sills associated with the bushveld complex south africa an estimate of the parental magma composition abstract four distinct ages of sills intrude the floor rocks under the bushveld complex the first is a metadolerite and pre dates the bushveld complex the middle two are generally related to the complex itself one has quench textured olivine and orthopyroxene crystals and chemically equates to the parental magma of the bushveld complex the second suite is a hypersthene microgabbro representative of the second major injection of magma into the complex the last is doleritic and does not obviously correlate with any portion of the layered sequence the sills genetically related to the complex display several features 1 lack of chilled margins coarse grain size ability to melt country rock xenoliths extreme heterogeneity 2 atypical of normal sills but explicable in their present context

spinels and mg ilmenites from the noril sk 1 and talnakh intrusions and other mafic rocks of the siberian flood basalt province spinels from the mineralized noril sk 1 and talnakh intrusions show a broad and distinctive compositional spectrum from normal chromite to ti enriched aluminous magnetite in the talnakh intrusion in particular the range of spinel compositions extends across a compositional space where spinels are rare in other layered intrusions spinel compositions vary widely on a scale of individual thin sections and are influenced by the nature of the enclosing silicate phase spinels enclosed in olivines are less ti enriched than those outside olivine grains compositional variance in the spinels is attributed to variable but generally advanced reaction between cumulus chromite grains and solidifying trapped intercumulus liquid the advanced nature of the process is attributed to the open nature of the intrusions continuous prolonged flow through of hot magma enabled cumulates within the intrusions to remain above their solidus temperature for longer than would be normal for sills of comparable thickness the barren chibacheka river intrusion shows some of the same features but differs from talnakh in that spinel olivine pairs record equilibration temperatures higher than those at talnakh temperatures recorded by spinel olivine pairs at talnakh are anomalously low for such a small intrusion again indicating prolonged slow cooling consistent with a flow through model for the sill spinel compositions support a hypothesized comagmatic relationship between the chibacheka river intrusion and the ti cr rich gudchichinsky picritic basalts spinels in the noril sk and talnakh intrusions imply a parent magma with a lower cr al ratio than the gudchichinsky picritic basalts spinels from the siberian intrusions show a similar range in compositions to the feeder intrusions of the karoo flood basalt province jinchuan spinels share the features of high ti contents and anomalously low equilibration temperatures with the noril sk talnakh spinels mg ilmenites are a feature of the siberian and jinchuan intrusions they are attributed to crystallization of ilmenite from evolved trapped intercumulus liquid buffered by surrounding mg rich cumulus phases their existence implies moderately but not unusually ti rich parent magmas consistent with compositions of the siberian traps lavas

cosmogenic 3he production rates from 39 n to 46 n latitude western usa and france the in situ production rate of cosmogenic 3he was determined in seven different olivine or pyroxene bearing rocks between the latitudes 39 n and 46 n the integrated production rate over the last 17 500 years is about 115 4 atoms gram yr for high latitude at sea level the variation in the integrated production rate between 2000 and 18 000 years is less than 30 and is probably on the order of 20 or less

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

experimental data at 3 kbars pressure on parental magma to the bushveld complex experimental studies mainly under 3 kbars pressure have been undertaken on representative samples to determine if any of these compositions could be parental magma to the bushveld complex one such composition with 12 5 mgo mg mg fe of 0 72 and quartz normative crystallizes olivine fo88 as liquidus mineral at about 1 300 c followed at only slightly lower temperature by orthopyroxene at 3 kbars pressure there is a temperature drop of over 100 c before the appearance of plagioclase and finally clinopyroxene this crystallization sequence is in excellent agreement with the observed sequence in the lower part of the bushveld complex

rates and timing of earth surface processes from in situ produced cosmogenic be 10 beryllium 10 is the longest lived of the seven known unstable isotopes of be it results mostly from the interaction of cosmic radiation primarily neutrons with a variety of target atoms by spallation the splitting of nuclei lal 1988 it can also be produced at very low levels by radio disintegration of u and th sharma and middleton 1989 although all radioactive be isotopes are produced in the atmosphere via cosmic ray reactions arnold 1956 lal and peters 1967 morris et al chapter 5 this volume in this paper we are most interested in 10be produced in rock which we refer to as in situ produced such in situ produced 10be is a cosmic ray dosimeter functioning as a quantitative monitor of near surface residence time in some geologic situations 10be can be used as a chronometer allowing one to estimate the duration of surface exposure in other settings 10be can be used as a tracer allowing one to estimate the rate distribution and behavior of earth surface processes rates of 10be production are very low in rocks exposed at earth s surface 102 atoms g 1 y 1 resulting in typical in situ produced 10be concentrations termed activities of 103 to 107 atoms g 1 the development of accelerator mass spectrometry ams in the 1980s allowed routine measurement of 10be at levels typical of rocks exposed near earth s surface elmore and phillips 1987 analytical refinements in the 1990s lowered detection limits by an order of magnitude allowing dating of exposure periods 103 years in favorable geologic and geographic situations simple exposure histories at high altitude and latitude most in situ 10be is measured in quartz mineral separates where the primary spallation target is o lal and arnold 1985

geology of the cenozoic namakwa sands heavy mineral deposit west coast of south africa a world class resource of titanium and zircon exploration for heavy minerals along the coastal strip of southwest africa led to the discovery and subsequent delineation of the namakwa sands deposit near brand se baai in 1987 currently tronox limited controls the namakwa sands operation which has developed into a significant global producer of titanium slag pig iron ceramics quality zircon and rutile concentrates the namakwa sands deposit is classified as a megaresource with 1 17 billion tons bt of in situ ore containing an estimated 92 6 million metric tons mt of total heavy minerals and the equivalent of 23 9 mt tio2 units and 9 4 mt zircon the siliciclastic arenaceous deposit is divided into two adjacent orebodies referred to as graauwduinen west and graauwduinen east the graauwduinen west orebody consists of three strandline dune couplets set in a transitional shallow marine aeolian environment whereas the graauwduinen east orebody comprises a largely unrelated dune deposit superimposed onto the ore bearing sequence is a pseudostratigraphy of duricrust that effectively cemented the bulk of the mineralized sands to various degrees of hardness the two orebodies are profoundly different in many aspects spatially the planar mineralization bears northeasterly in the graauwduinen west orebody strikingly perpendicular to the southeasterly trend observed in the graauwduinen east orebody contrasts in their bulk chemistry and mineralogy are statistically significant the graauwduinen west orebody features noticeably better total heavy mineral grades accompanied by conspicuously high proportions of the gangue minerals garnet and pyroxene the valuable mineral fraction is predominated by ilmenite fetio3 followed in abundance by zircon zrsio4 leucoxene an alteration product of ilmenite and rutile tio2 their geochemistry mineralogy and petrography are extremely variable but the bulk of the current valuable mineral fraction is recoverable to premium grade products the proximal mesoproterozoic namaqualand metamorphic province is considered the main contributor to the current heavy mineral population the graauwduinen west orebody received source sediment via established fluvial marine courses whereas the source of the graauwduinen east orebody is considered to originate from a distinct fluvial aeolian corridor the genesis of the namakwa sands deposit is strongly entrenched in the geomorphic evolution of the west coast of southern africa since the cenozoic which was driven by regional tectonism fluctuating sea levels and alternating climatic patterns

a cosmic trip 25 years of cosmogenic nuclides in geology terrestrial cosmogenic nuclides produced by secondary cosmic ray interactions in the atmosphere and in situ within minerals in the shallow lithosphere are widely used to date surface exposure of rocks and sediments to estimate erosion and weathering rates and to date sediment deposition or burial their use has transformed geomorphology and quaternary geology for the first time allowing landforms to be dated and denudation rates to be measured over soil forming time scales the application of cosmogenic nuclides to geology began soon after the invention of accelerator mass spectrometry ams in 1977 and increased dramatically with the measurement of in situ produced nuclides in mineral grains near earth s surface in the 1980s the past 25 yr have witnessed the development of cosmogenic nuclides from their initial detection to their prevalence today as a standard geochronological and geochemical tool this review covers the major developments of the past 25 yr by comparing the state of the field in 1988 with that of today and by identifying key advances in that period that moved the field forward we emphasize the most commonly used in situ produced nuclides measured by ams for geological applications but we also discuss other nuclides where their applications overlap our review covers ams instrumentation cosmogenic nuclide production rates the methods of surface exposure dating measurement of erosion and weathering and burial dating and meteoric 10 be in memoriam devendra lal 1929 2012 whose vision inspired the field

convection generated high frequency gravity waves as observed by mst radar and simulated by wrf model over the indian tropical station of gadanki using a mesosphere stratosphere troposphere mst radar operating at 53 mhz the present work reports that during the development of convection over the indian tropical station of gadanki at 16 38 17 30 ut universal time on 06 june 2011 all these three source mechanisms played important roles in determining the characteristics of high frequency internal atmospheric gravity waves generated 1 latent heating of convection 2 mechanical oscillation of up and down drafts associated with convection and 3 obstacle effect of wind flows over convective towers near the tropopause level in general it is found that while the depth of latent heating determines the vertical wavelength of gravity waves the oscillation frequency of up and down drafts determines the observed frequency of waves from the study of vertical structure of vertical wind velocities and phases of three waves 13 17 and 26 min generated during this event it is observed that while the 13 min and 26 min oscillations are associated with mechanical oscillator mechanism the 17 min oscillation is associated with obstacle effect analyses fourier morlet wavelet transforms and maximum entropy method mem of all the three components of wind velocities measured by the mst radar show that there is a clear association of gravity waves generated with convection and the vertical propagation characteristics of the gravity waves are found to be in good agreement with theoretical expectations with the back ground atmospheric information obtained by using the data of gps radiosondes era interim and ncep ncar reanalyses high resolution wrf model simulations support the present observations that in turn will help in a large way to the progress of parameterization of convection generated high frequency gravity waves in general circulation models the present work also finds that water molecules induced distinct atmospheric polarized refractive index structures are in existence which is in accordance with an earlier report on this subject

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

sorption and diffusion of helium in garnet implications for volatile tracing and dating abstract we present data on he sorption in garnet obtained by loading garnet samples with he at 250 10 bar in the temperature range 800 990 c for this temperature range we find a three fold decrease in he sorption from 2 1 10 5 800 c to 0 72 10 5 cm 3 stpg 1 bar 1 990 c we use the sorption data of partially saturated samples to obtain the activation energy e a 660 120 190 kj mol and frequency factor d 0 10 15 6 8 5 2 cm 2 s for he diffusion in garnet the results of our study suggest that garnet is probably one of the most retentive silicate minerals for he which is in agreement with its very low ion porosity the high retentivity results in a high closure temperature t c for the u th he system for cooling rates as low as 0 1 1 c ma t c lies between 590 and 630 c accordingly u th he dating of garnet may provide information about the medium to high temperature cooling path of a rock at the lower end of the temperature range of currently used isotopic systems in garnet in addition the high retentivity for he makes garnet a potentially useful mineral in the field of exposure age dating currently it is the only known common silicate mineral occurring in upper crustal rocks which probably retains cosmogenic noble gases under all climatic conditions for geochemical studies using noble gases as tracers for geofluids garnet is a perfect container if temperatures never exceed 600 c that is all originally trapped noble gases are quantitatively retained

diffusion of cosmogenic3he in olivine and quartz implications for surface exposure dating abstract thein situ production of3he in surface rocks by cosmic ray induced nuclear reactions offers an important geochronological tool to evaluate helium loss problems in this technique cosmogenic3he diffusivities were measured in quartz and olivine by incremental heating at 150 600 c arrhenius temperature dependences were observed in both minerals with similar activation energies ea 25 4 kcal mole logd0 3 7 0 9 in olivine andea 25 2 0 9 logd0 0 2 0 4 in quartz and imply very low diffusivities when extrapolated to environmental temperatures less than 10 18 cm2 s in quartz and 10 22 cm2 s in olivine at 20 c these low diffusivities suggest helium loss will not significantly affect cosmogenic helium exposure dating for time scales on the order of 106 years in quartz and 109 years in olivine provided large 2 mm sample grains are used exposure ages obtained with smaller grains need to be corrected for diffusive helium loss and equations are provided for this purpose cosmogenic3he diffuses more than 100 times faster than trapped magmatic3he in olivine or radiogenic4he in quartz the mechanisms responsible for these differences have yet to be determined but may involve both trapping processes at mineral defects and enhanced mobility associated with spallation produced crystal damage therefore it is possible that helium diffusivities may depend on a sample s cosmic ray exposure or radiogenic production history whatever the precise mechanism the higher mobility of3he suggests that incremental heating will be useful in separating cosmogenic helium from magmatic or radiogenic helium in future studies comparison of the diffusivity results with total cosmogenic3he contents of different quartz size fractions for an antarctic quartzite suggests that he loss may have occurred more rapidly than predicted by the laboratory measurements nonetheless retention of cosmogenic3he is quite high equivalent to more than a million years of exposure and cosmogenic3he geochronology is feasible in this and other quartz containing rocks

a new look at the super outbreak of tornadoes on 3 4 april 1974 abstract the outbreak of tornadoes from the mississippi river to just east of the appalachian mountains on 2 5 april 1974 is analyzed using conventional techniques and the pennsylvania state university national center for atmospheric research fifth generation mesoscale model mm5 the mm5 was run for 48 h using the ncep ncar reanalysis dataset for initial conditions it is suggested that the first damaging squall line within the storm of 2 5 april 1974 herein referred to as the super outbreak storm was initiated by updrafts associated with an undular bore the bore resulted from the forward advance of a pacific cold front into a stable air mass the second major squall line within the super outbreak storm which produced the strongest and most numerous tornadoes was directly connected with the lifting associated with a cold front aloft this second squall line was located along the farthest forward protrusion of a pacific cold front as it occluded with a lee trough dryline an important factor in the

experimental study on the effects of h 2 o upon crystallization in the lower and critical zones of the bushveld complex with an emphasis on chromitite formation mafic ultramafic layered rocks of the lower and critical zones of the bushveld complex are believed to have formed from a parental magma usually labelled b1 with sio2 content at 55 wt mgo at about 12 wt and cr2o3 concentration at 0 15 wt previous experimental study at 300 mpa and nominally dry conditions showed that this composition had orthopyroxene on liquidus between 1250 and 1300 c and the mineral was joined by plagioclase and clinopyroxene at and below 1166 c the objective of this study was to investigate the effects of variable additions of h2o 2 4 and 6 wt in starting compositions on liquidus and sub liquidus phase equilibria main attention was given to the solubility of cr spinel and cr distribution between melt and crystalline phases six experiments were carried out in the internally heated pressure vessel ihpv at 300 mpa and in the temperature interval 1100 1300 c redox conditions were not controlled and thermal dissociation of the h2o component followed by hydrogen diffusion through container walls resulted in a noticeable oxidation of run products the effective values of fo2 in experiments were calculated using spinmelt 2 0 software bases on the regression of experimental data on the spinel melt equilibrium according to the calculations h2o additions in isothermal series of experiments increased fo2 by 0 8 1 6 log units from the level in a nominally dry run overall the experiments showed that h2o additions suppressed crystallization of silicates shifted olivine orthopyroxene peritectic equilibrium towards higher olivine stability olivine first appears in h2o bearing compositions at 1175 c and increased the proportion of magnetite component in spinel solid solution the addition of 4 wt h2o to the starting composition at 1200 c resulted in complete melting of orthopyroxene which left cr spinel as the only liquidus phase equal amount of h2o at 1125 c leads to complete melting of plagioclase orthopyroxene and clinopyroxene which leaves only olivine and spinel stable and increases the liquid mass fraction from 0 62 in the nominally dry compositions to 0 9 the results of this study appear to support the idea that chromitite layers in the critical zone of the bushveld complex were formed by episodes of hydration melting of silicate cumulates

a case study of the interaction of a mesoscale gravity wave with a mesoscale convective system abstractthis study documents the high amplitude mesoscale gravity wave mgw event of 7 march 2008 in which two mgws strongly impacted the sensible weather over a large portion of the southeast united states these mgws exhibited starkly contrasting character despite propagating within similar environments the primary i e long lived mgw was manifest by a solitary wave of depression associated with rapid sinking motion and adiabatic warming while the secondary short lived mgw was manifest by a solitary wave of elevation mgwel dominated by rising motion and moist adiabatic cooling genesis of the primary mgw occurred as a strong cold front arrived at the foot of mexico s high terrain and perturbed the appreciable overriding flow the resulting gravity wave became ducted in the presence of a low level frontal stable layer and caused surface pressure falls up to 4 hpa the mgw later amplified as it became coupled with a stratiform precipitation system which led to its evolution into an intense

cosmogenic 3he in terrestrial rocks the summit lavas of maui we have identified terrestrial cosmic rayproduced 3he in three lava flows on the crest of haleakala volcano on maui 3 km above sea level and 0 5 million years old although these lavas like all oceanic basalts contain primordial 3he from the mantle the cosmogenic component 3hec can be identified unambiguously because it is extractable only by high temperature vacuum fusion in contrast a large fraction of the mantle helium resides in fluid inclusions and can be extracted by vacuum crushing leaving a residual component with 3he 4he ratios as high as 75 those in the atmosphere which can be liberated by melting the crushed grains cosmogenic 3he is present in both olivines and clinopyroxenes at 0 8 1 2 10 12 ml stp g and constitutes 75 5 of the total 3he present the observed 3hec levels require a cosmic ray exposure age of only some 64 000 years much less than the actual age of the lavas if there is no erosion using a model that includes effects of uplift or submergence as well as erosion we calculate an apparent erosion rate of the order of 8 5 m 106 years for the western rim of the summit crater as an example of the application of measurements of cosmogenic rare gases to terrestrial geological problems

the distribution of platinum group elements in the insizwa lobe mount ayliff complex south africa implications for ni cu pge sulfide exploration in the karoo igneous province the mount ayliff complex of the eastern cape province of south africa is a layered intrusion of some 800 km2 surface area and up to 1 200 m thickness on the basis of compositional similarities and spatial association it is generally interpreted to form part of the karoo igneous province similarities between the mount ayliff complex and the staging chambers and feeder conduits to flood basalts that host magmatic sulfide ores elsewhere in the world suggest that the mount ayliff complex may have an enhanced potential for noril sk talnakh type massive ni cu sulfide ores an idea that is supported by the well known sulfide occurrence at waterfall gorge here we present major trace and noble element data for 30 samples of cumulate rocks from a continuous 1 200 m drill core through the insizwa lobe of the complex as well as six samples of a footwall sill the taylor s koppie dike that was considered a possible feeder zone to the lobe and through a massive sulfide lens in the footwall of the lobe at ndzongiseni we show that most of the insizwa cumulate rocks contain small amounts of cumulus sulfides the sulfides including those at waterfall gorge can be explained by a model in which the sulfides were segregated from magma having chilled margin composition on the basis of the composition of the chilled margins the insizwa magma contained no entrained sulfides and was depleted in platinum group elements pge although not in cu and ni before emplacement these compositional characteristics are also observed in lavas of the central karoo igneous province and suggest only limited sulfide segregation from these magmas at depth our findings thus indicate relatively little potential for economic magmatic ni cu pge sulfide deposits in the insizwa lobe

mesoscale gravity waves and their environment in the central united states during storm fest abstract in an effort to better understand mesoscale gravity waves in winter storms in the central united states their frequency of occurrence wave characteristics the general conditions under which they occur and their effects upon the weather mesoscale surface and rawinsonde data as well as radar and satellite imagery collected during the storm scale operational and research meteorology fronts and experimental system test are analyzed in addition factors affecting the ability of objective surface map analysis to properly represent the waves are investigated thirteen coherent pressure pulse events with amplitudes of 0 2 4 0 mb and periods of 1 6 h were identified in the surface pressure data during the 6 weeks of the project involving 34 of the total hours investigated a variety of wave types occurred including wavelets wave trains and singular waves the three largest amplitude events were analyzed in detail using autospectral analysis and a barnes time to space conversion objective analysis

quantifying glacial moraine age denudation and soil mixing with cosmogenic nuclide depth profiles 1 glacial boulders and soils on moraines are often dated to quantify the timing of glaciations and or rates of chemical weathering in moraine chronosequences a common assumption is that moraine crest erosion and soil mixing are unimportant however several studies suggest moraine denudation may be substantial we evaluate the magnitude of moraine denudation and soil mixing in the pinedale 21 ka and bull lake 140 ka moraines at fremont lake wyoming united states using cosmogenic nuclide depth profiles and a numerical model of moraine erosion and nuclide production and decay depth profiles indicate mixing of the surface layer from 40 to 60 cm depth age constraints from depth profile dating of cosmogenic nuclides result in ages of 17 24 ka for the pinedale moraine and 70 127 5 ka for the bull lake moraine these ages are comparable to ages based on the lal and chen approach which uses a sample from the mixed layer and the unmixed layer however the lal and chen approach is difficult to apply in real environments as the mixing depth needs to be determined independently we find our best fit model based ages for the bull lake moraine are younger than independent age constraints from boulder exposure dating due to incomplete mixing exposure age constraints from boulders are more straightforward for moraines than ages based on depth profile dating finally we find that moraine crests do significantly erode and are mixed suggesting that previous weathering and dust accumulation rate studies on moraines provide minimum estimates for these processes

ilmenite composition in the tellnes fe ti deposit sw norway fractional crystallization postcumulus evolution and ilmenite zircon relation major and trace element xrf and in situ la icp ms analyses of ilmenite in the tellnes ilmenite deposit rogaland anorthosite province sw norway constrains a two stage fractional crystallization model of a ferrodioritic fe ti p rich melt stage 1 is characterized by ilmenite plagioclase cumulates partly stored in the lower part of the ore body lower central zone lcz and stage 2 by ilmenite plagioclase orthopyroxene olivine cumulates upper central zone ucz the concentration of v and cr in ilmenite corrected for the trapped liquid effect 1 defines the cotectic proportion of ilmenite to be 17 5 wt during stage 1 and 2 implies an increase of dvilm during stage 2 most likely related to a shift in fo2 the proportion of 17 5 wt is lower than the modal proportion of ilmenite ca 50 wt in the ore body implying accumulation of ilmenite and flotation of plagioclase the fraction of residual liquid left after crystallization of tellnes cumulates is estimated at 0 6 and the flotation of plagioclase at 26 wt of the initial melt mass the increasing content of intercumulus magnetite with stratigraphic height from 0 to ca 3 wt results from differentiation of the trapped liquid towards magnetite saturation the mgo content of ilmenite 1 4 4 4 wt is much lower than the expected cumulus composition it shows extensive postcumulus re equilibration with trapped liquid and ferromagnesian silicates correlated with distance to the host anorthosite the zr content of ilmenite provided by in situ analyses is low 114 ppm and uncorrelated with stratigraphy or cr content the data demonstrate that zircon coronas observed around ilmenite formed by subsolidus exsolution of zro2 from ilmenite the u pb zircon age of 920 3 ma probably records this exsolution process

complex exposure histories of chert clasts in the late pleistocene shorelines of lake lisan southern israel activities of 26 al and 10 be in five chert clasts sampled from two beach ridges of late pleistocene lake lisan precursor of the dead sea in southern israel indicate low rates of chert bedrock erosion and complex exposure burial and by inference transport histories the chert clasts were derived from the senonian mishash formation a chert bearing chalk which is widely exposed in the nahal zin drainage basin the drainage system that supplied most of the material to the beach ridges simple exposure ages assuming only exposure at the beach ridge sampling sites range from 35 to 354 ky using the ratio 26 al 10 be total clast histories range from 0 46 to 4 3 my unrelated to the clasts current position and exposure period on the late pleistocene beach ridges 160 177 m below sea level optically stimulated luminescence dating of fine sediments from the same and nearby beach ridges yielded ages of 20 0 s 1 4 ka and 36 1 s 3 3 ka these ages are supported by the degree of soil development on the beach ridges and correspond well with previously determined ages of lake lisan which suggest that the lake reached its highest stand around 27 000 cal years bp if the clasts were exposed only once and than buried beyond the range of significant cosmogenic nuclide production then the minimum initial exposure and the total burial times before delivery to the beach ridge are in the ranges 50 1300 ky and 390 3130 ky respectively alternatively the initial cosmogenic dosing could have occurred during steady erosion of the source bedrock back calculating such rates of rock erosion suggests values between 0 4 and 12 m my 1 t he relatively long burial periods indicate extended sediment storage as colluvium on slopes and or as alluvial deposits in river terraces some clasts may have been stored for long periods in abandoned pliocene and early pleistocene routes of nahal zin to the mediterranean before being transported again back into the nahal zin drainage system and washed on to the shores of lake lisan during the late pleistocene copyright 2003 john wiley sons ltd

climate forcing of volcano lateral collapse evidence from mount etna sicily in this study we present evidence for early holocene climatic conditions providing circumstances favourable to major lateral collapse at mount etna sicily the volcano s most notable topographic feature is the valle del bove a 5 x 8 km cliff bounded amphitheatre excavated from the eastern flank of the volcano its origin due to prehistoric lateral collapse is corroborated by sturtzstrom deposits adjacent to the amphitheatre s downslope outlet but the age nature and cause of amphitheatre excavation remain matters for debate cosmogenic 3 he exposure ages determined for eroded surfaces within an abandoned watershed flanking the valle del bove support channel abandonment ca 7 5 ka bp as a consequence of its excavation in a catastrophic collapse event watershed development was largely dictated by pluvial conditions during the early holocene which are also implicated in slope failure a viable trigger is magma emplacement into rift zones in the eastern flank of a water saturated edifice leading to the development of excess pore pressures consequent reduction in sliding resistance detachment and collapse such a mechanism is presented as one potential driver of future lateral collapse in volcanic landscapes forecast to experience increased precipitation or melting of ice cover as a consequence of anthropogenic warming

accelerator mass spectrometry in geologic research the ability of accelerator mass spectrometry ams to measure very small concentrations of the nuclides 10 be 14 c 26 al 36 cl and 129 i has led to many innovative applications in geologic research to take advantage of this opportunity in the geosciences it is important to understand how ams works how these nuclides are produced and how they can be applied to geologic problems we first discuss the basics of ams explaining what gives the method its ability to count small numbers of these nuclides we review how these nuclides are produced and transported in the atmosphere hydrosphere and lithosphere we then explain the ways that ams is being used to solve a wide range of problems in geologic research by discussing specific applications in areas such as geomorphology tectonics climatology hydrology and geochronology

radiocarbon and cation radio ages for rock varnish on tioga and tahoe marainal boulders of pine creek eastern sierra nevada california and their paleoclimatic implications abstract accelerator mass spectrometry 14 c analyses of organic matter extracted from rock varnishes on morainal boulders yield limiting minimum ages for three crests of the tioga glaciation at pine creek in the eastern sierra nevada varnish started to form on boulders of the outermost tioga moraine before 19 000 yr b p and varnish originated on the innermost tioga moraine before 13 200 yr b p comparisons with lake level paleohydrological paleoecological colluvial and rock varnish micromorphological data indicate that central eastern california and western nevada experienced a moisture effective period during the late pleistocene but after the tioga maximum and perhaps as tioga glaciers receded from the mouth of pine creek canyon varnishes on tahoeage morainal boulders at pine creek have cation ratio ages of about 143 000 156 000 yr b p suggesting that the tahoe glaciation should not be correlated with oxygen isotope stage 4 in the early wisconsin but rather with stage 6 varnishes on morainal boulders of an older glaciation at pine creek are dated by cation ratio at about 182 000 187 000 yr b p

effective attenuation lengths of cosmic rays producing 10be and 26al in quartz implications for exposure age dating we have measured cosmic ray produced 10be t 1 5 my and 26al t 0 72 my as functions of depth in a core of quartz sandstone bedrock collected in south victoria land antarctica these data were used to place limits on the exposure age and erosion rate of the exposed surface and to calculate the effective attenuation lengths of cosmic rays producing these nuclides these latter results indicate that the production attenuation length for 26al is comparable to that for 10be this suggests that the production ratio 26al 10be does not vary substantially with depth in a rock and that its value at the surface of exposed rock is not strongly affected by erosional losses allowing wider use of the ratio in determining exposure ages

examination of surface exposure ages of antarctic moraines using in situ produced 10be and 26al concentrations of 10be t12 1 5 106 y and 26a1 t12 0 72 106 y have been determined by accelerator mass spectrometry ams in a suite of quartz samples taken from sandstone boulders in several moraines in arena valley a dry valley adjacent to the taylor glacier in the quatermain mountains southern victoria land east antarctica these isotopes are produced in surficial quartz by cosmic ray spallation of o and si the concentrations in these samples ranged from 6 1 105 to 3 0 107 at g 1 for 10be and from 9 4 106 to 1 2 108 at g 1 for 26a1 depending upon the extent of exposure at the surface analyses of 10be in several samples from a single moraine at altitudes ranging from 1300 to 1650 m permitted an estimation of the air attenuation pathlength of cosmic ray neutrons the value of 142 96 41 g cm 2 is consistent with other published values production rates of 17 6 4 at g 1 y 1 for 10be and 113 54 16 at g 1 y 1 for 26a1 at 1300 m and 87 s and a 26al 10be production ratio of 6 5 1 3 1 3 were calculated from the data these values correspond to sea level production rates at high geomagnetic latitude of 6 4 at g 1 y 1 and 41 7 at g 1 y 1 for 10be and 26a1 respectively consistent with determinations based on 11 ky glacially polished surfaces in the sierra nevada in california these production rates imply exposure ages for the various moraines ranging from 50 ky to 2 5 my in accordance with other geological evidence the 10be and 26al ages of these rocks compare favorably with those found using a similar dating method based on in situ production of 3he examination of the 3he concentrations in conjunction with the present data set yields an estimate of 3he production of 230 8540 at g 1 y 1 corresponding to 85 31 15 at g 1 y 1 at sea level this is lower than previously reported values of 240 60 at g 1 y 1 and 190 at g 1 y 1 scaled to sea level and high geomagnetic latitude based respectively on volcanic flows less than 2000 years old at low latitude and samples collected from 14 400 year old volcanic flows this discrepancy is likely due to diffusive losses which may become significant up to 50 for surface exposure ages on the order of 2 0 my nevertheless the general correlations among the three isotopes suggest that 3he does not suffer gross losses through diffusion

numerical simulations of a gravity wave event over ccope part iii the role of a mountain plains solenoid in the generation of the second wave episode abstract mesoscale model simulations have been performed of the second episode of gravity waves observed in great detail in previous studies on 11 12 july 1981 during the cooperative convective precipitation experiment the dominant wave simulated by the model was mechanically forced by the strong updraft associated with a mountain plains solenoid mps as this updraft impinged upon a stratified shear layer above the deep well mixed boundary layer that developed due to strong sensible heating over the absaroka mountains the gravity wave was created this wave rapidly weakened as it propagated eastward however explosive convection developed directly over the remnant gravity wave as an eastward propagating density current produced by a rainband generated within the mps leeside convergence zone merged with a westward propagating density current in eastern montana the greatly strengthened cool pool resulting from this new convection then generated a bore wave that appeared to be continuous with the move

dating geomorphologic surfaces using cosmogenic 3he cosmogenically produced 3he in rocks can be used to date geomorphologic surfaces using the late pleistocene bonneville flood event the subsequent development of the provo shoreline and associated volcanic rocks a production rate of 432 atoms g yr in olivine is determined for 38 56 n latitude present geomagnetic latitude 46 5 at 1445 m altitude integrated over the last 14 400 years measurements of cosmogenic helium in flood deposits on river scoured surfaces and lava flows show that it is possible to date directly large scale events such as volcanic eruptions and catastrophic floods olivine and pyroxene are both satisfactory minerals for cosmogenic dating by the 3he method however in most cases plagioclase and quartz are unsuitable because they do not quantitatively retain helium isotopes in their lattices

modeling of the temperature sensitivity of the apatite u th he thermochronometer apatite u th he apparent ages will generally reflect residence for extended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion to characterize the response of apatite he ages to thermal histories involving partial he retention we explored solutions to the he production diffusion equation under thermally static conditions the analytical solution to this equation coupled with published diffusivity data demonstrates that the zone of partial he retention extends from about 40 c to 85 c this zone lies at temperatures 35 c cooler than the analogous fission track partial annealing zone he ages within the partial retention zone ultimately achieve a balance between he production and loss yielding a steady state age both the ultimate age and the time it takes to achieve this age are temperature dependent for example an apatite held at 75 c equilibrates to an age of 2 ma after 17 myr regardless of whether equilibrium is approached from a higher or a lower initial he age for representative dynamic thermal histories we evaluated apatite he ages using a numerical solution to the ingrowth diffusion equation the results illustrate the sensitivity of he ages to various geologic histories and are useful for understanding he age elevation relationships and for testing time temperature paths derived from apatite fission track length distributions in addition although he diffuses rapidly from apatite at shallow crustal temperatures modeling of ambient temperature fluctuations indicates that he ages are nearly unaffected by surficial processes

the life cycle of a mesoscale gravity wave as observed by a network of doppler wind profilers abstract for the first time an analysis has been made of the evolving vertical structure of a long lived mesoscale gravity wave that exerted a strong influence upon the precipitation distribution across a large area this paper describes this gravity wave system on 14 february 1992 which was observed using a combination of a surface mesonetwork digital satellite and radar imagery and several doppler wind profilers the resulting vertical structures are compared to the predictions of linear stability theory since the signature of the gravity waves in the profiler vertical beam data was often complicated by the presence of precipitation a kinematic method was developed for estimating the vertical air motions during these periods the resultant time height fields show vertical and horizontal winds that are consistent with a gravity wave conceptual model the microbarograph traces and the cloud and precipitation patterns in the early stages of development a strong vertically erect wave of depression

rock to sediment slope to sea with 10be rates of landscape change abstract measurements of cosmogenic nuclides predominately 10be allow new insights into the ways in which and the rates at which sediment is generated transported and deposited over timescales ranging from 103 to 106 years samples from rock exposures are used to estimate erosion rates at points on the landscape whereas samples of fluvial sediment provide estimates of basin scale rates of denudation integrated over 104 km2 nuclide data show that hilltop bare rock outcrops erode more slowly than basins as a whole suggesting the potential for relief to increase over time as well drained outcrops grow higher more elaborate experiments and interpretive models provide insight into the distribution of hillslope processes including the bedrock to soil conversion rate which appears to increase under shallow soil cover and then decrease under deeper soils changes in average nuclide activity down slopes can be used to estimate grain speed over millennia suggesting for example that sediment o

beryllium 10 terrestrial cosmogenic nuclide surface exposure dating of quaternary landforms in death valley quaternary alluvial fans and shorelines spits and beach bars were dated using 10 be terrestrial cosmogenic nuclide tcn surface exposure dating in death valley the 10 be tcn ages show considerable variance on individual surfaces samples collected in the active channels date from 6 ka to 93 ka showing that there is significant inheritance of 10 be tcn within cobbles and boulders that were dated which suggests that slopes erode and sediment is transferred very slowly in most regions within death valley comparisons of 10 be tcn ages on alluvial fan surfaces with chronostratigraphies based on soil development and optically stimulated luminescence dating show that minimum 10 be tcn ages within sample sets on individual surfaces most closely approximate to the age of landforms that are younger than 70 ka alluvial fan surfaces older than 70 ka have begun to undergo significant erosion so that the majority of 10 be tcn ages for data sets on individual surfaces underestimate the true age of the surface due to erosion and exhumation of fresh cobbles and boulders the spread of 10 be tcn ages for beach bars near beatty junction and shorelines 8 km south of furnace creek is large ranging from 119 ka to 385 ka and 109 ka to 465 ka respectively however new and previously published luminescence ages soil development and geomorphic context suggest that these landforms formed during high lake level stands in marine isotope stage 2 22 18 ka but these younger ages my reflect soil development and material elluviated into the bar deposit long after deposition this suggests that the cobbles and boulders dated with 10 be methods have inherited 10 be tcns which have been derived from older shorelines or associated landforms these results highlight the problems associated with using surface cobbles and boulders to date quaternary surfaces in death valley and other similar dryland settings moreover this study emphasizes the need to combine multiple different dating methods to accurately date landforms in death valley and similar settings

numerical simulations of a gravity wave event over ccope part ii waves generated by an orographic density current a mesoscale numerical model and detailed observations are used to investigate the generation and maintenance of a mesoscale gravity wave event observed in eastern montana on 11 july 1981 during the cooperative convective precipitation experiment ccope it is shown that the interaction between an orographic density current and a mountain barrier leads to the generation of the gravity waves the simulation results suggest the following four stage conceptual model during stage i shortly after sunset the remnant up branch of a thermally driven upslope flow east of the rockies was driven back toward the mountain by the pressure gradient force associated with a cool pool over north dakota the nocturnal stable layer over eastern montana was strengthened during passage of this density current during the 1 2 h transition period of stage ii the advancing density current became blocked as it encountered the higher terrain an isentropic ridge developed above the original warm lee trough due to strong adiabatic cooling caused by the sustained upward motion in the presence of orographic blocking during stage iii an even stronger upward motion center formed to the east of the density current head updraft in response to an eastward horizontal pressure gradient force produced by the isentropic ridge in stage iv as the density current head collapsed and downward motion developed to the west of the original updraft in quadrature phase with the isentropic perturbation a gravity wave was generated this wave propagated eastward with the mean wind opposite to the motion of the earlier density current and was maintained by the strong wave duct established earlier by the density current thus the mountain plains circulation may at times generate mesoscale gravity waves and deep convection hours after diurnal heating has ended

the interactions of gravity waves with mesocyclones preliminary observations and theory abstract apparent interactions between ducted gravity waves and preexisting mesocyclones are investigated preliminary analyses of weather surveillance radar 1988 doppler wsr 88d observations from several cases reveal that the intersection of fine lines whose propagation speed is consistent with that of gravity waves and existing mesocyclones leads to an increase in the rotational velocity of the mesocyclone utilizing simplified ducted wave kinematics and the vorticity equation changes in vorticity associated with convergence divergence and perturbation wind shear within the gravity wave are examined convergence ahead of wave ridges may be significant causing mesocyclone intensification through vorticity stretching it will also be shown that a wave may significantly change the vertical wind shear and streamwise vorticity in storm inflow a simple one dimensional model is presented which shows that vorticity decreases temporarily ahead of the wave ridge then increases rapidly behind the ridge as

directly dating postglacial greenlandic land surface emergence at high resolution using in situ 10be postglacial emergence curves are used to infer mantle rheology delimit ice extent and test models of the solid earth response to changing ice and water loads such curves are rarely produced by direct dating of land emergence rather most rely on the presence of radiocarbon datable organic material and inferences made between the age of sedimentary deposits and landforms indicative of former sea level here we demonstrate a new approach 10 be dating to determine rates of postglacial land emergence in two different settings in southern greenland narsarsuaq igaliku we date directly the exposure as relative sea level fell of gravel beaches and rocky outcrops allowing determination of rapid post younger dryas emergence in western greenland kangerlussuaq we constrain holocene isostatic response by dating the sequential stripping of terrace sediment driven by land surface uplift relative sea level fall and resulting fluvial incision the technique we employ provides high temporal and elevation resolution important for quantifying rapid emergence immediately after deglaciation and less rapid uplift during the middle holocene 10 be constrained emergence curves can improve knowledge of relative sea level change by dating land emergence along rocky coasts at elevations and locations where radiocarbon datable sediments are not present and without the lag time needed for organic material to accumulate

cosmic ray produced neon in antarctic rocks we studied three antarctic rocks and quartz separates thereof for their records of cosmic ray produced 21ne all three samples reveal an excess of 21ne and in two of them a cosmic ray origin is documented by the spallation ratios 22ne 21ne 3he is shown to be incompletely retained in quartz 10 previous studies have shown the suitability of quartz as a monitor for cosmic ray produced radionuclides 10be and 26al and therefore as a tracer for geological processes 26al 10be ratios allow the calculation of minimum exposure ages and maximum erosion rates and measured ratios 21ne 10be and 21ne 26al add significant information to exposure histories especially in the case of a complex exposure this information is necessary for an evaluation of erosion rates and to define geological processes such as uplift and variations in the antarctic ice cover

a valid margules formulation for an asymmetric ternary solution revision of the olivine ilmenite thermometer with applications abstract the olivine ilmenite thermometer of andersen and lindsley 1979 was based on an incorrect formulation for the excess free energy of an asymmetric ternary solution a valid formulation is derived and used to revise the parameters of the olivine ilmenite thermometer for olivine and ilmenite that have equilibrated above 700 c temperature can be calculated from t c 273 12549 p 0 03x fa 0 01099 x gk x il 0 062 10496 x fa 5767 x gk x il x hem 38602 141550x il 47183x gk 5 67 r ln k d 6 52x fa 3 09 x gk x il x hem 16 49 109 46 x il 36 49x gk with k d x il x fo x gk x fa the revised model gives wil gkg 5767 3 09t 0 011p and gexch 7301 8 9t 0 047p t in k p in bars applications include apollo 17 breccias and kimberlites

relative and absolute dating of land surfaces abstract the dating of land surfaces has long posed problems for geomorphologists relative methods stratigraphic geomorphic topographic are sound and convincing exhumed forms may complicate identification and relationships for both epigene and etch forms have been buried and exhumed but in tectonically undisturbed areas the higher surfaces are older than those preserved at lower levels also surfaces have an age range the relationship of surfaces with volcanic deposits old shorelines and genetically related sedimentary sequences provides sound ages and correlation with dated duricrusts and faults is also useful there are no temporal limits to relative dating for the methods are equally applicable to the dating say of proterozoic surfaces as of those of pleistocene age the disadvantage of such methods is that the necessary evidence is frequently either not preserved or not exposed the so called absolute physical numerical methods and especially those based on exposure age dating with in situ cosmogenic radionuclides are appealing because they produce direct numerical ages and appear to be widely applicable but there are severe temporal limitations and sampling problems complicate and may invalidate interpretation absolute age determinations must be consistent with the stratigraphic and geomorphologic settings the best results are obtained when physicists and earth scientists pool their knowledge and experience a background in local and regional geology is especially important

cosmogenic noble gas paleothermometry we present a theoretical basis for reconstructing paleotemperatures from the open system behavior of cosmogenic noble gases produced in minerals at earth s surface experimentally determined diffusion kinetics predicts diffusive loss of cosmogenic 3he and 21ne from common minerals like quartz and feldspars at ambient temperatures incomplete retention has also been observed empirically in field studies we show that the theory of simultaneous production and diffusion that applies to radiogenic noble gases in minerals the basis of thermochronology can also be applied to cosmogenic noble gases to reconstruct past surface temperatures on earth we use published diffusion kinetics and production rates for 3he in quartz and 21ne in orthoclase to demonstrate the resolving power of cosmogenic noble gas paleothermometry with respect to exposure duration temperature and diffusion domain size calculations indicate that when paired with a quantitatively retained cosmogenic nuclide such as 21ne or 10be observations of cosmogenic 3he in quartz can constrain temperatures during surface exposure in polar and high altitude environments likewise 21ne retention in feldspars is sensitive to temperatures at lower latitudes and elevations expanding the potential geographic applicability of this technique to most latitudes as an example we present paired measurements of 3he and 10be in quartz from a suite of antarctic sandstone erratics to test whether the abundances of cosmogenic 3he agree with what is predicted from first principles and laboratory determined diffusion kinetics we find that the amounts of cosmogenic 3he present in these samples are consistent with the known mean annual temperature mat for this region of antarctica between 25 and 30 c these results demonstrate the method s ability to record paleotemperatures through geologic time

geomagnetic poles over the past 10 000 years locations of the geomagnetic pole over the past 10 000 years have been calculated by averaging the vgp positions obtained from paleomagnetic data the distribution of the geomagnetic pole was elongated to the direction parallel to the meridian of 45 and 225 longitude and westward movement of the pole was predominant throughout this period the time sequence of the polar motion can be divided into three intervals the intervals between ca 10 000 b p and ca 7000 b p between ca 7000 b p and ca 3700 b p and between ca 3700 b p and the present during the period between ca 7000 and ca 3700 b p the range of the movement of the geomagnetic pole was limited within 5 degrees around the geographical pole before and after this period the movement was very active fluctuating over 10 degrees the results of the last 2000 years show good consistency with the geomagnetic pole calculated from archaeomagnetic data by merrill and mcelhinny 1983

surface pressure features of landfalling typhoon rainbands and their possible causes abstract this study uses temporally high resolution surface observations doppler radar and micro rain radar to document the finescale features of the two landfalling rainbands associated with typhoon longwang 2005 as they passed over northern taiwan the present case allows a unique opportunity to investigate well defined convectively active tropical cyclone rainbands over land in particular the surface pressure fluctuations observed during the passage of the two rainbands and their possible causes are explored the rainbands were predominantly convective in nature with embedded stratiform precipitation outside their inner outer edge analyses of surface observations show similar surface pressure fluctuations during the rainband s passage low high pressure with relatively strong weak cross band flow and warmer colder temperature was located inside the outer inner edge maximum minimum pressure perturbations were observed to be 1 5 1 mb with smaller magnitudes 0 4 mb outside th

cosmogenic 10be and 3he accumulation in pleistocene beach terraces in death valley california u s a implications for cosmic ray exposure dating of young surfaces in hot climates to investigate the history of pleistocene lake levels in the southwestern u s a and to improve our understanding of the systematics of in situ produced cosmogenic nuclide accumulation in hot climates and young rocks we examined the 3he 10be and 26al contents of quartz separates from metamorphic quartzites and quartz rich felsic rocks incorporated in a series of stranded conglomerate beach ridges ranging from 3 to 160 m above sea level in death valley california calculated 10be ages range from 17 to 135 kyr with most ages between 20 and 60 kyr in contrast cosmogenic 3he contents indicate exposure ages of 1 kyr except for one sample at 18 4 kyr for which the 10be age is still significantly greater 44 11 kyr 26al10be ratios close to the cosmogenic production ratio rule out the presence of meteoric 10be and point to 3he loss as the cause of the age discrepancies comparison to laboratory diffusive loss measurements using a diurnal direct solar heating model implies the 3he loss occurs because the effective quartz grain sizes are 100 m far smaller than the 1 3 mm fracture spacings estimated from thin sections because direct insolation enhances 3he loss sampling from shallow depths 20 40 cm may be useful in some cases 4he and 20ne measurements demonstrate that small amounts of radiogenic and atmospheric 3he are also present at levels equivalent to a few kiloyears sea level exposure which should be taken into account when dating surfaces of similar age the lack of any clear progression with terrace altitude and the occurrence of two fold variations in apparent exposure ages within individual terraces suggest that the 10be contents do not solely represent accumulation in the present sample locations instead accumulation probably also occurred during sample exposure in the higher altitude beach rock source regions and during fluvial transport to the ancient lake shore thus the beach terrace exposure ages of 20 60 kyr are probably maximum ages for the times of these high lake level stands in death valley

crystallization of titaniferous chromite magnesian ilmenite and armalcolite in tholeiitic suites in the karoo igneous province titaniferous chromite up to 8 wt tio2 and magnesian ilmenite up to 10 wt mgo coexist at the base of the differentiated tholeiitic mount ayliff intrusion in the karoo province of southern africa suggesting that the original magma was tio2 rich picritic lavas with 3 tio2 from the lebombo monocline of the karoo province also contain microphenocrysts of magnesian ilmenite up to 6 wt mgo and armalcolite up to 7 wt mgo these oxide mineral associations and compositions are atypical of tholeiitic magmas in which chromite usually has less than 1 wt tio2 ilmenite less than 3 wt mgo and armalcolite is rarely a primary mineral experiments have been conducted at one atmosphere pressure on a range of compositions to determine the effect of tio2 on the crystallization and composition of chromite ilmenite and armalcolite the results indicate that increasing the tio2 content of picritic magmas increases the tio2 content of the spinel mainly at the expense of al2o3 whereas cr2o3 is not affected spinel compositions in the mount ayliff intrusion with over 45 wt cr2o3 less than 10 wt al2o3 and 8 wt tio2 were duplicated in experiments on a picrite at temperatures of about 1 200 c at the ni nio buffer increasing fo2 from fayalite magnetite quartz to ni nio buffer is shown to increase the crystallization temperature of armalcolite and to decrease that of ilmenite the total feo content of the liquid has little influence on the crystallization temperature of these phases the tio2 content of the liquid when either ilmenite or armalcolite crystallizes varies inversely with sio2 content the mgo content of the liquid at which ilmenite or armalcolite crystallizes depends upon the tio2 content of the starting composition with naturally occurring and experimetally determined saturation being demonstrated for liquids with 5 wt mgo and 5 5 wt tio2 the partition coefficent for mgo between armalcolite or ilmenite and liquid is about 1 5 observed magnesian armalcolite and ilmenite compositions in picrite lavas both minerals and in the mount ayliff intrusion ilmenite only are consistent with crystallization from a tio2 rich magma with approximately 5 wt mgo the fe23 tio5 component of armalcolite in the picrite lavas matches those formed experimentally at temperatures of 1 150 1 110 c and fo2 of the ni nio to ni nio 1 log unit similarities also exist between the compositions of chromite ilmenite and armalcolite and liquid fraction ation trends of some hawaiian high tio2 lavas and the experimental studies presented here

a mesoscale gravity wave event observed during ccope part ii interactions between mesoscale convective systems and the antecedent waves abstract this paper presents the results of a very detailed investigation into the effects of preexisting gravity waves upon convective systems as well as the feedback effects of convection of varying intensity upon the waves the analysis is based on the synthesis of synoptic surface and barograph data with high resolution surface mesonetwork radar and satellite data collected during a gravity wave event described by koch and golus in part i of this series of papers use is also made of the synoptic barograph data and satellite imagery to trace the waves beyond the mesonetwork and thus determine their apparent source region just upstream of the mesonetwork it is shown that two of the gravity waves modulated convection within a weak squall line as they propagated across the line the other six waves remained closely linked with convective systems that they appeared to trigger however it is shown that the waves were not excited by convection furthermore the waves retained their signatures in the su

mesoscale dynamics in the palm sunday tornado outbreak abstract radar and satellite imagery suggest that strong mesoscale forcing occurred in the palm sunday tornado outbreak on 27 march 1994 parallel lines of severe thunderstorms within each of three mesoscale convective systems developed just ahead of a cold front in mississippi and alabama on this date analyses of routine meteorological observations barograph data and forecasts from the eta and ngm models and a mesoscale research model mass are used to examine the relative roles of large scale dynamics and mesoscale processes in triggering and organizing the mesoscale convection quasigeostrophic forcing was absent in the outbreak region likewise a thermally direct circulation system transverse to the upper level jet that was present to the northwest of the outbreak region was decoupled from the strong low level ascent occurring in northern alabama and mississippi at the time of the outbreak strong ageostrophic frontogenesis in the presence of conditional symmetric instability csi was the chief

the accumulation of cosmogenic chlorine 36 in rocks a method for surface exposure dating because of the reaction of cosmic rays with elements in minerals chlorine 36 accumulates in rocks exposed at the earth s surface this suggests that the ratio of chlorine 36 to stable chloride can be used as a geochronometer reasonable agreement has been obtained between measurements of chlorine 36 in volcanic rocks of known age and calculations of chlorine 36 production based on rock composition and cosmicray flux the buildup of chlorine 36 should be a useful means for measuring the surface exposure time of young volcanic rocks and geomorphic features in the age range of 10 3 to 10 6 years

chronology of taylor glacier advances in arena valley antarctica using in situ cosmogenic 3he and 10be abstract in situ produced cosmogenic nuclides provide a new technique for constraining exposure ages of glacial deposits in situ3he and 10be in quartz sandstone boulders from arena valley southern victoria land antarctica provide chronological constraints for a sequence of moraines taylor ii ivb moraines related to expansions of taylor glacier and the east antarctic ice sheet mean 3he ages are 113 000 45 000 yr 208 000 67 000 yr 335 000 187 000 yr and 1 2 0 2 myr for taylor ii iii iva and ivb moraines respectively mean 1 corresponding mean 10be ages for taylor ii and ivb moraines are 117 000 51 000 yr and 2 1 0 1 myr for the older deposits the 3he ages are probably lower limits due to diffusive loss although the exposure ages appear consistent with the few previous age estimates particularly with an isotope stage 5 age for taylor ii each moraine exhibits a broad age distribution the distribution probably results from a variety of factors which may include prior exposure to cosmic rays 3he loss erosion postdepositional boulder movement and radiogenic production of 3he nonetheless the exposure ages provide direct chronological constraints for the moraine sequence and suggest a maximum thickening of taylor glacier relative to the present ice surface of 500 m since the late pliocene early pleistocene

late pleistocene basaltic ash and volcanic eruptions in the bonneville basin utah microprobe analyses of glass and radiocarbon dating of samples associated with three basaltic volcanic ashes in the deposits of lake bonneville make the ashes useful stratigraphic markers for correlation within the bonneville basin two of the ashes were derived from tuff cones pavant butte and tabernacle hill that erupted into lake bonneville in what is now the black rock desert of south central utah near kanosh utah the pavant butte ash is interbedded with barrier beach lagoon marl that is slightly less than 16 000 yr old five radiocarbon dates on carbonate materials stratigraphically associated with the ash elsewhere in the sevier desert average 15 300 yr b p lake bonneville was in its transgressive phase about 15 m below the bonneville shoreline when the pavant butte eruption began at the tabernacle hill tuff cone a basalt flow that has pillows wave rounded cobbles and tufa on its outer margins was erupted into lake bonneville shortly after the tuff cone at or near the provo shoreline the tabernacle hill tuff cone and basalt flow are older than a radio carbon date of about 14 300 yr b p on tufa collected from the outer margin of the basalt flow but younger than the bonneville flood at about 14 500 yr b p the third ash informally termed the thiokol ash has been found interbedded with lake bonneville deposits in two exposures in northern utah and in sediment cores from the great salt lake it is about 25 000 yr old and may have been erupted from a volcanic field on the northwest shore of great salt lake or from the snake river plain microprobe analyses of glass from samples of the black rock desert ashes show them to have similar chemical compositions but glass from tabernacle hill can be distinguished from pavant butte glass by its higher concentrations of cao and p 2 o 5 systematic differences in chemical composition between samples of pavant butte glass can be explained by comagmatic processes thiokol glass has less sio 2 ai 2 o 3 mgo and na 2 o but greater total iron and p 2 o 5 than do the black rock desert glasses

mesoscale ascent in nocturnal low level jets abstracta theory for gentle but persistent mesoscale ascent in the lower troposphere is developed in which the vertical motion arises as an inertia gravity wave response to the sudden decrease of t

chrome spinels from the jinchuan ni cu sulfide deposit gansu province people s republic of china chrome spinels occur in olivine rich cumulates in the jinchuan intrusion host to one of the largest known accumulations of magmatic ni cu in the world the chemistry and mode of occurrence of these spinels is compared with those in two other similar but unmineralized mafic ultramafic intrusions in the same belt the jinchuan intrusion contains mg poor chromites with widely varying tio 2 contents showing many features that are typical of chromites in normal tholeiitic layered intrusions wide compositional variance occurs on the scale of single thin sections and is attributed to reaction between cumulus chromite and trapped intercumulus liquid this process operated extensively even on grains that were armored by cumulus olivine crystals different reaction paths correlate with the abundance of sulfides in the rock the unmineralized intrusions show similar reaction trends but are offset from the jinchuan trends yejili spinels are generally higher in al and ga while zangbutai chromites are systematically higher in fe super 3 jinchuan spinel olivine pairs record equilibration temperatures well below the solidus and are slightly but significantly lower than blocking temperatures recorded at yejili the jinchuan ore zone contains low cr high fe super 3 chromites that are unusual and distinctive a plausible model for these unusual compositions is that these grains originated as normal aluminous spinels crystallizing from the parent silicate melt and subsequently became enlarged and modified during crystallization of the sulfide melt ti enriched chromites enclosed within sulfides record a prehistory of extensive reaction with trapped liquid sulfide associated chromites from jinchuan are strongly nickel depleted relative to expected ni values for their fe super 3 content when compared with the barren intrusions this feature combined with the distinctive chemistry of the sulfide related spinels may have applications in the use of heavy resistate minerals during exploration distinctively high ti chromites are also a feature of intrusions associated with the karoo flood basalts suggesting that jinchuan may have a similar affinity however tio 2 enrichment in chromite can be a product of interaction with differentiated trapped liquid as indicated by similar trends in lava lakes layered intrusions and at sudbury and therefore cannot be taken to prove unusually ti rich parent magmas

a nonclassical cold front observed during cops 91 frontal structure and the process of severe storm initiation abstract this case study addresses the issue of gravity current and bore development at surface cold fronts and the role of these phenomena in the generation of severe frontal convection the event investigated occurred on 27 april 1991 during the cooperative oklahoma profiler studies 1991 field project the development of a bore from a gravity current like structure along a cold front the subsequent propagation of the bore ahead of the front on a low level inversion and the process of severe thunderstorm development along the front are revealed by a dense network of remote sensing and other special observations evidence for the gravity current and bore is strengthened by comparisons made between the synthesized observations and theory the bore developed after a nocturnal inversion which acted as a waveguide had become established the bore and gravity current were both evident as fine lines in the radar reflectivity displays a microscale envelope of enhanced water vapor with an embedded roll cl

origin evolution and finescale structure of the st valentine s day mesoscale gravity wave observed during storm fest part iii gravity wave genesis and the role of evaporation abstract on 14 february 1992 a long lived moderate amplitude mesoscale gravity wave formed in kansas during the storm scale operational and research meteorology fronts experiment systems test storm fest wave formation was evident in correlated surface pressure and wind fields the wave of depression accompanied by a weak rainband tracked across the state a wealth of data was collected on the mature wave as it passed over the storm fest dual doppler domain however the mechanism of genesis remained difficult to ascertain since wave formation occurred in a region of less comprehensive observations the genesis of the storm fest gravity wave is successfully simulated using the fifth generation pennsylvania state university national center for atmospheric research penn state ncar mesoscale model mm5 which was run at 6 km grid spacing in the innermost domain the lee cyclone movement dry airmass development and gravity wave formation over kansas were successfully captured by the model results

isotopic insights into smoothening of abandoned fan surfaces southern california cosmogenic nuclide concentrations measured on abandoned fan surfaces along the mojave section of the san andreas fault suggest that sediment is generated transported and removed from the fans on the order of 30 40 kyr we measured in situ produced cosmogenic 10 be and in some cases 26 al in boulders n 15 surface sediment n 15 and one depth profile n 9 nuclide concentrations in surface sediments and boulders underestimate fan ages suggesting that 10 be accumulation is largely controlled by the geomorphic processes that operate on the surfaces of the fans and not by their ages field observations grain size distribution and cosmogenic nuclide data suggest that over time boulders weather into grus and the bar sediments diffuse into the adjacent swales as fans grow older the relief between bars and swales decreases the sediment transport rate from bars to swales decreases and the surface processes that erode the fan become uniform over the entire fan surface the nuclide data therefore suggest that over time the difference in 10 be concentration between bars and swales increases to a maximum until the topographic relief between bars and swales is minimized resulting in a common surface lowering rate and common 10 be concentrations across the fan during this phase the entire fan is lowered homogeneously at a rate of 10 15 mm kyr 1

are the benches at mormon point death valley california usa scarps or strandlines the benches and risers at mormon point death valley usa have long been interpreted as strandlines cut by still stands of pluvial lakes correlative with oxygen isotope stage ois 5e 6 120 000 186 000 yr b p and ois 2 10 000 35 000 yr b p this study presents geologic mapping and geomorphic analyses gilbert s criteria longitudinal profiles which indicate that only the highest bench at mormon point 90 m above mean sea level msl is a lake strandline the other prominent benches on the north descending slope immediately below this strandline are interpreted as fault scarps offsetting a lacustrine abrasion platform the faults offsetting the abrasion platform most likely join downward into and slip sympathetically with the mormon point turtleback fault implying late quaternary slip on this low angle normal fault our geomorphic reinterpretation implies that the ois 5e 6 lake receded rapidly enough not to cut strandlines and was 90 m deep consistent with independent core studies of the salt pan no evidence of ois 2 lake strandlines was found at mormon point which indicates that the maximum elevation of the ois 2 lake surface was 30 m msl thus as measured by pluvial lake depth the ois 2 effective precipitation was significantly less than during ois 5e 6 a finding that is more consistent with other studies in the region the changed geomorphic context indicates that previous surface exposure dates on fault scarps and benches at mormon point are uninterpretable with respect to lake history

surface exposure chronology using in situ cosmogenic 3he in antarctic quartz sandstone boulders abstract in situ produced cosmogenic helium 3 he c provides a new tool for constraining histories of quaternary geomorphic surfaces before general application of the technique however the systematics and production rates of 3 he c must be well understood in a companion study 3 he and 10 be data from sandstone and granite boulders in the dry valleys region of antarctica have been used to constrain the ages of an important moraine sequence formed by the taylor glacier data from these deposits also provide information about the systematics of 3 he in quartz that has important implications for geochronology based on 3 hec in contrast to previous results from olivine and clinopyroxene crushing quartz in vacuo releases helium with high 3 he 4 he ratios up to 148 r a where r a is the atmospheric 3 he 4 he ratio indicating that crushing cannot be used to determine the isotopic composition of trapped i e noncosmogenic helium in quartz analysis of 3 he in different size fractions of the same samples indicates significant 3 he loss not predicted by existing 3 he diffusion data for quartz the origin of the discrepancy is not clear but loss from these samples is not as significant as suggested by the limited data of previous studies

he diffusion in olivine helium diffusion in olivine dunite xenolith has been measured in the temperature range 1180 1460 c a linear arrhenius function was obtained with an activation energy of 120 27 32 kcal mole and a pre exponential factor d0 2 2 108 cm2 s diffusion mechanisms are not a viable means of degassing he from the mantle olivine phenocrysts can be expected to retain previously trapped he during cooling in extrusive basalts provided the flow units are thinner than 50 m xenoliths will retain mantle he signatures only if magma transport times are less than 50 years or if the he fugacity in the magma is high enough to prevent xenolith degassing the lower oceanic crust is probably substantially degassed of he trapped he will be qualitatively retained in quenched submarine basalt glass only if the cooling rate is faster than 5 1014 c m y glass at several centimeters depth in a basalt flow near the spherulite zone will have cooling rates lower than this so he loss may be significant in many basalt glass samples

emplacement and crystallization time for the bushveld complex the bushveld complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years for example the columbia river basalts hooper injections of magma which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988 were erupted in the period 17 12 ma with minor eruptions for a further 5 my although most outpouring significantly before the addition of the next one to constrain the emplacement and crystallization times a thermal model is presented occurred within the first 2 my it is now recognized that large intrusions were not emplaced in a single pulse but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition and subresult from multiple magma injection the question is how rapidly were magma chambers such as the bc traction such modelling indicates that magmas injected into the bushveld complex were emplaced within 75 000 years at that filled and how much magma was involved answers are relevant to the dynamics of melt production storage and time injection into the complex ceased the volume of rock in the eastern and western limbs is 370 000 600 000 km however transport in the mantle and crust this paper describes a thermal modelling technique not previously applied a quantitative evaluation of the cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers which can be used to analyse this process and to obtain an estimate of the emplacement accounted for in the preserved rock sequence similarly an evaluation of the incompatible trace element abundances such as those for zr time to present this model it is necessary to discuss the stratigraphy size and connectivity of the different limbs and k suggests that the chamber was open and that large volumes of differentiated magma escaped the volume of magma therefore of the bushveld complex and to consider the extent of tapping of the magma chamber as well as its filling greatly exceeded the preserved volume of cumulate rocks giving an estimated magma volume of over 1 10 km an average the term bushveld complex has been given several meanings in the literature and according to the south emplacement rate of 13 km year is indicated by these calculations african commission on stratigraphy 1980 includes not only the ultramafic mafic layered rocks but also the sills beneath the intrusion volcanic rocks which pre date the

pluvial lakes in the great basin of the western united states a view from the outcrop abstract paleo lakes in the western united states provide geomorphic and hydrologic records of climate and drainage basin change at multiple time scales extending back to the miocene recent reviews and studies of paleo lake records have focused on interpretations of proxies in lake sediment cores from the northern and central parts of the great basin in this review emphasis is placed on equally important studies of lake history during the past 30 years that were derived from outcrop exposures and geomorphology in some cases combined with cores outcrop and core records have different strengths and weaknesses that must be recognized and exploited in the interpretation of paleohydrology and paleoclimate outcrops and landforms can yield direct evidence of lake level facies changes that record details of lake level fluctuations and geologic events such as catastrophic floods drainage basin changes and isostatic rebound cores can potentially yield continuous records when sampled in stable parts of lake basins and can provide proxies for changes in lake level water temperature and chemistry and ecological conditions in the surrounding landscape however proxies such as stable isotopes may be influenced by several competing factors the relative effects of which may be difficult to assess and interpretations may be confounded by geologic events within the drainage basin that were unrecorded or not recognized in a core the best evidence for documenting absolute lake level changes lies within the shore nearshore and deltaic sediments that were deposited across piedmonts and at the mouths of streams as lake level rose and fell we review the different shorezone environments and resulting deposits used in such reconstructions and discuss potential estimation errors lake level studies based on deposits and landforms have provided paleohydrologic records ranging from general changes during the past million years to centennial scale details of fluctuations during the late pleistocene and holocene outcrop studies have documented the integration histories of several important drainage basins including the humboldt amargosa owens and mojave river systems that have evolved since the miocene within the active tectonic setting of the great basin these histories have influenced lake levels in terminal basins many pre late pleistocene lakes in the western great basin were significantly larger and record wetter conditions than the youngest lakes outcrop based lake level data provide important checks on core based proxy interpretations we discuss four such comparisons in some cases such as for lakes owens and manix outcrop and core data synthesis yields stronger and more complete records in other cases such as for bonneville and lahontan conflicts point toward reconsideration of confounding factors in interpretation of core based proxies

contrasting parental magma compositions for the hongge and panzhihua magmatic fe ti v oxide deposits emeishan large igneous province sw china many 260 ma mafic ultramafic layered intrusions including hongge and panzhihua in the emeishan large igneous province southwestern china host world class fe ti v oxide ore deposits these two most important ore bearing intrusions show differences in lithology and mineral chemistry the most important orebodies in the hongge intrusion occur as concordant layers in the middle part of the intrusion closely associated with clinopyroxenites titanomagnetite and mg rich ilmenite are the major ore minerals of the hongge deposit coexisting clinopyroxene contains 1 7 wt tio2 these data indicate high ti parental magma for the hongge ore bearing clinopyroxenites in the panzhihua intrusion the most important orebodies also occur as concordant layers but in its lower part instead of middle part in contrast with the hongge deposit the most important host rocks of the panzhihua deposit are gabbros not clinopyroxenites in addition ilmenite is rare and titanomagnetite is predominant in the panzhihua deposit coexisting clinopyroxene in the panzhihua deposit contains 1 6 wt tio2 the contrasting lithologic and mineral compositions indicate that the parental magma for the hongge deposit has higher tio2 than that for the panzhihua deposit the compositions of clinopyroxene from the panzhihua and hongge deposits indicate that their parental magmas are also different in mgo feo and trace element ratios as well calculations using average experimental mg fe exchange coefficient and trace element partition coefficients between clinopyroxene and magma show that the hongge parental magma has higher mgo feo ratios and more fractionated mantle normalized trace element patterns than the panzhihua parental magma the estimated compositions of the parental magmas for the hongge and panzhihua ore bearing lithologies resemble the average compositions of the longzhoushan type high ti basalts and ertan type intermediate ti basalts in the emeishan large igneous province respectively the new trace element data from this study together with available sr nd isotope data for the ore bearing intrusions from literature support a new petrogenetic model involving selective assimilation of newly subducted stagnant oceanic gabbroic slab above the deep seated emeishan mantle plume this process and subsequent contamination with the upper crust played an important role in the variation of parental magma compositions between the hongge and panzhihua magmatic oxide ore deposits abundant fe ti oxide ore deposits associated with less evolved basaltic magma in the emeishan large igneous province than elsewhere in the world are attributed to selective assimilation of newly subducted stagnant oceanic lithospheric slab by the ascending mantle plume derived picritic magma that was originally undersaturated with fe ti oxides

the petrology of some picrites from mauna loa and kilauea volcanoes hawaii the mineralogy and chemistry of picrites from mauna loa and kilauea have been investigated to evaluate for hawaiian tholeiitic picrites the contrasting genetic models which have been proposed for these mg rich volcanics namely products of direct crystallization of high mg melts 20 25 mgo or the result of accumulation of olivine phenocrysts into less mg rich melts genetic interpretations rely heavily on mg fe partitioning relations between olivines and their picrite hosts although the 100 mg mg fe2 ratios m of picrites are wide ranging m 73 6 82 9 for fe2o3 feo 0 15 with mgo as high as 27 8 the average 100 mg mg fe ratios mg of the cores of olivine phenocrysts megacrysts show only restricted compositional variation mg 87 2 89 0 successive olivine generations are progressively more fe rich olivine liquid mg fe partitioning data and the mn and ni abundances in olivine phenocrysts collectively indicate that they were precipitated by mg rich basaltic melts with 12 14 mgo spinel compositions range from liquidus magnesiochromites occurring mainly as inclusions in olivine phenocrysts to groundmass titanomagnetites which crystallized at nearsolidus temperatures the cr2o3 contents and m values of liquidus magnesiochromites suggest that their parent melts were neither mg rich picritic mgo 20 nor relatively mg poor basaltic types