the origin of fluoride in groundwater supply to hermosillo city sonora mexico anomalous high fluoride concentration up to 7 59 mg dm3 is found in groundwater from la victoria area this water is used to supply drinking water to hermosillo city sonora geochemistry of groundwater relationship between physicochemical parameters hydrogeology and geologic setting were correlated to define the origin and the geochemical mechanisms of groundwater fluorine enrichment high fluoride concentration is associated with high bicarbonates ph and temperature and it decreases toward the west and south of the area fluoride is in negative correlation to calcium concentration sodium sulphate facies of regional deep water flow are related to high fluoride concentration high electric resistivity rocks associated with granites from the sierra bachoco basement might be the deep source of fluoride outcropping of sierra bachoco in the west causes upward regional flow groundwater of longer residence time can be pumped there the anomalous area is restricted to la victoria because calcareous paleozoic rocks outcrop to the south

active seismic deformation in the grabens of northern central america and its relationship to the relative motion of the north america caribbean plate boundary abstract the active seismic deformation along the grabens of northern central america is calculated using a method which relates relative plate velocity with the seismic strain rate tensor the latter is in turn obtained from seismic moment tensors we calculate the typical or average moment tensor from available centroid moment tensors and fault plane solutions this average tensor is then used to obtain the sum of moment tensors of historical and modern earthquakes reported in the literature both historical and modern earthquakes yield an average extension rate of 8 mm yr along the grabens of northern central america while the relative motion between the north america and caribbean plates expressed along the motagua polochic fault system is 20 mm yr this result suggests that part of the seismic deformation related to the plate boundary is taken up as extension along the grabens which play the role of fault termination structures for the motagua polochic system

rotation of the chortis block causes dextral slip on the guayape fault the chortis block is undergoing deformation as a direct result of large magnitude sinistral strike slip faulting on the north america caribbean plate boundary we have studied the chortis block using geologic mapping remote imagery and earthquake seismology to evaluate the effect that large magnitude strike slip faulting has on an adjacent microcontinental block in the eastern part of the chortis block recent geologic mapping has shown that the northeast striking guayape fault is dextral instead of sinistral as previously assumed because this is a major fault that cuts most of the central america isthmus the new evidence for dextral slip necessitates a reinterpretation of the active tectonics of the chortis block the presence of several north trending rifts in the western part of the chortis block has led previous workers to propose that the chortis block is divided into smaller independently rotating blocks we use earthquake seismology and remote imagery data to extend this interpretation to the central portion of the chortis block however we have modified their model to account for dextral slip on the guayape fault we believe that rotation of the chortis block causes dextral slip on the guayape fault even though central america is undergoing large sinistral displacement with respect to north america

light elements and li isotopes across the northern portion of the central american subduction zone 1 quaternary mafic rocks have been erupted from volcanoes both on and behind the volcanic front in southeastern guatemala and western el salvador behind the front volcanism extends almost continuously some 110 km from the front b and li contents as well as b nb and li yb ratios are generally higher in the mafic volcanic rocks of the volcanic front and lower in those behind the front beryllium concentrations show the opposite relationships with generally higher values behind the front in addition many behind the front samples have lower 7li 2 3 versus 4 6 for the volcanic front guatemalan volcanic rocks in general have lower 7li than those found along the remainder of the central american margin the light element variations across northern central america clearly indicate that the volcanic front receives a much more pronounced fluid contribution from dehydrating subducting cocos lithosphere than the behind the front region this dehydration pulse occurs at slab depths between 85 and 105 km and selective enrichments of b cs and cl in volcanic rocks of the volcanic front suggest that the source of this pulse is dehydrating serpentinite in addition the dehydration pulse from subducted serpentinite is likely superimposed on a continuous dehydration signal provided by subducting oceanic crust which continues behind the volcanic front to slab depths 200 km the enhanced fluid contributions at the front produce higher degrees of wedge melting thus explaining the lower be contents of front samples there are two possible explanations of the lower 7li of many behind the front volcanic rocks from southeastern guatemala western el salvador selective loss of 7li during progressive dehydration of subducting cocos lithosphere or diffusive fractionation accompanying late stage selective crustal contamination since the slightly lower 7li of guatemalan volcanic rocks in general is consistent with a minor crustal overprint the second explanation is favored and could be relevant to the few subduction zones where lower li isotopic values characterize back arc regions light element variation across southeastern guatemala and western el salvador is consistent with the operation of decompression melting behind the volcanic front be zr ratios across the northern portion of the central american subduction zone may indicate some mobility of be

volcanic history and magmatic evolution of seguam island aleutian island arc alaska new 40 ar 39 ar dating coupled with detailed field mapping stratigraphy and chemical analyses have established an eruptive chronology that reveals and constrains the compositional and volumetric evolution of seguam island in the aleutian island arc alaska sixty new 40 ar 39 ar ages from lavas domes and pyroclastic deposits were obtained using furnace incremental heating techniques on replicate samples of whole rock and ground mass separates and they constrain the duration of pleistocene to holocene subaerial volcanism to 318 k y the 40 ar 39 ar plateau ages indicate that over 85 of the complex 68 km 3 of material was erupted almost continuously between 318 ka and 9 ka at ca 9 ka a stratocone on the eastern half of the island partially collapsed producing a 4 km wide crater rhyolitic dome forming eruptions followed from vents in the newly created crater and were likely contemporaneous with 8 0 km 3 of basaltic and basaltic andesitic effusions from pyre peak and a 1 4 km 3 basaltic eruption from a monogenetic cone on the far eastern end of the island geochemical changes over the last 318 k y are subtle most notably the earliest eruptions from 318 to 142 ka produced no andesite and basalt from this period has larger ranges in zr rb and la yb than younger basalts small volumes of dacitic to rhyolitic magma were produced from basalt by a monotonic crystal liquid fractionation process that varied only slightly in successive eruptive phases over 318 k y we identified minor geochemical changes in magma composition during each of the three main stages of volcanism but overall the monotonic variations in major and trace element compositions of basaltic andesitic to rhyolitic lavas are consistent with an origin via closed system fractional crystallization of basalt using the 40 ar 39 ar geochronology and estimates of individual flow volumes we calculated a time averaged eruptive rate at seguam that is similar to growth rates of other well dated arc volcanoes in the cascades and chilean southern volcanic zone but less than that of mount katmai and mount mageik which are located on the alaska peninsula the eruptive flux at seguam has been highly variable fluctuating more than an order of magnitude from 0 07 km 3 k y during the early history of bimodal volcanism to 1 18 km 3 k y over the past 9 k y

geochemistry of fluoride rich groundwater in kolar and tumkur districts of karnataka groundwater is a significant water resource in india for domestic irrigation and industrial needs by far the most serious natural groundwater quality problem in india in terms of public health derives from high fluoride arsenic and iron concentrations hydrogeochemical investigation of fluoride contaminated groundwater samples from kolar and tumkur districts in karnataka are undertaken to understand the quality and potability of groundwater from the study area the level of fluoride contamination the origin and geochemical mechanisms driving the fluoride enrichment majority of the groundwater samples did not meet the potable water criteria as they contained excess 1 5 mg l fluoride dissolved salts 500 mg l and total hardness 75 924 mg l hydrogeochemical facies of the groundwater samples suggest that rock weathering and evaporation crystallization control the groundwater composition in the study area with 50 67 of samples belonging to the ca hco3 type and the remaining falling into the mixed ca na hco3 or ca mg cl type the saturation index values indicated that the groundwater in the study area is oversaturated with respect to calcite and under saturated with respect to fluorite the deficiency of calcium ion concentration in the groundwater from calcite precipitation favors fluorite dissolution leading to excess fluoride concentration

geochemical provenance and spatial distribution of fluoride in groundwater of taiyuan basin china hydrogeochemistry data were utilized to understand origin distribution and geochemical evolution of the high fluoride groundwater in taiyuan basin china in the study area the spatial distribution of the high fluoride groundwater are strictly controlled by the host rock and geomorphic conditions three types of groundwater with the f concentration of 2 mg l are located in the areas bordering the limestone zones in the areas bordering the sandstone of permian and carboniferous and in the depressions of the central parts of the basin respectively the high fluoride groundwater mostly have the high values of tds and its values of ph range from 7 2 to 8 8 the most common water types of the high fluoride groundwater are na ca hco3 and na mg hco3 the geochemical mode reveals that the dissolution of the fluorine containing minerals and the evaporation effect of the shallow groundwater control the evolution of high f concentration in taiyuan basin

hydrogeochemical study in the main ethiopian rift new insights to the source and enrichment mechanism of fluoride the central main ethiopian rift suffers a severe water quality problem characterized by an anomalously high fluoride f content that causes an endemic fluorosis disease the current study conducted in the ziway shala lakes basin indicates that the f content exceeds the permissible limit for drinking prescribed by the world health organization who 1 5 mg l in many important wells up to 20 mg l with even more extreme f concentration in hot springs and alkaline lakes up to 97 and 384 mg l respectively the groundwater and surface water from the highlands typically characterized by low total dissolved solids tds and ca mg hco3 hydrochemical facies do not show high f content the subsequent interaction of these waters with the various rocks of the rift valley induces a general increase of the tds and a variation of the chemical signature towards na hco3 compositions with a parallel enrichment of f the interacting matrixes are mainly rhyolites consisting of volcanic glass and only rare f bearing accessory minerals such as alkali amphibole comparing the abundance and the composition of the glassy groundmass with other mineral phases it appears that the former stores most of the total f budget this glassy material is extremely reactive and its weathering products i e fluvio volcano lacustrine sediments further concentrate the fluoride the interaction of these weathered reworked volcanic products with water and carbon dioxide at high ph causes the release of fluoride into the interacting water this mainly occurs by a process of base exchange softening with the neo formed clay minerals i e ca mg uptake by the aquifer matrix with release of na into the groundwater this is plausibly the main enrichment mechanism that explains the high f content of the local groundwater as evidenced by positive correlation between f ph and na and inverse correlation between f and ca mg saturation indices si have been calculated using phreeqc 2 for the different water groups highlighting that the studied waters are undersaturated in fluorite in these conditions fluoride cannot precipitate as caf2 and so mobilizes freely without forming other complexes these results have important implications for the development of new exploitation strategies and accurate planning of new drilling sites

hydrogeochemistry of sodium bicarbonate type bedrock groundwater in the pocheon spa area south korea water rock interaction and hydrologic mixing the pocheon spa land area south korea occurs in a topographically steep fault bounded basin and is characterized by a hydraulic upwelling flow zone of thermal water up to 44 c in its central part hydrogeochemical and environmental isotope data for groundwater in the study area suggested the occurrence of two distinct water types a ca hco3 type and a na hco3 type the former water type is characterized by relatively high concentrations of ca so4 and no3 which show significant temporal variation indicating a strong influence by surface processes in contrast the na hco3 type waters have high and temporally constant temperature ph tds na cl hco3 and f indicating the attainment of a chemical steady state with respect to the host rocks granite and gneiss oxygen hydrogen and tritium isotope data also indicate the differences in hydrologic conditions between the two groups the relatively lower 18o d and tritium values for na hco3 type waters suggest that they recharged at higher elevations and have comparatively long mean residence times considering the geologic and hydrogeologic conditions of the study area na hco3 type waters possibly have evolved from ca hco3 type waters mass balance modeling revealed that the chemistry of na hco3 type water was regulated by dissolution of silicates and carbonates and concurrent ion exchange particularly low ca concentrations in na hco3 water was mainly caused by cation exchange multivariate mixing and mass balance modeling m3 modeling was performed to evaluate the hydrologic mixing and mass transfer between discrete water masses occurring in the shallow peripheral part of the central spa land area where hydraulic upwelling occurs based on q mode factor analysis and mixing modeling using phreeqc an ideal mixing among three major water masses surface water shallow groundwater of ca hco3 type deep groundwater of na hco3 type was proposed m3 modeling suggests that all the groundwaters in the spa area can be described as mixtures of these end members after mixing the net mole transfer by geochemical reaction was less than that without mixing therefore it is likely that in the hydraulic mixing zone geochemical reactions are of minor importance and therefore that mixing regulates the groundwater geochemistry

epeirogenic uplift above a detached slab in northern central america p wave tomographic images reveal that the northern central america highlands east of the modern volcanic arc overlie a detached slab hypsometric analysis of the highlands in honduras demonstrates that the region is a dissected plateau that is disrupted by normal faults near the north american caribbean plate margin the dissected central american plateau contains a network of superimposed rivers with meanders cut into bedrock such a geomorphic character indicates that the regional uplift occurred in the absence of tilting we propose that the epeirogenic uplift of northern central america is the buoyant upper plate response to the influx of mantle asthenosphere following the break off and sinking of the slab

cenozoic extension along the reactivated aurora fault system in the east antarctic craton abstract the east antarctic craton is characterized by major intracontinental basins and highlands buried under the 34 ma east antarctic ice sheet their formation remains a major open question paleozoic to cenozoic intraplate extensional tectonic activity has been proposed for their development and in this work the latter hypothesis is supported here we focus on the aurora trench at within the aurora subglacial basin latitude 75 77 s longitude 117 118 e whose origin is still poorly constrained the at is an over 150 km long 25 km wide subglacial trough elongated in the nnw sse direction geophysical campaigns allowed better definition of the at physiography showing typical half graben geometry the rounded morphology of the western flank of the at was simulated through tectonic numerical modelling we consider the subglacial landscape to primarily reflect the locally preserved relict morphology of the tectonic processes affecting the interior of east antarctica in the cenozoic the bedrock morphology was replicated through the activity of the listric aurora trench fault characterized by a basal detachment at 34 km considered the base of the crust according to available geophysical interpretations and vertical displacements ranging between 700 and 300 m the predicted displacement is interpreted as the partial reactivation of a weaker zone along a major precambrian crustal scale tectonic boundary we propose that the aurora trench fault is the southern continuation of the 1000 km long aurora fault independently recognized by previous studies together they form the aurora fault system a long lived tectonic boundary with poly phased tectonic history within the eac that bounds the eastern side of the aurora subglacial basin the younger cenozoic reactivation of the investigated segment of the aurora fault system relates to the intraplate propagation of far field stresses associated to the plate scale kinematics in the southern ocean

geochemical appraisal of fluoride contamination of groundwater in the nayagarh district of orissa india the occurrence of high concentrations of fluoride in groundwater 1 5 mg l in the villages of singhpur and sagaragan in the nayagarh district of orissa and its relation with the fluoride rich hot spring water 10 mg l located nearby have been studied the composition of the hot spring water is very comparable to the other thermal waters of the world thermodynamic considerations show that more than 65 of the groundwater and hot spring water samples are oversaturated with calcite and only four samples are oversaturated with fluorite fluoride concentration shows a positive relation to sodium bicarbonate and ph and a negative relation to calcium and magnesium in fluoride rich groundwater and hot spring water principal component analysis shows that the fluoride rich groundwater was produced due to a mixing of hot spring water with the surrounding groundwater studies of the saturation index of fluorite with log electrical conductivity also indicate mixing of hot spring water with the surrounding groundwater the spatial distribution of fluoride in groundwater follows the topography of the area

origin of silicic magmas along the central american volcanic front genetic relationship to mafic melts abstract silicic pyroclastic flows and related deposits are abundant along the central american volcanic front these silicic magmas erupted through both the non continental chorotega block to the southeast and the paleozoic continental chortis block to the northwest the along arc variations of the silicic deposits with respect to diagnostic trace element ratios ba la u th ce pb oxygen isotopes nd and sr isotope ratios mimic the along arc variation in the basaltic and andesitic lavas this variation in the lavas has been interpreted to indicate relative contributions from the slab and asthenosphere to the basaltic magmas carr m j feigenson m d bennett e a 1990 incompatible element and isotopic evidence for tectonic control of source mixing and melt extraction along the central american arc contributions to mineralogy and petrology 105 369 380 patino l c carr m j and feigenson m d 2000 local and regional variations in central american arc lavas controlled by variations in subducted sediment input contributions to mineralogy and petrology 138 3 265 283 with respect to along arc trends in basaltic lavas the largest contribution of slab fluids is in nicaragua and the smallest input from the slab is in central costa rica similar trends are observed in the silicic pyroclastic deposits data from melting experiments of primitive basalts and basaltic andesites demonstrate that it is difficult to produce high k 2 o na 2 o silicic magmas by fractional crystallization or partial melting of low k 2 o na 2 o sources however fractional crystallization or partial melting of medium to high k basalts can produce these silicic magmas we interpret that the high silica magmas associated central america volcanic front are partial melts of penecontemporaneous mantle derived evolved magmas that have ponded and crystallized in the mid crust or are melts extracted from these nearly completely crystallized magmas

controls on elevated fluoride and arsenic concentrations in groundwater from the yuncheng basin china chemical analysis of groundwater and sediments was carried out to investigate causes of elevated f 1 5 6 6 mg l and as concentrations 10 27 mu g l one sample affected by local contamination with 4870 mu g l as in groundwater from the yuncheng basin northern china groundwater from 9 out of 73 wells contains both f and as concentrations above world health organisation safe drinking guidelines 1 5 mg l and 10 mu g l respectively f concentrations above safe levels are more widespread than as 27 vs 12 wells the elevated as and f concentrations represent a significant health risk as groundwater is widely used to supply agricultural and domestic water in the region high f and as concentrations occur in shallow groundwater affected by agriculture and deep groundwater with long residence times 13 ka that shows little sign of anthropogenic influence the strong positive correlation between groundwater f cl and as cl ratios r 2 0 98 and 0 77 in shallow and deep groundwater respectively indicates that these elements are mobilized and enriched by common processes positive correlations between f and as concentrations and na ca ratios r2 0 67 and 0 46 respectively indicate that groundwater major ion chemistry plays a significant role in mobilizing f and as mobilization likely occurs via de sorption of as and f anions e g haso 4 2 and f from hydrous metal oxides moderate positive correlations between ph and as and f concentrations r 2 0 36 and 0 17 respectively indicate that high ph may favour de sorption while hco 3 may act as a sorption competitor high groundwater na ca ratios likely result from cation exchange while ph and hco 3 contents are predominantly controlled by carbonate weathering reactions sediments from the area were reacted with various water solutions producing f concentrations between 0 49 and 2 7 mg l and as concentrations between 0 51 and 16 7 mu g l up to 45 more f and 35 more as were released when sediments were reacted with a na rich ca poor solution compared to a ca rich solution this is consistent with increased mobilization of f and haso 4 2 by na rich ca poor groundwater increasing f and as concentrations across a wide area caused by high levels of pumping is a potential future health concern c 2011 elsevier ltd all rights reserved

constraints from finite element modeling on the active tectonics of northern central america and the middle america trench 1 we have developed an elastic finite element model in order to study the role of the different forces acting on the northwestern part of the central american volcanic arc and the chortis block we present synthetic focal mechanisms maps of tectonic regime and strain crosses to analyze the results the models show that to achieve the observed state of stress on the volcanic arc the arc must be modeled as a lithospheric weak zone also the forces related to the eastward drift of the caribbean plate must be higher than those related to the subduction of the cocos plate the coupling on the subduction interface must be low with or without slip partitioning due to the obliquity of the subduction at the trench at guatemala the western edge of the chortis block is pinned against north america even with low trench normal forces making the triple junction between the cocos north american and caribbean plates a zone of diffuse deformation the extension in the western part of the chortis block from guatemala to the honduras depression is explained by the geometry of the north american caribbean plate boundary and the direction of motion of the caribbean plate with respect to north america the direction of extension in the chortis block is always e w regardless of the magnitude of the applied forces and the main part of the deformation is absorbed between the ipala graben and the honduras depression both features being consistent with our models

10be imaging of sediment accretion and subduction along the northeast japan and costa rica convergent margins sediment accretion and subduction at convergent margins play an important role in the nature of hazardous interplate seismicity the seismogenic zone and the subduction recycling of volatiles and continentally derived materials to the earth s mantle identifying and quantifying sediment accretion essential for a complete mass balance across the margin can be difficult seismic images do not define the processes by which a prism was built and cored sediments may show disturbed magnetostratigraphy and sparse biostratigraphy this contribution reports the first use of cosmogenic 10be depth profiles to define the origin and structural evolution of forearc sedimentary prisms biostratigraphy and 10be model ages generally are in good agreement for sediments drilled at deep sea drilling project site 434 in the japan forearc and support an origin by imbricate thrusting for the upper section forearc sediments from ocean drilling program site 1040 in costa rica lack good fossil or paleomagnetic age control above the decollement low and homogeneous 10be concentrations show that the prism sediments are older than 3 4 ma and that the prism is either a paleoaccretionary prism or it formed largely from slump deposits of apron sediments low 10be in costa rican lavas and the absence of frontal accretion imply deeper sediment underplating or subduction erosion

fluoride ion contamination in the groundwater of mithi sub district the thar desert pakistan groundwater samples were collected from various localities of mithi sub district of the thar desert of pakistan and analysed for fluoride ion along with other chemical parameters the area is mainly covered by sand dunes and kaolin granite at variable depths results showed that collected water samples were severely contaminated by the presence of fluoride ion and most of the samples have higher concentration than prescribed who standards 1 5 mg l for drinking water fluoride ion concentrations ranged between 0 09 and 11 63 mg l with mean and median values of 3 64 and 3 44 mg l respectively in this area whereas distribution pattern showed high concentrations in the vicinity of islamkot and mithi towns the content of f has also been correlated with other major ions found in the groundwater of the study area the positive correlation of f with na and hco3 showed that the water with high na and hco3 stabilizes f ions in the groundwater of the thar desert the ph versus f plots signifies high fluoride concentration at higher ph values implying that alkaline environment favours the replacement of exchangeable oh with f in the groundwater of mithi area the saturation indices si of fluorite caf2 and calcite caco3 in the groundwater samples showed that most of the samples are oversaturated with respect to calcite whereas majority of samples have been found under saturated with respect to fluorite the log tds and na na ca ratio reflected supremacy of weathering of rocks which promotes the availability of fluoride ions in the groundwater piper diagram has been used to classify the hydrofacies in the cation triangle all samples are na type while the anion triangle reflects major dominance of cl type with a minor influence of hco3 and so4

geochemical control on spatial variability of fluoride concentrations in groundwater from rural areas of gujrat in punjab pakistan spatial variability and geochemical factors controlling fluoride f concentrations were evaluated in five villages of subdistrict gujrat in pakistan groundwater samples n 70 were collected along the river chenab and examined for f concentrations the average f concentrations in groundwater of village chhani nikoani 1 8 mg l and dhool khurd 2 2 mg l were crossing the world health organization who permissible limit of 1 5 mg l while village kalra khasa ghazi chak and kula chor were below the who guidelines about 64 of the samples contain fluoride concentrations that exceed the 1 5 mg l drinking water standard set by who spatially f concentrations in groundwater increased as moving away from the river chenab because more residence time for water rock interaction was available high f concentrations were associated with high electric conductivity ec total dissolved solids tds ph and na values the overall groundwater quality is controlled by silicate mineral weathering and less by evaporation fluoride originates from anion exchange oh for f on clays and weathered micas under high ph conditions high fluoride levels are associated with nacl and nahco3 type water produced by calcite precipitation and or base ion exchange the groundwater with higher fluoride concentrations is accompanied by lower ca 2 levels this study enhances our understanding on geochemical behavior of f and highlights the importance of spatial variability on f release in aquifers of punjab in pakistan as well as in other countries of asia region

source composition and distribution of the fluid in the kurile mantle wedge constraints from across arc variations of b nb and b isotopes b and nb concentrations and b isotope composition were determined for lavas covering the entire length of the kurile arc b nb and 11b of these lavas show clear across arc variations in which both are highest at the trench side and continuously decrease as the slab depth increases b nb in the volcanic front lavas 8 25 is significantly higher than those of morb and oib indicating that the mantle wedge is extensively metasomatized by b rich fluid derived from the subducting slab the frontal 11b values are also out of the range of morb and oib and systematic difference exists between northern kurile nk 5 4 0 5 and central and southern kurile csk 4 3 0 3 however these two parameters approach or coincide with mantle values at the most back arc side the 11b and 87sr 86sr linearly correlate with nb b suggesting control by simple mixing of two isotopically homogeneous components that is slab derived fluid and mantle wedge the estimated 11b and 87sr 86sr of the fluid are 7 0 0 6 and 0 70326 0 00005 for nk and 6 3 0 3 and 0 7033 for csk respectively these fluid compositions are consistent with a source constituted by 95 altered oceanic crust and 5 oceanic sediment with the slight difference between nk and csk resulting from the different lithologies of subducting sediments the mantle wedge beneath the kurile arc has b sr and nd isotope compositions identical with morb but shows significantly higher b nb the above trends bear a close resemblance to those observed in the izu arc strongly suggesting their universality in the arc system the across arc decline of b nb observed both in kurile and izu implies that the fluid flux from the slab continuously decreases as the slab depth increases however this relates only to a final stage of slab dehydration since most of the volatile components are lost at the fore arc region in contrast to b k is uniformly enriched across the arc this requires control by a k bearing mineral phase either in the slab phengite or the mantle wedge phlogopite although such a mineral is responsible for only a small part of the whole fluid transport

controls on the genesis of some high fluoride groundwaters in india india has an increasing incidence of fluorosis dental and skeletal with some 62 million people at risk high fluoride groundwaters are present especially in the hard rock areas south of the ganges valley and in the arid north western part of the country the phenomenon is related to groundwater with residual alkalinity ca2 hco3 fluoride concentrations are governed by adsorption equilibria and by fluorite solubility evapotranspiration leads to a precipitation of calcite a lowering of ca activity and increase in na ca ratios and this allows an increase in f levels in southern india mg seems to be controlled by dolomite while sepiolite and palygorskite are mg sinks in rajasthan but may then release f under alkaline conditions the latter two minerals are probably also important sources and sinks for f in the hydroxy positions the increase in the extent of sodic soils as a result of irrigation is a contributing factor to the increasing incidence of fluorosis remedial measures including addition of gypsum and rainwater harvesting are needed even in areas where the sodicity does not cause structural problems in the soil

genesis of arsenic fluoride enriched soda water a case study at datong northern china abstract the high as and f groundwaters from datong basin are mostly soda waters with a na cl so4 meq ratio greater than unity as and f up to 1550 g l and 10 4 mg l respectively and with ph between 7 6 and 9 1 geochemical modeling indicates that the waters are oversaturated with respect to calcite and clay minerals such as kaolinite and undersaturated with respect to primary rock forming minerals such as anorthite and albite the water chemistry also is affected by evapotranspiration the degree of evaporative enrichment is up to 85 in terms of cl results of the hydrogeochemical studies indicate that the occurrence of soda water at datong is the result of incongruent dissolution of aluminosilicates at one stage of their interaction with groundwater when the water is oversaturated with respect to calcite and evapotranspiration related salt accumulation is not too strong studying the genesis of soda waters provides new insights into mechanism of as and f enrichment in the aquifer system due to caf2 solubility control and oh f exchange reactions f can be enriched in soda water and the high ph condition of soda water favors as desorption from oxyhydroxide surfaces thereby increasing the concentration of as in the aqueous phase

monogenetic behind the front volcanism in southeastern guatemala and western el salvador 40ar 39ar ages and tectonic implications abstract eighteen lavas erupted from cinder cones behind the volcanic front in southeastern guatemala and western el salvador have been dated using the 40 ar 39 ar method on groundmass separates plateau ages range between 1069 5 22 1and 37 12 0 ka the majority of dated lavas are younger than 600 ka importantly cinder cone lavas become progressively younger toward the volcanic front trench in addition the lavas become systematically depleted in cs ba rare earth elements cs zr la zr and 18 o with time the baseline values of these same geochemical parameters increase with distance from the volcanic front the cinder cone lavas are closely associated with rifting south of the north american caribbean plate boundary the trenchward migration of behind the front volcanism may be linked to a westward migration of this rifting existing evidence suggests there was an important change in the face of volcanism behind the volcanic front in southeastern guatemala from a small number of large polygenetic volcanoes to abundant smaller monogenetic cones the temporal spatial changes in the geochemistry of cinder cone lavas is attributed to decreasing contamination by paleozoic basement rocks both with time and with diminishing distances to the volcanic front

lead isotope composition of central american volcanoes influence of the galapagos plume 1 lead isotopic analyses of lavas from central america both along and behind the volcanic arc help to clarify source components in the mantle wedge analysis of previous pb isotopic data had implied that little or no marine sediment lead was added to the central american source region as all samples fell within the morb field in contrast to other information e g ba la 10be 87sr 86sr that indicated a high subduction component the data presented here include several analyses of local marine sediment showing it to be exceptionally unradiogenic in pb and thus permitting high sediment contributions to the mantle source region without significant changes in pb isotopes combined pb nd and pb sr isotopic diagrams clearly illustrate the influence of crustal contamination for samples from guatemala and honduras and of subducted sediment for all lavas of the volcanic front samples collected behind the volcanic front are derived from mixing between enriched and depleted mantle sources and in central costa rica extending to the back arc overlap pb sr and nd isotope values for both cocos island and some components of the galapagos hot spot the restricted geographical occurrence of the enriched mantle signature in central america coupled with the persistence of the signal well into the back arc region imply that these lavas are sampling galapagos plume influenced mantle the presence of this plume component beneath southern central america and extending to the northeast beneath the caribbean confirms a galapagos hot spot origin for this part of caribbean plate