clasts were derived from a depleted source we performed partial melt modeling for ci and cm precursor compositions and compared the results to the observed trace element k ba and sr abundances in the plagioclase our results indicate that 1 the upb evolved from a alkali rich carbonaceous chondritic precursor 2 parent melts of porphyritic clasts could have formed by 5 20 equilibrium partial melting and subsequent fractional crystallization and 3 parent melts of the incompatible trace element depleted clasts could be derived from fractional melting where low degree 10 partial melts were repeatedly extracted from their solid sources thus both the oxygen isotopic and trace element compositions of the plagioclase bearing clasts in dag 319 suggest that the upb underwent localized low degree partial melting events the partial melts could have been repeatedly extracted from the precursor resulting in the formation of the olivine pigeonite monomict ureilites as the final residue

the system fayalite albite anorthite and the syenite problem the presence in a magma of fayalite the iron end member of the olivine binary series affects the feldspars at pressure by lowering the temperatures at which they crystallize from the magma starting with estimates from published literature it becomes obvious that at pressure fayalite becomes important because the pressure effects on the melting temperatures are very different large for albite and small for anorthite in this experimental study a powder of fayalite composition was combined with six finely ground natural feldspars from ab to an 97 to make six bulk compositions using graphite crucibles in piston cylinder apparatus at a pressure of 5 kbar a cotectic in the ternary system was found to range from 1141 c at an fa to 1124 c at ab fa with fayalite contents from 68 to 17 wt respectively the results can be used to show that ternary feldspars saturated with fayalite and fe monoclinic pyroxene will crystallize at a 5 kbar multiphase eutectic 1010 c 56 c below a calculated azeotropic point on the ab or join the results are used to compare the end points of two very different layered intrusions skaergaard and kiglapait and to illuminate the nature and origins of syenite and trachyte which are leucocratic rocks unsaturated with mafic minerals because fayalite saturated melts are responsive to pressure unequally on the feldspar end members olivine of intermediate composition will have a damped but potentially significant effect on feldspar fractionation in the lower crust of the earth possibly affecting the origin of anorthosite and syenite

the origin of anorthosites and related rocks from the lofoten islands northern norway i field relations and estimation of intrinsic variables these calculations indicate that the calcic flakstadoy anorthosite introduction and geological fbc cpx ol opx mtss ilmss plag an57 47 setting crystallized under polybaric conditions at pressures between 4 and 9 kbar and at temperatures between 1140 and 1185 c the sodic the commonly observed association of proterozoic maseidsfjord complex cpx opx mtss ilmss plag sif type anorthosites see ashwal 1993 and references an48 44 crystallized at 1100 1135 c at a maximum pressure therein with jotunitic and mangeritic or charnockitic of 7 3 kbar this technique may provide a means to estimate rocks is still not satisfactorily explained some models crystallization pressure and asio2 in many types of intrusive and consider them to be comagmatic and relate these rocks extrusive rocks coeval mangerites and charnockites intruded subby processes of liquid immiscibility philpotts 1981 or a sequently at 4 kbar and temperatures between greater than 925 c specific fractionation trend owens et al 1993 others and 800 c respectively indicated by the succession of the mafic describe them as coeval but not comagmatic duchesne phase assemblages cpx opx cpx opx ol cpx et al 1989 duchesne 1990 a typical proterozoic pig ol cpx ol that reflect continuous fractionation to anorthosite mangerite charnockite association is exhigher fe mg ratios the evidence for polybaric crystallization of posed in the lofoten islands off the nw coast of norway the fbc quantitatively supports the common model that generation which is dominated by mangerites and charnockites in of proterozoic anorthosites involves initial crystallization at depth this paper the first of a series on these rocks we present crust mantle boundary and intrusion as a crystal rich mush the field observations and phase petrology and determine the conditions under which the intrusions were emplaced detailed estimation of intrinsic parameters p t fo2 asio2 fhcl

evidence for an enriched mantle source for jotunite orthopyroxene monzodiorite associated with the st urbain anorthosite quebec abstract mafic orthopyroxene monzodiorite jotunite lithologies are exposed in the st urbain plutonic suite as a marginal facies to quartz mangerite and massif anorthosite intrusive bodies and as dikes within a variety of host rocks high concentrations of ti fe p k ba nb la ce zn ga zr and y characterize these rocks and are distinctive of many mafic lithologies associated with anorthosite massifs worldwide characteristically low concentrations of ni and cr in conjuction with low mg numbers have been used by previous investigators as evidence for either partial melting of mafic granulitic lower crust or extensive fractional crystallization of a mantle derived magma in an attempt to distinguish between these competing models we note that jotunite display many features that bear a strong resemblance to continental tholeiitic flood basalts including chemical signatures on normalized multi element spider diagrams ratios of incompatible trace elements and patterns on rare earth and spider diagrams collectively indicate that the jotunite rocks were derived from an enriched rather than depleted mantle source enrichment may have occured by subduction derived fluids or by mixture with a plume component prior to partial melting so that isotopic and trace element compositions are decoupled small amounts of partial melting of mafic granulite has been advanced as an alternative model we show however that the experimental data on which this model is built are not applicable our preferred model begins with partial melting of a trace element enriched mantle source that fractionates olivine at high to moderate pressures increasing concentrations of p and ti eventually caused a contraction of the olivine stability field in favor of orthopyroxene fractional crystallization may yield the series of rocks from anorthosite leuconorite oxide apatite gabbronorite to jotunite mafic magmas emplaced into continental crust are typically attributed to incipient rifting or mantle upwelling which are features common to many models for the genesis of anorthosite and related rocks

the geochemical evolution of anorthosite residual magmas in the laramie anorthosite complex wyoming olivine and pyroxene bearing fit enriched dioritic rocks in the 1434 ma laramie anorthosite complex are interpreted to represent variably fractionated and contaminated magmas residual after the crystallization of anorthosite geochemical characteristics of this suite include the following high contents ofti02 fe20s and p2o5 high incompatible trace element contents rare earth element patterns with a large range of eu anomalies and isotopic compositions that reflect the geographic location of individual samples with i r increasing and ejy decreasing from south to north after extraction from anorthosite fractionation offerrodioritic residual magmas resulted in secondary residual monzpdioritic melts and complementary oxide rich ferrodiorite cumulates geographic trends in isotopic composition reflect an increasing archean crustal component from south to north dioritic dikes and cumulates with isotopic compositions similar to associated anorthosites were derived locally large isotopic discrepancies between some diorites and their hosting anorthosites reflect preferential contamination of residual magma during ascent and emplacement of mantle derived plagioclase rich diapirs followed by subsequent extraction and isolation of fe enriched interstitial melt strong isotopic contrasts between anorthosite and associated fe enriched rocks in anorthosite complexes do not preclude a direct relationship between them and reflect the diversity and complexity of processes during their petrogenesis

petrogenesis of the red mountain pluton laramie anorthosite complex wyoming implications for the origin of a type granite abstract the red mountain pluton the youngest intrusion in the laramie anorthosite complex is one of five monzonitic to granitic a type plutons intruded at 1 43 ga in the laramie mountains of southeastern wyoming the red mountain pluton ranges in composition from fayalite monzonite clinopyroxene quartz monzonite and biotite hornblende quartz syenite to granite the red mountain pluton is petrologically and geochemically distinct from the northern and southern sherman batholith the sybille intrusion and the maloin ranch pluton it has higher feo t feo t mgo and higher k 2 o is more strongly enriched in rees and at any given silica content it has lower abundances of tio 2 feo mgo cao and p 2 o 5 the fayalite monzonite in the red mountain pluton crystallized at temperatures around 1000 c and oxygen fugacities 1 2 log units below that of the fmq buffer similar to the crystallization conditions of the sybille and maloin ranch plutons the biotite hornblende quartz syenite crystallized at oxygen fugacities near or slightly below those of fmq most of the units in the pluton carry mantle like nd sr and pb isotopic compositions similar to the least contaminated anorthositic and ferrodioritic rocks of the lac indicating that the pluton evolved mainly via differentiation substantial crustal assimilation occurred only in the late dikes and the granite we suggest that all of the 1 43 ga a type intrusions in southeastern wyoming are ultimately derived from tholeiites extreme differentiation or partial melting produced monzonitic magmas that have assimilated varying amounts of siliceous crust the differences between the red mountain pluton and other a type granitoids in southeastern wyoming are related to the extent of differentiation prior to crustal assimilation the compositional ranges of a type granitoids in southeastern wyoming suggest that the geochemical spectrum of a type granites worldwide similarly are produced from evolved mantle derived melt that has assimilated varying proportions of crustal material

paleomagnetism of the middle proterozoic laramie anorthosite complex and sherman granite southern laramie range wyoming and colorado we present the results of a combined paleomagnetic and 40ar 39ar geochronologic investigation of the middle proterozoic laramie anorthosite complex and sherman granite in the southern laramie range of wyoming and colorado anorthosites and monzosyenites of the laramie anorthosite complex yield a well defined characteristic magnetization of northeast declination d and moderate negative inclination i although antipodal normal and reverse polarity magnetizations are present at three sites a grand mean direction from 29 of 35 sites in the complex is d 44 6 i 48 7 k 77 4 95 3 1 alternating field af and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low ti titanomagnetite of single or pseudo single domain character that occurs as elongate to rod shaped inclusions in plagioclase and potassium feldspar the sherman granite contains a dual polarity magnetization that is less well defined than that of the laramie anorthosite complex but similar in declination and inclination d 53 1 i 48 1 k 46 5 95 7 6 n 8 14 sites rock magnetic data indicate the primary carrier of remanence in sherman granite is magnetite the 40ar 39ar geochronologic data from sherman granite hornblende biotite and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 ma microcline data indicate that the laramie anorthosite complex and sherman granite have probably not been thermally remagnetized paleomagnetic poles from the laramie anorthosite complex and sherman granite are indistinguishable at the 95 confidence level and individual virtual geomagnetic poles vgps from both units are combined to provide a mean pole at 215 0 e 6 7 s k 46 9 a95 3 5 n 37 vgps the location of this pole is similar to paleomagnetic poles derived from 1480 ma to 1450 ma intrusions elsewhere in north america but it plots significantly north of those from middle proterozoic sedimentary strata of the belt supergroup and sibley group in addition to the characteristic magnetization samples from some sites in sherman granite contain a remanence of southeast declination and shallow negative inclination d 154 9 i 16 0 k 90 6 95 9 7 n 4 sites this secondary magnetization was probably acquired during late paleozoic time

northwest africa 1500 plagioclase bearing monomict ureilite or ungrouped achondrite northwest africa nwa 1500 is an ultramafic meteorite dominated by coarse 100 500 m olivine 95 96 augite 2 3 and chromite 0 6 1 6 in an equilibrated texture plagioclase 0 7 1 8 occurs as poikilitic grains up to 3 mm in vein like areas that have concentrations of augite and minor orthopyroxene other phases are cl apatite metal sulfide and graphite olivine ranges from fo 65 73 with a strong peak at fo 68 69 most grains are reverse zoned and also have 10 30 m reduction rims in terms of its dominant mineralogy and texture nwa 1500 resembles the majority of monomict ureilites however it is more ferroan than known ureilites fo 75 and other mineral compositional parameters are out of the ureilite range as well furthermore neither apatite nor plagioclase have ever been observed and chromite is rare in monomict ureilites nevertheless this meteorite may be petrologically related to the rare augite bearing ureilites and represent a previously unsampled part of the ureilite parent body upb the mn mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation however its petrogenesis differs from that of known augite bearing ureilites in that 1 it formed under more oxidized conditions 2 plagioclase appeared before orthopyroxene in its crystallization sequence and 3 it equilibrated to significantly lower temperatures 800 1000 c from two pyroxene and olivine chromite thermometry formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the upb than previously sampled however its significantly different thermal history compared to ureilites may more plausibly be explained if it formed on a different parent body this conclusion is consistent with its oxygen isotopic composition which suggests that it is an ungrouped achondrite nevertheless the parent body of nwa 1500 may have been compositionally and petrologically similar to the upb and may have had a similar differentiation history

fe mg fe mn relations of meteorites and primary heterogeneity of primitive achondrite parent bodies abstract we examine fe mg mn relations of meteorites as represented on plots of molar fe mg ratio vs molar fe mn ratio and use them to consider whether it is meaningful to classify achondrites as primitive or evolved first we demonstrate that fe mg fe mn plots can be used to identify some of the major processes that have affected meteorite parent bodies during both the nebular condensation and accretion and the planetary igneous stages of their evolution examination of compositions of chondrites on this diagram reveals distinct patterns that result from three nebular processes 1 volatility controlled mn mg fractionation 2 metal silicate fractionation under highly reducing conditions and 3 subsequent oxidation leading to variations in feo mgo ratio ordinary chondrites ocs show a characteristic pattern that records all three of these processes and suggests that the latter two are coupled examination of the compositions of achondrites on this diagram shows that all of those that have been classified as primitive lodranites acapulcoites winonaites iab silicate inclusions brachinites and ureilites are easily distinguished from those that have been classified as evolved sncs heds and lunar basalts the mn mg ratios of the former are chondritic while those of the latter are superchondritic hence they plot in two graphically distinct regions of the fe mg fe mn diagram a simple melting calculation reveals that this distinction results principally because the former are residues and the latter are melts or cumulate products of melts rather than because the latter are more evolved additionally however some of the groups classified as primitive achondrites show fe mg fe mn patterns similar to that of ocs constant but distinct mno mgo ratios over large ranges of feo mgo ratio we argue that these trends reflect primary heterogeneity resulting from the same nebular processes as those that affected ocs and therefore preserve evidence that more of the materials that accreted to form planetesimals experienced these processes than is apparent from known chondrites this evidence is preserved principally because these meteorites are residues in contrast groups commonly classified as evolved achondrites show fe mg fe mn patterns that reflect only igneous processing and any evidence for primary heterogeneity of their parent bodies has been obliterated again this is principally because these meteorites represent melts rather than beause they are more evolved usage of the terms primitive and evolved to describe achondrites should be clearly defined

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

petrogenesis of the labrieville alkalic anorthosite massif grenville province quebec the labrieville massif 1010 ma is an eroded dome consisting mineral plagioclase feldspar the circumstances whereby such nearly monomineralic plutons can be produced of three zones 1 a foliated inner core of pink anorthosite an30or13 2100 ppm sr 1300 ppm ba with minor have puzzled geologists for more than a century and massif anorthosites remain petrologic enigmas in adleuconorite 2 a foliated outer core of pink anorthosite an35or10 1900 ppm sr 850 ppm ba with more abundant leuconorite dition to this fundamental aspect of the anorthosite problem see bowen 1917 scoates 2000 the petro 3 a border of foliated to massive green leucogabbro antiperthitic plagioclase orthopyroxene hemoilmenite and small amounts of genesis of massif anorthosites was apparently restricted in time and space specifically virtually all of the world s biotite are present in all rocks the border zone contains additional clinopyroxene magnetite and apatite labrieville is more alkalic supply of massif anorthosite was formed during the proterozoic and much of it is found in the grenville and srand ba rich than almost all other massif anorthosites the widespread presence of biotite and the high hematite content of province of quebec and labrador kranck 1961 anderson 1969 ashwal 1993 hemoilmenite suggest crystallization from a relatively oxidizing and water bearing magma collectively these features imply that despite their similar plagioclase rich nature it is now apparent that massif anorthosites worldwide are minlabrieville originated from a magma that differed considerably from typical tholeiitic liquids majorand trace element variations suggest eralogically and compositionally diverse see xue that all rock types represent mixtures of cumulus minerals with morse 1993 implying involvement of a wide range of negligible trapped melt the compositionally distinct zones imply parental magma compositions or petrogenetic processes differentiation during crystallization although plagioclase comin their formation however comprehensive studies on positions become more evolved inward other indicators e g xmg individual anorthosite complexes that can elucidate their in pyroxene suggest differentiation outward from core to border we petrogenesis combining field and petrographic obfavor the latter interpretation and suggest that the increase in servations with mineral chemical data and with complete plagioclase an content reflects a decline in pressure during crysmajorand trace element data on a wide range of rock tallization types are few in number admittedly the immense size of certain massifs e g lac st jean complex woussen et al 1988 and the remote or inhospitable locations of

petrogenesis of a mid proterozoic anorthosite mangerite charnockite granite amcg complex isotopic and chemical evidence from the nain plutonic suite the nain plutonic suite nps about 1 30 ga underlies some 19 000 km intruding the boundary between middle to late archean rocks in the east nain province and late archean and early proterozoic rocks in the west churchill province nd isotopic compositions in most igneous units of the nps reflect their geographic position relative to the inferred boundary between the nain and churchill provinces rare exceptions are local intrusions of olivine bearing basic rocks with highest epsilon nd 1 3 ga near 3 0 present in both eastern and western sectors of the nps anorthositic rocks have variable and significant contributions of crustal nd but notably lower i sr than granitoid rocks ferrodiorites are similar isotopically to the anorthositic group and have chemical and mineral compositions more akin to the anorthositic group than the granitoids aluminous orthopyroxene megacrysts in nps anorthositic rocks indicate crystallization pressures of 6 2 to 11 0 kbar consistent with lower cru

radar enabled recovery of the sutter s mill meteorite a carbonaceous chondrite regolith breccia doppler weather radar imaging enabled the rapid recovery of the sutter s mill meteorite after a rare 4 kiloton of tnt equivalent asteroid impact over the foothills of the sierra nevada in northern california the recovered meteorites survived a record high speed entry of 28 6 kilometers per second from an orbit close to that of jupiter family comets tisserand s parameter 2 8 0 3 sutter s mill is a regolith breccia composed of cm mighei type carbonaceous chondrite and highly reduced xenolithic materials it exhibits considerable diversity of mineralogy petrography and isotope and organic chemistry resulting from a complex formation history of the parent body surface that diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to c class asteroids are interpreted

strategies for high temperature oxygen isotope thermometry a worked example from the laramie anorthosite complex wyoming usa abstract an understanding of the factors that affect the magnitude and direction of isotopic exchange is fundamental to the application of oxygen isotope geothermometers this is particularly true in high temperature rocks where retrograde exchange between minerals commonly results in the calculation of spurious isotopic temperatures herein we present an approach to isotope thermometry that combines numerical modelling techniques with a directed sampling strategy to retrieve peak or near peak temperature information from rocks that have behaved as closed systems on the hand sample scale during cooling the isotopic composition of minerals in such samples changes in predictable fashion on cooling these so called isotope exchange trajectories iet can be calculated for any mineral given the whole rock isotopic composition the modal abundance of each mineral in the rock and the fractionation factors between the minerals model temperatures can be obtained for both single and multiple hand samples through the calculation of iets temperatures for single samples are determined by locating the intersection between a mineral s calculated iet and its measured isotopic composition on plots of 18 o vs 1 t 2 the resulting temperature represents either the closure temperature of the mineral or the maximum temperature attained by the rock whichever is lower the multiple sample method relies on the premise that at high temperatures isotopic equilibrium is effectively achieved over a large area typically on the scale of meters or larger this method permits the retrieval of temperatures that are significantly higher than the closure temperature of any individual mineral multiple sample temperatures are calculated by determining the intersection of the iets of a given mineral in two or more samples the intersection of these iets represents the temperature at which the isotopic composition of the mineral was the same in both samples i e the last temperature of large scale isotopic equilibrium our methods allow for a rigorous determination of uncertainties in the calculated temperatures these numerical techniques can be used on any samples precise temperature estimates however require samples with specific mineralogic and modal characteristics the best samples for use with the single sample method are those that contain small amounts of a slow diffusing mineral and large fractionation factors between this mineral and the other minerals in the rock the multiple sample method requires two or more samples with widely different modal proportions of a mineral that strongly fractionates oxygen isotopes such samples are present in many terrains but are commonly overlooked because their mineral assemblages are not well suited for conventional thermobarometry these methods yield excellent results when applied to rocks from the sybille fe ti oxide quarry a late magmatic segregate of the laramie anorthosite numerical modelling of single sample and multiple samples from this quarry yield temperatures of 1000 and 1080 c respectively the single sample temperatures record the closure temperature of ilmenite whereas the multiple sample temperatures correspond closely to the experimentally determined solidus for these rocks these model temperatures are higher than any other conventional oxygen isotope temperatures yet reported from slowly cooled rocks

crustal differentiation in the early solar system clues from the unique achondrite northwest africa 7325 nwa 7325 the unique achondrite nwa 7325 is an unusual olivine gabbro composed chiefly of calcic plagioclase an85 93 diopsidic pyroxene en50 1 54 0 wo44 8 49 3 fs0 6 1 3 and forsteritic olivine fo97 it is al and mg rich and fe and na poor and displays very low concentrations of incompatible trace elements much below 0 3 x ci abundances for many of them it is also characterized by huge eu and sr anomalies eu eu 65 srn cen 240 although the o isotopic composition of nwa 7325 and some ureilites those with olivine cores in the range fo75 to fo88 are similar a genetic relationship between them is unlikely due to the fe poor composition of nwa 7325 it is almost certainly derived from a distinct planetesimal not previously sampled by other achondrites the low na al ga al zn al ratios as well as the low k rb and cs shown by nwa 7325 suggest a volatile depleted parent body this unique gabbro is demonstrably a cumulate but the composition of its parental melt cannot be precisely assessed however the liquid from which nwa 7325 crystallized would have been very poor in incompatible trace elements yb in the range of 0 25 to 1 5 x ci abundance with a very large positive eu anomaly such a melt cannot be the product of the early magmatic activity on a small parent body instead we propose that the parental melt to nwa 7325 formed as a consequence of the total melting of an ancient gabbroic lithology possibly upon impact in agreement with the systematics of 26al 26mg based on recent dating the crustal material that was parental to nwa 7325 must have been older than 4562 8 ma and formed possibly 4566 ma ago if this scenario is correct nwa 7325 provides evidence of one of the earliest crusts on a differentiated body so far studied

petrology and isotope geochemistry of the mesoproterozoic anorthosite and related rocks of the kunene intrusive complex nw namibia abstract the intrusion of the large c 18 000 km2 anorthosite body of the kunene intrusive complex angola namibia which is associated with minor marginal granite bodies and syenite syenodiorite dykes marked the beginning of mesoproterozoic igneous activity in nw namibia anorthosites display an igneous mineral assemblage of dominating plagioclase together with minor olivine orthopyroxene or clinopyroxene fe ti oxides whereas biotite and pargasite are late magmatic phases apatite is a common accessory phase in regionally associated felsic rocks k feldspar plagioclase clinopyroxene and or hastingsite crystallized first followed by interstitial quartz hastingsite fe ti oxides and titanite the general geochemical characteristics for the anorthositic rocks of the kunene intrusive complex i e high al2o3 cao sr and eu intermediate to high mg numbers of 0 37 0 74 positive nd t values of 3 0 to 1 0 and initial sr ratios of 0 7028 0 7041 are in excellent agreement with their derivation from fractionated basaltic liquids the 18o values obtained for plagioclase 5 88 0 19 18o support their development from mantle derived magmas relic crustal xenoliths of calc silicate rocks and sr and nd isotope data however suggest that the anorthosite magma underwent minor crustal contamination which was most prominent during the early intrusion stage a u pb zircon age of 1376 2 ma was obtained for a felsic rock associated with the 1385 25 ma anorthosites which suggests that the two suites were emplaced during the same igneous event the geochemical and isotope signatures of the granites syenites and syenodiorites provide convincing evidence against consanguinity of the anorthositic and the felsic rock suites i e 1 a compositional gap exists between the major and trace element contents of the two suites 2 trace element and sr and nd isotopic data of the felsic rocks point to a crustal source nd 2 0 to 0 4 initial sr ratio 0 7024 0 7063 3 the ree patterns of the felsic rocks do not exhibit ubiquitous negative eu anomalies which would be expected from fractionation products of melts that previously formed extensive plagioclase cumulates and 4 18o values of magmatic feldspar from the felsic rock suite fall in a restricted range 7 20 7 92 that is about 1 6 higher than the average igneous plagioclase 18o of the anorthosites and suggests a formation of the felsic melts by anatexis of the lower crust

a strontium and neodymium isotopic investigation of the laramie anorthosites wyoming usa implications for magma chamber processes and the evolution of magma conduits in proterozoic anorthosites abstract a strontium and neodymium isotopic investigation of plagioclase rich cumulate rocks in the 1 43 ga laramie anorthosite complex lac provides new insights into the evolution of magma chambers and underlying magma plumbing systems during the crystallization of proterozoic anorthosites the 725 km2 lac intruded the boundary between archean rocks of the wyoming province in the north and proterozoic island arc terranes in the south the cheyenne belt at pressures of 3 4 kilobars initial strontium and end isotopic ratios from the three large composite anorthositic intrusions poe mountain chugwater and snow creek and late troctolitic intrusions range from 0 7034 0 7055 and 0 9 to 4 9 respectively the isotopic ratios in the cumulates vary as a result of 1 the isotopic composition of the parental mantle derived high al gabbroic magmas 2 the amount of contamination by archean rocks during ascent through the crust and 3 mixing of replenishing magmas and resident magmas with different isotopic compositions in the magma chamber at the final level of emplacement strontium isotopic and normative an variations across the 5 7 km thick stratigraphic section of layered plagioclase rich cumulates in the 200 km2 poe mountain anorthosite indicate that multiple inputs of magma are needed to construct even relatively small intrusions in proterozoic anorthosites the earliest and stratigraphically lowest intrusion was emplaced as a plagioclase rich magma that crystallized large volumes of compositionally homogeneous anorthosite at least three subsequent chamber wide episodes of magma replenishment followed by mixing are indicated by abrupt shifts in isr and or normative an in the stratigraphically higher levels of the poe mountain anorthosite strontium and neodymium isotopic disequilibrium between a high al clinopyroxene megacryst and the layered cumulates is consistent with a high pressure origin for al rich pyroxene megacrysts in proterozoic anorthosites the megacryst crystallized in a magma chamber at pressures of 10 12 kilobars from relatively uncontaminated basaltic parent magma high al gabbro and was transported through the crust in progressively contaminated anorthositic magma the range of observed isotopic compositions in the anorthositic rocks is nearly identical to that of high al gabbroic dikes in the lac supporting the proposition that high al gabbros are parental to anorthosite mitchell et al 1995 the isotopic data require that the anorthositic parental magmas were contaminated during ascent through the crust by archean orthogneisses and or metapelitic rocks decreasing isr and increasing end with a relative decrease in age of the intrusions snow creek poe mountain chugwater troctolites indicates that the magma conduit became increasingly insulated from crustal contamination over time this study indicates that the strontium and neodymium isotopic systems can be used to distinguish between processes that occurred at lower crustal upper mantle pressures during the ascent of magma diapirs and within individual magma chambers in proterozoic anorthosites and underscores the need for stratigraphic control when addressing the origin of plagioclase rich cumulates

oxygen isotope systematics of chondrule phenocrysts from the co3 0 chondrite yamato 81020 evidence for two distinct oxygen isotope reservoirs high precision oxygen three isotope measurements of olivine and pyroxene were performed on 33 chondrules in the yamato 81020 co3 0 chondrite by secondary ion mass spectrometry in chondrules where oxygen isotopes were measured in both olivine and pyroxene the majority of grains have similar values indicating co magmatic crystallization however many chondrules contain relict grains with unique oxygen isotope ratios a striking feature of yamato 81020 chondrules is a bimodal distribution of oxygen isotope ratios as those with mg 97 phenocrysts range in 17o from 4 8 to 6 5 5 5 group and those with mg 96 36 phenocrysts have 17o values of 2 1 to 3 0 2 5 group a single mg 99 6 barred olivine chondrule has a 17o of 3 3 we discuss that 17o 5 5 chondrules are derivative of a reservoir with limited dust enrichment 100 solar system which yielded a relatively reduced chondrule forming environment in contrast the 17o 2 5 chondrules may have been influenced by 16o poor h2o ice that sublimed and then homogenized with precursor material the addition of h2o when combined with high dust enrichment 1000 solar system and greater bulk fe content could have induced an oxidized environment at high temperatures forming mg 96 36 chondrules among the 33 chondrules studied the al mg relative ages of 20 had been obtained previously comparing the oxygen isotope ratios and the 26al ages of these chondrules it is likely that the 5 5 and 2 5 oxygen isotope reservoirs existed contemporaneously this implies that the snow line was spatially fixed during chondrule formation and separated the co chondrite accretion region into two distinct volumes of precursors

crystal fractionation and the evolution of intra plate hy normative igneous suites insights from their feldspars feldspars and normative feldspar constituents of bulk magma show complexes and anorogenic granites is associated with hotspot and continental rift environments a major chartrends supportive of fractional crystallization in the three main types of hy normative intraplate suites that contain qz oversaturated acteristic of this magmatism is the production of suites of compositionally diverse rocks that are temporally and rocks ocean island and continental alkalic suites ocean island tholeiitic suites and continental tholeiitic suites these suites are spatially associated much is still unknown about the processes that have led to the diversity of magmas within characterized by the presence of a single feldspar in each suite member a shift of this feldspar from plagioclase to alkali feldspar such suites or to the global diversity of intra plate suite types and k enrichment of alkali feldspar with decreasing temperature in the trachytic members the modal feldspars provide evidence for a many ocean island and continental hotspot and continental rift related suites are characterized by rocks with reaction relationship between feldspars and indicate a build up of magmatic volatile content towards saturation with progressive high alkali silica ratios that fall into the alkalic field of irvine baragar 1971 fig 1 regardless of their state fractionation of a parental magma having low initial volatile content the feldspar and normative feldspar evolutionary paths are unique of silica saturation such alkalic suites range from those that are strongly silica undersaturated containing rocks for each of the three suite types but similar for different suites within the same type this characteristic extends to the felsic members such as nephelinites or leucitites in the more potassic making it easy to distinguish between rhyolitic or granitic rocks suites to mildly silica undersaturated suites the kenfrom the different suite types the feldspars in natural volcanic nedy series of miyashiro 1978 containing alkali basalts suites commonly show evidence for a polybaric history particularly silica undersaturated trachytes and phonolites and in the least evolved suite members late stage feldspars of the finally to hy normative suites the coombs trend of intermediate members and feldspars of the most evolved members miyashiro 1978 that include olivine basalts trachytes show paths indicative of significantly lower temperature and pressure and rhyolites hy normative alkalic suites that contain regimes silica oversaturated rocks on ocean islands are widespread and include those from ascension island e g harris 1983 the samoan islands macdonald 1968 terceira azores e g white et al 1979 and clarion island mexico

magmatic co 2 and saline melts from the sybille monzosyenite laramie anorthosite complex wyoming there are three populations of fluid inclusions in quartz from the sybille monzosyenite early co2 secondary co2 and rare secondary brines the oldest consist of low density co2 0 70 inclusions that appear to be co magmatic the densities of these inclusions are consistent with the inferred crystallization conditions of the sybille monzosyenite namely 3 kilobars and 950 1000 c the other types of inclusions are secondary they contain co2 0 50 and secondary brine inclusions that form trains radiating out from a decrepitated inclusion the sites of these decrepitated inclusions are now marked by irregularly shaped fluid inclusions and solid inclusions of salt and carbonate rather than fluid inclusions feldspar contain abundant solid inclusions these consist of magmatic minerals hedenbergite hornblende ilmenite apatite and graphite intimately associated with k na chlorides we interpret these relations as follows the sybille monzosyenite formed from a magma that contained immiscible droplets of a halide rich melt along with a co2 vapor phase the salt was trapped along with the other obvious magmatic minerals during growth of the feldspars co2 may have also been included in the feldspars but it probably leaked later during exsolution of the feldspars and was not preserved both the saline melt and the co2 vapor were trapped in the quartz the melt inclusions in the quartz later decrepitated perhaps due to progressive exsolution of fluids to produce the secondary h2o and co2 inclusions these observations indicate that the sybille monzosyenite which is a markedly anhydrous rock was actually vapor saturated rather than being h2o however the vapor was co2 rich and possibly related to an immiscible chloride rich melt

petrogenesis and provenance of ungrouped achondrite northwest africa 7325 from petrology trace elements oxygen chromium and titanium isotopes and mid ir spectroscopy northwest africa nwa 7325 is an ungrouped achondrite that has recently been recognized as a sample of ancient differentiated crust from either mercury or a previously unknown asteroid in this work we augment data from previous investigations on petrography and mineral compositions mid ir spectroscopy and oxygen isotope compositions of nwa 7325 and add constraints from cr and ti isotope compositions on the provenance of its parent body in addition we identify and discuss notable similarities between nwa 7325 and clasts of a rare xenolithic lithology found in polymict ureilites nwa 7325 has a medium grained protogranular to poikilitic texture and consists of 10 15 vol mg rich olivine fo 98 25 30 vol diopside wo 45 mg 98 55 60 vol ca rich plagioclase an 90 and trace cr rich sulfide and fe ni metal we interpret this meteorite to be a cumulate that crystallized at 1200 c and very low oxygen fugacity similar to the most reduced ureilites from a refractory incompatible element depleted melt modeling of trace elements in plagioclase suggests that this melt formed by fractional melting or multi stage igneous evolution a subsequent event likely impact resulted in plagioclase being substantially remelted reacting with a small amount of pyroxene and recrystallizing with a distinctive texture the bulk oxygen isotope composition of nwa 7325 plots in the range of ureilites on the ccam line and also on a mass dependent fractionation line extended from acapulcoites the e54cr and e50ti values of nwa 7325 exhibit deficits relative to terrestrial composition as do ordinary chondrites and most achondrites its e54cr value is distinct from that of any analyzed ureilite but is not resolved from that of acapulcoites as represented by acapulco in terms of all these properties nwa 7325 is unlike any known achondrite however a rare population of clasts found in polymict ureilites the magnesian anorthitic lithology are strikingly similar to nwa 7325 in mineralogy and mineral compositions oxygen isotope compositions and internal textures in plagioclase these clasts are probably xenolithic in polymict ureilites and could be pieces of nwa 7325 like meteorites using constraints from chromium titanium and oxygen isotopes we discuss two possible models for the provenance of the nwa 7325 parent body 1 accretion in the inner solar system from a reservoir similar to that of acapulcoites in 17o e54cr and e50ti or 2 early 1 ma after cai formation accretion in the outer solar system beyond the snow line before 54cr and 50ti anomalies were introduced to this region of the solar system the mid ir emission spectrum of nwa 7325 obtained in this work matches its modal mineralogy and so can be compared with spectra of new meteorites or asteroids planets to help identify similar materials and or the parent body of nwa 7325

ultra high temperature and subsolidus shear zones examples from the poe mountain anorthosite wyoming abstract ultra high temperature shear zones formed with interstitial melts present during the emplacement of the poe mountain anorthosite wyoming the shear zones are characterized by coarse grained plagioclase with flat extinction and rare deformation microstructures dissected grain microstructures suggest that plagioclase underwent recrystallization by fast grain boundary migration orthopyroxene crystallized from interstitial melts as strain free foliation parallel tablet grains plagioclase has a magmatic petrofabric characterized by poles to 010 normal to the shear foliation and 100 parallel to the foliation and movement direction poles to 001 have a broad distribution with a point maximum roughly parallel to the foliation and normal to the movement direction orthopyroxene petrofabrics result from oriented growth parallel to the shear foliation 100 defines a point maximum perpendicular to the shear foliation and 010 and 001 have great circle distributions approximately parallel to the shear foliation microstructures of the ultra high temperature shear zones contrast with those of granulite grade shear zones cutting across the intrusion the granulite grade shear zones initiated as fractures which evolved into shear zones by dynamic recrystallization deformed primary plagioclase and orthopyroxene recrystallized by grain boundary rotation into smaller polygonal grains

brachina a new type of meteorite not a chassignite the brachina meteorite previously classified as a chassignite is found to be a nearly primitive and unique meteorite it is a fine grained unshocked equigranular igneous rock containing small melt inclusions it contains 80 olivine fo70 5 5 clinopyroxene wo38en49fs13 10 plagioclase an22 0 5 chromite 0 5 chlorapatite 3 fe sulfide 0 3 pentlandite and a trace of ni rich metal orthopyroxene en70 and anorthoclase rich glass are found only in melt inclusions siderophiles are high in the sulfides and the oxidation state is similar to that of ll chondrites major element and mineralogic data of brachina are similar to chassigny but trace element data are dramatically different the major differences are higher feo mno ga al and k la ratios w depletion high siderophile content near horizontal ree pattern at about 1 chondrite level high refractory nonrefractory metal ratios e g ir au and chalcophile element depletion compositionally brachina is close to c2 chemistry but has undergone some segregation of fe ni s melt these data in addition to its near 4 5 b y age and oxygen isotopic signature within the eucritic field suggest that brachina is not related to chassigny or snc meteorites it may be related to primitive meteorites such as silicate inclusions in iab irons and winonaites but is more oxidized reduction would make it very similar to winonaites the assemblage may have experienced the beginning of melting in its parent body since it has characteristics that differ from any other meteorite we suggest it be called a brachinite

neodymium strontium and lead isotopes in the maloin ranch pluton wyoming implications for the origin of evolved rocks at anorthosite margins abstract neodymium strontium and lead isotopic data are used in this study to investigate the origin of chemically evolved rocks in the maloin ranch pluton a composite body that borders and intrudes the laramie anorthosite the range of rock types present is similar to that found at the margins of proterozoic anorthosites throughout the world in the maloin ranch pluton these include ferrodiorite at the base of the intrusion overlain progressively by fine grained monzonite monzosyenite and porphyritic granite biotite gabbro and fine grained granitic dikes are present locally at various levels of this sequence the origin of the evolved rocks and their possible relation to associated anorthositic bodies has been much debated in the maloin ranch pluton each rock type has distinct isotopic characteristics which together with trace element data previously reported suggest different source characteristics for each member strontium and neodymium isotopic data for maloin ranch ferrodiorite and laramie anorthositic rocks show considerable overlap consistent with a comagmatic relation biotite gabbro is chemically and isotopically the most primitive rock type in the maloin ranch pluton the data suggest that biotite gabbro has a mantle source but has undergone extensive fractionation in the crust our results suggest that the remainder and bulk of the intrusion formed by partial melting of the lower crust due to the emplacement of the laramie anorthosite maloin ranch fine grained monzonite is chemically more evolved than the underlying ferrodiorite i e lower mg higher silica and alkalis yet has more primitive isotopic characteristics lower isr higher epsilonnd formation of fine grained monzonite by in situ fractionation of ferrodiorite is precluded by both the isotopic data and by mass balance the isotopic data do not rule out formation of fine grained monzonite by fractionation or contamination of ferrodiorite at depth but this model requires that the ferrodiorite is isotopically heterogeneous and that very large volumes of ferrodiorite were present alternatively trace element and isotopic characteristics of the fine grained monzonite are explained by partial melting of mantle derived lower crust added to the margin of the archean wyoming craton at about 1 8 ga neodymium strontium and lead isotope data for maloin ranch monzosyenite and porphyritic granite also suggest a lower crustal source these data indicate a 1 8 ga source component for the maloin ranch monzosyenites but do not require input of archean crust from the wyoming province to the north

on the controls of oxygen fugacity in the generation and crystallization of peralkaline melts the formation of peralkaline melts is generally recognized to be related to enrichment of the mantle in sodium or potassium during a metasomatic event that predated magma generation the wide range in mineral assemblages mineral compositions and fluid compositions in peralkaline melts are to a large extent a function of the sodium potassium content of these mantle metasomatizing fluids which also governs redox conditions we propose that sodic fluids will reduce the mantle assemblage by depleting ferric iron from garnet to form the aegirine component in pyroxene in contrast potassic fluids would oxidize the mantle assemblage by extracting al2o3 from the garnet to make phlogopite we suggest that during differentiation of peralkaline melts once fe ti oxides have been depleted from the assemblage simple crystallization reactions of common solid phases such as aegirine or arfvedsonite control the oxygen fugacity by equilibria such as

some phase equilibrium constraints on the origin of proterozoic massif anorthosites and related rocks experimental data in the range of 1 bar to 13 kbar enable us to introduction map the liquidus equilibria relevant to proterozoic massif anorthere is hardly a consensus on the petrogenesis of thosites and related mafic rocks massif anorthosites are widely proterozoic massif anorthosites but many investigators believed to have formed by accumulation of plagioclase into highbelieve in a petrogenetic scheme that involves at least al basaltic liquids mantle derived basaltic liquids fractionating two major stages 1 extensive crystallization asat pressures sufficiently high 10 13 kbar to crystallize the highly similation of a mantle derived magma ponded at or aluminous orthopyroxene megacrysts typically observed in anorthosite near the base of the crust that produces suspensions of massifs reach plagioclase saturation at low normative silica contents plagioclase in fe rich high al gabbroic liquids followed peritectic like equilibria e g liq opx plag cpx sp by 2 intrusion of these suspensions into the mid upper and a thermal divide on the plagioclase pyroxene liquidus surface crust where they form complex magma chambers e g ensure that mantle derived liquids become nepheline normative with duchesne 1984 emslie 1985 ashwal 1993 wiebe further crystallization and crustal assimilation at depth such 1994 a key component of the scheme is the presence liquids cannot produce the full range of troctolitic noritic to trocin most massifs of high al orthopyroxene megacrysts tolitic gabbroic mineral assemblages observed in anorthosite massifs haom emslie 1975 that appear to preserve a record without extensive low pressure granite assimilation conversely the of the high pressure stage maquil duchesne 1984 array of plausible anorthosite parental liquids not only lies along longhi et al 1993 the structures of massifs vary from the trace of the plagioclase two pyroxene cotectic from 10 to 13 those that are composites of multiple diapiric intrusions kbar but also straddles the thermal divide on the plagioclase harp lake emslie 1980 rogaland duchesne 1984 pyroxene liquidus surface this condition requires mafic source to intrusive bodies with well developed layering of regions such as lower continental crust or foundered mafic plutons leucocratic troctolites and leuconorites or leucogabbros for liquids parental to massif anorthosites and associated mafic overlain by masses of anorthosite michikamau emslie intrusions 1970 laramie frost et al 1993 so there may be

stable isotopes and the noncarbonaceous derivation of ureilites in common with nearly all differentiated planetary materials abstract the abundant diverse ureilite meteorites are peridotitic asteroidal mantle restites that have remarkably high bulk carbon contents average 3 wt and have long been linked to the so called carbonaceous chondrites although this term is potentially misleading because the high petrologic type carbonaceous chondrites are if anything c poor compared to ordinary chondrites ureilite oxygen isotopic compositions i e diversely negative ccam like 17o viewed in isolation have long been viewed as confirming the carbonaceous chondritic derivation hypothesis however a very different picture emerges through analysis of a compilation of recently published high precision isotopic data for chromium titanium and nickel for ureilites and various other planetary materials ureilites have lower e62ni and far lower e50ti and e54cr than any known variety of carbonaceous chondrite on a plot of e50ti vs e54cr and similarly 17o vs e54cr ureilite compositions cluster far from and in a direction approximately orthogonal to a trend internal to the carbonaceous chondrites and the carbonaceous chondrites are separated by a wide margin from all other planetary materials i conclude that notwithstanding the impressive resemblance to carbonaceous chondrites in terms of diversely negative 17o the ureilite precursors accreted from preponderantly noncarbonaceous sensu stricto materials despite total depletion of basaltic matter the ureilites retain moderate pyroxene olivine ratios which is an expected outcome from simple partial melting of moderate sio2 feo mgo noncarbonaceous chondritic material but would imply an additional process of major reduction of feo if the precursor material were carbonaceous chondritic the striking bimodality of planetary materials on the e50ti vs e54cr and 17o vs e54cr diagrams may be an extreme manifestation of the effects of episodic accretion of early solids in the protoplanetary nebula however an alternative admittedly speculative explanation is that the bimodality corresponds to a division between materials that originally accreted in the outer solar system carbonaceous and materials that accreted in the inner solar system noncarbonaceous including the ureilites

an exsolution silica pump model for the origin of myrmekite myrmekite as defined here is the microscopic intergrowth between vermicular quartz and modestly anorthitic plagioclase calcic albite oligoclase intimately associated with potassium feldspar in plutonic rocks of granitic composition hypotheses previously invoked in explanation of myrmekite include 1 direct crystallization 2 replacement 3 exsolution the occurrence of myrmekite in paragneisses and its absence in rocks devold of discrete grains of potassium feldspar challenge those hypotheses based on direct crystallization or replacement however several lines of evidence indicate that myrmekite may in fact originate in response to kinetic effects associated with the exsolution of calcic alkali feldspar into discrete potassium feldspar and plagioclase phases exsolution of potassium feldspar system projected from alsi2o8 involves the exchange caalk 1si 1 in which the alsi 1 tetrahedral couple is resistant to intracrystalline diffusion by contrast diffusion of octahedral k proceeds relatively easily where it remains uncoupled to the tetrahedral exchange we suggest here that where the ternary feldspar system is open to excess silica the exchange reaction that produces potassium feldspar in the ternary plane is aided by the net transfer reaction k si orthoclase leaving behind indigenous si that reports as modal quartz in the evolving plagioclase as the caal component is concomitantly incorporated in this same phase thus silica is pumped into the reaction volume from a silica reservoir a process that enhances redistribution of both si and al through the exsolving ternary feldspar

a new source of basaltic meteorites inferred from northwest africa 011 eucrites are a class of basaltic meteorites that share common mineralogical isotopic and chemical properties and are thought to have been derived from the same parent body possibly asteroid 4 vesta the texture mineralogy and noble gas data of the recently recovered meteorite northwest africa nwa 011 are similar to those of basaltic eucrites however the oxygen isotopic composition of nwa011 is different from that of other eucrites indicating that nwa011 may be derived from a different parent body the presence of basaltic meteorites with variable oxygen isotopic composition suggests the occurrence of multiple basaltic meteorite parent bodies perhaps similar to 4 vesta in the early solar system

the relationship between a type granites and residual magmas from anorthosite evidence from the northern sherman batholith laramie mountains wyoming usa abstract anorthosite complexes commonly are spatially and temporally associated with a type granite batholiths but their genetic relationship if any has been difficult to determine this study focuses on the northern portion of the 1 43 1 44 ga sherman batholith which is located adjacent to the 1 44 ga laramie anorthosite complex laramie mountains wyoming usa the northern sherman batholith is an ideal place to resolve the relationship of a type granite with anorthosite because of the age of the crust into which it is emplaced most a type granites are intruded into crust only slightly older than the granites themselves and there is little isotopic contrast between potential mantle and crustal source rocks the northern sherman batholith intrudes archean crust that by 1 44 ga had developed nd sr and pb isotopic compositions that differed greatly from contemporaneous mantle the northern sherman batholith is an iron enriched metaluminous alkalic to alkali calcic batholith most of the batholith is composed of fayalite granite with minor volumes of monzodiorite and olivine gabbro hypabyssal dikes of these compositions crop out in the highest structural levels exposed in the area the northern sherman batholith is less differentiated and has assimilated less felsic crust than the southern portions which intrude more fertile proterozoic crust northern sherman batholith rocks are compositionally and isotopically very similar to monzonitic and dioritic rocks of the laramie anorthosite complex none of these rocks have isotopic compositions similar to their archean host rocks precluding derivation solely from such sources instead northern sherman batholith granites like laramie anorthosite complex rocks are related to mantle derived tholeiitic magmas emplaced at the base of the crust the tectonic environment and magmatic processes that lead to the formation of massif anorthosite are also conducive to the formation of voluminous a type granite of which the northern sherman granite is a prime example

the northwest africa 1500 meteorite not a ureilite maybe a brachinite the northwest africa nwa 1500 meteorite is an olivine rich achondrite containing approximately 2 3 vol augite 1 2 vol plagioclase 1 vol chromite and minor orthopyroxene cl apatite metal and sulfide it was originally classified as a ureilite but is currently ungrouped we re examined the oxygen three isotope composition of nwa 1500 results of ultra high precision 0 03 for d 17 o laser fluorination analyses of two bulk chips and high precision 0 3 secondary ion mass spectrometry sims analyses of olivine and plagioclase in a thin section show that the oxygen isotope composition of nwa 1500 d 17 o 0 22 from bulk samples and 0 18 0 06 from 16 mineral analyses is within the range of brachinites we compare petrologic and geochemical characteristics of nwa 1500 with those of brachinites and other olivine rich primitive achondrites including new petrographic mineral compositional and bulk compositional data for brachinites hughes 026 reid 013 nwa 5191 nwa 595 and brachina modal mineral abundances texture olivine and pyroxene major and minor element compositions plagioclase major element compositions rare earth element abundances and siderophile element abundances of nwa 1500 are within the range of those in brachinites and in most cases well distinguished from those of winonaites iab silicates acapulcoites lodranites ureilites and divnoe nwa 1500 shows evidence of internal reduction in the form of reversely zoned olivine fo 65 73 core to rim and fine grained intergrowths of orthopyroxene metal along olivine grain margins the latter also occur in reid 013 hughes 026 nwa 5191 and nwa 595 we argue that reduction olivine fi enstatite fe 0 o 2 is the best hypothesis for their origin in these samples as well we suggest that nwa 1500 should be classified as a brachinite which has implications for the petrogenesis of brachinites fe mn mg compositions of brachinite olivine provide evidence of redox processes among bulk samples nwa 1500 provides evidence for redox processes on a smaller scale as well which supports the interpretation that these processes occurred in a parent body setting sims data for 26 al 26 mg isotopes in plagioclase in nwa 1500 show no 26 mg excesses beyond analytical uncertainties 1 2 the calculated upper limit for the initial 26 al 27 al ratio of the plagioclase corresponds to an age younger than 7 ma after cai compared to 53 mn 53 cr data for brachina wadhwa et al 1998b this implies either a much younger formation age or a more protracted cooling history however brachina is atypical and this comparison may not extend to other brachinites

the plagioclase magma density paradox re examined and the crystallization of proterozoic anorthosites intermediate composition plagioclase an40 60 is typically less dense than the relatively evolved basaltic magmas from which it crystallizes and the crystallization of plagioclase produces a dens residual liquid thus plagioclase should have a tendency to float in these magmatic systems there is however little direct evidence for plagioclase flotation cumulates either in layered intrusions or in proterozoic anorthosite complexes the layered series of the poe mountain anorthosite southeast wyoming contains numerous anorthosite leucogabbro blocks that constrain density relations during differentiation all blocks are more mafic than their hosting anorthositic cumulates their plagioclase compositions are more calcic and each block is in strong sr isotopic disequilibrium with its host cumulate associated structures disrupted and deformed layering indicate that 1 a floor was present during crystallization and that plagioclase was accumulating and or crystallizing on the floor 2 compositional layering and plagioclase lamination formed directly at the magma crystal pile interface and 3 the upper portions of the crystal pile contained significant amounts of interstitial melt liquid densities are calculated for proposed high al olivine gabbroic parental magmas and fe enriched ferrodioritic and monzodioritic residual magmas of the anorthosites taking into account pressure oxygen fugacity p2o5 estimated volatile contents and variable temperatures of crystallization for all reasonable conditions calculated block densities are greater than those of the associated melt the liquid densities however are greater than those for an40 60 plagioclase which cannot have settled to the floor plagioclase must either have been carried to the floor in relatively dense packets of cooled liquid plus crystals or have crystallized in situ a sloping floor possibly produced by diapiric ascent of relatively light plagioclase rich cumulates is required to allow for draining and removal of the dense interstitial liquid produced in the crystal pile and may be a characteristic feature during the crystallization of many proterozoic anorthosites and layered intrusions

the origin of anorthosites and related rocks from the lofoten islands northern norway ii calculation of parental liquid compositions for anorthosites clinopyroxene orthopyroxene and plagioclase in the proterozoic introduction lofoten anorthosites are inferred to be in equilibrium with ferrodioritic proterozoic massif type anorthosites are huge intrusive melts on the basis of literature partition coefficients for these phases bodies that contain hundreds to thousands of km of in equilibrium with basaltic melts ferrodiorites and high al gabbros nearly pure 90 plagioclase cumulates of intermediate occur as fine grained dykes in spatial association with the 1 8 composition an50 10 ashwal 1993 and references ga lofoten anorthosites they are linked to each other by an fetherein most proterozoic anorthosites are not associated enrichment fractionation trend typical of tholeiites and they are with significant amounts of mafic or ultramafic cumulates characterized by al2o3 contents of 14 18 5 wt and xfe they contain plagioclase that is relatively sodic usually feo feo mgo between 0 5 and 0 8 the association around an50 and this makes them distinct from other of high al gabbros ferrodiorites and anorthosites is common in anorthositic rocks archean anorthosites lunar anormany proterozoic anorthosites we infer that the high al gabbros thosites anorthosites in layered intrusions which contain to ferrodiorites represent the parental liquids from which these more calcic plagioclase proterozoic anorthosite comproterozoic anorthosites formed the lofoten rocks show fractionation plexes contain anorthosite sensu stricto 90 plagioclase trends that are similar to those of high al gabbros and ferrodiorites as well as variable amounts of leuco gabbroic troctolitic from other proterozoic anorthosite complexes we interpret these trends and noritic rocks which respectively may contain up to in terms of incorporation of intermediate composition plagioclase into 35 clinopyroxene olivine or orthopyroxene a typical tholeiitic magma with subsequent fractionation the first the proterozoic anorthosites of the lofoten islands in process produces positive eu anomalies and the high al2o3 contents northern norway heier 1960 green jorde 1971 of the melts and explains the lack of negative eu anomalies in romey 1971 griffin et al 1978 wade 1985 are most of the ferrodiorites from lofoten and laramie associated with large amounts of temporally related intrusive mangerites charnockites and gabbros markl et al 1998 there are two different anorthosite intrusions present one is a relatively calcic complex which occurs on the island of flakstadoy flakstadoy basic complex fbc and the other is a relatively sodic complex which

brachinites igneous rocks from a differentiated asteroid we have done petrologic studies of brachinites allan hills alh 84025 elephant moraine eet 99402 and eet 99407 bulk geochemical studies of eet 99402 and eet 99407 ar ar studies of brachina and eet 99402 and a xe isotopic study of brachina textural mineral compositional and bulk compositional evidence show that eet 99402 and eet 99407 are paired alh 84025 eet 99402 and eet 99407 have igneous textures petrofabric analyses of alh 84025 and eet 99407 demonstrate the presence of lineations and probable foliations of olivine grains that support formation as igneous cumulates mineral minor element chemistry and bulk rock incompatible lithophile element contents of the brachinites are distinct from those of acapulcoite lodranite clan meteorites a suite of high grade metamorphic rocks and anatectic residues the differences demonstrate a higher blocking temperature of equilibration for the brachinites and that cumulus plagioclase is present in eet 99402 eet 99407 and probably alh 84025 thus indicating an igneous origin brachinites are differentiated ultramafic achondrites and are not part of a suite of primitive achondrites we infer that their parent asteroid is a differentiated body brachina has an excess of 129xe correlated with reactor produced 128xe demonstrating that short lived 129i was present at the time of formation this plus literature data attests to early formation of the brachinites within a few ma of the formation of chondrites ar ar age data show that brachina and eet 99407 were degassed about 4 13 ga ago possibly by a common impact event eet 99402 and eet 99407 show petrographic evidence for shock including possible conversion of plagioclase to maskelynite followed by devitrification brachina is unshocked making a direct association between the ar ar age and textures ambiguous

open system dehydration of amphibolite morton pass wyoming elemental and nd and sr isotopic effects we have undertaken a geochemical and sm nd and rb sr isotopic study of banded metabasites in the contact aureole of the 1 43 ga laramie anorthosite complex wyoming in order to determine the isotopic effects of open system high temperature metamorphism samples were collected from two locations along strike one that experienced peak temperatures of 720 c and one that was subjected to temperatures up to 810 c at the higher temperature location metabasites adjacent to marble have undergone extreme dehydration presumably due to fluxing of co 2 rich fluids these hornblende poor rocks also show textural evidence of high temperature shearing element mobility is most evident within this zone of dehydration variations in chemical composition are homogenized and elements including ti fe k na rb sr y sm and nd have been removed metabasites at greater distances from the marble also provide evidence of open system behavior albeit less dramatic rb k and sr contents are low throughout the hi

the tafassasset primitive achondrite insights into initial stages of planetary differentiation abstract tafassasset is an exceptional meteorite that has been linked to both the cr chondrites and the primitive achondrites because previous evidence suggests it might be a primitive achondrite from a known chondrite type we have undertaken a study of the petrology geochemistry and formation history of the meteorite tafassasset is predominantly feo rich olivine 58 yet contains abundant fe ni metal 10 vol and sulfide 3 vol other phases include high and low ca pyroxene plagioclase chromite and phosphate it has a recrystallized texture containing equigranular grains that often meet at 120 triple junctions there are no relict chondrules in the thin sections examined although they have been reported previously electron microprobe analyses reveal homogeneous olivine fa 28 6 both low and high ca pyroxene fs 23 6 wo 3 7 and fs 12 2 wo 39 3 1 a range of plagioclase composition an 23 47 fe ni metal with 5 3 36 6 wt ni and 0 1 0 8 wt co troilite chromite and ca phosphate bulk composition analyses reveal two chips depleted in refractory lithophile and some siderophile elements compared to ci chondrites exceptions are enrichments in fe ni and co a third chip is essentially chondritic in bulk composition different stones of the meteorite have slightly different oxygen isotope composition yet all lie in the cr chondrite trend with one in the acapulcoite lodranite field thermodynamic calculations show that tafassasset equilibrated at a temperature above the fe ni fes eutectic and at an oxygen fugacity of iw 1 the texture heterogeneous distribution of mineral phases plagioclase composition two mineral closure temperatures and bulk composition all provide evidence that tafassasset partially melted on its parent body a comparison with the cr chondrites the brachinites and two anomalous achondrites indicates that tafassasset is most similar to ungrouped primitive achondrites lewis cliff lew 88763 and divnoe and to the brachinites in overall petrography modal mineralogy mineral compositions oxidation state and bulk composition the comparison also excludes the possibility that tafassasset formed by partial melting of a cr chondrite tafassasset is a primitive achondrite and likely evolved on a parent body that experienced incomplete melting never reached isotopic homogeneity and was from the same oxygen isotopic reservoir as the cr chondrite parent body

oxygen isotope constraints on the petrogenesis of the sybille intrusion of the proterozoic laramie anorthosite complex the origin of monzonitic intrusions that are associated with proterozoic massif type anorthosite complexes is controversial a detailed oxygen isotope study of the sybille intrusion a monzonitic intrusion of the laramie anorthosite complex wyoming indicates that either derivation from a basaltic magma of mantle origin with a metasedimentary component 20 incorporated early in its magmatic history or a partial melt of lower crustal rocks is consistent with the data the oxygen isotope compositions of plagioclase pyroxene and zircon from the sybille monzosyenite the dominant rock type in the sybille intrusion were analyzed in order to establish the isotopic composition of the source of the magma plagioclase 18o values range from 6 77 to 9 17 we interpret the higher plagioclase 18o values average 8 69 0 30 n 19 to be magmatic in origin lower plagioclase 18o values average 7 51 0 44 n 22 to be the result of variable subsolidus alteration and pyroxene 18o values average 6 34 0 38 n 19 to be the result of closed system diffusional exchange during cooling low magnetic zircons which have been shown to retain magmatic oxygen isotope values despite high grade metamorphism and extensive subsolidus hydrothermal alteration have 18o values 7 40 0 24 n 11 which are consistent with our interpretation of the plagioclase and pyroxene results oxygen isotope data from all three minerals indicate that the magmatic oxygen isotope composition of the sybille intrusion is enriched in 18o relative to the composition of average or normal mantle derived magmas this enrichment is approximately twice the oxygen isotope enrichment that could result from closed system fractionation rendering a closed system comag matic petrogenetic model between the sybille intrusion and the mantle derived anorthositic lithologies of the laramie anorthosite complex improbable

marginal mafic intrusions as indicators of downslope draining of dense residual melts in anorthositic diapirs abstract geochemical and isotopic investigation of three small mafic intrusions loyning 1250 150 m hogstad 2000 200 m koldal 1250 500 m in the marginal zones of the egersund ogna loyning koldal and ana sira massif type anorthosites hogstad rogaland anorthositic province south norway 930 ma provides new insights into the late evolution of anorthositic diapirs these layered mafic intrusions are essentially of norite gabbronorite as well as leuconorite and display conspicuous evidence of subsolidus recrystallization in loyning and hogstad the modal layering is parallel to the subvertical foliation in the enclosing anorthosite the northern part of the koldal intrusion cuts across the foliation of the anorthosite whereas in its southern part the subvertical layering is parallel to the anorthosite s foliation the regularity of the layered structures suggests that the layering was initially acquired horizontally and later tilted during the final movements of the diapirs the least differentiated compositions of plagioclase and orthopyroxene in the three intrusions an 59 en 68 in loyning an 49 en 64 in hogstad and an 44 en 61 in koldal and the ree contents in apatite hogstad indicate that their parent magmas were progressively more differentiated in the sequence loyning hogstad koldal isotopic data loyning 87 sr 86 sr 0 70376 0 70457 e ndt 6 8 to 2 7 hogstad 87 sr 86 sr 0 70537 0 70588 e ndt 2 1 to 0 5 koldal 87 sr 86 sr 0 70659 0 70911 e ndt 3 5 to 1 6 also indicate that in this sequence parent magmas were characterized by a progressively more enriched sr and nd isotopic signature in loyning the parent magma was slightly more magnesian and anorthitic than a primitive jotunite in hogstad it is a primitive jotunite and in koldal an evolved jotunite given that plagioclase and orthopyroxene of the three intrusions display more differentiated compositions than the orthopyroxene and plagioclase megacryts of the enclosing anorthosites it is suggested that the parent magmas of the small intrusions are residual melts after anorthosite formation which were entrained in the anorthositic diapir during its rise from lower crustal chambers calculated densities of primitive jotunites 2 73 2 74 at fmq 0 15 h 2 o 200 ppm co 2 435 ppm f 1150 c 3 kb and evolved jotunites 2 75 2 76 at fmq 0 30 h 2 o 400 ppm co 2 870 ppm f 1135 c 3 kb demonstrate that they are much denser than the plagioclase of the surrounding anorthositic crystal mush 2 61 2 65 efficient migration and draining of dense residual melts through the anorthositic crystal mush could have taken place along sloping floors zones of lesser permeability in the mush which occur along the margins of the rising anorthositic diapirs this process takes into account the restricted occurrence of the mafic intrusions in the margins of the massif anorthosites in a later stage when the anorthosite was nearly consolidated the residual melts were more differentiated evolved jotunites and could have been extracted into extensional fractures in the cooling and contracting anorthositic body in a similar way as aplitic dikes are emplaced in granitic plutons as in the rogaland anorthositic province these dikes are much more abundant than the small mafic intrusions collection and transport along dikes was probably more efficient than draining through the crystal mush

the jugsaipatna anorthosite complex eastern ghats belt india magmatic lineage and petrogenetic implications abstract the jugsaipatna anorthosite complex jac in the eastern ghats mobile belt india comprises a central anorthosite leuconorite troctolitic norite core mantled by a rim of high al gabbronorite hag small dykes of ferrodiorite occur in the core region of the jac a lensoidal body of eucritic gabbronorite occurs in contact with leuconorite and outer gabbronorite rim baring ferrodiorites the mafic members such as hag troctolitic norite and eucritic gabbronorite are not known in other anorthosite complexes of this high grade belt the hag is characterised by high al2o3 content 17 wt enriched ree and lree absence of eu anomaly and negative sr anomaly and is therefore inferred to be the source melt for the jac suite of rocks the eucritic gabbronorite comprising an rich plagioclase and high al pyroxenes has a komatiitic affinity with high cr ni contents and primitive ree patterns troctolitic norite leuconorite and anorthosite are interpreted to be cumulate rocks fractionated from the hag the ferrodiorite with enriched fe ti p zr and ree abundances and a prominent negative eu anomaly corresponds to the residual melt of the plagioclase pyroxene dominated fractionates that comprise anorthosite leuconorite

precambrian basement structure and laramide deformation revealed by seismic reflection profiling in the laramie mountains wyoming the university of wyoming conducted seismic reflection profiling in the laramie mountains of wyoming to investigate the nature of precambrian crustal structure and laramide deformation in the region here the imaging of layering in the laramie anorthosite complex confirms that layered intrusions can be reflective and establishes an analog for interpreting events in other seismic reflection data sets where layered intrusions could be the cause of multicyclic events moreover the presence of prominent primary layering in the complex supports a conclusion that it formed in situ and not as the result of the emplacement of a diapiric crystal mush imaging of laramide thrusts in the laramie mountains adds credence to the idea that brittle faults can be reflective in some situations migrated apparent dips on these faults are 30 westward basement involved compressive structures consisting of monoclinal folding and a fault triple junction demonstrate that compression controlled the style of deformation during the laramide orogen in wyoming west dipping precambrian structures imaged in these data have a similar attitude to the laramide structures suggesting that laramide deformation was influenced by precambrian features these precambrian structures may be subthrust slices of exotic proterozoic terranes

oxygen isotope variation in primitive achondrites the influence of primordial asteroidal and terrestrial processes a detailed oxygen isotope study of the acapulcoites lodranites winonaites brachinites and various related achondrites has been undertaken to investigate the nature of their precursor materials high levels of terrestrial alteration displayed by many of these samples have been mitigated by leaching in ethanolamine thioglycollate eatg solution due to their high metal and sulphide content acapulcoite lodranite and winonaite samples show much greater isotopic shifts during weathering than brachinites as observed in previous studies antarctic weathered finds are displaced to lighter oxygen isotope compositions and non antarctic finds to heavier values leached primitive achondrite residues continue to show high levels of oxygen isotope heterogeneity this variation is reflected in the 2 error on group mean 17o values which decrease in the following order acapulcoite lodranite clan brachinites winonaites on an oxygen three isotope diagram the acapulcoite lodranite clan define a limited trend with a slope of 0 61 0 08 and an intercept of 1 43 0 27 r2 0 78 a broad positive correlation between 17o and olivine fayalite contents displayed by both acapulcoite and lodranite samples may be the result of early aqueous alteration and subsequent dehydration winonaites experienced a greater degree of differentiation than the acapulcoite lodranite clan and define a distinct mass fractionation line with a slope of 0 53 0 01 and an intercept of 0 53 0 04 r2 1 a number of samples currently classified as acapulcoites nwa 725 nwa 1052 and dho 1222 have oxygen isotope compositions indicating that they are winonaites the relatively high level of oxygen isotope heterogeneity displayed by the brachinites supports their designation as primitive achondrites a number of ungrouped olivine rich achondrites divnoe nwa 4042 nwa 4363 nwa 4518 nwa 5400 zag b as well as the unique plagioclase rich achondrites gra 06128 and gra 06129 have similar oxygen isotope compositions to the brachinites it remains unclear whether the brachinites and related olivine rich achondrites are from a single or multiple parent bodies the primitive achondrites and related samples represent material from at most only 18 parent bodies compared to an estimated 65 for the iron meteorites this suggests that asteroidal mantle material is underrepresented in the meteorite record early fragmentation of differentiated asteroids followed by preferential destruction of their silicate rich mantles offers a possible explanation for this discrepancy on an oxygen three isotope diagram primitive chondrule bearing winonaites dho 1222 nwa 725 nwa 1052 nwa 1463 mt morris wisconsin plot close to the young russell y r slope 1 line with more evolved samples extending away from it towards the ccam line a similar relationship is shown by the cr chondrites the acapulcoite lodranite clan plots between the slope 1 and ccam lines however the precursor material to the clan may have had a composition close to the slope 1 line prior to parent body processing these relationships support the view that primordial oxygen isotope variation in the early solar system is best represented by the slope 1 y r line

isotopic constraints on emplacement age of anorthositic rocks of the marcy masiff adirondack mts new york the igneous emplacement age of anorthositic rocks comprising the marcy massif have been constrained by fixing minimum and maximum ages for their intrusion minimum ages have been fixed by a u pb dating of cores of air abraded zircons from the anorthosite 1113 ma b u pb dating of baddeleyite within the anorthosite 1087 ma c u pb dating of baddeleyite from an olivine metagabbro crosscutting marcy massif anorthosite 1109 ma d u pb zircon dating of jotunitic rocks crosscutting the marcy anorthosite massif 1143 pm 31 ma maximum emplacement ages are based upon a sheets of anorthositic rock crosscutting mangeritic charnockitic gneiss dated by u pb zircon at 1125 pm 10 ma 1134 pm 4 ma and 1135 pm 43 ma and b anorthosite related pyroxenite dikes crosscutting mangerite that contains xenoliths of identical pyroxenite these results constrain emplacement of the marcy anorthosite to 1113 1138 ma with the most probable age in the interval 1125 1135 ma this time of in

oxygen isotope ratios of feo poor chondrules in cr3 chondrites influence of dust enrichment and h2o during chondrule formation abstract we present detailed electron microprobe analyses and oxygen three isotope measurements by high precision secondary ion mass spectrometry on 45 type i feo poor chondrules fragments and 3 type ii feo rich chondrule fragments from meteorite hills 00426 and queen alexandra range 99177 two of the most primitive cr3 chondrites type i chondrules fragments have mg s defined as the mg of constituent olivine and or low ca pyroxene ranging from 94 2 to 99 2 type ii chondrule fragments have mg s of 53 63 oxygen three isotope measurements plot on the slope 1 primitive chondrule mineral pcm line within chondrules 17 o 17 o 0 52 18 o values of coexisting olivine pyroxene and plagioclase are homogeneous with propagated uncertainties of 0 3 this indicates each phase crystallized from the final chondrule melt and that efficient oxygen isotope exchange occurred between ambient gas and chondrule melt among type i chondrules there is a well defined increase in 17 o from 5 9 to 1 as mg s decrease from 99 2 to 96 type ii chondrule fragments are comparatively 16 o poor 17 o 0 2 0 6 the relationship between mg and 17 o among type i chondrules confirms that addition of a 16 o poor oxidizing agent to the highest mg chondrule precursors resulted in forming lower mg cr chondrules using aspects of existing equilibrium condensation models and a mass balance we estimate that type i cr chondrules formed at dust enrichments of 100 200 from dusts with 0 0 8 times the atomic abundance of ice relative to ci dust the type ii chondrule fragments are predicted to have formed at ci dust enrichments near 2500

mineral chemistry and the petrogenesis of eucrites ii cumulate eucrites abstract we carried out an ion microprobe study of rees and other selected trace and minor element concentrations in orthopyroxene augite lamellae and plagioclase of two cumulate eucrites moama and moore county in order to reevaluate the relationships between cumulate and noncumulate eucrites and among cumulate eucrites pyroxene and plagioclase in these two meteorites are more homogeneous in major minor and trace elements than in noncumulate eucrites and trace and minor element concentrations in moore county are higher than in moama although subsolidus equilibration erased and original igneous zoning it did not affect the ree patterns of these minerals both pigeonite and plagioclase of moama and moore county have higher lree hree ratios than those phases in noncumulate eucrites this indicates that cumulate eucrites derived from melts with fractionated ree patterns and thus are not related to noncumulate eucrites in addition the inconsistency between degrees of fractional crystallization deduced from major and trace elements and the differences of lree hree ratios recorded in pigeonite and plagioclase also suggest that moama and moore county did not form in the same fractional crystallization sequence this study confirms earlier observations and conclusions of ma and schmitt 1979 and poses problems for prevalent models

kiglapait mineralogy v feldspars in a hot dry magma the lithology of the 1 305 ga kiglapait intrusion is dominated by a lower zone of troctolite succeeded by an upper zone of olivine gabbro ferrodiorite and syenite with olivine composition of pure fayalite the feldspar composition of the intrusion varies from an 68 to an 9 over a thickness of 8 4 km from the base to a sandwich horizon under an upper border zone the anhydrous nature of the kiglapait syenites is shown by their high temperature by the loss of minor biotite up stratigraphy in the intrusion and the absence of amphibole the end stage feldspar of the kiglapait syenites is that of a solidus embedded in a solvus in a 3 kbar eutectic at 1000 c the end member assemblage at temperature and pressure is invariant the final bulk composition is relatively an rich an 11 with a composition of x or 1 3 when projected to the ab or sideline the experimental feldspar solvus when corrected for the effects of an and ba and referred to 3 kbar penetrates the solidus and fits the experimental tie lines these conditions precede a stage of local coarsening under subsolidus conditions that is found in colloform symplectites invading mesoperthite the oligoclase orthoclase symplectites are iso compositional with their host mesoperthites the coarsening is assumed to be related to a plausibly f rich vapor phase that is locally consumed with time the observed phase compositions indicate the end of exsolution at 800 c at 3 kbar on the binodal solvus

do fe ti oxide magmas exist probably not many fe ti oxide bodies associated with anorthosite suites and with some tholeiitic plutonic bodies have cross cutting relationships with their host rocks suggesting that they may have been emplaced as oxide melts pure fe ti oxides melt at temperatures much higher than is considered to be geologically realistic so various fluxes mainly apatite fluorine or carbon have been called upon to stabilize the melts down to plausible temperatures this review traces our experimental attempts to test the effectiveness of proposed fluxes and therefore to demonstrate the existence of such melts at geologically realistic temperatures neither f apatite nor carbon act to stabilize ti rich fe ti oxide melts at 1300 c and below and we conclude that unless some totally unforeseen material does serve as a flux fe ti oxide magmas almost certainly do not exist although our data are not conclusive it appears that increasing contents of feo and possibly tio 2 and p 2 o 5 mutually enhance their solubilities in silicate melts allowing extensive buildup of those components in melts residual to anorthosite we interpret that oxide orebodies form by gravitational accumulation of crystalline oxides from such liquids once those melts become saturated with either fe ti oxides or apatite both phases will tend to co precipitate thus explaining the common occurrence of apatite with oxide orebodies nelsonites cross cutting oxide bodies were probably emplaced as crystalline oxides possibly lubricated by small amounts of residual silicate liquid oxidation of the fe 2 tio 4 component in initially ulvospinel rich spinel and concomitant formation of ilmenite grains by granule oxy exsolution may have weakened the crystalline oxide and facilitated its flow during emplacement it seems clear though that the presence of carbon does stabilize ti poor iron oxide melts to very low temperatures at and even below 1000 c consistent with the disputed magmatic origin of the magnetite lavas at el laco chile

the specific case of the mid proterozoic rapakivi granites and associated suite within the context of the columbia supercontinent abstract rapakivi granites and the associated suite anorthosite mangerite and charnockite that developed during the mid proterozoic escape the global rules that control usual granitic magma formation about 40 points that characterise the proterozoic magmatism confirm the originality of the magmatism rapakivi granites developed without a global orogenic context within the supercontinent columbia the intrusions extend from the east european shield to the western us through fennoscandia greenland and labrador other occurrences in the amazonian shield australia or south china are less well documented they show no definite trend in age or chemistry that would explain large scale mantle plume effects a mantle upwelling of material is contradictory with the re assembly of a supercontinent because it occurred before this magmatic episode 1 9 1 8 ga at 1 7 ga the supercontinent columbia amalgamated with the collision of the yapavai and mazatzal provinces a model is suggested that takes into account the supercontinent re assembly defining a downwelling flow in the mantle that anchors the continent above it in contrast to a material flow heat is still delivered to the base of the continental lithosphere and is focused toward the juvenile suture zone the base of the crust must reach 1200 1300 c before producing anorthosite magmas under such a high temperature gradient lasting over a long time magmas are transferred toward the upper crust giving the thin 5 km and square shape 100 km that the intrusions presently have heat delivery is essentially conductive leading to long time spans for intrusions the presence of the supercontinent immobile over a descending cell poloidal mode of convection developed a tangential force toroidal mode of convection that partly split the continent through strike slip deformation due to plate rotation it developed progressively between 1 57 and 1 3 ga starting from fennoscandia and then passing to amazonia western us and labrador in a clockwise sense the associated rotation induced sinistral shear manifested by small scale shear zones and the orientation of late magmatic facies topaz bearing granites in each province the proterozoic magmatism appears to be unique because it requires a supercontinent with a zone of juvenile crust surrounded by older cratons the present moho still shows remnants of this process having bumpy undulations that may reach 22 km in amplitude over a distance of 200 km

micro textures and chemical compositions of metamorphic magnetite and ilmenite insights from the mianhuadi mafic complex in sw china abstract the compositions of titano magnetite in magmatic and hydrothermal ore deposits have shown the potential to discriminate mineral deposit types however primary fe ti oxides were sometimes overprinted by metamorphic event s over the long geologic history to what extent the fe ti oxide aggregates in mafic rocks can be modified either by re equilibration or interaction with the silicates during high grade metamorphism has not been systematically studied so far the mianhuadi mafic complex intruded at 260 ma as a layered intrusion in the emeishan large igneous province sw china has undergone granulite facies metamorphism in eocene during the development of the ailao shan red river shear zone the primary igneous titanomagnetite underwent oxyexsolution and reacted with the silicates to form magnetite ilmenite and pleonaste resulting in an increased volume proportion of ilmenite and pleonaste a retrograde chlorite carbonate assemblage was found to replace the magnetite whereas alo oh sometimes replaced pleonaste redistribution of elements associated with the oxyexsolution occurred that trace elements with larger partition coefficients in magnetite e g al ga v and cr than in ilmenite are more enriched in magnetite after metamorphism and vice versa iron ti oxide equilibrium calculations indicate that the increased oxygen fugacity is mainly responsible for the oxyexsolution of ilmenite and therefore the geochemical variation of magnetite metamorphic magnetite from the mianhuadi mafic complex can be clearly distinguished from its magmatic precursor by high ga zr and v ti ratios we further propose that v versus fe ti diagram can be used to discriminate between magmatic metamorphic and metasomatised titano magnetite

magmatic oxidation in the syenitic murdock creek intrusion kirkland lake ontario evidence from the ferromagnesian silicates the syenitic murdock creek intrusion of late archean age lies within one of the major lode gold camps of the world at kirkland lake ontario there is a strong spatial association between syenitic intrusions gold mineralization and carbonatization hydrothermal fluids that carried the gold had an unusually high oxygen fugacity f o 2 notwithstanding a wide range in rock compositions within the intrusion from clino pyroxenite to alkali feldspar syenite clinopyroxene and biotite have consistently low fe fe mg and high fe 3 fe 2 fe 3 ratios these compositional features indicate a deficiency of fe 2 in the magma during crystallization together with the presence of abundant early formed magnetite and titanite this is interpreted to result from an intrinsically high magmatic f o 2 that remained constant during pluton evolution f o 2 approx 10 12 bars mineral compositions proportions of crystallizing mineral phases and textural features i

petrogenesis of the 1 43 ga sherman batholith se wyoming usa a reduced rapakivi type anorogenic granite the 1 43 ga sherman batholith southeastern wyoming usa origin that are found throughout the proterozoic anorogenic granite provinces of the southwestern usa adirshows extreme a type petrochemical characteristics compared with other mid proterozoic granite batholiths of north america it ondacks eastern canada southern greenland and the fennoscandian shield anderson 1983 phanerozoic consists of 1 the sherman granite a coarse grained biotite hornblende granite that locally contains fayalite and pyroxenes 2 examples include paleozoic granites of southeastern australia collins et al 1982 and of corsica poitrasson et the lincoln granite a medium grained biotite granite 3 a porphyritic biotite hornblende granite that probably formed by interaction al 1995 and the mesozoic nigerian younger granites jacobsen et al 1958 and the white mountain magma of granitic and mafic magmas and 4 iron enriched mafic dikes and pods the ilmenite series metaluminous sherman granite series foland allen 1991 anorogenic granite refers to plutons that are not associated with compressional exhibits extreme values of feo feo mgo and is rich in structures and that were emplaced long after any known k ree nb and y it crystallized at temperatures exceeding orogenic event anorogenic or a type granites loiselle 900 c and a pressure of 2 5 kbar with water activity of 0 7 wones 1979 are characterized by low h2o and o2 and dlog fo2 of 0 1 to 0 5 the lincoln granite which is fugacities along with high feo feo mgo k2o peraluminous and has less extreme a type geochemical characteristics na2o and k2o contents a type granites typically are rich crystallized at temperatures as low as 750 c and dlog fo2 of in incompatible elements including rare earth elements around 0 5 units above fmq fayalite magnetite quartz the ree zr nb and ta but poor in co sc cr ni ba rocks of the sherman batholith are chemically equivalent to lavas sr and eu minimum magma liquidus temperatures are from the yellowstone hotspot like the yellowstone magmas the 900 1000 c clemens et al 1986 creaser white sherman batholith probably originated by partial melting of under1991 some workers suggest these rocks are derived plated mantle derived mafic rocks from melting of tonalitic or more felsic crust anderson cullers 1978 collins et al 1982 clemens et al 1986 creaser et al 1991 whereas others relate the granites

fe ti oxide chronometry with application to granulite formation abstract ilmenites occurring in amphibolite and granulite facies orthogneisses in sri lanka yield precise u pb ages coexisting ilmenite magnetite in the granulite facies rocks indicate temperature conditions of 680 60 c and log o 2 17 0 1 2 the u pb age of 498 2 3 3 ma obtained for ilmenite is indistinguishable from the pb pb age of 501 5 ma derived for coexisting silicate minerals and suggests that oxide silicate equilibration has been preserved in contrast fe ti oxides in adjacent amphibolitefacies rocks indicate a temperature of 830 60 c and log o 2 12 7 0 8 and record u pb ages of 1093 7 7 6 ma and 1095 4 4 2 ma these ages are some 600 ma older than those obtained for coexisting silicate phases in the same sample or for both oxide and silicate minerals in the nearby charnockite and relate either to the time of emplacement of the granitic precursor or to some earlier metamorphic episode these results suggest that temperature o2 and fluid conditions estimated from fe ti oxides should be interpreted with extreme caution