a simple magma driven thermal balance model for the formation of volcanogenic massive sulphides abstract a simple model is used to examine the balance of heat water flow and temperature in a long lived single pass hydrothermal system heated by magmatic heat the geometry of the system is based on mid ocean ridge observations cold water percolates downward to the top of a shallow magma chamber is heated by conduction through a thin boundary layer and discharges through a permeable fault zone modelled as a bundle of narrow rough pipes as heat input is increased the model shows a catastrophic temperature transition at an exit water temperature of between 340 and 410 c depending on seafloor pressure which in nature would probably be represented by surging flow and sudden changes in the specific volume of the circulating fluids to generate a 3 million tonne sulphide deposit with 70 efficiency of deposition from water at 350 c within 4000 years would require a heat flux of 110 w m 2 from the magma chamber for a mass flow rate of 140 kg s 1 if the plan area of the reacting part of the system is 2 km 2 the total heat input would be 3 10 19 j under these conditions the thickness of the solid boundary layer between liquid magma and circulating water will be about 10 m and the integrated reactive water rock ratio over the lifetime of the system will be about 30 decreasing the water temperature to 250 c would require a heat flux of only 60 w m 2 but the time to form a 3 million tonne deposit is increased to 33 000 years and the overall heat requirement is more than quadrupled

melt viscosity temperature and transport processes troodos ophiolite cyprus abstract the lava section in the troodos ophiolite cyprus is chemically stratified and divided into a shallow lava sequence with low tio 2 content and a deeper lava sequence with high tio 2 content we calculate the viscosity at magmatic temperature based on major element chemistry of lavas in cyprus crustal study project ccsp holes cy 1 and 1a we find that typical shallow low ti lavas have a magmatic viscosity that is two to three orders of magnitude lower than that of the deeper high ti lavas this implies that after eruption on axis troodos low ti lavas would have been able to flow down the same slope faster and farther than high ti lavas the calculated lava viscosity increases systematically from the lava sediment interface to the bottom of the composite hole cy 1 1a this suggests that an efficient process of lava segregation by viscosity on the upper flanks of the paleo troodos rise may have been responsible for the chemical stratification in the troodos lava pile calculated magmatic temperature and molar mg mg fe or mg decrease systematically down section while sio 2 content increases correlation of mg in the lavas with mg in the underlying lower crustal plutonic rocks sampled by ccsp hole cy 4 shows that the shallow lavas came from a high temperature lower crustal magma reservoir which is now represented by high mg pyroxenite cumulates while the deeper lavas were erupted from a lower temperature mid crustal reservoir which is now represented by gabbroic cumulates with lower mg

allochthonous oceanic basalts within the mesozoic accretionary complex of alexander island antarctica remnants of proto pacific oceanic crust the mesozoic lemay group accretionary complex of alexander island antarctica contains thrust bound slices of accreted ocean floor ocean islands and seamounts they represent fragments of proto pacific oceanic crust of which only a tiny remnant the phoenix plate remains off northern antarctic peninsula they therefore provide an excellent opportunity to sample the ancient oceanic crust that formerly occupied the southern pacific ocean all the basalts experienced sea floor and subduction accretion metamorphism ranging from zeolite to transitional blueschist facies on the basis of rare arth and other immobile trace element characteristics the basalts are divided into depleted morb n morb e morb and tholeiitic and alkaline oib oceanic basalts occur within two rock associations on alexander island basalt volcaniclastite chert and basalt volcaniclastite tuff the basalt volcaniclastite chert rock association is dominated by pillow lavas which have light ree depleted n morb geochemical characteristics and is interpreted as representing ocean floor formed at spreading centres locally sills of tholeiitic oib intrude the sequence the basalt volcaniclastite tuff rock association exposed in the lully foothills was formed in shallow water during the early jurassic it is geochemically varied consisting of basalts with n morb e morb and tholeiitic oib characteristics the association is interpreted to have been formed on a large seamount or ocean island

petrology and oxygen isotope geochemistry of a fossil seawater hydrothermal system within the solea graben northern troodos ophiolite cyprus hydrothermal mineral zonations and o isotope patterns of the northern troodos complex do not parallel the ophiolite pseudostratigraphy but reflect the convective geometry of an upper cretaceous seawater hydrothermal system large areas of the sheeted intrusive complex sic including the subaxial region of the solea graben are composed of 18o rich subgreenschist mineral assemblages and may represent regions of diffuse seawater recharge other areas of the sic are recrystallized to distinctive epidosite rocks granular high variance assemblages of epidote quartz chlorite that are depleted in 18o al2o3 na2o k2o zr cu and zn and are enriched in cao and sr compared with other mafic volcanic and dike rocks of the solea graben epidosite alteration occurred at temperatures of 310 370 c and involved fluids with 18o values and salinities similar to those of upper cretaceous seawater the epidosite zones are discordant with earlier mineral o isotope zonations and with the axis of spreading in the solea graben suggesting a postspreading off axis origin the seawater hydrothermal system responsible for solea graben massive sulfide deposits was probably driven by hypabyssal intrusions not exposed emplaced in a terminal failed spreading episode the geometries of o isotope surfaces within the solea graben imply that the epidosites formed in fossil upflow and deep recharge conduits depletions in base metals show that epidosite alteration liberated cu and zn to mineralizing fluids within the fossil upflow zone

an integrated chemical and stable isotope model of the origin of midocean ridge hot spring systems chemical and isotopic changes accompanying seawater basalt interaction in axial midocean ridge hydrothermal systems are modeled with the aid of chemical equilibria and mass transfer computer programs incorporating provision for addition and subtraction of a wide range of reactant and product minerals as well as cation and oxygen and hydrogen isotopic exchange equilibria the models involve stepwise introduction of fresh basalt into progressively modified seawater at discrete temperature intervals from 100 to 350 c with an overall water rock ratio of about 0 5 being constrained by an assumed 18 o h 2o at 350 c of 2 0 per mil h craig personal communication 1984 this is a realistic model because 1 the grade of hydrothermal metamorphism increases sharply downward in the oceanic crust 2 the water rock ratio is high 50 at low temperatures and low 0 5 at high temperatures and 3 it allows for back reaction of earlier formed minerals during the course of reaction progress the results closely match the major element chemistry von damm et al 1985 and isotopic compositions craig et al 1980 of the hydrothermal solutions presently emanating from vents at 21 n on the east pacific rise the calculated solution chemistry for example correctly predicts complete loss of mg and so 4 and substantial increases in si and fe however discrepancies exist in the predicted ph 5 5 versus 3 5 measured and state of saturation of the solution with respect to greenschist facies minerals the calculated d h 2o is 2 6 per mil in excellent agreement with analytical determinations the calculated chemical mineralogic and isotopic changes in the rocks are also in good accord with observations on altered basalts dredged from midocean ridges humphris and thompson 1978 stakes and o neil 1982 as well as with data from ophiolites gregory and taylor 1981 predicted alteration products include anhydrite and clay minerals at low temperatures and a typical albite epidote chlorite tremolite greenschist assemblage at 350 c the models demand that the major portion of the water rock interaction occur at temperatures of 300 350 c interaction at temperatures below approximately 250 c results in negative 18 o h 2o shifts contrary to the observed positive 18 o values of the fluids exiting at midocean ridge vents hydrogen isotope fractionation curves by suzuoki and epstein 1976 lambert and epstein 1980 and liu and epstein 1984 among others are compatible with the model and require d h 2o to increase at all temperatures as a result of seawater basalt interaction

boron isotope systematics of a fossil hydrothermal system from the troodos ophiolite cyprus water rock interactions in the oceanic crust and subseafloor ore deposits abstract we determined concentrations and isotopic compositions of boron in a complete section of the hydrothermally altered cretaceous oceanic crust of the troodos ophiolite the boron content and 11 b value for each lithological section are pillow lava 3 8 206 8 g g 63 g g average 11 b 0 17 to 15 6 8 1 average sheeted dike complex 0 6 18 0 g g 4 0 g g average 11 b 3 3 to 10 6 6 0 average and plutonic complex 0 3 8 4 g g 1 7 g g average 11 b 1 7 to 18 5 4 5 average these boron contents are higher than the estimated original igneous values throughout the oceanic crust indicating uptake of boron from seawater and hydrothermal fluid at temperatures ranging from 50 c to 300 c although our boron data for the troodos ophiolite are generally consistent with those for the oman ophiolite of similar age the distinctly low 11 b values of the lower gabbro section in the troodos ophiolite 11 b depleted fluid at a very small water rock ratio the boron content of the bulk oceanic crust 12 3 g g estimated for the troodos ophiolite is relatively high as a result of strong boron enrichment in the pillow lava section which underwent prolonged seafloor weathering despite these differences the weighted average 11 b value of the bulk oceanic crust 7 6 is similar to that of the oman ophiolite 7 9 we also analyzed the boron isotope geochemistry of a subseafloor hydrothermal stockwork sulfide deposit in the troodos ophiolite to investigate its formation processes in contrast to the normal upper oceanic crust the 11 b values of the rocks below the ore body decrease with increasing depth and have large negative values 6 in the highly altered uppermost dike section these low 11 b values are coupled with high boron contents 2 5 17 g g and high and uniform 87 sr 86 sr ratios 0 7064 average and are unlikely to have resulted from interactions with fluids at a small water rock ratio these characteristics are better explained by interaction of ore forming hydrothermal fluids with oceanic crust that had previously been enriched in boron through hydrothermal alteration at low temperatures these observations demonstrate that boron and boron isotopes are useful for quantitative evaluation of fluid related processes with multiple stages including petrogenesis of hydrothermal ore deposits

a multiple sulfur isotope study through the volcanic section of the troodos ophiolite abstract multiple s isotope systematics 34s and 33s and high resolution in situ s isotope measurements offer new perspectives on the study of biological and abiotic cycling of sulfur in hydrothermal systems we applied these techniques to the tethyian troodos ophiolite 91 ma from cyprus one of the best preserved remnants of oceanic crust in the world using materials from deep drill cores and surface sampling we focused on the volcanic section of the ophiolite including the hydrothermal massive sulfide deposit at agrokipia which represents a fossil zone of high temperature fluid upwelling and the akaki river section which displays a range of lower temperature alteration types the 34s and 33s values of bulk and sims secondary ion mass spectrometry analyses from the agrokipia sulfide deposits show that the sulfide minerals are largely derived from thermochemical reduction of entrained seawater sulfate and leached h2s from the root zone of hydrothermal upwelling the contributions of these two sources can vary substantially within individual sulfide grains indicating a very dynamic mixing between these sulfur sources microbial reworking of the sulfide mound is recorded in a sample with very elevated 33s values 0 22 the akaki and agrokipia volcanics experienced low temperature sulfur loss and removal of heavier sulfur isotopes due to partial oxidation by microbes while some intervals gained sulfur and have 34s and 33s values indicative of microbial sulfate reduction ree data of vein quartz containing pyrite with 34s 21 implies local ephemeral hydrothermal upwelling in the lower akaki volcanics possibly associated with the late stage boninitic magmatic activity in the troodos ophiolite suggesting that microbial sulfate reduction in oceanic crust may continue for 10 15 ma in crustal sections with prolonged igneous activity such as troodos

k ar and rb sr ages of celadonites from the troodos ophiolite cyprus k ar dating and rb sr isochrons of two celadonite veins from the troodos ophiolite cyprus consistently yield precipitation ages of 77 80 and 87 92 ma respectively their k ar bulk rock age data 84 7 2 3 ma combined with the oldest celadonite age and recently published radiolarian ages suggest that the troodos ophiolite was formed between 85 and 92 ma providing an estimate for the duration of celadonite formation of at least 7 m y but possibly up to 15 m y these data are consistent with data from deep sea drilling project holes suggesting that precipitation of the most significant portion of vein minerals in the oceanic crust occurs within about 12 m y but possibly up to 19 m y after crustal formation leaching experiments suggest that k ar rb and sr in exchangeable and nonexchangeable sites of the structure of stoichiometric celadonite maintain their isotope characteristics and are resistant to change from diagenetic processes or surface alteration

seawater sediment basalt interactions stable isotope h o and elemental fluxes within the ordovician volcano sedimentary sequence of erquy brittany france the ordovician volcano sedimentary succession of erquy northern brittany is made of immature sediments thermally metamorphosed at the contact of intruding basic sills pillow lavas constitute the upper part of the sequence the trace element geochemistry of sills and pillow lavas suggests that they were derived from a tholeiitic source located beneath a passive margin this volcanic sequence was metamorphosed under low to moderate greenschist facies conditions in this study the direction and amplitude of chemical and isotopic fluxes in the basalt sediment water system were established and the oxygen and hydrogen isotope compositions of the aqueous fluid that reacted with the volcanic rocks were characterized cationic thermometry on chlorites and isotopic thermometry on plagioclase chlorite pairs indicate closure metamorphic temperatures in the range 200 250 c for the basaltic sills stable isotope compositions of iron rich chlorites 18o 5 5 d from 60 to 50 and plagioclases 18o from 9 to 10 reveal that the source of the fluid was certainly seawater the 18o variations within the sills are strongly correlated with the rate of progress of the main metamorphic reaction clinopyroxene plagioclase ilmenite chlorite albite epidote quartz sphene that produced major element mobility at the scale of the volcanosedimentary sequence calculation of elemental fluxes by mass balance combined with oxygen isotopic compositions of basalts shows that the highest water rock ratios 1 were at sill sediment boundaries and within pillow lavas at the top of the pile the volcanosedimentary sequence of erquy was a net sink for na and a source for ca no mg uptake could be detected whereas the hydrothermal alteration of the sediments released fe si and k trapped by the volcanic rocks the 18o value of the fluid reacting with sills appears to have shifted no more than 4 after percolation at low temperature through immature sediments 18o 12 the erquy volcano sedimentary sequence represents a marine hydrothermal system dominated by low temperature exchange which allowed a general 18o enrichment of the volcanic rocks and a 18o depletion of sediments

east pacific rise at 21 n the volcanic tectonic and hydrothermal processes of the central axis abstract photographs obtained by the angus survey system at 21 n reveal many similarities to the geological processes delineated at other spreading centers and in particular those observed in the galapagos rift at 86 w the region of recent volcanism is restricted to a narrow zone zone 1 approximately 1 km wide this suggests that the width of the magma chamber feeding these flows is also narrow at the top variations in sediment cover are used to subdivide the flows within zone 1 into three sheet flow pillow flow pairs the youngest pair appears to have erupted form a linear fissure 8 km long running parallel to the valley axis this fissure is part of a larger en echelon pattern of eruptive fissures active hydrothermal vents associated with the youngest flows are situated directly above the apparent eruptive fissure the high 350 c temperature for some of the fluids exiting from the vents suggests a highly restricted circulation system involving at times little to no mixing with cold seawater the lava terrain in the remainder of zone 1 and bordering regions is characterized by extensive fracturing further suggesting a narrow upper dimension to the magma chamber and also that subsurface hydrothermal deposition of massive sulfides may lead to a reduction in the mixing processes

volcanic glass compositions of the troodos ophiolite cyprus fresh volcanic glass is preserved throughout the extrusive section of the troodos ophiolite which indicates that the lavas have not been pervasively metamorphosed glass compositions reveal the existence of two major magma suites apparently corresponding to distinct stratigraphic intervals the basal 400 500 m of the sequence consists of an andesite dacite rhyolite assemblage containing abundant hyaloclastites the remainder of the section comprises a basalt basaltic andesite assemblage with high mgo and low tio 2 and total iron the lower sequence is interpreted as an evolved arc tholeiite suite the upper has some similarities to boninitic lavas the close association in time and space of these two suites is similar to that observed in the mariana and bonin arcs and suggests that all of the troodos lavas were erupted in a subduction zone environment most probably in an incipient arc or fore arc

systematics of and postulated controls on submarine hydrothermal solution chemistry the chemical compositions of seafloor hydrothermal solutions from the galapagos spreading center 21 n and 13 n on the east pacific rise the southern juan de fuca ridge and the guaymas basin gulf of california are examined for systematic variations several elemental ratios elemental products and empirical geothermometers show much consistency among the various areas in spite of large absolute differences in elemental concentrations the results suggest that most of the major cations in these solutions are solubility controlled with respect to a similar phase assemblage at all of the sites examination of the solution chemistry in conjunction with experimental results and seafloor rock alteration data permits identification of phases which may be important in controlling the solution chemistry as potassium appears to be solubility controlled its use to calculate water rock ratios is inappropriate lithium appears to be a better element to use in these calculations a model to explain the observed solution chemistries as a result of three component mixing is discussed all of the solutions are postulated to be either the result of mixing between a seawater hydrothermal solution which has not undergone a phase separation and is hence gas containing a gas rich vapor phase and a gas poor brine phase or any one of these individual components alone the vapor and brine phases are the result of phase separation which may have occurred at some time in the past essentially all of the reported data fall within the field defined by these three components the solutions apparently have had sufficient time after the mixing event s to reequilibrate with the rock this is true for quartz as well as other chemical components the best test of this model is chloride together with gas and isotropic data as the model predicts that solutions with elevated chlorinities will be depleted in gases and enriched in 18 o and low chlorinity solutions will have elevated gas contents and lower 18 o

multi year tracking of sediment sources in a small agricultural watershed using rare earth elements rare earth elements rees have been successfully used as a sediment tracer but the ree technique has never been used for studying sediment sources for a multi year period a nearly four year field experiment was conducted on a small agricultural watershed near coshocton oh usa to assess the applicability of the ree technique for a multi year period and to evaluate the relative contributions of sediment sources in the watershed tracer depletion and tracer enrichment ratio ratio of the tracer concentrations in sediment to the concentrations in the soil in the areas of application were evaluated to examine the applicability and accuracy of the technique a minimum of 91 per cent of the mass of the applied elements was still available on any individual morphological element at the end of the experimental period the tracer enrichment ratio varied from 0 4 to 2 3 and it was not significantly related to time the relative contributions of six morphological elements within the watershed were evaluated as proportions to total sediment yield the relative contribution of the lower channel was significantly increased as a function of the amount of sediment yield while that of the lower backslope was significantly decreased the relative contribution of the lower channel significantly decreased as a function of cumulative sediment yield while the contributions of the shoulder and the upper backslope significantly increased our results showed that the ree technique can be used to track sediment sources for a relatively long period with two limitations or potential sources of error associated with a selective depletion of tracers and a contamination of downslope areas with tagged sediments from upslope areas copyright 2006 john wiley sons ltd

a field chemical and stable isotope study of subseafloor metamorphism of the josephine ophiolite california oregon the josephine ophiolite is a large complete ophiolite generated in a late jurassic back arc basin along western north america the ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and 18o values characteristic mineral zonation includes mica orthoclase and hematite in the lower extrusives greenschist facies assemblages in the upper sheeted dikes and amphibolite facies assemblages in the lower sheeted dikes and high level gabbro cumulate gabbros are only incipiently altered but are depleted in 18o indicating high temperature alteration with fluids that evolved at low water w rock r ratios a second off axis circulation system is recorded by metalliferous sediments that occur 8 21 m above the ophiolite alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy chemistry and 18o values within individual pillows and dikes the heterogenity is due to 1 variations in w r ratio and starting material during alteration by downwelling seawater e g pillow cores versus glassy rims and 2 localized discharge mineralization mostly epidosites superimposed on the background recharge alteration epidosites occur largely as dike parallel stringers irregular replacement of pillow and massive lava and replacement of interpillow and pillow breccia matrices the chemistry of most extrusives and sheeted dikes is characterized by loss of ca and gain in na and mg consistent with alteration by downwelling fluids in addition the sheeted dikes are depleted in k2o and the lower sheeted dikes and high level gabbros are depleted in zn and cu the 18o and d values for most samples from the extrusive sequence indicate alteration by seawater at 50 rotation of the entire crustal sequence at the spreading axis suggests that the ophiolite formed at a slow spreading center where magma chambers were episodic

chemistry of hydrothermal solutions from the southern juan de fuca ridge fluids from three vent fields on the southern juan de fuca ridge were sampled in september 1984 using the dsrv alvin the fluids are uniquely high in both chloride which ranges up to twice the seawater value and in metal content simple vapor liquid phase separation could not have produced both the high chlorinity and gas concentrations observed in these fluids the cause of the elevated chlorinity can not be uniquely identified but may be the result of either or a combination of two processes 1 subsurface formation of a degassed brine and subsequent mixing of a small amount of this brine with a hydrothermal seawater which has not undergone a phase separation and 2 dissolution of a chloride rich phase combined with a possible small loss of gas during sampling procedures although measured temperatures were all less than 300 sup 0 c quartz geothermometry suggests that the fluids have equilibrated at greater than 340 sup 0 c quartz geobarometry is also in agreement with geophysical estimates of depth to the local magma chamber copyright american geophysical union 1987

low temperature alteration of the upper oceanic crust and the alkalinity budget of seawater abstract while the input of river alkalinity into seawater is relatively well known the complementary acidity production is poorly understood using the major element budget of seafloor alteration of the upper 500 m of 120 ma old oceanic crust at dsdp odp sites 417a 417d and 418a in the central western atlantic we estimate the acidity flux associated with the low temperature weathering of the upper oceanic crust the acidity flux is calculated based on major element fluxes and charge balance considerations the relevant chemical fluxes from seawater to the upper crust are 4 1 0 1 1 4 1 4 2 2 0 6 and 12 2 1011 mol yr 1 for k mg na and silicate ca respectively the associated acidity flux is 3 5 3 1011 eq yr 1 relative to continental weathering these fluxes are significant for k and silicate ca but are minor for na mg and acidity thus riverine fluxes of alkalinity are not significantly balanced by acidity fluxes from low temperature upper ocean crust alteration

mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocks mineralogical and isotopic variations observed in altered glassy and crystalline rocksfrom the east pacific rise and the mid atlantic ridge provide information about the temperatures of alteration and seawater rock ratios for various hydrothermal regimes within the oceanic crust a systematic increase in alteration temperature is evident for the glassy rocksin the sequence 1 nontronite and celadonite vesicle fillings 35 c 2 saponite rich pillow breccias 130 170 c 3 calcite rich greenstone breccias and epidote rich greenstone 200 350 c theseresults include the highest temperatures thus far reported for saponite formation the seawater dominated hydrothermal alteration process that formed the saponite rich pillow breccias is characterized by high water rock ratios 0 1 low to moderate temperatures a seawater origin of most of the carbon in vein calcites 13 c 0 and the predominance of fe rich saponite and calcite as secondary phases greenstones chlorite quartz epidote and greenstone breccias chlorite quartz albite calcite are altered in a rock dominated system with lower water rock ratios 50 1 to 13 c 4 the crystalline rocks diabase gabbro and metagabbro are affected to varying degrees by pervasive high temperature seawater interactions that commence soon after solidification producing varying proportions of fine grained secondary minerals including talc smectite chlorite vermiculite actinolite and sodicplagioclase hydrothermal solutions derived from alteration of the crystalline rocks are of the appropriate temperature and isotopic composition to alter the overlying glassy rocks to the observed mineralogies as well as being the source of metal rich deposits associated with the oceanic spreading centers

weathering dynamics and geosphere mixing with reference to the potassium cycle abstract continental weathering and erosion are discussed from a planetary perspective as important links in the chain of chemical and physical exchanges between surface geospheres atmosphere hydrosphere crust and the mantle the influence of weathering on geosphere chemistry is explored by considering potassium exchanges between geospheres k is transferred from the continents by weathering and erosion to the oceans from which dissolved k is removed during spilitization and by incorporation into sediments k carried to the mantle by subduction of spilites and sediments 8 10 13 g a 1 could balance the dissolved k added to the oceans from continental river runoff and from seafloor hydrothermal springs the largest k flux is that 2 7 10 14 g a 1 physically carried to the oceans in suspended sediments the weakest link in quantifying geosphere exchanges is the lack of understanding of the construction and restoration of continents and hence in defining the parameters involved in their chemical and physical renewal

mass transfer during sub seafloor alteration of the upper troodos crust cyprus five major alteration zones in the extrusive series and the sheeted dike complex of the troodos ophiolite are each characterized by a distinct elemental changes compared to the original composition and b secondary mineralogy the upper ca 300 m of the extrusive crust the highly oxidatedcold seawater alteration zone csa is strongly enriched in k2o and depleted in na2o it is followed downwards by alow temperature alteration zone 170 c which is most widespread in the troodos extrusives and where na2o and k2o are enriched the latter less strongly than in the csa zone three types ofhigh temperature alteration zones 440 c hta i iii are found in the sheeted dike complex all are marked by thorough leaching of k2o while the behavior of na2o e g unchanged in type iii and cao depleted in type i enriched in types ii iii is variable mass budgets of elemental changes are quantified by calibration of whole rock analyses via systematic stable element variations of fresh glasses found throughout the extrusive section the troodos extrusive crust and upper sheeted dike complex are a major sink for mgo k2o and na2o and a source for cao the overall scale of fluxes drastically exceeds estimates based on fresh basalt compositions from present ocean crust

low temperature oceanic crust alteration and the isotopic budgets of potassium and magnesium in seawater abstract low temperature 41 k 0 which is 0 50 enriched relative to bulk silicate earth bse 41 k 0 54 here we present a suite of isotopic systems 41 k 26 mg 7 li 87sr 86sr and major minor elements in bulk rock veins and mineral separates from the upper volcanic section of cretaceous troodos ophiolite and jurassic ocean drilling program hole 801c oceanic crust we use these data to estimate the k isotopic fractionation associated with low temperature oceanic crust alteration and provide new constraints on the role of this process in the global geochemical cycles of mg and k in seawater we find that hydrothermally altered basalts from the troodos ophiolite and odp hole 801c most of which are enriched in k relative to the unaltered glass compositions have 41 k values both higher and lower than bse ranging from 0 01 to 1 07 n 83 and 0 04 to 0 88 n 17 respectively average 41 k values of bulk rock samples from troodos and hole 801c are indistinguishable from each other at 0 50 indicating that low temperature basalt alteration is a sink of 39k from seawater and explaining in part why seawater has a higher 41k 39k than bse in contrast to k average 26 mg values for both troodos 0 00 and hole 801c 0 20 indicate that altered oceanic crust aoc is a sink of 26mg from seawater likely contributing to the light 26 mg composition of seawater 0 8 relative to bse 0 2 we observe isotopically heavy 26 mg values in basalt samples characterized by small to no changes in bulk mg content consistent with extensive isotopic exchange of mg between seawater and oceanic crust during low temperature oceanic crust alteration finally we find that variability in 7 li and 41 k across three sites in the troodos ophiolite can be explained by different styles of alteration that appear to be related to the timing of sedimentation and its effects on chemical and isotopic exchange between seawater and oceanic crust

distribution of alteration zones in the upper oceanic crust aging of the upper oceanic crust produces alteration zones that have distinct mineralogical and geochemical characteristics five zones are recognized in the troodos ophiolite and in situ oceanic crust 1 a sea floor weathering zone swz 2 a low temperature zone ltz 3 a transition zone tz 4 an upper dike zone udz and 5 a mineralized zone mz the mz may be overlain by the swz ltz or tz but in the known examples it is always underlain by the udz the development of the swz is dependent upon the duration of cold seawater circulation which is dependent upon seafloor topography and the rate and nature of sedimentation the boundary between the low and high temperature zones is largely controlled by permeability contrasts lateral variation in the depth of this boundary indicates that the thermal gradient is not uniform within the crust this along with the evidence for disequilibrium assemblages implies that regional metamorphic zones are not appropriate to describe the distribution of alteration effects

hydrothermal alteration of a 1 km section through the upper oceanic crust deep sea drilling project hole 504b mineralogy chemistry and evolution of seawater basalt interactions deep sea drilling project hole 504b penetrates 1076 m into oceanic layer 2 and is the first hole to pass through the transition from pillow basalts altered at low temperatures into hydrothermally metamorphosed sheeted dikes alteration of the crust at site 504 occurred in four stages related to the movement of the crust away from the spreading axis 1 dikes reacted with seawater 200 300 c in the upwelling zone of an axial convection cell at the spreading axis resulting in the formation of greenschist facies parageneses in veins and host rocks mixing of the upwelling hydrothermal fluids with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the lithologic transition zone causing a steep temperature gradient at the base of the pillow section secondary minerals formed in the lower pillow section from the resultant reducing mixed fluids at temperatures of around 100 c at the same time the initial effects of seafloor weathering began in the upper 320 m of the pillow section at low temperatures 50 c and under conditions of open circulation of oxidizing seawater 2 following refracturing of the dikes a second stage of axial upwelling occurred hydrothermal fluids 200 380 c were probably similar to those presently sampled from spreading ridges on the seafloor mixing of these fluids with seawater in the lithologic transition zone caused deposition of quartz epidote and sulfides in veins and formation of a sulfide rich stockworklike zone within this transition zone alteration of the pillow section proceeded under the prior conditions with the effects of seafloor weathering extending progressively downward into the crust 3 seawater recharge penetrated to depths of at least 1075 5 m subbasement and deposited anhydrite locally in veins 4 off axis alteration of the dikes was characterized by formation of zeolites calcite and prehnite in veins and host rocks from more highly evolved and lower temperature 100 250 c fluids alteration in the upper pillow section evolved from seafloor weathering conditions to more reducing and rock dominated as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment zeolites and calcite were the last phases to form throughout the pillow section sealing of the crust to convective cooling also allowed conductive reheating of the crust from below calculations of seawater crustal chemical fluxes from whole rock data are complicated by the low recovery and heterogeneity of alteration effects but indicate that basalt seawater interactions are a sink for seawater mg and k and a source for si and ca

hydrothermal alteration processes at midocean ridges experimental and theoretical constraints from ca and sr exchange reactions and sr isotopic ratios diabase and basalt were reacted with na ca k cl fluids of seawater chlorinity at 375 425 c 375 400 bars and fluid rock mass ratios of 0 5 1 0 to assess the role of basalt chemistry and texture on sr and ca mobility during high temperature hydrothermal alteration an additional experiment utilizing an 84sr spiked fluid was performed to help quantify reaction rates of processes affecting sr mobility the experimental results help to constrain reaction processes responsible for the chemistry of hot spring fluids at midocean ridges alteration of basalt and diabase is characterized by formation of tremolite actinolite smectite chlorite and clinozoisitic epidote smectite chlorite respectively diabase alteration produced dissolved sr ca ratios similar to those observed for ridge crest hot spring fluids whereas alteration of a mostly crystalline basalt produced significantly lower ratios this observation supports the premise that the sr ca ratios observed in vent fluids may be produced during deep seated reaction of the hydrothermal fluids with diabase dikes and or gabbro at relatively low fluid rock ratios and suggests that hydrothermal alteration of primary igneous minerals at relatively high temperatures 400 c leads to formation of a mineral with low dsr ca possibly hydrothermal plagioclase results of the 84sr spiked experiment indicated that only 4 of the sr in basalt was mobilized after 800 hours of reaction despite the fact that b and li were nearly quantitatively leached it is thus suggested that b and li are good indicators of the amount of fresh rock encountered by fluids while sr concentration and isotopic data can be used to estimate the degree of alteration that the sr bearing primary minerals have undergone the degree of alteration of subsurface reservoirs may be estimated by dividing the fluid rock ratio obtained for highly mobile elements by the fluid rock ratio obtained from path integrated sr isotope and concentration data based on this method for example it can be estimated that only 5 of the primary sr bearing minerals encountered by fluids in the subsurface at epr 21 n are actually converted to secondary phases

metal depleted root zones of the troodos ore forming hydrothermal systems cyprus the cupriferous pyrite deposits of cyprus were precipitated from hydrothermal solutions derived by interaction of contemporaneous seawater with hot mafic rock at the ancient troodos spreading centre here we identify the zones in which this interaction took place the zones occur in the lower part of the sheeted dyke complex and within them 30 50 of the rock is made up of epidosite an epidote quartz rock replacing the dykes as sheets and pipes the epidosites contain abundant fluid inclusions which give trapping temperatures of 350 400 c or even higher and contain water normally near seawater in salinity zones of epidosite are elongate parallel to the strike of the sheeted dykes and are up to 1 km wide the rocks throughout these zones are strongly depleted in cu and zn and the metals removed are sufficient to supply the ore deposits in fact several large ore deposits lie along strike from zones of epidosite all of these features support the identification of the epidosites as the hydrothermal reaction zones the location of the epidosite zones immediately above the gabbros of the plutonic complex supports the hypothesis that the heat to drive the ore forming systems came from the underlying magma as is also likely for modern black smoker springs

isotopic and trace element composition of volcanic glasses from the akaki canyon cyprus implications for the origin of the troodos ophiolite abstract we report major elements k rb cs sr ba sc cr ni hf ta th and ree concentrations and isotopic compositions of sr nd pb and o of carefully handpicked volcanic glasses from the akaki river section of the troodos ophiolite complex on the basis of sr and o isotopic composition and fe 2 o 3 feo ratios nine of the ten glasses analyzed are considered to be fresh even with respect to those elements that are easily affected by alteration the glasses range in composition from basaltic andesite to dacite incompatible element concentrations in the mafic compositions are low 10 chondritic and the light ree are strongly depleted however the ratios of low field strength elements k rb cs sr th to high field strength elements ta hf ti and rare earth elements are higher than those in n morb nd isotopic compositions show a very small range with e nd t 80ma varying from 6 5 to 7 5 initial 87 sr 86 sr ratios range from 0 7033 to 0 7040 pb isotope ratios have a restricted range with 206 pb 204 pb 18 60 18 70 207 pb 204 pb 15 55 15 59 and 208 pb 204 pb 38 08 38 51 the oxygen isotopic composition for fresh glasses falls within the range reported for fresh mafic mantle derived volcanics with 18 o varying from 5 4 to 6 5 all geochemical characteristics of the troodos volcanic glasses are similar to those observed in supra subduction zone volcanics but we cannot distinguish between an early stage island arc or back arc origin for the troodos ophiolite on a geochemical basis only however considering our data and previously published geologic and geochemical data we suggest that the troodos ophiolite formed in the earliest stages of island arc development

troodos extrusive series of cyprus a comparison with oceanic crust the extrusive section on the northern flank of the troodos ophiolite is 1 000 to 1 500 m thick and consists of about 12 lithologic units of pillowed and massive lava and breccia some pillowed units which average 100 to 200 m in thickness vary systematically from lava tubes up to 6 m in diameter at the base through common pillows less than 1 m in diameter to small pillows and extrusive breccias at the top these assemblages appear to be individual volcanoes suggesting episodic volcanic accretion the upper 100 to 200 m of the section is pervasively altered and oxidized but fresh volcanic glass is preserved throughout most of the extrusive section the lavas are subdivided on geochemical evidence into a lower differentiated andesite to dacitic andesite series resembling evolved island arc tholeiites and an upper series of picrite mafic basalt and basaltic andesite very low in tio2 k2o p2o5 nb rb and ba the highly depleted basalt and basaltic andesite resemble lavas from ocean crust near island arcs lateral continuity of lithologic units interpreted as individual volcanoes and the relatively constant dips of the extrusive rocks suggest that tilting and rotation have not played major roles in the accretion process the occurrence of fractionated lavas at the base and primitive lavas at the top suggests a broad zone of accretion located above a subduction zone

a comparative analysis of potential biosignatures in basalt glass by fib tem abstract bulk analyses of fresh and altered subseafloor basalts have elucidated the large scale geochemical fluxes that result from water rock interactions which ultimately affect both seawater and mantle compositions but these analyses do not provide insights into the physical and chemical processes occurring on the scale of microorganisms micron sized tubules that may represent microbial boring into basalt glass are ubiquitous throughout the seafloor and can potentially be preserved over geological time these putative biosignatures give clues into the history of life on earth and may be significant for the search for life on other planets however their formation and mineralization are still poorly understood we have used a combination of focused ion beam fib milling and transmission electron microscopy tem to analyze the textural and geochemical characteristics of tubular alteration features from four basalt glass samples representing different sample locations collection depths and ages including one sample from the cretaceous troodos ophiolite high resolution tem imaging consistently identified platy layers of partially crystalline material infilling the tubules the lattice spacings and diffraction patterns showed that this material comprises a mix of two layer and three layer silicates closer inspection of the contact point between the tubules and the surrounding basalt glass commonly revealed a thin leached rim around the tubules electron energy loss spectroscopy mapping demonstrated that these rims are depleted in everything but si and o which implies that they are formed by a leaching process likely the incongruent dissolution of the glass during formation of the tubule the infilling material is depleted in ca and enriched in k and in most cases also enriched in fe which is consistent with their identification as fe bearing phyllosilicates these observations are consistent across the samples which implies that the processes that form and mineralize tubular glass alteration features are similar throughout the oceanic subsurface and through time

chemical fluxes during hydrothermal alteration of a 1200 m long section of dikes in the oceanic crust dsdp odp hole 504b abstract chemical interactions between seawater and the oceanic crust have been widely investigated during recent years however most of these studies concern the uppermost volcanic part of the crust the contribution of the underlying sheeted dike complex to the global budget of the oceans is inferred solely from some ophiolite studies and from the 500 m high level dike section of dsdp odp 504b which was drilled in 1981 hole 504b is the only place where a continuous and long 1260 m section in the sheeted dike complex has been cored and it is now regarded as a reference section for the upper oceanic crust many petrological and chemical data from these dolerites are available including the relative proportions of veins extensively altered adjacent rocks and less altered host rocks for these three reasons considering the entire dike section penetrated by hole 504b is a unique chance to study chemical fluxes related to hydrothermal alteration of this part of the oceanic crust the calculation of any chemical flux implies knowledge of the chemical composition of the fresh precursor protolith previously mean compositions of glasses p1a or basalts from the hole 504b volcanics have been used as protoliths in this paper we calculate and discuss the use of various protoliths based on dolerites from hole 504b we show that the most adequate and realistic protolith is the mean of individual protoliths that we calculated from the acquisition by automatic mode of about 1000 microprobe analyses in each thin section of dolerite from the hole 504b lower dikes consequently pfm is further used to calculate chemical fluxes in the dike section of hole 504b the chemical compositions of the host rocks adjacent to alteration halos tend to converge to that of pfm with depth except for fe2o3t and tio2 because the volume percent of alteration halos increases with depth the total fluxes related to these halos increase with depth this explains why the mean flux host rocks halos veins of the upper dikes is roughly similar to the mean flux of the lower dikes during the alteration of the entire hole 504b dike section the dolerites gained relatively large quantities of fe2o3t 4 0 g 100 cm3 and released much sio2 6 8 g 100 cm3 cao 5 8 g 100 cm3 and tio2 1 6 g 100 cm3 and minor al2o3 0 7 g 100 cm3 and mgo 0 7 g 100 cm3 we show the importance of the choice of the protolith in the calculation of chemical budget particularly for elements showing low flux values in hole 504b the mg uptake by the volcanics during low temperature alteration added to the mg release by the dikes gives a net flux of 0 07 1014 g year we propose that part of the mg uptake by the oceanic crust which is necessary to compensate the rivers input 1 33 1014 g year occurs in the underlying gabbros and or in sections which are altered such as trinity and troodos ophiolites compared with ophiolites fluxes calculated for elements other than mg for the entire crust are generally similar in tendency if not in absolute value to that we obtained from hole 504b

the role of sedimentation history and lithology on fluid flow and reactions in off axis hydrothermal systems a perspective from the troodos ophiolite abstract off axis hydrothermal systems are thought to carry globally significant chemical fluxes but different ocean crust sections show widely differing extents of alteration making the quantification of these fluxes complex with the aim of better understanding the origin of this diversity in alteration extent we have studied two sections of the lava pile in the troodos ophiolite with distinct sedimentation history and volcanic architecture the akaki section is dominated by pillow lavas and the oldest sediments overlying the crust are 20 myr younger than the ophiolite here there is a 300 m thick zone at the top of the lavas that is enriched in co2 and alkali elements and has high 87sr 86sr and 7li these features indicate extensive chemical exchange with seawater in contrast to the akaki area the onophrious section is dominated by sheet flows and the oldest sediments are of the same age as the ophiolite here the co2 and alkali element enriched zone is much thinner

the direction correction tilt test an all purpose tilt fold test for paleomagnetic studies abstract the tilt or fold test in paleomagnetism is used to infer whether paleomagnetic remanence was acquired before or after tectonic tilting while several tilt test formulations have been proposed none fully satisfies the requirements of statistical validity applicability to all bedding geometries and ease of use this paper introduces the direction correction dc test which examines the relationship between the paleomagnetic site mean directions and their corresponding bedding tilt corrections the dc test is similar to mcfadden s correlation test because both test whether or not the site directions contain information about the bedding tilts however the dc test gives greater weight to sites with greater counter bedding the dc test is similar to watson s numerical tilt tests because they determine the degree of untilting which gives optimal concentration of site directions however the dc test uses analytical rather than numerical methods graphical output aids the researcher in recognizing problem sites using both real and simulated data the dc test is demonstrated to be more discriminating than other tilt test formulations for all bedding geometries the simulations show that the power of the tilt test is inversely proportional to the 95 confidence interval 95 of the overall mean as a rule of thumb paleomagnetists should attempt to sample sufficient sites to obtain an 95 less than 1 6 of the bedding attitude difference