upper mantle s velocity structure and moho depth variations across zagros belt arabian eurasian plate boundary the collision of the arabian and eurasian plates in the early miocene after the subduction of neo tethys ocean beneath eurasia formed the zagros belt a seismically active continental continental plate boundary in southwest iran the zagros suture zone is an important seismotectonic boundary indicating an abrupt cutoff between the intense seismicity of the zagros and the almost aseismic central iran plateau compared with other more evolved plate boundaries little is known about the deep lithospheric structure of the zagros belt such as the fate of subducted neo tethys plate and ambiguity in the presence of intermediate and deep earthquakes under the zagros suture zone in this study we use the partitioned waveform inversion pwi method to image the upper mantle s velocity structure and moho depth variations across zagros collisional zone the resulting moho depth along the profile shows the average moho depth value of order of 40 45 km for most parts of the profile with abrupt crustal thickening in the middle of the profile up to about 65 km as expected the derived models show that the relatively old and cold arabian plate has higher velocity at depth than the younger lithosphere farther north in central iran a sharp and steep subcrustal boundary is found roughly coincident with the surficial expression of the main zagros thrust mzt separating two different mantle domains a high velocity anomaly possibly representing a fragment of subducted lithosphere has been imaged beneath central iran at a depth between 350 and 600 km which is quite similar to the case of indo asian collision in qinghai tibetan plateau these observations as well as the sudden changes of shallow velocities along the cross section with a sharp boundary under the zagros suture zone support the idea that subducted oceanic lithosphere has broken off under the region of maximum moho depth a relatively low velocity region beneath the arabian plate is also imaged in our high resolution tomography model which is interpreted as an indication of lithospheric delamination within the arabian lithosphere

chemical weathering of basalts southwest iceland effect of rock crystallinity and secondary minerals on chemical fluxes to the ocean river ground and peat water from a basaltic catchment area laxa in kjos southwest iceland were sampled and analyzed for major element concentrations to define the effect of glassy versus crystalline basalt the formation of secondary minerals and runoff on fluxes of dissolved elements to the ocean the proportions of the dissolved solids in the water samples derived from precipitation air and rock weathering were estimated the amounts of na mg ca k and s originating from precipitation were calculated from the respective solute cl marine ratios and the estimated aqueous cl originated from precipitation which was calculated using measured b content of the waters and b cl molal marine and rock ratios the rock contribution from weathering ranged from 22 to 46 percent of the total dissolved solids of the waters iron and al showed low mobility compared to na this was primarily due to consumption of these elements by ferrihydroxides allophane imogolite and clay minerals also si and to a lesser degree ca show apparent slightly lower mobility than na these results are in good agreement with these phases being dominant in the soils of the study area wada and others 1992 and the main secondary minerals of basaltic glass in iceland crovisier and others 1992 reaction progress calculations support these findings according to a model interaction of basaltic glass and meteoric water al fe and si are consumed by amorphous fe and al hydroxides and imogolite and allophane at low reaction progress with increasing reaction progress ca fe mg smectite became the primary secondary minerals limiting the mobility of ca fe mg si and al these findings suggest that the weathering in the laxa in the kjos catchment area has evolved into the beginning of the smectite weathering stage and that it is dominated by a relatively low reaction progress and a high water to rock ratio in the present study the effect of runoff and rock crystallinity was quantified for dissolved elemental fluxes temperature lithology other than rock crystallinity and rock age were similar in all of the catchment areas in this study silica ca f s al k mg and b had a very similar dependence on runoff on the other hand na fluxes were found to be less dependent on runoff and fe fluxes were found to be independent of runoff basaltic glass dissolves faster than fully crystallized basaltic rocks gislason and eugster 1987a therefore increased glass content of the primary rocks is thought to increase the leaching and elemental fluxes at constant runoff however because some of the solutes particularly al and fe were highly influenced or even controlled by the formation of secondary minerals this increasing dissolution with increasing glass content was hidden on the other hand basaltic glass was observed to enhance the fluxes of more mobile elements like na si ca f and s by a factor of 2 to 5 at a constant runoff of 200 cm yr constant vegetative cover and 0 to 100 percent glass content of the rocks the k and mg fluxes were found to be independent of rock crystallinity however runoff alone cannot explain the fluxes of these elements and it seems that vegetative cover and seasonal variations in biomass activities influence the k and mg fluxes

properties weathering and classification of some soils formed in peralkaline volcanic ash in kenya abstract physical chemical and mineralogical properties were determined for six profiles in and west of the great rift valley in kenya data include morphology information on gross composition of horizons organic carbon cation exchange capacities extractable cations etc and detailed characterization of the clay fractions by means of transmission electron microscopy x ray diffraction infrared spectroscopy and additional techniques the three least weathered profiles nos 1 3 of table i have morphologies typical of andepts but lack certain other characteristics common to such soils their clay fractions were largely amorphous to x rays and consisted mainly of poorly ordered siliceous feoxides with virtually no allophane all of the clays contained halloysite the peralkaline nature of the ash parent material seems responsible for the clay mineralogy of the soils which differs from that generally found in andepts the high proportions of iron in the amorphous materials depart from the composition widely reported for such soils one distinctive feature was the presence of pantelleritic trachyte found in ash from a number of sites in addition to those of the profiles how the six profiles may best be classified is an open question none of the profiles qualifies as andept nor would they be included in the proposed order of andisols as presently defined modifications of the definition of andisols so as to include some of these soils would seem desirable

growth of an intermittent ice sheet in iceland during the late pliocene and early pleistocene abstract deep sea paleoclimatic records show that the quaternary climate around iceland was and probably still is very sensitive to rapid shifts in north atlantic oceanic circulation studies of several key sections in iceland indicate that similar oscillations are reflected in the late pliocene and early pleistocene terrestrial stratigraphy correlations between six rock sequences in western northern eastern and southern iceland show a fairly distinct trend during the pliocene pleistocene transition indicating the progressive growth of an ice sheet from southeast toward the north and west the correlations are based on k ar dates and paleomagnetic studies a total of 11 glacial horizons are recorded in a stratigraphic column from eastern iceland extending back 6 5 myr in western iceland 7 glacial horizons are preserved in a rock section dated from 7 0 to 1 8 myr and in northern iceland 14 glacial horizons are identified in a section that extends back to 9 0 myr well over 20 glacial horizons have been identified in the stratigraphic column in iceland full scale glacial interglacial cyclicity with regional ice cover is indicated at approximately 2 6 myr a further amplification leading to islandwide glaciations is identified at 2 2 2 1 myr these results show that changes in the ice cover in iceland correlate with the deep sea oxygen isotope records from benthic and planktonic foraminifera as well as ird studies from the north atlantic

glacial steady state topography controlled by the coupled influence of tectonics and climate glaciers and rivers are the main agents of mountain erosion while in the fluvial realm empirical relationships and their mathematical description such as the stream power law improved the understanding of fundamental controls on landscape evolution simple constraints on glacial topography and governing scaling relations are widely lacking we present a steady state solution for longitudinal profiles along eroding glaciers in a coupled system that includes tectonics and climate we combined the shallow ice approximation and a glacial erosion rule to calculate ice surface and bed topography from prescribed glacier mass balance gradient and rock uplift rate our approach is inspired by the classic application of the stream power law for describing a fluvial steady state but with the striking difference that in the glacial realm glacier mass balance is added as an altitude dependent variable from our analyses we find that ice surface slope and glacial relief scale with uplift rate with scaling exponents indicating that glacial relief is less sensitive to uplift rate than relief in most fluvial landscapes basic scaling relations controlled by either basal sliding or internal deformation follow a power law with the exponent depending on the exponents for the glacial erosion rule and glen s flow law in a mixed scenario of sliding and deformation complicated scaling relations with variable exponents emerge furthermore a cutoff in glacier mass balance or cold ice in high elevations can lead to substantially larger scaling exponents which may provide an explanation for high relief in high latitudes

quantification of the effect of plants on weathering studies in iceland the weathering of calcium and magnesium silicate minerals on the continents has exerted a major control on atmospheric co 2 over geologic time and vascular plants may have played an important role in this process in western iceland we have examined the role of plants in weathering by measuring the chemistry of waters draining adjacent areas of basaltic rocks that are either barren having a partial cover of mosses and lichens or populated by trees the study area was chosen to maximize vegetational differences and to minimize differences in microclimate slope and lithology while avoiding hydrothermal waters and anthropogenic acid rain results including data on cation uptake by growing trees indicate that the rate of weathering release of ca and mg to streams and vegetation is two to five times higher in vegetated areas than the release of ca and mg to streams in barren areas this finding suggests a major role for vascular plants in accelerating weathering and thereby lowering atmospheric co 2 as they invaded upland areas of the continents between 380 and 350 ma

major and trace elements of river borne material the congo basin the congo river basin is the second largest drainage basin in the world after the amazon the materials carried by its main rivers provide the opportunity to study the products of denudation of a large fraction of the upper continental crust of the african continent this paper presents the chemical composition of the different phases carried in the congo rivers and is followed by a companion paper devoted to the modelling of major and trace elements the congo river between bangui and brazzaville as well as its main tributaries including a few organic rich rivers also called black rivers were sampled during the 1989 high water stage the three main phases suspended load dissolved load and bedload were analysed for twenty five major and trace elements concentrations normalized to the upper continental crust show that in each river suspended sediments and dissolved load are chemical complements for the most soluble elements ca na sr k ba rb and u while these elements are enriched in the dissolved loads they are considerably depleted in the corresponding suspended sediments this is consistent with their high mobility during weathering another type of complementarity is observed for zr and hf between suspended sediments and bedload related to the differential velocity of suspended sediments and zircons which are concentrated in bedloads compared to other rivers absolute dissolved concentrations of ca na sr k ba rb and u are remarkably low surprisingly high dissolved concentrations are found in the congo waters for other trace elements e g rees especially in the black rivers on a world scale these concentrations are among the highest measured in rivers and are shown to be ph dependent for a number of dissolved trace elements the dissolved loads are systematically normalized to the suspended loads for each river in order to remove the variations of the element abundances owing to source rock variations normalized diagrams for rees are presented and extended to the other elements they strongly support the argument that the apparent higher solubility of trace elements in the congo waters is due to the presence in the dissolved load of a colloidal phase as a result of 0 2 m filtration an important result is that these colloids are strongly depleted in fe and al with respect to the other elements finally the comparison of the dissolved suspended and sandy transport fluxes of each element in the congo basin rivers shows that although the proportions of for example the rees in the dissolved loads of the majority of the congo basin rivers is close to 10 of the total transport flux up to 80 of the rees are transported by the so called dissolved load in the black rivers

dissolution of primary basaltic minerals in natural waters saturation state and kinetics abstract the state of saturation of olivine orthopyroxene and plagioclase of variable composition has been assessed in various types of natural waters in iceland including river waters groundwater and geothermal waters with temperatures up to 250 c the stability of olivine and orthopyroxene decreases with increasing mg content similarly the stability of plagioclase decreases with increasing anorthite content the river waters which are representative of the weathering environment are always undersaturated with both olivine and orthopyroxene the degree of undersaturation being dominated by the water ph all river waters tend to dissolve olivine of the composition encountered in basalts according to the linear rate law and orthopyroxene when the ph is 200 c are undersaturated with olivine orthopyroxene and plagioclase the first two particularly when fe rich and sufficient to cause olivine to dissolve according to the linear rate law at intermediate temperatures 50 150 c the geothermal waters are close to equilibrium with these minerals except for mg rich olivine it seems likely that dissolution of glass from basalt which is very reactive will favor stability of the igneous minerals

dissolution of primary minerals in natural waters abstract the saturation state of olivine plagioclase clinopyroxene orthopyroxene fe ti oxides and apatite of variable composition has been assessed in natural waters in iceland with temperature ranging from 0 c to 300 c and in situ ph from below 5 to above 10 cold waters are undersaturated with respect to olivine orthopyroxene clinopyroxene and plagioclases indicating that all these minerals tend to dissolve under weathering conditions with increasing ph and temperature the waters approach saturation with pyroxene olivine and plagioclases also the degree of undersaturation of olivine and orthopyroxene decreases with increasing fe content of the minerals and fe rich olivine and orthopyroxene are stable between 50 c and 150 c whereas mg rich ones tend to dissolve natural waters in iceland are saturated with respect to pure albite when above 50 c they are on the other hand undersaturated with ca rich plagioclase up to 250 c where the waters reach saturation pure magnetite and hematite are stable at all temperatures with increasing titanium content the minerals become unstable and the waters are undersaturated with respect to pure ulvospinel at temperatures up to 300 c pure ilmenite is however close to saturation under weathering conditions but undersaturated above 200 c f apatite is close to saturation at all temperatures on the other hand undersaturation with respect to oh apatite is observed at all temperatures the weathering susceptibilities of primary minerals of basalt in icelandic waters in increasing order are mg olivine fe olivine ti rich magnetite ca plagioclase mg orthopyroxene fe orthopyroxene clinopyroxene na plagioclase f apatite ti rich ilmenite ti poor magnetite ti poor hematite

solute flux and mineral mass balance approaches to the quantification of plant effects on silicate weathering the weathering of calcium and magnesium silicate minerals on the continents has exerted a major control on atmospheric co 2 over geologic time and vascular plants may have played an important role in this process we have examined the role of plants in weathering in western iceland by two methods a solute flux approach and a mineral mass balance approach using the chemistry of weathering products and of waters draining adjacent areas of basaltic rocks that are either barren with a partial cover of mosses and lichens or populated by trees the study area was chosen to maximize vegetational differences and to minimize differences in microclimate slope and lithology while avoiding hydrothermal waters and anthropogenic acid rain solute flux results including data on cation uptake by growing trees indicate that the rate of weathering release of ca and mg to streams is about 4 times higher in vegetated areas than in bare areas mineral mass balance results show that trees increase plagioclase weathering by a factor of 2 and pyroxene weathering by a factor of 10 possibly as a result of preferential weathering of pyroxene by the vegetation biomass normalized weathering fluxes from the birch trees are greater than those from the conifers suggesting that angiosperms enhance weathering to a greater degree than gymnosperms results indicate a perpetual short term sink for atmospheric c as tree induced leakage of hco super 3 from the forest ecosystem to rivers and streams in addition to the finite sink of c in forest biomass the findings also suggest a major role for vascular plants as they invaded upland portions of the continents between 410 and 360 my ago in accelerating weathering and thereby lowering atmospheric co 2

lithologic and climatologic controls of river chemistry abstract the chemistries of rivers draining a variety of lithologic and climatic regions have been surveyed for the purpose of quantifying the fluxes of bicarbonate and silica from rivers with respect to bedrock lithology and runoff in all 101 different rivers each draining a primary lithology were examined across the united states puerto rico and iceland to minimize seasonal effects only rivers with at least two years of data were used basaltic catchments were examined in the greatest detail in a survey of hawaiian island watersheds the average river chemistries could be related to the distribution of soil associations within each catchment an analysis of cation activity relationships among rivers draining basaltic catchments shows that the river compositions define slopes which are consistent with an equilibrium ion exchange control on cation ratios among different lithologies unique weathering rate relationships were developed with yields at typical present day runoff rates 1 100 cm y increasing in the order sandstones granites basalts shales and carbonates the bicarbonate and silica fluxes for each of these lithologies have been quantified for use in global studies of chemical denudation our study confirms that the dissolved yield of a given drainage basin is determined by a balance between physical and chemical weathering thus a warm wet climate or the presence of abundant vegetation cannot guarantee high rates of chemical denudation unless accompanied by high rates of physical removal

the effect of land plants on weathering rates of silicate minerals abstract land plants and their associated microbiota directly affect silicate mineral weathering in several ways by generation of chelating ligands by modifying ph through production of co2 or organic acids and by altering the physical properties of a soil particularly the exposed surface areas of minerals and the residence time of water in laboratory experiments far from equilibrium 1 mm oxalate a strong chelator of al has a negligible effect on the dissolution rate of alkali feldspars but some effect on calcic feldspars and olivine by analogy to oxalate the overall effect of organic ligands on the weathering rate of silicate minerals in nature is likely to be small except perhaps in microenvironments adjacent to roots and fungal hyphae the effect of ph on silicate mineral dissolution rate depends on ph below ph 4 5 the rate increases with decreasing ph in the circumneutral region the rate is ph independent and at ph values above around 8 the rate increases with increasing ph vegetation should thus cause an increase in weathering rate through the ph effect only where the ph is below 4 5 as an overall generalization the effect of plants on weathering rate through changes in soil solution chemistry is probably small for granitic rocks it may be greater for more mafic rocks it is the release of ca and mg from mafic rocks that has the greatest influence on the global co2 budget the effect of changes in soil physical properties on weathering rate can be major by binding fine particles plants can greatly increase weathering rates in areas of high physical erosion where erosion rates are lower the effect of plants is less clear on long timescales plants may decrease chemical weathering by binding secondary products and isolating unweathered minerals from meteoric water a major unknown in estimating the effect of the advent of land plants on weathering rates is the nature thickness particle size distribution permeability of the regolith on the pre silurian continents the indirect effect of vegetation through changing the regional distribution of precipitation may be as important as the direct effects

crustal spreading due to dikes and faults in southwest iceland abstract a 30 km long profile extending from 3 million year old pleistocene rocks to the holocene rocks of the rift zone in southwest iceland is dissected by 443 dikes and 156 normal faults nearly all the dikes are basaltic with an average strike of n26 e which is subparallel to the trend of fractures in the nearby rift zone their average dip is 69 and average thickness 1 37 m both values much lower than averages for regional tertiary dike swarms in iceland many dikes are seen to change into sills the great majority of the faults are pure dip slip with an average throw of 10 2 m their average strike n37 e is different from the average strike of the dikes in this area and the fractures in the nearby rift zone their average dip is 75 2 which is somewhat steeper than in the tertiary areas but the dips are less in the interbedded sedimentary layers than in the lava flows growth faults are very rare in the area and most of the faults appear to have completed their growth in less than 10 000 years in subsections crustal dilation due to dikes is 0 4 3 7 that due to faults is 0 6 5 3 and that due to both dikes and faults is 1 04 6 44 in these subsections average fault spacing is one fault per 170 563 m spacing decreases toward the active rift zone it is concluded that in typical fissure swarms of the rift zone the dilation at the surface is almost entirely due to extensional fractures and gaping vertical normal faults in the depth range of a couple of hundred metres to perhaps 1 km the non vertical faults contribute most to the dilation at deeper crustal levels however dilation due to dikes becomes dominant

direct effects of co2 and temperature on silicate weathering possible implications for climate control a critical uncertainty in models of the global carbon cycle and climate is the combined effect of organic activity temperature and atmospheric co2 on silicate weathering here we present new dissolution rates of anorthite and augite which indicate that silicate weathering in organic rich solutions is not directly affected by soil co2 but is very sensitive to temperature apparently co2 accelerates silicate weathering indirectly by fertilizing organic activity and the production of corrosive organic acids the weathering dependencies highlight the ability of silicate weathering to act as a global thermostat and damp out climate change when used as input in steady state carbon cycle and climate models

quartz sillimanite leucosomes in high grade schists black hills south dakota a perspective on the mobility of al in high grade metamorphic rocks in the black hills south dakota anatectic migmatites that formed above the second sillimanite isograd comprise mesosomes dominated by quartz biotite sillimanite plagioclase and sporadic poikiloblasts of k feldspar melanosomes dominated by biotite and sillimanite and leucosomes some leucosomes have near minimum granite composition but many lack alkali elements and have fibrolitic sillimanite instead of feldspar the proportion of si al is the same in both leucosome types the sillimanite containing leucosomes include fluorapatite which indicates high activity of f the lack of alkali feldspar in these leucosomes is attributed to its instability relative to sillimanite quartz in the presence of fluids with high a h a k ratios that evolved from the crystallizing leucosomes it is suggested that quartz aluminosilicate segregations and veins in other high grade terranes may also represent pegmatitic melts in which alkali feldspar crystallization was inhibited by high activities of hydrogen resulting in loss of alkali elements

clay minerals of four soils formed in eolian and tephra materials in iceland abstract clay minerals in a vitricryand and three haplocryands derived from eolian and tephra materials in different parts of iceland and in glacial till samples representative of underlying strata were studied by a combination of methods the soils are primarily used as rangelands and are highly susceptible to wind erosion allophanes imogolite and poorly crystalline ferrihydrite are abundant 700 g kg of the clay throughout the soils including a horizons this clay mineralogy would give rise to stable low density aggregates that influence the susceptibility of the soils to wind erosion it also affects chemical properties such as high phosphate retention of the soils even of the cryands with relatively low clay contents 160 275 g kg imogolite was found in some but not all horizons of all soils no particular relationship of imogolite to measured si al molar ratios of the oxalate oxalic acid soluble fractions of the clays 0 51 to 1 13 was evident layer silicate minerals and laminar opaline silicas are either not present or occur in small quantities plagioclase augite and or their weathering residues were found in the clay and silt separates dark colored glass is predominant in the fine sand throughout the soils and the tills one till sample had a clay mineral composition similar to the eolian andic soils while the other contained smectite and possibly a spherical halloysite smectite intermediate but not allophane and imogolite as major weathering products the clay mineral composition of these soils and tills was compared with those derived from basic tephras in other regions

fluorine in basalts from iceland the fluorine content of icelandic tholeiitic and alkaline basalts matches values found in similar rocks from other areas covariation between fluorine and incompatible minor elements such as potassium or phosphorus is found in evolved tholeiites and alkali basalts lack of such covariation in primitive olivine tholeiites indicates that fluorine and other incompatible minor and trace elements are not controlled by minerals such as amphibole mica or apatite in the mantle residue and that the covariation between these elements in the evolved basalts cannot be inherited from the mantle model calculations on rocks from the langjokull area show that olivine tholeiite suites are if derived by simple fractional crystallization enriched in incompatible elements much in excess of the increase due to crystal removal these observations are taken to indicate that the well documented covariation between fluorine and other incompatible elements is not established until evolution of the basaltic magma has started in crustal holding chambers the constancy of element ratios and enrichment in excess of what can be accounted for by crystal fractionation or incremental addition of new batches of primitive magmas does indicate 1 mineral control involving amphibole mica or apatite and 2 addition of fluorine potassium and phosphorous from an external source it is argued that this source is the crustal envelope of the holding chamber

the fluvial geochemistry and denudation rate of the guayana shield in venezuela colombia and brazil abstract the guayana shield is composed of early to mid precambrian igneous and metamorphic basement rocks with a quartzitic platform cover the complete absence of limestones and evaporites allows a clear chemical expression in the stream data of the primary weathering of the basement in a humid tropical environment total erosion rates are extremely slow 10 m m y with equal contributions from the dissolved and suspended loads however the former is largely silica with ratios of si to total cation equivalents si tz ranging to in excess of three weathering is complete to kaolinite and gibbsite i e the environment is one of active laterisation with a penetration rate of the weathering front into the fresh substrate about twice the denudation rate in basins of relatively homogeneous lithology rb sr isochrons constructed from the river data agree with the whole rock ages from the drainages thus all the common refractory rb containing minerals k feldspar mica are completely dissolved the thick lateritic regolith that is accumulating as a result of this intense weathering is a common relict feature on other southern hemisphere shields in the absence of active tectonics or greatly accelerated mechanical erosion the weathering rates of these basement rocks must be quite insensitive to environmental change

meteoric water basalt interactions ii a field study in n e iceland the compositions of rain snow melt spring and geothermal waters from the rift zone of n e iceland can be explained by seaspray addition chemical fractionation at the seawater air interface burning of fossil fuel farming activities purification by partial melting of snow and ice dissolution of basalts and buffering by alteration minerals the dissolution of the rocks appears to be incongruent during solute acquisition spring compositions move through the stability fields of kaolinite and smectite to the laumontite and illite fields all but four of the springs are undersaturated with respect to calcite silica concentrations are compatible with the solubility of basaltic glass the reactions reflected in the spring waters appear to have taken place sealed off from atmospheric co2 after initial saturation the geothermal waters which are recharged by the spring waters are depleted in mg and ca but enriched in carbon and sulfur with respect to dissolution of primary rocks expressions are derived relating dissolution rates of rocks age of groundwaters physical properties of groundwaters and mass transfer the characteristic rock particle radii in the cold water aquifers range from 0 2 to 2 cm and the characteristic crack openings are of the order 0 04 to 0 4 cm using laboratory studies on the icelandic lavas as a guide the residence times of the cold waters in the aquifers can be estimated at 60 days to 4 years the average active surface area of the aquifers enclosing 1000 g of spring water is of the order of 0 6 to 6 m2 and these 1000 g of water have reacted with 0 1 to 1 g of basaltic rocks the same mass of thermal water has interacted with 100 to 300 g of unaltered rocks

thermodynamic properties for bunsenite nio magnetite fe 3 o 4 and hematite fe 2 o 3 with comments on selected oxygen buffer reactions

manganese rich rock coatings from iceland manganese rich rock coatings are widespread in iceland especially in fractures in basalt lavas three common types of coating are described a thick subsurface coating is found on whaleback forms on high plateaux the most common type is thinner and occurs in hydrothermal clay filled fracture systems which are especially well developed in tholeiite lavas a third type occurs in younger rocks and is associated with tephra rich loessic sediment which fills open fractures and vesicles there are differences in the chemistry morphology and mineralogy of these coatings which are interpreted as being due to the different microenvironments in which they form the fracture microenvironment is regarded as being of much more importance to coating thickness and chemistry than simple age and in this respect the hydrothermal clays seem to favour rapid manganese concentration on the other hand there are variations in coating type both laterally and vertically through the lava succession which are related to regional distribution of hydrothermal alteration and rock type the regional patterns are the result of iceland s lateral drift and as a result there is an incidental connection between coating type and rock age there is some evidence to suggest that the manganese coatings may be both forming and being degraded rapidly in the present environment

the chemistry of geothermal waters in iceland i calculation of aqueous speciation from 0 to 370 c abstract a computer programme has been developed to calculate the composition and aqueous speciation of geothermal reservoir waters including ph redox potential and gas partial pressures the programme is specifically suited to handle geochemical data from wet steam wells hot water wells and boiling hot springs but it may also be used for non thermal waters solubility data for selected geothermal minerals are incorporated to facilitate the study of solution mineral equilibria the programme may also be used to study chemical changes in water chemistry accompanying boiling variable degassing and cooling and how these changes disturb solution mineral equilibria

dissolution of subglacial volcanic glasses from iceland laboratory study and modelling abstract subglacial hyaloclastites from iceland with ages ranging from 2 ka to 2 2 ma have been studied from a mineralogical and geochemical standpoint the chemical composition of palagonite alteration crust formed on the surface of the glass is almost identical with that of the clayey material filling the intergranular spaces of the rock the clayey material is made up of two particle populations the first is si mg and ca rich with a smectite structure while the second is amorphous fe ti and al rich and has a smectite like morphology it is suggested that these two types of particles can be formed simultaneously in the same solution such that it is not necessary to explain their existence by local or temporary equilibria the mineralogical sequences observed in natural samples were reproduced using the geochemical computer code dissol the geochemical mass balances calculated with dissol also fit quite well with those calculated from icelandic samples illustrating the predictive capability of such a calculation code and gives us confidence in applying a similar approach to nuclear waste form glass problems

processes controlling the distribution of boron and chlorine in natural waters in iceland abstract the concentrations of b and cl in natural waters in iceland lie in the range 0 001 10 and 1 20 000 ppm respectively lowest concentrations occur in surface and groundwaters in the central highlands highest cl concentrations are found in high temperature waters on the reykjanes peninsula and highest b concentrations in well waters from high temperature geothermal systems in the axial zones of the active volcanic belts the b and cl contents of icelandic basalts are 0 1 6 6 and 75 750 ppm respectively these large variations are considered to result from variable degassing during consolidation as well as concentration variations in the initial magma tholeftes are lowest in both elements av 1 2 ppm b and 170 ppm ci transitional basalts intermediate av 2 1 ppm b and 240 ppm ci and alkali basalts highest av 3 8 ppm b and 340 ppm cl cl b ratios in tholeiites are most often within 25 50 but somewhat lower in alkali basalts most geothermal waters contain b and cl within 0 05 1 and 10 100 ppm respectively these rather low values are attributed to the low content of b and cl in the basaltic rock the b and cl distribution in the icelandic natural waters indicate that these elements act as essentially incompatible their sources of supply include 1 the atmosphere seawater spray and aerosols 2 the rock with which the water interacts 3 seawater groundwater and 4 magma intrusions in surface water and nonthermal groundwater the dominant source of supply of cl is seawater spray and aerosols but the soil and rock contribute often significantly to the b in these waters their cl b ratio is similar to that of seawater 1330 or somewhat lower boron and cl concentrations generally increase with water temperature warm waters cl b ratios intermediate between those of seawater and basalt with increasing temperature this ratio decreases and gradually approaches that of the rock some high temperature geothermal waters have as low cl b ratios as 1 and b concentrations as high as 10 ppm the cause is either b degassing of the magmatic intrusive heat source or phase separation in the producing aquifer some low temperature waters

importance of volcanic glass alteration to sediment stabilization offshore japan a minor amount ca 1 wt of amorphous silica cement sourced from volcanic glass inhibits consolidation of hemipelagic sediment approaching the nankai trough subduction zone throughout the shikoku basin the distribution and nature of the cement were examined via secondary and backscattered electron imaging the amorphous silica occurs as altered material in contact with volcanic glass coating grains including grain contacts and filling pores based on chemical and petrographic evidence the cement is probably sourced from volcanic glass this is in contrast to a previous suggestion that this silica cement is sourced dominantly from biogenic silica amorphous silica sourced from disseminated volcanic glass shards has the ability to form a thin coating on clay dominated sediment throughout the shikoku basin measured amorphous silica content in hemipelagic sediments suggests that the cementing process is active throughout the shikoku basin at sites separated by 500 km the cementation process may occur in other locations where sediment containing hydrated disseminated volcanic glass is buried sufficiently for heat to facilitate alteration i e central america cascadia and the gulf of alaska

cool climate lateritic and bauxitic weathering it is commonly held that laterites and bauxites form under tropical climates although some workers have suggested lateritic weathering was not representative of tropical climates the assertion is still almost universally made early tertiary 56 34 ma basalts are widespread in the monaro region of new south wales they overlie and enclose fluvial and lacustrine sediments hyaloclastites and tuffs and the upper parts of many flows are bauxitized the basalts are up to about 200 m thick and in one section comprise 22 flows eight tuffs and hyaloclastites two dolerites and nine interflow bauxitic profiles this region in the eastern australian highlands was tectonically stable and had probably been at its present elevation for some 50 100 ma immediately before the beginning of volcanism and during the extrusion and bauxitization of the earlier flows the region was at 57 5 s latitude climates were wet cool to cold and thermally seasonal and the region was well forested paleomagnetic studies sh