annual layers revealed by gpr in the subsurface of a prograding coastal barrier southwest washington u s a abstract the southwest washington coastline has experienced extremely high rates of progradation during the late holocene subsurface stratigraphy preserved because of progradation and interpreted using ground penetrating radar gpr has previously been used successfully to document coastal response to prehistoric storm and earthquake events new gpr data collected at ocean shores washington suggest that the historic stratigraphy of the coastal barrier in this area represents a higher resolution record of coastal behavior than previously thought gpr records for this location at 200 mhz reveal a series of gently sloping seaward dipping reflections with slopes similar to the modern beach and spacings on the order of 20 45 cm field evidence and model results suggest that thin 1 10 cm possibly magnetite rich heavy mineral lags or low porosity layers left by winter storms and separated by thick 20 40 cm summer progradational sequences are responsible for generating the gpr reflections these results indicate that a record of annual progradation is preserved in the subsurface of the prograding barrier and can be quantified using gpr such records of annual coastal behavior where available will be invaluable in understanding past coastal response to climatic and tectonic forcing

multiple events in the neo tethyan oceanic upper mantle evidence from ru os ir alloys in the luobusa and dongqiao ophiolitic podiform chromitites tibet in situ analysis of major elements platinum group elements and os isotope composition data for 170 grains of ru os ir alloys from podiform chromitites in the luobusa and dongqiao ophiolites tibet provide new constraints on the sources of the alloy grains the os isotope composition of the neo tethyan oceanic upper mantle and the timing of melt rock reactions in the ultramafic peridotites of the host ophiolites most alloy grains are osmiridium and iridosmine based on elemental and isotopic compositions two populations can be identified group i found in both ophiolite bodies has fractionated pge patterns typical of alloys and homogeneous 187 os 188 os 0 12645 4 2s n 145 group ii alloys from the dongqiao ophiolites show smooth pge patterns and 187 os 188 os ranges from 0 12003 to 0 12194 yielding re depletion ages 1 1 ga both the pge patterns and the os isotopic compositions of group ii resemble those of sulfides residual after melting group i alloys represent an early cumulate phase during the crystallization of magmas and are interpreted as derived from an asthenospheric melt in contrast group ii alloys are inferred to derive from the breakdown of residual sulfides from the mantle section of the ophiolite which may represent relics of a sub continental lithospheric mantle the coexistence of the two groups of alloys in dongqiao samples reflects interaction between asthenosphere derived melts and old depleted lithospheric mantle and suggests that the chromitites originated through melt rock reactions the data on the group i alloys support the hypothesis that the re os evolution of the convective oceanic mantle resembles that of enstatite chondrites 2007 elsevier b v all rights reserved

the ryonchon podiform chromitite from the chongjin ophiolite northern korea peninsula abstract ryonchon chromitite is located structurally in the chongjin area the northern part of the korean peninsula which crops out at chongjin terrane the eastern end of the jilin yanbian hambuk fold belt we made on site geological survey of this deposit and conducted the analyses on the investigated chromitite by means of microscopes single crystal x ray diffraction sxrd and electron probe micro analysis epma to elucidate the phase composition the chemical composition and the texture and structure of the chromitite the chromian spinels from the study area are separated into four groups according to chemical composition of spinel as ordinary spinel group minerals are the studied chromites have different compositions according to their occurrences which is not owing to the precipitation of different pararental melts but the subsolidus re equilibration during the hydrous alternation the massive semi massive unaltered chromite in chromitite from dunite have consistently low cr cr

gpr in sediments advice on data collection basic processing and interpretation a good practice guide within sedimentological studies ground penetrating radar gpr is being used with increasing frequency because it yields images of the shallow subsurface that cannot be achieved by any other non destructive method the purpose of this paper is to provide an introduction to the collection processing and interpretation of gpr data so that future sedimentary studies can be improved with gpr equipment now being lightweight robust and portable proper data collection and survey design methods need to be followed in order to acquire high resolution subsurface digital data various factors are discussed including reflection profiling velocity soundings test surveys topography logistics data quality and extreme environments basic data processing and visualization are then reviewed followed by a discussion on gpr interpretation strategies including a background to radar stratigraphy for the sedimentary geologist or geomorphologist gpr offers unique data of the shallow subsurface including stratigraphy geometry architecture and structure

accommodation space controls on incised valley sediment accumulation rates during the holocene marine transgression 0 11 ka in grays harbor a large meso tidal estuary washington usa abstract holocene deposits 0 11 ka that were drill cored 275 samples from 17 sites in the meso tidal grays harbor estuary were analyzed for 1 sediment texture and source components and 2 calibrated radiocarbon age these data were used with basin 10 m contour interval volumes to establish relations between rates of increasing basin accommodation space and sediment volume accumulation rates of increasing accommodation space 0 3 2 1 106 m3 yr 1 are based on interval volumes and a sea level curve 0 11 5 ka for the grays harbor basin total sediment accumulation rates 0 41 1 4 106 m3 yr 1 are computed for the same 10 m depth intervals 0 60 m depth msl and corresponding 10 m interval sedimentation durations 0 7 5 0 103 ka from a sediment level curve the rates of basin increasing accommodation space and total sediment accumulation are positively correlated r2 0 81 but sediment accumulation lagged behind increasing accommodation space 23 37 during early and middle holocene time 7 10 ka slowing rates of sea level rise after 8 ka permitted combined river and littoral inputs of sediment to reduce basin subtidal water depths 10 m to intertidal water depths 1 m the ratios between sand and mud components in the analyzed holocene deposits are negatively correlated to basin water depth shallowing tidal flats promoted the export of river mud out of the estuary and the import of littoral sand into the central estuary areas variable rates of increasing basin accommodation space 1 directly controlled the total sediment accumulation rate in the evolving basin and 2 indirectly controlled the relative proportions of mud and littoral sand accumulation due to changing water depths in the basin the late holocene accumulation of littoral sand 52 of total sediment volume is used to compute the net littoral sand sink 100 106 m3 in the grays harbor basin from a potential 1 0 m rise of future sea level following predicted global warming the contribution of littoral sand from adjacent barrier sand spits 20 km in length to fill the increasing accommodation space in the grays harbor estuary could result in 0 5 1 0 km of shoreline retreat following a 1 0 m rise in sea level

ground penetrating radar antennae frequencies and transmitter powers compared for penetration depth resolution and reflection continuity1 twelve ground penetrating radar gpr experiments were conducted on the modern wave influenced william river delta on the southern shore of lake athabasca in northern saskatchewan canada the delta is a well sorted quartzoserich clean sand dominated water saturated geomorphic feature which provided an ideal site to test gpr penetration depths resolution and continuity of reflections were compared for different antennae frequencies 25 50 100 200 mhz and transmitter powers pulser voltage 400 v 1000 v the data show significant variations in vertical resolution from 0 15 m to 0 76 m 200 25 mhz depth of penetration from 14 m 28 m 200 25 mhz and continuity of reflections increasing the transmitter power from 400 v to 1000 v increases the depth of penetration by 5 to 14 and improves the continuity of reflections with little effect on the resolution

petrology and origin of primitive lavas from the troodos ophiolite cyprus parental magmas to the troodos ophiolite are characterised by low tio2 and al2o3 and high sio2 extremely fresh and chemically primitive high mgo rocks are found within the upper pillow lavas and along the arakapas fault belt of cyprus and contain forsteritic olivine enstatite and groundmass clinopyroxene set in glass or plagioclase with accessory magnesiochromite and sometimes hornblende they are quartz normative and may have originally contained up to 3 wt h2o geochemically there are three distinct groups of primitive lavas based on tio2 and zr contents but also reflected by cao na2o and ree abundances these groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region the parental magma to the least depleted group group i was that of the major portion of the troodos plutonic complex and is similar to those postulated for other low ti ophiolites chemically it has close affinities with komatiitic basalts the most depleted lavas group iii all have u shaped ree profiles and variable 143nd 144nd ratios interpreted in terms of metasomatism of the source region by an incompatible element enriched component which was probably derived from a subducted slab these lavas represent an intermediate step in the development of boninite series rocks

the genesis of refractory melts in the formation of oceanic crust refractory primary liquids arising in various oceanic plate tectonic settings are characterized by high mgo sio2 ca na low tio2 and generally low incompatible element abundances relative to primary liquids parental to morb we propose that the former melts segregate from upper mantle peridotite which has earlier been depleted by extraction of picritic melts which were parental to morb a compositional range in the second stage melts is expected depending on the extent of previous depletion of the peridotite the temperature and pressure of melt segregation and the possible influence of volatile phases c h o present during melting

accommodation space in a high wave energy inner shelf during the holocene marine transgression correlation of onshore and offshore inner shelf deposits 0 12 ka in the columbia river littoral cell system washington and oregon usa abstract the columbia river littoral cell crlc a high wave energy littoral system extends 160 km alongshore generally north of the large columbia river and 10 15 km in across shelf distance from paleo beach backshores to about 50 m present water depths onshore drill holes 19 in number and 5 35 m in subsurface depth and offshore vibracores 33 in number and 1 5 m in subsurface depth constrain inner shelf sand grain sizes sample means 0 13 0 25 mm and heavy mineral source indicators 90 holocene columbia river sand of the inner shelf facies 90 fine sand stratigraphic correlation of the transgressive ravinement surface in onshore drill holes and in offshore seismic reflection profiles provide age constraints 0 12 ka on post ravinement inner shelf deposits using paleo sea level curves and radiocarbon dates post ravinement deposit thickness 1 50 m and long term sedimentation rates 0 4 4 4 m ka 1 are positively correlated to the cross shelf gradients 0 36 0 63 of the transgressive ravinement surface the total post ravinement fill volume of fine littoral sand 2 48 1010 m3 in the inner shelf represents about 2 07 106 m3 year 1 fine sand accumulation rate during the last 12 ka or about one third of the estimated middle to late holocene columbia river bedload or sand discharge 5 6 106 m3 year 1 to the littoral zone the fine sand accumulation in the inner shelf represents post ravinement accommodation space resulting from 1 geometry and depth of the transgressive ravinement surface 2 post ravinement sea level rise and 3 fine sand dispersal in the inner shelf by combined high wave energy and geostrophic flow down welling drift currents during major winter storms

hybridization of mingling magmas with different densities abstract it is usually assumed that chemical exchange between mafic enclaves considered as blobs of mafic liquids and their host silicic melt proceeds through static diffusion however the chemical boundary layer should keep the interior of magmatic enclaves nearly pristine a feature which is not observed the surface layer of a mafic blob falling in a more silicic melt is likely to be continuously folded in with the inner part a numerical model shows that buoyancy driven forced convection is an efficient way of enhancing the rate at which melt inclusions become hydrous and hybridized by the host melt

latest pleistocene and holocene 2 16ka sedimentation in the columbia river estuary oregon usa abstract a deep borehole drilled at warrenton oregon in the ancestral valley of the columbia river represents a geologic record that extends from 2 ka to 16 ka prior to the onset of the holocene marine transgression at 16 ka the incised columbia river valley was cut to 112 m below present sea level at this location the onset of estuarine circulation in the columbia river estuary occurred at 11 5 ka as determined by the first appearance of brackish water diatoms in sediments from borehole cores at a depth of 70 m grain size and heavy mineral analyses indicate that the columbia river tidal basin served initially 16 11 ka as a bedload bypassing conduit to the continental shelf and or the astoria canyon with the ongoing holocene transgression the columbia river tidal basin became more efficient as a river sediment sink between 11 5 and 9 0 ka after 9 0 ka the filling tidal basin again served as a sediment bypassing conduit source of sand to the coastline and continental shelf heavy mineral analyses indicate that between 16 and 11 ka the columbia river tidal basin was dominated by sediment from the metamorphic interior basins of the columbia drainage basin the dominant sediment source changed from the metamorphic interior basins of the columbia drainage basin to the cascade volcanic arc between 11 5 and 9 0 ka after 9 0 ka the tidal basin was dominated by cascade volcanic arc derived sediments the total volume of holocene sediment primarily bedload which has accumulated in the lower columbia river valley is 73 km 3 this compares to only 13 km 3 accumulation during the last 5 ka the tidal basin accumulation rate gradually increased from 0 6 million m 3 yr to slightly over 18 million m 3 yr between the depths of 112 m and 30 m 16 to 9 ka above the 30 m depth corresponding to 8 2 ka the rate of basin volume fill dramatically decreased to just over 4 million m 3 yr such a large decrease in sediment accumulation rate suggests that after 9 ka sediments were bypassing the nearly full tidal basin to the beaches and inner shelf the rates of bedload bypassing the lower columbia river valley substantially greater than 2 4 million m 3 yr supported the shoreface progradation in the littoral cell during the late holocene

coesite and clinopyroxene exsolution lamellae in chromites in situ ultrahigh pressure evidence from podiform chromitites in the luobusa ophiolite southern tibet abstract we report in situ petrological evidence of deep mantle origin from podiform chromitites in the luobusa ophiolite southern tibet analytical transmission electron microscopy measurements reveal that chromites in podiform chromitites from the luobusa ophiolite have numerous exsolution lamellae of diopsidic clinopyroxene and coesite which indicate an ultrahigh pressure origin of over 3 gpa 100 km deep the presence of these lamellae coupled with abundant micro inclusions of clinopyroxene requires high solubility of sio2 and cao in the host chromite and suggests a precursor of chromite a cafe2o4 structured high pressure polymorph stable at pressures over 12 5 gpa 380 km deep these nano scale observations and geological occurrence indicate that the mantle peridotite under the tibetan mid ocean ridge was transported from the deep mantle at least 100 km probably more than 380 km deep by the mantle convection it implies that the root of mantle upwelling has a much deeper origin than previously believed

tidal marsh stratigraphy sea level change and large earthquakes i a 5000 year record in washington u s a abstract many of the estuaries of the pacific northwest of the u s a and canada contain stratigraphic sequences typified by alternating peat mud couplets recent studies in this region interpret such couplets as the product of repeated large magnitude s or 9 earthquakes on the cascadia subduction zone the resultant pattern of land level movements is described by a model the earthquake deformation cycle of coseismic land subsidence followed by land uplift during interseismic strain accumulation however peat mud couplets similar to those recorded in the pacific northwest are found on other less tectonically active temperate latitude coasts such as northwest europe and the atlantic coast of the u s a where they have been interpreted as the product of non seismic coastal processes in this paper we apply the methods and scientific framework common to sea level investigations in northwest europe to a sequence of peat mud couplets recorded in the lower johns river an estuary in southern washington to provide a test of the earthquake deformation cycle stratigraphic investigations of the intertidal sediments along the lower johns river using lithological pollen diatom and foraminiferal data show evidence for eight coastal submergence events during the last 5000 years to evaluate the earthquake deformation cycle we assess the lateral extent of peat mud couplets the synchroneity of submergence the presence of tsunami deposits accompanying submergence and the suddenness and amount of submergence each submergence is shown to be accompanied by changes in coastal sedimentation broadly commensurate with those predicted by the earthquake deformation cycle demonstrating the continued intermittent seismic activity of the cascadia subduction zone throughout the mid and late holocene quantitative analyses of contemporary and fossil biostratigraphic data using twinspan and detrended correspondence analysis enable us to estimate the magnitude of submergence accompanying each peat mud couplet one event was accompanied by submergence of about 1 5 m or more four events by intermediate submergence of about 1 0 5 m and a further three events by submergence of

composition age and depositional rates of shoreface deposits under barriers and beach plains of the columbia river littoral cell usa abstract the columbia river littoral cell crlc consists of four subcells totaling 160 km in length that are unique in the west coast of the united states in that they contain prograded barriers and beach plains reaching 0 5 3 km in width fig 1 the prograded beach deposits 1 5 ka in age overlie shoreface deposits 1 8 ka in age as identified in 18 ground penetrating radar profiles and sampled from 24 boreholes two competing hypotheses were initially proposed to account for the origins of these unique progradative shorelines 1 cross shore feeding by onshore wave transport of pre holocene sand from the submerged shelf and 2 longshore dispersal of nearshore sand that was supplied to the littoral system by bedload sediment discharge from the columbia river during the holocene the crlc sand forming the shoreface deposits is fine diameter 0 2 0 02 mm and rich in lithic fragments 20 40 by volume gravel and shell lag layers are uncommon in most of the crlc shoreface deposits but they show greater abundance locally near ravinement surfaces tidal inlets and in the clatsop subcell located south of the columbia river mouth gravel and granule layers increase upsection in barriers south of the columbia river and downsection in barriers at the northern end of the littoral system these trends suggest different mechanisms of shoreface sediment feeding within the four subcells however borehole samples from all four subcells show the same sand provenance i e post glacial columbia river sand which is identified by high ratios of hypersthene augite in heavy mineral fractions selected shoreface sections were dated 0 5 8 ka by ams radiocarbon analysis of articulated shell and wood fragments recovered from auger flights 3 22 m depth subsurface relatively young shoreface deposition 2 5 ka at 6 5 m elevation ngvd88 in the clatsop subcell south of the columbia river shows a net southward beach transport that fed shoreface and beachface progradation into deeper water older and deeper shoreface deposition 4 4 ka at 7 1 m elevation in the long beach subcell north of the columbia river was a result of the filling of innermost shelf accommodation space prior to beachface progradation the total volume of shoreface sand deposited under the barrier spits and beach plains of the crlc is estimated to be 6 7 km3 deposited since 6 8 ka there was a net northward transport of littoral sand 1 106 m3 year 1 along the nearshore and inner shelf subsequently some of this sand was transported onshore to feed beaches of the northernmost subcells columbia river sand was also the source for the formation of the offshore shelf wedge above the transgressive ravinement surface and for the filling of major tidal basins located north of the columbia river in summary the unique progradational history of the crlc barriers and beach plains derives from the combination of 1 longshore dispersal of fine sand discharged from the columbia river during holocene time and 2 across shore feeding of beaches at the northern end of the littoral system from fine sand carried north along the nearshore and the inner shelf

coseismic coastal subsidence associated with unusually wide rupture of prehistoric earthquakes on the kamchatka subduction zone a record in buried erosional scarps and tsunami deposits abstract the prograding strand plain of avachinsky bay kamchatka russia along the highly active kamchatka subduction zone exhibits geological evidence buried erosional scarps for coseismic subsidence only three times in the last four millennia the last event about 1200 years ago this same coast has a historical record since a d 1737 of five subduction zone earthquakes with large tsunami runup 5 m the last of which was the 1952 mw 9 kamchatka earthquake and a geological record of more than 30 large tsunamis in the last 4000 years this rarity of buried scarps relative to large earthquakes contrasts with the cascadia strand plain in sw washington state where most or all large events are represented by buried scarps a strong factor in the amplitude and sign of coseismic deformation is distance from the seaward edge of a subduction zone the trench the avachinsky bay coastline is 180 200 km from the trench with 25 slab dip requiring unusually wide ruptures to generate significant coseismic subsidence this coastal zone is undergoing net subsidence approximately equivalent to the total of the three coseismic subsidence events generating a sequence of beach ridges that increase in elevation seaward each of the three unusual coseismic subsidence events comprises a an earthquake whose deformation field caused b onshore coseismic subsidence thus local sea level rise and c sufficient deformation offshore to produce a large tsunami a b c followed by d a period of coastal erosion and shoreline retreat leaving e an erosional beach scarp that was f subsequently buried once progradation resumed we identified dated and correlated the scarps and tsunami deposits from these events with several field methods including trenching tephrostratigraphy and ground penetrating radar the scarps were correlated over an alongshore distance of 50 70 km the most recent event event 1 occurred 800 cal ad 1100 1250 14 years bp event 2 600 cal bc 2400 2450 14 years bp and event 3 1700 cal bc 3300 3500 14 years bp we developed methods for quantifying subsidence coastal erosion and tsunami size for each of these events all three retain evidence of 0 4 1 2 m of coseismic subsidence coastal erosion in the case of event 1 averaged more than 100 m all three event tsunamis were amongst the largest in the last 4000 years

podiform chromitites of the tari misaka ultramafic complex southwestern japan as mantle melt interaction products petrological and geochemical data for chromian spinel from the tari misaka ultramafic complex southwest japan are discussed in this complex the spinels form a chromitite pod coated by a dunite envelope and are collectively enclosed by clinopyroxene bearing harzburgite chromitite and its dunite envelope are distinctly different from the surrounding harzburgite in terms of the chromian spinel chemistry the ti content is five to ten times higher in spinels of the enveloped chromitite than in those of the harzburgite although the cr ratio cr cr al the atomic ratio for both is almost constant or varies only slightly 0 4 0 5 this eliminates a comagmatic origin for the harzburgite dunite chromitite association spinel chemistry for both major and trace elements is almost the same in the chromitite and its dunite envelope except for the mg ratio and the co and mn contents which have a changed subsolidus as a function of the spinel mode thus the dunite and chromitite could be comagmatic but fractional crystallization of the dunite after or before chromitite deposition is not possible the chemistry of the chromian spinel and the mode of occurrence of the dunite chromitite association are consistent with an origin by interaction of an exotic melt with wall rock harzburgite the dunite may be of replacive origin the exotic melt would react with the harzburgite at the new contact to form dunite and a secondary si rich melt the melt thus formed could mix with a successive inflow of a more primitive melt the mixed melt would be expected to have chromite chromian spinel as the only liquidus phase precipitating chromitite next to the replacive dunite the dunite envelope podiform chromitite is most commonly formed in the mantle beneath island arcs the spread in cr ratios of spinel in podiform chromitites is almost comparable with that of spinel in primitive arc related magmas the presence of chromitite in the upper mantle beneath the japan arcs is consistent with this interpretation

dating of late holocene beach shoreline positions by regional correlation of coseismic retreat events in the columbia river littoral cell usa abstract beaches and barrier spits of the columbia river littoral cell crlc 160 km in length experienced substantial progradation 0 5 2 5 km in width during the late holocene the accreted beach plains and barriers preserved evidence of episodic catastrophic retreat that was forced by coseismic subsidence the abrupt subsidence events 0 5 1 5 m submergence are associated with slip on the underlying cascadia megathrust with a mean recurrence interval of 500 yr ground penetrating radar gpr profiles taken across the barriers and beach plains show high angle beach retreat scarps 3 15 m vertical relief and foredune ridge slump scarps 5 15 m vertical relief the upper shoreface retreat features are correlated throughout the littoral cell by radiocarbon age position relative to abandoned foredune ridges and distinct sequences of large and small scarps particularly large retreat scarps 10 15 m in height are associated with the first oldest and fourth oldest prehistoric scarps a and d respectively a maximum of 10 retreat scarps in the long beach subcell date from 216 211 calrcybp to 4756 114 calrcybp the youngest seven scarps a g are correlated between all four subcells showing a regional response to the coseismic strain cycles backshore disturbance features 1 3 m vertical relief are observed in the gpr profiles but they are not identified here due to their discontinuities between adjacent profiles at 5 km longshore spacing the seven regional scarps a g are directly correlated to seven coseismic subsidence events reported to date between 0 3 and 3 2 ka the correlation of scarps to earthquake events provides high resolution proxy ages for shoreline positions in late holocene time the proxy shoreline positions are dated at 0 3 1 1 1 3 1 7 2 5 2 8 and 3 2 ka based on buried wetland sequences from adjacent tidal basins the paleoshoreline dating demonstrates that the onset of net shoreface progradation between 4 7 and 0 3 ka was delayed with increasing distance 0 100 km from the columbia river sand source the history of beach progradation reflects the sequential filling of bay and shoreface accommodation spaces located down drift of the columbia river mouth these late holocene shoreline changes portend future redistributions of existing sand within the littoral system as the limited sand reserves continue to migrate away from the columbia river mouth source

dynamics of the foreshock compressional boundary and its connection to foreshock cavities 1 we use several global hybrid kinetic ions fluid electrons simulation runs for steady and time varying interplanetary magnetic field imf conditions to examine the dynamics of the foreshock compressional boundary fcb and its connection to foreshock cavities the results demonstrate that for steady imf conditions the fcb forms and evolves over a long period of time due to the dynamics of the bow shock and ion foreshock formation of the fcb is tied to the generation and nonlinear evolution of ulf waves associated with large amplitude fluctuations in magnetic field and density within the foreshock as a result even during steady imf conditions the transitions in the magnetic field strength and direction across an fcb evolve although the fcb itself is associated with increases in the magnetic field strength and density these quantities are reduced on the turbulent side of the fcb as compared to the pristine solar wind hybrid simulations with time varying imf have been performed to examine the relationship between the fcb and foreshock cavities generated under two possible scenarios in the first scenario a bundle of field lines connects to an otherwise quasi perpendicular bow shock and results in the formation of a finite sized foreshock region that travels with this bundle of field lines as it connects to different parts of the bow shock surface two fcbs bound the traveling foreshock region in the second scenario solar wind discontinuities cause the imf cone angle angle between the imf and the solar wind flow direction to vary and thereby modify the foreshock geometry and the position of the fcb we demonstrate that structures similar to foreshock cavities bounded by fcbs form in both scenarios we show that the two scenarios cannot be distinguished based on convecting or nonconvecting fcbs we also demonstrate that depending on spacecraft location and the nature of the solar wind discontinuities foreshock cavities may be bounded by an fcb on one side and a foreshock bubble on the other

experimental constraints on the formation of the tibetan podiform chromitites abstract the discovery of diamonds and highly reduced minerals in podiform chromitites which have generally been interpreted as magmatic rocks formed from partial melts of upper mantle peridotites under low pressure conditions has raised many questions about the origin of these enigmatic bodies in order to provide experimental constraints on the formation and emplacement of podiform chromititesin ophiolites we carried out a number of multi anvil experiments in the magnesiochromite sio 2 system at temperatures of 1000 1600 c and pressures of 5 15 gpa the experimental results demonstrate that magnesiochromite is stable up to 14 gpa and decomposed into eskolaite cr 2 o 3 together with a quenchable modified ludwigite structured phase fe mg 2 al cr 2 o 5 at higher pressures thus placing an approximate maximum depth for chromite crystallization and or metamorphism this depth corresponds to the top of the mantle transition zone mtz at 410 km the ludwigite structured post chromite phase has significant implications for understanding phase transformations and cr incorporation partitioning of minerals in the mtz on the basis of our results we propose a multi stage model for the formation of podiform chromitites that incorporates the geochemical textural and mineralogical features of these bodies

archaeological evidence for a former bay at seaside oregon cultural shell middens a common feature of coastal landscapes preserve a record of past human use of coastal resources and often provide important paleoenvironmental information molluscan remains from two shell midden sites located 0 7 km from the modern pacific coast at seaside oregon suggest the former presence of a small sheltered bay at this locality the modern sand beaches in the seaside area stretching for 30 km southward from the mouth of the columbia river are exposed to high energy surf where razor clams siliqua patula are today the only common bivalve by contrast over 90 of molluscan remains by weight recovered from the archaeological sites represent species that favor sheltered water geologic studies in the seaside area have shown that the beach prograded some 2 km over the last 4 millennia the midden evidence suggests that the progradation was accompanied by the infilling of a former bay

large scale coastal change in the columbia river littoral cell an overview abstract this overview introduces large scale coastal change in the columbia river littoral cell crlc covering 165 km of the southwest washington and northwest oregon coasts the littoral cell is made up of wide low sloping dissipative beaches broad coastal dunes and barrier plains three large estuaries and is bounded by rocky headlands the beaches and inner shelf are composed of fine grained sand from the columbia river and are exposed to a high energy winter wave climate throughout the holocene the crlc has undergone large fluctuations in shoreline change trends responding to a variety of coastal change drivers including changing rates of sea level rise infrequent yet catastrophic co seismic subsidence events a large regional sediment supply inter annual climatic fluctuations el nino cycles seasonally varying wave climate and numerous anthropogenic influences human influences on the crlc include construction of over 200 dams in the columbia river drainage basin dredging of navigation channels removing sand to upland sites and offshore deep water sites and construction of large inlet jetties at the entrances to the columbia river and grays harbor the construction of these massive entrance jetties at the end of the 19th century has been the dominant driver of coastal change through most of the littoral cell over the last hundred years presently some beaches in the littoral cell are eroding in response to nearshore sediment deficits resulting from a ebb jets of the confined entrances pushing the previously large shallow ebb tidal deltas offshore into deeper water and b waves dispersing the nearshore delta flanks initially onshore and then alongshore away from the jetties this overview describes 1 the motivation for developing a system wide understanding of sediment dynamics in the littoral cell at multiple time and space scales 2 the formation and approach of the southwest washington coastal erosion study and 3 an introduction to the papers in this special issue

magma mingling in dikes and sills dikes in which two different liquids flowed simultaneously usually called composite dikes naturally fall into two classes depending on which lithology forms the contact with the country rock and hence which liquid was the first to enter the fracture in these two kinds of dikes the structures formed by the mingling of the two liquids differ dikes in which the more basic liquid entered first have contacts between the two lithologies that are nearly planar and parallel to the dike walls whereas the more basic lithology forms discrete pillows in those dikes in which the more silicic liquid entered first experiments indicate that these pillows probably form from a flow front instability that develops when a liquid invades another of higher viscosity between two parallel rigid walls we provide scalings for the critical flow rate for the onset of this instability the time required for the instability to develop and the wavelength that is selected these scalings are consistent with field observations

compositions of chromite associated minerals and parental magmas of podiform chromite deposits the role of slab contamination of asthenospheric melts in suprasubduction zone environments abstract podiform chromitites in the mantle sections of ophiolites belong to either high cr metallurgical or high al refractory varieties their highly variable compositions are reflected by different cr s 100cr cr al and cr2o3 and al2o3 contents of the chromite falling in the boninitic and morb fields respectively parental magmas of high cr chromitites have higher sc mn co and ni and lower ti v zn and ga concentrations than morb melts their trace element patterns are similar to those of boninites except for ni and zn in contrast high al chromitites have parental magmas characterized by generally flat morb normalized patterns showing slight enrichments in v mn and co and depletion in ni and zn regardless of their compositions both types of chromitites have chondrite normalized platinum group element pge patterns showing enrichment in ipge and depletion in ppge a variety of platinum group minerals are typically present in both types occurring either as euhedral inclusions or along fractures in chromite grains these minerals have a wide span of re os isotopic compositions reflecting a variety of origins there is a diversity of unusual minerals and mineral inclusions associated with podiform chromitites the presence of these minerals suggest that grains of amphibolite plagioclase amphibole and zircon and eclogite coesite kyanite and garnet were present in the magmas from which chromite crystallized multiphase mineral inclusions demonstrate that podiform chromitites form from hydrous mafic magmas in suprasubduction zone environments ssz we propose a new model in which chromitite formation was involved in intra oceanic subduction zones initiated in closing oceanic basins continued subduction carries oceanic and possibly continental crustal materials to deep levels where they are metamorphosed under greenschist amphibolite and eclogite facies conditions the tearing and breakoff of the subducted slab possibly along the transitional contact between amphibolites and eclogites create a slab window through which the underlying asthenosphere rises and melts to generate cr rich mafic magmas these upward migrating magmas pass through the subduction zone and assimilate the subducted slab as a result of slab contamination these magmas become more siliceous more oxidized and more hydrous rapidly triggering chromite crystallization minute grains of chromite are suspended in the upward moving magmas as they migrate through the overlying metasomatized mantle wedge such chromite bearing magmas eventually deposit chromite in magma conduits in the uppermost mantle close to the moho where the upward flow changes from vertical to subhorizontal and velocity is greatly reduced highly reduced and ultrahigh pressure minerals including diamonds are reported in literature both in podiform chromitites and host peridotites of ophiolites some of these minerals in association with host peridotites may have been brought by the uprising asthenosphere at mid oceanic ridges due to the mantle convection it is also possible that some diamonds may have formed in the subducted slab below about 150 km some minerals of subducted slabs are preserved because they are encapsulated in chromite grains where they are protected from the ssz melts some of these ssz mantle wedges are emplaced on land to become podiform chromitite bearing ophiolites

kinetics of crystallization of igneous rocks the geochemistry of igneous rocks is discussed with the primary objectives of bringing together the theories underlying the kinetics of crystallization of igneous rocks and illustrating the use of these theories in understanding experimental and observational data the primary purpose of the chapter is to introduce current thinking about the kinetics of igneous rocks and to provide a basis for understanding other work a basic assumption made in the discussion is that the rate of any chemical reaction including the crystallization of igneous rocks is zero at equilibrium and proceeds at a finite rate only at a finite deviation from equilibrium as such an understanding of the processes operating in igneous rocks requires an understanding of how deviation from equilibrium affects the rates and mechanisms of the processes occurring during crystallization these processes are detailed with special emphasis given to nucleation and crystal growth jmt

morphodynamics of prograding beaches a synthesis of seasonal to century scale observations of the columbia river littoral cell abstract findings from nearly two decades of research focused on the columbia river littoral cell crlc a set of rapidly prograding coastal barriers and strand plains in the u s pacific northwest are synthesized to investigate the morphodynamics associated with prograding beaches due to a large sediment supply from the columbia river the crlc is the only extensive stretch of shoreline on the u s west coast to have advanced significantly seaward during the late holocene since the last cascadia subduction zone csz earthquake in 1700 with associated co seismic subsidence and tsunami much of the crlc has prograded hundreds of meters however the rates of progradation and the processes most responsible for sediment accumulation vary depending on time scale and the morphological unit in question remarkably the 20th and early 21st century shoreline change rates were more than double the late prehistoric rates that include recovery from the last major csz event most likely due to an increase in sediment supply resulting from inlet jetty construction in some locations detailed beach morphology monitoring reveals that at interannual to decadal scale the upper shoreface aggraded about 2 cm yr subtidal sandbars migrated offshore and decayed while intertidal bars migrated onshore and welded to the shoreline the shoreline prograded about 4 m yr and 1 to 2 new foredune ridges were generated a detailed meso scale sediment budget analysis in one location within the littoral cell shows that approximately 100 m3 m yr accumulated between 12 m seaward limit of data and 9 m crest of landward most foredune gradients in alongshore sediment transport net onshore directed cross shore sediment transport within the surf zone and cross shore feeding from a shoreface out of equilibrium with forcing conditions are each partially responsible for the significant rates of sediment supplied to the beaches and dunes of the crlc during the observational period direct observations of beach progradation at seasonal to decadal scale are put in context of measured or inferred changes over time scales of decades to centuries

evidence for eight great earthquake subsidence events detected with ground penetrating radar willapa barrier washington a new approach to detect holocene subduction zone earthquakes combines the results from ground penetrating radar gpr vibracores and accelerator mass spectrometry ams dates from a barrier spit located west of willapa bay southwest washington gpr data show a 10 m thick facies of beach sand within which we identify and vibracores confirm beach parallel wave eroded buried scarps mantled with multiple beds of magnetite the eight gpr detected buried scarps are interpreted to be eroded by minor transgressions caused by instantaneous barrier subsidence during earthquakes associated with the juan de fuca plate subducting under the north american plate of these scarps four have been ams dated at 300 1110 2540 and 4250 radiocarbon yr b p no datable material has yet been found for the other four radar detected scarps but we interpolate and extrapolate dates of 1800 3400 5000 and 5800 yr b p

morphology and microstructure of chromite crystals in chromitites from the merensky reef bushveld complex south africa the merensky reef of the bushveld complex consists of two chromitite layers separated by coarse grained melanorite microstructural analysis of the chromitite layers using electron backscatter diffraction analysis ebsd high resolution x ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals fine grained idiomorphic and large silicate inclusion bearing crystals the lower chromitite layer contains both populations whereas the upper contains only fine idiomorphic grains most of the inclusion bearing chromites have characteristic amoeboidal shapes that have been previously explained as products of sintering of pre existing smaller idiomorphic crystals two possible mechanisms have been proposed for sintering of chromite crystals 1 amalgamation of a cluster of grains with the same original crystallographic orientation and 2 sintering of randomly orientated crystals followed by annealing into a single grain the ebsd data show no evidence for clusters of similarly oriented grains among the idiomorphic population nor for earlier presence of idiomorphic subgrains spatially related to inclusions and therefore are evidence against both of the proposed sintering mechanisms electron backscatter diffraction analysis maps show deformation related misorientations and curved subgrain boundaries within the large amoeboidal crystals and absence of such features in the fine grained population microstructures observed in the lower chromitite layer are interpreted as the result of deformation during compaction of the orthocumulate layers and constitute evidence for the formation of the amoeboid morphologies at an early stage of consolidation an alternative model is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals formed as a result of supercooling during emplacement of the lower chromite layer against cooler anorthosite during the magma influx that formed the merensky reef the upper chromite layer formed from a subsequent magma influx and hence lacked a mechanism to form dendritic chromite this accounts for the difference between the two layers

upper crustal podiform chromitite from the northern oman ophiolite as the stratigraphically shallowest chromitite in ophiolite and its implication for cr concentration a new type of podiform chromitite was found at wadi hilti in the northern oman ophiolite it is within a late intrusive dunite body possibly derived from olivine rich crystal mush between the sheeted dike complex and upper gabbro this chromitite forms small 30 cm in thickness pods with irregular to lenticular shapes neither layering nor graded bedding is observed within the pods the chromitite is in the upper crust by far shallower in ophiolite stratigraphy than the other podiform chromitites that have ever been found in the moho transition zone to the upper mantle it is dis tributed along a small felsic to gabbroic melt pool within the dunite body which was formed by melting of gab broic blocks captured by the mush chromian spinel was precipitated due to mixing of two kinds of melt a basaltic interstitial melt from the mush and an evolved possibly felsic melt formed by the melting of gabbro blocks the podiform chromitite reported here is strik ingly similar in petrography and spinel chemistry to the stratiform chromitite from layered intrusions the for mer contains plagioclase and clinopyroxene as matrix silicates instead of olivine as well as includes euhedral and fine spinel with solid mineral inclusions chromian spinel of the upper crustal podiform chromitite from oman has relatively low content of cr2o3 a l 2o3 the cr cr al atomic ratio of around 0 6 and the relatively high tio2 content ranging from 1 to 3 wt we conclude that assimilation of relatively si rich materials crustal rocks or mantle orthopyroxene by olivine spinel saturated melts can explain the genesis of any type of chromitite

apatite hosted melt inclusions from the panzhihua gabbroic layered intrusion associated with a giant fe ti oxide deposit in sw china insights for magma unmixing within a crystal mush the 2 km thick panzhihua gabbroic layered intrusion in sw china is unusual because it hosts a giant fe ti oxide deposit in its lower zone the deposit consists of laterally extensive net textured and massive fe ti oxide ore layers the thickest of which is 60 m to examine the magmatic processes that resulted in the fe enrichment of parental high ti basaltic magma and the formation of thick fe ti oxide ore layers we carried out a detailed study of melt inclusions in apatite from a 500 m thick profile of apatite bearing leucogabbro in the middle zone of the intrusion the apatite hosted melt inclusions are light to dark brown in color and appear as polygonal rounded oval and negative crystal shapes which range from 5 to 50 m in width and from 5 to 100 m in length they have highly variable compositions and show a large and continuous range of sio2 and feot with contrasting end members one end member being fe rich and si poor 40 2 wt feot and 17 7 wt sio2 and the other being si rich and fe poor 74 0 wt sio2 and 1 20 wt feot this range in composition may be attributed to entrapment of the melt inclusions over a range of temperature and may reflect the presence of m scale and immiscible fe rich and si rich components in different proportions simulating results for the motion of si rich droplets within a crystal mush indicate that si rich droplets would be separated from fe rich melt and migrate upward due to density differences in the interstitial liquid when the magma unmixed migration of the si rich immiscible liquid component from the interstitial liquid caused the remaining fe rich melt in the lower part to react with plagioclase primocrysts an59 60 as evidenced by fine grained lamellar intergrowth of an rich plagioclase an79 84 clinopyroxene in the oxide gabbro of the lower zone therefore magma unmixing within a crystal mush combined with gravitationally driven loss of the si rich component resulted in the formation of fe rich melagabbro and major fe ti oxide ores in the lower part and si rich leucogabbro in the upper part of the intrusion

high cr and high al chromitites from the sagua de tanamo district mayari cristal ophiolitic massif eastern cuba constraints on their origin from mineralogy and geochemistry of chromian spinel and platinum group elements article i nfo several chromitite bodies of variable sizes are hosted in dunite harzburgite of the small b3 km mining district of sagua de tanamo in the mayari cristal ophiolitic massif eastern of cuba the chromian spinel in these bodies displays a large range in cr cr cr al atomic ratio from 0 74 to 0 45 in high cr chromitites cr and ti contents increase from harzburgite to dunite to chromitite a roughly opposite trend of variation in cr is observed in high al chromitites although in this case ti distributes randomly the differences in the type of chromian spinel i e high cr or high al in the chromitite coincide with a different behavior of the platinum group elements pge whereas high cr chromitites are rich in pge and contain abundant grains of platinum group minerals pgm high al chromitites are systematically poor in pge and in pgm the calculated melts in equilibrium with chromian spinel of high cr chromitite are island arc thoeliites iat with boninitic affinity whereas those in equilibrium with chromian spinel of high al chromitites are back arc basin basalts babb the formation of high cr chromitites is interpreted as a result of the extensive reaction of harzburgite with migrating island arc tholeiite melts of boninitic affinity melt rock reaction produces boninitic melts with variable composition and porous dunitic channels in which the mixing mingling of melts promotes crystallization of mononomineralic high cr chromian spinel in contrast high al chromitites formed by the mixing mingling of babb melts within conduits not in equilibrium with dunite percolation of primitive babb melts through pre existing dunite dissolved olivine producing melt conduits in which babb melts mixed and formed high al chromian spinel the higher pge and pgm in high cr chromitites is not only a function of the degree of partial melting but is linked to interaction between the migrating melt and mantle harzburgite during the formation of the chromitite the coexistence of both types of chromitites in one small mining district reflects the temporal and or spatial variations of separate melt intrusions emplaced in different subarc mantle domains during the opening of a back arc basin in a supra subduction zone environment

sea level rise and anthropogenic activities recorded in the late pleistocene holocene sedimentary infill of the guadiana estuary sw iberia abstract this study reviews data on sea level rise during the last 13000 yr cal bp 13 kyr as recorded in the estuarine sediments of the guadiana river se portugal sw spain we combined new data from a 63 m long borehole drilled through the entire postglacial sedimentary sequence with information on five previously studied cores by integrating sedimentological geochemical and palaeontological proxies we were able to make a palaeoenvironmental reconstruction of the guadiana terminal palaeovalley during the last 13 kyr and propose a curve of sea level rise for the sw iberian atlantic margin our foraminifera based palaeoecological reconstruction anchored to a 14 c age model reveals rapid sea level rise from 13 kyr interrupted during the younger dryas and resuming ca 11 5 kyr the pace of marine transgression slackened ca 7 5 kyr and since then has progressed upwards at a rate of 1 2 mm yr 1 holocene anthropocene sediments from two boreholes were also analysed to assess the timing levels and sources of trace metals produced by acid mine drainage from the iberian pyrite belt study of metal aluminium ratios through the profiles allowed background metal concentrations to be estimated from lithostratigraphic units older than ca 5 kyr i e unaffected by anthropogenic activities human activities are especially evident from 4 5 kyr the beginning of the copper age with anthropogenic sources of metal fluxes prevailing over natural sources especially pb co ni and mn and to a lesser extent zn cu and ni mining activities became particularly intensive between the late bronze age and the roman period 3 1 5 kyr when the highest metal enrichment factors were recorded ef pb 2 ef cd 10 ef cr 2 ef cu 3 ef zn 1 4 this study reveals the utility of postglacial sedimentary records for reconstructing historical changes in regional water sediment quality and separating natural and anthropogenic sources of geochemical contaminants

on the formation of peridotite derived os rich pge alloys osmium rich pt group element pge alloys occur worldwide in association with chromite in ultramafic peridotite complexes it has been suggested that these os rich alloys formed under extreme p t conditions in the lowermost mantle before the metallic core of the earth formed or later in the outer core and have been transported to the upper mantle as xenoliths in deep rooted mantle plumes

chromite and pge geochemistry of the elekdag ophiolite kastamonu northern turkey implications for deep magmatic processes in a supra subduction zone setting abstract the geochemistry of chromium spinel and platinum group elements pge provides significant information about conditions in the mantle during the emplacement and genesis of ophiolitic complexes the elekdag ophiolite is a greenschist to amphibolite facies ophiolite that was emplaced onto the sakarya zone in the central part of the pontides of northern turkey nw se trending elekdag ophiolite hosts small scale chromitite occurrences in serpentinized harzburgite and dunite and have maximum resource of several thousands t cr2o3 the chromitites the elekdag ophiolite can be divided in to high cr and high al types the calculated melt composition of the high cr chromitites is 9 4 13 2 wt al2o3 0 2 0 4 wt tio2 with feo mgo ratios of 0 4 1 9 which are indicative of boninitic compositions however the high al chromitites have 14 7 17 3 wt al2o3 and 0 3 0 8 wt tio2 with feo mgo ratios of between 0 3 and 0 8 which are consistent with morb like tholeiitic affinities the total pge content of the chromitites range between 82 and 1466 ppb and the average pge content of the high cr and high al chromitites are 267 ppb and 195 ppb respectively the chromitites are characterized by low pd ir ratios of 0 02 0 20 that is characteristic of non fractionated chromitites the chondrite normalized pge pattern of the chromitites resemble those of mantle derived cr rich chromitites in ophiolites of the supra subduction zone ssz elsewhere mineral and bulk rock pge data for the elekdag chromitites indicate that high cr chromitites formed from cr rich and al and ti poor boninitic melts most probably in the fore arc setting in the deeper part of the mantle this was succeeded by the mineralization of high al chromitites from a differentiated boninitic melt at shallow depths close to the moho discontinuity

effects of magnetite on high frequency ground penetrating radar large concentrations of magnetite in sedimentary deposits and soils with igneous parent material have been reported to affect geophysical sensor performance we have undertaken the first systematic experimental effort to understand the effects of magnetite for ground penetrating radar gpr characterization of the shallow subsurface laboratory experiments were conducted to study how homogeneous magnetite sand mixtures and magnetite concentrated in layers affect the propagation behavior velocity attenuation of high frequency gpr waves and the reflection characteristics of a buried target important observations were that magnetite had a strong effect on signal velocity and reflection at magnitudes comparable to what has been observed in small scale laboratory experiments that measured electromagnetic properties of magnetite silica mixtures magnetite also altered signal attenuation and affected the reflection characteristics of buried targets our results indicated important implications for several fields including land mine detection martian exploration engineering and moisture mapping using satellite remote sensing and radiometers

historical evolution of the columbia river littoral cell abstract this paper details the historical coastal evolution of the columbia river littoral cell in the pacific northwest of the united states geological data from a d 1700 and records leading up to the late 1800s provide insights to the natural system dynamics prior to significant human intervention most notably jetty construction between 1885 and 1917 all reliable surveys charts and aerial photos are used to quantify decadal scale changes at the three estuary entrances and four sub cells of the littoral cell shoreline bathymetric and topographic change over three historical intervals 1870s 1920s 1920s 1950s and 1950s 1990s are integrated to provide an understanding of sediment sharing relationships among the littoral cell components regional morphological change data are developed for alongshore segments of approximately 5 km enabling comparisons of shoreline change to upper shoreface and barrier volume change within common compartments the construction of entrance jetties at the columbia river 1885 1917 and grays harbor 1898 1916 has profoundly affected the evolution of the littoral cell and has accentuated the morphological coupling between the inlets ebb tidal deltas shorefaces and barriers the jetties induced erosion of the inlets and offshore migration of ebb tidal deltas the change in boundary conditions at the entrances enabled waves to rework the flanks of ebb tidal deltas and supply enormous quantities of sand to the adjacent coasts over several decades the initial sand pulses have been dispersed alongshore up to tens of kilometers from the estuary entrances winter waves and coastal currents produce net northward sediment transport across the shoreface while summer conditions tend to induce onshore sediment transport and accumulation of the upper shoreface and barriers at relatively high rates historical shoreline progradation rates since jetty construction are approximately double the late prehistoric rates between 1700 and the 1870s erosion rates of the mid to lower shoreface to the south of the jettied estuary entrances have typically been greater than the accumulation rates of the upper shoreface and barrier suggesting that the lower shoreface has been an important source of littoral sediments over decadal and longer time scales until recent decades sediment supply from the ebb tidal delta flanks and lower shoreface has largely masked the decline in columbia river sediment supply resulting from flow regulation and dredging disposal practices with the contemporary onset and expansion of coastal erosion adjacent to the jettied estuary entrances proper management of dredged sediment is imperative to mitigate the effects of a declining sediment budget

noble metal enrichment processes in the merensky reef bushveld complex we have analysed sulphides silicates and chromites of the merensky introduction reef for platinum group elements pges re and au using laser the enrichment of noble metals in layered gabbroic ablation inductively coupled plasma mass spectrometry and synthetic intrusions to form stratiform merensky type ore horizons pyrrhotite standards annealed with known quantities of noble metals is an extremely efficient yet poorly understood process os ir and ru reside in solid solution in pyrrhotite and pentlandite metal enrichment must be related to the formation of rh and part of the reef s pd in pentlandite whereas pt au re magmatic layering as the ore horizons form an integral and some pd form discrete phases olivine and chromite often part of the magmatic stratigraphies of their host insuspected to carry os ir and ru are pge free all phases analysed trusions metal concentration is also related to the prescontain noble metals as discrete micro inclusions with diameters ence of magmatic sulphides because the noble metals are either dissolved in base metal sulphides or occur as typically 100 nm inclusions in sulphides commonly have the discrete noble mineral phases intergrown with sulphides element combinations os ir pt and pt pd au inclusions in vermaak hendriks 1976 ballhaus ryan 1995 olivine and chromite are dominated by pt au pd few inclusion ballhaus ulmer 1995 spectra can be related to discrete noble metal phases and few there is considerable controversy as to where the inclusions have formed by sub solidus exsolution rather some metals came from and how metal enrichment worked pge inclusions notably those in olivine and chromite are earlyfor a stratiform platinum group element pge deposit magmatic nuggets trapped when their host phases crystallized we inside the magmatic stratigraphy of a layered intrusion suggest that the silicate melt layer that preceded the merensky reef there are two possibilities either the metals were derived was pge oversaturated at early cumulus times experiments from the melt column above the ore horizon or they came combined with available sulphide silicate partition coefficients sugfrom the magma equivalent to the cumulate package gest that a silicate melt in equilibrium with a sulphide melt underlying the ore owing to the large density contrast containing the pge spectrum of the merensky ore would indeed be between silicate and sulphide melt a popular view is that oversaturated with respect to the least soluble noble metals sulphide the metals were enriched by gravitational settling of melt apparently played little role in enriching the noble metals in sulphide melt i e out of the magma column above the the merensky reef rather its role was to immobilize a pre existing ore campbell et al 1983 barnes naldrett 1985 in situ stratiform pge anomaly in the liquid stratified magma naldrett et al 1987 on the other hand it is well chamber established that gravitational crystal settling plays little role in producing magmatic layering and magmatic stratigraphies mcbirney noyes 1979 morse 1986 campbell 1987 and this cannot remain without consequence for stratiform noble metal deposits a third alternative is noble metal enrichment by late

platinum group element pge mineralisation and chromite geochemistry in the berit ophiolite elbistan kahramanmaras se turkey abstract the high al chromitite bearing stratigraphic levels cr ranges from 0 38 to 0 47 are located at the mantle crust transition zone mtz and in the cumulate dunites in the berit ophiolite in se turkey iridium group elements ipge ir os and ru and related mineralisation were previously reported from ophiolitic high cr chromitites in the berit elbistan area palladium group elements ppge pt pd and rh and associated mineralisation are predominantly found in the high al chromitites and they are presented in this study for the first time our results indicate that ipge enriched phases of the pgms such as laurite are present in both the ppge enriched and the ppge poor chromitites in berit the high al chromitite chondrite normalised patterns in the berit ophiolite show an enrichment of ppge with values of up to 1700 ppb pt and up to 4469 ppb pd during their occurrence sulphur saturation is consistent with the production of minor quantities of base metal sulphides bms which took place in the form of droplets enclosed in chromite commonly with ppge enriched patterns these ppgm assemblages include pd pt tellurides sperrylite platarsite pd antimonide pd sb as and pt pd cu au alloys the ppge enrichment in the berit ophiolite is unusual and the presence of such occurrences is known only in a few other ophiolites worldwide the geochemical evolution of the high cr ratios of the high cr chromitites indicates an origin due to high degrees of partial melting whereas the high al chromitites indicate lower degrees of partial melting both high degree and low degree melts may have been fluxed through the upper mantle either synchronously or possibly with a hiatus between the generation of different melts

significance of chromian spinels from the mantle sequence of the andaman ophiolite india paleogeodynamic implications abstract the mantle section of the andaman ophiolite between rutland island in the south and north andaman in the north has been studied the restitic peridotite of rutland island is comprised of depleted harzburgite to clinopyroxene bearing harzburgite whereas that in middle and north andaman is mostly less depleted lherzolite dominated mantle chromian spinels from the mantle section of the andaman ophiolite belong to four major groups group 1 spinels from rutland chromitite pods characterized by high cr cr cr a1 atomic ratio 0 71 group 2 spinels from north andaman chromitite pods with two subgroups 2a characterized by high cr 0 70 0 79 and 2b characterized by medium cr 0 48 0 51 and high mg mg mg fe 2 atomic ratio 0 74 0 77 group 3 residual accessory grains from rutland peridotite characterized by medium cr 0 37 0 55 and low mg 0 41 0 54 and group 4 residual accessory grains from middle and north andaman peridotites characterized by low cr 0 09 0 23 and high mg 0 71 0 81 group 1 chromian spinels of rutland island and group 2a of north andaman are likely to have crystallized from boninitic melts formed by relatively high degrees of partial melting whereas group 2b varieties from north andaman are from tholeiitic melts formed by lower degrees of melting the chemistry of the residual accessory chromian spinels groups 3 and 4 suggests that the mantle peridotites of rutland island towards the south are similar to fore arc peridotites of suprasubduction zone environments whereas those of north andaman are less depleted this spatial and or temporal directional change in spinel compositions may reflect variations linked to the melting history where the same sliver of oceanic mantle underwent different styles of melting in different tectonic settings at different points in time the coexistence of both chromitite types with high and low cr spinels in the same area from north andaman possibly reflects temporal variations of separate melt intrusions produced through specific melting stages it is inferred that a similar geodynamic setting to the present day also existed during the cretaceous period with a fore arc at rutland island and a back arc towards the north at north andaman

late holocene shoreline responses to competing shelf bay and beach accommodation spaces under conditions of relative sea level change and the potential for future catastrophic beach retreat in the columbia river littoral cell washington and oregon usa abstract the columbia river littoral cell crlc 160 km in length provides opportunities to compare competing accommodation space relations under different conditions of relative sea level change the crlc system includes abundant littoral sand supply from the large columbia river late holocene prograded beach plains and barrier spits 0 5 5 km in width two large marine dominated estuaries willapa bay and grays harbor and a high wave energy inner shelf littoral sand accumulation rates in prograded beach plains and barrier deposits are based on paleo shoreline positions that are dated by great earthquake catastrophic beach retreat scarps n 10 from 0 3 to 5 0 ka the retreat scarp timelines are mapped in across shore gpr transects n 79 yielding timeline bounded polygons n 247 the polygons are evaluated for littoral sand volume and bounding ages yielding volume accretion rates m3 ka 1 which are summed for the four crlc subcells clatsop plains 27 4 106 m3 ka 1 long beach 19 8 106 m3 ka 1 grayland plains 15 9 106 m3 ka 1 north beaches 11 7 106 m3 ka 1 major submarine sinks of littoral sand including the inner shelf and large marine dominated estuaries are evaluated for increased littoral sand accommodation space that could result from potential future sea level rise of 1 2 and 3 m during the next century or two the estimated beach and nearshore sand erosion needed to fill the increased submarine accommodation space from a 2 0 m rise in relative sea level would result in averaged beach retreat values of 0 7 km clatsop plains 1 2 km long beach and 1 3 km grayland plains about 30 50 larger than previous estimates based on the brunn method these catastrophic shoreline retreat distances 0 7 1 3 km represent 25 50 of the present widths of prograded barrier spits and beach plains they serve as warnings about future catastrophic beach erosion resulting from potential future slr in other similar barrier spit and beach plain shorelines worldwide

origin of podiform chromite deposits by magma mingling abstract i have oversaturated the immiscible system sio2 al2o3 feo k2o with fecr2o4 to study the fractionation of chromite between conjugate siliceous and fayalitic melts although both melts are oversaturated with chromite chromite only nucleates in the fayalite rich melt where the crystal melt interfacial energy sl is lowest in less than three days at 1150 c all excess chromium in the bulk composition is concentrated as cumulus chromite in the fayalitic melt pools producing nodular cumulate textures as observed in podiform chromite ore when these results are applied to the lithospheric mantle podiform chromite deposits may form when droplets of olivine normative high pressure low viscosity melt mingle with siliceous low pressure more viscous melt in shallow magma conduits it is inferred from the experimental analogue that chromite only nucleates in the melt where sl is lowest which in the mantle would be the high pressure melt the low pressure melt only serves as the diffusive chromium reservoir the richest chromite ores form when the volume ratio of the melt nucleating chromite is small relative to the ambient melt

time and size of a giant earthquake in cascadia inferred from japanese tsunami records of january 1700 geological evidence shows that great earthquakes have occurred in the recent prehistoric past in the cascadia subduction zone off the pacific coast of north america the most recent event or series of events is dated at about 300 years ago1 4 but the precise date and magnitude have not been determined geological investigations have not been able to distinguish a single giant earthquake from a series of great earthquakes occurring over a timespan of a decade or two4 although this information is important for the assessment of future hazard5 we have found several tsunami records in japan from ad 1700 with no indication of a local cause historical earthquake records and palaeoseismic evidence indicate the absence of a large earthquake in 1700 in south america alaska or kamchatka leaving cascadia as the most likely source of this tsunami the estimated time of the earthquake is the evening about 21 00 local time of 26 january 1700 the magnitude is estimated as 9 from the tsunami heights in which case the earthquake ruptured the entire length of the cascadia subduction zone2 these estimates are consistent with native american legends that an earthquake occurred on a winter night6

modeling the effects of wave climate and sediment supply variability on large scale shoreline change abstract the application of an integrated data analysis and modeling scheme reveals that decadal scale shoreline evolution along a u s pacific northwest littoral cell is highly dependent on both sediment supply and wave climate variability in particular accurate estimates of columbia river sediment supply and sediment feeding from the lower shoreface are critical components of balancing the barrier beach sediment budget and are therefore essential to making sensible shoreline change hindcasts and forecasts a simple deterministic one line shoreline change model applied in a quasi probabilistic manner enables evaluation of the influence of sediment supply and wave climate variability through simulation of historical shoreline change through iteration a range of realistic scenarios are developed to constrain decadal scale shoreline change predictions modeled shoreline changes are significantly sensitive to directional changes in the incident waves and therefore sensitive to the occurrence of interannual climatic fluctuations such as major el nino events a predicted increase in the intensity of the east pacific wave climate 1 0 m increase in significant wave height in 20 yr affects forecast shoreline positions only when this increase occurs during the winter storm season however the effect of this increase in storm power during any given year is small relative to the impact of major el nino events the model has significant skill in decadal scale hindcasts suggesting that alongshore gradients in sediment transport dominate coastal change at this scale at this site however both data and model results suggest that net onshore feeding from the lower shoreface is responsible for approximately 20 of the decadal scale coastal change field measurements and poor model skill at annual scale indicate that cross shore processes likely dominate coastal change at shorter time scales

high pressure phase transitions of natural chromitite from tibetan ophiolites abstract the podiform chromitites in the tibetan ophiolites have been thought to have experienced metamorphism corresponding to depths at the top of the mantle transition zone mtz however little is known about the phase relations of natural podiform chromitite in the mtz as well as the implications for processes related to formation of the ultra high pressure uhp chromitite an experimental study is presented to constrain the phase relations of natural chromitite at pressures and temperatures of 11 18 gpa and 1400 1600 c the experimental results show that the chromitite transforms into phase assemblages composed of chromite eskolaite garnet for pressures at the base of the upper mantle and chromite eskolaite garnet mc phase at the top of the mtz chromitite is replaced by a cati2o4 structured phase ct phase eskolaite garnet in the middle of the mtz the mc phase has an average composition of mg fe 3 cr al 2 2x 4 3y mg fe xsix y o6 x 0 2 y 0 3 with a cubic space group of fd3m it is suggested that the mc phase is a new silicon bearingmg and cr rich phase with idealized stoichiometry of mg3cr2o6 the observation of the mc phase in a natural rock system is helpful for understanding the incorporation of mg and cr minerals in the mtz and helps to constrain the related geological processes that controlled the formation of the tibetan chromitites

exploring linkages between coastal progradation rates and the el nino southern oscillation southwest washington usa 1 climate oscillations such as the el nino southern oscillation enso affect storm tracks wave climate precipitation and sea level in the u s pacific northwest the impacts of these changes on coastal behavior have not been investigated in detail beyond the study of recent el nino events largely because existing historical records of coastal behavior are not of sufficient resolution to study annual responses to climatic forcing we compare a newly developed annual record of coastal progradation for a location on the washington coast generated using high resolution subsurface ground penetrating radar gpr with enso indices this analysis reveals higher rates of seaward coastal growth following the warm el nino enso phase and lower rates of coastal growth following the cold la nina enso phase the observed relationship between enso and progradation although weak is hypothesized to result from differences in sediment transport patterns and beach recovery rates following el nino and la nina events

spinel compositions of mantle hosted chromitite from the eastern anatolian ophiolite body turkey implications for deep and shallow magmatic processes abstract the eastern anatolian accretionary complex includes several ophiolitic megablocks and or tectonic slivers within a 150 180 km long east west trending complex formed during the late cretaceous tertiary in eastern turkey the alabayir mollatopuz yukaribalcikli and mehmetalan ophiolites are megablocks or tectonic slivers containing locally massive nodular or schlieren banded chromitite layers these podiform chromitites formed in mantle sequences and are classified as high cr chromitites cr 0 63 0 88 mg 0 50 0 64 0 01 0 5 wt tio 2 5 7 18 8 wt al 2 o 3 calculated parental melt compositions of these chromitites indicate boninite magma characteristics 8 2 13 4 wt al 2 o 3 0 64 1 50 feo mgo the pge patterns also support the view that they crystallized from a boninitic melt the total pge contents of chromitites vary between 79 and 390 ppb pd ir ratios of chromitites vary from 0 07 to 0 28 and are consistent with an ipge fractionated nature the examined chromitites were divided into two groups in terms of their mineral chemistry group i alabayir mollatopuz yukaribalcikli and mehmetalan i chromitites group ii mehmetalan ii chromitites mineral chemistry of these chromitites and their parental melt composition indicate that group i chromitites were probably formed at shallow mantle depths and that group ii chromitites formed in deeper parts of the mantle mineral chemistry of these eastern anatolia high cr chromitites indicate that they formed not only in the deeper mantle but also in shallower parts of the moho transition zone