the changing geomorphology of the atchafalaya river louisiana a historical perspective abstract the atchafalaya river in south central louisiana is of geomorphic interest because of the large volumes of flow and sediment transported and its role as a major floodway for the lower mississippi river notwithstanding its potential to become an avulsion pathway at old river since its origin in the sixteenth century it has undergone many transformations and currently receives flow from the red river and partial flow of the mississippi river through multiple pathways flow increased progressively beginning in the mid 1800s and continued until concerns of diversion spurred placement of engineering structures to regulate flows at the juncture between the mississippi and atchafalaya rivers in 1963 prior studies that looked at reaches or six to seven rangelines or cross sections on hydrographic surveys document areal width and depth increases in the uppermost 80 km 50 mi of the atchafalaya river from 1880 to 1974 this study assesses how channel geometry of the atchafalaya river has changed with flow variations since the 1880s by evaluating instantaneous discharge measurements that document change at superior temporal resolution and differences in thalweg values from coupled modern hydrographic surveys which document change at better spatial resolution than prior works while structures have regulated and limited flow into the atchafalaya since 1963 the highest values of discharge cross sectional area and width at simmesport upstream follow river regulation associated geometry data at simmesport and morgan city downstream document that mean bed and thalweg elevations have risen comparison of thalweg elevations on hydrographic surveys from 1963 and 2006 show pronounced local variability in bed elevation change but also reaches dominated by degradation including the red river and uppermost section of the atchafalaya near the old river diversion aggradation occurs in the lower atchafalaya a zone where along channel lakes are filling flow and sediment quantities are changing in part caused by the constructed wax lake outlet and deltas have been growing in atchafalaya bay findings are relevant to river engineering and flood management especially navigation and help in better understanding the evolution of large distributaries and the role of human impacts

disequilibrium partial melting model and its implications for trace element fractionations during mantle melting abstract a model to describe trace element evolutions during partial melting is proposed by assuming surface equilibrium instead of complete equilibrium between mineral grains and melt and by considering the following factors 1 chemical diffusion in minerals 2 melting and melt extraction for mantle melting rates of 10 7 yr 1 inferred from mid ocean ridge spreading rates elements with diffusivities d in minerals of 10 15 cm 2 s be at equilibrium between minerals of 0 5 cm in diameter and melt during mantle melting however the anomalous radioactive excess of 236 ra over 230 th up to 300 in young basalts which cannot be explained by equilibrium melting because of the highly incompatible nature of both elements can be accounted for by the present model only when melting rate is in the orders of 10 6 to 10 5 yr 1 the disequilibrium effect cannot be ignored for elements with d 13 cm 2 s if the melting rate is 10 5 yr 1 disequilibrium melting lowers the incompatibility of an incompatible element the high the incompatibility the more obvious this effect is fractionation of trace elements during disequilibrium melting depends both on their equilibrium partition coefficients k and their diffusivities this has two consequences 1 fractionation of two highly incompatible elements k 0 01 by more than 20 is still possible at melting degree as high as 15 if their diffusivities differ by a factor of 5 or more as compared to the requirement for less than 5 partial melting for the same extent of fractionation by equilibrium melting and 2 the incompatibility order of two incompatible elements can be reversed under certain circumstances i e k 1 k 2 but d 1 d 2 the pb paradox namely that the u pb ratio in mantle sources inferred from pb isotopes has increased during the differentiation of the earth even though pb is less incompatible than u is explainable if the diffusivity of pb is higher than that of u e g by a factor of 10 and if the melting rate is in the order of 10 5 yr 1

comparison of 230th 238u disequilibrium systematics in lavas from three hot spot regions hawaii prince edward and samoa 230th 238u disequilibrium systematics reveal several important characteristics of the mantle source regions and petrogenesis of volcanic rocks in the presumed hot spots of hawaii marion island prince edward hot spot and samoa the 230th232th activity ratios of lavas from these three hot spots 1 06 0 07 1 04 0 08 and 0 81 0 06 respectively imply that the source regions are each nearly homogeneous with thu weight ratios of 2 9 3 0 and 3 8 for marion island and mauna kea hawaii negligible secular variation occurs in the 230th232th initial ratios this supports other evidence for very short transfer time between source and surface significant residence time at depth prior to eruption cannot be ruled out for the samoan lavas we have studied however the data for one of these flows deviate from the proposed 230th232th 87sr86sr correlation condomineset al 1981a in the opposite sense from that expected for such residence if it is assumed that the measured 230th232th ratios of the young lavas reflect thu in their mantle sources then the observed variations among these three hot spots combined with those reported by other workers for iceland the azores and tristan de cunha suggest that these sources are characterized by thu ratios ranging from values similar to that of morb source 2 5 to values similar to those of bulk earth 3 8 mixing of different proportions of depleted and enriched mantle may be responsible for the observed range

fluid geochemistry of natural manifestations from the southern poroto rungwe hydrothermal system tanzania preliminary conceptual model abstract the south poroto rungwe geothermal field in the northern part of the malawi rift tanzania divides in two main areas the relatively high altitude northern area around the main ngozi rungwe tukuyu and kyejo volcanoes is characterised by cold and gas rich springs in contrast hot springs occur in the southern and low altitude area between the kyela and livingstone faults the isotopic signature of the almost stagnant cold springs of the northern district is clearly influenced by h2o co2 g exchange as evidenced from negative oxygen shifts in the order of few deltas permil in contrast the isotopic signature of waters discharged from the hot springs of the southern district is markedly less affected by the h2o co2 g interaction this evidence is interpreted as an effect of the large permanent outflow of these springs which supports the hypothesis of a regional scale recharge of the major thermal springs measurements of carbon isotope variations of the dissolved inorganic carbon of waters and co2 g from the northern and southern springs support a model of co2 g driven reactivity all over the investigated area our combined chemical and isotopic results show that the composition of hot springs is consistent with a mixing between i cold surface fresh sfw and ii deep hot mineralised dhmw water indicating that the deep originated fluids also supply most of the aqueous species dissolved in the surface waters used as local potable water based on geothermometric approaches the temperature of the deep hydrothermal system has been estimated to be higher than 110 c up to 185 c in agreement with the geological and thermal setting of the malawi rift basin geochemical data point to i a major upflow zone of geothermal fluids mixed with shallow meteoric waters in the southern part of the province and ii gas absorption phenomena in the small perched aquifers of the northern volcanic highlands

constraints on the sedimentary input into the loki s castle hydrothermal system amor from b isotope data abstract loki s castle at 73 30 n along the arctic mid ocean ridge amor provides a natural laboratory to study the evolution of hydrothermal circulation in an ultraslow spreading environment in addition a sedimentary input into the loki s castle hydrothermal circulation cell is indicated by vent fluid and gas chemistry here we present b and sr isotope data to investigate interaction between fluid morb and sediment and to constrain mass transfer during hydrothermal circulation of a sediment influenced black smoker hydrothermal system the high boron concentrations 1770 2170 mol l and the relatively low boron isotope composition of the vent fluids 11 b values ranging from 11 6 to 15 2 reflect not only fluid rock interaction with mafic crust but indicates a significant contribution from sediments in agreement with other chemical and isotopic 87 sr 86 sr analyses we present a model of b extraction from sediments at a temperature of 300 c which suggests a water sediment ratio of approximately 2 to 3 at loki s castle at these ratios relative b extraction from sediment ranges from 35 to 55

rare earth elements geochemistry in springs from taftan geothermal area se iran abstract concentrations of rare earth elements rees were determined in springs and andesitic dacitic rocks of taftan geothermal field hydrochemical results of major ions indicate that thermal springs are na so 4 cl and ca so 4 cl types concentrations of rees are in ranges of 10 4 to 1 2 and 49 to 62 times of chondrite for springwater and rock samples respectively the thermal sts and tts and the cold aps springs with low ph values exhibit a very high ree contents 0 64 to 3 15 mg l saturation index indicates that fe and al phases can control dissolved ree concentration in fts and pf cold springs the speciation of ree complexes indicates dominant presence of lnso 4 and free ion in the taftan thermal springs in aps cold spring with ph 4 fluoride complexes are dominate over the free ion and sulfate species while in pf and fts cold springs with ph 6 4 and 7 respectively carbonate complexes lnco 3 are predominant species chondrite normalized pattern for the low ph waters show very distinctive gull wing patterns characteristic feature of acid sulfate geothermal systems and are similar to those of the host rocks chemical characteristics of rare earth elements in spring and volcanic rock samples indicate that rees are originated from the andesitic dacitic host rocks whole rock normalized ree patterns and petrographic evidences show that rare earth elements leached mainly from marginal alteration of minerals and matrix decomposition in volcanic rocks in chondrite normalized ree patterns significant negative eu anomaly in the cold springs compare to the thermal and acidic springs indicates that alteration of plagioclase is more intense in the later corresponding to increasing in temperature and acidic state of reactant water

230th 238u radioactive disequilibria in tholeiites from the famous zone mid atlantic ridge 36 50 n th and sr isotopic geochemistry we analyzed u th and230th 232th activity ratios for a few tholeiites from the mid atlantic ridge famous zone at 36 50 n the results show a fairly wide scatter for both th u and 230th 232th ratios seawater contamination appears to be responsible for this scatter and for the uranium produces an increase in content yielding a 234u 238u ratio greater than 1 and for the th an increase of the 230th 232th ratio which is a very sensitive indicator for contamination also the latter often is selective u th and sr are not affected in the same manner when discarding all data for contaminated samples the famous zone appears to be very homogeneous with ath u ratio value of 3 05 and a 230th 232th ratio value of 1 24 comparison with other active volcanic areas reveals a negative correlation between 230th 232th and87sr 86sr ratios for present lavas which is indicative of a consistency in th u and rb sr fractionation in the source regions of these magmas the th isotopic geochemistry can thus provide useful information for the study of present volcanism information as valuable as that from sr pb or nd isotopes

226 ra excesses in mid ocean ridge basalts and mantle melting the discovery of radioactive disequilibrium among isotopes of the 238u decay chain in recent volcanic rocks1 7 has enhanced our understanding of both the timescale and processes of melting in the mantle excesses of 230th half life 75 200 yr relative to its parent 234u in some of these rocks indicate that thorium is preferentially partitioned into the melt phase and constrain the combined duration of melting and magma transport to less than a few hundred thousand years recent observations of 226ra and 228ra excesses over their respective parents 230th and 228th in individual subaerial volcanoes have led to even more precise determinations of the timescale of magmatic processes3 6 here we report the observation and implications of 226ra half life 1 620 yr excesses in mid ocean ridge basalts these excesses are probably produced during melting in the mantle and the timescale they impose suggests that they are controlled by kinetic processes rather than chemical equilibrium regardless of the process the 226ra excesses constrain the length of the magma transfer times from melting through eruption and sample collection to less than about a thousand years

the chemistry of geothermal waters in iceland iii chemical geothermometry in geothermal investigations abstract new data from geothermal wells in iceland have permitted empirical calibration of the chalcedony and na k geothermometers in the range of 25 180 c and 25 250 c respectively the temperature functions are t c 1112 4 91 log sio 2 273 15 t c 933 0 993 log na k 273 15 concentrations are expressed in ppm these temperature functions correspond well with the chalcedony solubility data of fournier 1973 and the thermodynamic data for low albite microcline solution equilibria of heloeson 1969 a new co2 geothermometer is proposed which is considered to be useful in estimating underground temperatures in fumarolic geothermal fields its application involves analysis of co2 concentrations in the fumarole steam the temperature function which applies in the range 180 300 c is logco2 37 43 73192 t 11829 103 t2 0 18923t 86 187 logt where t is in k and co2 in moles per kg of steam

recent decadal growth of the atchafalaya river delta complex effects of variable riverine sediment input and vegetation succession abstract the mississippi river delta plain has experienced substantial wetland loss from subsidence erosion and sea level rise threatening coastal communities and the ecosystems that support them the atchafalaya river the largest distributary of the mississippi river has one of the few prograding delta features along the 200 km deltaic coastline understanding changes in the atchafalaya river delta complex ardc development has critical implications for future prediction and management strategy for the mississippi river delta plain this study was organized to answer two major questions 1 how did development of the ardc respond to fluctuation in riverine sediment supply over the period 1989 2010 and 2 has vegetation succession helped stabilize subaerial land the study quantified annual total suspended sediment yields to the two ardc subdeltas atchafalaya river subdelta arsd and wax lake outlet subdelta wlsd classified delta land cover using satellite imagery over 5 year intervals into three classes barren land vegetation and open water and investigated the relationship of delta land change with sediment yield and vegetation succession over the entire 21 year study period we found a net land gain of 59 km 2 with the arsd accounting for 58 of this gain and wlsd 42 sediment yield to the subdeltas decreased from an average annual of 38 megatonnes mt for arsd and 18 mt for wlsd during 1989 1995 to an average annual of 24 mt for arsd and 17 mt for wlsd during 2004 2010 corresponding to the decrease in riverine suspended sediment concentration concurrently total land growth rate decreased from 2 4 km 2 y 1 to 1 6 km 2 y 1 for arsd and 3 2 km 2 y 1 to 0 6 km 2 y 1 for wlsd however the ardc had a net land loss of 2 1 km 2 during 1999 2004 because of tropical system effects in conjunction with the lack of large river floods defined as discharge 13 800 m 3 s 1 on average more than 60 of newly vegetated land remained vegetated in subsequent years and when compared with barren areas vegetated land was less likely 7 3 vs 32 to be converted to water indicating vegetative stabilization effect however during the period without a major flood vegetation buffering against tropical system erosion was limited this indicates that over the period 1989 to 2010 land growth of the ardc was dictated by large flood events

87sr86sr and ree variations along the easter microplate boundaries south pacific application of multivariate statistical analyses to ridge segmentation abstract we report 87 sr 86 sr ree sc v cr and co analyses on 48 basaltic glasses dredged at 47 different localities along the spreading boundaries of the easter microplate emp the latitudinal 87 sr 86 sr variation along the east and west rifts closely parallels that of the pb isotope variation previously published and confirms the strong abnormality of the mantle underlying this part of the east pacific rise epr the data also further support the binary mantle mixing model previously proposed we evaluate the potential of multivariate statistical analyses in studying this and previously published geochemical data obtained on these very same 48 basaltic glasses the data base includes major and trace elements ranging from compatible e g cr to highly incompatible e g la and the pb and sr isotope ratios multivariate classification of these geochemical variables in q mode sample space allows to clearly distinguish major element grouping from those of moderate to highly incompatible elements and the isotope ratios reflecting mantle source heterogeneities present in the region the power of this multivariate geochemical classification is best represented in two dimensional space by combining principal components analysis pca with cluster analysis ca illustrated in the form of a dendrogram the method is further illustrated by classifying elements of the lanthanide series at different statistical level of affinities using the variance covariance matrix and correlation matrix in r mode we further demonstrate a close relationship between segmentation revealed by the multiple variation of the four isotope ratios and the actual tectonic segmentation both discriminant analysis and pca further indicate a close relationship between the geochemical anomaly located on the east rift of the emp and sala y gomez and easter island to a lesser extent the tuamotu chain whereas there is no close affinity between the emp and the more distant juan fernandez san felix or society islands in the isotopic space considered this provides further support that the geochemical anomaly located on the east rift of the emp may be related to the influence of a plume located either beneath the east rift or in the vicinity of sala y gomez or easter island in the last two models the plume would be laterally deflected at shallow depth towards the epr emp spreading complex an analysis of statistical generalized distances against geographical distances rules out that such influence is caused by radial dispersion of this plume in the upper mantle the flow between the plume and the east rift of the emp must be more confined or channelled the possibility that the plume would be located directly beneath the east rift cannot be ruled out but appears less likely

synergism of riverine and winter storm related sediment transport processes in louisiana s coastal wetlands abstract the roles of various mechanisms that supply sediments from major sources including rivers and nearshore shelf to coastal louisiana are not well understood or quantified temporally or spatially recent studies reveal that an important association between riverine sediment input and the cyclic passage of winter storms results in a periodic supply of suspended sediments to coastal marshlands the fact that these two mechanisms coincide in winter and early spring maximizes the availability of particulate matter for counteracting coastal land loss by enhancing substrate accretion and plant productivity identification of these optimum periods of suspended sediment activity when overbank sedimentation riverine concentrations flood processes and cold front passages are synergistic can maximize the performance of wetland restoration efforts because mineral matter and nutrients are critical for marsh vitality when mitigation of coastal land loss is planned it would be advantageous for weekend restoration to consider this timing when both sediment availability and constructive processes of sediment transport are optimal

observations of nearshore wave dissipation over muddy sea beds 1 wave propagation on the louisiana inner shelf is studied using concurrent measurements at two shallow water ocean observatory sites in sedimentary environments dominated by mud at one site and sand at the other contrary to the widely accepted hypothesis that mud induced wave dissipation is important only for long waves observations show significant damping of high frequency short waves which interact weakly with the bottom the mechanism of short wave dissipation is not understood numerical simulations show that other dissipative processes such as refraction or depth limited breaking do not account for the magnitude of the observed effects independent observations of strong sediment re working during storms suggest that these effects are related to sediment resuspension processes

230th 238u disequilibrium systematics in oceanic tholeiites from 21 n on the east pacific rise significant disequilibrium occurs between 230 th and its parent 238 u in a suite of fresh basalt glasses from the rise project study area at 21 n on the east pacific rise the 230 th 232 th activity ratios observed for eight of nine samples from the crest of the axis at this site are constant within analytical uncertainty with a value of 1 22 this observed homogeneity of 230 th 232 th has two possible interpretations first the measured 230 th 232 th can be considered to indicate a mantle source for the rise basalts with th u of 2 5 this interpretation however conflicts with the proposed correlation between 230 th 232 th and 87 sr 86 sr 1 which predicts that 230 th 232 th should equal 1 33 at the rise site a second possible interpretation is that radioactive decay of 230 th in the basalts themselves or in a magma chamber has decreased 230 th 232 th from 1 33 to the observed values the required time span is 11 000 to 100 000 years however petrologic arguments rule against long residence time in a magma chamber and the spreading rate of this section of the east pacific rise 6 cm yr predicts that the maximum age for axis basalts is 27 000 years both interpretations of the measured 230 th 232 th imply a low th u ratio for the rise basalt source and suggest that the morb source at this location is depleted in th with respect to u relative to primitive mantle or bulk earth in spite of their constant 230 th 232 th the basalts from 21 n have wide ranges of measured th u and thorium and uranium concentrations in apparent conflict with the common assumption that these two elements have very small similar crystal liquid partition coefficients participation of an accessory phase with high th and u concentrations during partial melting or fractional crystallization appears to be required to explain this anomaly major and other trace element compositions provide more information about petrogenetic processes although fractional crystallization can explain the variations in major element composition a more complex process such as continuous melting is required to account for the observed trends in trace element compositions in addition to its use as a tracer the 230 th 238 u disequilibrium technique is capable of putting some constraints on the timing of fractionation events and it may eventually lead to a method for dating very young mid ocean ridge basalts

hydrogeochemistry of the simav geothermal field western anatolia turkey thermal waters hosted by menderes metamorphic rocks emerge along fault lineaments in the simav geothermal area thermal springs and drilled wells are located in the eynal citgol and nasa locations which are part of the simav geothermal field studies were carried out to obtain the main chemical and physical characteristics of thermal waters these waters are used for heating of residences and greenhouses and for balneological purposes bottom temperatures of the drilled wells reach 163 c with total dissolved solids around 2225 mg kg surface temperatures of thermal springs vary between 51 c and 90 c all the thermal waters belong to na hco3 so4 facies the cold groundwaters are ca mg hco3 type dissolution of host rock and ion exchange reactions in the reservoir of the geothermal system shift the ca mg hco3 type cold groundwaters to the na hco3 so4 type thermal waters thermal waters are oversaturated at discharge temperatures for aragonite calcite quartz chalcedony magnesite and dolomite minerals giving rise to a carbonate rich scale gypsum and anhydrite minerals are undersaturated with all of the thermal waters boiling during ascent of the thermal fluids produces steam and liquid waters resulting in an increase of the concentrations of the constituents in discharge waters steam fraction y of the thermal waters of which temperatures are above 100 c is between 0 075 and 0 119 reservoir ph is much lower than ph measured in the liquid phase separated at atmospheric conditions since the latter experienced heavy loss of acid gases mainly co2 assessment of the various empirical chemical geothermometers and geochemical modelling suggest that reservoir temperatures vary between 175 c and 200 c

galapagos hot spot spreading center system 2 87sr 86 sr and large ion lithophile element variations 85 w 101 w thirty eight samples of basalts dredged from the galapagos spreading center gsc between 85 w and 101 5 w longitudes have been analyzed for k rb cs ba and sr contents as well as for 87sr 86sr ratios basalts between 85 w and 89 w segment of the gsc range from 0 7025 to 0 7028 in 87sr 86sr and those between 95 5 w and 101 5 w range from 0 7026 to 0 7027 these basalts are characterized by lree depleted patterns basalts from 89 w to 95 5 w segment having slightly lree depleted to lree enriched patterns la sm e f between 0 8 and 2 4 range from 0 7027 to 0 7031 in 87sr 86sr the longitudinal 87sr 86sr variation shows a maximum at about 92 w where the gsc is nearest to darwin island the alkali and alkaline earth elements as well as several element ratios also show maxima in their longitudinal profiles but these occur at about 91 w the apparent shift of 87sr 86sr maximum with respect to that of the trace elements may be related to differences in the degree of partial melting the highest 87sr 86sr from the gsc is only slightly lower than the highest 87sr 86sr for tholeiitic basalts from the galapagos archipelago several possible hypotheses have been examined to account for these geochemical and sr isotopic variations in the light of the presently available data the most promising explanation seems to be a binary mixing of hot spot or plume type material with the source of normal mid ocean ridge basalts and resulting rift propagation both end members seem to be heterogeneous and therefore it is at present difficult to put severe constraints on this binary mixing nevertheless the flow pattern of mantle material from the hot spot does not appear to be radial but is preferentially channeled along the 91 w and 92 5 w fracture zones toward the rift

investigation of 238u 230th 226ra and 232th 228ra 228th radioactive disequilibria in volcanic rocks from trindade and martin vaz islands brazil southern atlantic ocean abstract this paper presents the first results of 238u 230th 226ra and 232th 228ra 228th radioactive disequilibria of alkaline volcanic rocks from trindade and martin vaz islands southern atlantic ocean brazil th and u concentrations reinforce the role of fractional crystallization in the genesis of trindade volcanics and allow dividing the rocks in three groups group i is characterized by phonolites depleted in u in relation to th whose genesis can be explained by mineral assemblage fractionation with high uranium partition coefficients magnetite apatite and or titanite group ii is composed of ultrabasic basic and intermediate rocks with relatively high u contents in respect to th whose enrichment is probably caused by the presence of small comagmatic xenoliths containing accumulation of those fractionated mineral phases group iii is composed of remaining rocks which tend to present despite of some variation homogeneous th u ratios average 4 6 0 8 very low degrees 1 2 of partial melting generated ultrabasic rocks of the last group which are highly sio2 undersaturated and together with ascension volcanics have the highest th u ratios of all oib from southern atlantic ocean activity ratios of 234u 238u 230th 238u 226ra 238u 226ra 230th 228ra 232th and 228th 232th are equal to unity in all phonolites and some basic and ultrabasic rocks belonging to trindade complex and desejado sequence agreeing with ages more than 300 ky martin vaz peralkaline phonolite also presents activity ratios indicating radioactive equilibrium except for 226ra 238u which is significantly smaller than unity as the result of 226ra steady mobilization rocks from the three youngest volcanic episodes of trindade morro vermelho formation valado formation and vulcao do paredao present 230th 238u and 226ra 238u ratios distinct from unity showing also a significant 230th enrichment and all pairs of activity ratios indicate system closure between 8 and 300 ky ago 230th 232th and 238u 232th data of morro vermelho rocks allowed determining an age of 111 20 ky radioactive disequilibrium between 238u and 226ra of valado formation rocks associated with literature information constrain the system was closed between 8 and 11 ky for ultrabasic rocks calculated 230th0 excesses along with th u ratios do not show significant correlations with k2o contents therefore if phlogopite remained as a residual phase during k poor magma generation this mineral did not fractionate th because of its very small partition coefficient the relatively low 230th 232th 0 in comparison with measured 87sr 86sr plotting slightly below the mantle array may be explained either by a relatively long time maximum of about 110 ky for morro vermelho formation of magma transfer towards surface or more probably by melting of heterogeneous mantle due to a metasomatic event occurred between 111 and 300 ky followed by a very short magma transfer time hundreds of years

chemical geothermometry and fluid mineral equilibria for the omer gecek thermal waters afyon area turkey abstract thermal waters of the omer gecek geothermal field turkey with temperatures ranging from 32 to 92 c vary in chemical composition and tds contents they are generally enriched in na cl hco 3 and suggest deep water circulation silica and cation geothermometers applied to the omer gecek thermal waters yield reservoir temperatures of 75 155 c the enthalpy chloride mixing model which approximates a reservoir temperature of 125 c for the omer gecek field accounts for the diversity in the chemical composition and temperature of the waters by a combination of processes including boiling and conductive cooling of deep thermal water and mixing of the deep thermal water with cold water it is also determined that the solubility of silica in most of the waters is controlled by the chalcedony phase equilibrium states of the omer gecek thermal waters studied by means of the na k mg triangular diagram na k mg ca diagram k mg ca geoindicator diagram activity diagrams in the systems composed of na 2 o cao k 2 o al 2 o 3 sio 2 co 2 h 2 o phases log si diagrams and finally the alteration mineralogy indicate that most of the spring and low temperature well waters in the area can be classified as shallow or mixed waters which are likely to be equilibrated with calcite chalcedony and kaolinite at predicted temperature ranges similar to those calculated from the chemical geothermometers it was also observed that mineral equilibrium in the omer gecek waters is largely controlled by co 2 concentrations

effects of partial melting on the uranium decay series distinction between possible partial melting processes using stable incompatible trace elements depends largely on the mass fraction of liquid produced at low degrees of melting less than 5 both incompatible element concentrations and interelement fractionations are greater for batch fractional and accumulated fractional melting than for dynamic or continuous melting at higher degrees of melting uncertainty in initial source composition and small fractionations between incompatible elements make it unlikely that the most realistic melting models batch and dynamic could be discriminated however it is possible for these melting processes to be distinguished by the magnitude of radioactive disequilibria that is produced within the 238u decay series even for durations of partial melting much longer than the half lives of 230th and 226ra both230th 238u and 226ra 230th disequilibria can occur because of differences in the bulk solid liquid partition coefficients for these elements the magnitude of these disequilibria depends on melting process the 230th 232th of a dynamic melt can differ significantly from that of a batch melt and will not necessarily indicate the original 238u 232th of the source if morb is formed by a dynamic melting process then the th u of the upper mantle near ridge axes may be higher than that calculated from the 230th 232th of the basalts

the evolution from miocene potassic to quaternary sodic magmatism in western turkey implications for enrichment processes in the lithospheric mantle abstract new k ar dating and major and trace element analyses from the usak selendi emet use area constrain the timing of changes in the nature of volcanism in the miocene in western turkey the data reveal a change from dominantly calc alkaline and silicic in the early miocene to largely alkaline and more mafic volcanism in the middle miocene this probably reflects a decreasing amount of crustal contamination with time a result of extensional tectonics high levels of various incompatible elements including k in the more mafic members suggest an enriched subcontinental lithospheric source region for the middle miocene use lavas highly variable nb y ti y and th nb ratios suggest a lithospheric mantle heterogeneously enriched by two processes 1 enrichment by subduction related processes producing high th nb but low nb y and ti y and 2 enrichment by small degree melts of depleted upper mantle producing low th nb but high nb y and ti y both of these enrichment processes have variably contributed to middle miocene k rich lavas in the use area the mechanism which initiated the melting of the enriched lithosphere is considered to be extension which produced decompression melting comparisons with the nearby kula lavas reveals that by the pliocene to quaternary volcanism although still enriched in incompatible elements had become sodic it seems likely that continued extension up to this time thinned the lithosphere to such an extent that asthenospheric melts were produced which ascended and mixed with previously enriched lithosphere

thsr isotopic relationships in morb abstract although morb are well known to exhibit a limited range in isotopic ratios such as 87sr 86sr a compilation of existing 230th 232th data which reflect present th u in the source shows a large dispersion this is in contrast to the admittedly limited data available for oceanic islands for which there is a broad correlation between 87sr 86sr and 230th 232th the distribution of data for morb is approximately bimodal with relatively depleted samples plotting from within to well above the band defined by the oceanic island correlation 87sr 86sr 0 70230 0 70270 and 230th 232th 1 1 2 3 and relatively enriched morb plotting from within to below this band 87sr 86sr 0 70270 70320 and 230th 232th 0 8 1 4 the data are consistent with the generally accepted idea that e morb are generated by the interaction of depleted mantle with more enriched material having higher th u and 87sr 86sr however the large range in source th u implied by the thorium isotopic compositions of both enriched and depleted morb requires complex melting and or other recent source modification processes much of this variability may be caused by multiple small degree melt extraction at the spreading ridge a process which would affect all incompatible trace element concentrations and ratios

low to intermediate subsurface temperatures calculated by chemical geothermometers abstract the concentrations of silica and proportions of sodium potassium lithium calcium and magnesium in water from hot springs and geothermal wells have been combined into 14 chemical geothermometers that are used successfully to estimate the subsurface temperatures of the reservoir rocks modified versions of these 14 geothermometers and a new chemical geothermometer based on the concentrations of magnesium and lithium were used to estimate the subsurface temperatures 40 c 200 c of more than 200 formation water samples from about 30 oil and gas fields located in coastal texas and louisiana central valley california and north slope alaska the new mg li geothermometer which can be used to estimate subsurface temperatures as high as 350 c is given by t 1 900 4 67 log cmg 0 5 cli 273 where t is temperature c and c is the concentration mg l of the subscripted cation quartz mg li na k ca mg and na li geothermometers give concordant subsurface temperatures that are within 10 c of the measured values for reservoir temperatures higher than about 70 c mg li na li chalcedony and na k geothermometers give the best results for reservoir temperatures from 40 c to 70 c subsurface temperatures calculated by chemical geothermometers are at least as reliable as those obtained by conventional methods chemical and conventional methods should be used together where reliable temperature data are required end of article last page 273

uth fractionation by fluids in k rich magma genesis the vico volcano central italy abstract the 230th 238u disequilibrium method has been applied to the potassic volcanic series of vico latium central italy the 3 main events of the recent volcanic activity have been dated using whole rock isochrons the lava flows of the main cone were erupted between 260 and 180 ky pyroclastic flows are dated around 90 ky and the final effusive activity mte venere around 60 ky these results are in good agreement with previous k ar ages the intense th u fractionation th u varying from 3 2 to 11 is apparently not related to processes of partial melting and crystal fractionation the th u fractionation is the most intense in basic terms of the first episode and is due to the percolation of a deep hydrothermal fluid which impoverishes u in different batches of primary magmas more limited th u fractionations in highly differentiated magma are due to u accompanied by sb and alkalies enrichments in the shallow magma chambers a time integrated model is proposed for the evolution of the 230th 232th ratio in the mantle source of vico it shows that th u and th ta fractionations are probably related to the bulk k u and th enrichment of the mantle of the roman province around 300 ky ago a metasomatic enrichment by a silicate melt is proposed to explain the anomalously high th u and th ta fractionations

uzth isotopic systematics at 13 n east pacific ridge segment fresh basaltic glasses have been analyzed for uzth disequilibrium systematics as part of an extensive study on the east pacific rise epr at 12 45 n these samples are well described in terms of major and trace elements as well as in nd pb and sr isotopes our results show significant heterogeneities in the mantle source at a small scale and show heterogeneities at larger scales also when compared to other epr data u and th concentration and isotopic data rule out fractional crystallization as a main process and support a mixing model in agreement with the marble cake model developed by alle gre and turcotte and constrained by trace elements and nd sr and pb isotopes on the same samples by prinzhofer et al based on the high 230th 232th isotopic ratios on recent tholeiites especially the th u values inferred for their sources clearly show that the upper mantle th u has decreased with time

thorium uranium ratio of the earth through the study of komatiites and ophiolites remnants of the old mantle we review the evolution of the lead system in the depleted mantle through time using the 230th232th vs 208pb 206pb diagram 208pb 206pb radiogenic lead ratio we constrain the th u ratio of the bulk silicate earth to 4 2 this value is also in full agreement with ratios inferred from the study of the various types of meteorites using this value in the 208pb 206pb 143nd144nd diagram bulk earth plots on the oceanic basalt correlation line as previously guessed a comparison with sr and nd data shows that the th u fractionation correlates with that of rb sr and nd sm whereas the u pb fractionation is independent of other ratios

submarine magma degassing and explosive magmatism at macdonald tamarii seamount macdonald seamount is an active volcanic centre located at 29 98 s 140 25 w in the south central pacific since its discovery1 in 1968 a number of expeditions have surveyed and dredged rocks from the seamount during a scripps institution of oceanography expedition in october 1987 the first direct observation of volcanic activity at macdonald was made when a small eruption occurred spewing forth scoriaceous basaltic rocks which briefly floated to the sea surface the rocks were accompanied by huge bubbles of volcanic gas that created large sea surface slicks h craig personal communication similar activity at macdonald was observed by a joint french german team in january 1989 using the submersible cyana2 here we look at the radio geochemical features of the lone rock recovered from the october 1987 eruption we present analyses of u series nuclides from the first observed eruption of macdonald also known as tamarii seamount with interpretations focusing on the timescales of mag matic events and the implications of uranium series data for magma degassing this highly gas charged magma lost all of its volatile 210po upon eruption based on this estimates of the fluxes of other volatile elements to sea water which may contribute significantly to their oceanic budget are calculated high 230th 232th and 87sr 86sr ratios common signatures of altered crust suggest that the explosive nature of the macdonald volcan ism results partly from the sealing of magma conduits by seawater circulation

coastal mudflat accretion under energetic conditions louisiana chenier plain coast usa mudflat accretion on the chenier plain coast of louisiana northern gulf of mexico is anomalous in an area that otherwise experiences widespread land loss due to rapid relative sea level rise accretion is shown to be related to energetic events winter cold fronts and occasional tropical depression storms using a 17 year record of meteorological conditions and aerial surveys the results indicate substantial differences between the behavior of sand and mud dominated coastal systems under energetic conditions comparison of the louisiana chenier plain to other mud rich coasts indicates that certain conditions are necessary for mudflat accretion to occur during energetic atmospheric activity these include an abundant supply of fine grained fluvial sediment and resuspension events that maintain an unconsolidated sea floor dominant onshore wind direction during energetic conditions particularly when onshore winds coincide with high fluvial sediment input to the coastal ocean and a low tidal range

forecasting landscape effects of mississippi river diversions on elevation and accretion in louisiana deltaic wetlands under future environmental uncertainty scenarios abstract large sediment diversions are proposed and expected to build new wetlands to alleviate the extensive wetland loss 5000 km 2 affecting coastal louisiana during the last 78 years current assessment and prediction of the impacts of sediment diversions have focused on the capture and dispersal of both water and sediment on the adjacent river side and the immediate outfall marsh area however little is known about the effects of sediment diversions on existing wetland surface elevation and vertical accretion dynamics in the receiving basin at the landscape scale in this study we used a spatial wetland surface elevation model developed in support of louisiana s 2012 coastal master plan to examine such landscape scale effects of sediment diversions multiple sediment diversion projects were incorporated in the model to simulate surface elevation and vertical accretion for the next 50 years 2010 2060 under two environmental moderate and less optimistic scenarios specifically we examined landscape scale surface elevation and vertical accretion trends under diversions with different geographical locations diverted discharge rates and geomorphic characteristics of the receiving basin model results indicate that small diversions 3 s 1 tend to have limited effects of reducing landscape scale elevation loss 1500 m 3 s 1 are required to achieve landscape level benefits to promote surface elevation via vertical accretion to keep pace with rising sea level