strontium isotope constraints on fluid flow in the upper oceanic crust at the east pacific rise strontium isotopes are useful tracers of fluid rock interaction in marine hydrothermal systems and provide a potential way to quantify the amount of seawater that passes through these systems we have determined the whole rock sr isotopic compositions of a section of upper oceanic crust that formed at the fast spreading east pacific rise now exposed at hess deep this dataset provides the first detailed comparison for the much studied ocean drilling program odp drill core from site 504b whole rock and mineral sr concentrations indicate that sr exchange between hydrothermal fluids and the oceanic crust is complex being dependent on the mineralogical reactions occurring in particular epidote formation takes up sr from the fluid increasing the 87sr 86sr of the bulk rock calculating the fluid flux required to shift the sr isotopic composition of the hess deep sheeted dike complex using the approach of bickle and teagle 1 m j bickle d a h teagle strontium alteration in the troodos ophiolite implications for fluid fluxes and geochemical transport in mid ocean ridge hydrothermal systems earth planet sci lett 113 1992 219 237 gives a fluid flux similar to that determined for odp hole 504b this suggests that the level of isotopic exchange observed in these two regions is probably typical for modern oceanic crust unfortunately uncertainties in the modeling approach do not allow us to determine a fluid flux that is directly comparable to fluxes calculated by other methods

tectonic rotations of dikes in fast spread oceanic crust exposed near hess deep oceanic crust that initially formed at the fast spreading 130 mm yr east pacific rise at 1 ma is exposed in the walls of hess deep rift at the northern edge of the galapagos microplate steep fault scarps expose a dismembered vertical sequence of pillow basalts sheeted diabase dikes and gabbroic rocks diabase dikes in these units are dominantly vertical however extensive outcrops of steeply to moderatley dipping dikes exist in some places the nonvertical attitude of the dikes may be the result of tectonic rotations attending sea floor spreading at the east pacific rise or the result of complex rotations of fault blocks during the opening of the hess deep rift at 0 65 ma

controls on hydrothermal alteration in a section of fast spreading oceanic crust abstract young oceanic crustal sections that formed at the fast spreading east pacific rise epr were mapped by submersible along steep fault scarps that bound the hess deep rift valley excellent exposures allowed the documentation of the spatial distribution of alteration within the lava sequence sheeted dike complex and uppermost plutonic sequence alteration assemblages record the initiation and subsequent cooling of an axial hydrothermal system s that was periodically perturbed by the injection of sheeted dikes mineralogical and geochemical data indicate that temperatures increased across the lava dike transition 50 180 c and that the grade of metamorphism was heterogeneous within the sheeted dike complex in that temperatures varied laterally from 100 300 c to 600 800 c within the upper dikes and were 400 450 c within the lower sheeted dikes during vigorous hydrothermal circulation at the epr the base of the hydrothermal system was located within or at the top of a sequence of gabbros that is interpreted to be the root zone of the sheeted dike complex the base of the hydrothermal cell did not penetrate into the subjacent plutonic sequence until the section of crust had migrated off axis and the magma lens had solidified the vigor of the hydrothermal system would have waned by this time and hydrothermal fluids migrated into the lower crust along broadly distributed microfracture networks at temperatures between 450 and 650 c although locally temperatures may have reached 800 c the highest temperature alteration assemblages within the sheeted dikes formed in response to the injection of a dike or swarm of dikes that initiated a short lived hydrothermal system the sheeted dikes and plutonic rocks experienced very little retrograde alteration and there is no evidence for a discrete off axis hydrothermal event s incipient alteration conditions in crustal sections that formed at mid ocean ridges with slower rates of spreading than the epr are similar to hess deep although there are differences in the distribution and mineralogical expression of incipient alteration in plutonic sequences the evolution of alteration as a section of crust cools and moves off axis reflects its particular magmatic tectonic history and thus may vary significantly in ridges with all spreading rates

relationship between the sr and o isotope compositions of hydrothermal fluids and the spreading and magma supply rates at oceanic spreading centers we find a first order correlation between the sr and o isotope compositions of hydrothermal fluids and spreading rate at oceanic spreading centers the isotope signatures of hydrothermal fluids at slow spreading ridges are more rock dominated than those at fast spreading ridges implying that they have undergone a greater extent of sr and o exchange with the crust the local spatial and temporal variability in sr and o isotope signatures within individual hydrothermal sites can be attributed to short term variations in the hydrothermal system in response to magmatic or tectonic events phase separation at depth and mixing with seawater in the upwelling zone however these processes cannot explain the systematic global relationship of an increasing rock signature in hydrothermal fluids with decreasing spreading rate a greater extent of sr and o exchange between rocks and the circulating fluids can be achieved by 1 a longer fluid flow pathway through the oceanic lithosphere 2 more fractured rock that exposes a greater reaction area 3 an increased residence time of the fluid due to slower hydrothermal circulation or 4 a combination of all three our preferred interpretation is that fluid flow paths are longer and depths of penetration are likely greater at slow spreading ridges

87sr 86sr ratios in hydrothermal waters and deposits from the east pacific rise at 21 n abstract 87 sr 86 sr ratios of three hydrothermal waters collected on the east pacific rise at 21 n define a mixing line between seawater and a hydrothermal end member at 0 7030 which is derived by seawater basalt interaction at ca 350 c and water rock ratio of about 1 5 sr concentrations are not affected in the process while mg uptake from seawater is almost complete up to2 3 of this hydrothermal component is involved in anhydrite precipitation while the sr isotopic ratio in sulfides chalcopyrite sphalerite cannot be distinguished from that of sulfate it is estimated that ca 1 10 10 moles of strontium are yearly cycled in the hydrothermal systems of mid oceanic ridges thereby affecting the 87 sr 86 sr budget of seawater mass balance between river runoff limestone precipitation and ridge basalt alteration suggests that the 87 sr 86 sr ratios of the river runoff are in the range 0 7097 0 7113 and are largely dominated by limestone alteration

petrology and geochemistry of the lower ocean crust formed at the east pacific rise and exposed at hess deep a synthesis and new results 1 the geochemistry and petrology of the lower oceanic crust record information about the compositions of melts extracted from the mantle how these melts mix and crystallize and the role of hydrothermal circulation in this portion of the crust unfortunately lower oceanic crust formed at fast spreading ridges is rarely exposed at the seafloor making it difficult to study these processes at hess deep crust formed at the east pacific rise epr is exposed due to the propagation of the cocos nazca spreading center westward here we review our state of knowledge of the petrology of lower crustal material from hess deep and document new mineral major and trace element compositions amphibole plagioclase thermometry and plagioclase crystal size distributions samples from the deeper parts of the gabbroic sequence contain clinopyroxene that is close to being in trace element equilibrium with erupted basalts but which can contain primitive moderate cr high mg orthopyroxene and very calcic plagioclase because primitive mid ocean ridge basalts morbs are not saturated with orthopyroxene or very calcic plagioclase this suggests that melts added to the crust have variable compositions and that some may be in major but not trace element equilibrium with shallow depleted mantle these apparently conflicting data are most readily explained if some of the melt extracted from the mantle is fully aggregated within the mantle but reacts with the shallow mantle during melt extraction the occurrence of cumulates with these characteristics suggests that melts added to the crust do not all get mixed with normal morb in the axial magma chamber amc but rather that some melts partially crystallize in isolation within the lower crust however evidence that primitive melts fed the amc along with steep fabrics in shallow gabbros from near the base of the dyke complex provides support for models in which crystallization within the amc followed by crystal subsidence is also an important process in lower crustal accretion more evolved bulk compositions of gabbros from the upper than lower parts of the plutonic section are due to greater amounts of reaction with interstitial melt and not because their parental melt had become highly fractionated through the formation of large volumes of cumulates deeper in the crust amphibole plagioclase thermometry confirms previous reports that the initial ingress of fluid occurs at high temperatures in the shallow gabbros tave 713 c and show that the temperature of amphibole formation was similar in deeper gabbros tave 722 c this thermometry also suggests that fracture and grain boundary permeability for seawater derived fluids was open over the same temperature interval

sr isotope evolution of seawater the role of tectonics we use a high resolution seawater sr isotopic evolution curve for the last 100 m y in conjunction with modern riverine sr flux measurements and also geologic tectonic and geochronological data to make the case for a close relationship between seawater sr isotopic composition and the india asia continental collision using a simple seawater sr budget model we begin by showing that the sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater sr isotopic curve of the last 100 m y the flux of dissolved sr carried by rivers originating in the himalaya tibet region on the other hand is presently a significant fraction of the global sr budget we calculate how this riverine flux would have had to change with time in order to match the observed seawater sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of india with asia but then increases very significantly after collision we note that the period of most rapid change in seawater sr isotopic ratio from 20 ma to 15 ma is also a period of exceptionally high erosion in parts of the himalayas and the tibetan plateau as further evidence that sr derived from the collision of india with asia plays a major role in the sr isotopic evolution of seawater we show that the total amount erosion of the himalaya tibetan plateau since collision which we calculate separately represents a total amount of sr that is very nearly the same as the cumulative amount required by the sr isotopic change of seawater since collision the relationship between erosion and riverine sr flux allows us to use the sr isotopic evolution of seawater to reconstruct a history of erosion since collision and we find that the erosion rate accelerates with time since collision with the present having the largest rate when we apply the sr budget model to the entire phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large scale structure of the seawater sr isotopic curve but fail to reproduce several local maxima and minima especially in the period 100 300 ma the present high s7sr s6sr of seawater and similar highs in the devonian and cambrian do correlate with extensive deformation on the continents

hydrogeochemistry and strontium isotopes in the arno river basin tuscany italy constraints on natural controls by statistical modeling summary in this paper the chemistry of major and trace elements and for the first time strontium isotopic ratios measured in running waters from the arno river basin tuscany central northern italy and thermal springs discharging in the same hydrographic system are presented and discussed classical graphical methods e g mixing diagrams have here been improved to identify in a correct statistical sample space extreme chemical compositions attributable to the action of geochemical processes and or inherited from specific lithologies namely contributions or components 1 2 and 3 to be used in inverse modeling procedures due to the absence of clear end members a linear least squares problem with non negativity constraints and distances as required for compositional data convex linear mixing was solved by considering the contribution of the most discriminant variables given by the 87 sr 86 sr isotopic ratios and the concentration of ca 2 mg 2 sr and rb following such approach it can be assumed p 2 mg 2 hco 3 radiogenic rich 87 sr 86 sr 0 71274 rb sr 0 039 represents a dominant feature at basin scale with a weight ranging from 69 to 100 much lower percentages are related to component 2 represented by a ca 2 mg 2 so 4 2 facies with intermediate 87 sr 86 sr 0 70874 and low rb sr 2 8 10 4 ratios and component 3 identified by ca 2 hco 3 facies with less radiogenic 87 sr 86 sr 0 70827 and low mg 2 ca 2 0 011 ratios these components are mainly dominated by the dissolution of evaporitic rocks and or mixing with thermal waters in the southern part of the arno river basin and by dissolution of the carbonatic fraction kinetically favored with respect to that of the silicatic minerals in the upper reaches of the main course and its tributaries respectively

fluid flow patterns in fast spreading east pacific rise crust exposed at hess deep tectonic exposures of a volcanic sequence and sheeted dike complex over a 4 km wide region at hess deep equatorial pacific reveal significant spatial heterogeneity 10 103 m in the extent and nature of hydrothermal alteration in young fast spreading east pacific rise crust the volcanic sequence is fairly uniformly altered with only minor oxidation and alteration to clay minerals sheeted dikes in the eastern part of the field area are highly fractured with narrow intervals of intact dikes that dip up to 60 their alteration characteristics show a simple depth trend such that with increasing depth the dominant secondary mafic mineral changes from chlorite to amphibole clinopyroxene replacement increases 40 whole rock 18o values decreases 4 4 5 5 to 3 5 4 5 and calculated peak metamorphic temperatures increase 250 c to 450 700 c within the deepest dikes localized zones up to 400 m wide are chlorite rich and have low 18o 2 9 4 1 and low peak metamorphic temperatures 345 c these alteration patterns likely formed within broad recharge zones whereby the low 18o zones developed in the regions with the highest fluid flux in the west massive slightly rotated sheeted dikes near the volcanic sheeted dike transition are 18o and cu depleted and display higher peak temperatures 345 c than elsewhere in the shallow dikes these characteristics are consistent with formation within a high temperature hydrothermal discharge zone we propose that the spreading history of a fast spreading ridge segment can create significant spatial heterogeneity in fluid flow and alteration patterns within sheeted dike complexes similar to those preserved in many ophiolites

a mid ocean ridge thermal model constraints on the volume of axial hydrothermal heat flux the depth to the axial magma chamber at three oceanic spreading centers as indicated by seismic reflection surveys is greater than that predicted by conductive cooling thermal models of ridge crests the additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust a theoretical model for the temperature distribution at spreading centers which includes distributed heat sources and sinks is presented by representing the hydrothermal heat loss as a series of heat sinks the depths to the axial magma chamber at the east pacific rise at latitude 9 n the southern juan de fuca ridge and the lau basin spreading center are modeled heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys the thermal modeling indicates that the axial hydrothermal heat flux is between 7 1 106 and 13 8 106 mj m2 or 10 to 20 of the total missing heat at the ridge axis requiring extensive low temperature circulation off axis high temperature vents that are spaced 1 km apart and are active between 4 and 10 of the time would account for the axial hydrothermal heat loss

near axis subsidence rates hydrothermal circulation and thermal structure of mid ocean ridge crests we systematically investigated near axis subsidence on the ridge flanks of intermediate and fast spreading mid ocean ridges using bathymetric data from well surveyed portions of the southeast indian ridge spreading at 72 76 mm yr the northern east pacific rise 91 96 mm yr and the southern east pacific rise 144 mm yr in all three regions the mean subsidence rate of young 1 1 4 ma seafloor is less than 220 m m y 1 2 the distribution of individual estimates shows a distinct peak at 180 220 m m y 1 2 with few profiles having subsidence rates greater than 275 m m y 1 2 the observed subsidence rates are significantly lower than both the worldwide average 350 m m y 1 2 and subsidence rates observed for older lithosphere at the same ridge segments intense hydrothermal circulation at the ridge axis can result in low subsidence rates on the adjacent ridge flanks provided the vigorous flow is confined to the immediate vicinity 5 km of the axis according to our model the extremely vigorous hydrothermal circulation ceases off axis and conductive heat flow becomes a primary mechanism of vertical heat transport on the ridge flanks the very low geothermal gradient within the cooled portion of the uppermost lithosphere retards conductive cooling and the cooled area needs to be heated from below before a geothermal gradient can be established which permits significant heat to be conducted out of the lithosphere as a result subsidence in very young 0 1 1 ma lithosphere is suppressed a simple one dimensional thermal model with a nusselt number parameterization was used to estimate the effect of hydrothermal circulation an upper layer with a high nusselt number and the half space that it overlies are initially at a temperature of 1300 c and the surface is maintained at 0 c after 0 1 m y of cooling about 3 5 to 7 km from the spreading axis the nusselt number of the top layer is set to 1 so that normal conduction is simulated in the cooled layer we used an explicit finite difference method to solve for the temporal changes in temperature with depth this model produces subsidence rates in the range that we observe for nu in the range of 15 30 isotherms resulting from the modeling imply rapid lithospheric thickening very near the axis which is incompatible with most current models for the formation of the bathymetric axial high observed at fast spreading mid ocean ridges

structural pattern of the galapagos microplate and evolution of the galapagos triple junctions a 13 000 km2 microplate intervenes to prevent the junction of the nazca cocos and pacific plates in the eastern equatorial pacific ocean west of the galapagos islands the microplate is rotating clockwise at 6 m y about an absolute pole 400 km southeast of its center so that although its northern margin has a similar motion to adjacent parts of the cocos plate its southern margin is 50 slower the perimeter of the galapagos microplate is best defined by bathymetry and has been surveyed with sea beam and examined by deep tow two of the three triple junctions on its margins are ridge ridge ridge rrr junctions 160 km apart on the crest of the east pacific rise epr the most like a classic rrr junction is the pacific nazca galapagos intersection at 1 10 n where the epr axis is joined by a narrow volcanic ridge 70 km long and 1 km high the cocos nazca galapagos junction at the west end of the cocos nazca spreading center is inferred to be of the rff type but the transform fault branches are represented by large nontransform offsets lack of stable transform faults is related to rapid westward migration of the junction caused by lengthening of the cocos nazca axis at 60 70 mm yr this rate which was once required to maintain a stable nazca cocos pacific rrr junction has persisted even after birth of the microplate about 1 ma continuing propagation of the cocos nazca axis has opened up the hess deep rift basin in the eastern epr flank in advance of the eruptive propagator tip this rift valley has a relief of more than 3 km from its uplifted rims to the floor where severe attenuation exposes basal layers of the epr crust the similar dietz deep rift basin at the southeastern margin of the microplate 70 km from hess deep was formed when a lengthening galapagos nazca spreading axis rifted through another part of the epr flank

mobilization of ree during crustal aging in the troodos ophiolite cyprus ree data for secondary minerals from low and high temperature alteration environments in the troodos ophiolite have been determined using a combined mass spectrometry neutron activation technique smectite and analcime ree mimic fresh rock profiles and have concentrations ranging from 1 5 times chondritic values celadonite is depleted in lree and total ree relative to the rocks groundmass replacing chlorite and epidote are similar to or enriched in ree relative to fresh rocks in contrast fault related chlorite and void filling epidote are lree enriched with large positive eu anomalies and yield comparable ree profiles to those of hydrothermal fluids venting at mid ocean ridges the zeolites clinoptilolite and laumontite are also lree enriched with ree contents significantly lower than rock values carbonate is enriched in hree relative to lree and concentrations vary from 0 2 to 18 times chondritic values ree data for these minerals indicate that seawater ree contents are modified by seawater rock interaction in volcanic sequences and sheeted dykes in oceanic crust during axial hydrothermal alternation and subsequent crustal aging the absolute and relative abundances of ree in seawater derived solutions evolve in response to the precipitation of secondary minerals and the prevailing water rock ratios rocks altered at low temperatures will show no change or a decrease in ree content the lree hree ratio will reflect the proportion of smectite celadonite and carbonate in the groundmass assemblage in contrast rocks altered at high temperatures will show no change or an increase in ree the lree hree ratio will reflect the proportion of void filling and groundmass replacing chlorite and epidote in the rock

drilling to gabbro in intact ocean crust sampling an intact sequence of oceanic crust through lavas dikes and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid ocean ridges but it has been an elusive goal of scientific ocean drilling for decades recent drilling in the eastern pacific ocean in hole 1256d reached gabbro within seismic layer 2 1157 meters into crust formed at a superfast spreading rate the gabbros are the crystallized melt lenses that formed beneath a mid ocean ridge the depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid ocean ridges melt lenses occur at shallower depths at faster spreading rates the gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by hole 1256d

recharge flux to ocean ridge black smoker systems a geochemical estimate from odp hole 504b we use the sr isotopic composition of rock and anhydrite as a monitor of fluid composition in dsdp odp hole 504b to calculate the recharge flux to the axial high temperature hydrothermal circulation the fluid and rock sr isotope profiles are well fit by a tracer transport mass balance model that approximates fluid solid exchange by linear kinetics the calculated time integrated flux of 1 7 0 2 x 106 kg m 2 is significantly less than the 5 x 106 kg m 2 calculated from thermal models that assume all magma is intruded into a high level magma chamber at the base of the sheeted dykes our low flux is consistent with the observed thermal structure as recorded by secondary alteration minerals in hole 504b and the intrusion of magma as lenses distributed through the lower oceanic crust it leaves open the questions as to how the lower oceanic crust cools and how seawater geochemical budgets balance

tracing the chemical evolution of fluids during hydrothermal recharge constraints from anhydrite recovered in odp hole 504b a unique record of the chemical evolution of seawater during hydrothermal recharge into oceanic crust is preserved by anhydrite from the volcanic sequences and sheeted dike complex in odp hole 504b chemical and isotopic analyses 87sr 86sr 18o 34s of anhydrite constrain the changing composition of fluids due to reaction with basalt there is a general trend of decreasing 87sr 86sr of anhydrite corresponding to the minor incorporation of basaltic strontium with depth in the volcanic rocks 87sr 86sr ratios decrease rapidly with depth in the dikes to values identical to host basalt 0 7029 sr ca ratios 0 1 mmol mol suggest that recharge fluids have very low sr concentrations and fluids evolve by first precipitating sr bearing phases before extensive exchange of sr with the host basalt there is a background trend of decreasing sulfate 18o with depth from 12 13 in the lower volcanics to 7 in the lower sheeted dikes recording an increase in recharge fluid temperature from 150 to 250 c and confirming the presence of sulfate in hydrothermal fluids at elevated temperatures from the amount of anhydrite recovered from hole 504b and the amount of seawater sulfur that has been reduced to sulfide a minimum seawater recharge flux can be calculated this value is 4 25 times lower than estimates of high temperature fluid fluxes based on either thermal constraints or global chemical budgets and suggests that there is significant deficit of seawater derived sulfur in the oceanic crust only a minor proportion of the seawater that percolates into the crust near the axis is heated to high temperatures and exits as black smoker type fluids a significant proportion of the axial heat loss must be advected at 200 250 c by sulfate bearing hydrothermal solutions that egress diffusely from the crust these fluids penetrate into the dikes and exchange both heat and chemical tracers without the extensive clogging of porosity by anhydrite precipitation which would halt hydrothermal circulation for any reasonable fluid flux the heating of the major proportion of hydrothermal fluids to only moderate temperatures 250 c reconciles estimates of hydrothermal fluxes derived from thermal models and global geochemical budgets the flux of hydrothermal sulfate would be of a magnitude similar to the riverine input and oxygen isotopic exchange at 200 250 c between dissolved sulfate and recharge fluids during hydrothermal circulation provides a mechanism to continuously buffer seawater sulfate oxygen to the light isotopic composition observed

tracing the evolution of hydrothermal fluids in the upper oceanic crust sr isotopic constraints from dsdp odp holes 504b and 896a

direct observation of a fossil high temperature fault hosted hydrothermal upflow zone in crust formed at the east pacific rise fault zones in the ocean crust are commonly hypothesized to act as high permeability conduits that focus fl uid fl ow in oceanic hydrothermal systems however there has been little direct study of faults in crust formed at fast spreading ridges here we describe the geology and geochemistry of an 40 m wide fault zone within the uppermost sheeted dike complex exposed at pito deep northeastern easter microplate titanium in quartz thermometry gives temperatures of 392 33 c for quartz precipitation indicating that this fault zone focused upwelling fl uids at temperatures similar to those of black smoker vent fl uids correlated enrichment in 87 sr 86 sr and mgo in fault breccias along with 87 sr 86 sr ratios higher than in average vent fl uids provide evidence for mixing between high temperature upwelling fl uids and a seawater like fl uid within the fault zone large high temperature fl uid fl uxes are required to maintain high temperatures during mixing if this fault zone is representative of upfl ow zones beneath hydrothermal vents on the east pacifi c rise then it is possible that vent fl uids evolve thermally and chemically during their ascent and may not record the precise conditions at the base of the hydrothermal system

transport mechanisms by fluid flow in metamorphic rocks oxygen and strontium decoupling in the trois seigneurs massif a consequence of kinetic dispersion

chemical and thermal constraints on focussed fluid flow in the lower oceanic crust the mechanism of heat extraction from the lower oceanic crust near the ridge axis is poorly constrained despite its importance for understanding both the process of accretion of the plutonic complex and the mass fluxes associated with ridge hydrothermal systems we have investigated the role of zones of focussed fluid flow in the plutonic complex of the oman ophiolite in the near axis cooling of the oceanic crust lineaments identified on aerial photographs that occur at 1 km spacing show evidence for extensive hydrothermal fluid flow through regions 10 to 50 m wide fluid flow is initiated in these regions at 800 c and continues at least into the lower greenschist facies strontium isotope analyses indicate that the fluid flux through these zones is sufficient to transport a metasomatic front from the base of the sheeted dike complex to close to the moho computed minimum fluid fluxes to transport a metasomatic front through the focussed fluid flow zones are 1x108 kgm2 modeling of diffusive exchange of calcium from olivine to clinopyroxene indicates enhanced cooling rates adjacent to the focussed fluid flow zones heat fluxes estimated from the enhanced cooling rates are broadly consistent with the fluid fluxes determined from modeling the sr isotopic composition of samples from the focussed fluid flow zones the combination of independent estimates of the fluid and heat fluxes such as these can provide more rigorous constraints on the thermal history than either approach used in isolation our results show that focussed fluid flow could play a major role in the cooling in the lower oceanic crust significant focussed fluid flow in the lower oceanic crust has important implications for predicting the total mass flux associated with hydrothermal circulation at mid ocean ridges this is because fluids flowing through channels become chemically rock buffered at smaller fluid fluxes than those flowing pervasively through a rock mass thus if focussed fluid flow is an important mechanism of heat loss from the lower oceanic crust the chemical fluxes from ridge hydrothermal systems into the oceans may be smaller than currently thought

the northern samail ophiolite an oxygen isotope microprobe and field study geological petrological and oxygen isotopic data are presented for 228 whole rock and mineral samples collected from a 100 20 km area of the northern samail ophiolite in oman most of these samples are from three detailed profiles through the pillow lavas sheeted dikes and layered gabbros of this laterally heterogeneous fragment of cretaceous oceanic crust down to and across the petrologic moho the profiles encompass a range of petrologic and tectonic styles and each profile exhibits distinctive 18 16 o variations compared to one another and to mid ocean ridge basalts as a result of pervasive seawater hydrothermal interaction that varied in intensity along strike in the ophiolite in general 18 o depletions are observed in the layered gabbros and 18 o enrichments in most of the sheeted dikes and pillow lavas similar to results previously observed in the southern part of the ophiolite ibra area where 18 o depletions within the gabbroic section are quantitatively balanced by 18 o enrichments in the shallower parts of the oceanic crust the wadi hilti profile selected as an example of relatively intact crust differs from ibra in having more uniform and slightly higher 18 o in the gabbros 5 4 to 6 3 as well as in containing more hydrous alteration minerals amphibole epidote chlorite and prehnite the profiles in the wadi kanut shafan and wadi rajmi sections are much more complex and reveal the impact of off axis intrusions and deep crustal shearing plagiogranite wehrlite intrusions in the shafan kanut area superimposed a local hydrothermal aureole on the ophiolite evident in dikes highly depleted in 18 o quartz sulfide veins abundant epidote thullite and chlorite in shallower rocks and low temperature hydrous alteration of deeper gabbroic rocks the latter produced an overall increase in whole rock 18 o 6 2 to 6 9 such late stage intrusions are found throughout the northern half of the samail ophiolite the wadi rajmi area which is a possible fossil transform or propagating rift represents the most complex of the three profiles it also contains the most abundant highly deformed and hydrothermally altered rocks together with the deepest and largest zone of 18 o depletion yet found in any ophiolite locally 18 o 2 0 conduits for large volumes of high temperature hydrothermal fluids were provided by fractures now occupied by low 18 o gabbro pegmatites and low 18 o dikes material balance estimates for the regional samples and from the various transects through the ophiolite give crustal bulk 18 o averages 5 9 to 6 3 that are within sampling error almost identical to the average morb basalt value of about 5 8 if both vertical and lateral crustal heterogeneities are integrated into a three dimensional model this supports and amplifies the conclusion of earlier workers that the 18 o of seawater is buffered and controlled by hydrothermal interaction with oceanic crust as long as the cumulative effects both spatial and temporal of all seafloor magmatic hydrothermal processes are considered the very slight out of balance enrichment of the integrated crustal average 18 o compared to morb may be explained by the ubiquitous mineralogical and isotopic evidence for a late low temperature alteration event in the basal gabbros these effects are prominent in the vicinity of the petrologic moho and may indicate exchange with low temperature aqueous fluids during or after detachment of the obducted slab

strontium isotopic composition of the po river dissolved load insights into rock weathering in northern italy abstract strontium isotopic composition 87sr 86sr of river water provides insight on the nature of rocks involved in the weathering processes and important constraints for reconstructing silicate versus carbonate weathering in large scale basins new data presented in the context of a geochemical map of the po river basin highlight that the po river water evolves from the upper part of the catchment where 87sr 86sr approaching 0 7097 relates to the weathering of igneous and metamorphic silicate rocks to the middle part where less radiogenic compositions 87sr 86sr 0 7089 are driven by the confluence of the tributaries draining mesozoic carbonate rocks of the south alpine domain and from inflow of apennine streams characterized by less radiogenic sr signatures due to weathering of marly sediments and mafic magmatic rocks down flow the sr isotopic compositions of po river water rises at 0 7091 either by the confluence of the last apennine tributary panaro 87sr 86sr 0 7095 and or by the possible contribution from hyporheic exchanges po river isotopic signatures reveal slightly more radiogenic 87sr 86sr in comparison to other important alpine river systems e g danube rhine and rhone likely suggesting that silicate rocks are comparatively contributing more weathering byproducts in the po basin

strontium alteration in the troodos ophiolite implications for fluid fluxes and geochemical transport in mid ocean ridge hydrothermal systems new and published strontium isotope analyses from the troodos ophiolite constrain fluid solid exchange processes and the magnitude and circulation paths of the hydrothermal fluids the87sr 86sr profile reflects alteration in the recharge zone of an evolving hydrothermal system fluid rock strontium isotope exchange in the upper 1 5 km of extrusive lavas was kinetically limited and seawater derived fluids emitted from the base of this zone were buffered to87sr 86sr ratios between 0 7047 and 0 7059 in contrast over the next 1 km depth interval of sheeted dykes and the uppermost plutonics 87sr 86sr values cluster about0 7054 7 2 and fluid flow is inferred to have been pervasive with near equilibrium fluid rock exchange quartz chlorite and epidosite zones the probable pathways of the concentrated high temperature upwelling fluids have identical87sr 86sr ratios to adjacent diabase dykes on troodos a time integrated fluid flux in excess of2 9 107 kg m 2 is required to transport the strontium isotope composition of 0 7054 set in the kinetically controlled exchange zone through the 1 km of sheeted dykes and into the zones of concentrated upwelling the uniformity of the87sr 86sr ratios in the diabase sheeted dykes and high temperature epidosite and quartz chlorite rocks indicate that the strontium isotopic alteration took place during the high temperature phase of hydrothermal circulation the inferred minimum time integrated fluid flux of2 9 107 kg m 2 substantially exceeds that of 5 106 kg m 2 inferred from thermal models of high temperature circulation but is comparable with estimates of the hydrothermal flux from oceanic budgets of3he mg and87sr the high flux estimate for troodos is consistent with the ophiolite venting fluids with87sr 86sr elevated significantly above rock values which contrasts with the near morb87sr 86sr ratios of fluids from active high temperature vents at mid ocean ridges and the87sr 86sr profile in dsdp hole 504b this requires that either the modern systems are immature and there is a protracted phase of lower temperature circulation or that circulation in the troodos ophiolite differed from that in normal mid ocean ridges and oceanic strontium budgets are balanced by another source the lack of very elevated87sr 86sr values in the vent fluids precludes significant channelling in the recharge zones of the active systems

petrology of the east pacific rise crust and upper mantle exposed in hess deep eastern equatorial pacific the hess deep a rifted oval shaped depression located east of the galapagos triple junction at the tip of the cocos nazca ridge about 101 w 2 n was explored in 1988 during 21 submersible dives a total of 11 dives were devoted to the exploration of the e w trending intrarift ridge 15 km in length 3000 5400 m in depth north of the hess deep depression the intrarift ridge represents an outcrop of recent 1 m y crustal and subcrustal material created at the axis of the east pacific rise epr and emplaced during the lithospheric extention responsible for the westward propagation of the cocos nazca rift francheteau et al 1990 the lithospheric block has undergone cataclastic deformation and was dislocated by tectonic activity as attested to by the mixed and erratic distribution of rock types and by the occurrence of polygenic breccias and gabbroic mylonites the samples are metamorphosed to varying degrees but their protolith textures are generally well preserved their relic mineralogy indicates that they consist of harzburgites dunites gabbroic cumulates gabbronorites and olivine gabbros isotropic gabbros dolerites and basalts some samples of refractory harzburgites and most dunitic cumulates with local accumulation of chromite have been impregnated by wehrlitic and gabbroic primitive melts similar to those described from the mantle crust transition zone of the samail ophiolite complex oman the mineral chemistry indicate that the ultramafics partly reequilibrated with the magmatic impregnations in the liquidus solidus temperature range of 980 1100 c the dolerites and basalts have been derived from mid ocean ridge basalt primary melts presenting a broad range of incompatible element composition which suggests intermittent cycles of magmatic processes involving a progressive melting of a composite source with discontinous extraction of liquids as proposed for the epr volcanics near 13 n hekinian et al 1989 most of the rocks underwent partial retrograde metamorphism and recorded several episodes of recrystallization from the upper greenschist facies ultramafics and gabbros to diagenetic alteration volcanics the cumulate gabbronorites the isotropic gabbros and some dolerites were partially albitized and amphibolitized during the penetration of seawater in the ocean crust prior to serpentinization several samples of unfoliated amphibolites are believed to be completely metamorphosed gabbroic rocks the gabbroic cumulates and the plagioclase rich melt impregnations were variably rodingitized presence of various ca silicates such as epidote prehnite hydrogarnet and zeolite in relation to the serpentinization of the peridotites one dive located on the scarps forming the northern wall of the hess deep to the east of the explored area revealed the presence of in situ outcrops of isotropic gabbros doleritic dikes and extrusives and permitted to observe the contact between the sheeted dike complex and the high level isotropic gabbros

mid ocean ridge sulfide deposits evidence for heat extraction from magma chambers or cracking fronts numerous seafloor observations show that the sizes of high temperature hydrothermal sulfide edifices vary dramatically with spreading rate on fast spreading ridges venting occurs from spindly chimneys which reach heights of about 10 m while on slow spreading ridges vents are frequently located near the tops of large sulfide mounds whose volumes may reach 105 106 m3 we argue that such variations are the result of a profound difference in the nature of hydrothermal heat extraction between fast spreading ridges where spreading occurs predominantly by magmatism and slow spreading ridges where there is a significant component of tectonic extension along fast spreading ridges a steady state axial magma chamber can be insulated by a relatively thick conductive boundary layer because heat extraction is limited by low permeability in such systems episodes of vigorous venting are linked to diking events not only because these introduce a significant heat source into the upper crust but also because the intrusion of a dike is the primary source of increased permeability near the ridge axis over a time scale of the order of a decade mineral deposition tends to clog the reaction and upflow zones and there is a high probability that young sulfide structures will be buried by subsequent eruptions on slow spreading ridges tectonic extension maintains the fluid pathways necessary to support vigorous convection in the waning stages of magmatism hydrothermal circulation driven by a downward migrating cracking front can cool the entire crust leading to the formation of very large sulfide deposits as the depth of circulation increases overburden pressures reduce the permeability and the hydrothermal heat fluxes decrease progressively once hydrothermal fluids penetrate the moho serpentinization clogs the fluid pathways and high temperature venting ceases until it is reactivated by a fresh magmatic intrusion

hydrothermal circulation anhydrite precipitation and thermal structure at ridge axes the thermal structure of the axial region of ridges spreading at 60 mm yr 1 was studied by comparing observations with numerical thermal models data from deep sea drilling project hole 504b constrain the temperature in the pillows and the upper part of the dike complex seismic reflection data constrain the depth to the magma chamber magnetic anomalies constrain the accumulation rate of pillows the thermal models assumed steady state and included the kinematics hydrothermal flow and the accumulation of pillows basalts in the successful thermal models hydrothermal fluid entered the dike intrusion zone at the ridge axis over a depth range from the base of the pillows to about 0 8 km above the top of the magma chamber the computed temperature at the axis in this depth range was nearly isothermal and suitable for the source regions of black smoker vents the computed temperature grid and assumed flow grid were used to model anhydrite precipitation and sulfate transport and thus more fully examine the implications of the flow pattern anhydrite was precipitated as seawater approached the hot axial zone the anhydrite was the carried outward with the spreading seafloor and redissolved when it reacted with pristine downwelling seawater the net effect was to deposit anhydrite near the base of vigorous hydrothermal circulation where it could not easily be leached by lower temperature off axis circulation and returned to the ocean this is a general feature of flow patterns where cold seawater vigorously enters the axial zone in broad downwellings that are not strongly heated by conduction at shallow depths alternatively the cold recharge is concentrated in the axial region and exchanges heat with nearby upwelling flow in the transition zone beneath the pillows anhydrite is then precipitated at shallow depths where it can later be leached back into the ocean the latter possibility is favored by studies of geochemical mass balances as well as the lack of evidence for a deep anhydrite layer in the oceanic crust

structure of uppermost fast spread oceanic crust exposed at the hess deep rift implications for subaxial processes at the east pacific rise the uppermost 2 km of the oceanic crust created at the fast spreading 135 mm yr 1 full rate equatorial east pacific rise epr is exposed for tens of kilometers along escarpments bounding the hess deep rift mosaics of large scale digital images from the remotely operated vehicle rov argo ii and direct observations from the submersible alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units these rock units are characterized by extensive faulting fine scale fracturing and rotations of coherent crustal blocks meters to tens of meters across the uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces through most of the basaltic lava unit however lava flow contacts dip 20 70 w toward the epr and generally increase in dip downward in the section dikes cutting the lavas and in the underlying sheeted dike unit generally dip 90 40 e away from the epr deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units we interpret this upper crustal structure as the result of subaxial subsidence within 1 2 km of the epr that accommodated the thickening of the basaltic lava unit to 500 m variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply these results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the hess deep rift if this type of structure is typical of uppermost oceanic crust generated at the epr these processes may be common along fast spreading mid ocean ridges

reconciling temperatures of metamorphism fluid fluxes and heat transport in the upper crust at intermediate to fast spreading mid ocean ridges 1 current models of the mineralogical and chemical modification of the crust within mid ocean ridge hydrothermal systems at intermediate to fast spreading ridges are difficult to reconcile with the temperature distribution predicted by the required fluid fluxes in particular the sharp increase in metamorphic grade passing down from the lavas into the sheeted dike complex places important constraints on the flow of fluid through the crust this distribution of metamorphic temperatures has typically been discussed in terms of a step in permeability in the downwelling limb of a single pass convection cell simple modeling of this system shows that downwelling fluid cools the crust too efficiently for high temperature metamorphism of the upper dikes to occur in the downflow zone in fact temperatures throughout most of the sheeted dike complex are too high three alternative models are considered fluid rock reaction during the cooling of each dike from magmatic temperatures is difficult to reconcile with cooling rate estimates for dikes reaction rates required to grow metamorphic minerals and the relatively constant composition of many hydrothermal vents metamorphism during reheating of the crust after fluid flow slows or stops is difficult to reconcile with the required fluid fluxes during metamorphism based on the sr isotopic composition of the sheeted dike complex and cannot produce high enough temperatures in the upper dikes and low temperatures in the lower lavas in contrast fluid rock reaction in the sheeted dike complex as hot fluids migrate upward after being heated by the underlying magma source appears to be consistent with the data in this model downwelling seawater cools the recharge zone and the fluid only becomes hot enough to significantly alter the crust once it approaches the conductive boundary layer overlying the magma chamber metamorphism of the sheeted dike complex then largely occurs in the discharge zone

strontium oxygen and hydrogen isotope geochemistry of hydrothermally altered and weathered rocks in dsdp hole 504b costa rica rift dsdp hole 504b was drilled into 6 ma crust about 200 km south of the costa rica rift galapagos spreading center penetrating 1 35 km into a section that can be divided into four zones zone i oxic submarine weathering zone ii anoxic alteration zones iii and iv hydrothermal alteration to greenschist facies in zone iii there is intense veining of pillow basalts zone iv consists of altered sheeted dikes isotopic geochemical signatures in relation to the alteration zones are recorded in hole 504b as follows zone depth m average87sr 86sr average 18o o average d o i 275 550 0 7032 7 3 63 ii 550 890 0 7029 6 5 45 iii 890 1050 0 7035 5 6 31 iv 1050 1350 0 7032 5 5 36 full size table table options view in workspace download as csv alteration temperatures are as low as 10 c in zones i and ii based on oxygen isotope fractionation strontium isotopic data indicate that a circulation of seawater is much more restricted in zone ii than in zone i fluid inclusion measurements of vein quartz indicate the alteration temperature was mainly 300 20 c in zones iii and iv which is consistent with secondary mineral assemblages the strontium oxygen and hydrogen isotopic compositions of hydrothermal fluids which were responsible for the greenschist facies alteration in zones iii and iv are estimated to be 0 7037 2 and 3 respectively strontium and oxygen isotope data indicate that completely altered portions of greenstones and vein minerals were in equilibrium with modified seawater under low water rock ratios in weight of about 1 6 this value is close to that of the end member hydrothermal fluids issuing at 21 n epr basement rocks are not completely hydrothermally altered about 32 of the greenstones in zones iii and iv have escaped alteration thus 1 g of fresh basalt including the 32 unaltered portion are required in order to make 1 g of end member solution from fresh seawater in water rock reactions

a model for atmospheric co 2 over phanerozoic time a new model has been constructed for calculating the level of atmospheric co sub 2 over phanerozoic time which is much simpler mathematically than the blag model but more complex geologically and biologically results suggest that there has been a notable pattern of varying atmospheric co sub 2 level over the past 570 my with high levels during the mesozoic and early paleozoic and low levels during the permo carboniferous and late cenozoic sensitivity analysis shows that within reasonable limits of the principal governing parameters this qualitative trend of varying co sub 2 is relatively insensitive to the actual values chosen for these parameters causes of the co sub 2 variation are multiple and no single geological or biological process can be called upon to explain all co sub 2 variation for all time the calculated trend of co sub 2 over time agrees well with independent estimations of paleoclimates thus the greenhouse theory of paleoclimate on a long geological time scale is supported by the results of the present study

geochemistry of dikes and lavas from the north wall of the hess deep rift insights into the four dimensional character of crustal construction at fast spreading mid ocean ridges 1 an investigation of 1 m y old dikes and lavas from the north wall of the hess deep rift 2 15 n 101 30 w collected during alvin expeditions provides a detailed view of the evolution of fast spreading oceanic crust the study area encompasses 25 km of an east west flow line representing 370 000 years of crustal accretion at the east pacific rise samples analyzed exhibit depleted incompatible trace element abundances and ratios la sm n 1 indices of fractionation mgo and incompatible element ratios la sm nb ti show no systematic trends along flow line rather over short 4 m and long 25 km distances significant variations are observed in major and trace element concentrations and ratios modeling of these variations attests to the juxtaposition of dikes of distinct parental magma compositions these findings combined with studies of segmentation of the subaxial magma chamber and lateral magma transport in dikes along rift dominated systems suggest a more realistic model of the magmatic system underlying the east pacific rise relative to the commonly assumed two dimensional model in this model melts from a heterogeneous mantle feed distinct portions of a segmented axial magma reservoir dikes emanating from these distinct reservoirs transport magma along axis resulting in interleaved dikes and host lavas with different evolutionary histories this model suggests the use of axial or flow line lava compositions to infer the evolution of axial magma chambers should be approached with caution because dikes may never erupt lava or may transport magma significant distances along axis and erupt lavas far from their axial magma chamber of origin

measuring sediment transport and bed disturbance with tracers on a mixed beach abstract a new tracer deployment method is applied to the problem of resolving rates and patterns of littoral sediment transport on mixed beaches we applied the method at the morphologically complex meso tidal beach fringing the elwha river delta washington clasts were transformed into tracers using radio frequency identifier rfid tags these tracers were deployed in and on the bed along a cross shore oriented beach transect along and cross shore variations in tracer velocity were used to estimate sediment transport patterns including the depth and cross sectional area of the disturbed bed and bulk alongshore transport rates we found that the peak bed disturbance averaged 22 of the tidal maximum significant wave height and that tracer velocity declined approximately logarithmically with depth in the bed across all deployments the maximum alongshore tracer velocity occurred between 1 0 and 2 0 m above mean lower low water mllw corresponding to elevations 0 3 m below and 0 7 m above mean sea level msl at this location alongshore bulk sediment transport rates estimated from the advection of tracers ranged from 1 1 to 173 1 m3 d under significant wave heights of 0 1 to 2 1 m and these transport rates related linearly to measured wave energy transport both the volumetric sediment transport rates and a cerc type k coefficient derived from the results agreed well with results from other published studies

o and h stable isotope compositions of oceanic crust and upper mantle rocks exposed in the hess deep near the galapagos triple junction abstract a large variety of rocks consisting of basalts dolerites gabbros and ultramafics are exposed on the intra rift ridge in the hess deep representing a section of the oceanic crust formed at the east pacific ridge 1 we conducted a stable isotope study of o and h to assess the nature of the interaction between seawater and the oceanic crust in this region fresh basalt and dolerite samples have magmatic morb values 18o 6 0 105 d 70 and low h2o content 57 and h2o contents 1 3 most gabbros have 18o 5 6 essentially identical to their initial magmatic values their d values are generally higher than 57 suggesting that they have reacted with some seawater derived fluids as is also suggested by their mineralogy in gabbros displaying little or no visual evidence of alteration of their plagioclases and pyroxenes 18oplagioclase pyroxene is small 0 3 and consistent with magmatic values the calcic an 50 plagioclases have magmatic 18o 5 8 albitized prehnitized plagioclases have variable 18o values 4 8 it also appears that the 18o modified gabbros are those in which plagioclases are transformed into albite and or prehnite which are characteristic of greenschist facies conditions the lack of calcic an 50 plagioclases depleted in 18o and hornblende suggests that most of the gabbros did not react extensively under amphibolite facies conditions with seawater derived fluids t 400 c or that they were subsequently altered to greenschist facies gabbros the serpentinized peridotites were depleted in 18o 3 3 4 9 in one sample the 18oserpentine magnetite 5 7 corresponds to a temperature of about 325 50 c these values are compatible with serpentinization by seawater derived fluids at temperatures between 200 and 350 c another sample has a higher 18o value of 10 corresponding to a serpentinization temperature of less than 50 c the 18o mass balance between seawater and the oceanic crust seems to have attained steady state where enrichment in 18o of metabasalts is balanced by the depletion in 18o of metadolerites the contribution of the gabbros is minor the depletion in 18o of the serpentinized peridotites is not taken into account since we believe that the high temperature serpentinization is characteristic of the hess deep and cannot be extrapolated to the normal oceanic crust

volatiles co2 s f cl br in the dike gabbro transition zone at iodp hole 1256d magmatic imprint versus hydrothermal influence at fast spreading mid ocean ridge abstract the dike gabbro transition zone of mid ocean ridges mors is a critical site for the accretion of oceanic crusts as it is the main zone of interactions between the hydrothermal and magmatic systems in this study volatiles contents of co 2 s f cl and br for a variety of lithologies from the dike gabbro transition zone of an intact oceanic crust were investigated in order to examine the magmatic imprint versus hydrothermal influence the studied samples include plutonic rocks gabbros diorites and tonalites hornfelses i e granoblastic dikes an amphibole rich vein and albitites these rocks were recently sampled by the iodp integrated ocean drilling program at site 1256 representing parts of an oceanic crust that formed at the fast spreading ridge of the east pacific rise the bulk co 2 contents 500 10 000 ppm show no lithological dependence and are similar to those of fresh lavas highly variable bulk s concentrations 3 900 ppm show significant depletions compared to undegassed morb melts which might be caused by exsolution of magmatic fluids during crystallization the plutonic samples have cl concentrations of 500 1000 ppm remarkably higher than the hornfelses 200 600 ppm cl and erupted lavas average 200 ppm cl except for the albitites that contain very low f 20 30 ppm and the hydrothermal amphibole rich vein that contains extremely high f 1000 ppm the other lithologies have similar f concentrations within 100 350 ppm the br concentrations of different lithologies are largely overlapping within 0 5 3 5 ppm petrological and mineralogical evidence indicates that bulk f concentrations are dominated by the presence of amphibole and apatite whereas cl and br are mainly hosted in fluid inclusions rather than in hydrous minerals the variation of f cl and br cl ratios may trace the mixing between morb magmas and seawater derived fluids crystallization of apatite and amphibole and or extraction of magmatic fluids our data support the current s budget of oceanic crust but indicate that the budgets of c and cl may have been underestimated

geology of oil shale deposits within the narrows graben queensland australia the narrows beds are a tertiary sedimentary sequence containing oil shale seams which were deposited in a narrow north northwest trending graben the graben is about 24 mi 38 km long by 3 mi 5 km wide and is located adjacent to the central queensland coast two similar oil shale deposits rundle and stuart containing 5 1 billion bbl of shale oil have been defined within the graben the tertiary sedimentary rocks which were deposited in a freshwater lacustrine environment are more than 3 280 ft 1 000 m thick and dip gently westward they consist of the worthington beds and the rundle and curlew formations the worthington beds range from conglomerate through sandstone to claystone the rundle formation consists of six kerogenous seams and a claystone unit the sedimentary rocks are consistent throughout the graben the curlew formation is an interbedded sequence of claystone and minor carbonaceous shale the worthington beds and the rundle formation have been intruded by olivine dolerite which is not extensive quaternary overburden blankets most of the area paleontologic studies indicate an early to late eocene age for the rundle and curlew formations

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

a cenozoic seawater sr ca record from benthic foraminiferal calcite and its application in determining global weathering fluxes a cenozoic multi species record of benthic foraminiferal calcite sr ca has been produced and is corrected for inter specific offsets typically less than 0 3 mmol mol and for the linear relationship between decreasing benthic foraminiferal sr ca and increasing water depth the water depth correction determined from holocene late glacial maximum and eocene paleowater depth transects is not vert similar0 1 mmol mol km the corrected cenozoic benthic foraminiferal sr ca record ranges from 1 2 to 2 0 mmol mol and has been interpreted in terms of long term changes in seawater sr ca enabling issues related to higher resolution variability in sr ca to be ignored we estimate that seawater sr ca was not vert similar1 5 times modern values in the late cretaceous but declined rapidly into the paleogene following a minimum in the eocene seawater sr ca increased gradually through to the present day with a minimum superimposed on this trend centered in the late miocene by assuming scenarios for changing seawater calcium concentration and using published carbonate accumulation rate data combined with suitable values for sr partition coefficients into carbonates the seawater sr ca record is used to estimate global average river sr fluxes these fluxes are used in conjunction with the seawater strontium isotope curve and estimates of hydrothermal activity tectonic outgassing to calculate changes in global average river 87sr 86sr through the cenozoic the absolute magnitude of sr fluxes and isotopic compositions calculated in this way are subject to relatively large uncertainties nevertheless our results suggest that river sr flux increased from 35 ma to the present day roughly two fold accompanied by an overall increase in 87sr 86sr by not vert similar0 to 0 001 between 75 and 35 ma river 87sr 86sr also increased by not vert similar0 001 to 0 002 but was accompanied by a decrease two to three fold in river sr flux

hydrothermal circulation through mid ocean ridge flanks fluxes of heat and magnesium thermally driven convection of seawater occurs through oceanic crust of all ages at the seafloor spreading axis on mid ocean ridge flanks and in the ocean basins at the ridge axis and on the flanks circulating seawater produces a major chemical exchange between the oceans and the crust based on heat budget constraints and the composition of hot springs between 10 and 40 of the river flux of mg can be taken up during high temperature alteration of the basaltic crust along the ridge axis most of the hydrothermal heat loss however occurs on the mid ocean ridge flanks where the temperatures are lower and the seawater flux correspondingly larger the estimated heat loss on the flanks is so large that upwelling must occur over a large fraction 5 30 of the seafloor less than 65 ma in age if temperatures are 20 c and seepage velocities are on the order of 10 to 100 cm y the circulating seawater needs to lose on average less than 1 2 of its mg content in order to solve the mg mass balance for the oceans chemical fluxes through mid ocean ridge flanks are poorly known because of the wide range of crustal conditions that prevail there and the paucity of study to date the most critical parameter for characterizing crustal conditions is temperature in basement which is a function of crustal age basement topography and sediment thickness and permeability basement temperature which can usually be inferred from heat flow and seismic reflection surveys largely determines the change in the composition of seawater circulating through basement this change can be inferred in turn from profiles of sediment porewater chemistry all sites studied to date with temperatures at the sediment basement interface 25 c have a large component of advective heat loss and show only a small 80 mg loss both types of sites may be important for chemical fluxes whether the cooler sites are important depends on how much the seawater changes in composition as it circulates through the crust only small changes are needed whether the warmer sites are important depends on how much heat is lost by advection in this type of setting the warmer sites could produce significant chemical fluxes even if they are scarce they could account for the entire river input of mg to the oceans even if they represent only 8 20 of the total advective heat loss on ridge flanks

evidence for glacial control of rapid sea level changes in the early cretaceous lower cretaceous bulk carbonate from deep sea sediments records sudden inputs of strontium resulting from the exposure of continental shelves strontium data from an interval spanning 7 million years in the berriasian valanginian imply that global sea level fluctuated about 50 meters over time scales of 200 000 to 500 000 years which is in agreement with the exxon sea level curve oxygen isotope measurements indicate that the growth of continental ice sheets caused these rapid sea level changes if glaciation caused all the rapid sea level changes in the cretaceous that are indicated by the exxon curve then an antarctic ice sheet may have existed despite overall climatic warmth

deep crustal seawater penetration and circulation at ocean ridges evidence from the oman ophiolite abstract detailed field mapping of the plutonic sequence and sheeted dyke complex in several areas of the sumail ophiolite nappe mainly in the haylayn block wadi haymiliyah area and comparisons with other well studied areas has shown that early hydrothermal circulation affected the whole crust down to the petrological moho five major vein systems have been distinguished in the field and their mineralogy is as follows amphibole zoisite epidote epidote quartz and prehnite vein systems the amphibole veins occur in the lower plutonic sequence and are interpreted as the fossil trace of a cracking front at the interface between a hydrothermal and a magmatic system the plutonic sequence and the zoisite epidote and epidote vein systems were formed in an off axis environment after the formation of the magma chamber in the sheeted dyke complex the epidote and quartz sulphide veins are restricted to the dyke margins and represent early hydrothermal circulation at the top of a magma chamber these two systems formed contemporaneously the quartz sulphide veins marking zones of mixing with colder seawater in the stockwork zone prehnite veins along with minor calcite veinlets are the fossil trace of the latest hydrothermal circulations and occur down to the petrological moho in a diffused system shear zones are rare in the whole ophiolite and are essentially developed in the northern fizh block diffusing from these veins seawater penetrates the crust and induces retrograde metamorphic transformations mineralogical and petrological data show that the entire crust down to the petrological moho has been affected by hydrothermal metamorphism pervasive in the sheeted dyke complex more localized around veins in the underlying plutonic unit the mineralogical parageneses in the sheeted dyke complex are typical of the greenschist facies albite chlorite actinolite and sphene whereas the high level isotropic gabbros are characterized by the association of calcic plagioclase and actinolite actinolitic facies retrogressive in the greenschist facies the almost unaltered underlying units although all samples show traces of metamorphic alteration exhibit mineral parageneses typical of the amphibolite to greenschist facies the main crustal transformations during this metamorphism are hydration a na gain and leaching of ca superimposed ca rich and na mg poor epidotised zones in the sheeted dyke complex are produced by ca rich upflowing discharge fluids high cl contents were found in the amphibole veins suggesting that they formed during the circulation of seawater or of its modified equivalent the hydration of the wadi haymiliyah area plutonic sequence and sheeted dyke complex haylayn block was calculated as requiring a minimum seawater consumption of a column of 260 m fluids trapped in minerals during their growth quartz epidote amphiboles calcite and sphalerite or during a fracturing event quartz and plagioclase consist of aqueous solutions with an average salinity close to that of seawater 3 2 nacl equivalent thermal gradients deduced from fluid inclusion microthermometric studies decrease exponentially with depth and vary between 300 c km in the volcanic unit and 30 c km in the plutonic sequence with temperatures up to 530 c circulation was active until cooling of the crust to a general thermal gradient of 30 c km