iron magnesium order disorder in an orthopyroxene crystal from the johnstown meteorite abstract equilibrium reversals of fe 2 mg distribution between the m1 and m2 sites of an orthopyroxene from the johnstown meteorite were achieved at several temperatures between 700 and 1000 c one single crystal was used for the whole thermal treatment and for collecting all the x ray data after quenching the intracrystalline ion exchange for the bulk chemical composition mg 1 453 fe 0 441 cr 0 024 ca 0 054 mn 0 015 fe 0 005 ti 0 003 al 0 005 si 1 960 al 0 040 o 6 is given by ln k d 3027 39 t k 0 872 0 013 where k d is the distribution coefficient for the reaction fe m2 2 mg m1 mg m2 fe m1 2 a transmission electron microscopy tem study of part of the crystal showed the presence of very thin augite lamellae and guinier preston zones indicating a relatively rapid cooling of the host rock at temperatures close to 1000 c the new temperature scale yields a relatively high quenching temperature of 379 8 c for the pyroxene which appears consistent with a rapid cooling estimated few degrees per hundred years of a magmatic cumulate excavated by an impact on its parental body

cation ordering of orthopyroxenes from the skaergaard intrusion implications for the subsolidus cooling rates and permeabilities we have determined the quenched cation ordering states of five orthopyroxene crystals collected from the marginal border group and the lower zone a and b of the skaergaard intrusion and modeled these data to retrieve their closure temperatures tc of fe mg ordering and cooling rates according to existing thermal models for the skaergaard pluton conductive cooling dominated the high and low temperature regimes which were separated by an intermediate temperature regime in which the cooling was controlled primarily by convective fluid circulation the cooling rates retrieved from the quenched cation ordering states of the orthopyroxene crystals strictly apply to temperatures around the closure temperatures of the ordering states 340 400 c which fall at the transition from convective to the lower temperature conductive cooling the cooling rates obtained from the cation ordering states of orthopyroxene vary from 1 to 270 k ka these results agree well with a thermal model calculated using an assumed average permeability of 10 12 cm2 for the pluton but not completely with a model calculated on the basis of an average permeability of 10 13 cm2 although both values produced shifts of 18o that are comparable to those observed in the pluton

inversion of combined gravity and bathymetry data for crustal structure a prescription for downward continuation abstract we propose a new and improved recipe for inverting for crustal thickness variations from combined gravity and bathymetry data the new ingredients with respect to previous studies are the posing of this problem as an inverse problem and the use of a downward continuation filter which can be tailored to find the crustal thickness solution which satisfies a minimum slope or maximal smoothness criteria or a weighted combination of these desired solution features we explore several suggestions for determining the optimum smoothing parameters for a given crustal thickness inversion then we compare this new method to that used previously in marine gravity studies we find it to be an improved recipe for dealing with gravity data with observation errors of 3 5 mgal but not by much this recipe will be most useful in extracting more geological information from marine gravity data where apparent observation errors are 1 mgal e g from future global positioning satellite navigated studies

antarctic fro90011 lodranite cooling history from pyroxene crystal chemistry and microstructure the antarctic fro90011 lodranite shows an achondritic coarse granoblastic texture with an approximately chondritic composition the crystal chemistry of pbca and c2 c pyroxene single crystals and the tem texture of pbca pyroxene were investigated to reconstruct the evolutionary history of the meteorite parent body the structural behaviour of orthopyroxene en85fs12 4wo2 6 is well constrained in the en fs trend orthopyroxene fe2 mg ordering in the m1 and m2 sites yields a closure temperature tc of 570 20 c and a cooling rate 10 c day near the tc clinopyroxene is a low iron augite en51fs5wo44 clinopyroxene fe2 mg ordering in the m1 and m2 sites yields a tc of 513 50 c which is compatible with the cooling rate derived from the orthopyroxene crystals tem investigation on orthopyroxene showed a few thin lamellae not larger than 20a parallel to 100 and rare one unit cell guinier preston zones tem eds analysis did not reveal significant chemical differences between lamella rich and lamella free zones the above data are interpreted as being due to fast cooling resulting from radiative heat loss of small fragments ejected from a parent body which were subsequently reassembled into a larger body

anomalous seismic crustal structure of oceanic fracture zones summary the seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the north atlantic the crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3 anomalous crust is present in both large and small offset fracture zones since they are among the most common tectonic features in the ocean basins and are particularly closely spaced on slow spreading ridges fracture zones represent a major source of seismic crustal heterogeneity we interpret the anomalous crust as a thin intensely fractured faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that in places are extensively serpentinized the unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness culminating in the very thin crust of the fracture zones at either end of the ridge segment

on the accuracy of marine gravity measurements we have assessed the accuracy of lamont doherty geological observatory s global marine gravity data bank by examining the crossover errors coes at intersecting ship tracks more than 63 000 coes were found having a standard deviation of 22 43 mgal mgal 10 5 m s 2 we use the coes to find and remove linear drifts and dc shifts present in the data set this adjustment reduces the standard deviation to 13 96 mgal coes generally decrease with latitude which we attribute to uncertainties in the eotvos correction high coes occur in areas of high gravity gradients these two features point to poor navigation as the principal source of error in marine gravity surveys coes have generally been decreasing during the last two decades reflecting improvements in instrumentation and quality of navigation a comparison of the shipboard gravity data to seasat derived gravity revealed a 9 mgal bias in the terrestrial data which probably reflects uncertainties in choice of reference field the adjusted data set was used to generate a gravimetric geoid for the nw atlantic ocean by removing this geoid from the seasat sea surface heights a residual geoid was obtained a special feature of this map is an elongate ene trend that appears to correlate with the edge of the gulf stream

single grain u th he ages from phosphates in acapulco meteorite and implications for thermal history the cooling history of the acapulco meteorite for 400 c is well established using various chronometers suggesting extremely fast cooling 1000 c ma in contrast the thermal history for low temperatures 400 c is poorly understood because of large uncertainties in the chronometers applicable to this temperature range to better constrain the cooling history for the low temperature range we applied u th he dating techniques to individual phosphate grains one whitlockite and 11 apatite grains yielded u th he ages ranging from 1272 22 1 analytical error only ma to 4584 51 ma with tight clustering at 4 55 ga the weighted mean of the five oldest ages 4538 32 ma 1 uncertainty including systematic error is suggested to be the minimum age representing primary cooling of the acapulco body passing through 120 c although it is impossible to precisely quantify the effects of energetic particle ejection from the outermost 20 m of the phosphates petrographic evidence suggests that most dated samples are fragments likely derived from the interior of larger grains thus greatly reducing this source of error indeed the five oldest samples cannot have suffered substantial ejection since the uncorrected ages are identical with the crystallization age of the acapulco meteorite the new u th he data suggest rapid cooling of acapulco down to 120 c this evidence suggests that the younger 40 ar 39 ar age 4507 9 1 ma obtained from acapulco plagioclase which should reflect cooling through 300 c is spuriously young due to systematic errors i e decay constants and or standard data in the 40 ar 39 ar method as suggested by comparison between high precision 40 ar 39 ar and u pb ages for terrestrial volcanic rocks the scattered he age distribution 4 0 ga implies very heterogeneous thermal disturbances after the primary cooling of the body

horizontal extent of anomalously thin crust near the vema fracture zone from the three dimensional analysis of gravity anomalies recent seismic refraction and reflection experiments have demonstrated that parts of oceanic fracture zones are characterized by very thin crust with total crustal thickness less than half that of normal oceanic crust the horizontal extent of this anomalous crust is not well known to map the lateral distribution of thin crust in a major fracture zone we have constructed free air anomaly bathymetry and sediment isopach maps of a 220 by 280 km region surrounding the western intersection of the vema fracture zone with the mid allantic ridge we isolate those components of the gravity field that are associated with variations in crustal thickness by first subtracting the attraction of the water crust interface the sediment layer and the crust mantle interface assuming constant crustal thickness and density these calculations are performed using a fully three dimensional model the remaining mantle bouguer anomaly is dominated by the effects of the cooling lithosphere variations in density of the crust and upper mantle associated with the cooling of the plate with increasing age of the seafloor are predicted for a thermal model that incorporates advection and the three dimensional conduction of heat the attraction of this three dimensional density structure is subtracted from the mantle bouguer anomaly to generate a map of the residual gravity anomaly which should directly reflect variations in crustal thickness or density the residual gravity anomaly map indicates that there is thin crust along much of the length of the vema fracture zone thin crust is concentrated primarily beneath the walls of the fracture zone rather than beneath the center of the fracture zone valley an interpretation confirmed by seismic refraction data the degree of thinning along the length of the fracture zone is variable ranging from one section with no appreciable thinning to others in which the crustal thickness is less than 3 km the excess depth of the fracture zone compared to surrounding seafloor is not isostatically compensated by local thinning of the crust other major topographic features such as the median valley and the elevated transverse ridge along the south wall of the fracture zone are also not locally compensated by changes in crustal thickness

isostatic compensation of tectonic features of the mid atlantic ridge 25 27 30 s we investigate the isostatic compensation of tectonic features along the mid atlantic ridge 25 27 30 s through a detailed examination of the relationship between bathymetry and gravity anomalies the study area includes three ridge segments spreading at an average full rate of 35 mm yr their flanks out to about 6 m y old crust and the intervening rio grande and moore fracture zones in a three dimensional analysis of gravity and sea beam bathymetry data we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models positive residual gravity anomalies over the northern wall of the rio grande fracture zone indicate that in spots the crust is 2 3 km thinner than average there in contrast to the moore fracture zone where little thinning is observed the greater than average depths of the fracture zones as a whole are not locally compensated by thin crust but the deepest basins within the active transform parts of the fracture zones are partially compensated by thin crust or cooler mantle a narrow linear ridge that crosses one of the inactive branches of the moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner not thicker crust there is no indication of thicker crust beneath topographic highs at the inside corners of ridge transform intersections presumably these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies there is a residual gravity low associated with an unusually shallow 15 to 20 km section of the ridge segment between the rio grande and moore offsets where the median valley virtually disappears our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation two dimensional fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented this implies that the tectonic processes that control the gross ridge transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average effective elastic plate thickness in the study area is about 6 km

thermal history of mesosiderites quantitative constraints from compositional zoning and fe mg ordering in orthopyroxenes abstract we have derived mathematical relations to calculate cooling rates from the extent of compositional zoning developed during cooling across the interface of a natural diffusion couple these relations were used to calculate the high temperature cooling rates of the mesosiderites lowicz and clover springs from the available data on compositional zoning across core overgrowth interface of orthopyroxene grains we have also determined the cation ordering in four selected orthopyroxene crystals from bondoc and estherville mesosiderites with very high precision and used these data to calculate their low temperature cooling rates the compositional zoning of orthopyroxene crystals reflects extremely rapid cooling rates at least 1 c 100 years in the temperature range 850 1150 c however simultaneous consideration of both metallographic and cation ordering data for estherville within the framework of either an asymptotic or an exponential cooling model requires a cooling rate of l c ma near 250 c the cation ordering data for bondoc for which no metallographic data are yet available are suggestive of even slower cooling rate which implies excavation from a somewhat greater depth in the parent body however within the limits of their uncertainties the measured site occupancies of the orthopyroxene crystals from bondoc can be reconciled with a cooling rate similar to that of estherville the calculated cooling rates at both high and low temperatures have been used to develop a thermal evolution model of mesosiderites the suggested model is not incompatible with an asteroidal parent body for these meteorites further it is shown that the closure temperature of ar ar age must be tied to the slow cooling rate below 500 c

crustal structure and isostatic compensation near the kane fracture zone from topography and gravity measurements i spectral analysis approach summary six gravity and bathymetry profiles perpendicular to the kane fracture zone each more than 300 km long were gathered to study the variation in crustal structure in the vicinity of a major fracture zone and the gravitational edge effect at the contact between lithosphere of two different ages a spectral analysis of the gravity and bathymetric series as a function of wavelength shows that the gravitational edge effect is only significant at the longest wavelengths for remaining wavelengths the admittance the ratio of the amplitude of the gravity anomaly to the amplitude of the bathymetry is best explained by a model of isostasy in which topographic loads are partially supported by the flexural rigidity of an elastic plate about 6 km in thickness after subtracting the gravitational attraction of the bathymetry and its compensation substantial isostatic anomalies remain we interpret these anomalies as being caused by variations in crustal thickness which have little correlation with surface topography except at very long wavelengths the apparent crustal thickness varies by as much as a factor of 2 but there is no evidence indicating systematic thinning of the crust beneath the fracture zone our data do suggest that such density variations within the plate are also compensated by the isostatic response of an elastic plate but with very different effect from those at the surface this indicates that there are two different modes of crustal formation with different gravity and topographic signatures effusive volcanism which loads the surface of the elastic plate producing both topographic relief and coherent gravity anomalies and intrusive volcanism or underplating producing gravity anomalies but little topographic relief

thermal history of acapulco and alha81261 acapulcoites constrained by fe2 mg ordering in orthopyroxene the quenched fe2 mg ordering states of four orthopyroxene crystals from the acapulco meteorite and two from the antarctic alha81261 acapulcoite were determined from x ray single crystal diffraction and electron microprobe analysis the closure temperatures of cation ordering were 49028 and 46543 c for acapulco and alha81261 respectively kinetic analysis of the fe2 mg intracrystalline reaction on the basis of mueller s theory 1 2 was carried out using the method developed by ganguly 3 to measure the cooling rates of the host rock assuming both asymptotic and exponential models the determined ordering states yield cooling rates near their closure temperatures of 0 17 and 0 05 c day for acapulco and alha81261 respectively slightly slower than that measured for the fro90011 lodranite 4 a break up of the a l parent body with subsequent radiative heat loss of small ejected fragments may explain such rapid cooling the closure temperatures obtained for the two acapulcoites were then compared with those calculated using the same method for other achondrites and their cooling rates with those measured for the fro90011 lodranite 4 and the estherville mesosiderite 5 the sequence of decreasing closure temperatures from the fro90011 lodranite to the landes iab iron meteorite reflects decreasing cooling rates silicates enclosed in iron meteorites and thus involved in core formation processes deeper in the interior of a parent asteroid seem to have cooled more slowly than meteorites that were affected by partial melting events

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

a two and three dimensional analysis of gravity anomalies associated with the east pacific rise at 9 n and 13 n we have used a unique data set collected during the 1985 east pacific rise multichannel seismic experiment to reevaluate the constraints that gravity data can place on the crustal structure of the east pacific rise epr the close spacing of track lines within the two main survey areas 8 45 n 9 55 n and 12 20 13 30 n allowed us to perform three dimensional analyses of high quality gravity data 1 mgal uncertainity obtained with the bell aerospace bgm 3 gravity meter our gravity modeling was enhanced by the availability of high resolution sea beam bathymetric data and by independent structural constraints provided by seismic reflection and refraction data to model the crustal and upper mantle density structure at the ridge axis we first calculated the gravity anomalies due to the density contrasts at the water crust and crust mantle boundaries and the changes in density caused by the cooling of the lithosphere with age and subtracted these predictable signals from the observed free air anomaly the residual anomalies were then used to place constraints on the magnitude and distribution of anomalous mass at the epr our results show that over 90 of the power in the observed free air anomaly can be modeled by these predictable components of the gravity signal the amplitude and wavelength of the small residual anomaly can be modeled by a broad region 20 km wide centered on the rise axis of slightly lower than normal crustal and or upper mantle densities 0 03 mg m 3 this region of anomalous mass and density variations in the underlying mantle provide the principle isostatic support for the axial topographic high the anomalies are consistent with but do not require the presence of a low density crustal magma chamber at the rise axis if a largely molten magma chamber exists then the gravity data require that it be a narrow volumetrieally small body there are first order differences in the distribution of anomalous mass between the two survey areas with the 9 n region generally being characterized by lower densities in the axial region along axis variations in anomalous mass within each of the survey areas correlate with first order changes in the depth and cross sectional shape of the epr axial high we attribute these differences in axial morphology and the amplitude of the associated gravity anomalies to along strike changes in the size of the crustal magma chamber the width of the surrounding zone of cooling crustal rocks and densities in the uppermost mantle beneath the rise axis

petrogenesis of acapulcoites and lodranites a shock melting model abstract acapulcoites are modeled as having formed by shock melting cr like carbonaceous chondrite precursors the degree of melting of some acapulcoites was low enough to allow the preservation of 3 6 vol relict chondrules shock effects in acapulcoites include veins of metallic fe ni and troilite polycrystalline kamacite fine grained metal troilite assemblages metallic cu and irregularly shaped troilite grains within metallic fe ni while at elevated temperatures acapulcoites experienced appreciable reduction because graphite is present in some acapulcoites and lodranites it seems likely that carbon was the principal reducing agent reduction is responsible for the low contents of olivine fa 4 14 mol and low ca pyroxene fs 3 13 mol in the acapulcoites the observation that in more than two thirds of the acapulcoites the fa value is lower than the fs value in contrast to the case for equilibrated ordinary chondrites the low feo mno ratios in acapulcoite olivine 16 18 compared to 32 38 in equilibrated h chondrites the relatively high modal orthopyroxene olivine ratios e g 1 7 in monument draw compared to 0 74 in h chondrites and reverse zoning in some mafic silicate grains lodranites formed in a similar manner to acapulcoites but suffered more extensive heating loss of plagioclase and loss of an fe ni s melt acapulcoites and lodranites experienced moderate post shock annealing presumably resulting from burial beneath material of low thermal diffusivity the annealing process repaired damaged olivine crystal lattices lending acapulcoites and lodranites the appearance of unshocked i e shock stage s1 rocks any high pressure phases that may have formed during initial shock reverted to their low pressure polymorphs during annealing some samples were subsequently shocked again several acapulcoites reached shock stage s2 levels alh 84190 reached s3 and the lodranite mac 88177 reached s5

orthopyroxene as a geospeedometer thermal history of kapoeta old homestead 001 and hughes 004 howardites 1dipartimento di scienze della terra universitti di pavia via ferrata 1 27100 pavia italy 2dipartimento di mineralogia e petrologia universitti di padova c so garibaldi 37 35122 padova italy 3dipartimento di scienze della terra universitil di messina salita sperone 31 98166 messina s agata italy 4centro grandi strumenti universitti di pavia via bassi 21 27100 pavia italy correspondence author s e mail address domeneghetti crystal unipv it

crustal structure of the mid atlantic ridge at 23 n from seismic refraction studies an explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120 km long section of the mid atlantic ridge mar median valley at latitude 23 n immediately south of its intersection with the kane fracture zone the data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques these solutions are combined using two dimensional ray tracing to produce a single model defining the major along axis structural changes occurring beneath this section of the median valley with the exception of slightly low layer 3 velocities the crust beneath several tens of kilometers of the mar median valley has all the characteristics of simple mature ocean crust including a well defined moho transition zone 8 km s upper mantle velocities and a total crustal thickness of 6 7 km no evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along axis we infer from these observations that the basic seismic characteristics of the ocean crust are frozen in in a time period less than that separating the major volcanic injection events 10 000 50 000 years by hydrothermal circulation penetrating to the base of the crust along the 100 km long ridge segment studied here two major along axis structural changes occur the first is a 10 to 15 km wide along axis zone of lowered velocities in the lower crust centered beneath a major along axis topographic high this is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities the second occurs abruptly at latitude 23 15 n and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly north of this boundary the 30 km long ridge segment that abuts the kane fracture zone has a crustal thickness of 4 5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust interpretation of the cause of this structural change is uncertain but may be related to local tectonic events perhaps a recent 10 to 20 km ridge jump to the east

rapid determination of initial 87sr 86sr and estimation of the rb sr age of plutonic rocks by la icpms of variably altered feldspars an example from the 1 14 ga great abitibi dyke ontario canada abstract sr isotope analysis of plutonic rocks is an important method for deriving the provenance and age of ancient magmatic episodes and the petrogenetic processes operating during intrusive activity we determined the 87 sr 86 sr isotopic compositions of fresh and variably altered feldspars from the 1 14 ga gabbroic great abitibi dyke ontario canada through the use of laser ablation inductively coupled plasma mass spectrometry la icpms the data show an alteration trend between fresh low rb plagioclase with 87 sr 86 sr ratios of 0 7045 and high rb alkali feldspar with 87 sr 86 sr ratios of 0 7059 which may be used to constrain the 87 sr 86 sr ratio of the mantle source this trend is incompatible with simple mixing and instead has age significance as post intrusive alteration through hydrothermal circulation starts immediately following emplacement la icpms analysis allows rapid estimation of initial 87 sr 86 sr ratios and intrusive age suitable for sample screening and selection for more precise dating for the great abitibi dyke the rb sr age estimate of emplacement of 1 10 0 23 ga 2 is concordant with the precise u pb emplacement age of the dyke 1140 6 2 0 ma the most altered high rb alkali feldspars return younger ages suggesting that post intrusive alteration lasted several hundred million years this study highlights the need for microanalysis to differentiate between alteration and magmatic processes on basis of sr isotopic signatures and outlines a rapid method to determine initial 87 sr 86 sr ratios and to estimate rb sr ages of plutonic rocks in terranes with long histories of multiple intrusive magmatic episodes

multichannel seismic evidence for anomalously thin crust at blake spur fracture zone a two ship wide aperture multichannel seismic reflection profile obtained across a small western north atlantic fracture zone provides convincing new evidence for unusually thin crust at oceanic fracture zones reflections from the crust mantle boundary typically observed at about 2 s below the basement reflection rise to only 0 8 s beneath the fracture zone trough this corresponds to crustal thicknesses of perhaps as little as 2 to 2 4 km in the fracture zone less than half the thickness of normal oceanic crust this is the first time that moho reflections have been observed across an oceanic fracture zone and the first convincing evidence that anomalously thin crust may be associated even with fracture zones of relatively small offset the existence of thin crust at both large and small offset fracture zones suggests that the magnitude of the thermal edge effect at ridge transform intersections is not the only factor responsible for the accretion of unusually thin crust in fracture zones

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

upb systematics of phosphates from equilibrated ordinary chondrites u pb systematics were determined from fifteen phosphate separates from equilibrated ordinary chondrites and from small bulk fragments of the same meteorites the high 238u204pb ratios of thirteen of these phosphate separates lead to extremely radiogenic pb whose 206pb204pb ratios range from 250 up to 3500 the pb pb model ages for these phosphates range from 4 563 to 4 502 ga with an analytical precision of 106 y and the u pb system is apparently concordant the time interval observed 60 106 y reflects the thermal processing of the equilibrated chondrites and is consistent with that previously derived from the rb sr k ar and pu chronologies the pb pb ages of the phosphates from the seven h chondrites show a negative correlation versus their metamorphic grade this is the first clear relationship ever observed between a long lived chronometer and the intensity of metamorphism as reflected by metamorphic grade assuming that the pb pb age indicates the accurate u pb closure time in phosphates the pb pb chronology is compatible with the model of a layered h chondrite parent body however this interpretation of the u pb systematics is not unique it postulates a slow cooling of the equilibrated materials at high temperature in apparent conflict with petrological observations except for the h chondrites which agree rather well with pu systematics comparison of the pb pb chronology with published radiochronometric data does not reveal simple correlations in the present debate concerning the thermal history of chondrites the chronometric information derived from each isotope system is interpreted as the time of its thermal closure however this basic assumption may not be correct for all the radiochronologies and must be evaluated before the radiochronometric data can be applied as compelling time constraints for the period of 4 56 4 4 ga of proto planetary history

cooling rates of diogenites a study of fe2 mg ordering in orthopyroxene by single crystal x ray diffraction the iron magnesium exchange between m1 and m2 sites in orthopyroxene is a reversible reaction that records the latest event in the thermal evolution of the host rock a kinetic analysis of this process has been applied to 16 orthopyroxene single crystals from 7 different diogenites to constrain the cooling history of their parent body the fe2 mg ordering degrees have been determined by single crystal x ray diffraction the fe2 mg ordering closure temperatures were very homogeneous within each sample and ranged for all diogenites studied between 311 29 c and 408 10 c cooling rates at these closure temperatures were calculated using a numerical method developed by ganguly 1982 these ranged between 5 c 104 year in johnstown and 0 8 c year in roda a comparison with other achondrites studied with the same method showed that increasing closure temperatures correspond to increasing cooling rates and that meteorites from a same parent body exhibit similar closure temperature and cooling rate values the cooling rates obtained for these diogenites at their low closure temperatures should probably be ascribed to a complex thermal history of their parent body thus confirming miyamoto and takeda s theory 1994a of excavation of deep crustal material due to impact events the differences on cooling rate values for different diogenites could be due to different burial depths in the fragment ejected from the parent body