na k li geochemistry of the prestige pluton in the slave province of the canadian shield in the southwestern slave province canadian precambrian shield a cluster of 14 muscovite biotite granite plutons dated at about 2 6 109 years was emplaced into a thick succession of archean gr

venus topography a harmonic analysis a model of venusian global topography has been obtained by fitting an eighteenth degree harmonic series to pioneer venus orbiter radar altimeter data the mean radius is 6051 45 0 04 km the corresponding mean density is 5244 8 0 5 kg m 3 the center of figure is displaced from the center of mass by 0 339 0 088 km towards 6 6 10 1 n 148 8 7 7 e the figure of venus is distinctly triaxial but the orientation and magnitudes of the principal topographic axes correlate rather poorly with the gravitational principal axes however the higher degree harmonics of topography and gravity are significantly correlated the topographic variance spectrum of venus is very similar in form to those of the moon mars and especially earth we suggest that this spectral similarity simply reflects a statistical balance between constructional and degradational geomorphic processes venus and earth are particularly similar and differ from the moon and mars in that the larger bodies both exhibit a significant low degree deficit relative to the extrapolated trend of the higher harmonics

isocrater impacts conditions and mantle dynamical responses for different impactor types abstract impactors of different types and sizes can produce a final crater of the same diameter on a planet under certain conditions we derive the condition for such isocrater impacts from scaling laws as well as relations that describe how the different impactors affect the interior of the target planet these relations are also valid for impacts that are too small to affect the mantle the analysis reveals that in a given isocrater impact asteroidal impactors produce anomalies in the interior of smaller spatial extent than cometary or similar impactors the differences in the interior could be useful for characterizing the projectile that formed a given crater on the basis of geophysical observations and potentially offer a possibility to help constrain the demographics of the ancient impactor population a series of numerical models of basin forming impacts on mercury venus the moon and mars illustrates the dynamical effects of the different impactor types on different planets it shows that the signature of large impacts may be preserved to the present in mars the moon and mercury where convection is less vigorous and much of the anomaly merges with the growing lid on the other hand their signature will long have been destroyed in venus whose vigorous convection and recurring lithospheric instabilities obliterate larger coherent anomalies

evolution of depleted mantle the lead perspective abstract isotopic data have established that compared to estimated bulk earth abundances the sources of oceanic basaltic lavas have been depleted in large ion lithophile elements for at least several billions of years various data on the tertiary mesozoic gorgona komatiite and cretaceous oka carbonatite show that those rocks also sample depleted mantle sources this information is used by analogy to compare pb isotopic data from 2 6 billion year old komatiite and carbonatite from the suomussalmi belt of eastern finland and munro township ontario that are with associated granitic rocks and ores that should contain marked crustal components within experimental error no differences are detected in the isotopic composition of initial pb in either of the rock suites these observations agree closely with sr and nd data from other laboratories showing that depleted mantle could not have originated in those areas more than a few tenths of billions of years before the rocks were emplaced on a world wide basis the pb isotope data are consistent with production of depleted mantle by continuous differentiation processes acting over approximately the past 3 billion years the data show that pb evolution is more complex than the simpler models derived from the rb sr and sm nd systems the nature of the complexity is still poorly understood

plume mantle source heterogeneity through time insights from the duarte complex hispaniola northeastern caribbean 1 located in the cordillera central of hispaniola the duarte complex offers an opportunity to study an on land fragment of the caribbean colombian oceanic plateau geochemical sr nd isotope and 40ar 39ar radiometric age data combined with detailed mapping have shown that the duarte complex includes two lithostratigraphic units composed of four geochemical groups of metavolcanic rocks group ia low ti high mg basalts and group ib high ti picrites and primitive high mg basalts occur interlayered in the lowermost levels of the lower unit group ii light rare earth element lree enriched picrites ferropicrites and high mg basalts forms the main lava sequence of the lower unit and group iii lree enriched fe ti basalts is present exclusively in the upper unit nd isotope and incompatible trace element patterns are diverse in the duarte complex metavolcanics and are consistent with mantle sources related to a heterogeneous plume mantle melt modeling suggests that an early extensive melting 10 20 of shallow mantle resulted in the formation of relatively depleted group i and ii magmas whereas the late more enriched group iii magmas were the product of deeper low degree 96 ma hence an early cretaceous phase of the caribbean colombian oceanic plateau construction is recorded in hispaniola

la isla de gorgona colombia a petrological enigma a wide range of intrusive wehrlite dunite gabbro and olivine gabbro and extrusive komatiites picrites and basalts igneous rocks are found on the small pacific island of gorgona the island is best known for its 90 ma spinifex textured komatiites the only true phanerozoic komatiites yet discovered early work led to suggestions that the rocks of the island formed at a mid ocean ridge however more recent research supports an origin as part of a hot mantle plume derived oceanic plateau one of the main lines of evidence for this origin stems from the inferred high mantle source temperatures required to form the high mgo n15 wt komatiites and picrites another remarkable feature of the island considering its small size 8 2 5 km is the degree of chemical and radiogenic isotopic heterogeneity shown by the rocks this heterogeneity requires a mantle source region with at least three isotopically distinctive source regions two depleted and one enriched although these mantle source regions appear to be derived in significant part from recycled oceanic crust and lithosphere enrichments in 187os 186os and 3he in gorgona lavas and intrusive rocks suggest some degree of transfer of material from the outer core to the plume source region at dw modelling reveals that the komatiites probably formed by dynamic melting at an average pressure of 3 4 gpa leaving residual harzburgite trace element depletion in gorgona ultramafic rocks appears to be the result of earlier deeper melting which produced high mgo trace element enriched magmas the discovery of a trace element enriched picrite on the island has confirmed this model gorgona accreted onto the palaeocontinental margin of northwestern south america in the eocene and palaeomagnetic work reveals that it was formed at 26 8s it has been proposed that gorgona is a part of the caribbean colombian oceanic plateau ccop however the ccop accreted in the late cretaceous and was derived from a more equatorial palaeolatitude this evidence and differing geochemical signatures strongly suggests that gorgona and probably other coastal oceanic plateau sequences in colombia and ecuador belong to a completely different oceanic plateau to the ccop

pb isotopic compositions of archean komatiites and sulfides abstract pb isotopic compositions were measured in komatiites and other volcanic rocks from several archean greenstone belts and in magmatic and exhalative sulfides associated with these volcanics thin komatiite flows generally yielded pb pb isochron ages consistent with other geochronological or geological information but thicker basaltic flows gave ages that are too young these results suggest that the thin komatiite flows were hydrothermally altered during eruption and that this event is recorded by the pb pb age thicker basaltic flows resisted alteration during eruption but were affected by later events their pb pb ages probably correspond to the time of metamorphism and not to the time of emplacement magmatic and exhalative sulfides from the abitibi belt in canada fall on the same trend in the 207 pb 204 pb 206 pb 204 pb diagram as komatiites basalts sedimentary rocks and k feldspars from granites the calculated 1 value is 7 8 0 1 a similar suite of rocks and minerals from the kambalda area in australia has distinctly higher 207 pb 204 pb and a higher 1 of 8 3 nd isotopic data demonstrate that some kambalda rocks are highly contaminated with older continental crust the differences in initial pb isotopic compositions are attributed to the presence or absence and the age of underlying granitoid basement kambalda greenstones are underlain and contaminated by old 3 0 ga granitoid crust but significantly older granitoid basement appears absent from the abitibi belt the pb isotopic compositions of kambalda greenstones are dominated by the crustal component and do not represent the composition of the mantle source of these rocks the abitibi greenstones are not highly influenced by crustal contamination and their pb isotopic compositions may reflect those of archean mantle

radar altimetry of mercury a preliminary analysis measurements of mercurian topography based on arecibo radar observations are presented the data which were obtained from 1978 to 1984 cover much of the equatorial zone of mercury between 12on and 5os latitude over thirty continuous altitude profiles were obtained each spanning from 20 to 90 degrees of longitude at a resolution of 0 15 o longitude by 2 5 o latitude radar depths for large craters support previous indications from imagery that mercurian craters are shallower than lunar craters of the same size one very large 800 km impact basin shows some distinct topographic structure although its relative shallowness suggests postimpact modification by isostatic relaxation or volcanic filling the plains of tir planitia appear topographically smooth to the radar these plains extend well into the hemisphere not imaged by mariner 10 possibly forming part of a large annulus of smooth plains around caloris basin the circum caloris smooth plains are strongly down bowed indicating subsidence under a load this and other similarities to lunar maria suggest a volcanic origin for these plains additional areas of topographically smooth terrain have been found in both the imaged and unimaged hemispheres several ridges scarps and fault zones have been identified in the altimetry three mapped arcuate scarps show heights of about 700 m and cross sectional widths of about 70 km one of these features is clearly a ridge while the other two scarps have a more ridge like appearance than is suggested by images one large scale topographic drop of 3 km correlates well with a mapped system of faults and intracrater scarps the equatorial zone of mercury shows 7 km of maximum relief although the typical elevation difference between highlands and lowlands is closer to 3 km three major highland areas are found the largest two of which are roughly antipodal and aligned within about 10 o of the hot poles of mercury the unimaged hemisphere possessing both large craters and topographically smooth areas does not appear to be markedly different in its topography from the imaged hemisphere no evidence has been found for another caloris type impact structure in the unimaged hemisphere

the low degree shape of mercury the shape of mercury particularly when combined with its geoid provides clues to the planet s internal structure thermal evolution and rotational history elevation measurements of the northern hemisphere acquired by the mercury laser altimeter on the mercury surface space environment geochemistry and ranging spacecraft combined with 378 occultations of radio signals from the spacecraft in the planet s southern hemisphere reveal the low degree shape of mercury mercury s mean radius is 2439 36 plusmn 0 02 km and there is a 0 14 km offset between the planet s centers of mass and figure mercury is oblate with a polar radius 1 65 km less than the mean equatorial radius the difference between the semimajor and semiminor equatorial axes is 1 25 km with the long axis oriented 15 g west of mercury s dynamically defined principal axis mercury s geoid is also oblate and elongated but it deviates from a sphere by a factor of 10 less than mercury s shape implying compensation of elevation variations on a global scale

the distribution and origin of smooth plains on mercury 1 orbital images from the messenger spacecraft show that 27 of mercury s surface is covered by smooth plains the majority 65 of which are interpreted to be volcanic in origin most smooth plains share the spectral characteristics of mercury s northern smooth plains suggesting they also share their magnesian alkali basalt like composition a smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope suggesting more ultramafic compositions an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation the knobby and hummocky plains surrounding the caloris basin known as odin type plains occupy an additional 2 of mercury s surface the morphology of these plains and their color and stratigraphic relationships suggest that they formed as caloris ejecta although such an origin is in conflict with a straightforward interpretation of crater size frequency distributions if some fraction is volcanic this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions smooth plains are widespread on mercury but they are more heavily concentrated in the north and in the hemisphere surrounding caloris no simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed a likely volcanic origin for some older terrain on mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large scale volcanic deposits rather than differences in crustal formational process

the equatorial shape and gravity field of mercury from messenger flybys 1 and 2 on 14 january and 6 october 2008 the messenger spacecraft passed within 200 km of the surface of mercury these flybys by messenger provided the first observations of mercury from a spacecraft since the mariner 10 flybys in 1974 and 1975 data from the mercury laser altimeter mla provided new information on the equatorial shape of mercury and doppler tracking of the spacecraft through the flybys provided new data on the planet s gravity field the mla passes were on opposite hemispheres of the planet and span collectively 40 of the equatorial circumference the mean elevation of topography observed during flyby 1 in the longitude range 0 90e is greater than that seen during flyby 2 in the longitude range 180 270e indicating an offset between centers of mass and figure having a magnitude and phase in general agreement with topography determined by earth based radar both mla profiles are characterized by slopes of 0 015 downward to the east which is consistent with a long wavelength equatorial shape defined by a best fitting ellipse the doppler tracking data show sensitivity to the gravitational structure of mercury the equatorial ellipticity of the gravitational field c 2 2 is well determined and correlates with the equatorial shape the s2 2 coefficient is 0 as would be expected if mercury s coordinate system defined by its rotational state is aligned along its principal axes of inertia the recovered value of the polar flattening of the gravitational potential j2 is considerably lower in magnitude than the value obtained from mariner 10 tracking a result that is problematic for internal structure models this parameter is not as well constrained as the equatorial ellipticity because the flyby trajectories were nearly in the planet s equatorial plane the residuals from the doppler tracking data suggest the possibility of mascons on mercury but flyby observations are of insufficient resolution for confident recovery for a range of assumptions on degree of compensation and crustal and mantle densities the allowable crustal thickness is consistent with the upper limit of about 100 km estimated from the inferred depth of faulting beneath a prominent lobate scarp an assumed ductile flow law for crustal material and the condition that temperature at the base of the crust does not exceed the solidus temperature the messenger value of c2 2 has allowed an improved estimate of the ratio of the polar moment of inertia of the mantle and crust to the full polar moment cm c a refinement that strengthens the conclusion that mercury has at present a fluid outer core

lead lead age of komatiitic lavas and limitations on the structure and evolution of the precambrian mantle abstract using lead isotopes we have studied komatiites and associated sulfides from three complexes at barberton the lead lead isochron gives an age of 3 46 0 07gy in good agreement with the sm nd age at munro township three distinct komatiitic flows yield significantly different ages pyke hill is 2 72 0 02gy theo s flow is 2 47 0 13gy and fred s flow is 2 58 0 02gy old 2 72 gy is the emplacement age while the alignment obtained on theo s flow and fred s flow clearly points out the effect of a metamorphism posterior to 2 7 gy at cape smith the age obtained by the lead lead method is 1 6 0 13gy this age is younger than that obtained by other methods in contrast the isochron obtained on sulfide gives an age of 1 88 0 17gy in agreement with sm nd methods these results together with others from the literature are used to describe the evolution of pb in the mantle the important result inferred is that the mantle 2 70 gy ago was very different from the primitive mantle according to both th u and u pb values indeed the latter are lower than the planetary values this evolution is the result of the formation of the continental crust

oceanic plateau model for continental crustal growth in the archaean a case study from the kostomuksha greenstone belt nw baltic shield abstract field studies combined with chemical and isotope data indicate that the kostomuksha greenstone belt in the nw baltic shield consists of two lithotectonic terranes one mafic igneous and the other sedimentary separated by a major shear zone the former contains submarine komatiite basalt lavas and volcaniclastic lithologies and the latter is composed of shelf type rocks and bif komatiitic and basaltic samples yield sm nd and pb pb isochron ages of 2843 39 and 2813 78 ma respectively their trace element compositions resemble those of recent pacific oceanic flood basalts with primitive mantle normalized nb th of 1 5 2 1 and nb la of 1 0 1 5 this is in sharp contrast with island arc and most continental magmas which are characterized by nb th la n 1 calculated initial nd isotope compositions nd t 2 8 to 3 4 plot close to an evolution line previously inferred for major orogens momo which is also consistent with the compositions of recent oceanic plateaux the high liquidus temperatures of the komatiite magmas 1550 c and their al depleted nature require an unusually hot 1770 c mantle source for the lavas 200 c hotter than the ambient mantle at 2 8 ga and are consistent with their formation in a deep mantle plume in equilibrium with residual garnet this plume had the thermal potential to produce oceanic crust with an average thickness of 30 km underlain by a permanently buoyant refractory lithospheric mantle keel nb u ratios in the komatiites and basalts calculated on the basis of th u pb relationships range from 35 to 47 and are thus similar to those observed in modern morb and oib this implies that some magma source regions of the kostomuksha lavas have undergone a degree of continental material extraction comparable with those found in the modern mantle the mafic terrane is interpreted as a remnant of the upper crustal part of an archaean oceanic plateau when the newly formed plateau reached the active continental margin its upper part collided with the sedimentary terrane but was too buoyant to subduct as a result the volcanic section of the plateau was imbricated and obducted thus becoming a new segment of continental crust the deeper zones were delaminated and tectonically underplated or subducted

gravity and tectonic patterns of mercury effect of tidal deformation spin orbit resonance nonzero eccentricity despinning and reorientation 1 we consider the effect of spin orbit resonance nonzero eccentricity despinning and reorientation on mercury s gravity and tectonic pattern large variations of the gravity and shape coefficients from the synchronous rotation and zero eccentricity values j2 c22 10 3 and b c a c 1 4 arise because of nonsynchronous rotation and nonzero eccentricity even in the absence of reorientation or despinning reorientation or despinning induces additional variations the large gravity coefficients j2 6 2 10 5 and c22 1 0 5 10 5 estimated from the mariner 10 flybys cannot be attributed to caloris alone since the required mass excess in this case would have caused caloris to migrate to one of mercury s hot poles similarly a large remnant bulge due to a smaller semimajor axis and spin orbit resonance can be dismissed since the required semimajor axis is unphysically small 0 1 au reorientation of a large remnant bulge recording an epoch of faster rotation without significant semimajor axis variations can explain the large gravity coefficients this requires initial rotation rates 20 times the present value and a positive gravity anomaly associated with caloris capable of driving 10 45 equatorward reorientation the required gravity anomaly can be explained by infilling of the basin with material of thicknesses 7 km or an annulus of volcanic plains emplaced around the basin with an annulus width 1200 km and fill thicknesses 2 km the predicted tectonic pattern due to these despinning and reorientation scenarios including some radial contraction is in good agreement with the lobate scarp pattern observed by mariner 10 we also predict that lobate scarps will follow a ne sw orientation in the eastern hemisphere and a positive gravity anomaly of a few hundred mgal associated with caloris

viscoelastic tides of mercury and the determination of its inner core size we computed interior structure models of mercury and analyzed their viscoelastic tidal response the models are consistent with mercury surface space environment geochemistry and ranging mission inferences of mean density mean moment of inertia moment of inertia of mantle and crust and tidal love number k2 based on these constraints we predict the tidal love number h2 to be in the range from 0 77 to 0 93 using an andrade rheology for the mantle the tidal phase lag is predicted to be 4 at maximum the corresponding tidal dissipation in mercury s silicate mantle induces a surface heat flux smaller than 0 16 mw m2 we show that independent of the adopted mantle rheological model the ratio of the tidal love numbers h2 and k2 provides a better constraint on the maximum inner core size with respect to other geodetic parameters e g libration amplitude or a single love number provided it responds elastically to the solar tide for inner cores larger than 700 km and with the expected determination of h2 from the upcoming bepicolombo mission it may be possible to constrain the size of the inner core the measurement of the tidal phase lag with an accuracy better than 0 5 would further allow constraining the temperature at the core mantle boundary for a given grain size and therefore improve our understanding of the physical structure of mercury s core

enceladus s and dione s floating ice shells supported by minimum stress isostasy enceladus gravity and shape have been explained in terms of a thick isostatic ice shell floating on a global ocean in contradiction of the thin shell implied by librations here we propose a new isostatic model minimizing crustal deviatoric stress and demonstrate that gravity and shape data predict a 38 4km thick ocean beneath a 23 4km thick shell agreeing with but independent from libration data isostatic and tidal stresses are comparable in magnitude south polar crust is only 7 4km thick facilitating the opening of water conduits and enhancing tidal dissipation through stress concentration enceladus resonant companion dione is in a similar state of minimum stress isostasy its gravity and shape can be explained in terms of a 99 23km thick isostatic shell overlying a 65 30km thick global ocean thus providing the first clear evidence for a present day ocean within dione

the size shape and orientation of the asteroid vesta based on data from the dawn mission abstract the aim of this paper is to study the size shape and orientation of the asteroid vesta based on the dawn spacecraft observations in this line three main reference surfaces are defined the geoid reference ellipsoid and best fit ellipsoid to be consistent with international astronomical union iau standards all the computations are done in the claudia double prime coordinate system the geoid and its potential value w 0 are computed by fitting an equipotential surface to the shape of vesta in a least squares sense regarding the topographic bias effect we find that the topographic bias effect on the geoid potential value as well as the geoidal and topographic heights is significant the geoid potential value w 0 is estimated equal to 68709 24 m 2 s 2 the reference ellipsoid is defined as an equipotential ellipsoid which best fits the geoid the reference ellipsoid is computed based on the fundamental geodetic constants which define a geodetic reference system grs the semi axes of the triaxial reference ellipsoid are found to be equal to 280413 104 m 274572 102 m and 231253 86 m with the equatorial semi major axis longitude 8 29 e while the semi axes of the biaxial reference ellipsoid are 278556 117 m and 229921 76 m the results show that the geoidal heights with respect to the triaxial reference ellipsoid are significantly different from the geoidal heights with respect to the biaxial reference ellipsoid the parameters of the best fit ellipsoid are estimated by fitting geometrical ellipsoids with various degrees of freedom to vesta s shape in a least squares sense we report the semi axes of the general best fit ellipsoid with 9 degrees of freedom equal to 284562 75 m 277248 73 m and 226405 57 m regarding the orientation of vesta we find that the angle between the equatorial semi major axis of the general best fit ellipsoid and the x axis of the coordinate system is 9 17 0 47 and the angle between its polar axis and the z axis is equal to 0 63 0 05 furthermore the spherical harmonic coefficients of the shape model of vesta up to degree 180 are computed giving the ability to estimate the parameters of the general best fit ellipsoid the significant differences observed between the parameters of the general best fit ellipsoid derived from the spherical harmonic coefficients of the shape model and those derived from the fitting are due to the large polar and equatorial flattenings of vesta we estimate that the offset of the center of figure from the center of mass is 1350 53 m and the mean radius of vesta is 261403 39 m

petrology and li be b geochemistry of muscovite biotite granite and associated pegmatite near yellowknife canada prosperous granite rb sr 2520 25 ma occurs as several plutons 1 380 km2 outcrop area in a thick succession of metamorphosed greywacke mudstone of the yellowknife supergroup the average mineral content of the sparrow pluton in vol is quartz 32 plagioclase 31 k feldspar 24 muscovite 9 biotite 3 and apatite 1 average trace element concentrations in ppm are li 140 be 4 b 28 zn 47 rb 250 sr 76 zr 75 and ba 360 the central portion of the pluton is slightly richer in k sr and ba than the margin li is concentrated in mica li in biotite li in muscovite 4 7 and be and b in muscovite and plagioclase countless pegmatite dikes occur in the sparrow pluton and in schist hornfels to the east the outer limit is marked by the cordierite isograd 9 km from the granite contact dikes vary greatly in size 1 km to a few cm in length in mineral content quartz albite k feldspar muscovite tourmaline beryl spodumene in major element composition especially the na k ratio and in trace element content li 18 5000 ppm be 5 260 ppm b 20 150 ppm compared with prosperous granite the pegmatite bodies are richer in p and rb and poorer in ti fe mg zr and ba dikes rich in tourmaline beryl and spodumene occur in overlapping zones situated progressively farther from the centre of the sparrow pluton the composition of tourmaline is related to host rock the highest concentrations of fe and zn occur in crystals from pegmetite and the highest concentrations of mg and v occur in crystals from tourmalinized schist while those from granite and quartz veins occupy on intermediate position complex compositional zoning is present in some tourmaline crystals in pegmatite estimates of temperature 500 600 c and pressure 2 4 kb of granite emplacement based on the distribution of andalusite and sillimanite in the contact rocks suggest that the final stage of granite emplacement occurred at sub solidus conditions a vaportransport model is proposed to explain the widespread distribution of the pegmatite dikes and their extreme compositional variability some of the pegmatite constituents including li be and b were possibly derived from yellowknife graywacke and mudstone

pyroclastic rocks another manifestation of ultramafic volcanism on gorgona island colombia tertiary ultramafic volcanism on gorgona island colombia is manifested not only by komatiite flows but also by a more voluminous sequence of tuff breccias which is cut by comagmatic picrite dikes the ultramafic pyroclastic rocks are chaotic to stratified mixtures of angular to subrounded glassy picritic blocks and a fine grained volcaniclastic matrix that consists primarily of plastically deformed glassy globules the entire deposit is interpreted to have formed by an explosive submarine eruption of phenocryst laden picritic magma mgo contents of tuff breccias and picrite dikes range from 21 to 27 wt relative to nearby komatiite flows these rocks are mgo rich and feo tio2 and ni poor hree concentrations are very low seven times chondrite and similar to those of the komatiites whereas the lree are extremely fractionated ce sm n 0 2 indicating derivation from a more highly depleted mantle source region nd isotopic data suggest a cogenetic relationship between the picritic magma that formed the tuff breccia and associated dikes and that which produced the komatiite flows nevertheless pb isotopic data as well as whole rock geochemistry preclude such a connection either due to olivine fractionation accumulation or to different degrees of partial melting these ultramafic rock types crystallized from magmas which most likely were extracted from distinct mantle source regions

pb and nd isotopic study of two archean komatiitic flows from alexo ontario isotopic analysis of two archean komatiitic flows from alexo ontario gives a pb pb isochron age of 2690 15 ma and a sm nd isochron age of 2752 87 ma these ages agree well with u pb zircon ages from underlying and overlying volcanics the variations in element ratios that define the isochrons were not produced during crystallization of the lavas the spread in u pb was caused by submarine alteration soon after eruption and the spread in sm nd resulted from a differences in the composition of the residue of melting and b contamination of the upper komatiite flow through thermal erosion of the lower flow the 147sm 144nd ratio of uncontaminated komatiite is 0 25 which reflects the depleted nature of its mantle source the th u ratio of about 3 4 is probably also representative of depleted mantle the initial nd of 2 44 0 51 indicates that the mantle depletion took place long before magma formation

the sm nd age of kambalda volcanics is 500 ma too old although sm nd isotopic analyses of volcanics from kambalda western australia form a 3 2 ga linear array pb pb analyses of the same suite give an isochron age of 2 73 0 03 ga and a 2 7 ga model age for the associated sulphides we suggest that the 3 2 ga age is incorrect and that the sm nd array results from mixing between depleted mantle and either older continental crust or enriched mantle

initial observations from the lunar orbiter laser altimeter lola as of june 19 2010 the lunar orbiter laser altimeter an instrument on the lunar reconnaissance orbiter has collected over 2 0 10 9 measurements of elevation that collectively represent the highest resolution global model of lunar topography yet produced these altimetric observations have been used to improve the lunar geodetic grid to 10 m radial and 100 m spatial accuracy with respect to the moon s center of mass lola has also provided the highest resolution global maps yet produced of slopes roughness and the 1064 nm reflectance of the lunar surface regional topography of the lunar polar regions allows precise characterization of present and past illumination conditions lola s initial global data sets as well as the first high resolution digital elevation models dems of polar topography are described herein

support of long wavelength topography on mercury inferred from messenger measurements of gravity and topography to explore the mechanisms of support of surface topography on mercury we have determined the admittances and correlations of topography and gravity in mercury s northern hemisphere from measurements obtained by nasa s mercury surface space environment geochemistry and ranging messenger spacecraft these admittances and correlations can be interpreted in the context of a number of theoretical scenarios including flexural loading and dynamic flow we find that long wavelength spherical harmonic degree l 15 surface topography on mercury is primarily supported through a combination of crustal thickness variations and deep mass anomalies the deep mass anomalies may be interpreted either as lateral variations in mantle density or as relief on compositional interfaces domical topographic swells are associated with high admittances and are compensated at 300 400 km depth in the lower reaches of mercury s mantle quasi linear topographic rises are primarily associated with shallow crustal compensation and are weakly correlated with positive mass anomalies in the mantle the center of the caloris basin features some of the thinnest crust on the planet and the basin is underlain by a large negative mass anomaly we also explore models of dynamic flow in the presence of compositional stratification above the liquid core if there is substantial compositional stratification in mercury s solid outer shell relaxation of perturbed compositional interfaces may be capable of creating and sustaining long wavelength topography

pb isotopes of gorgona island colombia isotopic variations correlated with magma type abstract lead isotopic results obtained on komatiites and basalts from gorgona island provide evidence of large isotopic variations within a restricted area 8 2 5 km the variations are correlated with differences in volcanic rock type the highest isotopic ratios 206 pb 204 pb 19 75 correspond to tholeiites which make up most of the island the lowest ratios 18 3 correspond to the komatiites of the west coast of the island other rock types komatiites of the east coast k tholeiites picrites and tuffs have isotopic characteristics intermediate between these two extreme values these results are explained by the existence of two distinct mantle source regions and by mixing or contamination between them

constraints on ceres internal structure and evolution from its shape and gravity measured by the dawn spacecraft ceres is the largest body in the asteroid belt with a radius of approximately 470 km in part due to its large mass ceres more closely approaches hydrostatic equilibrium than major asteroids pre dawn mission shape observations of ceres revealed a shape consistent with a hydrostatic ellipsoid of revolution the dawn spacecraft framing camera has been imaging ceres since march 2015 which has led to high resolution shape models of the dwarf planet while the gravity field has been globally determined to a spherical harmonic degree 14 equivalent to a spatial wavelength of 211 km and locally to 18 a wavelength of 164 km we use these shape and gravity models to constrain ceres internal structure we find a negative correlation and admittance between topography and gravity at degree 2 and order 2 low admittances between spherical harmonic degrees 3 and 16 are well explained by airy isostatic compensation mechanism different models of isostasy give crustal densities between 1 200 and 1 400 kg m3 with our preferred model giving a crustal density of 1 287 70 87 kg m3 the mantle density is constrained to be 2 434 5 8 kg m3 we compute isostatic gravity anomaly and find evidence for mascon like structures in the two biggest basins the topographic power spectrum of ceres and its latitude dependence suggest that viscous relaxation occurred at the long wavelengths 246 km our density constraints combined with finite element modeling of viscous relaxation suggests that the rheology and density of the shallow surface are most consistent with a rock ice salt and clathrate mixture

mafic pegmatites intruding oceanic plateau gabbros and ultramafic cumulates from bolivar colombia evidence for a wet mantle plume

venus center of figure center of mass offset abstract magellan altimetry data reveal that the center of figure cf of venus is displaced approximately 280 m from its center of mass cm toward 4 4 s 135 8 e a location in aphrodite terra this offset is smaller than those of other terrestrial planets but larger than the estimated error which is no more than a few tens of meters we examine the possibility that the cf cm offset is related to specific geologic provinces on venus by deriving three simple models for the offset a thick crust model a hotspot model and a thick lithosphere model the offset caused by a region of thick crust depends upon the region s extent the crust mantle density contrast and the thickness of excess crust a hotspot related offset depends on the extent of the thermally anomalous region and the magnitude of the thermal anomaly offset due to a region of thick lithosphere depends upon the extent of the region the average temperature contrast across the lithosphere and the amount of excess lithosphere we apply the three models to venus plateau shaped highlands volcanic rises and lowlands respectively in an attempt to match the observed cf cm offset location and magnitude the influence of most volcanic rises and of ishtar terra on the cf cm offset must be quite small if we are to explain the direction of the observed offset the lack of influence of volcanic rises can be explained if the related thermal anomalies are limited to a few hundred degrees or less and are plume shaped i e characterized by a flattened sublithospheric head with a narrow cylindrical feeder tail the unimportance of ishtar terra is most easily explained if it lies atop a significant mantle downwelling

topography of the northern hemisphere of mercury from messenger laser altimetry laser altimetry by the messenger spacecraft has yielded a topographic model of the northern hemisphere of mercury the dynamic range of elevations is considerably smaller than those of mars or the moon the most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains within this lowland is a broad topographic rise that experienced uplift after plains emplacement the interior of the 1500 km diameter caloris impact basin has been modified so that part of the basin floor now stands higher than the rim the elevated portion of the floor of caloris appears to be part of a quasi linear rise that extends for approximately half the planetary circumference at mid latitudes collectively these features imply that long wavelength changes to mercury s topography occurred after the earliest phases of the planet s geological history

thorium uranium ratio of the earth through the study of komatiites and ophiolites remnants of the old mantle we review the evolution of the lead system in the depleted mantle through time using the 230th232th vs 208pb 206pb diagram 208pb 206pb radiogenic lead ratio we constrain the th u ratio of the bulk silicate earth to 4 2 this value is also in full agreement with ratios inferred from the study of the various types of meteorites using this value in the 208pb 206pb 143nd144nd diagram bulk earth plots on the oceanic basalt correlation line as previously guessed a comparison with sr and nd data shows that the th u fractionation correlates with that of rb sr and nd sm whereas the u pb fractionation is independent of other ratios

characterizing and navigating small bodies with imaging data recent advances in the characterization of small body surfaces with stereophotoclinometry are discussed the principal data output is an ensemble of landmark maps l maps high resolution topography albedo maps of varying resolution that tile the surface of the body because they can have a resolution comparable to the best images and can be located on a global reference frame to high accuracy l maps provide a significant improvement in discriminatory power for studies of small bodies ranging from regolith processes to interior structure these techniques are now being used to map larger bodies such as the moon and mercury l maps are combined to produce a standard global topography model gtm with about 1 57 million vectors and having a wide variety of applications they can also be combined to produce high resolution topography maps that describe local areas with much greater detail than the gtm when combined with nominal predictions from other data sources and available data from other instruments such as lidar or radar solutions for the spacecraft position and camera pointing are the most accurate available examples are drawn from studies of phobos eros and itokawa including surface characterization gravity analysis spacecraft navigation and incorporation of lidar or radar data this work has important implications for potential future missions such as deep interior and the level of navigation and science that can be achieved

multiple plume events in the genesis of the peri caribbean cretaceous oceanic plateau province the oceanic crust fragments exposed in central america in north western south america and in the caribbean islands have been considered to represent accreted remnants of the caribbean colombian oceanic plateau ccop on the basis of trace element and nd sr and pb isotopic compositions we infer that cumulate rocks basalts and diabases from coastal ecuador have a different source than the basalts from the dominican republic the latter suite includes the 86 ma basalts of the duarte complex which are light rare earth element ree enriched and display relative to normal mid ocean ridge basalts nmorb moderate enrichments in large ion lithophile elements together with high nb ta pb and low th contents moreover they exhibit a rather restricted range of nd and pb isotopic ratios consistent with their derivation from an ocean island type mantle source the composition of which includes the himu high 238u 204pb component characteristic of the galapagos hotspot in contrast the 123 ma ecuadorian oceanic rocks have flat ree patterns and relative to nmorb are depleted in zr hf th and u moreover they show a wide range of nd and pb isotopic ratios intermediate between those of ocean island basalts and nmorb it is unlikely on geochemical grounds that the plume source of the ecuadorian fragments was similar to that of the galapagos in addition because of the nne motion of the farallon plate during the early cretaceous the ecuadorian oceanic plateau fragments could not have been derived from the galapagos hotspot but were likely formed at a ridge centered or near ridge hotspot somewhere in the se pacific

implications of 187os isotopic heterogeneities in a mantle plume evidence from gorgona island and curacao recent work has suggested that the mafic ultramafic volcanism in evidence throughout portions of the caribbean central america and northern south america including the islands of gorgona and curacao was generated as part of a middle cretaceous large igneous province new re os isochron results for tholeiitic basalts from gorgona and curacao indicate crystallization ages of 89 2 5 2 and 85 6 8 1 ma respectively consistent with reported ar ages the gorgona ultramafic suite shows a large range in initial os isotopic composition with os values ranging from 0 5 to 12 4 this large range reflects isotopic heterogeneities in the mantle source similar to those observed for modern ocean island basalts in contrast to ocean island basalts however os isotopic compositions do not correlate with variations in nd sr or pb isotopic compositions which are within the range of depleted mid ocean ridge basalts the processes that produced these rocks evidently resulted in the decoupling of os isotopes from the nd sr and pb isotopic systems picrites from curacao have very uniform chondritic initial os isotopic compositions with initial os values ranging only from 0 4 to 1 4 basalts from curacao however define an isochron with a 187os enriched initial isotopic composition os 9 5 in contrast to the 187os enriched ultramafic rocks from gorgona the enrichment in these basalts could have resulted from lithospheric contamination if the gorgona and curacao rocks were derived from the same plume os results combined with sr nd and pb data indicate a heterogeneous plume with multiple compositionally and isotopically distinct domains the os isotopic results require derivation of os from a minimum of two distinct reservoirs one with a composition very similar to the chondritic average and one with long term enriched re os oceanic crustal recycling has been invoked to explain most of the 187os enrichments that have been observed in ocean island basalt sources and could potentially apply to the gorgona suite crustal recycling however requires large proportions of very ancient recycled basaltic crust in the sources of the 187os enriched ultramafic rocks to explain the magnitude of 187os enrichments observed for example addition of 20 oceanic crust to fertile mantle and nearly 3 billion years are necessary to generate a reservoir with the os isotopic composition of the most radiogenic komatiites if the recycled oceanic crust was added to basalt depleted mantle as may be indicated by nd values for the komatiites averaging about 10 even larger proportions of older crust are required large proportions of oceanic mafic crust in the sources of the 187os enriched komatiites although petrologically conceivable under certain melting conditions is unlikely here given the limited trace element and lithophile isotope system variations these results raise questions about the efficacy of using os isotopes to constrain the proportion of recycled oceanic crust in other plumes other possible mechanisms for generating 187os enriched mantle include invoking the existence of a 187os enriched lower mantle and minor outer core lower mantle interactions

gravity field and internal structure of mercury from messenger radio tracking of the messenger spacecraft has provided a model of mercury s gravity field in the northern hemisphere several large gravity anomalies including candidate mass concentrations mascons exceed 100 milli galileos mgal mercury s northern hemisphere crust is thicker at low latitudes and thinner in the polar region and shows evidence for thinning beneath some impact basins the low degree gravity field combined with planetary spin parameters yields the moment of inertia c mr2 0 353 0 017 where m and r are mercury s mass and radius and a ratio of the moment of inertia of mercury s solid outer shell to that of the planet of cm c 0 452 0 035 a model for mercury s radial density distribution consistent with these results includes a solid silicate crust and mantle overlying a solid iron sulfide layer and an iron rich liquid outer core and perhaps a solid inner core

fertile granites of precambrian rare element pegmatite fields is geochemistry controlled by tectonic setting or source lithologies abstract a total of twenty fertile granites and their rare element pegmatite aureoles was examined to correlate their tectonic position relative to the preceding orogenic event with their geochemical signature namely li rb cs be ga sn nb ta y ree sc ti zr be th u f nyf the range of t time difference between metamorphism and or deformation and granite intrusion is 20 to 465 ma for the examined cases there is some tendency toward syn to late orogenic origin of lct s as opposed to post to anorogenic timing of nyf s but the correlation is poor compared to the previously accepted stereotype similarly both metaluminous and peraluminous fertile granites are about equally spread among syn late and anorogenic environments lct systems are largely associated with peraluminous granites whereas all metaluminous to peralkaline plutons bear the nyf signature tectonic control on geochemistry of granite pegmatite suites is poor and secondary to the fundamental influence of the source lithologies yielding granitic melts the lct type suites are derived from undepleted upper crustal lithologies suffering their first anatectic event which mobilizes the most volatile components into low temperature low percentage melts supracrustal sequences as well as ortho and para lithologies of their basement serve as protoliths with variable degrees of involvement the nyf type suites whose plutons have the geochemical signature of a granites may be derived by melting of depleted lower crustal sources residual after preceding anatectic events or possibly by melting of juvenile lithologies with short crustal residence differentiation from mantle derived magmas is highly improbable volatile extraction of nyf elements from the mantle and their pre enrichment in protoliths or melts seems to be required the few mixed systems with variable expressions of a nyf lct signature are derived basically as nyf suites but contaminated by the lct elements from undepleted upper crust via several possible mechanisms

the global topography of mars and implications for surface evolution elevations measured by the mars orbiter laser altimeter have yielded a high accuracy global map of the topography of mars dominant features include the low northern hemisphere the tharsis province and the hellas impact basin the northern hemisphere depression is primarily a long wavelength effect that has been shaped by an internal mechanism the topography of tharsis consists of two broad rises material excavated from hellas contributes to the high elevation of the southern hemisphere and to the scarp along the hemispheric boundary the present topography has three major drainage centers with the northern lowlands being the largest the two polar cap volumes yield an upper limit of the present surface water inventory of 3 2 to 4 7 million cubic kilometers

the petrogenesis of gorgona komatiites picrites and basalts new field petrographic and geochemical constraints gorgona island colombia is remarkable not only because it contains the only phanerozoic komatiites but also because it has mafic to ultramafic lavas with a wide range of compositions from moderately enriched to extremely depleted relative to bulk earth the komatiite flows are in many respects similar to archaean komatiites they formed from mgo rich 18 liquids and have upper spinifex zones and lower cumulate zones the cumulate zones of archaean komatiites contain many solid grains in contrast more than 90 of the olivine in the gorgona cumulates is highly skeletal this combined with the fact that the gorgona cumulate zones are thinner than those in archaean komatiites suggests that the komatiite magma became strongly superheated en route to the surface the komatiites have trace element contents intermediate between those of the basalts and the ultramafic tuffs some basalts have isotope compositions indicative of long term enrichment in incompatible elements whereas other basalts and ultramafic volcanics have isotopic signatures that imply corresponding depletion it is apparent that the plume source region of the gorgona magmas was markedly heterogeneous with at least two source components contributing to the observed variation in composition this heterogeneity may have resulted from the incorporation of different components into the plume source or it may be the result of complex melting and melt extraction processes during the ascent of a heterogeneous plume despite earlier suggestions that there may have been a significant age gap between depleted komatiite and basalt flows and the enriched basal new 4oar 39ar dating of basalts and gabbros are more consistent with all being generated at 87 ma during formation of the caribbean colombian plateau possibly at the galapagos hotspot