explosive volcanic eruptions on mercury eruption conditions magma volatile content and implications for interior volatile abundances images obtained by the messenger spacecraft have revealed evidence for pyroclastic volcanism on mercury because of the importance of this inference for understanding the interior volatile inventory of mercury we focus on one of the best examples determined to date a shield volcano like feature just inside the southwestern rim of the caloris impact basin characterized by a near central irregularly shaped depression surrounded by a bright deposit interpreted to have a pyroclastic origin this candidate pyroclastic deposit has a mean radius of 24 km greater in size than the third largest lunar pyroclastic deposit when scaled to lunar gravity conditions from the extent of the candidate pyroclastic deposit we characterize the eruption parameters of the event that emplaced it including vent speed and candidate volatile content the minimum vent speed is 300 m s and the volatile content required to emplace the pyroclasts to this distance is hundreds to several thousands of parts per million ppm of the volatiles typically associated with pyroclastic eruptions on other bodies e g co co2 h2o so2 h2s for comparison measurements of the exsolution of volatiles h2o co2 s from basaltic eruptive episodes at kilauea volcano hawaii indicate values of 1300 6500 ppm for the terrestrial mantle source evidence for the presence of significant amounts of volatiles in partial melts derived from the interior of mercury is an unexpected result and provides a new constraint on models for the planet s formation and early evolution

magmatic precursors to the 18 may 1980 eruption of mount st helens usa perhaps the most difficult task facing volcanologists today is that of distinguishing between low level volcanic restlessness and activity that presages a full scale eruption we illustrate these difficulties by reexamining the sequence of events that led to the 18 may 1980 eruption of mount st helens activity that is often presented as a classic example of early phreatic explosions leading to a magmatic eruption this interpretation is problematic however given the known ascent of juvenile magma to within 250 m of the surface prior to the climactic event reexamination of ash samples from several precursory events shows that small amounts of juvenile material were erupted as early as 28 march 1980 just two weeks after the first seismic signals of reawakening and that the juvenile content of the ash had probably increased by early may the magmatic component of these eruptions was not recognized at the time because of the high crystallinity of the juvenile material a signature of extensive degassing induced crystallization during magma ascent identification of juvenile material in the precursory ash from mount st helens has important implications for initiation of eruptive activity

multiphase petrography of volcanic rocks using element maps a method applied to mount st helens 1980 2005 quantitative textural analyses including crystal size distributions csds provide insights into crystallisation kinetics of magmatic systems investigations of volcanic crystal textures often rely on greyscale variations on backscattered electron images to identify crystal phases which must then be thresholded and or traced manually a laborious task and investigations are typically restricted to a single crystal phase a method is presented that uses energy dispersive x ray element maps to generate textural data each pixel is identified as a crystal phase glass or vesicle according to relative chemical composition enabling concurrent acquisition of multiphase csd crystallinity and mineral mode data data processing is less time intensive for the operator but considerable instrument time is required to generate element maps the method is applied to 17 dacite samples from the 1980 1986 and 3 from the 2004 2005 eruptive periods of mount st helens volcano usa to provide quantitative insights into multiphase textural evolution all of the csds are curved and concave up in the standard csd plot with curvature increasing with plagioclase content to facilitate comparisons with previous studies csds for microlites 50 m length crystals are approximated as straight lines the line intercepts and slopes provide information on n 0 nucleation density and characteristic length or g the product of growth rate g and residence time respectively these parameters as well as the total groundmass crystallinity show distinct differences between explosive deposits from summer 1980 and post summer 1980 domes post summer 1980 microlite n 0 values are mostly at the lower end of the range of those measured for summer 1980 samples total groundmass crystallinities during summer 1980 are between 10 and 30 vol whereas post summer 1980 crystallinity increases to between 39 and 51 vol the range of n 0 values is similar to those previously published for mount st helens but g is consistently higher g of a may 1985 sample analysed in this study is approximately 2 m higher compared with previously published data for the same sample when processed using similar methodologies groundmass crystallinity data show similar trends to those previously published for the 1980 to 1986 eruption increasing sharply after summer 1980 then increasing more gradually during the dome building phase of the eruption the effects of varying l the apparent crystal size and crystal aspect ratio on resultant csds are also investigated whilst relative temporal variations in csds for a given dataset can be investigated absolute values from different studies cannot be compared unless methods of data acquisition and processing are exactly the same

pit floor craters on mercury evidence of near surface igneous activity abstract high resolution images obtained during the first mercury surface space environment geochemistry and ranging messenger flyby of mercury show evidence for igneous modification of the surface through the formation of pit craters on the floors of several impact craters pit craters are rimless steep sided depressions that are inferred to have formed by non impact processes among the arguments in favor of this interpretation are that the features lack evident rims exhibit no observable ejecta are irregularly shaped e g subcircular elliptical or arcuate with the long axis of the pit crater often concentric to the rim of the host impact crater and are superposed on smooth plains deposits within the craters pit craters documented to date do not exhibit signs of extrusive flows impact craters hosting pit craters termed pit floor craters vary in size from 55 to 120 km in diameter while the associated pit craters range in maximum horizontal dimension from 20 to almost 40 km pit crater radius and host crater radius are not well correlated for pit floor craters on mercury in contrast to the situation for otherwise similar features on mars on the basis of morphology structural association relative age and proximity to smooth plains for pit craters on mercury we test mechanisms of pit crater formation previously proposed for such features on other planetary bodies e g earth moon mars and venus we conclude that pit craters on mercury formed through collapse into an underlying drained magma chamber pit floor craters thus provide evidence for near surface magmatic activity on mercury and extend the range of evidence for magmatism beyond such surface expressions as smooth plains and pyroclastic deposits

magma ascent at floor fractured craters diagnoses the lithospheric stress state on the moon abstract on the moon floor fractured craters ffcs present evidence of horizontal crater centred magmatic intrusions crater floor uplift and moat formation indicate that these sill intrusions occur at shallow depths

volcanism on mercury evidence from the first messenger flyby for extrusive and explosive activity and the volcanic origin of plains the first messenger flyby of mercury obtained images of 21 of the surface not seen by mariner 10 including the center and western half of the caloris basin and regions near the terminator that show details of the nature of smooth and intercrater plains these new data have helped to address and resolve a series of longstanding questions on the existence and nature of volcanism on mercury and the distribution of volcanic materials data from the mercury dual imaging system mdis on the messenger spacecraft have shown the following 1 numerous volcanic vents in the form of irregularly shaped rimless depressions are concentrated around the interior edge of the caloris basin 2 these vents appear to be sources for effusive volcanism that in one case built a shield in excess of 100 km in diameter and in some cases formed bright haloes around the vents that are interpreted to represent pyroclastic eruptions 3 lobate margins of plains units seen previously in mariner 10 data are documented in messenger images with more clarity and are often distinctive in morphology and color properties supporting the interpretation that these features are the edges of lava flow units 4 the interior of the caloris basin is filled with plains units spectrally distinctive from the rim deposits and comparison with the lunar imbrium basin and superposed impact crater stratigraphy provide evidence that these units are volcanic in origin detailed differences in the mineralogy of lava flow units so prominent in imbrium are not seen in the caloris interior 5 some of the smooth plains surrounding the exterior of the caloris basin show distinct differences in color and morphological properties supporting a volcanic origin 6 some smooth and intercrater plains units distant from the caloris basin show evidence of flooding and embayment relations unrelated to caloris ejecta emplacement local and regional geological and color relationships support a volcanic origin for these plains 7 large impact craters show a sequence of embayment of interior floor and exterior ejecta deposits that supports a volcanic origin for the embayment and filling processes 8 crater embayment and flooding relationships in selected areas suggest volcanic plains thicknesses of many hundreds of meters and local thicknesses inside impact craters of up to several kilometers 9 impact crater size frequency distributions for caloris exterior deposits including the facies of the caloris group and relatively high and low albedo smooth plains show that they are younger than plains interior to caloris and thus must be dominantly the product of post caloris volcanism these new data provide evidence that supports and confirms earlier hypotheses from mariner 10 data that volcanism was important in shaping the surface of mercury the emerging picture of the volcanic style of mercury is similar to that of the moon the other small one plate planetary body there are no major shield volcanoes e g comparable to tharsis montes on mars shallow magma reservoirs are rare and there is little evidence for surface deformation or long lived volcanic sources related to sites of upwelling mantle the close association of volcanic plains and surface deformation features suggests that future observations and analyses can help document the relation between the volcanic flux and the evolving state and magnitude of stress in the lithosphere of mercury

basaltic magma reservoirs factors controlling their rupture characteristics and evolution the manner in which magma enters and leaves a magma reservoir is a fundamental aspect of magmatic activity a quantitative model is developed to evaluate the variation of stress acting on the wall of a magma reservoir as a function of depth factors assessed include the size of the reservoir its depth of burial the strength of the country rocks and the variation with depth of the densities of the magma and country rocks it is shown that small magma reservoirs with halfheights less than 1 km centered at levels of neutral buoyancy can grow in all directions with nearly equal ease by injecting dikes into their surroundings at the sites of wall ruptures in larger reservoirs with half heights greater than 2 km however the stress required to cause wall failure is much less at the depth corresponding to the center of the reservoir than it is near the top or bottom thus larger reservoirs are likely to grow predominantly sideways by lateral dike injection in these cases vertical growth will not be important unless significant accumulation of low density material can occur in the upper part of the reservoir the formation of a low density layer due to gas exsolution or chemical differentiation of the magma can facilitate failure of the roof and upward dike migration from a mature reservoir as the reservoir grows and evolves through time the increasing magma pressure head at the depth corresponding to the center of the reservoir means that the amount of gas exsolution or chemical differentiation required to allow the occurrence of vertical dike emplacement also increases ultimately a vertical reservoir size is reached beyond which the excess stress required to cause lateral dike injection is so much less than that required to cause vertical injection that essentially no further vertical growth of the reservoir will occur the reservoir will thus develop a more laterally elongate shape with time the final aspect ratio will then depend mainly on the supply rate of magma to the reservoir from the mantle

mercury s global contraction much greater than earlier estimates observations of compressional structures on mercury have fallen short of accommodating the global contraction that is required owing to cooling of the planet s interior mapping of folds and faults across mercury s surface using messenger spacecraft images reveals deformation consistent with a planet that has contracted radially as much as seven kilometres over its history

stressing of locked patches along a creeping fault an analysis is presented of the stressing of locked patches along a fault zone which is creeping elsewhere the model consists of strike slip faulted elastic lithospheric plates loaded in a manner equivalent to imposition of a remotely uniform tectonic shear stress the fault zone has an inhomogeneous strength distribution both depth wise and along strike it is modeled as being composed of locked patches and freely slipping parts treated as cracks the solution is given with the use of the line spring model which analyzes the problem by thickness averaged plane stress theory for lithospheric plates which slip along a discontinuity cut at the plate boundary as a boundary condition thickness averaged stress and slip at each local section along the cut was related to one another by the result of an antiplane strain analysis of slip for the crack or crack pair which describes the slipping and locked depth ranges at that section the analysis indicates that the slip distribution along creeping parts of the fault as well as the stress distribution along locked patches depends strongly on the geometry of these zones the model is used to examine stress concentrations associated with a slip deficient seismic gap along strike and to study the effect of local irregularities in the margin of a locked region it is also used to simulate slip and stressing processes associated with the creeping portion of the san andreas fault in central california between the presently locked zones of the great 1906 and 1857 ruptures and to constrain the nature of an apparently locked zone at the southeastern end which ruptures in characteristic parkfield earthquakes near fault creep and broadscale displacement data along the fault since the 1966 parkfield earthquake and inferences from seismicity distributions are used limitations of the modeling procedure at short spatial wavelengths prohibit an accurate description of the parkfield locked patch but results suggest that it may be localized and occupy a small fraction of area of the normal seismogenic zone an effective remote stressing rate of order 0 3 10 6 shear modulus yr is inferred together with a less well constrained 30 to 40 km lithospheric thickness for consistency with the displacement data results enable estimates of stress accumulation along the locked 1857 rupture zone and the build up of fracture energy release capability of order 107 j m2 in 150 yr at its lower margin

numerical simulations of impact crater formation with dilatancy impact induced fracturing creates porosity that is responsible for many aspects of the geophysical signature of an impact crater this paper describes a simple model of dilatancy the creation of porosity in a shearing geological material and its implementation in the isale shock physics code the model is used to investigate impact induced dilatancy during simple and complex crater formation on earth simulations of simple crater formation produce porosity distributions consistent with observations dilatancy model parameters appropriate for low quality rock masses give the best agreement with observation more strongly dilatant behavior would require substantial postimpact porosity reduction the tendency for rock to dilate less when shearing under high pressure is an important property of the model pressure suppresses impact induced dilatancy in the shock wave at depth beneath the crater floor and in the convergent subcrater flow that forms the central uplift consequently subsurface porosity distribution is a strong function of crater size which is reflected in the inferred gravity anomaly the bouguer gravity anomaly for simulated craters smaller than 25 km is a broad low with a magnitude proportional to the crater radius larger craters exhibit a central gravity high within a suppressed gravity low lower crustal pressures on the moon relative to earth imply that impact induced dilatancy is more effective on the moon than earth for the same size impact in an initially nonporous target this difference may be mitigated by the presence of porosity in the lunar crust

propagation of an aseismic dislocation through asperities with smooth borders abstract a 2 d model is presented for the propagation of a somigliana dislocation along a fault with nonuniform friction fault slip is driven by a uniform ambient shear stress slowly increasing with time the dislocation is nucleated in the lowest friction region of the fault plane and is confined by the surrounding higher friction regions asperities the case studied involves asperities which have smooth borders characterized by a constant friction gradient for values of ambient shear stress near to the weak zone friction the propagation of dislocation is slowed down by the presence of asperities only when it goes beyond the border does the dislocation front move at increasing velocity the model shows that at a given value of ambient shear stress the slip amplitude is larger in the case of finite and constant friction gradient than in the case of asperities with sharp borders unlike the propagation velocity slip rate is ever increasing during the dislocation process the model shows to what extent a dislocation is influenced by the distribution of friction on the fault a detailed knowledge of slip rate and slip history is needed to understand the mechanism of frictional instability on faults

fracture roughness and gouge distribution of a granite shear band 1 localization of deformation during fracture mechanical tests leads to the development of shear bands we performed triaxial tests using sidobre granite at four different confining pressures from 20 to 80 mpa we compared two sets of tests one set was stopped immediately after the formation of the shear band a second one included additional shear deformation from the analysis of thin sections of these laboratory samples we characterize the typical microstructures in the shear band mode i and ii cracks riedel cracks cataclastic flow statistical properties of rupture surface roughness and gouge grain size reveals scaling invariance using a mechanical profiler the fracture roughness is measured along parallel profiles and shown to be correctly described over up to 3 orders of magnitude by self affine geometry with a roughness exponent close to 0 80 this property is very similar to tensile crack even if local processes are different the influence of the slip is observed fracture surfaces are rougher along the slip direction 0 74 than perpendicular to it 0 80 the confining pressure is shown to have a weak effect on the fracture roughness it smoothes the surface slight increase of the roughness exponent gouge particles extracted from the shear band present a power law distribution with an exponent ranging from 1 44 to 1 91 this exponent appears to increase with the shearing displacement and the confining pressure when a significant shear of the band is combined with a high confining pressure i e impeded dilation of the band the hallmark of fragmentation is observed for the particle distribution and related to a smoothing of the band boundary

the morphology of craters on mercury results from messenger flybys topographic data measured from the mercury laser altimeter mla and the mercury dual imaging system mdis aboard the messenger spacecraft were used for investigations of the relationship between depth and diameter for impact craters on mercury results using data from the messenger flybys of the innermost planet indicate that most of the craters measured with mla are shallower than those previously measured by using mariner 10 images mdis images of these same mla measured craters show that they have been modified the use of shadow measurement techniques which were found to be accurate relative to the mla results indicate that both small bowl shaped and large complex craters that are fresh possess depth to diameter ratios that are in good agreement with those measured from mariner 10 images the preliminary data also show that the depths of modified craters are shallower relative to fresh ones and might provide quantitative estimates of crater in filling by subsequent volcanic or impact processes the diameter that defines the transition from simple to complex craters on mercury based on messenger data is consistent with that reported from mariner 10 data

regional controls on magma ascent and storage in volcanic arcs understanding the controls for magma ascent and storage depth is important for volcanic hazard assessment regional differences in the depths of magma storage between volcanic arcs suggest that the settings of subduction zones and of overriding plates influence how magma ascends through the crust here we use a compilation of data for 70 volcanoes in 15 volcanic regions to better understand the geodynamic controls on magma storage we describe the subduction system which consists of the subducting slab the mantle wedge and the upper plate with 12 parameters encompassing the kinematics of the subduction the structure and geometry of the slab the timing of the subduction the thermal structure of the slab the upper plate crustal structure its stress regimes and its thermal structure we find that the magma reservoir depths correlate with the upper plate crustal structure and with the stress regimes shallow reservoirs 45 km similarly shallow magma reservoirs are 33 69 more common in extensional and strike slip stress regimes that in compressional regimes this illustrates the effect of buoyancy for magma ascent as well as the importance of stress regime and preexisting structures

trace element evidence for plagioclase recycling in calc alkaline magmas abstract trace element zoning in plutonic plagioclase feldspars from the val fredda complex southern adamello massif italy has been measured using secondary ion mass spectrometry sims the studied samples range from hornblende gabbro through diorite to granitoid with an overall range in plagioclase composition from an96 to an15 individual plagioclases display extensive zonation in anorthite through a considerable variety of zoning textures there is equally extensive zoning in trace elements notably sr and ba broadly similar trace element zoning patterns and concentrations are displayed by all plagioclases irrespective of host rock composition using a relationship betweenxan and the distribution coefficientsdsr anddba trace element concentrations in plagioclase are converted to those of the melt which precipitated each crystal zone the calculated variations in ba and sr are greater than can be attributed to changes in pressure or temperature alone indicating that compositional variation in the coexisting melt was the principal cause of zoning the ba sr melt trend for each crystal describes a two stage crystallisation history the calcic cores of all crystals show melt trends of increasing sr and ba which are attributed to mixing and fractionation in parental hydrous picrobasalt magmas in contrast the melt trends recorded by the rims vary from sample to sample but essentially show decreasing sr and ba consistent with fractionation of sodic plagioclase alkali feldspar and or biotite the plagioclase cores are thought to have crystallised in a common magma chamber at 6 10 kbar pressure while the rims crystallised at the emplacement level 1 5 kbar from interstitial melt whose composition is determined by that of the host magma the data suggest that early formed calcic plagioclase was recycled by the magma during fractionation vigorous convection is invoked to both retain relatively dense calcic plagioclase within the evolving magma and to periodically re entrain cumulus crystals into more evolved magmas the occurrence of calcic plagioclase cores in felsic derivative magmas indicates that fractionation from picrobasalt to tonalite occurred on a relatively short time scale

lithospheric flexure and the evolution of the dichotomy boundary on mars 1 the boundary of the martian crustal dichotomy in the eastern hemisphere is one of the most striking topographic features on the planet the long wavelength topography of much of the boundary is expressed by a broad rise and an arched ramp that slopes downward from the southern highlands into the northern lowlands and often ends in a steep scarp lithospheric flexure of the southern highlands for both a continuous and broken plate boundary with the northern lowlands lithosphere is modeled we find that the long wavelength topography of the boundary is best fit by a lithospheric deflection profile of a broken lithosphere for reasonable values of the flexural parameter the lithosphere near the dichotomy boundary may have been weakened by tectonic stress associated with the formation of the crustal dichotomy by subcrustal transport caused by overturn of an early magma ocean

high resolution quantitative imaging of plagioclase composition using accumulated backscattered electron images new constraints on oscillatory zoning oscillatory zoning in plagioclase is investigated at small scale 10 m using accumulated backscattered electron bse images as high resolution imaging method combined with electron microprobe quantitative analysis gray value profiles across these images can be calibrated for an content with a resolution of 0 5 mol an applied to oscillatory zoned crystals this new application of bse imaging allows better characterization of zoning patterns along a profile and quantification of wavelength amplitude and shape of the oscillations we also obtain high resolution information on the morphology of growth zones boundaries this approach allows us to better classify the different types of oscillations and concentric zoning dissolution is more frequent than usually recognized major resorption surfaces crosscut several growth zones irregular 5 10 m saw tooth zones are delimited by faint wavy dissolution surfaces and must be distinguished from small scale oscillations 1 3 m with straight boundaries this suggests at least two mechanisms for the formation of these zoning patterns faint oscillations are probably caused by local kinetic control whereas wavy dissolution surfaces involve magma chamber dynamics

the apparent absence of kilometer sized pyroclastic volcanoes on mercury are we looking right spacecraft data reveal that volcanism was active on mercury evidence of large volume effusive and smaller scale explosive eruptions has been detected however only large 15 km volcanic features or vents have been found so far despite abundant high resolution imagery on other volcanic planets the size of volcanoes is anticorrelated with their frequency small volcanoes are much more numerous than large ones here we present results of a numerical model that predicts the shapes of ballistically emplaced volcanic edifices and hence can explain the lack of kilometer sized constructional explosive volcanoes on the surface of mercury we find that due to the absence of the atmosphere particles are spread on this planet over a larger area than is typical for earth or mars erupted volumes are likely insufficient to build edifices with slope angles that enable their easy recognition with currently available data or that could survive destruction by subsequent impact bombardment

roughness of fault surfaces over nine decades of length scales 1 we report on the topographic roughness measurements of five exhumed faults and thirteen surface earthquake ruptures over a large range of scales from 50 m to 50 km we used three scanner devices lidar laser profilometer white light interferometer spanning complementary scale ranges from 50 m to 10 m to measure the 3 d topography of the same objects i e five exhumed slip surfaces vuache sillingy bolu corona heights dixie valley magnola a consistent geometrical property i e self affinity emerges as the morphology of the slip surfaces shows at first order a linear behavior on a log log plot where axes are fault roughness and spatial length scale covering five decades of length scales the observed fault roughness is scale dependent with an anisotropic self affine behavior described by four parameters two power law exponents h constant among all the faults studied but slightly anisotropic h 0 58 0 07 in the slip direction and h 0 81 0 04 perpendicular to it and two pre factors showing variability over the faults studied for larger scales between 200 m and 50 km we have analyzed the 2 d roughness of the surface rupture of thirteen major continental earthquakes these ruptures show geometrical properties consistent with the slip perpendicular behavior of the smaller scale measurements our analysis suggests that the inherent non alignment between the exposed traces and the along or normal slip direction results in sampling the slip perpendicular geometry although a data gap exists between the scanned fault scarps and rupture traces the measurements are consistent within the error bars with a single geometrical description i e consistent dimensionality over nine decades of length scales

the role of magma buoyancy on the eruption of lunar basalts it has long been recognized that mare basalts on the moon are preferentially located both on the earth facing hemisphere and within large impact basins a popular model that accounts for this observation assumes that these magmas were denser than the lunar crust that they accumulated at the crust mantle interface and that eruptions occurred only when this magma chamber became overpressurized in this paper we re evaluate this model and argue that it is not consistent with the available data nor with models of dike propagation as an alternative hypothesis we propose that magma buoyancy is the predominant factor that determines whether mare basalts erupt at the surface or form crustal intrusions instead we have computed the densities of mare basaltic magmas and find that some are in fact less dense than the moon s upper anorthositic crust based on the widely accepted view that the lunar crust becomes more mafic with depth we also show that all mare basaltic magmas should be less dense than the lower portion of the crust thus if the upper anorthositic crust was regionally removed by an impact event then any mare basaltic magma could have risen to the surface there based on buoyancy considerations alone in support of this model we note that mare basalts are indeed found wherever geophysical crustal thickness models predict the upper crust to be absent furthermore many of the basalts that erupted within the anorthositic highlands are predicted to be less dense than the underlying crust based on remote sensing data the high titanium flows within oceanus procellarum are somewhat problematical to our model in that an anorthositic crust is predicted to be present beneath them using results from recent lunar thermal models we suggest that these magmas may have overcome their negative buoyancy in the crust by possessing superliquidus temperatures if magma buoyancy does indeed control whether or not a basaltic eruption will occur then this implies that the quantity of magma produced beneath the south pole aitken basin was about 10 times less than that of the nearside fl 2001 elsevier science b v all rights reserved

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

structural geology of impact craters abstract the formation of impact craters is a highly dynamic and complex process that subjects the impacted target rocks to numerous types of deformation mechanisms understanding and interpreting these styles of micro meso and macroscale deformation has proved itself challenging for the field of structural geology in this paper we give an overview of the structural inventory found in craters of all size ranges on earth and look into the structures of craters on other planetary bodies structural features are discussed here that are caused by i extremely high pressures and temperatures that occur during the initial passage of the shock wave through the target rock and projectile ii the resulting flow field in the target that excavates and ejects rock materials and iii the gravitationally induced modification of the crater cavity into the final crater form a special focus is put on the effects that low angle impacting bodies have on crater formation we hope that this review will help both planetary scientists and structural geologists understand the deformation processes and resulting structures generated by meteorite impact

prolonged eruptive history of a compound volcano on mercury volcanic and tectonic implications a 27 13 km rimless depression 100 km inside the southwest rim of the caloris basin is revealed by high resolution orbital imaging under a variety of illuminations to consist of at least nine overlapping volcanic vents each individually up to 8 km in diameter it is thus a compound volcano indicative of localised migration of the site of the active vent the vent floors are at a least 1 km below their brinks but lack the flat shape characteristically produced by piston like subsidence of a caldera floor or by flooding of a crater bottom by a lava lake they bear a closer resemblance to volcanic craters sculpted by explosive eruptions and or modified by collapse into void spaces created by magma withdrawal back down into a conduit this complex of overlapping vents is at the summit of a subtle edifice at least 100 km across with flank slopes of about only 0 2 degrees after correction for the regional slope this is consistent with previous interpretation as a locus of pyroclastic eruptions construction of the edifice could have been contributed to by effusion of very low viscosity lava but high resolution images show that the vent facing rim of a nearby impact crater is not heavily embayed as previously supposed on the basis of lower resolution flyby imaging contrasts in morphology sharpness versus blurredness of the texture and different densities of superposed sub km impact craters inside each vent are consistent with but do not prove substantial differences in the age of the most recent activity at each vent this suggests a long duration of episodic magmagenesis at a restricted locus the age range cannot be quantified but could be of the order of a billion years if each vent was fed from the same point source geometric considerations suggest a source depth of at least 50 km however the migration of the active vent may be partly controlled by a deep seated fault that is radial to the caloris basin other rimless depressions in this part of the caloris basin fall on or close to radial lines suggesting that elements of the pantheon fossae radial fracture system that dominates the surface of the central portion of the caloris basin may continue at depth almost as far as the basin rim 2013 elsevier b v

magma emplacement and remobilization timescales beneath montserrat insights from sr and ba zonation in plagioclase phenocrysts received november 22 2001 accepted december 12 2002since 1995 the soufri ere hills volcano on montserrat lesserantilles island arc has erupted crystal rich andesite magmathat is geochemically very similar to the volcanic products of atleast the last 174 3kyr 1s nomarski images of plagio clase phenocrysts from six andesites erupted between 151 4ka 1s and

vapor transfer prior to the october 2004 eruption of mount st helens washington dome lavas from the 2004 eruption of mount st helens show elevated li contents in plagioclase phenocrysts at the onset of dome growth in october 2004 these cannot be explained by variations in plagioclase melt partitioning but require elevated li contents in coexisting melt a fact confirmed by measurements of li contents as high as 207 g g in coexisting melt inclusions similar li enrichment has been observed in material erupted prior to and during the climactic may 1980 eruption and is likewise best explained via pre eruptive transfer of an exsolved alkali rich vapor phase derived from deeper within the magma transport system unlike 1980 however high li samples from 2004 show no evidence of excess 210pb 226ra implying that measurable li enrichments may occur despite significant differences in the timing and or extent of magmatic degassing diffusion modeling shows that li enrichment occurred within 1 yr before eruption and that magma remained li enriched until immediately before eruption and cooling this short flux time and the very high li contents in ash produced by phreatomagmatic activity prior to the onset of dome extrusion suggest that vapor transfer and accumulation were associated with initiation of the current eruption overall observation of a high li signature in both 1980 and 2004 dacites indicates that li enrichment may be a relatively common phenomenon and may prove useful for petrologic monitoring of mount st helens and other silicic volcanoes lithium diffusion is also sufficiently rapid to constrain vapor transfer on similar time scales to short lived radionuclides

rates of hydrothermal cooling of new oceanic upper crust derived from lithium geospeedometry episodic emplacement and cooling of lavas and dikes at mid ocean ridges leads to large fluctuations in hydrothermal fluxes and biological activity however the processes operating beneath the seafloor during these transient events such as permeability creation and dike cooling are poorly understood we have developed a new approach to determine the cooling rate of the sheeted dike complex based on the extent of diffusion of lithium from plagioclase into clinopyroxene during cooling we have calibrated this li geospeedometer using new high temperature experiments to determine both the temperature dependence of the partitioning of li between plagioclase and clinopyroxene and the diffusion coefficient for li in clinopyroxene application of this method to lavas and dikes from odp hole 504b shows that cooling rates vary dramatically with depth in the upper oceanic crust extremely rapid cooling rates 450 c hr 1 in the upper part of the sheeted dike complex are sufficient to power hydrothermal megaplume formation within the overlying water column

tof sims and electron microprobe investigations of zoned magmatic orthopyroxenes first results of trace and minor element analysis with implications for diffusion modeling zoned phenocrysts in volcanic rocks potentially provide an archive of magmatic processes as a crystal grows and comes into contact with different melt batches the chemical and textural signature of this journey is recorded within its crystal lattice the timescale of some magmatic processes can be investigated through the relaxation of chemical gradients across crystal growth zones through the application of diffusion modeling techniques one of the current limitations to diffusion modeling is the spatial and analytical resolution of the chemical profile that conventional techniques such as electron probe microanalyzer epma dynamic secondary ion mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icpms can achieve here for the first time we present time of flight tof sims tof sims data for zoning of orthopyroxene crystals from the may 1982 eruption of mount st helens volcano u s a and cross calibrate these data between backscattered electron images and epma tof sims has the advantage of being able to achieve micrometer to nanoscale spatial resolution of major elements as well as analyses of light elements such as li and trace and minor elements na k and ni at concentrations that cannot be achieved by epma provided that convolution overlap effects and polyatomic mass inferences are carefully considered with tof sims analyses we identified zoning of li on a spatial scale ca 5 10 m that would be inaccessible to most other conventional analytical techniques preliminary results indicate that li a fast diffusing element may be introduced to the crystals in the minutes hours or days prior to eruption and may provide insights into pre eruptive magmatic processes thus tof sims has the potential to be a powerful tool for obtaining minor and trace element profiles across compositional interfaces within crystals at high spatial resolution

using amphibole phenocrysts to track vapor transfer during magma crystallization and transport an example from mount st helens washington abstract in order to evaluate and further constrain models for volatile movement and vapor enrichment of magma stored at shallow levels amphibole phenocrysts from 2004 2005 mount st helens dacite were analyzed for major and selected trace elements li cu zn mn and ree and li isotopes several recent studies have examined fluid mobile trace element abundances in phencryst phases and melt inclusions as a means of tracking volatile movement within subvolcanic magmatic systems and high li contents in plagioclase phenocrysts from 1980 and 2004 mount st helens dacites have been interpreted as evidence that shallow magma was fluxed by a li bearing vapor phase prior to eruption in amphibole phenocrysts zn and mn behave compatibly correlating to feo and al 2 o 3 and show no systematic change with time in contrast li and cu abundances in amphibole vary by up to 3 orders of magnitude 7 6 1140 g g and 1 7 to 94 g g respectively and do not generally correlate with either major or trace elements however they do correlate moderately well r 2 0 54 95 confidence with each other and show systematic temporal variations that are opposite to those observed for plagioclase precluding a simple 1 step diffusion model for li enrichment we propose a diffusion crystallization multi stage dcms model to explain the temporal variations and co variations of li and cu in early erupted dacite october december 2004 profiles of li isotopes in conjunction with measured 7 li intensities and core to rim increases in li concentration are characteristic of li diffusion into the amphiboles consistent with prior models of plagioclase enrichment in amphiboles from 2005 dacite average li and cu concentrations are high 260 660 g g and 29 45 g g respectively and in contrast to amphiboles from earlier erupted dacite correlate weakly with al 2 o 3 wt amphibole al 2 o 3 concentrations are an indicator of pressure with high al amphiboles crystallizing at higher pressures and we suggest that li and cu are partitioned into a fluid phase during ascent and crystallization of the magma so that amphiboles crystallizing at lower pressure have correspondingly lower li and cu concentrations however low li and cu in amphiboles from the dacite at the start of the eruption also require crystallization from a low li cu bearing melt or residence times long enough for amphiboles to re equilibrate with a li cu depleted melt estimated residence times suggest that amphiboles in early dacite could have been present since the end of the 1980 1986 eruptive episode at mount st helens

breaking of a single asperity rupture process and seismic radiation the problem of spontaneous shear rupture of a single circular asperity on an infinite fault plane is studied initially the fault plane is broken everywhere except at a circular asperity an applied displacement at infinity results in a stress concentration along the bounding edge of the asperity the frictional stress on the broken part of the fault plane is taken to be a constant once a point on the asperity breaks the stress there drops to the same value as on the main fault surface the rupture is started by relaxing the shear stress at a point on the asperity edge and is then allowed to propagate spontaneously using a critical stress level fracture criterion the rupture process is calculated numerically it is found that for asperities of constant strength the rupture first propagates around the edge of the asperity and then inward a phenomenon best described by the well known term of classic military maneuver the double encircling pincer movement in the appendix the expressions for the far field seismic radiation due to the rupture of such an asperity are derived it is shown that the nth cartesian component of the far field displacement at x t for p sv and sh waves using the notation of aki and richards 1980 is given by un x t dni 4 c2r s0 i3 t r c ds thus the far field pulses can be directly found from the stress drops on the fault plane this formula is also true for crack or dislocation problems the directivity function dni for displacement for the asperity problem is found to be that for the double couple modified by some factor in particular the fault plane is a nodal plane for sv waves for the rupturing of asperities on a finite fault these directivity functions are applicable only to the initial part of observed pulses at a receiver provided the receiver is not located on the fault plane outside the broken part of the main crack edge in which case it is inapplicable for all time

lunar floor fractured craters as magmatic intrusions geometry modes of emplacement associated tectonic and volcanic features and implications for gravity anomalies lunar floor fractured craters are a class of 170 lunar craters with anomalously shallow fractured floors two end member processes have been proposed for the floor formation viscous relaxation and subcrater magmatic intrusion and sill formation recent morphometric analysis with new lunar reconnaissance orbiter laser altimeter lola and image lroc data supports an origin related to shallow magmatic intrusion and uplift we find that the distribution and characteristics of the ffc population correlates strongly with crustal thickness and the predicted frequency distribution of overpressurization values of magmatic dikes for a typical nearside lunar crustal thickness dikes with high overpressurization values favor surface effusive eruptions medium values favor intrusion and sill formation and low values favor formation of solidified dikes concentrated lower in the crust we develop a model for this process make predictions for the morphologic morphometric volcanic and geophysical consequences of the process and then compare these predictions with the population of observed floor fractured craters in our model the process of magmatic intrusion and sill formation begins when a dike propagates vertically towards the surface as the dike encounters the underdense brecciated region beneath the crater the magmatic driving pressure is insufficient to continue vertical propagation but pressure in the stalled dike exceeds the local lithostatic pressure the dike then begins to propagate laterally forming a sill which does not propagate past the crater floor region because increased overburden pressure from the crater wall and rim crest pinch off the dike at this boundary the sill then continues to inflate further raising and fracturing the brittle crater floor when the intrusion diameter to intrusion depth ratio is smaller than a critical value the intrusion assumes a laccolith shape with a domed central region when the ratio exceeds a critical value the intrusion concentrates bending primarily at the periphery resulting in a flat tabular intrusion we predict that this process will result in concentric fractures over the region of greatest bending this location is close to the crater wall in large flat floored craters as observed in the crater humboldt and interior to the crater over the domed floor in smaller craters as observed in the crater vitello a variety of volcanic features are predicted to be associated with the solidification and degassing of the intrusion these include 1 surface lava flows associated with concentric fractures e g in the crater humboldt 2 vents with no associated pyroclastic material from the deflation of under pressurized magmatic foam e g the crater damoiseau and 3 vents with associated pyroclastic deposits from vulcanian eruptions of highly pressurized magmatic foam e g the crater alphonsus the intrusion of basaltic magma beneath the crater is predicted to contribute a positive component to the bouguer gravity anomaly we assess the predicted bouguer anomalies associated with ffcs and outline a process for their future interpretation we conclude that our proposed mechanism serves as a viable formation process for ffcs and accurately predicts numerous morphologic morphometric and geophysical features associated with ffcs these predictions can be further tested using grail gravity recovery and interior laboratory data

timescales of magmatic processes at ruapehu volcano from diffusion chronometry and their comparison to monitoring data abstract recent eruptions from mt ruapehu have been difficult to predict despite the presence of a multi parametric monitoring network as a result it is necessary to assess precursory signals prior to an eruption and align those to magmatic processes at depth fortuitously scoria from all historical ruapehu eruptions contains pyroxene crystals that are strongly reversely zoned in the form of a thin 2 to 3 m outermost rim these crystals therefore preserved changes in the magmatic system soon before their eruption we used experimentally determined diffusion coefficients to assess the timescales of magma magma interaction and compared those to the monitoring record four of the five eruptions analysed 1969 1971 1977 1995 gave diffusion timescales 3 to 5 months before their eruption with an increased number of crystals recording timescales within 1 month of eruption pyroxene crystals from the 1996 eruption record events that occurred prior to and during the 1995 eruption suggesting that the bulk of the 1996 crystals was derived from the 1995 magma these diffusion timescales do not compare well to a change in any monitoring signal before historical eruptions however an examination of recent seismicity 2005 2013 since a significant upgrade both in number of stations and type of seismometers showed that two phreatic eruptions in 2006 and 2007 were preceded by a seismic swarm from 5 to 15 km depth 3 to 5 months before each eruption consistent with the diffusion timescales based on this correlation deep seismic swarms likely indicate a period of pressurisation in the magmatic system which may lead to gas rich phreatic eruptions

mechanisms of explosive volcanism on mercury implications from its global distribution and morphology the identification of widespread pyroclastic vents and deposits on mercury has important implications for the planet s bulk volatile content and thermal evolution however the significance of pyroclastic volcanism for mercury depends on the mechanisms by which the eruptions occurred using images acquired by the mercury surface space environment geochemistry and ranging spacecraft we have identified 150 sites where endogenic pits are surrounded by a relatively bright and red diffuse edged spectral anomaly a configuration previously used to identify sites of explosive volcanism we find that these sites cluster at the margins of impact basins and along regional tectonic structural trends locally pits and deposits are usually associated with zones of weakness within impact craters and or with the surface expressions of individual thrust faults additionally we use images and stereo derived topographic data to show that pyroclastic deposits are dispersed up to 130 km from their source vent and commonly have either no relief or low circumpit relief within a wider thinner deposit these eruptions were therefore likely driven by a relatively high concentration of volatiles consistent with volatile concentration in a shallow magma chamber prior to eruption the colocation of sites of explosive volcanism with near surface faults and crater related fractures is likely a result of such structures acting as conduits for volatile and or magma release from shallow reservoirs with volatile overpressure in these reservoirs a key trigger for eruption in at least some cases our findings suggest that widespread long lived explosive volcanism on mercury has been facilitated by the interplay between impact cratering tectonic structures and magmatic fractionation

the formation of floor fractured craters in xanthe terra floor fractured craters ffc are a peculiar form of degradation of impact craters defined by the presence of crevice networks and mesas affecting crater floors they are preferentially distributed near chaotic terrains and outflow channels the scope of this paper is to present a detailed systematic analysis of ffc at xanthe terra ffc morphologies in this region are classified into five types making a picture of different stages of the same degradation process ffc are geographically intermixed with un fractured normal craters non ffc young craters are less prone to show this type of degradation as suggested by fresh ejecta layer with preserved crater floor size distributions of ffc and non ffc indicate that larger craters are preferentially fractured careful examinations of the crater floor elevations reveal that the crevices often extend deeper than the original crater cavity furthermore an onset depth for the formation of ffc is evidenced from the difference of spatial distributions between ffc and non ffc roof collapsed depressions observed in the same region have been also documented and their characteristics suggest the removal of subsurface material at depth from about 1200 to 4000 m these observations taken together suggest a subsurface zone of volume deficit at depth from 1 to 2 km down to several kilometers responsible for ffc formation then a scenario of ffc formations is presented in the context of groundwater discharge events at the late hesperian this scenario involves two key processes earth fissuring and piping erosion known to occur with rapid groundwater migrations on earth

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

earthquakes as self organized critical phenomena a cellular automation model with threshold elements previously proposed by bak et al 1988 which generates 1 f noise and self organizes into a critical state is studied as a model for earthquake occurrence the model explains some of fractal features of earthquakes such as the power law distribution of the size of the earthquake the model is modified so that every earthquake may trigger aftershocks the modified trigger model explains other fractal features of earthquakes such as the power law decay of aftershock frequency and the power law decay of aftershock frequency and the power law correlation of the hypocenter distribution

mercury s hollows constraints on formation and composition from analysis of geological setting and spectral reflectance 1 landforms unique to mercury hollows are shallow flat floored irregular depressions notable for their relatively high reflectance and characteristic color here we document the range of geological settings in which hollows occur most are associated with impact structures simple bowl shaped craters to multiring basins and ranging from kuiperian to calorian in age hollows are found in the low reflectance material global color unit and in low reflectance blue plains but they appear to be absent from high reflectance red plains hollows may occur preferentially on equator or hot pole facing slopes implying that their formation is linked to solar heating evidence suggests that hollows form because of loss of volatile material we describe hypotheses for the origin of the volatiles and for how such loss proceeds intense space weathering and solar heating are likely contributors to the loss of volatiles contact heating by melts could promote the formation of hollows in some locations lunar ina type depressions differ from hollows on mercury in a number of characteristics so it is unclear if they represent a good analog we also use messenger multispectral images to characterize a variety of surfaces on mercury including hollows within a framework defined by laboratory spectra for analog minerals and lunar samples data from messenger s x ray spectrometer indicate that the planet s surface contains up to 4 sulfur we conclude that nanophase or microphase sulfide minerals could contribute to the low reflectance of the low reflectance material relative to average surface material hollows may owe their relatively high reflectance to destruction of the darkening agent sulfides the presence of alteration minerals and or physical differences in particle size texture or scattering behavior

hollows on mercury materials and mechanisms involved in their formation recent images of the surface of mercury have revealed an unusual and intriguing landform sub kilometre scale shallow flat floored steep sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters these hollows appear to form by the loss of a moderately volatile substance from the planet s surface and their fresh morphology and lack of superposed craters suggest that this process has continued until relatively recently and may be on going hypotheses to explain the volatile loss have included sublimation and space weathering and it has been suggested that hollow forming volatiles are endogenic and are exposed at the surface during impact cratering however detailed verification of these hypotheses has hitherto been lacking in this study we have conducted a comprehensive survey of all messenger images obtained up to the end of its fourth solar day in orbit in order to identify hollowed areas we have studied how their location relates to both exogenic processes insolation impact cratering and solar wind and endogenic processes explosive volcanism and flood lavas on local and regional scales we find that there is a weak correlation between hollow formation and insolation intensity suggesting formation may occur by an insolation related process such as sublimation the vast majority of hollow formation is in localised or regional low reflectance material within impact craters suggesting that this low reflectance material is a volatile bearing unit present below the surface that becomes exposed as a result of impacts in many cases hollow occurrence is consistent with formation in volatile bearing material exhumed and exposed during crater formation while in other cases volatiles may have accessed the surface later through re exposure and possibly in association with explosive volcanism hollows occur at the surface of thick flood lavas only where a lower reflectance substrate has been exhumed from beneath them indicating that this form of flood volcanism on mercury lacks significant concentrations of hollow forming volatiles

evidence for intrusive activity on mercury from the first messenger flyby images from messenger s first flyby of mercury have shown convincing evidence for surface volcanism here we report on evidence in the new data for several features that are characterized by fractures and graben rare features on a planet dominated by contractional deformation that may be linked to intrusive activity these features include 1 a floor fractured crater interpreted to have been the site of laccolith like sill intrusions the feature is similar to some floor fractured craters on the moon and shows evidence for individual fractured dome like uplifts on the floor 2 a concentric complex of graben observed inside the peak ring on the floor of the 250 km diameter raditladi basin and associated with dark plains and possibly embayed by them the feature may represent an unusual type of floor fracturing associated with deeper intrusions and related ring dikes or cone sheets or the graben may instead be the product of non magmatic uplift of the basin floor 3 a large radial graben swarm pantheon fossae located near the center of the caloris basin thus far unique on mercury and characterized by hundreds of individual graben segments ranging from 5 km to 110 km in length in the nexus graben crosscut one another and produce a local polygonal pattern others curve away from the center as the nexus is approached two scales of graben length are observed the radius of the dense radially symmetric plexus of graben is 175 km and a few graben extend to greater radial distances to the north and southwest out to distances that intersect with a ring of generally concentric graben around the outer basin floor two width scales of graben are observed a large graben about 8 km wide emerges from the nexus and extends for 100 km most graben are less than half this width some graben walls appear cuspate with convex outward wall segments that resemble crater chain segments one crater chain with distinctive raised rims parallels nearby graben locally some graben appear in en echelon patterns and smaller graben sometimes show cross cutting superposition relationships abundant impact craters the most prominent being apollodorus and secondary crater clusters and chains are superposed on the graben system there is little evidence that craters greater than 5 km in diameter have been cut by a graben this relation implies that the graben swarm formed soon after the emplacement of the caloris floor plains these graben are interpreted to be the surface expression of a radial dike swarm emanating from a subsurface magma reservoir similar features in which the dikes contribute to a near surface stress field that favors radial graben are known on the earth venus and mars the location of pantheon fossae in the center of the caloris basin suggests that formation of the radial graben structure is linked to basin evolution

evidence for geochemical terranes on mercury global mapping of major elements with messenger s x ray spectrometer abstract we have mapped the major element composition of mercury s surface from orbital messenger x ray spectrometer measurements these maps constitute the first global scale survey of the surface composition of a solar system body conducted with the technique of planetary x ray fluorescence full maps of mg and al together with partial maps of s ca and fe each relative to si reveal highly variable compositions e g mg si and al si range over 0 1 0 8 and 0 1 0 4 respectively the geochemical variations that we observe are consistent with those inferred from other messenger geochemical remote sensing datasets but they do not correlate well with units mapped previously from spectral reflectance or morphology location dependent rather than temporally evolving partial melt sources were likely the major influence on the compositions of the magmas that produced different geochemical terranes a large 5 10 6 km 2 region with the highest mg si ca si and s si ratios as well as relatively thin crust may be the site of an ancient and heavily degraded impact basin the distinctive geochemical signature of this region could be the consequence of high degree partial melting of a reservoir in a vertically heterogeneous mantle that was sampled primarily as a result of the impact event

analysis of peak ground motion in terms of a model of inhomogeneous faulting measurements of peak ground velocity v and acceleration a for earthquakes and mine tremors with local magnitudes ranging from 0 8 to 6 4 have been analyzed in terms of a model of inhomogeneous faulting the fault model involves the failure of a circular asperity of radius ri surrounded by a previously faulted annular region of outer radius r0 the failure of the asperity results in seismic radiation with a characteristic frequency proportional to 1 ri followed by lower frequency radiation proportional to 1 r0 as static equilibrium is regained over the larger region for r0 ri 1 both v and a are associated primarily with the failure of the asperity but also depend on the large scale source parameters specifically v r0 r 0 10 r0 ri 0 15 and a r 0 30 r0 ri 2 0 45 where is the shear wave velocity is the overall stress drop is the modulus of rigidity and is density the terms involving r0 ri correspond to the high frequency radiation associated with the failure of the asperity and the other terms indicate the peak parameters due to the broad scale readjustment observations of peak ground motion for events with seismic moments ranging from 5 1016 to 1026 dyn cm indicate that r0 ri is normally in the range of 1 10 and appears to be independent of earthquake size the inhomogeneous fault model also yields convenient expressions for the small scale displacement and stress drop of the asperity failure as well as for the level of regional stress available to cause slip

the scaling of complex craters the empirical relation between the transient crater diameter dg and final crater diameter dr of complex craters and basins is estimated using cumulative terrace widths central uplift diameters continuous ejecta radii and transient crater reconstructions determined from lunar and terrestrial impact structures the ratio dg dr is a power law function of dr decreasing uniformly from unity at the diameter of the simple complex crater morphology transition to about 0 5 for large multiring basins like imbrium on the moon the empirical constants in the dg dr relation are interpreted physically to mean that the position of the final rim relative to the transient crater and hence the extent of collapse is controlled or greatly influenced by the properties of the zone of dissociated material produced by the impact shock the continuity of the dg dr relation over the entire spectrum of morphologic types from complex craters to multiring basins implies that the rims of all these structures form in the same tectonic environment despite morphologic differences

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

penetration of a quasi statically slipping crack into a seismogenic zone of heterogeneous fracture resistance this paper is concerned with some aspects of nonuniform stressing above a deep creeping portion of a fault zone prior to a large crust breaking earthquake the model that we use involves a slipping crack representing the deeper more stably sliding portions of the fault zone which penetrates upward from depth and is blocked in the lower region of the seismogenic zone when conditions are uniform along strike the upward penetration at the crack front is by mode iii in strike slip fault zones but by mode ii in thrust or normal fault zones two major results are reported first we analyze approximately via a linear perturbation formulation how a tectonic crack front encounters and ultimately shears through arrays of localized asperities that are distributed parallel to the crack front and have a toughness which is greater than that of adjoining segments of the fault zone using a fast fourier transform based numerical procedure to simulate crack penetration into asperities we find a notable difference between mode ii and mode iii crack fronts in that the former penetrates approximately twice as far between the asperities as the latter under the same loading level this is interesting because observations of slip distribution in large earthquakes suggest that there is a significant aseismic component to the total slip budget in subduction zone earthquakes which in contrast does not seem to be present in strike slip zone earthquakes and also that the surface slip distribution in continental dip slip faulting is typically much more irregular than for strike slip faulting in a simulation involving multiple rows of periodic asperities we note that the more deeply penetrating mode ii crack front contacts more asperities simultaneously while breaking them at different load levels compared to the less flexible mode iii crack front which simply breaks one row of asperities and jumps unstably to the next the second major result concerns whether a straight crack front in the lithosphere along a strike slip fault zone is configurationally stable that is whether the crack front will tend to remain straight as the crack penetrates upward from depth it is found that for infinitesimal perturbations of the straight front beyond a critical wavelength of the order of the crustal lithosphere thickness the stress intensity factor is higher at the most advanced portions of the crack front rather than at the least advanced the opposite is true at shorter wavelengths when resistance to crack growth is essentially uniform over the fault plane this means that the straight crack front is configurationally unstable at long wavelengths the issue of configurational stability is related to the concept of fault segmentation which is based on the observation that fault zones particularly long ones do not rupture along their entire length during a single earthquake effect of a vertical gradient of fracture resistance is discussed in the appendix where it is shown that a significant upward gradient of resistance to crack growth may completely stabilize the straight crack configuration

lunar floor fractured craters evidence for viscous relaxation of crater topography in this paper we evaluate quantitatively the hypothesis that topographic modification of floor fractured craters on the moon was accomplished predominantly by viscous relaxation adopting the simple assumption that the moon may be modeled as having a uniform newtonian viscosity we compare the observed topographic profiles for a number of floor fractured craters with the profiles predicted from the viscous relaxation of topography of fresh craters of similar diameter despite the simplicity of the rheological model the comparison is quite good the floor uplift the rim subsidence and the apparent subsidence outside the rim for the several floor fractured craters considered are well matched by the viscous relaxation hypothesis floor fractures while indicating that a purely viscous model is not strictly valid can be explained by the effects of isostatic adjustment on a thin brittle lithosphere the association of many floor fractured craters with impact basins and with the time of mare volcanism can be understood in terms of a pronounced acceleration of crater relaxation in local regions of anomalously high near surface temperatures and therefore of low effective viscosity and thin lithosphere the quantitative extent of relaxation of floor fractured craters can be interpreted in terms of a limited time interval of substantial relaxation for each crater that time interval ended for each crater after local cooling had been sufficient for the viscosity to rise for the lithosphere to thicken and for the present topographic relief to be frozen in thus viscous relaxation is a viable hypothesis to explain the topographic profiles of a number of lunar floor fractured craters and the extent of viscous relaxation of crater topography may serve as a tool to map lateral and temporal variations in the shallow thermal structure of the moon and of other planets and satellites

interplay of seismic and aseismic deformations during earthquake swarms an experimental approach observations of earthquake swarms and slow propagating ruptures on related faults suggest a close relation between the two phenomena earthquakes are the signature of fast unstable ruptures initiated on localized asperities while slow aseismic deformations are experienced on large stable segments of the fault plane the spatial proximity and the temporal coincidence of both fault mechanical responses highlight the variability of fault rheology however the mechanism relating earthquakes and aseismic processes is still elusive due to the difficulty of imaging these phenomena of large spatiotemporal variability at depth here we present laboratory experiments that explore in great detail the deformation processes of heterogeneous interfaces in the brittle creep regime we track the evolution of an interfacial crack over 7 orders of magnitude in time and 5 orders of magnitude in space using optical and acoustic sensors we explore the response of the system to slow transient loads and show that slow deformation episodes are systematically accompanied by acoustic emissions due to local fracture energy disorder features of acoustic emission activities and deformation rate distributions of our experimental system are similar to those in natural faults on the basis of an activation energy model we link our results to the rate and state friction model and suggest an active role of local creep deformation in driving the seismic activity of earthquake swarms

textural and chemical variation in plagioclase phenocrysts from the 1980 eruptions of mount st helens usa this study presents major and trace element chemistry of plagioclase phenocrysts from the 1980 eruptions of mount st helens volcano despite the considerable variation in textures and composition of plagioclase phenocrysts distinct segments have been cross correlated between crystals the variation of sr and ba concentration in the melt as calculated from the concentration in the phenocrysts using partition coefficients suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector in both of these melts sr and ba are correlated despite the abundance of plagioclase in the 1980 dacites we propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower sr and higher ba as documented in the rims of the phenocrysts although the cores of the phenocrysts crystallised at relatively shallow depths they preserve the sr and ba of the deep seated melts as they ascended from a deeper region further magma ascent resulted in microlite nucleation which is responsible for a similar shift to even lower sr concentration as observed in the groundmass of post 18 may 1980 samples

geochemical precursors to volcanic activity at mount st helens usa the importance of the interplay between degassing and crystallization before and after the eruption of mount st helens washington usa in 1980 is well established here we show that degassing occurred over a period of decades to days before eruptions and that the manner of degassing as deduced from geochemical signatures within the magma was characteristic of the eruptive style trace element lithium and short lived radioactive isotope lead 210 and radium 226 data show that ascending magma stalled within the conduit leading to the accumulation of volatiles and the formation of lead 210 excesses which signals the presence of degassing magma at depth

lunar mare volcanism stratigraphy eruption conditions and the evolution of secondary crusts abstract lunar mare basalt deposits cover 17 of the lunar surface occur preferentially in topographic lows on the nearside and have a total volume estimated at 1 107 km3 analysis of returned samples and photogeologic and remote sensing studies shows that mare volcanism began prior to the end of heavy bombardment the period of cryptomare formation in pre nectarian times and continued until the copernican period a total duration approaching 3 5 4 ga stratigraphic analyses show that the flux was not constant but peaked in early lunar history during the imbrian period which spans the period 3 85 3 2 ga average volcanic output rate during this peak period was about 10 2 km 3 a very low relative to the present global terrestrial volcanic output rate and comparable to the present local output rates for individual volcanoes such as vesuvius and kilauea hawaii volcanic landforms indicate that many eruptions were of high volume and long duration some eruptions associated with sinuous rules may have lasted of the order of a year and emplaced 103 km3 of lava representing the equivalent in one year of about 70 000 years at the average flux shallow magma chambers were uncommon the nearsidefarside mare deposit asymmetry can be readily explained by differences in crustal thickness magma ascending from the mantle or from a buoyancy trap at the base of the crust should preferentially extrude to the surface on the nearside but should generally stall and cool in dikes in the farside crust extruding only in the deepest basins the occurrence of farside maria within craters whose diameter is generally near to or less than the thickness of the crust may be accounted for by the difference between local and regional compensation dikes that establish pathways to the surface on the nearside should have very high volumes comparable to the volumes associated with many observed flows and sinuous rilles an abundance of dikes should exist in the lower crust of the moon many more than those feeding surface eruptions the upper limit is 37 50 of the crust by volume the presence and abundance of such dike swarms have important implications for the interpretation of the average composition of the lunar crust and the composition of basin and crater ejecta the interplay between thermal contraction and differentiation leads to net cooling and ultimate contraction of the outer portions of the moon resulting in a regional horizontal compressive stress acting on the lunar crust in addition overall cooling and deepening of sources require the production of ever larger volumes of magma in order to reach the surface with time stresses became high enough so that few dikes could open to the surface causing eruptive activity to be severely diminished in the eratosthenian and perhaps to cease in the copernican period lower stress levels are required to terminate eruptive activity on the lunar farside consistent with the imbrian age of the farside maria and the nearside location of the youngest maria lunar mare deposits provide an example of the transition from primary crusts to secondary crusts and illustrate the significance of several factors in the evolution of secondary crusts such as crustal density variations in crustal thickness presence of impact basins state and magnitude of stress in the lithosphere and general thermal evolution these factors are also responsible for the extremely low volcanic flux even during periods of peak extrusion

compositional analyses of lunar pyroclastic deposits abstract the 5 band clementine uvvis data at 100 m pixel were used to examine the compositions of 75 large and small lunar pyroclastic deposits lpds and these were compared to representative lunar maria and highlands deposits results show that the albedo spectral color and inferred composition of most lpds are similar to those of low titanium mature lunar maria these lpds may have consisted largely of fragmented basalt with substantial components of iron bearing mafic minerals pyroxenes olivine and smaller amounts if any of volcanic glass several smaller lpds also show substantial highland components three classes of very large deposits can be distinguished from most lpds and from each other on the basis of crystallinity and possible titanium content of their pyroclastic components one class has spectral properties that are dominated by high titanium crystallized black beads e g taurus littrow a second consists of a mixture of high titanium glasses and beads with a higher glass bead ratio sulpicius gallus than that of taurus littrow and a third has a significant component of quenched iron bearing volcanic glasses aristarchus with possible moderate titanium contents although areally extensive these three classes of very large pyroclastic deposits compose only 20 of the 75 deposits studied 27 and eruption of such materials was thus likely to have been less frequent on the moon

plagioclase residence times at two island arc volcanoes kameni islands santorini and soufriere st vincent determined by sr diffusion systematics the diffusive relaxation of trace element profiles in plagioclase phenocrysts may provide important constraints on magma residence times in crustal magma chambers initial trace element profiles in plagioclase phenocrysts are governed by variations in the concentration of a trace element in the melt and by the plagioclase melt partition coefficient trace element diffusion will subsequently act to modify this initial profile and given enough time produce a profile that is in equilibrium with the anorthite variations within the crystal we argue that the trace element partition coefficient da b between two parts a and b of a plagioclase crystal of variable anorthite content is equal to the ratio of their crystal liquid partition coefficients and that the equilibrium profile of the crystal can be calculated the time required to establish diffusive equilibrium is dependent on the wavelength and amplitude of the initial trace element concentration range and on the diffusivity of the trace element in plagioclase strontium plagioclase melt partition coefficients and diffusivities are calculated for a range of magmatic temperatures and plagioclase compositions a one dimensional diffusion model is developed that describes the diffusive destruction of oscillatory trace element zoning with time and allows the calculation of upper limits for plagioclase crystal residence times in a magma reservoir the model is tested using major and trace element concentrations measured along crystal traverses of plagioclase phenocrysts from the kameni island dacites santorini and from the 1979 soufriere andesite st vincent three out of eight plagioclase phenocrysts have sr concentration profiles that are not in diffusive equilibrium for these the diffusion model is employed to calculate maximum crystal residence times from incomplete diffusive equilibration of trace element zoning in plagioclase maximum crystal residence times range from 100 to 450 years this is in good agreement with estimates from crystal size distribution and from ra th disequilibrium studies for the kameni islands for soufriere however such short residence times are incompatible with u th mineral errorchron data that suggest residence times of 40 ka in a thermally buffered magma reservoir to reconcile these apparently different ages we invoke a more complicated magmatic history for soufriere where an initially buffered magma reservoir is disturbed by magma mixing and suffers limited additional crystal fractionation prior to eruption

diversity of basaltic lunar volcanism associated with buried impact structures implications for intrusive and extrusive events abstract relatively denser basalt infilling and the upward displacement of the crust mantle interface are thought to be contributing factors for the quasi circular mass anomalies for buried impact craters in the lunar maria imagery and gravity observations from the lunar reconnaissance orbiter lro and dual gravity recovery and interior laboratory grail missions have identified 10 partially or fully buried impact structures where diversity of observable basaltic mare volcanism exists with a detailed investigation of the characteristics of associated volcanic landforms we describe their spatial distribution relationship with respect to the subsurface tectonic structure of complex impact craters and propose possible models for the igneous processes which may take advantage of crater related zones of weakness and enable magmas to reach the surface we conclude that the lunar crust having been fractured and reworked extensively by cratering facilitates substance and energy exchange between different lunar systems an effect modulated by tectonic activities both at global and regional scales in addition we propose that the intrusion caused contribution to gravity anomalies should be considered in future studies although this is commonly obscured by other physical factors such as mantle uplift and basalt load

pronounced central uplift identified in the bosumtwi impact structure ghana using multichannel seismic reflection data the lake bosumtwi impact structure is the youngest and best preserved complex terrestrial impact crater and serves as an important reference site for the study of cratering processes because the impacting body struck continental crystalline target rocks and not a submerged sedimentary platform no significant backwash processes have modified the crater morphology not only may bosumtwi contain the best preserved central uplift structure on earth but it is the most accessible relatively large young crater in the solar system generated in a large gravity field there is a well established link between the lake bosumtwi impact structure and the ivory coast tektite field and the lacustrine sediments within the crater contain a unique 1 m y record of paleoclimate in the continental tropics south of the sahel eight profiles of marine type multichannel seismic reflection mcs data were acquired from the 8 km diameter similar to75 m deep lake that fills much of the crater these were augmented by wide angle seismic data acquired with ocean bottom hydrophones mcs data reveal a well defined central uplift near the northwest central part of the lake and a maximum postimpact lacustrine sediment thickness of similar to310 m the central uplift structure has a diameter of 1 9 km and a maximum height of 130 m above the annular moat inside the crater an intermediate velocity layer 3200 m s beneath the lacustrine sediment is interpreted as fallback breccia or a breccia melt horizon the measured apparent depth of the crater d a is 500 m implying a slightly higher aspect ratio for the structure than predicted from published empirical relationships the bosumtwi structure is a small complex crater that deviates slightly from trends predicted from classical scaling laws perhaps because of the effects of a large gravity field

density and compressibility of the molten lunar picritic glasses implications for the roles of ti and fe in the structures of silicate melts abstract the density and compressibility of four synthetic molten lunar picritic glasses was investigated from 0 to 10 gpa and 1748 to 2473 k the picritic glasses were collected from the lunar surface during the apollo missions and they are hypothesized to have rapidly quenched as glass beads during pyroclastic fire fountain eruptions the specific melt compositions investigated in the present study are the apollo 15 green glass type c a15c tio 2 0 26 wt the apollo 14 yellow glass a14y tio 2 4 58 wt the apollo 17 orange glass 74220 type a17o tio 2 9 12 wt and the apollo 14 black glass a14b tio 2 16 40 wt these glasses are reported to represent primary unfractionated melts making them a prime candidate for experimental studies into lunar basalt density and compressibility during partial melting of the lunar mantle sink float experiments were conducted on the synthetic molten lunar glass compositions using a piston cylinder apparatus p p 2 5 gpa in order to bracket the density of the melts new sink float data are reported for a15c a14y and a17o which are combined with previously published density and compressibility data on a15c a17o and a14b although the ti rich liquids are highly compressible at lower pressures they become nearly incompressible at much higher pressures when compared to the molten low ti glasses consequently the melts with the most tio 2 a14b are the least dense at higher pressures a reversal of what is seen at lower pressures this change in density and compressibility is attributed to changes in coordination of ti and fe in the silicate melt structure as ti 4 abundances in the silicate melt increase predominantly iv ti 4 and iv fe 2 change to vi ti 4 and vi fe 2 in the melt structure all of the data from the present study were used to calculate a birch murnaghan equation of state bm eos for each melt composition the bm eos model for each composition was then combined with previously published estimates for the residual mantle source mineralogy and depth of origin for each of the glasses to assess the density of the partial melt with respect to its point of origin this information was used to determine whether or not the melt would rise or sink with respect to its source region we determined that all melt compositions with the exception of the a17o melt should have been able to rise to the crust mantle boundary as a result of buoyancy forces alone although different mechanisms are likely required for magma ascent through the lunar crust for the rise of a17o other modes of ascent through the lunar mantle are required to extract this melt composition from the mantle and volatiles are not a plausible solution on the grounds of melt density alone

floor fractured craters in mare smythii and west of oceanus procellarum implications of crater modification by viscous relaxation and igneous intrusion models endogenic modification in lunar floor fractured craters can constrain spatial variations in early lunar conditions the nature of these constraints however depends on the assumed mecha nism of crater modification for viscous relaxation the extent of crater modification depends on the surrounding crustal viscosity and thus provides loose constraints on the history of crustal heat ing within a region for igneous intrusion models the extent of crater modification reflects mag matically driven deformation and can be inverted to estimate both local magma pressure and intru sion depth both models indicate clear differences between regional conditions at mare smythii and in the highlands west of oceanus procelm the uniformly shallow crater depths in mare smythii probably indicate a long lived period of extreme crater relaxation whereas the wide range of modified crater depths in the western highlands suggest a much shorter period of partial crater relaxation for comparable relaxation times the average viscosity derived for mare smythii is over 10 times lower than the average viscosity inferred for the western highlands alternatively if modification reflects deformation over crater centered laccolithlike intrusions the derived magma pressures indicate a broad uniform magma source beneath mare smythii whereas the spatial vari ation of estimated magma pressures in the western highlands suggests the presence of several smaller magma sources the derived intrusion depths are partly a function of crater size but range from 1 to 10 km in depth for both regions and may be slightly greater on average in the western highlands while both viscous relaxation and igneous intrusions can explain the modification of individual craters the regional variations in these derived modification conditions also allow fur ther testing of each modification model in particular the correlation of the lowest viscosities or the longest relaxation times with the smallest craters in both mare smythii and the western high lands seems inconsistent with crater modification by the relaxation mechanism the onset of total relaxation at progressively smaller topographic wavelengths over time might produce such a trend in mare smythii but total relaxation cannot be invoked for the western highlands where many large craters still preserve significant fractions of their initial relief in addition the relaxation of the smallest craters in both regions suggests the presence of exceptionally high near surface ther mal gradients 150 200 k km since the observed regional variations in crater modification can be easily attributed to variations in magma pressure as a function of mantle topography we con clude that crater floor fracturing in both regions is more consistent with the igneous intrusion mechanism than with viscous relaxation during a crustal heating event

alkali diffusion in plagioclase feldspar tracer diffusion kinetics at 1 atm pressure under dry conditions using 6li 41k and 87rb in four plagioclase feldspars have been determined diffusion profiles were measured using an ion microprobe sims the arrhenius parameters and 2 uncertainties for li k and rb respectively in albite are pre exponential factor log do 3 8 1 0 3 4 1 4 and 5 8 3 8 in m2 s and activation energies 146 14 296 30 and 283 85 kj mol the diffusion rate of k in albite at 700 c and 100 mpa water pressure equals that at 1 atm dry suggesting that diffusion of k is not dependent on water fugacity between 0 004 and 100 mpa the k diffusion rate depends on plagioclase anorthite content and is approximately 20 times slower in labradorite than albite li diffusion rates in albite and anorthite are identical a linear relation between log d for the alkalis measured and their ionic radius squared permits an estimate of cs diffusion kinetics diffusion rates are isotropic or nearly so in albite

a model for the dynamics of crater centered intrusion application to lunar floor fractured craters lunar floor fractured craters are a class of craters modified by post impact mechanisms they are defined by distinctive shallow floors that are convex or plate like sometimes with a wide floor moat bordering the wall region radial concentric and polygonal floor fractures suggest an endogenous process of modification two mechanisms have been proposed to account for such deformations viscous relaxation and spreading of a magma intrusion at depth below the crater to test the second assumption and bring more constraints on the intrusion process we develop a model for the dynamics of magma spreading below an elastic overlying layer with a crater like topography as predicted in earlier more qualitative studies the increase in lithostatic pressure at the crater wall zone prevents the intrusion from spreading laterally leading to the thickening of the intrusion additionally our model shows that the final crater floor appearance after the uplift which can be convex or flat with or without a circular moat bordering the wall zone depends on the elastic thickness of the layer overlying the intrusion and on the crater size our model provides a simple formula to derive the elastic thickness of the overlying layer hence a minimum estimate for the intrusion depth finally our model suggests that crust redistribution by cratering must have controlled magma ascent below most of these craters

explosive volcanism on mercury analysis of vent and deposit morphology and modes of eruption abstract the messenger mission revealed for the first time conclusive evidence of explosive volcanism on mercury several previous works have cataloged the appearance and location of explosive volcanism on the planet using a variety of identifying characteristics including vent presence and deposit color as seen in multispectral image mosaics we present here a comprehensive catalog of vents of likely volcanic origin our classification scheme emphasizes vent morphology we have analyzed the morphologies of all vents in our catalog and recognize three main morphologies simple vent pit vent and vent with mound the majority of vents we identify are located within impact craters the spatial distribution of vents does not correlate with the locations of volcanic smooth plains deposits in contrast to the moon nor do vents correlate with the locations of large impact basins except for the caloris and tolstoj basins using the degradation state of the vent host crater as a proxy for maximum age we suggest that vent formation has been active through the mansurian and into the kuiperian periods although the majority of vents were likely formed much earlier in mercurian history the morphologies and locations of vents are used to investigate a set of plausible formation geometries we find that the most likely and most prevalent formation geometry is that of a dike stalled at depth which then explosively vents to the surface we compare the vent and deposit size of mercurian pyroclastic deposits with localized and regional lunar pyroclastic deposits and find a range of possible eruption energies and corresponding variations in eruption style localized lunar pyroclastic deposits and the majority of mercurian pyroclastic deposits show evidence for eruption that is consistent with the magmatic foam at the top of a dike reaching a critical gas volume fraction a subset of mercurian vents including the prominent copland rachmaninoff vent to the northeast of the rachmaninoff basin indicates eruption at enhanced gas volume fractions this subset of vents shows a similar eruptive behavior to the lunar orientale dark mantle ring deposit suggesting that the dikes that formed these vents and deposits on mercury underwent some form of additional volatile build up either through crustal volatile incorporation or magma convection within the dike there also exists a population of mercurian vents that no longer retain a visible associated pyroclastic deposit we hypothesize that the visible signature of the pyroclastic deposit has been lost through space weathering and regolith mixing processes together these results provide a comprehensive analysis of explosive volcanism on mercury and inform continued research on the thermal history of mercury and magma composition and evolution

constructional volcanic edifices on mercury candidates and hypotheses of formation mercury a planet with a predominantly volcanic crust has perplexingly few if any constructional volcanic edifices despite their common occurrence on other solar system bodies with volcanic histories using image and topographical data from the mercury surface space environment geochemistry and ranging messenger spacecraft we describe two small 15 km diameter prominences with shallow summit depressions associated with volcanically flooded impact features we offer both volcanic and impact related interpretations for their formation and then compare these landforms with volcanic features on earth and the moon though we cannot definitively conclude that these landforms are volcanic the paucity of constructional volcanic edifices on mercury is intriguing in itself we suggest that this lack is because volcanic eruptions with sufficiently low eruption volumes rates and flow lengths suitable for edifice construction were highly spatiotemporally restricted during mercury s geological history we suggest that volcanic edifices may preferentially occur in association with late stage post impact effusive volcanic deposits the esa jaxa bepicolombo mission to mercury will be able to investigate further our candidate volcanic edifices search for other as yet unrecognized edifices beneath the detection limits of messenger data and test our hypothesis that edifice construction is favored by late stage low volume effusive eruptions

physical controls and depth of emplacement of igneous bodies a review the formation and growth of magma bodies are now recognised as involving the amalgamation of successive discrete pulses such as sills sills would thus represent the building blocks of larger plutons sensu lato mechanical and thermal considerations on the incremental development of these plutons raise the issue of the crustal levels at which magma can stall and accumulate as sills reviewing the mechanisms that could a priori explain sill formation it is shown that principal physical controls include rigidity contrast where sills form at the interface between soft strata overlaid by comparatively stiffer strata rheology anisotropy where sills form within the weakest ductile zones and rotation of deviatoric stress where sills form when the minimum compressive stress becomes vertical comparatively the concept of neutral buoyancy is unlikely to play a leading control in the emplacement of sills although it could assist their formation these different controls on sill formation however do not necessarily operate on the same length scale the length scale associated with the presence of interfaces separating upper stiffer layers from lower softer ones determines the depth at which rigiditycontrolled sills will form on another hand the emplacement depths for rheology controlled sills is likely to be determined by the distribution of the weakest ductile zones whereas the emplacement depth of stress controlled sills is determined by a balance between the horizontal maximum compressive stress which favours sill formation and the buoyancy of their feeder dykes which drives magma vertically ultimately the depth at which a sill forms depends on whether crustal anisotropy or stress rotation is the dominant control i e which of these processes operates at the smallest length scale using dimensional analysis it is shown that sill formation controlled by remote stress rotation would occur on length scales of hundreds of meters or greater this therefore suggests that crustal heterogeneities and their associated anisotropy are likely to play a larger role than remote stress rotation in controlling sill emplacement unless these heterogeneities are several hundred meters or more apart this also reinforces the role of local stress barriers owing to interactions between deviatoric stress and crustal heterogeneities in the formation of sills

crustal deformation in great california earthquake cycles periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model a depth l h extending downward from the earth s surface at a transform boundary between uniform elastic lithospheric plates of thickness h is locked between earthquakes it slips an amount consistent with remote plate velocity vpl after each lapse of earthquake cycle time tcy lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress the plates are coupled at their base to a maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions compatible with plate velocity are transmitted to the surface plates the coupling is described approximately through a generalized elsasser model we argue that the model gives a more realistic physical description of tectonic loading including the time dependence of deep slip and crustal stress buildup throughout the earthquake cycle than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone parameters of the model are chosen to fit seismic and geologic constraints and the apparent time dependence of surface strain rates along presently locked traces of the 1857 and 1906 san andreas ruptures we fix vpl 35 mm yr tcy 160 years and l 9 11 km based on earthquake nucleation depths the geodetic data are then found to be described reasonably within the context of a model that is locally uniform along strike and symmetric about a single san andreas fault strand by lithosphere thickness h 20 30 km and elsasser relaxation time tr 10 16 years we therefore suggest that the asthenosphere appropriate to describe crustal deformation on the earthquake cycle time scale lies in the lower crust and perhaps crust mantle transition zone and has an effective viscosity between about 2 1018 and 1019 pa s depending on the thickness assigned to the asthenospheric layer predictions based on the chosen set of parameters are also consistent with data on variations of contemporary surface strain and displacement rates as a function of distance from the 1857 and 1906 rupture traces although the fit is degraded by asymmetry relative to the fault and by slip on adjacent fault strands

explosive volcanism in complex impact craters on mercury and the moon influence of tectonic regime on depth of magmatic intrusion vents and deposits attributed to explosive volcanism occur within numerous impact craters on both the moon and mercury given the similarities between the two bodies it is probable that similar processes control this spatial association on both however the precise morphology and localization of the activity differs on the two bodies indicating that the nature of structures beneath impact craters and or volcanic activity may also be different to explore this we analyze sites of explosive volcanism within complex impact craters on the moon and mercury comparing the scale and localization of volcanic activity and evidence for post formation modification of the host crater we show that the scale of vents and deposits is consistently greater on mercury than on the moon indicating greater eruption energy powered by a higher concentration of volatiles additionally while the floors of lunar craters hosting explosive volcanism are commonly fractured those on mercury are not the most probable explanation for these differences is that the state of regional compression acting on mercury s crust through most of the planet s history results in deeper magma storage beneath craters on mercury than on the moon the probable role of the regional stress regime in dictating the depth of intrusion on mercury suggests that it may also play a role in the depth of sub crater intrusion on the moon and on other planetary bodies examples on the moon and also on mars commonly occur at locations where flexural extension may facilitate shallower intrusion than would be driven by the buoyancy of the magma alone

crustal forensics in arc magmas abstract the geochemical characteristics of continental crust are present in nearly all arc magmas these characteristics may reflect a specific source process such as fluid fluxing common to both arc magmas and the continental crust and or may reflect the incorporation of continental crust into arc magmas either at source via subducted sediment or via contamination during differentiation resolving the relative mass contributions of juvenile mantle derived material versus that derived from pre existing crust of the upper plate and providing these estimates on an element by element basis is important because 1 we want to constrain crustal growth rates 2 we want to quantitatively track element cycling at convergent margins and 3 we want to determine the origin of economically important elements and compounds traditional geochemical approaches for determining the contributions of various components to arc magmas are particularly successful when applied on a comparative basis studies of suites from multiple magmatic systems along arcs for which differentiation effects can be individually constrained can be used to extrapolate to potential source compositions in the lesser antilles arc for example differentiation trends from individual volcanoes are consistent with open system evolution however such trends do not project back to a common primitive magma composition suggesting that differentiation modifies magmas that were derived from distinct mantle sources we propose that such approaches should now be complemented by petrographically constrained mineral scale isotope and trace element analysis to unravel the contributing components to arc magmas this innovative approach can 1 better constrain true end member compositions by returning wider ranges in geochemical compositions among constituent minerals than is found in whole rocks 2 better determine magmatic evolution processes from core rim isotopic or trace element profiles from the phases contained in magmas and 3 constrain rates of differentiation by applying diffusion controlled timescales to element profiles an example from nguaruhoe volcano new zealand underscores the importance of such a microsampling approach showing that mineral isotopic compositions encompass wide ranges that whole rock isotopic compositions are consequently simply element weighted averages of the heterogeneous crystal cargo and that open system evolution is proved by core rim variations in sr isotope ratios nguaruhoe is just one of many systems examined through microanalytical approaches the overwhelming conclusion of these studies is that crystal cargoes are not truly phenocrystic but are inherited from various sources the implication of this realization is that the interpretation of whole rock isotopic data including the currently popular u series needs careful evaluation in the context of petrographic observations

preseismic rupture progression and great earthquake instabilities at plate boundaries we present a procedure for modeling the initially quasi static upward progression of a zone of slip from some depth in the lithosphere toward the earth s surface along a transform plate margin culminating in a great crustal earthquake stress transmission in the lithosphere is analyzed with a generalized elsasser model in which elastic lithospheric plates undergo plane stress deformation and are coupled by an elementary foundation model to a maxwellian viscoelastic asthenosphere upward progression of rupture over a finite length of plate boundary corresponding to a seismic gap along strike is analyzed by a method based on the line spring concept whereby a two dimensional antiplane analysis of the upward progression provides the relation between lithospheric thickness averaged stress and slip used as a boundary condition in the generalized elsasser plate model the formulation results in a nonlinear integral equation for the rupture progression as a function of time and distance along strike a simpler approximate single degree of freedom analysis procedure is described and shown to lead to instability results that can be formulated in terms of the slip softening slope at the boundary falling below the elastic unloading stiffness of the surroundings the results also indicate a delay of ultimate seismic instability due to the stiffer short versus long time asthenospheric response and predict a final period of self driven creep toward instability the procedures for prediction of rupture progression and instability are illustrated in detail for an elastic brittle crack model of slip zone advance and parameters of the model are chosen consistently with great earthquake slips and stress drops for example an effective crack fracture energy of the order 4 106 j m2 at the peak 7 to 10 km below surface of a gaussian bell shaped distribution of fracture energy with depth with variance of the order 5 km simulating strength build up in a seismogenic layer leads to prediction of nominal seismic stress drops of 30 to 60 bars and slips of 2 to 5 m in great strike slip earthquake ruptures breaking 100 to 400 km along strike precursory surface straining in the self driven stage is predicted to proceed at a distinctly higher rate over time intervals beginning 3 to 10 months before such an earthquake this interval being greater for longer distances along strike over which the preseismic upward rupture progression takes place

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

slip patterns and earthquake populations along different classes of faults in elastic solids numerical simulations of slip instabilities on a vertical strike slip fault in an elastic half space are performed for various models belonging to two different categories the first category consists of inherently discrete cellular fault models such are used to represent fault systems made of segments modeled by numerical cells that can fail independently of one another their quasi independence is assumed to provide an approximate representation of strong fault heterogeneity due to geometric or material property disorder that can arrest ruptures at segment boundaries the second category consists of models having a well defined continuum limit these involve a fault governed by rate and state dependent friction and are used to evaluate what types of property heterogeneity could lead to the quasi independent behavior of neighboring fault segments assumed in the first category the cases examined include models of a cellular fault subjected to various complex spatial distributions of static to kinetic strength drops and models incorporating rate and state dependent friction subjected to various spatial distributions of effective stress normal stress minus pore pressure the results indicate that gradual effective stress variations do not provide a sufficient mechanism for the generation of observed seismic response strong and abrupt fault heterogeneity as envisioned in the inherently discrete category is required for the generation of complex slip patterns and a wide spectrum of event sizes strong fault heterogeneity also facilitates the generation of rough rupture fronts capable of radiating high frequency seismic waves the large earthquakes in both categories of models occur on a quasi periodic basis the degree of periodicity increases with event size and decreases with model complexity however in all discrete segmented cases the models generate nonrepeating sequences of earthquakes and the nature of the large quasi periodic events is highly variable the results indicate that expectations for regular sequences of earthquakes and or simple repetitive precursory slip patterns are unrealistic the frequency size fs statistics of the small failure episodes simulated by the cellular fault models are approximately self similar with b 1 2 and ba 1 where b and ba are b values based on magnitude and rupture area respectively for failure episodes larger than a critical size however the simulated statistics are strongly enhanced with respect to self similar distributions defined by the small events this is due to the fact that the stress concentrated at the edge of a rupture expanding in an elastic solid grows with the rupture size when the fault properties e g geometric irregularities are characterized by a narrow range of size scales the scaling of stress concentrations with the size of the failure zone creates a critical rupture area terminating the self similar earthquake statistics in such systems events reaching the critical size become on the average unstoppable and they continue to grow to a size limited by a characteristic model dimension when however the system is characterized by a broad spectrum of size scales the above phenomena are suppressed and the range of apparent self similar fs statistics is broad and characterized by average b and ba values of about 1 the simulations indicate that power law extrapolations of low magnitude seismicity will often underestimate the rate of occurrence of moderate and large earthquakes the models establish connections between features of fs statistics of earthquakes range of self similar regimes local maxima and structural properties of faults dominant size scales of heterogeneities dimensions of coherent brittle zones the results suggest that observed fs statistics can be used to obtain information on crustal thickness and fault zone structure

floor fractured impact craters on venus implications for igneous crater modification and local magmatism regional tectonism and volcanism affect crater modification and crater loss on venus but a comparison of venusian craters to lunar floor fractured craters suggests that a third style of more localized crater controlled magmatism also may occur on venus based on lunar models for such magmatism venusian crustal conditions should generally favor crater filling volcanism over crater centered floor fracturing nevertheless three craters on venus strongly resemble extensively modified craters on the moon where deformation can be attributed to failure over large crater centered intrusions models for crater modification over igneous intrusions indicate typical magmatic pressures beneath these three craters of 200 300 bars and intrusion depths of the order of 1 6 km all three craters also share common settings and low elevations whereas craters embayed by regional volcanism preferentially occur at much higher elevations on venus we suggest that the style of igneous crater modification on venus thus may be elevation dependent with crater centered intrusions primarily occurring at low elevations on venus this interpretation is consistent with theoretically predicted variations in magmatic neutral buoyancy depth as a function of atmospheric pressure suggested by other authors

geochemical variations in late stage growth of volcanic phenocrysts revealed by sims depth profiling in the petrologic and geochemical study of volcanic rocks microlites have become an important tool with which to determine magma decompression rate and ascent path prior to eruption however studies in experimental petrology and kinetic modeling indicate that growth of pre existing phenocrysts will occur over a much wider range of decompression induced undercoolings than microlite nucleation consequently we have developed a method using secondary ion mass spectrometry sims depth profiling to measure geochemical trends recorded during the final stage of phenocryst growth to test and demonstrate the new method we examined explosive and effusive eruptive products collected from soufriere hills volcano montserrat plagioclase feldspar crystals were removed from clasts of pyroclastic deposits phenocrysts were individually selected on the basis of euhedral morphology and a relatively homogeneous distribution of surface contamination such as volcanic glass crystals were cleaned embedded in in and analyzed by sims in depth profiling mode we used an o 2 primary ion beam which provides a faster sputtering rate than the typically utilized o primary beam a normal incidence electron gun is used for charge compensation ten isotopes were examined over 2 10 h periods and the resulting crater depth was determined data show variable trends in an k and fe that we interpret to represent late stage physical variations within the magma prior to eruption the continuous chronology of geochemical data obtained from the sims analyses presented here provide information with unprecedented 0 1 m resolution allowing researchers to determine physical conduit conditions during magma ascent from the chamber to eruption at the surface

bang month scale eruption triggering at santorini volcano the time lag between intrusion of fresh hot magma and an ensuing eruption is of critical importance in both understanding the triggering and mitigating the consequences of volcanic eruptions this work looks at material erupted during 1925 28 at the nea kameni volcanic center in santorini greece to determine this time scale by exploiting fe mg diffusion in olivine crystals we constrained the intrusion to eruption time lag to between 3 and 10 weeks these techniques have potential application at many volcanic centers previously erupted material can be used to calibrate records of the short time scale processes common to many volcanic centers

lunar floor fractured craters classification distribution origin and implications for magmatism and shallow crustal structure 1 floor fractured craters ffcs are a class of lunar craters characterized by anomalously shallow floors cut by radial concentric and or polygonal fractures additional interior features are moats ridges and patches of mare material two formation mechanisms have been hypothesized floor uplift in response to shallow magmatic intrusion and sill formation and floor shallowing in response to thermally driven viscous relaxation this study combines new lunar orbiter laser altimeter lola and lunar reconnaissance orbiter camera lroc data to characterize and categorize the population of ffcs and map their distribution on the moon and uses variations in floor fractured crater morphology and regional distribution to investigate the proposed formation mechanisms the population of ffcs was categorized according to the classes outlined by schultz 1976 the distribution of these ffc categories shows an evolution of crater morphology from areas adjacent to lunar impact basins to areas in the lunar highlands we propose that this trend is supportive of formation by shallow magmatic intrusion and sill formation crustal thickness determines the magnitude of magmatic driving pressure and thus either piston like floor uplift for high magnitude or a convex floor profile for low magnitude predictions from previous studies modeling viscous relaxation are inconsistent with the observed altimetric profiles of ffcs hence our analysis favors ffc formation by shallow magmatic intrusion with the variety of ffc morphologies being intimately linked with location and crustal thickness and the driving pressure of the intrusion data from the grail gravity recovery and interior laboratory mission will help to test these conclusions

deep earthquakes beneath mount st helens evidence for magmatic gas transport small magnitude earthquakes began beneath mount st helens 40 days before the eruption of 20 march 1982 unlike earlier preeruption seismicity for this volcano which had been limited to shallow events less than 3 kilometers many of these earthquakes were deep between 5 and 11 kilometers the location of these preeruptive events at such depth indicates that a larger volume of the volcanic system was affected prior to the 20 march eruption than prior to any of the earlier dome building eruptions the depth time relation between the deep earthquakes and the explosive onset of the eruption is compatible with the upward migration of magmatic gas released from a separate deep reservoir

nanoscale characterisation of crystal zoning abstract advances in analytical techniques are fundamental to the enhanced understandings of many geological processes zoned volcanic crystals have been analysed by low 5 kv field emission gun electron probe micro analyser feg epma and nanosims to obtain sub micrometre chemical profiles and compared to time of flight sims tof sims and high 15 20 kv epma profiles plagioclase and orthopyroxene crystals have been analysed by feg epma at accelerating voltages of 5 kv providing a spatial resolution step size of 350 nm the resolution of the lowest energy x ray for orthopyroxene crystals using a 30 nm beam and ca 750 nm for plagioclase crystals which at low voltages are unstable and require a 500 nm defocused beam step sizes are comparable in size to interaction volumes analytical protocols are detailed that permit quantitative major and minor element compositions to be acquired at similar precision and accuracy as traditional epma analyses at 15 20 kv nanosims analysis of the same crystals provides a greater spatial resolution of up to 200 nm and allows the measurement of li also the nanosims profiles however cannot currently be quantified the ability to analyse crystals at sub micrometre scales is demonstrated by the good agreement between nanosims feg epma conventional epma and tof sims data feg epma nanosims and tof sims techniques have broad applications within the earth sciences in petrologic studies for example these methods have the ability to analyse small crystals in experimental charges and provide chemical profiles of crystal zoning at a spatial resolution of ca 200 300 nm such profiles are important in crystal forensics and diffusion chronometry studies the implications for the latter application are that timescales of volcanic processes that occur in the days years immediately prior to the eruption can now be studied

forecast model for great earthquakes at the nankai trough subduction zone an earthquake instability model is formulated for recurring great earthquakes at the nankai trough subduction zone in southwest japan the model is quasistatic two dimensional and has a displacement and velocity dependent constitutive law applied at the fault plane a constant rate of fault slip at depth represents forcing due to relative motion of the philippine sea and eurasian plates the model simulates fault slip and stress for all parts of repeated earthquake cycles including post inter pre and coseismic stages calculated ground uplift is in agreement with most of the main features of elevation changes observed before and after the m 8 1 1946 nankaido earthquake in model simulations accelerating fault slip has two time scales the first time scale is several years long and is interpreted as an intermediate term precursor the second time scale is a few days long and is interpreted as a short term precursor accelerating fault slip on both time scales causes anomalous elevation changes of the ground surface over the fault plane of 100 mm or less within 50 km of the fault trace

magma recharge contamination and residence times revealed by in situ laser ablation isotopic analysis of feldspar in volcanic rocks we illustrate a rapid and effective method for determining variations in 87sr 86sr across single plagioclase feldspar crystals using laser ablation interfaced to a multicollector inductively coupled plasma source mass spectrometer the speed of analysis proves to be a particular advantage permitting representative populations of crystals to be characterized or enabling reconnaissance analysis of rocks to test for mineral scale isotopic heterogeneity our results from magmatic crystals in lavas from el chichon volcano confirm trends previously obtained by the more time consuming microdrilling method for the first time with confidence the laser data allow us to associate abrupt changes in 87sr 86sr with distinct petrographic discontinuities such as dissolution surfaces in the crystal because the isotopic variations within crystals would be expected to equilibrate diffusively with time the width over which abrupt changes in 87sr 86sr occur can also be used to constrain timescales at which the crystal has been held at magmatic temperatures

evidence of recent thrust faulting on the moon revealed by the lunar reconnaissance orbiter camera lunar reconnaissance orbiter camera images reveal previously undetected lobate thrust fault scarps and associated meter scale secondary tectonic landforms that include narrow extensional troughs or graben splay faults and multiple low relief terraces lobate scarps are among the youngest landforms on the moon based on their generally crisp appearance lack of superposed large diameter impact craters and the existence of crosscut small diameter impact craters identification of previously known scarps was limited to high resolution apollo panoramic camera images confined to the equatorial zone fourteen lobate scarps were identified seven of which are at latitudes greater than 60 indicating that the thrust faults are globally distributed this detection coupled with the very young apparent age of the faults suggests global late stage contraction of the moon

diffusion coupling between trace and major elements and a model for calculation of magma residence times using plagioclase residence times of plagioclase crystals in magma reservoirs can be determined by modeling the compositional zoning of trace elements in these crystals we present a formulation to model diffusion of trace elements in plagioclase paying special attention to certain thermodynamic and kinetic aspects in particular we account for the compositional dependence on anorthite content of the chemical potential and diffusion coefficients of trace elements e g mg the choice of suitable boundary conditions and potential effects of diffusion in more than one dimension we show that contrary to intuition diffusive fluxes of trace elements may be coupled to major element concentration gradients and ignoring such coupling can lead to incorrect estimates of timescales we illustrate application of the model using plagioclase crystals from a suite of gabbroic xenoliths from a holocene dacitic lava flow of volcan san pedro chilean andes 36 s the inferred timescale for metasomatism of the xenoliths by evolved liquids is on the order of 100 30 to 148 yr and serves to illustrate how trace element zoning in plagioclase provides a window into timescales of magmatic processes inaccessible by isotopic or other methods