climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

potassic dyke swarm in the sapucai graben eastern paraguay petrographical mineralogical and geochemical outlines abstract the western side of the parana basin of brazil extends to central paraguay where repeated and widespread magmatic activity developed from lower cretaceous to oligocene associated with late mesozoic crustal extension trending ne sw in central paraguay this trend is characterized by a zone of nw se normal faults which formed the asuncion sapucai graben up to 45 km wide and 200 km long where alkaline rocks occur as volcanic domes complexes lava flows and dykes these rocks 128 ma aged are dominantly potassic and ne normative a swarm of at least 200 mainley nw se trending dykes occurs in the sapucai region and seems to be formed by two main lineages tephrite to phonolite and peralkaline phonolite and alkali basalt to trachyphonolite they are characterized by ubiquitous diopside to ferrosalite consistently yielding al enrichment trends common olivine fo 81 69 in tephrites and alkaline basalts and up to fo 65 in phonolites zoned megacrysts of hastingsitic hornblende core to kaersutite rim associated with accessory groundmass pargasite in tephrites and phonotephrites k rich hastingsite and k rich ferroan pargasite in the phonolites accessory groundmass mica falls in the annite phlogopite range and consistently yields insufficient si al to satisfy the expected t site occupancy of 8 00 a f u fe ti oxides are ti magnetite rarely ilmenite or haematite phenocrystal i e xenocrystal plagioclase is an 70 20 and an 74 42 in the tephrites and phonolites respectively coexisting groundmass microlites are an 22 14 associated with sodasanidine and sanidine feldspathoids include analcimized leucite and nepheline accessories ti andradite and sphene the two main lineages recognized by distinctive mineralogical variations are consistent with the petrochemical variations complex interaction of discrete and independently evolving magma batches are indicated by intra and or interphase chemical variations suggesting multiple equilibrations of the crystallizing phases under shallow level volcanic pressure regime the observed geochemical trends are quite similar to those of roman region type magma with the same negative anomalies of ta nb zr and ti the most likely mantle source is a garnet peridotite characterized by different enrichment in incompatible elements and which suffered low degree of partial melting 3 7 which has geochemical and isotopic features distinct from those of the adjoining tholeiitic basalts 130 ma and nephelinites 61 39 ma the similarities of the sapucai dyke suite with barton s roman region type magma supports the view that this magma type may not be formed as a result of orogenic and or subduction driven activity in this region therefore a causal relationship of the latter activity with roman region type magma is not supported and remains questionable

characterization of late glacial continental dust in the greenland ice core project ice core twelve glacial dust samples extracted from the greenland ice core project ice core have been characterized in terms of mineralogy sr and nd isotopic composition and rare earth element concentrations the ice samples are taken from the younger dryas the bolling the last glacial maximum and both cold and mild glacial periods back to 44 kyr b p a comparison with samples from possible dust source areas determines eastern asia as being the main source area at all times hereby confirming and extending earlier results biscaye et al 1997 this finding is partly in agreement with recent atmospheric general circulation model simulations of glacial atmospheric dust transport andersen et al 1998 mahowald et al 1999 a low intersample variability in tracer characteristics is discussed in the context of a volcanogenic influence grain size dependence and variability in the conti nental dust source areas the clay mineralogy of the ice core dust shows a weak climatic de pendence while the isotopic composition of sr and nd does not correlate with climate this is interpreted as a geographical variation of source areas on a regional scale within asia two ice dust samples were size fractionated at 2 gm and the grain size dependence on tracer characteristics is in accordance with the literature

geometry and p and s velocity structure of the african anomaly 1 we constrain the geometry and p and s velocity structure of a low velocity anomaly in the lower mantle beneath southern africa we term it the african anomaly on the basis of forward traveltime and waveform modeling of seismic data sampling a great arc across the anomaly from the east pacific rise to the japan sea our collected data set consists of direct s direct p sdiff scs pcp sks and skks phases recorded by three temporary broadband passcal seismic arrays deployed in africa between 1994 and 2002 the tanzania seismic array 1994 1995 the kaapvaal seismic array 1997 1999 and the ethiopia kenya seismic array 2000 2002 for earthquakes occurring in the east pacific rise drake passage south sandwich islands iran hindu kush xinjiang and the japan sea the seismic data provide excellent sampling of the african anomaly in the lower mantle along the specific great arc in order to accurately account for the contributions from the african anomaly we relocate all the events using a global seismic shear velocity tomographic model and seismic data recorded by the global seismographic network and correct for the contributions from the seismic heterogeneities outside the african anomaly the seismic observations suggest that the african anomaly locally extends 1300 km above the core mantle boundary beneath southern africa around 25 n 27 e and exhibits a bell like geometry with both the southwestern and the northeastern flanks dipping toward its center with the lateral dimension of the anomaly increasing with depth the base is about 4000 km wide extending broadly in both the southwestward and the northeastward directions the seismic data can best be explained by a shear velocity structure with average velocity decreases of 5 in the base and 2 to 3 in the mid lower mantle above the base and a compressional velocity structure with a uniform s to p velocity perturbation ratio of 3 1 for the entire african anomaly these geometric and velocity features suggest that the mid lower mantle portion of the african anomaly is an integral component of the very low velocity province and the african anomaly is compositionally distinct and geologically stable

surficial properties in gale crater mars from mars odyssey themis data abstract we examine the nature of the surface layer in gale crater as determined from high resolution thermal and visible mars odyssey thermal emission imaging system themis data as well as how our conclusions compare to past analyses at themis resolution the thermal surface structure is dominated by local control thus providing us with detailed images that contain thermophysical information as well using these data sets we have created a map of the area defining units based primarily on their geomorphology as determined from the daytime thermal and visible images and then using the nighttime thermal data to interpret the nature of the surface layer within each unit seven units have been defined i partially blanketed knobby plateaus ii crater walls with terrain similar to that on the plateaus on the upper half and exposed rocky surfaces on the lower half iii v three floor units with varying combinations of bedrock and indurated and or particulate deposits vi sand sheets and vii a central mound consisting of indurated and or rocky material forming layers terraces and slides covered by particulate material that tapers in thickness downslope additionally dozens of channels have been observed on the crater walls and central mound the results indicate that aeolian processes have played a major role in shaping much of the present surface layer within gale and may still be active today because of the dramatic size and structure of gale the winds are most likely controlled by the local topography additionally the presence and frequency of channels within gale bolster hypotheses involving aqueous episodes in the history of the crater

mantle tomography and its relation to temperature and composition we propose a new method to constrain lateral variations of temperature and composition in the lower mantle from global tomographic models of shear and compressional wave speed we assume that the mantle consists of a mixture of perovskite and magnesio wustite in a first stage we directly invert vp and vs anomalies for variations of temperature and composition using the appropriate partial derivatives or sensitivities of velocities to temperature and composition however uncertainties in the tomographic models and in the sensitivities are such that variations in composition are completely unconstrained inferring deterministic distributions of temperature and composition being currently not possible we turn to a statistical approach which allows to infer several robust features comparison between synthetic and predicted ratios of the relative shear to compressional velocity anomalies indicates that the origin of seismic anomalies cannot be purely thermal but do not constrain the amplitude of the variations of temperature and composition we show that we can estimate these variations using histograms of the relative vp and vs anomalies at a given depth we computed histograms for a large variety of cases and found that at the bottom of the mantle variations in the volumic fraction of perovskite from 14 to 10 are essential to explain seismic tomography in the mid mantle anomalies of perovskite are not required but moderate variations up to 6 can explain the observed distributions equally well these trade offs between anomalies of temperature and composition cannot be resolved by relative velocity anomalies alone an accurate determination of temperature and composition requires the knowledge of density variations as well we show that anomalies of iron can then also be resolved 2003 elsevier b v all rights reserved

carbonatites and primary carbonates in the rio apa and amambay regions ne paraguay abstract in the rio apa and amambay regions north eastern paraguay npaa potassic alkaline carbonatitic rocks 138 139 ma predate the eruption of tholeiitic flood basalts 133 1 ma these rocks mainly outcropping as dykes or ring like complexes intrude a cambro ordovician carbonate platform and precambrian metamorphic rocks along with their silurian and permo carboniferous sediments the main rock types range from basanite to trachyte and trachyphonolite and intrusive equivalents to carbonatite in addition to glimmeritic and pyroxenitic veins geological and geophysical evidence indicate that the npaa magmatism is related to extensional tectonics like the early cretaceous alkaline carbonatitic complexes from central eastern paraguay 127 1 ma and the eastern magmatic occurrences of the parana basin oxygen and carbon isotope compositions whole rocks and carbonates vary from values close to the field of continental lithospheric mantle or that of primary carbonatites up to values typical of a hydrothermal environment an isotope exchange model implies that the main isotope variations ranging from a magmatic e g 1200 c to a low temperature 2 h 2 o ratios between 0 8 and 1 0 in particular the o c isotopic variations in combination with the la vs la yb ratios suggest an increasingly higher level of carbonate in the silicate liquids with increasing evolution i e basanite phonotephrite trachyphonolite phonolite trachyte carbonatite sr nd isotopes show that the carbonatites are mantle derived without significant crustal contamination and that they can be related to isotopically enriched sources where newly formed veins enriched component and peridotite matrix depleted component underwent differing isotopic evolution t dm model ages for npaa range from 1 2 to 2 3 ga considering that in the whole parana basin isotopically distinct k alkaline and tholeiitic magmas were generated following the enrichment of the subcontinental mantle mainly between 1 0 and 2 3 ga paleo mesoproterozoic events the mantle sources preserved the isotopic heterogeneities over long time periods suggesting a non convective i e lithospheric mantle source beneath different cratonic or intercratonic areas

stratigraphic analysis of the distributary fan in eberswalde crater using stereo imagery the eroded remains of a fluvial distributary network in eberswalde crater are uniquely well preserved among similar structures on mars a quantitative analysis of the exposed stratigraphy has been performed to investigate the internal structure of the deposit using topographic information derived from stereo pairs of high resolution mars orbiter camera images we have for the first time quantified the orientation of individual layers exposed along the distal end of the distributary network in combination with topographic data from the mars orbiter laser altimeter we have examined plausible scenarios for the formation of this structure we find that the evidence is inconsistent with formation both as an alluvial fan and as a progradational delta instead we find that an aggradational delta best fits the observed characteristics of the channel network and the eberswalde basin as a whole we conclude that the delta likely formed not in a stable long lived lake but over the course of a small number of shorter lacustrine episodes which were not sustained at equilibrium conditions

compositionally diverse magmas erupted close together in space and time within a karoo flood basalt crater complex geochemical data and mapping from a karoo flood basalt crater complex reveals new information about the ascent and eruption of magma batches during the earliest phases of flood basalt volcanism flood basalt eruptions at sterkspruit south africa began with emplacement of thin lava flows before abruptly switching to explosive phreatomagmatic and magmatic activity that formed a nest of craters spatter and tuff rings and cones that collectively comprise a crater complex 40 km2 filled by 9 18 km3 of volcaniclastic debris rising magma flux rates combined with reduced access of magma to external water led to effusion of thick karoo flood basalts burying the crater complex beneath the 1 5 km thick lesotho lava pile geochemical data is consistent with flood basalt effusion from local dikes and some lava flows likely shared or re occupied vent sites active during explosive eruptions at sterkspruit flood basalt magmas involved in sterkspruit eruptions were chemically heterogenous this study documents the rapid perhaps simultaneous eruption of three chemically distinct basaltic magmas which cannot be simply related to one another from one vent site within the sterkspruit crater complex stratigraphic and map relationships indicate that eruption of the same three magma types took place from closely spaced vents over a short time during formation of the bulk of the crater complex two magma types recognized there have not been recognized in the karoo province before the variable composition of flood basalts at sterkspruit argues that magma batches in flood basalt fields may be small 0 5 1 km3 and not simply related to one another this implies in turn that heterogeneities in the magma source region may be close to each other in time and space and that eruptions of chemically distinct magmas may take place over short intervals of space and time without significant hybridisation in flood basalt fields

basalt magmatism along the passive continental margin of se brazil campos basin the se brazil passive continental margin is characterized by tholeiitic magmatism that is particularly widespread in the marginal campos basin facing the inland flood basalts of the adjacent parana basin campos magmatism is represented by early cretaceous ec 134 122 ma flood basalts and minor upper cretaceous early tertiary basalt flows and intrusives which were emplaced in a basin with attenuated crustal thickness 20 km petrography mineral chemistry wholerock geochemistry and sr nd isotope composition emphasize that the ec campos basalts have suffered extensive seawater interaction which caused enrichment in mgo feo total k2o rb and ba and depletion in sio2 and cao while zr nb y and ree remained virtually constant in samples with loss on ignition values less than 4 wt in general campos basalts have bulkrock chemistry similar to those of the inland parana tholeiites 140 130 ma with relatively low concentrations of incompatible elements and tio2 16 morb mid ocean ridge basalt mantle in simple binary mixing models on the whole the early cretaceous campos basalts appear as an easterly younger extension of the northern parana volcanism and probably erupted during early stages of the major riftingprocesses which caused continental thinning it is notable that in the campos marginal basin both the basalt magmatism contemporaneous with the continental break up as well as that which occurred after the s america africa separation appears substantially related to subcontinental lithosphere and a dupal like oib ocean island basalt e g tristan da cunha source components

lower cretaceous tholeiitic dyke swarms from the ponta grossa arch southeast brazil petrology sr nd isotopes and genetic relationships with the parana flood volcanics the lower cretaceous dykes of the ponta grossa arch the most important dyke swarms in brazil are associated with the flood basalts and rare acid flows of the northern parana basin the ponta grossa pg dykes are formed by two pyroxene tholeiites and rare acid rocks the basaltic dykes may be distinguished into two main groups a dominant high tio2 2 wt hti group and a subordinate low tio2 2 wt lti group characterized for similar mgo content by high and low incompatible element contents respectively most pg dykes do not show chemical and isotope evidence supporting important crustal contamination pg dykes with 87sr86sr 0 0 7060 plot in the mantle array sr 17 and nd 3 and can be considered virtually uncontaminated intra dyke chemical variations suggest that the normal or reverse differentiation trends may be related to pre emplacement low pressure differentiation processes the important chemical differences between incompatible element poor and incompatible element rich basaltic dykes cannot be accounted for in terms of fractional crystallization but are compatible with different melting degrees of a garnet peridotite mantle source e g 9 and 20 melting for the hti and lti tholeiites respectively field distribution of the magmatism chemistry and sr nd isotope compositions support that the pg dykes may be related to the flood volcanics of northern parana and not to those of the central and southern parana paleomagnetic data indicate that pg dykes are younger than the parana volcanics all the results suggest that pg dykes were probably feeders of the stratoid volcanics erupted in northern parana towards the continental margin and later eroded chemistry and isotopic data reveal that asthenospheric mantle components were not significant factors in the genesis of pg dykes dyke emplacement occurred during early phases of rifting and or flexuring cutting flood volcanic suites

north polar cap of mars polar layered deposit characterization and identification of a fundamental climate signal of the cap 6 10 5 km 2 in the 13 images analyzed to date 3 assessment of geometric relationships shows that layers are not horizontal but rather have an apparent dip of approximately 0 5 degrees we interpret these results as follows 1 the fundamental 30 m signal is interpreted as a climate signal that may correspond to a 51 kyr insolation cycle 2 the lateral correlation and broad distribution of these layer sequences strongly imply that layer accumulation processes are widespread across the cap rather than confined within a single trough or region 3 local to regional variability in individual layer thicknesses and thus accumulation and sublimation rates is typically less than a factor of 2 5 providing the ability to study regional trends but often making it difficult to correlate visually the vertical sequences in individual cores finally initial examination of layers located deeper in the stratigraphic sequence within the north polar cap than the 300 m thick zone 1 provides evidence for a unit less than 100 m thick zone 2 in which the fundamental 30 m sequence is not detected we interpret this as a deposit having formed during a recent high obliquity phase of mars during which time polar volatiles underwent mobilization and were transport equatorward leaving a polar lag of dust rich material the most recent ice age 0 5 2 ma offers a plausible candidate for this period of ice cap removal and lag deposit formation an underlying zone 3 200 m contains a dominant 35 m signal and a lowermost zone 4 200 m contains multiple signals but no dominant one together these four zones represent 800 m of vertical stratigraphic section about one fourth of the total thickness of the cap these findings support earlier interpretations that orbital parameter variations could cause significant erosion and possibly complete removal of the polar caps the interpreted crater retention ages of the layered terrain are consistent with the correlations and vertical sequences described here suggesting that the polar caps wax and wane throughout geological history depending on the evolution of orbital parameters definition of the 30 m unit signal holds promise for determining 1 the detailed origin of individual layer types 2 the nature of deposition and sublimation processes and their relation to insolation geometry across the polar cap and 3 correlation with and comparison to the south polar layered terrain record

structural features and shear velocity structure of the pacific anomaly 1 we constrain structural features and shear velocity structure of a low velocity anomaly in the lower mantle beneath the pacific we term it the pacific anomaly on the basis of forward travel time and waveform modeling of the observed direct s sdiff scs sks and skks phases sampling a great arc across the anomaly from eastern eurasia to southern south america after correction for the effects of earthquake mislocation and the seismic heterogeneities outside the pacific anomaly seismic observations suggest that the pacific anomaly along the great arc consists of at least two separated portions with a 740 km wide gap between them the western portion of the anomaly is about 1050 km wide extends at least 740 km above the core mantle boundary cmb and exhibits a trapezoidal shape with lateral dimensions increasing slightly with depth the velocity structure of the western portion varies from 3 0 at the top 740 km above the cmb to 3 5 at 100 km above the cmb and an average shear velocity reduction of 5 in the bottom 100 km of the mantle the eastern portion of the anomaly reaches at least 340 km above the cmb beneath the mid pacific with an 1800 km wide base and has a uniform velocity reduction of 3 waveform modeling further suggests a very low velocity layer with a shear velocity reduction of 10 located at the edge of the western portion of the anomaly combining the latest results from others we present a general picture of structural and velocity structures of the pacific anomaly the structural and velocity features suggest that the anomaly represents a cluster of metastable thermo chemical piles

occurrence and origin of rhythmic sedimentary rocks on mars sedimentary rocks preserved on the surface of mars represent a natural archive of past climate conditions although the details of their formation often remain poorly constrained the recent detection of rhythmic bedding patterns in the arabia terra region suggests the influence of orbital variations on sedimentary deposition here we detail a number of new sites which exhibit quasiperiodic stratigraphic variations demonstrating their occurrence throughout the equatorial region of the planet we characterize these recorded signals as well as the local geomorphic context and structural attributes two cyclic units are identified within gale crater the landing site of the mars science laboratory mission enabling estimation of possible formation timescales for the geologic units that may be studied in situ by the rover we find a general lack of fluvial features in connection with rhythmic geologic units contrasting these sites with the aperiodic deltaic stratigraphy found at eberswalde crater possible formation scenarios and their climatic implications are discussed for the diverse set of quasiperiodic sedimentary units we propose multiple depositional pathways for recording cyclic climate changes including repeated evaporitic precipitation from groundwater discharge in topographic lows as well as largely anhydrous accumulation of atmospheric dust for deposits outside of confined basins the preservation of orbital signals in sediments distributed across a wide range of geographic settings suggests a pervasive influence on martian climate conditions through time

testing the tracer ratio method for modeling active compositional fields in mantle convection simulations 1 tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion composition is typically taken to be proportional to the absolute local concentration of tracers but an increasingly popular method is to have dense and regular tracers with composition being equal to the local fraction of dense tracers this paper tests this ratio method using established benchmarks and by comparing the performance of the two tracer methods and grid based methods for simulating the long term evolution of a convecting mantle with a thick dense stable layer for this scenario the ratio method is found to have several advantages giving sharp stable long term layering with no tracer settling minimal statistical noise and low entrainment even with only 5 tracers per cell the method is equally applicable to finite volume and finite element treatments of the underlying flow entrainment in grid based advection methods is heavily dependent on resolution and numerical details and is reduced 1 order of magnitude by the filter proposed by a lenardic numerical determination of physically correct entrainment rates remains a challenging problem comparing tracer and grid based methods the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids however the estimated entrainment differs significantly

geochemistry of the little dal basalts continental tholeiites from the mackenzie mountains northwest territories canada analyses of the little dal lavas and the 779 ma tsezotene sills both of the mackenzie mountains supergroup northwest territories canada show them to be continental tholeiites that are geochemically related the plagioclase and clinopyroxene phyric lavas are geochemically evolved and enriched in iron up to 20 2 wt as fe2o3 two magmatic lineages are identifiable and may represent different degrees of partial melting in the same source region but cannot be related by fractional crystallization within each lineage geochemical variation can be explained by fractional crystallization involving up to 60 crystallization of the original magma the most evolved lavas occur at the base of the pile less fractionated lavas occur toward the top of the sequence the nd isotopic composition of the little dal lavas averages e 1 4 at 780 ma trace element and isotopic compositions are permissive of contamination by continental crust but do not require a crustal component the preserved volume of the littl

hydrogeologic evolution of gale crater and its relevance to the exobiological exploration of mars abstract the presence of an amazonian impact crater lake in the noachian crater gale located in the aeolis northwest subquadrangle of mars is indicated by evidence from young floor deposits streamlined terraces layers and channels observed on the central sedimentary deposit evidence for the filling of this lake by two processes is described a the drainage of the aquifer in the aeolis mensae region supported by extended mass wasting and rim sliding in the crater at the contact with the mensae and b the overspilling of the northern rim by an amazonian south transgression of the elysium basin this last hypothesis is supported by hydrologic features such as channels and channel like depressions north of the crater and by the crescent like shape of the central sedimentary deposit the presence of an impact melt sheet and uplifted central peak may have also generated hydrothermal activity including an early crater lake shortly after the formation of the crater in the noachian period with time decreasing heat flux and changing climates gale may have experienced transitions in aqueous environments from warm and wet to cold and ice covered water that could have provided suitable oases for various communities of microorganisms preservation of the biological and climatic record may have been favored in this paleolacustrine environment which probably occured episodically over two billion years

the impact of early cretaceous gateway evolution on ocean circulation and organic carbon burial in the emerging south atlantic and southern ocean basins abstract organic carbon burial is an important driver of carbon cycle and climate dynamics on geological and shorter time scales ocean basins emerging during the early cretaceous break up of gondwana were primary sites of organic carbon burial implying that their tectonic and oceanographic evolution may have affected trends and perturbations in global climate via changes in local organic carbon burial assessing the role of individual ocean basins in the global carbon climate context requires a sound understanding of the processes that induced large scale changes in carbon burial and the timing of these changes here we reconstruct the oceanographic evolution and its links to organic carbon burial in the barremian to albian south atlantic and southern ocean basins which may have acted as carbon sinks of global importance our reconstruction is based on combined seawater neodymium isotope and sedimentological records obtained from multiple deep sea drill sites and a new general circulation model deep water circulation within and between those basins was primarily controlled by the opening of the shallow falkland plateau gateway between 118 ma and 113 ma and the deep georgia basin gateway by 110 ma for which we provide new age constraints based on biostratigraphic and carbon isotope data the opening of these gateways was accompanied by local to basin wide decreases in organic carbon burial suggesting that ocean circulation affected the oxygenation state via changes in deep water ventilation although our data do not provide quantitative information on the impact of changes in regional organic carbon burial on the global carbon cycle the synchronicity between the reduction of organic carbon burial in the south atlantic basin and global warming during the early albian points to a strong causal relationship

a reappraisal of the high ti and low ti classification of basalts and petrogenetic linkage between basalts and mafic ultramafic intrusions in the emeishan large igneous province sw china abstract based on published data we reappraise the classification of high ti and low ti basalt from the emeishan large igneous province elip and the correlations between basalts and mafic ultramafic intrusions because of the lack of clear spatial and temporal variations of different types of basalts we suggest that the basalts in the elip cannot be classified into high ti and low ti groups by tio 2 contents and or ti y ratios the distinctive characteristics of these high ti and low ti lavas probably result largely from the different fractionating assemblages whether or not fractional crystallization of the fe ti oxides occurred probably is the key factor that controls the ti abundances and ti y ratios in the residual melts e g lavas although the nature of the mantle sources variable degrees of partial melting of mantle and crustal contamination also influence the geochemical signatures of the lavas therefore neither ti abundance nor ti y ratios in basalts can reflect the nature of their mantle source moreover the different types of mafic ultramafic intrusions in the elip cannot simply be attributed to be genetically related special types of basalts either high ti or low ti basalts it is likely that they are merely the cumulus phases i e chamber or conduit of the basaltic lavas hence caution should be exercised in the use of high ti or low ti basalts as prospecting vectors for ore deposits in the region potential implications are proposed that both the fe v ti oxide and cu ni pge sulfide mineralization in the elip intrusions is largely due to the variable differentiation and crustal contamination during magmatic processes

iron spin transition controls structure and stability of llsvps in the lower mantle abstract seismic tomography models have revealed that there exist two large low shear velocity provinces llsvps beneath africa and the pacific ocean in the earth s lower mantle waveform modeling results suggest both llsvps have steep sharp side boundaries which imply that they probably are compositionally heterogeneous from the ambient mantle when applying the surface plate motion history in the last a few hundred million years ma as the driving mechanism numerical modeling has successfully reproduced the geographical distribution of the two llsvps in thermochemical mantle convection however two prominent seismic features of the llsvps the steep side boundaries and the high elevation can hardly be obtained in previous geodynamic models here we include in our mantle convection model the effects of iron spin transition of ferropericlase which substantially change physical properties of the mantle our results show that iron spin transition plays a dominant role in controlling the structure and stability of llsvps large chemical blocks with steep side boundaries and high elevations up to 1200 km above core mantle boundary cmb emerge in our models with the volume content of ferropericlase 20 such blocks cause a shear wave velocity decrease of 3 5 which is consistent with the seismic observations our results also show that these llsvps are transient structures in the lower mantle which can typically last for a few hundred million years before destroyed by large scale mantle motion

radar images of mars full disk images of mars have been obtained with the use of the very large array vla to map the radar reflected flux density the transmitter system was the 70 m antenna of the deep space network at goldstone california the surface of mars was illuminated with continuous wave radiation at a wavelength of 3 5 cm the reflected energy was mapped in individual 12 minute snapshots with the vla in its largest configuration fringe spacings as small as 67 km were obtained the images reveal near surface features including a region in the tharsis volcano area over 2000 km in east west extent that displayed no echo to the very low level of the radar system noise the feature called stealth is interpreted as a deposit of dust or ash with a density less than about 0 5 gram per cubic centimeter and free of rocks larger than 1 cm across the deposit must be several meters thick and may be much deeper the strongest reflecting geological feature was the south polar ice cap which was reduced in size to the residual south polar ice cap at the season of observation the cap image is interpreted as arising from nearly pure co 2 or h 2o ice with a small amount of martian dust less than 2 percent by volume and a depth greater than 2 to 5 m only one anomalous reflecting feature was identified outside of the tharsis region although the elysium region was poorly sampled in this experiment and the north pole was not visible from earth

geologic context of the mars radar stealth region in southwestern tharsis stealth is a region on mars that has no distinguishable radar return stealth was discovered in 1988 by muhleman et al 1991 in 3 5 cm earth based radar images and it was found to extend 2000 km along the martian equator west of arsia and pavonis montes stealth was proposed to be a deposit of unconsolidated granular material with a minimum thickness of several meters with the most likely explanation being that stealth is a volcanic tephra deposit muhleman et al 1991 1995 stealth overlies some of the youngest geologic units on mars including lava flows and a giant curvilinear ridged lobate unit on the west flank of arsia mons stealth also overlies part of a region of smooth and wind scoured materials called the medusae fossae formation examination of high resolution viking orbiter images suggests that stealth is the youngest of several discontinuous wind worked mantles of fine grained 1 cm sediment older deposits of fine sediment appear to be interbedded with lava flows and show evidence of aeolian deposition dunes and deposits in the lee of topographic obstacles and erosion yardangs and pedestal craters the geologic context supports a volcanic ash origin for stealth if stealth is composed of tephra then its extent and the nature of landforms in the region indicate that it resulted from multiple eruptions that occurred relatively late in martian history possible source vents include linear depressions flanked by constructive volcanic landforms that are located on the lower western slopes of arsia and pavonis montes

gypsum a joint tomographic model of mantle density and seismic wave speeds 1 gypsum is a 3 d model of mantle shear wave s speeds compressional wave p speeds and density the model is developed through simultaneous inversion of seismic body wave travel times p and s and geodynamic observations while using realistic mineral physics parameters linking wave speeds and density geodynamic observations include the global free air gravity field divergence of the tectonic plates dynamic topography of the free surface and the flow induced excess ellipticity of the core mantle boundary gypsum is built with the philosophy that heterogeneity that most closely resembles thermal variations is the simplest possible solution models of the density field from earth s free oscillations have provided great insight into the density configuration of the mantle but are limited to very long wavelength solutions alternatively scaling higher resolution seismic images to obtain density anomalies generates density fields that do not satisfy geodynamic observations the current study provides a 3 d density model for the mantle that directly satisfies geodynamic and seismic observations through a joint seismic geodynamic inversion process notable density field observations include high density piles at the base of superplume structures supporting the general results of past normal mode studies however we find that these features are more localized and have lower amplitude than past studies would suggest when we consider both fast and slow seismic anomalies in gypsum we find that p and s wave speeds are strongly correlated throughout the mantle however we find a low correlation of fast s wave zones in the deep mantle 1500 km depth with the corresponding p wave anomalies suggesting a systematic divergence from simplified thermal effects in ancient subducted slab anomalies the cratonic lithosphere and d regions are shown to have strong compositional signatures however we argue that temperature variations are the primary cause of p wave speed s wave speed and density anomalies throughout most of the mantle

martian layered fluvial deposits implications for noachian climate scenarios 1 fluvial deposits in a basin north of holden crater exhibiting a sinuous anabranching pattern are of deltaic or fan origin channel width and meander wavelength indicate persistent flow magnitudes of about 700 m3 s if these flows were produced by quasi periodic climate episodes involving typical terrestrial style precipitation the deposit required thousands to millions of years to accumulate however a scenario of deposition during a few short episodes of constant heavy precipitation resulting from large impacts cannot be excluded

a community benchmark for 2 d cartesian compressible convection in the earth s mantle benchmark comparisons are an essential tool to verify the accuracy and validity of computational approaches to mantle convection six 2 d cartesian compressible convection codes are compared for steady state constant and temperature dependent viscosity cases as well as time dependent constant viscosity cases in general we find good agreement between all codes when comparing average flow characteristics such as nusselt number and rms velocity at rayleigh numbers near 106 and dissipation numbers between 0 and 2 the results differ by approximately 1 per cent differences in discretization and use of finite volumes versus finite elements dominate the differences there is a small systematic difference between the use of the anelastic liquid approximation ala compared to that of the truncated ala in determining the onset of time dependence there was less agreement between the codes with a spread in the rayleigh number where the first bifurcation occurs ranging from 7 79 10 5 to 1 05 10 6

spectral analysis of the lower eocene wilkins peak member green river formation wyoming support for milankovitch cyclicity abstract this study is the first to employ spectral analysis to examine meter scale sedimentary cyclicity in the wilkins peak member of the lower eocene green river formation of wyoming generally regarded as the classic example for orbital forcing of lacustrine sediments at eccentricity and precession time scales this long standing interpretation was recently contested with a much shorter duration 10 ky inferred for the dominant cyclicity earlier work lacked adequate age control or spectral analysis or both our analysis is based upon an evaluation in the frequency domain of oil yield values from four boreholes accuracy estimation for suggested orbital interpretations and comparison to independent geochronology cored intervals 266 364 m thick represent a span of 1 2 1 7 m y with temporal resolution of 3 5 ky 1 m for oil yield values variations in spectral power with depth within the original records are interpreted to reflect changes in the rate of sediment accumulation these changes are corrected prior to testing the orbital forcing hypothesis by using two methods 1 a minimal adjustment three segments accounting for the dominant changes of spectral frequency with depth and 2 correlating the published definitions of precessional cycles in these records to a 21 ky cosine curve orbital age models resulting from the two tuning methods are compared to available chronology and the tuned records are tested for the expected spectral peaks from orbitally forced records we conclude that the dominant cyclicity of the wilkins peak member is orbitally forced orbital age models overlap 40 ar 39 ar ages and inferred periods include long and short eccentricity weak obliquity and precession eccentricity is resolved in the analyzed records but the expected 95 and 125 ky periods are not resolved controlling the range of possible tuning periods and the accuracy of orbital age models sub milankovitch variability exists and can be resolved to a minimum period of 3 5 ky by the analyzed records however it cannot be characterized fully with the available chronology or by the previously calculated mean cycle duration

thermodynamic properties and phase relations in mantle minerals investigated by first principles quasiharmonic theory this research has been motivated by geophysics and materials physics the objective of this research has been to advance materials theory and computations for high pressure and high temperature applications to the point that it can make a difference in our understanding of the earth understanding of the mineralogy composition and thermal structure of the earth evolves by close interaction of three fields seismology geodynamics and mineral physics earth s structure is imaged by seismology which obtains body wave velocities and density throughout the earth s interior fig 1 interpretation of this data relies on knowledge of aggregate properties of earth forming materials either measured in laboratory or calculated in many cases by both however the conditions of the earth s interior especially at the core may be challenging for important experiments and materials computations have emerged to contribute decisively to this field earth s evolution is simulated by geodynamics but these simulations need as input information about rheological and thermodynamic properties of minerals including phase transformation properties such as clapeyron slopes our research has concentrated on the phases of the earth s mantle particularly the deep mantle whose conditions are more challenging for experiments the mantle accounts for 83 of the earth s volume in this article we will review the essential first principles approach used to investigate solids at high temperatures and pressures summarize their performance for mantle minerals fig 2 and point to critical results that have stirred us in the current research path the success is remarkable but some limitations point the way to future developments in this field figure 1 a velocities and density profiles kennett et al 1995 b earth s cross section and c composition and mineralogy of earth s layers figure 2 phases proportions versus depth and or pressure ol mg fe 2sio4 olivine also known as phase mg fe

discovery of bridgmanite the most abundant mineral in earth in a shocked meteorite meteorites exposed to high pressures and temperatures during impact induced shock often contain minerals whose occurrence and stability normally confine them to the deeper portions of earth s mantle one exception has been mgsio 3 in the perovskite structure which is the most abundant solid phase in earth here we report the discovery of this important phase as a mineral in the tenham l6 chondrite and approved by the international mineralogical association specimen ima 2014 017 mgsio 3 perovskite is now called bridgmanite the associated phase assemblage constrains peak shock conditions to 24 gigapascals and 2300 kelvin the discovery concludes a half century of efforts to find identify and characterize a natural specimen of this important mineral

mars white rock feature lacks evidence of an aqueous origin results from mars global surveyor the white rock feature on mars has long been viewed as a type example for a martian playa largely because of its apparent high albedo along with its location in a topographic basin a crater data from the mars global surveyor thermal emission spectrometer tes demonstrate that white rock is not anomalously bright relative to other martian bright regions reducing the significance of its albedo and weakening the analogy to terrestrial playas its thermal inertia value indicates that it is not mantled by a layer of loose dust nor is it bedrock the thermal infrared spectrum of white rock shows no obvious features of carbonates or sulfates and is in fact spectrally flat images from the mars orbiter camera show that the white rock massifs are consolidated enough to retain slopes and allow the passage of saltating grains over their surfaces material appears to be shed from the massifs and is concentrated at the crests of nearby bedforms one explanation for these observations is that white rock is an eroded accumulation of compacted or weakly cemented aeolian sediment

relationships between crustal contamination and crystallisation in continental flood basalt magmas with special reference to the deccan traps of the western ghats india the upper part of the deccan traps sequence bushe to mahabaleshwar formations shows a statistically significant tendency for the most mafic lavas to be the most contaminated by crustal materials this is the reverse of the relationship shown by suites evolving by contamination accompanied by fractional crystallisation afc the observed correlations e g between mg number and sr isotope initial ratios are partly an accidental consequence of the fact that the most mafic lavas are more abundant in the lower part of the sequence while contaminant availability declines in the upper part it is probable however that the correlations are augmented by increased contamination of hotter magma batches during ascent through dykes a process during which fractional crystallisation is suppressed by magmatic turbulence the absence of afc relationships suggests that most of the contamination took place during the ascent stage rather than in a magma chamber other continental flood basalt provinces such as the parana and etendeka do show afc relationships and it is speculated that this may be a result of magma chamber contamination coupled with flow rates which prevent contamination during ascent

mid mantle heterogeneities and iron spin transition in the lower mantle implications for mid mantle slab stagnation abstract recent high pressure experimental results reveal that the elastic and transport properties of mantle materials are impacted by the electronic spin transition in iron under lower mantle pressure and temperature conditions the electronic transition in ferropericlase fp the second major constituent mineral of the lower mantle material is associated with a smooth increase in density starting from the mid mantle depth to the core mantle boundary cmb the transition also yields softening in the elastic moduli and an increase in the thermal expansivity over the transition zone in the lower mantle although there is not yet robust experimental evidence for spin transition induced density change in the perovskite pv phase the major constituent mineral in the lower mantle the spin transition in the octahedral b site in al free perovskite causes a bulk modulus hardening increase in the bulk modulus in the mineral we have incorporated these physical processes into high resolution 3d spherical control volume models for mantle convection a series of numerical experiments explore how the electronic spin transition in iron modifies the mantle flow and in particular the fate of sinking cold slabs such mid mantle stagnations are prevalent globally in seismic tomographic inversions but previous explanations for their existence are not satisfactory employing density anomalies from the iron spin transition in ferropericlase and density anomaly models for perovskite we study the influence of the spin transition in the minerals of the lower mantle on mantle flow our model results reveal that while the spin transition induced property variations in ferropericlase enhance mixing in the lower depths of the mantle the density anomaly arising from the hardening in the bulk modulus of al free perovskite can be effective in slowing the descent of slabs and may cause stagnation at mid mantle levels a viscosity hill in the lower mantle may further enhance the stagnation effect cold descending slabs can stall in the mid mantle for tens of million years or even longer before penetrating to the lower mantle

electronic spin transitions in iron bearing mgsio3 perovskite the electronic spin state of iron in perovskites with the chemical composition mg0 9375fe0 0625sio3 mg0 8750fe0 1250sio3 and mg0 9375fe0 0625si0 9375fe0 0625o3 have been determined at 0 k and various pressures between 40 160 gpa using ab initio methods the results indicate that ferric iron exhibits a wide range of spin transition pressures between about 60 160 gpa while ferrous iron has a much narrower span between about 130 145 gpa in general the lowest spin transition pressures are associated with the most energetically favorable substitution configurations where iron atoms are closest together the effect of spin state on calculated elastic properties is found to be small c 2006 elsevier b v all rights reserved

geochemistry of the miocene mount noel volcanic complex british columbia and comparison with the columbia river basalt the 19 6 ma mount noel volcanic complex mnvc is located about 150 km east of the garibaldi volcanic belt and adjacent to the chilcotin plateau basalts in southwestern british columbia the mnvc unconformably overlies jurassic to cretaceous metasedimentary rocks and lower cretaceous coast plutonic rocks the complex consists of flat lying columnar jointed basalt flows along with volcaniclastic debris flows and minor pyroclastic rocks basaltic flows have olivine fo 60 to fo 85 and plagioclase an 60 to an 80 phenocrysts with clinopyroxene microphenocrysts major trace and rare earth element analyses of the mnvc indicate the flows can be divided into two groups the largest group comprises tholeiitic to transitional basalt whereas a smaller subset is alkaline basalt rare earth element patterns are slightly enriched in the light rare elements in the transitional group and more strongly enriched in the alkaline basalts in tectonic discriminant diagrams all basalts plot in the within plate fields or in overlap areas typical of continental flood basalts alkaline basalts found only at the base of the thick sequence of tholeiitic to transitional basalts are characterized by high p 2 o 5 sr and ba contents the mnvc tholeiitic to transitional basalts are comparable to the chilcotin group basalts and to the lowermost basalts of the columbia river group the origin of mnvc basalts is best interpreted as volcanism behind the pemberton volcanic arc

evidence of magmatic activity related to middle jurassic and lower cretaceous rifting from northeastern brazil ceara mirim k ar age palaeomagnetism petrology and sr 1bnd isotope characteristics the mesozoic magmatic activity in the easternmost part of ne brazil ceara mirim is mainly represented by two pyroxene tholeiitic dykes only few dykes have alkaline character k ar ages and palaeomagnetism data indicate that ceara mirim dykes are of middle jurassic 175 160 ma and early cretaceous 140 130 ma age both middle jurassic and lower cretaceous dykes have high incompatible element concentrations and tio2 with the lower contents generally confined to the middle jurassic dykes rare tholeiitic dykes low in tio2 and incompatible elements are present sr 1bnd isotopic and other chemical data do not support appreciable crustal contamination and in a 87sr86sr vs 143nd144nd diagram almost all the dykes plot in a narrow area of the enriched quadrant of the mantle array tholeiitic and alkaline ceara mirim dykes are related to different parent magmas probably originating by different degrees of melting of garnet peridotite mantle sources characterized by distinct sr 1bnd isotope compositions and small scale chemical heterogeneities ceara mirim tholeiitic dykes show close mineralogical chemical and isotopic similarities with the lower cretaceous high tio2 tholeiites from maranhao sardinha intrusives and parana northern province basins in general chemical and sr 1bnd isotope data for high and low tio2 brazilian mesozoic tholeiites appear to be related to time integrated enriched and depleted mantle components respectively subcontinental mantle heterogeneity is believed to be due to variable enriched components related to metasomatic processes the ceara mirim jurassic dykes correlate well in terms of composition and tectonic setting with the analogue tholeiitic dyke swarms from the benue trough and can be related to the opening of the central atlantic ocean the lower cretaceous ceara mirim magmatic activity corresponds to the coeval one of the benue trough and can be related to the early rifting events responsible for the opening of the equatorial domain of the south atlantic whose oceanic crust formed between 120 and 100 ma

a habitable fluvio lacustrine environment at yellowknife bay gale crater mars the curiosity rover discovered fine grained sedimentary rocks which are inferred to represent an ancient lake and preserve evidence of an environment that would have been suited to support a martian biosphere founded on chemolithoautotrophy this aqueous environment was characterized by neutral ph low salinity and variable redox states of both iron and sulfur species carbon hydrogen oxygen sulfur nitrogen and phosphorus were measured directly as key biogenic elements by inference phosphorus is assumed to have been available the environment probably had a minimum duration of hundreds to tens of thousands of years these results highlight the biological viability of fluvial lacustrine environments in the post noachian history of mars

thermochemical structures beneath africa and the pacific ocean large low velocity seismic anomalies have been detected in the earth s lower mantle beneath africa and the pacific ocean that are not easily explained by temperature variations alone the african anomaly has been interpreted to be a northwest southeast trending structure with a sharp edged linear ridge like morphology the pacific anomaly on the other hand appears to be more rounded in shape mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes it has not been shown however that such models can lead to thermochemical structures that satisfy the geometrical constraints as inferred from seismological observations here we present numerical models of thermochemical convection in a three dimensional spherical geometry using plate velocities inferred for the past 119 million years we show that earth s subduction history can lead to thermochemical structures similar in shape to the observed large lower mantle velocity anomalies we find that subduction history tends to focus dense material into a ridge like pile beneath africa and a relatively more rounded pile under the pacific ocean consistent with seismic observations

sr isotopic evidence for ion exchange buffering in tropical laterites from the parana brazil sr isotopes have been measured in laterite sequences and in associated waters from the region of ribeirao preto parana basin brasil in order to characterize the water rock interactions corresponding to the successive stages of weathering of these continental flood basalts we analyzed detailed sections of concentric rims of alteration in ball shaped spheroids developed around cores of fresh basalts waters were also analyzed on the different types of facies in this region unaltered basalts thick weathered profiles of lateritic material and surrounding sandstones the results show that a major part of the initial sr is removed from the basalt during the earliest stages of weathering within the narrow grey rims surrounding the unweathered core radiogenic sr is also leached from hydrothermal minerals located in basalt fractures or disseminated in the basalt groundmass at this early stage very high water rock ratios of at least 100 000 are calculated for this process from the sr 87 sr 86 in the related waters a general trend of increasing sr 87 sr 86 ratios is observed in the increasingly weathered facies inner yellow cortexes red kaolinitic soils and matrices and in their related waters the very high sr 87 sr 86 ratios of the red kaolinitic horizons probably result from isotopic exchange with radiogenic rainwaters or sandstone waters and also from the presence of residual or neoformed radiogenic minerals budget calculations as well as experimental leaching of kaolinite samples suggest that significant amounts of radiogenic sr can be leached from these facies during weathering thus the kaolinitic facies of tropical laterite profiles are able first to trap strontium from surficial waters then to provide radiogenic sr to the waters that weather the fresh basalts at the weathering front therefore the thick weathered horizons must be considered as a natural cation exchange layer that play an active role in the weathering process

hesperian age for western medusae fossae formation mars the medusae fossae formation mff on mars is an intensely eroded deposit north of the cratered highlands it is widely thought that mff materials were emplaced through ignimbrite eruptions recent geologic mapping of western mff identified outliers of mff materials well beyond the previously mapped western extent for the deposit including outliers close to gale crater we report counts of impact craters on the mff units that have implications for our understanding of the general history of mff and the uppermost layered materials on the gale crater mound

formation and erosion of layered materials geologic and dust cycle history of eastern arabia terra mars 1 eastern arabia terra is mantled in a layer of dust a few centimeters to a meter thick yet contains morphologic features that suggest a history of multiple events of deposition and consolidation of fine grained material and a significant amount of erosion early in martian history this region was affected by volcanic and fluvial activity but has since been dominated by aeolian processes five craters in this region contain interior mound material that ranges in height from 1600 to 2100 m above the crater floor the fluted erosional pattern and the thermal inertia are suggestive of a weakly indurated material and the extensive layering implies that these mounds were formed by a repeated process or processes although these materials primarily occur within craters there are materials outside craters that have similar erosional features and fine laminations suggesting a more extensive deposit the most likely process to form this material is the deposition and cementation of air fall dust and is potentially related to martian obliquity changes the significant amount of erosion of the intracrater mounds unit indicates a dramatic change from a depositional environment to an erosional regime over the past 106 108 years currently dust is accumulating in this region in years with planet encircling dust events but global circulation model results indicate that dust devils may be removing slight amounts of dust from arabia terra these observations suggest that the thickness of the dust mantle may not be currently increasing and may instead be in equilibrium

paleoclimate of mars as captured by the stratigraphic record in gale crater a kilometers thick sedimentary sequence in gale crater exhibits stratigraphic changes in lithology that are consistent with transitions in aqueous and climatic conditions purported to be global in scale the sequence is divided into two formations where the lower formation exhibits a net transition in mineralogy from clay sulfate to sulfate oxide assemblages and is separated from the overlying upper formation by an erosional unconformity superposition and crater counts suggest strata in the lower formation lie along the noachian hesperian time stratigraphic boundary whereas beds in the upper formation which lack signatures indicative of clay minerals or sulfates are thinner more regularly spaced and clearly younger the observed stratigraphic trends are consistent with the rocks at gale crater recording a global transition from a climate favorable to clay mineral formation to one more favorable to forming sulfates and other salts

on the vertical extent of the large low shear velocity province beneath the south pacific superswell the three dimensional s wave velocity structure beneath the south pacific superswell is obtained from joint broadband seismic experiments on the ocean floor and islands we collected only approximately 800 relative times of long period teleseismic sh waves by using a waveform cross correlation from 76 events occurring from january 2003 to may 2005 we conducted relative time tomography to obtain a 3d structure to depths of 1600 km in the resultant image we find a characteristic distribution of low velocity regions the most prominent features are a large doughnut shaped low velocity region at 800 km depth and an elongated large low velocity region beneath the society to pitcairn hotspots at 1200 km depth our model suggests that a large low shear velocity province rooted in the d00 extends upwards and culminates near the top of the lower mantle beneath the central part of the south pacific superswell although its perfect continuity is not still confirmed

compressible thermochemical convection and application to lower mantle structures a new finite element code for compressible thermochemical convection is developed to study the stability of a chemical layer at the base of the mantle using composition dependent compressibility and a density difference between compositions at a reference pressure a composition dependent density profile is derived together with depth dependent thermal expansion this combination of parameters yields a wide range of dynamic evolutions for the chemical layer the chemical structures are classified into five major categories classical plumes mushroom shaped plume domes ridges and continuous layers and a few abnormal cases such as hourglass shaped plumes and columnar plumes several models have a chemical structure morphologically similar to the african low v s structure in the lower mantle at least at a single time guided by our models several dynamic scenarios are proposed for the dynamic nature of the lower mantle low velocity structures a k a superplumes including plumes at an early stage plume clusters ridges passive piles sluggish domes and high bulk modulus domes we predict seismic velocity anomalies from these dynamic models the thermoelastic parameters used in the conversion are additional constraints we compare the density structure with normal mode inversion the predicted seismic signature observations and the required thermoelastic parameters with mineral physics data among the proposed scenarios only the scenario of high bulk modulus domes satisfies all constraints simultaneously the implication on the geochemistry and mineralogy of lower mantle chemical structures is discussed

hirise images of yardangs and sinuous ridges in the lower member of the medusae fossae formation mars abstract hirise images of the lower member of the medusae fossae formation mff were used to identify characteristics of two specific landforms that are well expressed in this particular geologic unit yardangs and sinuous ridges yardangs are wind eroded ridges that are usually confined to arid environments where the bedrock materials can be easily eroded by windblown sand yardangs are common in the lower member of mff where many individual yardangs show evidence of a caprock unit overlying a more friable unit most consistent with an ignimbrite origin for these mff deposits heights of the yardangs in the lower member materials are generally less than a few tens of meters in contrast to yardangs in the thicker middle member mff materials to the east of the study area the yardangs may form in materials comprised of discrete depositional units and there is good evidence that at least a dozen such depositional events contributed to the emplacement of the lower member of mff the lower member yardang heights indicate aeolian erosion has removed at least 19 000 km 3 of lower member mff materials sinuous ridges are elongate positive relief landforms that have been attributed to a variety of possible fluvial flow processes on mars sinuous ridges are very common within exposures of the lower member of mff multiple ridge types are present but all forms seen at hirise scale are most consistent with some form of aqueous channel flow rather than other possible origins the results from this initial examination of hirise images indicate the potential utility of comparing yardangs and sinuous ridges in the lower member to other members of mff although it remains to be determined if sinuous ridges are abundant in the younger mff members

age of the ponta grossa dike swarm brazil and implications to parana flood volcanism the ponta grossa dike swarm pgds occurs in a nw trending 200 km wide zone exposed just east of the parana basin in southeastern brazil the predominantly basaltic dikes intrude crystalline basement paleozoic mesozoic sediments and rarely flows of the parana etendeka flood volcanic province pep the pgds resembles the failed arm of a rift rift rift triple junction related to the separation of south america and africa detailed geochemical studies of the dikes including major minor trace element and sr nd isotopic analyses indicate that they probably represent feeders for the voluminous phase of flood volcanism represented by relatively uncontaminated predominantly high tio2 lavas of the northern pep where lava accumulations reach 1700 m thick 40ar 39ar stepwise degassing data using both laser and radiofrequency induction furnace on plagioclase separates from eighteen dikes and one sill yield seventeen plateau ages three are between 120 7 1 3 ma and 125 8 0 6 ma and fourteen are clustered between 129 2 0 4 ma and 131 4 0 5 ma isochron ages are not significantly different from the plateau ages and plateau ages are adopted in all but two cases as being the most accurate age estimates the age probability distribution for the dominant pulse 131 4 0 4 to 129 2 0 4 ma shows a pronounced peak at 130 5 ma this distribution probably reflects the magma production history in the region the new geochronologic data are consistent with conclusions based on paleomagnetic and chemical stratigraphic data that the pgds is younger than the volumetrically dominant majority of volcanism in the southern pep which occurred at 133 132 ma the younger commonly ne trending dikes may reflect the initiation of full drift which was coincident with major basin development in the adjacent continental borderland at 125 120 ma the pgds may indeed represent the failed arm of a rift rift rift triple junction but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact

density and p wave velocity structure beneath the parana magmatic province refertilization of an ancient lithospheric mantle we estimate density and p wave velocity perturbations in the mantle beneath the southeastern south america plate from geoid anomalies and p wave traveltime residuals to constrain the structure of the lithosphere underneath the parana magmatic province pmp and conterminous geological provinces our analysis shows a consistent correlation between density and velocity anomalies the p wave speed and density are 1 and 15 kg m3 lower respectively in the upper mantle under the late cretaceous to cenozoic alkaline provinces except beneath the goias alkaline province gap where density 20 kg m3 and velocity 0 5 are relatively high underneath the pmp the density is higher by about 50 kg m3 in the north and 25 kg m3 in the south to a depth of 250 300 km these values correlate with high velocity perturbations of 0 5 and 0 3 respectively profiles of density perturbation versus depth in the upper mantle are different for the pmp and the adjacent archean sao francisco sfc and amazonian ac cratons the paleoproterozoic pmp basement has a high density root the density is relatively low in the sfc and ac lithospheres a reduction of density is a typical characteristic of chemically depleted archean cratons a more fertile proterozoic and phanerozoic subcontinental lithospheric mantle has a higher density as deduced from density estimates of mantle xenoliths of different ages and composition in conjunction with re os isotopic studies of the pmp basalts chemical and isotopic analyses of peridodite xenoliths from the gap in the northern pmp and electromagnetic induction experiments of the pmp lithosphere our density and p wave speed models suggest that the densification of the pmp lithosphere and flood basalt generation are related to mantle refertilization metasomatic refertilization resulted from the introduction of asthenospheric components from the mantle wedge above proterozoic subduction zones which surrounded the parana lithosphere the high density pmp lithosphere is presently gravitationally unstable and prone to delamination

martian dust storms 1999 mars orbiter camera observations the mars orbiter camera on board the mars global surveyor from march 9 1999 ls 107 where ls is the areocentric longitude of the sun measured in degrees from mars northern spring equinox to december 31 1999 ls 274 has obtained 7 5 km pixel 1 daily global maps of the martian surface in two wavelength bands blue 400 450 nm and red 575 625 nm visual inspection of these maps during the 1999 dust storm season has resulted in the detection of 783 dust storms ranging in size from local 102 km2 to regional 1 6 106 km2 no global storms were observed the difference in the numbers of dust storms occurring in the two hemispheres north 343 south 440 was not statistically significant fifty percent of the regional size dust storms formed as a result of the merger of two or more smaller local storms the regional storms from ls 207 to ls 223 were observed to move from the northern hemisphere into the southern hemisphere between 30 w and 50 w longitude this cross equatorial transport of dust suggests the location of the low lying southward flowing branch of the hadley circulation observations show that dust storms occur in several regions on mars at the two polar cap edges at the base of high elevation regions in the northern hemisphere near the polar hood during northern fall and at mid latitudes in both hemispheres specific regions such as solis planum and hesperia which were regions of significant dust activity in the past showed almost no dust storm activity in 1999 suggesting that regional dust sources are variable over a 20 year time frame one region of exceptional activity not noted previously was the arcadia amazonis border however other regions such as near elysium mons acidalia chryse hellas noachis argyre cimmeria and sirenum again were active dust storm regions relative dust opacities were modeled for 117 red wavelength events and ranged from 0 54 diffuse haze to 2 13 concentrated local events from the dust opacities we estimated the north polar sedimentation rate to be 6 0 10 10 4 g cm 2 yr 1 a factor of 40 smaller than previous estimates from the two 1977 global dust storms pollack et al 1979 along with the global dust mass loading 2 6 4 0 10 4 g cm 2 and cross equatorial dust mass loading 3 6 10 4 g cm 2 the 1999 cross equatorial mass dust loading suggests that the southern hemisphere subtropical latitudes require at least 2 3 martian years to replenish their dust sources before global storms can form

evolution of the lithospheric mantle beneath the southeastern north china craton constraints from mafic dikes in the jiaobei terrain abstract two stages of mafic dikes with ages of 117 116 ma and 95 87 ma respectively are identified in the jiaobei terrain based on zircon u pb ages the early cretaceous mafic intermediate dikes ecmds and late cretaceous mafic dikes lcmds show distinct geochemical and isotopic features the ecmds are enriched in large ion lithophile elements lile rb sr ba u and th and light rare earth elements lree la ce pr nd sm and eu with a strong depletion in high field strength elements hfse nb ta ti and p they show uniformly high 87sr 86sr i 0 70841 0 71387 low end t values 18 1 to 13 2 positive 206pb 204pb i 17 07 17 90 and negative ehf t 21 0 to 6 1 values these rocks were derived from partial melting of an enriched lithospheric mantle and the enrichment of the mantle source resulted from source metasomatism imparted by the melts generated from remelting of the subducted lower crust of the yangtze craton yc in contrast the lcmds show lower sio2 41 5 50 1 wt lack of hfse depletion and are depleted in whole rock nd and zircon hf isotope they are interpreted to have sourced from a newly accreted depleted lithospheric mantle generated through continuous erosion of older enriched lithospheric mantle by asthenospheric upwelling leaving out minor remnants of the older mantle in the newly accreted lithospheric mantle the geochemical and isotopic characteristics reported in our study demonstrate that the lithospheric mantle beneath the southeastern north china craton ncc was generally enriched during the early cretaceous 120 ma and the mechanism of the enrichment may be related to source metasomatism caused by the subducted lower crust of the yc however in late cretaceous the lithospheric mantle beneath southeastern ncc was highly heterogeneous which might be related to asthenospheric non uniform erosion the 116 87 ma marks the transition from enriched lithospheric mantle to depleted asthenospheric mantle

rb sr chronology and crystal chemistry of celadonites from the parana continental tholeiites brazil abstract celadonites from the parana flood basalts have been dated by internal rb sr isochrons using the ammonium acetate leaching technique previously used by hart and staudigel 1986 for celadonites from dsdp holes the residues after leaching have very high 87rb 86sr ratios 18 7 1900 and very radiogenic 87sr 86sr 0 73 2 96 electron microprobe analyses show that potassium and thus also presumably rubidium became enriched in these minerals during the transformation of a proto celadonite smectite phase into celadonite our leaching experiment suggests that radiogenic 17sr and rubidium are very resistant to chemical fractionation during leaching and thus may have remained in closed system for rb sr chronology ages varying from 107 1 to 81 1 ma have been obtained at two different sites one on high ti basalts in the northern parana basin and the other on low ti basalts in the southern basin these ages are significantly younger than the eruption of the flood basalts 127 137 ma and in this respect are in good agreement with results obtained on celadonites from oceanic ridges initial 87sr 86sr ratios are distinctly higher in the celadonites than in the host basalts probably due to contamination by radiogenic strontium transported by the hydrothermal fluids our results suggest that mineralogy and isotopic composition are different in celadonites from the northern and southern parana basins although this needs to be confirmed on a larger dataset

optical signatures of low spin fe3 in nal at high pressure the iron spin transition directly affects properties of lower mantle minerals and can thus alter geophysical and geochemical characteristics of the deep earth while the spin transition in ferropericlase has been documented at p 60 gpa and 300 k experimental evidence for spin transitions in other rock forming minerals such as bridgmanite and post perovskite remains controversial multiple valence spin and coordination states of iron in bridgmanite and post perovskite are difficult to resolve with conventional spin probing techniques optical spectroscopy on the other hand can discriminate between high and low spin and between ferrous and ferric iron at different sites here we establish the optical signature of low spin fe3 o6 a plausible low spin unit in bridgmanite and post perovskite by optical absorption experiments in diamond anvil cells we show that the optical absorption of fe3 o6 in new aluminous phase nal is very sensitive to the iron spin state and may represent a model behavior of bridgmanite and post perovskite across the spin transition specifically an absorption band centered at 19 000 cm 1 is characteristic of the 2t2g 2t1g 2a2g transition in low spin fe3 in nal at 40 gpa constraining the crystal field splitting energy of low spin fe3 to 22 200 cm 1 which we independently confirm by first principles calculations together with available information on the electronic structure of fe3 o6 compounds we show that the spin pairing energy of fe3 in an octahedral field is 20 000 23 000 cm 1 this implies that octahedrally coordinated fe3 in bridgmanite is low spin at p 40 gpa

dynamic changes and processes in the mississippi river delta research in the modern delta of the mississippi river has revealed short term changes and processes that are of significant magnitude deltaic lobes each lobe covering an area of 30 000 sq km and having an average thickness of 35 km switch sites of deposition on an average of every 1 500 yr through short periods of geologic time this process results in a relatively thick accumulation of stacked deltaic cycles covering extremely large areas within a single delta lobe and operating on an even higher frequency are bay fills and overbank splays bay fills having areas of 250 sq km and thickness of 15 m require only 150 yr to accumulate four major events have taken place in the modern balize delta since 1838 overbank splays are much smaller covering areas of less than 2 sq km and having thicknesses of 3 m but are associated with high floods on the river at the river mouth continued progradation of the distributary channel can form distributary mouth sand bodies that have dimensions of 17 km long 8 km wide and a thickness of 80 m in a period of only 200 yr differential sedimentary loading at the river mouth results in formation of diapirs that display vertical movements in excess of 100 m in a period of 20 yr on the subaqueous delta platform sediment instabilities operate nearly continuously mass moving large quantities of shallow water deposits to deeper water environments via arcuate rotational slides and mudflow gullies and depositional lobes all of these changes and processes operate at differing spatial and temporal scales but all result in deposition of large volumes of sediment over extremely short periods of time

stable intermediate spin ferrous iron in lower mantle perovskite iron has the ability to adopt different electronic configurations and transitions in its spin state in the lower mantle can significantly influence mantle properties and dynamics experimental results for two lower mantle perovskite compositions show that the intermediate spin state of iron is stable throughout the bulk of the lower mantle

ultrahigh resolution topographic mapping of mars with mro hirise stereo images meter scale slopes of candidate phoenix landing sites 1 the objectives of this paper are twofold first to report our estimates of the meter to decameter scale topography and slopes of candidate landing sites for the phoenix mission based on analysis of mars global surveyor mgs mars orbiter camera moc images with a typical pixel scale of 3 m and mars reconnaissance orbiter mro high resolution imaging science experiment hirise images at 0 3 m pixel 1 and second to document in detail the geometric calibration software and procedures on which the photogrammetric analysis of hirise data is based a combination of optical design modeling laboratory observations star images and mars images form the basis for software in the u s geological survey integrated software for imagers and spectrometers isis 3 system that corrects the images for a variety of distortions with single pixel or subpixel accuracy corrected images are analyzed in the commercial photogrammetric software socet set bae systems yielding digital topographic models dtms with a grid spacing of 1 m 3 4 pixels that require minimal interactive editing photoclinometry yields dtms with single pixel grid spacing slopes from moc and hirise are comparable throughout the latitude zone of interest and compare favorably with those where past missions have landed successfully only the mars exploration rover mer b site in meridiani planum is smoother moc results at multiple locations have root mean square rms bidirectional slopes of 0 8 4 5 at baselines of 3 10 m hirise stereopairs one per final candidate site and one in the former site yield 1 8 2 8 slopes at 1 m baseline slopes at 1 m from photoclinometry are also in the range 2 3 after correction for image blur slopes exceeding the 16 phoenix safety limit are extremely rare

extended boussinesq thermal chemical convection with moving heat sources and variable viscosity we have studied with an aspect ratio four box the thermal chemical convective evolution with strongly temperature and depth dependent viscosity and moving heat sources within the extended boussinesq framework in which both adiabatic and viscous heating are included and a depth dependent thermal expansivity is assumed in the equation of state our focus is to show how this type of mantle evolution with an averaged ra of 0 106 may develop with a linear chemical stratification and a uniformly hot mantle as an initial condition the effects of extended boussinesq and depth dependent thermal expansivity are to prevent the effective destruction of the chemical heterogeneities our results show that this initial condition would after the age of the earth lead to a lava lamp mode consisting of a thick chemically stratified and intensely internally heated layer with a thickness of around a quarter of the whole mantle thickness however in this isolated internally convecting layer exceedingly high temperatures greater than 4500 k would be reached in the deep mantle plumes can be launched from the top of this thick denser layer this lava lamp stage would give way to the formation of denser hill like structures at the core mantle boundary then upwellings with deep lower mantle origins can be induced by the interaction of the downwellings with the d layer our simulations show the possibility for some long range mass transfer interaction between these widely separated chemical hills promoted by the fast horizontal flow induced by the sinking currents along the low viscosity zone due to temperature dependent rheology at the core mantle boundary

intermediate spin ferrous iron in lowermost mantle post perovskite and perovskite the earth s lowermost mantle displays an important mineralogical transition from perovskite to post perovskite but the spin state of iron in these phases remains poorly known experimental results suggest that iron occurs in the intermediate spin state in both of these phases which implies that changes in physical properties of the lower mantle must be governed by factors other than spin transitions in iron

crustal signatures in the serra geral flood basalt province southern brazil o and sr isotope evidence strontium and oxygen isotopes in igneous rocks from a 35 000 km 2 region of the serra geral parana continental flood basalt province southern brazil indicate that certain mantle derived magmas that formed during the rifting and breakup of gondwana incorporated continental crust sr o and trace element assimilation fractional crystallization models point to variable amounts and types of warm archean granulitic crust 18 o 11 15 and 87 sr 86 sr i 0 735 having contaminated ascending mafic magmas resulting products include evolved rocks basaltic andesite to rhyolite that have 18 o 11 to 15 and 87 sr 86 sr i 0 714 to 0 729 and basaltic rocks that have 18 o 8 to 10 and 87 sr 86 sr i 0 710

the pulse of calm fan deltas abstract at the heart of interpreting the history of earth surface evolution preserved in the rock record is distinguishing environmental allogenic forcing from internally generated autogenic noise allogenic deposits classically have been recognized by their cyclic nature which apparently results from periodic changes in base level sediment supply or tectonics autogenic deposits which are quite variable in their origin and scale are caused by the nonlinearity of sediment transport and might be expected to have a random or scale free fractal signature here we describe a robust mechanism that generates cyclic deposits by an autogenic process in experimental fan deltas sheet flow over the fan surface induces deposition and an increase in fluvial slope and curvature to a point where the surface geometry is susceptible to a channelization instability similar to channel initiation on hillslopes channelized flow results in incision and degrading of the fan surface to a lower slope releasing a

the high pressure electronic spin transition in iron potential impacts upon mantle mixing 1 at the rayleigh number appropriate to earth s mantle radial heat transport is dominated by solid state thermal convection because of the large number of physical properties required to determine the rayleigh number and because these properties are expected to be perhaps strong functions of pressure and temperature p t laboratory measurements of them under the high pressure and temperature conditions that occur in the deep earth are of fundamental importance recent experimental data demonstrate that an electronic spin transition in iron that occurs at midmantle depths results in significant changes in the physical properties of the ferropericlase component of mantle mineralogy additional recent results suggest that it may also exist in the dominant perovskite component using control volume based numerical models we investigate the impacts on mantle mixing of this spin transition through its influence on the most important subset of these physical properties namely density thermal expansivity bulk modulus and heat capacity our numerical model results demonstrate that this electronic transition enhances mixing in the lower regions of the lower mantle by enhancing the vigor of rising plumes the lowermost region of the mantle is slightly warmed and the upper mantle slightly cooled by spin induced effects however the spin crossover in the lower mantle appears not to significantly influence mantle layering due to the competition that could exist between the strength of the spin induced thermodynamic properties of ferropericlase and perovskite cold descending thermal anomalies could stagnate at middle to lower mantle depths and lead to the occurrence of mid mantle avalanches

geochemistry and petrogenesis of upper cretaceous basaltic rocks from southern malagasy abstract upper cretaceous basaltic rocks of southern malagasy consist of tholeiitic flows and dykes and alkali basaltic dykes continental tholeiites include relatively primitive lava types with mg numbers 0 6 0 5 and voluminous evolved basaltic lavas and dykes with mg number 0 4 the evolved tholeiites have compositions similar to the high ti or enriched basalts from the karoo and parana flood basalt provinces they are characterized by a high content of strongly and moderately incompatible trace elements and a relative depletion of nb and sr the evolved tholeiites were probably derived from subcontinental lithosphere the primitive tholeiites resemble enriched morb and are not directly related to the evolved types alkali basalts 1 15 normative nepheline possess compositional characteristics of oceanic island basalts and show some geochemical similarities to the primitive tholeiites both rock types were probably derived from a mantle plume the upwelling plume induced lithospheric melting which produced the parental magmas of the evolved tholeiites the mesozoic volcanism of malagasy displays many similarities to other flood basalt provinces including its large volume rock composition and emplacement near plate margins

regional variations within the parana flood basalts southern brazil evidence for subcontinental mantle heterogeneity and crustal contamination abstract continental flood volcanism of the parana basin lower cretaceous is represented by two pyroxene tholeiitic basalts 90 vol the northern parana province npp is dominated by basalts high in tio2 and incompatible elements htib while the southern parana province spp is dominated by basalts low in tio2 and incompatible elements ltib npp basalts show relatively small variations of initial 120 ma 87 sr 86 sr r0 and 143 nd 144 nd nd ratios r0 0 7051 0 7062 and nd 0 5124 0 5125 respectively relative to those occurring in spp r0 0 7046 0 7120 and nd 0 5122 0 5128 respectively the latter basalts show significant positive correlations between r0 vs sio2 k2o rb and ba and negative correlations between r0 vs cr ni and mg value believed to be due to crustal granitic contamination in general the effects of contamination are pronounced in spp and tend to vanish towards npp 18o values range from 6 5 to 10 0 in the basalts from npp and spp and essentially reflect water magma interactions uncontaminated ltib and htib basalts from npp are isotopically distinct from the uncontaminated analogues from spp ltib r0 0 7055 vs 0 7046 nd 0 5124 vs 0 5128 respectively all chemical data consistently indicate distinct sources and a large scale mantle heterogeneity the npp mantle source is expected to have relatively high content of enriched components possibly related to small volume melts and metasomatic fluids tentatively the last stabilization age of the parana mantle heterogeneity is 0 5 1 0 ga old the existence of high and low tio2 basalt suites in both parana and karoo provinces in brazil and southern africa respectively indicates a large scale heterogeneity in the subcontinental mantle and suggests that basalt generation occurred in the lithospheric mantle

a comparative study of the geochemical and isotopic systematics of late archaean flood basalts from the pilbara and kaapvaal cratons geochemical and sm nd isotopic data are reported for igneous rocks of the 2 76 to 2 69 ga fortescue group of the pilbara craton western australia and the 2 70 ga ventersdorp supergroup of the kaapvaal craton southern africa the metamorphic history of the fortescue group has also been investigated using whole rock rb sr and pb pb isochron techniques igneous of both sequences have mixed tholeiitic and calc alkaline affinities and show immobile element correlations indicating derivation from chemically heterogeneous lree enriched sources both sequences have similar negative nd values between 1 5 and 4 4 for the fortescue group and 0 and 3 4 for the ventersdorp supergroup which are not correlated with mg mg fe2 the negative nd values of the fortescue group rocks probably do not result from crustal assimilation within crustal level magma chambers during or following differentiation as the fortescue group felsic differentiates have nd values within the ranges of values found in associated mafic rocks and zircon xenocrysts are absent in the felsic differentiates of the fortescue group instead these results are consistent with either extensive crustal contamination of primitive komatiitic or picritic fortescue group parent magmas prior to their differentiation or derivation from negative nd or enriched mantle sources the presence of zircon xenocrysts in some ventersdorp samples provides compelling evidence of some crustal contamination although correlations between immobile element ratios such as la yb ti zr ti sc and v zr and nd indicate derivation from chemically heterogeneous enriched mantle sources a whole rock sm nd isochron date of 3308 138 ma 600 ma older than the time of eruption indicated by u pb zircon data and within 300 ma of the formation of the kaapvaal granite greenstone terrane may date an sm nd fractionation event within the subcontinental lithospheric mantle sources of the ventersdorp rocks rb sr and pb pb isochrons obtained for several flows from the fortescue group register hydrothermal events occurring between 2 45 and 2 0 ga and probably associated with burial metamorphism similarities in the field geology geochemistry and isotopic characteristics of the fortescue and ventersdorp sequences and phanerozoic examples of continental flood basalt volcanism suggest a common mode of origin possibly involving the interaction of asthenospheric mantle plumes with subcontinental lithospheric mantle sources which have been modified by subduction processes