lateral velocity resolution from three dimensional seismic refraction data summary a method for estimating the lateral velocity resolution from 3 d seismic refraction traveltimes is presented the method is non linear in that synthetic data calculated from checkerboard models are inverted using an iterative tomographic approach incorporating smoothness constraints and updated ray paths at each iteration two applications are presented 1 real data from the faeroe basin experiment using the ray coverage and noise level of approximately 50 000 picks and 2 ideal data from the same experiment corresponding to 100 per cent data recovery equivalent to about three times as many ray paths the refraction data constrain the sedimentary and basement structure to approximately 12 km depth a comparison of the results from the two data sets provides a qualitative check on the resolution method illustrates the pitfalls of using ray coverage as an indicator of resolution and demonstrates the potential resolution of currently feasible 3 d refraction experiments 32 checkerboard models consisting of alternating positive and negative velocity anomalies superimposed on the preferred final model of the basin were tested each with a diverent cell size position or orientation the problem of ray bending associated with the 5 per cent checkerboard anomalies is shown to be small using models with four cell sizes allowed for resolution estimates from 3 to 15 km this range being imposed by the frequency content of the data and the areal dimensions of the study area 44 26 km an operator centred on each model node measures the local semblance between the known and recovered checkerboard models a determination of the smallest well resolved cell size at each model node is then made using a 0 7 semblance threshold to provide a spatially dependent lateral resolution estimate resolution for the real data set varies from an average of just over 3 km at 2 km depth to 10 km at 10 km depth where there is a local maximum in ray coverage due to a concentration of ray bottoming points for the ideal data set the average lateral resolution drops ov roughly linearly from better than 3 km at 2 km depth to 6 3 km at 10 km depth the largest diverence between the resolution associated with the ideal and real data occurs at 7 2 km depth where it averages 4 6 km for the former and 10 8 km for the latter due to the near absence of ray bottoming points between 7 and 9 km the results show that the final model for the faeroe basin obtained by regularized inversion with smoothness constraints is consistent with the resolution estimates in the sense that the model does not contain structure with a wavelength smaller than the resolution estimate at any point the results for the ideal data show that good quality 3 d refraction experiments can provide a lateral resolution of no worse than the receiver spacing shot spacing for land data throughout most of the sampled volume

inverse square root dependence of mid ocean ridge flank roughness on spreading rate the topographic roughness of mid ocean ridge flanks is known to increase with decreasing spreading rate1 4 but the exact form of this relationship has not been established as the topographic features that make up ridge flank roughness abyssal hills are created near the axes of mid ocean ridges by faulting and volcan ism5 7 the relationship between roughness and spreading rate may shed light on the process of crustal accretion at spreading centres here i present measurements of ridge flank roughness on profiles crossing the world mid ocean ridge system which show that roughness is proportional to the inverse square root of the spreading rate this result is consistent with some very simple inferences of how the topographic roughness of mid ocean ridge flanks scales with the lithospheric thickness near ridge axes as determined by thermal models

deep crustal structure and magmatic processes the inception of seafloor spreading in the norwegian greenland sea summary an extensive investigation into the deep crustal structure of the conjugate norwegian and e greenland margins utilizing two ship expanded spread and wide aperture cdp profiling has led to the identification of a unique suite of crustal structures developed during the onset of seafloor spreading on both margins a region of low velocity crust was found proximal to marginal escarpments immediately seaward of the low velocity crustal block seaward dipping reflectors occur within thick crust that is interpreted to be wholly igneous a product of voluminous melt production enhanced by convective partial melting processes this earliest formed oceanic crust is actually several kilometres thicker than the low velocity crust or the adjacent continental crust normal oceanic crustal thicknesses are not produced until about 5 my after the initiation of spreading when convective partial melting has abated and the magma budget has reduced the extrusive unit reduces to normal thicknesses long before the intrusive and plutonic section indicating that the thickening or thinning of oceanic crust is accomplished primarily by variations in the latter

a model for the thermal and chemical evolution of the moon s interior implications for the onset of mare volcanism crystallization of the lunar magma ocean creates a chemically stratified moon consisting of an anorthositic crust and magma ocean cumulates overlying the primitive lunar interior within the magma ocean cumulates the last liquids to crystallize form dense ilmenite rich cumulates that contain high concentrations of incompatible radioactive elements the underlying olivine orthopyroxene cumulates are also stratified with later crystallized denser more fe rich compositions at the top this paper explores the chemical and thermal consequences of an internal evolution model accounting for the possible role of these sources of chemical buoyancy rayleigh taylor instability causes the dense ilmenite rich cumulate layer and underlying fe rich cumulates to sink toward the center of the moon forming a dense lunar core after this overturn radioactive heating within the ilmenite rich cumulate core heats the overlying mantle causing it to melt in this model the source region for high tio2 mare basalts is a convectively mixed layer above the core mantle boundary which would contain small and variable amounts of admixed ilmenite and kreep this deep high pressure melting as required for the generation of mare basalts occurs after a reasonable time interval to explain the onset of mare basalt volcanism if the content of radioactive elements in the core and the chemical density gradients above the core are sufficiently high but within a range of values that might have been present in the moon regardless of details implied by particular model parameters gravitational overturn driven by the high density of magma ocean fe rich cumulates should concentrate high tio2 mare basalt sources and probably a significant fraction of radioactive heating toward the center of the moon this will have important implications for both the thermal evolution of the moon and for mare basalt genesis

s type asteroids ordinary chondrites and space weathering the evidence from galileo s fly bys of gaspra and ida new observations of the s type asteroids gaspra and ida especially by the galileo spacecraft demonstrate that a space weathering process operates which modifies the reflectance spectra of fresh material to be redder straighter and have shallower absorption bands the weathering process appears similar to but less potent than that observed on the moon it operates in the sense that it would tend to convert spectra of ordinary chondrites oc to having the spectral traits of s type asteroids these results appear to resolve the major obstacles of the long standing s type conundrum about the provenance of ordinary chondrite meteorites a wide body of recent research which is reviewed here builds on previous meteoritical evidence to support a new developing consensus that the larger s type asteroids are a diverse assortment of silicaceous assemblages which includes the ordinary chrondite parent bodies recent fairly realistic laboratory simulations of space weathering processes have changed oc spectra to resemble s type spectra j f bell s highly regarded earlier paradigm that oc parent bodies would be found only among sub 10 km main belt asteroids has been tested by an extensive observational survey by r binzel and his proteges the result is that no small main belt analog for the ocs has been found not even the somewhat oc like object boznemcova this article reviews the history of the s type conundrum which set the stage for galileo s historic flybys i review the findings about gaspra and ida including results about their geology their geophysical structure and probable origins and about ida s small moonlet dactyl density constraints on ida set by dactyl s orbit rule out for ida at least the classic view of s types as metallic stripped cores of differentiated precursor asteroids new analysis of the galileo spectral images of ida is presented that provides strong evidence that space weathering occurs on ida and that ida and the koronis family asteroids in general are plausibly oc like in composition after reviewing 1990s developments on the s type conundrum i advocate a new perspective that the ordinary chondrite parent bodies are among the s type asteroids a diverse grouping that also contains other silicaceous and silicate metal assemblages presumably including various stony irons and primitive achondrites represented in meteorite collections gaspra may be such a metal rich assemblage

major lunar crustal terranes surface expressions and crust mantle origins in light of global remotely sensed data the igneous crust of the moon can no longer be viewed as a simple globally stratified cumulus structure composed of a flotation upper crust of anorthosite underlain by progressively more mafic rocks and a residual melt kreep sandwich horizon near the base of the lower crust instead global geochemical information derived from clementine multispectral data and lunar prospector gamma ray data reveals at least three distinct provinces whose geochemistry and petrologic history make them geologically unique 1 the procellarum kreep terrane pkt 2 the feldspathic high lands terrane fht and 3 the south pole aitken terrane spat the pkt is a mafic province coincident with the largely resurfaced area in the procellarum imbrium region whose petrogenesis relates to the early differentiation of the moon here some 40 of the th in the moon s crust is concentrated into a region that constitutes only about 10 of the crustal volume this concentration of th average 5 ppm and by implication the other heat producing elements u and k led to a fundamentally different thermal and igneous evolution within this region compared to other parts of the lunar crust lower crustal materials within the pkt likely interacted with underlying mantle materials to produce hybrid magmatism leading to the magnesian suite of lunar rocks and possibly kreep basalt although rare in the apollo sample collection widespread mare volcanic rocks having substantial th enrichment are indicated by the remote data and may reflect further interaction between enriched crustal residues and mantle sources the fht is characterized by a central anorthositic region that constitutes the remnant of an anorthositic craton resulting from early lunar differentiation basin impacts into this region do not excavate significantly more mafic material suggesting a thickness of tens of kilometers of anorthositic crust the feldspathic lunar meteorites may represent samples from the anorthositic central region of the fht ejecta from deep penetrating basin impacts outside of the central anorthositic region however indicate an increasingly mafic composition with depth the spat a mafic anomaly of great magnitude may include material of the upper mantle as well as lower crust thus it is designated a separate terrane whether the spa basin impact simply uncovered lower crust such as we infer for the fht remains to be determined

re examination of the lunar magma ocean cumulate overturn hypothesis melting or mixing is required abstract there is a long standing hypothesis that the last fraction of the lunar magma ocean crystallized into a layer of dense ti rich cumulate minerals at shallow depths 100 km early in the moon s history many questions remain about the stability of these high ti cumulates it has been suggested that the cumulates subsequently sank deep into the moon because of gravitational instability but high ti material is required at shallower depths by 3 5 ga to create the high ti mare basalts and picritic glasses the high ti material may have re erupted from depth or some or all of it may have remained at shallow depths throughout lunar history data on phase stabilities bulk compositions densities and temperatures of melting and crystallizing in addition to results from numerical modeling suggest that the high ti cumulates would sink only under highly specific conditions five scenarios for sinking high ti cumulate materials are examined and only two are found plausible in particular it is found that simple sinking of solidified high ti cumulates is unlikely because the temperature at which the cumulates solidify is low and viscosity under these conditions is very high it is however possible that high ti cumulates mixed with a substantial fraction of olivine would have viscosity low enough to allow them to sink as solids further because clinopyroxene and ilmenite melt in a ratio of 2 1 remelted high ti cumulates would be negatively buoyant and sink as liquids percolating downward through the underlying mantle and beginning to recrystallize ilmenite at 200 km depth making a hybrid heterogeneous mantle

variations in thickness of layer 3 dominate oceanic crustal structure abstract seismic refraction studies have determined that while much of the crust underlying the world s oceans is about 7 km thick it also occurs in lesser and considerably greater thicknesses here we show that vastly different thicknesses of oceanic crust formed at ridges by seafloor spreading are generally characterized by the two gradient velocity structure recognized as typical of 6 7 km thick oceanic crust adopting a standard we study ninety published seismic velocity structures of oceanic crust ranging in thickness from 2 to 37 km several structures formed in midplate settings ocean islands and seamounts are also considered for comparison regardless of origin ridge or midplate we find that the percentage of the whole crust formed by layer 2 systematically decreases with increasing total crustal thickness while the average velocity of layer 2 varies widely for all crust the average velocity of layer 3 increases systematically with increases in the average whole crustal thickness and velocity crust formed in midplate settings differs in that the thickness of layer 2 velocity crust is generally two to three times that of crust formed at ridges residual depths were calculated for oceanic crust by removing the effects of sediment loading and subsidence due to lithospheric cooling most of the data fall close to a model whereby different crustal thicknesses result from different extents of partial melting of oceanic upper mantle the crustal sections were restored to zero age assuming the average depth of emplacement for 7 km thick oceanic crust is 2 5 km below sea level from this restoration it appears that the depth to the top of layer 3 at the time it was formed is 4 25 1 25 km below sea level regardless of whole crustal thickness or spreading rate these results suggest that the mechanism of construction of oceanic crust is in many ways remarkably uniform despite very large changes in the total thickness of the crust produced very basic questions remain it is for instance not at all clear why layer 2 and layer 3 exist in the proportions they do while neutral buoyancy or melt trapping at the base of a brittle lid may account for local properties of the layer 2 3 boundary they do not seem adequate to account for the global phenomenon

oceanic crustal thickness versus spreading rate a compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness away from plateaus is 6 km no systematic increase of crustal thickness with spreading rate is observed instead the data show large variations in crustal thickness at slow spreading ridges 3 8 km for half rates 30 mm yr the large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid ocean ridges both data sets support the speculation of a transition from a 3 d structure of crustal accretion at slow ridges to a 2 d accretion pattern at fast ridges

modes of faulting at mid ocean ridges abyssal hill bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the earth the recognition that these faults form at plate spreading centres came with the plate tectonic revolution recent observations reveal a large range of fault sizes and orientations numerical models of plate separation dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast spreading centres conversely plate stretching with differing amounts of constant rate magmatic dyke intrusion can explain the great variety of fault offset seen at slow spreading ridges very large offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion

analysis of lunar samples implications for planet formation and evolution the analysis of lunar samples returned to earth by the apollo and luna missions changed our view of the processes involved in planet formation the data obtained on lunar samples brought to light the importance during planet growth of highly energetic collisions that lead to global scale melting this violent birth determines the initial structure and long term evolution of planets once past its formative era the lunar surface has served as a recorder of more than 4 billion years of interaction with the space environment the chronologic record of lunar cratering determined from the returned samples underpins age estimates for planetary surfaces throughout the inner solar system and provides evidence of the dynamic nature of the solar system during the planet forming era

asymmetric thermal evolution of the moon 1 the moon possesses a clear dichotomy in geological processes between the nearside and farside hemispheres the most pronounced expressions of this dichotomy are the strong concentration of radioactive heat sources on the nearside in a region known as the procellarum kreep terrane pkt and the mare basaltic lava flows that erupted in or adjacent to this terrane we model the thermochemical evolution of the moon using a 3 d spherical thermochemical convection code in order to assess the consequences of a layer enriched in heat sources below the pkt on the moon s global evolution we find that in addition to localizing most of the melt production on the nearside such an enriched concentration of heat sources in the pkt crust has an influence down to the core mantle boundary and leaves a present day temperature anomaly within the nearside mantle moderate gravitational and topographic anomalies that are predicted in the pkt but not observed may be masked either by crustal thinning or gravitational anomalies from dense material in the underlying mantle our models also predict crystallization of an inner core for sulfur concentrations less than 6 wt

characterization of previously unidentified lunar pyroclastic deposits using lunar reconnaissance orbiter camera data 1 we used a lunar reconnaissance orbiter camera lroc global monochrome wide angle camera wac mosaic to conduct a survey of the moon to search for previously unidentified pyroclastic deposits promising locations were examined in detail using lroc multispectral wac mosaics high resolution lroc narrow angle camera nac images and clementine multispectral ultraviolet visible or uvvis data out of 47 potential deposits chosen for closer examination 12 were selected as probable newly identified pyroclastic deposits potential pyroclastic deposits were generally found in settings similar to previously identified deposits including areas within or near mare deposits adjacent to highlands within floor fractured craters and along fissures in mare deposits however a significant new finding is the discovery of localized pyroclastic deposits within floor fractured craters anderson e and f on the lunar farside isolated from other known similar deposits our search confirms that most major regional and localized low albedo pyroclastic deposits have been identified on the moon down to 100 m pix resolution and that additional newly identified deposits are likely to be either isolated small deposits or additional portions of discontinuous patchy deposits

understanding the lunar surface and space moon interactions the surface of the moon is a critical boundary that shapes our understanding of the moon as a whole all geologic mapping and remote sensing techniques utilize only the outermost portion of the moon before leaving the moon for study in our laboratories all lunar samples that have been studied existed at or very near the surface with the exception of the deeply probing geophysical techniques our understanding of the interior of the moon is derived from surficial but not superficial information coupled with boundary of the lunar crust it is the lower boundary layer of the tenuous lunar atmosphere and constitutes both a source and a sink for atmospheric gases the surface is also where the moon interacts with the space environment causing changes in the physical nature of lunar materials and provides a laboratory for the study of processes that occur on all airless bodies the data obtained remotely by the galileo clementine and lunar prospector missions as well as data derived from lunar meteorites have resulted in major changes to our understanding of global distributions of chemistry and rocks this chapter summarizes the current understanding of this critical interface the surface of the moon in its role as the lower boundary of the lunar atmosphere the upper boundary of the crust and the window through which we view through remote sensing the composition of the crust and the history of the moon in this post lunar prospector time the view of the moon has changed lending new perspectives to lunar samples and lunar processes but the new view will likely remain in flux as we continue to digest the results from these recent space missions and move forward to a new era of lunar exploration despite the freshness of our perspective this is an important moment to capture

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

a young solidification age for the lunar magma ocean abstract the time at which the lunar magma ocean solidified can be determined from the lu hf isotope systematics of lunar rocks derived from magma sources that formed during crystallization of the lunar magma ocean the final magma ocean crystallization product termed urkreep is enriched in incompatible elements including k ree and p we have determined the initial hf isotopic compositions of four samples two kreep basalts and two mg suite norites the incompatible trace element compositions of these samples are controlled by an urkreep component and therefore the initial hf isotopic compositions of these samples represent the hf isotopic evolution of urkreep in order to correct the effects of neutron irradiation on the hf isotopic compositions of these samples we have developed a model that uses the stable hf and sm isotopic compositions measured on an irradiated sample to determine and correct for the thermal and epithermal neutron fluence that has modified the hf isotopic composition of the sample we use our corrected results to calculate a 176 lu 176 hf urkreep model age of 4353 37 ma and the 176 lu 177 hf of urkreep to be 0 0153 0 0033 the lu hf model age is concordant with the re calculated sm nd urkreep model age of 4389 45 ma and we take the average of these ages 4368 29 ma to represent the time at which urkreep formed this age is concordant with the age of the most reliably dated ferroan noritic anorthosite as well as 142 nd model ages for the formation or re equilibration of mare basalt sources taken together these ages indicate that the moon experienced a widespread large scale magmatic event around 4370 ma most plausibly attributed to solidification of the lunar magma ocean

targeting the impactors siderophile element signatures of lunar impact melts from serenitatis abstract highly siderophile element compositions of lunar impact melt breccias provide a unique record of the asteroid population responsible for large cratering events in the inner solar system melt breccias associated with the 3 89 ga serenitatis impact basin resolve at least two separate impact events kreep rich melt breccias representing the apollo 17 poikilitic suite are enriched in highly siderophile elements 3 6 15 8 ppb ir with ci normalized patterns that are elevated in re ru and pd relative to ir and pt the restricted range of lithophile element compositions combined with the coherent siderophile element signatures indicate formation of these breccias in a single impact event involving an eh chondrite asteroid probably as melt sheet deposits from the serenitatis basin one exceptional sample a split from melt breccia 77035 has a distinctive lithophile element composition and a siderophile element signature more like that of ordinary chondrites indicating a discrete impact event the recognition of multiple impact events and the clear signatures of specific types of meteoritic impactors in the apollo 17 melt breccias shows that the lunar crust was not comprehensively reworked by prior impacts from 3 9 to 4 5 ga an observation more consistent with a late cataclysm than a smoothly declining accretionary flux late accretion of enstatite chondrites during a 3 8 4 0 ga cataclysm may have contributed to siderophile element heterogeneity on the earth but would not have made a significant contribution to the volatile budget of the earth or oxidation of the terrestrial mantle siderophile element patterns of apollo 17 poikilitic breccias become more fractionated with decreasing concentrations trending away from known meteorite compositions to higher re ir and pd pt ratios the compositions of these breccias may be explained by a two stage impact melting process involving 1 deep penetration of the serenitatis impactor into meteorite free lower crust followed by 2 incorporation of upper crustal lithologies moderately contaminated by prior meteoritic infall into the melt sheet trends to higher re ir with decreasing siderophile element concentrations may indicate an endogenous lunar crustal component or a non chondritic late accretionary veneer in the pre serenitatis upper crust

near far side asymmetry in the tidally heated moon abstract using viscoelastic mass spring model simulations to track heat distribution inside a tidally perturbed body we measure the near far side asymmetry of heating in the crust of a spin synchronous moon in eccentric orbit about the earth with the young moon within 8 earth radii of the earth we find that tidal heating per unit area in a lunar crustal shell is asymmetric due to the octupole order moment in the earth s tidal field and is 10 to 20 higher on its near side than on its far side tidal heating reduces the crustal basal heat flux and the rate of magma ocean crystallization assuming that the local crustal growth rate depends on the local basal heat flux and the distribution of tidal heating in latitude and longitude a heat conductivity model illustrates that a moderately asymmetric and growing lunar crust could maintain its near far side thickness asymmetry but only while the moon is near the earth

global prediction of abyssal hill root mean square heights from small scale altimetric gravity variability 1 abyssal hills which are pervasive landforms on the seafloor of the earth s oceans represent a potential tectonic record of the history of mid ocean ridge spreading however the most detailed global maps of the seafloor derived from the satellite altimetry based gravity field cannot be used to deterministically characterize such small scale 10 km morphology nevertheless the small scale variability of the gravity field can be related to the statistical properties of abyssal hill morphology using the upward continuation formulation in this paper i construct a global prediction of abyssal hill root mean square rms heights from the small scale variability of the altimetric gravity field the abyssal hill related component of the gravity field is derived by first masking distinct features such as seamounts mid ocean ridges and continental margins and then applying a newly designed adaptive directional filter algorithm to remove fracture zone discontinuity fabric a noise field is derived empirically by correlating the rms variability of the small scale gravity field to the altimetric noise field in regions of very low relief and the noise variance is subtracted from the small scale gravity variance suites of synthetically derived abyssal hill formed gravity fields are generated as a function of water depth basement rms heights and sediment thickness and used to predict abyssal hill seafloor rms heights from corrected small scale gravity rms height the resulting global prediction of abyssal hill rms heights is validated qualitatively by comparing against expected variations in abyssal hill morphology and quantitatively by comparing against actual measurements of rms heights although there is scatter the prediction appears unbiased

horst and graben structures on the flanks of the mid atlantic ridge in the mark area 23 22 n submersible observations abstract south of the kane fracture zone 23 40 n the mid atlantic ridge is segmented into 20 100 km long spreading cells in which morphology seismic and gravity signatures change along and across the ridge axis the rift valley considered as a typical structure along slow spreading ridges can be symmetric or asymmetric and segment centers experience strong magmatic activity whereas low magma supply results in thinning of the crust at segment tips along the segment centered on the snake pit neovolcanic ridge 23 22 n the rift valley is clearly asymmetric and mantle and lower crustal rocks are commonly exposed on the steepest and highest ridge flanking wall in this area we analyzed submersible observations gravinaute cruise collected up to 50 km on both sides of the axis to discuss the proceses responsible for the axial valley shape its possible relationship with deep rock exposures and to determine the origin of off axis structures our data reveal that the structure and the geology vary on both walls of the axial valley and is different from the volcanic and tectonic organization observed on the neovolcanic ridge itself furthermore the structure of the off axis domain in different from the axial valley and rift mountains are organized in a set of regularly spaced relief about 10 km on both flanks of the ridge scarps which built these topographic highs face either toward the axis or away thus the off axis morphology appears as a set of horsts and grabens which could partly result from off axis tectonic processes and or from a shifting of the volcano tectonic activity out of the axial valley during the last 3 ma off axis data show that serpentinized peridotites more commonly observed on the western rift valley wall crop out on the western flank up to 30 away from the axis models proposed to account for such rock exposure are usually related to the formation of the deep axial valley along slow spreading ridges here we propose that mantle rocks are exposed only in a very particular structural context which is directly controled by the ridge segmentation and is independent of the shape of the axial valley symmetric or not serpentinites crop out within a v shaped structure corresponding with the boundary between two adjacent segments where seismic and gravity data reveal that the crust is thin and the lithosphere is thick here the crust is fed by a lower magma supply than at the segment center snake pit area thus we interpret this segment end to be a region of extremely thin crust where modest normal fault can bring mantle rocks to the surface

lunar surface geochemistry global concentrations of th k and feo as derived from lunar prospector and clementine data abstract accurate estimates of global concentrations of th k and feo have an important bearing on understanding the bulk chemistry and geologic evolution of the moon we present empirical ground truth calibrations transformations for lunar prospector gamma ray spectrometer data k and th and a modified algorithm for deriving feo concentrations from clementine spectral reflectance data that incorporates an adjustment for tio 2 content the average composition of soil samples for individual landing sites is used as ground truth for remotely sensed data among the apollo and luna sites apollo 12 and 14 provide controls for the incompatible element rich compositions apollo 16 and luna 20 provide controls for the feldspathic and incompatible element poor compositions and apollo 11 15 and 17 and luna 16 and 24 provide controls for fe rich compositions in addition to these apollo and luna sample data we include the composition of the feldspathic lunar meteorites as a proxy for the northern farside highlands to extend the range of the calibration points thus providing an additional anorthositic end member composition these empirical ground truth calibrations for lunar prospector th and k provide self consistency between the existing derived data and lunar sample data clementine spectral reflectance data are used to construct a tio 2 sensitive feo calibration that yields higher feo concentrations in areas of high fe low ti mare basalt rich surfaces than previous feo algorithms the data set so derived is consistent with known sample compositions and regolith mixing relationships

the constitution and structure of the lunar interior the current state of understanding of the lunar interior is the sum of nearly four decades of work and a range of exploration programs spanning that same time period missions of the 1960s including the rangers surveyors and lunar orbiters as well as earth based telescopic studies laid the groundwork for the apollo program and provided a basic understanding of the surface its stratigraphy and chronology through a combination of remote sensing surface exploration and sample return the apollo missions provided a general picture of the lunar interior and spawned the concept of the lunar magma ocean in particular the discovery of anorthite clasts in the returned samples led to the view that a large portion of the moon was initially molten and that crystallization of this magma ocean gave rise to mafic cumulates that make up the mantle and plagioclase flotation cumulates that make up the crust smith et al 1970 wood et al 1970 this model is now generally accepted and is the framework that unifies our knowledge of the structure and composition of the moon the intention of this chapter is to review the major advances that have been made over the past decade regarding the constitution of the moon s interior much of this new knowledge is a direct result of data acquired from the successful clementine and lunar prospector missions as well as the analysis of new lunar meteorites as will be seen results from these studies have led to many fundamental amendments to the magma ocean model much of what we know from sample analyses has been previously summarized elsewhere and only their most important aspects will be discussed in this chapter the reader is referred to the relevant chapters in the books basaltic volcanism on the terrestrial planets basaltic volcanism study project 1981 the

volcanic growth faults and the origin of pacific abyssal hills the topographic features known as abyssal hills characterize 30 of the ocean floor and yet their origin has been the subject of vigorous debate for over 40 years submersible based investigations show that pacific abyssal hills are created on the flanks of the east pacific rise as horsts and grabens which lengthen with time hills are bounded on one side by ridge facing scarps produced by normal faulting and on the other by more gentle slopes produced by volcanic growth faulting

the isotopic record of lunar volcanism abstract the timing and extent of lunar volcanism was determined primarily by the internal thermal evolution of the moon a late heavy bombardment produced most of the observable lunar basins easily recognized stratigraphic markers and also may have influenced lunar volcanism most of the nearside basins seem to have been produced 3 8 4 0 ga ago during a period often referred to as the lunar cataclysm the pre cataclysm period was characterized by formation of the lunar crust plutonic rocks within the crust and pre mare volcanism isotopic analysis of breccia clasts pre dating the cataclysm has yielded an outline of this period the isotopic record of post cataclysm mare volcanism is clear but incomplete because samples from the youngest mare areas are lacking specifically the isotopic data show that some lunar anorthosites formed 4 45 ga ago consistent with early formation of an anorthositic lunar crust some mg suite norites troctolites and dunites also formed approximately contemporaneously with the oldest anorthosites but others are up to 200 300 ma younger more evolved rocks granites and quartz monzodiorites also formed between 4 4 and 4 0 ga ago the earliest known pre mare volcanism at 4 25 ga ago included aluminous mare like basalts and an olivine gabbronorite similar to apollo 12 basalts kreep basalt volcanism began at least 4 1 ga ago approximately contemporaneously with that of the ancient mare like basalts and continued during the cataclysm recorded instances of high ti mare basalt volcanism began during or immediately after the cataclysm low ti mare basalt volcanism is the most recent for which samples are available the isotopic data record an early primary lunar differentiation during which mare basalt sources formed probably by crystallization of cumulus minerals from a lunar magma ocean the isotopic record of post cataclysm basalts indicates a heterogeneous lunar mantle and can be explained by partial melting of cumulate sources but more complex petrogenesis also has been suggested more complex petrogenetic processes including assimilation and assimilation fractional crystallization afc seem to be required for pre mare volcanics the isotopic data contain a hint that petrogenesis of the oldest pre mare volcanics may be linked to formation of the so called procellarum basin possibly the oldest and largest lunar basin such a basin would provide a geologic context for the pre mare volcanics and its formation may have been a defining moment in lunar history

contributors to chromium isotope variation of meteorites we report the results of a comprehensive high precision survey of the cr isotopic compositions of primitive chondrites along with some differentiated meteorites to ensure complete dissolution of our samples they were first fused with lithium borate tetraborate at 1050 1000 c relative to the nist cr standard srm 3112a carbonaceous chondrites exhibit excesses in 54cr 52cr from 0 4 to 1 6e 1e 1 part in 10 000 and ordinary chondrites display a common 54cr 52cr deficit of 0 4e analyses of acid digestion residues of chondrites show that carbonaceous and ordinary chondrites share a common 54cr enriched carrier which is characterized by a large excess in 54cr 52cr up to 200e associated with a very small deficit in 53cr 52cr 2e we did not find 54cr anomalies in either bulk enstatite chondrites or in leachates of their acid digestion residues this either requires that the enstatite chondrite parent bodies did not incorporate the 54cr anomaly carrier phase during their accretion or the phase was destroyed by parent body metamorphism chromium in the terrestrial rocks and lunar samples analyzed here show no deviation from the nist srm 3112a cr standard the eucrite and martian meteorites studied exhibit small deficits in 54cr 52cr the 54cr 52cr variations among different meteorite classes suggest that there was a spatial and or temporal heterogeneity in the distribution of a 54cr rich component in the inner solar system we confirm the correlated excesses in 54cr 52cr and 53cr 52cr for bulk carbonaceous chondrites but the new data yield a steeper slope 6 6 than that reported in shukolyukov and lugmair 2006 the correlated excesses may affect the use of the mn cr chronometer in carbonaceous chondrites we could not confirm the bulk carbonaceous chondrite mn cr isochron reported by shukolyukov and lugmair 2006 and moynier et al 2007 mostly because we find much smaller total variations in e53cr 0 2 all bulk chondrites have small e53cr excesses up to 0 3 relative to the earth most likely reflecting the sub chondritic mn cr ratio of the earth the e53cr variations in chondrites do seem to grossly correlate with mn cr and yield an initial solar system 53mn 55mn value of 5 4 2 4 10 6 corresponding to an absolute age of 4566 4 2 2 ma nuclear interactions with cosmic rays result in coupled excesses in e54cr and e53cr with a 4 1 ratio in phases with high fe cr these are most dramatically demonstrated in the iron meteorite carbo showing excesses in e54cr of up to 140e these new results show that the mn cr chronometer should be used with caution in samples minerals with high fe cr and long cosmic ray exposure ages

osmium isotope and highly siderophile element constraints on ages and nature of meteoritic components in ancient lunar impact rocks abstract the concentrations of highly siderophile elements hse re os ir ru pt rh pd au and 187os 188os isotope compositions have been determined for 67 subsamples of six lunar impact rocks from the apollo 14 16 and 17 landing sites and the lunar meteorite dar al gani dag 400 using inductively coupled plasma mass spectrometry icp ms and negative thermal ionization mass spectrometry n tims we report the first re os isochron age on a lunar impact melt rock 187re 187os isotope systematics for apollo 16 sample 67935 define an isochron age of 4 21 0 13 ga mswd 1 5 which is interpreted to reflect localized partitioning processes between solid metal liquid metal as this rock melted the new age adds further constraints on the significance of pre 4 0 ga basin forming impacts on the moon and possible mixing of ancient impactor compositions in lunar impact rocks linear correlations displayed by subsamples of a given impact rock in plots of hse versus ir concentrations are explained by dilution processes through essentially hse free anorthositic lunar crustal target rocks or binary mixing between a high hse meteoritic end member and a low hse end member composition slope derived hse ratios and 187os 188os of the meteoritic component in granulitic impactites 67915 67955 and 79215 are similar to slightly volatile element depleted carbonaceous chondrites suprachondritic ratios of ru ir pt ir rh ir and pd ir for apollo 14 impact melt rock 14310 are similar to ratios observed for other apollo 14 samples and apollo 17 poikilitic impact melt rocks apollo 16 poikilitic and subophitic impact melt rocks 60315 and 67935 show slightly subchondritic os ir and suprachondritic ratios of 187os 188os ru ir pt ir rh ir pd ir and au ir their strongly fractionated hse compositions are similar to some members of the iva iron meteorite group and provide further evidence for an iron meteorite impactor component in apollo 16 impact melt rocks the range of chondritic to suprachondritic ratios of 187os 188os ru ir pt ir pd ir and chondritic to slightly subchondritic os ir of apollo 17 and 14 impact melt rocks may be explained by variable mixing of a carbonaceous chondrite like hse composition as in granulitic impactites with a suprachondritic iron meteorite like impactor composition as inferred from 67935 and 60315 and subsequent homogenization by younger impact events the inferred dispersal of these two components across the lunar nearside and their antiquity 4 2 ga suggest at least two basin forming impacts that predate the 3 9 3 8 ga basin forming era the hse composition of the silicate earth can be matched by a late veneer comprised of 80 volatile depleted carbonaceous chondrite like material and 20 differentiated meteoritic metal similar to what has been identified in ancient lunar impact rocks

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data

lunar magma ocean crystallization revisited bulk composition early cumulate mineralogy and the source regions of the highlands mg suite abstract crystallization of the lunar magma ocean lmo has been numerically modeled and its products inferred from sample observations but it has never been fully tested experimentally this study is a reexamination of the lmo hypothesis by means of the first experimental simulation of lunar differentiation two end member bulk moon compositions are considered one enriched in refractory lithophile elements relative to earth and one with no such enrichment a two stage model of magma ocean crystallization based on geophysical constraints is simulated and features early crystal suspension and equilibrium crystallization followed by fractional crystallization of the residual magma ocean an initially entirely molten moon is assumed part 1 of this study presented here focuses on stage 1 of this model and considers the early cumulates formed by equilibrium crystallization differences in mantle mineralogy resulting from different bulk moon compositions and implications for the source regions of the highlands mg suite refractory element enriched bulk moon compositions produce a deep mantle that contains garnet and trace cr spinel in addition to low ca pyroxene and olivine in contrast compositions without refractory element enrichment produce a deep dunitic mantle with low ca pyroxene but without an aluminous phase the differences in bulk composition are magnified in the residual melt the residual lmo from the refractory element enriched composition will likely produce plagioclase and ilmenite earlier and in greater quantities both compositions produce mg rich early cumulate piles that extend from the core mantle boundary to 355 km depth if 50 equilibrium crystallization and whole moon melting are assumed these early lmo cumulates provide good fits for the source regions for a component of the high mg ni and co poor parental magmas of the mg suite cumulates if certain conditions are called upon the olivine in early lmo cumulates produced by either bulk moon composition is far too rich in cr to be reasonable for the source regions of the mg suite meaning either core formation in the presence of s and or c must be invoked to deplete the lmo and the crystallizing olivine in cr or that current estimates of the bulk lunar cr content are too high we infer that melts meeting the criteria of the mg suite parents could be produced from early lmo cumulates by solid state kreep and plagioclase hybridization near the base of the crust and subsequent partial melting additionally we propose a revised model for mg suite petrogenesis

ages and stratigraphy of lunar mare basalts in mare frigoris and other nearside maria based on crater size frequency distribution measurements 1 we report on ages derived from impact crater counts for exposed mare basalt units in the northern part of the lunar nearside hemisphere mare frigoris the eastern and northeastern part of the nearside hemisphere lacus temporis joliot hubble goddard mare marginis and mare smythii the central part of the nearside hemisphere palus putredinis mare vaporum and sinus medii and the southwestern part of the nearside hemisphere grimaldi cruger rocca a lacus aestatis and schickard in mare frigoris we dated 37 basalt units showing ages from 2 61 to 3 77 gyr with most units being formed in the late imbrian period between 3 4 and 3 8 gyr ago in mare vaporum we dated six spectrally homogeneous units that show model ages of 3 10 to 3 61 gyr our model ages of basalts in mare marginis range from 3 38 to 3 88 gyr and are mostly older than basalts in mare smythii 3 14 3 48 gyr the model ages of four units in sinus medii indicate that the basalts in this region formed 3 63 to 3 79 gyr ago we find an excellent agreement of our crater size frequency model ages of the palus putredinis area which contains the apollo 15 landing site with the radiometric ages of apollo 15 samples according to our crater counts basalts in palus putredinis are 3 34 gyr old and this compares favorably with the radiometric ages of 3 30 3 35 gyr of the olivine normative and quartz normative basalts of the apollo 15 landing site lacus aestatis is a small irregular shaped mare patch in the southwestern nearside and shows an imbrian age of 3 50 gyr basalts in lacus temporis in the northeastern nearside formed between 3 62 and 3 74 gyr ago and are therefore older than the basalts in lacus aestatis we found that basalts in craters of the southwestern nearside schickard grimaldi cruger and rocca a are also mostly younger than basalts in craters of the northeastern nearside hubble joliot and goddard while basalt ages vary between 3 16 and 3 75 gyr in the southwest basalts in the northeast are 3 60 3 79 gyr old these results confirm and extend the general distribution of ages of mare basalt volcanism and further underline the predominance of older mare basalt ages in the eastern and southern nearside and in patches of mare peripheral to the larger maria in contrast to the younger basalt ages on the western nearside oceanus procellarum

velocity and attenuation in young oceanic crust new downhole log results from dsdp odp iodp holes 504b and 1256d 1 to characterize the acoustic properties of oceanic crust and investigate the nature of the oceanic layer 2 3 seismic boundary we analyze sonic logging waveforms recorded in the two deepest holes ever drilled in intact oceanic crust ocean drilling program odp integrated ocean drilling program iodp hole 1256d and deep sea drilling program dsdp odp hole 504b hole 1256d penetrated 15 ma old crust on a superfast spreading segment of the cocos plate while hole 504b was drilled in a 6 ma old section of an intermediate spreading segment of the nazca plate careful reprocessing of different sets of waveforms allows us to draw depth profiles of the primary parameters controlling wave propagation in the crust compressional and shear velocities vp and vs and attenuation the results underline the importance of proper filtering and processing parameters for the acquisition of acoustic logs in hard rocks while the reprocessed vp logs improve previous data particularly at the bottom of hole 1256d the vs logs represent significant enhancements in quality and in extent of previous profiles in the two holes attenuation results provide the first measure of intrinsic attenuation in these holes the velocity and attenuation logs confirm that hole 504b had reached oceanic layer 3 without recovering gabbros conversely while gabbros were recovered in hole 1256d no layer 3 velocity values were recorded in the interval logged however the reprocessed data suggest that the transition to layer 3 might occur within the deepest 87 m of the hole that were not logged or immediately below accordingly the layer 2 3 seismic boundary does not appear to be related to the top of the plutonic crust despite different ages spreading rates and mineralogy however the two sites display remarkably similar velocity and attenuation profiles through the entire crustal sections logged suggesting that they can be reliably considered representative of the acoustic properties of the nazca cocos spreading system and more generally of undisturbed oceanic crust

crustal thickness along the western galapagos spreading center and the compensation of the galapagos hotspot swell wide angle refraction and multichannel reflection seismic data show that oceanic crust along the galapagos spreading center gsc between 97 w and 91 25pw thickens by 2 3 km as the galapagos plume is approached from the west this crustal thickening can account for v52 of the 700 m amplitude of the galapagos swell after correcting for changes in crustal thickness the residual mantle bouguer gravity anomaly associated with the galapagos swell shows a minimum of 325 mgal near 92 15pw the area where the gsc is intersected by the wolf darwin volcanic lineament wdl the remaining depth and gravity anomalies indicate an eastward reduction of mantle density estimated to be most prominent above a compensation depth of 50 100 km melting calculations assuming adiabatic passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of v30 c the associated thermal expansion and increase in melt depletion reduce mantle densities but to a degree that is insufficient to explain the geophysical observations the largest density anomalies appear at the intersection of the gsc and the wdl our results therefore require the existence of compositionally buoyant mantle beneath the gsc near the galapagos plume possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the galapagos islands that is later transported to the gsc our estimate for the buoyancy flux of the galapagos plume 700 kg s 31 is lower than previous estimates while the total crustal production rate of the galapagos plume 5 5 m 3 s 31 is comparable to that of the icelandic and hawaiian plumes

the miocene pliocene macusani volcanics se peru i mineralogy and magmatic evolution of a two mica aluminosilicate bearing ignimbrite suite

forming a moon with an earth like composition via a giant impact in the giant impact theory the moon formed from debris ejected into an earth orbiting disk by the collision of a large planet with the early earth prior impact simulations predict that much of the disk material originates from the colliding planet however earth and the moon have essentially identical oxygen isotope compositions this has been a challenge for the impact theory because the impactor s composition would have likely differed from that of earth we simulated impacts involving larger impactors than previously considered we show that these can produce a disk with the same composition as the planet s mantle consistent with earth moon compositional similarities such impacts require subsequent removal of angular momentum from the earth moon system through a resonance with the sun as recently proposed

a review of lunar chronology revealing a preponderance of 4 34 4 37 ga ages in this study data obtained from sm nd and rb sr isotopic measurements of lunar highlands samples are renormalized to common standard values and then used to define ages with a common isochron regression algorithm the reliability of these ages is evaluated using five criteria that include whether 1 the ages are defined by multiple isotopic systems 2 the data demonstrate limited scatter outside uncertainty 3 initial isotopic compositions are consistent with the petrogenesis of the samples 4 the ages are defined by an isotopic system that is resistant to disturbance by impact metamorphism and 5 the rare earth element abundances determined by isotope dilution of bulk of mineral fractions match those measured by in situ analyses from this analysis it is apparent that the oldest highlands rock ages are some of the least reliable and that there is little support for crustal ages older than 4 40 ga a model age for ur kreep formation calculated using the most reliable mg suite sm nd isotopic systematics in conjunction with sm nd analyses of kreep basalts is 4389 45 ma this age is a good match to the lu hf model age of 4353 37 ma determined using a subset of this sample suite the average modelmore age of 4353 25 ma determined on mare basalts with the 146sm 142nd isotopic system with a peak in pb pb ages observed in lunar zircons of 4340 20 ma and the oldest terrestrial zircon age of 4374 6 ma the preponderance of ages between 4 34 and 4 37 ga reflect either primordial solidification of a lunar magma ocean or a widespread secondary magmatic event on the lunar nearside the first scenario is not consistent with the oldest ages reported for lunar zircons whereas the second scenario does not account for concordance between ages of crustal rocks and mantle reservoirs less

a highly depleted moon or a non magma ocean origin for the lunar crust abstract coupled 146 147 sm 142 143 nd systematics were analyzed on lunar samples that span the range of source compositions created in the early lunar differentiation event ferroan anorthosites high mg suite norite kreep and low and high ti basalts if fit with a single isochron the data agree with previous suggestions of a 250 ma duration for closure of the sm nd system in the lunar interior the two crustal rocks 60025 and 78236 among oldest samples found on the lunar surface have high initial 142 nd 144 nd ratios e 142 nd 0 24 0 27 relative to o chondrites which indicates that they derive from a source strongly depleted in light rare earth elements lree 147 sm 144 nd 0 23 this result suggests either that the moon formed with a strongly superchondritic sm nd ratio or that the lunar crust formed as a product of a post magma ocean remelting of lree depleted cumulates in the lunar interior the basalt data measured here add support to the idea that the lunar interior evolved with a superchondritic sm nd similar to that of the early earth addition of differentiation steps adds complexity and inherit non uniqueness to the interpretation of the lunar nd isotope data but our preferred model has the first stage of lunar evolution 4 567 to 4 44 ga occurring on an already differentiated earth with the moon forming shortly before 4 45 ga and undergoing rapid magma ocean evolution to form the crust the high sm nd source reservoirs of many mare basalt types and kreep by 4 44 ga continued differentiation of the lunar interior through the increasing depletion of the source of high ti mare basalts then continued until 4320 ma

global elemental maps of the moon the lunar prospector gamma ray spectrometer lunar prospector gamma ray spectrometer spectra along with counting rate maps of thorium potassium and iron delineate large compositional variations over the lunar surface thorium and potassium are highly concentrated in and around the nearside western maria and less so in the south pole aitken basin counting rate maps of iron gamma rays show a surface iron distribution that is in general agreement with other measurements from clementine and the lunar prospector neutron detectors

a global and regional stochastic analysis of near ridge abyssal hill morphology this paper presents the results of a global and regional stochastic analysis of near ridge abyssal hill morphology the analysis includes the use of sea beam data for the estimation of stochastic parameters up to order 4 these parameters provide important quantitative physical information regarding abyssal hills including their rms height azimuthal orientation characteristic width aspect ratio hausdorff dimension skewness tilt and peakiness the global data set consists of 64 sea beam swaths near the rivera cocos and nazca spreading sections of the east pacific rise the mid atlantic ridge and the central indian ridge in one form of analysis the parameters are averaged among spreading rate bins each of the spreading rate subsets can be identified as unique from the others in at least one aspect the slowest spreading rate subset mid atlantic data exhibit the largest scales rms height and characteristic width and length of abyssal hills these parameters generally decrease as spreading rate increases up to the fast spreading rate data pacific cocos but increase going from fast to very fast pacific nazca spreading rate data this indicates some complexity in the relationship between spreading rate and abyssal hill morphology the plan view aspect ratio is nearly twice as large for the fast spreading rate data than for any of the other subsets and is smallest for the very fast spreading rate data the fractal dimension is nearly identical for all spreading rate subsets the vertical skewness is positive for the slow and medium spreading rate data indicating larger peaks than troughs and negative for the fast spreading rate data indicating larger troughs than peaks the kurtosis or peakiness is everywhere larger than the gaussian value of 3 and tends to be larger in the atlantic than the pacific the tilting parameter provides substantial evidence indicating steeper inward facing slopes in the medium and fast spreading rate data but only marginal evidence for it in the slow spreading rate data from an analysis of correlations among parameters it is found that subsets sometimes behave differently from the entire data set in particular while over the global data set the characteristic width exhibits a well resolved positive trend when plotted versus rms height these parameters exhibit a more gradual positive trend in the mid atlantic data and a negative trend in the pacific cocos data in addition the plan view aspect ratio while generally uncorrelated with rms height for the global data set is positively correlated with rms height for the pacific cocos data set these results emphasize a strong uniqueness of the pacific cocos data relative to the rest of the data global set the pacific cocos data consist of 27 swaths concentrated between the siquieros and orozco fracture zones these data provide very good abyssal hill coverage of this well mapped and well studied region and form the basis of a regional analysis of the correlation between ridge morphology and stochastic abyssal hill parameters in this analysis it is found that abyssal hill parameters are correlated with the depth of the adjacent ridge axis indicating that the relative abundance in magma supply which likely controls the ridge axis depth may also be an important influence on the formation of abyssal hills

on spreading modes and magma supply at slow and ultraslow mid ocean ridges abstract the ultraslow eastern southwest indian ridge swir offers an opportunity to study the effect of magma supply on an ultraslow mid ocean ridge starting from quasi melt free detachment dominated spreading and transitioning to volcanic spreading as one nears prominent axial volcanos detachments in the quasi melt free mode extend along axis 60 to 95 km and have a lifetime of 0 6 to 1 5 myrs they cut into their predecessor s footwall with an opposite polarity causing part of the footwall lithosphere to experience further deformation hydrothermal alteration sparse magmatism and possibly thermal rejuvenation in a hanging wall position the accretion of the oceanic lithosphere in this context therefore occurs in two distinct stages over the lifetime of two successive detachment faults we examine the transition from this nearly amagmatic detachment dominated mode to the more common volcanic mode of spreading showing that it occurs along axis over distances 30 km it involves a significant thinning of the axial lithosphere and a gradual decrease of the amount of tectonic displacement on faults as the magmatic contribution to the divergence of the two plates increases we develop a conceptual model of this transition in which magma plays a double role it fills the space between the diverging plates thus reducing the need for displacement along faults and it modifies the thermal state and the rheology of the plate boundary affecting its thickness and its tectonic response to plate divergence based on a comparison of the ultraslow eastern swir with the faster spreading mid atlantic ridge we show that the activation of the volcanic or of the detachment dominated modes of spreading is connected with the volume of magma supplied per increment of plate separation over a range of axial lithosphere thickness and therefore over a range of the m ratio defined by buck et al 2005 as the relative contribution of magma and faults to plate divergence m is smaller for a given volume of melt per increment of plate separation if the plate is thicker we therefore propose that m does not fully explain the variability in faulting styles observed at slow and ultraslow ridges and propose that rheological changes induced by magma also play a key role melt itself is weak hydrothermally altered gabbro peridotite mixtures are weak and melt heat sustains more vigorous hydrothermal circulation resulting in contrasted potentials for strain localization footwall flexure on faults and the development of detachment faults

146sm 142nd formation interval for the lunar mantle now extinct 146sm t1 2 103ma was present in the early solar system and lunar basalts might be expected to have inherited small anomalies in 142nd abundances from mantle sources with variable sm nd ratios established during the early global differentiation of the moon 142nd 144nd ratios were measured for several lunar basalts and values of nd142 deviations from terrestrial 142nd 144nd in parts in 104 calculated for them the nd142 values show very small variations attributable to 146sm decay neutron capture near the lunar surface also modified both the samarium and neodymium isotopic compositions of the basalt samples the calculated neutron fluences range from 0 to 7 8 1016 n cm2 both thermal and epithermal neutron fluences were calculated from the sm isotopic compositions of the basalts and used to correct the nd142 values for neutron capture by neodymium well resolved radiogenic enrichments nd142 and 0 17 0 08 2 respectively were measured for lunar meteorite asuka 881757 and apollo 17 basalt 74255 respectively for which the neutron fluences were nil basalts 70135 and 75075 which were exposed to small measureable neutron fluences of 2 5 1015 and 7 7 1015 n cm2 respectively have neutron corrected nd142 values of 0 25 0 15 and 0 29 0 11 respectively the average value of radiogenic nd142 for the three high ti basalts from apollo 17 is 0 22 0 06 neutron capture effects were greater for the other basalts but can be corrected using the measured neutron fluences derived from the samarium isotopic data the neutron corrected nd142 for apollo 12 ilmenite basalt 12056 consistent with its derivation from a highly depleted mantle source also three low ti basalts from apollo 12 and apollo 15 12038 15076 15555 have neutron corrected nd142 which are not resolved from zero and average 0 04 0 06 a fourth low ti basalt 12039 has neutron corrected nd142 but is subject to the largest neutron correction of 0 32 e units kreep basalt 14078 was exposed to only a small neutron fluence of 6 4 1015 n cm2 and has a small deficit of 142nd corresponding to nd142 the average neutron corrected nd142 of three kreep basalts is 0 05 0 04 the nd142 values of the basalts correlate with 147sm 144nd values for their source regions as calculated from their ages and initial nd142 values 147sm 143nd and 146sm 142nd systematics were combined in a three stage model yielding an isochron equation for the lunar mantle formation interval which was calculated to be 238 40 56 ma 2 sm nd ratios for the source regions also are derived from the model and compare favorably with values from conventional geochemical models of mare basalt petrogenesis if the moon were formed by impact of a large mars sized planetesimal with the earth the impact was early enough that the lunar mantle cooled to neodymium isotopic closure by 4 32 ga ago the bulk lunar moon142 evaluated from the mantle isochron at 147sm 144nd chur 0 1967 is 0 01 0 03 2 consistent with derivation of lunar and terrestrial neodymium from a common reservoir

constraints on the formation age and evolution of the moon from 142 nd 143 nd systematics of apollo 12 basalts the moon likely formed as a result of a giant impact between proto earth and another large body the timing of this event and the subsequent lunar differentiation timescales are actively debated new high precision nd isotope data of apollo mare basalts are used to evaluate the low ti high ti and kreep mantle source reservoirs within the context of lunar formation and evolution the resulting models are assessed using both reported 146sm half lives 68 and 103 myr the linear relationship defined by 142nd 143nd systematics does not represent multi component mixing and is interpreted as an isochron recording a mantle closure age for the sm nd system in the moon using a chondritic source model with present day 142nd of 7 3 the mare basalt mantle source reservoirs closed at 4 45 09 10 ga t1 2sm146 68 myr or 4 39 14 16 ga t1 2sm146 103 myr in a superchondritic 2 stage evolution model with present day nd142 of 0 mantle source closure ages are constrained to 4 41 08 10 t1 2sm146 68 myr or 4 34 14 15 ga t1 2sm146 103 myr the lunar mantle source reservoir closure ages 4 5 ga may be reconciled by 3 potential scenarios first the moon formed later than currently favored models indicate such that the lunar mantle closure age is near or at the time of lunar formation second the moon formed ca 4 55 to 4 47 ga and small amounts of residual melts were sustained within a crystallizing lunar magma ocean lmo for up to ca 200 myr from tidal heating or asymmetric lmo evolution third the lmo crystallized rapidly after early moon formation thus the sm nd mantle closure age represents a later resetting of isotope systematics this may have resulted from a global wide remelting event while current earth moon formation constraints cannot exclusively advocate or dismiss any of these models the fact that u pb ages and hf isotopes for jack hills zircons from australia are best explained by an earth that re equilibrated at 4 4 ga or earlier following the moon forming impact does not favor a later forming moon if magma oceans crystallize in a few million years as currently advocated then a global resetting possibly by a large impact at 4 40 to 4 34 ga such as that which formed the south pole aitken basin best explains the late mantle closure age for the coupled sm nd isotope systematics presented here

osmium isotope and highly siderophile element systematics of lunar impact melt breccias implications for the late accretion history of the moon and earth abstract to characterize the compositions of materials accreted to the earth moon system between about 4 5 and 3 8 ga we have determined os isotopic compositions and some highly siderophile element hse re os ir ru pt and pd abundances in 48 subsamples of six lunar breccias these are apollo 17 poikilitic melt breccias 72395 and 76215 apollo 17 aphanitic melt breccias 73215 and 73255 apollo 14 polymict breccia 14321 and lunar meteorite nwa482 a crystallized impact melt plots of ir versus other hse define excellent linear correlations indicating that all data sets likely represent dominantly two component mixtures of a low hse target presumably endogenous component and a high hse presumably exogenous component linear regressions of these trends yield intercepts that are statistically indistinguishable from zero for all hse except for ru and pd in two samples the slopes of the linear regressions are insensitive to target rock contributions of ru and pd of the magnitude observed thus the trendline slopes approximate the elemental ratios present in the impactor components contributed to these rocks the 187 os 188 os and regression derived elemental ratios for the apollo 17 aphanitic melt breccias and the lunar meteorite indicate that the impactor components in these samples have close affinities to chondritic meteorites the hse in the apollo 17 aphanitic melt breccias however might partially or entirely reflect the hse characteristics of hse rich granulitic breccia clasts that were incorporated in the impact melt at the time of its creation in this case the hse characteristics of these rocks may reflect those of an impactor that predated the impact event that led to the creation of the melt breccias the impactor components in the apollo 17 poikilitic melt breccias and in the apollo 14 breccia have higher 187 os 188 os pt ir and ru ir and lower os ir than most chondrites these compositions suggest that the impactors they represent were chemically distinct from known chondrite types and possibly represent a type of primitive material not currently delivered to earth as meteorites

the southeast indian ridge between 88 e and 118 e gravity anomalies and crustal accretion at intermediate spreading rates although slow spreading ridges characterized by a deep axial valley and fast spreading ridges characterized by an axial bathymetric high have been extensively studied the transition between these two modes of axial morphology is not well understood we conducted a geophysical survey of the intermediate spreading rate southeast indian ridge between 88 e and 118 e a 2300 km long section of the ridge located between the amsterdam hot spot and the australian antarctic discordance where satellite gravity data suggest that the southeast indian ridge seir undergoes a change from an axial high in the west to an axial valley in the east a basic change in axial morphology is found near 103 30 e in the shipboard data the axis to the west is marked by an axial high while a valley is found to the east although a well developed axial high characteristic of the east pacific rise epr is occasionally present the more common observation is a rifted high that is lower and pervasively faulted sometimes with significant 50 m throw faults within a kilometer of the axis a shallow axial valley 1200 m deep valley across a transform at 114 e the changes in axial morphology along the seir are accompanied by a 500 m increase in near axis ridge flank depth from 2800 m near 88 e to 3300 m near 114 e and by a 50 mgal increase in the regional level of mantle bouguer gravity anomalies over the same distance the regional changes in depth and mantle bouguer anomaly mba gravity can be both explained by a 1 7 2 4 km change in crustal thickness or by a mantle temperature change of 50 c 90 c in reality melt supply crustal thickness and mantle temperature are linked so that changes in both may occur simultaneously and these estimates serve as upper bounds the along axis mba gradient is not uniform pronounced steps in the regional level of the mba gravity occur at 103 30 e 104 e and at 114 e 116 e and correspond to the changes in the nature of the axial morphology and in the amplitude of abyssal hill morphology suggesting that the different forms of morphology do not grade into each other but rather represent distinctly different forms of axial structure and tectonics with a sharp transition between them the change from an axial high to an axial valley requires a threshold effect in which the strength of the lithosphere changes quickly the presence or absence of a quasi steady state magma chamber may provide such a mechanism the different forms of axial morphology are also associated with different intrasegment mba gravity patterns segments with an axial high have an mba low located at a depth minimum near the center of the segment at epr like segments the mba low is about 10 mgal with along axis gradients of 0 15 0 25 mgal km similar to those observed at the epr rifted highs have a shallower low and lower gradients suggesting an attenuated composite magma chamber and a reduced and perhaps episodic melt supply segments with a shallow axial valley have very flat along axis mba profiles with little correspondence between axial depth and axial mba gravity

extremely thin crust in the indian ocean possibly resulting from plume ridge interaction the thickness of the crust created at ocean spreading centres depends on the spreading rate and melt production in the mantle it is 5 8 km for a crust formed at slow and fast spreading centres and 2 4 km at ultra slow spreading centres away from hotspots and mantle anomalies the crust is generally thin at the fracture zones and thick beneath hotspots and large igneous provinces here we present results for the crust generated at the fast wharton spreading centre 55 58 ma ago using seismic reflection and refraction data we find that the crust over a 200 km segment of the wharton basin is only 3 5 4 5 km thick the thinnest crust ever observed in a fast spreading environment a thin crust could be produced by the presence of depleted and or cold mantle numerical simulations and recent laboratory experiments studying the impact of a hot plume under a lithosphere show that a curtain of weak cold downwellings of depleted mantle material is likely to develop around the edges of the hot plume pond because of a strongly temperature dependent viscosity of lithospheric material the hotter therefore less viscous bottom of the lithosphere can be mobilized by an impinging plume if sampled by a spreading centre the locally cold and depleted mantle should result in low production of melt we suggest that the observed thin crust in the wharton basin is likely to have been formed by the interaction between the kerguelen mantle plume and the wharton spreading centre 55 ma ago

seismic structure of the upper crust from 0 75 ma in the equatorial atlantic ocean on the african plate using ultralong offset seismic data

emag2 a 2 arc min resolution earth magnetic anomaly grid compiled from satellite airborne and marine magnetic measurements 1 a global earth magnetic anomaly grid emag2 has been compiled from satellite ship and airborne magnetic measurements emag2 is a significant update of our previous candidate grid for the world digital magnetic anomaly map the resolution has been improved from 3 arc min to 2 arc min and the altitude has been reduced from 5 km to 4 km above the geoid additional grid and track line data have been included both over land and the oceans wherever available the original shipborne and airborne data were used instead of precompiled oceanic magnetic grids interpolation between sparse track lines in the oceans was improved by directional gridding and extrapolation based on an oceanic crustal age model the longest wavelengths 330 km were replaced with the latest champ satellite magnetic field model mf6 emag2 is available at http geomag org models emag2 and for permanent archive at http earthref org cgi bin er cgi s erda cgi n 970

petrogenesis of mare basalts a record of lunar volcanism returned rock and soil samples from our nearest planetary neighbor have provided the basis for much of our understanding of the origin and evolution of the moon of particular importance are the mare basalts which have revealed considerable information about lunar volcanism and the nature of the mantle as well as post magma generation processes this paper is a critical review of the petrogenetic models for the generation of mare basalts formulated over the last twenty years we have used all available mare basalt analyses to define a six fold classification scheme using tio2 contents as the primary division i e 1 wt very low ti or vlt 1 6 wt low ti 6 wt high ti a secondary division is made using al2o3 contents i e 11 wt low al 11 wt high al and a tertiary division is defined using k contents i e 2000 ppm low k 2000 ppm high k such divisions and subdivisions yield a classification containing twelve categories of which six are accounted for by the existing apollo and luna collections therefore we present our discussions in the form of six mare basalt rock types 1 1 high ti low al low k referred to as high ti low k 2 2 high ti low al high k referred to as high ti high k 3 3 low ti low al low k referred to as low ti 4 4 low ti high al low k referred to as high alumina 5 5 low ti high al high k referred to as vhk 6 6 vlt low al low k basalts referred to as vlt a variety of post magma generation processes have been invoked such as fractional crystallization either alone or combined with wallrock assimilation to explain the compositional ranges of the various mare basalt suites in order to evaluate these proposed petrogenetic processes this review is by rock type and is non site specific but for each rock type reference to particular lunar sample return missions is brought forth this permits a comparison of similarities and differences of broadly similar rock types correlated with geography on the moon which in turn allows a more thorough petrogenetic evaluation high ti mare basalts i e high ti low al low k are found at apollo 11 and apollo 17 sites however the a 11 basalts contain lower tio2 abundances a considerably larger range in trace element contents and the only occurrence of high ti high k mare basalts fractional crystallization and source heterogeneity within each site are the keys to understanding the petrogenesis of the high ti basalts low ti basalts including both low al k and high al k varieties are found at apollo 12 14 and 15 and luna 16 sites the low ti basalts exhibit a wide range of major and trace element compositions and require source heterogeneity fractional crystallization and some assimilation the high alumina mare basalts i e low ti high al low k are found at apollo 14 and luna 16 sites and exhibit a wide range of major and trace element compositions however in these examples source heterogeneity is not a major factor indeed fractional crystallization coupled with kreep assimilation particularly for the apollo 14 variants can explain the compositional ranges of these high alumina basalts the vhk mare basalts i e low ti high al high k have been sampled only at the apollo 14 locale and are products of a parental highalumina magma assimilating lunar granite very low ti vlt mare basalts i e vlt low al low k are found at apollo 17 and luna 24 sites fractional crystallization has had a major influence upon the range in vlt compositions but luna 24 vlt basalts have been derived from a source slightly different in composition from that for apollo 17 vlt varieties for example the luna 24 vlt basalts generally exhibit positive eu anomalies a unique property for mare basalts which almost always have negative eu anomalies the concept of a lunar magma ocean lmo is generally accepted and source modelling of all basalts invokes a mafic lmo cumulate source this is the only unifying model for mare basalt petrogenesis but the semantics and logistics of it are and will be debated for many years for example major convective overturn of the lmo appears plausible but whether this occurred on a local or planet wide scale to produce source heterogeneity remains to be determined

the late heavy bombardment heavily cratered surfaces on the moon mars and mercury show that the terrestrial planets were battered by an intense bombardment during their first billion years or more but the timing sources and dynamical implications of these impacts are controversial the late heavy bombardment refers to impact events that occurred after stabilization of the planetary lithospheres such that they could be preserved as craters and basins lunar melt rocks and meteorite shock ages point toward a discrete episode of elevated impact flux between 3 5 and 4 0 4 2 ga and a relative quiescence between 4 0 4 2 and 4 4 ga evidence from precambrian impact spherule layers suggests that a long lived tail of terrestrial impactors lasted to 2 0 2 5 ga dynamical models that include populations residual from primary accretion and destabilized by giant planet migration can potentially account for the available observations although all have pros and cons the most parsimonious solution to match constraints is a hybrid mode

thermal and magmatic evolution of the moon as with all science our continually developing concepts of lunar evolution are firmly tied to both new types of observations and the integration of these observations to the known pool of data this process invigorates the intellectual foundation on which old models are tested and new concepts are built just as the application of new observational tools to lunar science in 1610 galileo s telescope and 1840 photography yielded breakthroughs concerning the true nature of the lunar surface the computational and technological advances highlighted by the apollo and post apollo missions and associated scientific investigations provided a new view of the thermal and magmatic evolution of the moon 1 1 pre apollo view of the thermal and magmatic evolution of the moon many of the early views of the moon manifested in mythology and art throughout the world were primarily tied to lunar and terrestrial cycles and the relationships between the sun and the moon prophetically myths involving the lunar deities mwuetsi from zimbabwe and coyolxauhqui from mexico told of rather violent or catastrophic events in which the moon was expunged from the earth numerous ancient scientific observations were made about the nature of the moon ranging from those uncovered in early neolithic sites that correctly identified mare crisium and mare humorum to the insights made by greek philosophers such as anaxagoras ca 500 428 b c and democritus ca 460 370 b c who attached terrestrial analogues to its character stone mountains with the advent of the telescope 1610 and photography 1840 as scientific tools for lunar exploration semiquantitative data could be collected that would provide an intellectual foundation for scientific interpretation initially modern terrestrial geological analogs were extended to the moon lunar highlands volcanic craters seas combined with the rigors of computational modeling these observational data were extended to predict the original thermal state of the moon and its thermal and magmatic history its proximity to the earth

a new isostatic model for the east pacific rise crest we have developed a new isostatic model for the east pacific rise epr that explains the origin of both the axial topographic high and the rise crest gravity anomaly our model involves a thin elastic plate with a constant thickness crust that is broken at the ridge crest we assume that a buoyant force beneath the plate bends the free edge of the plate upward at the rise axis forming the topographic high and the rise crest gravity anomaly we have used this model and two conventional isostatic models to examine the isostasy of three segments of the epr at 9 l4 n 6 11 s and 16 21 s rather than modeling individual profiles we have stacked summed the gravity and bathymetry data in each area and computed a single 200 km long mean mirrored profile for each different ridge segment these stacked profiles are a remarkable improvement over even the best individual profiles and clearly show both the axial topographic high and its associated free air gravity anomaly in all three areas the best fitting model parameters are similar and give average compensation depths of about 6 7 km below the seafloor near the base of the oceanic crust and an upper limit on the flexural rigidity of the plate that increases from about 1018 n m at the ridge axis to about 3 1019 n m 5 km from the axis and 1020 n m 25 km or more from the rise crest these results indicate that the epr crest is compensated at much shallower depths than suggested by previous investigators our results are consistent with the presence of a small low density magma chamber in the lower crust although at least part of the compensating root for the axial high must extend into the upper mantle we interpret the low upper mantle densities below the rise crest as a zone of partial melt which accumulates at the base of the oceanic crust between episodic rifting events when the overlying crustal magma chamber is replenished

tungsten isotopic evidence for disproportional late accretion to the earth and moon examination of three lunar samples reveals that the moon s mantle has an excess of the tungsten isotope 182w of about 20 parts per million relative to the present day earth s mantle this suggests that the two bodies had identical compositions immediately following the formation of the moon and that the compositions then diverged as a result of disproportional late accretion of chondritic material to the earth and moon two papers published in this issue of nature present precise measurements of tungsten isotope composition in lunar rocks that are best explained by the earth and moon having had similar composition immediately following formation of the moon and then having diverged as a result of disproportional late accretion of material to the two bodies mathieu touboul et al found small 182w excess of about 21 parts per million relative to the present day earth s mantle in metals extracted from two kreep rich apollo 16 impact melt rocks while thomas kruijer et al measured tungsten isotopes in seven kreep rich whole rock samples that span a wide range of cosmic ray exposure ages and found a 182w excess of about 27 parts per million over the present day earth s mantle characterization of the hafnium tungsten systematics 182hf decaying to 182w and emitting two electrons with a half life of 8 9 million years of the lunar mantle will enable better constraints on the timescale and processes involved in the currently accepted giant impact theory for the formation and evolution of the moon and for testing the late accretion hypothesis uniform terrestrial mantle like w isotopic compositions have been reported1 2 among crystallization products of the lunar magma ocean these observations were interpreted to reflect formation of the moon and crystallization of the lunar magma ocean after 182hf was no longer extant that is more than about 60 million years after the solar system formed here we present w isotope data for three lunar samples that are more precise by a factor of 4 than those previously reported1 2 the new data reveal that the lunar mantle has a well resolved 182w excess of 20 6 5 1 parts per million 2 standard deviations relative to the modern terrestrial mantle the offset between the mantles of the moon and the modern earth is best explained by assuming that the w isotopic compositions of the two bodies were identical immediately following formation of the moon and that they then diverged as a result of disproportional late accretion to the earth and moon3 4 one implication of this model is that metal from the core of the moon forming impactor must have efficiently stripped the earth s mantle of highly siderophile elements on its way to merge with the terrestrial core requiring a substantial but still poorly defined level of metal silicate equilibration

re evaluating 142nd 144nd in lunar mare basalts with implications for the early evolution and bulk sm nd of the moon abstract the moon likely accreted from melt and vapor ejected during a cataclysmic collision between proto earth and a mars sized impactor very early in solar system history the identical w o k and cr isotope compositions between materials from the earth and moon require that the material from the two bodies were well homogenized during the collision process as such the ancient isotopic signatures preserved in lunar samples provide constraints on the bulk composition of the earth two recent studies to obtain high precision 142nd 144nd ratios of lunar mare basalts yielded contrasting results in one study after correction of neutron fluence effects imparted to the nd isotope compositions of the samples the coupled 142nd 143nd systematics were interpreted to be consistent with a bulk moon having a chondritic sm nd ratio rankenburg k brandon a d and neal c r 2006 neodymium isotope evidence for a chondritic composition of the moon science 312 1369 1372 the other study found that their data on the same and similar lunar mare basalts were consistent with a bulk moon having a superchondritic sm nd ratio boyet m and carlson r w 2007 a highly depleted moon or a non magma origin for the lunar crust earth planet sci lett 262 505 516 delineating between these two potential scenarios has key ramifications for a comprehensive understanding of the formation and early evolution of the moon and for constraining the types of materials available for accretion into large terrestrial planets such as earth to further examine this issue the same six lunar mare basalt samples measured in rankenburg et al rankenburg k brandon a d and neal c r 2006 neodymium isotope evidence for a chondritic composition of the moon science 312 1369 1372 were re measured for high precision nd isotopes using a multidynamic routine with reproducible internal and external precisions to better than 3 ppm 2 for 142nd 144nd ratios the measurements were repeated in a distinct second analytical campaign to further test their reproducibility evaluation of accuracy and neutron fluence corrections indicates that the multidynamic nd isotope measurements in this study and the 3 in boyet and carlson boyet m and carlson r w 2007 a highly depleted moon or a non magma origin for the lunar crust earth planet sci lett 262 505 516 are reproducible while static measurements in the previous two studies show analytical artifacts and cannot be used at the resolution of 10 ppm to determine a bulk moon with either chondritic or superchondritic sm nd ratios the multidynamic data are best explained by a bulk moon with a superchondritic sm nd ratio that is similar to the present day average for depleted morb hafnium isotope data were collected on the same aliquots measured for their 142nd 144nd isotope ratios in order to assess if the correlation line for 142nd 143nd systematics reflect mixing processes or times at which lunar mantle sources formed based on the combined 142nd 143nd 176hf obtained we conclude that the 142nd 143nd correlation line measured in this study is best interpreted as an isochron with an age of 229 24 20ma after the onset of nebular condensation the uncertainties in the data permit the sources of these samples to have formed over a 44 ma time interval these new results for lunar mare basalts are thus consistent with a later sm nd isotope closure time of their source regions than some recent studies have postulated and a superchondritic bulk sm nd ratio of the moon and earth the superchondritic sm nd signature was inherited from the materials that accreted to make up the earth moon system although collisional erosion of crust from planetesimals is favored here to remove subchondritic sm nd portions and drive the bulk of these bodies to superchondritic in composition removal of explosive basalt material via gravitational escape from such bodies or chondrule sorting in the inner solar system may also explain the compositional features that deviate from average chondrites that make up the earth moon system this inferred superchondritic nature for the earth similar to the modern convecting mantle means that there is no reason to invoke a missing subchondritic reservoir to mass balance the earth back to chondritic for sm nd ratios however to account for the subchondritic sm nd ratios of continental crust a second superchondritic sm nd mantle reservoir is required

sea floor spreading in the presence of the iceland plume the structure of the reykjanes ridge at 61 40 n abstract we present results from a combined normal incidence and wide angle seismic survey of the reykjanes ridge at 61 40 n in the north atlantic the seismic data show that the crust is 8 4 10 0 km thick on the ridge axis we attribute this thicker than normal oceanic crust to a high degree of melting caused by hot mantle from the iceland plume beneath the speading centre layer 2a of the oceanic crust is significantly thicker at 0 ma than in crust of age 4 ma our results combined with other seismic data from the north atlantic show that the crustal thickness decreases away from the plume centre to normal oceanic crustal thickness at a distance of approximately 1000 km

preservation potential of implanted solar wind volatiles in lunar paleoregolith deposits buried by lava flows abstract the lunar surface is bathed in a variety of impacting particles originating from the solar wind solar flares and galactic cosmic rays these particles can become embedded in the regolith and or produce a range of other molecules as they pass through the target material the moon therefore contains a record of the variability of the solar and galactic particle fluxes through time to obtain useful temporal snapshots of these processes discrete regolith units must be shielded from continued bombardment that would rewrite the record over time one mechanism for achieving this preservation is the burial of a regolith deposit by a later lava flow the archival value of such deposits sandwiched between lava layers is enhanced by the fact that both the under and over lying lava can be dated by radiometric techniques thereby precisely defining the age of the regolith layer and the geologic record contained therein the implanted volatile species would be vulnerable to outgassing by the heat of the over lying flow at temperatures exceeding 300 700 c however the insulating properties of the finely particulate regolith would restrict significant heating to shallow depths we have therefore modeled the heat transfer between lunar mare basalt lavas and the regolith in order to establish the range of depths below which implanted volatiles would be preserved we find that the full suite of solar wind volatiles consisting predominantly of h and he would survive at depths of 13 290 cm for 1 10 m thick lava flows respectively a substantial amount of co co2 n2 and xe would be preserved at depths as shallow as 3 7 cm beneath meter thick flows given typical regolith accumulation rates during mare volcanism the optimal localities for collecting viable solar wind samples would involve stacks of thin mare lava flows emplaced a few tens to a few hundred ma apart in order for sufficient regolith to develop between burial events obtaining useful archives of solar system processes would therefore require extraction of regolith deposits buried at quite shallow depths beneath radiometrically dated mare lava flows these results provide a basis for possible lunar exploration activities

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

late cretaceous and cenozoic seafloor and oceanic basement roughness spreading rate crustal age and sediment thickness correlations abstract single channel seismic data from the south australian basin and argentine basin and bathymetry data from the flanks of the mid atlantic ridge east pacific rise and southwest indian ridge are analysed to determine the root mean square rms roughness of the seafloor and oceanic basement created at seafloor spreading rates ranging from 3 to 80 km ma half rate for these data crustal ages range from near zero to 85 ma and sediment thicknesses range from near zero to over 2 km our results are consistent with a negative correlation of basement roughness and spreading rate where roughness decreases dramatically through the slow spreading regime

stochastic modeling of seafloor morphology inversion of sea beam data for second order statistics at scale lengths less than 100 km or so statistical descriptions of seafloor morphology can be usefully employed to characterize ridge crest processes off ridge tectonics and vulcanism sedimentation and postdepositional transport we seek to develop methods for the estimation of seafloor statistics that take into account the finite precision resolution and sampling obtained by actual echo sounding systems in this initial paper we restrict our attention to the problem of recovering second order statistics from data sets collected by multibeam devices such as sea beam the seafloor is modeled as a stationary zero mean gaussian random field completely specified by its two point covariance function we introduce an anisotropic two point covariance function that has five free parameters describing the amplitude orientation characteristic wave numbers and hausdorff fractal dimension of seafloor topography we formulate the general forward problem relating this model to the statistics of an ideal multibeam echo sounder in particular the along track autocovariance functions of individual beams and the cross covariance functions between beams of arbitrary separation using these second moments as data functionals we then pose the inverse problem of estimating the seafloor parameters from realistic noisy data sets with finite sampling and beamwidth and we solve this inverse problem by an iterative linearized least squares method the inversion method is applied to sea beam transit data from both the pacific and atlantic oceans sea beam system noise stands out as a sharp spike on the along track autocovariance function and can be modeled as a white noise process whose amplitude generally increases with beam angle the five parameters in our second order model can be estimated from the inversion of data sets comprising 100 200 km of track length in general the cross track wave number is the most poorly determined although uncertainties in the assumed sea beam response may bias the values of the fractal dimension using the assumed beamwidth the measured noise values and the seafloor parameters recovered from the inversion we generate sea beam synthetics whose statistical character can be directly compared with raw sea beam data for most of the track segments we have processed thus far the synthetics are similar to the data in the case of one atlantic profile however the comparison clearly indicates the necessity of incorporating higher order statistics the space domain procedures described in this paper can be extended for this purpose

inferring crustal structure in the aleutian island arc from a sparse wide angle seismic data set compressional seismic travel times from a relatively sparse wide angle data set hold key information on the structure of a 800 km long section of the central aleutian arc since the source and receiver locations form a swath along the arc crest that is similar to50 km wide we trace rays in 3 d for a collection of 8336 seismic refraction and reflection arrivals we investigate variations in seismic velocity structure parallel to the aleutian arc assuming that our result represents average crustal structure across the arc we explore seismic velocity models that consist of three crustal layers that exhibit smooth variations in structure in the 2 d vertical plane we consider the influence of additional constraints and model parameterization in our search for a plausible model for aleutian arc crust a tomographic inversion with static corrections for island stations reduces the data variance of a 1 d starting model by 91 our best model has seismic velocities of 6 0 6 5 km s in the upper crust 6 5 7 3 km s in the middle crust and 7 3 7 7 km s in the lower crust and a total crustal thickness of 35 37 1 km a resolution analysis shows that features having a horizontal scale less than 20 km cannot be imaged but at horizontal length scales of similar to50 km most model features are well resolved the study indicates that the aleutian island arc crust is thick compared to other island arcs and strongly stratified and that only the upper 60 of the arc crust has seismic velocities that are comparable to average seismic velocities in continental crust

crustal structure of the propagating tammar ridge segment on the mid atlantic ridge 21 5 n active ridge propagation frequently occurs along spreading ridges and profoundly affects ridge crest segmentation over time the mechanisms controlling ridge propagation however are poorly understood at the slow spreading mid atlantic ridge at 21 5 n a seismic refraction and wide angle reflection profile surveyed the crustal structure along a segment controlled by rapid ridge propagation tomographic traveltime inversion of seismic data suggests that the crustal structure along the ridge axis is controlled by melt supply thus crust is thickest 8 km at the domed segment center and decreases in thickness toward both segment ends however thicker crust is formed in the direction of ridge propagation suggesting that melt is preferentially transferred toward the propagating ridge tip further while seismic layer 2 remains constant along axis seismic layer 3 shows profound changes in thickness governing variations in total crustal thickness this feature supports mantle upwelling at the segment center thus fluid basaltic melt is redistributed easily laterally while more viscose gabbroic melt tends to crystallize and accrete nearer to the locus of melt supply the onset of propagation seems to have coincided with the formation of thicker crust suggesting that propagation initiation might be due to changes in the melt supply after a rapid initiation a continuous process of propagation was established the propagation rate seems to be controlled by the amount of magma that reaches the segment ends the strength of upwelling may govern the evolution of ridge segments and hence ultimately controls the propagation length

magma oceans in the inner solar system theory and observations point to the occurrence of magma ponds or oceans in the early evolution of terrestrial planets and in many early accreting planetesimals the apparent ubiquity of melting during giant accretionary impacts suggests that silicate and metallic material may be processed through multiple magma oceans before reaching solidity in a planet the processes of magma ocean formation and solidification therefore strongly influence the earliest compositional differentiation and volatile content of the terrestrial planets and they form the starting point for cooling to clement habitable conditions and for the onset of thermally driven mantle convection and plate tectonics this review focuses on evidence for magma oceans on planetesimals and planets and on research concerning the processes of compositional differentiation in the silicate magma ocean distribution and degassing of volatiles and cooling

hydrothermal recharge and discharge across 50 km guided by seamounts on a young ridge flank hydrothermal circulation within the sea floor through lithosphere older than one million years myr is responsible for 30 of the energy released from plate cooling and for 70 of the global heat flow anomaly the difference between observed thermal output and that predicted by conductive cooling models 1 2 hydrothermal fluids remove significant amounts of heat from the oceanic lithosphere for plates typically up to about 65 myr old3 4 but in view of the relatively impermeable sediments that cover most ridge flanks5 it has been difficult to explain how these fluids transport heat from the crust to the ocean here we present results of swath mapping heat flow geochemistry and seismic surveys from the young eastern flank of the juan de fuca ridge which show that isolated basement outcrops penetrating through thick sediments guide hydrothermal discharge and recharge between sites separated by more than 50 km our analyses reveal distinct thermal patterns at the sea floor adjacent to recharging and discharging outcrops we find that such a circulation through basement outcrops can be sustained in a setting of pressure differences and crustal properties as reported in independent observations and modelling studies

stochastic analysis of seafloor morphology on the flank of the southeast indian ridge the influence of ridge morphology on the formation of abyssal hills in this study we estimate the statistical properties of abyssal hill morphology adjacent to the southeast indian ridge in a region where the axial morphology changes from axial high to axial valley without a corresponding change in spreading rate we explore the influence of axial morphology on abyssal hills and place these results within the context of response to spreading rate two cruises aboard the r v melville collected sea beam 2000 multibeam data along the southeast indian ridge providing continuous multibeam coverage of the axis from 89 w to 118 w and 100 coverage within four survey regions extending out to 45 km 1 2 ma from the axis sempere et al 1997 cochran et al 1997 we apply the statistical modeling method of goff and jordan 1988 to gridded data from the four survey areas examining in particular estimates of abyssal hill rms height characteristic width and length aspect ratio and skewness two analyses are performed 1 comparison of the along axis variation in abyssal hill characteristics to ridge segmentation and 2 a calculation of population statistics within axial high intermediate and axial valley data populations of this study and comparison of these results to population statistics derived from studies adjacent to the mid atlantic ridge and east pacific rise we find that abyssal hills generated along axial high mid ocean ridges are very different from those generated along axial valley mid ocean ridges not only with respect to size and shape but also in their response to such factors as spreading rate and segmentation

evidence from hole 504b for the origin of dipping events in oceanic crustal reflection profiles as out of plane scattering from basement topography dipping reflectors in oceanic crustal seismic reflection profiles have been attributed to either faults cutting through the crustal section or magmatic layering in the mid to lower crust using closely spaced

cratering history and lunar chronology the moon is exceptional and important because it is the only planetary body besides the earth for which we have both a detailed stratigraphic history and datable rock samples that can be related to specific geomorphologic units fig 5 1 the moon has preserved much of its magmatic and impact record of at least the last 4 billion years while its endogenic history is of great interest for the fundamentals of planetary interiors and surfaces the moon has become a calibration plate for the cratering record of the earth moon system and by extrapolation of the entire inner solar system if one assumes a heliocentric origin for impactor populations these populations range from asteroids through long and short period comets to interplanetary dust and cover a size range from hundreds of kilometers to micrometers figure 5 1 telescopic view of the nearside of the earth s moon with landing sites of the apollo and luna missions this chapter reviews the presently available data sets in support of this paradigmatic assumption as follows 1 the phenomenology of lunar impact craters 2 the terrestrial record of the impact cratering process and the interpretation of terrestrial impactites as far as this ground truth is relevant for the interpretation of lunar impact craters and datable lunar impact breccias and melt rocks 3 the theory and numerical simulation of the cratering process and the characteristics of the earth moon crossing population of impactors asteroids and comets 4 the principles of relative age dating of lunar surface units and the general lunar stratigraphy 5 the stratigraphic significance and ages of lunar samples impactites and basalts and based on this data set the absolute ages of lunar surface units 6 the cratering rate of the moon as a function of time and 7 the time calibration of this cratering rate based on the

lunar feldspathic meteorites constraints on the geology of the lunar highlands and the origin of the lunar crust abstract the composition of the lunar crust provides clues about the processes that formed it and hence contains information on the origin and evolution of the moon current understanding of lunar evolution is built on the lunar magma ocean hypothesis that early in its history the moon was wholly or mostly molten this hypothesis is based on analyses of apollo samples of ferroan anorthosites 90 plagioclase molar mg mg fe mg 75 and the assumption that they are globally distributed however new results from lunar meteorites which are random samples of the moon s surface and remote sensing data show that ferroan anorthosites are not globally distributed and that the apollo highland samples used as a basis for the model are influenced by ejecta from the imbrium basin in this study we evaluate anorthosites from all currently available adequately described lunar highland meteorites representing a more widespread sampling of the lunar highlands than apollo samples alone and find that 80 of them are significantly more magnesian than apollo ferroan anorthosites interestingly luna mission anorthosites collected outside the continuous imbrium ejecta are also highly magnesian if the lunar highland crust consists dominantly of magnesian anorthosites as suggested by their abundance in samples sourced outside imbrium ejecta a reevaluation of the lunar magma ocean model is a sensible step forward in the endeavor to understand lunar evolution our results demonstrate that lunar anorthosites are more similar in their chemical trends and mineral abundance to terrestrial massif anorthosites than to anorthosites predicted in a lunar magma ocean this analysis does not invalidate the idea of a lunar magma ocean which seems a necessity under the giant impact hypothesis for the origin of the moon however it does indicate that most rocks now seen at the moon s surface are not primary products of a magma ocean alone but are products of more complex crustal processes

a chemical model for generating the sources of mare basalts combined equilibrium and fractional crystallization of the lunar magmasphere abstract it is generally considered that mare basalts were generated by the melting of a cumulate mantle formed in an early moon wide magma ocean or magmasphere however the nature and chemistry of this cumulate mantle and the logistics of its origin have remained elusive extensive studies of terrestrial layered mafic intrusions over the past sixty years have emphasized the imperfection of fractional crystallization and attendant crystal crystal and crystal liquid separation in a convecting magma chamber crystal liquid and crystal crystal separations were similarly inefficient during evolution of the lunar magma ocean lmo allowing for the trapping of interstitial liquid and entrainment of a small proportion of less dense plagioclase into the denser mafic cumulate mush indeed petrography of lunar highlands samples demonstrates this for anorthosites with 1 10 olivine the residual liquid after 80 90 crystallization was very evolved in fact kreepy and even in small proportions 1 5 would have a noticeable effect on the trace element chemistry of melts generated from these cumulates this trapped residual liquid would elevate total ree abundances in the cumulate pile while synchronously deepening the already negative eu anomaly essentially this trapped liquid will make the cumulate more fertile for melting to generate both kreep basalt and mare basalt magmas plagioclase entrained in the mafic cumulate pile adds an essential al component to the high ti basalt source and will moderate the requisite negative eu anomaly in the cumulate early in the evolution of the lunar mantle when the lmo still was largely liquid it is likely that vigorous convection was an important factor in crystallization such convection would allow crystals to remain suspended and in equilibrium with the lmo liquid for relatively long periods of time this extended period of equilibrium crystallization would then have been followed by fractional crystallization once plagioclase became a liquidus phase and began to float to form the lunar highlands crust previous authors have proposed a three component model for the evolution of high ti mare basalt source regions this model includes kreep early olivine rich high mg cumulates and late ilmeniterich low mg cumulates in various proportions however we propose a model for high ti basalt parent magmas which is in accord with studies of terrestrial layered intrusions this model for the high ti source includes trapped instantaneous residual liquid tirl 1 3 and entrainment of a small 2 5 proportion of plagioclase into the late stage cumulate pile in order to account for both the observed al compositions and trace element characteristics of high ti mare basalts melting of this relatively shallow ilmenite and clinopyroxene bearing late stage cumulate can generate high ti mare basalt magmas furthermore we are in agreement with other workers that only through a process of nonmodal melting will the high ti values for the parent magmas be realized large scale convective overturn of the cumulate pile and mixing of kreep with early and late stage cumulates is not required however localized overturn of the upper tenth of the cumulate pile is likely and in fact required to achieve an appropriate major element balance for the high ti mare basalt source region

chang e 4 initial spectroscopic identification of lunar far side mantle derived materials over 60 years of spacecraft exploration has revealed that the earth s moon is characterized by a lunar crust1 dominated by the mineral plagioclase overlying a more mafic richer in iron and magnesium mantle of uncertain composition both crust and mantle formed during the earliest stages of lunar evolution when late stage accretional energy caused a molten rock magma ocean flotation of the light plagioclase sinking of the denser iron rich minerals such as olivine and pyroxene and eventually solidification2 very large impact craters can potentially penetrate through the crust and sample the lunar mantle the largest of these craters is the approximately 2 500 kilometre diameter south pole aitken spa basin3 on the lunar far side evidence obtained from orbiting spacecraft shows that the floor of the spa basin is rich in mafic minerals4 but their mantle origin is controversial and their in situ geologic settings are poorly known china s chang e 4 lunar far side lander recently touched down in the von karman crater5 6 to explore the floor of the huge spa basin and deployed its rover yutu 2 here we report on the initial spectral observations of the visible and near infrared spectrometer vnis 7 onboard yutu 2 which we interpret to represent the presence of low calcium ortho pyroxene and olivine materials that may originate from the lunar mantle geological context6 suggests that these materials were excavated from below the spa floor by the nearby 72 km diameter finsen impact crater event and transported to the landing site continued exploration by yutu 2 will target these materials on the floor of the von karman crater to understand their geologic context origin and abundance and to assess the possibility of sample return scenarios initial spectral observations by china s chang e 4 far side lunar rover suggest the presence of materials that may originate from the moon s mantle

magmatic tectonic conditions for hydrothermal venting on an ultraslow spread oceanic core complex hydrothermal venting an important cooling mechanism of the earth supports a diverse array of seafloor and sub seafloor ecosystems that are sustained by large thermal and chemical fluxes vents have been found along even the slowest and coldest spreading centers calling into question the driving heat source for these vents the ultraslow spreading mid cayman spreading center in the caribbean sea which hosts the axial flank von damm vent field vdvf provides an opportunity to probe the mechanisms for venting at ultraslow spreading rates using active source seismic data from the 2015 cayseis cayman seismic experiment we determined the seismic velocities in the large massif beneath the vdvf we propose that this massif was produced by a pulse of on axis magmatism at ca 2 ma which was then followed by exhumation cooling and fracturing a low seismic velocity anomaly 5 km below the vdvf is evidence for either a cracking front mining lithospheric heat or intrusive magmatic sills both of which could drive ongoing deep hydrothermal fluid circulation we conclude that the transient magmatism and variable crustal thickness at ultraslow spreading centers create conditions for long lived hydrothermal venting that may be widespread and other vdvf like vents may be common in these areas

crustal thickness variations along the southeast indian ridge 100 116 e from 2 d body wave tomography 1 axial morphology along the southeast indian ridge seir systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100 116 e west of the australian antarctic discordance aad basalt geochemistry has a consistent indian mid ocean ridge basalt morb type isotopic signature so changes in axial topography are attributed to variations in both mantle temperature and melt supply wide angle seismic refraction lines were shot to four ocean bottom hydrophones within seir segments p1 p2 s1 and t where each segment is characterized by a different morphology we constructed 2 d crustal velocity models by jointly inverting hand picked p wave refraction pg and moho reflection pmp traveltime data using a top down minimum structure methodology the results show a 1 5 km eastward decrease in crustal thickness across the study area with segment averages ranging from 6 1 km at p1 to 4 6 km at t melt generation models require a 30 c decrease in mantle temperature toward the aad to account for the crustal thickness trend significant changes in axial morphology accompany small scale variations in crustal thickness consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature melt supply and cooling from hydrothermal circulation layer 3 thins by 3 0 km as layer 2 thickens by 1 4 km between segments p1 and t reflecting the eastward decrease in melt supply and increase in melt lens depth the trade off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material

crustal structure of north atlantic fracture zones seismic studies have established that large offset transforms along the slow spreading mid atlantic ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust seismically fracture zone crust in the north atlantic is extremely heterogeneous in both thickness and internal structure it is frequently quite thin 1 2 km thick and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3 a more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the mid atlantic ridge the geological nature of the seismically anomalous crust found within atlantic fracture zones and how this crust forms are still controversial one interpretation consistent with available seismic observations is that the crust within north atlantic fracture zones consists of a thin intensely fractured and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust the existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large scale detachment faults that form preferentially in the cold brittle lithosphere near the ends of segments