palaeomagnetic data suggest link between the archaean proterozoic boundary and inner core nucleation it is generally believed that the earth s magnetic field arises from dynamo action in the convecting outer core1 3 thus the geomagnetic field and especially its strength can provide information about core processes recent determinations of the geomagnetic palaeointensity using archaean and early proterozoic samples from the kaapvaal and superior cratons are consistent with a sharp increase in the magnitude of the geomagnetic field between 2 7 and 2 1 gyr analysis of published late archaean and early proterozoic apparent polar wander paths shows that an abrupt change in the rate and style of continental motion occurred at about 2 5 2 6 gyr ago if this change is also interpreted to be related to the growth of an inner core then an intriguing conjecture arises that the archaean proterozoic transition is related to inner core nucleation

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

re os and 40ar 39ar isotope measurements of inclusions in alluvial diamonds from the ural mountains constraints on diamond genesis and eruption ages the re os isotope data for 20 syngenetic sulphide inclusions recovered from 15 diamonds and the 40ar 39ar laser probe eruption ages of 7 syngenetic clinopyroxenes recovered from 5 diamonds all from alluvial placer deposits in the ural mountains have been determined six eclogitic sulphide inclusions two of which coexist in the same diamond yield an isochron age of 1280 310 ma 2 with an unusually high initial 187os 188os ratio of 2 10 0 58 2 the age is interpreted to date remobilisation of carbon and sulphur and homogenisation of os during rift related thermal magmatic events that affected the east european craton eec at ca 1 3 ga the high initial os ratio suggests re os evolution over a 100 to 500 ma period within previously metasomatised lithosphere most likely the eec five eclogitic clinopyroxenes recovered from four diamonds yielded similar 40ar 39ar ages averaging 472 28 ma which likely approximate the time of source kimberlite lamproite eruption this age suggests that the ural diamonds are not likely to have derived either from the well known diamond bearing kimberlites of the siberian craton nor from presently known russian and finnish kimberlite provinces on the eec the urals placer deposits are mainly confined to 397 407 ma sedimentary rocks along the western side of these mountains with sediment transportation at that time generally from the north west present evidence suggests the existence of an undiscovered kimberlite lamproite source probably on the volgo uralia crustal segment of the eec which gave rise to the urals diamond deposits

sm nd chronology and petrogenesis of mesosiderites we have obtained sm nd data from four mesosiderite silicate clasts including three clasts with a variety of textures from the vaca muerta type 1a mesosiderite and one gabbroic clast from the mt padbury mesosiderite the gabbroic vaca muerta pebble 12 and basaltic pebble 16 yield identical 147 sm 143 nd ages of 4 48 0 19 ae and 4 48 0 09 ae respectively while the highly recrystallized pebble 5 gives an age of 4 42 0 02 ae mt padbury yields an age of 4 52 0 04 ae all clasts show a correlation of 142 nd 144 nd with 147 sm 144 nd and provide unequivocal evidence for the presence of live 146 sm at the time of their formation calculated initial 146 sm 144 sm values range from 0 004 pebble 5 to 0 006 pebbles 12 16 and mt padbury and are generally consistent with the 147 sm 143 nd ages however discordance of whole rock leach and residue data and some disagreement between 146 sm 142 nd relative ages and 147 sm 143 nd absolute ages indicate small but significant disturbances to the sm nd systematics the ranges of ages and initial 146 sm 144 sm and 143 nd 144 nd values suggest that each of these silicate clasts underwent a separate protracted evolution on its parent body prior to mixing with metal textural and trace element criteria indicate that individual clasts from the same mesosiderite often had very different igneous sources and thermal histories prior to their incorporation in the meteorite pebble 12 is extremely lree depleted probably a result of several melt extraction events whereas pebbles 5 and 16 and mt padbury have nearly chondritic sm nd with bulk ree concentrations higher than chondrites by factors of 5 to 15 in general the sm nd systematics of mesosiderite silicates require formation of the silicates on a parent planet which underwent relatively early and extreme differentiation preservation of diverse old ages and the presence of 146 sm imply that metal silicate mixing did not seriously alter the sm nd isotopic memories of these clasts we present a model for metal silicate mixing which combines the cooling history with isotopic reequilibration for the case of thermal blanketing we show that the total amount of isotopic reequilibration in a sample can be related to the initial temperature depth of burial grain size and diffusion parameters application of this model to the silicate clasts measured in this study indicates that if the metal and silicate were thermally equilibrated above the metal solidus temperature during mixing then the clasts must have been buried no deeper than 1 10 m in regolith during the initial high temperature cooling phase in order to prevent the sm nd systems from being extensively reset in order to reconcile these results with the slow cooling rates at lower temperatures determined from studies of exsolution in the metal phase we infer that heat was transferred rapidly from hot metal to cold silicate material during initial metal silicate mixing and that the deeply buried portions of the mixture then cooled slowly after reaching thermal equilibrium at 600 700 c the data from this study point to the following history for the mesosiderite parent body 1 differentiation of a silicate parent body within the first 50 m y of solar system history to form diverse parent magmas 2 emplacement of primitive and differentiated mafic magmas near the planetary surface and extensive differentiation in the near surface environment 3 formation and reworking of regolith breccias through impact gardening at the near surface of the body 4 mixing of molten fe ni metal with the regolith followed by rapid cooling 100 150 m y after the origin of the solar system 5 slow cooling from temperatures of 700 c to produce the observed nickel diffusion profiles in the iron phase 6 mild impact metamorphism and brecciation to obtain the much younger k ar ages and 7 recent collisions or perturbations sending mesosiderite fragments into earth crossing trajectories

formation of mesosiderites by low velocity impacts as a natural consequence of planet formation mesosiderites are an enigmatic class of stony iron meteorites composed of roughly equal amounts of pyroxenitic and basaltic silicates and metallic iron nickel olivine is rare we suggest here that during the period of planet formation mesosiderites originated by the low velocity collisions of large metallic core fragments with the surface of a differentiated asteroid size body relative velocities of the asteroids native to this region were low 1 km s 1 because it was distant from massive protoplanets however a few differentiated asteroids were destroyed by high velocity collisions with interlopers perturbed by protoplanets into highly eccentric orbits these collisions reduced mantles and crusts to small silicate fragments but left cores in the form of large durable metal fragments mesosiderite like pyroxenite basalt metal mixtures were formed when large core fragments accreted at low velocities to the regolith of an intact asteroid the metal contents of other regolithic regions remained low most of the olivine in mesosiderite and howardite breccias we attribute to the mantles of the disrupted parent bodies the low 12 20 mg g 1 olivine contents indicate that the amount of debris accreted from disrupted asteroids was small relative to the volumes of the regoliths

on the suitability of baked clay for archaeomagnetic studies as deduced from detailed rock magnetic studies summary extensive rock magnetic investigations have been carried out on baked clays from four kilns two from bulgaria and two from switzerland found in archaeological sites of different age knowledge of the magnetic characteristics of the grains responsible for the archaeomagnetic signal enables us to determine which baked clays have the stablest magnetization and why this is so this is important in directional studies but even more so in painstaking palaeointensity studies that require a very careful evaluation of the suitability of the burnt clay material the proposed rock magnetic experiments enable the identification of the carriers responsible for the remanence and an adequate interpretation of the experimental results connected with the palaeointensity evaluation the experimental methods employed are illustrated with the particular results obtained from each of the four kilns studied the preliminary elucidation of the magnetic mineralogy of the archaeological samples helps first by obtaining a more reliable palaeointensity result and secondly by explaining some of the discrepancies in the palaeodirectional results examples of successful and failed palaeointensity experiments are given in relation to the magnetic properties previously established for each oven the burnt clay materials in this present study satisfy the essential condition of carrying a thermoremanence in spite of that it is shown that there are many factors that can produce undesirable magnetic properties and thus restrict the suitability of these materials for archaeomagnetic analysis the most important factors influencing the magnetic behaviour during magneto diagnostic experiments are the degree of heating in antiquity the initial composition of the unbaked material and the burial conditions the large difference in heating temperatures within a particular archaeological feature is a major cause of variation in magnetic behaviour amongst individual specimens and so preventing a successful pre selection of specimens for palaeointensity experiments nevertheless the study has shown a very good coincidence between the determined rock magnetic characteristics and the success rate in palaeointensity evaluation

diamond provenance studies from 40ar 39ar dating of clinopyroxene inclusions an example from the west coast of namibia abstract the west coast of namibia is host to substantive detrital diamond deposits located in onshore and offshore beach gravels desert deflation deposits and lower orange river terraces the origin of the namibian diamonds is controversial with some studies favouring derivation from distal cretaceous jurassic kimberlites on the kaapvaal craton and others arguing that most diamonds originated from proximal dwyka glacial deposits 300 ma which incorporated diamonds from older 500 ma pre karoo kimberlites previous studies have demonstrated that clinopyroxene inclusions extracted from their host diamonds give 40ar 39ar ages approaching the time of source kimberlite eruption this behaviour is attributed to diffusion of argon to lattice defect sites and the diamond inclusion interface region during mantle residence with subsequent loss of the latter component on cleaving of the diamond to release the inclusion s in this study we measured 40ar 39ar ages of extracted clinopyroxene inclusions from namibian detrital diamonds in order to determine potential kimberlite sources craton erosion histories and palaeo drainage evolution in southern africa 40ar 39ar step heating data were obtained for eclogitic and peridotitic clinopyroxene inclusions from 50 namibian diamonds low temperature steps produced older apparent ages than high temperature fusion steps consistent with partial retention of pre eruption argon in defect sites with one exception fusion steps yielded younger ages ranging from 62 30 ma to 1441 700 ma the majority 80 of inclusions have fusion ages based on the ages of known southern african kimberlites the current results suggest that very few

magmatic fabric acquisition mechanisms in a syenite results of a combined anisotropy of magnetic susceptibility and image analysis study the structural significance of the anisotropy of magnetic susceptibility ams and acquisition mechanisms of silicate and ferrimagnetic mineral fabrics are investigated in the archean lebel syenite intrusion ontario canada bulk mineral subfabrics are determined by intercept counting of mineral phases in classified digital images of thin sections cut parallel to the principal planes of the magnetic fabric ellipsoids of the samples intensities shapes and principal direction orientations of the resulting intercept ellipsoids are generally similar to the corresponding ams ellipsoids differences are due mainly to bimodal distribution of mineral fabrics observed on foliation planes of some samples showing pre full crystallization textures and structural overprinting in samples deformed after crystallization subfabrics of clinopyroxene and magnetite in samples showing pre full crystallization microstructure are investigated by measuring inertia tensors of individual grain shapes the resulting inertia tensor ellipsoids indicate that magnetic fabrics in these rocks are strongly correlated to the shape preferred orientation of magnetite which is similar to the overall petrofabric of the samples crystal size distributions of clinopyroxene and magnetite indicate that they formed during in situ crystallization and that magnetite probably appeared late in the crystallization and flow history of the magma analysis of preferred orientations of these grains suggests that the petrofabrics of the samples were acquired during flow with strongly interacting grains analogous to a logjam in a river the final characteristics of the ams and silicate petrofabric are the end points of a continuous crystallization and flow history in the magma rather than transient late features

the oldest zircons in the solar system abstract here we report the occurrence chemistry and u th pb isotopic systematics of three meteoritic zircon assemblages two from the vaca muerta mesosiderite and one from the simmern h5 chondrite one of the vaca muerta zircons occurs in the mesosiderite proper the other in a eucritic clast associated with chromite ilmenite and tridymite whereas the simmern zircon occurs in a chondrule composed predominantly of chromite and feldspar like terrestrial zircons the meteoritic zircons are enriched in the heavy rare earth elements but unlike terrestrial zircons they do not show a positive ce anomaly this feature is also absent in one lunar zircon analyzed and probably reflects the oxidation state of the formation environment under oxidising conditions ce4 which can substitute for zr4 in the zircon structure is stabilized whereas under relatively reducing conditions ce3 is stable the zircon from simmern has depletions in the relative abundances of tm and yb a characteristic of volatility fractionation of the ree the u concentrations of the two vaca muerta zircons are quite different at 1 and 50 ppm respectively both zircons have extremely low initial lead and give radiogenic ages that are the same albeit with relatively large errors on vm 1 zircon because of its much lower u concentration and hence radiogenic pb concentration the mean of the two analyses made on vm 2 zircon is concordant with a207pb 206pb age of 4563 15 ma 2 the simmern zircon has an exceptionally low u concentration of around 180 ppb and only a poorly constrained 207pb 206pb age of 4100 700 ma could be obtained

preliminary palaeomagnetic results of an archaean dolerite dyke of west greenland geomagnetic field intensity at 2 8 ga summary the geomagnetic field intensity during archaean times is evaluated from a palaeomagnetic and chronological study of a dolerite dyke intruded into the 3000 ma nuuk gneisses at nuuk 64 2 n 51 7 w west greenland plagioclase from the dolerite dyke yields a mean k ar age of 2752 ma palaeomagnetic directions after thermal demagnetization of the dyke and the gneiss reveal a positive baked contact test indicating that the high temperature component magnetization of the dyke is primary thellier experiments on 12 dyke specimens yield a palaeointensity value of 13 5 4 4 pt the virtual dipole moment at ca 2 8 ga is 1 9 0 6 x 10 am2 which is about onequarter of the present value the present study and other available data imply that the earth s magnetic field at 2 7 2 8 ga was characterized by a weak dipole moment and that a fairly strong geomagnetic field similar to the present intensity followed the weak field after ca 2 6 ga

81kr terrestrial ages and grouping of yamato eucrites based on noble gas and chemical compositions the ages and the groupings of yamato eucrites recovered in antarctica as well as terrestrial ages of nine yamato eucrites were determined based on cosmic ray produced noble gases including the radionuclide 81kr the terrestrial ages of y 792511 y 793164 and y 794043 are 0 14 0 11 and 0 28 ma respectively due to the relatively long storage time in ice the ce anomaly caused by the terrestrial alteration was significant in these three eucrites short terrestrial ages of 0 08 ma and long cosmic ray exposure ages of about 65 ma as well as similar noble gas and ree compositions for six eucrites y 75011 791826 793547 793548 793570 and 794002 strongly suggest that they belong to a single fall for these eucrites negative eu anomalies were found considering the data of yamato eucrites previously reported 1 fall s are required for monomict and 5 falls for polymict eucrites during the past 0 3 ma fissiogenic xe from 244pu was observed in nine eucrites in which the concentration of 244pu at the beginning of xe retention was calculated to be 0 5 2 2 ppb the 244pu nd weight ratio of 1 1 2 1 10 4 agrees with that reported for angra dos reis 244pu nd 1 5 10 4 within 40 244pu u ratios of y 793164 and y 793548 are 0 0027 0 0004 and 0 0045 0 0011 respectively assuming the u abundances occurred at 4 5 ga ago these ratios are somewhat lower than the ratio of 0 004 0 007 for the solar system

petrogenesis of mesosiderites i origin of mafic lithologies and comparison with basaltic achondrites abstract new petrologic and trace element data on basaltic and gabbroic clasts in mesosiderites and basaltic achondrites combined with existing petrologic and trace element data have served as a basis for interpretation of the petrogenesis of mesosiderite clasts compared with the basaltic achondrites the mesosiderite basaltic and gabbroic clasts contain more abundant modal tridymite and merrillite and more commonly contain augite as a late magmatic phase their pyroxenes tend to be more mgo rich and have lower fe mn ratios which are positively correlated with the fe mg ratio some of the mesosiderite basaltic and gabbroic clasts contain xenocrystic plagioclase the basaltic clasts commonly show super chondritic eu sm ratios and slight lree depletions the cumulate gabbro clasts are extremely depleted in lil have very high eu sm ratios and high lu sm ratios all of these features can be explained by a model in which the mafic lithologies of mesosiderites were formed by remelting of a mixed basalt cumulate gabbro metal source region near the parent body surface reducing agents p c s from the metallic fraction reduce feo out of the silicate melt and result in the positive correlation of fe mn with fe mg observed in pyroxene analyses and the modal abundances of tridymite merrillite and augite xenocrystic plagioclase is a natural consequence of remelting a cumulate gabbro rich source as are the unique ree patterns observed

paleointensity determination on the late precambrian tudor gabbro ontario paleointensity determination by the coe modified thellier method was carried out on the 1100 ma tudor gabbro of southern ontario the tudor has a bivectorial natural remanent magnetization nrm the c nrm is a grenvillian uplift thermoviscous remagnetization which overprints the a nrm the surviving primary thermoremanent magnetization trm of the tudor intrusion a sharp junction at 530 c between the a and c vectors in laboratory thermal demagnetization matches the peak metamorphic reheating temperature of 480 500 c in nature when the difference in time scales is taken into account the unblocking temperature distributions of the a nrm and of laboratory trm also match a is carried by single domain or small pseudo single domain grains probably magnetite rods exsolved in plagioclase or pyroxene at temperatures above 580 c arai plots of nrm versus partial trm are linear from 520 c or 530 c to 580 c for 45 specimens of 19 samples from 9 sites the 45 reliable paleointensity values from these specimens give a virtual axial dipole moment vadm of 4 6 0 8 1022 a m2 around 1100 ma this is about one half of the modern field intensity but is similar to the mean cretaceous cenozoic intensity and to most archean and proterozoic vadms keweenawan rocks whose ages are within 20 myr of the tudor gabbro have a vadm of 11 4 1022 a m2 the difference between the tudor and keweenawan vadms is similar to differences recorded over similar time intervals in the cretaceous and seems to represent normal variation in geomagnetic field strength throughout earth s history

effect of anisotropy on the paleomagnetic contact test for a grenville dike we report a detailed paleomagnetic contact test for a mafic dike and its tonalitic gneiss host rock in the precambrian grenville province of canada directions of natural remanent magnetization nrm in the country rock were corrected for assumed deflection by the strong magnetic anisotropy due to the rock fabric using the measured anisotropy of laboratory induced thermoremanent magnetization trm the trm ellipsoids have the same principal axes as the anisotropy of magnetic susceptibility ams ellipsoids but the anisotropy of remanence is more pronounced than the ams anisotropy correction moves nrm directions in the unbaked zone almost 20 bringing them close to typical grenvillian paleofield directions the anisotropy seems to be controlled by multidomain rather than single domain magnetic minerals nrm directions from the baked gneiss near the dike contact after anisotropy correction agree well with the nrm directions from the chilled margin of the early cambrian 570 3 ma dike this positive baked contact test is augmented by the presence of a hybrid zone in which country rock nrms are increasingly overprinted by the dike nrm as one moves from the unbaked toward the baked zone the dike nrm is therefore primary however the virtual geomagnetic pole vgp for the dike nrm 51 n 131 w dp 10 dm 12 n 24 samples or 41 n 153 w dp 13 dm 18 n 4 chilled margin samples does not fall near most established cambrian paleopoles post emplacement tectonic movement of the dike is unlikely it may be that the dike cooled and acquired trm during a geomagnetic field excursion because of its geologically short cooling blocking period of a few tens of years

u pb and sm nd ages for the stillwater complex and its associated sills and dikes beartooth mountains montana identification of a parent magma seven analyses from two samples of the lower banded series of the stillwater complex montana yielded a u pb zircon baddeleyite age of 2 705 plus minus 4 ma a more precise age of 2 704 plus minus 1 ma is obtained if the authors regress only five of seven analyses in either case the age is in very good agreement with the previously reported sm nd mineral isochron age of 2 701 plus minus 8 ma for a gabbro from the lower banded series noncumulate mafic sills and dikes that immediately underlie the stillwater complex are separated into five compositional groups u pb zircon ages for a group 1 gabbronorite 2 711 plus minus 1 ma and a group 3 mafic norite 2 703 plus minus 10 ma are comparable to previously reported u pb zircon ages for a group 1 diabase 2 713 plus minus 3 ma and a group 4 high ti norite 2 712 plus minus 3 ma although group 2 high mg gabbronorite and group 6 olivine gabbro have not yet yielded significant amounts of zircon baddeleyite or sphene mineral separates from group 2 yielded a four point pb pb internal isochron age of 2 704 plus minus 25 ma and separates from group 6 yielded a sm nd internal isochron age of 2 706 plus minus 64 ma indicating that these groupsmore are at least contemporaneous with intrusion of the stillwater complex these results indicate that groups 1 and 4 of the associated sills and dikes are perhaps slightly older than the complex and that groups 2 3 and 6 may be coeval with it supporting the idea that these three groups are the best candidates for a stillwater parent magma a conclusion previously presented on the basis of detailed mineralogic and geochemical evidence less

electron microscopy of iron oxides and implications for the origin of magnetizations and rock magnetic properties of banded series rocks of the stillwater complex montana the origins of multiple magnetizations of the archean stillwater complex have been investigated through scanning electron microscopy and scanning transmission electron microscopy observations of mineralogical relations using representative samples from nine sites in mafic banded series rocks on the basis of directional grouping and demagnetization behavior three magnetizations here labeled a b and c have been recognized the natural remanent magnetizaton nrm is lypically dominated by only one of these magnetizations and multicomponent behavior in individual specimens is rare the a remanence resides in magnetic grains of high median destructive fields and high discrete laboratory unblocking temperatures and is inferred to be a primary thermoremanent magnetization of circa 2 71 ga age the b and c magnetizations of lower median destructive fields and more distributed unblocking temperatures are inferred to be secondary and related to alteration including serpentinization involving limited moderate to low temperature fluid interaction perhaps in response to thermotectonic events e g mafic dike emplacement samples with nrm dominated by the a magnetization contain titanium free magnetite needles width 1 m and maximum dimension between about 2 and 50 m with preferred orientations in cumulus plagioclase crystals these oxides are interpreted to have formed during initial crystallization of cumulus phases in samples containing only b or c magnetizations such needles are absent in cumulus plagioclase whereas secondary fe bearing silicates particularly zoisite and chlorite are conspicuous the samples dominated by the b or c magnetizations contain anhedral low chromian magnetite fecr2 xfexo4 1 5 x 2 0 along fractures in both cumulus plagioclase and pyroxene and in zones of more pervasive alteration of pyroxenes the paragenesis of these oxides is consistent with an origin related to hydrothermal alteration hysteresis data show that plagioclase crystals containing magnetite needles have higher remanent coercivities hcr 60 to 100 mt whereas plagioclase crystals with anhedral chromian magnetite have remanent coercivities less than 60 mt consistent with their alternating field response because oxygen isotope data show no indication of appreciable alteration of host plagioclase we infer that the absence of magnetite needles and their associated a magnetization are caused by selective alteration of the magnetite needles to nonmagnetic phases in the remagnetized gabbros in this case rock magnetic and paleomagnetic data appear to be higher resolution indicators of fluid rock interaction than the stable isotope data anhedral chromian magnetite precipitated during hydrothermal alteration and therefore carries secondary magnetizations

40ar39ar laser probe dating of individual clinopyroxene inclusions in premier eclogitic diamonds abstract the ages of seven individual clinopyroxene inclusions in premier diamonds of eclogitic association have been determined using the 40 ar 39 ar dating technique syngenetic inclusions weighing between 10 and 130 g were exposed on cleaved surfaces of the diamonds and analysed using a laser probe the inclusion ages were found to be in the range 1111 35 to 1254 38 ma with an average of 1185 94 ma the ages obtained are in good agreement with previous determinations made on aggregates of eclogitic inclusions from premier diamonds and demonstrate the applicability of the laser probe to dating individual diamond inclusions

paleointensity during periods of rapid reversal a case study from the middle jurassic shamrock batholith western nevada paleointensity is one of the least deter mined parameters in geomagnetism yet this information has the potential to address many fundamental geophysical problems that bear on the evolution of the earth s core and lower mantle we test two important hy potheses that affect our understanding of how paleointensity has changed in earth s past 1 the mesozoic dipole low mdl hypoth esis and 2 the inverse relationship between geomagnetic polarity reversal rate and paleo intensity we report paleointensity results determined using both the coe modified thellier method and the izzi protocol on the medium grained hornblende biotite granite of the middle jurassic shamrock batholith in the southern singatse range western nevada usa previous zircon u pb dating gives an age of batholith emplacement at ca 165 8 0 4 ma this age estimate is coincident with a period of unusually high reversal frequency 10 reversals ma in the middle jurassic remanence in the shamrock batholith is held primarily by exsolved submicron magnetite inclusions in plagioclase the labora tory un blocking temperature range for the well de fined thermal remanent magnetization is nar row between 540 and 575 c and median destructive fields are between 40 and 50 mt thus we infer that the age of rema nence ac quisition is approximated by the u pb age estimate for pluton emplacement two sets of quality criteria were used to assess the paleo intensity results the first set yielded an 87 success rate while the second set using more stringent criteria resulted in only a 20 suc cess rate mean paleointensity values for the shamrock batholith using the loose and strict quality criteria are 34 0 6 4 t n 13 and 33 6 9 6 t n 3 respectively with errors reported at one standard deviation after correction for both remanence anisot ropy and cooling rate the mean values that pass loose and strict criteria are respec tively 17 9 2 7 t and 17 5 2 6 t the uncorrected and corrected paleointensity values for the strict estimates yield respec tive mean virtual dipole moments vdms of 62 7 18 zam 2 z 10 21 and 32 7 4 9 zam 2 the uncorrected value is within error of the vdms reported for plagioclase grains isolated from ca 160 ma basalts from ocean drilling program site 801 however these values have an absolute difference ranging from 6 to 20 zam 2 while the corrected vdm value is more consistent with the esti mated average jurassic field strength of 29 zam 2 using a recent estimate for the long term stable field strength of 42 zam 2 our corrected values add further credence to both the mdl and the inverse relationship between geomagnetic polarity reversal rate and dipole moment hypotheses

galactic cosmic ray produced 129xe and 131xe excesses in troilites of the cape york iron meteorite the flux of galactic cosmic rays gcr in the solar system appears to change with time based on the abundances in iron meteorites of cosmogenic nuclides of different half lives lavielle et al 1999 found that the gcr flux increased in recent times 100 ma by about 38 compared to average flux in the past 150 ma to 700 ma ago a promising technique for calibrating the gcr flux during the past 50 ma based on the 129i and 129xe pair of nuclides was discussed earlier marti 1986 murty and marti 1987 the 129i 129xen chronometer provides a shielding independent system as long as the exposure geometry remained fixed it is especially suitable for large iron meteorites te rich troilite because of the effects by the gcr secondary neutron component although gcr produced xe components were identified in troilites several issues require clarifications and improvements some are reported here we developed a procedure for achieving small xe extraction blanks which are required to measure indigenous xe in troilites the 129xe and 131xe excesses 129xen 131xen due to neutron reactions in te are correlated in a stepwise release run during the troilite decomposition our data show that indigenous xe in troilite of cape york has isotopic abundances consistent with ordinary chondritic xe oc xe in contrast to a terrestrial signature which was reported earlier two methods are discussed which assess and correct for an interfering radiogenic 129xer component from extinct 129i the corrected 129xen concentration in troilite d4 of cape york yields a cosmic ray exposure cre age of 82 7 ma consistent within uncertainties with reported data murty and marti 1987 marti et al 2004

39ar 40ar ages and thermal history of mesosiderites determinations of 39ar 40ar ages for plagioclase separates of three clasts from the vaca muerta and mount padbury mesosiderites give similar results ages increase monotonically by 0 2 0 3 ga when plotted against increasing 39ar release from the samples and give average values of 3 95 ga when combined with revised ages for mesosiderite analyses previously reported nineteen analyses of thirteen mesosiderites give an average ar ar age of 3 94 0 10 ga the average age at 90 39ar release is 4 13 0 14 ga and the ages at 10 39ar release range over 3 32 3 97 ga it seems unlikely that the 39ar 40ar ages were produced by direct impact heating but they probably are independent evidence for slow cooling deep within the mesosiderite parent body we conclude that the sloped age plateaus and the significant range in ar ar closure temperatures calculated for these mesosiderites 75 340 c are due to significant differences in ar diffusion properties among samples from the ar diffusion data we estimate an approximate mesosiderite cooling rate of 0 2 c ma which is similar to the preferred cooling rate of 0 1 0 5 c ma derived by others from ni diffusion and metal textures the 39ar 40ar ages seem incompatible with very slow cooling rates of 0 2 c ma unless the closure temperatures for ar diffusion and formation of metal cooling textures on the parent body substantially overlap any possible role of fragmentation of the mesosiderite parent body in producing the younger ar ar ages and the very slow cooling rates is unclear

the effect of remanence anisotropy on paleointensity estimates a case study from the archean stillwater complex paleomagnetism of archean rocks potentially provides information about the early development of the earth and of the geodynamo precambrian layered intrusive rocks are good candidates for paleomagnetic studies such complexes are commonly relatively unaltered and may contain some single domain magnetite armored by silicate mineral grains however layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected here we examine the magnetic anisotropy of an anorthosite sample with a well developed magmatic foliation the effect of the sample s remanence fabric on paleointensity determinations is significant paleointensities estimated by the method of thellier and thellier range from 17 to 55 t for specimens magnetized in a field of 25 t we describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the stillwater complex mt usa of 32 t adjusted for the effects of slow cooling

mineral deposits and earth evolution mineral deposits are not only primary sources of wealth generation but also act as windows through which to view the evolution and interrelationships of the earth system deposits formed throughout the last 3 8 billion years of the earth s history preserve key evidence with which to test fundamental questions about the evolution of the earth these include the nature of early magmatic and tectonic processes supercontinent reconstructions the state of the atmosphere and hydrosphere with time and the emergence and development of life the interlinking processes that form mineral deposits have always sat at the heart of the earth system and the potential for using deposits as tools to understand that evolving system over geological time is increasingly recognized this volume contains research aimed both at understanding the origins of mineral deposits and at using mineral deposits as tools to explore different long term earth processes

common asteroid break up events of eucrites diogenites and howardites and cosmic ray production rates for noble gases in achondrites we determined the cosmic ray produced noble gas abundances in eight diogenites four eucrites and four howardites using these and additional noble gas and chemical data from the literature composition and shielding dependent production rates of achondrites for 3he 21ne 38ar 78kr 83kr and126xe were derived based on 81krkr exposure ages the cosmogenic ratio 22ne 21ne is found to be a valid shielding indicator for all howardites eucrites and diogenites heds this ratio correlates with other shielding dependent ratios such as 78kr 83kr 80kr 83kr 124xe 131xe 126xe 131xe and 128xe 131xe we show that strong correlations for these ratios also exist for basaltic lunar rocks the new production rates were used for the calculation of exposure ages for all heds with known noble gas abundances two major exposure age clusters at 21 ma and 38 ma are observed in all three classes we find that five asteroid break up events characterized by exposure ages 6 1 ma 12 2 ma 21 4 ma 38 8 ma and 73 3 ma produced all diogenites fifteen out of seventeen howardites and twenty eight out of thirty nine eucrites we conclude that asteroidal or cometary impact events at such times on one or several parent bodies are responsible for the ejection of eucrites howardites and diogenites probable parent bodies are multiple 4 vesta derived kilometer sized objects of basaltic achondritic composition reaching the 3 1 resonance region a proposed source area for meteorites the direct ejection from vesta and capture by earth of basaltic meteorites has been shown to be dynamically difficult in such a scenario large events 38 ma and 21 ma ago are required liberating simultaneously material representing vesta s surface eucrites howardites and interior regions diogenites

compositional differences between basaltic and gabbroic clasts in mesosiderites two gabbroic pebbles and one pebble with basaltic textural affinities from the vaca muerta mesosiderite were analyzed by instrumental neutron activation analysis and electron microprobe these analyses plus literature data reveal remarkable compositional differences between basaltic and gabbroic clasts in mesosiderites on average basaltic clasts have small 15 depletions in light relative to heavy rare earth elements rees and moderate positive eu anomalies i e h chondrite normalizedla lu 0 85 eu sm 1 30 the meanla lu andeu sm abundance ratios of gabbroic clasts are at least 3 3 smaller and 28 greater respectively intense meteoroid bombardment probably persisted until after the mesosiderite parent body had differentiated near surface mixtures of major amounts of basalt and lesser amounts of cumulate eucritic rocks were essentially totally melted and rapidly cooled accounting for the moderately fractionated ree patterns of most basaltic clasts at greater depths impact heating partially melted cumulate gabbros incompatible element rich eutectic liquids drained away creating large ree fractionations in the residual rocks these rocks cooled slowly and developed gabbroic textures

the estherville mesosiderite upb rbsr and smnd isotopic study of a polymict breccia abstract a systematic u pb sm nd and rb sr isotopic study shows that the estherville mesosiderite was formed between 4 56 and 4 43 ga observed isotopic heterogeneity is in agreement with multiple generations of meteoritic impacts described in other mesosiderites at least part of the estherville silicate fraction was formed early in solar system history as indicated by the pb pb 4555 35 ma u pb 4560 31 ma rb sr 4542 203 ma and sm nd 4533 94 ma ages mesosiderites therefore present not only petrological but also geochronological similarities with eucrites the pb isotopic composition of the metal phase plots on the same isochron as the silicates indicating formation and subsequent mixing with silicates early in the history of the solar system this is consistent with previous observations indicating that iron was reduced during the silicate magmatic stage most likely a consequence of mixing with metal in addition to these more ancient portions of the estherville breccia other parts were formed later as suggested by the pb pb 4422 50 ma and u pb 4437 11 ma ages observed in a second group of leaches and residues this age is similar to some cumulate eucrite ages and may represent the formation of a second mesosiderite component the sm nd and the rb sr ages obtained on estherville show large errors that may be a consequence of the mixing between the 4 56 and 4 43 ga endmembers estherville like most mesosiderites was affected by a major heating event around 3 5 3 7 ga as shown by the ar ar ages bogard et al 1990 this heating event partially disturbed the rb sr isotopic system rb sr metamorphic ages range between 3 81 and 4 08 ga

the record of cosmogenic radiogenic fissiogenic and trapped noble gases in recently recovered chinese and other chondrites abstract we performed a comprehensive study of the noble gas isotopic abundances in thirty six chondrites including twenty seven chondrites recovered in china the large data base allows us to recognize some new characteristics of the nuclear record in chondritic matter the comparison of the trapped noble gas release pattern for ordinary and carbonaceous chondrites shows that the planetary trapped noble gases in ordinary chondrites are released mainly above 1200 c whereas 85 of these gases in carbonaceous chondrites are degassed at 200 c there exists a clear correlation of the fraction of trapped xe released at 1200 c and petrologic type of chondrites it thus appears that the carrier phases of the trapped noble gases in ordinary and in carbonaceous chondrites may not be the same the ngawi ll3 chondrite is solar gas rich the solar gases in meteorites and lunar surface material are mixtures of solar wind sw and solar energetic particles sep we show that the variations of the 3 he 4 he and 20 ne 22 ne ratios for solar gas rich meteorites and lunar surface material could be either due to preferential diffusive losses of the lighter isotopes of he and ne or due to a change of the sw sep flux ratio with time furthermore we demonstrate that the 81kr concentration is a function of the shielding depth of a sample within the meteoroid deviations from this depth dependency curve are observed for jilin h5 lishui l5 suizhou l6 and dongtai ll6 a complex exposure history for jilin is thus confirmed and is possible for the other three chondrites cosmic ray exposure ages are calculated based on six different nuclides 3he 21ne 38ar 83kr 126xe and 81kr kr in most cases good agreement is observed for the ages derived from the different methods quality classes are assigned to the exposure ages and the age distributions are discussed in some meteorites we observe effects induced by secondary cosmicray produced neutrons epithermal neutron fluxes jn 30 300 ev and fast neutron fluxes jn 5 mev are derived based on the reactions 79br n 80kr and 24mg n 21ne respectively and preatmospheric masses of the meteoroids are estimated we show that the ratio j n 30 300 ev j n 5 mev increases with increasing preatmospheric mass we introduce a 3he exposure age 21ne exposure age vs 4he gas retention age 40ar gas retention age diagram that is a powerful tool for distinguishing different thermal histories of meteoroids finally in some chondrites we observe xe produced by 244pu fission and calculate the time span between fission xe retention in these chondrites and that of the angra dos reis achondrite we find that the ordinary chondrites started to retain fission xe 48 30 ma earlier than angra dos reis we do not observe systematic differences between h l and ll or type 5 and 6 chondrites with respect to the time of fission xe retention

terrestrial 81kr kr ages of antarctic meteorites the production rate of 38ar in meteorites p 38 has been determined as a function of the sample s chemical composition from 81kr kr exposure ages of four eucrite falls the cosmogenic 78kr 83kr ratio is used to estimate the shielding dependence of p 38 from the true 38ar exposure ages and the apparent 81kr kr exposure ages of nine antarctic eucrite finds terrestrial ages are calculated they range from about 3 105a pecora escarpment 82502 to very recent falls thiel mountains 82502 polymict eucrites from the allan hills a78132 a79017 and a81009 have within the limits of error the same exposure age 15 2 106a and the same terrestrial age 1 1 105a this is taken as strong evidence that these meteorites are fragments of the same fall a similar case are the elephant moraine polymict eucrites a79005 a79006 and 82600 with an exposure age of 26 106a and a terrestrial age of 1 8 105a eeta79004 may be different from this group because its exposure age and terrestrial age are 21 106a and 2 5 105a respectively the distribution of terrestrial ages of allan hills meteorites is discussed meteorites from this blue ice field have two sources directly deposited falls and meteorites transported to the allan hills inside the moving antarctic ice sheet during the surface residence time meteorites decompose due to weathering processes the weathering half life is about 1 6 105a from the different age distributions of allan hills and yamato meteorites it is concluded that meteorite concentrations of different antarctic ice fields need different explanations

39ar 40ar dating of mesosiderites evidence for major parent body disruption 4 ga ago 39ar 40ar ages were measured on 17 samples of 14 different mesosiderites and indicate major degassing of ar by one or more thermal events less than 4 0 ga ago three samples gave release patterns of constant age whereas the other samples typically showed 200 400 ma increases in 39ar 40ar age with increasing gas extraction temperature these stepwise releases indicate ages of 3 4 3 8 ga 1 ga 109 a averaged ages for individual meteorites are 3 45 3 82 ga no substantial evidence exists in the temperature releases for 39ar 40ar ages older than 4 0 ga and the ar ages appear to be in contradiction to 244pu fission track ages previously reported for a few mesosiderites the ar ages probably date the event that caused disturbance in previously reported rb sr sm nd and pb pb ages of estherville and other mesosiderites data on release of ar as a function of extraction temperature were used in arrhenius plots to estimate activation energies for ar diffusion of 18 52 kcal mol these values were then used to calculate closure temperatures against ar loss of 2 340 c and 180 600 c for assumed cooling rates of 1 c ma and 1 c a respectively the ar data suggest that for certain assumed models of mesosiderite formation the 39ar 40ar ages may date the time of formation of those metal textures in mesosiderites which have led others to calculate cooling rates of 1 c ma models which call for metal silicate mixing 4 5 ga ago followed by cooling at 1 c ma from temperatures 1150 c seem unlikely single event models which produce the silicate metal mixture at high temperatures rapidly cool the objects to temperatures of 500 c followed by cooling of 1 c ma and are not consistent with the ar data unless the thermal event occurred 500 c and left them deeply buried to undergo slow cooling

intensity of the earth s magnetic field in late archean obtained from diabase dikes of the slave province canada abstract the geomagnetic field intensity in late archean times is evaluated from archean diabase dikes intruded into the yellowknife greenstone belt of the slave province n w t canada the dike set used is referred to as the 8a dikes and their age is bracketed between 2620 and 2642 ma by crosscutting relationships between the 8a dikes and adjacent granitic rocks dated by u pb method paleomagnetic directional data after thermal demagnetization thd and results of some field tests suggest that the characteristic remanent magnetizations chrm of the dikes have been acquired at ca 2 6 ga several rock magnetic studies indicate that the dike samples are suitable for paleointensity determinations thelliers method with consistency checks was used for the paleointensity determinations and two of the 8a dikes yielded the mean paleointensities of 30 9 1 1 and 43 9 1 2 t respectively the corresponding virtual dipole moments vdm of 6 3 0 2 1022 a m2 and 9 0 0 2 1022 a m2 are similar to the present day value suggesting that in the earth s core the dynamo process of comparable activity to that of the present day has already existed at ca 2 6 ga

mineral and fluid inclusions in diamonds from the urals placers russia evidence for solid molecular n2 and hydrocarbons in fluid inclusions abstract the compositions of mineral inclusions from a representative collection more than 140 samples of diamonds from the placer deposits in the ural mountains were studied to examine their compositional diversity the overwhelming majority of rounded octahedral and dodecahedral stones typical of placers contain eclogitic e type mineral inclusions up to 80 represented by garnets with mg 40 75 and ca 10 56 including the unique high calcic grospydite composition omphacitic pyroxenes containing up to 65 of jadeite as well as kyanite coesite sulfides and rutile peridotitic p type inclusions are represented by olivine subcalcic cr pyrope chrome diopside enstatite and magnesiochromite that are typical for diamonds worldwide comparing the chemical composition of olivine pyrope and magnesiochromite in diamonds of the urals north east of the siberian platform placers and arkhangelsk province kimberlites show striking similarity there are significant differences only in the variations of carbon isotopic composition of the diamonds from the placers of the urals and north east of the siberian platform one typical rounded dodecahedral diamond was found to contain abundant primary oriented submicrometer sized

ar 40 ar 39 laser probe dating of diamond inclusions from the premier kimberlite inclusions encapsulated by diamonds at the time of their formation provide a means for determining diamond crystallization ages and the chemistry of the surrounding upper mantle at that time sm nd studies of peridotitic inclusions from cretaceous age kimberlites in southern africa suggest that the diamonds formed 3 3 gyr ago1 by contrast eclogite suite inclusions generally yield younger ages sometimes approaching the time of kimberlite erup tion2 4 here we report the results of 40ar 39ar laser probe analy ses of individual eclogitic clinopyroxene inclusions from premier diamonds which yield a mean age of 1 198 14 myr this age agrees well with sm nd ref 2 and 40ar 39ar ref 3 analyses on similar premier inclusions and is indistinguishable from the inferred time of emplacement of the host kimberlite 1 150 1 230 myr 2 which implies that diamond formation was essentially synchronous with kimberlite generation the extrapolated non radiogenic 40 ar 36 ar ratio of 334 102 is similar to the present day atmospheric composition this value is inconsistent with sr and nd isotopic signatures from premier eclogite inclusions2 which suggest a depleted mantle source 40ar 36ar 20 000 pre entrap ment equilibration of the inclusions with an 36ar rich fluid is the most probable explanation for the low non radiogenic 40ar 36ar composition

luminescence of diamonds from metamorphic rocks this study explores the effects of metamorphism on luminescence of diamond four diamond suites extracted from metamorphic rocks were characterized using optical cathodoluminescence ftir spectroscopy photoluminescence at the liquid nitrogen temperature and cathodoluminescence spectroscopy the studied diamonds are from sedimentary conglomerate and lamprophyric breccia metamorphosed to the greenschist facies wawa canada and from the ultrahigh pressure uhp terranes of kokchetav kazakhstan and erzgebirge germany the majority of studied diamonds that experienced metamorphism have distinct cathodoluminescence and photoluminescence when compared to diamonds from unmetamorphosed rocks i e kimberlites or placers diamonds in metamorphic rocks do not show the prevalent blue cathodoluminescence emittance at 415 440 and 480 490 nm typical of common kimberlite and placer diamonds ultrahigh pressure diamonds are mostly non luminescent whereas diamonds from greenschists display green yellow orange and red cathodoluminescent colors emitting light at 520 and 576 nm and between 586 and 664 nm the shift of cathodoluminescence in octahedrally grown type ia diamonds metamorphosed in the greenschist facies results from the presence of optical centers with zero phonon lines at 575 and 637 nm detected via photoluminescence spectroscopy the centers may have formed due to irradiation of diamonds in the upper crust followed by annealing at t 500 c this process creates vacancies that diffuse through the diamond lattice and get trapped at n sites creating nv o and nv optical centers the distinct luminescence of metamorphosed diamonds can persist for billions of years in the crust but is annealed in the mantle

differentiation history of the mesosiderite parent body constraints from trace elements and manganese chromium isotope systematics in vaca muerta silicate clasts abstract we report here the results of a study of trace element microdistributions and 53mn 53cr systematics in several basaltic and orthopyroxenitic clasts from the vaca muerta mesosiderite ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that following igneous crystallization these clasts underwent extensive metamorphic equilibration that resulted in intra and inter grain redistribution of elements there is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents such as phosphorus derived from the mesosiderite metal at the time of metal silicate mixing furthermore our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal silicate mixing event after taking into account the effects of these postmagmatic secondary processes comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites respectively suggests that the primary igneous petrogenesis including parent magma and source compositions of vaca muerta silicates were similar to those of achondritic meteorites of the howardite eucrite diogenite hed association internal 53mn 53cr isochrons obtained for two basaltic pebble 16 and 4679 and two orthopyroxenitic 4659 and 4670 clasts show that chromium isotopes are equilibrated within each clast nevertheless just as for noncumulate eucrites and diogenites 53cr excesses in whole rock samples of the basaltic clasts 1 01 e in pebble 16 1 07 e in 4679 are significantly higher than in the orthopyroxene rich clasts 0 62 e in 4659 0 53 e in 4670 as in the case of the hed parent body this suggests that mn cr fractionation in the parent body of the vaca muerta silicate clasts occurred very early in the history of the solar system when 53mn was still extant however the slope of the 53mn 53cr isochron defined by the whole rock samples of vaca muerta clasts corresponding to a 53mn 55mn ratio of 3 3 0 6 10 6 is distinctly lower than that defined by the hed whole rock samples corresponding to a 53mn 55mn ratio of 4 7 0 5 10 6 indicating that the global mn cr fractionation event that established mantle source reservoirs on the parent body of the vaca muerta silicate clasts occurred 2 ma after a similar event on the hed parent body

trace element composition and u pb age of zircons from estherville constraints on the timing of the metal silicate mixing event on the mesosiderite parent body abstract mesosiderites are a group of stony iron meteorites which are thought to be the result of mixing of silicates with fe ni metal in this study we combined textural observations with geochemical and chronological studies of two zircon grains found in the estherville mesosiderite one of the zircons zrc1 occurs with pyroxene plagioclase troilite and silica and the other zrc2 is located at a boundary between fe ni metal and a silicate part mainly composed of pyroxene and plagioclase the textural observations demonstrate that zrc1 is relatively homogenous whereas zrc2 is composed of at least two chemically distinct domains trace element analyses of zrc2 resolve large concentration gradients within this single grain with variations that are an order of magnitude for rare earth elements ree and two orders of magnitude for u and th the lowest trace element concentration in zrc2 is more than an order of magnitude lower than those of lunar and eucritic zircons however it is similar to those of zrc1 and a zircon from the vaca muerta mesosiderite the calculated ree composition of the melt in equilibrium with zrc2 shows that zrc2 and perhaps also zrc1 did not crystallize from a melt that was produced by fractional crystallization of the primary magmatic mineral assemblages the zircons with low ree u and th concentrations can be interpreted to have formed in a residual melt after incorporation of large amounts of ree u and th into secondary phosphate minerals which formed during the metal silicate mixing event the large concentration gradients observed in zrc2 suggest significant heterogeneities in the melt from which the zircon crystallized alternatively either mixing or diffusion between a relict zircon and a newly formed zircon could explain the observed concentration gradients however the ree patterns of zrc2 cannot be explained by mixing or diffusion between the two distinct generations of zircons these considerations suggest that zrc1 and zrc2 formed during a high temperature reheating event which is probably related to the metal silicate mixing event the weighted average 207 pb 206 pb age obtained by sims from both zircons is 4521 26 ma 2 this age is younger than that of a primary magmatic zircon from vaca muerta 4563 15 ma and probably corresponds to the timing of the metal silicate mixing event or a later impact event

origin of anorthosite by textural coarsening quantitative measurements of a natural sequence of textural development the textures of plagioclase crystals within large olivine oikocrysts preserve the sequence of the formation of anorthosite such textures have been quantified in a troctolite anorthosite from the proterozoic lac st jean anorthosite complex canada crystal size distributions csds indicate that initially plagioclase nucleated and grew in an introduction environment of linearly increasing undercooling producing a straight the textures of plutonic rocks are one of their most line csd during this phase latent heat of crystallization was striking aspects but they have been little used to elucidate largely removed by circulation of magma through the porous crystal petrological processes as compared with chemical and mush by about 25 solidification the crystallinity was such that isotopic methods where used discussion has tended it reduced but did not eliminate the circulation of magma resulting to be qualitative in contrast to quantitative chemical in the retention of more latent heat within the crystal pile the methods hence reducing the weight of this approach temperature rose until it was buvered by the solution of plagioclase indeed it is not possible to verify physical models if close to the liquidus temperature of plagioclase nucleation of quantitative data are not available quantitative textural plagioclase was inhibited and conditions were suitable for textural studies of volcanic rocks are becoming more common coarsening of both plagioclase and olivine to occur ostwald and these methods are applied here to the development ripening in this process small crystals were resorbed whereas of anorthosite larger crystals grew from both material recycled by the resorption of quantitative textural studies of plutonic rocks really crystals smaller than the critical size and new material brought in started with jackson s studies of the stillwater complex by the circulating magmatic fluid the results of this process resemble in 1961 jackson 1961 he measured crystal size disthose of high temperature metasomatism but there is no necessity for tributions but did not record the area that he measured a magmatic fluid with a diverent composition the shapes of the hence his data cannot be completely compared with plagioclase csds fit better the communicating neighbours equation of modern data his pioneering work was not really followed textural coarsening rather than the classic lifshitz slyozov wagner up and since then most studies have looked only at equation as do other examples drawn from the literature if olivine average or typical crystal sizes these data are much started to nucleate at a higher temperature than plagioclase then less revealing of petrological processes than crystal size during the textural coarsening phase olivine would have been more distributions partly because they only give one parameter undercooled than plagioclase and would have had a higher maximum for each sample the few existing studies of crystal size growth rate in these conditions olivine would coarsen more rapidly distributions in plutonic rocks have not treated than plagioclase and engulf it hence the order of crystallization plagioclase determined from the textures would be the reverse of the order of one problem especially pertinent to quantitative texfirst nucleation of the two phases from equilibrium phase diagrams tural studies is the way that many igneous petrologists maintenance of the temperature near the plagioclase liquidus may look at rocks they commonly try to reconstruct how a rock has formed by examining the final fully crystallized also inhibit the nucleation and growth of other phases

noble gases 81kr kr exposure ages and 244pu xe ages of six eucrites bereba binda camel donga juvinas millbillillie and stannern noble gases including radioactive 81kr t1 2 2 1 105 yr have been measured for the cumulate eucrite binda and five noncumulate eucrites bereba camel donga juvinas millbillillie and stannern some of which have been repeatedly analyzed concentrations of 81kr which range from 4 2 10 14 to 3 2 10 13 cm3 stp g roughly correlate with abundances of the main target elements sr y and zr 81kr kr cosmic ray exposure ages are obtained as 24 5 0 6 ma 21 0 1 6 ma 36 6 1 4 ma 10 6 0 8 ma 20 8 0 5 ma and 35 1 0 7 ma for bereba binda camel donga juvinas millbillillie and stannern respectively all the measured eucrites have 244pu derived fission xe on the basis of fission 136xe concentrations 244pu abundances are calculated to range from 0 40 ppb for binda to 1 08 ppb for juvinas following the method proposed by shukolyukov and begemann 1996a 244pu xe ages relative to the absolute crystallization age of 4 5578 ga for angra dos reis are determined as 46 18 ma bereba 29 34 ma binda 51 16 ma camel donga 10 23 ma juvinas 8 24 ma fine grained portion of millbillillie 51 21 ma coarse grained portion of millbillillie and 124 13 ma stannern which correspond to absolute ages ranging from 4 434 ga stannern to 4 566 ga fine grained portion of millbillillie millbillillie is a mixture of materials with different 244pu xe ages stannern s unusually young 244pu xe age might reflect a secondary shock disturbance but otherwise it tends to confirm a distinctive mode of igneous petrogenesis in comparison to most other eucrites the 244pu xe ages of the cumulate eucrites obtained so far binda this work moore county shukolyukov and begemann 1996a do not differ systematically from those of noncumulate eucrites the four eucrites with the older 244pu xe ages bereba binda juvinas and the fine grained portion of millbillillie show significant amounts of radiogenic 129xe 0 6 2 3 10 12 cm3 stp g after correction for fission 129xe which imply earlier retention of radiogenic 129xe from extinct 129i than other eucrites with young 244pu xe ages

81kr kr age and multiple cosmic ray exposure history of the vaca muerta mesosiderite abstract noble gas isotopic compositions were measured for a eucritic pebble and bulk material of a silicate metal mixture from the vaca muerta mesosiderite as well as pyroxene and plagioclase separated from the eucritic pebble by total melting and stepwise heating methods trapped noble gases were degassed completely by a high temperature thermal event probably at the formation of the vaca muerta parent body vmpb the presence of fissiogenic xe isotopes from extinct 244pu in the bulk samples might be a result of rapid cooling from an early high temperature metamorphism high concentrations of cosmogenic noble gases enabled us to determine precise isotopic ratios of cosmogenic kr and xe spallogenic ne from na and unique ar isotopic compositions were observed the 81kr kr exposure age of 168 8 myr for the silicate pebble is distinctly longer than the age of 139 8 myr for the bulk samples the precursor of the pebble had been irradiated on the surface of the vmpb for more than 60 myr first stage irradiation with subsequent incorporation into bulk materials approximately 4 gyr ago the vaca muerta meteorite was excavated from the vmpb 140 myr ago second stage irradiation relative diffusion rates among the cosmogenic ar kr and xe based on data obtained by stepwise heating indicate that kr and xe can be partially retained in pyroxene and plagioclase under the condition that resets the k ar system this result supports the presence of fission xe and of excess concentration of cosmogenic kr which could have survived the thermal event approximately 3 8 gyr ago

riphean rifting and major palaeoproterozoic crustal boundaries in the basement of the east european craton geology and geophysics abstract the east european craton which occupies the northeastern half of the european continent is characterized by the presence of giant even transcratonic systems of riphean meso to neoproterozoic rifts aulacogens the largest of these riphean rift systems tend to follow the course of palaeoproterozoic sutures and junction zones that subdivide the craton into three crustal segments with autonomous development histories these segments named fennoscandia volgo uralia and sarmatia are marked by distinct t magsat anomalies genetically the intersegment junction zones represent different types of collisional and accretional interaction between the component crustal segments of the craton in accordance with this the riphean rift systems vary in width the presence or absence of moho uplifts and other properties like for instance heat flow values however all of them follow belts of weakly magnetized crust studded with local magnetic maxima due to the presence of mafic rocks associated with the rifting the rift system intersegment junction zone relationships in consequence similarly represent a range of variation and cannot be referred to one single model

classification of mafic clasts from mesosiderites implications for endogenous igneous processes we have analyzed thirteen igneous pebbles from the vaca muerta eet87500 and bondoc mesosiderites by electron microprobe and instrumental neutron activation and combined these data with literature data for forty three analyzed mesosiderite clasts we classify these well characterized clasts into the following five principal groups 1 1 polygenic and monogenic cumulates 39 are coarse grained gabbros that are highly depleted in incompatible elements relative to h chondrites they formed at moderate depth either as residues of low degree partial melting of pre existing cumulate eucrites or as cumulates from parent melts similar to cumulate eucrites 2 2 polygenic basalts 30 are finer grained rocks with positive europium anomalies la lu ratios 1 and lower rare earth element abundances than basaltic eucrites it seems likely that these rocks were formed near their parent body surface by remelting mixtures of major amounts of basaltic eucrites and lesser amounts of cumulate eucrites 3 3 quench textured rocks comprise two compositional groups a those which resemble basaltic eucrites 5 and b those which resemble cumulate eucrites 2 the quench textured rocks are probably monogenic they formed most likely when small scale impacts at their parent body surface totally melted small amounts of basaltic or cumulate eucrite material 4 4 monogenic basalts 11 resemble basaltic eucrites and formed by endogenous igneous processes on the mesosiderite parent body mpb the monogenic basalts and those quench textured rocks that resemble basaltic eucrites in composition plot subparallel to the nuevo laredo trend eucrites on diagrams of incompatible element concentration vs mg previous work has shown that the nuevo laredo trend eucrites were probably produced by fractional crystallization it seems likely that the monogenic basalts from the mpb had a similar origin 5 5 ultramafic rocks are cumulates consisting mainly of large crystals of orthopyroxene 9 or olivine 4 orthopyroxenite clasts closely resemble diogenites and were formed most likely by endogenous igneous processes olivine crystals vary significantly in major element composition some may be endogenous but others may have been derived from the disrupted mantle of a different parent body

a view into the cretaceous geomagnetic field from analysis of gabbros and submarine glasses the nature of the geomagnetic field during the cretaceous normal polarity superchron cns has been a matter of debate for several decades numerical geodynamo simulations predict higher intensities but comparable variability during times of few reversals than times with frequent reversals published geomagnetic paleointensity data from the cns are highly scattered suggesting that additional studies are required here we present new paleointensity results from 18 sites collected from the lower oceanic crust of the troodos ophiolite cyprus 92 1 ma old together with recently published data from the troodos upper crust we obtain three independent paleointensity time series these sequences reveal quasi cyclic variations of intensities about a mean value of 54 20 z am 2 providing insight into the fluctuating nature of the cretaceous magnetic field our data suggest the cns field was both weaker and more variable than predicted by geodynamo simulations the large amplitudes of these variations may explain the wide range of dipole moments previously determined from the cns 2007 elsevier b v all rights reserved

extensional basins of the former soviet union structure basin formation mechanisms and subsidence history abstract we review the structure and evolution of a number of riphean phanerozoic rifts and extensional basins within the territory of the former soviet union fsu horst and graben formation in strong crustal and subcrustal lithosphere layers can explain the multi trough character of rift systems observed in the russian platform the vilyuy rift the west siberian rift system the pechora kolva rift system and the laptev sea rift many features in the evolution of these rifted basins are incompatible with predictions of classical stretching models basin subsidence often occurs in the absence of any noticeable stretching and over time scales much longer than predicted by models of thermal subsidence other observations include a time gap between rifting and the onset of post rift basin subsidence of tens to hundreds ma and a correlation in timing of subsidence phases of rifted basins and platforms with opening and closure events of adjacent ocean basins these observations point to an important role for mechanisms such as ecologite formation within or beneath the lithosphere as well as intraplate compression and stress induced lithospheric deflection

evidence of fluid inclusions in metamorphic microdiamonds from the kokchetav massif northern kazakhstan abstract microdiamonds from garnet clinopyroxenites of the kokchetav massif northern kazakhstan and associated alluvial diamonds have been investigated using infrared spectroscopy all diamonds have high nitrogen contents from 747 up to 4488 20 at ppm and are of type ib iaa nitrogen data are consistent with a diamond formation over a narrow temperature range microdiamonds from garnet clinopyroxenites are characterized by water and carbonate inclusions similar to those ocurring in fibrous coated diamonds from kimberlite which suggest that diamonds grew from a c h o fluid in contrast alluvial microdiamonds do not contain any carbonate inclusions and h2o is absent or present in only minor amounts alluvial diamonds are considerably more n rich but have a similar range of nitrogen aggregation states it is suggested that alluvial diamonds belong to a separate diamond population compared with the microdiamonds from garnet clinopyroxenites the kokchetav microdiamonds are distinct from kimberlitic fibrous diamonds in that they contain abundant ib centres have high nitrogen contents and show different populations depending on host rock type these characteristics support a metamorphic origin for the microdiamonds from the uhpm rocks of the kokchetav massif

effects of internal mineral structures on the magnetic remanence of silicate hosted titanomagnetite inclusions an electron holography study 1 titanomagnetite inclusions in pyroxene and plagioclase are carriers of stable magnetic remanence in some slowly cooled rocks such as gabbros anorthosites granulites and diorites needle shaped inclusions with average dimensions of 1 1 25 m form epitaxially by exsolution from their host silicate close examination of clinopyroxene hosted inclusions reveals an internal microstructure which consists of magnetite fe3o4 prisms and ulvospinel fe2tio4 lamellae that formed as a result of phase unmixing during initial cooling this internal structure exerts a profound influence on the magnetic remanence properties of each inclusion primarily by transforming it from a multidomain grain into an assemblage of magnetostatically interacting single domain prisms here we use off axis electron holography to image the magnetization states of individual prisms and the magnetostatic interactions between them we show that the inclusions exhibit both single domain and collective magnetic states that depend primarily on the shape anisotropies of individual magnetite prisms magnetostatic interactions between closely spaced prism stacks and the shape anisotropy of the needle itself prisms that are separated by thick ulvospinel lamellae show uniformly magnetized and or vortex states in contrast closely spaced magnetite prisms behave as multipart vortices or as long composite columns whose strong net magnetization may not be related directly to the orientation and shape of either the needle or the constituent prisms the overall remanence direction recorded by clinopyroxene crystals containing finely exsolved inclusions is a reflection of both the inclusions elongation directions and the prism arrangements within them

magnetic anisotropy in the trenton limestone results of a new technique anisotropy of anhysteretic susceptibility a new method for determining magnetic anisotropy using anhysteric remanence susceptibility is described the magnetic fabric of a collection of trenton limestone specimens has been determined using this method as well as by conventional anisotropy of magnetic susceptibility the results demonstrate the usefulness of the new method for finding the magnetic fabric of rock units such as the trenton in which the bulk magnetic susceptibilities are low a model is proposed to explain the observed foliated and lineated fabric as a consequence of overburden compaction and regional horizontal stresses the original fabric is inferred to have been isotropic the anisotropy resides in secondary magnetite of late paleozoic age it is argued that the observed magnetic fabric must therefore be alleghenian or younger in age our method has the potential to determine paleostress directions in carbonates elsewhere provided our assumptions are correct

popigai crater origin and distribution of diamond bearing impactites about three decades ago a formerly unknown genetic type of natural diamonds was discovered in impact craters impact diamonds are currently known from a number of impact structures in europe asia and north america and it s likely that the number of finds will increase with time the popigai crater northern siberia where impact diamonds were first found was specifically investigated in terms of geology geophysics petrography and mineralogy large resources of industrial impact diamonds were discovered and these minerals were studied in detail authigenic impact diamonds occur in situ in shocked graphite bearing gneisses that are found as inclusions in impact melt rocks tagamites and suevites according to the observed transformation of coexisting minerals the lower estimated pressure of the coherent martensite transition of graphite to diamonds is 35 gpa impact diamonds inherit the original shape of graphite crystals and are composed of a polycrystalline structured aggregate of cubic and probably hexagonal carbon microcrystals 1 5 m across numerous properties of diamonds depend on the high density of defects in the crystal lattice allothigenic impact diamonds occur in rocks produced by the homogenization and solidification of impact melt which originated from the complete fusion of graphite bearing precursor gneisses these diamonds usually reflect the influence of the hot melt and are strongly corroded diamond bearing tagamites and suevites in the popigai crater interior occur as extended lens and sheets bodies and also as irregular small bodies diamond distribution depends on the original abundance of precursor graphite in the target rocks on the superimposed shock metamorphic zonation and on the character of the ejection of shocked and melted material along different trajectories and azimuths this has resulted in radial and concentric inhomogeneities in diamond distribution in the crater interior on a second order the distribution depends on the scale of melt contamination by clasts and fragments and by the duration of cooling of certain melt bodies and their constituents enrichment in diamonds at the margins of thick tagamite sheets is the result of rapid cooling which prevents combustion of diamonds a positive correlation between diamond content and the amount of phosphorus pentoxide in impactites indicates links to c and p probably in organic matter of the primary sedimentary rocks which were subjected to granulitic metamorphism 2 4 ga ago and melted at the time of impact 35 7 ma ago

the transport and sorting of diamonds by fluvial and marine processes this paper describes the systematic variations in size and quality of diamonds that result from transport by fluvial and marine processes it is shown that diamonds originating in rather restricted source areas can be spread across many tens of thousands of square kilometers downstream or alongshore transport can be followed for several hundred kilometers with a systematic reduction in average diamond size accompanying such transport the diamond size distribution is progressively modified with increasing travel distance and the quality of the diamonds improves as inferior types are destroyed the accompanying variations in crystal form are outlined certain implications for exploration programs are mentioned

a theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals summary susceptibility thermo remanent magnetization trm and isothermal remanent magnetization irm anisotropy ellipsoids have been determined for several rock samples the results indicate that the ellipsoid of initial susceptibility is less anisotropic than the trm and low field irm ellipsoids which are found experimentally to be of identical shape this suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non destructive low field irm measurements such irm measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured the results for a series of artificial anisotropic samples containing magnetite particles of different sizes in the range 0 2 90 m were very similar to those for the rocks in contrast a comparison of the susceptibility and irm ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory such results imply that susceptibility and irm ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single domain particles magnetization confined to the easy axis or whether the particles are essentially multidomain

diverse eucritic pebbles in the vaca muerta mesosiderite abstract seven 5 cm basaltic pebbles from the vaca muerta mesosiderite were analyzed by instrumental neutron activation analysis and electron microprobe three additional pebbles were only examined petrographically two pebbles are impact melt breccias four are similar texturally to main group eucrites or some clasts in polymict eucrites three are similar to cumulate eucrites and one is a fragmental breccia similar to certain polymict eucrites cumulate pebbles have extremely low ree concentrations and high eu sm ratios reflecting the virtual absence of intercumulus liquid siderophile interelement ratios in the pebbles are similar to those in vaca muerta metal except for anomalously low ir higher silica and merrillite in the pebbles relative to eucrites may reflect reduction of feo in the silicates by p from mesosiderite metal pebbles were annealed following incorporation into the vaca muerta host impacts were the dominant heat source the occurrence of 20 impact melt breccias among the pebbles and 35 40 melt breccias among mesosiderite whole rocks indicate that mesosiderites were more extensively impact melted than howardites we suggest three alternative models to account for this 1 mesosiderites were derived from a separate parent body that experienced a higher meteoroid fluence than the basaltic achondrite parent body 2 the low velocity accretion of large metallic core fragments from disrupted differentiated asteroids led to metal enrichment of a few regions of the regolith and heat retention in these mesosideritic regions and 3 mesosideritic regolith regions were less likely to be reduced to fine particles by extensive shock than howarditic regions

compaction of igneous cumulates part ii compaction and the development of igneous foliations quantitative petrofabric analysis of 66 samples from olivine bearing zones iii and iv of the middle banded series of the stillwater complex montana was conducted to allow direct comparison among textural and chemical variations systematic variations in foliations with stratigraphic height are observed in both zones statistical analysis of the orientations of inequant grains was used to calculate an alignment factor af corresponding to the extent of the development of the foliation both petrofabric and petrographic evidence indicates that the foliation is best explained by compaction of the crystal pile comparison of afs with estimates of the amount of compaction based on whole rock compositions shows approximate agreement variations in mole fraction cl of interstitial apatite mimic the observed trends in the af indicating that local concentrations of fluids exsolved from interstitial liquid were important in the development of strong foliations we conclude that 1 differing amounts of co

re os dating of sulphide inclusions zonally distributed in single yakutian diamonds evidence for multiple episodes of proterozoic formation and protracted timescales of diamond growth the timing of diamond formation in the siberian lithospheric mantle was investigated by re os isotope dating of sulphide inclusions from eclogitic and lherzolitic diamonds from the mir 23rd party congress and udachnaya kimberlite pipes in yakutia the diamonds contained multiple sulphide inclusions distributed over their core to rim zones cathodoluminescence carbon isotope and nitrogen aggregation studies demonstrate that the diamonds are zoned and that the distinct zones are associated with different diamond growth episodes there are coherent relationships between carbon isotope composition nitrogen concentration and aggregation state of the diamond hosts and major and trace element compositions re os compositions and initial os isotope ratios of the included sulphides this suggests that the different diamond and sulphide populations formed at different times from fluids melts with different chemical compositions based on the re os isochron ages and the nitrogen aggregation states we conclude that the sulphides are co genetic with their diamond hosts the ages obtained for the different diamond populations demonstrate two major periods of yakutian diamond formation for both eclogitic and lherzolitic suites i e 2 1 1 8 ga mir 23rd party congress and udachnaya and 1 0 0 9 ga mir and 23rd party congress these two episodes correspond to major tectono magmatic events collision between the different terranes of the siberian craton during the formation of the palaeoproterozoic supercontinent 2 0 1 8 ga and accretion leading to formation of the supercontinent rodinia 1 1 ga importantly re os isotope dating demonstrates that some single diamonds have protracted timescales of growth with different ages associated with core and rim zones zoned single eclogitic crystals from mir and 23rd party congress in the current study crystallised over intervals that were irresolvable 200 ma up to 1 ga apart the very radiogenic initial os isotope ratios of the eclogitic and lherzolitic sulphides 187os 188os 0 14 2 22 imply incorporation of radiogenic os from subducted oceanic lithosphere a conclusion that is also supported by the anomalously light carbon isotope compositions of some diamonds subducted lithosphere is therefore considered a significant component in proterozoic diamond formation beneath the siberian craton

the origin of laminated and massive anorthosite sept iles layered intrusion quebec canada well laminated plagioclase rich rocks anorthosite occur as layers in mineralogically similar but massive rocks in the sept iles intrusion several phases of crystallization of poikilitic clinopyroxene have preserved the intermediate stages in the production of these rocks plagioclase nucleated and initially grew in random orientations in the stationary part of the boundary layer there in the absence of other events crystallization produced the massive anorthosite the laminated anorthosite may have been produced by episodic increases in the velocity of the magma adjacent to the boundarylayer induced by magmatic density currents such currents would have caused the zone of simple shear of the boundary layer to migrate outwards into the previously stationary part this simple shear would then have rotated the existing plagioclase crystals as well as crystals that nucleated at that time towards the shear plane some rocks show evidence of several periods of enhanced flow velocities the production of the lamination may have weakened the crystal mush and enabled slumping to occur producing folds and contorted layers finally compaction liberated large quantities of intercumulus magma of which some was trapped as granophyric segregations and some escaped by overturing and churning the crystal pile earlier formed granophyric segregations and laminated layers were disrupted during this process to produce a range of more complex rocks including igneous breccias

multiple growth events processes and fluid sources involved in diamond genesis a micro analytical study of sulphide bearing diamonds from finsch mine rsa abstract twenty one sulphide inclusion bearing diamonds from the finsch mine south africa were analysed for nitrogen abundances and carbon isotope compositions by microbeam methods on the basis of sulphide ni contents one diamond is of peridotitic affinity the rest belongs to the eclogitic suite ftir analyses show nitrogen abundances and aggregation states from 21 to 1093 at ppm and 0 to 83 iab statistically indistinguishable from previous results for finsch eclogitic silicate inclusion bearing diamonds appleyard et al 2004 but significantly higher than observed before for diamonds of the peridotitic suite deines et al 1989 detailed analyses revealed marked variations in nitrogen characteristics within individual diamonds demonstrating a complex mantle residence consistent with multiple episodes of diamond growth over time linked to the growth stratigraphy of the diamond sims micro analyses show variations in 13 c from 8 90 to 2 80 with a mean value of 5 54 1 80 1 standard deviation closely overlapping the typical worldwide value the c isotopic variability within individual diamonds ranges up to 3 26 sims based nitrogen abundances are 3 2221 at ppm with heterogeneous distribution within individual diamond from the 13 c n co variations within individual diamonds three major processes of diamond growth for sulphide inclusion bearing samples at finsch are proposed 1 some diamonds were precipitated during a single event of open system isotopic fractionation in fluids that varied from oxidised carbonatitic to reduced ch 4 rich in this growth scenario nitrogen is either compatible or incompatible during diamond growth 2 other diamonds show abrupt 13 c n changes indicative of diamond growth involving mixing of several fluid sources 3 some diamonds grow from a combination of the two previous processes the models are consistent with metasomatic diamond growth involving single and multiple fluid sources multiple growth stages for individual diamonds may have taken place over extended time periods and which have important implications for diamond dating studies

a new approach for geomagnetic archaeointensity research insights on ancient metallurgy in the southern levant we present results from an archaeointensity investigation based on a relatively unexploited recording medium copper slag deposits together with a recently improved experimental design for the archaeointensity experiment we demonstrate the applicability of this medium as well as other archaeometallurgical artifacts for the study of the ancient geomagnetic field intensity in addition to archaeointensity data from well dated archaeological contexts we obtained reliable archaeointensity results from poorly dated or contentious archaeometallurgical sites in the southern levant these results shed new light on the dating of these sites among them the copper smelting installation of timna 39b a site that has important implications for the beginning of metallurgy during the fifth millennium bce the paper also aims to introduce archaeointensity research to the archaeologist scholar and to encourage further collaboration between the disciplines in future research

40ar 39ar analyses of clinopyroxene inclusions in african diamonds implications for source ages of detrital diamonds clinopyroxene inclusions in diamond contain elevated potassium contents and can potentially be dated by 40ar 39ar techniques previous 40ar 39ar studies of clinopyroxene inclusions contained in cleaved diamonds have suggested that argon produced from the decay of potassium prior to eruption of the host kimberlite magma diffuses to the diamond clinopyroxene interface under mantle conditions after intrusion and cooling below the closure temperature for argon diffusion radiogenic argon is retained by the clinopyroxene inclusions this behaviour complicates efforts to date diamond crystallisation events however extraction of inclusions from their host diamond should induce loss of all interface argon thus raising the possibility of determining kimberlite emplacement ages this possibility has important implications for constraining the source localities of detrital diamond deposits worldwide with concomitant benefits to diamond exploration to investigate this premise 40ar 39ar laser probe results are presented for single clinopyroxene inclusions extracted from a total of fifteen gem quality diamonds from the mbuji mayi kimberlite in the democratic republic of congo and the jwaneng and orapa kimberlites in botswana initial fusion analyses of clinopyroxene inclusions from mbuji mayi diamonds yielded ages older than the time of host kimberlite intrusion indicating partial retention of extraneous argon by the clinopyroxene inclusions themselves step heating analyses of clinopyroxene inclusions from orapa and jwaneng diamonds produced older apparent ages from lower temperature steps and the rim fragment of one orapa inclusion high temperature fusion analyses yielded younger apparent ages commonly approaching the times of host kimberlite eruption total gas integrated 40ar 39ar ages are mostly intermediate between the times of inferred diamond crystallisation and kimberlite eruption ca k ratios for each sample are uniform across step heating increments indicating that age variations are not due to compositional mineralogical or alteration effects the favoured explanation for these results is partial retention of extraneous argon in primary and or secondary fluid inclusions this component is then preferentially outgassed in lower temperature heating steps yielding older apparent ages the partial retention of extraneous argon by clinopyroxene inclusions clearly restricts efforts to determine source ages for detrital diamond deposits results from individual samples must necessarily be interpreted as maximum source emplacement ages nonetheless step heating analyses of several clinopyroxene inclusions from a detrital diamond deposit may provide reasonable constraints on the ages of source kimberlites lamproites however minor age populations as well as those closely spaced in time may be difficult to resolve it is argued that the majority of older 40ar 39ar ages can be explained in terms of the partial retention of inherited argon produced between the times of diamond crystallisation and kimberlite eruption although the presence of excess argon in some clinopyroxene inclusions cannot be excluded available evidence e g no excess argon in premier eclogitic inclusions or potassium poor inclusions suggests that this is not a factor for most samples three possible mechanistic models are forwarded to account for the uptake of inherited excess argon in fluid inclusions the first envisages negligible interface porosity and diffusion of extraneous argon exclusively to primary fluid inclusions which subsequently partially decrepitated during eruption causing accumulation of argon at the diamond clinopyroxene interface the second model permits diffusive loss of extraneous argon to both the interface region and primary fluid inclusions the third involves diffusion of extraneous argon to the interface region with later entrapment of some interface argon in secondary fluid inclusions produced by fracture annealing processes active during eruption the first model can account for all 40ar 39ar results whereas the latter two mechanisms require the presence of an excess argon component to explain older integrated ages up to 2 9 ga from two jwaneng samples excess argon contamination would compromise efforts to determine diamond genesis ages using the 40ar 39ar dating technique however if the first model is valid then the older 40ar 39ar integrated ages support previous re os age results for the crystallisation of jwaneng diamonds

magmatic processes of unzen volcano revealed by excess argon distribution in zero age plagioclase phenocrysts abstract excess argon 40 ar is one of the most important problems in k ar geochronometry because it renders incorrect the simple assumption that all initial argon in rock samples is derived from the atmosphere using the laser microprobe analytical method we investigate and reveal the source and trapping site of excess argon in zero age plagioclase phenocrysts extracted from the lavas erupted during 1990 1995 activity of the unzen volcano kyushu japan by combining the results of laser microprobe analysis and conventional stepwise heating of separated plagioclase phenocrysts we reveal that the majority of the excess argon is retained in melt inclusions in the dusty zone of the phenocrysts we propose a model for the isotopic evolution of argon in the phenocryst bearing magma of the unzen volcano by taking into consideration the noble gas data from unzen volcanic gases and the chemical composition of the melt inclusions in the phenocrysts in our model plagioclase phenocrysts crystallize from or are captured by a low temperature t rhyodacitic magma this magma has a comparatively high 40 ar 36 ar ratio for an arc magma due to assimilation of old cretaceous granitic wall rock mantle derived argon is replenished in the rhyodacitic magma over the life of the volcano when the magma chamber is recharged at intervals by high t juvenile magma the plagioclase phenocrysts crystallize rapidly trapping excess argon 40 ar 36 ar 970 during successive dissolution crystallization events caused by intrusion of the high t magma or water rich fluids the phenocryst bearing mixed magma ascends at a relatively rapid rate following a final injection of the high t magma and interacts with shallow groundwater causing the 40 ar 36 ar ratio of the magma to decrease to the atmospheric ratio contamination of the phenocrysts by atmospheric argon in the surrounding magma is enhanced by rapid short circuit diffusion in which dislocations and or micro cracks in the plagioclase crystals are used for argon transport the remnant magma has been continuously degassing for a decade since the recent 1990 1995 eruptive activity without significant change to 3 he 4 he and 40 ar 36 ar ratios

characteristics of diamondiferous plio pleistocene littoral deposits within the palaeo orange river mouth namibia abstract the orange river the principal conduit transporting diamonds from hinterland sources to the namibian coast in post cretaceous times is characterised by an extreme wave dominated delta that has given rise to a progression of coarse rudaceous littoral deposits preserved onshore for 150 km north of the mouth under the long lived prevailing vigorous wave wind and northward longshore drift regimes the orange river outfall has been reworked into amongst others a series of economically viable diamondiferous plio pleistocene onshore gravel beach deposits these placers comprise spits and barrier beaches in the proximal reach within the palaeo orange river mouth that after ca 5 km northwards merge into extensive but narrow linear beaches that in turn after ca 70 km give way to pocket beaches gravel and diamond size decreases northwards away from the ancestral orange river mouth the linear and pocket beach types have considerably higher diamond content but lower average diamond stone size than the two proximal units that are characterised by low diamond grade but comparatively large average diamond size given the risk of delineating low grade alluvial diamond deposits accurately we present here sedimentological reconstructions of the subtidal intertidal and supra tidal facies that constitute the spit and barrier beach sequences based largely on face mapping of exploration trenches and open cast mine cuts as well as the results of large tonnage sampling campaigns diamond distribution is also linked convincingly to basic littoral processes that were operational within the palaeo orange river mouth during the complex transgression that gave rise to the 30 m package in plio pleistocene times in both the spit and barrier beach settings the intertidal deposits prove to be the most promising targets whereas the subtidal sediments are the least economic the constant raking associated with coarse cobble boulder sized gravel foreshore deposits in an energetic micro tidal wave regime increased the average diamond stone size in the intertidal deposits to 1 to 2 carats per stone cts stn but the lack of fixed trapsites no competent footwall within the palaeo orange river mouth at that level prohibited the accumulation of substantially enriched diamondiferous gravels consequently grades of only 1 5 to 6 carats per 100 tons cpht are realised the highest grades 2 to 6 cpht are found in the landward facing intertidal beach deposits on the spits where gentle reworking in that sheltered environment had somewhat enriched and preserved the diamond content significantly the low average stone size of ca 0 5 cts stn in this lower energy setting probably reflects that of the general diamond population available at that time in contrast the sand rich subtidal deposits in the spit sequence return the lowest grades 0 1 to 0 5 cpht similar to those in the slightly younger subtidal transgressive boulder lags of the barrier beaches however the stone size in the spit subtidal sediments is also low 0 1 to 0 5 cts stn due to the highly mobile fine grained character of those deposits whereas that in the subtidal transgressive lag is large 2 to 3 cts stn as a result of the local semi permanent turbulence associated with the boulder sized clasts in these gravel sheets diamond distribution is therefore also influenced by littoral facies and associated beach types in addition to the spatial and temporal parameters that have already been documented for the onshore marine placers of the southern namibian coast

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

trm in low magnetic fields a minimum field that can be recorded by large multidomain grains abstract thermally acquired remanent magnetization is important for the estimation of the past magnetic field present at the time of cooling rocks that cool slowly commonly contain magnetic grains of millimeter scale this study investigated 1 mm sized magnetic minerals of iron iron nickel magnetite and hematite and concluded that the thermoremanent magnetization trm acquired by these grains did not accurately record the ambient magnetic fields less than 1 t instead the trm of these grains fluctuated around a constant value consequently the magnetic grain ability to record the ambient field accurately is reduced above the critical field trm acquisition is governed by an empirical law and is proportional to saturation magnetization m s the efficiency of trm is inversely proportional to the mineral s saturation magnetization m s and is related to the number of domains in the magnetic grains the absolute field for which we have an onset of trm sensitivity is inversely proportional to the size of the magnetic grain these results have implications for previous reports of random directions in meteorites during alternating field demagnetization or thermal demagnetization of trm extraterrestrial magnetic fields in our solar system are weaker than the geomagnetic field by several orders of magnitude extraterrestrial rocks commonly contain large iron based magnetic minerals as a common part of their composition and therefore ignoring this behavior of multidomain grains can result in erroneous paleofield estimates

evolutionary history of the mesosiderite asteroid a chronologic and petrologic synthesis abstract recent petrologic chemical and isotopic analyses of mesosiderite clasts and whole rocks allow reconstruction of the evolutionary history of the mesosiderite parent body mpb chronologic measurements indicate that initial melting of the mpb occurred shortly after accretion 4 56 ga ago at about the same time that the eucrites and angrites formed significant crustal remelting 4 47 ga ago probably associated with metal silicate mixing is responsible for forming abundant gabbroic clasts with extreme positive eu anomalies localized impact melting 4 5 3 9 ga ago produced quench textured clasts and matrices and may be responsible for causing mesosiderite metamorphism as well as the extensive melting of type 4 mesosiderites collisional disruption and gravitational reassembly 3 9 ga ago is indicated by thorough ar degassing at this time metallographic cooling rates of 1 c ma were likely initiated by this event the history of higher temperature events measured by thermoluminescence was annealed out impact excavation and ejection of deeply buried breccias occurred 3 9 ga ago the 10 160 ma cosmic ray exposure ages of mesosiderites represent the epoch when individual mesosiderites were excavated from the mpb or from a 10 m size fragment it is unlikely that mesosiderites and hed howardite eucrite diogenite samples come from the same parent body there is no evidence for metal silicate mixing with attendant redox effects or major crustal remelting on the hed body bulk howardites have flat chondrite normalized rare earth element ree patterns whereas many bulk mesosiderites have ree patterns with positive eu anomalies and relative enrichments in light ree the nature of the heating mechanism which caused the initial differentiation of mesosiderites and howardites is problematic

trace elements in sulfide inclusions from yakutian diamonds sulfide inclusions in diamonds may provide the only pristine samples of mantle sulfides and they carry important information on the distribution and abundances of chalcophile elements in the deep lithosphere trace element abundances were measured by proton microprobe in 50 sulfide inclusions sdi from yakutian diamonds about half of these were measured in situ in polished plates of diamonds providing information on the spatial distribution of compositional variations many of the diamonds were identified as peridotitic or eclogitic from the nature of coexisting silicate or oxide inclusions known peridotitic diamonds contain sdis with ni contents of 22 36 consistent with equilibration between olivine monosulfide solid solution mss and sulfide melt whereas sdis in eclogitic diamonds contain 0 12 ni a group of diamonds without silicate or oxide inclusions has sdis with 11 18 ni and may be derived from pyroxenitic parageneses eclogitic sdis have lower ni cu and te than peridotitic sdis the ranges of the two parageneses overlap for se as and mo the mo and se contents range up to 700 and 300 ppm respectively the highest levels are found in peridotitic diamonds among the in situ sdis significant zn and pb levels are found in those connected by cracks to diamond surfaces and these elements reflect interaction with kimberlitic melt significant levels of ru 30 1300 ppm and rh 10 170 ppm are found in many peridotitic sdis sdis in one diamond with wustite and olivine inclusions and complex internal structures have high levels of other platinum group elements pges as well and high chondrite normalized ir pd comparison with experimental data on element partitioning between crystals of monosulfide solid solution mss and sulfide melts suggests that most of the inclusions in both parageneses were trapped as mss while some high cu sdis with high pd rh may represent fractionated sulfide melts spatial variations of sdi composition within single diamonds are consistent with growth histories shown by cathodoluminescence images in which several stages of growth and resorption have occurred within magmatic environments that evolved during diamond formation

archean emplacement of eclogitic components into the lithospheric mantle during formation of the kaapvaal craton re os data for selected whole rock eclogites and eclogitic sulfide inclusions in diamonds from seven kimberlites on the kaapvaal zimbabwe cratons yield ages around 3 ga the presence of such old eclogitic components in the subcontinental lithospheric mantle is more common than previously thought overlap of these ages with those of many cratonic peridotites demonstrates that incorporation of eclogitic materials accompanied or closely followed cratonic lithosphere stabilization many of these eclogitic components have fractionated c n o and s isotopic compositions which could have been created at low temperatures implying that some style of archean subduction involving emplacement of oceanic lithosphere was part of craton keel development

40ar 39ar laser probe studies of clinopyroxene inclusions in eclogitic diamonds abstract 40 ar 39 ar laser probe ages are reported for individual clinopyroxene inclusions in thirty two eclogitic diamonds from four localities jwaneng and orapa both in botswana udachnaya siberia and argyle australia ages obtained from cleaved diamonds from jwaneng 244 ma orapa 102 ma and udachnaya 425 ma are similar to those of their host kimberlites however there is considerable variation in age between individual laser extractions obtained from the same inclusion which we suspect is mainly due to partial retention of radiogenic argon that diffused to the inclusion diamond interface while the diamond was in the mantle some of this argon was lost when the diamonds were cleaved as part of the experimental procedure the presence of an ambient mantle 40 ar component trapped in inclusions may be an additional cause of scatter in ages however this cannot be validated with the present data in support of the diffusive loss process the results from an udachnaya inclusion completely enclosed within host diamond with the argon being released by laser drilling yielded an age of 1149 37 ma which is considerably older than the host kimberlite about 340 400 ma likewise the scatter in 40 ar k ratios for eclogitic diamonds from jwaneng and orapa indicates that they too are probably xenocrysts therefore a combination of sample preparation technique and laser drilling can be used to offer a useful extension of the original method enabling both diamond formation from buried inclusions and host rock emplacement ages from cleaved inclusions to be determined argyle clinopyroxenes similarly cleaved to the diamond surface also show a considerable range of inclusion ages 1159 1540 ma only in this case there is a relationship between age and k ca ratio of the inclusions such a relationship is difficult to reconcile in terms of partial loss of 40 ar and instead indicates that there was a protracted period of diamond growth

a lead isotopic study of the stillwater complex montana constraints on crustal contamination and source regions analyses of the pb isotopic compositions of plagioclase from 23 samples covering the stratigraphic thickness of the stillwater complex indicate a narrow range of apparent initial isotopic compositions 206pb 204pb 13 95 207pb 204pb 14 95 15 01 208pb 204pb 33 6 the uniformity of our data is in contrast to but not necessarily contradictory to other recent investigations which give indications that the complex formed by repeated injection of magmas with at least two distinct compositions that were presumably derived from different source regions samples from the basal series of the complex have consistently higher 207pb 204pb ratios suggesting either minor contamination from adjacent country rocks or a slight distinction between parental magmas apparent initial pb isotopic compositions of the complex are very radiogenic compared to late archean model mantle values but are nearly identical to initial pb isotopic compositions found for the the adjacent slightly older 2 73 2 79 ga late archean crustal suite in the beartooth mountains contamination of magmas parental to the stillwater complex by the late archean crustal suite is rejected for two reasons 1 th and u concentrations in stillwater rocks and plagioclase are very low about 0 08 and 0 02 ppm respectively yet th u ratios are uniform at about 4 in contrast to the highly variable 2 26 but often high th u ratios found for the late archean crustal complex 2 it seems improbable that any contamination process would have adjusted the isotopic compositions of the diverse magmas entering the stillwater chamber to near identical values the preferred hypothesis to explain the pb isotopic data for the stillwater complex and the associated late archean crustal suite involves a major late archean crust forming event that resulted in a compositionally complex crust mantle system with relatively homogeneous and unusual pb isotopic compositions the parental magmas of the stillwater complex were generated at different levels within this crust mantle system before isotopic contrasts could develop by radioactive decay within compositionally discrete reservoirs this situation limits the utility of all isotopic tracer systems in discriminating among the various mantle and crustal reservoirs that may have affected the final isotopic character of the stillwater magmas the late archean crustal complex and the stillwater complex melts were ultimately derived from the same distinct mantle without obvious direct interaction with the middle to early archean crust present in the region

provenance studies from 40ar 39ar dating of mineral inclusions in diamonds methodological tests on the orapa kimberlite botswana abstract previous studies indicate that clinopyroxene inclusions extracted from their host diamonds give 40ar 39ar ages approaching the time of source kimberlite eruption this behaviour is attributed to diffusion of argon to the diamond inclusion interface region during mantle residence with subsequent loss of this component on cleaving of the diamond to release the inclusion s in this study we investigate the potential of the 40ar 39ar inclusion dating method as a niche provenance tool as diamondiferous kimberlites are uncommon igneous rocks largely restricted to archaean cratons and have limited age ranges provenance studies of detrital diamond occurrences should provide unique information on craton erosional histories palaeo drainage evolution and related basin development as a methodological test 40ar 39ar step heating and in vacuo crushing experiments were conducted on 50 eclogitic clinopyroxene inclusions extracted from diamonds from the 93 ma orapa kimberlite in botswana low temperature steps and crushing experiments produced anomalously old ages up to 2 98 0 10 ga consistent with partial retention of pre eruption inherited argon in sub micron defect sites high temperature fusion steps yielded significantly younger ages although only 35 are within error of the time of orapa kimberlite eruption however 77 of results are within 50 ma and 92 within 100 ma of the orapa eruption age these results mandate that individual fusion ages be treated as maximum estimates of source kimberlite eruption ages with analyses of several 6 inclusions required to date single age detrital populations for multi source diamond deposits only the youngest detrital population or populations separated by 100 ma will be resolvable despite these limitations we conclude that the 40ar 39ar inclusion dating method is still capable of resolving key provenance questions such as the source of detrital diamond deposits in southern africa where controversy exists as to whether the diamonds originated from proximal permo carboniferous glacial deposits or distal cretaceous jurassic kimberlites finally a model is presented to account for the variable retention of pre eruption argon in clinopyroxene inclusions extracted from their host diamonds the proportion of this component retained by inclusions is considered dependent on interface porosity volume partitioning of argon into defect structures and the timing of stress fracture development around inclusions during entrainment to surface

new evidence for extraordinarily rapid change of the geomagnetic field during a reversal palaeomagnetic results from lava flows recording a geomagnetic polarity reversal at steens mountain oregon suggest the occurrence of brief episodes of astonishingly rapid field change of six degrees per day the evidence is large systematic variations in the direction of remanent magnetization as a function of the temperature of thermal demagnetization and of vertical position within a single flow which are most simply explained by the hypothesis that the field was changing direction as the flow cooled