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

the process of formation of ocean crust ocean crust is the outermost layer of earth under the oceans it is separated from the underlying mantle by a seismic transition zone called the moho a widely held view is that the moho represents a petrologic change from basaltic type rocks to a mantle composed mostly of olivine and pyroxene according to this view crust is formed by a steady segregation of basaltic melt derived from partial melting of the mantle into a crustal magma chamber wherein cooling and crystallization bring about steady state accretion to the continuously spreading plates there is sufficient disagreement between the predictions of this hypothesis and marine geophysical data to cause one to doubt the validity of this formation process at least two other processes are more compatible with the geophysical data in one the crust is formed from the episodic injection of basaltic dikes from a mantle reservoir and the moho is a primary petrologic boundary in the other the crust is treated as a mechanical boundary layer in which thermal contraction results in cracking by comparison in the mantle thermal contraction is accommodated by flow the upper part of the crust is formed from episodic extrusion and intrusion of basaltic melt the lower crust is formed by rapid hydrothermal alteration of mantle that may be continuously or episodically injected by viscous flow at temperatures below the melting temperature

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

seismic reflection evidence for a dangerous grounds miniplate no extrusion origin for the south china sea the collision of india and asia has caused large strike slip faults to form in east asia resulting in the extrusion of crustal blocks toward the southeast since the eocene as a result of the indentation of rigid india into asia it has been suggested that the south china sea opened as a result of relative motion between a rigid indochina sundaland block and china alternative models propose that rifting and seafloor spreading were driven by trench forces to the south we test these competing models by analysis of seismic reflection profiles across the boundary between sundaland and the southern rifted margin known as the dangerous grounds we show that the southern boundary of the dangerous grounds is a subduction zone that jammed in the middle miocene to the west the dangerous grounds are bounded by a strike slip zone also active until similar to 16 ma that becomes diffuse south of the now inactive south china sea seafloor spreading center we place the western edge of the dangerous grounds just to the east of the natuna arch lupar line the west baram line is confirmed as originating as a major strike slip fault within the dangerous grounds and is continuous with the red river fault zone because the dangerous grounds were independent of sundaland until similar to 6 ma its motion cannot have been constrained by motion of this block making extrusion impossible as a mechanism to rift the south china sea se motion by both the dangerous grounds and sundaland suggests subduction forces were the primary trigger for plate motions our reconstruction places a similar to 280 km upper limit on the motion on the red river fault and a similar to 1400 km width to the paleo south china sea

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

elemental fluxes during hydrothermal alteration of the trinity ophiolite california u s a by seawater abstract the trinity ophiolite has undergone hydrothermal alteration in greenschist to transitional amphibolite facies conditions the mineralogical assemblages and sr isotopic results reveal a widespread seawater circulation which has principally affected the upper part of the oceanic crust the interface between sheeted dikes and isotropic gabbros has reacted with the largest amount of heated seawater with water rock ratios w r up to 16 w r ratios and temperature are the most important parameters in determining the direction and amplitude of chemical changes during seawater rock interaction in addition calculation of elemental fluxes by mass balance on a crustal section reveals the important role of magma chemistry and related variations of primary model compositions the large chemical fluxes calculated in trinity rocks agree with their strong degree of alteration the w r of 8 in the trinity basalts are larger than those from hole 504b and east pacific rise 1 5 this is a possible consequence of a longer life time convective hydrothermal system in relation with a slow spreading center generating a thin crust contrasting with hole 504b this hydrothermal pattern is likely able to equilibrate the chemical budget for mg na and partly for k between seawater and oceanic crust the strong degree of alteration recorded by ophiolites corresponds to the time integrated effects of both initial and off axis hydrothermal activity ophiolites certainly represent valuable probes in tracking the chemical budget of the oceanic reservoir

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

a sulfur isotopic profile through the troodos ophiolite cyprus primary composition and the effects of seawater hydrothermal alteration the sulfide mineralogy sulfur contents and sulfur isotopic compositions were determined for a section through the troodos ophiolite as represented by drillcore from holes cy1a cy2a and cy4 plus selected outcrops with the goals of understanding the geochemistry of s during hydrothermal alteration of the crust and obtaining a mass balance for sulfur in altered ophiolitic crust primary 34s values of the ophiolite are 0 1 consistent with a morb or bab like sulfur source the volcanic rocks have low sulfur contents mean 40 ppm and generally negative 34s values to 26 1 these are the results of sulfur loss through oxidation by seawater at low temperatures 100 c and isotopic fractionation during partial oxidation of igneous sulfides the sheeted dike complex is uniformly enriched in 34s 34s 5 4 as the result of mixing of reduced cretaceous seawater sulfate 17 with primary crustal sulfide sulfate was reduced through oxidation of ferrous iron in the dikes and conversion of igneous pyrrhotite to secondary pyrite decreasing water rock ratios with depth in the plutonic section led to a general decrease in 34s values downward to igneous values in the lower gabbros seawater effects are present locally to the base of the section however cooling of diffuse upwelling hydrothermal fluids t 250 c produced s and metal enrichments in the upper dikes 1 26 wt s whereas sulfur was lost from the lower dikes and upper plutonic rocks which contain 200 ppm and 10 1640 ppm s respectively through higher temperature 350 c alteration reactions epidosites at the dikes gabbro transition contain 30 ppm s with 34s 5 2 6 5 these rocks lost sulfur and metals during reaction with 34s enriched deep hydrothermal fluids at high temperatures 350 400 c and high water rock ratios in basal hydrothermal upflow zones the ultimate effect of alteration of ophiolitic crust is redistribution of igneous sulfur within the crust and exchange of crustal sulfur for seawater sulfur with little net change in the sulfur content of the crust overall the bulk altered troodos crust is enriched in 34s having 34s 3 6 fluxes of sulfur between seawater and ophiolitic crust are an order of magnitude smaller than modern riverine input to and sedimentary sulfide output from the oceans the amount of seawater sulfur that reacted with troodos ophiolite crust is approximately twice that for a composite section of oceanic crust consistent with the generally greater intensity of alteration of ophiolitic crust compared to modern oceanic crust

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

seafloor spreading and the ophiolitic sequences of the troodos complex a principal component analysis of lava and dike compositions the troodos complex of cyprus has been interpreted as a slice of oceanic crust formed from ultramafic magma in a back arc basin recent investigations however have suggested that only the upper gabbros the sheeted dike complex and the lower lava sequence formed as a direct consequence of seafloor spreading and that the primary magma was of basaltic composition according to this model the upper lava suite and related ultramafic cumulates are not represented in the sheeted dike complex a multivariate statistical analysis of major element compositions of 480 dikes and 130 volcanic glasses supports this suggestion statistically 98 the dikes are similar to the glasses of the lower lava suite this supports the model that the seafloor spreading related sequence of the troodos ophiolite is relatively thin and of basaltic composition the magma chambers represented by the ultramafic cumulates and associated gabbros and the upper lavas appear to have been emplaced after or during the termination of the major spreading events

lava deposition history in odp hole 1256d insights from log based volcanostratigraphy a log based volcanic stratigraphy of ocean drilling program hole 1256d provides a vertical cross section view of in situ upper crust formed at the east pacific rise epr with unprecedented resolution this stratigraphy model comprises ten electrofacies principally identified from formation microscanner images in this study we build a lava flow stratigraphy model for the extrusive section in hole 1256d by correlating these electrofacies with observations of flow types from the modern epr such as sheet flows and breccias and pillow lavas and their distribution the resulting flow stratigraphy model for the hole 1256d extrusive section represents the first realization of detailed in situ epr upper oceanic crust construction processes that have been detected only indirectly from remote geophysical data we correlated the flow stratigraphy model with surface geology observed from the southern epr 14 s by shinkai 6500 dives in order to obtain the relationship between lava flow types and ridge axis ridge slope morphology this dive information was also used to give a spatial time reference frame for modeling lava deposition history in hole 1256d in reconstructing the lava deposition history we interpreted that the origins of the 100 m thick intervals with abundant pillow lavas in hole 1256d are within the axial slope where pillow lavas were observed during the shinkai 6500 dives and previous epr surveys this correlation could constrain the lava deposition history in hole 1256d crust using the lateral scale of ridge axis ridge slope topography from the shinkai 6500 observations and assuming the paleospreading rate was constant 50 of the extrusive rocks in hole 1256d crust were formed within 2000 m of the ridge axis whereas nearly all of the remaining extrusive section was formed within 3000 m of the ridge axis these results are consistent with the upper crustal construction model previously suggested by seismic studies

hydrochemical evolution of groundwater in gabbro of the troodos fractured aquifer a comprehensive approach abstract ophiolites often form complex fractured multi lithological and highly compartmentalized aquifers that are difficult to characterize groundwater chemistry reflects water rock interactions and can provide insights to groundwater recharge mechanisms flow paths and relative residence time a comprehensive hydrogeochemical modelling approach with phreeqc has been developed to identify water rock interactions and simulate groundwater evolution and mixing with connate water in fractured aquifers the methodology was applied in the gabbro aquifers of troodos massif cyprus the geochemical processes generating the most frequently observed water types were reproduced through modelling based on the overall results a generic evolution path has been identified camghco3 mgcahco3 camgnahco3 ca mg na hco3 so4 namghco3so4 naso4 cl group camghco3 and mgcahco3 types were modelled to occur predominantly in early stages of recharge deep unsaturated zones and longer flow paths result in gradual removal of ca 2 and mg 2 from groundwater through secondary mineral precipitation and in an increase of na 2 and so4 2 the most frequently occurring water types camgnahco3 and mgcanahco3 were modelled to occur at later stages of hydro chemical evolution after secondary mineral precipitation and in closed conditions in regional flow where closed conditions with secondary mineral precipitation prevail water types progressively shift from the hco3 to the naso4 end member group in 38 of the 536 samples from gabbro aquifers in troodos groundwater mixing with connate water at a ratio of 0 999 0 001 occurs

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

glacial advance and stagnation caused by rock avalanches abstract new model results show that a glacier advance caused by deposition of a rock avalanche on the ice will be followed by stagnation of the advanced ice lobe producing distributed hummocky deposits quite different from the single moraine ridges typically dated in paleoclimatic reconstructions we obtain these results using a numerical flow line glacier model with superimposed rock debris in particular our results imply that the single moraine ridge known as the waiho loop of the franz josef glacier new zealand is not the result of an advance triggered by a rock avalanche else there would be a widespread hummocky glacial stagnation deposit associated with it

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

neotectonic regime on the passive continental margin of the northern south china sea abstract between 1989 and 1994 more than 6600 km of reflection seismic profiles were obtained in the south china sea off hong kong with the german research vessel sonne during cruises so 50b so 72a and so 95 a seismo stratigraphic interpretation of this data set leads to a new age assignment of the unconformity t0 which we place within the pleistocene both neogene unconformities t1 and t0 are generated by uplift of the dongsha rise and truncation of their overlying strata this uplift is caused by intrusion of magma into the upper crust our seismic profiles show plutons which have penetrated the sedimentary cover whereby their original stratification in the contact zone is eliminated these magmato tectonic events may be correlated to the two main collision phases between taiwan and the continental margin of east china 5 3 and 3 0 ma ago the collisional events subsequent to the nnw to wnw drift of taiwan transformed the compression into strike slip movements along the continental margin of southeastern china the accompanying stress regime is transtensional with subsidence of the cooling oceanic crust since the cessation of rifting and its consumption beneath the manila trench providing the extensional stress the strike slip movements remobilized many of the rift and drift faults providing pathways for magma ascent the tectonic framework of the northern south china sea is characterized by miocene faults trending ne sw these faults are scarce but are distributed throughout the study area pliocene faults striking ene wsw to ne sw are concentrated west of the dongsha islands and are mostly strike slip in character recent faults are generally oriented ne sw subparallel to the synrift faults they result in part from local uplifts where they are normal in character but strike slip motion also occurs most of the faults involve the basement and represent reactivated zones of weakness of the rift and drift phases

alteration and mineralization of an oceanic forearc and the ophiolite ocean crust analogy mineralogical chemical and isotopic o c s and sr analyses were performed on minerals and bulk rocks from a forearc basement section to understand alteration processes and compare with mid ocean ridges mor and ophiolites ocean drilling program hole 786b in the izu bonin forearc penetrates 103 m of sediment and 725 m into volcanic flows breccias and basal dikes the rocks comprise boninites and andesites to rhyolites most of the section was affected by low temperature 100 c seawater alteration with temperatures increasing downward the rocks are partly 5 25 altered to smectite fe oxyhydroxide calcite and phillipsite and exhibit gains of k rb and p loss of ca variable changes in si na mg fe sr and y and elevated 18o and 87sr 86sr higher temperatures 150 c in the basal dikes below 750 m led to more intense alteration and formation of chlorite smectite corrensite albite k feldspar and quartz chlorite a 5 m thick hydrothermally altered and pyritized zone at 815 m in the basal dikes reacted with mixtures of seawater and hydrothermal fluids to mg chlorite albite and pyrite and gained mg and s and lost si and ca focused flow of hydrothermal fluids produced serialization halos na k sericite quartz pyrophyllite k feldspar and pyrite along quartz veins at temperatures of 200 250 c high 87sr 86sr ratios of chloritized 0 7055 and sericitized 0 7065 rocks indicate involvement of seawater via mixing with hydrothermal fluids low 34s of sulfide 2 to 5 5 and sulfate 12 5 are consistent with input of magmatic so2 into hydrothermal fluids and disproportionation to sulfide and sulfate alteration processes were generally similar to those at mors but the arc section is more intensively altered in part because of the presence of abundant glassy rocks and mafic phases the increase in alteration grade below 750 m and the mineralization in the basal dikes are analogous to changes that occur near the base of the volcanic section in mor and the troodos ophiolite

seismic imaging of the transitional crust across the northeastern margin of the south china sea abstract crustal structure across the passive continental margin of the northeastern south china sea scs is presented based on a deep seismic survey cooperated between taiwan and china in august 2001 reflection data collected from a 48 hydrophone streamer and the vertical component of refraction reflection data recorded at 11 ocean bottom seismometers along a nw se profile are integrated to image the upper 1 6 2 4 km s lower 2 5 2 9 km s and compacted 3 4 5 km s sediment the upper 4 5 5 5 km s middle 5 5 6 5 km s and lower 6 5 7 5 km s crystalline crust successively the velocity model shows that the thickness 0 5 3 km and the basement of the compacted sediment are strongly varied due to intrusion of the magma and igneous rocks after seafloor spreading of the scs furthermore several volcanoes and igneous rocks in the upper middle crust 7 10 km thick and a high velocity layer 0 5 km thick in the lower crust of the model are identified as the ocean continent transition oct below the lower slope in the northeastern margin of the scs a thin continent nw of the oct and a thick oceanic crust se of the oct in the continental margin of the northeastern scs are also imaged but these transitional crusts cannot be classified as the oct due to their crustal thickness and the limited amount of the volcano the magma and the high velocity layer the extended continent next to the gravity low and a sag zone extended from the sw taiwan basin may have resulted from subduction of the eurasian plate beneath the manila trench whereas the thick oceanic crust may have been due to the excess volcanism and the late magmatic underplating in the oceanic crust after seafloor spreading of the scs

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

hydrothermal contributions to global biogeochemical cycles insights from the macquarie island ophiolite hydrothermal circulation is a fundamental process in the formation and aging of the ocean crust with the resultant chemical exchange between the crust and oceans comprising a key component of global biogeochemical cycles sections of hydrothermally altered ocean crust provide time integrated records of this chemical exchange unfortunately our knowledge of the nature and extent of hydrothermal exchange is limited by the absence of complete oceanic crustal sections from either submarine exposures or drill core sub antarctic macquarie island comprises 10 ma ocean crust formed at a slow spreading ridge and is the only sub aerial exposure of a complete section of ocean crust in the ocean basin in which it formed hydrothermally altered rocks from macquarie island therefore provide a unique opportunity to evaluate the chemical changes due to fluid rock exchange through a complete section of ocean crust here we exploit the immobile behavior of some elements during hydrothermal alteration to determine the precursor compositions to altered macquarie whole rock samples and evaluate the changes in bulk rock chemistry due to fluid rock interaction throughout the macquarie crust the extent to which elements are enriched or depleted in each sample depends upon the secondary mineral assemblage developed and hence the modal abundances of the primary minerals in the rocks and the alteration conditions such as temperature fluid composition and water rock ratios consequently the chemical changes vary with depth most notably within the lava dike transition zone where enrichments in k s rb ba and zn are observed our results indicate that hydrothermal alteration of the macquarie crust resulted in a net flux of si ti al and ca to the oceans whereas the crust was a net sink for h2o mg na k and s our results also demonstrate the importance of including the contribution of elemental uptake by veins for some elements e g si fe mg s extrapolation of our results assuming a crustal production rate of 3 km2 yr yields estimates of the hydrothermal contribution to global geochemical cycles for example the mg flux to the crust is estimated to be 3 3 1 1 1012 mol year sufficient to balance the riverine mg input to the oceans given the uncertainties involved however the relationship between spreading rate and hydrothermal chemical exchange fluxes remains poorly understood and the approach described here should be applied to crust produced at a range of spreading rates to refine the global hydrothermal flux estimates

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

trace element mobility during sub seafloor alteration of basaltic glass from ocean drilling program site 953 off gran canaria trace element concentrations of altered basaltic glass shards layer silicates and zeolites in volcaniclastic sediments drilled in the volcanic apron northeast of gran canaria during ocean drilling program odp leg 157 document variable element mobilities during low temperature alteration processes in a marine environment clay minerals saponite montmorillonite smectite replacing volcanic glass particles are enriched in transition metals and rare earth elements ree the degree of retention of ree within the alteration products of the basaltic glass is correlated with the field strength of the cations the high field strength elements are preferentially retained or enriched in the alteration products by sorption through clay minerals most trace elements are enriched in a boundary layer close to the interface mineral altered glass this boundary layer has a key function for the physico chemical conditions of the subsequent alteration process by providing a large reactive surface and by lowering the fluid permeability the release of most elements is buffered by incorporation into secondary precipitates sodium rich zeolites phillipsite fe and mn oxides as shown by calculated distribution coefficients between altered glasses and authigenic minerals chemical fluxes change from an open to a closed system behavior during prograde low temperature alteration of volcaniclastic sediments with no significant trace metal flux from the sediment to the water column

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

cenozoic plate reconstruction of the south china sea region abstract reconstructions of the south china sea region at 60 ma 40 ma 30 ma 20 ma 10 ma and 5 ma are presented we have attempted to place the south china sea basin in a regional tectonic framework the tectonic evolution of the major blocks surrounding the south china sea were analyzed as well as the relative motions of the indian and australian plates we have tried to correct the tectonic models available in this region a 3 d graphics terminal was used to derive rotation poles for the different tectonic blocks and our model was then tested to determine its self consistency when the model conflicted with previous interpretations the input data were evaluated for alternative explanations at least two and possibly three stages of extension can be recognized in this region the earliest one active in the late cretaceous to eocene involved nw se extension the second one active from the late eocene to early miocene involved north south extension the third stage of extension which probably trended nw se can be dated as post oligocene the first extensional event produced the ne sw trending proto south china sea and a series of sedimentary basins along the south china margin following the southeastward extrusion of indochina the proto south china sea was mostly consumed at the palawan trough renewed north south extension in the south china continental margin started the present day south china sea spreading in the oligocene the southeastward extrusion of indochina blocked by sundaland resulted in the nw se trending opening of the south china sea basin in the early miocene collision of the north palawan microcontinental block with the west philippines block stopped the opening of the south china sea at the end of early miocene spreading activity switched to the sulu sea basin in the middle miocene but collision between the sulu ridge and the west philippines at mindanao halted the opening of the sulu sea at the end of the middle miocene in the late miocene greater india continued its northward path and seems to have ripped open the andaman sea in the pliocene subduction along the northern manila trench placed the north luzon arc on a collision path with the east asia continental margin at taiwan our reconstructions along with detailed geological and geophysical information may be used as a predictive tool for basin evolution models and block interactions in this region the development of the south china sea basin the gulf of thailand the malay basin and the central thailand basins are the result of collision induced extensional forces the sulu celebes and sumatra basins were formed as a consequence of prolonged subduction the opening of the pearl river mouth west natuna south china sea sulu and possibly celebes basins were terminated by various plate collisions during the course of plate reorganizations major boundary faults have changed their slip senses during different stages of evolution

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

mapping crustal thickness using marine gravity data methods and uncertainties crustal thickness is a critical parameter for understanding the processes of continental rifting and breakup and the evolution of petroleum systems within passive margins however direct measurements of crustal thickness are sparse and expensive highlighting the need for methodologies using gravity anomaly data jointly with other geophysical data to estimate crustal thickness we evaluated alternative gravity inversion methodologies to map crustal thickness variations at rifted continental margins and adjacent oceanic basins and we tested our methodology in the south china sea scs different strategies were investigated to estimate and remove the gravity effect of density variations of sediments and the temperature and pressure variations of the lithospheric mantle from the observed free air gravity anomaly data sediment density was calculated using a relationship between sediment thickness porosity and density we found that this method is essential for crustal thickness inversion in the presence of a thick sedimentary cover by comparing the moho depths obtained from gravity inversion and seismic interpretation in the yinggehai basin where sediments are up to 13 km thick the inversion accuracy depended on the parameters of the exponential equation between porosity and the buried depth we modeled the lithospheric mantle temperature field based on oceanic crustal age continental crustal stretching factors and other boundary conditions we tested three different methods to calculate the thermal expansion coefficient which is either held constant or is a linear polynomial function of temperature for applying a thermal correction and found that the inversion results were relatively insensitive to alternative methods we compared inversion results with two recent deep seismic profiles that image the rifted continental edge at the northern margin of the scs and the continental liyue bank reed bank at the southern margin and we found that the inversion accuracy was improved considerably by removing sediment thermal and pressure gravity effects

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

a crustal structure profile across the northern continental margin of the south china sea an ocean bottom seismometer obs geophysical experiment was carried out across the northern continental margin of the south china sea two dimension kinematic ray tracing was used to construct synthetic records and to define the geological model along the obs profile according to the calculation and interpretation of refracted p wave phases the crustal thickness decreases from 22 km obs 1 in littoral area to 8 km obs 15 in deep sea the lower slope structured by volcanism changes sharply in moho depth from 22 to 9 km the volcanoes over the lower slope were markers of ocean continental transition zone a high velocity layer hvl with an average thickness of 4 km was found in the base of the lower crust over the shelf and slope the south china sea was originated by the passive rifting of south china continent and seafloor spreading however geological and geophysical data collected in the northern margin of the south china sea show a small amount of magma intruded into or erupted onto the crust during pull apart and breakup further analysis shows that the hvz in the base of the thinned continental crust was related to magmatism that occurred after the cessation of seafloor spreading of the south china sea magmatism during the crustal extension and breakup is very weak therefore the northern continental margin of the scs is classified as non volcanic passive margin the southern margin of the south china sea has a crust with a steady thickness of 18 24 km which is different from the nor them margin of the south china sea with a gradually thinned crust they constitute an asymmetrically conjugated pair of passive margins c 2001 elsevier science b v all rights reserved

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

crustal scale seismic profiles across taiwan and the western philippine sea abstract we have used combined onshore and offshore wide angle seismic data sets to model the velocity structure of the taiwan arc continent collision along three cross island transects although taiwan is well known as a collisional orogen relatively few data have been collected that reveal the deeper structure resulting from this lithospheric scale process our southern transect crosses the hengchun peninsula of southernmost taiwan and demonstrates characteristics of incipient collision here 11 km thick transitional crust of the eurasian plate eup subducts beneath a large rapidly growing accretionary prism this prism also overrides the n luzon forearc to the east as it grows just west of the arc axis there is an abrupt discontinuity in the forearc velocity structure because this break is accompanied by intense seismicity we interpret that the forearc block is being detached from the n luzon arc and philippine sea plate psp at this point our middle transect illustrates the structure of the developing collision steep and overturned velocity contours indicate probable large scale thrust boundaries across the orogen the leading edge of the coherent psp appears to extend to beneath the east coast of taiwan deformation of the psp is largely limited to the remnant n luzon arc with no evidence of crustal thickening to the east in the huatung basin our northern transect illustrates slab continent collision the continuing collision of the psp and eup as the psp subducts the collisional contact is below 20 km depths along this transect ne of hualien this transect shows elements of the transition from arc continent collision to ryukyu arc subduction both of our models across the central range suggest that the paleozoic to mesozoic basement rocks there may have been emplaced as thick coherent thrust sheets this suggests a process of partial continental subduction followed by intra crustal detachment and buoyancy aided exhumation although our models provide previously unknown structural information about the taiwan orogen our data do not define the deepest orogenic structure nor the structure of western taiwan additional seismic active and passive geologic and geodynamic modeling work must be done to fully define the structure the active deformation zones and the key geodynamic process of the taiwan arc continent collision

petrological and chemical evolution of the northeastern troodos extrusive series cyprus four magma series are distinguished in the northeastern troodos extrusive series a a lower low ti series lo lts of basaltic andesites b a high ti series hts of basaltic andesites to rhyodacites c a low ti series lts and d a depleted low ti series dlts the last two being of basaltic andesite trace element characteristics vary systematically from series a to d and are interpreted in terms of a variable contribution of three major source components scs lile enriched water rich fluids sci derived from dehydration of a subducted lithosphere slab were continuously added to the overlying mantle wedge increasing lile hfse and lile ree ratios and decreasing absolute hfse and ree concentrations from series a to d indicate progressive depletion of the actual mantle source scii a negative ta anomaly in the lavas decreases from series a to d and is interpreted to have resulted from partial melting of the lower crust scii where ta nb ti may be fractionated by ti rich accessory phases the contribution of sciii decreases when the eruptive sites successively move away from the central axial zone and the temperature of the lower crust decreases preventing partial melting of the lower crust chemical compositions of fresh glass separates and phenocrysts indicate a change of major petrogenetic processes from series a to d lo lts and hts lavas are intrepreted to be directly related by open system fractional crystallization in crustal magma chambers removal of observed phenocryst phases clinopyroxene orthopyroxene plagioclase and magnetite and repeated subsequent mixing of dacitic to rhyodacitic magmas with batches of replenishing basaltic andesites are the major processes possibly induced by vesicle formation in the mafic layer after a period of some crystallization lts and dlts magmas were directly fed to the surface without stagnating at crustal levels with feeder dykes positioned marginal to the central rift zone and thus by passing the central magma chambers these magmas apparently experienced only limited fractional crystallization of 10 15 wt olivine clinopyroxene chromite probably at the mantle crust boundary

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

shallow anatomy of a continent ocean transition zone in the northern south china sea from multichannel seismic data the cenozoic tectonic evolution of the south china sea is reflected in diverse tectonic processes including continental rifting seafloor spreading subduction terrane collision and strike slip fault movement a continent ocean transition zone in the northern south china sea caused by the lithospheric extension when the continent underwent break up rifting and later seafloor spreading is clearly imaged in the multi channel seismic data presented in this study the morphological units of the continent ocean transition zone are the rift depression the volcanic zone and tilted fault blocks the volcanic zone represents a highest extension zone within the continent ocean transition zone and is mainly distributed in the southern slope uplift zone along the northern passive margin of the south china sea the large listric normal faults bounding the dongsha rise and baiyun sag are evidenced in the seismic image the passive margin in the northern south china sea underwent the wide rift to narrow rift process in the transition zone as inferred from the relation between the surface heat flow and initial rifted crustal thickness the continent ocean transition zone in the continental margin of the south china sea is consistent with high heat flow zone average 90 mw m 2 observed in the previous heat flow measurements and at odp site 1148 and is manifested in a sharp change of the p wave velocity the rifted margin of the south china sea is a non typical magma poor passive margin or an intermediary form between the iberian type non volcanic and the greenland type volcanic margin compared to the world s typical passive margins c 2012 elsevier b v all rights reserved

distribution of porosity in a section of upper oceanic crust exposed in the troodos ophiolite the porosity structure of a section of old oceanic crust was characterized by a detailed study of the volcanic sequence of the troodos ophiolite the distribution and abundance of porosity was compared to the amount of secondary minerals in order to calculate how porosity created by crustal accretion is modified by alteration as a section of crust ages eight sites with one dominant lithology and contrasting alteration histories were studied two types of porosity were quantified macroscopic macrofractures vesicles vugs interpillow zones and total macroscopic and laboratory measured microscopic porosity crustal accretion produced a vertically zoned porosity structure due to differences between pillows that formed during the main and waning stages of crustal construction mean initial macroscopic porosity ranges from 14 to 17 in uppermost pillows and from 6 to 10 in the underlying pillows and massive flows macroscopic porosity was reduced by 10 11 where alteration was most pervasive i e 250 m of the paleoseafloor beneath this zone pillow and flow outcrops showed decreases of 1 3 and 2 5 respectively the combined effects of volcanic morphology and alteration resulted in no systematic variation with depth in the final macroscopic porosity 4 8 the greatest decrease in macroscopic porosity was localized where abyssal hill topography or volcanic edifices and sedimentation rates facilitated the growth of the seafloor weathering zone which comprises 10 of the volcanic pile thus in modern ocean basins alteration would have the greatest effect on crustal porosity in areas of rough topography where crests of abyssal hills remain unsedimented for long periods of time porosity data derived from deep sea drilling project ocean drilling program cores and borehole logs show similar relationships which implies that the evolution of upper crustal porosity is controlled by the local environment and that the evolution of upper crustal porosity cannot be uniquely described by seismic data

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

crustal structure across the xisha trough northwestern south china sea located at the northwestern part of the south china sea scs between the hainan and xisha paracel islands the xisha trough represents a failed rift in conjunction with the opening of the scs between 32 and 17 ma from west towards east within a scale of several hundred kilometers it presents all major stages of the rifting process and thus provides an ideal place to study the rifting process in great details in the autumn of 1996 a joint team of sino german scientists carried out a wide angle seismic experiment across the xisha trough with 10 ocean bottom hydrophones obh along a 237 km nnw sse oriented profile which was surveyed in 1987 with multi channel seismic mcs method by bgr of germany and soa of china favorable weather conditions and the powerful 4 x 12 1 air gun array rendered very good quality data with seismic signals observed at the offset of up to 110 km a detailed velocity depth model was obtained by using an interactive trial and error 2d ray tracing method interpretation of the nics data published by bgr provides very good geometrical constraints of the complex upper crustal structure which is characterized by fault blocks half horsts and half grabens filled with syn and post rift cenozoic sediments the velocity model in turn confirms the major structure outlined by the interpretation of the mcs data showing a varying sedimentary layer between i and 4 km of thickness and velocities between 1 7 and 4 5 km s the p wave velocity of 5 5 km s on the top of the crystalline basement is relatively low suggesting strong weathering within the crystalline crust the velocity increases downward continuously to 6 8 km s at the bottom of the crust without a clear differentiation in the middle crust showing clearly its continental nature even beneath the xisha trough the moho is marked by a sharp first order interface with a velocity of 8 0 8 1 km s at the uppermost mantle the moho depth is 15 km beneath the center of the trough and increases gradually to more than 25 kin towards north and south corresponding to an extreme thinning of the pre rift continental crust from more than 25 kin under the coast line to only 8 kin beneath the xisha trough the similar velocity structure of the continental nature on both sides of the xisha trough suggests a homogeneous pre rift continental setting the sharp moho and the lack of high velocity body hvb in the lower crust imply no magmatic underplating which is very different to interpretations across the eastern part of the continental margin in northern south china sea the intense faulting in the upper crust the strong but rather symmetrical crustal thinning centered at the xisha trough and the close neighborhood to the open scs nw subbasin suggest a pure shear rifting which failed most likely at the phase shortly before the continental breakup c 2001 elsevier science b v all rights reserved

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

bioalteration of basaltic glass in the oceanic crust 1 bioalteration of quaternary to early cretaceous basaltic glass from pillow lavas of the upper oceanic crust can be documented in deep sea drilling project ocean drilling program dsdp odp samples from shallow to deep drill holes from the north to central atlantic ocean lau basin and costa rica rift a wide range of marine settings biogenerated textures are rooted in fractures and occur as two main types a granular type and a tubular type the granular type common at all depths within the volcanic pile appears as solid bands semicircles or irregular patches of individual and or coalesced spherical bodies mostly 0 2 0 6 m in diameter with irregular protrusions into the fresh glass the tubular type is more common at deeper levels in the crust and consists of thin tubes sometime branching bodies mostly 20 30 m long and are more common at deeper levels the upper crust displays a large variability in the relative importance of biotic to abiotic alteration and the degree of bioalteration appears to decrease with depth thus the fraction of bioalteration of the total alteration of the glass ranges from 20 90 in the upper 300 m down to a maximum of 10 at about 500 m depth this might be due to a natural variability in the abundance of bioaltered glass or to biased sampling from low drilling recovery of relatively young crust the proportion of bioaltered to abiotically altered glass does not show any systematic variations with age of the crust thus bioalteration lasts as long as abiotic alteration i e for as long as water is available to the hydration of the oceanic crust evidence from heat flow measurements suggests that hydrothermal circulation lasts until at least 70 ma and thus the deep biosphere is likely to expand at least into crust of this age

displacement along the red river fault constrained by extension estimates and plate reconstructions significant e w extension and or compression must have been generated by displacements along the red river fault rrf since its curvature does not match a small circle centered at the euler pole for the indochina south china plate pair the amount of extension perpendicular to the rrf offshore vietnam depends on the magnitude of left lateral displacement along the rrf in general the larger the left lateral displacement along the fault the smaller the amount of e w extension all purely strike slip models of the opening of the south china sea that assume large displacements 250 km along the rrf encounter major problems because they imply little extension or even considerable shortening offshore east vietnam this is inconsistent with the presence of large elongated basins offshore vietnam using a plate tectonic model we compare continental extension values implied by different magnitudes of displacement along the rrf with crustal stretching estimates derived from 2 d profiles modeled from gravity data we utilize 2 d gravity forward models to restore the extended continental margin crust to its original position prior to extension we find that substantial amounts of extension for offshore vietnam can only be modeled assuming moderate displacements along the rrf compatible with the presence of a southward subducting proto south china sea the total amount of ene wsw extension offshore northern vietnam constrained by our 2 d gravity profiles and gravity inversion increases southward from 36 to 89 km along the yinggehai basin these values of ene wsw extension are consistent with 250 km of left lateral displacement along the rrf

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